Interaction between functionalized gold nanoparticles in physiological saline

Abstract

The interactions between functionalized noble-metal particles in an aqueous solution are central to applications relying on controlled equilibrium association. Herein, we obtain the potentials of mean force (PMF) for pair-interactions between functionalized gold nanoparticles (AuNPs) in physiological saline. These results are based upon >1000 ns experiments in silico of all-atom model systems under equilibrium and non-equilibrium conditions. Four types of functionalization are built by coating each globular Au₁₄₄ cluster with 60 thiolate groups: GS–AuNP (glutathionate), PhS–AuNP (thiophenol), CyS–AuNP (cysteinyl), and p-APhS–AuNP (para-amino-thiophenol), which are, respectively, negatively charged, hydrophobic (neutral-nonpolar), hydrophilic (neutral-polar), and positively charged at neutral pH. The results confirm the behavior expected of neutral (hydrophilic or hydrophobic) particles in a dilute aqueous environment, however the PMF curves demonstrate that the charged AuNPs interact with one another in a unique way—mediated by H₂O molecules and an electrolyte (Na⁺, Cl⁻)—in a physiological environment. In the case of two GS–AuNPs, the excess, neutralizing Na⁺ ions form a mobile (or ‘dynamic’) cloud of enhanced concentration between the like-charged GS–AuNPs, inducing a moderate attraction (∼25 kT) between them. Furthermore, to a lesser degree, for a pair of p-APhS–AuNPs, the excess, neutralizing Cl⁻ ions (less mobile than Na⁺) also form a cloud of higher concentration between the two like-charged p-APhS–AuNPs, inducing weaker yet significant attractions (∼12 kT). On combining one GS– with one p-APhS–AuNP, the direct, attractive Coulombic force is completely screened out while the solvation effects give rise to moderate repulsion between the two unlike-charged AuNPs.