Chemically irreversible redox mediator for SECM kinetics investigations: determination of the absolute tip-sample distance.

The use of a chemically irreversible redox probe in scanning electrochemical microscopy (SECM) was evaluated for the determination of the absolute tip-substrate distance. This data is required for a quantitative use of the method in the analysis of functional surfaces with an unknown redox response. Associated with the relevant model curves, the electrochemical response allows an easy positioning of the tip versus the substrate that is independent of the nature of the materials under investigation. The irreversible oxidation of polyaromatic compounds was found to be well adapted for such investigations in organic media. Anthracene oxidation in acetonitrile was chosen as a demonstrative example for evaluating the errors and limits of the procedure. Interest in the procedure was exemplified for the local investigations of surfaces modified by redox entities. This permits discrimination between the different processes occurring at the sample surface as the permeability of the probe through the layer or the charge transfer pathways. It was possible to observe small differences with simple kinetic models (irreversible charge transfer) that are related to permeation: charge transport steps through a permeable redox layer.