A novel, effective, antifouling membrane-covered voltammetric microsensor has been developed. It combines the unique properties of microelectrodes and diffusion in gels. An agarose gel, with thickness varying in the range 0.4−0.9 mm, is used as the membrane and acts as a dialysis membrane, i.e. allows diffusion of small ions and molecules and retains colloidal materials. The voltammetric microelectrode measures the test compounds within the gel after equilibration. Diffusion of ions through various types of gels has been investigated by diffusion cell, square wave cathodic sweep voltammetry (SWCSV), and square wave anodic stripping voltammetry (SWASV) to characterize the diffusion properties of the agarose gel membranes. These studies revealed that (i) the agarose gel used is totally inert toward target compounds and (ii) the anticonvection properties of the agarose gel ensure purely diffusion-controlled transport within the membrane. Diffusion coefficient values of ions in the agarose membrane were found to be lower than those in free solution. Considering the inertness of the agarose gel used toward the ions measured, this discrepancy can be ascribed to the intrinsic physical properties of the agarose gel which influence the ion mobility. Reproducible values have been obtained for equilibration times and diffusion coefficients for gels prepared under controlled conditions with a given type and concentration of agarose. The agarose membrane-covered, Hg-plated, Ir-based microelectrode (μ-AMMIE) was applied to lead and cadmium analysis by SWASV in the presence of 10−31 mg/L of fulvic and humic compounds and in raw river waters containing high concentrations of suspended matters (50−78 mg/L). Results of these measurements under such drastic conditions confirm the efficiency of the agarose gel membrane against adsorption of organic and inorganic matters on the Hg surface of the voltammetric microsensor. Thus, these results demonstate that direct measurements of analytes in complex media can be made with μ-AMMIE without physical and chemical interferences of the test solution, in particular due to the relatively large gel thickness compared to the diffusion layer thickness of the microelectrode and of the properties of the agarose gels used.

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Construction of a reproducible, reliable and rugged iridium-based microelectrode is described. Perfect electrical contact, Ir-glass sealing and Ir disk morphology are the key points for obtaining reproducible voltammetric sensors. These points are discussed in detail and optimal fabrication conditions are given. Electron bombardment under vacuum yielded good soldering between Ir and Cu electrical cable. Reproducible polishing of the microelectrode to a mirror like Ir disk surface is obtained with silicon carbide pads and diamond paste using an automatic home made polisher. Cyclic voltammetry and optical microscopy have been used to characterize this microelectrode. Hgplated Ir-based microelectrodes are prepared by electrodeposition of mercury on the iridium disk substrate. Reproducibility and reliability close to 100% have been obtained for Ir-based microelectrode preparation, mercury layer deposition and trace metal measurements by differential pulse anodic stripping voltammetry (DPASV) and square wave anodic stripping voltammetry (SWASV) in synthetic solutions even at low ionic strength M). Continuous measurements over long periods of time indicate that the Hg-plated Ir-based microelectrodes can be used for several days without renewal of the mercury layer. These microelectrodes were applied to lead and cadmium speciation studies directly in river waters by SWASV without any separation. The results show that free Pb" and Cd" concentrations as low as 0.5nM and O . l n~ respectively can be determined by direct measurements, without perturbing the medium. The lifetime of these microelectrodes in the present state of the art is more than 2 years.

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The state of the art and challenges in developing in situ voltammetric probes for direct measurements in natural waters are presented. Articles dealing with this topic are reviewed. Use of voltammetric probes in the laboratory and in the field is outlined. An up-to-date account of the results reported in the literature on the development of existing in situ voltammetric systems for such measurements is given. Emphasis is placed on the need for such systems to gain a better understanding of the mechanisms of processes occurring in natural aquatic systems. Suggestions are made for future development.