Voltammetry and spectroelectrochemistry of solid indigo dispersed in carbon paste

Cyclic voltammetry, square wave voltammetry and double potential step UV–vis spectroelectrochemistry were used to study the redox processes of indigo microparticles dispersed in a solid carbon paste electrode. The spectroelectrochemical measurements were performed in a home-made long-path-length thin-layer electrolysis cell. Both the indigo/leucoindigo and indigo/dehydroindigo redox couples underwent reversible 2e−/2H+ surface-confined reactions. Alkaline electrolytes showed more negative effect on the reduction of indigo to leucoindigo than on its oxidation to dehydroindigo. A new species (probably indolone) was monitored in the re-oxidation of leucoindigo, while isatin was found in the oxidation of indigo at enough positive potential. More detailed electrochemical mechanisms were proposed for the two redox systems, respectively. The present work shows that the microparticle-dispersed carbon paste is an attractive electrode material not only for solid state voltammetry but also for stripping spectroelectrochemistry.