Eumelanin, a brown-black pigment ubiquitous in the human body, can be oxidatively polymerized of 5,6-dihydroxyindole (DHI) and/or 5,6-dihydroxyindole-2 carboxylic acid (DHICA) . This redox-active pigment features interesting functional properties, such as photoprotection, antioxidation, metal binding affinity and free radical scavenging.
The electrochemical reduction of oxygen on quinone-modified electrodes, including iron-melanin complexes, has been demonstrated [2,3]. Here we report on the possibility to use eumelanin as catalyst for oxygen reduction. In our study, we used chemically controlled melanin, obtained by solid-state polymerization of the DHI and DHICA building blocks (to give respectively DHI- and DHICA-eumelanin) . Controlling the (supra)molecular structure of eumelanin is imperative to gain insight on its catalytic properties and to fully exploit its technological potential. Indeed, due to the limited processability of eumelanin, fundamental aspects related to the effect of the (supra)molecular structure on the catalytic properties of the pigment are largely undiscovered.
For our oxygen-reduction experiments we made use of eumelanin solid state polymerized on glassy carbon rotating disk electrodes from the two building blocks, DHI and DHICA. The experiments were conducted in a wide range of experimental conditions such that a dramatic effect on the catalytic properties were observed as a function of the chemical composition of the electrolyte, pH and – above all – (supra)molecular structure of the biopigment.
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