Hot electron-induced electrochemiluminescence of fluorescein in aqueous solution

Strong electrogenerated chemiluminescence (ECL) of fluorescein is generated in aqueous solution during cathodic pulse polarization of oxide-covered aluminum electrodes. The present method is based on the injection of hot electrons into the aqueous electrolyte solution, which probably results in the generation of hydrated electrons as reducing mediators. The successive one-electron redox reactions result in fluorescein in its lowest excited singlet state, i.e., the emission spectrum is similar to the corresponding fluorescence emission spectrum. This ECL can be enhanced by added coreactants, e.g., by peroxodisulfate, peroxodiphosphate, hydrogen peroxide and azide ions. The method can detect fluorescein over several orders of magnitude of concentration with detection limit below nanomolar concentration level in the presence of azide ions. The results suggest that the derivatives of fluorescein, such as fluorescein isothiocyanate, can be used as electrochemiluminescent labels in aqueous solution in bioaffinity assays carried out on the surface of thin aluminum oxide film-coated aluminum electrodes. This cathodic ECL is mainly based on the one-electron oxidation of fluorescein by the cathodically produced oxidizing radicals followed by reduction of the formed semioxidized fluorescein by hot or hydrated electrons back to the original oxidation state which finally emits light.