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 amperomet ric electrodes, is developed and examined experimentally. Under the co nditions of interest, this mode involves the diffusion-limited electro lysis of a solution species, R, to produce O, at a macroscopic substra te (generator) electrode. As O diffuses away, a fraction of the specie s is intercepted by a tip (collector) ultramicroelectrode (UME), posit ioned above the substrate, where the reverse reaction occurs, leading to the formation and diffusional feedback of R to the substrate. The t ime-dependent tip current response is shown to be sensitive to the rat io of diffusion coefficients of the O/R couple. Moreover, when the lon g-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, differe nces in the diffusion coefficients of the reduced and oxidized forms o f the couple can be determined with high precision. Measurements on th e ferrocene (Fc)/Fc(+) couple in acetonitrile solution at Pt generator and collector electrodes demonstrate that the diffusion coefficient r atio 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 a nd the ferrocyanide/ferricyanide couple in aqueous solution. For these systems, and the Fc/Fc(+) couple, it is shown that, by making measure ments of the steady-state tip current in both the SG/TC and feedback m odes at a particular tip/substrate distance, the ratio of diffusion co efficients of the mediator couple may be determined without any knowle dge of the sizes of, or the separation between, the two electrodes.