Covalent Functionalization of Graphene with Polysaccharides

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Covalent Functionalization of Graphene with Polysaccharides Qiang Yang,† Xuejun Pan,*,† Kimmy Clarke,‡ and Kecheng Li‡ †

Department of Biological Systems Engineering, University of Wisconsin—Madison, 460 Henry Mall, Madison, Wisconsin 53706, United States ‡ Department of Chemical Engineering, University of New Brunswick, P.O. Box 69000, Fredericton, NB, Canada E3B 6C2 ABSTRACT: Stable aqueous suspension of graphenes was fabricated from chemical reduction of graphene oxide with the assistance of hydroxypropyl cellulose or chitosan covalently grafted on the graphenes. First, pristine graphite was oxidized using a two-step method (H2SO4 + HNO3 followed by KMnO4). The oxidation-introduced carboxyl groups in graphene oxide were then converted into acyl chloride with thionyl chloride. Subsequently, hydroxypropyl cellulose or chitosan was covalently grafted onto the activated graphene oxide through esterification reactions. Finally, the functionalized graphene oxide was chemically reduced to graphene nanosheets with hydrazine.

exhibited no cytotoxicity to three mammalian cell lines.22 More recently, reducing sugars such as glucose, fructose, and sucrose were proved to be good stabilizers of graphene in aqueous suspension.23 Furthermore, some biocompatible macromolecules with functional groups such as amine and hydroxyl can be covalently grafted to graphene to prepare a stable aqueous suspension of graphenes. For example, when branched polyethylene glycol (PEG) was covalently grafted onto the graphene oxide surface, the resultant PEGylated nanographene oxide was water-soluble and could be used for delivering water-insoluble cancer drugs.19 Water-soluble graphene was also prepared through covalent functionalization using poly-L-lysine (PLL). The resultant graphene behaved amplified-biosensing toward H2O2.24 Because of their nontoxic, hydrophilic, biocompatible, and biodegradable nature, polysaccharides such as cellulose and chitosan have also been used in the functionalization of graphene. The resultant graphene has potential applications in the areas of biocomposites, biomedical areas, and biosensors.18,19,25 27 For example, cellulose (or chitosan)/graphite oxide (or graphene) composites (films) with enhanced mechanical strength properties have been fabricated by several research groups.28 31 In our previous work, water-soluble polysaccharide derivatives including sodium carboxymethyl cellulose (CMC) and pyrene-containing hydroxypropyl cellulose (HPC) were successfully used to stabilize graphenes noncovalently.12 However, to our knowledge, covalent functionalization/stabilization of graphene with polysaccharides has not been reported. In this study, we fabricated a stable aqueous suspension of graphene nanosheets by covalently grafting polysaccharides on the graphene through an esterification reaction. In brief, to introduce enough carboxyl groups for the polysaccharide-grafting esterification reaction, graphite was oxidized using a two-step oxidation method, concentrated sulfuric and nitric acids followed by potassium permanganate, which was proved to be an efficient method to introduce carboxyl groups

1. INTRODUCTION Graphene (G) is a one-atom-thick and two-dimension planar monolayer of sp2-hybridized carbon.1 Graphene has been prepared by different techniques, such as chemical vapor deposition, micromechanical exfoliation of graphite, carbon sweating from solutions in metals or from carbides, and reduction of exfoliated graphite oxide suspension.2 4 Of these fabrication techniques, chemical reduction of exfoliated graphite oxide (EGO) is believed as a promising method for low-cost and large-scale production of graphene from graphite.5,6 The method includes the steps of oxidation of graphite, dispersion/suspension of oxidized graphite sheets, and reduction of the graphite oxide to graphene. The oxidation introduces hydroxyl, carboxyl, and epoxide groups into graphene, which increases the hydrophilicity of the graphene and the layer distance between graphene oxide sheets, and therefore facilitates the exfoliation and dispersion of graphene oxide sheets. The widely used three oxidation methods are the Brodie method7 using potassium chlorate and fuming nitric acid, the Staudenmaier method8 using concentrated sulfuric and nitric acids with potassium chlorate, and the Hummers method9 using a water-free mixture of concentrated sulfuric acid, sodium nitrate, and potassium permanganate. To obtain a stable suspension of graphene oxide in a solvent, a stabilizer,10 12 noncovalent functionalization,13,14 or covalent functionalization15 17 is usually required. The reduction of the dispersed graphene oxide to remove the oxygen-containing groups introduced by the oxidation and to restore the CdC bonds can be achieved using strong reductants such as hydrazine, dimethylhydrazine, NaBH4, and hydroquinone.4 Graphene holds great potential in biological and medical applications,18,19 which essentially requires that the graphene must have aqueous stability and biocompatibility. These properties can be obtained by using water-soluble and biocompatible stabilizers or functionalizers when the graphene oxide suspension is prepared. For example, Patil et al. found that a single-stranded DNA could disperse graphene into water.20 Pyrene-labeled single-stranded DNA could do the same thing.21 Park et al. fabricated biocompatible graphene using polyoxyethylene sorbitan laurate (Tween) as a stabilizer, and the resultant graphene r 2011 American Chemical Society

Received: June 29, 2011 Accepted: November 19, 2011 Revised: November 16, 2011 Published: November 19, 2011 310

dx.doi.org/10.1021/ie201391e | Ind. Eng. Chem. Res. 2012, 51, 310–317

Industrial & Engineering Chemistry Research

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Scheme 1. Preparation Routes of Polysaccharides-Functionalized Graphenes

to carbon nanotubes.32 After the carboxyl groups were activated by thionyl chloride, water-soluble hydroxypropyl cellulose or chitosan oligosaccharide was covalently grafted to the graphene oxide through the esterification reaction. At the end, the dispersed graphene oxide was chemically reduced with hydrazine to obtain a stable aqueous suspension of graphene nanosheets.

(Ward Hill, MA). Thionyl chloride (SOCl2), hydroxypropyl cellulose (HPC, average Mn = 10, 000, molar substitution (MS) of propylene oxide was 2.9),33 low molecular weight chitosan (LMC, chitosan oligosaccharide lactate, average Mn = 5, 000, >90% deacetylation), potassium permanganate (KMnO4), hydrazine, and lithium chloride (LiCl) were obtained from SigmaAldrich (Saint Louis, MO). Hydrogen peroxide solution (H2O2, 30%), hydrochloric acid (HCl), 96% sulfuric acid (H2SO4), nitric acid (HNO3), anhydrous tetrahydrofuran (THF), anhydrous N,Ndimethylacetamide (DMAc), and anhydrous dimethylformamide (DMF) were purchased from Fisher Scientific (Pittsburgh, PA).

2. EXPERIMENTAL SECTION 2.1. Materials. Graphite microcrystalline (powder, 300 mesh, 75 82% C, 18 25% ash) was purchased from Alfa Aesar 311

dx.doi.org/10.1021/ie201391e |Ind. Eng. Chem. Res. 2012, 51, 310–317


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Figure 1. (A) UV vis spectrum of GO; (B) ATR FTIR spectrum of GO; (C) Raman spectra of graphite (a) and GO (b); (D) TGA curves of graphite (a) and GO (b).

2.2. Preparation of Graphite Oxide. The pristine graphite was preoxidized by nitric acid and sulfuric acid in order to form carboxyl groups on its surface. Briefly, graphite powder (3 g) was oxidized in 60 mL of mixture of sulfuric acid and nitric acid (3:1, v/v) under reflux for two days. The preoxidized graphite was separated by centrifugation, washed with water, and dried under vacuum. The preoxidized graphite was further oxidized by a mixture of 96% sulfuric acid (120 mL) and KMnO4 (15 g). The oxidation condition and separation procedure were adapted for literature.12 At the end, the lightly yellow graphite oxide (GO, 81% yield) was collected and dried under vacuum. 2.3. Preparation of Acyl Chloride-Functionalized Graphene Oxide. The as-prepared GO (1 g) was well dispersed in 20 mL of DMF by sonification for 1 h. To convert the carboxyl group ( COOH) on the GO surface to acyl chloride ( COCl), the dispersed GO reacted with SOCl2 (60 mL, 0.82 mol) at 80 °C for three days. The acyl chloride-functionalized GO (GO COCl, 78% yield) was separated by centrifugation, washed with anhydrous THF, and dried under vacuum. 2.4. Preparation of Hydroxypropyl Cellulose-Functionalized Graphene. The hydroxypropyl cellulose-functionalized graphene oxide (GO-HPC) was prepared through an esterification reaction between GO COCl and hydroxyl-containing HPC. Briefly, GO COCl (0.1 g) was dispersed in 80 mL of DMF containing HPC (2.5 g, 0.25 mmol), and the reaction lasted for two days at 120 °C. The resultant GO-HPC was filtered out, washed with DMF for five times, and dried under vacuum. Finally, GO-HPC (79% yield) was chemically reduced in water using 2 mL of hydrazine for three days.34 2.5. Preparation of Chitosan-Functionalized Graphene. Similarly, chitosan-functionalized graphene oxide (GO-LMC)

was obtained through an esterification reaction between GO COCl and amine-containing chitosan oligosaccharide (LMC). Typically, LMC (0.5 g, 0.1 mmol) was dissolved in 20 mL of DMAc containing LiCl (1.0 g) at 130 °C for 2 h. When completely dissolved, the LMC solution was cooled down to room temperature. Then, GO COCl (50 mg) was added into the LMC solution, reacting at 120 °C for two days. The resulting GO-LMC was filtered out, washed with DMAc and water for several times, and dried under vacuum. The GO-LMC (82% yield) was then chemically reduced in water by 2 mL of hydrazine for three days. 2.6. Analytical Methods. UV vis spectra were collected using a viable-temperature UV vis spectrophotometer (Cary 50 Bio, Varian). Attenuated total reflectance micro-Fourier transform infrared (ATR-FTIR) spectra were recorded on a PerkinElmer Spectrum 100 Series FT-IR spectrophotometer with a universal ATR sampling accessory (Waltham, MA). Thermogravimetric analysis (TGA) was performed on a Thermogravimetric Analyzer (PerkinElmer Q500, USA) under nitrogen atmosphere with a heating rate of 20 °C/min. Raman spectra were collected on a Horiba Jobin-Yvon LabRAM ARAMIS Raman confocal microscope (532 nm, Aramis CRM, Horiba Jobin Yvon, Edison, NJ). Typical tapping-mode atomic force microscopy (AFM) measurements were performed with an MFP-3D AFM (AsylumResearch, USA) using TESP-SS (Veeco, USA) super sharp probes at 320 kHz frequency. Samples were first sonicated in a sonic mixer (Bransonic, B-2200R-4) at 30 °C and then transferred onto a freshly cleaved mica (9.9 mm diameter, Ted Pella, INC) surface and allowed to airdry for AFM observation. X-ray photoelectron spectra (XPS) were recorded on a Thermo Scientific K-Alpha XPS spectrometer (ThermoFisher, E. Grinstead, UK) with a monochromatic Al Kα 312



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X-ray source. The samples were run at a takeoff angle (relative to the surface) of 90°. Data were processed using the software (Avantage) provided with the instrument.

3. RESULTS AND DISCUSSION Existing oxidation techniques (Brodie,7 Staudenmaier,8 and Hummers9 methods) of graphite introduce hydroxyl and epoxide groups to graphite, but they only generate limited carboxyl groups on the resultant graphite oxide (GO) needed for the subsequent functionalization by esterification. In this study, a two-step oxidation procedure was used to create more carboxyl groups on GO, as illustrated in Scheme 1. Natural graphite was first preoxidized by a mixture of sulfuric acid and nitric acid (3:1, v/v).16 Then, the preoxidized graphite was further oxidized with KMnO4.12 The as-prepared GO was characterized by UV vis, ATR FTIR, Raman, and TGA, as shown in Figure 1. In the UV vis spectrum (Figure 1A), GO showed a characteristic peak at 231 nm (corresponding to π π* transitions of CdC bonds) and a shoulder at 300 nm (due to n π* transitions of COOH groups).35 Presence of carboxyl groups was also verified by ATR FTIR (Figure 1B). The characteristic peak of carboxyl was clearly observed at 1720 cm 1. Raman (Figure 1C) was used to detect structural changes of graphite before and after the two-step oxidation. The pristine graphite showed three characteristic peaks at 1324 (D-band, C C), 1572 (G-band, CdC), and 2673 cm 1 (2D-band).36 The ID/IG ratio (intensity ratio of the D-band to G-band) was 0.10, which indicated the graphite had a nearly defect-free structure. Compared with the pristine graphite, the D-band and G-band of GO red-shifted to 1334 and 1575 cm 1, respectively. Because of the presence of some defects, GO had a much higher ID/IG ratio of 0.85. The structural defects in GO were further investigated by thermogravimetric analysis (TGA) (Figure 1D). The pristine graphite was very stable and had no observable weight loss when heated to 600 °C. However, GO became thermally unstable after the oxidation. It lost 11.4% weight up to 150 °C. The weight loss was possibly caused by the evaporation of adsorbed water and thermal decompositions of oxygen-containing functional groups such as carboxyl, hydroxyl, epoxy, nitrogen dioxide, and ketone.37 GO continued decomposing between 150 and 170 °C with additional 46.6% weight loss, suggesting the presence of structural defects in GO caused by the strong acid oxidation. So far, covalent functionalization of graphite oxide has been implemented mostly through condensation reactions between the functional groups such as carboxyl and epoxide on GO and amine or hydroxyl groups of the functionalizers. Before the covalent functionalization, the carboxyl usually needs to be activated by thionyl chloride or condensation agents to improve its reactivity.38 In the present study, functionalization of the graphite oxide with polysaccharides was carried out by a two-step procedure, as illustrated in Scheme 1. First, carboxyl groups in GO were partially converted into acyl chloride (GO COCl) by treating GO with thionyl chloride. After the activation, surplus thionyl chloride was removed by washing with anhydrous THF. Second, LMC or HPC was covalently grafted onto the GO COCl through esterification reaction. The grafting reactions of HPC and LMC were carried out in DMF and DMAc containing 5% (w/w) LiCl, respectively. The functionalized graphene oxide was then chemically reduced by hydrazine, which formed stable aqueous suspensions

Figure 2. UV vis absorption spectra of GO (a), G-HPC (b), and G-LMC (c) (top). UV vis absorption spectra of the G-HPC aqueous suspension (inset: correlation curve of absorbance at 273 nm vs concentration) at varied concentrations (bottom).

of graphene nanosheets, G-LMC and G-HPC, respectively. The G-HPC and G-LMC suspensions at a 0.5 mg/mL concentration were stable after one-month standing at room temperature, and no conspicuous aggregates were observed. It is worth noting that the suspensions of G-HPC and G-LMC had their limitations in stabilities. The G-HPC suspension is sensitive to salts. For example, the attempt to disperse G-HPC into phosphate-buffered saline failed. Although the LMC was water-soluble, the G-LMC was only dispersible and stable in an acidic aqueous medium. Dispersibility of the G-HPC in water was investigated by UV vis spectrometry as well. As shown in Figure 2 (bottom), absorbance at 273 nm of the G-HPC aqueous suspension was linearly correlated to its concentration (see inset), which was consistent with Beer’s law.12,39,40 The result indicated that G-HPC had good dispersibility in water. Similarly, G-LMC was well dispersible in an acidic aqueous medium (pH 4.3). ATR FTIR spectra of GO, G-HPC, G-LMC, pure HPC, and LMC are shown in Figure 3 (top). Compared to the original GO, peaks of C H stretching band at 3035 2774 cm 1 and C O-Cstretching at 1055 cm 1 were observed in G-HPC and G-LMC. These new peaks verified the presence/introduction of HPC and LMC in G-HPC and G-LMC, respectively. Contents of the grafted HPC and LMC in the functionalized graphenes were estimated by TGA. The TGA curves of polysaccharides, graphite, GO, G-HPC, and G-LMC are shown in Figure 4. It is apparent that G-HPC and G-LMC had different decomposition patterns, compared to pure polysaccharides, graphite, and GO. HPC started decomposing at 342 °C and lost 96% weight below 400 °C. Though the decomposition of 313



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Figure 3. ATR-FTIR spectra (top) of GO (a), G-HPC (b), G-LMC (c), HPC (d), and LMC (e), and Raman spectra (bottom) of GO (a), G-HPC (b), G-LMC (c), R-GO (d), R-G-HPC (e), and R-G-LMC (f).

Table 1. Raman Analysis Results of Graphite, GO, G-HPC, G-LMC, and Their TGA Residues at 600 °C sample

graphite GO R-GO G-HPC R-G-HPC G-LMC R-G-LMC

D (cm 1)

1324

1334 1336

1327

1327

1330

G (cm 1)

1572

1575 1573

1565

1587

1567

1579

ID/IG

0.10

0.85

1.06

1.17

1.01

1.09

0.78

1326

Figure 4. TGA traces of GO (a), LMC (b), HPC (c), G-HPC (d), G-LMC (e), and graphite (f).

G-HPC began at 228 °C, its weight loss was only ∼30% up to 340 °C, which was attributed to the thermal decomposition of the grafted HPC. LMC started decomposing at 123 °C, and approximately 20% weight remained even when heated to 600 °C. Different from pure LMC, G-LMC had higher onset decomposition temperature at 210 °C and lost 20% weight below 392 °C mainly from the thermal decomposition of the grafted LMC. The TGA analysis suggested that G-HPC and G-LMC contained approximately 30% HPC and 20% LMC, respectively. Raman spectra of GO, G-HPC, G-LMC, and their TGA residues at 600 °C are shown in Figure 3 (bottom). Detailed results were summarized in Table 1. Since the functionalization results in the increase of sp3-hybridized sidewall carbons, it is

Figure 5. XPS survey and high resolution N1s (inset) scan spectra of GO (a), G-HPC (b), and G-LMC (c).

expected that the blue (or red) shift at the D-band (C C) (or G-band, CdC) can be observed, and the degree of the defect or functionalization (ID/IG ratio) is enhanced to some extent. 314



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Figure 6. Typical AFM images and height profiles of G-LMC (top) and G-HPC (bottom).

As shown in Figure 3 and Table 1, shifts at D-bands (1 7 cm 1) and G-bands (4 10 cm 1) were indeed observed in G-LMC and

G-HPC, compared with GO. The ID/IG ratios of G-HPC and G-LMC, 1.06 and 1.01, respectively, were higher than that of GO 315


Industrial & Engineering Chemistry Research (0.85). In addition, it was expected that when the grafted HPC or LMC in G-HPC or G-LMC was thermally decomposed, the ID/ IG ratio of the residues would return to or be close to the value of GO or even the pristine graphite. The TGA residues of GO, G-HPC, and G-LMC (named R-GO, R-G-HPC, and R-G-LMC, respectively) were prepared by thermal decomposition at 600 °C under a nitrogen atmosphere. The ID/IG ratio of the GO residue slightly decreased from 0.85 to 0.78, probably due to the further thermal decomposition of oxygen-containing functional groups (see Figure 1D). On the contrary, it was out of expectation that the ID/IG ratios of the G-HPC and G-LMC residues (R-G-HPC and R-G-LMC, respectively) increased, which was presumably because the products from the HPC and LMC decomposition partially covered the graphite surface.41 The formation of graphene by the reduction of graphite oxide was verified by UV vis spectrometry3 according to the difference between graphene oxide and the functionalized graphenes (G-HPC and G-LMC) on UV vis spectra. As shown in Figure 2 (top), the GO had a characteristic peak at 231 nm, corresponding to π π* transitions of CdC bonds, and a shoulder at 300 nm, due to n π* transitions of carboxyl bonds. After the reduction, the G-HPC and G-LMC showed a characteristic absorption peak at 273 nm, which is the result of a red-shift of the characteristic absorption peak at 231 nm of CdC bonds. More importantly, the absorption peak at 300 nm disappeared. Raman spectroscopy provided additional evidence of formation of the graphene. It is expected that when graphite is oxidized to GO, the graphitic G band red shifts, while a blue shift should be observed when GO is reversely reduced to graphene.42 As shown in Figure 3 and Table 1, the red and then blue shifts were clearly observed. Compared with the graphite, the G band of GO redshifted from 1572 cm 1 to 1575 cm 1, while after reduction by hydrazine, the G band of GO blue-shifted back from 1575 cm 1 to 1565 1567 cm 1. The reduction of GO was further confirmed by the elemental composition of G-HPC and G-LMC using X-ray photoelectron spectroscopy (XPS). Figure 5 shows XPS survey scan spectra of GO, G-HPC, and G-LMC. GO contained 68.5% C and 31.2% O, and its O/C atomic ratio was 0.46. The oxygen is from the oxygen-containing groups introduced during the oxidation. G-HPC had 78.6% C and 20.9% O, and the O/C atomic ratio was 0.27. G-LMC contained 76.9% C and 19.7% O, and the O/C atomic ratio was 0.26. Theoretically, O/C ratio of HPC is 0.71 (with degree of substitution 2.9). Assume that HPC content in G-HPC is 30% (see the TGA data above), if the GO was not reduced at all, the calculated O/C ratio of the resultant G-HPC would be 0.52. Similarly, if the GO was completely reduced to graphene, the O/C ratio of the resultant G-HPC would be 0.13. The measured O/C ratio of G-HPC was 0.27, which suggests that the oxygen-containing groups of graphene oxide were approximately reduced by 65%. The result is consistent with the previous observation that a significant amount of oxygen exists in reduced graphene oxide.3 It was reported that it is difficult to completely remove oxygen-containing groups from graphene oxide.43 The GO, G-HPC, and G-LMC contained a small amount of N, which was observed at 400 eV in the N1s spectrum (inset of Figure 5). GO and G-HPC had 0.30% and 0.55% N, respectively, which came from the -NO2 formed in the nitric acid oxidation of graphite. Compared to GO and G-HPC, G-LMC had as high as 3.35% nitrogen content, which was from the grafted LMC. The thickness of G-HPC and G-LMC was estimated using an atomic force microscope (AFM), as shown in Figure 6. Typically,

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average thickness of G-HPC and G-LMC was 4 and 3 nm, respectively, which was consistent with the literature value of 2.5 4 nm.11,23,34 Considering the facts that the thickness of a single graphene nanosheet usually ranges from 0.8 to 1.3 nm estimated by AFM3 and that the grafted polymers have their own thickness, G-HPC and G-LMC were presumably graphene nanosheets comprising two layers of single graphene.

4. CONCLUSIONS In summary, two water-soluble and biocompatible polysaccharides (hydroxypropyl cellulose and chitosan) were covalently grafted onto graphene oxide by simple esterification reactions. After chemical reduction by hydrazine, stable aqueous suspensions of graphenes were prepared with the help of the grafted polysaccharides. The prepared graphenes presumably comprised two layers of single graphene. The polysaccharide-functionalized graphenes have potential applications in biological and medical fields, such as drug delivery, transplant device, and biosensor.18,19,25 27 ’ AUTHOR INFORMATION Corresponding Author

*Tel: 608-262-4951. Fax: 608-262-1228. E-mail: [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge the starting-up support from the Department of Biological Systems Engineering and the College of Agriculture and Life Science at University of Wisconsin— Madison and USDA McIntire-Stennis Fund (WIS01243). ’ REFERENCES (1) Kane, C. L. Materials science-erasing electron mass. Nature 2005, 438 (7065), 168–170. (2) Gengler, R. Y. N; Spyrou, K.; Rudolf, P. A roadmap to high quality chemically prepared graphene. J. Phys. D: Appl. Phys. 2010, 43, 374015 (19pp). (3) Park, S.; Ruoff, R. S. Chemical methods for the production of graphenes. Nat. Nanotechnol. 2009, 4 (4), 217–224. (4) Tkachev, S. V.; Buslaeva, E. Y.; Gubin, S. P. Graphene: a novel carbon nanomaterial. Inorg. Mater. 2011, 47 (1), 1–10. (5) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The chemistry of graphene oxide. Chem. Soc. Rev. 2010, 39 (1), 228–240. (6) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-based composite materials. Nature 2006, 442 (7100), 282– 286. (7) Brodie, B. C. On the atomic weight of graphite. Philos. Trans. R. Soc. 1859, 149, 249–259. (8) Staudenmaier, L. Verfahren zur darstellung der graphitsaure. Ber. Deutsch. Chem. Ges. 1898, 31, 1481–1487. (9) Hummers, W. S.; Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80 (6), 1339. (10) Bai, H.; Xu, Y. X.; Zhao, L.; Li, C.; Shi, G. Q. Non-covalent functionalization of graphene sheets by sulfonated polyaniline. Chem. Commun. 2009, 1667–1669. (11) Stankovich, S.; Piner, R. D.; Chen, X. Q.; Wu, N. Q.; Nguyen, S. T.; Ruoff, R. S. Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate). J. Mater. Chem. 2006, 16 (2), 155–158. (12) Yang, Q.; Pan, X. J.; Huang, F.; Li, K. C. Fabrication of highconcentration and stable aqueous suspensions of graphene nanosheets by noncovalent functionalization with lignin and cellulose derivatives. J. Phys. Chem. C 2010, 114 (9), 3811–3816. 316



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