Electrochimica Acta, Vol. 36, No. 516, pp. 163-111, 1991 0013-4686/91 $3.00 + 0.00 Printed in Great Britain. Pergamon Press pk. THIN FILM ELECTRODE: A NEW METHOD FOR THE FABRICATION OF SUBMICROMETER BAND ELECTRODES B. J. SEDWN,* M. J. EDDowEs,t A. FIRTH,$ A. E. OWENS and H. H. J. GIRAULT* *Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 355, U.K. tThom EMI, Central Research Laboratories, Dawley Road, Hayes UB3 IHH, U.K. IDepartment of Electrical Engineering, University of Edinburgh, Mayfield Road, Edinburgh EH9 355, U.K. (Received 25 June 1990) Abstract-A method is presented for the construction of nanometer scale band electrodes based on thin Iihn laminate materials. The electrodes are made using established thin film deposition techniques including metal evaporation and polymer spin coating. Such methods permit fabrication of highly uniform submicrometer metal and insulator layers. The physical nature of the Shn composite, which is highly flexible and robust, makes it suitable for the assembly of band electrodes in novel designs with a view to electroanalytical applications. The laminate band electrodes fabricated by this film process have been characterized by chronoamperometry and cyclic voltammetry for the reduction of Fe(W):- in aqueous KC1 solution. A discussion is presented detailing the experimental i-t behaviour with respect to electrode preparation. Extension of thin laminate band electrodes to electroanalysis is also included. Key wora!r: microelectrode, electrochemistry, thin film. INTRODUCTION The amperometric behaviour of micrometer band electrodes continues to be an active area of micro- electrode research as indicated by a steady flow of publications on the subject since the mid-1980s, ([l-26] and references therein). The interest follows earlier physiological investigations concerned with the development of a stable oxygen electrode where the analytical advantages of microband electrodes were put forward[27,28]. Current investigations have dealt principally with time-dependent aspects of mass trans- port problems at isolated and interacting electrode ensembles in stationary solution and flow systems. The focus of recent theoretical work, however, has been concerned with a fuller analytical description of the time-dependent diffusion field at single or isolated microband electrodes[2,3]. In this context, comparison has been made between the limiting diffusion current response for band and hemicylindrical electrode geometries[4-61. The i-t response for a microband electrode of bandwidth W, approximates that of a micrometer hemicylinder of radius r, where w = nr. The numerical and limiting analytical expressions derived for (hemi)cylinder electrodes are applicable to the band geometry. A useful development in this area was the introduction of an empirical expression for the chronoamperometric response of reversible redox species at microband electrodes[7l [see equation (4)]. This relation presented by Aoki et n1.[7] pre- dicted accurately i-t data for micrometer scale band electrodes of bandwidth greater than 20 pm. The diffusion controlled chronoamperometric characteristics of micrometer scale band electrodes in static solution are by now well documented. The main features of these electrodes which are of importance to analytical amperometry are listed as follows. (a) Large faradaic currents are observed as a consequence of one macroscopic dimension, ie the band length 1. This typically results in the measure- ment of a limiting diffusion current of the order of PA mM-’ in comparison with microdisc electrodes where the currents observed are the order of pA mM-‘. Microband electrodes are usually several millimeters long. However, if the band length is reduced below ca 200pm edge effect at the extremity of the band increase the current density and the limiting current response will not conform to standard hemicylindrical flux expressions. This behaviour has been published recently without emphasis on the small length of the microband electrodes used, ie 50 pm[20]. (b) An increased current density over that of macro- scopic electrodes is obtained. This feature confers greater amperometric sensitivity per unit electrode area, permitting determination of analyte at low micromolar concentration by quasi-steady state current measurement. (c) For sufficiently small bandwidth, cu w < 5 pm [or more precisely for small values of the dimension- less width w/,/(Dt)] a current response is observed which obeys an inverse logarithmic time function at long electrolysis time. Being given the nature of this function, the signal is a diffusion current in virtual steady-state. (d) The chronoamperometric response time, T, for a given D, is dependent on bandwidth, w. In this instance, T, is defined as the time for the onset of a quasi-steady-state current which may be set at a given rate of change di/dt applicable to the analytical requirements. 163