Voltammetric Determination of the Geometrical Parameters of Inlaid Microdisks with Shields of Thickness Comparable to the Electrode Radius

The cyclic voltammetric behavior of disk microelectrodes surrounded by thin insulating shields (TSM) was investigated from both theoretical and experimental points of view. In particular, microdisks with shields of thickness (b - a), a few electrode radii (a) were considered. A finite difference simulation procedure with a nonuniform, expanding spatial grid, already available in the literature, was employed for predicting shape and height of the voltammograms. The parameters of this numerical simulation were optimized again, and the steady-state limiting currents found in this work for a range of TSMs compared well with previous publications. The steady-state limiting current at TSMs was enhanced with respect to microelectrodes surrounded by thick insulating sheaths (i.e., b >> a), and steady-state conditions were achieved faster. Under these conditions, the difference in potential observed on the forward and backward waves, when the current is half of its maximum value (deltaE(1/2)), was almost equal to zero. Nonsteady-state cyclic voltammograms were also simulated using the optimized parameters, and the effect of scan rate on deltaE(1/2) was examined in detail. Based on this dependence, a voltammetric procedure for the simultaneous determination of microdisk radius and its insulator thickness was proposed. Experimental measurements were performed by using platinum wires, 10-12.5-microm radius, and carbon fibers, 4-microm radius, coated with 4-6-microm-thick electrophoretic paint. The shield thickness produced around the wires was such that b/a < 3. The experimental results obtained were in general congruent with the theory and demonstrated the validity of the method proposed here for the simultaneous determination of radius and shield thickness of a TSM.