When only ethanol is used as the source of reducing agent, and when 20 v.% ethanol is used, the NPs show a λspr value around 524 nm which correspond to 13.8 ± 3.9 nm NPs. As the amount of ethanol decreases to 19 or 18 v.% the λspr values increase to 524 nm and 526 nm respectively, corresponding to 12.1 ± 4.5 nm and 16.5 ± 4.4 nm, respectively, see Figure . The trend that the NP size increases and/or the size distribution increases when the amount of ethanol decreases is confirmed when 10 v.% of ethanol is used, for which the λspr value increases to 558 nm corresponding to 18.6 ± 12.1 nm NPs. This corresponds to a case where there is not enough reducing agent and the NP synthesis is less controlled. The poorer control over the synthesis is even more pronounced for 2 and 1 v.% ethanol where there is not enough alcohol to perform the reduction of Au III , as indicated by relatively featureless UV-vis spectra with an overall low absorption, e.g. very low A400 values compared to the other samples, see Figure and S3. As a result, no STEM characterization was performed on those samples obtained with 1 or 2 v.% ethanol.