MERCURY-PLATED IRIDIUM-BASED MICROELECTRODE ARRAYS FOR TRACE-METALS DETECTION BY VOLTAMMETRY - OPTIMUM CONDITIONS AND RELIABILITY

An amperometric microsensor for the detection of trace metals in the l ow or sub nanomolar range is presented. It is obtained by successively evaporating iridium (2000 Angstrom) and Si3N4 (2000 Angstrom) on a si licon wafer, followed by a photolithographic pattering procedure. It c onsists of an array of 10x10 iridium microdisc electrodes with a reces sed depth of 0.2 mu m, separated by 50 or 150 mu m. The electrical con tacts are isolated by a layer of Agolit or Epoxy resin. Scanning elect ron microscopy and Atomic force microscopy have been used to control t he regularity of the microelectrode array geometry and morphology. For the analysis of trace metals, mercury is deposited on the iridium-bas ed microelectrode array. A given array is able to sustain the operatio ns of Hg deposition/dissolution at least 10 times. The reliability of the mercury-plated iridium-based microelectrode arrays has been tested by a series of systematic Square Wave Anodic Striping Voltammetry (SW ASV) analyses in synthetic solutions of lead and cadmium ions in the c oncentration range 1-10 nM. Repeated measurements over long periods of time on a given mercury layer showed good stability when the Epoxy re sin was used and good reproducibility (+/-4%) for at least 5 h. A good reproducibility was also found between different arrays. Finally, the mercury-plated iridium-based microelectrode arrays were applied to th e lead and cadmium speciation in river water, by direct SWASV measurem ents, without any separation. A detection limit of 50 pM was establish ed for a preconcentration time of 15 min. The results were compared wi th other techniques, in particular a similar procedure using a single Hg-plated Ir-based microelectrode.