Molecular Dynamics Study of Functionalized Gold Nanoparticles: Structural and Aggregation Behavior under Varying Ionic Strength

This study employed classical molecular dynamics simulations to explore the interactions between functionalized gold nanoparticles (AuNPs) and monovalent ions in aqueous environments. Two AuNP models were analyzed: type I, positively charged (Au₁₄₄(SRNH₃ ⁺)₆₀), and type II, negatively charged (Au₁₄₄(SRCOO^-)₆₀), both functionalized with 60 organic thiolate ligands (SR). Four systems were constructed to examine the effects of ionic strength and nanoparticle concentration: (i) one AuNP of each type in pure water; (ii) the same system with 60 Na⁺ and 60 Cl^- ions; (iii) two AuNPs of each type in pure water; and (iv) the same configuration as (iii), with 120 Na⁺ and 120 Cl^- ions. Simulations focused on interparticle interactions, hydrogen-bonding dynamics, and the roles of electrostatic and van der Waals forces. Results show that ionic strength and nanoparticle concentration significantly affect the system's energy distribution and structural organization. Ionic screening reduces electrostatic interactions, modifies hydrogen bond lifetimes, and induces the rearrangement of hydration shells around the nanoparticles. Additionally, variations in ion distribution impact the spatial organization and mobility of solvated species. These findings provide molecular-level insights into ion-mediated nanoparticle interactions and are crucial for the rational design of functional nanomaterials in biomedical, catalytic, and materials science applications.