Gold Nanoparticle-Embedded Porous Graphene Thin Films

May 27, 2012 - This work opens up a new and facile way for direct preparation of metal or metal oxide nanoparticle–embedded porous graphene composit...
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Gold Nanoparticle-Embedded Porous Graphene Thin Films Fabricated via Layer-by-Layer Self-Assembly and Subsequent Thermal Annealing for Electrochemical Sensing Qian Xi, Xu Chen,* David G. Evans, and Wensheng Yang* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: A uniform three-dimensional (3D) gold nanoparticle (AuNP)−embedded porous graphene (AuEPG) thin film has been fabricated by electrostatic layer-by-layer assembly of AuNPs and graphene nanosheets functionalized with bovine serum albumin and subsequent thermal annealing in air at 340 °C for 2 h. Scanning electron microscopy (SEM) investigations for the AuEPG film indicate that an AuNP was embedded in every pore of the porous graphene film, something that was difficult to achieve with previously reported methods. The mechanism of formation of the AuEPG film was initially explored. Application of the AuEPG film in electrochemical sensing was further demonstrated by use of H2O2 as a model analyte. The AuEPG film-modified electrode showed improved electrochemical performance in H2O2 detection compared with nonporous graphene−AuNP composite film-modified electrodes, which is mainly attributed to the porous structure of the AuEPG film. This work opens up a new and facile way for direct preparation of metal or metal oxide nanoparticle−embedded porous graphene composite films, which will enable exciting opportunities in highly sensitive electrochemical sensors and other advanced applications based on graphene−metal composites.

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performance of this 3D hybrid film was relatively poor. Jung and co-workers20 combined LbL assembly and vacuum filtration methods to prepare graphene−gold nanoparticle (AuNP) composite multilayer films. The AuNPs in this work were formed by spontaneous reduction of gold ions on the graphene films. However, the sizes of the as-prepared AuNPs ranged from subnanometer to ∼200 nm. Such a wide range of sizes may result in low reproducibility and controllability of the composite films and adversely affect their applications in functional devices, especially in highly sensitive sensors. Therefore, both the properties and performance of such 3D graphene−metal nanoparticle composite films need to be further improved. Recently, there has been growing interesting in porous graphene due to its potential applications in gas separation,21 nanoelectronics, hydrogen storage, 22 and many other fields.23−25 Several methods have been successfully developed to prepare porous graphene, including “top-down” block copolymer lithography,23 activation of exfoliated graphene oxide (GO) with KOH,24 hydrothermal steam etching of GO nanosheets,25 and “bottom-up” polymerization of a macrocycle.26 Compared with nonporous graphene, porous graphene gives improved performance in electronic devices,23 supercapacitors24 and gas sensors.25 Porous graphene also should be very useful as a new support since it is well-known that porous supports can not only provide high surface area but also

INTRODUCTION Graphene−metal nanoparticle composites have attracted much attention in recent years due to their great promise in various applications ranging from sensors1,2 to fuel cells3,4 and energy conversion.5 The unique physical properties of the twodimensional (2D) exfoliated graphene sheets, such as their extremely large surface area and high charge mobility, as well as high mechanical strength and low manufacturing cost, make graphene an excellent substrate for immobilizing metal nanoparticles. Considerable progress has been made in the synthesis and preparation of 2D graphene−metal nanoparticle composites.6−10 However, some challenges still remain; for practical applications, it is essential to tailor the macroscopicscale as well as the molecular-scale assembly of these nanocomposites in order to integrate them into device structures. Thus good control of the orientation and spatial distribution of graphene−metal nanoparticle composites is a prerequisite for many applications.11,12 To address this problem, several methods have been used to fabricate large-scale assembled graphene composite films.13−16 Compared with other methods, layer-by-layer (LbL) assembly is simple and more efficient in allowing precise control over the structure and thickness of the films on both micro- and nanoscales.17,18 Graphene nanosheet as a building block has recently been combined with metal nanoparticles to prepare three-dimensional (3D) composite films by the LbL assembly method.12,19 For example, graphene−platinum nanoparticle 3D hybrid nanostructures have been constructed by the LbL assembly method with an ionic liquid as a linker and have been employed in electrocatalytic oxygen reduction,12 but the © 2012 American Chemical Society

Received: April 9, 2012 Revised: May 26, 2012 Published: May 27, 2012 9885

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facilitate the diffusion and mass transport of reactants.27−30 Recently, Reunchan and Jhi31 investigated H2 adsorption by various metal atom−dispersed porous graphene materials using first-principles calculations. High storage capacity for Cadecorated porous graphene was predicted. However, there have been few reports of the synthesis of metal nanoparticledecorated porous graphene.32 In this report, we describe a new and facile approach to fabricate a novel 3D AuNP−embedded porous graphene (AuEPG) thin film by electrostatic LbL assembly of bovine serum albumin- (BSA-) functionalized graphene nanosheets and AuNPs, with subsequent thermal annealing in air at 340 °C. To the best of our knowledge, such a nanostructured film is reported for the first time, in which AuNPs are embedded in a nanoporous graphene multilayer film. The formation mechanism of the AuEPG film was initially explored, and electrochemical application of the AuEPG film was further investigated. The AuEPG film-modified fluorine-doped SnO2 (FTO) electrode displayed prominent electrochemical properties and high electrocatalytic activity toward H2O2 oxidation (as a model analyte). Furthermore, significant enhancement in the sensitivity of the sensor for electrochemical determination of H2O2 was observed for the AuEPG film-modified electrode relative to those of nonannealing and nonporous graphene− AuNP composite film-modified electrodes. Moreover, low detection limit and wide linear range were obtained via the amperometric-time technique at the AuEPG film-modified electrode. These results clearly suggest that the AuEPG film is an effective platform for electrochemical sensors. The present work provides a new ideal for the structural design and highperformance applications of graphene−metal nanoparticle composites.

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platinum wire were used as reference electrode and counterelectrode, respectively. Preparation of BSA-G. GO dispersions were prepared from the graphite powder by a modified Hummers’ method.33 BSA was used as both reductant and stabilizer to prepare BSA-functionalized graphene (BSA-G) according to the literature.34 Briefly, 10 mL of 1.0 mg·mL−1 GO, 50 mL of H2O, and 10 mL of 20 mg·mL−1 BSA were mixed and the pH of the mixture was rapidly raised to around 12 by the addition of 1.0 M NaOH. The solution was kept in a water bath at 90 °C for 2 h. Then the obtained solution was subsequently centrifuged and washed with water several times to ensure the successful removal of free unbound BSA. The resulting black precipitates were redispersed in H2O by ultrasonication to obtain BSA-G suspension (1.0 mg·mL−1). Positively charged BSA-G (BSA-G+, 1.0 mg·mL−1) was prepared by adjusting the pH of BSA-G dispersion to 3. Synthesis of Citrate-Stabilized AuNPs. Citrate-stabilized AuNPs were synthesized according to the literature.35 Typically, 100 mL of 1 mM HAuCl4·4H2O 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 refluxed for an additional 15 min. The final concentration of AuNPs was fixed to 1 mM by evaporating excess water. Preparation of PEI/BSA-G/(BSA-G+/AuNPs)n Multilayer Films. Quartz slides or FTO electrodes were used as the substrates for building of multilayer film. Before use, the quartz slide was treated in hot piranha solution (mixture of 98% H2SO4 and 30% H2O2, 7:3 v/v) for about 30 min and then washed with water. FTO was cleaned by sequential sonication for 20 min each in acetone, 10% KOH in ethanol, and H2O. The freshly cleaned quartz slide or FTO electrode was first immersed in PEI solution (2.5 mg·mL−1) for 20 min, followed by rinsing with water and N2 drying. Then the PEI-modified electrode was immersed in BSA-G suspension for 20 min to form a negatively charged surface. After washing and N2 drying, the modified substrate was alternately immersed in the BSA-G+ and AuNPs suspensions each for 20 min, followed by the same washing and drying procedure. By repetition of the depositions between BSA-G+ and AuNPs, the PEI/ BSA-G/(BSA-G+/AuNPs)n film was prepared. In control experiments, the PEI/BSA-G/(BSA-G+/BSA-G)5 and PEI/BSA/(BSA+/AuNPs)5 multilayer films were fabricated by a similar process. Annealing of Assembled Multilayer Films. The PEI/BSA-G/ (BSA-G+/AuNPs)5 film was placed in a furnace and annealed at different temperatures in air for 2 h. The AuEPG film was prepared after annealing at 340 °C for 2 h. Annealed PEI/BSA-G/(BSA-G+/ BSA-G)5 and PEI/BSA/(BSA+/AuNPs)5 films were prepared under the same conditions.

EXPERIMENTAL SECTION

Materials. Graphite powder (99.9%, 325 mesh) was purchased from Alfa Aesar. Poly(ethylenimine) (PEI, 50% in water, Mw = 750 000 by LS) was obtained from Sigma−Aldrich. HAuCl4·4H2O, trisodium citrate, BSA (≥98% purity, Mw = 68 000), and other chemicals (analytical grade) were obtained from Sinopharm Chemical Reagent Co. Ltd., China, and used as received. FTO electrodes were obtained from Nippon Sheet Glass Co. Ltd., Japan. All aqueous solutions were prepared with deionized, doubly distilled water. Apparatus. Transmission electron microscopy (TEM) images were obtained on a Hitachi H-800 transmission electron microscope operating at 20 kV. Atomic force microscopy (AFM) images were taken by using a Veeco Nanoscope III atomic force microscope in the tapping mode with NSG01 Au-coated silicon cantilevers. Zeta potential measurements were performed on a Brookhaven ZetaPlus ζ potential analyzer. UV−vis absorbance measurements were carried out on a Shimadzu UV-2501PC spectrometer. X-ray photoelectron spectroscopic (XPS) measurement was performed on a Thermo VG Escalab 250 instrument equipped with a monochromatic Al Kα X-ray source (1486.6 eV). Scanning electron microscopy (SEM) images were recorded on a Zeiss Supra 55 field emission scanning electron microscope by using an accelerating voltage of 20 kV. Energydispersive X-ray analysis (EDS) was obtained from an Oxford Instruments INCAx-act EDS detector attached to the SEM microscope. X-ray diffraction (XRD) pattern was recorded om a Shimadzu XRD-6000 X-ray diffractometer with Cu Kα radiation (λ = 0.154 nm). Thermogravimetric analysis (TGA) was carried out on a Mettler Toledo TGA/DSC thermogravimetric analyzer in air at a heating rate of 5 °C·min−1. Electrochemical measurements were performed with CHI660B electrochemical workstation (ChenHua Instruments Co., Shanghai, China). A three-electrode system was used in the experiment with a bare or modified FTO electrode as the working electrode (1 cm2). An Ag/AgCl (saturated KCl) electrode and a

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RESULTS AND DISCUSSION Citrate-stabilized AuNPs were used as a building block. Figure 1A shows a TEM image of as-prepared AuNPs, indicating that their average size was ca. 14 nm. A characteristic absorption peak at 521 nm for the AuNPs was also observed, as shown in Supporting Information, Figure S1. Since the citrate-stabilized AuNPs are negatively charged, LbL assembly based on electrostatic interactions requires positively charged graphene nanosheets. BSA was chosen as a reducing and decorating agent to prepare highly dispersed functionalized reduced GO solutions. BSA has previously been shown to be an effective reagent for the reduction of graphene oxide since the phenolic groups in its tyrosine (Tyr) residues can be readily oxidized to quinone groups.34 Moreover, the amphiphilic property of proteins allows the charge on the BSA-G to be tailored by simply adjusting the pH of the solution. Finally, BSA has been demonstrated to be an efficient adhesive that promotes adsorption of nanoparticles.34 Figure 1B shows the UV−vis spectra of GO and BSA-G dispersions. The absorption maximum at 232 nm for GO was shifted to 268 nm for BSA9886

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Figure 1. (A) TEM image of AuNPs. (B) UV−vis absorption spectra of (a) GO and (b) BSA-G dispersions. (C, D) AFM images of (C) GO and (D) BSA-G.

G, indicating that the electronic conjugation within the graphene sheets was restored after reduction.33 The C 1s XPS spectrum of BSA-G (Supporting Information, Figure S2) shows less C−O content and increased CO signal (from BSA) relative to that of GO, indicating that the reduction of GO has taken place. AFM images and their corresponding height profiles show that BSA-G nanosheets (Figure 1D) were rougher than, and had an increased mean thickness relative to, pristine GO (Figure 1C). These results suggest that GO was successfully reduced and decorated by BSA. BSA-G+ was further prepared by adjusting the solution pH to 3 (BSA has an isoelectric point of ∼4.7). The mean ζ potentials for the asprepared BSA-G and BSA-G+ were measured to be −26.3 and +29 mV, respectively (Supporting Information, Figure S3). BSA-G and BSA-G+ nanosheets also could be stably dispersed in water for more than 3 weeks, suggesting that they are suitable building blocks for electrostatic LbL assembly. The assembly process of the PEI/BSA-G/(BSA-G+/AuNP)n film was monitored by UV−vis absorption spectra (Figure 2A). Two broad absorption peaks at 268 and 521 nm were observed in all the UV−vis absorption spectra, which are characteristic of graphene and AuNPs, respectively. This indicates that both graphene and AuNPs have been successfully assembled in multilayer films. The absorbances at 268 and 521 nm linearly increased with the number of bilayers as shown in the inset of Figure 2A, suggesting that almost the same amount of each unit was loaded in each assembly step. In addition, a new broad band appeared in the 600−650 nm region when the number of bilayers was ≥5 and the absorbance increased with increasing number of bilayers. This suggests that the increase in density of AuNPs with increasing number of bilayers resulted in a collective surface plasmon resonance from interparticle coupling.36 In order to avoid the possible aggregation of AuNPs when a large number of bilayers is used, the PEI/BSA-

Figure 2. (A) UV−vis absorption spectra following LbL assembly of PEI/BSA-G/(BSA-G+/AuNPs)n multilayer films on a quartz slide. Inset: Absorbance at 268 and 521 nm vs number of bilayers assembled. (B) SEM image and (C) EDS signal of PEI/BSA-G/(BSA-G+/ AuNPs)5 film on a FTO slide. (*) Peaks are attributed to the FTO glass substrate.

G/(BSA-G+/AuNP)5 film was used in the remainder of this study. The morphology of the PEI/BSA-G/(BSA-G+/AuNP)5 film was characterized by SEM. A typical top-view SEM image of the assembled film is shown in Figure 2B. The surface of the film was continuous and uniform. The graphene nanosheets have been decorated with AuNPs and most of the AuNPs were uniformly distributed on the surface of the graphene nanosheets. The outline of an individual graphene nanosheet was not clearly observed, possibly due to overlapping of the sheets. However the wrinkles on the graphene nanosheets can be seen in Figure 2B. Interestingly, due to the very thin functionalized graphene nanosheets, the AuNPs below (which appear darker) were also clearly observed and the good dispersion was still maintained after the next layer of graphene nanosheet was assembled. Side-view SEM images of the PEI/BSA-G/(BSAG+/AuNP)n film were also recorded, illustrating that the AuNPs were LbL-stacked on the surface of the FTO electrode and the film thickness linearly increased with the number of bilayers (Supporting Information, Figure S4). In addition, EDS 9887

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Figure 3. SEM images of annealed PEI/BSA-G/(BSA-G+/AuNPs)5 film-modified FTO electrodes in air for 2 h at (A) 300, (B) 320, (C) 340, and (D) 450 °C annealing temperatures. Inset: Magnification image of selected area in white box. Scale bar: 100 nm.

in a selected domain of the PEI/BSA-G/(BSA-G+/AuNP)5 film (Figure 2C) further revealed the presence of both Au and C elements. The XRD pattern of the PEI/BSA-G/(BSA-G+/ AuNPs)5 film (Supporting Information, Figure S5) also confirmed the existence of AuNPs and graphene in the assembled film. Taken together, these results indicate that a uniform 3D graphene/AuNP hybrid multilayer film has been constructed. To improve the conductivity of the PEI/BSA-G/(BSA-G+/ AuNP)5 film for electrochemical applications, a process of thermal annealing was introduced in order to decompose BSA on the surface of the graphene nanosheets. Initially thermal annealing of the multilayer film was carried out in an N2 atmosphere at 400 °C. Although the morphology of the film, including the size and dispersion of the assembled AuNPs, was well retained after thermal annealing, the electrochemical properties of the PEI/BSA-G/(BSA-G+/AuNP)5 modified FTO electrode were not significantly improved relative to that of the nonannealed film-modified electrode (data not shown). Therefore, thermal annealing in air was carried out. TGA of BSA, BSA-G, and BSA-G/AuNP composites in air is shown in Supporting Information, Figure S6. The visible decomposition of BSA in the BSA-G/AuNP composites appeared at temperatures >250 °C. When the temperature was higher than 450 °C, the graphene obviously began to decompose. Therefore thermal annealing temperatures between 300 and 450 °C were employed. Figure 3 shows typical SEM images of the annealed PEI/BSA-G(/BSA-G+/AuNPs)5 film-modified FTO electrodes at different temperatures. After annealing, close-packed AuNPs in the multilayer films tended to aggregate, which is similar to previously reports.37,38 When annealed at 300 °C, some neighboring AuNPs fused into larger AuNPs with particle size 30−45 nm (Figure 3A). As the annealing temperature was increased to 320 °C (Figure 3B),

interestingly, some holes near AuNPs started to appear in multilayer film besides the larger AuNPs were observed. Upon further increasing the annealing temperature to 340 °C, surprisingly, a uniform and porous multilayer film over a large scale with almost every pore containing one AuNP was observed (see Figure 3C and a low-magnification SEM image of the film shown in Supporting Information, Figure S7). That is to say, pits or pores could be formed in the area surrounding nearly every AuNP after the film was annealed at 340 °C, and some neighboring pits were even interconnected. This structure was formed reproducibly each time the annealing process was repeated with different samples. Such nanostructure has not been previously reported. Porous graphene nanosheets have been prepared,23−26 but even if these porous nanosheets could be fabricated into a uniform porous graphene film, it is difficult to imagine how one AuNP could be deposited in each pore of the film by other processes. When the annealing temperature of the PEI/BSA-G/(BSA-G+/AuNP)5 film was increased to 450 °C, graphene could not be clearly observed in the SEM image of the film (Figure 3D) and only larger particles formed by coalescence of the AuNPs could be seen. This is presumably due to the decomposition of most of the graphene at this higher temperature. In order to explore the mechanism of formation of the above AuEPG film, a control experiment was carried out in which a film of BSA-G and BSA-G+ was assembled and annealed under the same conditions, without any AuNPs being incorporated. No pores were observed in the SEM image of this film (Supporting Information, Figure S8A). In a second experiment, a multilayer film of AuNPs and BSA molecules was assembled and annealed at 340 °C for 2 h; in this case, only the size of AuNPs became larger but no porous structure was present (Supporting Information, Figure S8B). Combined with the observed morphological evolution of the annealed PEI/BSA-G/ 9888

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(BSA-G+/AuNP)5 films at 300−450 °C, we speculate that with increasing annealing temperature, the stability of graphene becomes lower and close-packed AuNPs in the assembled 3D graphene hybrid multilayer films tend to fuse into larger particles. At the optimum annealing temperature, the enlarged AuNPs in the 3D hybrid multilayer films will etch the neighboring graphene nanosheet and so the structure of the AuEPG film is formed. A similar highly structured layered graphene/AuNP hybrid paper has been prepared by vacuumassisted self-assembly,39 and its thermal and electrical conductivity were investigated after thermal annealing at 340 °C in air for 2 h. However, in this case such an AuEPG structure was not observed. It was noted from the SEM images of the graphene/AuNPs hybrid paper that the graphene in the hybrid paper was much thicker than that used in the present work, so that the graphene might not be etched into pits or pores by the fused AuNPs. Therefore, the thickness of the graphene sheets in the 3D graphene/AuNP hybrid films may be a key factor that influences the formation of AuEPG structure. However, the detailed mechanism of formation of the AuEPG film is not clear at present and needs further study. The novel nanostructure of the 3D AuEPG film inspired us to investigate its electrochemical properties since graphene− AuNP composites are attractive for applications in electrochemical (bio)sensing.40−42 K3Fe(CN)6 was used as an electrochemical probe to measure the properties of the modified electrodes. Figure 4 shows the cyclic voltammograms

important contribution to the improvement of electrochemical properties. It is well-known that modified electrodes with a porous structure have higher surface area and more effective electron transfer than a nonporous modified electrode of the same material.30 When the annealing temperature was increased to 450 °C, the peak currents at the resulting modified electrode (Figure 4f) were lower than those for the electrode annealed at 340 °C. This is possibly a result of partial decomposition of the graphene nanosheets. These results clearly demonstrate that the AuEPG film-modified electrode shows better electrochemical reactivity than nonannealed and nonporous modified electrodes. The electrochemical sensing performance of the AuEPG film-modified electrode was examined by use of H2O2 as a representative analyte. The determination of H2O2 is of practical importance in chemical, biological, clinical, and many other fields.43 A sensitive H2O2 sensor is also useful for enzyme-based biosensing because H2O2 is the product of most oxidase enzyme reactions.44 Figure 5 shows the typical CVs of 1

Figure 5. CVs of (a) bare FTO, (b) PEI/BSA-G/(BSA-G+/AuNPs)5 film, and (c−f) annealed PEI/BSA-G/(BSA-G+/AuNPs)5 filmmodified FTO electrodes at (c) 300, (d) 320, (e) 340, and (f) 450 °C annealed temperatures in the presence of 1 mM H2O2 in N2saturated 0.1 M PBS (pH 7.4). Scan rate: 50 mV·s−1. Inset: Electrocatalytic responses of annealed film-modified electrodes at +0.5 V vs annealing temperatures. Figure 4. CVs of (a) bare FTO, (b) PEI/BSA-G/(BSA-G+/AuNPs)5 film, and (c−f) annealed PEI/BSA-G/(BSA-G+/AuNPs)5 filmmodified FTO electrodes at (c) 300, (d) 320, (e) 340, and (f) 450 °C annealing temperatures in 5 mM K3Fe(CN)6 in 0.1 M KCl. Scan rate: 50 mV·s−1. Inset: Redox peak currents of annealed film-modified electrodes vs annealing temperatures.

mM H2O2 at different modified electrodes. In the absence of H2O2, the CVs of the bare FTO and modified electrodes (Supporting Information, Figure S9) exhibited no obvious electrochemical response. However, it can be seen from Figure 5 that remarkable oxidation peaks appeared at about +0.5 V, corresponding to the oxidation of H2O2. The largest catalytic currents and earliest onset potentials were observed at the AuEPG film-modified electrode, which suggests that the AuEPG film had much better electrocatalysis toward H2O2 than the other electrodes. This can be attributed to the porous structure, which provides higher surface area and better permeability. Amperometric detection of H2O2 at the AuEPG filmmodified electrode was further studied by current−time measurements. Figure 6A shows the amperometric response of the AuEPG film-modified electrode at +0.5 V. The oxidation currents at the AuEPG film-modified electrode increased gradually during successive additions of H2O2 into stirred phosphate-buffered saline solution (PBS) and reached the maximum steady-state current within 3 s (curve b in Figure 6A and its inset). The corresponding calibration curve is shown in Figure 6B (curve b). The linear range of H2O2 concentration was from 0.5 μM to 4.9 mM (R = 0.998) with a sensitivity of

(CVs) of 5 mM K3Fe(CN)6 obtained at different electrodes. Compared with the bare FTO electrode (Figure 4a), the PEI/ BSA-G/(BSA-G+/AuNP)5 modified electrode (Figure 4b) showed increased redox peak currents and decreased peak separation, indicating that the assembly of graphene and AuNPs on the FTO electrode significantly improved the electrochemical reactivity of the electrodes. The electrochemical properties of the PEI/BSA-G/(BSA-G+/AuNP)5 modified electrode annealed at 300 °C were further slightly improved (Figure 4c), possibly due to the removal of some of the BSA. However, significantly enhanced peak currents were observed for the film-modified electrodes annealed at 320 °C (Figure 4d), and the highest peak currents were obtained for the filmmodified electrodes annealed at 340 °C (Figure 4e). In addition to the removal of more BSA, the formation of porous structure in films annealed at 320 and 340 °C probably makes an 9889

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Figure 6. (A) Current−time response of (a) PEI/BSA-G/(BSA-G+/AuNPs)5 film and (b) AuEPG film-modified FTO electrodes on successive injection of different concentrations of H2O2 into stirred PBS (0.1 M, pH 7.4). Applied potential: +0.5 V. (B) Corresponding calibration curves of H2O2.

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75.9 μA·mM−1. Compared with the nonannealed assembled film-modified electrode (curve a in Figure 6), the AuEPG filmmodified electrode was more sensitive to the change in H2O2 concentration. The sensitivity of the sensor based on the AuEPG film-modified electrode was approximately 7 times as large as that of the nonannealed assembled film-modified electrode (9.8 μA·mM−1). As mentioned above, formation of the AuEPG structure significantly increased the surface area and permeability of the modified electrode, which accounts for the dramatic increase in electrochemical activity for H2O2. Moreover, the sensitivity at the AuEPG film-modified electrode is much higher than the values previously reported for 2D graphene−AuNP hybrid modified electrodes40−42 and LbLassembled graphene multilayer film-modified electrodes,45 showing that the combination of graphene and AuNPs in a 3D ordered assembly is more effective for electrochemical applications. The detection limit of the AuEPG film-modified electrode was estimated to be 0.1 μM based on signal/noise (S/ N) = 3, which is lower than the corresponding values for some enzyme-based H2O2 sensors.41,46,47 Therefore, the AuEPG filmmodified electrode exhibited higher electrochemical performance toward H 2 O 2 than nonannealed and nonporous graphene−AuNP composite film-modified electrodes.

ASSOCIATED CONTENT

S Supporting Information *

Nine figures showing UV−vis absorption spectrum of AuNPs; C1s XPS analyses of GO and BSA-G; ζ potential analyses of BSA-G and BSA-G+; thickness of PEI/BSA-G/(BSA-G+/ AuNPs)n multilayer films vs number of bilayers assembled; XRD pattern of PEI/BSA-G/(BSA-G+/AuNPs)5 film on a FTO slide; TGA curves of BSA, BSA-G, and BSA-G/AuNP composites in air; low-magnification SEM image of AuEPG film; SEM images of PEI/BSA-G/(BSA-G+/BSA-G)5 and PEI/ BSA/(BSA+/AuNP)5 films annealed at 340 °C; and CVs of bare FTO and modified electrodes in the absence of H2O2. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; fax +86-10-64425385; tel +86-10-64435271. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2011CBA00508), the National Natural Science Foundation of China (21175009), and the State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (SKLEAC201203).

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CONCLUSIONS A novel and uniform 3D AuEPG film has been fabricated by LbL assembly and subsequent thermal annealing in air. The new and facile fabrication method avoids the usual need when preparing porous graphene materials for porogens, posttreatment, and deposition steps. More importantly, it was found that an AuNP was embedded in every pore of the nanoporous graphene films, which was very difficult to achieve by previously reported methods. A possible mechanism of formation of the AuEPG film has been proposed. Furthermore, the AuEPG film-modified electrode showed excellent electrochemical properties. Obviously enhanced electrochemical sensing performance for H2O2 was obtained at the AuEPG film-modified electrode relative to nonannealed film-modified electrodes and previously reported 2D nonporous graphene/ AuNP hybrid modified electrodes. Our results demonstrate the efficiency of nanoporous graphene films as a support for metal nanoparticles in electrochemical applications. Finally, we believe that this method should also be useful for fabricating other metal or metal oxide−porous graphene composites with applications in a variety of fields.

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REFERENCES

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dx.doi.org/10.1021/la301440k | Langmuir 2012, 28, 9885−9892

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dx.doi.org/10.1021/la301440k | Langmuir 2012, 28, 9885−9892