Vibrational and Nanoimaging of Eumelanin Superstructures modulated by Functionalized Micronized Graphene Oxide

Natural organic/inorganic materials with rational cooperative formations have long been of enormous interest owing to their hybrid self-assembling properties. Natural biomolecules are expected to produce attractive superstructures capable of sensing their environment, following its inherent biological functions and high biocompatibility. However, understanding their assembly strategies with inorganic material often poses a major challenge. Herein, we investigated the bioactive assembling of natural eumelanin superstructures modulated by chemical functionalization of the micronized graphene oxide to research their strong structural affinity by analysing their vibrational-structural correlations. The application of complementary experiments of high-resolution electron nanoimaging coupled with self-healing vibrational Raman spectroscopy revealed intriguing and unique features of this complex hybrid material. In particular, high resolution nanodiffraction/imaging analysis evidence new nanocrystalline domains of pure natural eumelanin with different and irregular orientations forming irregular nanosheets. Interestingly, a disassembly and reassembly route of eumelanin units are actually evident not only on the oxide graphene surface but also located in high amounts on the edge of vertical graphene oxide, concretely supported by the analytical changes of the predominant resonance bands (D, D**, and G). This confirms the ability of eumelanin to reassemble in spherical and elongated nanostructures induced by the external stimuli of the graphene oxide in aqueous solution at room temperature. This work thus highlights the assembling mechanisms for designing strategy to control bioactive molecules through environment modification.