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 low or sub nanomolar range is presented. It is obtained by successively evaporating iridium (2000 Angstrom) and Si3N4 (2000 Angstrom) on a silicon wafer, followed by a photolithographic pattering procedure. It consists of an array of 10x10 iridium microdisc electrodes with a recessed depth of 0.2 mu m, separated by 50 or 150 mu m. The electrical contacts are isolated by a layer of Agolit or Epoxy resin. Scanning electron microscopy and Atomic force microscopy have been used to control the regularity of the microelectrode array geometry and morphology. For the analysis of trace metals, mercury is deposited on the iridium-based microelectrode array. A given array is able to sustain the operations 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 (SWASV) analyses in synthetic solutions of lead and cadmium ions in the concentration range 1-10 nM. Repeated measurements over long periods of time on a given mercury layer showed good stability when the Epoxy resin 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 the lead and cadmium speciation in river water, by direct SWASV measurements, without any separation. A detection limit of 50 pM was established for a preconcentration time of 15 min. The results were compared with other techniques, in particular a similar procedure using a single Hg-plated Ir-based microelectrode.