Particle Size Evolution during the Synthesis of Gold Nanoparticles Using In Situ Time-Resolved UV–Vis Spectroscopy: An Experimental and Theoretical Study Unravelling the Effect of Adsorbed Gold Precursor Species

The quantitative analysis of nanomaterials synthesis kinetics is valuable both for understanding the synthesis and for development of manufacturing processes, and it usually requires the use of synchrotron-based instrumentation, making it challenging to perform experiments in the large parametric space needed to develop quantitative kinetic models. UV–vis spectroscopy represents a convenient technique to circumvent such difficulties, as it is available in most chemistry laboratories and allows fast data acquisition. This technique can in theory be used for the characterization of plasmonic nanomaterials synthesis kinetics. However, linking UV–vis spectra with characteristic features of the produced nanomaterials, such as size and shape, is a challenging task. This work presents a detailed spectroscopic analysis of gold nanoparticles syntheses via in situ time-resolved UV–vis spectroscopy, with emphasis on the role of gold precursor adsorption on the particle surface during nanoparticle growth. We show that the classic Turkevich synthesis and the growth of preformed gold nanoparticles with two different methods exhibit significant commonalities in the spectra evolution, explained in terms of the interaction between gold precursor species and the nanoparticle surface. Such interaction was accounted for in a model based on the Mie theory, describing the growing nanoparticles as core–shell spheres with an outer shell of few Ångströms characterized by reduced conductivity and increased electron damping rate. The proposed model led to the determination of the nanoparticle size and concentration evolution throughout the synthesis with good quantitative agreement against literature SAXS data. Furthermore, the core–shell model enabled the reproduction of the progressive blue-shift in the SPR peak position observed during the synthesis. Thus, this work reconciles the “seed-mediated growth” mechanism for the Turkevich synthesis with the temporal evolution of the spectra within the framework of the Mie theory, showing that the distinctive purple-greyish hue observed during the synthesis can be related to the interaction of gold precursor with the growing gold nanoparticles