Characterization of Dopamine-Melanin Growth on Silicon Oxide

It has recently been demonstrated that dopamine solutions put in contact with a variety of solid substrates allow the production of thin coatings probably made of melanin (Science 2007, 318, 426). In this article, we show that the thickness of these coatings can be controlled to allow a growth regime that is proportional to the reaction time if fresh dopamine is regularly provided. We propose that the growth is initiated by the adsorption of a radical compound. When dopamine polymerization or aggregation has reached a steady state in solution, the produced species do not adhere anymore to the substrate, emphasizing the role played by unoxidized dopamine. X-ray photoelectron spectroscopy showed that the thickness of the deposit increases linearly with the number of immersion steps, but the thickness measured in ultravacuum is about 4 times smaller than the thickness measured by ellipsometry in conditions of ambient humidity. This suggests that the drying of the deposit has a considerable influence on its properties. The Si2p signal characteristic of the silicon substrate decreases progressively when the number of deposition steps increases but does vanish even after 32 deposition steps. This observation will be discussed with respect to the formation of a continuous film. Cyclic voltammetry experiments showed that a deposit impermeable to ferrocyanide is obtained after the immersion in nine freshly prepared dopamine solutions, demonstrating the formation of a film. The atomic composition of the film determined by X-ray photoelectron spectroscopy is compatible with that of melanin. Finally, we show that the deposit can be quantitatively removed from the substrate when put in a strongly alkaline solution.