Chinese Chemical Letters  2017, Vol. 28 Issue (1): 143-148   PDF    
Graphene oxide-MnO2 nanocomposite-modified glassy carbon electrode as an efficient sensor for H2O2
Hui-Li Xu, Wei-De Zhang     
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China
Abstract: In this study, a new facile preparation method of nanocomposites consisting of graphene oxide and manganese dioxide nanowires (GO/MnO2 NWs) was developed. The morphology, structure and composition of the resulted products were characterized by transmission electron microscopy, X-ray diffraction and N2 adsorption and desorption. The GO/MnO2 nanocomposite was used as an electrode material for non-enzymatic determination of hydrogen peroxide. The proposed sensor exhibits excellent electrocatalytic performance for the determination of hydrogen peroxide in phosphate buffer solution (PBS, pH 7) at an applied potential of 0.75 V. The non-enzymatic biosensor for determination of hydrogen peroxide displayed a wide linear range of 4.90 ÇŒmol L-1-4.50 mmol L-1 with a correlation coefficient of 0.9992, a low detection limit of 0.48 ÇŒmol L-1 and a high sensitivity of 191.22 ÇŒA (mmol L-1)-1 cm-2 (signal/noise, S/N=3). Moreover, the non-enzymatic biosensor shows an excellent selectivity.
Key words: MnO2     Graphene oxide     Electroanalysis     Nanowires     Hydrogen peroxide    
1. Introduction

Recently, graphene oxide (GO) which was usually derived from exfoliation of graphite oxide [1, 2], has attracted tremendous attraction due to its special surface properties and layered structure for synthesis of GO-containing nanocomposites [3-5]. Specifically, GO exhibits good dispersibility in many solvents, particularly in water [6-9] and ease of post-functionalization [10] owing to introduction of abundant oxygen-containing functional groups on the surface, such as epoxy, hydroxyl, carbonyl and carboxyl groups [11]. The functional groups act as anchor sites and enable nanostructures attaching on the surfaces of GO sheets [12], forming different composites. These composites often possess unusual properties as compared with their individual components and can be used as excellent electrode materials [5, 13-17].

Meanwhile, MnO2 as one of the most promising electrode materials, has drawn particular attention due to its low cost, high electrochemical activity and nontoxicity [18-20]. MnO2 exhibits excellent electrocatalytic activity toward oxidation or reduction of H2O2 [23-31]. For example, a recently published paper has reported the application of MnO2-graphene oxide composite for determination of H2O2 [29]. The MnO2/graphene oxide was constructed by in situ deposition of MnO2 on the surface of GO sheets. However, the detection was conducted in strong alkaline solution and a narrow linear response range and relative low sensitivity were obtained.

In this study, GO/MnO2 composite was prepared by grinding and ultrasonic processing of graphite oxide and MnO2 NWs. During the ultrasonic process, the graphite oxide was exfoliated into graphene oxide sheets [2]. At the same time, MnO2 NWs were attached onto the surface of the GO sheets through oxygencontaining functional groups to form GO/MnO2 nanocomposite. The combination of MnO2 NWs with GO sheets greatly inhibits the aggregation of MnO2 NWs and the stacking of individual GO sheets [21]. The weight ratio of graphite oxide and MnO2 NWs was optimized to make the best use of each ingredient’s advantages and the synergetic effect between them. The GO/MnO2 modified electrode reported in this study shows wider linear range, lower detection limit and higher sensitivity toward determination of H2O2 compared with the previous work [29], which is promising for the development of non-enzymatic H2O2 sensor.

2. Experimental

All chemicals were of analytical grade and used without further purification. Hydrogen peroxide (H2O2, 30%), potassium permanganate (KMnO4) and manganous sulfate (MnSO4) were purchased from Jiangsu Chemical Reagent Company, Tianjin Chemical Reagent Company and Guangzhou Chemical Reagent Company, respectively. Graphite oxide was provided by XFNANO Chemical Reagent Company. Deionized water (Resistivity >18.4 MΩcm-2) was produced using a pure water system (GWA-UN, Beijing, China).

MnO2 NWs were prepared as follows: 10 mL of 1 mol L-1 MnSO4 was added to 20 mL of 1 mol L-1 KMnO4 and mixed thoroughly. After deposition, 10 mL of 0.5 mol L-1 HCl was added into the above solution. The mixture was stirred for 2 h at room temperature. Then, the as-prepared product was washed with ethanol and water, dried at 80 ℃ for 12 h. Finally, the resulting powder was calcined in a muffle furnace with a heating rate of 2 ℃ min-1 from room temperature to 200 ℃, and maintained at 200 ℃ for 4 h. Graphite oxide was mixed with MnO2 NWs on a weight ratio of 1:1 in an agate mortar and ground for 1 h, producing a black-brown powder with uniform color. The resulting black-brown powder was dispersed in doubly distilled water and sonicated for 12 h, forming a black-brown mixture of GO/MnO2 NWs. The mixture solution was dried under vacuum at 80 ℃ for 24 h. Finally, GO/MnO2 NWs composite was dispersed in water to produce homogenous suspension (3.0 mg mL-1). The whole process was repeated for the synthesis of different weight ratio samples, which were 3:1 (3GO/MnO2), 2:1 (2GO/MnO2) and 1:2 (GO/2MnO2).

Prior to use, bare GCE (3 mm in diameter) was polished with 0.3 μm and 0.05μm alumina slurries separately, followed by sonication in ethanol and distilled water, each for 1 min. 3μL of 3 mg mL-1 GO/MnO2 NWs suspension was dropped onto a GCE and dried under infrared lamp in ambient air. Next, 3μL nafion solution (0.25 wt% in ethanol) was cast on the modified electrode, and then dried in an infrared lamp in ambient air. For comparison, the MnO2 NWs-modified electrode was also prepared under the same conditions.

The phase composition and crystal structures of the samples were analyzed by X-ray diffraction (XRD, Bruker GADDS diffractometer) with an area detector operating under a voltage of 40 kV and a current of 40 mA using Cu Kα radiation (λ=0.15418 nm). The morphology of the samples was observed by transmission electron microscopy (TEM, Tecnai G220, FEI).

All electrochemical measurement was performed on a CHI660C electrochemical workstation (Chenhua, Shanghai, China) with a standard three-electrode cell composed of a modified GCE (3mm in diameter) as a working electrode, a platinum wire as a counter electrode and a Ag/AgCl (saturated KCl) as a reference electrode. The supporting electrolyte solution for the electrochemical experiment was phosphate buffer saline (PBS, 10 mL, 0.2molL-1, pH 7.0) which was purged with high purity nitrogen for 20min prior to cyclic voltammetric and amperometric measurement. Impedancepotential measurement was carried out at a frequency ranging from 0.10 Hz to 100 kHz in 0.10 molL-1 KCl solution containing equimolar [Fe (CN)6]3-/[Fe (CN)6]4- (0.01 mol L-1/0.01 mol L-1).

3. Results and discussion 3.1. Characterization of GO/MnO2 nanocomposite

Fig. 1shows the XRD patterns of the graphite oxide, MnO2 NWs and GO/MnO2 nanocomposite. A clear XRD peak of graphite oxide is centered at around 2θ of 10°, which corresponds to the (0 01) diffraction of stacked GO sheets. Its spacing is much larger than that of pristine graphite, due to the introduction of oxygen-containing groups on the GO sheets [11]. All the diffraction peaks of the assynthesized MnO2 NWs can be indexed to tetragonal α-MnO2 (JCPDS44-0141). The XRD peak positions and shapes of GO/MnO2 are similar to those of MnO2, while the (0 01) peak of layered GO almost disappeared. It is possible that the diffraction signals of oxygen-containinggroupsmaybecoveredbyMnO2.Itwasreported that the diffraction peaks become weakened or even disappear whether the graphite oxide content was increased or decreased [21]. Another reason for the disappearance of graphite oxide signals may be the exfoliation of the regular lamellar structure of graphite oxide sheets, forming exfoliated GO sheets [22].

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Figure 1. XRD patterns of GO, MnO2 and GO/MnO2 samples.

The morphologies of the prepared MnO2 NWs and GO/MnO2 nanocomposite were observed by TEM. From Fig. 2A, nanowires of MnO2 were prepared, which are seriously aggregated. Fig. 2B shows that well-distributed MnO2 NWs on the GO surface, which may result from the strong interaction of hydrogen bond between GO and MnO2. Specifically, this is probably attributed to the fact that the surface of GO is decorated mostly with many active groups, such as epoxy, hydroxyl, carbonyl and carboxyl groups [11], the functional groups serve as anchor sites and make the assynthesized MnO2 NWs attach on the surface and edges of GO sheets through hydrogen bonding [12], which inhibits the aggregation of MnO2 NWs. On the other hand, the MnO2 NWs in return lead to the exfoliation of the lamellar GO, and avoid the re-stacking of the nanosheets [21]. Owing to the synergistic effect of the exfoliated GO and MnO2 NWs, the composite provides rich conducting channels for electron transfer between the electrolyte and electrode, which contributes to the enhancement of the electrochemical response and decreases the detection limit.

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Figure 2. TEM images of (A) MnO2 NWs, (B) GO/MnO2 nanocomposite.

Fig. 3 displays the electrochemical impedance spectroscopy (EIS) of the bare GCE, GO/MnO2/GCE and MnO2/GCE, which was conducted in 0.10 mol L-1 KCl solution containing equimolar [Fe (CN)6]3-/[Fe (CN)6]4- (0.01 mol L-1/0.01 mol L-1). In the Nyquist plots, the semicircle portion at high frequency corresponds to the electron transfer process, while the semicircle’s diameter of the Nyquist plot is equivalent to the electron transfer resistance (Rct) of an electrode. Compared with the bare GCE which exhibits an almost straight line, the MnO2 NWs electrode shows larger diameter in the EIS profile, indicating poor conductivity of MnO2 NWs. However, when GO was introduced, the diameter of the EIS profile becomes smaller, owing to the good conductivity of GO that decreases the impedance of the electrode. Obviously, GO serves as the conducting element in such an electrode.

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Figure 3. Nyquist plots of bare GCE, MnO2/GCE, GO/MnO2/GCE in 0.1 mol L-1 KCl solution containing equimolar [Fe (CN)6]3-/[Fe (CN)6]4- (0.01 mol L-1/0.01 mol L-1) at 100 mV s-1.

3.2. Amperometric detection of H2O2 on GO/MnO2/GCE

Fig. 4A shows the CVs of GO/GCE, GO/MnO2/GCE and MnO2/GCE in the presence of 0.20 mmol L-1 H2O2 in PBS (pH 7.0) and at a scan rate of 100 mV s-1. It was clearly observed that the peak current of MnO2/GCE is lower than that of GO/MnO2/GCE, no obvious current of GO/GCE for the oxidation of H2O2 is observed. Fig. 4B shows the CVs of the 3GO/MnO2 (1), 2GO/MnO2 (2), GO/MnO2 (3) and GO/ 2MnO2 (4) in PBS containing the same concentration of H2O2. The result clearly reveals that the oxidation peak increases with the presence of GO and the optimum weight ratio of GO and MnO2 is 1:1. The GO promotes electron transfer. However, further increasing the GO percentage results in the agglomeration, which decreases the available active sites for attaching MnO2 NWs. These results indicated that the modification of MnO2 on the surface of GO significantly improved the electrocatalytic activity toward H2O2.

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Figure 4. (A) CVs of GO/GCE, GO/MnO2/GCE and MnO2/GCE and (B) CVs of 3 GO/MnO2/GCE (1), 2 GO/MnO2/GCE (2), GO/MnO2/GCE (3), GO/2MnO2/GCE (4), in the presence of 0.2 mmol L-1 H2O2 in 0.1 mol L-1 PBS (pH 7.0). scan rate: 100 mV s-1.

Fig. 5 shows the CVs of the GO/MnO2/GCE in PBS (pH 7.0) containing 0.30 mmol L-1 H2O2 at scan rates from 20 mV s-1 to 100 mV s-1. The anodic peak current increases upon the increasing scan rate. The inset in Fig. 5 shows that the peak current is proportional to the scan rate with a correlation coefficient of 0.9993, indicating the process on the surface of electrode is typically surface-controlled. The CVs of the GO/MnO2/GCE in 0.10 mol L-1 PBS (pH 7.0) with different concentrations of H2O2 is presented in Fig. 6. The oxidation peak current increases upon the increase of H2O2 concentration from 5 μmol L-1 to 200 mmol L-1. The excellent electrocatalytic performance may be owing to synergistic effects of the composites.

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Figure 5. CVs of the GO/MnO2/GCE in the presence of 0.3 mmol L-1 H2O2 at different scan rates (20, 40, 60, 80, 100 mV s-1) in 0.1 mol L-1 PBS (pH 7.0). Inset is the plot of oxidation peak current with scan rate.

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Figure 6. CVs of GO/MnO2/GCE in 0.1 mol L-1 PBS (pH 7.0) with different concentrations of H2O2.

Fig. 7A shows the typical amperometric response of the GO/ MnO2 nanocomposite electrode upon the addition of varying amounts of H2O2 into the PBS solution (pH 7.0) under stirring at 0.75 V. The GO/MnO2 nanocomposite electrode displays distinct current response toward H2O2. It takes less than 5 s for the GO/ MnO2 modified electrode to reach 90% of the maximum current, indicating a fast amperometric response to the oxidation of H2O2. With the addition of H2O2 into the PBS solution under stirring, the catalytic current increases linearly. The sensor displays a linear relationship with the concentration of H2O2 from 4.9 mmol L-1 to 4.5 mmol L-1 with a correlation coefficient of 0.9992 (Fig. 7B). The regression equation is I (μA)=0.34 + 17.21X, where X is the concentration of H2O2 in mmol L-1. The detection limit is determined to be 0.48 mmol L-1 (S/N=3). The sensitivity is 191.22 μA (mmol L-1)-1 cm-2. On the basis of the high catalytic activity of the GO/MnO2-modified electrode to H2O2, a nonenzymatic sensor was constructed for the detection of H2O2.

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Figure 7. (A) Amperometric current curve of the GO/MnO2/GCE with the successive addition of H2O2 into the 0.1 mol L-1 PBS (pH 7.0) under stirring at 0.75 V. Inset is the amplification of the marked rectangle region shown in the curve, the unit for the added H2O2 is mmol L-1. (B) The corresponding calibration curves of the current response versus H2O2 concentration.

To evaluate the as-prepared sensor, other materials and methods were discussed for comparison (Table 1). Compared with the previous work [29], the linear range in this study is much wider while the detection limit is lower. Moreover, the sensitivity in our study is 5 times than that of the reported [29]. Table 1 lists several typical non-enzymatic sensors based on MnO2 nanostructures reported previously. The present GO/MnO2/GCE displayed better performance to the determination of H2O2 in terms of its high sensitivity, wide linear range, and low limit of detection. These results can be explained by the high surface area, excellent catalytic activity and improved electric conductivity of GO/MnO2/ GCE.

Table 1
Comparison of the present GO/MnO2 sensor with other H2O2 sensors based on MnO2.

3.3. Selectivity, stability and reproducibility of GO/MnO2/GCE

The interference experiments were conducted in N2-saturated 0.10 mol L-1 PBS (pH 7.0) at an applied potential of 0.75 V by comparing the current response to 25 μmol L-1 H2O2. Several interfering compounds were investigated under optimal conditions for the determination of H2O2. Here, we investigated the interferences from NaCl, oxalic acid, critic acid and glucose (Fig. 8). There is obvious catalytic current observed with the addition of 25 μmol L-1 H2O2. On the contrary, almost no catalytic response is observed in the presence of 1.0 mmol L-1 NaCl, 1.0 mmol L-1 oxalic acid, 1.0 mmol L-1 glucose. These results confirm that the interferences from aforementioned ones could be completely avoided. The addition of 1.0 mmol L-1 critic acid shows weak interference which can also be ignored, thus the GO/MnO2/GCE could be applied in the non-enzymatic hydrogen peroxide sensing.

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Figure 8. Amperometric responses of GO/MnO2/GCE on successive injections of 25 mmol L-1 H2O2, 1.0 mmol L-1 NaCl, 1.0 mmol L-1 H2C2O4, 1.0 mmol L-1 citric acid, 1.0 mmol L-1 glucose and 25 μmol L-1 H2O2 in N2-saurated 0.1 mol L-1 PBS (pH 7.0) at 0.75 V.

The reproducibility of GO/MnO2/GCE is also studied by amperometric measurement. Five electrodes are prepared independently for recording the current response in the presence of 0.10 mmol L-1 H2O2. The standard deviation of the response currents was 4.9% (Fig. 9), indicating high reproducibility of the prepared GO/MnO2 modified electrode. For one GO/MnO2 modified electrode, the R.S.D. was estimated to be 2.3% to 0.10 mmol L-1 H2O2 in seven successive measurements. After storage at ambient conditions for two weeks, the GO/MnO2 electrode still retains 93.4% of the initial current response value, which clearly demonstrates a high stability of the GO/MnO2 electrode.

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Figure 9. The current response of five different GO/MnO2 electrodes to 0.10 mmol L-1 H2O2.

4. Conclusion

In summary, GO/MnO2 nanocomposites were successfully prepared for the fabrication of a novel non-enzymatic H2O2 sensor. The resulting sensor exhibited wide linear range from 4.9μmol L-1 to 4.5 μmol L-1, high sensitivity (191.22μA L mmol-1 cm-2), fast response (within 5 s), low detection limit (0.48 μmol L-1) and longterm storage stability for the determination of H2O2. The nanocomposites based on GO nanosheets and MnO2 NWs present several advantages: (1) GO with abundance oxygen-containing functional groups displayed strong adsorption ability, which was helpful for the attachment of MnO2 NWs onto the surface of the GO sheets firmly and provided more active sites for electrochemical reactions; (2) the MnO2 NWs, with large surface area, enabled rapid diffusion of analytes and large contact area, resulting in an improved utilization efficiency and enhanced catalytic performance; (3) no chemical reducing agents and no organic linkers were used in the method, representing a facile, efficient and green approach to fabricate highly active electrocatalysts. With these advantages, the study may provide a feasible approach to develop new electrochemical sensors and detection of other biochemical reagent.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21273080) and Guangdong Natural Science Foundation (No. 2014A030311039)
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