(Invited) Thin Films of Eumelanin Pigment: Charge Carrier Transport, Ion Storage, and Interaction with Metal Electrodes
We characterized eumelanin films grown on substrates patterned with gold, platinum, and palladium hydride electrodes using hydration-dependent transient electrical current and atomic force microscopy measurements. We discovered an electrochemical interaction among metal-chelating catechol groups of hydrated eumelanin, Cl- traces in the eumelanin material and Au electrodes, assumed to be electrochemically stable in previously published works on the electrical properties of eumelanin. This interaction leads to the growth of highly conductive Au-eumelanin dendrites between the electrodes, ultimately leading to sudden resistive changes of the sample. This phenomenon suggests new possibilities for biocompatible memory devices and has to be taken into account when integrating eumelanin-like materials in electronic devices .
Electrical characterization of eumelanin films interfaced with ion-blocking Pt and proton-transparent electrodes  complemented by cyclic voltammetry and impedance spectroscopy measurements give new insights into the charge transport properties of eumelanin films. Our results indicate the presence of protonic currents in eumelanin thin films, increasing with sample hydration, and cannot exclude the presence of electronic currents. These results support recently published data obtained by spectroscopic measurements on eumelanin pellets  and underline the attractiveness of eumelanin films for applications at the interface of electronics and biology.
 M. D’Ischia et al., Angew. Chem. Int. Edit. 48, 3914 (2009).
 A. B. Mostert et al., Proc. Natl. Acad. Sci. 109, 8943 (2012).
 J. Wünsche et al., Adv. Funct. Mater. 23, 5569 (2013).
 C. Zhong, Y. Deng, A. F. Roudsari, A. Kapetanovic, M. P. Anantram, M. Rolandi, Nat. Commun. 2, 476 (2011).