Influence of Chain Length and Branching on the Structure of Functionalized Gold Nanoparticles

Functionalized gold nanoparticles (GNPs) in aqueous NaCl solutions have been studied using molecular dynamics simulations to assess the suitability of various functionalization chemistries to effectively shield the metallic core. Alkane thiol chains of various chain length (Cl) containing 6, 12, 18, and 24 carbon atoms are grafted onto the surface of the gold core. We compare the properties of GNPs functionalized with nonpolar CH3-terminated and polar COO–- and NH3+-terminated chains, where the nanoparticle charge is compensated by appropriate numbers of excess Na+or Cl–counterions. In addition to linear chains, we also investigate branched Y-shaped chains with the branching sites at the 4th, 8th, or 12th carbon atom from the sulfur atom that connects the chain to the gold core. The penetration depth of water and ions into the diffuse hydrocarbon shell region and its dependence on chain length, branching, and terminating group is found to increase with decreasing chain length irrespective of termination. Long linear chains, however, tend to form bundles independent of terminal group and can thus leave fractions of the nanoparticle surface exposed to small molecules, whereas shorter and branched chains do not form bundles and can cover the GNPs more homogeneously.