How does the size of gold nanoparticles depend on citrate to gold ratio in Turkevich synthesis? Final answer to a debated question

The dependence between the size of gold nanoparticle (AuNP) and the citrate to gold molar ratio (X) is still a matter of debate 65years after the seminal work by Turkevich et al. for high X values. We assume that this dispersion of results is due to the variation of certain parameters that are often not mentioned in the protocols, and to the use of a single characterization technique (dynamic light scattering (DLS) or transmission electron microscopy (TEM)). To adress definitely the question of this dependence, we have synthesized AuNPs with very precise protocols ensuring that the only parameters to be modified are X and the sequence of reagent addition. We have then studied, for the first time, the dependence of the size with X quantitatively with a multimodal approach (UV-Visible spectroscopy, DLS and TEM) for 2 synthetic routes differing only by the sequence of reagent addition. We show unambiguously that AuNPs' size decay monotonically, with X whatever the order of reagent addition. It allows us to exclude the occurrence of a measurable discontinuity for a peculiar value of X that prompted some authors to postulate the existence of two different reaction pathways when the citrate to gold molar ratio is around this value. In contrast, our result is in line with one reaction pathway, likely a "seed-mediated" growth mechanism, which should leads to monotonic size decrease. Also, we note that our result agrees with the sole available theoretical prediction (Kumar et al., 2007) on the whole range of X. Despite this apparent agreement, we point some contradictions between recent experimental results and basal hypothesis of this model.