Positive Thinking: Countercation Effects in Colloidal Syntheses of Gold Nanoparticles

Gold nanoparticles (Au NPs) are intensively studied and widely applicable to catalysis, sensing, medical applications, and many more. In particular, citrate- and borohydride- mediated colloidal syntheses of Au NPs are extremely popular. While it can be reasonably expected that countercations have a role to play, there is surprisingly almost no study on the effect of countercations in citrate- and borohydride-mediated colloidal syntheses of Au NPs. It is here shown that the countercation (Li+, Na+, K+) from citrate, borohydride, but also from hydroxide species, plays an overlooked role in the stabilization of gold colloidal dispersions. The stability, size, and degree of shape control over the NP decrease in the order Li+ > Na+ > K+, due to a stronger interaction between the smaller cations and metal surfaces. The findings are directly relevant for further fundamental studies, an improved control of the syntheses and scale-up.