Substrate-dependent morphology of synthetic melanins and local electrical response of ultrathin DOPA-melanin films.

Substrate interactions can influence the morphology of melanin deposits and complicate the interpretation of nanoscale measurements. Here, atomic force microscopy was used to compare polydopamine (PDA) and synthetic DOPA-derived eumelanin deposited from aqueous preparations onto mica, highly oriented pyrolytic graphite (HOPG), and surface-modified HOPG (GM-HOPG). Both pigments formed predominantly isolated globular features on mica and GM-HOPG, whereas larger lateral aggregates were observed on bare HOPG. For PDA, the dominant apparent heights were approximately 0.55, 0.95, and 1.05 nm on GM-HOPG, mica, and HOPG, respectively; the corresponding mean projected areas were 36, 184, and 482 nm. DOPA-melanin also showed the greatest lateral aggregation on bare HOPG, with projected areas extending to approximately 3200 nm and a broad height-distribution maximum at 1.5-2.3 nm. Conductive AFM measurements of an ultrathin DOPA-melanin deposit on HOPG, with an apparent step height of approximately 1 nm, yielded a nonlinear, polarity-dependent current-voltage response. Current magnitudes were approximately 0.3 nA at -0.2 V and 2 nA at +0.2 V. An exploratory measurement after magnetic-field exposure showed reduced current-voltage asymmetry, although the origin of this change remains unresolved. These observations identify substrate choice as an important variable in melanin deposition and show that an ultrathin DOPA-melanin layer modifies the local electrical response of the probe-film-HOPG junction.

Keywords

Adsorption, Aggregation, Atomic force microscopy, DOPA-melanin, Electrical properties, Nanoscale morphology, Polydopamine

Conflict of interest statement

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.