Lights on the synthesis of surfactant-free colloidal gold nanoparticles in alcohol and water mixtures

Surfactant-free colloidal syntheses in aqueous media are attractive to develop nanomaterials relevant for various applications, e.g. catalysis or medicine. However, controlled green syntheses without surfactants of metal nanoparticles in aqueous media remain scarce. Here, room temperature syntheses of gold (Au) nanoparticles (NPs) that require only HAuCl4, alkaline water and an alcohol, i.e. relatively benign chemicals and mild reaction conditions, are developed. The findings of a comprehensive multi-parameters screening performed in small volumes (< 3 mL) over 1000+ experiments, that pave the way to greener high throughput screening of large parametric spaces and leads to scalable (100 mL) synthetic strategies, are summarized. A rational selection of the alcohol is proposed. The influence of lights with defined wavelengths (222-690 nm) is investigated. It is found that lights with lower wavelengths favor the formation of smaller 5 nm NPs. Different kinetics and formation pathways are observed for different alcohols and for lights with different wavelengths. The sensitivity to various experimental parameters increases in the order glycerol < ethylene glycol < ethanol < methanol. New strategies for a rational fine size control, and to some extend shape control, are identified. The results lead to more sustainable and reproducible surfactant-free colloidal syntheses of NPs.