Scanning electrochemical microscopy (SECM) allows for the direct high-resolution vizualization of local variations in the (electro)chemical reactivity of solid/liquid, liquid/liquid, and even liquid/air interfaces. For a solid/liquid interface, as the electrode probe approaches the surface in the solution, its current, due to reduction or oxidation of freely-diffusing electroactive species, decreases over an insulating material (negative feedback) and increases over conductive material (positive feedback). These phenomena are often used in classical SECM to determine the tip-to-sample distance for relatively flat and homogeneously conductive samples. The SECM feedback mode has also been used for high resolution imaging of topographic changes and conductivity variations [1].
However, use of current feedback information for positioning and imaging is only possible for well-defined systems and is inappropriate for surfaces exhibiting complex topography and complex conductivity or for probes that do not consume (and produce) electroactives species (i.e. potentiometric probes).
Moreover, current feedback is strongly dependant on the radius of the electrode/probe. The gap between the electrode and sample surface must be within one radius of the electrode to achieve enough current feedback to control positioning. Thus, small electrodes required for higher spatial resolution must be positioned closer to the surface. The experiment is very delicate on flat surface and almost impossible on a sample showing complex topographic features.
One possible strategy to overcome these limitations is the implementation of a shearforce-based distance control in SECM, that allows to dissociate tip-positioning and electrochemical measurement. Tip-to-sample distance is then determined with a good accuracy using shearforce dampening of the electrode tip vibration. We used a non-optical detection based on two piezoelectric plates mechanically attached to the body of the electrode [2]. The first one stimulates the tip oscillation, while the second one is used to measure the response of the system. The setup is rather easy to implement on potentiometric [3,7] or amperometric [4] electrodes, and can be used to perform SECM experiments in feedback [2,3] or generation/collection modes [4-6]. One important question to be discussed is the performance of the tip positioning as it is a very critical parameter for the good operation of the electrochemical measurement.

[1] A.J. Bard, M.V. Mirkin, Scanning Electrochemical Microscopy. Marcel Dekker: New York, 2001.

[2] B. Ballestero-Katemann, A. Schulte, W. Schuhmann, Chem. Eur. J., 9, 2025 (2003).

[3] M. Etienne, A. Schulte, W. Schuhmann, Electrochem. Commun., 6, 288 (2004).

[4]M. Etienne, A. Schulte, S. Mann, G. Jordan, I. Dietzel-Meyer, W. Schuhmann, Anal. Chem., 76, 3682 (2004)

[5] M. Etienne, E.C. Anderson, S.R. Evans, W. Schuhmann, I. Fritsch, Anal. Chem., 78, 7317 (2006)

[6] M. Etienne, P. Dierkes, T. Erichsen, W. Schuhmann, I. Fritsch, Electroanalysis, 19, 318 (2007)