b Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, China
Bisphenol A (BPA) has been listed as a typical endocrine disruptor and used mainly in the broad production of epoxy resins, flame retardants,and polycarbonate. It can be mimic the body’s own hormones and may lead to negative effects [1, 2]. It has been proven that BPA can cause a decrease of sperm quality in humans ,neural and behavioral changes in infants and children,and negative effects in pregnant women and their unborn children or in other adults [4, 5],such as induced prolactin secretion [6, 7]. Furthermore,many kinds of abnormalities,including cancers and other diverse pleiotropic actions in the brain and cardiovascular system have been found with the prolonged exposure to BPA . Therefore,as the safety of products containing BPA has been brought to global public attention,an easy and sensitive analytical method for the detection of BPA has become an essential issue in environmental monitoring.
Many traditional methods such as enzyme-linked immunosorbent assay ,chemiluminescence immunoassay ,high-performance liquid chromatography ,molecular imprinting technique ,moresophisticatedliquidchromatography-massspectrometry ,andgas chromatography-massspectrometry[14, 15]havebeen widely reported for the determination of BPA content. Satisfactory resultswereobtainedwithhighsensitivity,excellent selectivity,and a low detection limit. However,these techniques also require technicians,complicated and expensive instruments,and large sample volumes,and are time-consuming. By comparison,electrochemicalmethods provide advantages of lowcost,easy preparation, fast response,convenience of surface renew,high sensitivity and excellent selectivity,andreal-timedetectionunderin situconditions. Zhu et al. used a layered double hydroxide modified glassy carbon electrode (GCE) to investigate the electrochemical behavior of BPA [16, 17]. Yan et al. explored the electroanalysis of BPA at a GCE modified with multi-wall carbon nanotubes . Brugnera and coworkers illustrated the effect of cetyltrimethylammonium bromide as an anti-fouling and pre-concentrating agent for electroanalysis of BPA on a screen printed carbon electrode . Gu’s group constructed an electrochemical sensor for the determination of BPA based on chitosan-Fe3O4 nanocomposite modified GCE . The single-walled carbon nanotubes/b-cyclodextrin conjugatemodified GCEandmulti-wall carbonnanotubes/melaminewere fabricatedfor the sensitivity and selective determination of BPA by Li and coworkers [21, 22]. Sun et al. reported a chitosan-graphene composite modified carbon ionic liquid electrode for the voltammetric detection of BPA . Recently,poly(3,4-ethylenedioxythiophene) (PEDOT) alone was also reported for the direct electrochemical detection of BPA .
PEDOT,one of the most stable conducting polymers,has been the main subject of extensive research in the past quarter century due to its versatile properties such as high electrical conductivity, low band gap,good redox activity,high thermal stability,and excellent transparency in the doped state,and it has been of particular significance because of its potential applications in rechargeable batteries,electrochromic display devices,organic light emitting diodes,supercapacitors,antistatic coatings,corrosion inhibitors,printed circuits,smart windows,microwave absorbing materials,and chem/bio sensors [25, 26, 27, 28, 29]. Simultaneously, in its well-known study,one of most attractive properties of PEDOT is that its properties can be tuned by grafting various functional groups. Carboxylic group functionalized 3,4-ethylenedioxythiophene (EDOT) derivatives were synthesized by Yu’s group,and electropolymerized corresponding thin polymer film (PC4) exhibited good biocompatibility,very low intrinsic cytotoxicity, and displayed no inflammatory response upon implantation, making them ideal for diode devices,ionic exchanges,cell capturing,biosensing,and bioengineering applications [30, 31, 32, 33, 34, 35, 36].
Owing to their high electrical catalytic properties,high chemical stability,and extremely high mechanical strength, SWCNT has been used in the field of modification of electrode surface. Moreover,as an electrode material,SWCNT not only enhances the electron transfer rate,selectivity,and sensitivity,but also minimizes overpotential and decreases the separation between the oxidation and reduction peaks [37, 38, 39, 40, 41, 42].
In this work,4-((2,3-dihydrothieno[3, 4, b][1, 4]dioxin-2-yl)- methoxy)-4-oxobutanoic acid (C4-EDOT-COOH) with good solubility in water was synthesized by an efficient five-step sequence (Fig. 1). Subsequently,a fast and easy method for the detection of BPA at the f-SWCNT/PC4/GCE has been developed. Electrochemical properties of the modified electrode for the voltammetric detection of BPA were studied in detail. It was found that the electrode exhibited excellent electrocatalytic activity to the oxidation of BPA.
BPA was obtained from Aladdin Chemistry Co.,Ltd. and sodium dodecyl sulfate (SDS,95%) was purchased from Sigma-Aldrich Co. LLC. BPA stock solution was prepared with absolute ethanol and stored at 277-281 K. Lithium perchlorate trihydrate (LiClO4·3H2O), disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O), and sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O) were obtained fromSinopharmChemical Reagent Co.,Ltd. 0.1 mol/L Phosphate-buffered solution (PBS,pH 7.0) was prepared from 0.1 mol/L NaH2PO4·2H2O and 0.1 mol/L Na2HPO4·12H2O. Carboxylic group functionalized SWCNT (f-SWCNT) suspension (SWCNT content 0.835 wt%,diameter 1-2 nm,length 5-30 mm) was purchased from Chengdu Institute of Organic Chemistry,Chinese Academy of Sciences. All reagentswere of analytical grade and used without further purification. All solutions were prepared using deionized distilled water.
Electrochemical experiments were carried out by using CHI660B connected to a conventional one-compartment threeelectrode electrochemical cell. A conventional three-electrode system was employed: the working electrode was the f-SWCNT/ PC4/GCE,a platinum wire was used as the auxiliary electrode,and the saturated calomel electrode SCE was used as the reference electrode. Scanning electron microscopy (SEM) measurements were taken using VEGA\\TESCAN Digital Microscopy Imaging. All potentials were given vs. SCE. The pH values of solutions were measured with a Delta 320 pH meter (Mettler-Toledo Instrument, Shanghai,China).
In a previous work,we chose 3,4-dibromothiophene as the starting material for the synthesis of 20-hydroxymethyl-3,4- ethylenedioxythiophene (EDOT-MeOH) through a five-step process . Then,a solution of succinic anhydride (1.231 g, 12.301 mmol),triethylamine (1.088 mL),and 4-dimethylaminopyridine (53.51 mg,0.438 mmol/L) in 21 mL anhydrous dichloromethane was added dropwise into a solution of 20- hydroxymethyl-3,4-ethylenedioxythiophene (1 g,5.807 mmol/L) in 54 mL anhydrous dichloromethane and stirred at room temperature overnight. The mixture was treated with 70 mL of an aqueous 10% hydrochloric acid solution thrice,70 mL deionized water five times,and 70 mL saturated sodium chloride aqueous solution twice. The remaining organic layer was dried by anhydrous sodium sulfate. C4-EDOT-COOH was obtained (2.19 g, 97.58%) as a white solid under reduced pressure. lH NMR (400 MHz,CDCl3): d 6.36 (s,2H),4.01-4.39 (overlapping m,5H), 2.66-2.72 (m,4H).
Prior to electrodeposition,the GCE (F = 3 mm) was carefully polished with chamois leather containing 0.05 mm alumina slurry and ultrasonically cleaned with deionized distilled water,absolute ethanol,and deionized distilled water for 5 min each. The platinum wires above were carefully polished with abrasive paper (1500 mesh) and cleaned by water and absolute ethanol successively before each examination. PC4 film was electrosynthesized by a constant potential of 0.94 V in an aqueous micellar solution containing 0.02 mol/L C4-EDOT-COOH,0.05 mol/L LiClO4,and 0.05 mol/L SDS at room temperature,and the deposition time was 70 s. Then,the obtained PC4 film modified GCE was washed repeatedly with deionized distilled water to remove the electrolyte and monomer. Finally,the modified electrode was dried in air. Subsequently,f-SWCNT was dispersed in double-distilled water to obtain an SWCNT suspension (1.0 mg/mL). Then,5 mL of the suspension was dropped onto the PC4/GCE surface to obtain the f- SWCNT/PC4/GCE,which was dried under an infrared lamp.
Ten milliliters of PBS (pH 7.0) with a specific amount of BPA was transferred into the electrochemical cell. Then,the three-electrode system was immersed into it. The standard equipment was used for cyclic voltammetry (CV) measurements,cyclic voltammograms (CVs) were recorded between 0.4 V and 0.8 V,the peak current was measured,and the scan rate was 50 mV s-1. All solutions were deoxygenated by purging with nitrogen for 20 min prior to each experiment. All the experiments were performed at room temperature.
The surface morphology of PC4 film and f-SWCNT/PC4 composite deposited on the indium-tin oxide (ITO) transparent electrode are observed by SEM (Fig. 2). The dedoped PC4 films (Fig. 2A) obtained from the aqueous micellar solution are rough, continuous,and homogeneous structure,which is different from the smooth surface morphology of PEDOT. The rough morphology of compact PC4 film is extremely beneficial for the improvement of adsorption performance. In addition,it can be clearly seen that some nanotubes are loosely and randomly entangled on the PC4 film (Fig. 2B). The SEM images of the f-SWCNT/PC4 composite film shows a three-dimensional (3D) network composed of fibers with similar diameters. It can be then concluded that the 3D network is formed with the f-SWCNT serving as the backbone,thus greatly enhancing the mechanical properties of the composite film. Moreover,the 3D network of composite film modified electrode may give higher sensitivity than a smooth surface due to increased surface area and therefore an increased concentration of the analytes .
|Fig. 2.The SEM image of the PC4 (A) film and f-SWCNT/PCE composite (B) (magnification: 1000×).|
CVs of the bare GCE (Inset) and modified GCE in PBS (pH 7.0) with 5 mmol/L BPA are shown in Fig. 3. In comparison to the bare GCE (Inset b),the PC4/GCE (Fig. 3b),f-SWCNT/GCE (Fig. 3c),and f- SWCNT/PC4/GCE (Fig. 3d) have favorable electrocatalytic activity with BPA. It can be seen that no peaks are observed for a blank determination at the f-SWCNT/PC4/GCE (Fig. 3a). An observable oxidation peak appeared at approximately 0.6 V at different modified GCEs,which is very different from in PBS without BPA, indicating that BPA can be oxidized by electrocatalytic oxidation. In addition,the corresponding cathodic peak does not appear, which indicates that this electrochemical reaction is an irreversible process. It is worth noting that the oxidation current of BPA at f-SWCNT/PC4/GCE increases as compared to PC4/GCE and f- SWCNT/GCE,which is attributed to both the excellent electrocatalytic property of f-SWCNT and the extraordinary conductivity of PC4.
|Fig. 3.CVs of 5.0 mmol/L BPA in 0.1 mol/L PBS (pH 7.0),on the PC4/GCE (b),f- SWCNT/GCE (c) and f-SWCNT/PC4/GCE (d). Absence of BPA in solution at f-SWCNT/ PC4/GCE is also shown (a). Scan rate: 50 mV s-1. Inset: CVs of the bare GCE in 0.1 mol/L PBS (pH 7.0) containing 0 (a) and 5.0 mmol/L (b) BPA.|
CVs of the fabricated sensor at different scan rates are recorded in Fig. 4. It can be seen that the anodic peak currents increase gradually with the increase of scan rates,and a linear relationship between ipa and v are constructed in the range of 30-500 mV s-1 with the linear regression equations as ipa (mA) = 22.8296 + 2.0116 v (mV s-1) (R2 = 0.9981) (shown in Fig. 5A),indicating that the oxidation of BPA is an adsorption-controlled electrode process. In addition,as shown in Fig. 5B,a linear relationship between Epa and Napierian logarithm of v (ln v) is also observed in the range of 30-500 mV s-1. The equation can be expressed as Epa (V) = 0.484 + 0.033 ln v (mV s-1) (R2 = 0.9777). As for an electron transfer controlled and totally irreversible electrode process,Epa is defined by the following equation:
where a is transfer coefficient,k0 is standard rate constant of the reaction,n is electron transfer number involved in rate-determining step,v is scanning rate,E0 is formal redox potential,R is the gas constant,T is the absolute temperature,and F is the faraday constant. According to the linear correlation of Epa vs. lnv as mentioned above,the slope of the line is equal to RT/anF,therefore an can be easily calculated from the corresponding slope of Epa - ln v. In this work,the slope of the line is 0.033. Therefore,the value of an is calculated to be 0.79 (taking T = 298,R = 8.314,and F = 96485). Generally,a is assumed to be 0.5 in totally irreversible electrode process,so the electron transfer number (n) is around 2 for the electrochemical oxidation of BPA. Considering that the number of electrons and protons involved in the BPA oxidation process is equal ,the electrooxidation of BPA on the f-SWCNT/ PC4/GCE is a two-electron and two-proton process.
|Fig. 4.CVs of f-SWCNT/PC4/GCE in 0.1 mol/L PBS (pH 7.0) containing 5 mmol/L BPA at various scan rates.|
|Fig. 5.(A) The plot of peak current vs. scan rate. (B) The relationship between the potential Epa and the natural logarithm of scan rate (ln v).|
Because the electrochemical reaction of BPA on the f-SWCNT/ PC4/GCE is an adsorption-controlled process,the accumulation step is usually a simple and effective way to enhance the sensitivity. The influence of the accumulation time on the oxidation peak current is also tested in 0.1 mol/L PBS (pH 7.0) containing 5 mmol/L BPA,and the solution is kept well-stirred by a magnetically rotated stirring rod in the enrichment process. The anodic current increases greatly within the first 120 s and then levels off (Fig. 6),suggesting that the accumulation of BPA on the f- SWCNT/PC4/GCE rapidly reaches saturation. Thus,the accumulation step in the experiments is performed under violent stirring for 150 s.
|Fig. 6.Effect of accumulation time on the oxidation peak current of 5 mmol/L BPA in 0.1 mol/L PBS (pH 7.0).|
Various concentrations of BPA solutions were prepared to explore the relationship between peak currents and concentrations of BPA. As could be seen from Fig. 7,the f-SWCNT/PC4/GCE displayed a linear function of the BPA concentration from 0.099 mmol/L to 5.794 mmol/L. The regression equation was Ipa (mA) = 38.897 + 7.560C (mmol/L) (R2 = 0.9989) (Fig. 7 Inset). The current sensitivity was calculated to be 7.560 mA mmol/L. At a signal-to-noise ratio of 3 (S/N = 3),the detection limit is found to be 0.03 mmol/L. The analytical performance of the f-SWCNT/ PC4/GCE was compared with other modified electrodes reported in the literature. As shown in Table 1,the proposed electrode also exhibited wider linear range and lower limit of detection. These results indicate that the proposed modified electrode is a good platform for the electrochemical detection of BPA.
|Fig. 7.CVs of the f-SWCNT/PC4/GCE in 0.1 mol/L PBS containing different concentration of BPA. (From inner to outer correspond to C = 0.099,0.591,0.884, 1.175,1.465,1.988,2.529,3.441,4.659,5.794 mmol/L.) Scan rate: 50 mV s-1. Inset: plot of cathodic peak current vs. concentration of BPA.|
The repeatability of the modified electrode is investigated by repetitively,successively measuring at a fixed BPA concentration of 5 mmol/L twenty times (Fig. 8). As a result,the relative standard deviation (RSD) is 0.61%,which suggests that the f-SWCNT/PC4/ GCE has good repeatability. Also,the electrode retains 92.7% of its initial peak current response after it is kept at room temperature for 2 weeks,which shows long-term stability of the f-SWCNT-PC4 hybrid film on the GCE surface.
|Fig. 8.Current responses of the f-SWCNT/PC4/GCE for 20 successive assays in 0.1 mol/L PBS (pH 7.0) containing 5 mmol/L BPA with the same modified electrode.|
To verify the applicability of the proposed f-SWCNT-PC4 modified electrode for real samples analysis,commercial mineral water was used to prepare PBS solution and then was analyzed with the proposed method. The results were shown in Table 2. The recovery was found to be 99.60%-103.35%,and the average recovery was 101.33%,which suggesting that the as-prepared electrode is able to predict the concentration of BPA in commercial mineral water.
BPA is successfully determined voltammetrically using the f- SWCNT-PC4 composite modified GCE,which is fabricated by layerby- layer self-assembly. BPA has remarkable electrocatalytic activity toward the as-prepared modified electrode,good linear range in a concentration range from0.099 mmol/L to 5.794 mmol/L, a low detection limit of 0.03 mmol/L,and a pronounced sensitivity of 7.560 mA mmol/L. Moreover,the stability and reproducibility of the f-SWCNT/PC4/GCE is also evaluated with satisfying results. These good results indicate that the prepared modified electrode is a promising candidate for studying electrochemical behaviors of BPA.
The authors would like to acknowledge the financial support of this work by the NSFC (Nos. 51272096,51263010),Jiangxi Provincial Department of Education (Nos. GJJ10678,GJJ11590), Natural Science Foundation of Jiangxi Province (Nos. 2010GZH0041,20114BAB203015),Jiangxi Science & Technology Normal University (No. KY2010ZY13),and Jiangxi Provincial Innovation Fund of Postgraduates (No. YC2012-S123).
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