Advent of Plasmonic Behavior: Dynamically Tracking the Formation of Gold Nanoparticles through Nonlinear Spectroscopy

The preparation of gold nanoparticles through reduction of chloroauric acid by trisodium citrate, also known as the Turkevich synthesis, was analyzed both ex situ and in situ. In situ experiments consist of dynamically tracking second and third harmonic light scattering and multiphoton luminescence. By complementing in situ data with ex situ quenching experiments, which enabled further UV-vis-NIR spectroscopy, as well as transmission electron microscopy (TEM) and dynamic light scattering (DLS) characterization, we obtained new insight into the mechanistic process of gold nanoparticle growth. Our results reveal that the growth proceeds through a metastable state of aggregation and offer additional evidence for a sharp transition from metallic molecular cluster to plasmonic nanoparticle behavior in the initial stage of the process. While multiphoton luminescence can be used as a marker for this transition, second and third harmonic scatterings reveal surface and bulk information such as size, shape, and the presence of aggregates.