Melanin Made by Dopamine Oxidation: Thin Films and Interactions with Polyelectrolyte Multilayers
The spontaneous oxidation of dopamine in slightly alkaline solutions was investigated on the basis of the work of Lee and others [Science, 318:426-430, 2007], and the reaction product was identified as dopamine-melanin. The ability of melanin to covalently bind amine functional groups was confirmed by quantification of the corresponding binding sites on dopamine-melanin aggregates. Furthermore it is possible to redissolve dopamine-melanin aggregates in strongly alkaline solutions. The obtained small melanin grains were used to build layer-by-layer deposits with poly(diallyldimethylammonium). Different methods of dopamine oxidation to grow melanin films at solid--liquid interfaces were developed. All examined methods lead to continuous dopamine-melanin films with very similar surface morphologies. The dopamine-melanin films become impermeable to electrochemical probes at a thickness in the 10 nm range. In this context a higher permeability for positively charged and neutral probes than for negatively charged ones was confirmed for one preparation method. The adsorption of proteins on dopamine-melanin coatings was explained as combination of electrostatic and strong, most probably covalent, interactions. To obtain this explanation, the zeta-potential of dopamine-melanin deposits has been measured as a function of pH. The formation of melanin by dopamine oxidation in layer-by-layer films of poly(L-lysine) (PLL) and hyaluronate (HA) was studied: Melanin is able to homogeneously fill $(\text{PLL-HA})_n$ films and the obtained polyelectrolyte-melanin composites can be detached from their substrate as free-standing membranes prepared by a biomimetic method in mild conditions.