Adsorption Dynamics and Structure of Polycations on Citrate-Coated Gold Nanoparticles

Despite the widespread application of engineered multilayered polyelectrolyte-coated gold nanoparticles (AuNPs), their interparticle interaction is not fully understood in part because of a lack of molecular scale observation of the layer-by-layer assembly of the polyelectrolyte coating. While top-down coarse-grained models of polyelectrolyte-coated AuNPs have focused on polyelectrolytes of short length, from 10 to a 100 monomers represented as one charged bead per monomer, here we use molecular dynamics and bottom-up coarse-grained approaches to access more typical polymer lengths on the order of 200 monomers. Specifically, we simulate the adsorption dynamics and structure of one or two such long polycations on negatively charged 4 nm citrate-coated AuNPs within implicit or explicit solvents. The first polycation coats approximately half of the AuNP surface regardless of solvent model and leaves a significant part of the anionic citrate layer exposed to absorption by a second polycation. We find that the most prevalent structural features across solvent conditions consist of 1-2-monomer loops or kinks. They extend radially from the nanoparticle surface and assemble into a double-layered coating of the AuNP. We compile a set of structural features of adsorbed polycations that would be ripe for coarse graining and outline a bottom-up coarse-graining scheme for simulations of polyelectrolyte-coated AuNPs with amphiphilic biomolecules that incorporates the hemispheric polycation coverage with exposed citrate layer, surface-bound bilayer-like segments, and long amphiphilic loops and tails.