Thin-film electrodes for trace metal analysis by dc resistance changes

The lateral d.c. resistivity of thin metal films with layer thicknesses of less than 30 nm is increased due to the adsorption of certain particles and is decreased by their desorption. The contribution of the adatoms to the film resistivity can be understood similarly to the effect of foreign atoms in a bulk metal. The magnitude of the resistivity increase is related to the surface coverage of the thin metal film. Using thin metal films of gold as working electrodes in a conventional three-electrode arrangement, a novel electrochemical microsensor, based on the described mechanism of the surface resistivity changes has been developed. The thin film sensor has been prepared by means of process steps of silicon planar technology. With this sensor the trace analysis of heavy metals, such as cadmium, lead, nickel, thallium, and zinc ions as well as cadmium-EDTA complexes in aqueous solutions is possible. The different species could be distinguished from each other due to their characteristic stripping potentials. For the investigated species a linear signal relation has been obtained over a wide range of concentrations from several ppb to some ppm.