Micrometer-Sized Water Droplets Induce Spontaneous Reduction

Abstract

Bulk water serves as an inert solvent for many chemical and biological reactions. Here, we report a striking exception. We observe that in micrometer-sized water droplets (microdroplets), spontaneous reduction of several organic molecules occurs, pyruvate to lactate, lipoic acid to dihydrolipoic acid, fumarate to succinate, and oxaloacetate to malate. This reduction proceeds in microdroplets without any added electron donors or acceptors and without any applied voltage. In three of the four cases, the reduction efficiency is 90% or greater when the concentration of the dissolved organic species is less than 0.1 μM. None of these reactions occurs spontaneously in bulk water. One example demonstrating the possible broad application of reduction in water microdroplets to organic molecules is the reduction of acetophenone to form 1-phenylethanol. Taken together, these results show that microdroplets provide a new foundation for green chemistry by rendering water molecules to be highly electrochemically active without any added reducing agent or applied potential. In this manner, aqueous microdroplets might have provided a route for abiotic reduction reactions in the prebiotic era, thereby providing organic molecules with a reducing power before the advent of biotic reducing machineries.

Ultrafast enzymatic digestion of proteins by microdroplet mass spectrometry

“…It is known that a higher H + concentration could facilitate acid hydrolysis of proteins and a higher OH − concentration would promote trypsin activity. The value of the surface potential at the air-water interface is still debatable and estimated to be on the order of tens of megavolts per centimeter 56 . A potential of approximately 3 V across the 5 Å air-water interface 57,58 exceeds a standard potential of 1.23 V for the electrolysis of water molecules.…”

Reaction of chloroauric acid with histidine in microdroplets yields a catalytic Au–(His)2complex

“…Recently, it has been proposed that as the size of a water microdroplet decreases, water molecules at and near the air-water interface autoionize more readily into H + and OH À than in bulk water. 55 This strong electric eld formed at the airwater interface is expected to promote the reduction of AuCl 4…”

Scale-up of microdroplet reactions by heated ultrasonic nebulization

“…† As the temperature was increased from 30 C to 50 C, the average droplet size decreased from 6.9 mm to 5.4 mm, which is closer to the microdroplet size (2-3 mm) in ESI. Previous reports 39,55,56 have indicated that the droplet size has a large effect on the reaction rate in microdroplets; thus we speculate that the increased reaction rate in HUN at higher temperature is caused by the smaller droplet size. The following three reactions were also conducted at 50 C. The reaction in the nebulization cell is accelerated by the continuously atomized microdroplets from the reactant solution.…”