Functional biocompatible interfaces for bioelectronics applications by the integration of eumelanin and graphene-like layers

In the manifold of available materials for functional biocompatible interfaces, the human pigment eumelanin (EU) is currently gaining increasing interest. The large EU non-solubility and the EU low electrical conductivity are the two main obstacles hampered a full fabrication and exploitation of eumelanin-based devices. Among the different strategies under investigation to improve electrical performance of eumelanin thin films, a clear-cut approach lies in hybridization with a suitable conductive counterpart. In this view, ? -conjugated systems molecules featuring conducting pathways appear a key choice in the production of new organic materials for electronic (nano)devices. Following this approach, conductive interfaces were designed and fabricated by an efficient integration of EU and graphene like (GL) layers [1,2,3]. The hybrid materials (EUGLs) exhibited quite good adhesion to hydrophilic and hydrophobic surfaces, water stability, biocompatibility and improved electrical conductivity compared to the sole EU pigment due to the presence of embedded GL layers [1]. EUGLs were easily produced allowing eumelanin precursors (5,6-dihydroxyindole (DHI) and/or 5,6- dihydroxyindole-2-carboxylic acid, DHICA [1]) to polymerize in a water suspension of GL layers [1]. Different EU:GL mass ratios were explored with the aim of deepening the comprehension of the interaction between the two conjugate ? systems. The chemical-physical, electrical and morphological analyses indicate that the actual composition of the EUGL hybrids could be considered as the outcome of quantitative merging of the starting materials. It can be speculated that both covalent bonds and ?-? stacking are expected to be involved in EU:GL interaction. To get more insights on this issue, more sophisticated analytical techniques were employed: X-ray absorption spectroscopy with synchrotron radiation and solid state nuclear magnetic resonance. Comparative AFM inspection of the morphologies of eumelanin, GL and EUGL thin films indicates a consistent modification induced by eumelanin to the GL self-assembling. The hybrid film has locally a more granular surface compared to the EU film (Fig. 1). Electrical dc resistivity was measured in a standard four contacts configuration. All the curves show an ohmic behavior, but with extremely different resistance (Fig. 1). Figure 1 AFM images at different magnifications and IV dc curves of GL, EU and EUGL (1:1 mass ratio) The study of the collected data is still ongoing however available evidences do suggest that electronic and structural properties are strictly interconnected. The deep understanding of structure-behavior relationship in EUGL hybrids is crucial for expanding the scope of eumelanin in bioelectronics and paving the way to advanced biocompatible organic electrochemical transistor-like interfaces capable to translate cellular activity in electrical signals.