Protonic and Electronic Transport in Hydrated Thin Films of the Pigment Eumelanin

Abstract

The electrical properties of eumelanin, a ubiquitous natural pigment, have fascinated scientists since the late 1960s. For several decades, the hydrationdependent 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-V) 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 −4 −10 −3 S cm −1 ), 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-V, 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.

Aqueous photo(electro)catalysis with eumelanin thin films

“…[20][21][22] showing the relationship between protons as the major charge carrier and the paramagnetic species observed in eumelanin, with the phenomenon of photoconductivity in eumelanin also being accounted for in ref. 23.…”

Natural melanin pigments and their interfaces with metal ions and oxides: emerging concepts and technologies

“…1) would make eumelanin a mixed ionic-electronic material. [21,23,36] Wünsche et al proposed an explanation of the charge-carrier transport properties of hydrated eumelanin thin films included between metal electrodes with an interplay between proton migration, interfacial metal electrode/ eumelanin charge transfer (redox) processes, and electronic transport [37,38] (Fig. 4).…”

Supplementing π-systems: eumelanin and graphene-like integration towards highly conductive materials for the mammalian cell culture bio-interface

“…25 In this scenario, a number of studies have recently reported eumelanin based functional devices 14,26,27 as well as eumelanin films for nerve tissue engineering 23 and nonwoven fibers for cardiac tissue engineering. 19 In a systematic investigation some of us addressed the issue of eumelanin processability focusing on thin film fabrication with the main goal of obtaining both a complete chemical-structural control of the material and high quality film morphology.…”