Structural study of citrate layers on gold nanoparticles: role of intermolecular interactions in stabilizing nanoparticles.

The structure of citrate adlayers on gold nanoparticles (AuNPs) was investigated. Infrared (IR) and X-ray photoelectron spectroscopy (XPS) analyses indicate citrate anions are adsorbed on AuNPs through central carboxylate groups. A unique structure of adsorbed citrate is determined, and a pH-induced structural transition is presented. IR analysis probes dangling dihydrogen anions (H2Citrate(-)) and hydrogen bonding of carboxylic acid groups between adsorbed and dangling citrate anions. A contribution of steric repulsion between citrate layers to particle stability is characterized. Structure-based modeling, which is consistent with scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) images in the literature, suggests organization details relating to the formation of self-assembled layers on (111), (110), and (100) surfaces of AuNPs. Adsorption characteristics of the citrate layer include the interaction between hydrogen-bonded citrate chains, bilayer formation, surface coverage, and chirality. The enthalpic gain from intermolecular interactions and the importance of molecular structure/symmetry on the adsorption are discussed. Combining the enthalpic factor with surface diffusion and adsorption geometry of (1,2)-dicarboxyl fragments on Au(111), H2Citrate(-) anions effectively stabilize the (111) surface of the AuNPs. The detailed understanding of intermolecular interactions in the molecular adlayer provides insight for nanoparticle formation and stabilization. We expect these findings will be relevant for other nanoparticles stabilized by hydroxy carboxylate-based amino acids and have broad implications in NP-based interfacial studies and applications.