(Invited) Thin Films of Eumelanin Pigment: Charge Carrier Transport, Ion Storage, and Interaction with Metal Electrodes

Abstract

The ubiquitous natural pigment eumelanin is widely studied for its photoprotective, thermoregulating, free-radical scavenging, and anti-oxidant functions in the human body. Recently, eumelanin received increased attention for potential applications in organic bioelectronics due to its unique set of physicochemical properties including strong broad-band UV-Vis absorption, metal chelation properties, and potentially mixed electronic-ionic conduction in a hydrated state [1,2]. To further explore the use of eumelanin in organic bioelectronic devices, the properties of eumelanin thin films interfaced with device components such as metal electrodes need to be investigated in presence of water and ionic species. A good understanding of the interaction of eumelanin with different metal electrodes is also essential to study the intrinsic charge transport properties of eumelanin films, which are still largely undiscovered. 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 [3]. Electrical characterization of eumelanin films interfaced with ion-blocking Pt and proton-transparent electrodes [4] 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 [2] and underline the attractiveness of eumelanin films for applications at the interface of electronics and biology. [1] M. D’Ischia et al., Angew. Chem. Int. Edit. 48, 3914 (2009). [2] A. B. Mostert et al., Proc. Natl. Acad. Sci. 109, 8943 (2012). [3] J. Wünsche et al., Adv. Funct. Mater. 23, 5569 (2013). [4] C. Zhong, Y. Deng, A. F. Roudsari, A. Kapetanovic, M. P. Anantram, M. Rolandi, Nat. Commun. 2, 476 (2011).