  Water microdroplets containing 100 micromolar HAuCl4 have been shown to
reduce gold ions into gold nanoparticles spontaneously. It has been suggested
that this chemical transformation is driven by ultrahigh electric fields at the
air-water interface, albeit without mechanistic insight. We investigated the
fate of several metallic salts in water, methanol, ethanol, and acetonitrile in
bulk and microdroplets. This revealed that when HAuCl4 (or PtCl4) is added to
bulk water (or methanol or ethanol), metal NPs appear spontaneously. Over time,
the nanoparticles grow in bulk, as evidenced by the solution's changing colors.
If the same bulk solution is sprayed pneumatically and collected, the NP size
has no significant enhancement. Interestingly, the reduction of metal ions is
accompanied by the oxidation of water (or alcohols); however, these redox
reactions are minimal in acetonitrile. We establish that the spontaneous
reduction of metal ions is (i) not limited to water or gold ions, (ii) not
driven by the air-water interface of microdroplets, and (iii) appears to be a
general phenomenon for solvents containing hydroxyl groups. These results
advance our understanding of liquids in general and should be relevant in soil
chemistry, biogeochemistry, electrochemistry, and green chemistry.
