Formation dynamics of gold nanoparticles in poly(vinylpyrrolidone) and other protective agent solutions

Intermediate chemical species involved in the formation dynamics of gold nanoparticles triggered by photoreduction were detected on the temporal range from microseconds to seconds by the near-field heterodyne transient grating method. In the dynamic processes, it was found that not only the ionic species of gold (H[Au^ICl₂], H[Au^IIICl₄]), but also the ionic species that formed complex species with the protective agent poly(vinylpyrrolidone) (PVP), (H[Au^ICl₂]–PVP, H[Au^IIICl₄]–PVP), were involved. In addition, another chemical species was observed at PVP concentrations greater than 0.4 mM with the HAuCl₄ concentration of 10 mM; its diffusion coefficient was two orders smaller than those for the other ionic species. From the reaction mechanism and the hydrodynamic diameter for the component (3.5 nm) estimated by the Einstein–Stokes equation, we concluded that this component corresponds to gold nanoparticle nuclei. Since the dependence of the nuclei concentration on the PVP concentration showed a similar dependence as those for the H[Au^ICl₂]–PVP and H[Au^IIICl₄]–PVP complex species, nucleation of gold nanoparticles occurred by way of the complex species. This reaction mechanism is true for the other four protecting agents (PVP K15, Triton X-100, Tween 20, Brij 58). Furthermore, we investigated the determining factor for the nucleation, and it was clarified that the determining factor for nuclei formation is the molecular ratio between HAuCl₄ and the protective agent, not the concentration of HAuCl₄ itself. Assuming a 2-step mechanism, this result suggests that the reaction is controlled by an initial reduction process aided by the protective agents, after which growth of the nuclei proceeds.