The lack of reports on the influence of light for glycerol mediated-syntheses can be at first surprisingly since reproducibility and replicability are general challenges in the nanomaterial sciences, and a direct matter of concerns for Green Chemistry to avoid wasting time, resources and generating waste while trying to reproduce or upscale reported NP syntheses. The empirical observation of a possible influence of light on mono-alcohol mediated syntheses pushed us to study in more details the influence of controlled light environment (i) to achieve more reproducible syntheses, (ii) to possibly achieve a fine size control, and (iii) to give new insights into the formation pathway of the NPs. Importantly, if light has an influence on the size of the NPs, this opens a range of opportunities to achieve size control using a very simple strategy starting with the same chemicals at the same concentrations. This strategy would be simpler than more common syntheses requiring different concentrations of precursors, reducing agents, base and/or surfactants, i.e. relatively more complex, diverse and often hazardous media from a chemical point of view, to achieve size control. The detailed experimental procedure and a detailed analysis is provided in section S9. In this section, methanol was included in the comparison for reasons detailed below, although it is toxic and not suitable to develop sustainable and safe syntheses. Different light environments are considered: syntheses are carried out in the dark (Dark), at ambient light (Room), under controlled white light environment (WL) or using lamps with 222, 365, 410, 450, 500, 525, 590 or 630 wavelengths (a specific discussion on the use of 222 nm in the context of Green syntheses is proposed in section S9). Syntheses carried under controlled light environment (WL) typically lead to more reproducible results. When the wavelength used for synthesis increases from 222 to 690 nm, the UV-vis data indicate an overall NP size increase. For instance, the size obtained for selected samples prepared using ethanol-mediated synthesis, NaOH and different light conditions indicated in parentheses, are 7.0 ± 2.2 (222 nm), 8.3 ± 2.4 (365 nm), 8.4 ± 2.2 (410 nm), 9.4 ± 3.9 (450 nm), 17.2 ± 5.5 (500 nm), 12.7 ± 6.2 (525 nm), 18.8 ± 9.0 (590 nm), 22.2 ± 9.0 nm (630 nm). Using LiOH the sizes are 7.2 ± 2.6 (222 nm), 5.3 ± 1.7 (365 nm), 6.2 ± 1.8 (410 nm), 11.7 ± 4.2 (450 nm), 10.7 ± 4.1 (500 nm), 12.7 ± 3.9 (525 nm), 15.6 ± 5.8 (590 nm), 17.3 ± 7.1 nm (630 nm), see more details in Figure . It is worth stressing that the use of lower wavelengths has a strong positive effect on methanol-mediated syntheses leading to ʎspr values as low as 518 nm when a 365 nm light is used. This is the lowest recorded value for this alcohol for a surfactant-free synthesis and this synthetic approach leads to ca. 10 nm NPs, which is a size about half of what is reported in previous reports or in the absence of controlled light environment. Ca. 5 nm Au NPs are also obtained here using LiOH in ethanol-mediated synthesis and a 365 nm light, which is about as small as what can be obtained with glycerolmediated syntheses. The effect of lights with different wavelengths is clearer when methanol or ethanol are used compared to when EG or glycerol are used. It is tempting to attribute this observation to the ability of the polyols themselves, or to the Au complex formed in polyols, to absorb light at higher wavelengths, see section S6. However, a detailed understanding remains challenging at this stage. First, it is important to stress that the correlation between the UV-vis absorption properties of HAuCl4 at higher wavelengths and the use of lights with low wavelengths to obtain smaller size NPs is here established and discussed based on results obtained in the absence of base. Therefore, the framework proposed does not take into account hydroxy-substituted Au complexes that are expected to formed under alkaline conditions, but that are featureless in the UV-vis region considered. Second, while light irradiation using different wavelengths clearly influences the NP optical properties and size, it is nontrivial to elucidate the role that light is playing on the particle formation and growth. Specifically, it is challenging to decouple a potential effect of heating of the reaction mixture as a consequence of light illumination and an actual photo chemical effect of light irradiation. However, all light sources utilized in this study, except the 222 nm, have low lamp powers: e.g. 310 mW for the 365 nm, 400 mW for the 410 nm and 480 mW for 450 nm, according to the manufacturer (Spectroline). This suggests that a possible effect of heating of the reaction mixture is minimal, even though the reaction volumes are small. Moreover, it would be expected that in case the dispersions are heated up, larger NPs would be obtained based on the results discussed in section S7. Since small NPs are obtained, this piece of evidence points towards a photochemical effect. Furthermore, different ambient lights (for which the light sources are placed far from the reaction mixture) also influences the NP size, as seen in particular for methanol-and for ethanol-mediated syntheses. This observation strengthens the hypothesis that heating has less of an effect on the NP size compared to the wavelength of the light used. A photochemical reaction is likely to account for the different NP sizes observed, but the mechanism of this reaction remains elusive at this stage. Third, the interpretation of the results does not consider at this stage the possible role(s) of radicals formed in the different solvents upon irradiation with lights with different wavelengths, that in turns can play the role of reducing agents. Promisingly, future studies will shine new lights of the more detailed reaction mechanism(s). Nevertheless, the findings reported are critical for the development of surfactant-free syntheses in several aspects. (i) It is now established that to best design a range of studies, controlling the light environment will be key. This is especially important for a range of in situ measurements, e.g. towards mechanistic studies. For experiments performed at synchrotrons, it will be necessary to take into account possible beam-related effects. (ii) It is also an important achievement here to prepare small size NPs with methanol. Methanol is a simpler molecule than ethanol and a reducing agent less prone to lead to organic by-products in alkaline conditions, i.e. a potentially simpler model system and solvent for fundamental studies. (iii) It is worth stressing that different starting chemical compositions are commonly used to achieve size / shape control. It can then be generally tedious to disentangle the effect(s) of different residual chemicals on the NP formation and the resulting NP properties, e.g. surfactants are typical additives with their own redox and/or photochemical and/or hazardous properties. This conceptual challenge for fundamental studies can be greatly alleviated in further studies given that size control is here achieved by starting with the same chemicals. In addition and promisingly, although this does not allow a shape control as fine as what is traditionally achieved with surfactants, some degree of shape control was possible by carefully tuning in particular the know cation-alcohol-light as reported in section 10.