Positive thinking: counter-cations 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. The controlled synthesis of Au NPs has been an intense area of research for decades and remains an important topic in nanoscience. In particular, citrate- and borohydride- mediated colloidal syntheses of Au NPs are extremely popular. More recently, so-called surfactant-free colloidal syntheses of Au NPs are emerging promising alternative strategies, for instance using alcohol-mediated syntheses in alkaline media. While anions have been reported to influence the synthesis and stability of colloidal Au NPs, and while it can reasonably be expected that counter-cations might also have a role to play, there is surprisingly almost no study on the effect of counter-cations in citrate- and borohydride- mediated colloidal syntheses of Au NPs. It is here shown that the counter cation (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+. This trend can be explained by a stronger interaction between the smaller cations and metal surfaces. The influence of the cations is more marked at higher concentrations of reducing agent and gold precursor. The findings are directly relevant for further fundamental studies, an improved control of the syntheses and scale up. It is anticipated that the findings will be broadly relevant for catalysis, sensing, biomedical applications and engineering of Au, Au-based and other nanomaterials.