Structure of arginine overlayers at the aqueous gold interface: implications for nanoparticle assembly.

Adsorption of small biomolecules onto the surface of nanoparticles offers a novel route to generation of nanoparticle assemblies with predictable architectures. Previously, ligand-exchange experiments on citrate-capped gold nanoparticles with the amino acid arginine were reported to support linear nanoparticle assemblies. Here, we use a combination of atomistic modeling with experimental characterization to explore aspects of the assembly hypothesis for these systems. Using molecular simulation, we probe the structural and energetic characteristics of arginine overlayers on the Au(111) surface under aqueous conditions at both low- and high-coverage regimes. In the low-density regime, the arginines lie flat on the surface. At constant composition, these overlayers are found to be lower in energy than the densely packed films, although the latter case appears kinetically stable when arginine is adsorbed via the zwitterion group, exposing the charged guanidinium group to the solvent. Our findings suggest that zwitterion-zwitterion hydrogen bonding at the gold surface and minimization of the electrostatic repulsion between adjacent guanidinium groups play key roles in determining arginine overlayer stability at the aqueous gold interface. Ligand-exchange experiments of citrate-capped gold nanoparticles with arginine derivatives agmatine and N-methyl-l-arginine reveal that modification at the guanidinium group significantly diminishes the propensity for linear assembly of the nanoparticles.