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

Surfactant-free syntheses of colloidal nanomaterials present several potential benefits for various applications, e.g. in catalysis or medicine. However, controlled colloidal syntheses of metal nanoparticles obtained by the reduction of a precursor in a low viscosity liquid phase without surfactants remain scarce. Here, the features of a room temperature synthesis of gold (Au) nanoparticles (NPs) using only HAuCl4 as gold precursor, alkaline water as solvent, and an alcohol as precursor of the reducing agents, are exploited to develop synthetic approaches compatible with the principles of Green Chemistry. To minimise the amount of chemicals needed and achieve the smallest Au NPs, the co-influences of the nature of the alcohol (ethanol, ethylene glycol, glycerol), the alcohol content (10, 20, 30, 40, 50, 80 and 99 v.%), the base content (Base/Au molar ratios of 3, 4, 5 and 10) and the nature of the base (LiOH, NaOH, KOH) are investigated in a unified parametric study. The optimal conditions to obtain small size nanomaterials are 20 v.% ethanol and a Base/Au molar ratio of 4 using NaOH as base for 0.5 mM HAuCl4. Second, it is shown that experiments performed without base, although they do not lead to stable colloids, can help identifying the most promising alcohols and alcohol contents to use. Third, it is established that higher temperatures (from room temperature to 50 °C) have little desirable effects on the size control of the Au NPs for ethanol-mediated syntheses. Fourth, the influence of lights with defined wavelengths in the range 222-690 nm on the formation of Au NPs is investigated. Lights with lower wavelengths favour the formation of smaller NPs. Fifth, using time resolved UV-vis studies it is established that the formation of small NPs is related to conditions favouring the rapid formation of the NPs, and different formation pathways are at stake when different alcohols and lights with different wavelengths are used. Overall, the sensitivity of the synthesis to these parameters increases in the order glycerol < ethylene glycol < ethanol < methanol. The results identify the pros and cons in selecting different alcohols and provide critical insights to achieve a fine size control, more reproducible and scalable syntheses. It is also shown than a certain degree of shape control can be achieved. The results obtained over more than 1000 experiments pave the way to high throughput screening of large parametric spaces and new approaches for a rational size and shape control in surfactant-free colloidal syntheses of NPs.