Eumelanin and graphene-like layers integration en route to functional interfaces for bioelectronics applications

We present here the design and fabrication of conductive interfaces by an efficient integration of two materials featuring conjugate ? systems, namely the human pigment eumelanin (EU) [1] and graphene like (GL) layers [2,3]. The hybrid materials (EUGLs) exhibited adhesion, water stability, biocompatibility and improved electrical conductivity with respect to the pure pigment due to the presence of embedded GL layers [2]. 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 [2]. 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. Electrical dc resistivity was measured in a standard four contacts configuration. All the curves show an ohmic behavior, but with extremely different resistance. The poor electrical conductivity (?) of EU is confirmed, while EUGL exhibited a ? value more than four orders of magnitudes greater than the one of the parent EU precursor. A time-decay of ? in the eumelanin and hybrid samples was also detected (while this phenomenon is absent in the GL film). On both EU and EUGL samples, the decay of ? seems to be composed by two contributions, exhibiting two different time scales (a clear two-scale behavior) and different quantitative amounts. The double time-scale of ? decay suggests the presence of a double contribution to the electrical transport in both eumelanin and EUGL sample: ionic and electronic. In the GL film the ionic transport (as well a huge trapping of electrons) is missing. The chemical-physical, electrical and morphological analyses indicate that the actual composition of the hybrid EUGL 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. The study of the data collected is still ongoing however available evidences do suggest that electronic and structural properties are strictly interconnected. The deep understanding of structure-behavior relationship in EUGLs 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.