Luminescence Onset and Mechanism of the Formation of Gold(I)- Thiolate Complexes as the Precursors to Nanoparticles

Gold(I) (Au(I))-thiolate complexes are widely believed as the precursors to Au nanoparticle formations. While the literature suggests that the Au(III)-to-thiol ligand stoichiometric ratio of 1:3 is required to reduce a Au(III) and yield a Au(I)-thiolate, other stoichiometric ratios are also known to produce Au nanoparticles upon reduction. Using the characteristic red luminescence of Au(I)-alkanethiolates, we examined the process of their formations and their implications on the Au nanoparticle synthesis in detail. The onset of the luminescence, correlated with the Au(I)-thiolate formation, as well as the kinetics of the luminophore formation were evaluated in terms of the Au(III)-to-alkanethiol ratios. The onset of the luminescence was affected significantly by the solvent polarity during reaction but not post reaction. We found that the kinetics of the luminophore formation can vary widely, requiring from minutes to 24 h for completion depending on the thiol ligands and molar ratios, as well as solvents. This information could help in designing Au nanoparticle syntheses with the logical choice of Au(III)-to-thiol ratio, solvent, and the timing of reduction.