ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2015, том 70, № 1, с. 58-62
ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
STUDY ON THE ELECTROCATALYTIC OXIDATION OF BISPHENOL A ON Au NANO-PARTICLES/CARBON NANOTUBES COMPOSITE
MODIFIED ELECTRODE © 2015 Anting Wang, Yingliang Wei1, Chao Wang
Department of Environment Engineering and Chemistry, Luoyang Institute of Science and Technology
Luoyang, 471023, China 1E-mail: Weihh1016@163.com Received 10.10.2012; in final form 20.12.2013
A novel modified electrode, Au nano-particles/multi-walled carbon nanotubes modified glassy carbon electrode (AuNPs/MWCNT/GCE) was fabricated in this work. In phosphate buffer solution (PBS-KCl), the electrochemical response of Bisphenol A (BPA) at this modified electrode was investigated by differential pulse voltammetry (DPV). A sensitive oxidation peak of BPA was observed at the potential of 0.396 V Comparing with the bare glassy carbon electrode, the oxidation peak potential shifted to less positive potentials, and the peak current enhanced 10 fold. The factors of the electrochemical response of BPA, such as supporting medium, solution pH, and surfactant were carefully optimized. Under the optimum conditions, a linear calibration curve of the peak current of BPA and concentration was obtained in the range 8.0 x 10-8—1.0 x 10-5 M, and the detection limit was 3 x 10-9 M. The oxidation peak current increases proportionally with scanning speed, which suggests that the electrochemical oxidation of BPA is adsorption-controlled. The recoveries of this method are from 96 to 103.4%, indicating that the proposed method is accurate and reliable.
Keywords: Bisphenol A, carbon nanotube, Au nano-particles, composite modified electrode, voltammetry.
DOI: 10.7868/S0044450215010041
Phenolic compounds are formed in many industrial processes and can be found in industrial waste from plastic, pharmaceuticaloil refinery, paint, and coal mining industry. Bisphenol A, 2,2-te-(4-hydroxyphenyl)pro-pane, as a kind of estrogenic phenolic compounds, is a so-called endocrine disrupting chemical [1], widely used as a raw material for the manufacture of polycarbonate and epoxy resins. In these production processes, some BPA can be involuntarily released into the environment to pollute rivers and ground waters. Additionally, trace amounts of BPA can also migrate into food and drinking water from nursing bottles, food can linings and beverage containers [2, 3]. BPA has been shown to affect hormonal control, sexual development, reproductive success, and behavior [4]. Therefore, it is necessary to determine the level of BPA in environment.
Nowadays, some analytical methods for the determination of BPA in environmental and biological samples have been reported, such as HPLC [5], gas chromatography—mass spectrometry [6], molecular imprinting technology [7]. In comparison to other methods of detection, electrochemical measurement possesses advantages, such as fast response, remarkable sensitivity, simple operation, and low cost. Some electrochemical methods have been reported, for in-
stance, a mesoporous silica-based electrochemical sensor for BPA that enhances the response signals due to the large active surface area and high accumulation efficiency [8]. Huanshun Yin [9] investigated electrochemical behavior of BPA at glassy carbon electrode modified with gold nanoparticles, silk fibroin, and PAMAM dendrimers with in a wide linear range (from 1 nM to 1.3 ^M). Though some satisfactory results have been reported, it is still a challenge to develop a novel electrochemical sensor based on composite nano-material with high sensitivity and simple fabrication. To the best of our knowledge, voltammetric determination of BPA by using AuNPs/MWCNT/GCE has not been reported yet.
Carbon-nanotubes (CNTs) represent an important group of nanomaterials with attractive mechanical, electronic, chemical properties and strong adsorptive ability. Owing to electronic structure and high electrical conductivity, CNTs has been proved to promote electron-transfer reactions when used as electrode material or modifier [10, 11]. Metal nano-particles (NPs) have emerged as a new class of compounds that are particularly interesting for materials science due to their unique electronic, optical, and catalytic properties. Over the past few years, metal NPs have received a lot of attention and have been extensively fabricated
as electrochemical sensors due to their interesting electrocatalytic properties [12, 13]. Owing to the stable chemical and physical properties of Au nano-par-ticles, they've been applied in electroanalysis and construction of electrochemical sensors [14, 15].
In this study, a novel electrochemical sensor was constructed based on the special characteristics of AuNPs and MWCNT. The sensor was successfully employed to catalyze the electro-oxidation of BPA at the reduced potential with high sensitivity. The proposed method has been successfully used to determine the content of BPA in simulated waste water.
EXPERIMENTAL
Apparatus and reagents. All electrochemical experiments were performed on a CHI 660D Electrochemical Analyzer (CH Instrument, USA). A three-electrode system was used, the glassy carbon electrode (GCE, A = 0.071 cm2) used as working electrode, a platinum wire and a saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. All the potentials were reported vs. SCE. The pH values of the solutions were recorded with a PHS-3C Digital pH-meter (Shanghai Yidian Science Instrument Company, Shanghai, China). All experiments were performed at the room temperature (25 ± 1°C).
MWCNT were obtained from Nano-material Company of Shenzhen, China. HAuCl4 • 4H2O was purchased from Shanghai Chemical Reagent Factory, China. A 1.0 x 10-3 M stock solution of Bisphenol A (Tianjin Damao Chemical Reagent Factory, China) was prepared in ethanol and then stored in the dark at 4°C. 0.2% (w/w) Chitosan (CTS) was fabricated by dissolving an appropriate amount of CTS in 1% (v/v) acetic acid solution. Other chemicals were of analytical grade without further purification. The water used in our experiment was redistilled water.
Fabrication of the modified electrode. AuNPs collo-sol was prepared according to the previous report [16], and then stored in a refrigerator under 4°C. MWCNT were purified by using procedure [17]: 4 mg purified MWCNT were dispersed in 4 mL mixing solution (containing 2.0 mL Au collosol, 1.2 mL anhydrous ethanol and 0.8 mL 0.2% CTS) and sonicated for about 30 min to produce a homogeneous AuNPs/MWCNT suspension. Prior to modification, GCE was polished with a 0.05 ^m aluminum slurry with bi-distilled water, then sonicated in HNO3—H2O (1 : 1, v/v), absolute ethyl alcohol and redistilled water each for 2 min. Finally, the GCE was coated with 10 ^L above-mentioned AuNPs/MWCNT suspension, and the solvent was evaporated at room temperature. Thus, a novel modified electrode, namely, AuNPs/MWCNT/GCE was fabricated successfully.
MWCNT/GCE was prepared by using the similar procedure.
Current, pA
Fig. 1. Cyclic voltammograms of BPA in pH 6.00 PBS— KCl at different electrodes: 1 — GCE + blank solution; 2 — GCE + 1.0 x 10-5 M BPA; scan rate 0.1 V/s.
Analytical method. In the subsequent experiments, AuNPs/MWCNT/GCE was used as the working electrode. The AuNPs/MWCNT/GCE was first activated in pH 6.00 PBS—KCl solution by cyclic voltammetric sweeps between 0.2—1.0 V until the cyclic voltammograms were stable, and then the three-electrode system was transferred into 10 mL electrolyte solution containing an appropriate amount of BPA. DPV curves of BPA at this modified electrode were recorded from 0.1 to 0.8 V, and the oxidation peak current of BPA at 0.396 V was measured. After each measurement, AuNPs/MWCNT/GCE was retransferred into pH 6.00 PBS—KCl to remove the adsorped substances and kept repeatability by 20 times cyclic voltammetric sweeps.
RESULTS AND DISCUSSION
Electrochemical behavior of BPA at the different electrodes. The electrochemical behavior of BPA at GCE was investigated by cyclic voltammetry, as shown in Fig. 1. Within the potential window from 0.2 to 1.0 V no redox peak was observed in pH 6.00 PBS—KCl (curve 1). An obvious oxidation peak was recorded at 0.480 V in the presence of 1.0 x 10-5 M BPA (curve 2). However, there was no corresponding reduction peak on the reverse scan, indicating that the electrode reaction was an irreversible process.
In this work, DPV was employed to certify the positive action of AuNPs/MWCNT/GCE. Figure 2 shows DPV response curves of BPA at different electrodes, and the optimized parameters of DPV are as follows: pulse amplitude 50 mV, scan rate 20 mV/s, and pulse width 50 ms. As can be seen from Fig. 2, there is a poor oxidation peak at a bare GCE for
60
ANTING WANG h gp.
Current, pA 0
1
2
3
4
5
6 7
X2
0.1 0.2
0.4 0.5 0.6 0.7 0.8 Potential, V
Fig. 2. DPV response curves of BPA in pH 6.00 PBS-KCl at different electrodes: 1 — GCE + blank solution, 2 — GCE + 1.0 x 10-5 M BPA, 3 - MWCNT/GCE + 1.0 x x10-5 M BPA, 4 - AuNPs/MWCNT/GCE + 1.0 x 10-5 M BPA; pulse amplitude 50 mV; scan rate 20 mV/s; pulse width 50 ms.
1.0 x 10 5 M BPA (curve 2). An obvious oxidation peak is recorded at 0.408 V on a MWCNT/GCE (curve 3). Simultaneously, a well-defined and sensitive oxidation peak is observed at 0.396 V on a AuNPs/MWCNT/GCE. Compared with that of bare GCE, the anodic peak current enhances 10 fold, and the peak potential shifts from 0.486 to 0.396. This phenomenon may be attributed to the synergic effect of the unique characteristic of MWCNT such as large specific area, strong adsorption ability and the catalytic characteristics of metal NPs [18]. From the experimental results, the conclusion can be drawn that AuNPs/MWCNT/GCE remarkably enhances the sensitivity of the determination of BPA.
Optimization of the measurement conditions. Optimizing of supporting electrolyte and choice of pH. The oxidation peak potential and
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