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Electrochemical properties of catechin at a single-walled carbon nanotubes–cetylramethylammonium bromide modified electrode
Bioelectrochemistry 75 (2009) 158–162
Contents lists available at ScienceDirect
Bioelectrochemistry j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / b i o e l e c h e m
Electrochemical properties of catechin at a single-walled carbon nanotubes–cetylramethylammonium bromide modified electrode Li-Jun Yang, Cheng Tang, Hua-Yu Xiong, Xiu-Hua Zhang, Sheng-Fu Wang ⁎ Ministry of Education, Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, Wuhan 430062, PR China College of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, PR China
a r t i c l e
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Article history: Received 8 January 2009 Received in revised form 16 March 2009 Accepted 18 March 2009 Available online 26 March 2009 Keywords: Catechin Single-walled carbon nanotubes Cetylramethylammonium bromide Determination
a b s t r a c t The electrochemical properties of catechin at single-walled carbon nanotubes (SWNTs)–cetylramethylammonium bromide (CTAB) modified glassy carbon electrodes (GCE) were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The SWNTs–CTAB film was characterized with transmission electron microscopy (TEM). A series of parameters including pH of supporting electrolyte, accumulation potential and accumulation time were optimized. There were three peaks of catechin at SWNTs–CTAB/GCE in the potential range of − 0.4–1.0 V in PBS (pH 7.0): a reversible pair of peaks and an irreversible peak of the anodic peak. The reductive peak current increased linearly with the concentration of catechin in the range from 3.72 × 10− 10 to 2.38 × 10− 9 M. The detection limit was 1.12 × 10− 10 M. The SWNTs–CTAB/GCE showed good stability and low detection limit, and could be applied to detect trace catechin. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Flavonoids are a group of naturally occurring polyphenolic compounds [1], which are well-known antioxidants due to their radicalscavenging abilities [2]. They are benzo-γ-pyrone derivatives containing several hydroxyl groups attached to the C6–C3–C6 ring and are found extensively in nature, in seeds, fruits and vegetables [3]. The antioxidant activities of flavonoids are much higher than that of vitamin C and E. Catechin is the most important members of flavonoids. It has been acknowledged to be an antioxidant and radical scavenger [4]. Therefore, catechin has been linked with many health benefits including prevention of DNA damage due to oxidation and improvement in blood flow and liver function [5]. On the other hand, catechin is susceptible to autoxidation to generate free radical intermediates, and active oxygen species, such as O•− 2 and hydrogen peroxide (H2O2) to induce fatty acid peroxidation [6], DNA damage and diseases [7]. Catechin has been separated by high-performance liquid chromatography (HPLC) from Oolong tea, green tea, black tea [8], red wine vinegar [9] and the seeds of red grape [10]. Fig. 1 shows the structure of catechin. It is constituted by benzene ring A and B. There is much evidence that the catechol group in B ring is the antioxidant active moiety [11]. Traditional catechin analysis is mainly carried out by instrumental analysis, such as thin-layer chromatography (TLC) [12], capillary electrophoresis (CE) [13,14], HPLC-UV [15]. However, such analysis is generally performed at centralized laboratories, requiring extensive labor and analytical resources, and often results in a lengthy turn⁎ Corresponding author. Tel.: +86 27 50865309; fax: +86 27 88663043. E-mail address: [email protected] (S.-F. Wang). 1567-5394/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.bioelechem.2009.03.009
around time [16]. Thus it is very important to establish a simple, fast, sensitive and low cost method for monitoring catechin. The electrochemical analysis has many advantages over conventional methods [17]. And the sensitivity and selectivity of electrochemical analysis can be enhanced using chemically modified electrodes [11]. Hence, various electrochemical modified electrodes have been developed for the determination of catechin [18–20]. Wu et al. successfully investigated the characteristics of electrochemical reduction of the intermediate produced in the process of catechin autoxidation using Ru(bpy)3+ 3 -modified oxidized BDD electrode [7]. Jarosz-Wilkołazka and co-workers immobilized laccase on the surface of graphite electrode for determination of catechin [18]. El-Hady fabricated HP-β-CD incorporated carbon paste modified electrode for simple monitoring of catechin in some commercial drinks and biological fluids [20]. Moreover, in the previous works, the oxidation mechanisms of catechin have been explored [19,21] via electrochemical methods. All these methods could not reach a low enough detection limit. And the way to fabricate the modified electrodes was comparably complicated. Therefore, a low and simpler analytical method is urgently required. Carbon nanotubes (CNTs) were discovered via transmission electron microscope (TEM) by Iijima in 1991 [22]. They exhibit a combination of electrical, mechanical, thermal, optical and structural properties that make them potentially important components of optoelectronic, high-strength and biomedical materials [23]. CNTs can behave as conductor or semiconductor, depending on the atomic structure, which present a broad range of potential application such as gas reservoirs, battery electrodes, and field emission displays [24,25]. The subtle electronic properties suggest that carbon nanotubes have the ability to promote electron-transfer reactions when used as an
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chemical workstation (CH instrument, USA), which was controlled by a personal computer. A three-electrode system was used in the measurements, with a bare GCE (3 mm diameter) or a SWNTs–CTAB/ GCE as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum wire as the auxiliary electrode. TEM was performed with a TEM-100CX11 (JEOL, Tokyo, Japan) operating at 200 kV. Ultrasonic cleaner (Branson2000, USA) and 320-S acidometer (Mettler-Toledo, Switzerland) were used in this experiment. All the data were obtained at room temperature. Fig. 1. Molecular structure of catechin.
electrode material in electrochemical reactions [26]. Contrasted with multi-wall carbon nanotubes, SWNT is a well-defined system in terms of electronic properties [27]. Individual SWNTs can be regarded as quantum wires, and SWNTs are the best systems for investigating intrinsic CNT properties because of their structural simplicity [28]. Compared with other carbon nanotube electrodes, the preparation of the SWNT-modified electrode was economical, simple, and convenient [25]. Thus, SWNTs modified electrodes have been used to detect large numbers of compounds [29,30]. The aim of this study is to explore a low cost and sensitive electrochemical method to detect catechin. The glassy carbon electrode (GCE) modified with SWNTs–cetylramethylammonium bromide (CTAB), which exhibited obvious electrocatalytic activities toward the redox of catechin. It has been found that the SWNTs–CTAB has strong accumulation effect on catechin. The electrochemical behaviors of catechin were explored in details. 2. Experimental 2.1. Chemicals and reagents Catechin was purchased from Shanghai U-sea Biotech Co., Ltd. (Shanghai, China). SWNTs were obtained from Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences (Chengdu, China). CTAB (analytical grade) was purchased from Lingfeng Chemical Reagent Co. (LTD., Shanghai, China). Phosphate buffer was comprised with Na2HPO4 and NaH2PO4. The pH value was adjusted with NaOH and H3PO4. Catechin solution was prepared with ethanol absolute. All other reagents were of analytical grade. All solutions were prepared with double-distilled water. 2.2. Apparatus Cyclic voltammetric (CV) and differential pulse voltammetric (DPV) measurements were performed by a model CHI 660A electro-
Fig. 2. TEM image of the SWNTs in the presence of CTAB.
2.3. Preparation of the SWNTs–CTAB modified electrode SWNTs crude material was agitated in an ultrasonic bath with 3 M nitric acid for an hour. Then it was refluxed in 5 M HCl solution for 4 h at 90 °C. After acid treatment, SWNTs were calcined in static air for 2 h at 90 °C. Here, SWNTs (0.5 mg) were dispersed in 1 mL CTAB solution (5 mg mL− 1) and the mixture was agitated in an ultrasonic bath for an hour to achieve a well-dispersed suspension. A cleaned GCE was coated by casting 10 µL of the suspension of SWNTs–CTAB and dried in the air for 8 h to remove the solvent. The effective surface areas of both bare and SWNTs–CTAB modified electrodes were obtained by CV with 1 mM K3Fe(CN)6 as a probe at different scan rates (T = 298 K). For a reversible process, the following equation applies: 5
3=2
ip = 2:69 × 10 An
1= 2
1=2
D R co v
ð1Þ
where ip refers to the peak current (A) and A is the electrode area (cm2). For 1 mM K3Fe(CN)6, n = 1, DR = 7.6 × 10− 6 cm2 s− 1 (0.1 M KCl) [17], co is the concentration of K3Fe(CN)6 (M), ν is the scan rate (V s− 1) and the unit of constant is C mol− 1 V− 1/2 [31], then the surface areas of the bare GCE and SWNTs–CTAB/GCE electrodes can be calculated from the slope of the ip versus ν1/2 plot to be 0.06191 cm2 and 0.1200 cm2, respectively. The effective working area is almost 2 times larger than that of the bare GCE. This proved that the bare electrode was modified efficiently by SWNTs–CTAB film. 3. Results and discussion 3.1. Characterization of the SWNTs–CTAB film TEM was used to confirm the morphologies of SWNTs–CTAB film. Fig. 2 shows the typical TEM image of the SWNTs. In the presence of CTAB, SWNTs were distributed evenly on the grids. TEM micrograph of SWNTs–CTAB film showed clearly tubular structures of SWNTs, which was symmetrical and simplex.
Fig. 3. Cyclic voltammograms of 2.5 × 10− 5 M catechin in PBS (pH 7.0), on a bare electrode (a) and SWNTs–CTAB/GCE (c). Absence of catechin in solution at SWNTs– CTAB/GCE is also shown (b). Scan rate: 0.05 V s− 1. Inset: Close-up of curve (a).
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3.2. Cyclic voltammetric behaviors of catechin at SWNTs–CTAB/GCE The electrochemical behaviors of catechin at SWNTs–CTAB/GCE were studied by CV. There was no redox peak observed in the absence of catechin at SWNTs–CTAB/GCE (Fig. 3b), suggesting the SWNTs– CTAB/GCE was nonelectroactive in the potential range of −0.4–1.0 V in PBS (pH 7.0). In 2.5 × 10− 5 M catechin solution, it exhibited three peaks at SWNTs–CTAB/GCE: a reversible pair of redox peaks (I) with ΔEp (the difference between the anodic peak potential and the cathodic peak potential) valued of 47 mV (EpcI = 0.151 V, EpaI = 0.198 V), and an irreversible oxidation (II) with a potential of 0.527 V (Fig. 3c). The bare electrode showed an irreversible oxidative peak and an irreversible redox peak with a ΔEp = 144 mV (EpcI = 0.088 V, EpaI = 0.232 V) (Fig. 3a and Inset). It's obviously that the reversibility of catechin was significantly improved at SWNTs–CTAB/GCE. The redox peak current was 10 times higher than that on the bare GCE. Considering the working area of the SWNTs–CTAB/GCE was only two times larger than that of the bare GCE, such current enhancement (10 times) may be due to not only the big working area of the SWNTs–CTAB/GCE, but also electrocatalysis of SWNTs–CTAB for catechin. These indicated that SWNTs could reduce the overpotential of catechin and greatly accelerate electron transfer rate, which due to their subtle electronic properties [25]. Furthermore, the background currents were higher than that of the film without SWNTs, which implied that SWNTs could increase the effective area of the electrode [32]. 3.3. Effects of pH on the reaction A CV study of catechin was performed over the pH range from 4.5 to 9.0 (Fig. 4). As can be seen, the ΔEp of the couple redox peaks reached its minimum at pH 7.0 PBS, namely the redox mechanism was approaching reversible reaction. Thus, pH = 7.0 was chosen in the following experiments as the best pH condition. In addition, the three peaks shift negatively with increasing of pH, suggesting that protons participate in the reaction. The equations between peak potential and pH values were: EpcI (V) = 0.3859–0.03792 pH, r = 0.996; EpaI (V) = 0.6497–0.06155 pH, r = 0.992; EpII (V) = 1.1134–0.07736 pH, r = 0.991. It indicated that the total number of electrons and protons taking part in the charge transfer was the same. The results were consistent with the previous studies [11,21,33].
Fig. 5. (A): Cyclic voltammograms of 2.5 × 10− 5 M catechin in PBS (pH 7.0) with different scan rates υ (V s−1): 0.01‚ 0.02‚ 0.03‚ 0.05‚ 0.08‚ 0.1‚ 0.12‚ 0.15‚ 0.18‚ 0.2‚ 0.23‚ 0.25‚ 0.28 and 0.3. (B): The linear relationship of ipI, ipII vs. υ.
2.45× 10− 5 υ (V s− 1), r =0.995. This indicated that the reaction of catechin at SWNTs–CTAB/GCE was a surface controlled process.
3.4. The influence of scan rate
3.5. Effect of accumulation potential and accumulation time
The influence of scan rates on the redox peaks current of catechin was studied using CV. The peak current increased with the scan rate in the range of 0.01–0.3 V s− 1 (Fig. 5). It was linear with υ: ipcI (A) = −1.12× 10− 6–4.37× 10− 5 υ (V s− 1), r =0.998; ipaI (A) =5.08 ×10− 6 +
Since the reversible reaction of catechin on SWNTs–CTAB/GCE was a surface controlled process, the accumulation potential and time had strong influence on the redox peak currents. Fig. 6 exhibits the effect of accumulation time (A) and potential (B) upon the cathodic currents of 2.5 × 10− 5 M catechin. It can be seen that the cathodic current increased with an increment in accumulation time. Then the current leveled off above 90 s (Fig. 6A). Therefore, a 90 s accumulation time was chosen. While the cathodic current increased with increasing accumulation potential at the range of 0.2 V–0.4 V, then decreased between 0.4 V and 0.5 V (Fig. 6B). Hence, a 0.4 V was used in the subsequent experiments. 3.6. Analytical applications
Fig. 4. Cyclic voltammograms of 2.5 × 10− 5 M catechin in PBS at different pH values: 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, and 9.0. Inset: plot of peak potential vs. pH.
The sensitivity of DPV is much higher than CV, and has been used for quantitative measurements. Fig. 7 shows the typical DPV for catechin in different concentrations. It was observed that well-defined cathodic peak currents increased with concentration of catechin in the range from 3.72 × 10− 10 to 2.38 × 10− 9 M. The linear regression equation was ipc (A) = 2.339× 103c (M)−8.90701 × 10− 8, with a correlation coefficient of 0.998 (n = 12). The detection limit is 1.12 × 10− 10 M (S/N = 3), which is much lower than that reported with laccase-modified graphite electrode (4.34 × 10− 6 M) [18] and self-assembled monolayers of a
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Table 1 Determination of catechin. Samples (1010 c/M)
Added (1010 c/M)
Found (109 c/M)
Recoveries (%)
8.00 8.00 8.00
4.29 7.92 9.11
1.24 1.53 1.67
102.6 92.2 95.5
In order to evaluate the validity of the proposed method, the recovery test was carried out by adding different known amounts of catechin into the pure solution of the catechin. As shown in Table 1, the recoveries were from 92.2% to 102.6%, indicating that the method was reliable for the quantitative determination of catechin. 3.7. Reproducibility and stability The reproducibility of the same SWNTs–CTAB/GCE was examined by the detection of 2.5 × 10− 5 M catechin in PBS (pH 7.0). A relation standard deviation (R.S.D.) value of 2.40% was obtained for five successive determinations, which showed that the SWNTs–CTAB/GCE had good reproducibility. The fabrication reproducibility was estimated with five different electrodes, which were constructed independently by the same procedure. R.S.D. was 5.41% for the steady-state current to 2.5 × 10− 5 M catechin, which demonstrated the reliability of the fabrication procedure. The stability of the SWNTs– CTAB/GCE was also investigated. The cathodic peak current of catechin on the modified electrode decreased only 14.4% of its original response after 3 weeks, which demonstrated that the SWNTs–CTAB/GCE had excellent stability. It was much more stable than self-assembled monolayers of a nickel (II) complex and 3-mercaptopropionic acid on a gold electrode [34], which showed apparent loss of activity after 20 days.
Fig. 6. (A) Influence of accumulation time on the cathodic peak current of 2.5 × 10− 5 M catechin in PBS (pH 7.0). Accumulation potential: 0.4 V. (B) Influence of accumulation potential on the cathodic peak current of 2.5 × 10− 5 M catechin in PBS (pH 7.0). Accumulation time: 90 s.
nickel (II) complex and 3-mercaptopropionic acid on a gold electrode (8.26 × 10− 7 M) [34]. The low detection limit was due to not only the big working area of the SWNTs–CTAB/GCE, but also adsorption of catechin at the SWNTs.
3.8. Interference with the determination The effects of some foreign species on the detection of 1.0 × 10− 6 M catechin in PBS (pH 7.0) were evaluated. We can conclude that 500 2− 2− folds higher levels of glucose, Gly, K+, Na+, Cl−, NO− 3 , CO3 , SO4 and Ac− did not interfere with the determination of catechin. 4. Conclusions SWNTs–CTAB/GCE were prepared and had been used to investigate the electrochemical behaviors of catechin. A reversible pair of redox peaks and an irreversible oxidative peak was obtained at the SWNTs–CTAB/GCE. The results showed SWNTs could enhance working area of the electrode, and efficiently accelerate electron transfer rate for catechin. The electrochemical behavior of catechin at the modified electrode is a surface-controlled electrochemical process. The outcome of the modified electrode exhibits the linear range of 3.72 × 10− 10–2.38 × 10− 9 M, especially low detection limit of 1.12 × 10− 10 M. The low detection limit obtained here indicates that SWNTs–CTAB/GCE is suitable for the determination of trace catechin with good reproducibility and stability. References
Fig. 7. DPVs of catechin on SWNTs–CTAB/GCE at different concentration (a → l): 3.72 × 10− 10, 4.34 × 10− 10, 4.95 × 10− 10, 5.56 × 10− 10, 6.78 × 10− 10, 9.20 × 10− 10, 1.10 × 10− 9, 1.34 × 10− 9, 1.57 × 10− 9, 1.81 × 10− 9, 2.04 × 10− 9, 2.38 × 10− 9 M. Inset: plot of cathodic peak current vs. concentration of catechin.
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Bioelectrochemistry j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / b i o e l e c h e m
Electrochemical properties of catechin at a single-walled carbon nanotubes–cetylramethylammonium bromide modified electrode Li-Jun Yang, Cheng Tang, Hua-Yu Xiong, Xiu-Hua Zhang, Sheng-Fu Wang ⁎ Ministry of Education, Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, Wuhan 430062, PR China College of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, PR China
a r t i c l e
i n f o
Article history: Received 8 January 2009 Received in revised form 16 March 2009 Accepted 18 March 2009 Available online 26 March 2009 Keywords: Catechin Single-walled carbon nanotubes Cetylramethylammonium bromide Determination
a b s t r a c t The electrochemical properties of catechin at single-walled carbon nanotubes (SWNTs)–cetylramethylammonium bromide (CTAB) modified glassy carbon electrodes (GCE) were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The SWNTs–CTAB film was characterized with transmission electron microscopy (TEM). A series of parameters including pH of supporting electrolyte, accumulation potential and accumulation time were optimized. There were three peaks of catechin at SWNTs–CTAB/GCE in the potential range of − 0.4–1.0 V in PBS (pH 7.0): a reversible pair of peaks and an irreversible peak of the anodic peak. The reductive peak current increased linearly with the concentration of catechin in the range from 3.72 × 10− 10 to 2.38 × 10− 9 M. The detection limit was 1.12 × 10− 10 M. The SWNTs–CTAB/GCE showed good stability and low detection limit, and could be applied to detect trace catechin. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Flavonoids are a group of naturally occurring polyphenolic compounds [1], which are well-known antioxidants due to their radicalscavenging abilities [2]. They are benzo-γ-pyrone derivatives containing several hydroxyl groups attached to the C6–C3–C6 ring and are found extensively in nature, in seeds, fruits and vegetables [3]. The antioxidant activities of flavonoids are much higher than that of vitamin C and E. Catechin is the most important members of flavonoids. It has been acknowledged to be an antioxidant and radical scavenger [4]. Therefore, catechin has been linked with many health benefits including prevention of DNA damage due to oxidation and improvement in blood flow and liver function [5]. On the other hand, catechin is susceptible to autoxidation to generate free radical intermediates, and active oxygen species, such as O•− 2 and hydrogen peroxide (H2O2) to induce fatty acid peroxidation [6], DNA damage and diseases [7]. Catechin has been separated by high-performance liquid chromatography (HPLC) from Oolong tea, green tea, black tea [8], red wine vinegar [9] and the seeds of red grape [10]. Fig. 1 shows the structure of catechin. It is constituted by benzene ring A and B. There is much evidence that the catechol group in B ring is the antioxidant active moiety [11]. Traditional catechin analysis is mainly carried out by instrumental analysis, such as thin-layer chromatography (TLC) [12], capillary electrophoresis (CE) [13,14], HPLC-UV [15]. However, such analysis is generally performed at centralized laboratories, requiring extensive labor and analytical resources, and often results in a lengthy turn⁎ Corresponding author. Tel.: +86 27 50865309; fax: +86 27 88663043. E-mail address: [email protected] (S.-F. Wang). 1567-5394/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.bioelechem.2009.03.009
around time [16]. Thus it is very important to establish a simple, fast, sensitive and low cost method for monitoring catechin. The electrochemical analysis has many advantages over conventional methods [17]. And the sensitivity and selectivity of electrochemical analysis can be enhanced using chemically modified electrodes [11]. Hence, various electrochemical modified electrodes have been developed for the determination of catechin [18–20]. Wu et al. successfully investigated the characteristics of electrochemical reduction of the intermediate produced in the process of catechin autoxidation using Ru(bpy)3+ 3 -modified oxidized BDD electrode [7]. Jarosz-Wilkołazka and co-workers immobilized laccase on the surface of graphite electrode for determination of catechin [18]. El-Hady fabricated HP-β-CD incorporated carbon paste modified electrode for simple monitoring of catechin in some commercial drinks and biological fluids [20]. Moreover, in the previous works, the oxidation mechanisms of catechin have been explored [19,21] via electrochemical methods. All these methods could not reach a low enough detection limit. And the way to fabricate the modified electrodes was comparably complicated. Therefore, a low and simpler analytical method is urgently required. Carbon nanotubes (CNTs) were discovered via transmission electron microscope (TEM) by Iijima in 1991 [22]. They exhibit a combination of electrical, mechanical, thermal, optical and structural properties that make them potentially important components of optoelectronic, high-strength and biomedical materials [23]. CNTs can behave as conductor or semiconductor, depending on the atomic structure, which present a broad range of potential application such as gas reservoirs, battery electrodes, and field emission displays [24,25]. The subtle electronic properties suggest that carbon nanotubes have the ability to promote electron-transfer reactions when used as an
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chemical workstation (CH instrument, USA), which was controlled by a personal computer. A three-electrode system was used in the measurements, with a bare GCE (3 mm diameter) or a SWNTs–CTAB/ GCE as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum wire as the auxiliary electrode. TEM was performed with a TEM-100CX11 (JEOL, Tokyo, Japan) operating at 200 kV. Ultrasonic cleaner (Branson2000, USA) and 320-S acidometer (Mettler-Toledo, Switzerland) were used in this experiment. All the data were obtained at room temperature. Fig. 1. Molecular structure of catechin.
electrode material in electrochemical reactions [26]. Contrasted with multi-wall carbon nanotubes, SWNT is a well-defined system in terms of electronic properties [27]. Individual SWNTs can be regarded as quantum wires, and SWNTs are the best systems for investigating intrinsic CNT properties because of their structural simplicity [28]. Compared with other carbon nanotube electrodes, the preparation of the SWNT-modified electrode was economical, simple, and convenient [25]. Thus, SWNTs modified electrodes have been used to detect large numbers of compounds [29,30]. The aim of this study is to explore a low cost and sensitive electrochemical method to detect catechin. The glassy carbon electrode (GCE) modified with SWNTs–cetylramethylammonium bromide (CTAB), which exhibited obvious electrocatalytic activities toward the redox of catechin. It has been found that the SWNTs–CTAB has strong accumulation effect on catechin. The electrochemical behaviors of catechin were explored in details. 2. Experimental 2.1. Chemicals and reagents Catechin was purchased from Shanghai U-sea Biotech Co., Ltd. (Shanghai, China). SWNTs were obtained from Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences (Chengdu, China). CTAB (analytical grade) was purchased from Lingfeng Chemical Reagent Co. (LTD., Shanghai, China). Phosphate buffer was comprised with Na2HPO4 and NaH2PO4. The pH value was adjusted with NaOH and H3PO4. Catechin solution was prepared with ethanol absolute. All other reagents were of analytical grade. All solutions were prepared with double-distilled water. 2.2. Apparatus Cyclic voltammetric (CV) and differential pulse voltammetric (DPV) measurements were performed by a model CHI 660A electro-
Fig. 2. TEM image of the SWNTs in the presence of CTAB.
2.3. Preparation of the SWNTs–CTAB modified electrode SWNTs crude material was agitated in an ultrasonic bath with 3 M nitric acid for an hour. Then it was refluxed in 5 M HCl solution for 4 h at 90 °C. After acid treatment, SWNTs were calcined in static air for 2 h at 90 °C. Here, SWNTs (0.5 mg) were dispersed in 1 mL CTAB solution (5 mg mL− 1) and the mixture was agitated in an ultrasonic bath for an hour to achieve a well-dispersed suspension. A cleaned GCE was coated by casting 10 µL of the suspension of SWNTs–CTAB and dried in the air for 8 h to remove the solvent. The effective surface areas of both bare and SWNTs–CTAB modified electrodes were obtained by CV with 1 mM K3Fe(CN)6 as a probe at different scan rates (T = 298 K). For a reversible process, the following equation applies: 5
3=2
ip = 2:69 × 10 An
1= 2
1=2
D R co v
ð1Þ
where ip refers to the peak current (A) and A is the electrode area (cm2). For 1 mM K3Fe(CN)6, n = 1, DR = 7.6 × 10− 6 cm2 s− 1 (0.1 M KCl) [17], co is the concentration of K3Fe(CN)6 (M), ν is the scan rate (V s− 1) and the unit of constant is C mol− 1 V− 1/2 [31], then the surface areas of the bare GCE and SWNTs–CTAB/GCE electrodes can be calculated from the slope of the ip versus ν1/2 plot to be 0.06191 cm2 and 0.1200 cm2, respectively. The effective working area is almost 2 times larger than that of the bare GCE. This proved that the bare electrode was modified efficiently by SWNTs–CTAB film. 3. Results and discussion 3.1. Characterization of the SWNTs–CTAB film TEM was used to confirm the morphologies of SWNTs–CTAB film. Fig. 2 shows the typical TEM image of the SWNTs. In the presence of CTAB, SWNTs were distributed evenly on the grids. TEM micrograph of SWNTs–CTAB film showed clearly tubular structures of SWNTs, which was symmetrical and simplex.
Fig. 3. Cyclic voltammograms of 2.5 × 10− 5 M catechin in PBS (pH 7.0), on a bare electrode (a) and SWNTs–CTAB/GCE (c). Absence of catechin in solution at SWNTs– CTAB/GCE is also shown (b). Scan rate: 0.05 V s− 1. Inset: Close-up of curve (a).
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3.2. Cyclic voltammetric behaviors of catechin at SWNTs–CTAB/GCE The electrochemical behaviors of catechin at SWNTs–CTAB/GCE were studied by CV. There was no redox peak observed in the absence of catechin at SWNTs–CTAB/GCE (Fig. 3b), suggesting the SWNTs– CTAB/GCE was nonelectroactive in the potential range of −0.4–1.0 V in PBS (pH 7.0). In 2.5 × 10− 5 M catechin solution, it exhibited three peaks at SWNTs–CTAB/GCE: a reversible pair of redox peaks (I) with ΔEp (the difference between the anodic peak potential and the cathodic peak potential) valued of 47 mV (EpcI = 0.151 V, EpaI = 0.198 V), and an irreversible oxidation (II) with a potential of 0.527 V (Fig. 3c). The bare electrode showed an irreversible oxidative peak and an irreversible redox peak with a ΔEp = 144 mV (EpcI = 0.088 V, EpaI = 0.232 V) (Fig. 3a and Inset). It's obviously that the reversibility of catechin was significantly improved at SWNTs–CTAB/GCE. The redox peak current was 10 times higher than that on the bare GCE. Considering the working area of the SWNTs–CTAB/GCE was only two times larger than that of the bare GCE, such current enhancement (10 times) may be due to not only the big working area of the SWNTs–CTAB/GCE, but also electrocatalysis of SWNTs–CTAB for catechin. These indicated that SWNTs could reduce the overpotential of catechin and greatly accelerate electron transfer rate, which due to their subtle electronic properties [25]. Furthermore, the background currents were higher than that of the film without SWNTs, which implied that SWNTs could increase the effective area of the electrode [32]. 3.3. Effects of pH on the reaction A CV study of catechin was performed over the pH range from 4.5 to 9.0 (Fig. 4). As can be seen, the ΔEp of the couple redox peaks reached its minimum at pH 7.0 PBS, namely the redox mechanism was approaching reversible reaction. Thus, pH = 7.0 was chosen in the following experiments as the best pH condition. In addition, the three peaks shift negatively with increasing of pH, suggesting that protons participate in the reaction. The equations between peak potential and pH values were: EpcI (V) = 0.3859–0.03792 pH, r = 0.996; EpaI (V) = 0.6497–0.06155 pH, r = 0.992; EpII (V) = 1.1134–0.07736 pH, r = 0.991. It indicated that the total number of electrons and protons taking part in the charge transfer was the same. The results were consistent with the previous studies [11,21,33].
Fig. 5. (A): Cyclic voltammograms of 2.5 × 10− 5 M catechin in PBS (pH 7.0) with different scan rates υ (V s−1): 0.01‚ 0.02‚ 0.03‚ 0.05‚ 0.08‚ 0.1‚ 0.12‚ 0.15‚ 0.18‚ 0.2‚ 0.23‚ 0.25‚ 0.28 and 0.3. (B): The linear relationship of ipI, ipII vs. υ.
2.45× 10− 5 υ (V s− 1), r =0.995. This indicated that the reaction of catechin at SWNTs–CTAB/GCE was a surface controlled process.
3.4. The influence of scan rate
3.5. Effect of accumulation potential and accumulation time
The influence of scan rates on the redox peaks current of catechin was studied using CV. The peak current increased with the scan rate in the range of 0.01–0.3 V s− 1 (Fig. 5). It was linear with υ: ipcI (A) = −1.12× 10− 6–4.37× 10− 5 υ (V s− 1), r =0.998; ipaI (A) =5.08 ×10− 6 +
Since the reversible reaction of catechin on SWNTs–CTAB/GCE was a surface controlled process, the accumulation potential and time had strong influence on the redox peak currents. Fig. 6 exhibits the effect of accumulation time (A) and potential (B) upon the cathodic currents of 2.5 × 10− 5 M catechin. It can be seen that the cathodic current increased with an increment in accumulation time. Then the current leveled off above 90 s (Fig. 6A). Therefore, a 90 s accumulation time was chosen. While the cathodic current increased with increasing accumulation potential at the range of 0.2 V–0.4 V, then decreased between 0.4 V and 0.5 V (Fig. 6B). Hence, a 0.4 V was used in the subsequent experiments. 3.6. Analytical applications
Fig. 4. Cyclic voltammograms of 2.5 × 10− 5 M catechin in PBS at different pH values: 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, and 9.0. Inset: plot of peak potential vs. pH.
The sensitivity of DPV is much higher than CV, and has been used for quantitative measurements. Fig. 7 shows the typical DPV for catechin in different concentrations. It was observed that well-defined cathodic peak currents increased with concentration of catechin in the range from 3.72 × 10− 10 to 2.38 × 10− 9 M. The linear regression equation was ipc (A) = 2.339× 103c (M)−8.90701 × 10− 8, with a correlation coefficient of 0.998 (n = 12). The detection limit is 1.12 × 10− 10 M (S/N = 3), which is much lower than that reported with laccase-modified graphite electrode (4.34 × 10− 6 M) [18] and self-assembled monolayers of a
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Table 1 Determination of catechin. Samples (1010 c/M)
Added (1010 c/M)
Found (109 c/M)
Recoveries (%)
8.00 8.00 8.00
4.29 7.92 9.11
1.24 1.53 1.67
102.6 92.2 95.5
In order to evaluate the validity of the proposed method, the recovery test was carried out by adding different known amounts of catechin into the pure solution of the catechin. As shown in Table 1, the recoveries were from 92.2% to 102.6%, indicating that the method was reliable for the quantitative determination of catechin. 3.7. Reproducibility and stability The reproducibility of the same SWNTs–CTAB/GCE was examined by the detection of 2.5 × 10− 5 M catechin in PBS (pH 7.0). A relation standard deviation (R.S.D.) value of 2.40% was obtained for five successive determinations, which showed that the SWNTs–CTAB/GCE had good reproducibility. The fabrication reproducibility was estimated with five different electrodes, which were constructed independently by the same procedure. R.S.D. was 5.41% for the steady-state current to 2.5 × 10− 5 M catechin, which demonstrated the reliability of the fabrication procedure. The stability of the SWNTs– CTAB/GCE was also investigated. The cathodic peak current of catechin on the modified electrode decreased only 14.4% of its original response after 3 weeks, which demonstrated that the SWNTs–CTAB/GCE had excellent stability. It was much more stable than self-assembled monolayers of a nickel (II) complex and 3-mercaptopropionic acid on a gold electrode [34], which showed apparent loss of activity after 20 days.
Fig. 6. (A) Influence of accumulation time on the cathodic peak current of 2.5 × 10− 5 M catechin in PBS (pH 7.0). Accumulation potential: 0.4 V. (B) Influence of accumulation potential on the cathodic peak current of 2.5 × 10− 5 M catechin in PBS (pH 7.0). Accumulation time: 90 s.
nickel (II) complex and 3-mercaptopropionic acid on a gold electrode (8.26 × 10− 7 M) [34]. The low detection limit was due to not only the big working area of the SWNTs–CTAB/GCE, but also adsorption of catechin at the SWNTs.
3.8. Interference with the determination The effects of some foreign species on the detection of 1.0 × 10− 6 M catechin in PBS (pH 7.0) were evaluated. We can conclude that 500 2− 2− folds higher levels of glucose, Gly, K+, Na+, Cl−, NO− 3 , CO3 , SO4 and Ac− did not interfere with the determination of catechin. 4. Conclusions SWNTs–CTAB/GCE were prepared and had been used to investigate the electrochemical behaviors of catechin. A reversible pair of redox peaks and an irreversible oxidative peak was obtained at the SWNTs–CTAB/GCE. The results showed SWNTs could enhance working area of the electrode, and efficiently accelerate electron transfer rate for catechin. The electrochemical behavior of catechin at the modified electrode is a surface-controlled electrochemical process. The outcome of the modified electrode exhibits the linear range of 3.72 × 10− 10–2.38 × 10− 9 M, especially low detection limit of 1.12 × 10− 10 M. The low detection limit obtained here indicates that SWNTs–CTAB/GCE is suitable for the determination of trace catechin with good reproducibility and stability. References
Fig. 7. DPVs of catechin on SWNTs–CTAB/GCE at different concentration (a → l): 3.72 × 10− 10, 4.34 × 10− 10, 4.95 × 10− 10, 5.56 × 10− 10, 6.78 × 10− 10, 9.20 × 10− 10, 1.10 × 10− 9, 1.34 × 10− 9, 1.57 × 10− 9, 1.81 × 10− 9, 2.04 × 10− 9, 2.38 × 10− 9 M. Inset: plot of cathodic peak current vs. concentration of catechin.
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