Mercury-electroplated-iridium microelectrode array based sensors for the detection of heavy metal ultratraces: optimization of the mercury charge

Mercury-electroplated-Ir microelectrode array based sensors have been developed; their electroanalytical performance is investigated for the simultaneous detection of ultratraces of Pb, Cd and Zn. Two Ir microelectrode array geometries were used, namely: (i) an array of 1089 microelectrodes of 3-μm diameter and (ii) an array of 1764 microelectrodes of 6-μm diameter. Prior to the Hg electroplating, the arrays were systematically characterized by means of cyclic voltammetry at various scan rates ranging from 5 to 500 mV s⁻¹ and found to exhibit a steady-state voltammetric behavior. The effect of the Hg charge (over a range as wide as 0.05–40 mC) on the electroanalytical performance (net peak current, peak potential and peak width) of the metal trace sensors has been systematically studied by using square wave anodic stripping voltammetry (SWASV). It is clearly shown that the net peak current not only depends on the magnitude of Hg charge, but also presents a maximum value for an optimum Hg charge. Optimum Hg charges of 4 and 15 mC were identified for the 3- and 6-μm diameter Ir microelectrodes, respectively. At the optimum Hg charges, calibration plots demonstrated good linearity for the three metal traces over a concentration range as wide as (100 ppt–1 ppm). By increasing the preconcentration time from 5 to 20 min, it is shown that the detection limit of metal ultratraces can be decreased from 100 to 20 ppt.