Abstract
The citrate reduction method is one of the simplest and most common methods used in the synthesis of gold nanoparticles. It has been thought that citrate acts as both a reducing agent for the gold salt and as the capping agent. However, it has recently been reported using density functional theory (DFT) that electron density builds up on uncomplexed apex gold atoms and the binding of formate (the simplest carboxylate and a model for citrate) becomes unfavorable after two additions, limiting citrate's utility as a capping agent. In this study, Au 20-formyloxyl radical interactions are investigated using DFT at the BP86/DZ level of theory to model neutral carboxylate-gold nanoparticle binding (corresponding to carboxylates interacting with a partially oxidized gold nanoparticle). Binding energies are refined using a TZP basis set. It is found that the incremental binding energies of formyloxyl radicals remain highly favorable through eight additions (the highest number tested). The addition of one formyloxyl radical is 56 kJ/mol less than the addition of one formate but becomes 210 kJ/mol more favorable for the second addition. The range of binding energies through the eight additions is 154-331 kJ/mol. Furthermore, after the third addition, the most favorable geometries feature distortion of the gold tetrahedron. These results suggest that oxidized species formed in the citrate reduction method are likely capping agents and that binding of these ligands may affect the properties of the nanoparticles through distortion of the gold structure. © 2012 American Chemical Society.