Noncovalently Functionalize Graphene Sheets with Water-Soluble

Sep 21, 2009 - electroactive dye, methylene green (MG), during chemical reduction of graphene oxide (GO) with hydrazine. Atomic force microscopic and ...
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Processing of Graphene for Electrochemical Application: Noncovalently Functionalize Graphene Sheets with Water-Soluble Electroactive Methylene Green Huan Liu,† Jian Gao,‡ Mianqi Xue,† Nan Zhu,† Meining Zhang,*,† and Tingbing Cao† ‡

† Department of Chemistry, Renmin University of China, Beijing 100872, P. R. China, and Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China

Received August 10, 2009. Revised Manuscript Received September 15, 2009 To explore graphene applications in various fields, the processability of graphene becomes one of the important key issues, particularly with the increasing availability of synthetic graphene approaches, because the direct dispersion of hydrophobic graphene in water is prone to forming agglomerates irreversibly. Here, a facile method is proposed to increase the dispersity of graphene through noncovalent functionalization graphene with a water-soluble aromatic electroactive dye, methylene green (MG), during chemical reduction of graphene oxide (GO) with hydrazine. Atomic force microscopic and UV-vis spectrophotometric results demonstrate that chemically reduced graphene (CRG) functionalized with MG (CRG-MG) is well-dispersed into water through the coulomb repulsion between MG-adsorbed CRG sheets. The electrochemical properties of the formed CRG-MG are investigated, and the results demonstrate that CRG-MG confined onto a glassy carbon (GC) electrode has lower charge-transfer resistance and better electrocatalytic activity toward the oxidation of NADH, in relation to pristine CRG (i.e., without MG functionalization). This method not only offers a facile approach to dispersing graphene in water but also is envisaged to be useful for investigations on graphene-based electrochemistry.

Introduction Graphene has been attracting much attention due to its unique electronic properties1 and potential applications in synthesizing nanocomposites2 and fabricating various microelectrical devices,3 such as electromechanical resonators and ultrasensitive sensors. Since the first isolation of graphene by mechanical exfoliation of graphite crystals,4 many chemical methods, such as epitaxial growth on silicon carbide5 or ruthenium,6 reduction of graphene oxide (GO),7 have been developed for large-scale synthesis of graphene. Nowadays, the processability of graphene turns out to be one of the most important factors that must be properly addressed before extending graphene into various research and industrial applications, especially with increasing availability of the methods for graphene preparation. As one kind of carbon nanostructure, graphene is a one-atomthick sheet with an extraordinary electronic transport property *Corresponding author. E-mail: [email protected]; Tel: þ86-10-62514332.

(1) (a) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (b) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S; Ahn, J.; Kim, P.; Choi, J.; Hong, B. Nature 2009, 457, 706. (2) (a) 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. Nature 2006, 442, 282. (b) Muszynski, R.; Seger, B.; Kamat, P. V. J. Phys. Chem. C 2008, 112, 5263. (d) Williams, G.; Seger, B.; Kamat, P. V. ACS Nano 2008, 2, 1487. (3) (a) Bunch, J. S.; van der Zande, A. M.; Verbridge, S. S.; Frank, I. W.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Science 2007, 315, 490. (b) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Nat. Mater. 2007, 6, 652. (c) Robinson, J. T.; Perkins, F. K.; Snow, E. S.; Wei, Z.; Sheehan, P. E. Nano Lett. 2008, 8, 3137. (4) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (5) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Naud, C.; Mayo, D.; Li, T. B.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Science 2006, 312, 1191. (6) Sutter, P. W.; Flege, J.; Sutter, E. Nat. Mater. 2008, 7, 406. (7) (a) Gilje, S.; Han, S.; Wang, M.; Kang, K. L.; Kaner, R. B. Nano Lett. 2007, 7, 3394. (b) Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Nano Lett. 2007, 7, 3499.

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and can thereby be potentially used as a new sort of electrochemical material.8 However, graphene is hydrophobic and easily forms agglomerates irreversibly or even restacks to form graphite in an aqueous solution with the absence of dispersing agents. Particularly, this is a case when the dispersion of graphene is dip-coated onto a solid electrode to form a graphene-modified electrode. Such a limitation essentially makes it difficult to investigate graphene-based electrochemistry with the as-formed graphene-modified electrodes. As a consequence, a strategy for rational functionalization of graphene to increase its processability remains very essential for electrochemical investigations on graphene nanostructure. Although various methods have so far been attempted to avoid graphene aggregation, such as covalent or noncovalent attachment of polymer,9 surfactant,10 aromatic molecule,11 or hydrophilic groups12,13 onto the graphene surface, of which noncovalent strategies are more favorable than the covalent ones since the former maintains the electronic stucture of graphene, the as-formed graphene nanocomposites are yet limited in electrochemical applications. This is because the methods reported so far could not endow graphene with electroactive properties, even though they could largely increase the dispersity of graphene. (8) (a) Shan, C.; Yang, H.; Song, J.; Han, D.; Ivaska, A.; Niu, L. Anal. Chem. 2009, 81, 2378. (b) Alwarappan, S.; Erdem, A.; Liu, C.; Li, C. J. Phys. Chem. C 2009, 113, 8853. (c) Zhou, M.; Zhai, Y.; Dong, S. Anal. Chem. 2009, 81, 5603. (9) Ramanathan, T.; Abdala, A. A.; Stankovich, S.; Dikin, D. A.; Herrera-Alonso, M; Piner, R. D.; Adamson, D. H.; Schniepp, H. C.; Chen, X.; Ruoff, R. S.; Nguyen, S. T.; Aksay, I. A.; Prud’homme, R. K.; Brinson, L. C. Nat. Nanotechnol. 2008, 3, 327. (10) Stankovich, S.; Piner, R. D.; Chen, X.; Wu, N.; Nguyen, S. T.; Ruoff, R. S. J. Mater. Chem. 2006, 16, 155. (11) (a) Xu, Y.; Bai, H.; Lu, G.; Li, C.; Shi, G. J. Am. Chem. Soc. 2008, 130, 5856. (b) Su, Q.; Pang, S.; Alijani, V.; Li, C.; Feng, X.; M€ullen, K. Adv. Mater. in press. (12) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101. (13) Si, Y.; Samulski, E. T. Nano Lett. 2008, 8, 1679.

Published on Web 09/21/2009

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Letter Scheme 1. Structure of Methylene Green

As reported previously, graphene can act as the basic building block for graphitic materials of all other dimensionalities. For instance, graphene could be wrapped into fullerenes and rolled into nanotubes. Consequently, it is reasonable to speculate that both the dispersity and the electrochemical property of graphene could well be improved by noncovalent functionalization of graphene with water-soluble electroactive aromatic molecule through π-π interaction.14 In this study, we chose an aromatic dye, methylene green (MG), to noncovalently functionalize chemically reduced graphene (CRG) to form a new kind of electroactive nanocomposite (CRG-MG). As shown in Scheme 1, MG is a water-soluble molecule with a positive charge. Moreover, MG possesses good electrochemical properties and has been widely used for basic electrochemical studies and applications, such as biosensors and biofuel cell.15 Therefore, we expect that MG could be adsorbed on the surface of CRG through π-π stacking to weaken the strong van der Waals interactions between CRG sheets. The adsorption of MG onto CRG not only greatly improves the dispersity of CRG in water, but also enhances the electrocatalytic activity of the CRG-MG nanocomposite toward the oxidation of NADH. We believe that the as-prepared CRG-MG nanocomposite is very useful for electrochemical studies and can serve as a new type of electronic nanodevice derived from graphene.

Experimental Section Materials. Hydrazine hydrate and a reduced form of nicotinamide adenine dinucleotide (NADH) were all purchased from Sigma and used without further purification. Graphite, MG, and potassium ferricyanide were purchased from Chemical Reagent Co. Ltd. (Beijing, China). Other chemicals were of at least analytical grade and used as received. Aqueous solutions were prepared with deionized water. Solutions of redox probes were freshly prepared using deionized water before experiments. Synthesis of CRG-MG Nanocomposite. Graphene oxide (GO) was prepared from graphite powders following the method described by Hummers.16 The obtained GO was dispersed in 0.01 M KOH to yield a yellow-brown suspension (0.1 mg mL-1). MG (8 mM) was added in this suspension, and the mixture was ultrasonicated for 40 min until it became clear with no particulate matter. Then, hydrazine hydrate (w/w, 7:10 with GO) was added into the mixture, and the resultant mixture was first sonicated at about 60 °C for about 6 h and then filtered. The precipitation was washed with a copious volume of deionized water and dried by continuous N2 flow for about 10 h to form CRG-MG nanocomposite. The nanocomposite was dispersed in water (0.1 mg mL-1) for subsequent experiments. The synthesis of CRG was performed with the same procedures as those for the CRG-MG, with the exception of the addition of MG into GO dispersion. Apparatus and Measurements. Glassy carbon electrodes (GC electrode, 3-mm diameter, Bioanalytical Systems Inc.) were used as substrate electrodes. The electrodes were first polished (14) Yan, Y.; Zhang, M.; Gong, K; Su, L.; Guo, Z.; Mao, L. Chem. Mater. 2005, 17, 3457. (15) (a) Lin, Y.; Zhu, N.; Yu, P.; Su, L.; Mao, L. Anal. Chem. 2009, 81, 2067. (b) Li, X.; Zhou, H.; Yu, P.; Su, L.; Ohsaka, T.; Mao, L. Electrochem. Commun. 2008, 10, 851. (16) Hummers, W.; Offeman, R. J. Am. Chem. Soc. 1958, 80, 1339.

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Figure 1. Photo of aqueous dispersion (0.1 mg mL-1) of CRGMG (left) and CRG (right). with 0.3 and 0.05 μm alumina slurry on a polishing cloth and then sonicated in the acetone and deionized water each for 10 min. All electrochemical experiments were conducted in a standard threeelectrode cell at room temperature using a computer-controlled CHI 660B electrochemical analyzer (CH Instruments, Shanghai Chenhua Instrument Corporation, China). A Pt spiral wire and an Ag/AgCl (saturated with KCl) were used as counter and reference electrode, respectively. A 2 μL aliquot of CRG-MG or CRG dispersion was dip-coated on GC electrode, and the solvent was evaporated at room temperature to get CRG-MG- or CRGmodified electrodes. For comparison, MG-modified electrodes were prepared by immersing GC electrodes into 8 mM MG solution for 2 h. The formation of CRG-MG nanocomposite was characterized with atomic force microscopy (AFM, Dimensional 3100, Veeco Co.) and UV-vis spectrophotometry (Cary 50 UV-vis spectrophotometer, Varian).

Results and Discussion Compared with the methods for synthesis of graphene, such as exfoliation of graphite and epitaxial growth on silicon carbide, the chemical reduction of exfoliated GO is a more feasible technique. Hence, in this study we use CRG to demonstrate the strategy for dispersing graphene through the MG functionalized approach. In this case, MG was added in GO solution before chemical reduction of GO, and such a compound would be expected to adsorb on the surface of CRG via a strong π-π stacking when the GO was reduced by hydrazine. The adsorbed MG enables CRG to bear a positive charge, which will avoid the aggregation of CRG produced by chemical reduction and thereby increase its dispersity in aqueous solution. As displayed in Figure 1, the noncovalent adsorption of aromatic MG greatly enhances the dispersity of CRG in water, and the CRG-MG dispersion (0.1 mg mL-1) could be stable for at least one month. In contrast, CRG without MG functionalization aggregated more readily under the same conditions than CRG-MG. The good dispersity of the CRG-MG nanocomposite could be mainly attributed to the enhanced electrostatic repulsion among CRG-MG sheets. Atomic force microscopy (AFM) images provide further information on the effect of MG functionalization on the dispersity of graphene, as shown in Figure 2. The samples for AFM studies were prepared by depositing the CRG-MG and CRG dispersion on the surface of new clear silicon wafer and drying the silicon wafer under vacuum at room temperature. As displayed in Figure 2A, graphene sheets, with a mean thickness of around 2 nm, do not aggregate. This thickness was somewhat larger than the theoretical value for a perfectly flat sp2-carbon-atom network (i.e., ∼0.34 nm), but was almost consistent with the reported values for the nanocomposite of graphene with aromatic molecule DOI: 10.1021/la9029613

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Figure 2. AFM images of CRG-MG (A) and CRG (B) on silicon wafer.

Figure 3. (A) UV-vis spectra of MG solution (red curve) and the filtrate of the mixture containing CRG and MG (black curve). (B) UV-vis spectra of CRG-MG (red curve), GO (black curve), CRG (violet) dispersion, and MG (green curve) solution.

(i.e., 1.7 nm) by taking into account the overestimation in the AFM measurements.11 In contrast, it was difficult to observe the well-separated graphene sheets in the image of CRG (Figure 2B) and the mean height of the CRG was around 15 nm, due to the aggregation of the CRG sheets. These demonstrations strongly suggest the MG adsorption on the CRG sheets. We used UV-vis spectrophotometry to further confirm that MG adsorbed onto CRG. Figure 3 (A, red curve) shows the UV-vis spectrum of aqueous solution of MG with the same 12008 DOI: 10.1021/la9029613

concentration as that in the GO dispersion in water. The spectrum of MG exhibits an absorption peak at about 616 nm with a shoulder at about 645 nm. This shoulder might be ascribed to the dimer of MG in the aqueous solution, which was similar to those of other kinds of dye molecules.17 The spectrum of the filtrate of the mixture containing CRG and MG also exhibits the same absorption peak at 616 nm (Figure 3, black curve), but the absorbance of this absorption peak was greatly decreased, in comparison with that of the red curve. This result clearly suggests that MG molecules are adsorbed onto CRG sheets. Figure 3B displays UV-vis spectra of GO, CRG, MG, and CRG-MG from 200 to 800 nm. The spectrum of GO shows an absorption peak at about 230 nm (violet), and this absorption peak shifts to 263 nm when GO is chemically reduced into CRG (blue curve). On the other hand, the spectrum of CRG also shows strong absorbance at 200-227 nm and the spectrum of MG has two small absorption peaks at 283 and 243 nm. The spectrum of CRG-MG displays a stong absorbance at 200-227 nm and two absorption peaks at 614 nm and in the range 243-283 nm, indicative of the formation of CRG-MG nanocomposite. As mentioned above, CRG easily forms an agglomerate when GO is treated with hydrazine in water. This property could decrease the effective surface area of CRG and increases the possibility of CRG restacking into graphite, both of which are eventually detrimental to the electrochemical applications of CRG. The adsorption of MG onto CRG to form the electroactive nanocomposite with improved dispersity is expected to improve electrochemical performance of the as-formed CRG-MG nanocomposite. Figure 4 compares cyclic voltammograms (CVs) obtained at the CRG-MG-modified (red curve), CRG-modified (blue curve), and bare (black curve) GC electrodes in 0.1 M KCl solution containing 5 mM Fe(CN)63-. The charging current and the redox peak currents obtained for the Fe(CN)63- probe obtained at the CRG-MG-modified electrode were larger than those at both the CRG-modified and the bare electrodes. The different currents obtained at the CRG-MG-modified electrode from those at the CRG-modified electrode suggest that the former (17) Sagara, T.; Niki, K. Langmuir 1993, 9, 831.

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Figure 4. CVs obtained at CRG-MG-modified (red curve), CRGmodified (blue curve), and bare (black curve) GC electrodes in 0.1 M KCl solution containing 5 mM Fe(CN)63-. Scan rate, 50 mV s-1.

Figure 5. Nyquist plots obtained at the CRG-MG-modified (red 9), CRG-modified (blue [), and bare (b) GC electrodes in 0.1 M KCl containing 5 mM Fe(CN)63-/4-. Frequency range, 105 to 0.1 Hz; potential, 0.24 V; perturbation signal, 5 mV.

electrode has a larger surface area than the latter one. Such a difference in the surface area was considered to result from the different dispersing property of CRG sheet and CRG-MG nanocomposite when they were dip-coated onto GC substrate. For instance, CRG sheet tends to aggregate at electrode surface upon solvent evaporation. This feature could, on one hand, make it possible for graphene sheets to restack into graphite and, on the other hand, reduce the surface area accessible for the redox probe. Differently, the noncovalent attachment of MG onto CRG substantially improves the stability of CRG-MG dispersion and decreases the possibility of CRG to restack into graphite during processing of CRG. Moreover, the as-formed CRG-MG was well-dispersed onto electrode surface to form the electrodes on which the dispersed graphene sheet could be easily accessible to the redox probe and ions. The good electrochemical property of CRG-MG nanocomposite was further verified with electrochemical impedance spectra as shown in Figure 5. The charge transfer resistance (Rct) values for the Fe(CN)63-/4- redox probe, measured as the diameter of the semicircle in the Nyquist plots, at the CRG-modified and bare GC electrodes were ca. 196 and 350 Ω, respectively. The Rct at the CRG-MG-modified electrode was smaller than those obtained at the CRG-modified and bare GC electrode, reflecting a fast electron transfer kinetics at the former electrode. The enhanced electron transfer kinetics for the Fe(CN)63-/4- redox couple at the CRG-MG-modified electrode might be considered to result from the unique electronic property of graphene sheet8 and from the electrostatic interaction between MG molecules attached onto Langmuir 2009, 25(20), 12006–12010

Figure 6. (A) CVs obtained at the CRG-MG-modified electrode in 0.1 M phosphate buffer (pH = 7.0) in the absence (dotted curve) and presence (solid curve) of 2 mM NADH. (B) CVs obtained at the MG-modified (black curve) and bare (red curve) GC electrodes in the same buffer in the absence (dotted curve) and presence (solid curve) of 2 mM NADH . Scan rate, 10 mV s-1.

CRG sheet and the Fe(CN)63-/4- redox couple employed in this study. The improved electrochemical activity of CRG-MG nanocomposite would be beneficial to its electrocatalytic performance toward the oxidation of NADH, as demonstrated below. NADH is an important coenzyme that takes part in more than 300 kinds of dehydrogenase enzymatic reactions.18 Electrocatalytic oxidation of NADH has been receiving great attention from the viewpoint of developing dehydrogenase-based biodevices. However, the electrochemical oxidation of NADH at a bare electrode in a neutral solution proceeds at a high overpotential (ca. þ0.5 V) because of slow electron transfer kinetics and electrode fouling.19 Thus, effective oxidation of NADH at a low potential is highly desirable to develop NADH-based biodevices. Although the use of redox-active dye molecules, like MG, offers an effective way for electrocatalytic oxidation of NADH, it is difficult to confine these water-soluble molecules on the electrode.14 Hence, the formation of CRG-MG nanocomposite not only avoids CRG aggregation, but also presents a facile approach to confinement of MG on electrode surface. Figure 6A illustrates the electrocatalytic oxidation of NADH at the CRGMG-modified electrode in phosphate buffer (pH = 7.0). For comparison, the oxidation of NADH at MG-modified and bare electrodes was also given (Figure 6B). At CRG-modified electrode, the oxidation of NADH occurs at ca. þ0.40 V (data not shown), which was more negative than that at bare GC electrode (ca. þ0.55 V), and remained quite close to that at carbon (18) Foster, J. W.; Park, Y. K.; Penfound, T.; Fenger, T.; Spector, M. P. J. Bacteriol. 1990, 172, 4187. (19) (a) Elving, P. J.; Bresnahan, W. T.; Moiroux, J.; Samec, Z. Bioelectrochem. Bioenerg. 1982, 9, 365. (b) Raj, C. R.; Ohsaka, T. Electrochem. Commun. 2001, 3, 633.

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nanotube electrodes.20 As displayed in Figure 6A, the addition of NADH in the buffer results in an increase in the oxidation peak current of MG adsorbed onto CRG. The potential for NADH oxidation obtained at CRG-MG electrode was ca. þ0.14 V, which was more negative than those at the CRG-modified and bare GC electrodes and carbon nanotube electrodes, suggesting the prepared electroactive CRG-MG nanocomposite possesses an excellent electrocatalytic activity toward the oxidation of NADH. Interestingly, we found that the adsorption of MG onto CRG to form the electroactive nanocomposite actually increases the electrocatalytic activity of MG toward the oxidation of NADH. Such a property could be evident from the more negative potential and reduced scan-rate dependence (data not shown) of NADH oxidation at the CRG-MG-modified electrode compared with those at the electrode modified only with MG. Such an enhancement could presumably benefit from the charge transfer (20) (a) Musameh, M.; Wang, Joseph.; Merkoci, A.; Lin, Y. Electrochem. Commun. 2002, 4, 743. (b) Valentini, F.; Amine, A.; Orlanducci, S.; Terranova, M. L.; Palleschi, G. Anal. Chem. 2003, 75, 5413. (c) Wang, J.; Musameh, M. Anal. Chem. 2003, 75, 2075. (d) Zhang, M.; Smith, A.; Gorski, W. Anal. Chem. 2004, 76, 5045.

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interaction between CRG sheet and MG molecule or other factors that are currently under investigation in our laboratory.

Conclusions In summary, by taking advantage of the interaction between CRG and MG, this study has demonstrated a facile method to process graphene nanosheet through noncovalent functionalization of CRG with water-soluble aromatic MG. The successful attachment of MG onto CRG not only solubilizes the as-formed CRG-MG nanocomposite into aqueous media, but also endows CRG nanosheet with excellent electrochemical property and electrocatalytic activity toward NADH oxidation. This study essentially paves a new way to solubilization and functionalization of graphene as well as development of graphene-based electronic devices with striking properties. Acknowledgment. We gratefully acknowledge the financial support from National Natural Science Foundation of China (20905076 for M. Zhang, 20733001 and 50773092 for T. Cao) and Renmin University of China (No. 22385005).

Langmuir 2009, 25(20), 12006–12010