Molecular Modeling of the Reduction Mechanism in the Citrate-Mediated Synthesis of Gold Nanoparticles

The synthesis of gold nanoparticles (Au NPs) via the reduction of tetrachloroauric acid by sodium citrate has become a standard procedure in nanotechnology. Simultaneously, gold-mediated reactions are gaining interest due to their catalytic properties, unseen in other metals. In this study, we have investigated the theoretical mechanism of this reaction under three different pH conditions (acid, mild acid, and neutral) and have corroborated our findings with experimental kinetic measurements by UV–vis absorption spectroscopy and transmission electron microscopy (TEM) analysis of the final particle morphology. We have demonstrated that, indeed, the pH of the medium ultimately determines the reaction rate of the reduction, which is the rate-limiting step in the Au NPs formation and involves decarboxylation of the citrate. The pH sets the dominant species of each of the reactants and, consequently, the reaction pathways slightly differ in each pH condition. The mechanism highlights the effect of the number of Cl– ligands in the metallocomplex, which ultimately originates the energetic differences in the reaction paths.