On the alkali instability of phenoxazine dyes as adsorbates of chemically modified electrodes

The alkali instability of phenoxazine dyes was investigated with respect to their application as catalysts for anodic NADH oxidation. Spectrophotometric measurements were used in aqueous solutions at different pH values together with cyclovoltammograms of the dyes adsorbed on a graphite electrode. It is shown that the reason for desactivation in the case of Meldola Blue is a cation-pseudobase equilibrium followed by a ring-opening tautomerism. The open-ring isomer, an unstable o,p′-quinoneanile dye, could be isolated as a dark blue precipitate. Also a by-product (“dimethyl”-Nile Red V) was identified. In the case of phenoxazine dyes substituted with an amino or acylamino group in p-position to the heterocyclic nitrogen, the reason for desactivation of the electrodes is the known reversible formation of a quinone diimine as an anhydrobase.