Antifouling membrane-covered voltammetric microsensor for in situ measurements in natural waters

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 anti-convection 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 (mu-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 mu-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.