pubs.acs.org/Langmuir © 2010 American Chemical Society
Self-Assembly of Cationic Polyelectrolyte-Functionalized Graphene Nanosheets and Gold Nanoparticles: A Two-Dimensional Heterostructure for Hydrogen Peroxide Sensing Youxing Fang, Shaojun Guo, Chengzhou Zhu, Yueming Zhai, and Erkang Wang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China, and Graduate School of the Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China Received February 8, 2010. Revised Manuscript Received March 7, 2010 We demonstrate the use of cationic polyelectrolyte poly(diallyldimethyl ammonium chloride) (PDDA) functionalized graphene nanosheets (GNs) as the building block in the self-assembly of GNs/Au nanoparticles (NPs) heterostructure to enhance the electrochemical catalytic ability. To ensure the GNs were modified with PDDA successfully, we study the PDDA/GNs with atomic force microscopy (AFM) and zeta potential measurements on the roughness and zeta potential changes relative to those of unmodified GNs, respectively. Then, the citrate-capped Au NPs are employed as the other model particles to construct two-dimensional GNs/NPs heterostructure. Here, the use of PDDA modifiers not only alters the electrostatic charges of graphene, but also probably provides a convenient self-assembly approach to the hybridization of graphene. Furthermore, we employ the high-loading Au NPs on graphene (GN/Au-NPs) as the electrochemical enhanced material for H2O2 sensing (as the model analyte). The wide linear ranges and low detection limits are obtained using the chronoamperometry technique at the GN/Au-NPs-modified glassy carbon electrode.
Introduction Graphene, a one-atom thick and two-dimensional closely packed honeycomb lattice, has received numerous investigations from both the experimental and theoretical scientific communities since the experimental observation of single layers by K. S. Novoselov and A. K. Geim in 2004.1 As a counterpart of graphite with well-separated 2D aromatic sheets, graphene possesses the high quality of the sp2 conjugated bond in the carbon lattice. Graphene has remarkably high electron mobility under ambient conditions with reported values in excess of 15000 cm2/(V s).2,3 Moreover, graphene has a very large specific surface area (theoretical value 2600 m2/g) with low manufacturing cost. These unique properties make graphene a promising additive or supporting component for potential applications in many technological fields such as nanocomposites,4,5 batteries,6 supercapacitors,7 nanoelectronics,8 and sensors,9,10 etc. To date, graphene-based hybrid materials have become a hot research topic in material science because the hybridization can be an effective strategy to enhance the functionality of materials,11 and the integration of nanomaterials on GNs potentially paves a new way to enhance their electronic, chemical, and electrochemical properties. *Corresponding author. E-mail:
[email protected]. Fax: þ86-431-85689711.
(1) 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. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197. (3) Zhang, Y. B.; Tan, Y. W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201. (4) Wang, D. H.; Choi, D. W.; Li, J.; Yang, Z. G.; Nie, Z. M.; Kou, R.; Hu, D. H.; Wang, C. M.; Saraf, L. V.; Zhang, J. G.; Aksay, I. A.; Liu, J. ACS Nano 2009, 3, 907. (5) Guo, S.; Dong, S.; Wang, E. ACS Nano 2009, 4, 547. (6) Guo, P.; Song, H. H.; Chen, X. H. Electrochem. Commun. 2009, 11, 1320. (7) Wang, Y.; Shi, Z. Q.; Huang, Y.; Ma, Y. F.; Wang, C. Y.; Chen, M. M.; Chen, Y. S. J. Phys. Chem. C 2009, 113, 13103. (8) Sui, Y.; Appenzeller, J. Nano Lett. 2009, 9, 2973. (9) Shan, C. S.; Yang, H. F.; Song, J. F.; Han, D. X.; Ivaska, A.; Niu, L. Anal. Chem. 2009, 81, 2378. (10) Wang, Y.; Li, Y.; Tang, L.; Lu, J.; Li, J. Electrochem. Commun. 2009, 11, 889.
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On the other hand, self-assembly is an important and effective strategy for nanofabrication that involves self-organizing the building blocks into functional structures by different driving forces. The most important driving force for self-assembly is the interaction between building blocks that range from atoms and small molecules to particles, macromolecules, and polymers.12 As an emerging very thin and large-area nanomaterial with unique properties, GNs can be a promising building block in the hybridization with various NPs. However, because of the unstable nature of graphene dispersion, it is difficult to graft NPs onto the graphene. Up to present, only a small number of examples for graphene/NPs hybrid have been reported via self-assembly approaches, such as the synthesis of TiO2/graphene nanocomposite using the sodium dodecyl sulfate-capped graphene,4 the linkage of Au NPs and graphene by π-π interactions,13 and Pt/graphene hybrid film via the layer-by-layer protocol,14 etc. Whereas with the demand of high loading and uniform distribution, it is still a challenge to synthesize graphene/NPs hybrids based on selfassembly strategy. Moreover, the chemical conversion method can produce largescale GNs that preserve residual defects with negative charges, such as carboxyl, hydroxyl, and epoxy groups,15 which can be helpful anchors for further modifications or hybridizations. Nevertheless, directly taking advantage of graphene nanosheets’ negative charges in the self-assembly process has two main difficulties: (1) due to a very high specific surface area of graphene, GNs are apt to aggregate (caused by strong van der Waals’ and π-π interactions between GNs); (2) the negative charge is too (11) Jasuja, K.; Berry, V. ACS Nano 2009, 3, 2358. (12) Zhang, J.; Wang, Z.; Liu, J.; Chen, S.; Liu, G. Self-Assembled Nanostructures; Kluwer Academic Publishers: New York, 2002; p 7. (13) Hong, W.; Bai, H.; Xu, Y.; Yao, Z.; Gu, Z.; Shi, G. J. Phys. Chem. C 2010, 114, 1822–1826. (14) Zhu, C.; Guo, S.; Zhai, Y.; Dong, S. Langmuir, published online January 14, 2010, http://dx.doi.org/10.1021/la904201j. (15) Tung, V.; Allen, M.; Yang, Y.; Kaner, R. Nat. Nanotechnol 2009, 4, 25.
Published on Web 03/16/2010
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weak to assemble NPs directly. To overcome the above two obstacles, we use poly(N-vinyl-2-pyrrolidone) (PVP) as the stabilizer and PDDA as the functional macromolecule to produce the stable building block dispersion of the cationic polyelectrolyte-functionalized GNs. In this paper, we demonstrate the use of functional graphene as a supporting material in constructing 2D heterostructures to enhance electrochemical catalytic ability of Au NPs. For their mature productive method and negative charges, the typical citrate-capped Au NPs were chosen as the model NPs, but this method can be applied to other materials with negative charge as well. To ensure the GNs were modified with PDDA successfully, we study the PDDA-functionalized GNs with AFM and zeta potential measurements on their roughness, thickness, and zeta potential changes. The use of PDDA modifiers not only alters the electrostatic charges of graphene, but also probably provides a convenient self-assembly approach for the hybridization of graphene. Furthermore, we employ the highdensity Au NPs supported on GNs (characterized by transmission electron microscope (TEM)) as the electrochemical enhanced material for H2O2 sensing (as the model analyte). The wide linear ranges and low detection limits are obtained using the amperometric-time technique at the GN/Au-NPs-modified glassy carbon electrode.
Experimental Section Chemical and Materials. Graphite was purchased from Alfa Aesar. PDDA (Mw = 400000-500000, 20 wt % in water) was obtained from Aldrich and used as received. HAuCl4 3 4H2O, PdCl2, PVP (K30, Mw = 30000-40000) and trisodium citrate dihydrate were purchased from the Shanghai Chemical Factory and used without further purification. All other chemicals were obtained from Beijing Chemical Factory and used as received. Unless otherwise stated, water used throughout all experiments was purified with the Millipore system. All the pieces of glassware were thoroughly cleaned with aqua regia and then washed repeatedly with Millipore water before use. Apparatus. TEM measurements were made on a HITACHI H-8100 EM (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. TEM samples were prepared on the carbon-coated copper grid. A drop of the GN/Au-NPs dispersion was carefully placed on the grid and dried in air. Zeta potential measurements were performed using a Zetasizer NanoZS (Malvern Instruments) and all the graphene samples were washed and diluted to 0.05 mg/ mL before measurements. The AFM image was taken by using a SPI3800N microscope (Seiko Instruments, Inc.) operating in the tapping mode with standard silicon nitride tips. Electrochemical experiments were performed with a CHI 830 electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, China). Thermogravimetric analysis (TGA) of the samples was performed on a Pyris Diamond TG/DTA thermogravimetric analyzer (PerkinElmer Thermal Analysis). Samples were heated under an N2 atmosphere from room temperature to 1000 at 10 °C/min-1. A conventional three-electrode cell was used, including a Ag/AgCl (saturated KCl) electrode as the reference electrode, a platinum wire as the counterelectrode, and a modified glassy carbon electrode (GCE) as the working electrode. Synthesis of Citrate-Capped Nanoparticles. Au NPs were synthesized according to the as-reported method.16 Briefly, 100 mL of 1 mM HAuCl4 was brought to reflux while stirring and then 10 mL of 38.8 mM trisodium citrate solution was added quickly, which resulted in a color change of the solution from pale yellow to deep red. After the color change, the solution was (16) Wang, Y.; Wei, H.; Li, B.; Ren, W.; Guo, S.; Dong, S.; Wang, E. Chem. Commun. 2007, 5220. (17) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353.
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refluxed for an additional 15 min. The final concentration of Au NPs was fixed to 1 mM by evaporating excess water. Au@Pd hybrid NPs (∼7 nm) were prepared according to a previous method.17,18 Briefly, 0.5 mL of 24.3 mM HAuCl4 was added to 50 mL of H2O with vigorous stirring, followed by the addition of 1.5 mL of 1% aqueous trisodium citrate; 1 min later, 0.5 mL of 0.075% NaBH4 was added. Stirring was continued for 12 h to obtain ∼3 nm Au NPs. Then 30 mL of 3 nm Au NPs (used as gold seeds) was first mixed with 0.129 mL of 56.4 mM H2PdCl4 (Au/Pd =1:1, mol/mol) and cooled to ca. 4 °C in an ice bath (the atomic ratio of Au to Pd was 1). Next, 0.365 mL of 100 mM ascorbic acid was also cooled in an ice bath and slowly dropped into the mixed solution under stirring. The color of the mixture turned to black-brown within several minutes. Stirring was continued for 30 min after finishing the addition of ascorbic acid. The final concentration of the Au@Pd hybrid NPs is 0.2425 mM.
Synthesis of Polyelectrolyte-Functional Graphene Nanosheets. 1). Preparation of PVP-Capped Graphene Nanosheets (PVP/GNs). First, the graphene oxide was synthesized from natural graphite powder by the modified Hummers method.19,20 Then, the as-prepared graphite oxides were exfoliated by ultrasonication in a water bath for more than 1 h. Finally, a homogeneous graphene oxide aqueous dispersion (0.5 mg/mL) was obtained. The PVP/GNs was prepared by the method according to a previous work.9 In a typical procedure for chemical conversion of graphene oxide to PVP/GNs, 400 mg of PVP was added into 100 mL of homogeneous GO dispersion (0.25 mg/mL), followed by stirring for 12 h. Then, to the resulting dispersion were added 35 μL of hydrazine solution (50% w/w) and 400 μL of ammonia solution (25% w/w). After being vigorously shaken or stirred for a few minutes, the mixture was stirred for 1 h at 95 °C. Finally, the stable black dispersion was centrifuged two times and dissolved in 25 mL of water (1 mg/mL).
2). Functionalization of Graphene Nanosheets with PDDA. The PVP/GNs were functionalized with PDDA by the following procedures. A 3 mL portion of 1 mg/mL PVP/GNs was dispersed in 12 mL of aqueous solution containing 0.625 M potassium chloride and 1.25 mg/mL PDDA. After being mixed well, the mixture was sonicated for 3 h to give a homogeneous black dispersion. Excess PDDA was removed by centrifugation, and the composite was rinsed with water for two times. Finally, the PDDA/GNs were redispersed in 6 mL of water. 3). Assembly of GN/Au-NPs Hybrid. A 160 μL portion of 0.5 mg/mL PDDA/GNs was added into 4 mL of Au NPs or 6.4 mL of Au@Pd NPs solution under stirring. Then, the mixture was sonicated for 2 min before standing overnight. Finally, the precipitate was collected by centrifugation and washed for several times. The as-prepared GN/Au-NPs mixture was redispersed in 1 mL of water and the concentration of Au NPs was 4 mM. Electrocatalytic Experiments. GCE (3 mm in diameter) was polished with 1.0 and 0.3 μm alumina slurry sequentially and then washed ultrasonically in water and ethanol for a few minutes, respectively. The cleaned GCE was dried with high-purity nitrogen steam for the next modification. For the H2O2 sensing, 5.0 μL of GN/Au-NPs solution was dropped on the clean surface of GCE, and then dried under an infrared lamp. Thereafter, 5 μL of Nafion (0.2%, ethanol solution) was dropped on the surface of the as-prepared nanomaterial-modified GCE and allowed to dry at ambient conditions. As a control experiment, 20 μL of Au NPs (1 mM) solution was dropped on bare GCE.
Results and Discussion The whole procedure for self-assembly of graphene and Au NPs is shown in Scheme 1. In the foremost step, PVP/GNs were (18) Hu, J. W.; Zhang, Y.; Li, J. F.; Liu, Z.; Ren, B.; Sun, S. G.; Tian, Z. Q.; Lian, T. Chem. Phys. Lett. 2005, 408, 354. (19) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (20) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771.
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Figure 1. Optical photographs of the PDDA/GNs dispersion (A) and the mixtures after PDDA/GNs (B) and PVP/GNs (C) were added into the Au NPs solution. Scheme 1. Procedure for the Self-Assembly of Au Nanoparticles and PDDA-Functionalized Graphene Nanosheets
prepared by a simple wet-chemical route (omitted in Scheme 1). Here, the introduction of PVP can remarkably increase the stability of GNs due to the strong hydrophobic interactions between the GNs and PVP.5,9 Then, the cationic functional macromolecules of PDDA were adsorbed at the PVP/GNs, which resulted in positive charges on the surface of GNs. Finally, the cationic graphene dispersion was directly mixed with the asprepared solution of Au NPs. Driven by the electrostatic interaction, the citrate-capped Au NPs quickly adhered to the surface of PDDA/GNs. The precipitate was found at the bottom of the vessel, and the supernatant was colorless, an indication that almost all the Au NPs were linked to graphene. An optical photograph of the PDDA/GNs dispersion is shown in Figure 1A. The black homogeneous aqueous dispersion was stable without precipitation, even more than 1 month at ambient condition. Figure 1 panels B and C are the optical photographs of the mixtures after PDDA/GNs and PVP/GNs were added into the Au NPs solution, respectively. As is seen, the mixture of PVP/GNs and Au NPs still kept the red color of the Au NPs solution overnight. However, precipitate has been obtained at the bottom in the mixture containing PDDA/GNs, which induced the colorless supernatant. These results indicate PDDA can be the linker between NPs and graphene. The typical TEM images of as-prepared GN/Au-NP hybrids at different magnifications were displayed in Figure 2. From an Langmuir 2010, 26(13), 11277–11282
Figure 2. Typical TEM images of GN/Au-NPs at different magnifications.
overlook image (Figure 2A), it is observed that all the graphene nanosheets have been decorated with NPs and nearly all the Au NPs were distributed uniformly at the surface of the GNs. The well circumscribed Au NPs were found in a zoomed TEM image (Figure 2B), which clearly indicated the NPs have been loaded on the PDDA/GNs. The size of Au NPs ranging from 11 to 18 nm was shown in Figure 2C. Moreover, because the NPs can be loaded at the both sides of graphene, we found the Au NPs being stacked as two layers of NPs. Thus, the TEM images showed the significant high-loading and uniform distribution of Au NPs on the PDDA/GNs, suggesting that our self-assembly method we used can effectively produce homogeneous high-loading NPs supported on graphene hybrids. Thereafter, another experiment was carried out to self-assemble the citrate-capped bimetallic Au-Pd nanoparticles (Au@Pd NPs, ∼7 nm in size) on the PDDA/GNs. The preparation procedure of graphene/Au@Pd hybrids is the same as the GN/Au-NPs except for the use of the asprepared Au@Pd NPs for self-assembly. From the TEM images (Figure 3), it is also found that all the Au@Pd NPs were uniformly anchored at the surface of PDDA/GNs. DOI: 10.1021/la100575g
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Figure 3. Typical TEM images of GN/Au@Pd-NPs at different magnifications. Table 1. Measured Average Zeta Potential of the Modified GNs at 25 °C
zeta potential (mV)
PVP/GNs
PDDA/GNs
GNs/Au-NPs
-35.8
34.9
-27.3
To study the linkage of PDDA between GNs and NPs, we further characterized the roughness and thickness of the modified GNs by AFM. Figure 4A shows the typical AFM image of PVP/ GNs. It is found that the substrate mica is covered with a number of nanosheets of high purity. From the corresponding crosssectional view (Figure S1A, Supporting Information), the typical AFM image of PVP/GNs indicates that the average thickness of nanosheets is about 1.8 nm. The dispersion of GNs decorated by PVP shows excellent stability and solubility in water (data not shown), which can be attributed to the strong interactions between graphene and PVP molecules.5 After being modified by PDDA, the GNs become slightly rough (Figure 4B) with the increased mean thickness to ∼2.8 nm (Figure S1B, Supporting Information). This can be ascribed to the adsorption of PDDA at the surface of GNs. With the mean thickness of the GN/Au-NPs hybrids increased to 35 nm (Figure S1C, Supporting Information), the more roughened hybrid graphene was obtained after the Au NPs were loaded (Figure 4C), which was coincident with the TEM results. The effect of electrostatic interaction on precursors was further investigated by zeta potential measurement and the results of the various modified GNs were summarized in Table 1. The mean zeta potential for the as-prepared PVP/GNs was negative (-35.8 mV). Then, the zeta potential dramatically changed from negative to positive value (þ34.9 mV), owing to the adsorption of PDDA at the graphene surface. After modification with Au NPs, the surface potential of modified GNs was significantly altered once again, and a negative surface potential (-38.4 mV) of GN/AuNPs was recorded, which was caused by the adsorption of citratecapped Au NPs on the graphene surface. As has been shown in TEM images, entire PDDA/GNs were covered by citrate capped Au NPs, which possess a strong negative-charged surface.21,22 The TGA technique was further used to estimate the accurate amounts of starting materials in the graphene hybrids. The TGA (21) Ji, Q. M.; Acharya, S.; Hill, J. P.; Richards, G. J.; Ariga, K. Adv. Mater. 2008, 20, 4027. (22) Han, W. K.; Hwang, G. H.; Hong, S. J.; Kim, S. S.; Yoon, C. S.; Kwak, N. J.; Yeom, S. J.; Kim, J. H.; Kang, S. G. Microelectron. Eng. 2009, 86, 374.
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of PVP/GNs (curve a), PDDA/GNs (curve b), and GN/Au NPs (curve c), as shown in Supporting Information, Figure S2, displays the weight losses with respect to temperature. It should be noted that the small weight loss at 80 °C is due to physically absorbed water. The weight losses at the temperatures of 395-500 °C and 600-800 °C could be ascribed to the decomposition of the polymers23,24 and GNs,25 respectively. Thus, the weight percentages of GNs were estimated to be 53.0, 54.7, and 7.3 wt % in the PVP/GNs, PDDA/GNs, and GNs/gold NPs hybrids, respectively. The accurate and rapid determination of H2O2 is of practical importance due to its application in food, pharmaceutical, clinical, industrial, and environmental analysis.26-28 To date, a great amount of biosensors were developed using enzymes immobilized on electrode to detect H2O2, which are based on the electrochemical methods. However, the enzymes always require relatively rigorous experimental conditions such as temperature, pH, and toxic chemicals. Moreover, enzymatic sensors cannot provide the admitted advantage of a complete long-term stability due to the intrinsic nature of enzymes.29 Therefore, it is necessary to develop a simple enzyme-free strategy for sensing H2O2 with high sensitivity. Self-assembly of NPs on GNs can be an alternative strategy for the construction of two-dimensional graphene hybrids. Their rough surfaces probably endow them with a higher electrochemical active area, resulting in a higher turnover for heterogeneous catalytic reactions or electrochemical reactions. Herein, H2O2 was selected as the model analyte using the amperometric methods. Figure 5A shows the typical cyclic voltammograms (CVs) of the Au NPs and GN/Au-NPs-modified GCE (GN/Au-NPs/GCE) in the N2-saturated phosphate buffer solution (PBS, pH = 7.4). After injection of 1 mM H2O2, it is observed that the currents at þ0.5 and -0.2 V of the GN/Au-NPs/GCE (red line) were two times larger than those at the Au NPs-modified GCE (black line). Moreover, the electrocatalytic behavior of GN/Au-NPs toward different concentrations of H2O2 was studied by CVs. In N2saturated PBS, the CVs of GN/Au-NPs/GCE in the absence and presence of H2O2 were shown in Figure 5B. Obviously, the reduction and oxidation currents increase gradually with increasing concentrations of H2O2 (0, 1, 2, 3, 4, 5, 6, 7, 8 mM; from a to i). Herein, þ0.5 and -0.2 V were chosen as the applied potentials for i-t measurements, respectively. The current response to H2O2 at the GN/Au-NPs/GCE was studied as shown in Figure 6A. At þ0.5 V, the oxidation current at the GN/Au-NPs/GCE increased gradually during the successive additions of H2O2 into the stirring PBS, and reached the maximum steady-state current within 5 s. A wide concentration range from 0.5 μM to 0.5 mM was studied (Figure 6B). At relative low concentration (from 0.5 to 50 μM), the calibration plot is steeper than that at high concentration (from 50 μM to 0.5 mM), indicating the GN/Au-NPs/GCE is more sensitive to H2O2 at low concentration. The resulting calibration curve (correlation coefficient = 0.996) is linear over the range from 0.5 to 50 μM with the detection limit of 0.22 μM. This detection limit is lower than (23) Zheng, M. P.; Jin, Y. P.; Jin, G. L.; Gu, M. Y. J. Mater. Sci. Lett. 2000, 19, 433. (24) Razdan, S.; Patra, P. K.; Kar, S.; Ci, L.; Vajtai, R.; Kukovecz, A.; Konya, Z.; Kiricsi, I.; Ajayan, P. M. Chem. Mater. 2009, 21, 3062. (25) Yang, H. F.; Li, F. H.; Shan, C. S.; Han, D. X.; Zhang, Q. X.; Niu, L.; Ivaska, A. J. Mater. Chem. 2009, 19, 4632. (26) Wolfbeis, O. S.; Durkop, A.; Wu, M.; Lin, Z. H. Angew. Chem., Int. Ed. 2002, 41, 4495. (27) Guo, S. J.; Wang, E. K. Anal. Chim. Acta 2007, 598, 181. (28) Guo, S. J.; Dong, S. J. Trends Anal. Chem. 2009, 28, 96. (29) Zou, G.; Ju, H. Anal. Chem. 2004, 76, 6871.
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Figure 4. AFM images of PVP/GNs (A), PDDA/GNs (B), and GN/Au-NPs (C).
Figure 5. (A) CVs of the as-prepared GN/Au-NPs-modified GCE (red) and Au NPs-modified GCE (black) in the absence (dot) and presence (solid) of 1 mM H2O2 in N2-saturated 0.1 M PBS (pH = 7.4). Scan rate: 50 mV/s. (B) CVs of the as-prepared GN/Au-NPs-modified GCE in the N2-saturated 0.1 M PBS (pH = 7.4) with different concentrations (0, 1, 2, 3, 4, 5, 6, 7, 8 mM; from a to i) of H2O2.
Figure 6. (A) Current-time response of the GN/Au-NPs-modified electrode on successive injection of different amounts of H2O2 into stirring PBS (0.1 M, pH = 7.4). Applied potential: þ0.5 V. (B) Plot of electrocatalytic current of H2O2 vs its concentrations.
certain enzyme-based H2O2 sensors30-32 and CNT-hybrids-based H2O2 sensors.33-35 The current response of H2O2 was also measured when a reduction potential was applied. Sometimes the electroreduction is preferable to the electrooxidation when the samples contain (30) Chen, H. J.; Dong, S. J. Biosens. Bioelectron. 2007, 22, 1811. (31) Radi, A. E.; Munoz-Berbel, X.; Cortina-Puig, M.; Marty, J. L. Electroanalysis 2009, 21, 1624. (32) Liu, Y.; Lei, J.; Ju, H. Talanta 2008, 74, 965. (33) Li, M. Y.; Zhao, G. Q.; Yue, Z. L.; Huang, S. S. Microchim. Acta 2009, 167, 167. (34) Zhao, W.; Wang, H.; Qin, X.; Wang, X.; Zhao, Z.; Miao, Z.; Chen, L.; Shan, M.; Fang, Y.; Chen, Q. Talanta 2009, 80, 1029.
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some interferents that are apt to be oxidized as the target analyte. Figure 7A illustrates i-t plot at the GN/Au-NPs/GCE with the successive increments of H2O2 concentration. As H2O2 was added into the N2-saturated PBS, the current at GN/Au-NPs/GCE responded rapidly to the concentration changes of H2O2, and steady-state signals were obtained in less than 6 s. The GN/AuNPs/GCE exhibited a wide linearity from 0.5 μM to 0.5 mM with a correlation coefficient of 0.999 (Figure 6B). The experimental detection limit was 0.44 μM at the signal-to-noise ratio of 3, which is also lower than that described in other reports.30-35 (35) Wang, Q.; Yun, Y. B.; Zheng, J. B. Microchim. Acta 2009, 167, 153.
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Figure 7. (A) Current-time response of the GN/Au-NPs-modified electrode on successive injections of different amounts of H2O2 into stirring N2-saturated PBS (0.1 M, pH = 7.4). Applied potential: -0.2 V. (B) Plot of electrocatalytic current of H2O2 vs its concentrations.
Conclusions We have presented a facile self-assembly method to synthesize 2D graphene/Au NPs hybrids using the cationic graphene nanosheets as the building block. The citrate-capped Au NPs were adsorbed to the graphene by electrostatic interaction, which was the driving force for the self-assembly process. The high-loading Au NPs distributed uniformly on the surface of PDDA/GNs. Thereafter, the GN/Au-NPs hybrid was employed as the electrochemical enhanced material for the H2O2 sensing, which showed wide linear ranges and low detection limits. In contrast to the reported in situ synthesis of hybrid materials based on graphene,
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this self-assembly method provides an alternative to obtaining the graphene/NPs hybrids with high-loading and uniform dispersion due to the wide adaptability of electrostatic interactions. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 20735003) and the 973 Project (Grant Nos. 2009CB930100 and 2010CB933600). Supporting Information Available: AFM cross section images and TGA curves of graphene hybrids. This material is available free of charge via the Internet at http://pubs.acs.org.
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