Fewer, but Better: On the Benefits of Surfactant-Free Colloidal Syntheses of Nanomaterials

Colloidal syntheses are common bottom-up synthetic approaches to obtain various nanomaterials, e.g., gold nanoparticles, where a precursor, e.g., HAuCl₄, is reduced in the presence of reducing agents in a solvent. It is often claimed, and almost dogmatically believed, that stabilizers, capping agents, ligands, surfactants, or other additives must be added to ensure the stability of the colloids. Although there is almost a systematic use of such chemicals in the literature, a range of surfactant-free, or additive-free, colloidal syntheses have been reported. In those syntheses, the solvent plays the role of source of reducing agents and/or stabilizers. Recently, the use of alkaline solutions of low-viscosity mono-alcohols, such as ethanol, has been shown to lead to stable surfactant-free colloids for various metal nanoparticles. Here, with the example of gold nanoparticles obtained at room temperature, it is shown that adding commonly reported stabilizers, such as trisodium citrate, PVP, SDS, poly(NIPAM), CTAB, or chemicals such as hydroquinone, actually does not lead to any advantages compared to the surfactant-free colloidal synthesis performed in an alkaline mixture of water and 20 vol.% ethanol. The results stress the potential of surfactant-free approaches compared to more conventional surfactant- and additive-assisted strategies to develop greener research studies and syntheses.