Nanoscale imaging and atomic vibrations of eumelanin superstructures modulated by functionalized micronized graphene oxide

Abstract

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 their inherent biological functions and high biocompatibility. However, understanding their assembly strategies with inorganic materials is often challenging. Herein, we investigated the bioactive assembly of natural eumelanin superstructures. modulated by chemical functionalization of micronized graphene oxide, to study their strong structural affinity by analysing their vibrational–structural correlations. The application of complementary experiments of high-resolution electron nanoimaging coupled with vibrational Raman spectroscopy revealed intriguing and unique features of this complex hybrid material. In particular, high-resolution nanodiffraction/imaging analysis provided evidence of new nanocrystalline domains of pure natural eumelanin with different and irregular orientations forming irregular nanosheets. Interestingly, a hierarchical reassembly process 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 confirmed the ability of eumelanin to reassemble in spherical and elongated nanostructures when induced by an external stimuli of graphene oxide in an aqueous solution at room temperature. Thus, this work highlights the assembling mechanisms for designing a strategy to control bioactive molecules through environment modification.