Electrochemical reactivity studies are usually performed by conventional electrochemical measurements carried out with a single electrode of either macroscopic (∼millimeter) or microscopic (∼micrometer) dimensions. Such kind of measurements provides information which can be time resolved down to very short time scales, but which is typically averaged over the entire electrode area and is thus insensitive to spatial heterogeneities in the x-y plane of the electrode. The measured current also gives access to the transport properties of the involved redox-active species, but this additional information corresponding to the z-direction axial to the electrode is rather indirect. Coupling electrochemistry with confocal fluorescence microscopy (EC-fluorescence) is an expedient way to obtain 3-dimensional spatial information on electrochemical reactions, in-situ and in real time. The optical sectioning enabled by confocal microscopy is particularly useful to map individual chemical species in the diffusion layer, especially for unstable electrogenerated species that further react homogeneously.

The present contribution focuses on the properties of the electroactive, fluorogenic molecule resorufin (see Figure), to illustrate the many potentialities of EC-fluorescence. The highly fluorescent resorufin can be generated electrochemically by the irreversible reduction of its N-oxide parent compound resazurin which is slightly fluorescent. It can also be reduced reversibly to the non-fluorescent dihydroresorufin, whose acetyl derivative is actually the popular Amplex^® Red that can be oxidized to generate resorufin. This exhaustive electrochemistry covers several electrochemical reaction mechanisms ranging from simple electron transfers to coupled electrochemical-chemical steps, with different degrees of reversibility. A comproportionation scheme is also evidenced for the first time. Each of these reactions is associated with specific characteristics of the reaction-diffusion layer that can be probed with an appropriate spatial resolution by EC-fluorescence.