Dihydroxy Indole-free Poly(catecholamine) for Smooth Surface Coating with Amine Functionality

The catecholamine of polydopamine(PDA) is associated with adhesionand self-polymerization. However, 5,6-dihydroxyindole (DHI) of PDAinevitably consumes primary amine groups, which are helpful for thepost-modification of PDA-coated surfaces. Furthermore, DHI leads toparticle aggregation by causing & pi;-& pi; interactionand cation-& pi; interaction. Here, we used 3,4-dihydroxybenzylaminehydrobromide to achieve DHI-free poly(catecholamine) (PCA) coatingby self-polymerization. The DHI-free nature renders a smooth and uniformsurface. Moreover, the suppressed indole ring formation preservesthe primary amine groups after the DHI-free PCA coating, suggestingan alternative surface amine functionalization strategy to the conventionalsilane-based coating that is only applicable to hydroxyl-containingsurfaces. Surface amine functionality of DHI-free poly(catecholamine)is confirmed by the fluorescence image of the tethered NHS ester-functionalizedpolydiacetylene liposome by the NHS ester-amine coupling reaction.Furthermore, DHI-free PCA was used to modify the surface propertiesof graphene to enhance the thermal conductivity of graphene-polymercomposites. Graphene nanoplatelets (GnPs) coated with DHI-free PCAformed adequate intermolecular bonding with the poly(acrylic acid)matrix, which reduces interfacial thermal resistance. The resultinggraphene-polymer composites achieved a sharp increase in thermal conductivity,reaching 2.9 W/mK at 15 wt % of graphene content, which is 87% largerthan the thermal conductivity of control composites with untreatedgraphene.