Mechanism of Gold-Thiol Interactions in Formation of Nano-Materials for Plasmonics

Optically coupled arrays of noble metal nanoparticles hold promise for enabling sub-diffraction limited waveguides and all-optical integrated circuits, due to their size-dependent optical properties and surface plasmon resonance. Realizations of these photonic waveguide structures require precise control of the nanoparticle size. A full understanding of the mechanisms of nanoparticle formations is critical, to develop protocols for ultra-monodisperse nanoparticles. Herein, we evaluate the prevailing hypothesis in the formations of thiol ligand-passivated monodisperse gold (Au) nanoparticles, that the formation of Au-thiol polymer [Au-SR]n is the necessary precursor to the nanoparticle formation. This hypothesis predicts a specific onset of the Au-thiol polymer formation at the Au-to-thiol molar ratio of 1:3. Using glutathione (GSH) as ligand, we correlated the onset of the Au-thiol polymer formation with the emergence of fluorescence associated with the polymer, and with the nanoparticle formation upon reduction. We observed the solvent-dependent onset of the fluorescence in methanol and water at the Au-to-thiol molar ratios of c.a. 1:4.5 and 1:2, respectively, deviating from the prevailing model.