Morphology of synthetic DOPA-eumelanin deposited on glass and mica substrates: An Atomic Force Microscopy investigation.

Bright Field Microscopy and Atomic Force Microscopy techniques are used to investigate morphological properties of synthetic eumelanin, obtained by oxidation of L-DOPA solution, deposited on glass and mica substrates. Deposits of eumelanin are characterized by aggregates with different shape and size. On a micrometric scale, filamentous as well as granular structures are present on glass and mica substrates, with a larger density on the former than on the latter. On a nanometric scale, filamentous aggregates, several microns long and about 100 nm wide and high, and granular aggregates, ~50 nm high and 100 nm wide, are found on both substrates, whereas point-like deposits less than 10 nm high and less than 50 nm wide are found on mica substrate. Dynamic Light Scattering measurements and Atomic Force Microscopy images support the evidence that eumelanin presents only nanometric point-like aggregates in aqueous solution, whereas such nanoaggregates organize themselves according to granular and filamentous structures when deposition occurs, as a consequence of interactions with the substrate surface.