Motivated by the recent nuclear magnetic resonance (NMR) analysis of functionalized gold nanoparticles (J. Am. Chem. Soc., 141, 2019, 4316-4327), we conduct explicit solvent atomistic simulations to characterize the conformational distribution and dynamics of surface ligands on a small gold nanoparticle of 2 nm diameter. Several quaternary alkyl amines are studied to probe the effect of chain length, and a gold slab system is studied to probe the effect of surface curvature. The simulations observe a higher degree of spatial heterogeneity as the ligand length increases, leading to a higher degree of local clustering of longer ligands. Due to the charged nature of the head groups, however, the degree of "ligand bundling"is minimal compared to previous studies of nanoparticles functionalized with charge-neutral ligands. Due to the considerable flexibility of long ligands, their local clustering is not long lived and rearranges at the time scale of 1-10 ns, suggesting that rearrangements of ligand conformation are unlikely to represent the kinetic bottleneck for nanoparticle-(bio)molecular interactions. The head group methyl proton T2 relaxation time is computed using a model-free approach, and the results are in general agreement with experimental data, providing essential validation of the nanoparticle model and the simulation protocol. Analysis of contributions to the computed T2 relaxation time suggests that to characterize the time scale of surface ligand dynamics, such measurements should focus on nanoparticles whose hydrodynamic radii are no larger than 3 nm; for larger particles, surface features such as ligand flexibility and heterogeneity can be qualitatively reflected through order parameters and T2/T2*. Nonequilibrium molecular dynamics simulations show that conformational features of the ligands impact the electrostatic properties of the nanoparticle, which suggests that nanoparticle/(bio)molecular interactions can be modulated by perturbing the conformational ensemble of surface ligands.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films