Electroreflectance study of the redox reaction of methylene blue adsorbed on a pyrolytic graphite electrode

The electrode reaction of methylene blue (MB) adsorbed on a pyrolytic graphite electrode was studied by using electroreflectance (ER) spectroscopy. It was found that ER spectra represent the difference absorption spectrum between reduced and oxidized forms of MB adsorbed on the electrode surface. The Stark effect does not contribute to the ER spectra at all. Detailed analysis of both ER spectra with the electrode potential modulation and the specular reflectance spectra at constant potentials leads to the conclusion that the reduced form of MB adsorbed on a pyrolytic graphite electrode is not colorless, though the fully-reduced form of MB in an aqueous solution, known as leuco methylene blue (leuco-MB), is colorless. A specific interaction between MB and the graphite electrode surface was suggested.