Theory and experiment for the substrate generation tip collection mode of the scanning electrochemical microscope: Application as an approach for measuring the diffusion coefficient ratio of a redox couple

A numerical model for the substrate generation/tip collection (SG/TC) mode of the scanning electrochemical microscope (SECM), with amperometric electrodes, is developed and examined experimentally. Under the conditions of interest, this mode involves the diffusion-limited electrolysis of a solution species, R, to produce O, at a macroscopic substrate (generator) electrode. As O diffuses away, a fraction of the species is intercepted by a tip (collector) ultramicroelectrode (UME), positioned above the substrate, where the reverse reaction occurs, leading to the formation and diffusional feedback of R to the substrate. The time-dependent tip current response is shown to be sensitive to the ratio of diffusion coefficients of the O/R couple. Moreover, when the long-time (steady-state) tip current, determined under these conditions, is combined with measurements of the diffusion-limited current for the electrolysis of R at a tip positioned far from the substrate, differences in the diffusion coefficients of the reduced and oxidized forms of the couple can be determined with high precision. Measurements on the ferrocene (Fc)/Fc(+) couple in acetonitrile solution at Pt generator and collector electrodes demonstrate that the diffusion coefficient ratio can readily be obtained from both the transient and steady-state tip responses. The simplicity of the technique is further demonstrated through steady-state measurements on the one-electron reduction of p-benzoquinone (BQ) to form the radical anion, BQ(.-), in acetonitrile and the ferrocyanide/ferricyanide couple in aqueous solution. For these systems, and the Fc/Fc(+) couple, it is shown that, by making measurements of the steady-state tip current in both the SG/TC and feedback modes at a particular tip/substrate distance, the ratio of diffusion coefficients of the mediator couple may be determined without any knowledge of the sizes of, or the separation between, the two electrodes.