Ion-exchange and permselectivity properties of poly(sodium 4-styrenesulfonate) coatings on glassy carbon: application in the modification of mercury film electrodes for the direct voltammetric analysis of trace metals in estuarine waters
Introduction
For over the last three decades, the development of chemically modified electrodes (CMEs) has been a continuously growing scientific area of renewed interest in diverse fields such as analytical chemistry and sensors, electrocatalysis and energy conversion [1], [2], [3], [4], [5], [6], [7]. Regarding the applications of CMEs in the voltammetric analysis of trace species, special attention has been paid to the improvement of both sensitivity and selectivity, especially for determinations in complex media, such as natural/industrial waters.
In natural or industrial waters containing significantly high concentrations of natural/anthropogenic organic matter or inorganic colloids, the anodic stripping voltammetric (ASV) analytical signals of trace metals at conventional mercury electrodes are often altered or even suppressed due to the adsorption of those surface active compounds onto the electrode [8], [9]. Fouling of the electrode surface can occur also for electrodes other than mercury (i.e., glassy carbon (GC) and graphite) in other complex media such as biological samples where proteins are usually present and thus they become a problem. The most promising procedure to minimize the electrode fouling by surfactants is to prevent the diffusion of the interfering species by coating the electrode surface with a thin layer of a semi-permeable material. The purpose is to preclude the income of the interfering species towards the electrode surface, either by size exclusion and/or by electrostatic exclusion. Several different approaches have been studied and applied: covering the electrode with a dialysis membrane [10], [11] or cellulosic coatings [12], [13] retains colloidal material (usually with molecular weights higher than 1000) but the diffusion of small molecules/ions is frequently hindered, limiting the sensitivity of the voltammetric determinations. On the other hand, coating the electrode surface with ion exchange polymers is widespread: Nafion, the most widely employed cation-exchange polymer [9], [14], [15], [16], [17], [18], [19], [20], [21], poly(ester sulfonic acid) [22], [23], [24], poly(ethyl 3-thiopheneacetate) [16], [21], poly(4-vinylpyridine) [25], [26] or overoxidised poly(pirrole) [27] are examples of such modifiers. In these cases, the target analyte diffuses selectively throughout the electrode surface by an ionic exchange mechanism. However, the anti-fouling properties of these systems are not completely satisfactory and the preparation procedures often show lack of reproducibility [11], [21]. Recently, poly(sodium 4-styrenesulfonate), PSS, was used for coating a thin mercury film electrode, TMFE, and applied to the ASV determination of lead and copper in estuarine water with very low organic matter content [28]. Stable PSS-coated electrodes could be prepared by a very reproducible and easy procedure and presented increased sensitivity to lead and copper cations. Besides, the mechanical stability of the mercury film was improved. The PSS coating prevented to some degree electrode fouling by non-ionic surfactants, e.g., Triton X-100 and agar, and also partially excluded the relatively small surface-active species sodium dodecyl sulfate. Apart from that study, only a brief mention to the use of the acidic form of PSS, poly(styrene sulfonic acid), mixed with Nafion for coating mercury film electrodes has been reported but the coating stability was very low due to the dissolution of poly(styrene sulfonic acid) in water-based electrolytes [20]. No further applications or characterization studies of PSS-coated mercury film electrodes have ever been presented.
On the other hand, PSS coatings on glassy carbon have been prepared by spin-coating of polystyrene and subsequent chemical sulfonation [29]. Also, cross-linked PSS-modified platinum and graphite electrodes were prepared by electropolymerization followed by chemical sulfonation [30]. For both procedures, the resulting electrodes were stable in water but the preparation procedures were intrinsically sluggish and no reproducibility data or permselectivity studies were presented. Modification of platinum electrodes with PSS [31], chlorosulfonated PSS [32] and organosilane–PSS copolymer [33] was applied in experiments in acetonitrile media.
Further, water-insoluble PSS-containing membranes have been applied in electrode modification for the production of amperometric biosensors for glucose, glutamate, H2O2 and l-lactic acid [34], [35], [36], [37], [38], and for the production of layer-by-layer assemblies [39], [40], [41], [42], [43]. However, no full characterization of PSS-modified glassy carbon electrodes (GCEs) has yet been reported, namely regarding its permselectivity, ion-exchange and morphological properties.
In the present work, a comprehensive characterization of the PSS coatings on glassy carbon and a detailed analysis of the performance of PSS-coated thin mercury film electrodes, PSS-TMFEs, are performed. Hence, the effects of some experimental parameters, namely the mercury concentration and the PSS loading, are evaluated. The morphology, the cation exchange ability and the permselectivity properties of the PSS coating are also assessed. The PSS-coated mercury film electrodes were successfully applied to the direct determination of dissolved heavy metals in estuarine water samples contaminated with moderate levels of dissolved organic matter. Filtration of the water samples was the only pre-treatment required in accordance to usual procedures for the determination of dissolved metal ions. Furthermore, the present methodology was checked using a certified seawater sample (NASS-5).
Section snippets
Instrumentation
All voltammetric measurements were performed with a BAS 100B/W electrochemical analyser (Bioanalytical Systems, West Lafayette, IN, USA) connected to a Cell Stand BAS-C2. The working electrode was a TMFE plated onto a glassy carbon disc, GCE1 (BAS, MF-2012); the auxiliary electrode was a Pt wire and the reference electrode was Ag/AgCl (saturated KCl). In some studies, three different glassy carbon electrodes were used: GCE1 and GCE2 from BAS and GCE3 from Metrohm. A combined glass electrode
Results and discussion
A previous work [28] has shown that a thin mercury film could be plated on a GC electrode coated with a layer of the cation-exchange polymer PSS. Fig. 1 presents an optical micrograph of a PSS-coated TMFE produced under the proposed conditions (PSS loading 4.8 μg mm−2 and qHg 7.36 μC mm−2 [28]). Small mercury droplets were formed onto the GC surface, beneath the PSS film, which looks as a rather open and transparent structure. In order to obtain further information on the characteristics of the
Conclusion
The use of a PSS coating onto glassy carbon improved not only the formation of the mercury film leading to high amounts of deposited mercury, but also the sensitivity and the reproducibility of the ASV determinations. PSS films thinner than ca. 10 μm presented no diffusion restrictions to lead and were able to pre-concentrate cations due to electrostatic interactions, namely by an ionic exchange mechanism. PSS coatings presented a rather loose and spongy structure, and are mechanically stable
Acknowledgements
Sandra Monterroso acknowledges the University of Aveiro for a Ph.D. grant. The authors are grateful to Eng. Augusto Lopes (Department of Ceramic and Glass, University of Aveiro) for assistance with the SEM examinations.
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