Capture of Hydroxyl Radicals by Hydronium Cations in Water Microdroplets

Enhanced Selectivity in Microdroplet-Mediated Enzyme Catalysis

Natural enzymes with enhanced catalytic activity and selectivity have long been studied by tuning the microenvironment around the active site, but how to modulate the active-site electric field in a simple fashion remains challenging. Here, we demonstrate that microdroplets as a simple yet versatile reactor can enhance the electric field at the active site of an enzyme. By using horseradish peroxidase as a model, improved selectivity in microdroplet-mediated enzyme catalysis can be obtained. Quantum mechanical/molecular dynamics calculations and vibrational Stark spectroscopy reveal that the electric field at the microdroplet interface can influence the electrostatic preorganization and orientation of the enzyme to enhance its internal electric field. As a result, the free energies of the substrate and heme can be tuned by the internal electric field, thereby changing its catalytic reaction pathway for a classical substrate, 3,3',5,5'-tetramethylbenzidine, and enabling selective C-N additions for specific substrates. This finding provides a green, simple, and effective way to modulate enzyme-catalyzed reactions and holds promise for a broad spectrum of biosensing and biosynthesis applications.

Catalyst-free selective oxidation of C(sp3)-H bonds in toluene on water

The anisotropic water interfaces provide an environment to drive various chemical reactions not seen in bulk solutions. However, catalytic reactions by the aqueous interfaces are still in their infancy, with the emphasis being on the reaction rate acceleration on water. Here, we report that the oil-water interface activates and oxidizes C(sp3)-H bonds in toluene, yielding benzaldehyde with high selectivity (>99%) and conversion (>99%) under mild, catalyst-free conditions. Collision at the interface between oil-dissolved toluene and hydroxyl radicals spontaneously generated near the water-side interfaces is responsible for the unexpectedly high selectivity. Protrusion of free OH groups from interfacial water destabilizes the transition state of the OH-addition by forming π-hydrogen bonds with toluene, while the H-abstraction remains unchanged to effectively activate C(sp3)-H bonds. Moreover, the exposed free OH groups form hydrogen bonds with the produced benzaldehyde, suppressing it from being overoxidized. Our investigation shows that the oil-water interface has considerable promise for chemoselective redox reactions on water without any catalysts.

Visualization of the Charging of Water Droplets Sprayed into Air

Water droplets are spraying into air using air as a nebulizing gas, and the droplets pass between two parallel metal plates with opposite charges. A high-speed camera records droplet trajectories in the uniform electric field, providing visual evidence for the Lenard effect, that is, smaller droplets are negatively charged whereas larger droplets are positively charged. By analyzing the velocities of the droplets between the metal plates, the charges on the droplets can be estimated. Some key observations include: (1) localized electric fields with intensities on the order of 109 V/m are generated, and charges are expected to jump (micro-lightening) between a positively charged larger droplet and the negatively charged smaller droplet as they separate; (2) the strength of the electric field is sufficiently powerful to ionize gases surrounding the droplets; and (3) observations in an open-air mass spectrometer reveal the presence of ions such as N2+, O2+, NO+, and NO2+. These findings provide new insight into the origins of some atmospheric ions and have implications for understanding ionization processes in the atmosphere and chemical transformations in water droplets, advancing knowledge in the field of aerosol science and water microdroplet chemistry.

Spontaneous Oxidation in Aqueous Microdroplets: Water Radical Cation as Primary Oxidizing Agent

Exploration of the unique chemical properties of interfaces can unlock new understanding. A striking example is the finding of accelerated reactions, particularly spontaneous oxidation reactions, that occur without assistance of catalysts or external oxidants at the air interface of both aqueous and organic solutions (provided they contain some water). This finding opened a new area of interfacial chemistry but also caused heated debate regarding the primary chemical species responsible for the observed oxidation. An overview of the literature covering oxidation in microdroplets with air interfaces is provided, together with a critical examination of previous findings and hypotheses. The water radical cation/radical anion pair, formed spontaneously and responsible for the electric field at or near the droplet/air interface, is suggested to constitute the primary redox species. Mechanisms of accelerated microdroplet reactions are critically discussed and it is shown that hydroxyl radical/hydrogen peroxide formation in microdroplets does not require that these species be the primary oxidant. Instead, we suggest that hydroxyl radical and hydrogen peroxide are the products of water radical cation decay in water. The importance of microdroplet chemistry in the prebiotic environment is sketched briefly and the role of partial solvation in reaction acceleration is noted.

Deciphering the Microdroplet Acceleration Factors of Aza-Michael Addition Reactions

Microdroplet chemistry is emerging as a great tool for accelerating reactions by several orders of magnitude. Several unique properties such as extreme pHs, interfacial electric fields (IEFs), and partial solvation have been reported to be responsible for the acceleration; however, which factor plays the key role remains elusive. Here, we performed quantum chemical calculations to explore the underlying mechanisms of an aza-Michael addition reaction between methylamine and acrylamide. We showed that the acceleration in methanol microdroplets results from the cumulative effects of several factors. The acidic surface of the microdroplet plays a dominating role, leading to a decrease of ∼9 kcal/mol in the activation barrier. We speculated that the dissociation of both methanol and trace water contributes to the surface acidity. An IEF of 0.1 V/Å can further decrease the barrier by ∼2 kcal/mol. Partial solvation has a negligible effect on lowering the activation barrier in microdroplets but can increase the collision frequency between reactants. With acidity revealed to be the major accelerating factor for methanol droplets, reactions on water microdroplets should have even higher rates because water is more acidic. Both theoretically and experimentally, we confirmed that water microdroplets significantly accelerate the aza-Michael reaction, achieving an acceleration factor that exceeds 107. This work elucidates the multifactorial influences on the microdroplet acceleration mechanism, and with such detailed mechanistic investigations, we anticipate that microdroplet chemistry will be an avenue rich in opportunities in the realm of green synthesis.

Pyrogenic Carbon Degradation by Galvanic Coupling with Sprayed Seawater Microdroplets

Surface waves are known for their mechanical role in coastal processes that influence the weather and climate. However, their chemical impact, particularly on the transformation of pyrogenic carbon, is poorly understood. Pyrogenic carbon is generally assumed to show negligible postformational alteration of its stable carbon isotope composition. Here we present an electrochemical interaction of pyrogenic carbon with the sprayed seawater microdroplets resulting from wave breaking, driven by the galvanic coupling between the microdroplet water-carbon interfaces and the microdroplet water-vapor interfaces. This enables refractory pyrogenic carbon to rapidly degrade via the oxygenation and mineralization reaction, which makes it ∼2.6‰ enriched in 13C, far exceeding the generally assumed postformation alteration values (<0.5‰) of pyrogenic carbon. The unique chemical dynamics of seawater microdroplets provide new insights into the discrepancy in carbon isotope signatures between riverine and marine black carbon, emphasizing the potential of coastal oceans for carbon sequestration in the global carbon cycle.

Molecular Mechanism for Converting Carbon Dioxide Surrounding Water Microdroplets Containing 1,2,3-Triazole to Formic Acid

Spraying water microdroplets containing 1,2,3-triazole (Tz) has been found to effectively convert gas-phase carbon dioxide (CO2), but not predissolved CO2, into formic acid (FA). Herein, we elucidate the reaction mechanism at the molecular level through quantum chemistry calculations and ab initio molecular dynamics (AIMD) simulations. Computations suggest a multistep reaction mechanism that initiates from the adsorption of CO2 by Tz to form a CO2-Tz complex (named reactant complex (RC)). Then, the RC either is reduced by electrons that were generated at the air-liquid interface of the water microdroplet and then undergoes intramolecular proton transfer (PT) or switches the reduction and PT steps to form a [HCO2-(Tz-H)]- complex (named PC-). Subsequently, PC- undergoes reduction and the C-N bond dissociates to generate COOH- and [Tz-H]- (m/z = 69). COOH- easily converts to HCOOH and is captured at m/z = 45 in mass spectroscopy. Notably, the intramolecular PT step can be significantly lowered by the oriented electric field at the interface and a water-bridge mechanism. The mechanism is further confirmed by testing multiple azoles. The AIMD simulations reveal a novel proton transfer mechanism where water serves as a transporter and is shown to play an important role dynamically. Moreover, the transient •COOH captured by the experiment is proposed to be partly formed by the reaction with H•, pointing again to the importance of the air-water interface. This work provides valuable insight into the important mechanistic, kinetic, and dynamic features of converting gas-phase CO2 to valuable products by azoles or amines dissolved in water microdroplets.

Direct electrochemical evidence suggests that aqueous microdroplets spontaneously produce hydrogen peroxide

Recent reports have detailed the striking observation that electroactive molecules, such as hydrogen peroxide (H2O2) and radical water species (H2O.+/H2O.-), are spontaneously produced in aqueous microdroplets. Stochastic electrochemistry allows one to study reactions in real-time occurring inside subfemtoliter droplets, one droplet at a time, when a microdroplet irreversibly adsorbs to an ultramicroelectrode surface (radius ~ 5 µm). Here, we use stochastic electrochemistry to probe the formation of hydrogen peroxide (H2O2) in single aqueous microdroplets suspended in 1,2-dichloroethane. The oxidation of H2O2 at alkaline pH (11.5) differs from near-neutral conditions (6.4), allowing us to create a digital, turn-off sensing modality for the presence of H2O2. Further, we show that the stochastic electrochemical signal is highest at the mass transfer limitation of the H2O2 couple and is dampened when the potential nears the formal potential. We validate these results by showing that the addition of a H2O2 selective probe, luminol, decreases the stochastic electrochemical response at alkaline pH (11.5). Our results support the observation that H2O2 is generated in water microdroplets at concentrations of ~100 s of µM.

The emerging chemistry of self-electrified water interfaces

Water is known for dissipating electrostatic charges, but it is also a universal agent of matter electrification, creating charged domains in any material contacting or containing it. This new role of water was discovered during the current century. It is proven in a fast-growing number of publications reporting direct experimental measurements of excess charge and electric potential. It is indirectly verified by its success in explaining surprising phenomena in chemical synthesis, electric power generation, metastability, and phase transition kinetics. Additionally, electrification by water is opening the way for developing green technologies that are fully compatible with the environment and have great potential to contribute to sustainability. Electrification by water shows that polyphasic matter is a charge mosaic, converging with the Maxwell-Wagner-Sillars effect, which was discovered one century ago but is still often ignored. Electrified sites in a real system are niches showing various local electrochemical potentials for the charged species. Thus, the electrified mosaics display variable chemical reactivity and mass transfer patterns. Water contributes to interfacial electrification from its singular structural, electric, mixing, adsorption, and absorption properties. A long list of previously unexpected consequences of interfacial electrification includes: "on-water" reactions of chemicals dispersed in water that defy current chemical wisdom; reactions in electrified water microdroplets that do not occur in bulk water, transforming the droplets in microreactors; and lowered surface tension of water, modifying wetting, spreading, adhesion, cohesion, and other properties of matter. Asymmetric capacitors charged by moisture and water are now promising alternative equipment for simultaneously producing electric power and green hydrogen, requiring only ambient thermal energy. Changing surface tension by interfacial electrification also modifies phase-change kinetics, eliminating metastability that is the root of catastrophic electric discharges and destructive explosions. It also changes crystal habits, producing needles and dendrites that shorten battery life. These recent findings derive from a single factor, water's ability to electrify matter, touching on the most relevant aspects of chemistry. They create tremendous scientific opportunities to understand the matter better, and a new chemistry based on electrified interfaces is now emerging.

Efficient catalyst-free N2 fixation by water radical cations under ambient conditions

The growth and sustainable development of humanity is heavily dependent upon molecular nitrogen (N2) fixation. Herein we discover ambient catalyst-free disproportionation of N2 by water plasma which occurs via the distinctive HONH-HNOH+• intermediate to yield economically valuable nitroxyl (HNO) and hydroxylamine (NH2OH) products. Calculations suggest that the reaction is prompted by the coordination of electronically excited N2 with water dimer radical cation, (H2O)2+•, in its two-center-three-electron configuration. The reaction products are collected in a 76-needle array discharge reactor with product yields of 1.14 μg cm-2 h-1 for NH2OH and 0.37 μg cm-2 h-1 for HNO. Potential applications of these compounds are demonstrated to make ammonia (for NH2OH), as well as to chemically react and convert cysteine, and serve as a neuroprotective agent (for HNO). The conversion of N2 into HNO and NH2OH by water plasma could offer great profitability and reduction of polluting emissions, thus giving an entirely look and perspectives to the problem of green N2 fixation.

Deciphering the Kinetics of Spontaneous Generation of H2O2 in Individual Water Microdroplets

Spontaneous generation of H2O2 in sub-10 μm-sized water microdroplets has received increasing interest since its first discovery in 2019. On the other hand, due to the short lifetime of these microdroplets (rapid evaporation) and lack of suitable tools to real-time monitor the generation of H2O2 in individual microdroplets, such a seemingly thermodynamically unfavorable process has also raised vigorous debates on the origin of H2O2 and the underlying mechanism. Herein, we prepared water microdroplets with a long lifetime (>1 h) by virtue of microwell confinement and dynamically monitored the spontaneous generation of H2O2 in individual microdroplets via time-lapsed fluorescence imaging. It was unveiled that H2O2 was continuously generated in the as-prepared water microdroplets and an apparent equilibrium concentration of ∼3 μM of H2O2 in the presence of a H2O2-consuming reaction can be obtained. Through engineering the geometry of these microdroplets, we further revealed that the generation rates of H2O2 in individual microdroplets were positively proportional to their surface-to-volume ratios. This also allowed us to extract a maximal H2O2 generation rate of 7.7 nmol m-2 min-1 in the presence of a H2O2-consuming reaction and derive the corresponding probability of spontaneous conversion of interfacial H2O into H2O2 for the first time, that is, ∼1 of 65,000 water molecules in 1 s. These findings delivered strong evidence that the spontaneous generation of H2O2 indeed occurs at the surface of microdroplets and provided us with an important starting point to further enhance the yield of H2O2 in water microdroplets for future applications.

Continuous ammonia synthesis from water and nitrogen via contact electrification

We synthesized ammonia (NH3) by bubbling nitrogen (N2) gas into bulk liquid water (200 mL) containing 50 mg polytetrafluoroethylene (PTFE) particles (~5 µm in diameter) suspended with the help of a surfactant (Tween 20, ~0.05 vol.%) at room temperature (25 °C). Electron spin resonance spectroscopy and density functional theory calculations reveal that water acts as the proton donor for the reduction of N2. Moreover, isotopic labeling of the N2 gas shows that it is the source of nitrogen in the ammonia. We propose a mechanism for ammonia generation based on the activation of N2 caused by electron transfer and reduction processes driven by contact electrification. We optimized the pH of the PTFE suspension at 6.5 to 7.0 and employed ultrasonic mixing. We found an ammonia production rate of ~420 μmol L-1 h-1 per gram of PTFE particles for the conditions described above. This rate did not change more than 10% over an 8-h period of sustained reaction.

Determination of C═C Positions of Unsaturated Fatty Acids in Foods via Ambient Reactive Desorption Ionization with Water Dimer Radical Cations

The positions of C═C bonds in unsaturated fatty acids (FAs) are one of the main factors determining the quality of food flavor. Herein, we developed an approach for the determination of C═C bonds of FAs by online epoxidation reaction with water dimer radical cations. The limit of detection for octenoic acid isomers was ∼9 μg/L. The positions of C═C bonds in trans-2/3-hexenoic acid, trans-2/3-octenoic acid, oleic acid, linoleic acid, and linolenic acid in black tea or olive oil samples were directly determined by the established method. These results indicate that the established method allows the rapid determination of unsaturated FAs in black tea and olive oil. The advantages of this approach include the analysis speed (∼1 min per sample), simple device, and no need for complex pretreatment. This study not only provides a strategy for the determination of C═C positions but also offers new possibilities for applications in the field of food chemistry.

Arc-Induced Electrospray Ionization Mass Spectrometry

Arc-induced electrospray ionization mass spectrometry (AESI-MS) was developed during which alternating current electrospray is simply achieved through the arc plasma. The AESI source exploits the arc's temperature and charge properties to generate aerosols consisting of charged microdroplets. The electrospray region, in which organic molecules are contained within microdroplets, partially overlaps with the arc plasma region. Guided by the electric field, these molecules undergo ionization, yielding ionic target analytes. AESI represents a soft ionization method that combines the mechanisms of atmospheric pressure chemical ionization and electrospray ionization, facilitating the ionization of analytes with wide ranging polarities. The precisely targeted spraying area enhances ion entry into the mass analyzer, thereby enabling excellent ionization efficiency. The AESI source exhibits several notable advantages over the electrospray ionization source, including an elevated but comparable level of active species concentrations and types, simplified mass spectra for direct amino acid analysis, high salt tolerance, versatile analysis of compounds with varying polarities, and reliable quantitative analysis of amino acids in complex matrices. Overall, AESI broadens the methodologies employed to generate microdroplets, providing a technological and scientific framework for creating distinctive electrospray ionization techniques.

Native top-down mass spectrometry for monitoring the rapid chymotrypsin catalyzed hydrolysis reaction

Enzymes play crucial roles in life sciences, pharmaceuticals and industries as biological catalysts that speed up biochemical reactions in living organisms. New catalytic reactions are continuously developed by enzymatic engineering to meet industrial needs, which thereby drives the development of analytical approaches for real-time reaction monitoring to reveal catalytic processes. Here, taking the hydrolase- chymotrypsin as a model system, we proposed a convenient method for monitoring catalytic processes through native top-down mass spectrometry (native TDMS). The chymotrypsin sample heterogeneity was first explored. By altering sample introduction modes and pHs, covalent and noncovalent enzymatic complexes, substrates and products can be monitored during the catalysis and further confirmed by tandem MS. Our results demonstrated that native TDMS based catalysis monitoring has distinctive strength on real-time inspection and continuous observation, making it a promising tool for characterizing more biocatalysts.

Harnessing the High Interfacial Electric Fields on Water Microdroplets to Accelerate Menshutkin Reactions

Even though it is still an emerging field, the application of a high external electric field (EEF) as a green and efficient catalyst in synthetic chemistry has recently received significant attention for the ability to deliver remarkable control of reaction selectivity and acceleration of reaction rates. Here, we extend the application of the EEF to Menshutkin reactions by taking advantage of the spontaneous high electric field at the air-water interfaces of sprayed water microdroplets. Experimentally, a series of Menshutkin reactions were accelerated by 7 orders of magnitude. Theoretically, both density functional theory calculations and ab initio molecular dynamics simulations predict that the reaction barrier decreases significantly in the presence of oriented external electric fields, thereby supporting the notion that the electric fields in the water droplets are responsible for the catalysis. In addition, the ordered solvent and reactant molecules oriented by the electric field alleviate the steric effect of solvents and increase the successful collision rates, thus facilitating faster nucleophilic attack. The success of Menshutkin reactions in this study showcases the great potential of microdroplet chemistry for green synthesis.

Is Reaction Acceleration of Microdroplet Chemistry Favorable to Controlling the Enantioselectivity?

Microdroplet chemistry has been proven to amazingly accelerate many chemical and biological reactions in the past 2 decades. Current microdroplet accelerated reactions are predominantly symmetric synthetic but minorly asymmetric synthetic reactions, where stereoselectivity is scarcely concerned. This study selected unimolecular and bimolecular reactions, multicomponent Passerini reactions, and enzymatic ketone reduction as the model reactions to illustrate whether reaction acceleration of microdroplet chemistry is favorable to retaining a chiral center and controlling the enantioselectivity or not. The results illustrated that microdroplet chemistry did not disrupt pre-existing stereogenic centers in chiral starting materials during reactions but did harm to stereospecificity in asymmetric catalysis by chiral catalysts and chiral organic ligands with the exclusion of enzymatic reactions. Our preliminary study reminds us of more cautions to the product enantioselectivity when conducting asymmetric catalysis in microdroplets. We also hope this study may promote more valuable further research on the stereoselectivity of microdroplet chemistry.

One-step Formation of Urea from Carbon Dioxide and Nitrogen Using Water Microdroplets

Water (H2O) microdroplets are sprayed onto a graphite mesh covered with a CuBi2O4 coating using a 1:1 mixture of N2 and CO2 as the nebulizing gas. The resulting microdroplets contain urea [CO(NH2)2] as detected by both mass spectrometry and 13C nuclear magnetic resonance. This gas-liquid-solid heterogeneous catalytic system synthesizes urea in one step on the 0.1 ms time scale. The conversion rate reaches 2.7 mmol g-1 h-1 at 25 °C and 12.3 mmol g-1 h-1 at 65 °C, with no external voltage applied. Water microdroplets serve as the hydrogen source and the electron transfer medium for N2 and CO2 in contact with CuBi2O4. Water-gas and water-solid contact electrification are speculated to drive the reaction process. This strategy couples N2 fixation and CO2 utilization in an ecofriendly process to produce urea, converting a greenhouse gas into a value-added product.

Prebiotic synthesis of mineral-bearing microdroplet from inorganic carbon photoreduction at air-water interface

The origin of life on Earth is an enigmatic and intricate conundrum that has yet to be comprehensively resolved despite recent significant developments within the discipline of archaeology and geology. Chemically, metal-sulfide minerals are speculated to serve as an important medium for giving birth in early life, while yet so far direct evidence to support the hypothesis for the highly efficient conversion of inorganic carbon into praxiological biomolecules remains scarce. In this work, we provide an initial indication that sphalerite, employed as a typical mineral, shows its enormous capability for promoting the conversion of inorganic carbon into elementary biomolecule formic acid (HCOOH) in airborne mineral-bearing aerosol microdroplet, which is over two orders of magnitude higher than that of the corresponding conventional bulk-like aqueous phase medium in the environment (e.g. river, lake, sea, etc.). This significant enhancement was further validated by a wide range of minerals and clays, including CuS, NiS, CoS, CdS, MnS, elemental sulfur, Arizona Test Dust, loess, nontronite, and montmorillonite. We reveal that the abundant interface of unique physical-chemical features instinct for aerosol or cloud microdroplets reduces the reaction energy barrier for the reaction, thus leading to extremely high HCOOH production (2.52 × 1014 kg year-1). This study unfolds unrecognized remarkable contributions of the considered scheme in the accumulation of prebiotic biomolecules in the ancient period of the Earth.

Aqueous microdroplets promote C-C bond formation and sequences in the reverse tricarboxylic acid cycle

The reverse tricarboxylic acid cycle (rTCA) is a central anabolic network that uses carbon dioxide (CO2) and may have provided complex carbon substrates for life before the advent of RNA or enzymes. However, non-enzymatic promotion of the rTCA cycle, in particular carbon fixation, remains challenging, even with primordial metal catalysis. Here, we report that the fixation of CO2 by reductive carboxylation of succinate and α-ketoglutarate was achieved in aqueous microdroplets under ambient conditions without the use of catalysts. Under identical conditions, the aqueous microdroplets also facilitated the sequences in the rTCA cycle, including reduction, hydration, dehydration and retro-aldol cleavage and linked with the glyoxylate cycle. These reactions of the rTCA cycle were compatible with the aqueous microdroplets, as demonstrated with two-reaction and four-reaction sequences. A higher selectivity giving higher product yields was also observed. Our results suggest that the microdroplets provide an energetically favourable microenvironment and facilitate a non-enzymatic version of the rTCA cycle in prebiotic carbon anabolism.

Electron Paramagnetic Resonance Tracks Condition-Sensitive Water Radical Cation

Oxidizing species or radicals generated in water are of vital importance in catalysis, the environment, and biology. In addition to several related reactive oxygen species, using electron paramagnetic resonance (EPR), we present a nontrapping chemical transformation pathway to track water radical cation (H2O+•) species, whose formation is very sensitive to the conditioning environments, such as light irradiation, mechanical action, and gas/chemical introduction. We reveal that H2O+• can oxidize the 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to the crucial epoxy hydroxylamine (HDMP=O) intermediate, which further reacts with the hydroxyl radical (•OH) for the formation of the EPR-active sextet radical (DMPO=O•). Interestingly, we uncover that H2O+• can react with dimethyl methylphosphonate (DMMP), 2-methyl-2-nitrosopropane (MNP), 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO), and α-phenyl-N-tert-butylnitrone (PBN) which contain a double-bond structure to produce corresponding derivatives as well. It is thus expected that both H2O+• and •OH are ubiquitous in nature and in various water-containing experimental systems. These findings provide a novel perspective on radicals for water redox chemistry.

Hydrocarbon Degradation by Contact with Anoxic Water Microdroplets

Oils are hydrophobic, but their degradation is frequently found to be accelerated in the presence of water microdroplets. The direct chemical consequences of water-oil contact have long been overlooked. We show that aqueous microdroplets in emulsified water-hexadecane (C16H34) mixtures can spontaneously produce CO2, •H, H2, and short-chain hydrocarbons (mainly C1 and C2) as detected by gas chromatography, electron paramagnetic resonance spectroscopy, and mass spectrometry. This reaction results from contact electrification at the water-oil microdroplet interface, in which reactive oxygen species are produced, such as hydrated hydroxyl radicals and hydrogen peroxide. We also find that the H2 originates from the water microdroplet and not the hydrocarbon it contacts. These observations highlight the potential of interfacial contact electrification to produce new chemistry.

Dual Tunability for Uncatalyzed N-Alkylation of Primary Amines Enabled by Plasma-Microdroplet Fusion

The fusion of non-thermal plasma with charged microdroplets facilitates catalyst-free N-alkylation for a variety of primary amines, without halide salt biproduct generation. Significant reaction enhancement (up to >200×) is observed over microdroplet reactions generated from electrospray. This enhancement for the plasma-microdroplet system is attributed to the combined effects of energetic collisions and the presence of reactive oxygen species (ROS). The ROS (e.g., O2 ⋅- ) act as a proton sink to increase abundance of free neutral amines in the charged microdroplet environment. The effect of ROS on N-alkylation is confirmed through three unique experiments: (i) utilization of radical scavenging reagent, (ii) characterization of internal energy distribution, and (iii) controls performed without plasma, which lacked reaction acceleration. Establishing plasma discharge in the wake of charged microdroplets as a green synthetic methodology overcomes two major challenges within conventional gas-phase plasma chemistry, including the lack of selectivity and product scale-up. Both limitations are overcome here, where dual tunability is achieved by controlling reagent concentration and residence time in the microdroplet environment, affording single or double N-alkylated products. Products are readily collected yielding milligram quantities in eight hours. These results showcase a novel synthetic strategy that represents a straightforward and sustainable C-N bond-forming process.

Detecting Early-Stage Intermediates of Free-Radical Oxidative Degradation in Charged Aqueous Microdroplets

We report the detection of early-stage intermediates of spontaneous free-radical oxidation of organic pollutants such as aliphatic amino alcohols and diamines in charged aqueous microdroplets in the ambient atmosphere. We propose that the intrinsic formation of reactive oxygen species at the air-water interface is responsible for the radical oxidation of the sp3 carbon. We suggest that our work will aid the understanding of the degradation mechanisms of organic molecules in the environment.

Evidence for the co-existence of isomers of water dimer radical cations and their inter-conversion in a linear ion trap

Water dimer radical cations are regarded as key intermediates in many aqueous reactions and biochemical processes. However, the structure of the water dimer radical cations, and particularly the inter-conversion between their isomers, remain difficult to investigate experimentally due to their short lifetime and low abundance under ambient conditions. Furthermore, the isomers cannot be distinguished in a full mass spectra. In this study, we report the experimental evidence for the hemi-bonded and proton-transferred isomers of gas-phase water dimer radical cations, and the inter-conversion process between them in a linear ion trap at low pressure and near room temperature. Multiple collisions of isolated water dimer radical cations with He inside the ion trap were systematically investigated; first, under different trapping times (i.e., reaction times) ranging from 0.03 to 800 ms, and then at a very low collision energies ranging from 0.1% to 10% normalized collision energy. The proton-transferred isomers were dominant at shorter trapping times (≤250 ms), while the hemi-bonded isomers were dominant at longer trapping times (250-800 ms). Moreover, the difference in symmetry of the shapes of the H₂O^•+ signal profiles and the H₃O⁺ signal profiles implied the existence of two kinds of isomers and there were small potential differences between them. Our results also suggested that by tuning the experimental parameters the hemi-bonded isomers would become dominant, which could allow the study of novel chemical reactions involving the hemi-bonded two-center-three-electron (2c-3e) structure in a linear ion trap.

Interface of biomolecular condensates modulates redox reactions

Biomolecular condensates mediate diverse cellular processes. The density transition process of condensate formation results in selective partitioning of molecules, which define a distinct chemical environment within the condensates. However, the fundamental features of the chemical environment and the mechanisms by which such environment can contribute to condensate functions have not been revealed. Here, we report that an electric potential gradient, thereby an electric field, is established at the liquid-liquid interface between the condensate and the bulk environment due to the density transition of ions and molecules brought about by phase separation. We find that the interface of condensates can drive spontaneous redox reactions in vitro and in living cells. Our results uncover a fundamental physicochemical property of the interface of condensates and the mechanism by which the interface can modulate biochemical activities.

Melatonin: Facts, Extrapolations and Clinical Trials

Melatonin is a fascinating molecule that has captured the imagination of many scientists since its discovery in 1958. In recent times, the focus has changed from investigating its natural role as a transducer of biological time for physiological systems to hypothesized roles in virtually all clinical conditions. This goes along with the appearance of extensive literature claiming the (generally) positive benefits of high doses of melatonin in animal models and various clinical situations that would not be receptor-mediated. Based on the assumption that melatonin is safe, high doses have been administered to patients, including the elderly and children, in clinical trials. In this review, we critically review the corresponding literature, including the hypotheses that melatonin acts as a scavenger molecule, in particular in mitochondria, by trying not only to contextualize these interests but also by attempting to separate the wheat from the chaff (or the wishful thinking from the facts). We conclude that most claims remain hypotheses and that the experimental evidence used to promote them is limited and sometimes flawed. Our review will hopefully encourage clinical researchers to reflect on what melatonin can and cannot do and help move the field forward on a solid basis.

Ion-Size Dependent Adsorption Crossover on the Surface of a Water Droplet

The adsorption of ionic and neutral spherical solutes on the surface of a liquid water droplet are investigated using molecular dynamics simulations and theoretical analyses. The results reveal a crossover in the sign of the adsorption free energy as a function of ion size, with ions larger than iodide predicted to be increasingly surface active. Adsorption free energies are decomposed into competing energetic and entropic contributions arising from direct solute-water interaction energy and its fluctuations. The entropically driven surface activity of large ions is predicted to increase with ion size, while small ions are typically driven away from the interface by a more delicate balance of energetic and entropic contributions, with a nonmonotonic ion size dependence linked to the ion's hydration-shell structure and stability. The physical interpretation of the results is illuminated by comparisons with dielectric linear response and cavity formation predictions and implications to interfacial acidity and enhanced chemical reactivity are discussed.

Spontaneous dark formation of OH radicals at the interface of aqueous atmospheric droplets

Hydroxyl radical (OH) is a key oxidant that triggers atmospheric oxidation chemistry in both gas and aqueous phases. The current understanding of its aqueous sources is mainly based on known bulk (photo)chemical processes, uptake from gaseous OH, or related to interfacial O3 and NO3 radical-driven chemistry. Here, we present experimental evidence that OH radicals are spontaneously produced at the air-water interface of aqueous droplets in the dark and the absence of known precursors, possibly due to the strong electric field that forms at such interfaces. The measured OH production rates in atmospherically relevant droplets are comparable to or significantly higher than those from known aqueous bulk sources, especially in the dark. As aqueous droplets are ubiquitous in the troposphere, this interfacial source of OH radicals should significantly impact atmospheric multiphase oxidation chemistry, with substantial implications on air quality, climate, and health.

One-Step Formation of Pharmaceuticals Having a Phenylacetic Acid Core Using Water Microdroplets

The properties of water microdroplets strikingly differ from bulk water. Using room-temperature water microdroplets, we find that toluene can react with CO₂ to form phenylacetic acid in one step without any catalyst with negative high voltage applied at the sprayer source. The chemical components of these microdroplets are identified by mass spectrometry, and product structures are confirmed by tandem mass spectrometry. In this manner, we generate three drug molecules in a single step: 4-aminophenylacetic acid (epithelial peptide transporter PepT1 inhibitor), 3,4-dihydroxyphenylacetic acid (dopamine metabolite neurotransmitter), and phenylacetic acid (sodium salt form; treatment of urea cycle disorder). Mechanistic studies show that benzyl radicals formed from hydroxyl radicals at the water microdroplet interface drive these carboxylation reactions. This water microdroplet chemistry is general, allowing activation and subsequent carboxylation of aryl α-C-H groups.

Spontaneous and Simultaneous Oxidation and Reduction of o-Quinones in Water Microdroplets

Microdroplet chemistry has been an emerging new field for its large plethora of unique properties, among which an especially intriguing one is the strong oxidizing and reducing powers. The hydroxide ion in water microdroplets is considered to split into a hydroxyl radical and an electron at the air-water interface, and the former is responsible for the oxidizing capability while the latter is responsible for the reducing power, making a unity of opposites. However, to date there are only two examples showing that oxidation and reduction occur simultaneously to the same substrates, which might be a result of the redox properties of the substrate per se. In this study, we carefully chose a group of ο-quinone compounds as the substrates in water microdroplets and discovered that they can be both oxidized by the hydroxyl radical and reduced by the electron. These results keep pushing the limit of the unique redox properties of microdroplet chemistry.

Spontaneous Reduction by One Electron on Water Microdroplets Facilitates Direct Carboxylation with CO2

Recent advances in microdroplet chemistry have shown that chemical reactions in water microdroplets can be accelerated by several orders of magnitude compared to the same reactions in bulk water. Among the large plethora of unique properties of microdroplets, an especially intriguing one is the strong reducing power that can be sometimes as high as alkali metals as a result of the spontaneously generated electrons. In this study, we design a catalyst-free strategy that takes advantage of the reducing ability of water microdroplets to reduce a certain molecule, and the reduced form of that molecule can convert CO₂ into value-added products. By spraying the water solution of C₆F₅I into microdroplets, an exotic and fragile radical anion, C₆F₅I^•-, is observed, where the excess electron counter-intuitively locates on the σ* antibonding orbital of the C-I bond as evidenced by anion photoelectron spectroscopy. This electron weakens the C-I bond and causes the formation of C₆F₅^-, and the latter attacks the carbon atom on CO₂, forming the pentafluorobenzoate product, C₆F₅CO₂^-. This study provides a good example of strategically making use of the spontaneous properties of water microdroplets, and we anticipate that microdroplet chemistry will be a green avenue rich in new opportunities in CO₂ utilization.

Significantly Accelerated Hydroxyl Radical Generation by Fe(III)-Oxalate Photochemistry in Aerosol Droplets

Fe(III)-oxalate complexes are ubiquitous in atmospheric environments, which can release reactive oxygen species (ROS) such as H₂O₂, O^•2-, and OH^• under light irradiation. Although Fe(III)-oxalate photochemistry has been investigated extensively, the understanding of its involvement in authentic atmospheric environments such as aerosol droplets is far from enough, since the current available knowledge has mainly been obtained in bulk-phase studies. Here, we find that the production of OH^• by Fe(III)-oxalate in aerosol microdroplets is about 10-fold greater than that of its bulk-phase counterpart. In addition, in the presence of Fe(III)-oxalate complexes, the rate of photo-oxidation from SO₂ to sulfate in microdroplets was about 19-fold faster than that in the bulk phase. The availability of efficient reactants and mass transfer due to droplet effects made dominant contributions to the accelerated OH^• and SO₄^2- formation. This work highlights the necessary consideration of droplet effects in atmospheric laboratory studies and model simulations.

Catalyst-Free Decarboxylative Amination of Carboxylic Acids in Water Microdroplets

Previous studies have shown that hydroxyl radicals can be formed at the water-gas surface of water microdroplets. We report the use of in situ generated hydroxyl radicals to carry out an organic transformation in one step, namely, the formation of anilines from aryl acids as well as both ammonia and primary/secondary amines via decarboxylation. Benzoic acids and amines are dissolved in water, and the solution is sprayed to form microdroplets whose chemical contents are analyzed mass spectrometrically. All intermediates and products are determined using mass spectrometry (MS) as well as in some cases tandem mass spectrometry (MS²). These results support the following reaction mechanism: NR₂OH, formed via reaction of the amine with •OH, reacts with benzoic acid to form an isocyanate via a Lossen rearrangement. Hydrolysis followed by liberation of CO₂ then delivers the aniline product. Notably, the scope of this transformation includes a variety of amines and aromatic acids and enables their conversion into aniline and N-substituted anilines, all in a single step. Additionally, this reaction occurs at room temperature and does not require metal catalysts or organic solvents.

Oxidation of Catechols at the Air-Water Interface by Nitrate Radicals

Abundant substituted catechols are emitted to, and created in, the atmosphere during wildfires and anthropogenic combustion and agro-industrial processes. While ozone (O₃) and hydroxyl radicals (HO^•) efficiently react in a 1 μs contact time with catechols at the air-water interface, the nighttime reactivity dominated by nitrate radicals (NO₃) remains unexplored. Herein, online electrospray ionization mass spectrometry (OESI-MS) is used to explore the reaction of NO₃(g) with a series of representative catechols (catechol, pyrogallol, 3-methylcatechol, 4-methylcatechol, and 3-methoxycatechol) on the surface of aqueous microdroplets. The work detects the ultrafast generation of nitrocatechol (aromatic) compounds, which are major constituents of atmospheric brown carbon. Two mechanisms are proposed to produce nitrocatechols, one (equivalent to H atom abstraction) following fast electron transfer from the catechols (QH₂) to NO₃, forming NO₃^- and QH₂^•+ that quickly deprotonates into a semiquinone radical (QH^•). The second mechanism proceeds via cyclohexadienyl radical intermediates from NO₃ attack to the ring. Experiments in the pH range from 4 to 8 showed that the production of nitrocatechols was favored under the most acidic conditions. Mechanistically, the results explain the interfacial production of chromophoric nitrocatechols that modify the absorption properties of tropospheric particles, making them more susceptible to photooxidation, and alter the Earth's radiative forcing.