In Situ UV/Vis, SAXS, and TEM Study of Single-Phase Gold Nanoparticle Growth

Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.