The significance of bromide in the Brust–Schiffrin synthesis of thiol protected gold nanoparticles

The mechanism of the two-phase Brust–Schiffrin synthesis of alkane thiol protected metal nanoparticles is known to be highly sensitive to the precursor species and reactant conditions. In this work X-ray absorption spectroscopy is used in conjunction with liquid/liquid electrochemistry to highlight the significance of Br⁻ in the reaction mechanism. The species [AuBr₄]⁻ is shown to be a preferable precursor in the Brust–Schiffrin method as it is more resistant to the formation of Au(I) thiolate species than [AuCl₄]⁻. Previous literature has demonstrated that avoidance of the Au(I) thiolate is critical to achieving a good yield of nanoparticles, as [Au(I)X₂]⁻ species are more readily reduced by NaBH₄. We propose that the observed behavior of [AuBr₄]⁻ species described herein explains the discrepancies in reported behavior present in the literature to date. This new mechanistic understanding should enable nanoparticle synthesis with a higher yield and reduce particle size polydispersity.