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.

Through-Space Electrochemiluminescence Reveals Bubble Forces at Remote Phase Boundaries.

Several groups have reported on the curious chemistry and reaction acceleration in confined volumes. These complex multiphase systems most closely resemble natural processes, and new measurement tools are necessary to probe chemistry in such environments. Generally, electrochemiluminescence (ECL) reports on processes immediately near (within a few micrometers) the electrode surface. Here, we introduce through-space ECL, reporting on dynamics of processes far away (100s of µm) from the electrode surface. We achieved this by collecting reflected ECL light. During the heterogeneous oxidation of C2O42- in an aqueous phase adjacent to a 1,2-dichlorethane droplet, CO2 accumulates in the 1,2-dichloroethane droplet. Upon buildup, we demonstrate that a CO2 bubble forms in the nonaqueous phase and is surprisingly trapped at the water|1,2-dichloroethane interface and continues to grow. The co-oxidation of tris(bipyridine)ruthenium(II) in the aqueous phase lights up the electrode surface and reflects off the edges of the bubble, revealing the bubble growth over time even when the bubble is fractions of a millimeter from the surface. We extend our results to quantifying bubble forces at the water-oil interface at remote distances from the electrode surface.

Levitating Droplet Electroanalysis.

Chemical reactions that occur in droplets proceed much differently compared to bulk phases. For instance, many groups have studied droplets during levitation by mass spectrometry and fluorescence to gain more detailed mechanistic insight. Such droplets maximize the probability of solution species interacting with the solution-air interface, an interface that is inherently difficult to probe electrochemically. In this Technical Note, we overcome this limitation by developing a laser-pulled dual-barrel electrode. Having two microwires sealed within the same glass capillary allows one to make two-electrode measurements. We show that the electrode can be positioned within a levitating water droplet and that the voltammetry of a redox indicator (hexacyanoferrate (II/III)) can be observed in real-time. Such foundational measurement tools are important to probe a variety of chemical reactions at complex interfaces.

Capture and Detection of Aerosolized Fentanyl in a Suspended Electrochemical Cell.

Fentanyl is an extremely potent opioid that is commonly laced into other drugs. Fentanyl poses a danger to users but also to responders or bystanders who may unknowingly ingest a lethal dose (∼2 mg) of fentanyl from aerosolized powder or vapor. Electrochemistry offers a small, simple, and affordable platform for the direct detection of illicit substances; however, it is largely limited to solution-phase measurements. Here, we demonstrate the hands-free capture and electroanalyzation of aerosols containing fentanyl. A novel electrochemical cell is constructed by a microwire (cylindrical working electrode) traversing an ionic liquid film that is suspended within a conductive loop (reference/counter electrode). We provide a quantitative finite element simulation of the resulting electrochemical system. The suspended film maintains a high-surface area:volume, allowing the electrochemical cell to act as an effective aerosol collector. The low vapor pressure (negligible evaporation) of ionic liquid makes it a robust candidate for in-field applications, and the use of a hydrophobic ionic liquid allows for the extraction of fentanyl from solids and sprayed aqueous aerosols.

Concentration Enrichment in a Dissolving Microdroplet: Accessing Sub-nanomolar Electroanalysis.

Droplet evaporation has previously been used as a concentration enrichment strategy; however, the measurement technique of choice requires quantification in rather large volumes. Electrochemistry has recently emerged as a method to robustly probe volumes even down to the attoliter (10-18 L) level. We present a concentration enrichment strategy based on the dissolution of a microdroplet placed on the surface of a Au ultramicroelectrode (radius ∼ 6.25 µm). By precisely positioning a 1,2-dichloroethane microdroplet onto the ultramicroelectrode with a microinjector, we are able to track the droplet's behavior optically and electrochemically. Because the droplet spontaneously dissolves over time, given the relative solubility of 1,2-dichloroethane in the water continuous phase, the change in volume with time enriches the concentration of the redox probe (Cp2*(Fe)II) in the droplet. We demonstrate robust electrochemical detection down to sub-nM (800 pM) concentrations of Cp2*(Fe)II. For this droplet, 800 pM constitutes only about 106 molecules. We extend the strategy in a single-blind study to determine unknown concentrations, emphasizing the promise of the new methodology. These results take voltammetric quantification easily to the sub-µM regime.

Single Liquid Aerosol Microparticle Electrochemistry on a Suspended Ionic Liquid Film.

Liquid aerosols are ubiquitous in nature, and several tools exist to quantify their physicochemical properties. As a measurement science technique, electrochemistry has not played a large role in aerosol analysis because electrochemistry in air is rather difficult. Here, a remarkably simple method is demonstrated to capture and electroanalyze single liquid aerosol particles with radii on the order of single micrometers. An electrochemical cell is constructed by a microwire (cylindrical working electrode) traversing a film of ionic liquid (1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide) that is suspended within a wire loop (reference/counter electrode). An ionic liquid is chosen because the low vapor pressure preserves the film over weeks, vastly improving suspended film electroanalysis. The resultant high surface area allows the suspended ionic liquid cell to act as an aerosol net. Given the hydrophobic nature of the ionic liquid, aqueous aerosol particles do not coalesce into the film. When the liquid aerosols collide with the sufficiently biased microwire (creating a complex boundary: aerosol|wire|ionic liquid|air), the electrochemistry within a single liquid aerosol particle can be interrogated in real-time. The ability to achieve liquid aerosol size distributions for aerosols over 1 µm in radius is demonstrated.

chem16S: community-level chemical metrics for exploring genomic adaptation to environments.

SUMMARY: The chem16S package combines taxonomic classifications of 16S rRNA gene sequences with amino acid compositions of prokaryotic reference proteomes to generate community reference proteomes. Taxonomic classifications from the RDP Classifier or data objects created by the phyloseq R package are supported. Users can calculate and visualize a variety of chemical metrics in order to explore the effects of redox, salinity, and other physicochemical variables on the genomic adaptation of protein sequences at the community level. AVAILABILITY AND IMPLEMENTATION: Development of chem16S is hosted at https://github.com/jedick/chem16S. Version 1.0.0 is freely available from the Comprehensive R Archive Network (CRAN) at https://cran.r-project.org/package=chem16S.

Electrochemical-Shock Synthesis of Nanoparticles from Sub-femtoliter Nanodroplets.

ConspectusNanoparticles have witnessed immense development in the past several decades due to their intriguing physicochemical properties. The modern chemist is interested not only in methods of synthesizing nanoparticles with tunable properties but also in the chemistry that nanoparticles can drive. While several methods exist to synthesize nanoparticles, it is often advantageous to put nanoparticles on a variety of conductive substrates for multiple applications (such as energy storage and conversion). Despite enjoying over 200 years of development, electrodeposition of nanoparticles suffers from a lack of control over nanoparticle size and morphology. There have been heroic efforts to address these issues over time. With an understanding that structure-function studies are imperative to understand the chemistry of nanoparticles, new methods are necessary to electrodeposit a variety of nanoparticles with control over macromorphology and also microstructure.This Account details our group's efforts in overcoming challenges of classical nanoparticle electrodeposition by electrodepositing nanoparticles from water nanodroplets. When a nanodroplet full of metal salt precursor is incident on the electrode biased sufficiently negative to drive electroplating, nanoparticles form at a fast rate (on the order of microseconds to milliseconds). We start with the general nuts-and-bolts of the experiment (nanodroplet formation and methods for electrodeposition). The deposition of new nanomaterials often requires one to develop new methods of measurement, and we detail new measurement tools for quantifying nanoparticle porosity and nanopore tortuosity within single nanoparticles. We achieve nanopore characterization by using Focused Ion Beam milling and Scanning Electron Microscopy. Owing to the small size of the nanodroplets and fast mass transfer (the contents of a femtoliter droplet can be electrolyzed in only a few milliseconds), the use of nanodroplets also allows the electrodeposition of high entropy alloy nanoparticles at room temperature.We detail how a deep understanding of ion transfer mechanisms can be used to expand the library of possible metals that can be deposited. Furthermore, simple ion changes in the dispersed droplet phase can decrease the cost per experiment by orders of magnitude. Finally, electrodeposition in aqueous nanodroplets can also be combined with stochastic electrochemistry for a variety of interesting studies. We detail the quantification of the growth kinetics of single nanoparticles in single aqueous nanodroplets. Nanodroplets can also be used as tiny reactors to trap only a few molecules of a metal salt precursor. Upon reduction to the zerovalent metal, electrocatalysis at very small metal clusters can be probed and evaluated with time using steady-state electrochemical measurements. Overall, this burgeoning synthetic tool is providing unexpected avenues of tunability of metal nanoparticles on conductive substrates.

Amplifying the electrochemical footprint of <1000 molecules in a dissolving microdroplet.

The ability of analytical strategies to detect and positively identify molecules under extremely dilute conditions is important for the growth and expansion of analytical techniques and instrumentation. At present, few measurement science techniques can robustly approach the measurement of just a few thousand molecules. Here, we present an electrochemical platform for the detection and positive identification of fewer than 1000 molecules of decamethylferrocene ((Cp*)2FeII). We achieve this remarkable detection threshold by trapping (Cp*)2FeII in a 1,2-dichloroethane microdroplet, which is allowed to dissolve into an aqueous continuous phase while on a gold microelectrode (radius ∼6.25 µm). Because electrochemistry is not sensitive enough to observe the charge of less than 1000 molecules, we dissolved µM amounts hexacyanoferrate(III) in the aqueous continuous phase. The biphasic reaction between hexacyanoferrate(III) and Cp2*(Fe)II allows for a feedback loop when the microelectrode is biased sufficiently negative to reduce Cp2*(Fe)III. This feedback loop, a typical EC' catalytic mechanism, amplifies the electrochemical signal of Cp2*(Fe)II when the droplet is of small enough dimensions for feedback to occur. Our results demonstrate that clever biphasic reactions can be coupled with dissolving microdroplets to access extremely low limits of quantitation in electroanalysis.

Understanding dynamic voltammetry in a dissolving microdroplet.

Droplet evaporation and dissolution phenomena are pervasive in both natural and artificial systems, playing crucial roles in various applications. Understanding the intricate processes involved in the evaporation and dissolution of sessile droplets is of paramount importance for applications such as inkjet printing, surface coating, and nanoparticle deposition, etc. In this study, we present a demonstration of electrochemical investigation of the dissolution behaviour in sub-nL droplets down to sub-pL volume. Droplets on an electrode have been studied for decades in the field of electrochemistry to understand the phase transfer of ions at the oil-water interface, accelerated reaction rates in microdroplets, etc. However, the impact of microdroplet dissolution on the redox activity of confined molecules within the droplet has not been explored previously. As a proof-of-principle, we examine the dissolution kinetics of 1,2-dichloroethane droplets (DCE) spiked with 155 µM decamethylferrocene within an aqueous phase on an ultramicroelectrode (r = 6.3 µm). The aqueous phase serves as an infinite sink, enabling the dissolution of DCE droplets while also facilitating convenient electrical contact with the reference/counter electrode (Ag/AgCl 1 M KCl). Through comprehensive voltammetric analysis, we unravel the impact of droplet dissolution on electrochemical response as the droplet reaches minuscule volumes. We validate our experimental findings by finite element modelling, which shows deviations from the experimental results as the droplet accesses negligible volumes, suggesting the presence of complex dissolution modes.

Considerations for dual barrel electrode fabrication and experimentation.

New electrochemical probes offer the opportunity to investigate new systems. A dual barrel electrode can be laser pulled to produce micron-sized platinum disk electrodes. Here, we detail several important considerations for both the fabrication process and for experimental implimentation of the probe. We provide parameters for a Sutter P-2000 laser puller, methods for optical and electrochemical characterization, tips for how to successfully bevel the microelectrodes, and how salt concentrations and electrostatic discharge affect the voltammetry. This paper serves as a guide for how to successfully implement dual barrel electrodes from fabrication to experimentation.

Electrochemical quantification of accelerated FADGDH rates in aqueous nanodroplets.

Enzymes are molecules that catalyze reactions critical to life. These catalysts are often studied in bulk water, where the influence of water volume on reactivity is neglected. Here, we demonstrate rate enhancement of up to two orders of magnitude for enzymes trapped in submicrometer water nanodroplets suspended in 1,2-dichloroethane. When single nanodroplets irreversibly adsorb onto an ultramicroelectrode surface, enzymatic activity is apparent in the amperometric current-time trace if the ultramicroelectrode generates the enzyme cofactor. Nanodroplet volume is easily accessible by integrating the current-time response and using Faraday's Law. The single nanodroplet technique allows us to plot the enzyme's activity as a function of nanodroplet size, revealing a strong inverse relationship. Finite element simulations confirm our experimental results and offer insights into parameters influencing single nanodroplet enzymology. These results provide a framework to profoundly influence the understanding of chemical reactivity at the nanoscale.

For Zinc Metal Batteries, How Many Electrons go to Hydrogen Evolution? An Electrochemical Mass Spectrometry Study.

Despite the advantages of aqueous zinc (Zn) metal batteries (AZMB) like high specific capacity (820 mAh g-1 and 5,854 mAh cm-3 ), low redox potential (-0.76 V vs. the standard hydrogen electrode), low cost, water compatibility, and safety, the development of practically relevant batteries is plagued by several issues like unwanted hydrogen evolution reaction (HER), corrosion of Zn substrate (insulating ZnO, Zn(OH)2 , Zn(SO4 )x (OH)y , Zn(ClO4 )x (OH)y etc. passivation layer), and dendrite growth. Controlling and suppressing HER activity strongly correlates with the long-term cyclability of AZMBs. Therefore, a precise quantitative technique is needed to monitor the real-time dynamics of hydrogen evolution during Zn electrodeposition. In this study, we quantify hydrogen evolution using in situ electrochemical mass spectrometry (ECMS). This methodology enables us to determine a correction factor for the faradaic efficiency of this system with unmatched precision. For instance, during the electrodeposition of zinc on a copper substrate at a current density of 1.5 mA/cm2 for 600 seconds, 0.3 % of the total charge is attributed to HER, while the rest contributes to zinc electrodeposition. At first glance, this may seem like a small fraction, but it can be detrimental to the long-term cycling performance of AZMBs. Furthermore, our results provide insights into the correlation between HER and the porous morphology of the electrodeposited zinc, unravelling the presence of trapped H2 and Zn corrosion during the charging process. Overall, this study sets a platform to accurately determine the faradaic efficiency of Zn electrodeposition and provides a powerful tool for evaluating electrolyte additives, salts, and electrode modifications aimed at enhancing long-term stability and suppressing the HER in aqueous Zn batteries.

Multiphase Chemistry under Nanoconfinement: An Electrochemical Perspective.

Most relevant systems of interest to modern chemists rarely consist of a single phase. Real-world problems that require a rigorous understanding of chemical reactivity in multiple phases include the development of wearable and implantable biosensors, efficient fuel cells, single cell metabolic characterization techniques, and solar energy conversion devices. Within all of these systems, confinement effects at the nanoscale influence the chemical reaction coordinate. Thus, a fundamental understanding of the nanoconfinement effects of chemistry in multiphase environments is paramount. Electrochemistry is inherently a multiphase measurement tool reporting on a charged species traversing a phase boundary. Over the past 50 years, electrochemistry has witnessed astounding growth. Subpicoampere current measurements are routine, as is the study of single molecules and nanoparticles. This Perspective focuses on three nanoelectrochemical techniques to study multiphase chemistry under nanoconfinement: stochastic collision electrochemistry, single nanodroplet electrochemistry, and nanopore electrochemistry.

The Microelectrode Insulator Influences Water Nanodroplet Collisions.

Studying chemical reactions in very small (attoliter to picoliter) volumes is important in understanding how chemistry proceeds at all relevant scales. Stochastic electrochemistry is a powerful tool to study the dynamics of single nanodroplets, one at a time. Perhaps the most conceptually simple experiment is that of the current blockade, where the collision of an insulating particle is observed electrochemically as a stepwise decrease in current. Here, we demonstrate that nanodroplet collisions on microelectrodes are not as simple as water droplets adsorbing to the electrode to block current and that the environment immediately around the microelectrode (glass insulator) plays a pivotal role in the electrochemical collision response. We use correlated opto-electrochemical measurements to understand a variety of electrochemical responses when water nanodroplets collide with a microelectrode during the heterogeneous oxidation of decamethylferrocene in oil. The amperometric current reports not only on current blockades but also on nanodroplet coalescence events and preferential wetting to the glass around the microelectrode. Treating the glass with dichlorodimethylsilane creates a hydrophobic environment around the working electrode, and the simple current blockade response expected from the absorption of insolating nanoparticles is observed. These results highlight the importance of the environment around the working electrode for nanodroplet collision studies.

Measuring Liquid-into-Liquid Diffusion Coefficients by Dissolving Microdroplet Electroanalysis.

Diffusion is a fundamental process in various domains, such as pollution control, drug delivery, and isotope separation. Accurately measuring the diffusion coefficients (D) of one liquid into another often encounters challenges stemming from intermolecular interactions, precise observations at the liquid interface, convection, etc. Here, we present an innovative electrochemical methodology for determining the diffusion coefficient of a liquid into another liquid. The method involves precisely tracking the lifetime of a nonaqueous droplet. An organic droplet is placed on an ultramicroelectrode surrounded by an aqueous solution of potassium hexacyanoferrate(II/III). The droplet initially blocks the reduction or oxidation of the redox species. As the droplet dissolves, giving access to the conductive microelectrode surface, a continuously increasing current is observed in voltammetry and the amperometric i-t response. The electrochemical response thus directly reports on the flux of redox species on the electrode surface, allowing us to precisely determine the lifetime of the droplet. D values are directly determined through a combination of electrochemical analysis and the principles of droplet dissolution. We demonstrate the quantification of 1,2-dichloroethane and nitrobenzene into water, yielding diffusion coefficients of (11.3 ± 1.2) × 10-6 cm2/s and (5.2 ± 1.1) × 10-6 cm2/s, respectively. This work establishes a reliable electrochemical approach for quantifying diffusion coefficients based on droplet lifetime analysis.

Triple-Barrel Ultramicroelectrodes for Multipurpose, Submilliliter Electroanalysis.

Here, we have developed and applied a triple-barrel microelectrode. This device incorporates a platinum disk working electrode, a platinum disk counter electrode, and a low-leakage Ag/AgCl reference electrode into a small probe. We demonstrate that the incorporated low-leakage reference electrode shows similar voltammetry, potentiometry, and drift when compared to a commercial reference electrode in bulk solution. We also demonstrate the versatility of such a small three-channel system via voltammetry in nanoliter droplets and through electroanalysis of captured aerosols. Finally, we demonstrate the probe's potential utility in single-cell electroanalysis by making measurements within salmon eggs.

On the mechanism of the bipolar reference electrode.

Commercial silver/silver chloride (Ag/AgCl) reference electrodes are some of the most commonly used reference electrodes, but they suffer from a number of issues due to their porous frits. Such issues include difficulty miniaturizing, silver and chloride ion leakage, charge screening effects at low ionic strength, frit drying if left unattended in air, and incompatibility with organic solvents. To solve these issues, we recently designed a reference electrode that is leakless in principle by replacing the porous frit with a sealed, conductive wire, where the ends of the wire are exposed to the reference electrode solution and the working electrode solution. We hypothesized that the reference electrode operated like a closed, bipolar electrochemical cell, and we termed the name bipolar reference electrode (BPRE). Here, we provide evidence that the BPRE can either act as a reference electrode by operating through an ion transfer mechanism via leakage through the imperfect seal, or it can act as a highly stable quasi-reference electrode through a bipolar electron transfer mechanism (BPQRE). Finally, we demonstrate the effectiveness of the BPRE in other types of common electrochemical studies, including chronoamperometry, linear sweep voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy.

A troubleshooting guide for laser pulling platinum nanoelectrodes.

While there are numerous publications on laser-assisted fabrication and characterization of Pt nanoelectrodes, the exact replication of those procedures is not as straightforward as following a single recipe across laboratories. Often, the working procedures vary by day, by laser puller, or by person. Only a handful of nanoelectrode fabrication papers record their parameters, and even fewer offer troubleshooting advice. Here, we provide a step-by-step guide for laser-assisted Pt nanoelectrode fabrication using low-cost equipment including a laser puller, voltammetry, and simple microscope images captured via cell phone. We also offer solutions for common failures experienced throughout the process to guide beginners as they troubleshoot their own fabrication procedures.

Chemical Links Between Redox Conditions and Estimated Community Proteomes from 16S rRNA and Reference Protein Sequences.

Environmental influences on community structure are often assessed through multivariate analyses in order to relate microbial abundances to separately measured physicochemical variables. However, genes and proteins are themselves chemical entities; in combination with genome databases, differences in microbial abundances directly encode for chemical variability. We predicted that the carbon oxidation state of estimated community proteomes, obtained by combining taxonomic abundances from published 16S rRNA gene sequencing datasets with reference microbial proteomes from the NCBI Reference Sequence (RefSeq) database, would reflect environmental oxidation-reduction conditions. Analysis of multiple datasets confirms the geobiochemical predictions for environmental redox gradients in hydrothermal systems, stratified lakes and marine environments, and shale gas wells. The geobiochemical signal is largest for the steep redox gradients associated with hydrothermal systems and between injected water and produced fluids from shale gas wells, demonstrating that microbial community composition can be a chemical proxy for environmental redox gradients. Although estimates of oxidation state from 16S amplicon and metagenomic sequences are correlated, the 16S-based estimates show stronger associations with redox gradients in some environments.