SCANNING ELECTROCHEMICAL MICROSCOPY - THEORY AND EXPERIMENT FOR THE POSITIVE FEEDBACK MODE WITH UNEQUAL DIFFUSION-COEFFICIENTS OF THE REDOXMEDIATOR COUPLE

Theory for the chronoamperometric positive feedback mode of the scanni ng electrochemical microscope (SECM) is extended to include the situat ion where the oxidised and reduced forms of the redox mediator couple have arbitrary diffusion coefficients. Under typical positive feedback conditions, the solution initially contains a redox-active species, R , along with excess supporting electrolyte. The potential of the tip u ltramicroelectrode (UME), positioned close to an interface of interest , is adjusted to a value where R is electrolysed to produce species O at a diffusion-controlled rate. O diffuses away from the tip towards t he interface, where the reverse redox reaction occurs leading to the p roduction, and diffusional feedback of R for electrolysis at the tip e lectrode. When positive feedback measurements are carried out under di ffusion-controlled chronoamperometric conditions, the form of the norm alised current-time behaviour, at a particular tip to interface distan ce, is found to be sensitive to the ratio of the diffusion coefficient s of the O/R couple. As a steady-state is established, the normalised current becomes independent of the diffusion coefficient ratio and dep ends only on the tip to interface distance. Experimental measurements on the chronoamperometric oxidation of ferrocene (Fc) in acetonitrile solution at a Pt tip UME positioned close to a Pt substrate electrode, provide support for the theoretical predictions and demonstrate that the diffusion coefficient ratio can readily be determined from SECM ch ronoamperometry. Although Fc and Fc(+) are often assumed to have the s ame diffusion coefficients, steady-state and chronoamperometric measur ements at a conventional UME yield a value of 2.0 X 10(-5) cm(2) s(-1) for the diffusion coefficient of Fc, while SECM chronoamperometry ind icates that the diffusion coefficient of Fc(+) is 1.6 X 10(-5) cm(2) s (-1). (C) 1997 Elsevier Science S.A.