Functional-Group Effect of Ligand Molecules on the Aggregation of Gold Nanoparticles: A Molecular Dynamics Simulation Study

In this paper, atomistic molecular dynamics simulations are performed for the systems consisting of functionalized gold nanoparticles (NPs) in a toluene medium. Gold NPs are coated with ligand molecules that have different terminal groups, that is, polar carboxyl (COOH), hydroxyl (OH), amine (NH₂), and nonpolar methyl (CH₃). These functional groups are selected to understand the relation between the aggregation behavior of functionalized gold NPs in toluene and the polarity of terminal groups of ligand molecules. The center-of-mass distances between NP pairs, the radial distribution functions, the mean square displacements, the radius of gyration, and the number of hydrogen bonds (H-bond) between ligand molecules are computed. Our simulation results indicate that functionalized gold NPs exhibit different aggregation/dispersion behaviors depending upon the terminal group of ligands.

Coalescence of Au Nanoparticles without Ligand Detachment

Repulsion of ligands is known as the key factor for hindering nanoparticle (NP) coalescence. Thus, during the past decade, it has generally accepted that the full removal of capping ligands of the contact surface is the first step for NP coalescence. Herein, using molecular dynamics simulations, we have identified a new mechanism for the coalescence of S(CH_{2})_{n}COOH-coated Au NPs in water without ligand detachment. In contrast to the traditional mechanism, the aggregation of the NPs is induced by the twined hydrophobic chains of the ligands rather than the hydrophilic carboxyl tails as believed previously. Next, the exposed surface atoms attach to form the neck, and extend with the atomic rearrangement of the contact interface to merge the NPs, which do not need the removal of ligands as expected from traditional supposition. This finding refreshes the understanding of the atomic mechanism of the coalescence of NPs, which paves the way for the rational design and synthesis of NPs.

A Unified AMBER-Compatible Molecular Mechanics Force Field for Thiolate-Protected Gold Nanoclusters

We present transferable AMBER-compatible force field parameters for thiolate-protected gold nanoclusters. Five different sized clusters containing both organo-soluble and water-soluble thiolate ligands served as test systems in MD simulations, and parameters were validated against DFT and experimental results. The cluster geometries remain intact during the MD simulations in various solvents, and structural fluctuations and energetics showed agreement with DFT calculations. Experimental diffusion coefficients and crystal structures were also reproduced with sufficient accuracy. The presented parameter set contains the minimum number of cluster-specific parameters enabling the use of these parameters for several different gold nanoclusters. The parameterization of ligands can also be extended to different types of ligands.