“On‐Droplet” Chemistry: The Cycloaddition of Diethyl Azodicarboxylate and Quadricyclane

The Structure of Carbon Dioxide at the Air-Water Interface and its Chemical Implications

The efficient reduction of CO2 into valuable products is a challenging task in an international context marked by the climate change crisis and the need to move away from fossil fuels. Recently, the use of water microdroplets has emerged as an interesting reaction media where many redox processes which do not occur in conventional solutions take place spontaneously. Indeed, several experimental studies in microdroplets have already been devoted to study the reduction of CO2 with promising results. The increased reactivity in microdroplets is thought to be linked to unique electrostatic solvation effects at the air-water interface. In the present work, we report a theoretical investigation on this issue for CO2 using first-principles molecular dynamics simulations. We show that CO2 is stabilized at the interface, where it can accumulate, and that compared to bulk water solution, its electron capture ability is larger. Our results suggest that reduction of CO2 might be easier in interface-rich systems such as water microdroplets, which is in line with early experimental data and indicate directions for future laboratory studies. The effect of other relevant factors which could play a role in CO2 reduction potential is discussed.

Characterization of 2-Butanone Peroxide Oligomeric Profiles and Their Associated Gas-Phase and Solution-Phase Rearrangement Products by Electrospray Ionization Mass Spectrometry for Forensic Applications

2-Butanone peroxide (also known as methyl ethyl ketone peroxide, MEKP) has applications as a cross-linker in the chemical industry and is also encountered as a homemade primary high explosive; therefore, it is of interest to both process chemists and forensic examiners. Specifically for forensic applications, we demonstrate that when traditional synthetic procedures, available to any hobbyist, are utilized to generate MEKP, oligomeric peroxide units (n ≤ 12), along with several other oligomeric byproduct distributions, are readily observed by liquid chromatography-mass spectrometry (LC-MS). These oligomeric byproducts correspond to the formation of methyl/ethyl ketone end group(s) at the oligomer end group (i.e., loss of ethanol(s) and/or methanol(s) from the oligomer termini). Based on the interpretation of the MS and MS/MS behavior along with the characterization of newly generated terminal alkyl ketone products, we propose that these byproducts are consistent with a Hock-like rearrangement of the primary MEKP distribution in the acidified reaction medium. Following a procedure for homemade preparation, triplicate lots were synthesized. Unique oligomeric and byproduct distributions provided discriminatory power between the synthetic lots. Furthermore, the distributions of MEKP oligomers and the various byproducts in the initiated MEKP match the intensity distributions observed in the intact material with remarkable accuracy. This observation suggests that the postinitiation residue of MEKP could be associated or dissociated from a separately collected intact material obtained during an investigation by examining these oligomeric and byproduct profiles.

Impact of interfacial curvature on molecular properties of aqueous interfaces

The curvature of soft interfaces plays a crucial role in determining their mechanical and thermodynamic properties, both at macroscopic and microscopic scales. In the case of air/water interfaces, particular attention has recently focused on water microdroplets, due to their distinctive chemical reactivity. However, the specific impact of curvature on the molecular properties of interfacial water and interfacial reactivity has so far remained elusive. Here, we use molecular dynamics simulations to determine the effect of curvature on a broad range of structural, dynamical, and thermodynamical properties of the interface. For a droplet, a flat interface, and a cavity, we successively examine the structure of the hydrogen-bond network and its relation to vibrational spectroscopy, the dynamics of water translation, rotation, and hydrogen-bond exchanges, and the thermodynamics of ion solvation and ion-pair dissociation. Our simulations show that curvature predominantly impacts the hydrogen-bond structure through the fraction of dangling OH groups and the dynamics of interfacial water molecules. In contrast, curvature has a limited effect on solvation and ion-pair dissociation thermodynamics. For water microdroplets, this suggests that the curvature alone cannot fully account for the distinctive reactivity measured in these systems, which are of great importance for catalysis and atmospheric chemistry.

Insulator-on-Conductor Fouling Amplifies Aqueous Electrolysis Rates

The chemical industry is a major consumer of fossil fuels. Several chemical reactions of practical value proceed with the gain or loss of electrons, opening a path to integrate renewable electricity into chemical manufacturing. However, most organic molecules have low aqueous solubility, causing green and cheap electricity-driven reactions to suffer from intrinsically low reaction rates in industry's solvent of choice: water. Here, we show that a strategic, partial electrode fouling with hydrophobic insulators (oils and plastics) offsets kinetic limitations caused by poor reactant solubility, opening a new path for the direct integration of renewable electricity into the production of commodity chemicals. Through electrochemiluminescence microscopy, we reveal for the oxidation of organic reactants up to 6-fold reaction rate increase at the "fouled" oil-electrolyte-electrode interface relative to clean electrolyte-electrode areas. Analogously, electrodes partially masked (fouled) with plastic patterns, deposited either photolithographically (photoresists) or manually (inexpensive household glues and sealants), outperform clean electrodes. The effect is not limited to reactants of limited water solubility, and, for example, net gold electrodeposition rates are up to 22% larger at fouled than clean electrodes. In a system involving a surface-active reactant, rate augmentation is driven by the synergy between insulator-confined reactant enrichment and insulator-induced current crowding, whereas only the latter and possibly localized decrease in iR drop near the insulator are relevant in a system composed of non-surface-active species. Our counterintuitive electrode design enhances electrolysis rates despite the diminished area of intimate electrolyte-electrode contact and introduces a new path for upscaling aqueous electrochemical processes.

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.

Hydroxylation of the indium tin oxide electrode promoted by surface bubbles

Adherent bubbles at electrodes are generally treated as reaction penalties. Herein, in situ hydroxylation of indium tin oxide surfaces can be easily achieved by applying a constant potential of +1.0 V in the presence of bubbles. Its successful hydroxylation is further demonstrated by preparing a ferrocene-terminated film, which is confirmed by cyclic voltammetry and X-ray photoelectron spectroscopy.

Direct access to carbamates via acylation of arylamines with dialkyl azodicarboxylates under metal-free conditions

A novel C-N coupling of various arylamines with dialkyl azodicarboxylates under metal-free conditions for the rapid assembly of carbamates has been achieved. This established protocol features mild reaction conditions, simple operation, broad substrate scope, moderate to excellent yields and good tolerance of functional groups. Moreover, the potential synthetic utility of products was exemplified by a series of intriguing chemical operations.

Review of air-water interface adsorption and reactions between trace gaseous organic and oxidant compounds

The surface chemistry of the atmospheric aerosol through homogeneous and heterogeneous catalytic reactions in the bulk water and the air-water surface is reviewed. Water plays a critical role as a substrate or an actual reactant in atmospheric reactions. The atmospheric aerosol differs in shape and surface area. Many gaseous reactive species and oxidants react at the air-water surface. Different thermodynamic methods to estimate partitioning coefficients are explored. The Gibbs free energy is reduced when reactant gaseous species react with oxidant at the air-water surface; this phenomenon is explained using examples. Langmuir-Hinshelwood reaction mechanism to quantify the heterogeneous reaction rate at the air-water interface is discussed. Critical comparisons of various sampling techniques used to analyze adsorption and reaction at the water surface are presented. The heterogeneous reaction rate at the air-water surface is significantly higher than in the bulk water phase due to a cage effect, higher rate of reactions, and lower Gibbs free energy of adsorption.

Rapid Characterization of Maillard Reaction Products in Heat-Treated Honey by Nanoelectrospray Ionization Mass Spectrometry

Amadori rearrangement products (ARPs) and α-dicarbonyl compounds (α-DCs) are critical intermediates in the Maillard chemistry. The screening of artificially heated honey (AH) is currently based on chromatography-mass spectrometry, which is commonly accompanied with the longer pretreatment and detection time. Here, low-abundance ARPs were detected directly in high-sugar environment by nanoelectrospray ionization mass spectrometry (nanoESI-MS) coupled with borosilicate glass capillaries (O-tips). When O-tips were replaced by borosilicate theta capillaries (θ-tips), the microdroplets allowed the derivatization of α-DCs to be accomplished on the millisecond timescale, rather than hours in conventional protocols. The results indicated that two ARPs and α-DCs of m/z 235 were significantly up-regulated in AH. Meanwhile, the straightforward differentiation between naturally matured honey (NH) and AH was achieved by nanoESI-MS fingerprints combined with multivariate analysis. The method may provide a rapid characterization of Maillard reaction products (MRPs), which exhibits the great application potential in other complex food matrix.

Role of an Ice Surface in the Photoreaction of Coumarins

Ice affects many chemical reactions in nature, which greatly influences the atmosphere, climate, and life. However, the exact mechanism of ice in these chemical reactions remains elusive. For example, it is still an open question as to whether ice can act as a catalyst to greatly enhance the reactivity and selectivity, which is essential for the production of some natural compounds in our planet. Here, we discover that ice can lead to high efficiency and stereoselectivity of the [2 + 2] photodimerization of coumarin and its derivatives. The conversion of the [2 + 2] photodimerization of coumarins enhanced by ice is dozens of times higher than that in the unfrozen saturated solution, and the reaction displays a high syn-head-head stereoselectivity (>95%) in comparison with those in the absence of the ice. Note that almost no reaction occurs in the crystal powder and melt of the coumarins, indicating that the role of ice in the photodimerization reaction is not simply due to the usual mechanisms found in the freezing concentration. We further reveal that the reaction rate is found to be proportional to the total area of the ice surface and follows Michaelis-Menten-like kinetics, indicating that the ice surface catalyzes the reaction. Molecular dynamics simulations demonstrate that ice surfaces can induce reactants to form a two-dimensional liquid-crystal-ordered layer with a suitable intermolecular distance and unique side-by-side packing, facilitating stereoselective photodimerization for syn-head-head dimers. These findings give evidence that ice-surface-induced molecular assembly may play an important role in atmospheric heterogeneous photoreaction processes.

In Situ Detection of Chemical Compositions at Nanodroplet Surfaces and In-Nanodroplet Phases

Small-volume nanodroplets play an increasingly common role in chemistry and biology. Such nanodroplets are believed to have unique chemical and physical properties at the interface between a droplet and its surrounding medium, however, they are underexamined. In this study, we present the novel technique of vibrational sum frequency scattering (VSFS) spectroscopy as an interface-specific, high-performance method for the in situ investigation of nanodroplets with sub-micron radii; as well as the droplet bulk through simultaneous hyper-Raman scattering (HRS) spectroscopy. We use laboratory-generated nanodroplets from aqueous alcohol solutions to demonstrate this technique's ability to separate the vibrational phenomena which take place at droplet surfaces from the underlying bulk phase. In addition, we systemically examine interfacial spectra of nanodroplets containing methanol, ethanol, 1-propanol, and 1-butanol through VSFS. Furthermore, we demonstrate interfacial differences between such nanodroplets and their analogous planar surfaces. The sensitivity of this technique to probe droplet surfaces with few-particle density at standard conditions validates VSFS as an analytical technique for the in situ investigation of small nanodroplets, providing breakthrough information about these species of ever-increasing relevance.

CHEMICAL REACTION MONITORING BY AMBIENT MASS SPECTROMETRY

Chemical reactions conducted in different media (liquid phase, gas phase, or surface) drive developments of versatile techniques for the detection of intermediates and prediction of reasonable reaction pathways. Without sample pretreatment, ambient mass spectrometry (AMS) has been applied to obtain structural information of reactive molecules that differ in polarity and molecular weight. Commercial ion sources (e.g., electrospray ionization, atmospheric pressure chemical ionization, and direct analysis in real-time) have been reported to monitor substrates and products by offline reaction examination. While the interception or characterization of reactive intermediates with short lifetime are still limited by the offline modes. Notably, online ionization technologies, with high tolerance to salt, buffer, and pH, can achieve direct sampling and ionization of on-going reactions conducted in different media (e.g., liquid phase, gas phase, or surface). Therefore, short-lived intermediates could be captured at unprecedented timescales, and the reaction dynamics could be studied for mechanism examinations without sample pretreatments. In this review, via various AMS methods, chemical reaction monitoring and mechanism elucidation for different classifications of reactions have been reviewed. The developments and advances of common ionization methods for offline reaction monitoring will also be highlighted.

In-Source Microdroplet Derivatization Using Coaxial Contained-Electrospray Mass Spectrometry for Enhanced Sensitivity in Saccharide Analysis

Online, droplet-based in-source chemical derivatization is accomplished using a coaxial-flow contained-electrospray ionization (contained-ESI) source to enhance sensitivity for the mass spectrometric analysis of saccharides. Derivatization is completed in microseconds by exploiting the reaction rate acceleration afforded by electrospray microdroplets. Significant improvements in method sensitivity are realized with minimal sample preparation and few resources when compared to traditional benchtop derivatizations. For this work, the formation of easily ionizable phenylboronate ester derivatives of several mono-, di-, and oligosaccharides is achieved. Various reaction parameters including concentration and pH were evaluated, and a Design of Experiments approach was used to optimize ion source parameters. Signal enhancements of greater than two orders of magnitude were observed for many mono- and disaccharides using in-source phenylboronic acid derivatization, resulting in parts-per-trillion (picomolar) limits of detection. In addition, amino sugars such as glucosamine, which do not ionize in negative mode, were detected at low parts-per-billion concentrations, and isobaric sugars such as lactose and sucrose were easily distinguished. The new in-source derivatization approach can be employed to expand the utility of ESI-MS analysis for compounds that historically experience limited sensitivity and detectability, while avoiding resource-intensive, bulk-phase derivatization procedures.

Accelerated synthesis of energetic precursor cage compounds using confined volume systems

Confined volume systems, such as microdroplets, Leidenfrost droplets, or thin films, can accelerate chemical reactions. Acceleration occurs due to the evaporation of solvent, the increase in reactant concentration, and the higher surface-to-volume ratios amongst other phenomena. Performing reactions in confined volume systems derived from mass spectrometry ionization sources or Leidenfrost droplets allows for reaction conditions to be changed quickly for rapid screening in a time efficient and cost-saving manner. Compared to solution phase reactions, confined volume systems also reduce waste by screening reaction conditions in smaller volumes prior to scaling. Herein, the condensation of glyoxal with benzylamine (BA) to form hexabenzylhexaazaisowurtzitane (HBIW), an intermediate to the highly desired energetic compound 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), was explored. Five confined volume systems were compared to evaluate which technique was ideal for forming this complex cage structure. Substituted amines were also explored as BA replacements to screen alternative cage structure intermediates and evaluate how these accelerated techniques could apply to novel reactions, discover alternative reagents to form the cage compound, and improve synthetic routes for the preparation of CL-20. Ultimately, reaction acceleration is ideal for predicting the success of novel reactions prior to scaling up and determining if the expected products form, all while saving time and reducing costs. Acceleration factors and conversion ratios for each reaction were assessed by comparing the amount of product formed to the traditional bulk solution phase synthesis.

On-Water Selectivity Switch in Microdroplets in the 1,2,3-Triazole Synthesis from Bromoethenesulfonyl Fluoride

Water profoundly affects many organic reactions by accelerating them or changing their selectivity. Performing reactions "on-water" offers an intriguing opportunity to influence chemical reactivity. A nebulizer plume is an efficient way of generating microdroplets─the uniquely complex reaction environment which opens alternative possibilities that are not readily accessible in bulk emulsions. We describe the on-water switch of chemoselectivity in the formation of triazoles controlled by the on-water environment in dual spray. These conditions facilitate elimination of H-SO₂F from the triazoline intermediate, whereas the reaction in organic solvents results in the exclusive HBr elimination. The influence of two-phase conditions was investigated to obtain the best reaction efficiency, and the crucial importance of the water/organic interface interactions was verified by pH variation and D₂O use.

Acceleration of the thermal degradation of PETN in the microdroplets flow reactor

Thermal degradation of pentaerythritol tetranitrate (PETN) was investigated in microdroplets within a heated capillary used as a flow reactor. The thermal degradation was monitored by aerodynamic thermal breakup droplet ionization mass spectrometry. It was shown that the PETN degradation in microdroplets occurs much faster than the bulk reaction (by 4-5 orders of magnitude). The effect of the capillary material [stainless steel (Fe, Cr), copper (Cu), or fused quartz (SiO₂)] on the thermal PETN degradation in microdroplets of water or acetonitrile was studied next. The capillary material affected the rate of thermal PETN degradation much more weakly than did the use of microdroplets (pure Cu was most conducive to the degradation). Kinetic parameters (activation energy and the frequency factor) of the PETN degradation for all the studied materials of the flow-through reactor and the solvents were estimated under the assumption that the thermal degradation is a first-order reaction. Implications of the acceleration of PETN degradation in microdroplets are discussed.

Synthetic Organic "Aquachemistry" that Relies on Neither Cosolvents nor Surfactants

There is a growing awareness of the underlying power of catalytic reactions in water that is not limited to innate sustainability alone. Some Type III reactions are catalytically accelerated without dissolution of reactants and are occasionally highly selective, as shown by comparison with the corresponding reactions run in organic solvents or under solvent-free conditions. Such catalysts are highly diversified, including hydrophilic, lipophilic, and even solid catalysts. In this Outlook, we highlight the impressive characteristics of illustrative catalysis that is exerted despite the immiscibility of the substrates and reveal the intrinsic benefits of these enigmatic reactions for synthetic organic chemistry, albeit with many details remaining unclear. We hope that this brief introduction to the expanding field of synthetic organic "aquachemistry" will inspire organic chemists to use the platform to invent new transformations.

Demystifying the Diffuse Vibrational Spectrum of Aqueous Protons Through Cold Cluster Spectroscopy

The ease with which the pH is routinely determined for aqueous solutions masks the fact that the cationic product of Arrhenius acid dissolution, the hydrated proton, or H⁺(aq), is a remarkably complex species. Here, we review how results obtained over the past 30 years in the study of H⁺⋅(H₂O)_n cluster ions isolated in the gas phase shed light on the chemical nature of H⁺(aq). This effort has also revealed molecular-level aspects of the Grotthuss relay mechanism for positive-charge translocation in water. Recently developed methods involving cryogenic cooling in radiofrequency ion traps and the application of two-color, infrared-infrared (IR-IR) double-resonance spectroscopy have established a clear picture of how local hydrogen-bond topology drives the diverse spectral signatures of the excess proton. This information now enables a new generation of cluster studies designed to unravel the microscopic mechanics underlying the ultrafast relaxation dynamics displayed by H⁺(aq).

Catalytic Mechanism of Interfacial Water in the Cycloaddition of Quadricyclane and Diethyl Azodicarboxylate

"On-water" catalysis, the unusual activity of water molecules at the organic solvent-water interface, has been demonstrated in many organic reactions. However, the catalytic mechanism has remained unclear, largely because of the irreproducibility of the organic-water interface under the common stirring condition. Here, the interfacial area was controlled by employing adsorbed water on mesoporous silica nanoparticles as the catalyst. Reliable kinetics of the cycloaddition reaction of quadricyclane and diethyl azodicarboxylate (DEAD) at the toluene-water interface within the nanoparticle pores were measured. Data reveal an Eley-Rideal mechanism, wherein DEAD adsorbs at the toluene-water interface via hydrogen bonds formed with interfacial water, which lower the activation energy of the cycloaddition reaction. The mechanistic insights gained and preparation of surface water in silica pores described herein may facilitate the future design of improved "on-water" catalysts.

Microdroplet Ultrafast Reactions Speed Antibody Characterization

Recently, microdroplet reactions have aroused much interest because the microdroplet provides a unique medium where organic reactions could be accelerated by a factor of 10³ or more. However, microdroplet reactions of proteins have been rarely studied. We report the occurrence of multiple-step reactions of a large protein, specifically, the digestion, reduction, and deglycosylation of an intact antibody, which can take place in microseconds with high reaction yields in aqueous microdroplets at room temperature. As a result, fast structural characterization of a monoclonal antibody, essential for assessing its quality as a therapeutic drug, can be enabled. We found that the IgG1 antibody can be digested completely by the IdeS protease in aqueous microdroplets in 250 microseconds, a 7.5 million-fold improvement in speed in comparison to traditional digestion in bulk solution (>30 min). Strikingly, inclusion of the reductant tris(2-carboxyethyl)phosphine in the spray solution caused simultaneous antibody digestion and disulfide bond reduction. Digested and reduced antibody fragments were either collected or analyzed online by mass spectrometry. Further addition of PNGase F glycosylase into the spray solution led to antibody deglycosylation, thereby producing reduced and deglycosylated fragments of analytical importance. In addition, glycated fragments of IgG1 derived from glucose modification were identified rapidly with this ultrafast digestion/reduction technique. We suggest that microdroplets can serve as powerful microreactors for both exploring large-molecule reactions and speeding their structural analyses.

Reaction Kinetics of Organic Aerosol Studied by Droplet Assisted Ionization: Enhanced Reactivity in Droplets Relative to Bulk Solution

Droplet Assisted Ionization (DAI) is a relatively new method for online analysis of aerosol droplets that enables measurement of the rate of an aerosol reaction. Here, we used DAI to study the reaction of carbonyl functionalities in secondary organic aerosol (SOA) with Girard's T (GT) reagent, a reaction that can potentially be used to enhance the detection of SOA in online measurements. SOA was produced by α-pinene ozonolysis. Particulate matter was collected on a filter, extracted, and mixed with GT reagent in water. While the reaction hardly proceeded at all in bulk solution, products were readily observed with DAI when the solution was atomized to produce micron-size droplets. Varying the droplet transit time between the atomizer and mass spectrometer allowed the reaction rate constant to be determined, which was found to be 4 orders of magnitude faster than what would be expected from bulk solution kinetics. Decreasing the water content of the droplets, either by heating the capillary inlet to the mass spectrometer or by decreasing the relative humidity of the air surrounding the droplets in the transit line from the atomizer to the mass spectrometer, enhanced product formation. The results suggest that reaction enhancement occurs at the droplet surface, which is consistent with previous reports of reaction acceleration during mass spectrometric analysis, where a bulk solution is analyzed with an ionization method that produces aerosol droplets.

Theoretical insight into the on-water catalytic effect in the biogenesis of triterpene dimers: from one-step to two-step hetero Diels–Alder reactions

An alternative pathway to the hetero Diels–Alder reaction for the biogenic origin of triterpene dimers is presented here. In this new pathway, the explicit water molecules take a fundamental role.

A critical analysis of electrospray techniques for the determination of accelerated rates and mechanisms of chemical reactions in droplets

Electrospray and Electrosonic Spray Ionization Mass Spectrometry (ESI-MS and ESSI-MS) have been widely used to report evidence that many chemical reactions in micro- and nano-droplets are dramatically accelerated by factors of ∼102 to 106 relative to macroscale bulk solutions. Despite electrospray's relative simplicity to both generate and detect reaction products in charged droplets using mass spectrometry, substantial complexity exists in how the electrospray process itself impacts the interpretation of the mechanism of these observed accelerated rates. ESI and ESSI are both coupled multi-phase processes, in which analytes in small charged droplets are transferred and detected as gas-phase ions with a mass spectrometer. As such, quantitative examination is needed to evaluate the impact of multiple experimental factors on the magnitude and mechanisms of reaction acceleration. These include: (1) evaporative concentration of reactants as a function of droplet size and initial concentration, (2) competition from gas-phase chemistry and reactions on experimental surfaces, (3) differences in ionization efficiency and ion transmission and (4) droplet charge. We examine (1-4) using numerical models, new ESI/ESSI-MS experimental data, and prior literature to assess the limitations of these approaches and the experimental best practices required to robustly interpret acceleration factors in micro- and nano-droplets produced by ESI and ESSI.

Accelerating Electrochemical Reactions in a Voltage-Controlled Interfacial Microreactor

Microdroplet chemistry is attracting increasing attention for accelerated reactions at the solution-air interface. We report herein a voltage-controlled interfacial microreactor that enables acceleration of electrochemical reactions which are not observed in bulk or conventional electrochemical cells. The microreactor is formed at the interface of the Taylor cone in an electrospray emitter with a large orifice, thus allowing continuous contact of the electrode and the reactants at/near the interface. As a proof-of-concept, electrooxidative C-H/N-H coupling and electrooxidation of benzyl alcohol were shown to be accelerated by more than an order of magnitude as compared to the corresponding bulk reactions. The new electrochemical microreactor has unique features that allow i) voltage-controlled acceleration of electrochemical reactions by voltage-dependent formation of the interfacial microreactor; ii) "reversible" electrochemical derivatization; and iii) in situ mechanistic study and capture of key radical intermediates when coupled with mass spectrometry.

Molecular reactions at aqueous interfaces

This Review aims to critically analyse the emerging field of chemical reactivity at aqueous interfaces. The subject has evolved rapidly since the discovery of the so-called 'on-water catalysis', alluding to the dramatic acceleration of reactions at the surface of water or at its interface with hydrophobic media. We review critical experimental studies in the fields of atmospheric and synthetic organic chemistry, as well as related research exploring the origins of life, to showcase the importance of this phenomenon. The physico-chemical aspects of these processes, such as the structure, dynamics and thermodynamics of adsorption and solvation processes at aqueous interfaces, are also discussed. We also present the basic theories intended to explain interface catalysis, followed by the results of advanced ab initio molecular-dynamics simulations. Although some topics addressed here have already been the focus of previous reviews, we aim at highlighting their interconnection across diverse disciplines, providing a common perspective that would help us to identify the most fundamental issues still incompletely understood in this fast-moving field.

Accelerated microdroplet synthesis of benzimidazoles by nucleophilic addition to protonated carboxylic acids

We report a metal-free novel route for the accelerated synthesis of benzimidazole and its derivatives in the ambient atmosphere. The synthetic procedure involves 1,2-aromatic diamines and alkyl or aryl carboxylic acids reacting in electrostatically charged microdroplets generated using a nano-electrospray (nESI) ion source. The reactions are accelerated by orders of magnitude in comparison to the bulk. No other acid, base or catalyst is used. Online analysis of the microdroplet accelerated reaction products is performed by mass spectrometry. We provide evidence for an acid catalyzed reaction mechanism based on identification of the intermediate arylamides. Their dehydration to give benzimidazoles occurs in a subsequent thermally enhanced step. It is suggested that the extraordinary acidity at the droplet surface allows the carboxylic acid to function as a C-centered electrophile. Comparisons of this methodology with data from thin film and bulk synthesis lead to the proposal of three key steps in the reaction: (i) formation of an unusual reagent (protonated carboxylic acid) because of the extraordinary conditions at the droplet interface, (ii) accelerated bimolecular reaction because of limited solvation at the interface and (iii) thermally assisted elimination of water. Eleven examples are shown as evidence of the scope of this chemistry. The accelerated synthesis has been scaled-up to establish the substituent-dependence and to isolate products for NMR characterization.

Facile and efficient preparation of organoimido derivatives of [Mo6O19]2- using accelerated reactions in Leidenfrost droplets

Reaction acceleration is a hot topic in recent years since it is very useful for rapid reaction screening and small-scale synthesis on a short timescale. It is known that the rates of chemical reactions are often accelerated in confined volumes (small droplets or thin films) where the unique chemical reactivities of molecules at the air-droplet/thin film interface, usually different from that in the bulk and gas phases, play a dominant role in acceleration. The Leidenfrost effect was employed to create small levitated droplets with no net charge. These droplets can accelerate many kinds of organic reactions. Our first accelerated synthesis of a series of organoimido-functionalized polyoxometalate (POM) clusters using Leidenfrost droplets with product analysis by electrospray ionization mass spectrometry (ESI-MS) demonstrated that this method can be successfully extended to the synthesis of inorganic/organic hybrids, a very promising area for developing POM-based functional materials. Comparable amounts of synthetic products [Mo₆O₁₈(NC₆H₄R)]^2- (R = H (6), m/z 477; p-i-C₃H₇ (7), m/z 498; p-OCH₃ (8), m/z 492; p-NO₂ (9), m/z 500) were prepared within minutes in Leidenfrost droplets versus in hours in the corresponding bulk reactions under the same reaction conditions in the presence of the DCC catalyst, suggesting that both concentration and interfacial effects are pivotal in causing reaction acceleration in the Leidenfrost droplet. Compared to the conventional bulk reactions, the acceleration factors (AFs) were 92, 136, 126, and 89 for the four model reactions (1)-(4), respectively. We also found out that substitution affects the rate of reactions occurring in droplets, and hence the magnitude of AF. The rates increase in the order of R = NO₂ < H < i-C₃H₇ < OCH₃, in which the electron-donating groups (i.e., R = OCH₃, i-C₃H₇) on the benzene ring are more favorable to the reaction than the electron-withdrawing group (i.e., R = NO₂). This experimental result is in good agreement with the DFT calculation which indicates that the free-energy barriers for the direct imidoylization of POM with RNH₂ are linearly correlated with the basicity constants (pK_b) of amines.

Brown Carbon Production by Aqueous-Phase Interactions of Glyoxal and SO2

Oxalic acid and sulfate salts are major components of aerosol particles. Here, we explore the potential for their respective precursor species, glyoxal and SO₂, to form atmospheric brown carbon via aqueous-phase reactions in a series of bulk aqueous and flow chamber aerosol experiments. In bulk aqueous solutions, UV- and visible-light-absorbing products are observed at pH 3-4 and 5-6, respectively, with small but detectable yields of hydroxyquinone and polyketone products formed, especially at pH 6. Hydroxymethanesulfonate (HMS), C₂, and C₃ sulfonates are major products detected by electrospray ionization mass spectrometry (ESI-MS) at pH 5. Past studies have assumed that the reaction of formaldehyde and sulfite was the only atmospheric source of HMS. In flow chamber experiments involving sulfite aerosol and gas-phase glyoxal with only 1 min residence times, significant aerosol growth is observed. Rapid brown carbon formation is seen with aqueous aerosol particles at >80% relative humidity (RH). Brown carbon formation slows at 50-60% RH and when the aerosol particles are acidified with sulfuric acid but stops entirely only under dry conditions. This chemistry may therefore contribute to brown carbon production in cloud-processed pollution plumes as oxidizing volatile organic compounds (VOCs) interact with SO₂ and water.

Acoustic levitation and infrared thermography: a sound approach to studying droplet evaporation

Herein we report a new technique combining acoustic levitation and infrared thermography to directly monitor droplet surface temperatures. Using it, temperature profiles were recorded during the evaporation of deionized water, methanol, n-propanol, and isopropanol. Results support the viability of this inexpensive and easily-accessed technique for studying chemical and physical changes in droplets.

Ultrafast enzymatic digestion of proteins by microdroplet mass spectrometry

Enzymatic digestion for protein sequencing usually requires much time, and does not always result in high sequence coverage. Here we report the use of aqueous microdroplets to accelerate enzymatic reactions and, in particular, to improve protein sequencing. When a room temperature aqueous solution containing 10 µM myoglobin and 5 µg mL⁻¹ trypsin is electrosonically sprayed (−3 kV) from a homemade setup to produce tiny (∼9 µm) microdroplets, we obtain 100% sequence coverage in less than 1 ms of digestion time, in sharp contrast to 60% coverage achieved by incubating the same solution at 37 °C for 14 h followed by analysis with a commercial electrospray ionization source that produces larger (∼60 µm) droplets. We also confirm the sequence of the therapeutic antibody trastuzumab (∼148 kDa), with a sequence coverage of 100% for light chains and 85% for heavy chains, demonstrating the practical utility of microdroplets in drug development.

Mass spectrometry (MS)-based protein sequencing usually relies on in-solution proteolytic digestion, which is time-consuming and inefficient for certain proteins. Here, the authors achieve full protein sequence coverage in less than 1 ms by subjecting protein-protease mixtures to electrosonic spray ionization-MS.

Nebulization prior to ionization for mechanistic studies of chemical reactions

Many important chemical transformations proceed by way of ionic and/or neutral intermediates. Great effort has been expended to understand the mechanism, with only minimum attention given to separate associated ionic and neutral intermediates. Herein, we present a nebulization method followed by on-line ionization to isolate and characterize the ionic and neutral intermediates. The separation of nebulization and ionization and electrical deflection of ionic species guarantee that only neutrals undergo the subsequent on-line ionization. We present data that show the formation of neutral intermediates and iminium ions with short lifetime in Eschweiler-Clarke methylation of di-n-butylamine, as well as data that provide evidence for the formation of carbocation and its isomer lactone products resulting from copper-mediated oxidative cyclization of 4-phenylbutyric acid. Experiments in which dissociation behavior of these two isomers varied at the same collision energy confirmed the carbocation during the cyclization. The nature of this process, which online isolates the ionic and neutral intermediates prior to ionization, greatly advances in mechanistic studies.

Diels-Alder Reactions in Water Are Determined by Microsolvation

Nanoconfined aqueous environments and the recent advent of accelerated chemistry in microdroplets are increasingly being investigated for catalysis. The mechanisms underlying the enhanced reactivity in alternate solvent environments and whether the enhanced reactivity due to nanoconfinement is a universal phenomenon are not fully understood. Here, we use ab initio molecular dynamics simulations to characterize the free energy of a retro-Diels-Alder reaction in bulk water at very different densities and in water nanoconfined by parallel graphene sheets. We find that the broadly different global solvation environments accelerate the reactions to a similar degree with respect to the gas-phase reaction, with activation free energies that do not differ by more than k_bT from each other. The reason for the same acceleration factor in the extremely different solvation environments is that it is the microsolvation of the dienophile's carbonyl group that governs the transition-state stabilization and mechanism, which is not significantly disrupted by either the lower density in bulk water or the strong nanoconfinement conditions used here. Our results also suggest that significant acceleration of Diels-Alder reactions in microdroplets or on-water conditions cannot arise from local microsolvation when water is present but instead must come from highly altered reaction environments that drastically change the reaction mechanisms.

Interfacial Water Mediates Oligomerization Pathways of Monoterpene Carbocations

The air-water interface plays central roles in "on-droplet" synthesis, living systems, and the atmosphere; however, what makes reactions at the interface specific is largely unknown. Here, we examined carbocationic reactions of monoterpene (C₁₀H₁₆ isomer) on an acidic water microjet by using spray ionization mass spectrometry. Gaseous monoterpenes are trapped in the uppermost layers of a water surface via proton transfer and then undergo a chain-propagation reaction. The oligomerization pathway of β-pinene (β-P), which showed prompt chain-propagation, is examined by simultaneous exposure to camphene (CMP). (CMP)H⁺ is the most stable isomer formed via rearrangement of (β-P)H⁺ in the gas phase; however, no co-oligomerization was observed. This indicates that the oligomerization of (β-P)H⁺ proceeded via ring-opening isomerization. Quantum chemical calculations for [carbocation-(H₂O)_n=1,2] complexes revealed that the ring-opened isomer is stabilized by hydrogen-π bonds. We propose that partial hydration is a key factor that makes the interfacial reaction unique.

Droplet Imbibition Enables Nonequilibrium Interfacial Reactions in Charged Microdroplets

A droplet imbibition experiment is proposed to study interfacial effects, which appears to be the main factor influencing reaction acceleration in charged microdroplets produced by electrospray ionization (ESI). One reagent is deposited onto the surface of rapidly moving microdroplets containing the second reagent to be reacted. In this manner, reactions are hindered from reaching equilibrium and monitored in real time by mass spectrometry. We demonstrated this phenomenon using Katritzky chemistry, which is known to proceed either by the solvent-stabilized 2H-pyran intermediate or via the surface-active pseudobase intermediate. Comparisons with reactions performed using ESI show obvious surface effects in favor of the droplet imbibition experiment. By keeping reactant mole ratio constant, it was demonstrated that similar interfacial effects observed in the droplet imbibition experiment can be reached by allowing ESI microdroplets containing premixed reagents to traverse a distance >16 mm. At such spray distance, molecular diffusion and droplet lifetime become comparable allowing reactants to be enriched at droplet surface. Reactions were also conducted in rapid mixing, theta capillary-based droplets, which showed markedly reduced yields compared with the interfacial droplet imbibition experiment.

Mechanism of the Oxidation of Heptafulvenes to Tropones Studied by Online Mass Spectrometry and Density Functional Theory Calculations

We have identified the most likely reaction mechanism for oxidizing heptafulvenes to the corresponding tropones by experimental and theoretical investigations. The experimental studies were done by coupling a three-dimensional printed miniaturized reactor with an integrated electrospray ionization needle to a mass spectrometer. Using the experimentally observed ions as a basis, nine alternative reaction pathways were investigated with density functional theory calculations. The lowest energy reaction pathway starts with the formation of an epoxide that is opened upon the addition of a second equivalent of the oxidizing species meta-chloroperoxybenzoic acid. The adduct formed then undergoes a Criegee-like rearrangement to yield a positively charged hemiketal, which on deprotonation dissociates into acetone and tropone. Overall, the reaction mechanism resembles a Hock-like rearrangement.

Reversible switching of arylazopyrazole within a metal-organic cage

Arylazopyrazoles represent a new family of molecular photoswitches characterized by a near-quantitative conversion between two states and long thermal half-lives of the metastable state. Here, we investigated the behavior of a model arylazopyrazole in the presence of a self-assembled cage based on Pd–imidazole coordination. Owing to its high water solubility, the cage can solubilize the E isomer of arylazopyrazole, which, by itself, is not soluble in water. NMR spectroscopy and X-ray crystallography have independently demonstrated that each cage can encapsulate two molecules of E-arylazopyrazole. UV-induced switching to the Z isomer was accompanied by the release of one of the two guests from the cage and the formation of a 1:1 cage/Z-arylazopyrazole inclusion complex. DFT calculations suggest that this process involves a dramatic change in the conformation of the cage. Back-isomerization was induced with green light and resulted in the initial 1:2 cage/E-arylazopyrazole complex. This back-isomerization reaction also proceeded in the dark, with a rate significantly higher than in the absence of the cage.

Screening of the Suzuki Cross-Coupling Reaction Using Desorption Electrospray Ionization in High-Throughput and in Leidenfrost Droplet Experiments

Suzuki cross-coupling is a widely performed reaction, typically using metal catalysts under heated conditions. Acceleration of the Suzuki cross-coupling reaction has been previously explored in microdroplets using desorption electrospray ionization mass spectrometry (DESI-MS). Building upon previous work, presented here is the use of a high-throughput DESI-MS screening system to identify optimal reaction conditions. Multiple reagents, bases, and stoichiometries were screened using the automated system at rates that approach 10,000 reaction mixture systems per hour. The DESI-MS system utilizes reaction acceleration in microdroplets to allow rapid screening. The results of screening of an array of reaction mixtures using this technique are presented as product ion images via standard MS imaging software, facilitating quick readout. Instructive comparisons are provided with another method of generating droplets for reaction acceleration-the Leidenfrost technique. Acceleration factors greater than 200 were measured for brominated substrates, paralleling the DESI-MS results. Acceleration factors dropped to near unity with highly substituted pyridines, attributable to a steric effect. The reaction proceeded in the absence of a base in Leidenfrost droplets although no product formation was seen without base in the bulk or in the DESI-MS screening experiments. These differences between Leidenfrost chemistry and the bulk and in droplets formed in high-throughput DESI are tentatively attributed to extremes of pH associated with the surfaces of Leidenfrost droplets.

Reaction Acceleration in Electrospray Droplets: Size, Distance, and Surfactant Effects

Phenylhydrazone formation from isatin is used to examine the effects on the reaction rate of (i) electrospray emitter distance from the mass spectrometer (MS) inlet, (ii) emitter tip diameter, and (iii) presence of surfactant. Reaction rates are characterized through measurement of conversion ratios. It is found that there is an increase in the conversion ratio as (i) the electrospray source is moved further from the inlet of the mass spectrometer, (ii) smaller sprayer diameters are used, and (iii) when surfactants are present. Each of these experimental operations is associated with an increase in reaction rate and with a decrease in average droplet sizes. The size measurements are made using super resolution microscopy from the "splash" on a collector surface produced by a fluorescent marker sprayed using conditions similar to those used for the reaction mixture. This measurement showed that droplets undergo significant evaporation as a function of distance of flight, thereby increasing their surface to volume ratios. Similarly, the effect of nanoelectrospray emitter size on conversion ratio is also found to be associated with changes in droplet size for which a 4 to 10 times increase in reaction rate is seen using tip diameters ranging from 20 μm down to 1 μm. Finally, the effects of surfactants in producing smaller droplets with corresponding large increases in reaction rate are demonstrated by splash microscopy. These findings point to reaction acceleration being strongly associated with reactions at the surfaces of microdroplets.

Influence of Inlet Capillary Temperature on the Microdroplet Chemistry Studied by Mass Spectrometry

Often, studies of microdroplet chemistry using electrospray ionization mass spectrometry (MS) either find a negligible effect of the heated inlet capillary of the mass spectrometer on the reaction rate or do not consider its effect. In this context, we studied two reactions in microdroplets, the Pomeranz-Fritsch synthesis of isoquinoline and the Combes quinoline synthesis. The reagents were electrosprayed with methanol and aqueous solutions forming small and large microdroplets at flow rates of 1 and 20 μL/min, respectively. We also varied the inlet capillary temperature from 100 to 350 °C. Contrary to the view that the inlet temperature has little to no influence on the reaction rate, we found that the Pomeranz-Fritsch reaction was markedly accelerated for both solvents and for both droplet sizes on increasing the temperature, whereas the Combes synthesis showed the opposite behavior. We propose that these strikingly different behaviors result from a competition of two effects, the evaporative cooling versus the heating of ejected bare ions from the droplet, both taking place inside the heated inlet. This finding suggests that these phenomena must be taken into account while interpreting the microdroplet reactions studied by electrospray or a similar kind of ambient ionization MS.

N-Substituted Auxiliaries for Aerobic Dehydrogenation of Tetrahydro-isoquinoline: A Theory-Guided Photo-Catalytic Design

Visible-light mediated aerobic dehydrogenation of N-heterocyclic compounds is a reaction with enormous potential for application. Herein, we report the first complete aerobic dehydrogenation pathway to large-scale production of isoquinolines. The discovery of this visible light photoredox reaction was enabled through the combination of mathematical simulations and real-time quantitative mass spectrometry screening. The theoretical calculations showed that hyper-conjugation, the main underlying factor hindering the aerobic oxidation of tetrahydroisoquinolines, is relieved both by π- and σ-donating substituents. This mechanistic insight provided a novel photocatalytic route based on N-substituted auxiliaries that facilitated the conversion of tetrahydroisoquinolines into the corresponding isoquinolines in just three simple steps (yield 71.7% in bulk-solution phase), using unmodified Ru(bpy)₃Cl₂ photocatalyst, sun energy, atmospheric O₂, and at ambient temperature.

A microdroplet-accelerated Biginelli reaction: mechanisms and separation of isomers using IMS-MS

Electrospray ionization (ESI) combined with ion mobility spectrometry (IMS) and mass spectrometry (MS) techniques is used to examine the Biginelli reaction in an ensemble of ions generated from droplets. We find evidence for rapid dihydropyrimidinone formation from condensation of ethyl acetoacetate, benzaldehyde, and urea on the very short timescales associated with the electrospray process (∼10 μs to ∼1.0 ms). Control bulk-solution reactions show no product formation even after several days. This implies that the in-droplet reaction rate is enhanced by an astonishing factor. Examination of the reaction conditions and characterization of the intermediates en route to product shows evidence for variations in the reaction mechanism. IMS separation shows that the Knoevenagel condensation intermediate from benzaldehyde and ethyl acetoacetate exists as both the cis- and trans-isomer, in a ∼5 to 1 ratio. We suggest that the dramatic acceleration arises because of increased reagent confinement as electrosprayed droplets shrink. The ability of IMS-MS to resolve intermediates (including isomers) provides a new means of understanding reaction pathways.

Methylene Group Transfer in Carbonyl Compounds Discovered in silico and Detected Experimentally

A previously unknown transformation of aldehydes, ketones, and carboxylic acid derivatives leads to the formation of substituted oxiranes, aziridines, and azirines as shown by DFT and MP2 computations. Formations of 2,2-dimethyloxirane-d₈ from acetone-d₆ , phenylazirine-d₂ from benzonitrile and 2-methyl-2-(4-hydroxyphenyl)-oxirane from 4-hydroxyacetophenone were detected experimentally by electrospray ionization mass-spectrometry with a heated desolvating capillary. This reaction is a truly concerted process characterized by high activation barriers (activation enthalpies 320-480 kJ mol^-1 ).

Studying Chemistry in Micro-compartments by Separating Droplet Generation from Ionization

Recent studies show that reactions inside micron-sized compartments (e.g., droplets, emulsions) can proceed at significantly accelerated rates and with different mechanisms compared to the same reactions in a macroscopic container. Many of these studies use electrospray ionization (ESI) to both generate droplets and to quantify, via mass spectrometry (MS), droplet reaction kinetics. The highly charged and rapidly evaporating droplets produced in ESI make it difficult to examine precisely the underlying cause for droplet-induced rate enhancements. Additionally, interpretation of the spectra from ESI-MS can be complicated by gas-phase ion-molecule and clustering reactions. Here, we use an approach where droplet generation is separated from ionization, in order to decouple the multiple possible sources of acceleration and to examine more closely the potential role of gas-phase chemistry. The production of sugar phosphates from the reaction of phosphoric acid with simple sugars (a reaction that does not occur in bulk solution but has recently been reported to occur in droplets) is measured using this approach to compare reactivity in droplets (i.e., with compartments) with that in the gas phase (i.e., without compartments). The same product ions that have been previously assigned to in droplet reactions are observed with and without compartmentalization. These results suggest that in some cases, gas-phase processes in the ionization region can potentially complicate the quantification and interpretation of accelerated reactions in droplets using ESI-MS (or one of its variants). In such cases, contributions from in-droplet chemistry cannot be ruled out, but we demonstrate that gas-phase processes can be a significant (and possibly dominant) reaction pathway. We suggest that future studies of rate acceleration in droplets be modified to better assess the potential for non-droplet-related processes. Graphical Abstract ᅟ.

Detecting Intermediates and Products of Fast Heterogeneous Reactions on Liquid Surfaces via Online Mass Spectrometry

One of the research priorities in atmospheric chemistry is to advance our understanding of heterogeneous reactions and their effect on the composition of the troposphere. Chemistry on aqueous surfaces is particularly important because of their ubiquity and expanse. They range from the surfaces of oceans (360 million km2), cloud and aerosol drops (estimated at ~10 trillion km2) to the fluid lining the human lung (~150 m2). Typically, ambient air contains reactive gases that may affect human health, influence climate and participate in biogeochemical cycles. Despite their importance, atmospheric reactions between gases and solutes on aqueous surfaces are not well understood and, as a result, generally overlooked. New, surface-specific techniques are required that detect and identify the intermediates and products of such reactions as they happen on liquids. This is a tall order because genuine interfacial reactions are faster than mass diffusion into bulk liquids, and may produce novel species in low concentrations. Herein, we review evidence that validates online pneumatic ionization mass spectrometry of liquid microjets exposed to reactive gases as a technique that meets such requirements. Next, we call attention to results obtained by this approach on reactions of gas-phase ozone, nitrogen dioxide and hydroxyl radicals with various solutes on aqueous surfaces. The overarching conclusion is that the outermost layers of aqueous solutions are unique media, where most equilibria shift and reactions usually proceed along new pathways, and generally faster than in bulk water. That the rates and mechanisms of reactions at air-aqueous interfaces may be different from those in bulk water opens new conceptual frameworks and lines of research, and adds a missing dimension to atmospheric chemistry.

Accelerated reactions in field desorption mass spectrometry

Field desorption mass spectrometry under ambient conditions is used to study solution-phase organic reactions in micro-volumes. Reagent solution is transferred onto the microdendrites of the field emitter, and reaction products are examined online by mass spectrometry. Three reactions, hydrazone formation by phenyl hydrazine and indoline-2,3-dione, the Katritzky reaction between a pyrylium salt and anisidine, and the Hantzsch synthesis of 1,4-dihydropyridine, were investigated, and reaction acceleration was observed to different extents. The increase in rate relative to the corresponding bulk reactions is attributed to solvent evaporation (simple concentration effect) and to the increase of surface-to-volume ratio (enhanced interfacial reactions). A distinguishing feature of this method of reaction acceleration, relative to that based on nano electrospray ionization, is the observation of radical cations and the formation of radical cation products. The study also breaks new ground in using field emitters at atmospheric pressure.

High yield accelerated reactions in nonvolatile microthin films: chemical derivatization for analysis of single-cell intracellular fluid

The identification of trace components from an individual cell can require derivatization under mild conditions for successful analysis by mass spectrometry (MS).

The identification of trace components from biological media can require derivatization under mild conditions for successful analysis by mass spectrometry (MS). When aqueous droplets (ca. 500 nL) containing a sugar and an amine as reagents are allowed to evaporate they may form long-lasting microthin films in which derivatization reactions can occur fast relative to reaction rates in bulk solution. Evidence is presented that these reactions are heterogeneous in nature and comparisons are made with reactions in pastes and in neat reagent mixtures. Moreover, these thin film reactors can be made stable for the long periods of time that may be necessary to give high product yields. The situation is typified by imine formation from reducing sugars which have reaction times of much more than 1 hour provided that small concentrations (e.g. 20 ppm) of nonvolatile solvents are included. After evaporation of almost all the water, the reaction occurs at an approximately constant rate for the first hour. The rate is two orders of magnitude faster than the reaction in the corresponding homogeneous saturated bulk solution. Conversion of the reagent to the Schiff base product is 67% to 96% efficient in these long-lasting thin films, in sharp contrast to the corresponding derivatization efficiencies in the bulk of less than 1%. This method was used to chemically derivatize and thus to identify, using tandem mass spectrometry, 29 reducing sugars in ca. 1 nL of intracellular fluid from a single onion epidermis cell. A formal description of the kinetics of reversible and irreversible second order reactions in thin films is provided. The effects of thermodynamic and kinetic factors are separated and the measured apparent acceleration factor is shown to represent the ratio of intrinsic rate constants for the microthin film reactor relative to the bulk reaction.