Scanning electrochemical microscopy: beyond the solid/liquid interface Anna L. Barker, Marylou Gonsalves, Julie V. Macpherson, Christopher J. Slevin, Patrick R. Unwin * Department of Chemistry, University of Warwick, Coventry CV4 7 AL, UK Received 23 July 1998; accepted 11 August 1998 Abstract Recent progress has seen scanning electrochemical microscopy (SECM) emerge as a powerful technique for probing physicochemical interfacial processes, beyond the solid/liquid and electrode/electrolyte interfaces that were originally of primary interest. This review (with 92 references) assesses recent developments in SECM as a methodology for investigating liquid/liquid and liquid/gas interfaces, along with processes of biochemical and biophysical signi®cance. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Ultramicroelectrodes; Liquid/liquid interface; Liquid/gas interface; Kinetics; Permeability 1. Introduction It is 10 years since the term ``scanning electroche- mical microscopy'' (SECM) was introduced by Bard et al. [1] to describe an instrument in which a mobile ultramicroelectrode (UME), in an electrolyte solution, was scanned in close proximity to a solid surface to characterise the topography and redox activity of the solid/liquid interface. The formulation of a theory for SECM [2] and further applications [3] provided a framework for a diversity of subsequent developments in this ®eld. SECM itself has its origins in earlier in situ electrochemical scanning tunnelling microscopy (STM) studies from Bard's group [4] and simulta- neous work by Engstrom et al. [5,6], who were the ®rst to show that an amperometric UME could be used as a local probe to map the concentration pro®le of a larger active electrode. The ®eld has now matured to a status where, at the time of writing, there are more than 280 published papers [7] and commercial instruments are available [8,9]. The original primary interest in SECM was as a tool for characterising and probing electrode/electrolyte interfaces, e.g. (i) mapping topography [3]; (ii) for modi®cation purposes [10] and (iii) in the investiga- tion of electrochemical dynamics. The latter topic encompasses reactivity imaging of electrodes [11], heterogeneous electron transfer kinetics [12±14], redox processes at semiconductors [15,16], coupled solution reaction kinetics [17], and processes in poly- mer modi®ed electrodes [18,19]. SECM studies of Analytica Chimica Acta 385 (1999) 223±240 *Corresponding author. Fax: +44-1203-524112; e-mail: p.r.unwin@warwick.ac.uk 0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0003-2670(98)00588-1