Precursor Ion–Ion Aggregation in the Brust–Schiffrin Synthesis of Alkanethiol Nanoparticles

Abstract

Tetraoctylammonium bromide is used in the Brust–Schiffrin nanoparticle synthesis to phase-transfer chloroaurate ions from the aqueous phase to the organic phase. While it is established that the quaternary ammonium complex self-associates in the organic phase, the actual self-assembled structure is poorly understood. We have confirmed the presence of ion–ion aggregates through quantitative 1H nuclear magnetic resonance spectroscopy (NMR), pulsed field gradient, diffusion-ordered NMR (DOSY-NMR), and density functional theory (DFT) based NMR chemical shift calculations. Tetraoctylammonium complexes (TOA-X, where X = Br, Cl, AuCl4–xBrx, AuBr4/Br, and AuBr4/Cl/Br) were investigated to measure the extraction of water into deuterated chloroform. 1H NMR- and DFT-based NMR shielding calculations indicated that deshielding of water is due to hydration of the anion and not the formation of the aqueous core of a reverse micelle. DOSY-NMR results were consistent with the formation of small aggregates at typical Brust...