Density, Structure, and Stability of Citrate(3-) and H(2)citrate(-) on Bare and Coated Gold Nanoparticles
GN Chong and ED Laudadio and M Wu and CJ Murphy and RJ Hamers and R Hernandez, JOURNAL OF PHYSICAL CHEMISTRY C, 122, 28393-28404 (2018).
We simulate the packing of citrate(3-) and H(2)citrate(-) 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 H(2)citrate(-) within polycation-wrapped cit-AuNPs with coordination between the H(2)citrate(-) layer and polycation layer in agreement with simulations. Intermolecular hydrogen bonding between terminal carboxylic acid groups of H(2)citrate(-) stabilizes the anionic layer at the interface between cit-AuNPs and adsorbing charged molecules. The calculated total density of H(2)citrate(-) on AuNPs decreases from 3.3 x 10(-10) to 3.0 x 10(-1) mol/cm(2) upon adsorption of a polycation due to some displacement of dangling H(2)citrate(-) hydrogen bonded to the surface-bound layer. The density of the surface- bound layer is consistently 2.8 x 10(-10) mol/cm(2) 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.
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