Scanning electrochemical microscopy: Diffusion controlled approach curves for conical AFM-SECM tips Kelly Leonhardt a , Amra Avdic b , Alois Lugstein b , Ilya Pobelov c , Thomas Wandlowski c , Bernhard Gollas d, e , Guy Denuault a, f, ⁎ a Chemistry, University of Southampton, Southampton, SO17 1BJ, UK b Solid State Electronics Institute, Vienna University of Technology, Floragasse 7, 1040 Vienna, Austria c Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland d CEST Competence Centre for Electrochemical Surface Technology, Viktor-Kaplan-Strasse 2, Wiener Neustadt, Austria e Institute for Chemistry and Technology of Materials, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria f Institute for Life Sciences, University of Southampton, Southampton, SO17 1 BJ, UK abstract article info Article history: Received 22 August 2012 Received in revised form 25 October 2012 Accepted 26 October 2012 Available online 2 November 2012 Keywords: SECM AFM Conical electrode Approach curve Diffusion control The diffusion controlled response of conical AFM-SECM probes is presented. Accurate expressions are given which describe the dependence of the probe current on the tip radius and aspect ratio, insulating sheath radius and tip–substrate distance for positive feedback and hindered diffusion. A procedure is proposed to determine the tip dimensions from the experimental approach curves. © 2012 Elsevier B.V. All rights reserved. 1. Introduction In SECM the current at a microelectrode is recorded as a redox mediator diffuses between probe and substrate [1]. Combining SECM with shear force positioning [2–4], ion conductance [5], intermittent contact [6] or atomic force microscopy provides reliable tip–substrate distance control. In AFM-SECM the force signal defines the contact point and allows scanning at constant tip–substrate distance thus decoupling the electrochemical response from the topography. This requires AFM tips with integrated electrodes [7–16]. The probes consid- ered here, Fig. 1, have a conical tip with conical insulation [17]; their bulk current, I T,∞ and the role of defects have been quantified [18]. Previous studies [19,20] compared diffusion controlled approach curves at discs and conical tips. The latter have reduced sensitivity towards the substrate in the feedback mode of SECM. Far away, diffusion is hemispherical and controlled by the cone radius. Near the substrate, diffusion is hindered and controlled by the aspect ratio. Large aspect ra- tios yield smaller change in feedback current for decreasing tip–substrate distances. To complement previous studies of AFM-SECM tips [15,21–23] this work presents expressions to analyse diffusion controlled approach curves for conical tips with conical insulation. Electrochemistry Communications 27 (2013) 29–33 ⁎ Corresponding author at: Chemistry, University of Southampton, Southampton, SO17 1BJ, UK. Tel.: +44 2380 592154. E-mail address: gd@soton.ac.uk (G. Denuault). Fig. 1. AFM-SECM conical tip with conical insulation. r tip , h tip , r g , α and l are the electroactive cone radius and height, the insulating cone radius and half angle and the tip–substrate distance. Assumption is made that insulating and electroactive cones have the same half-angle with respect to the vertical axis. Please cite this article as: K. Leonhardt, et al., Scanning electrochemical microscopy: Diffusion controlled approach curves for conical AFM-SECM tips, Electrochemistry Communications (2012), http://dx.doi.org/10.1016/j.elecom.2012.10.034 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.10.034 Contents lists available at SciVerse ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom