Characteristic bleaching profiles of cyanine dyes depending on active oxygen species in the controlled Fenton reaction.

The bleaching of cyanine-type pentamethine trinuclear dyes by active oxygen species (AOS), superoxide and hydroxyl radical, were studied under the xanthine-xanthine oxidase system and the Fenton reaction, respectively. The gradual bleaching of dyes occurred as the result of superoxide produced in the xanthine-xanthine oxidase system. However, the bleaching of cyanine dyes by Fenton reagent varied depending on the reaction conditions. If a normal Fenton reaction of Fe(II) and hydrogen peroxide (H2O2) took place, a hydroxyl radical was generated instantly. A Fenton reaction with an excess of [H2O2] over [Fe(II)] resulted in a gradual bleaching of the dye initiated by the hydroxyl radical, Fe(III) and H2O2. In this reaction, cyanine dyes with shorter side chains were bleached faster than those with longer ones. We controlled the Fenton reaction condition (Fe(II) or Fe(IIO)/H2O2 at pH 3.5 in the dark) to generate a specific AOS such as a hydroxyl radical (.OH) or superoxide (.O2-). Studies using xanthine-xanthine oxidase (pH 7.8), the Fe(II)-dipyridyl complex and various scavengers such as superoxide dismutase (SOD) and hyaluronic acid, revealed that .O2- was the primary radical responsible for this controlled Fenton reaction. This finding shows that this controlled Fenton reaction would be an effective AOS generation method, and that cyanine dyes may be hopeful probes for the detection of AOS.