Comparison of synthetic dopamine-eumelanin formed in the presence of oxygen and Cu2+ cations as oxidants.

Eumelanin is not only a ubiquitous pigment among living organisms with photoprotective and antioxidant functions, but is also the subject of intense interest in materials science due to its photoconductivity and as a possible universal coating platform, known as "polydopamine films". The structure of eumelanin remains largely elusive, relying either on a polymeric model or on a heterogeneous aggregate structure. The structure of eumelanin as well as that of the closely related "polydopamine films" can be modified by playing on the nature of the oxidant used to oxidize dopamine or related compounds. In this investigation, we show that dopamine-eumelanins produced from dopamine in the presence of either air (O2 being the oxidant) or Cu(2+) cations display drastically different optical and colloidal properties in relation with a different supramolecular assembly of the oligomers of 5,6 dihydroxyindole, the final oxidation product of dopamine. The possible origin of these differences is discussed on the basis of Cu(2+) incorporation in Cu dopamine-eumelanin.