Water, ethanol (alternatively methanol), an aqueous solution of NaOH at 50 mM, and an aqueous solution of gold precursor (e.g. HAuCl4) at 50 mM were mixed together. The gold precursor solution was added last typically under stirring. The final concentrations were 0.5 mM HAuCl4, 2 mM NaOH and 20 v.% ethanol (before volume contraction) for a total volume of typically 2 mL. The syntheses were typically performed in 1 cm wide rectangular UV-vis cuvette made of polystyrene . Upon addition of the gold precursor, the reaction mixtures slowly turn grey, then deep blue, then purple, then red in ca. 1-2 hours, which indicates the formation of Au NPs characterized by a surface plasmon resonance around 520 nm in UV-vis characterization. This corresponds to NPs with a size range around 10 nm, as illustrated in The role of the chemicals and the influence of different parameters such as the concentrations of the various chemicals and alcohol content has been extensively detailed elsewhere . In a nutshell, under alkaline conditions, the alkoxides from the alcohol play the role of reducing agents and allow the formation of the Au NPs by reduction of the Au III in HAuCl4 . The synthesis is best controlled if a fast nucleation is achieved which can be favoured by adding HAuCl4 last from a concentrated solution. This can be achieved by using different chemicals in different concentrations, where typically too high and too low amount of base and/or alcohol leads to larger unstable colloidal dispersions. A fast nucleation can also be achieved using lights with different wavelengths (where lower wavelengths lead to the faster formation of the Au NPs) . The overall formation of the NPs proceeds via a relatively fast formation of the first seeds, likely favoured by the absence of surfactant, followed by a relatively slow growth, favoured by the RT process . The overall reaction is light sensitive and lights with lower wavelengths favour the formation of smaller NPs . Stirring is not required but facilitates a faster and more uniform nucleation across the sample, as discussed below.