Density, Structure, and Stability of Citrate^3- and H₂citrate^- on Bare and Coated Gold Nanoparticles

We simulate the packing of citrate^3- and H₂citrate^- onto gold nanoparticles (AuNPs) to understand how citrate anions cap and stabilize AuNPs. We determine the molecular configurations of citrate on 4, 6, and 8 nm AuNP surfaces as a function of charge state and packing density and find that both the distribution of configurations and maximum packing density are independent of AuNP size. A combination of molecular dynamics simulations and in situ Fourier transform infrared spectroscopy (FTIR) is employed to compare the molecular configurations, stability, and density of citrate on 4 nm citrate-coated (cit-AuNPs) and within polycation-wrapped 4 nm cit-AuNPs. FTIR experiments indicate the presence of H₂citrate^- within polycation-wrapped cit-AuNPs with coordination between the H₂citrate^- layer and polycation layer in agreement with simulations. Intermolecular hydrogen bonding between terminal carboxylic acid groups of H₂citrate^- stabilizes the anionic layer at the interface between cit-AuNPs and adsorbing charged molecules. The calculated total density of H₂citrate^- on AuNPs decreases from 3.3 × 10^-10 to 3.0 × 10^-10 mol/cm² upon adsorption of a polycation due to some displacement of dangling H₂citrate^- hydrogen bonded to the surface-bound layer. The density of the surface-bound layer is consistently 2.8 × 10^-10 mol/cm² with and without polycation adsorption. We provide all-atom level insight into the distribution and organization of experimentally derived binding modes of citrate on bare and coated cit-AuNPs. The citrate density and surface charge density are determined for all-atom and coarse-grained modeling of cit-AuNPs, their functionalization, and transformations in complex environments.