Effect of the insulating shield thickness on the steady-state diffusion-limiting current of sphere cap microelectrodes.

The effect of the insulating shield thickness on the steady-state diffusion-limiting current of sphere cap microelectrodes is investigated. Theoretical steady-state limiting currents are obtained by using a simulation procedure, which relies on the explicit finite difference method with a fixed time grid and an exponentially spatial grid. The results obtained indicate that the current increases by decreasing the thickness of the insulating sheath or by increasing the aspect ratio of the sphere cap (h/a, where h is the height of the sphere cap and a is the electrode basal radius), similarly to other types of microelectrodes with different electrode geometry, such as disks and finite cones. The simulated data are fitted to approximate analytical expressions to describe the dependence of the limiting current on both h/a and RG (RG=b/a, where b is the overall tip radius) parameter. Theoretical currents are also compared with experimental data, which are obtained with a range of mercury-coated platinum microelectrodes having different RG and h/a values. The measurements are performed by using cyclic voltammetry at 1 mVs(-1), in aqueous solutions containing Ru(NH3)6-Cl3 as electroactive species. A good agreement (within 3%) between theoretical and experimental steady-state currents is found. Finally, SECM operating in the feedback mode is used to assess the validity of the shape parameters found by voltammetry for sphere cap microelectrodes, whose insulating shields are of a thickness comparable to the electrode radius.