The
electrical properties of eumelanin, a ubiquitous natural pigment,
have fascinated scientists since the late 1960s. For several decades,
the hydration-dependent electrical properties of eumelanin have mainly
been interpreted within the amorphous semiconductor model. Recent
works undermined this paradigm. Here we study protonic and electronic
charge carrier transport in hydrated eumelanin in thin film form.
Thin films are ideal candidates for these studies since they are readily
accessible to chemical and morphological characterization and potentially
amenable to device applications. Current–voltage (<i>I</i>-<i>V</i>) measurements, transient current measurements
with proton-transparent electrodes, and electrochemical impedance
spectroscopy (EIS) measurements are reported and correlated with the
results of the chemical characterization of the films, performed by
X-ray photoelectron spectroscopy. We show that the electrical response
of hydrated eumelanin films is dominated by ionic conduction (10<sup>–4</sup>–10<sup>–3</sup> S cm<sup>–1</sup>), largely attributable to protons, and electrochemical processes.
To propose an explanation for the electrical response of hydrated
eumelanin films as observed by EIS and <i>I</i>-<i>V</i>, we considered the interplay of proton migration, redox
processes, and electronic transport. These new insights improve the
current understanding of the charge carrier transport properties of
eumelanin opening the possibility to assess the potential of eumelanin
for organic bioelectronic applications, e.g. protonic devices and
implantable electrodes, and to advance the knowledge on the functions
of eumelanin in biological systems