Coverage Induced Confinement Effects on the Solvation Structure and Thermodynamics of Functionalized Gold Nanoparticles: A Molecular Dynamics Simulation and Grid Inhomogeneous Solvation Theory Study

Gold nanoparticles (GNPs) have become indispensable in various fields of science and technology, including biotechnology and nanomedicine, here the surface functionalization is the key to make them suitable candidates in these field. However, the mechanisms by which the functionalization of gold nanoparticles influences their solvation structure and thermodynamics in water are not completely understood. In this article, we studied the solvation structure and thermodynamics around anionic and cationic functionalized gold nanoparticles by systematically varying the coverage through a combined molecular dynamics simulation (MD) and Grid Inhomogeneous Solvation Theory (GIST) approach. Our results reveal that the cationic functional groups organize and pack themselves tightly around the metal surface as compared to anionic functional groups. There is a significant disruption of the tetrahedral hydrogen bond network of water under the confinement as well as the outer surface of the functionalized nanoparticle. The solute-water interactions are relatively stronger in the case of anionic functionalized GNPs than in the case of cationic counterparts. There is a significant gain in the free energy near the nanoparticle-water interface with an increase in coverage, this gain is markedly stronger in case of anionic GNPs compared to the cationic ones. Although the gain in free energy is predominantly contributed by the solute-water energy, the contribution from the orientational entropy loss is notable. We strongly believe that the insights provided by our study are critical to design and engineer the nanomaterial candidates for chemical and bio-medical applications, such as carriers in drug delivery and bio-medical imaging applications.