(420y) Simulations of Phenol-Capped Alkanethiol-Coated Gold Nanoparticles in Organic Solvents

Introduction: Gold nanoparticles (AuNP's) represent a sub-micrometer form of colloidal gold. Nanoparticles composed of gold were first used in association with the preparation of colored glass. AuNP's have since been researched in areas such as of materials science and biomedicine. However, fundamental interactions between coated AuNP's and solvents are not completely understood. Molecular dynamics simulations were conducted to gain insights into the influence of (1) the thickness of an alkanethiol AuNP coating and (2) different organic solvents on the behavior of AuNP's.

Methods: Phenol-capped alkanethiol-coated gold nanoparticles were composed of a 1.5-nm-diameter gold core coated with 60 phenol-capped alkanethiol ligands. All simulations were conducted with the Assisted Model Building with Energy Refinement (AMBER) 12 software package using the General Amber Force Field (GAFF) parameters. Simulations were conducted at 300 K for 40 ns using a timstep of 2 fs. Data analysis was used to assess the effective size of the NPs (radius of gyration), the distance from the ligand end groups to the gold surface atoms, and the radial distribution of the phenol end groups.

Results and Discussion: The size and polarity of the solvent molecules influenced the conformations of the alkanethiol ligands and nanoparticle-solvent interactions. Smaller, polar solvents (acetonitrile and methanol) led to a greater distribution of phenol groups positioned further away from the gold core (i.e., more accessible to the solvent molecules), the largest radius of gyration, and the greatest distance from the phenol groups to the gold surface atoms. In contrast, the non-polar solvents (chloroform and toluene) led to a greater distribution of phenol groups positioned closer to the gold core and less accessible to the solvent, the smallest radius of gyration, and the smallest distance from the phenol groups to the gold surface atoms. The larger, polar solvents (acetic acid, acetone, and dimethyl sulfoxide) afforded results that were between the smaller, polar solvents and the non-polar solvents. These results suggest that the polar ligand end groups (phenols) had the most favorable interactions with the smaller, polar solvents; followed by the larger, polar solvents; and then by the non-polar solvents. Interestingly, the results were more sensitive to the sizes of the solvent molecules than to the dipole moments of the solvent molecules. Regarding ligand length, it was found that the aforementioned effects of the solvents were more prominent with longer alkanethiol chains.

References:
Louis, C. & O. Pluchery, O. (2012). Gold Nanoparticles for Physics, Chemistry and Biology. London: Imperial College Press.