Article pubs.acs.org/JPCC
Simultaneous Reduction and Surface Functionalization of Graphene Oxide by Natural Cellulose with the Assistance of the Ionic Liquid Hongdan Peng,† Lingjie Meng,† Lvye Niu,† and Qinghua Lu*,†,‡ †
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai, 200240, P. R. China State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China
‡
ABSTRACT: An environmentally friendly method for preparation of multifunctional free-standing papers composed of reduced graphene oxide (RGO) and cellulose has been developed. The results show that the cellulose can effectively reduce the graphene oxide (GO) nanosheets in ionic liquids and the cellulose can be adsorbed onto the basal planes of the resulted RGO at the same time. Since cellulose is a natural, inexpensive, renewable, nontoxic, and biodegradable polymer, this approach presents a facile and cost-effective method to synthesize highly water-dispersible and stable functionalized graphene nanosheets (termed as RGO-CL) on a large scale. Furthermore, highly ordered RGO-CL composite papers are fabricated by flow-directed assembly of individual hybrid RGO-CL nanosheets and show a robust mechanical flexibility and significantly improved biocompatibility and conductivity, and have therefore potential applications in biomedical scaffolds for tissue engineering, medical devices, and so on.
1. INTRODUCTION
Cellulose, the most abundant natural polysaccharide in the world, is attracting more and more attention in fundamental research and applications as a green material due to its widespread availability, low cost, minimal environmental impact, and sustainability.16 Nevertheless, cellulose is difficult to process in solution or as a melt, because of its large proportion of intra- and intermolecular hydrogen bonds caused by the hydroxyl groups in glucose residues. This property causes difficulties in improving the processability, fusibility, and usability of the cellulose. Recently, it has been reported that the room-temperature ionic liquids (ILs), which are considered as desirable green solvents, can be used to effectively dissolve cellulose for a variety of applications.17−21 Actually, previous studies also showed that a large number of polar hydroxyl groups in cellulose could endow it with mild reducing power.22,23 Therefore, we presume the abundant functional groups in cellulose may be harnessed to the deoxygenation of exfoliated GO, and provide strong interactive sites to bind RGO nanosheets as well.24 Herein, we report a facile but highly efficient and environmentally friendly approach for the synthesis of biocompatible RGO (termed as RGO-CL) in IL media using natural cellulose as the reductant and stabilizer. In addition, large-area free-standing RGO-CL composite papers were readily fabricated by vacuum-assisted self-assembly. The morphology, structure, conductivity, hydrophilicity, and biocompatibility of this new composite paper are also investigated.
Emerging as atomically thin two-dimensional carbon materials, graphene nanosheets have attracted considerable attention recently in many prospective applications, such as nanocomposites, high-performance devices, biosensors, and energy storage and conversion, due to their exceptional electronic, optical, mechanical, and thermal properties.1−4 When targeting the practical implementation of graphene in these applications, a persistent challenge is realizing its high economical accessibility and easy processing. Up to now, various synthetic methods, such as micromechanical exfoliation, chemical vapor deposition, epitaxial growth, and chemical reduction of graphene oxide (GO), have been successfully developed to fabricate graphene.5,6 Among them, the chemical reduction of GO holds the great advantage of low cost and bulk quantity production. More importantly, it is well suited for the production of RGO bearing various desired functional groups amenable for further application. Early work indicated that deoxygenation of GO to restore the conjugate of graphene could be effectively accomplished using various strong reductants such as anhydrous hydrazine,7 hydrazine monohydrate,8 phenylhydrazine,9 and sodium borohydride,10 and even some mild reductants including dopamine,11 hydroquinone,12 L-ascorbic acid,13 alcohols,14 the protein bovine serum albumin (BSA),15 and so forth. However, either most of the reported reductants such as hydrazine and its derivatives are highly toxic or hazardous or some of them including BSA are quite expensive. In addition, extra stabilizers or surfactants are commonly needed to prevent the irreversible aggregation of graphene layers during the reduction process. Therefore, it is still very attractive to explore an inexpensive and eco-friendly material acting as both the reductant and the stabilizer for mass production of graphene. © 2012 American Chemical Society
Received: May 7, 2012 Revised: July 4, 2012 Published: July 9, 2012 16294
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Figure 1. Schematic illustration of the preparation of cellulose-induced reduction of exfoliated GO.
the hydrolysis reaction was carried out at 50 °C for 24 h under magnetic stirring. The mixture was then filtered and washed repeatedly with deionized water (200 mL × 3) to remove the digested cellulose. The black solid obtained was dried overnight at 40 °C under a vacuum for 12 h. Preparation of GO or RGO-CL Papers. GO or RGOcellulose aqueous suspension (0.1 mg/mL, 200 mL) was filtered through a nylon membrane (35 mm in diameter, 220 nm pore size) by vacuum at room temperature. Followed by air drying, the GO or the RGO-cellulose paper could be easily peeled off from the substrate of the membrane. The thickness of the GO or RGO-cellulose paper could be controlled by adjusting the concentration and volume of the suspension. AO/EB Double Staining for Biocompatibility Test. HeLa cells were seeded on the films at 40,000 cells per well of a 24-well culture plate (about 50−70% confluence for the experiments). After being cultured on RGO-CL or RGO film for 24 h at 37 °C, the cells were washed with sterilized phosphate buffered saline (PBS) and stained with a mixture of AO (5 μg/mL) and EB (5 μg/mL) at room temperature for 5 min. The stained cells were studied by a converted fluorescence microscope (IX71, Olympus), and images were taken by a charge coupled device (CCD, Cascade 650).
2. EXPERIMENTAL SECTION Materials and Instruments. Graphite powder (spectral requirement) was purchased from Shanghai Chemicals Co., Ltd. GO sheets were synthesized from graphite powder by a modified Hummers method.25,26 1-Butyl-3-methylimidazolium chloride ([Bmim]Cl) was obtained from Shanghai Chengjie Chemical Co. Ltd. Cellulose, acridine orange (AO), and ethidium bromide (EB) were purchased from Sigma-Aldrich. Fetal bovine serum (FBS) was obtained from Hyclone. Human cervical cancer HeLa cells were cultured in high glucose Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% FBS in a humidified incubator (MCO-15AC, Sanyo) stored at 37 °C (95% room air, 5% CO2). An aqueous solution of NaOH/thiourea (9.5 wt % NaOH and 4.5 wt % thiourea) was prepared according to a literature method and stored at −5 °C prior to use.27 Ultrapure water (18 MΩ) was produced by a Millore System (Millipore Q. USA). All other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. (SCRC) and used as received. UV−vis spectra were recorded by a Shimadzu 2550 UV−vis spectrometer. Fourier transform infrared (FT-IR) spectra were carried out on a Perkin-Elmer Paragon 1000 FTIR spectrometer. Raman spectra were taken with a Ramlab-010 Micro-Raman spectrometer at 632.8 nm. X-ray photoelectron (XPS) spectra were recorded on an ESCA LAB 250 spectrometer with Al Kα radiation (1486.6 eV). X-ray powder diffraction (XRD) spectra were performed on a D/max-2200/ PC (Japan Rigaku Corporation) using Cu Ka radiation. Atomic force microscopy (AFM) images were obtained using a digital Nanoscope IIIa atomic force microscope in tapping mode. Transmission electron microscopy (TEM) images were obtained using JEOL2100F. Scanning electron microscopy (SEM, Tescan) was performed at an acceleration voltage of 20 kV. Preparation of RGO-CL Composite. In a typical procedure, [Bmim]Cl (30 g) was added to the GO aqueous dispersion (0.8 mg/mL, 100 mL) and irradiated in an ultrasonic bath for 1 h. The mixture of [Bmim]Cl/H2O/GO was distilled by using a rotary evaporator to entirely remove water. Then, the [Bmim]Cl solution of cellulose (5% w/w, 20 mL) and ammonia solution (25 w%, 25 μL) were added to the GO suspension and the mixture was heated at 80 °C for 12 h. After adding ultrapure water (100 mL), the black sample was collected by filtration, and then dispersed in 9.5 wt % NaOH and 4.5 wt % thiourea aqueous solution (200 mL) at −5 °C under sonication for 20 min and recollected by centrifugation at 11000 rpm for 30 min. The obtained centrifugations were further purified twice by dissolution and centrifugation in precooled NaOH/thiourea aqueous solution (200 mL) and then washed extensively with water, followed by vacuum drying at room temperature. Preparation of Pure RGO Sample. The dried RGO-CL nanocomposite (100 mg) was dispersed in 50 mL of sodium citrate buffer (pH 4.7, 50 mM). After adding cellulose enzyme,
3. RESULTS AND DISCUSSION With the increasing emphasis on the environment and sustainability, the approach toward the mass production of processable graphene from graphite oxide using naturally produced bioderived reducing agent is a very attractive method. As a known renewable reducing agent,28 cellulose should be an excellent candidate for deoxygenation of GO. However, it is almost impossible to dissolve cellulose in water and common organic solvents, because multiple hydrogen bonds between cellulose molecules results in the formation of highly ordered crystalline regions. We decided to dissolve cellulose in an ionic liquid, [Bmim]Cl, a method recently developed by Rogers and his co-workers.17 The obtained cellulose−[Bmim]Cl solution can then be used for deoxygenation of GO. We started off with a GO aqueous dispersion and mixed it with [Bmim]Cl. After removal of water,29 a homogeneous viscous GO suspension formed. The GO nanosheets were reduced and functionalized after adding a stock solution of cellulose in [Bmim]Cl. The yellow-brown color of GO solution gradually changed into the characteristic black of RGO under mild conditions, and the presence of ammonia can accelerate the reduction reaction due to a synergistic augmentation of the deoxygenation reaction rate.30 The [Bmim]Cl was removed by filtrating and washing with ultrapure water, which was confirmed by adding AgNO3 aqueous solution into the filtrate without white precipitate generated. After further removal of unadsorbed cellulose by repeated dispersing in NaOH/thiourea aqueous solution and centrifugation,27 the black and flexible RGO-CL papers were fabricated by filtration of the homogeneous aqueous suspension of RGO-CL nanocomposite (Figure 1). For comparison, the 16295
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Figure 2. (a) UV−vis absorption spectra of GO (0.1 mg/mL) before and after the reduction with cellulose. (b) FTIR spectra of GO, RGO, cellulose, and RGO-CL. (c, d) The C1s XPS spectra of (c) GO and (d) pure RGO. (e) Raman spectra of GO and RGO-CL. (f) XRD pattern of GO, RGOCL, cellulose, and RGO.
pure RGO samples as a control were obtained by hydrolyzing the cellulose in RGO-CL nanocomposite with the help of the commercially available cellulose enzyme.31 The reduction and functionalization of GO by cellulose was demonstrated by the UV−vis, Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), Raman, and Xray diffraction (XRD) spectra, as shown in Figure 2. In the UV−vis spectra (Figure 2a), the GO has a characteristic peak at 231 nm, corresponding to π−π* transitions of carbon−carbon bonds, and a shoulder at about 300 nm, due to the n−π* transition of carboxyl bonds.32 After reduction, the peak at 231 nm was red-shifted to 269 nm and no absorption peak at 300 nm was observed, indicating the highly conjugated electronic structure is restored in the resulting RGO.8 Compared to the GO sample, the intensities of the stretching vibration bands of CO at 1730 cm−1 and the stretching vibration bands CO of epoxy and alkoxy at 1220 and 1058 cm−1, respectively, all decreased evidently in the RGO sample (Figure 2b). In
addition, the FT-IR spectrum of the RGO-CL is very similar to that of cellulose, giving direct evidence that the cellulose has been successfully functionalized onto RGO nanosheets. The deconvoluted C 1s spectra of GO show four types of carbon with different chemical states, which appear at 284.5 eV (graphite, CC/CC), 286.6 eV (COH), 287.8 eV (C O), and 288.4 eV (OCO), correspondingly (Figure 2c). Although four types of carbon with different chemical valences remain for RGO (Figure 2d), it is obviously seen that the intensities of all C1s peaks of the carbon binding to oxygen, especially the peak of CO, decreased tremendously and the content of CC/CC increases dramatically, supporting that most of the oxygen-containing functional groups were removed after the reduction. The C1s XPS spectra of GO and pure RGO were also used to quantitatively study the deoxygenation. The atomic percentage of C and O in GO was found to be 69.5% (C) and 30.2% (O), whereas it was 83.7% (C) and 15.3% (O) in RGO. The significant decrease of the atomic oxygen 16296
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Figure 3. AFM images and the corresponding height measurement of GO (a and c) and RGO-cellulose (b and d). TEM image (e) and corresponding selected electron diffraction pattern (f) of the as-made RGO-CL.
are absent in the RGO sample,37 showing the successful removal of the absorbed cellulose by the enzymatic method. For further characterization of the morphology and structure of the RGO-CL nanosheets, atomic force microscopy (AFM) and transmission electron microscopy (TEM) were carried out (Figure 3). As shown in Figure 3a and c, the average thickness of the single-layer GO sheet with various shapes and sizes was about 1.0 nm, which was consistent with the value reported in the previous literature.38 And the average thickness of RGO-CL slightly increased to 1.9 nm because the cellulose chains have already been attached on the RGO (Figure 3b and d). TEM of the as-prepared RGO-CL material shown in Figure 3e exhibits a wrinkle-like thin sheet with many folds at the edge, which is the intrinsic feature of graphene nanosheets. The corresponding SAED yields well-defined 6-fold-symmetry diffraction patterns matching those expected for graphene nanosheets (Figure 3f), indicating that GO has been successfully reduced to graphene nanosheets.39 Uniform RGO-CL composite papers were fabricated by vacuum filtration of as-prepared RGO-CL dispersion, and their bending, conductive, and hydrophilic properties and microscopic structures were investigated. The photographs of RGOCL are shown in Figure 4a and b. The composite paper is black and flexible, and can be folded dozens of times without noticeable breaking. The RGO content in the nanocomposite is
percentage in RGO further confirms the removal of oxygen containing functionalities. The restoration degree of sp2 conjugated structure of GO are also carried out by Raman spectra (Figure 2e). The Raman spectra of GO displayed two prominent peaks at 1591 and 1331 cm−1, which correspond to the well-documented G (corresponding to highly crystalline sp2 graphite structure) and D (resulting from amorphous carbon and defects) bands, respectively.33 These bands still existed in the Raman spectrum of RGO-CL, but the ratio of D/G intensity increased significantly from 1.32 to 1.53 after reduction by cellulose, which was actually attributed to the increase in the number of small in-plane sp2 domains. The changes in Raman spectra agree well with the hydrazineassisted reduction of GO,34 and definitely confirm the deoxygenation ability of cellulose toward GO. We further characterized the structure of RGO-CL by XRD (Figure 2f). The feature diffraction peak of GO appears at 11.9° (002) with an interlay space (d-spacing) of 0.74 nm due to the interlamellar water trapped between the hydrophilic GO sheets.35 However, the peak at 11.9° disappears in the XRD spectra of RGO-CL and RGO, confirming the significant reduction of GO and the exfoliation of the layered GO nanosheets.36 Compared to RGO-CL, the peaks at 15.1, 16.8, and 22.5° corresponding to the crystalline phase of the cellulose 16297
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Figure 4. (a, b) Optical images of RGO-CL paper. (c, d) Surface (c) and cross-section (d) SEM images of RGO-CL paper. (e, f) Fluorescence microscopy images of HeLa cells cultured for 24 h on the RGO-CL (e) and the pure RGO (f). The cells were stained by AO and EB.
necrosis or a late stage of apoptosis have red nuclei with a damaged cell membrane. As demonstrated in Figure 4e, very fewer necrotic cells were observed on RGO-CL paper after culturing for 24 h. In contrast, some HeLa cells cultured on RGO paper were in necrosis with red nuclei (Figure 4f), suggesting that the cellulose coating remarkably improved cell biocompatibility of RGO. It can be attributed to the intrinsic biocompatibility and hydrophilic nature of cellulose materials.41
about 24 wt % by weighing the samples before and after biodegradation of the cellulose part. The SEM image showed that the surfaces of the composite paper form a compact and rough morphology (Figure 4c). The cross sections exhibit a layered structure (Figure 4d) that should be caused by the directional flow induced by vacuum filtration and self-ordering of high aspect ratio RGO sheets. To evaluate the electrical conductivity of RGO-CL paper, four-point probe analyses were carried out at three different sites of each sample to get an average value. Though the GO papers are not electrically conductive (3 × 10−6 S m−1), the conductivity of the RGO-CL paper in our experiment is 3.2 S m−1. Moreover, it further increased to 700 S m−1 after enzymatic hydrolysis to remove the adsorbed cellulose in RGO paper, and this value is comparable to the conductivity of the non-surface-modified RGO paper from GO reduced by Vitamin C.40 The results demonstrated that the cellulose-induced reduction method can effectively recover the electronic conjugation of GO sheets and therefore enhance the conductivity of the RGO-CL composite. Due to the presence of huge hydrophilic groups on cellulose molecules, the RGO-CL composite paper appears much more hydrophilic with a contact angle of 49.2°, compared to nude RGO with a contact angle of about 81.6°. In order to examine the biocompatibility of RGO-CL paper, the cell viability of HeLa cells was studied by an acridine orange/ethidium bromide (AO/EB) double staining experiment. Generally, healthy cells have green nuclei and uniform chromatin with an intact cell membrane, whereas the cells in
4. CONCLUSIONS In summary, an inexpensive and facile approach for simultaneous reduction and functionalization of GO by natural cellulose in [Bmim]Cl was demonstrated, which should facilitate the large scale production of processable RGO and further applications. Moreover, the RGO-CL paper obtained by vacuum filtration shows robust mechanically flexible, comparatively highly conductive, dramatically improved hydrophilic and outstanding biocompatible properties, and may therefore have great potential applications in biomaterial scaffolds for tissue engineering and medical devices.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest. 16298
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ACKNOWLEDGMENTS This work was supported by National Science Fund for Distinguished Young Scholars (50925310), the National Science Foundation of China (20874059, 20904030, 21174087), the Major Project of Chinese National Programs for Fundamental Research and Development (973 Project: 2009CB930400), the High Technology Research and Development Program of China (863 Project: 2009AA03Z329), the Key Fundamental Research Project of Science and Technology Commission of Shanghai Municipal Government (08JC1412300), and the Shanghai Leading Academic Discipline Project (No. B202).
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