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Graphene modified basal and edge plane pyrolytic graphite electrodes for electrocatalytic oxidation of hydrogen peroxide and β-nicotinamide adenine dinucleotide
Electrochemistry Communications 11 (2009) 2153–2156
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Graphene modified basal and edge plane pyrolytic graphite electrodes for electrocatalytic oxidation of hydrogen peroxide and b-nicotinamide adenine dinucleotide Wei-Jhih Lin a, Chien-Shiun Liao b, Jia-Hao Jhang b, Yu-Chen Tsai a,* a b
Department of Chemical Engineering, National Chung Hsing University, 250, Kuo Kuang Road, Taichung 402, Taiwan Department of Chemical Engineering and Materials Science, Yuan Ze University, Taoyuan 320, Taiwan
a r t i c l e
i n f o
Article history: Received 30 August 2009 Received in revised form 14 September 2009 Accepted 15 September 2009 Available online 18 September 2009 Keywords: Graphene Edge plane pyrolytic graphite Electrocatalysis NADH Hydrogen peroxide
a b s t r a c t Graphene was cast on basal and edge plane pyrolytic graphite electrodes for electrochemical applications. The morphology of the resulting graphene modified electrode was investigated by atomic force microscopy. In cyclic voltammetric responses, both anodic and cathodic peak currents varied linearly with the square root of scan rates over the range of 25–600 mV in 0.1 M KCl containing 5 mM FeðCNÞ4 6 at graphene modified basal and edge plane pyrolytic graphite electrodes, which suggests a diffusion-controlled process. The graphene modified basal and edge plane pyrolytic graphite electrodes exhibited the abilities to lower the electrooxidation potentials of b-nicotinamide adenine dinucleotide and hydrogen peroxide in comparison with bare basal and edge plane pyrolytic graphite electrodes. The electrocatalytic behavior obtained at the graphene modified basal and edge plane pyrolytic graphite electrodes may led to new applications in electroanalysis. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Several forms of carbon electrodes such as carbon nanotubes and carbon nanofibers have been employed for electroanalytical applications [1–5]. Recently, graphene has attracted much attention because of its unique properties and potential applications in electroanalysis. Graphene is a single-layer two-dimensional material which composed of carbon atoms forming six-membered rings. Graphene samples with layers from two to ten are equally of interest [6]. Methods for the preparation of graphene samples include chemical vapor deposition of camphor [7], conversion of nanodiamond [8], and exfoliation of graphene oxide [9]. Graphene-based electrodes have been applied to the electrochemical stripping determination of cadmium [10], the selective detection of dopamine [11], and a platform for electrochemical sensing and biosensing [12,13] by virtue of their electrocatalytic ability and ease of modification. Electrocatalysis for a variety of electrochemical processes was though to be the main contribution of graphene when used in electrochemistry. Compton et al. demonstrated that the apparent electrocatalytic effects of carbon nanotubes can be attributed to the ends of carbon nanotubes, which structurally resemble the behavior of edge plane pyrolytic graphite [14]. They also showed that the electrocatalytic hydrogen peroxide detection * Corresponding author. Tel.: +886 4 22857257; fax: +886 4 22854734. E-mail address: [email protected] (Y.-C. Tsai). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.09.018
at carbon nanotubes modified electrode is due to iron oxide particles [15,16]. The use of edge plane pyrolytic graphite for electrocatalytic determinations of thiols, b-nicotinamide adenine dinucleotide (NADH), and ascorbic acid has been reported in the literature [17–20]. Here we compare the oxidation of hydrogen peroxide and NADH at basal plane pyrolytic graphite electrode (BPPGE) and edge plane pyrolytic graphite electrode (EPPGE) before and after modification with graphene in an attempt to investigate the electrocatalysis mechanism. 2. Experimental 2.1. Reagents All chemicals were used without further purification. Potassium hexacyanoferrate (II) trihydrate and NADH were purchased from Sigma. Hydrogen peroxide (30%) was obtained from Showa. All solutions were prepared with demineralized and filtered water of resistivity not less than 18 MX cm taken from a Milli-Q water purification system (Milli-Q, USA). 2.2. Instrumentation Electrochemical experiments were performed with an Autolab PGSTA30 Electrochemical Analyzer (Eco Chemie, Netherlands). A
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conventional three-electrode electrochemical cell, with a 3 mm diameter BPPGE or EPPGE working electrode (BAS, Japan), an Ag/ AgCl (3 M KCl) reference electrode (Metrohm Ltd., Switzerland), and a platinum wire counter electrode (Metrohm Ltd., Switzerland). Atomic force microscopy (AFM) images were obtained using a SPA-400 (Seiko, Japan) multiple function units together with SPI3800N control station in tapping mode in the air. 2.3. Preparation of graphene Graphene was prepared by reduction of graphene oxide by hydrazine. Graphene oxide was synthesized from graphite powder by a modified Hummers method [21,22]. A 2.5 g of KMnO4 was slowly added into a mixture of 5 g of graphite, 3.75 g of NaNO3, and 375 mL of concentrated H2SO4 with stirring for 2 h at 0 °C. After 5 days of vigorous stirring at room temperature, a 700 mL of 5 wt.% H2SO4 solution was slowly added and stirred for 2 h at 98 °C. Then, a 15 mL of 30 wt.% hydrogen peroxide solution was added in the cooled mixture and stirred for 2 h at room temperature. Repeating cycles of centrifugation and redispersing in a mixed solution of 3 wt.% of H2SO4 and 0.5 wt.% of hydrogen peroxide were employed to purify the precipitate. Further washing used 3 wt.% HCl and water and passed through a weak basic ion-exchange resin to remove the remaining acid. The resultant solution was filtrated and dried to obtain the graphene oxide. The stable suspension of chemically reduced graphene oxide in N,N-dimethylformamide (DMF) was prepared according to the Ruoff procedure [23]. First, 30 mg of graphene oxide was dispersed in 1 mL of water by sonication for 1 h. Then, 9 mL of DMF was added in 1 mL of water containing 30 mg of graphene. Finally, 10 lL of hydrazine was added in the prepared 10 mL mixed solution with vigorous stirring for 12 h at 80 °C and a black suspension of graphene was obtained. The product was characterized using transmission electron microscopy and AFM (not shown). The graphene samples contained 4–6 layers on average which is similar to other work [24]. 2.4. Preparation of graphene modified BPPGE and EPPGE The EPPGE was polished with 0.3 and 0.05 lm alumina slurries and sonicated in water. The BPPGE was polished with carborundum paper (P100 grade) and pressed with cellotape on the BPPGE surface. Then the BPPGE was cleaned in acetone. The graphene modified BPPGE and EPPGE were fabricated by casting a 4 lL aliquot of the prepared graphene solution on the BPPGE and EPPGE. The solvent was allowed to evaporate at room temperature in the air for 2 h. 3. Results and discussion The AFM images of the graphite substrate before and after modification with graphene are shown in Fig. 1a and b, respectively. The root-mean-square values were 9.5 nm and 18.9 nm for the graphite substrate and graphene modified graphite substrate, respectively. The surface roughness is larger for the graphene modified graphite substrate in comparison to the graphite substrate. This is attributed to the attachment of graphene on the surface of graphite. From these data, we can conclude that the graphene was successfully homogeneously coated onto the surface of graphite. The prepared graphene modified electrodes can be used for further applications in electroanalysis. Cyclic voltammetry was carried out in order to study the surface electrochemical behavior at the graphene modified electrodes 4 in a solution containing FeðCNÞ6 . Cyclic voltammograms obtained with different scan rates ranging from 25 mV to 600 mV at the
Fig. 1. AFM images of the graphite substrate (a) before and (b) after modification with graphene.
graphene modified EPPGE in 0.1 M KCl containing 5 mM FeðCNÞ4 6 are shown in Fig. 2. Well-defined oxidation and reduction peaks are observed at the graphene modified EPPGE. The well-defined peaks suggest that the graphene film is highly homogeneous, which is consistent with the AFM results. The increase of potential scan rate promoted an increase of peak current in anodic and cathodic reactions. The detailed description of the relationship between peak currents and scan rates is shown in the inset of Fig. 2. Both anodic and cathodic peak currents vary linearly with the square root of the scan rate over the range of 50–600 m Vs1, which suggests that the process is predominantly diffusion-controlled. The surface electrochemical behavior at the graphene modified BPPGE is similar to that of the graphene modified EPPGE. To evaluate the anti-fouling ability and electrocatalytic behavior toward the electrochemical oxidations of NADH and hydrogen peroxide at the graphene modified BPPGE and EPPGE, several electrochemical measurements were carried out at the graphene modified BPPGE and EPPGE. The cyclic voltammograms measured using the graphene modified BPPGE and EPPGE in 0.1 M phosphate buffer solution (pH 7) containing 0.5 mM hydrogen peroxide are shown in Fig. 3. On the graphene modified BPPGE and EPPGE, the onsets of electrooxidation of hydrogen peroxide were observed at around +0.6 V. On the contrary, there is no obvious oxidation at
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Fig. 4. Cyclic voltammograms obtained using (a) bare BPPGE, (b) graphene modified BPPGE, (c) bare EPPGE, and (d) graphene modified EPPGE in 0.1 M phosphate buffer solution (pH 7) containing 2 mM NADH. Fig. 2. Cyclic voltammograms at the graphene modified EPPGE in 0.1 M KCl containing 5 mM FeðCNÞ4 6 at scan rates of (a) 25, (b) 50, (c) 100, (d) 150, (e) 200, (f) 300, (g) 400, (h) 500, and (i) 600 m Vs1. The inset shows the anodic and cathodic peak currents as a function of the square root of scan rates.
Fig. 3. Cyclic voltammograms obtained using (a) bare BPPGE, (b) graphene modified BPPGE, (c) bare EPPGE, and (d) graphene modified EPPGE in 0.1 M phosphate buffer solution (pH 7) containing 0.5 mM hydrogen peroxide.
both the bare BPPGE and EPPGE. The cyclic voltammograms obtained using graphene modified BPPGE and EPPGE in 0.1 M phosphate buffer solution (pH 7) containing 2 mM NADH are shown in Fig. 4. For the electrochemical oxidation of NADH, the graphene modified BPPGE and EPPGE electrodes exhibited a substantial negative shift of the anodic peak potential when compared with bare BPPGE and EPPGE. The electrocatalytic oxidation is evident from the oxidation peak potentials of NADH are 0.564 V and 0.652 V for graphene modified EPPGE and bare EPPGE, respectively. The decrease of activation energy at the graphene modified BPPGE and EPPGE allow the detection of NADH and hydrogen peroxide at lower potentials. This might be attributed to the unique electronic structure and properties of graphene [6]. The bare BPPGE and EPPGE showed similar behaviors for the electrooxidation of hydrogen peroxide and NADH which can be seen in Figs. 3 and 4. The amperometric responses recorded over a continuous 2500 s period of 2 mM NADH in 0.1 M phosphate buffer solution (pH 7) with an operating potential of 0.6 V at the bare and graphene modified
Fig. 5. Amperometric responses recorded over a continuous 2500 s in 0.1 M phosphate buffer solution (pH 7) containing 2 mM NADH at (a) bare EPPGE and (b) graphene modified EPPGE. Working potential: +0.6 V (vs. Ag/AgCl).
EPPGE are shown in Fig. 5a and b, respectively. The bare EPPGE showed decay of 41%. There was 23% for the graphene modified EPPGE. This indicates that the EPPGE modified with graphene is able to alleviate surface passivation during the oxidation of NADH. These characteristics make the graphene modified BPPGE and EPPGE very attractive for electrocatalytic oxidation of NADH or hydrogen peroxide. It is noteworthy, that the electrocatalytic behavior and anti-fouling ability obtained at the graphene modified electrodes cannot be seen at the bare EPPGE. 4. Conclusion We have demonstrated the electrocatalytic oxidation of hydrogen peroxide and NADH using graphene modified BPPGE and
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EPPGE. Comparison of the cyclic voltammetric responses at the bare and graphene modified BPPGE and EPPGE suggests that the graphene modified BPPGE and EPPGE are effective electrocatalysts. The graphene modified BPPGE and EPPGE offer lower electrooxidation potentials of NADH and hydrogen peroxide and alleviate the electrode surface fouling due to oxidation product of NADH on the electrode surface. Considering the results and bearing in mind that the EPPGE is not responsible for the electrocatalytic effects of hydrogen peroxide and NADH at the graphene modified electrodes. A further study is needed to throw more light on the electrocatalysis mechanism of hydrogen peroxide and NADH at graphene modified electrodes. The proposed graphene modified electrodes might be useful for a simple and effective way to develop electrochemical sensors and biofuel cells. Acknowledgements The authors thank the National Science Council, Taiwan for financial support. This work is supported in part by the Ministry of Education, Taiwan under the ATU plan. References [1] R.R. Moore, C.E. Banks, R.G. Compton, Anal. Chem. 76 (2004) 2677. [2] A. Salimi, C.E. Banks, R.G. Compton, Analyst 129 (2004) 225.
[3] J. Wang, Electroanalysis 17 (2005) 7. [4] F. Marken, M.L. Gerrard, I.M. Mellor, R.J. Mortimer, C.E. Madden, S. Fletcher, K. Holt, J.S. Foord, R.H. Dahm, F. Page, Electrochem. Commun. 3 (2001) 177. [5] Y.C. Tsai, M.C. Tsai, C.C. Chiu, Electrochem. Commun. 10 (2008) 749. [6] C.N.R. Rao, K. Biswas, K.S. Subrahmanyam, A. Govindaraj, J. Mater. Chem. 19 (2009) 2457. [7] P.R. Somani, S.P. Somani, M. Umeno, Chem. Phys. Lett. 430 (2006) 56. [8] O.E. Andersson, B.L.V. Prasad, H. Sato, T. Enoki, Y. Hishiyama, Y. Kaburagi, M. Yoshikawa, S. Bandow, Phys. Rev. B. 58 (1998) 16387. [9] H.C. Schniepp, J.L. Li, M.J. McAllister, H. Sai, M. Herrera-Alonso, D.H. Adamson, R.K. Prud’homme, R. Car, D.A. Saville, I.A. Aksay, J. Phys. Chem. B 110 (2006) 8535. [10] J. Li, S. Guo, Y. Zhai, E. Wang, Electrochem. Commun. 11 (2009) 1085. [11] Y. Wang, Y. Li, L. Tang, J. Lu, J. Li, Electrochem. Commun. 11 (2009) 889. [12] C.H. Lu, H.H. Yang, C.L. Zhu, X. Chen, G.N. Chen, Angew. Chem. Int. Ed. 48 (2009) 4785. [13] M. Zhou, Y. Zhai, S. Dong, Anal. Chem. 81 (2009) 5603. [14] C.E. Banks, R.R. Moore, T.J. Davies, R.G. Compton, Chem. Commun. (2004) 1804. [15] C.E. Banks, A. Crossley, C. Salter, S.J. Wilkins, R.G. Compton, Angew. Chem. Int. Ed. 45 (2006) 2533. [16] B. Šljukic´, C.E. Banks, R.G. Compton, Nano Lett. 6 (2006) 1556. [17] C.E. Banks, R.G. Compton, Anal. Sci. 21 (2005) 1263. [18] C.E. Banks, R.G. Compton, Analyst 130 (2005) 1232. [19] R.R. Moore, C.E. Banks, R.G. Compton, Analyst 129 (2004) 755. [20] F. Wantz, C.E. Banks, R.G. Compton, Electroanalysis 17 (2005) 1529. [21] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [22] M. Hirata, T. Gotou, S. Horiuchi, M. Fujiwara, M. Ohba, Carbon 42 (2004) 2929. [23] S. Park, J. An, I. Jung, R.D. Piner, S.J. An, X. Li, A. Velamakanni, R.S. Ruoff, Nano Lett. 9 (2009) 1593. [24] N. Varghese, U. Mogera, A. Govindaraj, A. Das, P.K. Maiti, A.K. Sood, C.N.R. Rao, Chem. Phys. Chem. 10 (2009) 206.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Graphene modified basal and edge plane pyrolytic graphite electrodes for electrocatalytic oxidation of hydrogen peroxide and b-nicotinamide adenine dinucleotide Wei-Jhih Lin a, Chien-Shiun Liao b, Jia-Hao Jhang b, Yu-Chen Tsai a,* a b
Department of Chemical Engineering, National Chung Hsing University, 250, Kuo Kuang Road, Taichung 402, Taiwan Department of Chemical Engineering and Materials Science, Yuan Ze University, Taoyuan 320, Taiwan
a r t i c l e
i n f o
Article history: Received 30 August 2009 Received in revised form 14 September 2009 Accepted 15 September 2009 Available online 18 September 2009 Keywords: Graphene Edge plane pyrolytic graphite Electrocatalysis NADH Hydrogen peroxide
a b s t r a c t Graphene was cast on basal and edge plane pyrolytic graphite electrodes for electrochemical applications. The morphology of the resulting graphene modified electrode was investigated by atomic force microscopy. In cyclic voltammetric responses, both anodic and cathodic peak currents varied linearly with the square root of scan rates over the range of 25–600 mV in 0.1 M KCl containing 5 mM FeðCNÞ4 6 at graphene modified basal and edge plane pyrolytic graphite electrodes, which suggests a diffusion-controlled process. The graphene modified basal and edge plane pyrolytic graphite electrodes exhibited the abilities to lower the electrooxidation potentials of b-nicotinamide adenine dinucleotide and hydrogen peroxide in comparison with bare basal and edge plane pyrolytic graphite electrodes. The electrocatalytic behavior obtained at the graphene modified basal and edge plane pyrolytic graphite electrodes may led to new applications in electroanalysis. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Several forms of carbon electrodes such as carbon nanotubes and carbon nanofibers have been employed for electroanalytical applications [1–5]. Recently, graphene has attracted much attention because of its unique properties and potential applications in electroanalysis. Graphene is a single-layer two-dimensional material which composed of carbon atoms forming six-membered rings. Graphene samples with layers from two to ten are equally of interest [6]. Methods for the preparation of graphene samples include chemical vapor deposition of camphor [7], conversion of nanodiamond [8], and exfoliation of graphene oxide [9]. Graphene-based electrodes have been applied to the electrochemical stripping determination of cadmium [10], the selective detection of dopamine [11], and a platform for electrochemical sensing and biosensing [12,13] by virtue of their electrocatalytic ability and ease of modification. Electrocatalysis for a variety of electrochemical processes was though to be the main contribution of graphene when used in electrochemistry. Compton et al. demonstrated that the apparent electrocatalytic effects of carbon nanotubes can be attributed to the ends of carbon nanotubes, which structurally resemble the behavior of edge plane pyrolytic graphite [14]. They also showed that the electrocatalytic hydrogen peroxide detection * Corresponding author. Tel.: +886 4 22857257; fax: +886 4 22854734. E-mail address: [email protected] (Y.-C. Tsai). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.09.018
at carbon nanotubes modified electrode is due to iron oxide particles [15,16]. The use of edge plane pyrolytic graphite for electrocatalytic determinations of thiols, b-nicotinamide adenine dinucleotide (NADH), and ascorbic acid has been reported in the literature [17–20]. Here we compare the oxidation of hydrogen peroxide and NADH at basal plane pyrolytic graphite electrode (BPPGE) and edge plane pyrolytic graphite electrode (EPPGE) before and after modification with graphene in an attempt to investigate the electrocatalysis mechanism. 2. Experimental 2.1. Reagents All chemicals were used without further purification. Potassium hexacyanoferrate (II) trihydrate and NADH were purchased from Sigma. Hydrogen peroxide (30%) was obtained from Showa. All solutions were prepared with demineralized and filtered water of resistivity not less than 18 MX cm taken from a Milli-Q water purification system (Milli-Q, USA). 2.2. Instrumentation Electrochemical experiments were performed with an Autolab PGSTA30 Electrochemical Analyzer (Eco Chemie, Netherlands). A
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conventional three-electrode electrochemical cell, with a 3 mm diameter BPPGE or EPPGE working electrode (BAS, Japan), an Ag/ AgCl (3 M KCl) reference electrode (Metrohm Ltd., Switzerland), and a platinum wire counter electrode (Metrohm Ltd., Switzerland). Atomic force microscopy (AFM) images were obtained using a SPA-400 (Seiko, Japan) multiple function units together with SPI3800N control station in tapping mode in the air. 2.3. Preparation of graphene Graphene was prepared by reduction of graphene oxide by hydrazine. Graphene oxide was synthesized from graphite powder by a modified Hummers method [21,22]. A 2.5 g of KMnO4 was slowly added into a mixture of 5 g of graphite, 3.75 g of NaNO3, and 375 mL of concentrated H2SO4 with stirring for 2 h at 0 °C. After 5 days of vigorous stirring at room temperature, a 700 mL of 5 wt.% H2SO4 solution was slowly added and stirred for 2 h at 98 °C. Then, a 15 mL of 30 wt.% hydrogen peroxide solution was added in the cooled mixture and stirred for 2 h at room temperature. Repeating cycles of centrifugation and redispersing in a mixed solution of 3 wt.% of H2SO4 and 0.5 wt.% of hydrogen peroxide were employed to purify the precipitate. Further washing used 3 wt.% HCl and water and passed through a weak basic ion-exchange resin to remove the remaining acid. The resultant solution was filtrated and dried to obtain the graphene oxide. The stable suspension of chemically reduced graphene oxide in N,N-dimethylformamide (DMF) was prepared according to the Ruoff procedure [23]. First, 30 mg of graphene oxide was dispersed in 1 mL of water by sonication for 1 h. Then, 9 mL of DMF was added in 1 mL of water containing 30 mg of graphene. Finally, 10 lL of hydrazine was added in the prepared 10 mL mixed solution with vigorous stirring for 12 h at 80 °C and a black suspension of graphene was obtained. The product was characterized using transmission electron microscopy and AFM (not shown). The graphene samples contained 4–6 layers on average which is similar to other work [24]. 2.4. Preparation of graphene modified BPPGE and EPPGE The EPPGE was polished with 0.3 and 0.05 lm alumina slurries and sonicated in water. The BPPGE was polished with carborundum paper (P100 grade) and pressed with cellotape on the BPPGE surface. Then the BPPGE was cleaned in acetone. The graphene modified BPPGE and EPPGE were fabricated by casting a 4 lL aliquot of the prepared graphene solution on the BPPGE and EPPGE. The solvent was allowed to evaporate at room temperature in the air for 2 h. 3. Results and discussion The AFM images of the graphite substrate before and after modification with graphene are shown in Fig. 1a and b, respectively. The root-mean-square values were 9.5 nm and 18.9 nm for the graphite substrate and graphene modified graphite substrate, respectively. The surface roughness is larger for the graphene modified graphite substrate in comparison to the graphite substrate. This is attributed to the attachment of graphene on the surface of graphite. From these data, we can conclude that the graphene was successfully homogeneously coated onto the surface of graphite. The prepared graphene modified electrodes can be used for further applications in electroanalysis. Cyclic voltammetry was carried out in order to study the surface electrochemical behavior at the graphene modified electrodes 4 in a solution containing FeðCNÞ6 . Cyclic voltammograms obtained with different scan rates ranging from 25 mV to 600 mV at the
Fig. 1. AFM images of the graphite substrate (a) before and (b) after modification with graphene.
graphene modified EPPGE in 0.1 M KCl containing 5 mM FeðCNÞ4 6 are shown in Fig. 2. Well-defined oxidation and reduction peaks are observed at the graphene modified EPPGE. The well-defined peaks suggest that the graphene film is highly homogeneous, which is consistent with the AFM results. The increase of potential scan rate promoted an increase of peak current in anodic and cathodic reactions. The detailed description of the relationship between peak currents and scan rates is shown in the inset of Fig. 2. Both anodic and cathodic peak currents vary linearly with the square root of the scan rate over the range of 50–600 m Vs1, which suggests that the process is predominantly diffusion-controlled. The surface electrochemical behavior at the graphene modified BPPGE is similar to that of the graphene modified EPPGE. To evaluate the anti-fouling ability and electrocatalytic behavior toward the electrochemical oxidations of NADH and hydrogen peroxide at the graphene modified BPPGE and EPPGE, several electrochemical measurements were carried out at the graphene modified BPPGE and EPPGE. The cyclic voltammograms measured using the graphene modified BPPGE and EPPGE in 0.1 M phosphate buffer solution (pH 7) containing 0.5 mM hydrogen peroxide are shown in Fig. 3. On the graphene modified BPPGE and EPPGE, the onsets of electrooxidation of hydrogen peroxide were observed at around +0.6 V. On the contrary, there is no obvious oxidation at
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Fig. 4. Cyclic voltammograms obtained using (a) bare BPPGE, (b) graphene modified BPPGE, (c) bare EPPGE, and (d) graphene modified EPPGE in 0.1 M phosphate buffer solution (pH 7) containing 2 mM NADH. Fig. 2. Cyclic voltammograms at the graphene modified EPPGE in 0.1 M KCl containing 5 mM FeðCNÞ4 6 at scan rates of (a) 25, (b) 50, (c) 100, (d) 150, (e) 200, (f) 300, (g) 400, (h) 500, and (i) 600 m Vs1. The inset shows the anodic and cathodic peak currents as a function of the square root of scan rates.
Fig. 3. Cyclic voltammograms obtained using (a) bare BPPGE, (b) graphene modified BPPGE, (c) bare EPPGE, and (d) graphene modified EPPGE in 0.1 M phosphate buffer solution (pH 7) containing 0.5 mM hydrogen peroxide.
both the bare BPPGE and EPPGE. The cyclic voltammograms obtained using graphene modified BPPGE and EPPGE in 0.1 M phosphate buffer solution (pH 7) containing 2 mM NADH are shown in Fig. 4. For the electrochemical oxidation of NADH, the graphene modified BPPGE and EPPGE electrodes exhibited a substantial negative shift of the anodic peak potential when compared with bare BPPGE and EPPGE. The electrocatalytic oxidation is evident from the oxidation peak potentials of NADH are 0.564 V and 0.652 V for graphene modified EPPGE and bare EPPGE, respectively. The decrease of activation energy at the graphene modified BPPGE and EPPGE allow the detection of NADH and hydrogen peroxide at lower potentials. This might be attributed to the unique electronic structure and properties of graphene [6]. The bare BPPGE and EPPGE showed similar behaviors for the electrooxidation of hydrogen peroxide and NADH which can be seen in Figs. 3 and 4. The amperometric responses recorded over a continuous 2500 s period of 2 mM NADH in 0.1 M phosphate buffer solution (pH 7) with an operating potential of 0.6 V at the bare and graphene modified
Fig. 5. Amperometric responses recorded over a continuous 2500 s in 0.1 M phosphate buffer solution (pH 7) containing 2 mM NADH at (a) bare EPPGE and (b) graphene modified EPPGE. Working potential: +0.6 V (vs. Ag/AgCl).
EPPGE are shown in Fig. 5a and b, respectively. The bare EPPGE showed decay of 41%. There was 23% for the graphene modified EPPGE. This indicates that the EPPGE modified with graphene is able to alleviate surface passivation during the oxidation of NADH. These characteristics make the graphene modified BPPGE and EPPGE very attractive for electrocatalytic oxidation of NADH or hydrogen peroxide. It is noteworthy, that the electrocatalytic behavior and anti-fouling ability obtained at the graphene modified electrodes cannot be seen at the bare EPPGE. 4. Conclusion We have demonstrated the electrocatalytic oxidation of hydrogen peroxide and NADH using graphene modified BPPGE and
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EPPGE. Comparison of the cyclic voltammetric responses at the bare and graphene modified BPPGE and EPPGE suggests that the graphene modified BPPGE and EPPGE are effective electrocatalysts. The graphene modified BPPGE and EPPGE offer lower electrooxidation potentials of NADH and hydrogen peroxide and alleviate the electrode surface fouling due to oxidation product of NADH on the electrode surface. Considering the results and bearing in mind that the EPPGE is not responsible for the electrocatalytic effects of hydrogen peroxide and NADH at the graphene modified electrodes. A further study is needed to throw more light on the electrocatalysis mechanism of hydrogen peroxide and NADH at graphene modified electrodes. The proposed graphene modified electrodes might be useful for a simple and effective way to develop electrochemical sensors and biofuel cells. Acknowledgements The authors thank the National Science Council, Taiwan for financial support. This work is supported in part by the Ministry of Education, Taiwan under the ATU plan. References [1] R.R. Moore, C.E. Banks, R.G. Compton, Anal. Chem. 76 (2004) 2677. [2] A. Salimi, C.E. Banks, R.G. Compton, Analyst 129 (2004) 225.
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