J. Electroanal. Chem., 130 (1981) 81-97 81 Elsevier Sequoia S.A., Lausanne--Printed in The Netherlands ELECTROCHEMICAL KINETICS AT MICROSCOPICALLY SMALL ELECTRODES BENJAMIN SCHARIFKER and GRAHAM HILLS Department of Chemist~, The University, Southampton S09 5NH (England) (Received 7th August 1980; in revised form 23rd June 1981). ABSTRACT Two applications of microscopically small electrodes are described, one relating to the kinetics of homogeneous redox reactions and thc other to the heterogeneous electrochemical nucleation of metals. Spherical diffusion to exposed areas of < 10 -6 cm 2 gives rise to very high rates of mass transfer (<[:50 A cm Z)~which allow steady-state fluxes to be determined without external control of convection. The study of electrochemical kinetics is therefore greatly simplified. :Fhe effectiveness of the new methodology is exemplified using the homogeneous system: KsFe(CN)36 - + e = Fe(CN)~- and the nucleation reaction: Hg~ + + 2e -, 2Hg INTRODUCTION Metal microelectrodes are widely used to measure or stimulate phenomena inside single biological cells or neural units [1,2]. The diameter of the exposed area is typically of the order of 1 #m, and their applications in biological studies range from the measurement [3] of intracellular pO E to the design of auditory prostheses that function by direct electrical stimulation of the auditory nerve [4]. Several techniques for making what might be termed milli-microelectrodes or nano-electrodes have been devised, ranging from coating wires with insulating films [5] to the use of integrated microcircuit technology [6]. The increasing efficiency of electronic instrumentation for the high-speed detection of low currents makes their application to electrochemi- cal kinetic studies attractive and, in particular, the microscopically small area of the microelectrode surface makes it possible to observe an isolated nucleation event. The chemical, physical and interfacial properties of such microscopic electrodes do not differ significantly from those of more conventional size, but their small dimensions merit special consideration. Studies of electrochemical kinetics invariably require either the control or the measurements of the potential of the working electrode with respect to a non- polarised reference electrode in equilibrium with the electrolytic solution. Thus, 0022-0728/81/0000-0000/$02.75 © 1981 Elsevier Sequoia S.A.