Electrochemica Acta

Electrochimica Acta 83 (2012) 283–287 Contents lists available at SciVerse ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta One-pot synthesis of Ag nanoparticles/reduced graphene oxide nanocomposites and their application for nonenzymatic H2 O2 detection Qingzhen Li a , Xiaoyun Qin a , Yonglan Luo a , Wenbo Lu a , Guohui Chang a , Abdullah M. Asiri b,c , Abdulrahman O. Al-Youbi b,c , Xuping Sun a,b,c,∗ a Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, School of Chemistry and Chemical Industry, China West Normal University, Nanchong 637002, Sichuan, China b Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia c Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, Saudi Arabia a r t i c l e i n f o Article history: Received 12 April 2012 Received in revised form 1 August 2012 Accepted 2 August 2012 Available online 10 August 2012 Keywords: Ag nanoparticles Reduced graphene oxide One-pot H2 O2 detection a b s t r a c t In this paper, we report on one-pot synthesis of Ag nanoparticles/reduced graphene oxide (AgNPs/rGO) nanocomposites by heating mixed solution of graphene oxide (GO) and AgNO3 with the use of diethylenetriamine as a reducing agent at 80 ◦ C for 30 min. Several analytical techniques including UV–vis spectroscopy, Raman spectroscopy and transmission electron microscopy (TEM) have been employed to characterize the resulting nanocomposites. It was found that such nanocomposites exhibit good catalytic activity toward the reduction of H2 O2 . This nonenzymatic H2 O2 sensor shows a fast amperometric response time of less than 2 s. The linear detection range is estimated to be from 0.1 to 100 mM (r = 0.999), and the detection limit is estimated to be 3.6 ␮M at a signal-to-noise ratio of 3. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Graphene, which has a fundamental 2D carbon structure, possesses exceptionally high surface area, high chemical stability, excellently catalytic properties, electronic quality and mechanical properties, and thus has already turned into a promising candidate material in the field of material science [1–3]. It is demonstrated that graphene has been successfully applied in various applications, such as, in the aspect of nanoelectronics, composites, Li-ion batteries, and sensors [4–7]. Based on the aforementioned factors, a reliable synthesis of graphene based materials also raised attention, and scored tremendous achievements. Among them, synthesis of inorganic materials, especially noble metal nanoparticles (NPs) decorated graphene, which can be expected to bring out novel electrocatalytic properties owe to the intrinsic catalytic activities of noble metal NPs [8–14]. A great quantity of noble metal NPs has been decorated on graphene and its derivatives, such as AuNPs [15–19], AgNPs [15,17,20–22], PdNPs [17,22,23], and PtNPs [17,23,24]. This kind of nanocomposites ∗ Corresponding author at: Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, School of Chemistry and Chemical Industry, China West Normal University, Nanchong 637002, Sichuan, China. Fax: +86 431 85262065. E-mail address: sun.xuping@hotmail.com (X. Sun). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.08.007 can further enhance the catalytic activities of noble metal NPs and obtain wide range of applications. For example, AgNPsdecorated graphene composites have been successfully employed as effective SERS substrate, a glucose sensor, and a H2 O2 sensor [25,26]. H2 O2 is not only a by-product of several highly selective oxidase but also plays a significant role in the fields of chemistry, biology, food, and environment protection [27–29]. Therefore, detection of H2 O2 is of very important. Compared with other detection techniques, electrochemical technique has been proven to be an inexpensive and effective way due to its intrinsic simplicity and high sensitivity and selectivity. Meanwhile, the electrochemical methods based on nonenzymatic technique avoid the possibility of protein denaturing [30–33]. In this paper, we report on one-pot synthesis of AgNPs/reduced graphene oxide (AgNPs/rGO) nanocomposites by heating mixed aqueous solution of graphene oxide (GO) and AgNO3 in the presence of diethylenetriamine serving as a reducing agent at 80 ◦ C for 30 min. We further demonstrate the construction of a nonenzymatic H2 O2 sensor using the resulting nanocomposites with a fast amperometric response time of less than 2 s. The linear detection range is estimated to be from 0.1 to 100 mM (r = 0.999), and the detection limit is estimated to be 3.6 ␮M at a signal-to-noise ratio of 3. 284 Q. Li et al. / Electrochimica Acta 83 (2012) 283–287 2. Experimental 2.1. Materials A number of reagents such as graphite powder, AgNO3 , NaH2 PO4 , Na2 HPO4 , and H2 O2 (30 wt%) were purchased from Aladin Ltd. (Shanghai, China). NaNO3 , H2 SO4 (98 wt%), diethylenetriamine, and KMnO4 were purchased from Beijing Chemical Corp. All chemicals were used as received without further purification. The water used throughout all experiments was purified through a Millipore system. Phosphate buffer saline (PBS) was prepared by mixing stock solutions of NaH2 PO4 and Na2 HPO4 and a fresh solution of H2 O2 was prepared daily. 2.2. Apparatus UV–vis spectra were obtained on a UV5800 spectrophotometer. Raman spectra were obtained on J-Y T64000 Raman spectrometer with 514.5 nm wavelength incident laser light. TEM measurements were made on a HITACHI H-8100 EM (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by placing a drop of the dispersion on carbon-coated copper grid and drying at room temperature. Electrochemical measurements are performed with a CHI 660D electrochemical analyzer (CH Instruments, Inc., Shanghai). The three electrode system consisted by a pretreated glassy carbon electrode (geometric area = 0.07 cm2 ) as the working electrode, an Ag/AgCl electrode as the reference electrode and platinum wire as the auxiliary electrode. The potentials are measured with an Ag/AgCl electrode as the reference electrode. All the experiments were carried out at ambient temperature. Fig. 1. UV–vis absorption spectra of aqueous dispersion of (a) GO and (b) the product. Then GCE was cycled in the potential range between −0.8 and 0.2 V at 50 mV s−1 in 5 mM K3 [Fe(CN)6 ] containing 1 M KCl until a reproducible cyclic voltammogram (CV) was obtained. Then it was rinsed with distilled water, and ultrasonicated in ethanol and water about 1 min, respectively. Finally the electrode was dried with nitrogen. The suspension of AgNPs/rGO nanocomposite (3 ␮L) was dropped onto the GCE. Chitosan (0.5 wt%) (2 ␮L) was used as fixative to form a strong film to modify the electrode. 2.3. Preparation of GO 3. Results and discussion GO was prepared from natural graphite powder through a modified Hummers’ method [34]. In a typical synthesis, 1 g of graphite was added into 23 mL of 98% H2 SO4 , followed by stirring at room temperature over a 24 h period. After that, 100 mg of NaNO3 was introduced into the mixture and stirred for 30 min. Subsequently, the mixture was kept below 5 ◦ C by ice bath, and 3 g of KMnO4 was slowly added into the mixture. After being heated to 35–40 ◦ C, the mixture was stirred for another 30 min. After that, 46 mL water was added into above mixture during a period of 25 min. Finally, 140 mL of water and 10 mL of 30% H2 O2 were added into the mixture to stop the reaction. After the unexploited graphite in the resulting mixture was removed by centrifugation, as-synthesized GO was dispersed into individual sheets in distilled water at a concentration of 0.5 mg/mL with the aid of ultrasound for further use. 2.4. Preparation of AgNPs/rGO nanocomposites AgNPs/rGO nanocomposites were synthesized directly by heating the mixture of 1 mL diethylenetriamine, 6 mL GO, and 80 ␮L of 0.1 M AgNO3 solutions in water bath at 80 ◦ C for 30 min. After that, the product was centrifuged and further washed with deionized water three times. The precipitate was re-dispersed in 5 mL deionized water for characterization and further use. As a control experiment, rGO was also synthesized in the same way as the synthesis of AgNPs/rGO nanocomposite but without the introduction of AgNO3 . 2.5. Preparation of the modified electrode Glassy carbon electrode (GCE) was polished with 1.0 and 0.3 ␮m alumina powders, respectively, and then ultrasonically cleaned in ethanol and double-distilled water for about 1 min, respectively. Fig. 1 shows the UV–vis absorption spectra of the aqueous dispersion of GO and the product. It is obviously seen that the GO dispersion exhibits two characteristic peaks, a peak at 230 nm, which corresponds to ␲→␲* transitions of aromatic C C bands, and a shoulder at 300 nm, which is attributed to n→␲* transitions of C O bands (curve a) [35]. It is seen that the adsorption peak gradually red-shifts from 230 to 267 nm after the heat treatment (curve b), indicating the reduction of GO [35]. An additional peak at 408 nm is also observed in curve b, which can be assigned to the colloidal silver surface plasmon resonance band, indicating the formation of AgNPs [35]. It should be noted that the color of the solution change from pale-yellow to black after the reaction, providing another piece of evidence to support the successfully transition from GO to rGO [36]. Fig. 2 shows the Raman spectra of aqueous dispersion of GO (curve a) and the product (curve b). It is established that rGO obtained by chemical reduction of GO exhibits two characteristic main peaks: the D band at ∼1350 cm−1 , arising from a breathing mode of ␬-point photons of A1g symmetry; the G band at ∼1575 cm−1 , arising from the first order scattering of the E2g phonon of sp2 C atoms [37]. In the present study, it is seen that GO exhibits a D band at 1356 cm−1 and a G band at 1603 cm−1 , while the corresponding bands of the product are 1351 and 1599 cm−1 , respectively. The G band of the product red-shifts from 1603 to 1599 cm−1 , which is attributed to the high ability for recovery of the hexagonal network of carbon atom [38]. It is also found that the product show relative higher intensity of D to G band (0.93) than that of GO (0.84). These observations confirm the formation of new graphitic domains after the heat treatment process and suggest the good reducing ability of diethylenetriamine. Q. Li et al. / Electrochimica Acta 83 (2012) 283–287 285 Fig. 2. Raman spectra of (a) GO and (b) the product. Fig. 4. CVs of different electrodes in N2 -saturated 0.2 M PBS at pH 6.5 in the presence and absence of 1 mM H2 O2 (scan rate: 50 mV s−1 ). Fig. 3 shows the typical TEM images of the product. From the low magnification image (Fig. 3A), a number of small black dots are observed on the rGO. A high magnification image further reveals that these dots are ranging from 10 to 150 nm in size and irregular in shape, as shown in Fig. 3B. The high resolution TEM (HRTEM) image in the inset of Fig. 3B reveals clear lattice fringes with an interplanar distance of 0.236 nm, corresponding to the (1 1 1) planes of the face-centered cubic structure of metallic Ag [39]. All the above observations suggest the formation of AgNPs decorated rGO. To demonstrate the sensing application of the AgNPs/rGO nanocomposites, a nonenzymatic H2 O2 sensor was constructed by deposition of the nanocomposites on GCE surface. Fig. 4 presents the CVs of bare GCE, rGO modified GCE (rGO/GCE) and AgNPs/rGO modified GCE (AgNPs/rGO/GCE) in N2 -saturated 0.2 M PBS at pH 6.5 in the presence of 1 mM of H2 O2 . It is indicated that the responses of the bare GCE and rGO/GCE toward the reduction of H2 O2 are very weak. In contrast, the AgNPs/rGO/GCE shows a typical catalytic current peak about 67 ␮A centered at −0.54 V vs Ag/AgCl in the presence of 1 mM H2 O2 . However, the AgNPs/rGO/GCE exhibits no electrochemical response in the absence of H2 O2 . All these observations demonstrate that the AgNPs/rGO nanocomposites exhibit excellent catalytic performance toward H2 O2 reduction. The influence of the pH value of the buffer solution on the sensor was examined. Fig. 5 shows the amperometric responses of the sensor in 0.2 M PBS with different pHs from 6.0 to 8.0 in the presence of 1.0 mM H2 O2 . It is presented that the current toward the reduction of H2 O2 reached a maximum at a pH value of 6.5. Consequently, the PBS at pH 6.5 was selected as the optimum value in this work. In addition, the effect of scan rate on the sensor response was also examined, as shown in Fig. 6A. It suggests the peak current increases with the scan rate in the range from 20 to 100 mV s−1 and is proportional to the square root of the scan rate (Fig. 6B), indicating a diffusion-controlled process. Fig. 7 shows a typical current–time curve of the sensor under the optimized conditions after the addition of successive H2 O2 concentration to the PBS under endless stirring at an applied potential of −0.3 V vs Ag/AgCl electrode. When an aliquot of H2 O2 was dropped into the stirring PBS solution, the reduction current rose steeply to reach a stable value. The inset displays the calibration curve of the biosensor for H2 O2 determination. The response current increased linearly with the H2 O2 concentration in the range from 0.1 mM to 100 mM with a correlation coefficient of 0.999, and the detection limit was estimated to be 3.6 ␮M based on the criterion of a signal-to-noise ratio of 3. Fig. 3. Typical TEM images of the product at (A) low and (B) high magnifications. Inset: a HRTEM image of one single nanoparticle on rGO. 286 Q. Li et al. / Electrochimica Acta 83 (2012) 283–287 Fig. 5. The amperometric responses of AgNP/rGO/GCE in 0.2 M PBS with different pHs for 6.0, 6.5, 7.0, 7.4 and 8.0 in the presence of 1.0 mM H2 O2 (scan rate: 50 mV s−1 ). Fig. 7. Typical steady-state response of the AgNP/rGO/GCE to successive injection of H2 O2 into the stirred N2 -saturated 0.2 M PBS at pH 6.5 (applied potential: −0.30 V). Inset: the fitting of the experimental data by the regression line. 4. 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