Water microdroplets containing 100 µM HAuCl4 have been shown to reduce gold ions into gold nanoparticles spontaneously. It has been suggested that this chemical transformation takes place at the air–water interface of microdroplets, albeit without mechanistic insights. We compared the fate of several metallic salts in water, methanol, ethanol, and acetonitrile in bulk phase and microdroplet geometry (sprays). Experiments revealed that when HAuCl4 (or PtCl4) is added to bulk water (or methanol or ethanol), metal NPs appear spontaneously. Over time, the nanoparticles grow, evidenced by the bulk solutions’ changing colors. If a bulk solution is sprayed pneumatically and microdroplets are collected, the NP size distribution is not significantly enhanced. We find that the reduction of metal ions is accompanied by the oxidation of water (or alcohols); however, these redox reactions are minimal in acetonitrile. This establishes 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) it appears to be a general phenomenon for solvents containing hydroxyl groups. These results advance our understanding of aquatic chemistry and liquids in general and should be relevant in soil chemistry, biogeochemistry, electrochemistry, and green chemistry.

中文翻译：

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为什么一些金属离子会在水微滴中自发形成纳米颗粒：解开气-水界面和本体氧化还原化学的贡献


含有 100 μM HAuCl4 的水微滴已被证明可以自发地将金离子还原成金纳米颗粒。有人提出，这种化学转化发生在微滴的空气-水界面处，尽管没有机制见解。我们比较了几种金属盐在水、甲醇、乙醇和乙腈中的本体相和微滴几何形状（喷雾）的命运。实验表明，当 HAuCl4（或 PtCl4）添加到体水（或甲醇或乙醇）中时，金属 NP 会自发出现。随着时间的推移，纳米颗粒会生长，本体溶液的颜色变化就是证明。如果以气动方式喷涂散装溶液并收集微滴，则 NP 粒度分布不会显著增强。我们发现金属离子的还原伴随着水（或醇）的氧化;然而，这些氧化还原反应在乙腈中很小。这确定了金属离子的自发还原是 （i） 不限于水或金离子，（ii） 不是由微滴的气-水界面驱动的，以及 （iii） 它似乎是含有羟基的溶剂的普遍现象。这些结果促进了我们对水生化学和液体的理解，并且应该与土壤化学、生物地球化学、电化学和绿色化学相关。