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C60 trianion-mediated electrocatalysis and amperometric sensing of hydrogen peroxide
Electrochemistry Communications 10 (2008) 1377–1380
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Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
C60 trianion-mediated electrocatalysis and amperometric sensing of hydrogen peroxide Wei Liu, Xiang Gao * State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 4 June 2008 Received in revised form 26 June 2008 Accepted 27 June 2008 Available online 5 July 2008 Keywords: Amperometric sensor Electrocatalytic reduction Fullerenes Hydrogen peroxide Lipid film
a b s t r a c t Heterogeneous electrocatalytic reduction of hydrogen peroxide (H2O2) by C60 is reported for the first time. C60 is embedded in tetraoctylammonium bromide (TOAB) film and is characterized by scanning electron microscopy and cyclic voltammetry. Electrocatalytic studies show that the trianion of C60 mediates the electrocatalytic reduction of H2O2 in aqueous solution containing 0.1 M KCl. Application of such film modified electrode as an amperometric sensor for H2O2 determination is also examined. The sensor shows a fast response within 1 s and a linear response is obtained (R = 0.9986) in the concentration range from 3.33 105 to 2.05 103 mol L1 for H2O2, with the detection limit of 2 105 mol L1 and the sensitivity of 1.65 lA mM1. A good repeatability and stability is shown for the sensor during the experiment. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Due to its unique electrochemical property to undergo six reversible one-electron reductions in nonaqueous solutions [1], C60 has been shown to be a promising electron transfer mediator for various species [2]. For example, it has been shown that anionic C60 has catalytic reducing ability towards halogenated hydrocarbons [3–5], certain organic functional groups [6], and nitrogen fixation [7]. Consequently, electrocatalysis of target analytes with C60 modified electrodes has attracted great interest because C60 is chemically stable, readily available with high purity and ease of preparation for the electrodes, where a large number of literature appears claiming that C60 acts as electron mediator in electrocatalytic processes towards biomolecules including glucose [8], hemoglobin [9], cytochrome c [10], ascorbic acid [11], coenzymes [12], L-cysteine [13] and uric acid [14]. In the mean time, research on using fullerenes as a novel mediator in amperometric biosensors has also been reported [15]. However, recent work by Compton and Banks [16–18] have drawn much attention by raising questions on some of reported studies, and they have shown that the observed electrocatalyses in some cases are caused by either the small amount of graphite impurity in C60 sample, or the oxygenated species formed on the surface of glassy carbon electrodes with ‘‘electrode pretreatment”, while electrocatalysis mediated by C60 is only likely ‘‘where C60 itself becomes oxidized * Corresponding author. Tel./fax: +86 431 85262946. E-mail address: [email protected] (X. Gao). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.06.031
or reduced” [16]. Inspired by the unusual electron transfer and electrocatalytic property of anionic C60, we studied the electrocatalytic reduction of hydrogen peroxide (H2O2) by C60. To the best of our knowledge, no work on the electrocatalytic reduction of H2O2 by C60 has appeared to date. Study on electrocatalytic reduction of H2O2 by C60 is of great interest for several reasons: first, fullerenes have demonstrated a biological antioxidant activity toward various reactive oxygen species [19], while H2O2 is an important biological oxidant produced by enzymatic reactions [20]; second, fullerenes have shown a promising potential in the development of fuel cells [21], where H2O2 is an important intermediate of the electrochemical reactions [22]. In addition, determination of H2O2 is of great importance because it is widely used in areas of environmental, industrial, food, pharmaceutical and clinical analyses. Electrochemical sensing of H2O2 is one of the most promising approaches to monitor H2O2 [23–26]. Chemically modified electrodes with electroactive mediators immobilized on the solid surfaces have been frequently used for determination of H2O2 for the high selectivity, good sensitivity, low cost and ease of construction. The use of C60 for modifying electrode surfaces has attracted much attention [18,27–30], however, the electrochemistry of fullerene films in aqueous media is unstable and irreversible [27,28], which has limited further studies on heterogeneous electrocatalysis by C60 modified electrodes in aqueous solution. Various methods have been explored to solve this problem, among them, C60 embedded in films of cationic lipids such as tetraoctylammonium bromide (TOAB) on electrodes has shown good electrochemical response
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in aqueous system, due to the partial inhibition of the aggregation of C60, the strong binding of electrochemically reduced C60 anions with the matrix lipid cations, and small electrostatic repulsion between the C60 anions in the suitable hydrophobic microenvironments of the cationic lipids [31–33]. The electrochemistry of electrodes modified in this way is stable, reversible, and multiple redox states up to C3 60 can be generated in aqueous solution, thus making the C60 embedded in cationic lipids modified electrodes suitable candidates for the application in the field of electrocatalysis. Herein, the electrocatalytic reduction of H2O2 mediated by C60 trianion embedded in TOAB film modified GCE (C60/TOAB/GCE) is reported, where C60 of 99.9% purity was used, and no ‘‘pretreatment” of C60 modified electrode was conducted. The C60/TOAB/ GCE shows a good electrocatalytic activity for the reduction of H2O2, where the electrocatalysis occurs at the reduction peak for generation of C60 trianion, indicating that C 3 60 mediates the electrocatalytic reduction of H2O2. Potential application of the electrode as an amperometric sensor for the detection of H2O2 is also studied. The results show that the sensor exhibits a fast response to H2O2 and possesses good linear range and high sensitivity. 2. Experimental 2.1. Reagents C60 (99.9%) was purchased from SES Research, Houston, USA Tetra-n-octylammonium bromide (TOAB) (98%) and potassium chloride (KCl) (99.8%) were purchased from Fluka. Hydrogen peroxide (H2O2) (30%) was purchased from Beijing Chemical Works, Beijing, China. Benzonitrile (PhCN) (99%) was purchased from Sigma–Aldrich and distilled over P2O5 under vacuum at 30 °C. Milli-Q water (18 MX cm1) was used for the preparation of solutions.
Fig. 1. SEM images of (a) C60 films; (b) C60/TOAB films.
2.2. Preparation of C60/TOAB/GCE Glassy carbon electrodes (GCE, 3 mm in diameter) were polished with 1.0 lm alumina slurry, ultrasonicated in water and ethanol, and blown dry with high purity nitrogen stream. C60 (0.15 mM) and TOAB (1.5 mM) were mixed and sonicated in PhCN to give a light purple solution. 20 lL of the solution was deposited onto GCE and evaporated in air for about 24 h. The modified electrodes were further dried under nitrogen stream before electrochemical experiments. 2.3. Apparatus Cyclic voltammetry and amperometric i–t curve were carried out on a CHI 630B potentiostat (CH Instruments Inc., Austin, TX, USA) under nitrogen atmosphere at 20 ± 2 °C. Bare or C60/TOAB film modified GCE was used as the working electrode with a platinum wire as the counter electrode and KCl-saturated calomel electrode (SCE) as the reference electrode. A XL30 ESEM FEG scanning electron microscope (SEM) operated at an accelerating voltage of 20 kV was used for morphology characterization. For SEM images, 15–20 lL of solution was deposited on an indium-tin oxidase (ITO) coated glass and dried by evaporation in air.
3. Results and discussion 3.1. Electrocatalytic reduction of H2O2 by the C60/TOAB/GCE The morphology of the C60/TOAB film was first examined since the response of C60/TOAB modified electrode is related to it. Fig. 1 shows the SEM images of C60 and C60/TOAB films prepared by
evaporation of the respective PhCN solution. As shown in Fig. 1a, for the film containing only C60 molecules, large aggregates of C60 are stacked compactly together to form nonhomogeneous three-dimensional structures. While in the case of C60/TOAB film as shown in Fig. 1b, severe aggregation of C60 is inhibited by the addition of TOAB, and C60 molecules are dispersed into much smaller microarray like structures throughout the TOAB matrix. The change of the microenvironment of C60 due to the presence of TOAB may enhance the electron transfer rate for C60 and facilitate the access of analytes to C60. The electrochemical response of the C60/TOAB/GCE is investigated by cyclic voltammetry in the range between 0.3 and 1.5 V vs SCE in deaerated KCl aqueous solution. The peak currents decrease slowly during the initial scans and become almost constant in subsequent scans, indicating a structural rearrangement as observed previously [32,33]. As shown in Fig. 2a, three reversible re 2 2 3 dox couples corresponding to C60 =C 60 , C60 =C60 and C60 =C60 are observed with Epc1 = 0.16 V, Epa1 = 0.00 V, Epc2 = 0.65 V, Epa2 = 0.57 V, Epc3 = 1.29 V and Epa3 = 1.20 V vs SCE. The peak potential separations for both the second and third couples are less than 90 mV, indicating reasonably fast electron transfer kinetics 2 2 3 for both C 60 =C60 and C60 =C60 . The average surface coverage of C60 on the electrode is about 2.6 1010 mol cm2 calculated by inte3 grating the area under the C2 60 =C60 cathodic peaks. After the peak currents became stable, potential scans at different scan rates were conducted. The peak currents, taking the second cathodic peak as shown in Fig. 2b for example, are proportional to the square root of the scan rate in the range of 100–500 mV s1, implying that the electrochemical redox reactions at the C60/TOAB film are diffusion-controlled processes, which is in agreement with the work of Nakashima and Echegoyen [32,33]. The observed
W. Liu, X. Gao / Electrochemistry Communications 10 (2008) 1377–1380
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Fig. 3. Cyclic voltammograms of C60/TOAB/GCE (a) in the absence of H2O2, (b) 0.55 mM H2O2, (c) 1.44 mM H2O2 and (d) 2.67 mM H2O2 in deaerated 0.1 M KCl aqueous solution. Scan rate: 100 mV s1.
Fig. 2. (a) C60/TOAB/GCE in 0.1 M KCl aqueous solution in the range between 0.3 2 and 1.5 V. Scan rate: 100 mV s1. (b) Plot of cathodic peak current (ipc) for C 60 =C60 vs scan rate (v = 100, 150, 200, 250, 300, 350, 400, 500 mV s1).
diffusion-controlled behavior for the C60/TOAB film was attributed to the diffusion of anions (Br, Cl) into and out from the films to maintain electroneutrality during the electrochemical reactions as shown by Echegoyen et al. using electrochemical quartz crystal microbalance (EQCM) technique [33]. Fig. 3 shows the CVs of the C60/TOAB/GCE in the absence and presence of H2O2 in the potential range from 0.30 to 1.50 V. With the addition of H2O2 to the solution, the reductive current at around 1.30 V increases and the corresponding oxidation current decreases at the same time, indicating that H2O2 is electrocatalytically reduced by C3 60 . The reduction current increases proportionally with H2O2 in the concentration range of 4.76 105–2.67 103 mol L1. Notably, the electroreduction currents for the first two peaks (the first peak is not shown in Fig. 3) are unaffected by the addition of H2O2, suggesting that C 60 and C2 60 are incapable of electrocatalytic reduction of H2O2. It has been shown previously that OH is the product for H2O2 reduction [26], while C3 60 is shown to reduce H2O2 electrocatalytically in the current work, a possible mechanism of the electrocatalytic reduction of H2O2 at C60/TOAB film modified GC electrode is therefore proposed as follows:
Fig. 4. (a) Amperometric response of the C60/TOAB/GCE with successive addition of 8.5 mM H2O2 dissolved in 0.1 M KCl into deaerated 5 mL 0.1 M KCl aqueous solution under stirring at 1.34 V vs SCE. (b) Calibration curve of the C60/TOAB/GCE for H2O2 determination in 0.1 M KCl aqueous solution at 1.34 V. Inset: linear calibration curve.
þ þ þ 3 C2 60 ðTOA Þ2 þ TOA þ e C60 ðTOA Þ3
ð1Þ
þ þ þ 2 2C3 60 ðTOA Þ3 þ H2 O2 ! 2C60 ðTOA Þ2 þ 2TOA þ 2OH
XðfilmÞ Xðsol:Þ
ð2Þ
X ¼ Br ; Cl ; OH
ð3Þ
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3.2. Amperometric sensing of H2O2 by the C60/TOAB/GCE
Acknowledgement
The C60/TOAB/GCE was used to construct an amperometric sensor for H2O2. Fig. 4a shows a typical i–t curve for the sensor upon successive step additions of H2O2 into the stirred 0.1 M KCl aqueous solution at an applied potential of 1.34 V. The reduction current rises steeply and achieves 96% of steady–state current within 1 s upon the addition of H2O2. The response time is much shorter than those reported previously [23–26], indicating that the electrocatalytic response is fast. Fig. 4b displays the calibration curve of the sensor. The catalytic currents are proportional to the concentration of H2O2 in the concentration range from 3.33 105 to 2.05 103 mol L1 with a correlation coefficient (R) of 0.9986. The sensitivity is 1.65 lA mM1 and the detection limit is calculated to be 2 105 mol L1 at a signal-to-noise ratio (S/N) of 3. As compared with system of lipid membrane immobilized horseradish peroxidase biosensor [23], the linear range is broader and the sensitivity is higher for the sensing of H2O2 by C60/TOAB/GCE. The repeatability of the current response of the sensor was examined in the presence of 1 103 mol L1 H2O2 with the same electrode. The relative standard deviation (R.S.D.) is 2.0% for seven successive determinations. The sensor can sustain long time storage in ambient atmosphere. The electrocatalytic current of the sensor measured after 1 week storage retains 86% of its activity in the detection of 1 103 mol L1 H2O2.
We thank Prof. Shaojun Dong for helpful discussions. Financial supports from the Hundred Talents Program of Chinese Academy of Sciences, NSFC International Collaboration Grants 00550110367 and 00650110171 are gratefully acknowledged.
4. Conclusion
[18]
Electrocatalytic reduction of H2O2 by C60/TOAB/GCE in aqueous solution has been studied. Unlike some previous reports where C60-mediated electron transfer were claimed to occur in the potential range within which no C60 redox process is observed, and have been shown to be caused by either impurity such as graphite or oxygenated species due to ‘‘pretreatment”, the observed electrocatalysis of H2O2 is one of the very reports where C60 actually mediates the analytical target of interest in aqueous solutions. The results show that the electrocatalytic reduction of H2O2 is mediated by C3 60 , while the mono- and dianion are unable to reduce H2O2. The C60/TOAB/GCE was used as an amperometric sensor for the determination of H2O2, and it shows fast response, broad linear range, and good sensitivity, indicating that C60 trianion is a promising electron transfer mediator for electrocatalytic reduction of H2O2. This work extends the scope of the electrocatalytic properties of fullerenes and sheds light on the promising application of C60/cationic lipid films in constructing novel chemical and biosensors.
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Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
C60 trianion-mediated electrocatalysis and amperometric sensing of hydrogen peroxide Wei Liu, Xiang Gao * State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 4 June 2008 Received in revised form 26 June 2008 Accepted 27 June 2008 Available online 5 July 2008 Keywords: Amperometric sensor Electrocatalytic reduction Fullerenes Hydrogen peroxide Lipid film
a b s t r a c t Heterogeneous electrocatalytic reduction of hydrogen peroxide (H2O2) by C60 is reported for the first time. C60 is embedded in tetraoctylammonium bromide (TOAB) film and is characterized by scanning electron microscopy and cyclic voltammetry. Electrocatalytic studies show that the trianion of C60 mediates the electrocatalytic reduction of H2O2 in aqueous solution containing 0.1 M KCl. Application of such film modified electrode as an amperometric sensor for H2O2 determination is also examined. The sensor shows a fast response within 1 s and a linear response is obtained (R = 0.9986) in the concentration range from 3.33 105 to 2.05 103 mol L1 for H2O2, with the detection limit of 2 105 mol L1 and the sensitivity of 1.65 lA mM1. A good repeatability and stability is shown for the sensor during the experiment. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Due to its unique electrochemical property to undergo six reversible one-electron reductions in nonaqueous solutions [1], C60 has been shown to be a promising electron transfer mediator for various species [2]. For example, it has been shown that anionic C60 has catalytic reducing ability towards halogenated hydrocarbons [3–5], certain organic functional groups [6], and nitrogen fixation [7]. Consequently, electrocatalysis of target analytes with C60 modified electrodes has attracted great interest because C60 is chemically stable, readily available with high purity and ease of preparation for the electrodes, where a large number of literature appears claiming that C60 acts as electron mediator in electrocatalytic processes towards biomolecules including glucose [8], hemoglobin [9], cytochrome c [10], ascorbic acid [11], coenzymes [12], L-cysteine [13] and uric acid [14]. In the mean time, research on using fullerenes as a novel mediator in amperometric biosensors has also been reported [15]. However, recent work by Compton and Banks [16–18] have drawn much attention by raising questions on some of reported studies, and they have shown that the observed electrocatalyses in some cases are caused by either the small amount of graphite impurity in C60 sample, or the oxygenated species formed on the surface of glassy carbon electrodes with ‘‘electrode pretreatment”, while electrocatalysis mediated by C60 is only likely ‘‘where C60 itself becomes oxidized * Corresponding author. Tel./fax: +86 431 85262946. E-mail address: [email protected] (X. Gao). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.06.031
or reduced” [16]. Inspired by the unusual electron transfer and electrocatalytic property of anionic C60, we studied the electrocatalytic reduction of hydrogen peroxide (H2O2) by C60. To the best of our knowledge, no work on the electrocatalytic reduction of H2O2 by C60 has appeared to date. Study on electrocatalytic reduction of H2O2 by C60 is of great interest for several reasons: first, fullerenes have demonstrated a biological antioxidant activity toward various reactive oxygen species [19], while H2O2 is an important biological oxidant produced by enzymatic reactions [20]; second, fullerenes have shown a promising potential in the development of fuel cells [21], where H2O2 is an important intermediate of the electrochemical reactions [22]. In addition, determination of H2O2 is of great importance because it is widely used in areas of environmental, industrial, food, pharmaceutical and clinical analyses. Electrochemical sensing of H2O2 is one of the most promising approaches to monitor H2O2 [23–26]. Chemically modified electrodes with electroactive mediators immobilized on the solid surfaces have been frequently used for determination of H2O2 for the high selectivity, good sensitivity, low cost and ease of construction. The use of C60 for modifying electrode surfaces has attracted much attention [18,27–30], however, the electrochemistry of fullerene films in aqueous media is unstable and irreversible [27,28], which has limited further studies on heterogeneous electrocatalysis by C60 modified electrodes in aqueous solution. Various methods have been explored to solve this problem, among them, C60 embedded in films of cationic lipids such as tetraoctylammonium bromide (TOAB) on electrodes has shown good electrochemical response
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in aqueous system, due to the partial inhibition of the aggregation of C60, the strong binding of electrochemically reduced C60 anions with the matrix lipid cations, and small electrostatic repulsion between the C60 anions in the suitable hydrophobic microenvironments of the cationic lipids [31–33]. The electrochemistry of electrodes modified in this way is stable, reversible, and multiple redox states up to C3 60 can be generated in aqueous solution, thus making the C60 embedded in cationic lipids modified electrodes suitable candidates for the application in the field of electrocatalysis. Herein, the electrocatalytic reduction of H2O2 mediated by C60 trianion embedded in TOAB film modified GCE (C60/TOAB/GCE) is reported, where C60 of 99.9% purity was used, and no ‘‘pretreatment” of C60 modified electrode was conducted. The C60/TOAB/ GCE shows a good electrocatalytic activity for the reduction of H2O2, where the electrocatalysis occurs at the reduction peak for generation of C60 trianion, indicating that C 3 60 mediates the electrocatalytic reduction of H2O2. Potential application of the electrode as an amperometric sensor for the detection of H2O2 is also studied. The results show that the sensor exhibits a fast response to H2O2 and possesses good linear range and high sensitivity. 2. Experimental 2.1. Reagents C60 (99.9%) was purchased from SES Research, Houston, USA Tetra-n-octylammonium bromide (TOAB) (98%) and potassium chloride (KCl) (99.8%) were purchased from Fluka. Hydrogen peroxide (H2O2) (30%) was purchased from Beijing Chemical Works, Beijing, China. Benzonitrile (PhCN) (99%) was purchased from Sigma–Aldrich and distilled over P2O5 under vacuum at 30 °C. Milli-Q water (18 MX cm1) was used for the preparation of solutions.
Fig. 1. SEM images of (a) C60 films; (b) C60/TOAB films.
2.2. Preparation of C60/TOAB/GCE Glassy carbon electrodes (GCE, 3 mm in diameter) were polished with 1.0 lm alumina slurry, ultrasonicated in water and ethanol, and blown dry with high purity nitrogen stream. C60 (0.15 mM) and TOAB (1.5 mM) were mixed and sonicated in PhCN to give a light purple solution. 20 lL of the solution was deposited onto GCE and evaporated in air for about 24 h. The modified electrodes were further dried under nitrogen stream before electrochemical experiments. 2.3. Apparatus Cyclic voltammetry and amperometric i–t curve were carried out on a CHI 630B potentiostat (CH Instruments Inc., Austin, TX, USA) under nitrogen atmosphere at 20 ± 2 °C. Bare or C60/TOAB film modified GCE was used as the working electrode with a platinum wire as the counter electrode and KCl-saturated calomel electrode (SCE) as the reference electrode. A XL30 ESEM FEG scanning electron microscope (SEM) operated at an accelerating voltage of 20 kV was used for morphology characterization. For SEM images, 15–20 lL of solution was deposited on an indium-tin oxidase (ITO) coated glass and dried by evaporation in air.
3. Results and discussion 3.1. Electrocatalytic reduction of H2O2 by the C60/TOAB/GCE The morphology of the C60/TOAB film was first examined since the response of C60/TOAB modified electrode is related to it. Fig. 1 shows the SEM images of C60 and C60/TOAB films prepared by
evaporation of the respective PhCN solution. As shown in Fig. 1a, for the film containing only C60 molecules, large aggregates of C60 are stacked compactly together to form nonhomogeneous three-dimensional structures. While in the case of C60/TOAB film as shown in Fig. 1b, severe aggregation of C60 is inhibited by the addition of TOAB, and C60 molecules are dispersed into much smaller microarray like structures throughout the TOAB matrix. The change of the microenvironment of C60 due to the presence of TOAB may enhance the electron transfer rate for C60 and facilitate the access of analytes to C60. The electrochemical response of the C60/TOAB/GCE is investigated by cyclic voltammetry in the range between 0.3 and 1.5 V vs SCE in deaerated KCl aqueous solution. The peak currents decrease slowly during the initial scans and become almost constant in subsequent scans, indicating a structural rearrangement as observed previously [32,33]. As shown in Fig. 2a, three reversible re 2 2 3 dox couples corresponding to C60 =C 60 , C60 =C60 and C60 =C60 are observed with Epc1 = 0.16 V, Epa1 = 0.00 V, Epc2 = 0.65 V, Epa2 = 0.57 V, Epc3 = 1.29 V and Epa3 = 1.20 V vs SCE. The peak potential separations for both the second and third couples are less than 90 mV, indicating reasonably fast electron transfer kinetics 2 2 3 for both C 60 =C60 and C60 =C60 . The average surface coverage of C60 on the electrode is about 2.6 1010 mol cm2 calculated by inte3 grating the area under the C2 60 =C60 cathodic peaks. After the peak currents became stable, potential scans at different scan rates were conducted. The peak currents, taking the second cathodic peak as shown in Fig. 2b for example, are proportional to the square root of the scan rate in the range of 100–500 mV s1, implying that the electrochemical redox reactions at the C60/TOAB film are diffusion-controlled processes, which is in agreement with the work of Nakashima and Echegoyen [32,33]. The observed
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Fig. 3. Cyclic voltammograms of C60/TOAB/GCE (a) in the absence of H2O2, (b) 0.55 mM H2O2, (c) 1.44 mM H2O2 and (d) 2.67 mM H2O2 in deaerated 0.1 M KCl aqueous solution. Scan rate: 100 mV s1.
Fig. 2. (a) C60/TOAB/GCE in 0.1 M KCl aqueous solution in the range between 0.3 2 and 1.5 V. Scan rate: 100 mV s1. (b) Plot of cathodic peak current (ipc) for C 60 =C60 vs scan rate (v = 100, 150, 200, 250, 300, 350, 400, 500 mV s1).
diffusion-controlled behavior for the C60/TOAB film was attributed to the diffusion of anions (Br, Cl) into and out from the films to maintain electroneutrality during the electrochemical reactions as shown by Echegoyen et al. using electrochemical quartz crystal microbalance (EQCM) technique [33]. Fig. 3 shows the CVs of the C60/TOAB/GCE in the absence and presence of H2O2 in the potential range from 0.30 to 1.50 V. With the addition of H2O2 to the solution, the reductive current at around 1.30 V increases and the corresponding oxidation current decreases at the same time, indicating that H2O2 is electrocatalytically reduced by C3 60 . The reduction current increases proportionally with H2O2 in the concentration range of 4.76 105–2.67 103 mol L1. Notably, the electroreduction currents for the first two peaks (the first peak is not shown in Fig. 3) are unaffected by the addition of H2O2, suggesting that C 60 and C2 60 are incapable of electrocatalytic reduction of H2O2. It has been shown previously that OH is the product for H2O2 reduction [26], while C3 60 is shown to reduce H2O2 electrocatalytically in the current work, a possible mechanism of the electrocatalytic reduction of H2O2 at C60/TOAB film modified GC electrode is therefore proposed as follows:
Fig. 4. (a) Amperometric response of the C60/TOAB/GCE with successive addition of 8.5 mM H2O2 dissolved in 0.1 M KCl into deaerated 5 mL 0.1 M KCl aqueous solution under stirring at 1.34 V vs SCE. (b) Calibration curve of the C60/TOAB/GCE for H2O2 determination in 0.1 M KCl aqueous solution at 1.34 V. Inset: linear calibration curve.
þ þ þ 3 C2 60 ðTOA Þ2 þ TOA þ e C60 ðTOA Þ3
ð1Þ
þ þ þ 2 2C3 60 ðTOA Þ3 þ H2 O2 ! 2C60 ðTOA Þ2 þ 2TOA þ 2OH
XðfilmÞ Xðsol:Þ
ð2Þ
X ¼ Br ; Cl ; OH
ð3Þ
1380
W. Liu, X. Gao / Electrochemistry Communications 10 (2008) 1377–1380
3.2. Amperometric sensing of H2O2 by the C60/TOAB/GCE
Acknowledgement
The C60/TOAB/GCE was used to construct an amperometric sensor for H2O2. Fig. 4a shows a typical i–t curve for the sensor upon successive step additions of H2O2 into the stirred 0.1 M KCl aqueous solution at an applied potential of 1.34 V. The reduction current rises steeply and achieves 96% of steady–state current within 1 s upon the addition of H2O2. The response time is much shorter than those reported previously [23–26], indicating that the electrocatalytic response is fast. Fig. 4b displays the calibration curve of the sensor. The catalytic currents are proportional to the concentration of H2O2 in the concentration range from 3.33 105 to 2.05 103 mol L1 with a correlation coefficient (R) of 0.9986. The sensitivity is 1.65 lA mM1 and the detection limit is calculated to be 2 105 mol L1 at a signal-to-noise ratio (S/N) of 3. As compared with system of lipid membrane immobilized horseradish peroxidase biosensor [23], the linear range is broader and the sensitivity is higher for the sensing of H2O2 by C60/TOAB/GCE. The repeatability of the current response of the sensor was examined in the presence of 1 103 mol L1 H2O2 with the same electrode. The relative standard deviation (R.S.D.) is 2.0% for seven successive determinations. The sensor can sustain long time storage in ambient atmosphere. The electrocatalytic current of the sensor measured after 1 week storage retains 86% of its activity in the detection of 1 103 mol L1 H2O2.
We thank Prof. Shaojun Dong for helpful discussions. Financial supports from the Hundred Talents Program of Chinese Academy of Sciences, NSFC International Collaboration Grants 00550110367 and 00650110171 are gratefully acknowledged.
4. Conclusion
[18]
Electrocatalytic reduction of H2O2 by C60/TOAB/GCE in aqueous solution has been studied. Unlike some previous reports where C60-mediated electron transfer were claimed to occur in the potential range within which no C60 redox process is observed, and have been shown to be caused by either impurity such as graphite or oxygenated species due to ‘‘pretreatment”, the observed electrocatalysis of H2O2 is one of the very reports where C60 actually mediates the analytical target of interest in aqueous solutions. The results show that the electrocatalytic reduction of H2O2 is mediated by C3 60 , while the mono- and dianion are unable to reduce H2O2. The C60/TOAB/GCE was used as an amperometric sensor for the determination of H2O2, and it shows fast response, broad linear range, and good sensitivity, indicating that C60 trianion is a promising electron transfer mediator for electrocatalytic reduction of H2O2. This work extends the scope of the electrocatalytic properties of fullerenes and sheds light on the promising application of C60/cationic lipid films in constructing novel chemical and biosensors.
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