(217dw) Combined Experimental and Computational Study of the Formation of Gold Tetrapod Nanostructures

Branched gold nanoparticles were synthesized via a soft-template directed process using a biological buffer, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES). These branched Au nanoparticles are mainly tetrapods and show distinct absorption in the range of 700-900 nm. A combined experimental and computational study suggests that at high concentration, the HEPES molecules self-assemble into structures with long-range order, serving as soft-templates to direct the formation of the anisotropic gold nanoparticles. Detailed analyses of surface chemistry and structure indicate the formation of a molecular bilayer structure for the stabilization of the branched Au nanostructures. Our density functional theory (DFT) calculations predict that the sulfonate group of the HEPES molecules prefer to bind to the Au surfaces, while the free hydroxyl groups facilitate the self-assembly and bilayer formation through the formation of hydrogen bonds. By comparing three different buffer molecules, our study demonstrates the critical importance of ligand chemistry in the directed formation of anisotropic metallic nanoparticles.