Insight into the Final Step of the Supramolecular Buildup of Eumelanin.

The final step in the supramolecular buildup of eumelanin particles is investigated regarding the involved species and mechanism. Time-resolved in situ light scattering and scanning electron microscopy reveal an aggregation of particles with a narrow size distribution around 40 nm, previously only observed as substructures. These form larger particles with again very uniform size and diameters around 200 nm. Aggregation of each single particle takes only a few minutes to complete, whereas the entire process goes on for at least 3 h, partly due to the kinetics of the precursors. The individual particles also undergo an additional consolidation step toward their final form, which takes up to 24 h. Atomic force microscopy shows that the size before consolidation is around twice the size of the final particles, due to free space between the substructures. Light scattering also reveals that the aggregation is random with respect to where the particles attach, as the shape of aggregates changes from sphere to coil, before it returns to a spherical shape at the end. Application of enzyme mediated autodeposition finally shows the potential to stop the supramolecular buildup at each level, and therefore enables isolation of the respective eumelanin particles at will. This may enable the full potential for melanin materials in nanotechnology deriving from its unique (for biological polymers) properties like paramagnetism, electrical conductivity, and many more.