Melanin granules extracted from Sepia ink: A nanoscale study of charge carrier transport

Eumelanin, the brown-black member of the melanin family of biopigments, has emerged as a promising material for sustainable organic electronics. Sepia eumelanin develops hierarchically from (5, 6)-dihydroxyindole (DHI) and (5, 6)-dihydroxyindole-2-carboxylic acid (DHICA) monomers, and its structure is made up of granules with typical size in the 100–300 nm range. Literature reports that Sepia eumelanin (from now on indicated as Sepia melanin), derived from the ink sac of cuttlefish, exhibits predominant electronic transport in dry state and mixed ionic–electronic transport in its hydrated state when studied at millimetric or micrometric distance ranges. To explore the upper limit of the conductivity of Sepia melanin and unlock its full technological potential, we investigated its electrical response at the nanoscale where the influence of granule boundaries is expected to be minimal. Using electrodes patterned at the nanoscale by e-beam lithography, we run current–voltage, current–time and electrochemical impedance spectroscopy measurements; we observed predominant electronic transport mechanisms in Sepia melanin, with conductivities that increase as the interelectrode distance decreases. Temperature-resolved experiments permitted us to deduce the transport activation energy. Our work highlights the importance of exploring the electrical response of natural materials across varying distance scales to exploit their full potential for sustainable organic electronics.