Full Paper. Full Paper. 1. Introduction. Z. M. Wang, H. W. Guo, E. Liu,* G. C. Yang, N. W. Khun

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1 Full Paper Bismuth/Polyaniline/Glassy Carbon Electrodes Prepared with Different Protocols for Stripping Voltammetric Determination of Trace Cd and Pb in Solutions Having Surfactants Z. M. Wang, H. W. Guo, E. Liu,* G. C. Yang, N. W. Khun School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore , Singapore * Received: May 13, 2009 Accepted: August 19, 2009 Abstract To improve reproducibility, stability and sensitivity, a bismuth (Bi) thin film was coated on glassy carbon (GC) substrates which surfaces were modified with a porous thin layer of polyaniline (PANI) via multipulse potentiostatic electropolymerization to form Bi/PANI/GC electrodes (Bi/PANI/GCEs). The Bi/PANI/GCEs were used successfully for simultaneous detection and determination of Cd 2þ and Pb 2þ ions, and various parameters were studied with reference to square wave anodic stripping voltammetric (SWASV) signals. The experimental results depicted that the environment-friendly Bi/PANI/GCEs had the ability to rapidly monitor trace heavy metals even in the presence of surface-active compounds. Keywords: Bi/PANI/GC electrode, Trace analysis, Stripping voltammetry, Thin films DOI: /elan Introduction Contamination of water by trace toxic and heavy metals represents a major current environmental problem, which results in an ever-increasing demand for the detection and determination of metal contaminants [1]. Square wave anodic stripping voltammetry (SWASV) is a widely used analytical technique for detection of trace heavy metals at low cost. SWASV is based on a preconcentration by electrodeposition of metallic ions from a sample solution onto a working electrode surface, followed by anodic stripping of the analyte from the electrode surface into the sample solution [2, 3]. Recently, bismuth-film (Bi-film) electrodes have become an attractive new subject of electroanalytical investigations as they could be a potential replacement for mercury and mercury film electrodes [4 8]. Several types of bismuth electrodes showed excellent advantages over mercury film electrodes when applied to detect trace heavy metals using stripping voltammetry [5, 6, 9 17]. One of main problems associated with Bi-film electrodes is the interferences that arise from various surface-active substances that are adsorbed onto the electrode surfaces and cause passivation of the electrodes [6, 18, 19]. Natural environmental samples, in which trace heavy metals need to be analyzed, usually contain some kinds of surface-active substances [20, 21]. The adsorption of the surfactants onto electrode surfaces may affect both deposition and stripping steps, leading to weaker or broader peaks and shifts in peak potentials. These effects depend upon specific surfactants and target metals, and reflect the interfacial properties of Bifilm electrodes [6, 7, 22]. To alleviate such interferences, efforts have been made by means of various surface manipulations on Bi-film electrodes, such as adsorbed and self-assembled monomolecular layers of ligands on gold electrodes [23 25], composite electrodes prepared by mixing ligands with carbon paste [26, 27], polymer film modified electrodes [7, 18 22, 28 30], and so on. The principle of these approaches is that modified films work like a membrane that can mechanically prevent surfaceactive substances from reaching electrode surfaces by hindering their diffusion through the films, while metal cations with smaller sizes can relatively easily diffuse through the films and eventually reach the electrode surfaces [18]. Recently, modified electrodes based on incorporation of conducting polymer (CP) films have received considerable attention for detection of trace heavy metals due to their superior electrical conductivities, good adhesive strengths and suitable structural characteristics [11, 31]. The electrical conductivity of CP films is dependent on their microstructures that can be controlled by fabrication methods and conditions [32]. CP films prepared by conventional potentiodynamic and potentiostatic methods are usually thick and compact. A compact film has relatively a small specific surface area and a poor electrical conductivity, which are unfavorable for the construction of electrochemical sensors. Recently, Gao et al. [33] proposed a method for the Electroanalysis 2010, 22, No. 2, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 209

2 fabrication of thin and nanoporous carbon nanotube/ poly(1,2-diaminobenzene) composite film coated electrodes by multipulse potentiostatic electropolymerization. Compared to conventional potentiodynamic and potentiostatic methods, multipulse potentiostatic electropolymerization can prepare composite film electrodes having a larger specific surface area and a higher electrical conductivity. It was reported that CP films deposited on electrode surfaces could enhance stripping voltammetric response in detection of trace metal ions, but their superiority was not obvious [34]. In Ref. [11], the anodic stripping behavior of bismuth/polyaniline/glassy carbon electrodes (Bi/PANI/ GCEs) fabricated using a cyclic voltammetric method was not studied in the presence of surfactants, and no systematic comparison with other electrodes was conducted. In this study, polyaniline/glassy carbon electrodes (PANI/ GCEs) and Bi/PANI/GCEs were prepared by electrodeposition using both cyclic voltammetric and multipulse potentiostatic methods, which were used to determine Cd 2þ and Pb 2þ ions in mixed H 2 SO 4 and KCl solutions with or without surfactants. 2. Experimental Aniline monomers (Fluka) were freshly distilled under a reduced pressure and stored under nitrogen atmosphere at a low temperature (48C). CH 3 COOH (Fluka) and CH 3 COO- Na (Fluka) were used for the preparation of 0.1 M acetate buffer solutions. Stock solutions of bismuth(iii) nitrate, lead(ii) nitrate, and cadmium(ii) nitrate, all having a concentration of 0.5 g/l, were prepared and stored at room temperature (22 8C). All the chemicals used were of analytical reagent grade. The solutions were prepared with distilled water and all the experiments were carried out at room temperature. Glassy carbon electrode (GCE) surfaces were polished thoroughly with 0.3 mm a-al 2 O 3 powder slurry on a soft cloth and then sonicated in ethanol and double distilled water for 3 min each to remove alumina particles and other possible contaminants. The polished GCEs were immersed into an aqueous solution containing 5 mm aniline monomers, 0.05 M sulfuric acid and 0.05 M potassium sulfate under nitrogen atmosphere. A polyaniline (PANI) film was then electrodeposited onto the polished GCE surfaces to form PANI/GCEs. Electropolymerization of PANI was performed using two different electrochemical methods, i.e., multipulse potentiostatic method that was carried out from 0.2 to 0.7 V with a pulse width of 0.5 s, and cyclic voltammetry that was performed from 0.2 to 0.7 V with a scan rate of 50 mv for 10 cycles. After the electrodeposition of PANI, the working electrodes (PANI/GCEs) were transferred into a measuring solution containing 20 mm H 2 SO 4 and 30 mm KCl, into which bismuth along with selected model metals was also introduced under nitrogen atmosphere. A three-electrode cell was used for electrochemical measurements, which had a working electrode, an Ag/ AgCl (saturated KCl) reference electrode, and a Pt coil counter electrode. SWASV was performed using a potentiostat/galvanostat (m-autolab Type II, Eco Chemie BV, Utrecht, Netherlands), and analyzed with GPES 4.9 software (Eco Chemie, Netherlands). A magnetic stirrer (Model 79-1) was used to stir the testing solutions during the precondition and deposition. A preconcentration (deposition) potential, typically 1.3 V, was applied to the PANI/GCEs for 120 s with the solutions being stirred, and an alloy mixture of bismuth and target metals was deposited on the PANI/GCE surfaces by using SWASV. After the preconcentration, the stirring was stopped, followed by equilibration (quiet) for 20 s. Then anodic square-wave stripping was taken with the frequency, potential step and amplitude being 8 Hz, 4 mv and 5 mv, respectively, while the voltammograms were recorded automatically. The potential scans were terminated at 0.6 V, followed by 30 s conditioning (cleaning) by holding the working electrodes at 0.6 V (with the solutions stirred) to remove the remaining target metals and bismuth films from the PANI/GCE surface. The cleaned PANI/GCEs were then ready for use in the next cycle. Bi/PANI/GCEs were fabricated only when bismuth was introduced into the electrolyte. Otherwise, only the PANI/ GCEs were used for stripping. For the GCEs that were used for anodic stripping in the electrolyte containing bismuth, the Bi-films were in-situ coated on the glassy carbon electrodes (Bi/GCEs). 3. Results and Discussion Z. M. Wang et al. Some crucial factors that determine the current response in SWASV are the composition of bulk solutions, structures of CP and bismuth films, condition of accumulation, and parameters of SWV mode. Figure 1 shows the SWASV responses of 25 nm Cd 2þ and 25 nm Pb 2þ at the four electrodes, namely, a bare GCE, two different PANI/GCEs modified using the cyclic voltammetric and multipulse potentiostatic methods, respectively, and a Bi/PANI/GCE. In Figure 1, relatively small current responses are observed at the bare GCE (curve a), which shows a difficulty for the metal ions to be adsorbed onto the GCE surface. However, the stripping peak currents of Cd 2þ and Pb 2þ at the PANI/GCE fabricated by the cyclic voltammetric method (curve b) are even lower than those measured at the GCE (curve a). This is due to the fact that the PANI films prepared by cyclic voltammetry are always thick and compact, and thus have a relatively small specific surface area and a poor electrical conductivity that are unfavorable [33]. On the other hand, for the PANI/GCE fabricated by the multipulse potentiostatic method (curve c), the stripping peak currents for the two metal ions are both larger than those obtained from the first two electrodes. This can be explained that the PANI films prepared on the GCE by the multipulse potentiostatic method possess unique porous structures that are three-dimensional with a large number of microgaps and micropores [33]. These Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2010, 22, No. 2,

3 Determination of Trace Cd and Pb porous structures can offer a larger specific surface area and better ionic and electronic conductivities, so the porous PANI films can be used as an excellent supporting material for chemical sensors. Therefore, the larger current responses observed from curve c are due to the favorable properties of the porous PANI films, which have promoted electrontransfer. Finally, the peak currents of Cd 2þ and Pb 2þ at the Bi/PANI/GCE are even greatly enhanced (curve d), which are attributed to bismuth that exhibits a strong adsorptive ability towards the metal ions and hence improves the surface sensitivity of the electrode. The peak current enhancement observed from the Bi/PANI/GCE as compared to the other three electrodes as shown in Figure 1 indicates a significant improvement in analytical sensitivity of the Bi/PANI/GCE to the trace metals concerned. The PANI film thickness can be increased by increasing the aniline concentration in the solutions or the deposition time. In this work, for the PANI films fabricated by the multipulse potentiostatic method, the thickness effect with respect to deposition time is illustrated in Figure 2a that shows the stripping responses to 25 nm Cd 2þ and 25 nm Pb 2þ in the solution containing Bi 3þ (1.25 mm). Below a certain PANI layer thickness, the stripping peak currents of the both metal ions increase when the PANI deposition time is increased. However, when the PANI layer is beyond a critical thickness, the stripping responses to Cd 2þ and Pb 2þ become weaker with a further increase of the PANI deposition time. This is in agreement with [11] that reported that the stripping peak intensity would first increase and then weaken as the aniline concentration was increased. The reason might be a competition between two effects, i.e., enhancing effect due to the porous structure of the PANI layers that can offer a bigger surface area to enhance the accumulation of the metals and suppressing effect due to the thicker PANI films that could reduce the conductivity of the films. To compromise between the two effects, a deposition time of about 160 s is selected. From Figure 2a, it can be expected that the Bi/ PANI/GCEs with the PANI deposition of 160 s can achieve a higher current response than the Bi/GCEs without PANI deposition. The nominal thickness of the Bi films can be controlled by varying the Bi 3þ concentration in the bulk solutions. The thickness effect of the Bi films on the stripping responses to 25 nm Cd 2þ and 25 nm Pb 2þ is investigated using the Bi/ PANI/GCEs with respect to Bi 3þ concentration ranging from 25 nm to 10 mm (Fig. 2b). The stripping peak currents for the two metal ions increase with increased Bi 3þ concentration when the Bi 3þ concentration is lower than 250 nm. However, the Pb 2þ stripping peak currents almost stabilize with further increased Bi 3þ concentration when the Bi 3þ concentration is higher than 250 nm, while the Cd 2þ peak currents achieve a maximum at 1.25 mm Bi 3þ and then turn Fig. 1. SWASVs of 25 nm Cd 2þ and 25 nm Pb 2þ in solutions containing 20 mm H 2 SO 4 and 30 mm KCl measured from: (a) bare GCE, (b) PANI/GCE prepared by cyclic voltammetry, (c) PANI/GCE prepared by multipulse potentiostatic method, and (d) Bi/PANI/GCE. Fig. 2. Effects of (a) PANI deposition time and (b) Bi 3þ concentration on stripping peak currents of 25 nm Cd 2þ (solid line) and 25 nm Pb 2þ (dashed line). Electroanalysis 2010, 22, No. 2, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 211

4 to decrease at higher Bi 3þ concentrations. It was reported [11, 14] that a Bi 3þ -to-target metal ion concentration ratio larger than 4 would be good enough to obtain high quality data from different electrodes. Thus, a Bi 3þ concentration of 1.25 mm will be used in the following sections. For the Bi/ PANI/GCEs used in a solution having a Bi 3þ concentration of 1.25 mm as shown in Figure 2b, the peak currents are higher than those measured from the PANI/GCEs in a solution having no Bi 3þ. From the above discussion, a major merit of the Bi/PANI/ GCEs can be concluded that they have higher current responses to the both Pb 2þ and Cd 2þ ions than other three types of electrodes. The effect of ph value of the solutions on the responding currents for the determination of Cd 2þ and Pb 2þ at the Bi/ PANI/GCE is depicted in Figure 3a where the supporting electrolytes are KCl, H 2 SO 4 and acetate buffer. The peak currents decrease with increasing ph values, which is in agreement with the literature [35] that reported that the electrochemical activity of PANI swiftly increased as ph was decreased. This is because the conductivity of PANI comes from doped hydrogen ions that serve as electron transfer mediators for the PANI films. At a lower ph, a higher concentration of hydrogen ions can enhance the PANI conductivity, which can greatly accelerate the redox reactions and current response of the electrodes. As shown in Figure 3a, the ph values between 1.5 and 2.5 can offer the highest current responses. Therefore, a mixed electrolytic solution of 20 mm H 2 SO 4 and 30 mm KCl is employed for the SWASV measurements. The influence of preconcentration potential varied in the range of 1.4 and 0.9 Von the current responses to 25 nm Cd 2þ and 25 nm Pb 2þ is shown in Figure 3b. The stripping peak currents of both Cd 2þ and Pb 2þ rapidly increase when the preconcentration potential decreases from 0.9 to 1.3 V due to more complete reductions of Cd 2þ and Pb 2þ to their neutral forms, but the peak currents drop when the preconcentration potential becomes more negative than 1.3 V due to hydrogen evolutions that can reduce the surface activities of the electrodes. Therefore, a preconcentration potential of about 1.3 V is considered optimal for the determination of Cd 2þ and Pb 2þ. The effect of preconcentration time on the stripping peak currents of both 25 nm Cd 2þ and 25 nm Pb 2þ is illustrated in Figure 3c where the stripping peak currents of the both metals increase with increased preconcentration time till about 320 s beyond which the curve slops are slightly lower, which may be due to the electrode saturation with the Cd and Pb in the reduced states. An accumulation time of 120 s is used to avoid the saturation of the electrodes for higher target metal concentrations. Figure 4 shows the SWASVs measured from the Bi/PANI/ GCEs for the detection of Cd 2þ and Pb 2þ with the concentrations varying from 25 to 150 nm each in the solutions having mixed electrolytes of 20 mm H 2 SO 4 and 30 mm KCl. With the deposition time of 160 s, the calibration curves for the Cd 2þ and Pb 2þ are linear in the range of nm and can be represented by Equations 1 and 2, respectively, Z. M. Wang et al. Fig. 3. Effects of (a) ph of solutions, (b) preconcentration potential and (c) preconcentration time on stripping peak currents of 25 nm Cd 2þ (solid line) and 25 nm Pb 2þ (dashed line) Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2010, 22, No. 2,

5 Determination of Trace Cd and Pb Fig. 4. SWASVs of Cd 2þ and Pb 2þ having concentrations of 25, 50, 75, 100, 125 and 150 nm from bottom to top, respectively, which were measured using Bi/PANI/GCE. The insets show the calibration curves for determination of Cd 2þ and Pb 2þ of different concentrations. I (ma) ¼ [Cd 2þ ] (nm) (1) and I (ma) ¼ [Pb 2þ ](nm) (2) The regression coefficients of Equations 1 and 2 are and , respectively, and the relative standard deviations from 10 measurements of 25 nm Cd 2þ and 25 nm Pb 2þ by using the same Bi/PANI/GCE are about 5.95% and 3.31%, respectively. Therefore, the detection limits of Cd 2þ and Pb 2þ are about 1.1 nm and 16.5 nm, respectively. Bi-film electrodes are particularly prone to interferences from surfactants that can be adsorbed onto and foul the electrode surfaces [36], and the effect of typical surfactants on Bi-film electrodes has also been reported [18]. In this study, the effects of different types of surfactants on the repeatability and stability of the Bi/PANI/GCEs are evaluated and compared with those on the Bi/GCEs that are a type of Bi-film electrodes. Table 1 presents the stripping currents normalized with the maximum stripping current, i.e., I p /I pmax, measured with respect to different concentrations of the three surfactants dissolved in the electrolytic solutions containing 25 nm Cd 2þ and 25 nm Pb 2þ ions using the two types of electrodes. Obviously, for the both kinds of electrodes in the same electrolytic solutions (with same type and same concentration of surfactants), the Cd stripping peak currents are more sensitive to the type of surfactants than the Pb ones. For the both electrodes, the cationic surfactant, cetyltrimethylammonium bromide (CTAB), gives the least decreases in stripping currents, the Triton X-100 induces moderate decreases in stripping currents, and the sodium dodecyl sulfate (SDS) induces the most significant drops in stripping currents. Compared to the Bi/GCEs, it is clear that the Bi/PANI/GCEs are much more tolerant to the presence of the surface-active compounds for the detection of both Pb and Cd. The resistance of the Bi/ PANI/GCEs to the surfactants is attributed to the presence of the PANI layers that have formed an effective barrier to prevent the macromolecules from transporting to the Table 1. Normalized current, I p /I pmax, for 25 nm Cd 2þ and 25 nm Pb 2þ ions vs. surfactant concentrations measured from Bi/GCEs and Bi/ PANI/GCEs. Metal ion Electrode I p /I pmax (%) Surfactant Triton X-100 (mm) DS (mm) CTAB (mm) Cd 2þ Bi/GCE Bi/PANI/GCE Pb 2þ Bi/GCE Bi/PANI/GCE Electroanalysis 2010, 22, No. 2, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 213

6 Z. M. Wang et al. surfactants on the stripping behaviors of the Bi/GCEs and Bi/PANI/GCEs were investigated. The results showed that the porous PANI interlayers could offer a high specific electrode surface area and diminish accumulating fouling on the electrode surfaces as caused by the surfactants. Thus, the developed Bi/PANI/GCE configuration provides an excellent platform for electroanalysis and has a good potential for the development of some other chemical sensors or biosensors in the presence of surfactants. 5. Acknowledgements Fig. 5. Repetitive SWASV tests for evaluation of stability of Bi/ PANI/GCE in a solution containing 25 nm Cd 2þ (solid line) and 25 nm Pb 2þ (dash line) in the presence of 8 mg/l of Triton X-100. electrode surfaces, which is another major advantage of the Bi/PANI/GCEs over the Bi/GCEs. Ref. [18] reported that a Nafion-coated Bi-film electrode (NCBFE) was used to detect target metals Cd 2þ, Pb 2þ and Zn 2þ in a solution containing Triton X-100 and the results indicated that the NCBFE was much more tolerant to the presence of nonionic surface-active compounds than Bi-film electrode. The merit of a porous polymeric film was that it formed an effective barrier to prevent the macromolecules from moving to the electrode surface. The stability of the Bi/PANI/GEs is tested for eight SWASV cycles in a solution containing 25 nm Cd 2þ and 25 nm Pb 2þ in the presence of 8 mg/l of Triton X-100 with the experimental results shown in Figure 5. The relative standard deviations from the 8 measurements of Cd 2þ and Pb 2þ are about 1.3% and 1.1%, respectively. Thus, the stability of the Bi/PANI/GCEs is satisfactory. For the Bi/ GCEs, the stripping peak currents exhibit a decreasing trend due to the accumulating fouling of the electrode surfaces. The mechanical robustness of the Bi/PANI/GCEs is excellent, especially compared with the Bi/GCEs. A single PANI membrane can be used for a few hours without apparent deterioration of the current signals. In addition, the PANI coatings also provide a good protection to the Bifilms from mechanical damage. 4. Conclusions Bismuth film coated glassy carbon electrodes modified with polyaniline interlayers (Bi/PANI/GCEs) that were developed using multipulse potentiostatic and cyclic voltammetric methods, were successfully used for simultaneous voltammetric determination of trace Cd 2þ and Pb 2þ ions at nm levels. The parameters for the fabrication of the electrodes and the determination of the two trace heavy metals were optimized and the influences of several This work was supported by the research grant from The Environment and Water Industry Development Council (EWI), Singapore. Z. M. Wang and N. W. Khun were also grateful for the PhD scholarships from The Nanyang Technological University (NTU), Singapore. 6. References [1] M. Heitzmann, L. Basaez, F. Brovelli, C. Bucher, D. Limosin, E. Pereira, B. L. Rivas, G. Yal, E. Saint-Aman, J. C. Moutet, Electroanalysis 2005, 17, [2] J. B. Jia, L. Y. Cao, Z. H. Wang, T. X. Wang, Electroanalysis 2008, 20, 542. [3] E. P. Achterberg, C. Braungardt, Anal. Chim. Acta 1999, 400, 381. [4] S. B. Hočevar, B. Ogorevc, J. Wang, B. Pihlar, Electroanalysis 2002, 14, [5] J. Wang, J. Lu, S. B. Hoeevar, P. Farias, B. Ogorevc, Anal. Chem. 2000, 72, [6] J. Wang, Electroanalysis 2005, 17, [7] C. Kokkinos, A. Economou, Curr. Anal. Chem. 2008, 4, 183. [8] A. Bobrowski, J. Zarebski, Curr. Anal. Chem. 2008, 4, 191. [9] N. A. Malakhova, N. Y. Stojko, K. Z. Brainina, Electrochem. Commun. 2007, 9, 221. [10] J. Wang, U. A. Kirgoz, J. Lu, Electrochem. Commun. 2001, 3, 703. [11] W. W. Zhu, N. B. Li, H. Q. Luo, Anal. Lett. 2006, 39, [12] J. Wang, U. A. Kirgoz, J. Lu, Electrochem. Commun. 2001, 3, 703. [13] J. Wang, J. Lu, S. B. Hoeevar, B. Ogorevc, Electroanalysis 2001, 13, 13. [14] J. Wang, J. Lu, S. B. Hoeevar, P. Farias, B. Ogorevc, Anal. Chem. 2000, 72, [15] E. A. Hutton, S. B. Hocevar, B. Ogorevc, M. R. Smyth, Electrochem. Commun. 2003, 5, 765. [16] L. Lin, N. S. Lawrence, S. Thongngamdee, J. Wang, Y. Lin, Talanta 2005a, 65, 144. [17] L. Lin, S. Thongngamdee, J. Wang, Y. Lin, O. Sadik, S. Ly, Anal. Chim. Acta 2005b, 53, 9. [18] G. Kefala, A. Economou, A. Voulgaropoulos, Analyst 2004, 129, [19] I. Adraoui, M. E. Rhazi, A. Amine, Anal. Lett. 2007, 40, 349. [20] C. Gouveia-Caridade, R. Pauliukaite, C. M. A. Brett, Electroanalysis 2006, 18, 854. [21] S. Daniele, M. A. Baldo, C. Bragato, Curr. Anal. Chem. 2008, 4, 215. [22] A. Economou, Trends Anal. Chem. 2005, 24, 334. [23] W. Yang, E. Chow, G. D.Willett, D. B. Hibbert, J. J. Gooding, Analyst 2003, 128, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2010, 22, No. 2,

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