Self-Assembly of Gold Nanoparticles into 2D Arrays Induced by Bolaamphiphilic Ligands

We performed synthesis and investigated the self-assembly properties of gold nanoparticles (NPs) with covalently attached bolaamphiphilic ligands (B-AuNPs). The judiciously designed coating rendered the NPs amphiphilic and induced their self-assembly. The B-AuNPs formed ordered two-dimensional structures over large areas upon simple drop-casting. The films exhibited an uncommon and applicable topography, consisting of densely packed rings of inner diameter of around 30 nm, with the B-AuNPs at the rim and an empty interior. We introduced and proved experimentally an explanation of how the structures were formed. The model involved elements of geometric packing and ligand reorganization. Upon contact with the hydrophilic surface, ligands rearranged at the surface of the metallic cores of the B-AuNPs so that the bolaamphiphilic moieties (constituting ca. 50% of the coating) were in proximity to the surface, while the hexanethiol moieties moved away from it. The described mechanism is of general relevance for the design of functional NPs capable of self-assembly.