Functionally Modified Cellulose Nanocrystals as an Adsorbent for Anionic Dyes
Ming Li, ShiYu Fu*
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, Guangdong Province, 510640, China
Abstract: Cellulose nanocrystal was modified with poly(N,N-diethy-laminomethyl methacrylate) to prepare an adsorbent containing amine groups for removing anionic dyes from waste water. The prepared adsorbent was characterized by Fourier-transform infrared spectrometry (FT-IR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The adsorption was affected by various factors, such as the contact time, adsorbent dosage, dye solution pH value, initial dye concentration, and ionic strength. The results revealed that amine functional groups mainly contribute to the adsorption of azo dyes (AO7). The adsorbent showed pseudo-second-order adsorption kinetics, indicating that the dye molecules were chemisorbed on the adsorbent. The adsorption isotherm was found to fit better with the Langmuir isotherm model than with the Freundlich isotherm model.
Keywords: cellulose nanocrystal; anionic dye; adsorption kinetic; adsorption isotherm
Ming Li, PhD candidate;
E-mail: li163ming@163.com
*Corresponding author:
ShiYu Fu, professor, PhD tutor; research interests: pulping chemistry and nanocellulose materials;
E-mail: shyfu@scut.edu.cn
1 Introduction
Water contamination due to the discharge of dyes into water bodies has grown drastically over the past few years. Dye waste water contains a large amount of toxic azo dyes. Azo dye molecules in dye waste water are decomposed into aromatic amines and aromatic derivatives. These aromatic compounds have long been identified as carcinogenic and mutagenic and caused grievous injury to aquatic organisms and human life[1-2]. Therefore, it is essential to remove azo dyes from dye waste water before discharging it into water bodies.
Many physical/chemical methods such as coagulation/flocculation[3], anion exchange[4-5], chemical precipitation[6], electrochemical processes[7], membrane technology[8], and adsorption processes[9] have been reported for removing azo dyes from waste water. However, some of these methods are not efficient for practical applications; for example, the electrochemical and membrane methods are expensive, while the coagulation/flocculation, precipitation, and oxidation processes produce large amounts of toxic sludge, causing secondary pollutant that need further treatment. The adsorption process is one of the most common and effective methods used to remove dyes from dye waste water. Adsorbents play the most important role in the adsorption process. Many materials such as activated carbon[2], chitosan[10], and cellulose[11] can be used as adsorbents via functional modification. Some modified materials possessing functional groups such as carboxylic[3], amine[11], and amidoxime[12] can be used as adsorbents for the removal of harmful ions from aqueous solutions. These functional groups promote both physical and chemical adsorption, and hence improve the adsorption efficiency of adsorbents. Recently, the application of grafted materials for dye adsorption has received considerable attention[1,8-10]. Zhou et al[13] synthesized ethylenediamine-modified magnetic chitosan nanoparticles via a complex route for removing AO7 from aqueous solutions. Kiransan et al prepared modified montmorillonite containing quaternary ammonium groups using cetyltrimethylammonium bromide[14]. Noorimotlagh et al[2] prepared active nano-porous carbon from milk vetch by a complex process. This carbon has nano-sized pore structure, and the pore size of 67% of the pores in the carbon is less than 100 nm. Unfortunately, the disadvantage of these adsorbent materials is that their preparation processes are very complicated and expensive. High-cost and stringent preparation conditions encounter significant challenges for widespread application of dye adsorbents. Hence, it is imperative to develop simple and cost-effective method to synthesize adsorbents for dye waste water treatment.
Cellulose Nanocrystal (CNC) is the most commonly used natural polymer material and possesses a large amount of active hydroxyl groups. Hence, functional monomers can be easily grafted onto it to synthesize multifunctional materials. N,N-diethylaminomethyl methacrylate (DMAEMA) is a tertiary amine-containing acrylic monomer that can be easily polymerized by various reactions such as Atom Transfer Radical Polymerization (ATRP)[15] and Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT)[16]. In this work, DMAEMA was polymerized on the surface of CNC by ATRP. The functionalized CNC was used as an adsorbent for acid orange 7 (AO7) dye. The modified CNC was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). Furthermore, the adsorption behavior of the CNC adsorbent was investigated under equilibrium and dynamic conditions.
2 Materials and methods
2.1 Chemicals and instrument
2-Bromoisobutyryl bromide (BIBB), N-methyl-2-pyrrolidone (NMP), CuBr (99%, Energy Chemical), 2-(Dimethyl amino) pyridine (DMAP), AO7, N, N, N′, N′′, N′′-pentamethyl diethylenetriamine (PMDETA), and 2-(diethylamino) ethylmethacrylate (DMAEMA, 99%) were purchased from Shanghai Aladdin Co., Ltd., China. N,N-dimethylformamide (DMF) and triethylamine (TEA, AR) were obtained from Tianjin Chemical Company, Tianjin, China. The chemical structure and characteristics of AO7 are shown in Table 1. DMF was dried by molecular sieves (4 ) before use. CuBr was purified by acetic acid and ethanol. CNC was prepared using a method reported by us previously[17]. FT-IR (VERTEX 70, Bruker, Germany), XPS (Kratos Axis Ultra DLD, Kratos, England), and TGA (TA Q500, USA) were used to determine the chemical structure and the surface elemental composition of the CNC samples. An ultraviolet-visible (UV-vis) spectrophotometer (G1369C, Agilent, China) was employed to determine the concentration of dye in the dye solution.
2.2 Preparation of CNC-based adsorbent
The CNC-based adsorbent was synthesized according to a previously reported method[18]. The synthesis route, which involved the ATRP of DMAEMA on CNC, is shown in Scheme 1. First, the initiator was fixed on the surface of CNC. CNC (3.24 g), NMP (200 mL), DMAP (0.02 mol), and TEA (0.03 mol) were added in a flask filled with Ar. BIBB (0.12 mol) was added to this flask over 1 h. The reaction system was then placed in an ice-water bath for 24 h. The resulting product, CNC-Br, was thoroughly washed and freeze-dried. CNC-Br (0.4 g) was then added to a flask containing DMF (30 mL), DMAEMA (0.05 mol), CuBr (0.5 mmol), and PMDETA (0.5 mmol). The polymerization reaction was carried out at 60℃ for 6 h under stirring after executing three freeze-evacuate-thaw cycles. The white product so obtained was thoroughly washed and extracted with a mixture of water and ethanol (water∶ethanol=1∶1 (V/V)) for 4 h and was finally freeze-dried.
Scheme 1 Synthesis route (a) and schematic (b) of the
CNC-based adsorbent
2.3 Adsorption experiment
Batch experiments were conducted to investigate the effects of the contact time, dye solution temperature, ionic strength, initial pH value, and adsorbent dosage on the adsorption capacity of the adsorbent. A certain amount of adsorbent was added to the dye solution under stirring at 400 r/min for a predetermined time and was then removed by centrifugation. The residual concentration of AO7 in the dye solution after the adsorption was determined at 485 nm using a UV-vis spectrophotometer. The residual dye concentration was determined from a standard curve derived from a series of dye solutions with a known dye content. The adsorbent dosage in the AO7 dye (400 mg/L) solution was varied from 10 mg to 80 mg and the solution temperature was maintained at 20℃. The effect of the initial pH value on the dye removal efficiency of the adsorbent was investigated by varying the initial pH value (which was adjusted with 0.1 mol/L HCl or NaOH at the beginning) of the solution. In order to investigate the effects of the contact time and ionic strength on the adsorption of AO7 molecules onto the CNC adsorbent, different contact times (4, 8, 12, 16, 20, 30, and 40 min) and NaCl concentrations (0.001~0.1 mol/L) were used. To determine the adsorption capacity of the CNC-based adsorbent at different temperatures (i.e. 20℃ and 30℃), the initial concentration of AO7 in the adsorption medium (pH=2.0, t=20 min) was varied from 100 mg/L to 700 mg/L. All the experiments were carried out in duplicate. The dye removal efficiency of the adsorbent was calculated using Equation (1):
Where, C0 and Ct are the concentrations (mg/L) of dye initially and at time t, respectively. The amount of dye adsorbed per unit mass of the adsorbent at equilibrium, Qe (mg/g), was calculated using Equation (2):
Where, Qe is the dye adsorbed at equilibrium, Ce is the equilibrium dye concentration (mg/L), and m and V are the mass of the adsorbent (g) and volume of the dye solution (L), respectively.
3 Results and discussion
3.1 Characterization of CNC-Br and CNC-PDMAEMA
FT-IR and XPS analyses were carried out to examine the chemical structure and surface elements of CNC-Br and CNC-PDMAEMA. BIBB was anchored on the surface of CNC by esterification to prepare the ATRP initiator. As shown in Fig.1(b), unlike CNC, CNC-Br showed a strong FT-IR peak at 1740 cm1. This peak corresponds to the stretching vibration of C=O from the ester group[19]. A strong peak at 1280 cm1 corresponding to C—O was observed in the FT-IR spectrum of CNC-Br (Fig.1(b))[20]. The XPS spectrum of CNC-Br (Fig.2) showed a Br3d peak at 68 eV. It can be observed from Table 2 that the Br content in CNC-Br was 3.01%. These results suggest that BIBB was successfully grafted onto the CNC surface (reaction is shown in Scheme 1).
Fig.1 FT-IR spectra of (a) CNC, (b) CNC-Br, and
(c) CNC-PDMAEMA
After the polymerization reaction, the intensity of the peak at 1740 cm1 increased (Fig.1(b) and Fig.1(c)) because of the additional carbonyl groups from PDMAEMA. The peaks at 2946, 2822, and 2773 cm1 can be attributed to the C—H groups of PDMAEMA. The peak at 1460 cm1 corresponds to the C—C stretching vibration, while that at 1150 cm1 corresponds to the stretching vibration of C—N from the —N(CH3)2— group. From Fig.2 it can be observed that after the polymerization, new resonance signals were generated attributing to the carbon of CNC-PDMAEMA. After the ATRP reaction of CNC-PDMAEMA, the characteristic peak of N was detected at 396 eV (Fig.2). In addition, the nitrogen content in CNC-PDMAEMA increased sharply (5.03%, N1s), compared with that in CNC-Br (Table 2). These results confirm that PDMAEMA was successfully grafted onto the surface of CNC.
Table 2 Elemental surface composition of CNC-Br and CNC-PDMAEMA, as determined by XPS
Samples Composition/%
C1s O1s N1s Br3d
CNC-Br 53.66 43.22 0.11 3.01
CNC-PDMAEMA 67.73 25.62 5.03 0.49
The TG/differential TG (DTG) curves for CNC, CNC-Br, and CNC-PDMAEMA are shown in Fig.3. For CNC, the initial weight loss was observed at 240℃. This weight loss can be mainly attributed to the carbonization and volatilization of small molecular compounds and water[21]. The thermal decomposition of CNC-Br began at 170℃ (lower than the initial decomposition temperature of CNC) because of the release of HBr formed during the heating. The TG and DTG curves of CNC-PDMAEMA show that the weight loss occurred in three stages. In the first stage (T1=220~290℃), almost 63% weight loss was observed because of the pyrolytic decomposition of PDMAEMA that was grafted onto CNC. In the second stage of decomposition (T2=330℃), the weight loss occurred because of the splitting of the cellulose structure and main-chain scission. A total weight loss of about 95% was observed in the final stage (T3=420℃) attributing to the formation of large amounts of volatile compounds and solid char from the decomposed cellulose units[22].
3.2 Dye adsorption properties
3.2.1 Effects of adsorbent dosage and dye solution pH value
The dye (AO7) removal efficiency of CNC at various adsorbent dosages (10~80 mg) and dye solution pH values was measured (Fig.4). The dye removal efficiency increased with an increase in the adsorbent dosage. For a fixed initial dye concentration, an increase in the adsorbent dosage increased the adsorption area and the number of adsorption sites[2,23]. It should be noted that at the pH values of 2.0 and 4.0, an increase in the adsorbent dosage beyond 60~80 mg resulted in an insignificant increase in the AO7 removal efficiency. These results are consistent with those reported previously[2,12,24].
Moreover, it was found that AO71R2) of PDMAEMA on the surface of CNC acted as the binding sites for anions. Under1R2H+ groups, and the electric repulsion between the —NR1R2H+ groups restrained the tendency of the polymer chains to collapse or aggregate in solution[19]. However, under alkaline conditions, the tertiary amine groups could not protonate and the uncharged polymer chains collapsed or aggregated. Hence, PDMAEMA grafted on the CNC adsorbent became positively charged and dissolved completely at pH=2.0 and 4.0. On the other hand, at pH=8.5, the PDMAEMA chains remained uncharged and collapsed. And acidic environment increased the number of adsorption sites and the contact area between the adsorbent and azo ions. Therefore, from Fig.4 it can be observed that the AO7 removal efficiency of the CNC-PDMAEMA adsorbent at pH=2.0 and 4.0 was far greater than that at pH=8.5 when the other conditions were the same.
3.2.2 Effect of ionic strength
The effect of ionic strength on the AO7 adsorption of CNC-PDMAEMA was investigated by carrying out dye degradation at various NaCl concentrations. As can be seen from Fig.5, as the ionic strength increased from 0.001 mol/L to 0.2 mol/L, the AO7 removal efficiency decreased from 73.6% to 26.3%. This can be attributed to the competition of Cl with azo anions and other negative ions for adsorption sites. The presence of NaCl in high concentrations suppressed the AO7 adsorption[22].
3.2.3 Effect of the contact time and kinetic study
The effect of the contact time on the AO7 removal efficiency of the CNC-based adsorbent was investigated at two different temperatures (i.e. 20℃ and 30℃) and the results are shown in Fig.6. As can be seen from Fig.6, at the beginning of the adsorption, the AO7 removal efficiency increased rapidly with time. After 16 min, the adsorption process reached equilibrium. At this equilibrium, the AO7 removal efficiency did not change significantly with time. Initially, the adsorption mainly occurred on the surface of the CNC-based adsorbent. After that, the adsorption occurred on the inner surface of PDMAEMA[25], which was dominated mainly by the electrostatic interactions between azo ions and the amine groups on PDMAEMA.
Adsorption kinetics provide essential information regarding the adsorptionQeQt)=lnQek1·t (3)
Where, k1 is the pseudo-first-order rate constant, Qe and Qt (mg/g) are the amounts of dye adsorbed at equilibrium and time t (min), respectively.
Pseudo-second-order: t/Qt=1/(k2·Qe2)+t/Qe (4)
Where, k2 is the rate constant for the pseudo-second-order kinetics.
The adsorption kinetics of CNC-PDMAEMA based on the pseudo first- and second-order models are shown in Fig.7, and the estimated kinetic parameters are listed in Table 3. At the tested temperatures, the R2 for the second-order model was >0.99, while that for the first-order model was found to be within the range 0.84~0.88. Moreover, the Qe,cal values (81.56 mg/g at 20℃, 89.58 mg/g at 30℃) for the pseudo-first-order model were much lower than the Qe,exp values (99.69 mg/g at 20℃, 114.80 mg/g at 30℃). On the other hand, the Qe,cal values (111.11 mg/g at 20℃, 125.00 mg/g at 30℃) for the second-order model matched well with the Qe,exp values. This indicates that the adsorption kinetics of CNC-PDMAEMA were better described by the pseudo-second-order model. This suggests that the adsorption of the anionic dye onto the CNC-based adsorbent was mainly controlled by chemisorption[25-26]. The pseudo-second-order kinetics of dye adsorption have also been reported for other biomass-based materials[25, 27].
3.2.4 Effect of the initial dye concentration and adsorption isotherm
Fig.8(a) shows the effects of the initial AO7 concentration and temperature on the adsorption. The adsorption of the dye at different concentrations increased rapidly in the initial stages and decreased gradually as the adsorption progressed until the equilibrium was reached. The initial concentration provided a driving force to overcome all the mass transfer resistances (between the aqueous and solid phases) of the dye. Hence, high initial dye concentrations increased the adsorption capacity[26]. Additionally, an increase in temperature (from 20℃ to 30℃) also showed a positive effect on the adsorption. This is because the temperature increase accelerated the dye molecular diffusion rate and promoted the adsorption of AO7.
Adsorption isotherm models are used to describe the interaction between an adsorbate and adsorbent. In this study, the adsorption of AO7 onto the CNC-based adsorbent was evaluated using the Langmuir and Freundlich isotherm models. The Langmuir isotherm model, which assumes monolayer adsorption with uniform energies of adsorption on the surface, can be expressed as Equation (5):
Ce/Qe=Ce/Qm+1/(Qm·KL) (5)
Where, Qe (mg/g) and Ce (mg/L) are the amounts of dye adsorbed per unit weight of biomass and dye equilibrium concentration in solution, respectively. Qm (mg/g) denotes the monolayer adsorption capacity of the adsorbent and KL is the Langmuir constant.
The Freundlich model is based on multilayer adsorption with the adsorption energy decreasing with the surface coverage. It is expressed as Equation (6):
lnQe=·lnCe·lnKF (6)
Where, KF is the Freundlich constant and n is an empirical parameter relating the adsorption capacity and adsorption intensity, which varies with the heterogeneity of the material.
The adsorption isotherms are shown in Fig.8(b) and Fig.8(c) and the model parameters are summarized in Table 4. As we can see, the Langmuir isotherm model fitted the experimental data well (high correlation coefficients, R2=0.99). The theoretical maximum adsorption capacity (Qe,cal=136.43 mg/g at 20℃, Qe,cal=162.87 mg/g at 30℃) matched well with the experimental values (Qe,exp=123.95 mg/g at 20℃, Qe,exp=152.93 mg/g at 30℃). Therefore, it can be stated that the adsorption of AO7 onto CNC-PDMAEMA showed a monolayer uniform adsorption behavior[28-29]. Moreover, the Freundlich isotherm model also described the adsorption accurately. The values of n were in the range of 1~10, indicating a favorable adsorption process[4,13,30].
4 Conclusions
In this study, we demonstrated that poly(N,N-diethylaminomethylmethacrylate) (PDMAEMA)-functionalized CNC (via ATRP) can be effectively used for removing AO7 from aqueous solutions. The CNC-PDMAEMA adsorbent was characterized by FT-IR and XPS. The results showed that the adsorbent was successfully prepared. The CNC-based adsorbent exhibited good adsorption properties. The adsorption process depended significantly on the initial pH value of the dye solution. The pH value of 2.0 favored the adsorption process. The adsorption isotherms of the CNC-PDMAEMA adsorbent could be well described by the Langmuir model, and the adsorption capacity was found to be 162.87 mg/g at 30℃. The kinetic study results showed that the adsorption process followed pseudo-second- order kinetics. Hence, CNC-PDMAEMA is a promising adsorbent for removing anionic dyes from waste water.
Acknowledgments
This work was supported by the Science and Technology Program of Guangzhou (No. 201704020038); the foundation of State Key Laboratory of Pulp and Paper Engineering (No. 2017QN01); and National Natural Science Foundation of China (No. 31570569).
References
[1] Liu Z, Zhang F, Liu T, et al. Removal of azo dye by a highly graphitized and heteroatom doped carbon derived from fish waste: adsorption equilibrium and kinetics[J]. Journal of Environmental Management, 2016, 182: 446-454.
[2] Noorimotlagh Z, Soltani R D C, Khataee A R, et al. Adsorption of a textile dye in aqueous phase using mesoporous activated carbon prepared from iranian milk vetch[J]. Journal of the Taiwan Institute of Chemical Engineers, 2014, 45(4): 1783-1791.
[3] Martínez-Quiroz M, López-Maldonado E A, Ochoa-Terán A, et al. Innovative uses of carbamoyl benzoic acids in coagulation-flocculation’s processes of wastewater[J]. Chemical Engineering Journal, 2017, 307: 981-988.
[4] Greluk M, Hubicki Z. Efficient removal of acid orange 7 dye from water using the strongly basic anion exchange resin amberlite IRA-958[J]. Desalination, 2011, 278(1): 219-226.
[5] Zhai F, Wang Z, Liu Y. Preparation of lignin based anion exchanger for nitrate and phosphate removal[J]. Paper and Biomaterials, 2016, 1(1): 22-30.
[6] Petta L, De Gisi S, Casella P, et al. Evaluation of the treatability of a winery distillery (vinasse) wastewater by UASB, anoxic-aerobic UF-MBR and chemical precipitation/adsorption[J]. Journal of Environmental Management, 2017, 201: 177-189.
[7] Cotillas S, Sáez C, Ca Vatanpour V, Salehi E, Sahebjamee N, et al. Novel chitosan/polyvinyl alcohol thin membrane adsorbents modified with detonation nanodiamonds: preparation, characterization, and adsorption performance[J]. Arabian Journal of Chemistry, 2018.
[9] Aljeboree A M, Alshirifi A N, Alkaim A F. Kinetics and equilibrium study for the adsorption of textile dyes on coconut shell activated carbon[J]. Arabian Journal of Chemistry, 2017, 10(S2): S3381-S3393.
[10] Chen Y, Ru J, Geng B, et al. Charge-functionalized and mechanically durable composite cryogels from Q-NFC and CS for highly selective removal of anionic dyes[J]. Carbohydrate Polymers, 2017, 174: 841-848.
[11] Jin L, Li W, Xu Q, et al. Amino-functionalized nanocrystalline cellulose as an adsorbent for anionic dyes[J]. Cellulose, 2015, 22(4): 2443-2456.
[12] Jin L, Sun Q, Xu Q, et al. Adsorptive removal of anionic dyes from aqueous solutions using microgel based on nanocellulose and polyvinylamine[J]. Bioresource Technology, 2015, 197: 348-355.
[13] Zhou L, Jin J, Liu Z, et al. Adsorption of acid dyes from aqueous solutions by the ethylenediamine-modified magnetic chitosan nanoparticles[J]. Journal of Hazardous Materials, 2011, 185(2/3): 1045-1052.
[14] K1ranan M, Soltani R D C, Hassani A, et al. Preparation of cetyltrimethylammonium bromide modified montmorillonite nanomaterial for adsorption of a textile dye[J]. Journal of the Taiwan Institute of Chemical Engineers, 2014, 45(5): 2565-2577.
[15] Wang M, Yuan J, Huang X, et al. Grafting of carboxybetaine brush onto cellulose membranes via surface-initiated ARGET-ATRP for improving blood compatibility[J]. Colloids and Surfaces B Biointerfaces, 2013, 103(1): 52-58.
[16] Roy D, Guthrie J T, Perrier S. Synthesis of natural-synthetic hybrid materials from cellulose via the raft process[J]. Soft Matter, 2008, 4: 145-155.
[17] Hu Z, Patten T, Pelton R, et al. Synergistic stabilization of emulsions and emulsion gels with water-soluble polymers and cellulose nanocrystals[J]. ACS Sustainable Chemistry & Engineering, 2015, 3: 1023-1031.
[18] Liu P, Chen Q, Wu S, et al. Surface modification of cellulose membranes with zwitterionic polymers for resistance to protein adsorption and platelet adhesion[J]. Journal of Membrane Science, 2010, 350(1): 387-394.
[19] Tang J, Lee M F, Zhang W, et al. Dual responsive pickering emulsion stabilized by poly[2-(dimethylamino)ethyl methacrylate] grafted cellulose nanocrystals[J]. Biomacromolecules, 2014, 15(8): 3052-3060.
[20] Yi, J, Xu Q, Zhang X, et al. Temperature-induced chiral nematic phase changes of suspensions of poly(N,N-dimethylaminoethyl methacrylate)-grafted cellulose nanocrystals[J]. Cellulose, 2009, 16(6): 989-997.
[21] Majoinen J, Walther A, McKee J R, et al. Polyelectrolyte brushes grafted from cellulose nanocrystals using Cu-mediated surface-initiated controlled radical polymerization[J]. Biomacromolecules, 2011, 12(8): 2997-3006.
[22] Anirudhan T S, Nima J, Divya P L. Adsorption of chromium(VI) from aqueous solutions by glycidylmethacrylate-grafted-densified cellulose with quaternary ammonium groups[J]. Applied Surface Science, 2013, 279(279): 441-449.
[23] Zhou Y, Jin Q, Zhu T, et al. Adsorption of chromium (VI) from aqueous solutions by cellulose modified with -CD and quaternary ammonium groups[J]. Journal of Hazardous Materials, 2011, 187(1): 303-310.
[24] Kousha M, Daneshvar E, Sohrabi M S, et al. Adsorption of acid orange II dye by raw and chemically modified brown macroalga stoechospermum marginatum[J]. Chemical Engineering Journal, 2012, 192(2): 67-76.
[25] Hamzeh Y, Ashori A, Azadeh E, et al. Removal of acid orange 7 and remazol black 5 reactive dyes from aqueous solutions using a novel biosorbent[J]. Materials Science and Engineering C, 2012, 32(6): 1394-1400.
[26] Kousha M, Daneshvar E, Sohrabi M S, et al. Adsorption of acid orange II dye by raw and chemically modified brown macroalga stoechospermum marginatum[J]. Chemical Engineering Journal, 2012, 192(2): 67-76.
[27] Lin Y, Fang G G, Deng Y J. Preparation and characteristics of a pH-sensitive glucose-based hydrogel[J]. Paper and Biomaterials, 2018, 3(3): 39-46.
[28] Zhu S, Yang N, Zhang D. Poly(N,N-dimethylaminoethyl methacrylate) modification of activated carbon for copper ions removal[J]. Materials Chemistry and Physics, 2009, 113(2): 784-789.
[29] Bayramoglu G, Yakuparica M. Adsorption of Cr(VI) onto PEI immobilized acrylate-based magnetic beads: isotherms, kinetics and thermodynamics study[J]. Chemical Engineering Journal, 2008, 139(1): 20-28.
[30] Mussatto S I, Fernandes M, Rocha G J M, et al. Production, characterization and application of activated carbon from brewer :s spent grain lignin[J]. Bioresource Technology, 2010, 101(7): 2450-2457.
Green Modification of Cellulose Nanocrystals and Their Reinforcement in Nanocomposites of Polylactic Acid
Xue Jiang1,2,3,*, Martin A. Hubbe4
1. Jiangsu Engineering and Technology Research Center for Functional Textiles, Jiangnan University, Wuxi, Jiangsu Province, 214122, China
2. Key Laboratory of Eco-textiles of Ministry of Education, School of Textile and Clothing, Jiangnan University, Wuxi, Jiangsu Province, 214122, China
3. State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, Guangdong Province, 510640, China
4. Department of Forest Biomaterials, College of Natural Resources, North Carolina State University, Raleigh, NC 27695-8005, USA
Abstract: Cellulose nanocrystals (CNCs) of rod-like shape were prepared from degreased cotton using sulfuric acid hydrolysis. The obtained CNC suspension was neutralized using a sodium hydroxide solution to remove the residual sulfuric acid and improve the thermal stability of the CNC particles. Then, poly(ethylene oxide) (PEO) was employed to modify the nanocrystals through entanglement and physical adsorption. The goal was to further improve the thermal stability and weaken the hydrophilicity of CNCs. Original and modified CNCs were dosed into a polylactic acid (PLA) matrix to prepare nanocomposites using a hot compression process. Results of the thermogravimetric analysis showed that the initial thermal decomposition temperature of the modified CNCs showed a 120℃ improvement compared to the original CNCs. That is, the thermal stability of the modified CNCs improved because of their shielding and wrapping by a PEO layer on their surface. Results from scanning electron microscopy and ultraviolet-visible spectrophotometry showed that the compatibility of the modified CNCs with organic PLA improved, which was attributed to the compatibility of the PEO chains adsorbed on the surface of the CNCs. Finally, the results of tensile tests indicated a significant improvement in terms of breaking strength and elongation at the break point.
Keywords: cellulose nanocrystals; thermal stability; hydrophilicity; polylactic acid; compatibility; nanocomposites
*Corresponding author:
Xue Jiang, PhD, professor;
research interests: preparation and applications of bio-based functional materials;
E-mail: jiangx@jiangnan.edu.cn
1 Introduction
During recent years, research of alternatives to petroleum-based materials has been a general trend to help alleviate the present energy shortage. Biocomposites may be regarded as an ideal type of product for the future, because they are biodegradable and recyclable. Polylactic acid (PLA) is among the most suitable matrix materials for such biocomposites because of its relatively low cost, biocompatibility, processability, and eco-friendly nature[1-2]. However, it has some drawbacks at the same time that limit its use in certain applications. Although its tensile strength and elastic modulus are comparable to poly(ethylene terephthalate) (PET), it is too brittle for most uses[3]. Therefore, it is essential to find appropriate fillers to increase the toughness of PLA materials to expand their applications.
Cellulose nanocrystals (CNCs) have aroused much attention in the field of nanocomposites given their biocompatibility, biodegradability, light weight, nano-scale effects, low cost, high specific strength and modulus, unique morphology, and relatively reactive surface[4-7]. Nevertheless, there are two challenges to overcome in the application of CNCs in the field of nanocomposites. First, a large number of hydroxyl groups on the surface of CNCs make the nanoparticles present hydrophilic property, which limits the compatibility between hydrophilic CNCs and a hydrophobic polymeric matrix[8-10]. In addition, the preparation of CNCs based on sulfuric acid hydrolysis is considered the most main stream approach. Negatively charged sulfate esters are introduced on the surface of the obtained CNC particles when using the sulfuric acid as a hydrolyzing agent. The charged sulfate esters can promote dispersion of the CNCs in water because the like charges repel each other. However, the introduction of charged sulfate esters diminishes the thermostability of the nanoparticles because of the catalytic nature of the sulfate esters[11-12]. The inferior thermostability of CNCs limits their use because most polymeric composites are processed at temperatures near 200℃ or above[5]. Therefore, these two challenges should be overcome in the preparation of nanocomposites reinforced by CNCs, i.e. inferior thermal stability and poor compatibility with non-polar materials.
To improve the thermal stability and weaken the hydrophilicity of CNCs, different chemical and/or physical modification strategies have been attempted. Chemical modification of the nanocrystal surface with polymer chains by “graft-onto” or “graft-from” procedures is the most common approach[13-17]. The “graft-onto” method is easier to complete compared to the “graft-from” method. However, it is difficult to achieve high grafting densities and control the length of the grafting chains[18]. The reaction process of “graft-from” is difficult to control, because it needs water- and oxygen-free conditions. An even more serious concern is that metal contamination may result because of the heavy metal catalyst. Compared to the chemical method, physical modification is much easier to complete and the environmental pollution can be greatly reduced.
Part of the approach used in the present work was based upon the concept that poly(ethyelene oxide) (PEO) may be able to function as a compatibilizer within composites prepared using a hydrophobic PLA matrix and the much more hydrophilic CNCs, particularly after modification of the CNCs to remove sulfate ester groups. Although PEO is soluble in water[19], it has well known unique characteristics. In principle, by changing the details of its helical conformation in solution[12], PEO can exhibit a more hydrophilic or hydrophobic character. Evidence of this dual nature becomes apparent in the ability of phenolic resins or other co-factors to destabilize solutions of PEO[20]. Conformational changes also might explain why PEO can drop out of solution when the temperature is increased[21]. Accordingly, it is proposed in this work that PEO, by changing its conformation, might be able to compatibilize the CNC-PLA composite system, on account of PEO’s combination of polar and non-polar characteristics, depending on the details of its conformation.
A combination of neutralization with sodium hydroxide and physical adsorption of PEO chains was innovatively adopted in this study to improve the thermal stability and reduce the hydrophilicity of CNCs. The period of the experiment was obviously shortened. Furthermore, PEO acted as a compatibilizing agent between the PLA matrix and CNCs.
2 Experimental
2.1 Materials
Medical adsorbent cotton, along with the analytical reagents sulfuric acid (98%) and sodium hydroxide (NaOH), were purchased from Sinopharm Chemical Reagent Co., Ltd. (SCRC). PEO with an average molecular weight (Mw) of 5×106 g/mol was supplied by Sigma-Aldrich and used for physical adsorption. PLA with a Mw=100,000 g/mol was purchased from Shanghai Yisheng Industry Ltd.
2.2 Preparation of CNCs
According to our previous research[22], CNCs were fabricated from cotton using sulfuric acid hydrolysis. In brief, approximately 10 g of cotton was mixed with 200 mL of sulfuric acid aqueous solution (64 wt%) in a three-neck flask, which was set with a stirrer, thermometer, and condenser. The reactive mixture was continuously stirred for 1 h at 45℃ for hydrolysis, then approximately 200 mL of cold water (approximately 0℃) was poured into the obtained suspension to stop the reaction. Subsequently, the suspension was centrifuged at 10,000 r/min until it had no obvious stratification and existed as a type of transparent dispersion. To remove the sulfate groups on the surface of CNCs, the mixture was neutralized using sodium hydroxide aqueous solution (1 wt%). Nanoparticles with a sulfate group content nSOsH=0.26 mol/kg were obtained.
2.3 Physical adsorption of PEO on the nanocrystals
PEO was added into the distilled water, followed by stirring at 500 r/min for 4 days at an ambient temperature under protection against light by aluminum foil. A specified amount of neutralized CNCs was added into the PEO solution. The mixture was freeze-dried after stirring for 5 h to obtain the CNC powder modified with PEO. The quality fraction of the PEO on the CNC surface was controlled at 35%.
2.4 Preparation of PLA nanocomposites
PLA nanocomposites reinforced with pure CNCs (p-CNC), neutralized CNCs (n-CNC) and CNCs modified with PEO (CNC/PEO) were fabricated throughout the processing. The process was conducted at 170℃ for 10 min under 40 MPa of pressure. The p-CNC, n-CNC, and CNC/PEO were utilized to reinforce the PLA matrix, which were marked as PLA/p-CNC, PLA/n-CNC, and PLA/CNC/PEO, respectively.
2.5 Characterization
Transmission electron microscopy (TEM) was performed using a JEM-2100 electron microscope at an acceleration voltage of 200 kV to characterize the morphology and distribution of CNCs before and after modification.
The thermostabilities of CNCs and PLA composites were analyzed using thermogravimetric analysis (TGA) and a differential scanning calorimeter (DSC). The thermal degradation of CNCs was analyzed using the thermal analyzer TGA/SDTA851e under nitrogen flow. Approximately 5 mg of dried samples were heated from 30℃ to 600℃ at a heating rate of 10℃/min. The thermal properties of the PLA composites were measured using a Pyris 1 DSC instrument. To eliminate the thermal history, the samples were scanned from 30℃ to 200℃ at a heating rate of 10℃/min, and maintained at 200℃ for 5 min, then cooled to 30℃ at a rate of 20℃/min and maintained for 5 min. Finally, the samples were scanned from 30℃ to 200℃ at a heating rate of 10℃/min.
X-ray diffraction (XRD) patterns of manufactured CNCs and modified CNCs were recorded using a Bruker Siemens D8 X-ray diffractometer operated at 3 kW with Cu K radiation (=0.154 nm) in the range of 2=3°~60°with a step of 0.02° and a scanning speed of 4°/min.
Contact angle measurement was performed to investigate the hydrophilicity of CNCs before and after modification, and was performed at room temperature using a DSA25S-Kruss contact angle measuring device. CNCs before and after modification were compacted under 20 MPa of pressure to obtain samples with smooth surfaces. A small drop of water (2 L) was dropped on the surface of the samples. Then, the contact angle was calculated using a sessile drop contact angle system.
The homogeneity of the PLA composites was observed using scanning electron microscopy (SEM) with a su1510 device (Hitachi Zosen Corporation) at 30 kV.
The mechanical properties of the PLA composites were investigated through tensile measurement using a universal material experiment machine. The samples were thin rectangular films with dimensions of approximately 100 mm×20 mm×0.5 mm. The drawing speed was 2 mm/min.
3 Results and discussion
3.1 Morphology and structure of the nanoparticles
TEM was performed to observe the morphology and dimensions of CNCs, as shown in Fig.1. Fig.1(a) shows TEM photos of the obtained CNC, while Fig.1(c) shows the static size distributions of the length of the nanoparticles. Rod-like nanoparticles of a length of 240~300 nm and a width of approximately 10 nm were separated from cotton using sulfuric acid hydrolysis, in agreement with the previous literature[23]. Aggregation appeared because of the hydrogen bonding between the hydroxyl groups on the surface of the nanoparticles, but many particles independently existed on account of the repelling interaction between the negative charges introduced on the surface of the CNC during sulfuric acid hydrolysis. However, the thermostability of CNCs declined with the introduction of these negatively charged sulfate ester groups. Therefore, to improve the thermostability and weaken the hydrophilicity of CNCs, the surfaces of the nanoparticles were modified to remove the sulfate ester groups and the hydrophilic hydroxyl groups.
The crystalline structures of CNCs before and after modification are shown in Fig.2. The crystallinity index (Ic) of the cellulose was approximately 87.3% for p-CNC, 87.1% for n-CNC, and 86.7% for CNC/PEO. Compared to p-CNC, the Ic of n-CNC was nearly unchanged, which showed that the neutralization with the sodium hydroxide solution did not affect the crystalline structure of CNCs. A new diffraction peak at 2=19.2°indicated the presence of PEO chains as it was attributed to the diffraction peak of PEO. The Ic of CNC/PEO declined because of the amorphous PEO chains, which corresponded to the emergence of a new diffraction peak at 2=19.2°. Nevertheless, the crystalline structures of all samples were not destroyed, because the diffraction peaks at 2 of 14.8°, 16.4°, 22.5°, and 34.5° appeared in the XRD spectra of all CNC samples, representing the crystalline structure of CNC[24].
Fig.2 XRD patterns of the p-CNC, n-CNC,
CNC/PEO, and PEO
3.2 Thermal stability of the CNCs
The thermal degradation of PEO and CNCs before and after modification are shown in Fig.3.
Fig.3 TGA thermograms for p-CNC, n-CNC,
CNC/PEO, and PEO
As shown in Fig.3, the weight loss below 100℃ for all samples was attributed to the removal of moisture. The p-CNC presented a stepwise degradation behavior. The degradation from 160℃ to 280℃ was attributed to the degradation of more accessible and therefore more highly sulfated amorphous regions, whereas the higher temperature degradation process was related to the breakdown of the unsulfated crystalline domains. The results correspond to the conclusions previously reported[14]. The onset of the thermal degradation of n-CNC was improved by approximately 120℃ in comparison to the p-CNC as a result of the sulfate groups being removed from their surface[6]. The CNC/PEO also presented a stepwise degradation behavior. The degradation from 300℃ to 350℃ corresponded to the breakdown of CNCs, and the higher temperature degradation was attributed to the breakdown of the PEO chains. The thermal stability of the nanoparticles modified with the PEO chains was further improved compared to those of p-CNC and n-CNC, which was attributed to the shielding effect of the PEO layer to the sulfate esters on the surface of CNCs.
3.3 Compatibility between CNCs and PLA matrix
To analyze the compatibility between CNCs and PLA matrix, dynamic contact angle measurement, SEM, transmittance measurement, and differential scanning calorimetry (DSC) were performed. As shown in Fig.4, the dynamic contact angle measurement was conducted to show the hydrophilicity of CNCs before and after modification. The initial contact angle of the CNC modified with PEO increased approximately 25° in comparison to the original CNC. Furthermore, the water drop sharply decreased on the surface of the orginal CNC in 5 s because of the swelling property of the cellulose by water. However, the contact angle of the CNC modified with PEO was maintained at approximately 60° over 40 s, which was attributed to the shielding effect of the PEO layer to the surface of the CNCs. The results showed that the hydrophilicity of the modified CNCs was reduced by the PEO layer absorbed on the surface of the nanoparticles.
As shown in Fig.5, SEM images were recorded to show the dispersion of CNCs in the PLA composites. Compared to pure PLA, some nanoscale dark dots appeared in the PLA/p-CNC nanocomposites (Fig.5(b) and Fig.5(c)), which were ascribed to the carbonization and thermal decomposition of the CNCs during the hot-compaction process because of the inferior thermostability of the p-CNCs. There were no dots presenting the PLA composites reinforced with n-CNC (Fig.5(d), Fig.5(e), and Fig.5(f)) in comparison to those of the PLA/p-CNC, which showed that the thermostability of the n-CNC improved corresponding with the results of the TGA. However, serious agglomeration was observed for these composites. The agglomeration was weakened for the PLA composites filled with CNC/PEO (Fig.5(g), Fig.5(h), and Fig.5(i)), but microphase separation appeared in PLA/5% CNC/PEO, such as the hole in the cross section as indicated by the arrow in Fig.5(i). The results of SEM imaging showed that the compatibility between the modified CNCs and PLA matrix obviously improved.
Transmittance measurements were performed to elaborate upon the dispersion of the CNCs in the PLA composites. As shown in Fig.6, the transmittance of the p-PLA matrix was near 75%, and sharply decreased with the addition of cellulose nanoparticles, which was ascribed to the uneven dispersion and degradation of the CNCs during hotpressing. The transmittance of PLA/CNC/PEO increased compared to that of PLA/n-CNC. To visually present the transparency of the composites, photographs of different types of PLA composites are shown in Fig.7. It is obvious that a large number of dark dots appeared in the PLA/p-CNC composites, which was attributed to the reduction in thermostability caused by the sulfate groups on the surface of the CNCs. Although dark dots were not observed in the composites filled with n-CNCs, the color of the composites gradually darkened with the increase in n-CNC content, which was ascribed to the aggregation of nanoparticles in the polymeric matrix. Compared to these composites, the transparency of the PLA/CNC/PEO composites obviously improved, which was attributed to the improvement in thermostability and the decrease in hydrophilicity of the CNCs modified with PEO chains.
As shown in the DSC curves in Fig.8, the glass transition temperature (Tg) of the composites nearly remained stable with the addition of the nanoparticles, which was attributed to the favorable compatibility of the nanofillers with the PLA matrix. The cold crystallization temperature (Tcc) of PLA/CNC/PEO slightly decreased, which indicated faster crystallization induced by the CNCs which acted as nucleating agents for the PLA[25]. CNCs modified with PEO chains, as nucleating agents, helped lower the free energy barrier, thus favoring faster nucleation[26-27].
The breaking strength and elongation of the films were tested to characterize the mechanical properties of the nanocomposites. As shown in Fig.9, the elongation of the pure PLA was 1.93%, indicating the obvious brittleness of the PLA matrix. The breaking strength of the PLA composites increased because of the intrinsic rigidness of the nanofillers. The elongation of the PLA composites decreased with the addition of unmodified CNCs because of the poor compatibility between the CNC controls and the PLA matrix. The elongation at the break of the PLA composites reinforced with CNCs modified with PEO chains improved in comparison to the unmodified CNCs, which was attributed to the cohesion at the phase interface produced by the addition of the modified CNCs. In contrast, the mechanical properties of the composites decreased with the addition of n-CNC because of the CNC aggregation, which generated points of stress concentration, leading to weakness.
4 Conclusions
Neutralization with alkaline and physical adsorption with PEO chains were adopted to improve the thermal stability and reduce the hydrophilicity of CNCs. The results of the characterization indicated that the degradation temperature of the modified CNCs increased by approximately 120℃ and the contact angle of the modified CNCs increased by approximately 25° in comparison to those of the original CNCs, , , which was attributed to the removal of sulfate ester groups and the PEO layers covering the surface of the CNCs. The mechanical properties of the PLA composites improved because of the reinforcement of the modified CNCs, which showed that the compatibility between the PLA matrix and the modified CNCs improved. However, the effects on the mechanical properties were not statistically significant, which may be ascribed to the slippage of the PEO chains on the surface of the CNCs because of the weak interaction between the PEO chains and CNC macromolecules. Therefore, it is important to research further modification methods to improve upon the effects achieved in the present work. These studies are in progress and will be presented in future publications.
Acknowledgments
The authors are grateful to the National Natural Science Foundation of China (grant Nos. 31570578 and 31270632), the Fundamental Research Funds for the Central Universities (grant No. JUSRP51622A) and the State Key Laboratory of Pulp and Paper Engineering (grant No. 201809).
References
[1] Rasal R M, Janorkar A V, Hirt D E. Poly(lactic acid) Modifications[J]. Prog. Polym. Sci., 2010, 35: 338-356.
[2] Goffin A L, Raquez J M, Duquesne E, et al. From Interfacial Ring-Opening Polymerization to Melt Processing of Cellulose Nanowhisker-Filled Polylactide-Based Nanocomposites[J]. Biomacromolecules, 2011, 12: 2456-2465.
[3] Rasal R M, Hirt D E. Toughness Decrease of PLA-Phbhhx Blend Films Upon Surface-Confined Photopolymerization [J]. J. Biomed. Mater. Res. A, 2009, 88: 1079-1086.
[4] Morelli C L, Belgacem N, Branciforti M C, et al. Nanocomposites of PBAT and Cellulose Nanocrystals Modified by in situ Polymerization and Melt Extrusion[J]. Polym. Eng. Sci., 2016, 56(12): 1339-1348.
[5] Hou L, Bian H, Wang Q L, et al. Direct Functionalization of Cellulose Nanocrystals with Polymer Brushes via UV-Induced Polymerization: Access to Novel Heterogeneous Visible-Light Photocatalysts[J]. RSC Adv., 2016, 6: 53062-53068.
[6] Lin N, Dufresne A. Physical and/or Chemical Compatibilization of Extruded Cellulose Nanocrystal Reinforced Polystyrene Nanocomposites[J]. Macromolecules, 2013, 46: 5570-5583.
[7] Liu Y, Li Y, Yang G, et al. Multi-Stimulus-Responsive Shape-Memory Polymer Nanocomposite Network Cross-Linked by Cellulose Nanocrystals[J]. ACS Appl. Mater. Inter., 2015, 7: 4118-4126.
[8] Habibi Y, Lucia L A, Rojas L A. Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications[J]. Chem. Rev., 2010, 110: 3479-3500.
[9] Bagheriasl D, Carreau P J, Dubois C, et al. Properties of Polypropylene and Polypropylene/Poly(Ethylene-co-Vinyl Alcohol) Blend/CNC Nanocomposites[J]. Compos. Sci. Technol., 2015, 117: 357-363.
[10] Mujica-Garcia A, Hooshmand S, Skrifvars M, et al. Poly(Lactic Acid) Melt-Spun Fibers Reinforced with Functionalized Cellulose Nanocrystals[J]. RSC Adv., 2016, 6: 9221-9231.
[11] Roman M, Winter W T. Effect of Sulfate Groups from Sulfuric Acid Hydrolysis on the Thermal Degradation Behavior of Bacterial Cellulose[J]. Biomacromolecules, 2004, 5: 1671-1677.
[12] Abraham E, Kam D, Nevo Y, et al. Highly Modified Cellulose Nanocrystals and Formation of EpoxyNanocrystalline Cellulose (CNC) Nanocomposites[J]. ACS Appl. Mater. Inter., 2016, 8(41): 28086-28095.
[13] Labet M, Thielemans W, Dufresne A. Polymer Grafting onto Starch Nanocrystals[J]. Biomacromolecules, 2007, 8: 2916-2927.
[14] ZoppeJ O, Habibi Y, Rojas O J. Poly(N-isopropylacrylamide) Brushes Grafted from Cellulose Nanocrystals via Surface-Initiated Single-Electron Transfer Living Radical Polymerization[J]. Biomacromolecules, 2010, 11: 2683-2691.
[15] Chen G, Dufresne A, Huang J, et al. A Novel Thermoformable Bionanocomposite Based on Cellulose Nanocrystal-graft-Poly(epsilon-caprolactone)[J]. Macromol. Mater. Eng., 2009, 294: 59-67.
[16] Zoppe J O, Osterberg M, Venditti R A, et al. Surface Interaction Forces of Cellulose Nanocrystals Grafted with Thermoresponsive Polymer Brushes[J]. Biomacromolecules, 2011, 12: 2788-2796.
[17] Zhang C M, Salick M R, Cordie T M, et al. Incorporation of Poly(Ethylene Glycol) Grafted Cellulose Nanocrystalsin Poly(Lactic Acid) Electrospun Nanocomposite Fibers as Potential Scaffolds for Bone Tissue Engineering[J]. Mater. Sci. Eng., 2015, 49: 463-471.
[18] Morandi G, Heath L, Thielemans W. Cellulose Nanocrystals Grafted with Polystyrene Chains through Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP)[J]. Langmuir, 2009, 25: 8280-8286.
[19] Devanand K, Selser J C. Polyethylene Oxide does not Necessarily Aggregate in Water[J]. Nature, 1990, 343(6260): 739-741.
[20] Xiao H N, Pelton R, Hamielec A. The Association of Aqueous Phenolic Resin with Polyethylene Oxide and Poly(Acrylamide-co-Ethylene Glycol)[J]. J. Polym. Sci. Pol. Chem., 1995, 33(15): 2605-2612.
[21] Pang P, Englezos P. Kinetics of the Aggregation of Polyethylene Oxide at Temperatures above the Polyethylene Oxide-Water Cloud Point Temperature[J].Colloid. Surface. A, 2002, 204: 23-30.
[22] Yin Y Y, Tian X Z, Jiang X, et al. Modification of Cellulose Nanocrystal via SI-ATRP of Styrene and the Mechanism of Its Reinforcement of Polymethylmethacrylate[J].Carbohyd. Polym., 2016, 142: 206-212.
[23] Lin N, Huang J, Dufresne A. Preparation, Properties and Applications of Polysaccharide Nanocrystals in Advanced Functional Nanomaterials: A Review[J]. Nanoscale, 2012, 4: 3274-3294.
[24] Wong S S, Stefan K, Tan Y F. Bacterial and Plant Cellulose Modification Using Ultrasound Irradiation[J]. Carbohyd. Polym., 2009, 77: 280-287.
[25] Frone A, Berlioz N S, Chailan J F, et al. Morphology and Thermal Properties of PLA-Cellulose Nanofibers Composites[J]. Carbohyd. Polym., 2013, 91: 377-384.
[26] Suksut B, Deeprasertkul C. Effect of Nucleating Agents on Physical Properties of Poly(lactic acid) and Its Blend with Natural Rubber[J]. J. Polym. Environ., 2011, 19(1): 288-296.
[27] Dong T, Shin K, Zhu B, et al. Nucleation and Crystallization Behavior of Poly(Butylene Succinate) Induced by Its Alpha-Cyclodextrin Inclusion Complex: Effect of Stoichiometry[J]. Macromolecules, 2006, 39(6): 2427-2428.
Facile Fabrication of Conductive Paper-based Materials from Tunicate Cellulose Nanocrystals and Polydopamine-decorated Graphene Oxide
XiaoZhou Ma1, YaoYao Chen2, Peter R. Chang3,*, Jin Huang1,2,*
1. School of Chemistry and Chemical Engineering, Joint International Research Laboratory of Bio-mass-Based Macromolecular Chemistry and Materials, Southwest University, Chongqing, 400715, China
2. School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan, Hubei Province, 430070, China
3. Bioproducts and Bioprocesses National Science Program, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, SK, S7N 0X2, Canada
Abstract: Conductive papers made from graphene and its derivatives are important for the development of electronic devices; however, elastomer-based matrices usually make it difficult for the conductive sheets to form continuous conductive networks. In this work, we used tunicate-derived cellulose nanocrystals (TCNC) instead of traditional elastomers as the matrix for polydopamine (PDA)-coated and reduced graphene oxide (GO) to prepare conductive paper, which, at a low concentration, were better for the formation of conductive networks from conductive sheets. It was found that the Young’s modulus of the conductive paper produced via this strategy reached as high as 7 GPa. Meanwhile, owing to the partial reduction of GO during the polymerization of dopamine, the conductivity of the conductive paper reached as high as 1.3×105 S/cm when the PDA-coated GO content was 1 wt%, which was much higher than the conductivity of pure GO (~4.60×108 S/cm). This work provides a new strategy for preparing environmentally friendly conductive papers with good mechanical properties and low conductive filler content, which may be used to produce high-performance, low-cost electronic devices.
Keywords: conductive paper; tunicate cellulose nanocrystal; graphene oxide; polydopamine coating; casting paper
XiaoZhou Ma, PhD;
E-mail: maxiaozhou@swu.edu.cn
*Corresponding author:
Peter R. Chang, professor;
E-mail: peter.chang@canada.ca
*Corresponding author:
Jin Huang, professor; research interests: soft-matter chemistry and nanomaterials, especially novel materials derived from biomass resources;
E-mail: huangjin2015@swu.edu.cn
1 Introduction
The high conductivity and light weight of conductive papers or films are important features for the development of electronic devices. A general strategy for the production of conductive papers or films is to build conductive networks using conductive materials in or on cellulose paper or a flexible matrix (usually macromolecular network-based elastomers)[1-2]. Nevertheless, continuous conductive networks are rather difficult to form because of high steric hindrance and percolation ability, which severely limits their conductivity performance in the resultant papers/films. For example, Choi H Y et al used a layer-by-layer strategy to dope silver nanowires and carbon nanotubes (CNTs) onto cellulose paper[3]. As the conductive network was built on the surface of the paper, the conductivity of the paper reached as high as 1 S/cm; however, the conductivity of the papers made using this strategy was easily affected by scratches on the paper surface that exposed the conductive layer. Chang-Jian C-W and coworkers used an aerogel of pre-formed multi-walled carbon nanotubes (MWCNTs) to improve the dispersion of the MWCNTs in either a polydimethylsiloxane (PDMS) matrix or a poly (3,4-ethylene-dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) matrix and prepared a conductive paper with low MWCNT content[4-5]. The results showed that as little as 5 wt% or 10 wt% of MWCNTs could be dispersed in the matrix to form conductive papers with conductivities of about 100 S/cm to 1000 S/cm, respectively. Furthermore, Gan and coworkers studied the percolation behavior of CNTs in a PDMS matrix in detail and successfully predicted the percolation threshold of CNTs in the PDMS matrix[6]. It was found that a high mass ratio of conductive fiber was required to make a conductive paper with high conductivity.
Graphene oxide (GO) and reduced graphene oxide (RGO) are layered carbon nanomaterials that have high surface areas and are flexible and conductive[7-11]. Their hydrophobic surfaces usually allow GO and RGO to disperse easily in organic solvents and be used to build in situ conductive networks in conductive papers. Compared to CNTs, which are typically seriously self-entangled and hard to disperse in solvents, GO can be easily dispersed in a uniform manner to form continuous networks[2,12]. Kang et al used graphene and CNTs to build a transparent electrode from films with a resistance of ~2.4 k/cm2[13]. Bai et al also used pure GO to make a transparent conductive paper having a conductivity of 3.7×105 S/cm[14]. The conductivity of the material could be further improved by reducing the GO (or RGO) content, which could then be better applied in the preparation of conductive films[15]. However, the load ratio of GO in these materials was still high relative to its large size; this needs to be addressed to make GO-containing papers conductive.
Tunicate-derived cellulose nanocrystals (TCNCs) have a high aspect ratio and exhibit high mechanical strength[16-17]. Their hydrophilic surfaces make the formation of hydrogen bonds among TCNCs easy, which suggests that TCNCs could be used to obtain conductive papers with high mechanical strength. Wen et al used CNCs and RGO to prepare a composite conductive paper with ultra-low resistance (1.48 /cm2)[18], in which the CNCs greatly enhanced the mechanical strength of the paper. The toughness of this paper was as high as 11 MJ/m3. Zhao et al used glucomannans with TCNCs to develop a tough biomass film[19]. The Young’s modulus of the paper and tensile strength of the film were found to be 11.71 GPa and 55.56 MPa, respectively. However, to the best of our knowledge, TCNCs have not been used before for the preparation of conductive papers.
Cellulose is a green material that has been previously employed to produce conductive papers[20-22]. Herein, inspired by the abovementioned results, we dispersed polydopamine-coated GO into an oxidized TCNC suspension, cast the mixture, evaporated the solvent, and prepared a conductive paper with good mechanical properties and a low GO content. In our strategy, GO was first coated with polydopamine (PDA) to improve its surface hydrophilicity. The electrostatic interaction of the amino groups on PDA molecules and carboxyl groups on the oxidized TCNC (OTCNC) surface further improved the stability of the GO sheets in the TCNC suspension. TCNCs were oxidized by 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) and the PDA-coated GO aqueous suspension was subsequently mixed with the OTCNC aqueous suspension, followed by casting and evaporation to make conductive papers. The advantage of this strategy is that the hydrogen-bond cross-linked particle-based network of OTCNC papers is better for the formation of a continuous GO conductive network, while the high mechanical strength of OTCNC provides the paper with good mechanical properties.
2 Experimental
2.1 Materials and methods
GO was obtained from Changchun Institute of Applied Chemistry, Chinese Academy of Science. Dopamine hydrochloride and 1,1,1-tris(hydroxymethyl)-methylamine (Tris, 99%) were purchased from Aladdin Chemical Co. Tunicates were collected at Weihai, Shandong province, China. H2SO4, KOH, NaClO, acetic acid, EtOH and NaBr were procured from Sinopharm Group Co. Ltd. and 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) was purchased from Sigma-Aldrich Co.
The morphologies of TCNC, GO, and their derivatives were characterized using an H-700FA (Hitachi) transmission electron microscope (TEM). The results of surface modification of TCNC and GO were characterized using a Nicolet 6700 (Nicolet Instruments) Fourier transform infrared spectrometer (FT-IR, wave number 400~4000 cm1). The crystal structures of TCNC, GO, and their derivatives were characterized using a D8 Advance (Bruker) X-ray diffraction spectrometer (XRD) and the constituent elements were characterized using ESCALAB 250Xi (Thermo Fisher Scientific) X-ray photoelectron spectroscopy (XPS). The mechanical properties of the conductive papers were characterized on a CMT6503 (SANS Co.) universal testing machine, and the conductivity was measured with a Keithley 6517B (Tektronix Inc.) high resistance meter.
2.2 Extraction of TCNC
To extract TCNCs, tunicates (10 g) were dried (60℃, 3 days), ground, and treated with 5 wt% KOH aqueous solution (300 mL) for 12 h, and then dried at 60℃ for 3 days. The KOH-treated tunicate was then bleached twice using acetic acid (10 mL) and NaClO (7.1 mL), and subsequently dried. The bleached tunicate was hydrolyzed using 40% H2SO4 at 40℃ for 4 h, then washed with distilled water via centrifugation, and dialyzed to yield TCNCs. The product was centrifuged and freeze dried to obtain purified TCNCs (white powder, yield of 50%).
2.3 Synthesis of oxidized TCNCs
The oxidized TCNCs were prepared following the literature procedure[23]. TCNCs (2 g) were dispersed in 200 mL of distilled water to make a 1 wt% TCNC suspension. NaBr (652 mg) and TEMPO (59 mg) were then dissolved in 200 mL of distilled water and added to the TCNC suspension. Subsequently, NaClO (15 g, Cl=14.5%) was added to the mixture and the pH value was adjusted to 12 using 0.1 mol/L NaOH aqueous solution. After stirring for 4 h, EtOH (4 mL) was added to the mixture to terminate the reaction. The product was washed thrice with distilled water and then added into 1 mol/L HCl and stirred for 30 min. This was followed by washing with 0.5 mol/L HCl and distilled water until the pH value of the solution decreased to 7. The solution was then freeze-dried to yield pure oxidized TCNC (OTCNC).
2.4 Synthesis of polydopamine-coated GO
To make polydopamine-coated GO (PDA-GO), GO (100 mg) and dopamine hydrochloride (50 mg) were added into 10 mmol/L Tris-HCl buffer (200 mL, pH value of 8.5) and ultrasonicated in an ice bath for 10 min to allow the GO to disperse uniformly. The mixture was then stirred at 60℃ for 24 h and filtered until the filtrate was clear and colorless. The filtrate was freeze dried to yield PDA-GO.
2.5 Preparation of conductive paper
Different amounts (50, 100, 150, and 200 mg) of PDA-GO were added to 1 wt% OTCNC suspension (20 mL) and stirred for 3 h. The mixtures were then poured into Teflon molds and heated for 12 h at 60℃ to obtain conductive papers with different GO mass ratios.
3 Results and discussion
3.1 Synthesis of OTCNC and PDA-GO
The results of the oxidation of TCNCs and coating of GO were easily determined by FT-IR spectra (Fig.1). After oxidation, a band appeared at 1724 cm1 and was attributed to the C=O of carboxyl groups. This band was not present in the FT-IR spectrum of TCNCs. TEMPO oxidized the C6—OH group into carboxyl, which then formed hydrogen bonds with other hydrophilic groups more easily than the hydroxyl groups. Meanwhile, the existence of carboxyl groups also made the TCNC surface more negative, which improved its stability in water. On coating GO with PDA, two new bands at 1568 cm1 and 1220 cm1 and attributed to the N—H and phenyl ring, respectively, were observed in the FT-IR spectrum of PDA-GO and indicated that PDA had successfully coated the GO surface.
TEM and XPS were used to study the effects of these modifications on the morphology of the nanomaterials (Fig.2). The TCNCs were 1~5 m long with a diameter of 10~20 nm (Fig.2a). After oxidation, the morphology of the TCNCs was not changed (Fig.2b). The XRD spectra of TCNC and OTCNC also showed the same trend (Fig.2c), with the XRD spectrum of OTCNC overlapping well with that of TCNC. The peaks located at 2=14.7°, 16.5°, 22.7°, and 34.4° indicated that both TCNC and OTCNC were in a good -crystal format. The crystallinities of TCNC and OTCNC were 87% and 81%, respectively, which also indicated that oxidation did not damage the crystal structure of TCNC. Coating GO with PDA obviously increased the thickness of the GO sheet, which was observed by TEM (Fig.2d and Fig.2e). The XRD spectra of GO and PDA-GO showed the same trend. The peak in the GO XRD spectrum located at 2=10.7° shifted to 8.9°, which indicated that the distance between the GO sheets had increased from 0.84 nm to 0.99 nm. Meanwhile, a new peak was found at 26.1° after the coating of GO, which indicated that GO was partially reduced.
3.2 Mechanical properties of the conductive paper
Benefiting from the multiple hydrogen bonds formed between PDA-GO and OTCNC and among OTCNCs, the cast conductive paper showed good mechanical strength. However, as a result of the rigidity of OTCNC, the extensibility of the paper was low. Table 1 shows that with an increase in the content of PDA-GO, the Young抯 modulus of the film first increased from 3.48 GPa to 6.78 GPa, and then decreased to 4.70 GPa. This was mainly because with an increase in the PDA-GO content, the catechol group on the PDA-GO surface interacted strongly with the carboxyl and hydroxyl groups on the OTCNC surface to form hydrogen bonds. This hybrid network made the paper more flexible and rigid; however, a further increase in the PDA-GO content led to self-assembling of PDA on the PDA-GO surface, causing the PDA-GO to aggregate. Aggregation led to an uneven dispersion of PDA-GO in the OTCNC matrix. Meanwhile, the continuous network of PDA-GO partially broke the OTCNC network and decreased the rigidity of the paper. It is worth noting that the interaction between OTCNC and PDA-GO was not specific. The OTCNC network could slide on the PDA-GO surface under an external force applied to the paper, thus decreasing the rigidity and extensibility of the paper.
3.3 Conductivity of the conductive paper
It has been reported that the conductivity of GO is 4.60×108 S/cm[24]. The prepared conductive paper benefited from the partial reduction of GO and the conductivity of PDA, and showed an increase in conductivity from about 109 S/cm to 105 S/cm with the increasing PDA-GO content (Fig.3). The catechol groups on PDA made the molecule conductive. Owing to the continuous network formed by PDA-GO, the conductive PDA molecules could further improve the conductivity of the paper. The low conductivity of GO could be improved by its reduction to RGO[15]. Polymerization of dopamine also induced the reduction of the GO sheets, and thus increased the conductivity of the PDA-GO network. As seen in Fig.3, the conductivity of the paper increased dramatically from 2×106 S/cm to 1.3×105 S/cm when the PDA-GO content increased from 0.75 wt% to 1 wt%, and increased slightly with further increase in the PDA-GO content. This change indicated that a continuous network was formed when the PDA-GO content was ~1 wt%. This concentration was low as compared to previous research[7-11]. Based on the observed changes in the conductivity of the paper when the PDA-GO content was more than 1 wt%, it was predicted that the conductivity of the paper would increase further with increasing PDA-GO content as more electrically conductive paths would form. This result indicated that the OTCNC matrix was better for the formation of continuous conductive networks in the conductive sheets, and for the production of conductive papers with good conductivity and mechanical properties.
4 Conclusions
In this study, the conductive paper was prepared by mixing OTCNC and PDA-GO suspensions and casting the mixture. As a matrix, the nanoparticle-sized OTCNC can make the PDA-GO easily to form a conductive network in the material, while the rigidity of OTCNC also imparted good mechanical properties to the paper. In this way, the conductivity of the paper reached as high as 105 S/cm, with a PDA-GO content of 1 wt%, while the Young’s modulus of the paper was ~4.5 GPa. It is proposed that this conductive paper could be used in soft electrical devices or electrical signal-based sensors.
Acknowledgments
Authors are grateful to the National Natural Science Foundation of China (51373131), Fundamental Research Funds for the Central Universities (XDJK2016A017 and XDJK2016C033), Project of Basic Science and Advanced Technology Research, Chongqing Science and Technology Commission (cstc2016, jcyjA0796), and the Talent Project of Southwest University (SWU115034).
References
[1] Wang M, Wang X, Moni P, et al. CVD Polymers for Devices and Device Fabrication[J]. Advanced Materials, DOI: 10.1002/adma.201604606.
[2] Zheng Q, Li Z, Yang J, et al. Graphene oxide-based transparent conductive films[J]. Progress in Materials Science, 2014, 64: 200-247.
[3] Choi H Y, Lee T-W, Lee S-E, et al. Silver nanowire/carbon nanotube/cellulose hybrid papers for electrically conductive and electromagnetic interference shielding elements[J]. Composites Science and Technology, 2017, 150: 45-53.
[4] Chang-Jian C-W, Cho E-C, Lee K-C, et al. Thermally conductive polymeric composites incorporating 3D MWCNT/PEDOT:PSS scaffolds[J]. Composites Part B: Engineering, 2018, 136: 46-54.
[5] Cho E-C, Chang-Jian C-W, Hsiao Y-S, et al. Three-dimensional carbon nanotube based polymer composites for thermal management[J]. Composites Part A: Applied Science and Manufacturing, 2016, 90: 678-686.
[6] Gan L, Dong M, Han Y, et al. Connection-Improved Conductive Network of Carbon Nanotubes in a Rubber Cross-Link Network[J]. ACS Applied Materials & Interfaces, 2018, 10(21): 18213-18219.
[7] Kobayashi T, Bando M, Kimura N, et al. Production of a 100-m-long high-quality graphene transparent conductive film by roll-to-roll chemical vapor deposition and transfer process[J]. Applied Physics Letters, 2013, 102(2): 023112-023112-4.
[8] Zhang L L, Zhao X, Stoller M D, et al. Highly Conductive and Porous Activated Reduced Graphene Oxide Films for High-Power Supercapacitors[J]. Nano Letters, 2012, 12(4): 1806-1812.
[9] Zheng Q, Ip W H, Lin X, et al. Transparent Conductive Films Consisting of Ultra Large Graphene Sheets Produced by Langmuir-Blodgett Assembly[J]. ACS Nano, 2011, 5(7): 6039-6051.
[10] Zhao J, Pei S, Ren W, et al. Efficient Preparation of Large-Area Graphene Oxide Sheets for Transparent Conductive Films[J]. ACS Nano, 2010, 4(9): 5245-5252.
[11] Geng J, Jung H-T. Porphyrin Functionalized Graphene Sheets in Aqueous Suspensions: From the Preparation of Graphene Sheets to Highly Conductive Graphene Films[J]. Journal of Physical Chemistry C, 2010, 114(18): 8227-8234.
[12] Zhu J-J, Sun H-B, Wu Y-Z, et al. Graphene: Synthesis, Characterization and Application in Transparent Conductive Films[J]. Acta Physico-Chimica Sinica, 2016, 32(10): 2399-2410.
[13] Kang C H, Shen C, Saheed M S M, et al. Carbon nanotube-graphene composite film as transparent conductive electrode for GaN-based light-emitting diodes[J]. Applied Physics Letters, 2016, 109(8): 97130.
[14] Bai M, Chen J, Wu W, et al. Preparation of stable aqueous dispersion of edge-oxidized graphene and its transparent conductive films[J]. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2016, 490: 59-66.
[15] Marquez C, Rodriguez N, Ruiz R, et al. Electrical characterization and conductivity optimization of laser reduced graphene oxide on insulator using point-contact methods[J]. RSC Advances, 2016, 6(52): 46231-46237.
[16] Sacui I A, Nieuwendaal R C, Burnett D J, et al. Comparison of the Properties of Cellulose Nanocrystals and Cellulose Nanofibrils Isolated from Bacteria, Tunicate, and Wood Processed Using Acid, Enzymatic, Mechanical, and Oxidative Methods[J]. ACS Applied Materials & Interfaces, 2014, 6(9): 6127-6138.
[17] Zhang T, Zuo T, Hu D, et al. Dual Physically Cross-Linked Nanocomposite Hydrogels Reinforced by Tunicate Cellulose Nanocrystals with High Toughness and Good Self-Recoverability[J]. ACS Applied Materials & Interfaces, 2017, 9 (28): 24230-24237.
[18] Wen Y, Wu M, Zhang M, et al. Topological Design of Ultrastrong and Highly Conductive Graphene Films[J]. Advanced Materials, 2017, DOI:10.1002/adma.201702831.
[19] Zhao Y, Zhang Y, Lindstrom M E, et al. Tunicate cellulose nanocrystals: Preparation, neat films and nanocomposite films with glucomannans[J]. Carbohydrate Polymers, 2015, 117: 286-296.
[20] Huang X, Li X, Li Y, et al. Biopolymer as Stabilizer and Adhesive To in Situ Precipitate CuS Nanocrystals on Cellulose Nanofibers for Preparing Multifunctional Composite Papers[J]. ACS Omega, 2018, 3(7): 8083-8090.
[21] Ye W, Li X, Zhu H, et al. Green fabrication of cellulose/graphene composite in ionic liquid and its electrochemical and photothermal properties[J]. Chemical Engineering Journal, 2016, 299: 45-55.
[22] Ling Y, Li X, Zhou S, et al. Multifunctional cellulosic paper based on quaternized chitosan and gold nanoparticle-reduced graphene oxide via electrostatic selfassembly[J]. Journal of Materials Chemistry A, 2015, 3(14): 7422-7428.
[23] Cheng F, Liu C, Wei X, et al. Preparation and Characterization of 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-Oxidized Cellulose Nanocrystal/Alginate Biodegradable Composite Dressing for Hemostasis Applications[J]. ACS Sustainable Chemistry & Engineering, 2017, 5(5): 3819-3828.
[24] Mrlik M, Ilcikova M, Plachy T, et al. Graphene oxide reduction during surface-initiated atom transfer radical polymerization of glycidyl methacrylate: Controlling electro-responsive properties[J]. Chemical Engineering Journal, 2016, 283: 717-720.
Illustration of Cover Photographs
Vertical-assembly of cellulose nanocrystals for information hiding and reading: Saving the information in the optical materials, which can display various designated states in alternating conditions, is an important method of information security. Fluorophores and stimulation-responsive dyes are two typical materials in this method, whereas they usually suffer problems of photobleaching and aggregation-induced quenching. By contrast, the assembled materials with the periodicities equal to the wavelength of visual light own unique structural colors, which are free of the problems mentioned above. In this case, the structural color of cellulose nanocrystal-assembled films were successfully controlled in ultraviolet region, which could hide the information of arrayed cellulose nanocrystals-based films under natural light. Periodical arrays of cellulose nanocrystals could even enhance scattering, which was similar to the photonic crystals. This feature made the hided information of the cellulose nanocrystal-assembled films readable as enhanced scattering blue light under an ultraviolet radiation. By an evaporation-induced method, cellulose nanocrystals (shown in the TEM image of front cover, the left block diagram) could arrange vertically to the solution plane, which removed the usual chirality and iridescence of assemble films based on rod-like cellulose nanocrystals. This ensured the specific designation of structural color as monochromatic light for the vertical-assembled cellulose nanocrystal films (the arrangement of cellulose nanocrystals can be seen in the AFM image of front cover, the right block diagram), which prevented the information from being misread due to iridescence.
Corresponding author: Jin Huang, professor.
School of Chemistry and Chemical Engineering, Joint International Research Laboratory of Biomass-Based Macromolecular Chemistry and Materials, Southwest University, Chongqing, 400715, China
E-mail: huangjin2015@swu.edu.cn; huangjin@iccas.ac.cn
Electro-conductive Nanocrystalline Cellulose Film Filled with TiO2-Reduced-Graphene Oxide Nanocomposite
RuoNan Zhao1, YanJun Tang1,2,*, XiaoChuang Shen1,
XingHua Hong2, YiMing Zhou1
1. Pulp and Paper Center, Zhejiang Sci-Tech University, Hangzhou, Zhejiang Province, 310018, China
2. National Engineering Laboratory of Textile Fiber Materials and Processing Technology, Zhejiang Sci-Tech University, Hangzhou, Zhejiang Province, 310018, China
Abstract: Imparting electro-conductive properties to nanocellulose-based products may render them suitable for applications in electronics, optoelectronics, and energy storage devices. In the present work, an electro-conductive nanocrystalline cellulose (NCC) film filled with TiO2-reduced-graphene oxide (TiO2-RGO) was developed. Initially, graphene oxide (GO) was prepared using the modified Hummers method and thereafter photocatalytically reduced using TiO2 as a catalyst. Subsequently, an electro-conductive NCC film was prepared via vacuum filtration with the as-prepared TiO2-RGO nanocomposite as a functional filler. The TiO2-RGO nanocomposite and the NCC/TiO2-RGO film were systematically characterized. The results showed that the obtained TiO2-RGO nanocomposite exhibited reduced oxygen-containing group content and enhanced electro-conductivity as compared with those of GO. Moreover, the NCC film filled with TiO2-RGO nanocomposite displayed an electro-conductivity of up to 9.3 S/m and improved mechanical properties compared with that of the control. This work could provide a route for producing electro-conductive NCC films, which may hold significant potential as transparent flexible substrates for future electronic device applications.
Keywords: nanocrystalline cellulose; nanocomposite film; graphene oxide; photocatalytic reduction; electro-conductivity
RuoNan Zhao, master candidate;
E-mail: 2424775817@qq.com
*Corresponding author:
YanJun Tang, PhD, professor;
research interests: specialty paper and converted paper, separation and high value utilization of lignocellulosic biomass;
E-mail: tangyj@zstu.edu.cn
1 Introduction
Nanocellulose,derivedfrom the biopolymercellulose, has received considerable attention owing to its many potential advantages over traditional materials such as the abundance of cellulose resources, renewability and biodegradability, exceptional mechanical properties, and nano-scaled dimensions[1]. In recent years, developing nanocellulose-based bionanomaterials with advanced functionalities for diversified applications has attracted increasing attention[2]. For instance, Henriksson et al[3] successfully achieved an eye-catching cellulose nanopaper with remarkably high toughness using wood nanofibrils. The as-prepared cellulose nanopaper exhibited a network composed of intertwined nanofibrils, with an aspect ratio exceeding 100 and a tensile index of 214 MPa[4]. Sun et al[5] fabricated transparent cellulose films based on 2,2,6,6-tetramethylpiperidine-1-oxyl radical oxidized cellulose nanocrystals, which exhibited a high tensile strength of 236.5 MPa, close to that of industrial steel (250 MPa). In this regard, the feasibility and possibility of producing nanocellulose-based films with advanced functionality is of significant interest to academic and industrial communities. In particular, a nanocellulose-based film imparted with electro-conductivity may be promising for applications in a wide range of fields such aselectronics, optoelectronics, and energy storage devices[6].
In the production of nanocellulose-based films, the added electro-conductive filler should be preferably considered to render the obtained film electro-conductive. The fillers currently used for conferring electro-conductivity to nanocellulose-based films mainly include graphite, carbon black, carbon fiber, carbon nanotubes, and graphene, as well as polypyrrole and polyaniline. Among these electro-conductive fillers, graphene is one of the most promising candidates owing to its unique nanostructure and excellent physical properties. The most common route obtaining bulk quantities of graphene begins with the oxidation of graphite to graphene oxide (GO). However, the abundance of oxygen groups imparts GO with low electrical conductivity. Therefore, prior to the desired application, it is generally necessary to reduce GO. Currently, GO reduction may be performed through chemical methods and/or heat treatment processes[7-9]. However, owing to the toxicity of chemical reductants such as hydrazine and the high temperature (>500℃) required in the thermal reductions, the usual chemical and thermal reductions are not compatible with the current electronic and chemistry technologies, and hence, their extensive applications have been limited. Therefore, in addition to improving the current reduction processes, other effective reduction methods, including microwave and photocatalytic reductions, are considered feasible for the production of high-quality reduced GO (RGO) nanosheets. Concerning the photocatalytic reduction processes, Xu et al[10] showed that Au, Pt, and Pd metallic nanoparticles adsorbed on GO sheets can reduce them with ethylene glycol in a catalytic process. Moreover, photocatalytic reduction of GO sheets using metal oxide semiconductors such as TiO2 was investigated[11-12]. Thus, the feasibility of photocatalytic reduction of GO and its application potential should be further explored.
Nanocrystallinecellulose(NCC),alsocalledce-llulosenanocrystals (CNC), istypicallyarigidrod-shaped monocrystallinecellulosedomain (whisker) with widths of approximately 5~80 nm and lengths of 200~500 nm.In our previous works, processes for the production of NCC from cotton microcrystalline cellulose (MCC)[13] and recycled paper fiber[14] were developed, and the application of NCC as a reinforcing phase in nanocomposite films was demonstrated[6]. Moreover, graphite[15], carbon nanotubes[16], and GO[17] were employed as fillers for the production of electro-conductive cellulosic paper. In the present work, photocatalytic reduction of GO was achieved in the presence of titanium dioxide (TiO2) as a catalyst. The microstructure of the as-prepared TiO2-RGO nanocomposite was characterized using Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Subsequently, the obtained TiO2-RGO nanocomposite was used as an electro-conductive filler for the production of NCC films via vacuum filtration. The rheological behavior of NCC suspensions and the electro-conductivity and mechanical properties of the NCC films as a function of the added TiO2-RGO nanocomposite were systematically investigated. This work offers an effective route for the production of electro-conductive NCC films, which may hold significant potential as transparent flexible substrates for future electronic device applications.
2 Experimental
2.1 Materials
Commercial MCC powder provided by Shanghai Tonnor Material Science Co., Ltd. (Shanghai, China) was used as the raw material for the extraction of NCC. Dialysis bags (Mw cut off 10,000) were purchased from Hangzhou Mike Chemical Instrument Co., Ltd. (Hangzhou, China). Graphite powder (particle size < 20 m) was purchased from Hangzhou Xinhua Paper Co., Ltd. Sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl, 37%), potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrogen peroxide (H2O2, 30%), and sodium hydroxide (NaOH) were purchased from Sigma-Aldrich. Distilled water was used for all the experiments.
2.2 Preparation of NCC
Rod-shaped NCC was prepared from cotton material according to the procedure described in our previous work[13]. Briefly, 10 g of MCC powder was gradually added to 100 mL of 65.0 wt% sulfuric acid aqueous solution. The hydrolysis was performed at 50℃ for 2 h under continuous stirring. Subsequently, deionized water was added to terminate the hydrolysis. Subsequently, the resulting mixture was centrifuged at 11,000 r/min for 10 min to separate the NCC, which was thereafter washed with deionized water and repeatedly centrifuged for four times. The obtained colloidal suspension of NCC was filtered with deionized water in a dialysis bag for one day to a constant pH value of 7.0. Finally, the suspensions were subjected to freeze-drying and vacuum drying prior to the subsequent processing.
2.3 Photocatalytic reduction of GO
A GO sample was prepared from graphite powder by following the modified Hummers method[18-19]. For the photocatalytic reduction of the GO sheets, first, 0.09 g GO powder was dispersed in 200 mL ethanol (C2H5OH, Merck, >99.9%). The mixture was first sonicated (Qsonica Q1376, USA) for 1.5 h at 100 W to obtain GO. Subsequently, 0.01 g TiO2 powder was added and sonication was continued for 0.5 h at 100 W. Subsequently, the suspension was slowly stirred for 1 h in a black box and irradiated with a 110 mW/cm2 mercury lamp (peak wavelengths at 275, 350, and 660 nm) for 1.5 h at room temperature (25℃). The suspension was slowly stirred during the reduction process to ensure uniform irradiation of the TiO2-RGO suspension.
2.4 Preparation of NCC/TiO2-RGO nanocomposite film
In a typical procedure, TiO2-RGO suspension (0.5 mg/mL) was added to the NCC suspension[20] in an ultrasonication bath for 10 min at room temperature, according to the mass ratio of NCC to TiO2-RGO of 99∶1. To prepare the NCC/TiO2-RGO nanocomposite film, each NCC/TiO2-RGO nanocomposite suspension was vacuum-filtered, followed by dissolution of the membrane filter and subsequent drying at room temperature to obtain dry films.
2.5 Characterization and analysis
The molecular analyses of pure GO, RGO, and TiO2-RGO nanocomposites were performed using FT-IR spectra (Nicolet 5700 spectrometer Thermo Fisher Scientific, USA). The FT-IR spectra in the range 500~4000 cm1 were obtained at a resolution of 0.09 cm1, for which the samples were palletized with KBr powder.
The XRD patterns of the GO, RGO, and TiO2-RGO nanocomposite samples were characterized using an X-ray diffractometer (X’TRA-055, ARL, Switzerland) with Cu K radiation (=0.154 nm) at 50 kV and 100 mA.
Scattered radiation was detected in the range 2=3°~80°, at a scan rate of 3°/min. TGA (PerKin Elmer, USA) was used to investigate the thermal weight loss of all the samples. The samples were analyzed under nitrogen atmosphere and heating up to 800℃ at a heating rate of 20℃/min.
The morphologies of all the samples were observed using SEM (FEI, SIRION 200) and TEM (JSM-2100, JEOL, Japan). The rheological properties of all the suspensions were evaluated using a cylinder rotary rheometer (Physica MCR301, Anton Paar, Austria) at 25℃. The electro-conductivity of the film samples was measured using the four-point probe method (SZ2258C). The mechanical properties of the NCC/TiO2-RGO nanocomposite film were evaluated using a universal tester for high-strength materials.
3 Results and discussion
3.1 Preparation of electro-conductive NCC film
The general process for the fabrication of an electro-conductive NCC film is illustrated in Fig.1. NCC was initially isolated from the hydrolysis of MCC with sulfuric acid and thereafter used as the matrix for the production of the electro-conductive composite film. Subsequently, a modified Hummers method was introduced to prepare GO. Subsequently, GO was photocatalytically reduced in the presence of TiO2 as a catalyst. Finally, the electro-conductive NCC film was fabricated via vacuum filtration using the TiO2-RGO nanocomposite as an electro-conductive filler and NCC as the base material. In general, NCC, TiO2-RGO nanocomposite, and the target electro-conductive NCC film are composed of recyclable bio-based and carbon-based materials, which can largely satisfy the requirements of sustainable development strategies. Thus, the proposed concept not only broadens the application field of NCC, but also provides a theoretical and practical route to the research and development of electro-conductive NCC films.
3.2Effect of photocatalyticreduction on the microstructure of RGO
3.2.1 FT-IR spectra
Fig.2 shows the FT-IR spectra of GO, RGO, and TiO2-RGO samples, which were obtained to determine the changes in the various oxygen-containing functional groups. The characteristic peak at 1726 cm1 corresponds to the C=O stretching of COOH groups located at the edges of the oxidized graphite for GO and the broad overlapped peaks in the range 2300~3700, 1726, and 1622 cm1 indicates that GO contains numerous —OH, —COOH, C=O, and other oxygen-containing functional groups. After reduction, for RGO, the peak around 1726 cm1 was weakened, whereas for the TiO2-RGO composite, the peak in this vicinity almost disappeared, indicating that, after the GO reduction under UV irradiation, a small portion of the C=O functional groups on the GO was removed, and the reduction under UV irradiation was limited. With regard to TiO2-RGO, most of the C=O groups on the GO base were effectively removed when the photocatalyst TiO2 was added for the photocatalytic reduction, which indicates enhanced reduction efficiency compared with that of UV reduction. The characteristic peaks at 1622 cm1 and 3400 cm1 are attributed to the vibrational absorption of the O—H functional group, and the characteristic peak at 3400 cm1 was absorbed by the vibration of the C—OH functional group. The apparent signs of weakening at 3400, 1726, and 1622 cm1 for TiO2-RGO indicate that most of the —OH and —COOH groups on the GO substrate were removed via the photocatalytic reduction, but some oxygen-containing groups remained in RGO. In addition, the residual OH bands of the spectrum may partly originate from the high and strongly bound humidity content of GO[21]. The vibrational absorption peak of the C=O functional group at 1726 cm1 can be regarded as the characteristic peak of GO rather than RGO, which is consistent with the result reported in the literature and may indicate that GO was effectively reduced to RGO[22].
Fig.2 FT-IR spectra of GO, RGO, and TiO2-RGO samples
3.2.2 XRD analysis
The XRD patterns of the GO, RGO, and TiO2-RGO nanocomposites samples are shown in Fig.3. The strong diffraction peak at 2=10° corresponds to the characteristic XRD peak of GO. However, the intensity of the diffraction peak of RGO was slightly weakened, which may be due to the poor reduction. The characteristic peak of the TiO2-RGO composite evidently decreased, indicating that UV irradiation had a certain reduction effect on GO. The addition of the photocatalyst nano-TiO2 to the light catalytic reaction could improve the reduction efficiency. The weaker diffraction peaks of the XRD curves of TiO2-RGO at 2=25°, 48°, 55°, and 62° are the characteristic diffraction peaks of TiO2[23], further illustrating the presence of TiO2 in the TiO2-RGO composite. In addition, the broad shoulder peak around 16°~17° is interpreted to be due to the incomplete intercalation and absorbed water[24].
3.2.3 Thermal stability analysis
The thermal decomposition behaviors of the GO, RGO, and TiO2-RGO samples were comparatively evaluated, and the corresponding curves are shown in Fig.4. It can be observed that, for the GO, RGO, and TiO2-RGO samples, the process of thermogravimetry can be roughly divided into three main stages. In the initial stage in the temperature range of 20~120℃, the surface moisture and interlayer moisture evaporation of GO, RGO, and TiO2-RGO naturally caused weight loss with the increase in temperature. The weight losses were 18.9%, 16.8%, and 13.5% for GO, RGO, and TiO2-RGO, respectively, and the weight loss rate was mainly affected by the degree of dryness. The second major weight loss phase was in the temperature range of 120~390℃, which was due to the thermal decomposition of the functional groups on the GO, RGO, and TiO2-RGO substrates. During this stage, the weight losses were 46.5%, 44.6%, and 36.2%, respectively, for GO, RGO, and TiO2-RGO, indicating that a large number of oxygen-containing functional groups such as —OH, —COOH, and C=O on GO were effectively reduced and removed. The weight loss rate of TiO2-RGO was much lower than those of GO and RGO, which is mainly attributed to the fewer functional groups on TiO2-RGO and also indicates that adding TiO2 enhanced the photocatalytic reduction and thermal stability significantly. The third stage was the gradual decomposition of graphene carbon skeleton in the temperature range of 390~660℃, and the final residual rates were 5.0%, 5.8%, and 12.9%, respectively, for GO, RGO, and TiO2-RGO. The TiO2-RGO composite contains a small amount of TiO2, resulting in its relatively high residual rate. In contrast, we can conclude that the thermal stability of GO is relatively poor and GO has strong water absorption. There was no significant improvement in the thermal stability of RGO obtained without the photocatalytic reaction with TiO2, indicating its low reduction efficiency. When nano-TiO2 was added to the photocatalytic reaction, the obtained TiO2-RGO composite had a remarkably improved thermal stability, indicating that GO was effectively reduced to RGO[25].
3.2.4 TEM images
The liquid micro-morphologies of GO and TiO2-RGO were determined using TEM (JSM-2100, JEOL, Japan) and the results are shown in Fig.5. After oxidative stripping, the multiple layers of graphite powder were decomposed into a single layer or a few layers of graphene sheet, which appeared brighter and less overlapped. From the TEM image of TiO2-RGO, it can be observed that RGO did not agglomerate or recombine in the layer after photocatalytic reduction, and the monolayer two-dimensional ho, ne, ycomb structure did not change. In contrast, the carbon atoms showed a trend of further decreasing layers. Simultaneously, a small amount of nano-TiO2 was compounded on the carbon atom layer of RGO, indicating the successful and effective preparation of TiO2-RGO nanocomposites. The GO and TiO2-RGO samples obtained in this study are similar to those reported in the previous work[26].
3.3 Application of TiO2-RGO nanocomposite in NCC film
3.3.1 Effect of TiO2-RGO on the rheological behavior of the NCC suspensions
Fig.6 shows the rheological properties of NCC, NCC/GO, and NCC/TiO2-RGO composite sols. As the shear rate increased, the viscosity of the three sols experienced a significant decrease. This was probably due to a large number of hydrogen bonding interactions between the solvated NCC particles; therefore, the NCC sol formed a three-dimensional network of cross-linked structures, exhibiting good thixotropy. The addition of 1% GO to the NCC sol had a limited effect on the rheological behavior. When 1% TiO2-RGO was added to the system, the viscosity of the NCC/TiO2-RGO sol decreased compared with that of the NCC sol. In other words, the addition of the TiO2-RGO composite helped improve the dispersibility of the NCC/TiO2-RGO composite sol because RGO has a large -conjugated system and can bind to small or high molecules with conjugated systems to enhance their dispersibility through - interactions and thus improve the dispersibility of the composite sol[27].
3.3.2 Effect of TiO2-RGO on the electro-conductivity of the NCC film
The electro-conductivity of the NCC film filled with various TiO2-RGO samples is shown in Fig.7. Upon adding 1% GO as a conductive filler, the conductivity of the composite NCC film became almost 0 S/m. The electro-conductivity of the composite NCC film increased slightly for the ratio of RGO to TiO2 of 10∶0 (1% RGO as the conductive filler). Moreover, when the ratio of RGO to TiO2 was 9∶1, the electro-conductivity of the composite NCC film was 9.3 S/m, indicating the low reduction efficiency of pure RGO without TiO2. With the increase in the proportion of TiO2 added to the photocatalytic reaction, the electro-conductivity of the composite NCC-based film decreased gradually. However, TiO2 is non-conductive.With the increase in TiO2 content, the amount of catalyst gradually increased, whereas the proportion of the conductive filler RGO in the composites relatively decreased, leading to the decrease in the electro-conductivity of the NCC film. Therefore, in the current experiment, when the ratio of RGO to TiO2 was 9∶1, the electro-conductivity of the NCC film appeared to achieve the desired value.
3.3.3 Effect of TiO2-RGO on the mechanical properties of the NCC film
The mechanical properties of the NCC/TiO2-RGO nanocomposite film were evaluated using an universal tester for high-strength materials. Various loadings of TiO2-RGO (with the control of the TiO2-RGO photocatalytic ratio of 9∶1) were measured, and the results are shown in Fig.8. It can be observed that the mechanical properties of the electro-conductive NCC film generally show a tendency to increase first and thereafter decrease with the addition of TiO2-RGO. Specifically, in the absence of TiO2-RGO, the elastic modulus of the NCC film was 3498 MPa, which increased to 3998 MPa with the addition of 1% TiO2-RGO. This is because the hydrogen bonds between NCC and RGO molecules enhance the interaction between the two substances, thus further improving the mechanical properties of the conductive NCC/TiO2-RGO film. Nevertheless, with the continuous increase in the content of TiO2-RGO, the elastic modulus of the conductive NCC/TiO2-RGO film decreased gradually, which may be largely attributed to the distribution of the conductive fillers. Fig.9 shows the tensile strength of the conductive NCC/TiO2-RGO film. The tensile strength of the NCC film was 16.0 MPa, whereas the tensile strength of the NCC film with 1% TiO2-RGO reached 18.1 MPa. Overall, the addition of TiO2-RGO can lead to enhanced mechanical properties of the conductive NCC/TiO2-RGO film; however, the addition level should be strictly controlled.
3.3.4 Morphology of the NCC film filled with TiO2-RGO
The morphologies of all the samples were observed using SEM (FEI, SIRION 200). Fig.10 shows the SEM surface morphologies of the NCC film and the conductive NCC/TiO2-RGO film. The NCC film (Fig.10(a)) and the conductive NCC/TiO2-RGO film (Fig.10(b)) are easily distinguishable. The surface of the conductive NCC/TiO2-RGO film was relatively smooth. In contrast to the apparent rod-shaped particles and relatively large gap observed on the NCC film (Fig.10(c)) at high magnification, the high-magnification surface of the conductive NCC/TiO2-RGO film (Fig.10(d)) displayed a uniform and compact surface, indicating that TiO2-RGO was favorable for improving the dispersion of the NCC sol. Moreover, the compatibility between the two parts appeared to be ideal. The addition of the TiO2-RGO nanocomposites also helped to improve the barrier properties of the composite NCC film.
4 Conclusions
A flexible transparent electro-conductive film was prepared via vacuum filtration with ultrasonic dispersion, using NCC as the matrix and TiO2-RGO as the electro-conductive filler. The TiO2-RGO nanocomposite was successfully obtained via photocatalytic reduction, and subsequently, the suitable conditions for the film fabrication were discussed. The microstructure, electrical conductivity, and mechanical properties were comprehensively investigated. The results showed that the composite electro-conductive films exhibited the desired conductivity and mechanical properties when RGO∶TiO2=9∶1 and the TiO2-RGO loading was 1%. The optimum electrical conductivity was 9.3 S/m, and the elastic modulus and tensile strength reached 3998 MPa and 18.1 MPa, respectively. SEM analysis showed that the surface of the electro-conductive NCC/TiO2-RGO film was smooth, uniform, and compact. This work offersa routefor the production of electro-conductive NCC films, which may hold significant potential as transparent flexible substrates for future electronic device applications.
Acknowledgments
This work was financially supported by the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY14C160003, LQ16C160002), the National Natural Science Foundation of China (Grant No. 31100442), the Public Projects of Zhejiang Province (Grant No. 2017C31059), Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Zhejiang Open Foundation of the Most Important Subjects (Grant No. 2016KF01), and 521 Talent Cultivation Program of Zhejiang Sci-Tech University (Grant No. 11110132521310).
References
[1] Fu L H, Liu S, Li S M, et al. Characterization of Hemicelluloses Extracted from Populus tomentosa Carr. by the Hydrothermal Method with Ethanol[J]. Paper and Biomaterials, 2017, 2(3): 1-11.
[2] Fang G G, Liu S S, Shen K Z, et al. Wet Oxidation Pretreatment of Poplar Waste for Enhancing Enzymatic Hydrolysis Efficiency[J]. Paper and Biomaterials, 2017, 2(2): 8-17.
[3] Henriksson M, Berglund L A, Isaksson P, et al. Cellulose nanopaper structures of high toughness[J]. Biomacromolecules, 2008, 9(6): 1579-1585.
[4] Du Y F, Liu J G, Wang J F, et al. Eco-friendly PVA/Kaolin Clay Coating for Barrier Paper[J]. Paper and Biomaterials, 2017, 2(2): 26-32.
[5] Sun X, Wu Q, Ren S, et al. Comparison of highly transparent all-cellulose nanopaper prepared using sulfuric acid and TEMPO-mediated oxidation methods[J]. Cellulose, 2015, 22(2): 1123-1133.
[6] Yang S, Tang Y, Wang J, et al. Surface Treatment of Cellulosic Paper with Starch-Based Composites Reinforced with Nanocrystalline Cellulose[J]. Industrial & Engineering Chemistry Research, 2014, 53(36): 13980-13988.
[7] Gilje S, Han S, Wang M, et al. A chemical route to graphene for device applications[J]. Nano Letters, 2007, 7(11): 3394.
[8] Akhavan O. The effect of heat treatment on formation of graphene thin films from graphene oxide nanosheets. Carbon[J]. Carbon, 2010, 48(2): 509-519.
[9] Yang D, Velamakanni A, Bozoklu G, et al. Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy[J]. Carbon, 2009, 47(1): 145-152.
[10] Xu C, Wang X, Zhu J. Graphene-Metal Particle Nanocomposites[J]. Journal of Physical Chemistry C, 2008, 112(50): 19841-19845.
[11] Akhavan O, Ghaderi E. Photocatalytic Reduction of Graphene Oxide Nanosheets on TiO2 Thin Film for Photoinactivation of Bacteria in Solar Light Irradiation[J]. Journal of Physical Chemistry C, 2009, 113(47): 20214-20220.
[12] Gaya U I, Abdullah A H. Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems[J]. Journal of Photochemistry & Photobiology C Photochemistry Reviews, 2008, 9(1): 1-12.
[13] Tang Y, Yang S, Zhang N, et al. Preparation and characterization of nanocrystalline cellulose via low-intensity ultrasonic-assisted sulfuric acid hydrolysis[J]. Cellulose, 2014, 21(1): 335-346.
[14] Tang Y, Shen X, Zhang J, et al. Extraction of cellulose nano-crystals from old corrugated container fiber using phosphoric acid and enzymatic hydrolysis followed by sonication[J]. Carbohydr Polym, 2015, 125, 360-366.
[15] Tang Y, Mosseler J A, He Z, et al. Imparting Cellulosic Paper of High Conductivity by Surface Coating of Dispersed Graphite[J]. Ind. Eng. Chem. Res, 2014, 53(24): 10119-10124.
[16] Tang Y, Zhou D, Zhang J. Novel Polyvinyl Alcohol/Styrene Butadiene Rubber Latex/Carboxymethyl Cellulose Nanocomposites Reinforced with Modified Halloysite Nanotubes[M]. Hindawi Publishing Crop., 2013.
[17] Tang Y, He Z, Mosseler J A, et al. Production of highly electro-conductive cellulosic paper via surface coating of carbon nanotube/graphene oxide nanocomposites using nanocrystalline cellulose as a binder[J]. Cellulose, 2014, 21(6): 4569-4581.
[18] Jr W S H, Offeman R E. Preparation of Graphitic Oxide[J]. J. Am. Chem. Soc, 1958, 80(6): 1339.
[19] N I K, P J O, B R M, et al. Layer-by-Layer Assembly of Ultrathin Composite Films from Micron-Sized Graphite Oxide Sheets and Polycations[J]. Chemistry of Materials, 1999, 11(3): 771-778.
[20] Ding M C, Li C W, Chen F S, et al. Preparation and Characterization of Nanocrystalline Cellulose/Poly(lactic acid) Composite Membranes[J]. Paper and Biomaterials, 2017, 2(3): 28-34.
[21] Hong X, Yu W, Chung D D L. Electric permittivity of reduced graphite oxide[J]. Carbon, 2017, 111, 182-190.
[22] Hong X, Yu W, Chung D D L. Significant effect of sorbed water on the electrical and dielectric behavior of graphite oxide[J]. Carbon, 2017, 119, 403-418.
[23] Sheha E. Studies on TiO2/Reduced Graphene Oxide Composites as Cathode Materials for Magnesium-Ion Battery[J]. Graphene, 2014, 03(3): 36-43.
[24] Hong X, Yu W, Wang A, et al. Graphite oxide paper as a polarizable electrical conductor in the through-thickness direction[J]. Carbon, 2016, 109: 874-882.
[25] Paredes J I, Villar-Rodil S, Martínez-Alonso A, et al. Graphene Oxide Dispersions in Organic Solvents[J]. Langmuir the Acs Journal of Surfaces & Colloids, 2008, 24(19): 10560.
[26] Fan W, Lai Q, Zhang Q, et al. Nanocomposites of TiO2 and Reduced Graphene Oxide as Efficient Photocatalysts for Hydrogen Evolution[J]. Journal of Physical Chemistry C, 2011, 115(21): 10694-10701.
[27] Li D, Müller M B, Gilje S, et al. Processable aqueous dispersions of graphene nanosheets[J]. Nature Nanotechnology, 2008, 3(2): 101-105.
Comparable Characterization of Nanocellulose Extracted from Bleached Softwood and Hardwood Pulps
Bin Li1, 2,*, WenYang Xu2, Dennis Kronlund3, Jan-Erik Eriksson4,
Anni Mttnen3, Stefan Willfr2, ChunLin Xu2,*
1. CAS Key Laboratory of Bio-based Materials, CAS Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, Shandong Province, 266101, China
2. Johan Gadolin Process Centre, c/o Laboratory of Wood and Paper Chemistry,
Abstract: In this study, the characteristics of nanocellulose extracted from bleached softwood and hardwood pulps by formic acid hydrolysis followed by TEMPO-mediated oxidation were compared using transmission electron microscopy (TEM), atomic force microscopy (AFM), Fourier transform infrared analysis (FT-IR), X-ray diffraction (XRD), and thermal gravimetric analysis (TGA). The experimental results showed that the nanocellulose products derived from spruce pulp exhibited a relatively larger particle size, higher crystallinity, and higher thermal stability, compared with the corresponding products obtained from aspen pulp under the same conditions. Furthermore, the study helped establish that the properties of the nanocellulose products were highly dependent on the nature of the starting materials under identical processing conditions.
Keywords: nanocellulose; cellulose nanocrystal; cellulose nanofibril; formic acid; TEMPO-mediated oxidation; comparison
*Corresponding author:
Bin Li, PhD, associate professor; research interests: green and high efficient utilization of lignocellulosic resources;
E-mail: libin@qibebt.ac.cn
*Corresponding author:
ChunLin Xu, adjunct professor; research interests: biorefinery, bio-based functional products;
E-mail: cxu@abo.fi
1 Introduction
Nanomaterials with at least one dimension having a size less than 100 nm are expected to revolutionize the field of materials science, considering that their chemical, physical, or biological properties are fundamentally different from bulk materials[1]. Nanocellulose is an organic nanomaterial that can be released from cellulosic resource, e.g. microcrystalline cellulose (MCC), cotton fiber, wood pulp, sisal, ramie, agricultural straw, tunicate cellulose[2]. Owing to its unique properties such as high aspect ratio, excellent strength, light weight, low coefficient of thermal expansion, biodegradability, and renewability, nanocellulose can be used to enhance the strength and barrier properties of composites, such as films, paper[3-4], as well as the rheological properties or stability of products, e.g. coatings, food products, cosmetics, and pharmaceutical products[5-6]. It can also be used to manufacture recyclable, highly porous, flexible, transparent, or conductive materials for a number of applications[7-8]. Furthermore, nanocellulose has the potential for large-scale industrial manufacturing at a relatively lower cost compared to an inorganic nanomaterial, e.g. graphene[9], considering that the cellulosic biomass (raw material) is available in abundance and that the existing pulping/bleaching technologies can be used to produce nanocellulose to some extent; therefore, there has been a remarkable increase in the research activities, driven by the opportunities in potential applications of nanocellulose.
Nanocellulose derived from cellulosic fiber mainly has two nanostructured forms with different morphologies, i.e. cellulose nanofibril (CNF) and cellulose nanocrystal (CNC). These forms are obtained using different methods. The noodle-like CNF with a diameter of 2~100 nm and a length of a few micrometers can be produced using mechanical methods such as high pressure homogenization[3], microfluidization[10], grinding[11], ultrasonication[12], and enzymatic or chemical pretreatment (e.g. TEMPO-mediated oxidation) adopted prior to the physical nano-fibrillation step, which can significantly lower the energy consumption[13]. In contrast, the rod-like CNC (referred to as crystals) has a diameter in the range of 2~30 nm and a length in the range of about 100~1000 nm[10], and it is typically prepared using hydrolysis by a strong inorganic acid.
It is well known that the properties of the nanocellulose products are highly dependent on fabrication methods and processing conditions[14]; for instance, increasing the acid concentration, reaction temperature, or time of hydrolysis can lower the size of the fabricated CNC. In our previous studies, we adopted formic acid (FA) hydrolysis for the purpose of developing a clean and sustainable approach for the preparation of nanocellulose products, considering that FA could be readily recovered and reused[15], and the esterification of the cellulose surface by FA could also take place simultaneously. Subsequently, we investigated this approach in detail including the tunable preparation of CNC by FeCl3-catalyzed FA hydrolysis[16], the tailored fabrication of CNF using FA pretreatment and homogenization[17], the simultaneous manufacture of hydrophobic CNC and CNF by FA treatment and homogenization in organic solvents[4], the detailed kinetic study of FA hydrolysis for the preparation of nanocellulose[18], as well as the controllable extraction of lignin-containing CNF (from tobacco stover) with a larger diameter compared to the CNF without lignin[7].
However, the impact of the starting materials on the properties of the nanocellulose products using FA hydrolysis has not been investigated in detail. In the present work, we measured and compared the characteristics of the nanocellulose products extracted from bleached softwood and hardwood pulps using FA hydrolysis and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation. The conditions adopted for the fabrication of nanocellulose from two kinds of starting materials (aspen pulp and spruce pulp) were maintained identically. The fabricated samples of the two kinds of nanocellulose were compared with regard to morphology, surface property, and thermal stability to evaluate the impact of the starting materials.
2 Experimental
2.1 Materials
The samples of the bleached softwood (spruce) and hardwood (aspen) pulps were gifts from UPM (Finland). The spruce pulp contained 80.1 wt% cellulose, 17.6 wt% hemicelluloses, and 0.4 wt% lignin, while the aspen pulp had 75.4 wt%, 20.2 wt%, and 4.8 wt% of the respective constituents. FA (98 wt%) and NaClO solution (active chlorine 14 wt%) were purchased from VWR International LLC., while NaBr (extra pure) was from Merck KGaA. TEMPO (98 wt%) was obtained from Sigma-Aldrich (Finland). All chemicals were used as received.
2.2 FA hydrolysis of pulp
The FA hydrolysis of the bleached pulp was carried out based on the previous method[15]. Bleached pulp (2 g) was added to 100 mL FA, and the mixture was vigorously agitated using a Mixstab TEFAL Swing (300 W, regular kitchen blender) for 1 min to obtain a pulp-like slurry. The reaction was then allowed to take place in a spherical flask (250 mL) placed in an oil bath at 95℃ for 6 h with a magnetic stirring at 900 r/min. The flask was then immediately cooled to room temperature using tap water, and the reaction mixture was centrifuged (SORVALL R77 Plus) at 3500 r/min for 10 min. After separation of the spent liquor for recovery of FA, the cellulosic solid residue was washed by mixing with 200 mL of distilled water and then centrifuged at 3500 r/min for 10 min. The washing procedure was repeated three times. The fiber suspensions obtained were stored in a cold room (4℃) for further treatments and analyses. The yield of the isolated fibers was calculated based on the volume and solid content of the resultant fiber suspensions. Considering that the diameter and the length of the isolated fibers were 2~10 nm and several micrometers (based on transmission electron microscopy (TEM) analyses), respectively, the fiber samples isolated by FA hydrolysis were identified as CNF (named Aspen F and Spruce F, respectively).
2.3 TEMPO-mediated oxidation of CNF
The CNF obtained by FA hydrolysis was further treated by TEMPO-mediated oxidation[19]. The desired amounts of TEMPO (0.1 mmol/g-fiber) and NaBr (1 mmol/g-fiber) were dissolved in a certain volume of water, and then added to the CNF suspensions (the final consistency of CNF was 1 wt%) followed by stirring at 700 r/min. Subsequently, NaClO solution (10 mmol/g-fiber) was added dropwise to the CNF suspension during the first 10 min of the total 1 h oxidation time. The reaction was conducted at a pH value of 9.5 by adding 0.5 mol/L NaOH dropwise at room temperature. Upon completion of the reaction, the mixture was washed by an equivalent volume of ethanol and then centrifuged at 3500 r/min for 10 min. Subsequently, the supernatant was decanted, and the obtained cellulose gel was washed by de-ionized water twice, using the same washing procedure as mentioned above. Finally, the resultant nanocellulose suspension was stored in a cold room (4℃) for further analyses. According to the TEM results, the morphology of the obtained nanocellulose was changed to rod-like CNC after the TEMPO-mediated oxidation, and the CNC products were named as Aspen T and Spruce T, respectively.
2.4 Preparation of CNC by sulfuric acid hydrolysis
The CNC prepared by sulfuric acid (64 wt% H2SO4, 45℃ for 30 min) was used for comparison, and the CNC products obtained from the aspen and spruce pulps were named as Aspen S and Spruce S, respectively.
2.5 Particle size distribution and Zeta potential
The particle size distribution and Zeta potential of the nanocellulose samples were measured using a Zetasizer Nano ZS instrument (Malvern Instruments Ltd., UK) based on dynamic light scattering. All the sample suspensions were ultrasonically treated (VWR, 45 kHz and 180 W) for 10 min before measurement, and the solid concentration was maintained at 0.3 wt% for all samples. Every sample was tested in triplicate, while 7~15 runs were performed for each test, and the average values were reported. In addition, the content of the carboxyl group in the nanocellulose samples was determined using conductometric titration method[19].
2.6 TEM analysis
The TEM images of the nanocellulose samples were captured using a JEM-1400 Plus TEM microscope (JEOL Ltd., Japan) with an accelerating voltage of 120 kV. Prior to imaging, the samples were prepared based on our previous report[15].
2.7 Atomic force microscopy (AFM) analysis
AFM images of the nanocellulose samples were carried out with a Nanoscope V (MultimodeTM series, Bruker) AFM microscope in Peak Force Tapping TM mode at a peak force of 790~1150 pN, a scan speed of 0.68 s/m, a peak force amplitude of 50 nm, and a peak force frequency of 2 kHz. The images were obtained at ambient conditions (33% RH, 25℃) using a SCANASYST-AIR cantilever (Bruker) with a spring constant of 0.406 N/m (thermal tune). The Scanning Probe Image Processor (SPIPTM, Image Metrology, Denmark) software was used for processing and analyzing the AFM images. Before the AFM analyses, one drop of nanocellulose dispersion (2 mg/mL) was applied on a freshly cleaned mica and air dried at room temperature.
2.8 Fourier transform infrared (FT-IR) analysis
FT-IR analyses of pulp and nanocellulose samples were performed using a PerkinElmer FTIR-1000 spectrometer (USA), and the spectra were recorded in the wave number range of 400~4000 cm1 with a resolution of 4 cm1. Before the analysis, KBr pellets were prepared for each sample with a weight ratio (KBr to freeze-dried sample) of 100∶1.
2.9 X-ray diffraction (XRD) measurement
The XRD patterns of the samples of the pulp and the nanocellulose samples were recorded at ambient conditions on an X-ray diffractometer (Bruker Discover D8, Germany), whereby the freeze-dried samples were pressed into flattened sheets on a sample holder before the tests. The detailed test conditions were maintained as the same as those in our previous report[15]. The crystallinity index (CrI) of each sample was calculated using the empirical Equation (1), and the crystal size (Dhkl) was estimated using the Scherrer Equation (2)[20] shown as follows:
Where, I200 is the maximum peak intensity at lattice diffraction (200), Iam is the minimum intensity between planar reflections (200) and (110), Dhkl is the crystal dimension perpendicular to the diffracting planes with Miller indices of hkl, is the wavelength of X-ray radiation (=1.5406 ), and 1/2 is the full width at half-maximum of the diffraction peaks.
2.10 Thermal gravimetric analysis (TGA)
TGA of the pulp and nanocellulose samples was carried out on a thermogravimetric analyzer (Netzsch STA 449 F1 Jupiter, Germany). The temperature was raised to 600℃ from room temperature at a heating rate of 10℃/min under N2 (25 mL/min). Both TGA and differentialthermogravimetric (DTG) curves were recorded.
3 Results and discussion
3.1 Comparison of the size and morphology of nanocellulose products
In this work, the bleached aspen and spruce pulps were used as starting materials for the preparation of nanocellulose using the same method under identical conditions. In step I, FA hydrolysis was conducted, and the resultant nanocellulose was found to be CNF, i.e. Aspen F and Spruce F. In step II, a TEMPO-mediated oxidation of the CNF (obtained from step I) was carried out, and the resultant nanocellulose was found to be CNC, i.e. Aspen T and Spruce T. Table 1 lists the yield, mean size, Zeta potential, mobility, and carboxyl group content of the nanocellulose products. As can be seen, the yields of Spruce F and Spruce T were higher compared to Aspen F and Aspen T, respectively; this phenomenon was attributed to the higher values of the density, fiber length, and cellulose crystallinity (Table 2) of the spruce pulp in comparison with the aspen pulp[21-22]. Consequently, the mean sizes of Spruce F (5193 nm) and Spruce T (523 nm) were also larger than those of Aspen F (3280 nm) and Aspen T (276 nm), respectively, and the detailed distribution of the particle size is shown in Fig.1. In other words, the aspen pulp with shorter fiber was easier to be hydrolyzed compared to spruce pulp under the same treatment conditions.
Table 1 also shows that both Aspen F and Spruce F had a relatively lower Zeta potential (absolute value). This was because no additional charge was introduced on the cellulose surface, and the hydrophobic ester groups were formed on the surface during FA hydrolysis[15]; thus, the obtained CNF (Aspen F and Spruce F) with hydrophobic surface could not be well dispersed in water, but they could be well dispersed in the solvents such as dimethylacetamide (DMAC), N,N-Dimethylformamide (DMF), and dimethyl sulfoxide (DMSO)[17]. After the TEMPO-mediated oxidation, the Zeta potentials (absolute values) of the obtained CNC (Aspen T and Spruce T) were found to be increased, which was attributed to the significant increase of surface carboxyl groups (Table 1). The carboxyl group content of Aspen T (1.5 mmol/g) was obviously higher than that of Spruce T (1.3 mmol/g), which was also probably due to the relatively looser structure and lower crystallinity (Table 2) of the aspen pulp[21]; for the same reason, the aspen pulp was found to be easier to be reacted.
Fig.2 shows the TEM images of the nanocellulose products. Although both Aspen F and Spruce F fibers were found to be aggregated, the individual fibers were distinctly identifiable; the fiber diameter of Aspen F was approximately 2~4 nm, while it was approximately 6~9 nm in the case of Spruce F. After the TEMPO-mediated oxidation, the fibers of the obtained Aspen T and Spruce T were found to be well dispersed owing to the increased surface charge density[15], and the fiber length was clearly reduced as presented in Table 1, Fig.1, and Fig.2. Interestingly, the diameter of Aspen T continued to be in the range of 2~4 nm, whereas the diameter of Spruce T was decreased to 3~6 nm after the oxidation. These results also indicated that the aspen pulp with shorter fiber length, smaller fiber diameter, and lower cellulose crystallinity could be easier to be hydrolyzed and reacted compared with spruce pulp[23], thereby yielding the nanocellulose products with smaller size.
Fig.3 exhibits the AFM topographical images of nanocellulose products. Both Aspen F and Spruce F fibers were found to be highly aggregated in line with the TEM images shown in Fig.2 but with a different appearance. In contrast, the individual fibers of Aspen T and Spruce T were distinctly identifiable, despite a few aggregations that were probably due to the high sample concentration. In addition, the calculated root mean square surface roughness (Sq values) for Aspen F, Aspen T, Spruce F, and Spruce T were 11.9, 5.1, 37.2, and 4.5 nm, respectively. These results indicated that smoother and thinner films could be formed with Aspen T and Spruce T after water evaporation compared to Aspen F and Spruce F. Similar results for birch pulp were also reported previously[15].
3.2Comparison of surface chemical and crystalline structure of nanocellulose
Surface chemical properties of the pulp and nanocellu-lose samples were characterized using FT-IR, and the corresponding spectra are shown in Fig.4. As can be seen, the wave number band between 3500 cm1 and 3250 cm1 was related to O—H stretching, and the band close to 2900 cm1 was related to the C—H stretching vibration in CH2 groups[24]. The small shoulder peak at 1725 cm1 in the spectrum of bleached aspen pulp was associated with the C=O stretching in acetyl groups of hemicelluloses[15], and the peak near 1635 cm1 was ascribed to O—H bending vibration of the adsorbed water[24]. In contrast, the sharp peak at 1725 cm1 in the spectrum of Aspen F was probably due to the C=O stretching vibration in the adsorbed FA on the surface of cellulose. On one hand, FA could be adsorbed onto the surface of cellulose owing to the formation of hydrogen bonds[25]. On the other hand, FA could swell the cellulose fibers and react with them, thus forming cellulose formate; therefore, the strong peak at 1725 cm1 was also partially attributed to the C=O stretching in cellulose formate[26]. As measured, the carboxyl group content of Aspen F was (0.525±0.015) mmol/g (Table 1). As for the spectrum of Aspen T, the peak at 1725 cm1 was related to the C=O stretching in COO, and the peak at 1610 cm1 was because of the antisymmetric stretching of COO in carboxylate salts. The presence of these two peaks indicated that the carboxyl groups were introduced on Aspen T after the modification of the surface of Aspen F by TEMPO-mediated oxidation, and the corresponding carboxyl group content of Aspen T was (1.501±0.022) mmol/g, as listed in Table 1. Comparable results were also obtained for the spruce samples, as presented in Fig.4(b), but the corresponding carboxyl group content of Spruce T was only (1.292±0.001) mmol/g (Table 1).
Table 2 gives the crystallinity and crystal size values of pulp and nanocellulose products. The total crystallinity index (TCI), lateral order index (LOI), and hydrogen-bond intensity (HBI) were calculated on the basis of the FT-IR absorbance ratio of A1372/A2900, A1430/A897, and A3308/A1330, respectively[27]. The TCI, LOI, and HBI were sensitive to the degree of the intermolecular regularity of cellulose and the crystal system[28]. The crystallinity index (CrI) and crystal size (Dhkl) were calculated based on the XRD determination.
As can be seen from Table 2, for the aspen samples, the TCI of Aspen F (1.44) increased compared to aspen pulp (1.22), which was attributed to the degradation of amorphous area of the cellulose during FA hydrolysis[7,15]; however, the TCI of Aspen T was lower than that of Aspen F, indicating that a part of the crystalline cellulose molecules in Aspen F developed a disorderly structure during the process of TEMPO-mediated oxidation. This was in agreement with the corresponding CrI values (Table 2). Similar results were also reported earlier[13]. Compared to the pulp sample, the successively increasing LOI of Aspen F and Aspen T indicated the orderly structure of the obtained nanocellulose products[28]. Similarly, the successively increasing HBI values of Aspen F and Aspen T indicated that more hydroxyl groups were exposed on the surface of CNF/CNC, thereby increasing the hydrogen bonding[15]. Furthermore, the crystal sizes of Aspen F and Aspen T were smaller than that of aspen pulp. The changes in crystallinity for spruce pulp had a similar trend, but the CrI values of Spruce F (74.3%) and Spruce T (66.9%) were higher in comparison with the corresponding Aspen F (72.8%) and Aspen T (60.4%). Also, the intensity peaks of XRD for spruce samples were stronger compared to the corresponding aspen samples, as shown in Fig.5. These results were probably due to the fact that the CrI of spruce pulp was higher than that of aspen pulp (Table 2). Thus, the CrI of the resultant nanocellulose was dependent on the original CrI of the starting material under identical preparation method and process conditions.
3.3 Comparison of thermal stability of nanocellulose
Fig.6 shows the TGA and DTG curves of pulp and nanocellulose samples. The corresponding thermal degradation onset temperature (Ton) and maximum decomposition temperature (Tmax) for all the samples are given in Table 3. Compared with aspen pulp, the thermal stability of Aspen F was clearly improved as evident from the increased value of Ton, and the increase was attributed to the removal of hemicellulose and part of amorphous regions of cellulose during FA hydrolysis[13,15]; however, the Ton and Tmax values of Aspen T and Aspen S were obviously lower compared with aspen pulp (Table 3) mainly due to the fact that the carboxyl and sulfate groups introduced on the surface of CNC products reduced the activation energies of the degradation of cellulose[14]; thus, the thermal stability of pulp and nanocellulose products could be sorted in an ascending order: Aspen S
The DTG curve of Aspen T shows two peaks in the vicinity of 224℃ and 282℃ related to the degradation of sodium anhydroglucuronate units and crystalline cellulose chains, respectively. The decreasing thermal stability of the crystalline cellulose chains in Aspen T was attributed to the presence of the thermally unstable anhydroglucuronate units (i.e. carboxyl groups)[29]. On the other hand, the smaller size of CNC could lead to exposure of a higher specific surface area to heat, thereby disrupting the crystal structure of the cellulose, and its lower crystallinity affected the thermal stability of cellulose adversely[13] . Fig.6 also exhibits the total weight losses of Aspen S, Aspen T, aspen pulp, and Aspen F as 72.4%, 73.6%, 88.8%, and 92.8%, respectively. The higher char yields of Aspen S and Aspen T at 600℃ (27.6% and 26.4%, respectively) compared with the aspen pulp and Aspen F were attributed to the presence of sulfate and carboxyl groups that facilitated the char formation[14,29]. In addition, Fig.6 and Table 3 indicated a relatively higher thermal stability of the spruce samples compared to the corresponding aspen samples, which was attributed to the higher crystallinity (Table 2) and larger particle size (Table 1) of the spruce samples. For instance, the Ton values of spruce pulp, Spruce F, Spruce T, and Spruce S were 314, 329, 213, and 153℃, respectively, while the corresponding values of aspen pulp, Aspen F, Aspen T, and Aspen S were 307, 326, 212, and 146℃, respectively.
4 Conclusions
This study investigated the properties of the nanocellulose products fabricated from the bleached aspen and spruce pulps under the same conditions by comparing their characteristics. It was found that the aspen pulp underwent hydrolysis and reaction more easily than the spruce pulp, and the resultant nanocellulose products derived from the aspen pulp exhibited a relatively smaller particle size, lower crystallinity, and lower thermal stability compared to the corresponding products obtained from the spruce pulp. This was attributed mainly to the relatively longer fiber length, higher crystallinity, and higher thermal stability of the spruce pulp compared to the aspen pulp; thus, the study helped establish that the starting materials influenced the properties of the resultant nanocellulose products under identical process conditions.
Acknowledgments
Financial support was from the Johan Gadolin Scholarship Programme at the Johan Gadolin Process Chemistry Centre at bo Akademi University (Finland), and the National Natural Science Foundation of China (No. 31470609). The authors would also like to thank Dr. Markus Peurla at Lab. of Electron Microscopy, University of Turku (Finland) for helping on the TEM measurements.
References
[1] Xia Y, Yang P, Sun Y, et al. One-dimensional nanostructures: synthesis, characterization, and applications[J]. Advance Materials, 2003, 15(5): 353-389.
[2] Zhu H, Luo W, Ciesielski P N, et al. Wood-derived materials for green electronics, biological devices, and energy applications[J]. Chemical Review, 2016, 116(16): 9305-9374.
[3] Liu C, Du H, Yu G, et al. Simultaneous extraction of carboxylated cellulose nanocrystals and nanofibrils via citric acid hydrolysis—a sustainable route[J]. Paper and Biomaterials, 2017, 2(4): 19-26.
[4] Du H, Liu C, Wang D, et al. Sustainable preparation and characterization of thermally stable and functional cellulose nanocrystals and nanofibrils via formic acid hydrolysis[J]. Journal of Bioresources and Bioproducts, 2017, 2(1): 10-15.
[5] Tang Y, Mosseler J A, He Z, et al. Imparting cellulosic paper of high conductivity by surface coating of dispersed graphite[J]. Industrial & Engineering Chemistry Research, 2014, 53(24): 10119-10124.
[6] Liu C, Du H, Lv D, et al. Properties of nanocelluloses and their application as rheology modifier in paper coating[J]. Industrial & Engineering Chemistry Research, 2017, 56(29): 8264-8273.
[7] Wang Q, Du H, Zhang F, et al. Flexible cellulose nanopaper with high wet tensile strength, high toughness and tunable ultraviolet blocking ability fabricated from tobacco stalk via a sustainable method[J]. Journal of Materials Chemistry A, 2018, 6: 13021-13030.
[8] Bian H, Zhu J, Chen L, et al. Toward sustainable, economic, and tailored production of cellulose nanomaterials[J]. Paper and Biomaterials, 2017, 2(4): 1-7.
[9] Postek M T, Moon R J, Rudie A W, et al. Production and applications of cellulose materials[M]. USA: TAPPI Press, 2013.
[10] Du H, Liu C, Zhang M, et al. Preparation and industrialization status of nanocellulose[J]. Progress in Chemistry, 2018, 30(4): 448-462.
[11] Wang C, Li L, Zhao M, et al. Co-effect of mechanical ball-milling and microenvironmental polarity on morphology and properties of nanocellulose[J]. Paper and Biomaterials, 2017, 2(4): 8-18.
[12] Chen W, Yu H, Lee S-Y, et al. Nanocellulose: a promising nanomaterial for advanced electrochemical energy storage[J]. Chemical Society Reviews, 2018, 47: 2837-2872.
[13] Liu C, Li B, Du H, et al. Properties of nanocellulose isolated from corncob residue using sulfuric acid, formic acid, oxidative and mechanical methods[J]. Carbohydrate Polymers, 2016, 151: 716-724.
[14] Liu Y, Wang H, Yu G, et al. A novel approach for the preparation of nanocrystalline cellulose by using phosphotungstic acid[J]. Carbohydrate Polymers, 2014, 110: 415-422.
[15] Li B, Xu W, Kronlund D, et al. Cellulose nanocrystals prepared via formic acid hydrolysis followed by TEMPO-mediated oxidation[J]. Carbohydrate Polymers, 2015, 133: 605-612.
[16] Du H, Liu C, Mu X, et al. Preparation and characterization of thermally stable cellulose nanocrystals via a sustainable approach of FeCl3-catalyzed formic acid hydrolysis[J]. Cellulose, 2016, 23(4): 2389-2407.
[17] Du H, Liu C, Zhang Y, et al. Preparation and characterization of functional cellulose nanofibrils via formic acid hydrolysis pretreatment and the followed high-pressure homogenization[J]. Industrial Crops and Products, 2016, 94: 736-745.
[18] Lv D. Kinetic study of formic acid hydrolysis for the preparation of nanocellulose and the application of nanocellulose[D]. Beijing: University of Chinese Academy of Sciences, 2018.
[19] Liu J, Korpinen R, Mikkonen K S, et al. Nanofibrillated cellulose originated from birch sawdust after sequential extractions: a promising polymeric material from waste to films[J]. Cellulose, 2014, 21(4): 2587-2598.
[20] Segal L, Greely J, Martin A, et al. An empirical method for estimating the degree of crystallinity of native cellulose using the X-ray diffractometer[J]. Textile Research Journal, 1959, 29(10): 786-794.
[21] Li B, Li H, Zha Q, et al. Review: Effects of wood quality and refining process on TMP pulp and paper quality[J]. BioResources, 2011, 6(3): 3569-3584.
[22] Li B, Bandeker R, Zha Q, et al. Fiber Quality Analysis: Fiber Quality Analyzer versus L&W Fiber Tester[J]. Industrial & Engineering Chemistry Research, 2011, 50(22): 12572-12578.
[23] Sixta H. Handbook of Pulp[M]. Weinheim: Wiley-VCH, 2006.
[24] Li B, Mou H, Li Y, et al. Synthesis and thermal decomposition behavior of zircoaluminate coupling agent[J]. Industrial & Engineering Chemistry Research, 2013, 52(34): 11980-11987.
[25] Sun Y, Lin L. Hydrolysis behavior of bamboo fiber in formic acid reaction system[J]. Journal of Agricultural Food & Chemistry, 2010, 58: 2253-2259.
[26] Fujimoto T, Takahashi S J, Tsuji M, et al. Reaction of cellulose with formic acid and stability of cellulose formate[J]. Journal of Polymer Science: Part C: Polymer Letter, 1986, 24(10): 495-501.
[27] Oh S Y, Dong I Y, Shin Y, et al. Crystalline structure analysis of cellulose treated with sodium hydroxide and carbon dioxide by means of X-ray diffraction and FTIR spectroscopy[J]. Carbohydrate Research, 2005, 340(15): 2376-2371.
[28] Spiridon I, Teacǎ C-A, Bodrlǎu R. Structural changes evidenced by FTIR spectroscopy in cellulose materials after pretreatment with ionic liquid and enzymatic hydrolysis[J]. BioResources, 2011, 6(1): 400-413.
[29] Fukuzumi H, Saito T, Okita Y, et al. Thermal stabilization of TEMPO-oxidized cellulose[J]. Polymer Degradation & Stability, 2010, 95(9): 1502-1508.
Review of Cellulose Nanocrystal-based Fluorophore Materials and Their Application in Metal Ion Detection
Ya Wang1, Alain Dufresne2, Peter R. Chang3, XiaoZhou Ma1,*,
Jin Huang1,*
1. School of Chemistry and Chemical Engineering, Joint International Research Laboratory of Biomass-Based Macromolecular Chemistry and Materials, Southwest University, Chongqing, 400715, China
2. Université Grenoble Alpes, CNRS, Grenoble INP, LGP2, F-38000 Grenoble, France
3. Bioproducts and Bioprocesses National Science Program, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, SK, S7N0X2, Canada
Abstract: Cellulose nanocrystals (CNCs), a unique and promising natural material extracted from native cellulose, have attracted considerable attention owing to their physical properties and special surface chemistry. This review focuses on chemical conjugation strategies that can be used for preparation of fluorescent-molecule labeled CNCs and the development of biomaterials. Furthermore, their application in the detection of metal ions and future development prospects are discussed. We hope to provide a clear view of the strategies for surface fluorescent modification of CNCs and their application in detection of metal ions.
Keywords: fluorescent cellulose nanocrystals; detection of metal ions; sensors; optical imaging; biomaterial
Ya Wang, master candidate;
E-mail: wy0206@163.com
*Corresponding author:
XiaoZhou Ma, PhD;
E-mail: maxiaozhou@swu.edu.cn
*Corresponding author:
Jin Huang, professor; research interests: soft-matter chemistry and nanomaterials, especially novel materials derived from biomass resources;
E-mail: huangjin2015@swu.edu.cn
1 Introduction
The development of nanomaterials with sensing capability has grown over the past decade to meet the demand for applications in metal ion detection, drug delivery, and biological and biomedical sciences, as well as agricultural production[1-4]. In order to avoid potential environmental safety issues with nanomaterials and meet the need for renewable resource utilization, sustainable development and application of cellulose-based nanomaterials from renewable resources or waste materials have increasingly become a focus of research[5]. Cellulose is a linear homopolysaccharide that consisted of D-glucopyranose units linked by 1,4--linkages[6-7]. During biosynthesis, van der Waals and intermolecular hydrogen bonds between hydroxyl groups and the oxygens of adjacent molecules promote parallel stacking of multiple cellulose chains, forming basic fibers that further aggregate into larger microfibers[8]. These fibrils act as reinforcing elements in trees, plants, some marine organisms (tunicates), algae, and bacteria[9-11]. In these cellulose fibers, highly ordered (crystalline) structures of cellulose chains and disordered (amorphous) regions can be found. The crystalline regions contained within cellulose microfibers can be extracted to produce cellulose nanocrystals (CNCs)[12].
CNCs, which are also called cellulose nanowhiskers or nanocrystalline cellulose in some reports, exhibit special morphological, self-assembly, and modification ability properties. Fabricating new high-performance functional nanocomposites that can be applied in detection, diagnosis, or other biomedical applications is a frontier and a hot issue in cellulose-based research. Compared to inorganic nanoparticles, CNCs have many desirable properties and advantages, such as high crystallinity, a large surface area, high tensile strength and modulus, renewability, biodegradability, excellent colloidal stability, and facile modification[13]. CNCs are typically rod-like nanoparticles with a length of 100 nm to 1000 nm and a width of 20 nm to 100 nm[14-16]. CNCs can be isolated from a variety of cellulose sources, including plants (e.g., cotton, hemp, wood)[17-18], marine animals (e.g., tunicates)[19-21], bacteria (e.g., Acetobacter xylinum)[22], and so on, and also could be isolated from microcrystalline cellulose via mechanical shearing and acid and enzymatic hydrolysis[23-24]. The dimensions, crystal structure, degree of crystallinity, surface chemistry, morphology, and aspect ratio of the extracted CNCs depend strongly on the nature of the raw material and the reaction conditions. The most popular strategy for producing CNCs is to use acid hydrolysis, which can effectively remove the disordered or paracrystalline regions of the cellulose fiber while releasing CNCs with high crystallinity. Commonly, sulfuric acid[25-26], hydrochloric acid[27], phosphoric acid[28], and hydrogen bromide[29] can be used in this strategy to produce CNCs with different functional groups on their surfaces. The conditions of acid hydrolysis (e.g., the acid type and concentration, hydrolysis time, and temperature) will seriously affect the physical properties of the produced CNCs (i.e., the surface charge, size, yield, and birefringence). For example, CNCs obtained using sulfuric acid exhibit significantly lower thermal stability and higher stability in suspension than their counterparts obtained from hydrochloric acid.
On the one hand, CNCs exhibit favorable characteristics including biocompatibility, biodegradability, renewability, and nanoscale size; consequently, they have been used extensively to prepare sensing nanomaterials for applications in various fields[30]. The hydroxyl groups and large specific surface area of CNCs enable modification by physical or chemical reactions. CNCs have a large number of hydroxyl functional groups in their macromolecular chains, and owing to their high specific surface area, they can be easily modified via various reactions, such as carboxylation[31], esterification[32], silanization[33-34], cationic interaction, [3, 5], and graft copolymerization[36]. Chemical modification can introduce new functional groups or molecules onto the CNC surface while retaining the basic structure and properties of the particle[37]. For example, Dong et al introduced fluorescent molecules onto the CNC surface, and the fluorescent-tagged CNCs exhibited good potential for application in bioimaging and related diagnostics[38]. On the other hand, functional cellulose nanocrystalline materials are used as filler in polymers to produce high-performance nanocomposites[39-40], stimulus-responsive films[41-43], hydrogels[44], or photoinduced materials[45]. They are also used as templates for the synthesis of chiral[46], nematic[47], and porous materials, silica films, or TiO2[48-49]. Furthermore, CNCs have been demonstrated to be biocompatible with living cells, thus they can also be used for in vivo testing[50].
Fluorescent molecules (small molecules or macromolecules) can be excited by light at a specific wavelength and radiate fluorescence at another wavelength that is different from the excitation wavelength. Thus, fluorescent molecules are often used as sensors or indicators to detect target molecules. By introducing fluorescent molecules on the CNC surface, fluorescent CNCs (fCNCs) can be obtained, which can be dispersed in an aqueous solvent and display stable luminosity[51]. The wavelength of fCNCs depends strongly on the fluorescent molecules that have been grafted onto their surfaces. A wide range of fluorescent molecules or quantum dots (QDs) were grafted onto CNCs using different binding mechanisms. A common strategy to produce a fCNC sensor is to modify fluorescent molecules that can be used for detection onto its surface[52-55]. Many fluorescent indicators of heavy metal ions and transition metal ions such as Cu2+, Hg2+, Pb2+, Cr3+, and so on have been reported[56-60]. For example, Chen et al synthesized CNCs with porphyrin pendants (CNC-SA-COOC6TPP) by a combination of carboxylation, extended esterification, and the dicyclohexylcarbodiimide reaction[61]. Owing to the good dispersion of fCNCs and the high selectivity and sensitivity of porphyrin derivatives, CNC-SA-COOC6TPP can efficiently detect Hg2+ in water. CNCs were converted into fluorescent-labeled nanoparticles (Py-CNCs) by a three-step procedure. This sensing nanomaterial can be employed as a chemosensor for Fe3+ and for many applications in chemical, environmental, and biological systems[62].
Many reviews have summarized the surface modification and application of CNCs[5,10-11,50]. This review focuses on chemical conjugation strategies that can be used for the development of biomaterials using fluorescent-molecule-labeled CNCs. In the first part, the reactions used for fluorescence modification of CNCs are introduced. Then, by considering the differences among the molecules used to modify CNCs, the use of fCNCs for sensors is analyzed, and the detection mechanism is briefly introduced. Finally, a short summary and prospective are given.
2 Methods for surface chemical modification of CNCs
The surface chemistry of CNCs, especially the surface groups, is governed by the type of acid used for the hydrolysis procedure[37]. Because of the highly ordered arrangement of the crystalline structure, the reactivity of CNCs is different from that of a macromolecular polysaccharide, so they should not be simply regarded as multi-hydroxyl alcohols. The reactions between hydroxyl groups and a reagent occur only on the surface of CNCs (Fig.1). The preparation of fluorescent-tagged CNCs depends mainly on the chemical activity of functional groups on the surface of the CNCs and the resulting physical interaction. Typically, fCNCs are prepared mainly by the formation of covalent bonds between fluorophores and hydroxyl groups on the surface of CNCs, including esterification, etherification, amidation, the formation of a Schiff base, and the nucleophilic substitution reaction. The extraction and pre-chemical modification of CNCs for the preparation of fCNCs are currently performed as follows.
Typical procedures for producing CNCs include the following steps:
(1) The cellulose raw material is purified under strict control of the temperature, stirring time, acid selection and concentration, and ratio of acid to cellulose. Subsequently, the pure cellulose material is hydrolyzed by a strong acid.
(2) Continuous centrifugation and repeated washing.
(3) Removal of free acid molecules by ultrafiltration.
(4) Concentration and drying of the suspension to obtain solid CNCs. The obtained CNCs are generally rod-shaped nanocrystals with a high aspect ratio.
The hydroxyl groups on the CNC surface can be easily used to react with carboxyl groups or isocyanate groups on the fluorescent molecules. However, to conjugate fluorescent molecules that cannot be reacted with hydroxyl groups on the CNC surface, it is necessary to modify CNC with suitable functional groups, such as carboxyl groups or aldehyde groups. The CNC surface can be decorated with any of these groups via oxidation or esterification. Here, we will briefly discuss the mechanism of oxidation, a fundamental procedure for producing fCNCs.
A common strategy for oxidizing hydroxyl groups on the CNC surface is to use 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) as a catalyst to facilitate oxidation of the CNCs by NaClO[63-67]. The strategy can selectively oxidize C6—OH on the CNC surface into carboxyl groups. The mechanism is shown in Fig.2(a).
The oxidation process is pH-dependent and can be monitored using the pattern of aqueous NaOH consumption. In addition, in TEMPO/NaBr/NaClO oxidation of CNCs, the pH value of the reaction obviously affects the efficiency or the time required for oxidation. The optimum pH value for shortening the oxidation time was found to be 10[63].
The oxidized products have almost homogeneous chemical structures of -1,4-linked polyglucuronic acid (cellouronic acid). Hence, the C6-primary hydroxyls of CNCs can be entirely converted to C6-sodium carboxyl or aldehyde groups by TEMPO-mediated oxidation, while the reaction also show a great selectivity to the C6-OH of CNC[70-72]. The maximum degree of oxidation (DOmax) of CNCs was calculated as 0.095 (roughly 0.1) using a simplified rectangle model. As the NaClO/anhydroglucose unit ratio changed, the DO of CNCs varied from partial oxidation (DO=0.029) to nearly complete oxidation (DO=0.097)[13, 73-74].
However, in TEMPO oxidation of CNCs, hypochlorite is used. Heating of hypochlorite in water can generate chlorine, which is extremely harmful for the environment; this can seriously limit the industrialization of the TEMPO oxidation process. APS is an inexpensive oxidant with low long-term toxicity and high water solubility[69,75] (Fig.2(c)). Persulfate oxidation is a novel method because the carboxylic acid functionality is a direct result of the particle extraction method. Under heating, persulfate hydrolyzes in aqueous solution to form hydrogen peroxide and persulfate free radicals, which are strongly oxidizing. Under their combined action, the lignin, hemicellulose, and amorphous regions of cellulose in raw plant fiber materials are oxidized and degraded, thereby releasing the crystalline cellulose region. In addition, the persulfate method can effectively oxidize the hydroxyl groups on the surface of CNCs into carboxyl groups, increase the surface charge of CNCs, and enhance the colloidal stability of CNCs. Consequently, the resulting CNCs are uniform and possess nanoscale dimensions.
Sodium periodate is another useful oxidant that can be used to introduce aldehyde groups onto the CNC surface[31,41,68] (Fig.2(b)). It can selectively oxidize the C2 and C3 hydroxyl groups into 2,3-dialdehyde units. The resulting product is generally used to react with substances that have a primary amine group to form a Schiff base. For example, Alexa Fluor dyes can be facilely conjugated onto CNCs by this strategy while retaining the overall crystalline structure of the CNCs. The amino groups on Alexa Fluor dyes can form a Schiff base with the aldehyde groups on oxidized CNCs under mild conditions[77]. Further, 7-hydrazino-4-methylcoumarin (HMC) or 7-amino-4-methylcoumarin (AMC) can also be prepared by attaching hydrazine- or amino-substituted fluorophores onto CNCs to form hydrazine and Schiff-base compounds[78].
Anhydrides are a type of commonly selected reagent for esterification of the CNC surface (Fig.1). Mild conditions and controllable anhydride esterification were used to produce CNCs with a highly carboxylated ethylenediaminetetraacetic dianhydride (EDTAD)-modified surface (ECNCs), which are expected to maintain their integration, in contrast to those produced by TEMPO oxidation, for easy regulation of the densities of molecules. EDTAD can also be used for esterification of hydroxyl groups on the CNC surface. Acetic anhydride has been proved to be an effective reagent for acetylation of the CNC surface. Alkenyl succinic anhydride has also been used to improve the compatibility of CNCs with non-polar materials[79-80]. Via esterification, the modified CNCs can achieve a higher carboxylation degree than TEMPO-oxidized CNCs and can be used to make fCNCs with higher fluorescence intensity. The application of these reactions in the preparation of fCNCs will be introduced in detail in the following section.
3 Methods of preparing fluorescent-tagged CNCs
Owing to the various functional groups on fluorescent molecules (small molecules or macromolecules) or QDs, e.g., the isocyanate group, carboxyl group, amino group, hydrazine group, and alkynyl group, many types of reactions can be applied. Thus, fCNCs can form chemical or physical interactions between fluorescent molecules or QDs and the surface-active groups of the CNCs. Here, we will focus mainly on methods of constructing fCNCs. Table 1 summarizes recent studies on surface immobilization of fluorescent molecules (small molecules or macromolecules) or QDs on CNCs using different strategies (chemical binding or physical absorption).
3.1 Carbamation reaction
Isocyanate or isothiocyanate can react with hydroxyl groups or amino groups on the surface of CNCs[38,96-97,99]. This reaction is typically used to modify the CNC surface with fluorescent molecules or other functional groups. Therefore, this reaction is very suitable for CNC surface modification in some cases. However, the base used in this reaction may damage biomacromolecules such as proteins, so the use of the resulting CNCs in living organisms is limited. In 2007, Roman’s group reported a three-step strategy for covalently attaching fluorescein-5′-isothiocyanate (FITC) molecules to the surface of CNCs[38,99]. First, epichlorohydrin was attached to the hydroxyl groups of CNCs by nucleophilic substitution. Subsequently, ammonium hydroxide was used to convert the epoxy group to an amino group. Finally, FITC was conjugated onto CNCs via the reaction between the isothiocyanate group and amino group. It was shown that the unlabeled suspension was colorless and slightly opaque, whereas the FITC-labeled CNC suspension appeared clear and yellow. Further, according to the results of UV/vis spectroscopy, the FITC-labeled nanocrystals showed the absorption maxima peak associated with both the dianionic (490 nm) and anionic (453 and 472 nm) forms of FITC, whereas unlabeled CNCs did not show any absorption peak in the wavelength range 200~600 nm. A calculation showed that approximately 30 mol/g of FITC was attached to each CNC, which was sufficient for application in bioimaging.
The method of modifying the surface of CNCs with two fluorescent molecules (FITC and RBITC) separately is similar to that described above[99-100]. A suspension of RBITC-labeled CNCs appeared wine red and exhibited emission under excitation at 540 nm, which is attributed to RBITC. Note that in comparison with a free RBITC aqueous solution, the fluorescence emission of RBITC-labeled CNCs was red-shifted (from 577 nm to 584 nm), which was attributed to the influence of the fluorescent molecules covalently attached to the nanocrystals. Although both molecules showed low cytotoxicity and strong fluorescence, further study demonstrated that FITC-CNCs and RBITC-CNCs were unsuitable for use as biomarkers. Compared to FITC-CNCs, RBITC-CNCs penetrated the cell membrane more easily at various pH values owing to their cationic surfaces. However, the FITC-CNCs could penetrate the cell membrane only at low pH values but aggregated on the cell surface in neutral and high pH environments, allowing the FITC-CNCs to better detect cancer cells.
Nielsen et al[97] developed two synthetic approaches to introduce two fluorescent or dye molecules onto CNCs for the preparation of dual-fluorescent-labeled CNCs. The first approach employed a procedure similar to that of Dong and Roman and involved the reaction between amino CNCs and both isothiocyanates, FITC and RBITC. The quantities of FITC and RBITC on the dual-fluorescent-labeled CNCs were estimated to be 2.8 mol/g and 2.1 mol/g, respectively. To avoid the use of scarce isothiocyanate fluorescent molecules, a new three-step procedure was performed on CNCs: the introduction of a double bond on the CNCs via esterification, followed by thiol-ene Michael addition and finally coupling with succinimidyl ester dyes. Two different pH-sensitive dyes, 5-(and-6)-carboxyfluorescein succinimidyl ester (FAM-SE) and Oregon green 488 carboxylic acid, succinimidyl ester (OG-SE), were grafted onto CNCs along with the reference, fluorophore 5,6-carboxytetramethyl rhodamine succinimidyl ester (TAMRA-SE) dye, which is brighter than Rhodamine B and is available only as a succinimidyl ester. The average quantities of the dyes on the labeled CNCs were 10.4 mol/g (FAM-SE), 4.7 mol/g (TAMRA-SE) and 7.3 mol/g (OG-SE), 4.2 mol/g (TAMRA-SE), respectively. The dual-fluorescent-labeled CNCs (FITC- and RBITC-labeled CNCs) and two dual-dye-labeled CNCs (FAM-SE- and TAMRA-SE-labeled CNCs as well as OG-SE- and TAMRA-SE-labeled CNCs) all exhibited pH sensing in McIlvaine buffers under various pH value conditions.
3.2 Nucleophilic substitution
Nucleophilic substitution is another strategy for modifying the surface of CNCs with fluorescent molecules[62,81,102-103]. Fluorescent molecules containing halogen atoms such as chlorine or bromine are generally more suitable for this strategy. Hydroxyl groups or amino groups on the surface of CNCs (modified by an amination reaction) act as a nucleophilic reagent and react with a bromomethyl group or a chloromethyl group on a fluorescent molecule and replace a halogen atom by a nucleophilic substitution reaction; thus, CNCs can be modified by functional molecules. Both carbazole[81] and pyrene[62] were introduced onto the CNC surface by this reaction. Furthermore, by using this strategy, 5-(4,6-dichlorotriazinyl) amino-fluorescein (DTAF) and RBITC were also attached to the CNC surface in an alkaline environment in one step[90]. Although the requirements for the reagents and environment are relatively high, this method of grafting fluorescent molecules onto the surface of CNCs is simple[62,102].
3.3 Amidation reaction
In some studies, many different molecules were grafted onto nanocrystals using the carboxylation-amidation reaction to create a covalent amide bond between a primary amine-terminated molecule and carboxylated CNCs by COOH-NH2 coupling[67,74,104-106]. It was reported that during this reaction, to make conjugation easier, N-hydromaleimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide can be used to activate the carboxyl group and facilitate the reaction between the carboxyl and primary amine in the first step (Fig.3). Then, the NHS-activated carboxyl group is directly reacted with the amine group in a neutral or alkaline environment to form the amide. By using this two-step reaction, the amidation reaction can be controlled, and no excess byproducts are produced. The advantages of this method are mild reaction conditions, a simple operation procedure, and convenient disposal. Amidation can be applied to studies in which rhodamine methacrylamide is immobilized on the surface of the CNC[61,76,91,98] (Fig.4(c) and Fig.4(e)). Rhodamine, a popular fluorophore probe, has been investigated in terms of its physical and chemical features owing to its excellent photophysical properties and low cost. Because rhodamine methacrylamide is heat-sensitive, the fluorescence signal of rhodamine-methacrylamide-modified CNCs (RhB-CNCs) can be controlled, because the isoporphyrin-a substructure destroys the binding of rhodamine and turns the fluorescence signal off. When RhB-CNCs are treated with ultraviolet light, the five-membered open ring re-forms the π bond and turns the fluorescence signal on. Huang’s group reported the use of the amino acid L-leucine as a spacer linker for attachment of 5 (and 6)-carboxy-2’, 7’-dichlorofluorescein to CNCs[98].
3.4 Schiff base reaction
A Schiff base is a compound in which carbon atoms and nitrogen atoms are bonded by double bonds. Although the formation of Schiff bases is reversible, the reaction is widely used for surface modification of CNCs[77-78,82,94] (Fig.4(f)). Reduction of a double bond between a carbon atom and a nitrogen atom into a single bond is a useful measure to immobilize a functional group on the surfaces of CNCs. However, because the glucoside at the end of the cellulose chain can form an aldehyde group, it may be much easier to decorate the CNC surface with a functional group using a Schiff base. The degree of substitution (DS) of the fluorescent-molecule-modified CNC obtained by this strategy is relatively low, but the reaction can be carried out in one step and stably under physiological conditions, and thus it is still used for some in vivo biomarkers. Huang et al used this strategy for fluorescent labeling of CNCs with HMC and AMC[78]. CNCs labeled with Alexa Fluor 633 were also prepared by this reaction[93].
3.5 Click reaction
Click chemistry has been utilized to prepare new fluorescent cellulose nanomaterials[88-89,94] (Fig.4(a)). Filpponen et al reported that the primary hydroxyl groups in CNCs were first selectively oxidized to carboxylic acids. In the next step, a compound having a terminal amine functional group (propargylamine) was grafted to the surface of the oxidized CNCs by an amidation reaction, and the alkynyl group was used as a reaction site for further modification. These surface-modified CNCs were finally subjected to click chemistry reaction conditions, that is, copper(I)-catalyzed azide-alkyne cycloaddition with an azide-containing fluorescent coumarin, to produce highly fluorescent-tagged CNCs[94].
4 Fabrication of fluorescent CNCs for ion detection
Among many green nanomaterials, CNCs are preferred owing to their excellent properties, such as the high density of surface active hydroxyl groups and low cost[22,37,69,107]. In addition, an important property of CNCs prepared by sulfuric acid hydrolysis is the grafting of sulfate esters, which are negatively charged, onto the surfaces of the CNCs. Owing to the presence of charged groups, the colloidal stability of aqueous suspensions of CNCs helps to block aggregation through electrostatic repulsion interactions[26]. This property is attractive for water-based dispersion and processing of CNCs. There are many reports on microcrystalline cellulose[53-54], fluorophore-labeled cellulose[55], and CNCs. This review will consider mainly works related to CNCs. In covalent conjunction with CNCs, fluorescent molecules (small molecules or macromolecules) or QDs can be used to label CNCs not only for fluorescence bioassay and bioimaging applications but also for the detection of metal ions and monitoring the bio-effects of nanoparticles inside cells or human beings. Owing to the small size of fluorescent molecules (which results in smaller spatial effects) and their higher reactivity (various functional groups and stable chemistry), it is easier to bind fluorescent molecules to the CNC surface than other macromolecules. Dong and Roman used a multistep method to tag NH2-CNCs with FITC at a level of one fluorophore group per 27 nm2 of CNC surface[38]. Mahmoud et al used a similar method to tag CNCs with fluorophores, including FITC and RBITC[99]. A simple, low-cost, and scalable two-step method was used to attach FITC and a pyrene-based fluorophore to aminosilane-treated CNCs[95-96]. Filpponen et al used TEMPO oxidation, carbodiimide amidation, and copper (I)-catalyzed azide–alkyne click chemistry to add coumarin and anthracene fluorophores to CNC surfaces[93]. Nielsen et al presented a thiol-ene reaction method to modify CNCs with pH-responsive fluorophores[97]. Abitbol et al prepared DTAF-tagged CNCs by nucleophilic substitution[90]. In addition, stimuli-responsive fluorescent sensors are also widely applied in the regulation of cell or tissue environments and other fields. Stimuli-responsive polymers have attracted attention in recent years. Yuan et al synthesized CNC-g-poly(AzoC6MA-co-DMAEMA) fluorescent nanosensors by the atom transfer radical copolymerization (ATRP) reaction[86]. In contrast to organic fluorophores, core-shell QDs are resistant to photobleaching and have high quantum yields, broad excitation, and narrow, symmetric emission bands[46, 76]. The size-dependent optical properties allow simultaneous excitation of QDs of different sizes using a single wavelength, such as simple UV light[92, 101, 108]. The interest in cellulose as a substrate for fluorescent molecules and nanoparticles is driven by its relative inertness, along with recognition of the potential of this combination to develop new functional materials that combine the unique optical properties of QDs with the properties contributed by cellulose, e.g., transparency, good mechanical properties, and self-assembly. Abitbol et al used a carbodiimide chemistry coupling approach to decorate carboxylated CNCs with Cd/ZnS QDs[76]. These fluorescent-group-modified nanomaterials are used in bioimaging, sensing, in vivo detection, and so on. Optical nanosensors are widely used today, and many of them use fluorescence as an indicator to measure changes in biological indicators owing to their high sensitivity, usefulness for remote monitoring, and real-time performance.
Chen et al synthesized a porphyrin-functionalized CNC fluorescence nanosensor (CNC-SA-COOC6TPP) by simple esterification of extended porphyrin (TPPC6-OH) with carboxylated CNCs (CNC-SA-COOH) (Fig.5)[61]. Owing to the specific formation of the Hg2+-porphyrin complex as well as the interaction stoichiometry of one Hg2+ per porphyrin moiety, Hg2+ was specifically detected with interference from other metal ions[109-113]. Porphyrin pendants could serve as Hg2+-reactive sensors, and the CNCs afforded mechanical support, acting as a backbone with good dispersion in water. Moreover, the CNC-SA-COOC6TPP nanomaterials could be easily separated from water by filtering. These features make CNC-SA-COOC6TPP nanomaterials a promising fluorescent chemical sensor. The synthesized chemical sensor realized high-selectivity detection of Hg2+ with a credible detection limit of 5.0×108 mol/L (50 nmol/L) at a concentration of 0.01 wt%. When combined with Hg2+, the CNC-SA-COOC6TPP nanomaterials exhibited a distinct blue shift of the fluorescence peak from 652.5 nm to 628.5 nm, which was assigned to stable coordination aggregates induced by Hg2+[114].
Fluorescent sensors for Fe3+ in aqueous environments are rare. A limiting factor in the design of such sensor molecules is the paramagnetism of Fe3+, which can cause fluorescence quenching[97, 115]. In addition, many Fe3+-selective fluorescent probes are hydrophobic, and this incompatibility with aqueous environments limits the use of these sensors in biological systems. To overcome fluorescence quenching, researchers utilized a “green” nanomaterial for this template. The fluorescent chromophore, pyrene, which can detect Fe3+, is grafted onto the surface of CNC to improve the compatibility of the chromophore with aqueous environments. CNCs grafted with pyrene (Py-CNCs) are well dispersed in water (Fig.6). The sensors were used to detect Fe3+ in environmental and biological systems[62]. The fluorescence emission of pyrene was enhanced after modification of CNCs. The Py-CNCs exhibited high selectivity toward Fe3+ among other screened metal ions, with good discrimination between different iron oxidation states (Fe2+ and Fe3+). The quenching reaction mechanism of Fe3+ on Py-CNC is ligand-metal charge transfer, in which the charge transfer transition is initiated by the unfilled shell of paramagnetic and iron ions. The coordination reaction between Fe3+ and Py-CNCs is mainly an additional coordination of the N—H group, which is well known for strong binding affinity for transition metals and 2-hydroxy groups[56, 116].
Terpyridine is a N heterocycle that has a strong binding affinity for many transition metal ions owing to its ability to chelate with the d-p* of the coordination metal ions[117-119]. Much research has been done on the use of transition metal-terpyridine complexes in optics, electricity, magnetism, etc., and this complex can be reversed by changing the pH value or temperature[103, 120-121]. CNCs were used as a potential scaffold for the construction of ordered arrays and networks with tunable properties. Terpyridine-modified CNCs (CTP), a new metallo- and supramolecular nanocellulosic material[122-123], is prepared by nucleophilic substitution (Fig.7(a)), and its corresponding transition metal complexes have unique physicochemical properties and offer the possibility of forming supramolecular derivatives with a wide range of functionalities and applications (Fig.7(b)). According to the valence and reactant ratio, CTP can form a mono- or bis-teridinium metal complex with different transition metal ions. The complexation of transition metal ions with CTP occurs instantaneously, resulting in the formation of metal-nanocellulose materials with different optical properties (Fig.8). Owing mainly to complexation with different transition metal ions, the apparent red shift of the -* absorption band of terpyridine is from 350 nm to 413 nm.
Coumarin derivatives are important fluorescent dyes with high photoluminescence quantum efficiencies and applications in optical materials; they also display antitumor, antiviral, and antioxidant properties. Because the carbon in the 3-position in coumarins has a partial negative charge, the intramolecular charge transfer (ICT) mechanism can be modulated by introducing an electron-donating substituent at the 7-position or an electron-withdrawing substituent at the 3-position. Attachment of a hydrazide chain to the coumarin moiety at the 3-position could both enhance the ICT and set up a selective donor set for copper via the carbonyl oxygen and amide nitrogen[124, 125]. Huang et al constructed a new type of coumarin-modified CNC probe for selective and quantitative detection of Cu2+ in solution via a two-step reaction strategy, i.e., esterification of EDTAD to produce CNCs with a highly carboxylated surface and subsequent amidation with AMC to give fluorescent AMC-amidated ECNCs (Fig.9). At the same time, the steric effect and colloidal stability of rigid fCNCs might contribute to the prevention of self-quenching of surface AMC, resulting in a stable fluorescence intensity regardless of the fCNC concentration[80].
Fig.9 Schematic illustration of molecular-level structural changes on the CNC surface during synthesis of fCNCs
5 Conclusions
In this review, we focused on fCNC-based metal ion detection. Strategies for surface modification of CNCs, an important step in preparation of fCNCs, were first introduced. Then, various works that used the rich hydroxyl groups on the CNC surface to prepare fCNCs with different emission wavelengths and luminous behavior were reviewed. Finally, we analyzed works on metal ion detection by fCNCs in detail, gave examples, and discussed the effect of the molecular structure of fCNCs on their detection ability. Compared with fluorescent-molecule-based metal ion detection, detection by fCNCs has advantages, as follows: 1) Because the raw material of fCNCs, CNCs, have hydrophilic surface, fCNCs can be stably dispersed in water, so they provide an ideal platform to detect metal ions in aqueous solution; 2) The facile surface modification of CNCs can provide more options for designing fCNC sensors; 3) fCNCs exhibit stable fluorescence intensity with respect to the fCNC concentration, and thus provide a reliable detection result. Thus, in conclusion, owing to the low toxicity and low cost of CNCs, fCNCs are expected to afford high-performance but inexpensive and environmentally friendly sensors with specific functional properties for in vivo detection, bioimaging, drug delivery, and so on.
Acknowledgments
Authors are grateful to the National Natural Science Foundation of China (51373131), Fundamental Research Funds for the Central Universities (XDJK2016A017 and XDJK2016C033 ), Project of Basic Science and Advanced Technology Research, Chongqing Science and Technology Commission (cstc2016, jcyjA0796), and the Talent Project of Southwest University (SWU115034). LGP2 is part of the LabEx Tec 21 (Investissements d’Avenir-grant agreement n°ANR-11-LABX-0030) and of the PolyNat Carnot Institut (Investissements d’Avenir-grant agreement n°ANR-11-CARN-030-01).
References
[1] Persin Z, Stana-Kleinschek K, Foster T J, et al. Challenges and opportunities in polysaccharides research and technology: The EPNOE views for the next decade in the areas of materials, food and health care[J]. Carbohydrate Polymers, 2011, 84(1): 22-32.
[2] Xuan L, Zhang P, Li X, et al. Trends for nanotechnology development in China, Russia, and India[J]. Journal of Nanoparticle Research, 2009, 11 (8): 1845-1866.
[3] Baruah S, Dutta J. Nanotechnology applications in pollution sensing and degradation in agriculture: a review[J]. Environmental Chemistry Letters, 2009, 7(3): 191-204.
[4] Ruiz-Palomero C, Soriano M L, Valcárcel M. Nanocellulose as analyte and analytical tool: Opportunities and challenges[J]. Trac Trends in Analytical Chemistry, 2017, 87: 1-18.
[5] Siqueira G, Bras J, Dufresne A. Cellulosic bionanocomposites: A review of preparation, properties and applications[J]. Polymers, 2010, 2(4): 728-765.
[6] Gopalan Nair K, Dufresne A, Gandini A, et al. Crab shell chitin whiskers reinforced natural rubber nanocomposites. 3. Effect of chemical modification of chitin whiskers[J]. Biomacromolecules, 2003, 4(6): 1835-1842.
[7] Gopalan Nair K, Dufresne A. Crab shell chitin whisker reinforced natural rubber nanocomposites. 2. Mechanical behavior[J]. Biomacromolecules, 2003, 4(3): 666-674.
[8] Yoshiharu N. Structure and properties of the cellulose microfibril[J]. Journal of Wood Science, 2009, 55(4): 241-249.
[9] Nishiyama Y, Langan P, Chanzy H. Crystal structure and hydrogen-bonding system in cellulose I from synchrotron X-ray and neutron fiber diffraction[J]. Journal of the American Chemical Society, 2003, 125(47): 14300-14306.
[10] Habibi Y, Lucia L A, Rojas O J. Cellulose nanocrystals: chemistry, self-assembly, and applications[J]. Chemical Reviews, 2010, 110(6): 3479-3500.
[11] Moon R J, Martini A, Nairn J, et al. Cellulose nanomaterials review: structure, properties and nanocomposites[J]. Chemical Society Review, 2011, 40(7): 3941-3994.
[12] Lin N, Huang J, Dufresne A. Preparation, properties and applications of polysaccharide nanocrystals in advanced functional nanomaterials: a review[J]. Nanoscale, 2012, 4(11): 3274-3294.
[13] Habibi Y, Chanzy H, Vignon M R. TEMPO-mediated surface oxidation of cellulose whiskers[J]. Cellulose, 2006, 13(6): 679-687.
[14] Chen J, Lin N, Huang J, et al. Highly alkynyl-functionalization of cellulose nanocrystals and advanced nanocomposites thereof via click chemistry[J]. Polymer Chemistry, 2015, 6(24): 4385-4395.
[15] Van d B O, Capadona J R, Weder C. Preparation of homogeneous dispersions of tunicate cellulose whiskers in organic solvents[J]. Biomacromolecules, 2007, 8(4): 1353-1357.
[16] Favier V, Chanzy H, Cavaille J Y. Polymer nanocomposites reinforced by cellulose whiskers[J]. Macromolecules, 1995, 28(18): 6365-6367.
[17] Sacui I A, Nieuwendaal R C, Burnett D J, et al. Comparison of the properties of cellulose nanocrystals and cellulose nanofibrils isolated from bacteria, tunicate, and wood processed using acid, enzymatic, mechanical, and oxidative methods[J]. ACS Applied Materials & Interfaces, 2014, 6(9): 6127-6138.
[18] Ditzel F I, Prestes E, Carvalho B M, et al. Nanocrystalline cellulose extracted from pine wood and corncob[J]. Carbohydrate Polymers, 2017, 157: 1577-1585.
[19] Cao L, Fu X, Xu C, et al. High-performance natural rubber nanocomposites with marine biomass (tunicate cellulose)[J]. Cellulose, 2017, 24(7): 2849-2860.
[20] Jorfi M, Roberts M N, Foster E J, et al. Physiologically responsive, mechanically adaptive bio-nanocomposites for biomedical applications[J]. ACS Applied Materials & Interfaces, 2013, 5(4): 1517-1526.
[21] Shanmuganathan K, Capadona J R, Rowan S J, et al. Stimuli-responsive mechanically adaptive polymer nanocomposites[J]. ACS Applied Materials & Interfaces, 2010, 2(1): 165-174.
[22] Vasconcelos N F, Feitosa J P, da Gama F M P, et al. Bacterial cellulose nanocrystals produced under different hydrolysis conditions: Properties and morphological features[J]. Carbohydrate Polymers, 2017, 155: 425-431.
[23] Siqueira G, Tapin-Lingua S, Bras J, et al. Morphological investigation of nanoparticles obtained from combined mechanical shearing, and enzymatic and acid hydrolysis of sisal fibers[J]. Cellulose, 2010, 17(6): 1147-1158.
[24] Filson P B, Dawson-Andoh B E, Schwegler-Berry D. Enzymatic-mediated production of cellulose nanocrystals from recycled pulp[J]. Green Chemistry, 2009, 11(11): 1808-1814.
[25] Roman M, Winter W T. Effect of sulfate groups from sulfuric acid hydrolysis on the thermal degradation behavior of bacterial cellulose[J]. Biomacromolecules, 2004, 5(5): 1671-1677.
[26] Abitbol T, Kloser E, Gray D G. Estimation of the surface sulfur content of cellulose nanocrystals prepared by sulfuric acid hydrolysis[J]. Cellulose, 2013, 20(2): 785-794.
[27] Araki J, Wada M, Kuga S, et al. Birefringent glassy phase of a cellulose microcrystal suspension[J]. Langmuir, 2000, 16 (6): 2413-2415.
[28] Camarero Espinosa S, Kuhnt T, Foster E J, et al. Isolation of thermally stable cellulose nanocrystals by phosphoric acid hydrolysis[J]. Biomacromolecules. 2013, 14(4): 1223-1230.
[29] Sadeghifar H, Filpponen I, Clarke S P, et al. Production of cellulose nanocrystals using hydrobromic acid and click reactions on their surface[J]. Journal of Materials Science, 2011, 46 (22): 7344-7355.
[30] Edwards J V, Prevost N T, French A D, et al. Kinetic and structural analysis of fluorescent peptides on cotton cellulose nanocrystals as elastase sensors[J]. Carbohydrate Polymers, 2015, 116: 278-285.
[31] Jauovec D, VogriniR, Kokol V. Introduction of aldehyde vs. carboxylic groups to cellulose nanofibers using laccase/TEMPO mediated oxidation[J]. Carbohydrate Polymers, 2015, 116: 74-85.
[32] Garcia-Astrain C, González K, Gurrea T, et al. Maleimide-grafted cellulose nanocrystals as cross-linkers for bionanocomposite hydrogels[J]. Carbohydrate Polymers, 2016, 149: 94-101.
[33] De Menezes A J, Siqueira G, Curvelo A A S, et al. Extrusion and characterization of functionalized cellulose whiskers reinforced polyethylene nanocomposites[J]. Polymer, 2009, 50(19): 4552-4563.
[34] Grubbstrm G, Holmgren A, Oksman K. Silane-crosslinking of recycled low-density polyethylene/wood composites[J]. Composites Part A: Applied Science and Manufacturing, 2010, 41(5): 678-683.
[35] Habibi Y, Hoeger I, Kelley S S, et al. Development of langmuir-schaeffer cellulose nanocrystal monolayers and their interfacial behaviors[J]. Langmuir, 2010, 26(2): 990-1001.
[36] Thielemans W, Belgacem M N, Dufresne A. Starch nanocrystals with large chain surface modifications[J]. Langmuir, 2006, 22(10): 4804-4810.
[37] Eyley S, Thielemans W. Surface modification of cellulose nanocrystals[J]. Nanoscale, 2014, 6(14): 7764-7779.
[38] Dong S, Roman M. Fluorescently labeled cellulose nanocrystals for bioimaging applications[J]. Journal of American Chemical Society, 2007, 129(45): 13810-13811.
[39] Zoppe J O, Peresin M S, Habibi Y, et al. Reinforcing poly(epsilon-caprolactone) nanofibers with cellulose nanocrystals[J]. ACS Applied Materials & Interfaces, 2009, 1(9): 1996-2004.
[40] Angellier H, Molina-Boisseau S, Dufresne A. Mechanical properties of waxy maize starch nanocrystal reinforced natural rubber[J]. Macromolecules, 2005, 38(22): 9161-9170.
[41] Yang H, Tejado A, Alam N, et al. Films prepared from electrosterically stabilized nanocrystalline cellulose[J]. Langmuir, 2012, 28(20): 7834-7842.
[42] Brioude M M, Roucoules V, Haidara H, et al. Role of Cellulose nanocrystals on the microstructure of maleic anhydride plasma polymer thin films[J]. ACS Applied Materials & Interfaces, 2015, 7(25): 14079-14088.
[43] Qi H, Chang C, Zhang L. Properties and applications of biodegradable transparent and photoluminescent cellulose films prepared via a green process[J]. Green Chemistry, 2009, 11(2): 177-184.
[44] Tummala G K, Felde N, Gustafsson S, et al. Light scattering in poly(vinyl alcohol) hydrogels reinforced with nanocellulose for ophthalmic use[J]. Optical Materials Express, 2017, 7(8): 2824-2837.
[45] Morandi G, Thielemans W. Synthesis of cellulose nanocrystals bearing photocleavable grafts by ATRP[J]. Polymer Chemistry, 2012, 3(6): 1402-1407.
[46] Qu D, Zhang J, Chu G, et al. Chiral fluorescent films of gold nanoclusters and photonic cellulose with modulated fluorescence emission[J]. Journal of Materials Chemistry C, 2016, 4(9): 1764-1768.
[47] Thérien-Aubin H, Lukach A, Pitch N, et al. Structure and properties of composite films formed by cellulose nanocrystals and charged latex nanoparticles[J]. Nanoscale, 2015, 7(15): 6612-6618.
[48] Song J, Fu G, Cheng Q, et al. Bimodal mesoporous silica nanotubes fabricated by dual templates of CTAB and bare nanocrystalline cellulose[J]. Industrial & Engineering Chemistry Research, 2013, 53(2): 708-714.
[49] Maeda H, Nakajima M, Hagiwara T, et al. Bacterial cellulose/silica hybrid fabricated by mimicking biocomposites[J]. Journal of Materials Science, 2006, 41 (17): 5646-5656.
[50] Seabra A B, Bernardes J S, Favaro W J, et al. Cellulose nanocrystals as carriers in medicine and their toxicities: a review[J]. Carbohydrate Polymers, 2018, 181: 514-527.
[51] Viet D, Beck-Candanedo S, Gray D G. Dispersion of cellulose nanocrystals in polar organic solvents[J]. Cellulose, 2006, 14(2): 109-113.
[52] Aylott J W. Optical nanosensors—an enabling technology for intracellular measurements[J]. The Analyst, 2003, 128 (4): 309-312.
[53] Helbert W, Chanzy H, Husum T L, et al. Fluorescent cellulose microfibrils as substrate for the detection of cellulase activity[J]. Biomacromolecules, 2003, 4(3): 481-487.
[54] Ferreira L F V, Cabral P V, Almeida P, et al. Ultraviolet visible absorption, luminescence, and X-ray photoelectron spectroscopic studies of a rhodamine dye covalently bound to microcrystalline cellulose[J]. Macromolecules, 1998, 31 (12): 3936-3944.
[55] Nawaz H, Tian W, Zhang J, et al. Cellulose-Based Sensor Containing Phenanthroline for the Highly Selective and Rapid Detection of Fe(2+) ions with naked eye and fluorescent dual modes[J]. ACS Applied Materials & Interfaces, 2018, 10(2): 2114-2121.
[56] Xu W, Mu L, Miao R, et al. Fluorescence sensor for Cu(II) based on R6G derivatives modified silicon nanowires[J]. Journal of Luminescence, 2011, 131(12): 2616-2620.
[57] Bag B, Pal A. Rhodamine-based probes for metal ion-induced chromo-/fluorogenic dual signaling and their selectivity towards Hg(II) ion[J]. Org Biomol Chem, 2011, 9 (12): 4467-4480.
[58] Panda S, Pati P B, Zade S S. Twisting (conformational changes)-based selective 2D chalcogeno podand fluorescent probes for Cr(III) and Fe(II)[J]. Chemical Communications, 2011, 47(14): 4174-4176.
[59] De Silva A P, Gunaratne H Q, Gunnlaugsson T, et al. Signaling recognition events with fluorescent sensors and switches[J]. Chemical Reviews, 1997, 97(5): 1515-1566.
[60] Li L, Pan S S, Dou X C, et al. Direct electrodeposition of ZnO nanotube arrays in anodic alumina membranes[J]. Journal of Physical Chemistry C, 2007, 111(20): 7288-7291.
[61] Chen J, Zhou Z, Chen Z, et al. A fluorescent nanoprobe based on cellulose nanocrystals with porphyrin pendants for selective quantitative trace detection of Hg2+[J]. New Journal of Chemistry, 2017, 41(18): 10272-10280.
[62] Zhang L, Li Q, Zhou J, et al. Synthesis and photophysical behavior of pyrene-bearing cellulose nanocrystals for Fe3+ sensing[J]. Macromolecular Chemistry and Physics, 2012, 213(15): 1612-1617.
[63] Saito T, Kimura S, Nishiyama Y, et al. Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose[J]. Biomacromolecules, 2007, 8(8): 2485-2491.
[64] Follain N, Marais M F, Montanari S, et al. Coupling onto surface carboxylated cellulose nanocrystals[J]. Polymer, 2010, 51(23): 5332-5344.
[65] Okita Y, Saito T, Isogai A. Entire surface oxidation of various cellulose microfibrils by TEMPO-mediated oxidation[J]. Biomacromolecules, 2010, 11(6): 1696-1700.
[66] Da Silva Perez D, Montanari S, Vignon M R. TEMPO-mediated oxidation of cellulose III[J]. Biomacromolecules, 2003, 4(5): 1417-1425.
[67] Mangalam A P, Simonsen J, Benight A S. Cellulose/DNA hybrid nanomaterials[J]. Biomacromolecules, 2009, 10 (3): 497-504.
[68] Kumari S, Chauhan G S. New cellulose-lysine Schiff-base-based sensor-adsorbent for mercury ions[J]. ACS Applied Materials & Interfaces, 2014, 6(8): 5908-5917.
[69] Leung A C, Hrapovic S, Lam E, et al. Characteristics and properties of carboxylated cellulose nanocrystals prepared from a novel one-step procedure[J]. Small, 2011, 7(3): 302-305.
[70] Shibata I, Yanagisawa M, Saito T, et al. SEC-MALS analysis of cellouronic acid prepared from regenerated cellulose by TEMPO-mediated oxidation[J]. Cellulose, 2006, 13(1): 73-80.
[71] Shibata I, Isogai A. Nitroxide-mediated oxidation of cellulose using TEMPO derivatives: HPSEC and NMR analyses of the oxidized products[J]. Cellulose, 2003, 10 (4): 335-341.
[72] Saito T, Yanagisawa M, Isogai A. TEMPO-mediated oxidation of native cellulose: SEC-MALLS analysis of water-soluble and -insoluble fractions in the oxidized products[J]. Cellulose, 2005, 12(3): 305-315.
[73] Lin N, Bruzzese C, Dufresne A. TEMPO-oxidized nanocellulose participating as crosslinking aid for alginate-based sponges[J]. ACS Applied Materials & Interfaces, 2012, 4(9): 4948-4959.
[74] Lin N, Dufresne A. Physical and/or chemical compatibilization of extruded cellulose nanocrystal reinforced polystyrene nanocomposites[J]. Macromolecules, 2013, 46(14): 5570-5583.
[75] Mascheroni E, Rampazzo R, Ortenzi M A, et al. Comparison of cellulose nanocrystals obtained by sulfuric acid hydrolysis and ammonium persulfate to be used as coating on flexible food-packaging materials[J]. Cellulose, 2016, 23(1): 779-793.
[76] Abitbol T, Marway H S, Kedzior S A, et al. Hybrid fluorescent nanoparticles from quantum dots coupled to cellulose nanocrystals[J]. Cellulose, 2017, 24(3): 1287-1293.
[77] Grate J W, Mo K F, Shin Y, et al. Alexa Fluor-labeled fluorescent cellulose nanocrystals for bioimaging solid cellulose in spatially structured microenvironments[J]. Bioconjugate Chemistry, 2015, 26(3): 593-601.
[78] Huang J L, Li C J, Gray D G. Cellulose nanocrystals incorporating fluorescent methylcoumarin groups[J]. ACS Sustainable Chemistry & Engineering, 2013, 1(9): 1160-1164.
[79] Zhou Y, Jin Q, Hu X, et al. Heavy metal ions and organic dyes removal from water by cellulose modified with maleic anhydride[J]. Journal of Materials Science, 2012, 47(12): 5019-5029.
[80] Zhang Y J, Ma X Z, Huang J, et al. Fabrication of fluorescent cellulose nanocrystal via controllable chemical modification towards selective and quantitative detection of Cu(II) ion[J]. Cellulose, 2018, 1-12.
[81] Srbu E, Eyley S, Thielemans W. Coumarin and carbazole fluorescently modified cellulose nanocrystals using a one-step esterification procedure[J]. The Canadian Journal of Chemical Engineering, 2016, 94(11): 2186-2194.
[82] Gorgieva S, Vivod V, Maver U, et al. Internalization of (bis) phosphonate-modified cellulose nanocrystals by human osteoblast cells[J]. Cellulose, 2017, 24(10): 4235-4252.
[83] Hassan M L, Moorefield C M, Elbatal H S, et al. New metallo-supramolecular terpyridine-modified cellulose functional nanomaterials[J]. Journal of Macromolecular Science, Part A, 2012, 49(4): 298-305.
[84] Wu W, Li J, Liu W, et al. Temperature-sensitive, fluorescent poly(N-Isopropyl-acrylamide)-grafted cellulose nanocrystals for drug release[J]. Bioresources, 2016, 11(3): 7026-7035.
[85] Wu W, Huang F, Pan S, et al. Thermo-responsive and fluorescent cellulose nanocrystals grafted with polymer brushes[J]. Journal of Materials Chemistry A, 2015, 3(5): 1995-2005.
[86] Yuan W, Wang C, Lei S, et al. Ultraviolet light-, temperature- and pH-responsive fluorescent sensors based on cellulose nanocrystals[J]. Polymer Chemistry, 2018, 9 (22): 3098-3107.
[87] Chen L, Cao W, Grishkewich N, et al. Synthesis and characterization of pH-responsive and fluorescent poly (amidoamine) dendrimer-grafted cellulose nanocrystals[J]. Journal of Colloid & Interface Science, 2015, 450(12): 101-108.
[88] Eyley S, Thielemans W. Imidazolium grafted cellulose nanocrystals for ion exchange applications[J]. Chemical Communications, 2011, 47(14): 4177-4179.
[89] Parsamanesh M, Tehrani A D. Synthesize of new fluorescent polymeric nanoparticle using modified cellulose nanowhisker through click reaction[J]. Carbohydrate Polymers, 2016, 136: 1323-1331.
[90] Abitbol T, Palermo A, Moran-Mirabal J M, et al. Fluorescent labeling and characterization of cellulose nanocrystals with varying charge contents[J]. Biomacromolecules, 2013, 14 (9): 3278-3284.
[91] Zhao L, Li W, Plog A, et al Multi-responsive cellulose nanocrystal-rhodamine conjugates: an advanced structure study by solid-state dynamic nuclear polarization (DNP) NMR[J]. Physical Chemistry Chemical Physics, 2014, 16 (47): 26322-26329.
[92] Guo J, Liu D, Filpponen I, et al. Photoluminescent hybrids of cellulose nanocrystals and carbon quantum dots as cytocompatible probes for in vitro bio-imaging[J]. Biomacromolecules, 2017, 18(7): 2045-2055.
[93] Filpponen I, Sadeghifar H, Argyropoulos D S. Photoresponsive cellulose nanocrystals[J]. Nanomaterials and Nanotechnology, 2011, 1(1): 34-43.
[94] Colombo L, Zoia L, Violatto M B, et al. Organ distribution and bone tropism of cellulose nanocrystals in living mice[J]. Biomacromolecules, 2015, 16(9): 2862-2871.
[95] Yang Q, Pan X. A facile approach for fabricating fluorescent cellulose[J]. Journal of Applied Polymer Science, 2010, 117(6): 3639-3644.
[96] Cai C, Wei B, Jin Z, et al. Facile method for fluorescent labeling of starch nanocrystal[J]. ACS Sustainable Chemistry & Engineering, 2017, 5(5): 3751-3761.
[97] Nielsen L J, Eyley S, Thielemans W, et al. Dual fluorescent labelling of cellulose nanocrystals for pH sensing[J]. Chemical Communications, 2010, 46(47): 8929-8931.
[98] Tang L, Li T, Zhuang S, et al. Synthesis of pH-sensitive fluorescein grafted cellulose nanocrystals with an amino acid spacer[J]. ACS Sustainable Chemistry & Engineering, 2016, 4(9): 4842-4849.
[99] Mahmoud K A, Mena J A, Male K B, et al. Effect of surface charge on the cellular uptake and cytotoxicity , , of fluorescent labeled cellulose nanocrystals[J]. ACS Applied Materials & Interfaces, 2010, 2(10): 2924-2932.
[100] Ding Q, Zeng J, Wang B, et al. Influence of binding mechanism on labeling efficiency and luminous properties of fluorescent cellulose nanocrystals[J]. Carbohydrate Polymers, 2017, 175: 105-112.
[101] Khabibullin A, Alizadehgiashi M, Khuu N, et al. Injectable shear-thinning fluorescent hydrogel formed by cellulose nanocrystals and graphene quantum dots[J]. Langmuir, 2017, 33(43): 12344-12350.
[102] Zhang L, Zhou J, Zhang L. Synthesis and fluorescent properties of carbazole-substituted hydroxyethyl celluloses[J]. Macromolecular Chemistry and Physics, 2012, 213(1): 57-63.
[103] Hassan M L, Moorefield C M, Elbatal H S, et al. Fluorescent cellulose nanocrystals via supramolecular assembly of terpyridine-modified cellulose nanocrystals and terpyridine-modified perylene[J]. Materials Science and Engineering: B, 2012, 177(4): 350-358.
[104] Harrisson S, Drisko G L, Malmstrom E, et al. Hybrid rigid/soft and biologic/synthetic materials: polymers grafted onto cellulose microcrystals[J]. Biomacromolecules, 2011, 12(4): 1214-1223.
[105] Way A E, Hsu L, Shanmuganathan K, et al. pH-responsive cellulose nanocrystal gels and nanocomposites[J]. ACS Macro Letters, 2012, 1(8): 1001-1006.
[106] Akhlaghi S P, Berry R C, Tam K C. Surface modification of cellulose nanocrystal with chitosan oligosaccharide for drug delivery applications[J]. Cellulose, 2013, 20(4): 1747-1764.
[107] Tang Y, Yang S, Zhang N, et al. Preparation and characterization of nanocrystalline cellulose via low-intensity ultrasonic-assisted sulfuric acid hydrolysis[J]. Cellulose, 2013, 21(1): 335-346.
[108] Gong B, Liu W, Chen X, et al. Stabilizing alkenyl succinic anhydride (ASA) emulsions with starch nanocrystals and fluorescent carbon dots[J]. Carbohydrate Polymers, 2017, 165: 13-21.
[109] Figueira F, Farinha A S, Muteto P V, et al. [28]Hexaphyrin derivatives for anion recognition in organic and aqueous media[J]. Chemical Communications, 2016, 52(10): 2181-2184.
[110] Shamsipur M, Sadeghi M, Beyzavi M H, et al. Development of a novel fluorimetric bulk optode membrane based on meso-tetrakis(2-hydroxynaphthyl) porphyrin (MTHNP) for highly sensitive and selective monitoring of trace amounts of Hg2+ ions[J]. Materials Science and Engineering: C, 2015, 48: 424-433.
[111] Motreff N, Le Gac S, Luhmer M, et al. Formation of a dinuclear mercury(II) complex with a regular bis-strapped porphyrin following a tunable cooperative process[J]. Angewandte Chemie International Edition, 2011, 50(7): 1560-1564.
[112] Suijkerbuijk B M, Klein Gebbink R J. Merging porphyrins with organometallics: synthesis and applications[J]. Angewandte Chemie International Edition, 2008, 47(39): 7396-7421.
[113] Caselli M. Porphyrin-based electrostatically self-assembled multilayers as fluorescent probes for mercury (II) ions: a study of the adsorption kinetics of metal ions on ultrathin films for sensing applications[J]. RSC Advances, 2015, 5(2): 1350-1358.
[114] Lv J, Ouyang C, Yin X, et al. Reversible and highly selective fluorescent sensor for mercury(II) based on a water-soluble poly(para-phenylene)s containing thymine and sulfonate moieties[J]. Macromolecular Rapid Communications, 2008, 29(19): 1588-1592.
[115] Hu Z Q, Feng Y C, Huang H Q, et al. Fe3+-selective fluorescent probe based on rhodamine B and its application in bioimaging[J]. Sensors and Actuators B: Chemical, 2011, 156(1): 428-432.
[116] Kumar R, Nandi G C, Verma R K, et al. A facile approach for the synthesis of 14-aryl- or alkyl-14H-dibenzo[a, j] xanthenes under solvent-free condition[J]. Tetrahedron Letters, 2010, 51(2): 442-445.
[117] Aamer K A, Tew G N. Supramolecular polymers containing terpyridine-metal complexes in the side chain[J]. Macromolecules, 2007, 40(8): 2737-2744.
[118] Morsali A, Monfared H H, Morsali A. Syntheses and characterization of nano-scale of the Mn (II) complex with 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine (pyterpy): the influence of the nano-structure upon catalytic properties[J]. Inorganica Chimica Acta, 2009, 362(10): 3427-3432.
[119] Chiper M, Hoogenboom R, Schubert U S. New terpyridine macroligands as potential synthons for supramolecular assemblies[J]. European Polymer Journal, 2010, 46(2): 260-269.
[120] Yeung C T, Lee W S, Tsang C S, et al. Chiral-symmetric 2,2′:6′,2′′-terpyridine ligands: synthesis, characterization, complexation with copper(II), rhodium(III) and ruthenium(II) ions and use of the complexes in catalytic cyclopropanation of styrene[J]. Polyhedron, 2010, 29(5): 1497-1507.
[121] Kamyabi M A, Narimani O, Monfared H H. Electrocatalytic oxidation of hydrazine using glassy carbon electrode modified with carbon nanotube and terpyridine manganese(II) complex[J]. Journal of Electroanalytical Chemistry, 2010, 644(1): 67-73.
[122] Hornig S, Manners I, Newkome G R, et al. Metal-containing and metallo-supramolecular polymers and materials[J]. Macromolecular Rapid Communications, 2010, DOI:10.1002/marc.201000196.
[123] Chiper M, Hoogenboom R, Schubert U S. Toward main chain metallo-terpyridyl supramolecular polymers: “the metal does the trick”[J]. Macromolecular Rapid Communications, 2009, 30(8): 565-578.
[124] Goncalves A C, Luis Capelo J, Lodeiro C, et al. A selective emissive chromogenic and fluorogenic seleno-coumarin probe for Cu2+ detection in aprotic media[J]. Photochemical & Photobiological Sciences, 2017, 16(7): 1174-1181.
[125] Karaoglu K, Yilmaz F, Mentese E. A new fluorescent “Turn-Off” coumarin-based chemosensor: synthesis, structure and Cu-selective fluorescent sensing in water samples[J]. Journal of Fluorescence, 2017, 27(4): 1293-1298.
Advances in Hard Tissue Engineering Materials—anocellulose-based Composites
HuiZe Luo, JuanJuan Li, FengShan Zhou*
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing, 100083, China
Abstract: Nanocellulose (NC) has attracted much interest in the tissue engineering (TE) field because of its properties including biocompatibility, renewability, non-toxicity, functionality, and excellent mechanical performance. This review mainly focused on the advanced applications of NC-based composites in hard TE including cartilage TE, bone TE, and dental TE, illustrated the processing methods for synthesizing scaffolds including electrospinning, freeze-drying, and 3D printing, reviewed the current status of hard TE, and presented perspective on the future of TE technology.
Keywords: nanocellulose; hard tissue engineering; scaffolds
HuiZe Luo, master candidate;
E-mail: 2003170043@cugb.edu.cn
*Corresponding author:
FengShan Zhou, professor, PhD tutor; research interests: polymer, clay minerals;
E-mail: zhoufs@cugb.edu.cn
1 Introduction
Nanocellulose (NC) is a kind of cellulose whose specific size is in the nanometer range, including bacterial cellulose (BC), nanocrystalline cellulose (NCC), and nanofibrillated cellulose (NFC)[1]. Among these, NCC is also known as cellulose nanocrystal (CNC) or cellulose nanowhisker (CNW), while NFC is also known as cellulose nanofibers (CNF). In addition, the nano-sized cellulose fibers prepared by electrospinning from cellulose and its derivatives, such as cellulose acetate (CA) and carboxymethyl cellulose (CMC), are defined herein as electrospun cellulose nanofibers (ESC). Since ESC is nanostructured, it is also classified as NC. Different from the macro-sized cellulose, NC generally combines several properties, including a high specific surface area, high mechanical strength, and light weight. NC also maintains the excellent characteristics of traditional cellulosic materials, which can be regenerated and is widely sourced and degradable. Therefore, the NC family exhibits appealing prospects for a wide range of applications, including drug delivery, food, biosensors, and tissue engineering (TE)[2-5].
TE is a special technology for repairing and reconstructing lesions by seeding cells onto scaffold materials under the regulation of growth factors to construct functional tissues or organs in vitro or in vivo[6]. It is expected to overcome the limits of clinical autologous tissue and organ shortage as well as immune rejection during transplantation[7]. TE involves a large scale of applications, which covers the repair and reconstruction of almost all body tissues including hard tissue and soft tissue, just like a “body parts factory”.
As a kind of biomass material, NC is biocompatible, non-toxic, and harmless, because of which its applications in TE scaffolds have drawn great interest in recent years. There are many hydroxyl groups in the structure of NC, and thus, intra-chain and inter-chain hydrogen bonds can easily form with orderly arrangement, thereby constituting a highly regular crystalline structure of NC, leading to a high mechanical strength, good water absorption and retention, and strong interaction with living cells, which endow bright prospects in TE technology[8-11]. In particular, when applied in hard TE, the scaffold materials usually require higher mechanical strength than that required in soft TE. Hence, the requirements for mechanical properties make NC the best choice for the preparation and reinforcement of TE scaffolds. The classification of NC and its applications in hard TE are shown in Fig.1.
Fig.1 NC and its applications in hard TE
In this review, we mainly described the applications of NC in hard TE represented by cartilage TE, bone TE, and dental TE. We also summarized the different processing methods for the preparation of scaffolds, and finally presented future perspectives on NC-based scaffold composites.
2 Applications of NC in hard TE
Hard tissue and soft tissue are relatively different, and the former is denser and harder than the latter. Hard tissue mainly includes skeletons and teeth that possess hard structures and are formed through biomineralization, and the skeletal system can be divided into two parts: bones and cartilages[12]. Compared to the functional requirements of soft TE materials, hard TE usually pays more attention to improving the mechanical properties and stability.
On the other hand, NC is composed of cellulose molecular chains with a large number of hydroxyl groups, and therefore, a hydrogen bonding network can be easily formed[9, 13]. Thus, NC has crystal characteristics and prominent mechanical properties, which are obvious advantages that enhance the composites. For example, a highly regulatable and injectable poly(oligoethylene glycol methacrylate)-based hydrogel (POEGMA-based hydrogel) was developed, and the rigid rod-shaped NCCs were physically bound to the hydrazone-crosslinked POEGMA-based hydrogel[14]. The strong adsorption of hydrazide and aldehyde-modified POEGMA precursor polymers on the surface of NCC contributed to the uniform dispersion of NCC and increased the overall mechanical strength significantly. The addition of as low as 5 wt% NCC could drastically increase the mechanical properties especially the storage modulus. In another research, electrospun nanofiber mats based on PLA-g-silane/NCC and PLA/NCC nanocomposites were synthesized and immersed in hot water (70℃) to crosslink the silane-grafted PLA[15]. The influence of NCC on the tensile strength was prominent, and the mechanical properties of the nanofibers were significantly improved by chemical crosslinking.
2.1 Cartilage TE
Cartilage tissue is a special type of tough and slightly elastic connective tissue, with a certain compressive rigidity and ability to facilitate moisture ingress, thus helping in cushioning the forces that bones are subjected to[16]. The cartilage tissue in an adult is distributed in the ears, nose, joints, ribs, and vertebrae, which lack lymphs, vessels and nerves; hence, the ability of cartilage to repair itself is greatly limited once damaged[17].
2.1.1 Applications of freeze-drying in cartilage TE
Freeze-drying is a commonly used processing method in TE technology, and the resulting scaffold materials generally have high porosity. For instance, crosslinked interpenetrating polymer network (IPN) hydrogels composed of sodium alginate and gelatin (SA/G) with 50 wt% NCC were prepared by freeze-drying (Fig.2)[18]. The crosslinking process was divided into two steps: the first step was to prepare a single crosslinked nanocomposite hydrogel (X SA/G/NCC) by crosslinking with CaCl2, and the next step was to obtain a double-crosslinked hydrogel (XX SA/G/NCC) by crosslinking with genipin. The results showed that X SA/G/NCC had high porosity (>96%), high interconnectivity, and uniform pore distribution. Further, NCC was combined with a matrix to serve as templates for surface crosslinking, thus resulting in an optimal nanopore wall roughness beneficial to cell adhesion and extracellular matrix (ECM) production. In XX SA/G/NCC, a tighter network formed between the matrix phase and NCC, resulting in collapse of the pores on the surface. The second crosslinking process showed no significant effect on porosity, while the tight gathering of pore walls led to the formation of a layered structure with a broad pore size distribution (12~192 m) and less interconnectivity compared with X SA/G/NCC. Therefore, XX SA/G/NCC could not be used as a template because the NCC network was destroyed. In general, the combination of NCC and crosslinking led to the improvement of mechanical properties of X SA/G/NCC. Compared with natural cartilage, X SA/G/NCC had a higher modulus and equivalent strain and strength, which enabled its use as cartilage substitutes.
Similarly, NCC-reinforced semi-IPNs of chitosan hydrogels were also developed by freeze-drying[19]. NCC was distributed uniformly within the chitosan matrix, combining the amorphous and crystalline regions of the hydrogels. In terms of performance, the maximum compression of the chitosan hydrogels increased from (25.9±1) kPa to (50.8±3) kPa as the NCC content increased from 0 to 2.5%. However, the maximum compression did not increase significantly when the NCC content exceeded 2.5%. On the other hand, the composite hydrogels exhibited prominent pH sensitivity and produced maximum swelling ratio under acidic conditions (pH=4.01). NCC-chitosan hydrogels with improved mechanical properties and pH sensitivity could be applied in TE.
2.1.2 Applications of 3D printing in cartilage TE
As a novel processing method, 3D printing stands out in the TE technology with many advantages such as flexibility, high efficiency, and accuracy. The bioink properties such as good printability and cell viability, are the key to the application of 3D printing, which has attracted great attention[20-22].
Importantly, NC shows promising properties in promoting printability and cell activity. A type of bioink that could be printed with high fidelity was created by combining NC with alginate sulfate, which could be employed to create complex three-dimensional (3D) structures[23]. NC was combined with alginate sulfate to improve the poor printability of alginate sulfate. The cartilage cells in alginate sulfate-NC gel discs were active, mitogenic, collagen II capable cells. Although the printing conditions affected the cell behavior greatly in this material, the conical needle with a wide diameter could best preserve the cellular function. Furthermore, a 3D bioprinting process for NC hydrogels loaded with human nasal chondrocytes (hNCs) was studied, which could be used for patient-specific auricular cartilage regeneration[24]. Besides, 3D printing using a NFC-alginate (NFC-A) bioink facilitated cell-supported biomanufacturing as well as patient-specific auricle constructs with open internal structures, high cell density, and uniform cell distribution. The NFC-A constructs loaded with cells exhibited excellent shape and dimensional stability as well as increased cell viability and proliferation properties in vitro. In addition, NFC-A bioink supported the redifferentiation of hNCs and the synthesis of cartilage-specific ECM components.
2.1.3 Applications of other methods in cartilage TE
In addition to freeze-drying and 3D printing, there are a number of processing methods that can be used in cartilage TE, including salt leaching, casting, and spin coating, to meet the requirements of different scaffold materials. Porous starch/NFC composites for cartilage TE were prepared by salt leaching[25]. The combination of nanofibers in the starch structures enhanced the attachment and proliferation of cells on the scaffolds. Furthermore, a high-strength microcrystalline cellulose (HS-MCC) hydrogel was synthesized from natural cellulose via three steps: dissolution, curing, and exchange[26]. Because of the physical entanglement of MCC in a solvent mixture of tetrabutylammonium fluoride/dimethyl sulfoxide (TBAF/DMSO), the viscosity was adjusted to synthesize the high-strength hydrogel. The highest viscosity was obtained when the contents of MCC and TBAF were 2.5% and 3.5% in cellulose solution, respectively, resulting in the strongest HS-MCC hydrogel, which had the potential for bio-related applications such as drug delivery vehicles and artificial cartilage.
Further, bio-elastic lubricating films of NFC and hyaluronic acid (HA) were developed, which showed appealing applications in cartilage implants[27]. The NFC dispersion was spin-coated on gold-plated quartz crystals to prepare thin films, and HA was covalently bonded to NFC membranes by esterification reaction, which improved the poor lubricating properties of the NFC membranes. In addition, the hydrogels were synthesized by forming a network of poly(2-hydroxyethyl methacrylate) (PHEMA) matrix enhanced with cellulose whiskers[28]. The hydrogels had enhanced toughness, increased viscoelasticity, and improved recovery behavior, which could be used for articular cartilage replacement. The applications of NC-based cartilage TE scaffolds are summarized in Table 1.
2.2 Bone TE
Bone is a kind of hard tissue that forms most of the skeletons of humans and vertebrates, and plays a major role in supporting physical activity and protecting internal organs. Bone tissue contains a large amount of calcium deposits in the cell matrix, which concentrates most calcium of the body. To maintain normal heart function, it is necessary to ensure a suitable concentration of blood calcium; here, bone tissue plays a crucial role in maintaining the calcium balance of blood[12]. The bone defects caused by trauma, surgery, infection, etc., are very common clinically, which bring great inconvenience and pain to patients. The functional requirements of bone need bone TE scaffolds to possess good biocompatibility and osteoinduction in addition to adequate mechanical support[29]. As a biomass material, NC can be extracted from different kinds of wastes including paper residue[30], which is green and renewable, and combines excellent mechanical properties with compatibility, opening a new avenue for bone TE.
2.2.1 Applications of electrospinning in bone TE
Electrospinning is one of the most important methods in the synthesis of bone TE scaffolds, which can be applied to prepare microfibers and even nanofibers. The obtained materials can possess high porosity, which are beneficial to the mineralization and deposition of inorganic mineral crystals, and mimicking the natural ECM. Calcium phosphate is the main inorganic component of bone, and most researches on calcium phosphate are focused on hydroxyapatite (HAp), -tricalcium phosphate (-TCP), and the mixture of both in the preparation of bone TE scaffolds[31]. Some researchers studied the bio-simulated composite scaffold with mineralization of HAp based on electrospun poly(-caprolactone) (PCL)/NC fibers[32]. With increasing content of NC, the average diameter of the composite fibers decreased and the conductivity of the mixture solution increased. On the other hand, NC could be applied as an additive to activate the biomineralization process, and might induce the deposition of HAp. The longer the mineralization time, the more the mineral crystals were formed. The scaffold could not only simulate the components of natural bone, but also improved the surface hydrophilicity, and could therefore be used for bone TE. By utilizing the electrospinning method, other nanocomposites were synthesized by incorporating poly(ethylene glycol) (PEG)-grafted NCC into poly(lactic acid) (PLA)[33]. The PLA/NCC-g-PEG scaffolds in which the weight percent of NCC-g-PEG was 5% on the basis of PLA had smaller nanofiber diameters with enhanced mechanical strength, and showed improved cell viability and proliferation, indicating an appealing application for bone TE.
Besides the ESC synthesized from NFC and NCC, the ESC prepared from electrospinning of cellulose derivatives also has important applications. By using the novel solvent combinations, such as acetone/ethanol and dimethylformamide (DMF)/tetrahydrofuran (THF)/acetone, electrospun nanofiber scaffolds of cellulose acetate phthalate (CAP) were produced[34]. The scaffolds combined several prominent characteristics, including sufficient stability and strength, proper porosity, and uniformly distributed nanofibers; thus, the CAP scaffolds had been standardized for 3D culture of chondrocytes. In addition, thick scaffolds could be developed by extending the collection time (90 min) to further optimize the mechanical properties of scaffdds. Furthermore, electrospun cellulose/nano-HAp nanocomposite nanofibers (ECHNN) were prepared, which had excellent mechanical properties and increased thermal stability when nano-HAp was added, and were beneficial to the adhesion and proliferation of human dental follicle cells[35]. Therefore, ECHNN might have potential applications in bone TE. In another research, randomly oriented nanofiber scaffolds of modified cellulose (MC) and polyvinyl alcohol (PVA) were synthesized by electrospinning[36]. Compared with pure PVA, the crystallinity of the nanofibers decreased as the PVA content decreased. The nanofibers with lower crystallinity were more advantageous for bone TE. For the (1→4) glycosidic bond of the MC structure, a peak appeared at 770 cm1, which exists for the natural skeletal structure; therefore, they were expected to be used for bone TE. However, cytotoxicity studies were not conducted in this work. Afterward, these researchers prepared hydroxyethyl cellulose (HEC)-based nanostructured scaffolds with a uniform fiber morphology, and PVA was employed as the ionic solvent to support the electrospinning of HEC[37]. The mechanical properties of composites were significantly affected by HEC. Increasing the HEC content resulted in an increase in elastic modulus and tensile strength, while the elongation at break decreased proportionally. In addition, the cell viability increased, and cell proliferation was significantly promoted at a high HEC concentration.
Overall, the ESC derived from CA plays an important role in promoting cell proliferation, differentiation, and mineralization. For instance, artificial bone tissue scaffolds based on CA and nano-HAp natural hybrids were prepared by electrospinning[38]. The CA-HAp nanoscaffolds promoted the adhesion and growth of osteoblasts and stimulated the cells to exhibit functional activity. The morphologies and structures of scaffolds played important roles in promoting cell activities and enhancing apatite mineralization. By utilizing a series of processes, including electrospinning, saponification, and in situ hydrolysis, 3D cellulose sponges based on CA were synthesized[39]. The cellulose sponges showed the ability of nucleating active calcium phosphate (Ca-P) crystals in simulated body fluid (SBF), and the minerals deposited on nanofibers had a similar composition to that of HAp. On the other hand, the sponges exhibited better cell infiltration and proliferation than 2D cellulose mats, and were therefore expected to be applied in bone TE. These researchers also prepared a cellulose-reinforced nylon 6 (N6) nanofibrous film by electrospinning and saponification[40]. Cellulose was regenerated in situ by the alkaline saponification of CA of hybrid fibers to synthesize cellulose-enhanced N6 (nylon 6/cellulose, i.e., N6/CL) nanofibers. As the content of CA in the electrospinning solution increased, the fiber diameter and pore diameter gradually decreased. The regeneration of cellulose improved the nucleating ability of bioactive calcium phosphate crystals in the SBF. However, in Joshi’s study, only in vitro mineralization experiments were performed, and the requirement of researches on compatibility and degradability still remains. In addition, graphene oxide (GO)-CA nanofiber scaffolds were prepared in another study, which might also be used in bone TE[41]. With the incorporation of GO, the Young’s modulus of the nanofibers increased, and the adhesion and proliferation of human mesenchymal stem cells (hMSCs) on the scaffolds were significantly improved. Biomineralization had also been prominently optimized with the doping of GO in nanofibers. Accelerated biomineralization on GO-CA nanofibers led to a significant increase in the activity of biomineralization-associated alkaline phosphatase, and therefore induced the osteogenic differentiation of hMSCs.
Apart from CA, the applications of ESC based on CMC in bone TE are also very extensive. In one research, polysaccharide-based electrospun nanofiber composites with HAp were prepared, and the formation conditions of nanofibers from CMC and polyethylene oxide (PEO) were systematically studied[42]. It was determined that a mixture of 7 wt% CMC and 5 wt% PEO at a ratio of 50/50 (V/V) under suitable electrospinning parameters (low humidity and high voltage) could obtain uniformly shaped nanofibers with a diameter distribution between 150~200 nm. The HAp nanoparticles (nHAp) were then incorporated into the nanofibers by preparing a nHAp dispersion in the CMC/PEO mixture and subsequently spinning into fibers. The nHAp/CMC/PEO composites were non-toxic and biocompatible at a concentration of 0.03 wt%. Moreover, the nanofibers were further hydrophobized by their reaction with alkenyl succinic anhydride (ASA), which rendered them insoluble in water; however, they partially retained their morphology. Cells growing on the hydrophobized fibrous webs showed similar viability but reduced cell attachment compared with the cells growing on commercially available collagen/apatite scaffolds. The results confirmed that CMC/PEO/nHAp nanocomposites would have potential applications in regenerative medicine if surface modification, degradation, solubility, and mechanical properties of the material were further optimized. In another research, a silk fibroin (SF) and CMC composite nanofibrous scaffold was developed by free liquid surface electrospinning[43]. The SF/CMC scaffold had excellent cell-supporting properties, with improved osteoblast differentiation ability compared with pure SF. Moreover, an electrospun HAp-coated CMC scaffold (HAp/CMC) was successfully prepared[44]. The HAp/CMC nanofibers showed different morphologies with the differences of the NaOH concentration during carboxymethylation, the HCO3 concentration of SBF, and the mineralization time. As the seeding time increased, the osteoblast MC3T3-E1 cells on the HAp/CMC mats proliferated. The results indicated that the HAp/CMC scaffold could promote bone regeneration. The NC-based bone TE scaffolds prepared by electrospinning are summarized in Table 2.
2.2.2 Applications of freeze-drying in bone TE
The scaffold materials synthesized by freeze-drying can also have a high porosity, with good mutual connectivity between the pores, which is beneficial for seeding cells for various physiological activities. In addition, the freeze-drying process can maintain the structure and morphology of scaffolds betterly, and thus has wide applications in bone TE. Kumar et al conducted a series of studies on the applications of NCC-reinforced composite scaffold materials in bone TE. They used the freeze-drying method to prepare a NCC-reinforced PVA/silica glass hybrid scaffold at first[45]. The scaffold combined a highly porous structure with good pore connectivity. With increasing content of NCC and the subsequent addition of silica-based bioactive glass, the stiffness of the hybrid scaffold significantly increased. In addition, the incorporation of bioactive glass did not alter the overall microstructure of the PVA/SG/NCC scaffold; however, the crystallinity decreased, making it suitable (as the amorphous area) for bone regeneration. Moreover, a PVA/SG/NCC scaffold possessed better cell adhesion and growth characteristics than the PVA/NCC scaffold. However, further analysis of biocompatibility in vivo is required. Afterward, a NCC-reinforced polyacrylamide (PAAm)/sodium alginate/silica glass hybrid hydrogel with good compression stiffness was also synthesized[46]. The obtained hydrogel exhibited a high porosity and an interconnected pore structure after freeze-drying. The addition of NCC (2.5~10.0 wt%) improved several properties of hydrogel, including thermal stability, mechanical properties, and degradation stability in vitro, and also afforded good cell activities in vitro on hydrogels without influencing the apatite-forming ability in SBF. Furthermore, this group prepared NCC and/or halloysite nanotube (HNT)-reinforced sodium alginate (Alg) and xanthan gum (XG)-based nanocomposite scaffolds by freeze-casting/drying[47]. All the scaffolds exhibited high porosity and pore interconnectivity. Compared with Alg and AlgX scaffolds without NCC and/or HNTs, the nanocomposite scaffolds had improved thermal stability, compressive strength, and cell compatibility. On the other hand, XG/bioactive silica glass (SG) hybrid scaffolds reinforced with NCC were found to be highly porous and had adjustable and improved mechanical stability under dry and wet conditions compared to pure XG scaffolds as well as good cell compatibility. Therefore, the scaffolds might show appealing applications in low-load bone TE[48].
In addition, NFC-enhanced gelatin scaffolds were synthesized by carbodiimide crosslinking chemistry and freeze-thawing[49]. The scaffolds possessed suitable microstructures, osteoid-like compressive strength, and elasticity, and could cause calcium deposition. Introducing 3-aminopropylphosphoric acid (ApA) moieties into the NFC further enhanced and facilitated the deposition of HAp-like crystals on the scaffolds. These materials had no cytotoxic effect on mesenchymal stem cells.
From the abovementioned researches, it can be confirmed that NC can not only enhance the scaffolds, but is also nontoxic, thus contributing to the adhesion and proliferation of cells. Further, NC is beneficial to the mineralization and deposition of inorganic components, and thus shows potential applications in bone TE. In addition, PAAm is commonly used to graft and modify cellulose to prepare nanocomposites. For example, a semi-IPN cellulose-graft PAAm/nano-HAp nanocomposite scaffold was synthesized by free radical polymerization and freeze-drying[50]. The pores on the scaffold were interconnected, and after soaking in SBF, apatite particles were well deposited on the interconnected irregular pores. The obtained semi-IPN nanocomposite scaffold could be combined with living bone by forming an apatite layer on its surface, and can thus be applied in bone TE. Moreover, a novel porous 3D nanocomposite scaffold was prepared, which was composed of cellulose-graft-PAAm (Cel-g-PAAm) and nanopowders of HAp (n-HAp)[51]. The n-HAp/Cel-g-PAAm scaffold exhibited higher mechanical strength than that of a trabecular bone, and the scaffold extraction was not cytotoxic and had good biocompatibility.
In another research, a simvastatin-loaded gelatin-NC- tricalcium phosphate hydrogel scaffold was developed by freeze-drying, which combined osteoconductivity with osteoinductive properties and could be applied in promoting bone regeneration[52]. NFC resulted in a slower rate of degradation and was beneficial to the sustained release of simvastatin. The GNTS.5 scaffold and its osteoconductive structure could release an optimal concentration of simvastatin to enhance osteogenic activity. The bone TE scaffolds prepared by freeze-drying are listed in Table 3.
2.2.3 Applications of other methods in bone TE
There are many other processing methods in bone TE technology to satisfy the requirements of different scaffolds. For instance, by employing the solvent casting/particulate leaching process, polyurethane/NCC bimodal foam nanocomposites were synthesized for osteogenic differentiation of hMSCs[53]. The incorporation of different NCCs led to the generation of tunable mechanical properties and biodegradable structures, and the elastic modulus and tensile strength of the highly porous composites increased significantly with the addition of NCC. All these nanocomposites were biodegradable and non-cytotoxic. Moreover, these researchers also extracted NCC from waste paper by acid hydrolysis and calcification, and prepared phosphorized calcified cellulose nanowhiskers (PCCNW), which were found to have positive effects on the osteogenic differentiation of hMSCs; thus, they showed promising prospects in the development of new bone TE technologies[30]. In addition, a study on alginate-gelatin-NCC injectable hydrogels was conducted, which intended to provide an environment for cell growth and proliferation, facilitate the exchange of nutrients, and achieve the mechanical properties that resemble natural tissues[54]. NCC affected the degradation and interaction between hydrogels and cells prominently, and enhanced the mechanical properties. The hydrogel possessed the required mechanical properties and was biocompatible, and therefore might be applied in bone regeneration.
In addition, to evaluate the use of waste paper-derived NCC in bone TE, the NCCs produced from the hydrolysis of bagasse-derived cellulose by hydrochloric acid, sulfuric acid, and phosphoric acid were also evaluated, and NCCs with different surface compositions were used to produce biomimetic growth HAp[55]. The sulfonate and phosphonate groups on the surface of the NCC had a direct effect on the nucleation and growth of HAp. The differences in HAp deposition contents and surface areas indicated that sulfonate and phosphonate groups were directly involved in the growth of HAp on NCC. The materials prepared by the biomimetic method had higher biocompatibility/biological activity compared with the materials synthesized by the wet chemical precipitation methods. On the other hand, NCC was obtained by extracting the integuments of tunicate by means of sulfuric acid hydrolysis, which were found to be useful for adhesion, growth, and differentiation of osteoblasts without cytotoxicity[56]. The results indicated that intracellular calcium flux was associated with the increased differentiation of NCC under mechanical stress, confirming the applicability of NCC in bone TE, and a complex system for osteoblast growth and differentiation based on NCC was developed.
A study was conducted to research the effects of cellulose-nanodiamond conjugates, biocompatibility, and surface functionalization on the activities of osteoblasts[57]. The cellulose-nanodiamond composites, which were called oxidized biocompatible interfacial nanocomposites (oBINC), had the capacity to act as bio-interface materials because of their osteoconductive properties and biocompatibility. The related properties of oxidized nanodiamonds (oND) were improved by their covalent bonding with silylated NCC, exhibiting better activities of human fetal osteoblastic (hFOB) 1.19 cells than the precursor materials. Moreover, the injectable heat-sensitive chitosan/glycerophosphate (CS/GP) hydrogels supplemented with TEMPO-oxidized cellulose nanofiber (TOCNF) were prepared for bone regeneration[58]. To resolve the limitation of the balance between biocompatibility and thermal gel properties of the CS/GP thermo-sensitive hydrogel system, TOCNF was added as an additive into the hydrogel. These hydrogels could undergo sol-gel transitions at body temperature through the interaction between chitosan and -glycerophosphate, and the addition of TOCNF led to faster gelling times and increased porosity. Furthermore, TOCNF could significantly improve the biocompatibility of chitosan hydrogel as a biomaterial for biomedical applications.
In addition, another special material can be utilized for bone regeneration, which is bioactive glass. Since Hench, a professor at the University of Florida, developed the first generation of bioactive glass, its use in bone repair has become increasingly widespread. For example, pure cellulose, methyl cellulose, and amine-grafted cellulose were used as templates for the synthesis of bioactive glass nanoparticles[59]. Amine grafted cellulose and pure cellulose promoted the formation of in situ nanoparticle composites, while methyl cellulose was considered an outstanding sacrificial template for synthesizings the uniform nanoparticles with diameters in the range of 55 nm, which resulted in the combined properties of the bioglass, including excellent biological activity, damping properties, and mechanical stiffness. Depending on the template types, the HAp of each phase treated by SBF was observed to be crystalline, showing strong bone bonding ability. The abovementioned researches are summarized in Table 4.
2.3 Dental TE and restoration
In oral and maxillofacial tissues, a tooth is the highly differentiated component in organisms and is slowly developed from the tooth germ tissue located in the jaw, including enamel, dentin, cementum, and dental pulp. Its formation is regulated by an extremely complex network of delicate signaling molecules, including a series of epithelial-mesenchymal interactions and tooth-generating molecular events. The tooth has received extensive attention owing to its structural complexity and functional specificity[60-62]. Teeth grow twice during the human life: the first growth is called deciduous teeth, and after they fall off, they are replaced with permanent teeth. Once the permanent teeth are lost, they cannot be regenerated. Due to aging or some oral and systemic diseases such as periodontitis, teeth loss often occurs, resulting in discounting of human oral health and causing a lot of inconvenience in life at the same time.
To solve the problem of teeth loss, denture restoration is often used in clinical practice. However, it is not an ideal solution because of its poor functionality and difficulty in retention, coupled with safety and service life issues. At present, with the development of materials science and biomedical technology, glass ionomer cement (GIC), which is a kind of material for repairing, bottoming, and bonding of teeth defects, is gradually being widely used in oral clinical practice. However, because of the poor mechanical properties of the cementitious materials, research and development in materials science has focused on improving the mechanical strength of materials while maintaining their excellent performance.
Because of insufficient mechanical strength, general GIC materials are mostly used for bonding, and filling and repair of anterior teeth and deciduous teeth. By adding cellulosic fibers to GIC, the material structures were changed and the mechanical properties were improved, rendering it applicable for chewing and repairing posterior teeth, especially in atraumatic restorative treatment (ART) technology[63]. The researchers analyzed GIC (control) and three different types of GIC modified with cellulose fibers (GICMF): GICMF1, GICMF2, and GICMF3. The composites exhibited similar water absorption and solubility as GIC, and no signs of disintegration were observed. Although GICMF2 provided better compression resistance, abrasion resistance, and adhesion, it did not interfere with the diametral tensile strength. The analysis of GICMF2 illustrated that the interaction between fibers/ionomer matrix/loading particles formed new stable composites. Afterward, cellulose microfibers (CMF) and NCC were employed to reinforce dental GIC[64]. The addition of CMFs to the matrix did not improve the mechanical properties of GIC significantly, but the addition of a small amount of NCC into the GIC significantly improved the mechanical properties, including elastic modulus, diametral tensile strength, and compressive strength.
However, the abovementioned studies are mainly aimed at improving the mechanical properties of the composites. Compatibility and cytotoxicity tests in vitro and in vivo are also required before clinical applications. Therefore, researchers evaluated the biocompatibility of commercial dental GIC mechanically reinforced with CMFs (GIC+CM) or NCC (GIC+CN)[65]. Their results confirmed that all biomaterials showed satisfactory biocompatibility; however, the GIC modified with NCC showed outstanding tissue repairing function.
In addition, composites of polyacrylonitrile-electrospun nanofibers containing NCC were prepared by electrospinning, which could be used as reinforced dental resins[66]. The addition of 3% NCC resulted in higher tensile properties of the electrospun fibers, which showed appealing applications in reinforcing dental composites by nanofiber incorporation. Similarly, an ESC web for dental materials was also synthesized by electrospinning and possibly used as an aesthetic orthodontic scaffold material or a veneer for restorative dentistry[67]. The non-defective nanofibers were successfully electrospun into flat membranes by using acetone and CA polymer and DMAc solutions. The increased charge density of the electrospinning jet resulted in a decrease in the average fiber diameter. However, the infiltration of nanofiber web with Concise resin and epoxy resin has not yet improved the flexural strength of composites. The reason is speculated to be incomplete wetting of the fibers by the resin component, and air inclusions. Therefore, further reduction in air inclusions is required to determine the enhanced effect of the CA nanofibers.
In view of the problems of teeth loss, apart from material repairing, there is another important approach to reconstruct biologically active teeth to repair the missing teeth by artificial methods. The rapid development of TE technology is a great opportunity to complete this vision. Dental TE, a kind of TE technology that can be applied to dental restorations and teeth regeneration, is demonstrating thriving vitality. For instance, doxycycline-loaded chitosan/HAp/hydroxypropylmethyl cellulose (HPMC) spongy scaffolds were prepared by freeze-drying, which were non-toxic and could promote the viability of pre-osteoblasts[68]. Moreover, the scaffolds showed compressive strength of 14 MPa/cm3 and could be used as a substitute for alveolar bone and promote the formation of mineralized tissue.
3 Conclusions and overview
Nanocellulose (NC), a kind of green biomass material, can be obtained by extracting and processing cellulose that is widely synthesized by animals, plants, and microorganisms in nature by means of mechanical and chemical methods. As NC has excellent mechanical properties, it can be compounded with gelatin, polyester, PVA, etc., and is widely used as a reinforcing phase for composite materials. With prominent biocompatibility and degradability, NC has attracted much attention in biomedical applications, especially in tissue engineering (TE) scaffolds. Unlike the functionalities required for soft TE materials, the hard TE scaffolds require high mechanical properties, which can enhance the performance of NC and broaden its application prospects in hard TE. In addition, NC can be used for the adhesion, growth, and differentiation of osteoblasts, and to promote ossification. When compounded with other substances, it may promote the mineralization and deposition of hydroxyapatite (HAp), the main inorganic component of bone. At the same time, composites with different shapes and the desired characteristics of hard TE scaffolds can be prepared by kinds of methods such as electrospinning, freeze-drying, and 3D printing, making the scaffolds more controllable and adaptable.
In general, TE, a cross-cutting-edge technology that integrates numerous sciences including life science, engineering, and materials science, aims to overcome the limited source of implants and post-implantation immune rejection that need to be urgently resolved. Therefore, its impact on biomedical science is profound. As a sustainable bio-based nanomaterial, NC combines the advantages of compatibility and degradability, which is superior to other artificial synthetic materials. The prospect of applying NC to the construction of TE scaffolds is obvious. However, the TE technology is currently immature. Moreover, there is a big difference between artificially induced tissues and normal tissues, and the understanding of growth, maturation, and function of native tissues is not deep enough. Thus, there is still a long way to explore TE in the future.
Acknowledgments
Financial support was provided by the special fund for Independent Innovation and Industry Development in the Core Area in Haidian District of Beijing (255-kjc-020).
References
[1] Moon R J, Martini A, Nairn J, et al. Cellulose nanomaterials review: structure, properties and nanocomposites[J]. Chemical Society Reviews, 2011, 40(7): 3941-3994.
[2] Niazi M B K, Jahan Z, Berg S S, et al. Mechanical, thermal and swelling properties of phosphorylated nanocellulose fibrils/PVA nanocomposite membranes[J]. Carbohydrate Polymers, 2017, 177: 258-268.
[3] Sirvio J A, Honkaniemi S, Visanko M, et al. Composite Films of Poly(vinyl alcohol) and Bifunctional Cross-linking Cellulose Nanocrystals[J]. ACS Applied Materials & Interfaces, 2015, 7 (35): 19691-19699.
[4] Ko S W, Soriano J P E, Lee J Y, et al. Nature derived scaffolds for tissue engineering applications: Design and fabrication of a composite scaffold incorporating chitosan-g-D,L-lactic acid and cellulose nanocrystals from Lactuca sativa L. cv green leaf[J]. International Journal of Biological Macromolecules, 2018, 110: 504-513.
[5] Ma C, Ma M G, Li Z W, et al. Nanocellulose Composites—Properties and Applications[J]. Paper and Biomaterials, 2018, 3(2): 51-63.
[6] Langer R, Vacanti J P. Tissue Engineering[J]. Science, 1993, 260 (5110): 920-926.
[7] Sensharma P, Madhumathi G, Jayant R D, et al. Biomaterials and cells for neural tissue engineering: current choices[J]. Materials Science & Engineering c-Materials for Biological Applications, 2017, 77: 1302-1315.
[8] Klemm D, Kramer F, Moritz S, et al. Nanocelluloses: A New Family of Nature-Based Materials[J]. Angewandte Chemie-International Edition, 2011, 50(24): 5438-5466.
[9] Ching Y C, Rahman A, Ching K Y, et al. Preparation and Characterization of Polyvinyl Alcohol-Based Composite Reinforced with Nanocellulose and Nanosilica[J]. Bioresources, 2015, 10(2): 3364-3377.
[10] Tang A, Li J, Zhao S, et al. Biodegradable Tissue Engineering Scaffolds Based on Nanocellulose/PLGA Nanocomposite for NIH 3T3 Cell Cultivation[J]. Journal of Nanoscience and Nanotechnology, 2017, 17(6): 3888-3895.
[11] Hao X N, Mou K W, Jiang X Y, et al. High-value Applications of Nanocellulose[J]. Paper and Biomaterials, 2017, 2(4): 58-64.
[12] El-Zainy M A, Halawa A M, Elsharkawy R T. Effect of the parotid salivary gland on calcium and amylase enzyme levels in blood and its influence on bone healing in albino rats (Histological and radiographic study)[J]. Future Dental Journal, 2017, 3(2): 73-79.
[13] Mao H Q, Gong Y Y, Liu Y L, et al. Progress in Nanocellulose Preparation and Application[J]. Paper and Biomaterials, 2017, 2(4): 65-76.
[14] De France K J, Chan K J, Cranston E D, et al. Enhanced Mechanical Properties in Cellulose Nanocrystal-Po, , ly(oligoethylene glycol methacrylate) Injectable Nanocomposite Hydrogels through Control of Physical and Chemical Cross-Linking[J]. Biomacromolecules, 2016, 17(2): 649-660.
[15] Rahmat M, Karrabi M, Ghasemi I, et al. Silane crosslinking of electrospun poly (lactic acid)/nanocrystalline cellulose bionanocomposite[J]. Materials Science & Engineering C-Materials for Biological Applications, 2016, 68: 397-405.
[16] Mardones R, Jofre C M, Minguell J J. Cell Therapy and Tissue Engineering Approaches for Cartilage Repair and/or Regeneration[J]. International Journal of Stem Cells, 2015, 8(1): 48-53.
[17] Camarero-Espinosa S, Rothen-Rutishauser B, Weder C, et al. Directed cell growth in multi-zonal scaffolds for cartilage tissue engineering[J]. Biomaterials, 2016, 74: 42-52.
[18] Naseri N, Deepa B, Mathew A P, et al. Nanocellulose-Based Interpenetrating Polymer Network (IPN) Hydrogels for Cartilage Applications[J]. Biomacromolecules, 2016, 17(11): 3714-3723.
[19] Sampath U G T M, Ching Y C, Chuah C H, et al. Preparation and characterization of nanocellulose reinforced semi-interpenetrating polymer network of chitosan hydrogel[J]. Cellulose, 2017, 24(5): 2215-2228.
[20] Park J, Lee S J, Lee H, et al. Three dimensional cell printing with sulfated alginate for improved bone morphogenetic protein-2 delivery and osteogenesis in bone tissue engineering[J]. Carbohydrate Polymers, 2018, 196: 217-224.
[21] Chen Y W, Shen Y F, Ho C C, et al. Osteogenic and angiogenic potentials of the cell-laden hydrogel/mussel-inspired calcium silicate complex hierarchical porous scaffold fabricated by 3D bioprinting[J]. Materials Science & Engineering C-Materials for Biological Applications, 2018, 91: 679-687.
[22] Li Y, Jiang X, Li L, et al. 3D printing human induced pluripotent stem cells with novel hydroxypropyl chitin bioink: scalable expansion and uniform aggregation[J]. Biofabrication, 2018, 10(4): 044101.
[23] Muller M, Ozturk E, Arlov O, et al. Alginate Sulfate-Nanocellulose Bioinks for Cartilage Bioprinting Applications[J]. Annals of Biomedical Engineering, 2017, 45 (1): 210-223.
[24] Martínez vila H, Schwarz S, Rotter N, et al. 3D bioprinting of human chondrocyte-laden nanocellulose hydrogels for patient-specific auricular cartilage regenera-tion[J]. Bioprinting, 2016, 1-2: 22-35.
[25] Nasri-Nasrabadi B, Mehrasa M, Rafienia M, et al. Porous starch/cellulose nanofibers composite prepared by salt leaching technique for tissue engineering[J]. Carbohydrate Polymers, 2014, 108: 232-238.
[26] Choe D, Kim Y M, Nam J E, et al. Synthesis of high-strength microcrystalline cellulose hydrogel by viscosity adjustment[J]. Carbohydrate Polymers, 2018, 180: 231-237.
[27] Valle-Delgado J J, Johansson L S, Osterberg M. Bioinspired lubricating films of cellulose nanofibrils and hyaluronic acid[J]. Colloids and Surfaces B-Biointerfaces, 2016, 138: 86-93.
[28] Karaaslan M A, Tshabalala M A, Yelle D J, et al. Nanoreinforced biocompatible hydrogels from wood hemicelluloses and cellulose whiskers[J]. Carbohydrate Polymers, 2011, 86(1): 192-201.
[29] Lin K, Xia L, Gan J, et al. Tailoring the nanostructured surfaces of hydroxyapatite bioceramics to promote protein adsorption, osteoblast growth, and osteogenic differentiation[J]. ACS Applied Materials & Interfaces, 2013, 5(16): 8008-8017.
[30] Shahrousvand M, Tabar F A, Shahrousvand E, et al. High aspect ratio phospho-calcified rock candy-like cellulose nanowhiskers of wastepaper applicable in osteogenic differentiation of hMSCs[J]. Carbohydrate Polymers, 2017, 175: 293-302.
[31] Yuan J, Zhen P. Research progress of bone tissue engineering scaffolds[J]. Medical Recapitulate, 2015, 21(12): 2126-2130.
[32] Si J, Cui Z, Wang Q, et al. Biomimetic composite scaffolds based on mineralization of hydroxyapatite on electrospun poly(epsilon-caprolactone)/nanocellulose fibers[J]. Carbohydrate Polymers, 2016, 143: 270-278.
[33] Zhang C, Salick M R, Cordie T M, et al. Incorporation of poly(ethylene glycol) grafted cellulose nanocrystals in poly(lactic acid) electrospun nanocomposite fibers as potential scaffolds for bone tissue engineering[J]. Materials Science & Engineering C-Materials for Biological Applications, 2015, 49: 463-471.
[34] Shrestha R, Palat A, Punnoose A M, et al. Electrospun cellulose acetate phthalate nanofibrous scaffolds fabricated using novel solvent combinations biocompatible for primary chondrocytes and neurons[J]. Tissue Cell, 2016, 48(6): 634-643.
[35] Ao C, Niu Y, Zhang X, et al. Fabrication and characterization of electrospun cellulose/nano-hydroxyapatite nanofibers for bone tissue engineering[J]. International Journal of Biological Macromolecules, 2017, 97: 568-573.
[36] Chahal S, Hussain F S J, Yusoff M B M. Characterization of Modified Cellulose (MC)/Poly (Vinyl Alcohol) Electrospun Nanofibers for Bone Tissue Engineering[J]. Procedia Engineering, 2013, 53: 683-688.
[37] Chahal S, Hussain F S J, Kumar A, et al. Fabrication, characterization and in vitro biocompatibility of electrospun hydroxyethyl cellulose/poly(vinyl) alcohol nanofibrous composite biomaterial for bone tissue engineering[J]. Chemical Engineering Science, 2016, 144: 17-29.
[38] Gouma P, Xue R, Goldbeck C P, et al. Nano-hydroxyapatite—cellulose acetate composites for growing of bone cells[J]. Materials Science & Engineering C-Materials for Biological Applications, 2012, 32(3): 607-612.
[39] Joshi M K, Pant H R, Tiwari A P, et al. Three-dimensional cellulose sponge: Fabrication, characterization, biomimetic mineralization, and in vitro cell infiltration[J]. Carbohydrate Polymers, 2016, 136: 154-162.
[40] Joshi M K, Tiwari A P, Maharjan B, et al. Cellulose reinforced nylon-6 nanofibrous membrane: Fabrication strategies, physicochemical characterizations, wicking properties and biomimetic mineralization[J]. Carbohydrate Polymers, 2016, 147: 104-113.
[41] Liu X, Shen H, Song S, et al. Accelerated biomineralization of graphene oxide - incorporated cellulose acetate nanofibrous scaffolds for mesenchymal stem cell osteogenesis[J]. Colloids and Surfaces B-Biointerfaces, 2017, 159: 251-258.
[42] Gasparic P, Kurecic M, Kargl R, et al. Nanofibrous polysaccharide hydroxyapatite composites with biocompati-bility against human osteoblasts[J]. Carbohydrate Polymers, 2017, 177: 388-396.
[43] Singh B N, Panda N N, Mund R, et al. Carboxymethyl cellulose enables silk fibroin nanofibrous scaffold with enhanced biomimetic potential for bone tissue engineering application[J]. Carbohydrate Polymers, 2016, 151: 335-347.
[44] Yamaguchi K, Prabakaran M, Ke M, et al. Highly dispersed nanoscale hydroxyapatite on cellulose nanofibers for bone regeneration[J]. Materials Letters, 2016, 168: 56-61.
[45] Kumar A, Negi Y S, Choudhary V, et al. Morphological, mechanical, and in vitro cytocompatibility analysis of poly(vinyl alcohol)–silica glass hybrid scaffolds reinforced with cellulose nanocrystals[J]. International Journal of Polymer Analysis and Characterization, 2016, 22(2): 139-151.
[46] Kumar A, Rao K M, Han S S. Synthesis of mechanically stiff and bioactive hybrid hydrogels for bone tissue engineering applications[J]. Chemical Engineering Journal, 2017, 317: 119-131.
[47] Kumar A, Rao K M, Han S S. Development of sodium alginate-xanthan gum based nanocomposite scaffolds reinforced with cellulose nanocrystals and halloysite nanotubes[J]. Polymer Testing, 2017, 63: 214-225.
[48] Kumar A, Rao K M, Kwon S E, et al. Xanthan gum/bioactive silica glass hybrid scaffolds reinforced with cellulose nanocrystals: Morphological, mechanical and in vitro cytocompatibility study[J]. Materials Letters, 2017, 193: 274-278.
[49] Gorgieva S, Girandon L, Kokol V. Mineralization potential of cellulose-nanofibrils reinforced gelatine scaffolds for promoted calcium deposition by mesenchymal stem cells[J]. Materials Science & Engineering C-Materials for Biological Applications, 2017, 73: 478-489.
[50] Saber-Samandari S, Saber-Samandari S, Kiyazar S, et al. In vitro evaluation for apatite-forming ability of cellulose-based nanocomposite scaffolds for bone tissue engineering[J]. International Journal of Biological Macromolecules, 2016, 86: 434-442.
[51] Beladi F, Saber-Samandari S, Saber-Samandari S. Cellular compatibility of nanocomposite scaffolds based on hydroxyapatite entrapped in cellulose network for bone repair[J]. Materials Science & Engineering C-Materials for Biological Applications, 2017, 75: 385-392.
[52] Sukul M, Min Y-K, Lee S-Y, et al. Osteogenic potential of simvastatin loaded gelatin-nanofibrillar cellulose- tricalcium phosphate hydrogel scaffold in critical-sized rat calvarial defect[J]. European Polymer Journal, 2015, 73: 308-323.
[53] Shahrousvand E, Shahrousvand M, Ghollasi M, et al. Preparation and evaluation of polyurethane/cellulose nanowhisker bimodal foam nanocomposites for osteogenic differentiation of hMSCs[J]. Carbohydrate Polymers, 2017, 171: 281-291.
[54] Wang K, Nune K C, Misra R D. The functional response of alginate-gelatin-nanocrystalline cellulose injectable hydrogels toward delivery of cells and bioactive molecules[J]. Acta Biomaterialia, 2016, 36: 143-151.
[55] Fragal E H, Cellet T S P, Fragal V H, et al. Hybrid materials for bone tissue engineering from biomimetic growth of hydroxiapatite on cellulose nanowhiskers[J]. Carbohydrate Polymers, 2016, 152: 734-746.
[56] Kim D S, Jung S M, Yoon G H, et al. Development of a complex bone tissue culture system based on cellulose nanowhisker mechanical strain[J]. Colloids and Surfaces B-Biointerfaces, 2014, 123: 838-844.
[57] Vega-Figueroa K, Santillán J, García C, et al. Assessing the Suitability of Cellulose-Nanodiamond Composite As a Multifunctional Biointerface Material for Bone Tissue Regeneration[J]. ACS Biomaterials Science & Engineering, 2017, 3(6): 960-968.
[58] Nguyen T H M, Abueva C, Ho H V, et al. In vitro and in vivo acute response towards injectable thermosensitive chitosan/TEMPO-oxidized cellulose nanofiber hydrogel[J]. Carbohydrate Polymers, 2018, 180: 246-255.
[59] Gupta N, Santhiya D. Role of cellulose functionality in bio-inspired synthesis of nano bioactive glass[J]. Materials Science & Engineering C-Materials for Biological Applications, 2017, 75: 1206-1213.
[60] Zheng Y, Wang X Y, Wang Y M, et al. Dentin Regeneration Using Deciduous Pulp Stem/Progenitor Cells[J]. Journal of Dental Research, 2012, 91(7): 676-682.
[61] Lymperi S, Ligoudistianou C, Taraslia V, et al. Dental Stem Cells and their Applications in Dental Tissue Engineering[J]. Open Dentistry Journal, 2013, 7(1): 76-81.
[62] Sujesh M, Rangarajan V, Ravi Kumar C, et al. Stem cell mediated tooth regeneration: new vistas in dentistry[J]. Journal of Indian Prosthodontic Society, 2012, 12(1): 1-7.
[63] Silva R M, Santos P H N, Souza L B, et al. Effects of cellulose fibers on the physical and chemical properties of glass ionomer dental restorative materials[J]. Materials Research Bulletin, 2013, 48(1): 118-126.
[64] Silva R M, Pereira F V, Mota F A, et al. Dental glass ionomer cement reinforced by cellulose microfibers and cellulose nanocrystals[J]. Materials Science & Engineering C-Materials for Biological Applications, 2016, 58: 389-395.
[65] Menezes-Silva R, Pereira F V, Santos M H, et al. Biocompatibility of a new dental glass ionomer cement with cellulose microfibers and cellulose nanocrystals[J]. Brazilian Dental Journal, 2017, 28(2): 172-178.
[66] Peres B U, Vidotti H A, Manso A P, et al. Experimental composites of polyacrilonitrile-electrospun nanofibers containing nanocrystal cellulose[J]. Dental Materials, 2016, 32(1): e22-e23.
[67] Boyd S A, Su B, Sandy J R, et al. Cellulose Nanofibre Mesh for Use in Dental Materials[J]. Coatings, 2012, 2(3): 120-137.
[68] Iqbal H, Ali M, Zeeshan R, et al. Chitosan/hydroxyapatite (HA)/hydroxypropylmethyl cellulose (HPMC) spongy scaffolds-synthesis and evaluation as potential alveolar bone substitutes[J]. Colloids and Surfaces B-Biointerfaces, 2017, 160: 553-563.
Nanocellulose Research Exchange Activities between China and Japan
Min Wu
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, China
Min Wu, PhD, professor;
research interests: nanocellu-lose, biomass resources, gra-hpenebased materials;
E-mail: wumin@mail.ipc.ac.cn
Japanese companies and research institutions have been working with cellulose nanofibers (CNFs) for more than 20 years and stand in the forefront of the materials’ technology and application development. In the last decade, the Chinese research institutions that major in pulp and paper, wood chemistry, and polymer research have been vigorously researching CNF. Here, a brief overview of the nanocellulose research exchange activities between China and Japan was given.
1 Research exchange activities
The Japan Nanocellulose Forum (JNCF) was established in 2014 in Japan with the focus of strengthening the cooperation between industries, universities, and the government. JNCF manages common problems such as global standardization and risk assessment as well as collaboration with stakeholders in foreign countries.
The Nanocellulose and Materials Committee of the China Technical Association of Paper Industry (NMC-CTAPI) was established in 2015 with the purpose of promoting the exchanges and co-operation among researchers in the fields of pulp and paper, forest products, and nanomaterials technologies.
JNCF, highly active in both research and academic communication, holds many international and national academic symposiums or exhibitions every year. The NMC-CTAPI hosted the first international symposium on nanocellulosic materials in May 2017, in Hangzhou, China. The second one will be held in May 2019 in Tianjin, China.
In November 2016, Dr. Satoshi Hirata, the secretary-general of JNCF, visited Dr. Ruitao Cha, the secretary-general of NMC-CTAPI. At that time, I hosted a mini-symposium in Technical Institute of Physics and Chemistry (TIPC), CAS. We had a particularly useful discussion about the China-Japan collaboration for R & D of nanocellulose.
In March 2017, JNCF organized the 9th technical seminar in Shinagawa, Tokyo. The program consisted of 2 invited reports by Professor Shiyu Fu, South China University of Technology, and me. The titles were “Research on nanocellulose-based nanocomposites” and “Nanocellulose research and industrialization in CAS”, respectively.
On December 79, 2017, I was invited to organize more than 20 Chinese members from both research institutions and companies to attend the Nanocellulose Exhibition 2017. In this exhibition, various kinds of research and development, manufacturing technologies, and nanocellulose utilization studies were presented. Further, a business zone was designed to promote business negotiations and matching of active exchanges in this industry, and the latest information and topics related to nanocellulose were provided in the form of seminars facilitated by experts, researchers, and corporate representatives.
On June 37, 2019, Technical Association of the Pulp and Paper Industry (TAPPI) will organize the TAPPI Nanotech Conference 2019 in Chiba, Japan. I have been invited as a co-chair of this conference with professor Akira Isogai, president of the Nanocellulose Forum, Japan, professor of the University of Tokyo, Alan Rudie, USDA Forest Products Laboratory, USA, Lars Berglund, KTH Royal Institute of Technology, Sweden, and Nathalie Lavoine, North Carolina State University, USA.
2 Progress in fabrication and application of nanocellulose
Since 2015, Mitsubishi Pencil Co. has been producing TEMPO oxidized CNF, trade name “Leo Christa” by DKS Co., which was used as an ink thickener.
Nippon Paper Industries developed a high deodorization seat, which holds the metal ions with the antibacterial deodorization effect on the huge surface of TEMPO oxidized CNF. The deodorization power is triple that of conventional products.
For ONKYO SC-3(B) 2-way speaker system, a new vibration board using CNF is applied. The response is good and encourages the upgrade of low tone reproduction. Pioneer Co. is sealing up type dynamic stereo headphones. A new driver is developed in which CNF is used for the vibration board. Broadband reproduction (5 Hz~85 kHz) in correspondence with a high-resolution sound source is possible.
For DAIO Paper Corporation, the originally developed CNF produced by the energy-saving process was combined with a toilet seat paper cleaner for the first time ever. It was marketed since April 2017. This product is hard to tear compared to other existing products.
Koyo Kasei Co. sells 3 kinds of cosmetics using rose essence and carboxymethylated CNF as a moisturizing ingredient. This nanofiber is supplied by Nippon Paper Industries. Koyo Kasei is a manufacturer of wet wipes, not cosmetics. ASICS, the largest manufacturer of sporting goods in Japan, announced that new running shoes, GEL-KAYANO 25, were developed and available for sale from August 2018. CNF composites are used as raw materials for the mid-sole (middle cushioning between former cover and the sole), which increase the strength and durability by approximately 20% and 7%, respectively, while maintaining lightness. The composite is supplied by Seiko PMC corporation.
Daicel Fine Chem Co. started industrial production of “CELISH”, a micro-fibrillated cellulose, in 1990. It is refined by only physical processing using the Gaulin homogenizer as 10% to 35% aqueous suspension. The diameter of the fiber is 10~100 nm. Application includes: binder for powders or fibers, reinforcing materials for paper, texture improvement of food, filtering aid for alcohol and beverages, water-holding improvement of cosmetics and so forth.
Universities and research institutes in China are actively engaged in the application of nanocellulose. Generally speaking, China has more research institutions and achievements but less enterprises participating in the production of nanocellulose compared to Japan.
Professor Haipeng Yu’s research group at Northeast Forestry University have achieved many good results related to the preparation of nanocellulose. Their research includes the extraction of CNF using chemical pre-treatments combined with high intensity ultrasonic techniques and application of nanocellulose in the environment and biomedical fields.
Professor Guigan Fang’s research group at the Institute of Chemical Industry of Forest Products has focused on the preparation of nanocellulose and environment-friendly bio-based thermosetting polymer materials. The products can be used as multi-function fillers, green catalysts, and water treatment materials or in some sensor devices.
Professor Shiyu Fu’s research group at South China University of Technology and Professor Jin Huang’s group at Southwest University focus mainly on the modification and functionalization of nanocellulose. Dr. Ruitao Cha et al also developed nanocrystalline cellulose films with highly transparent and oxygen barrier properties.
Based on basic research, our group at TIPC, CAS, is co-operating with Ji’nan Shengquan Group Co. to make full use of agricultural wastes for industrial application of nanocellulose.
We are looking forward to displaying more Chinese nanocellulose products in the future.
|
