Step-by-step deposition of synthetic dopamine-eumelanin and metal cations

• Step-by-step assembly of dopamine-eumelanin and metal cations (Cu2+, Fe3+, La3+) is possible.
• The coatings made from trivalent cations growth faster than those made from Cu2+.
• Fe3+ does not undergo a change in its redox state upon incorporation in the coatings.

The photoprotection of skin depends mostly on a balance between two natural pigments: the black–brown eumelanin and the yellow-reddish pheomelanin. These pigments as well as their counterpart in the central nervous system, neuromelanin, interact strongly with metal cations like Fe3+. In the mussel foot proteins, the coordination between catechol groups and these ions is also responsible for the strong hardness the mussel’s cuticle. These examples are suggestive that coatings and materials can be made based on such materials, for instance, synthetic eumelanin colloids and metal cations. Herein, we demonstrate that films made from synthetic dopamine-eumelanin colloids and metal cations can be deposited on surfaces in a step-by-step manner. As typical metal cations, we used Cu2+, Fe3+, and La3+ which are known to interact with eumelanins. In all cases, step-by-step deposition is only possible in the absence of water rinse between two deposition steps. It was found that trivalent cations allow for a faster increase in film deposition than Cu2+. Complementary in PDADMAC-(eumelanin-Fe3+)m films, iron III was found not to be reduced.

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