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Abstract Background Proteins of various compositions are required by organisms inhabiting different environments. The energetic demands for protein formation are a function of the compositions of proteins as well as geochemical variables including temperature, pressure, oxygen fugacity and pH. The purpose of this study was to explore the dependence of metastable equilibrium states of protein systems on changes in the geochemical variables. Results A software package called CHNOSZ implementing the revised Helgeson-Kirkham-Flowers (HKF) equations of state and group additivity for ionized unfolded aqueous proteins was developed. The program can be used to calculate standard molal Gibbs energies and other thermodynamic properties of reactions and to make chemical speciation and predominance diagrams that represent the metastable equilibrium distributions of proteins. The approach takes account of the chemical affinities of reactions in open systems characterized by the chemical potentials of basis species. The thermodynamic database included with the package permits application of the software to mineral and other inorganic systems as well as systems of proteins or other biomolecules. Conclusion Metastable equilibrium activity diagrams were generated for model cell-surface proteins from archaea and bacteria adapted to growth in environments that differ in temperature and chemical conditions. The predicted metastable equilibrium distributions of the proteins can be compared with the optimal growth temperatures of the organisms and with geochemical variables. The results suggest that a thermodynamic assessment of protein metastability may be useful for integrating bio- and geochemical observations.

The leukemia stem cell (LSC) compartment is a complex reservoir fueling disease progression in acute myeloid leukemia (AML). The existence of heterogeneity within this compartment is well documented but prior studies have focused on genetic heterogeneity without being able to address functional heterogeneity. Understanding this heterogeneity is critical for the informed design of therapies targeting LSC, but has been hampered by LSC scarcity and the lack of reliable cell surface markers for viable LSC isolation. To overcome these challenges, we turned to the patient-derived OCI-AML22 cell model. This model includes functionally, transcriptionally and epigenetically characterized LSC broadly representative of LSC found in primary AML samples. Focusing on the pool of LSC, we used an integrated approach combining xenograft assays with single-cell analysis to identify two LSC subtypes with distinct transcriptional, epigenetic and functional properties. These LSC subtypes differed in depth of quiescence, differentiation potential, repopulation capacity, sensitivity to chemotherapy and could be isolated based on CD112 expression. A majority of AML patient samples had transcriptional signatures reflective of either LSC subtype, and some even showed coexistence within an individual sample. This work provides a framework for investigating the LSC compartment and designing combinatorial therapeutic strategies in AML., (© 2024. The Author(s).)

We have discovered a strategy to synthesize reactive radical salts, effectively bottling up radicals in space and time for future use. We apply this new principle to electrochemiluminescence (ECL) through the simultaneous electro-reduction of peroxydisulfate, S 2 O 8 2- , and tris(bipyridine)ruthenium(II), [Ru(bpy) 3 ] 2+ in a water/acetonitrile mixture. The electrode generates a concentration profile exceeding the solubility of the cation and anion pair, promoting precipitation. After the application of a potential, leads are disconnected, and the crystals electrolessly chemiluminesce during dissolution and can be transported to other solutions for later chemiluminescence uses. Our method extends ECL hundreds of micrometers from the electrode surface and increases the ECL lifetime by orders of magnitude. Control experiments, including electron spin resonance, validate the crystallization of SO 4 •- , allowing detailed mechanistic insight. We demonstrate platform generalizability by precipitating a radical salt made of calcium and SO 4 •- , and we demonstrate the salt's ability to drive chemiluminescence. Our results emphasize the elegant chemical tenet that extremely reactive radicals can be bottled up as solids to be used as future reagents if precipitation occurs more quickly than the radical lifetime.

Electrochemical aptamer-based sensors provide a highly modular platform for real-time monitoring of small molecules. Their ability to selectively recognize target molecules in complex environments like biological fluids makes them an attractive technology for the analysis of micro- and nanoscale systems. The signal-to-noise of the measurement depends on the electroactive surface (i.e., how many aptamers one can place), which has previously precluded miniaturization of aptamer-based sensors to planar disk ultramicroelectrodes ( r ∼ 5-10 μm). Here, we employ a concentration enrichment strategy based on the active dissolution of an aqueous, aptamer-containing microdroplet on an ultramicroelectrode submerged in an organic continuous phase (1,2-dichloroethane). We show consistent voltammetric signal increase as a function of droplet lifetime, indicating the successful immobalization of the thiol-terminated aminoglycoside aptamers to the electrode surface. We observe a diagnostic methylene blue peak and 10-fold increase in current magnitude as compared to bare microelectrodes. We report robust sensor behavior with a linear dynamic range extending from milli- to micromolar concentrations of kanamycin in buffer. This research offers a successful method for optimized electrochemical aptamer-based sensor fabrication and miniaturization on ultramicroelectrodes without the need for electrode surface area enhancement.

Abstract: Loss of long-term hematopoietic stem cell (LT-HSC) function ex vivo hampers the success of clinical protocols that rely on culture. However, the kinetics and mechanisms through which this occurs remain incompletely characterized. In this study, through time-resolved single-cell RNA sequencing, matched in vivo functional analysis, and the use of a reversible in vitro system of early G1 arrest, we defined the sequence of transcriptional and functional events that occur during the first ex vivo division of human LT-HSCs. We demonstrated that the sharpest loss in LT-HSC repopulation capacity happens early on, between 6 and 24 hours of culture, before LT-HSCs commit to cell cycle progression. During this time window, LT-HSCs adapt to the culture environment, limit the global variability in gene expression, and transiently upregulate gene networks involved in signaling and stress responses. From 24 hours, LT-HSC progression past early G1 contributes to the establishment of differentiation programs in culture. However, contrary to the current assumptions, we demonstrated that the loss of HSC function ex vivo is independent of cell cycle progression. Finally, we showed that targeting LT-HSC adaptation to culture by inhibiting the early activation of JAK/STAT signaling improves HSC long-term repopulating function ex vivo. Collectively, our study demonstrated that controlling early LT-HSC adaptation to ex vivo culture, for example, via JAK inhibition, is critically important to improve HSC gene therapy and expansion protocols., (© 2024 American Society of Hematology. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

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*) 2 Fe II ). We achieve this remarkable detection threshold by trapping (Cp*) 2 Fe II 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 Cp 2 *(Fe) II allows for a feedback loop when the microelectrode is biased sufficiently negative to reduce Cp 2 *(Fe) III . This feedback loop, a typical EC' catalytic mechanism, amplifies the electrochemical signal of Cp 2 *(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.

Aims: The heterogeneous phenotype of hypertrophic cardiomyopathy (HCM) is still not fully understood. Clonal haematopoiesis (CH) is emerging as a cardiovascular risk factor potentially associated with adverse clinical events. The prevalence, phenotype and outcomes related to CH in HCM patients were evaluated., Methods and Results: Patients with HCM and available biospecimens from the Peter Munk Cardiac Centre Cardiovascular Biobank were subjected to targeted sequencing for 35 myeloid genes associated with CH. CH prevalence, clinical characteristics, morphological phenotypes assessed by echocardiogram and cardiac magnetic resonance and outcomes were assessed. All patients were evaluated for a 71-plex cytokines/chemokines, troponin I and B-type natriuretic peptide analysis. Major adverse cardiovascular events (MACE) were defined as appropriate implantable cardioverter-defibrillator shock, stroke, cardiac arrest, orthotopic heart transplant and death. Among the 799 patients, CH was found in 183 (22.9%) HCM patients with sarcomeric germline mutations. HCM patients with CH were more symptomatic and with a higher burden of fibrosis than those without CH. CH was associated with MACE in those HCM patients with sarcomeric germline mutations (adjusted hazard ratio [HR] 6.89, 95% confidence interval [CI] 1.78-26.6; p = 0.005), with the highest risk among those that had DNMT3A, TET2 and ASXL1 mutations (adjusted HR 5.76, 95% CI 1.51-21.94; p = 0.010). Several cytokines (IL-1ra, IL-6, IL-17F, TGFα, CCL21, CCL1, CCL8, and CCL17), and troponin I were upregulated in gene-positive HCM patients with CH., Conclusions: These results indicate that CH in patients with HCM is associated with worse clinical outcomes. In the absence of CH, gene-positive patients with HCM have lower rates of MACE., (© 2024 The Author(s). European Journal of Heart Failure published by John Wiley & Sons Ltd on behalf of European Society of Cardiology.)

Clonal hematopoiesis (CH) arises when hematopoietic stem cells (HSCs) acquire mutations, most frequently in the DNMT3A and TET2 genes, conferring a competitive advantage through mechanisms that remain unclear. To gain insight into how CH mutations enable gradual clonal expansion, we used single-cell multi-omics with high-fidelity genotyping on human CH bone marrow (BM) samples. Most of the selective advantage of mutant cells occurs within HSCs. DNMT3A- and TET2-mutant clones expand further in early progenitors, while TET2 mutations accelerate myeloid maturation in a dose-dependent manner. Unexpectedly, both mutant and non-mutant HSCs from CH samples are enriched for inflammatory and aging transcriptomic signatures, compared with HSCs from non-CH samples, revealing a non-cell-autonomous effect. However, DNMT3A- and TET2-mutant HSCs have an attenuated inflammatory response relative to wild-type HSCs within the same sample. Our data support a model whereby CH clones are gradually selected because they are resistant to the deleterious impact of inflammation and aging., Competing Interests: Declaration of interests J.E.D. receives royalties from Trillium Therapeutics Inc./Pfizer and a commercial research grant from Celgene/BMS., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

Hypothesis: This paper develops a new measurement method to answer the question: How does one measure the interfacial tension of adsorbed droplets?, Experiments: This measurement is based on the placement of a bubble at a water|organic interface. To prove the concept, a bubble was formed by pipetting gas below the water|1,2-dichloroethane interface. Our values agree well with previous reports. We then extended the measurement modality to a more difficult system: quantifying interfacial tension of 1,2-dichloroethane droplets adsorbed onto conductors. Carbon dioxide was generated in the aqueous phase from the electro-oxidation of oxalate. Given carbon dioxide's solubility in 1,2-dichloroethane, it partitions, a bubble nucleates, and the bubble can be seen by microscopy when driving the simultaneous oxidation of tris(bipyridine)ruthenium (II), a molecule that will interact with CO 2 .-. and provide light through electrochemiluminescence. We can quantify the interfacial tension of adsorbed droplets, a measurement very difficult performed with more usual techniques, by tracking the growth of the bubble and quantifying the bubble size at the time the bubble breaks through the aqueous|1,2-dichloroethane interface., Findings: We found that the interfacial tension of nanoliter 1,2-dichloroethane droplets adsorbed to an electrified interface in water, which was previously immeasurable with current techniques, was one order of magnitude less than the bulk system., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

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.

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.

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., (© 2024 The Authors. Small published by Wiley‐VCH GmbH.)

Analytical techniques operating at the nanoscale introduce confinement as a tool at our disposal. This review delves into the phenomenon of accelerated reactivity within micro- and nanodroplets. A decade of accelerated reactivity observations was succeeded by several years of fundamental studies aimed at mechanistic enlightenment. Herein, we provide a brief historical context for rate enhancement in and around micro- and nanodroplets and summarize the mechanisms that have been proposed to contribute to such extraordinary reactivity. We highlight recent electrochemical reports that make use of restricted mass transfer to enhance electrochemical reactions and/or quantitatively measure reaction rates within droplet-confined electrochemical cells. A comprehensive approach to nanodroplet reactivity is paramount to understanding how nature takes advantage of these systems to provide life on Earth and, in turn, how to harness the full potential of such systems.

Interfacial effects are well-known to significantly alter chemical reactivity, especially in confined environments, where the surface to volume ratio increases. Here, we observed an inhomogeneity in the electrogenerated chemiluminescence (ECL) intensity decrease over time in a multiphasic system composed of femtoliter water droplets entrapping femtoliter volumes of the 1,2-dichloroethane (DCE) continuous phase. In usual electrochemiluminescence (ECL) reactions involving an ECL chromophore and oxalate ([C 2 O 4 ] 2- ), the build-up of CO 2 diminishes the ECL signal with time because of bubble formation. We hypothesised that relative solubilities of chemical species in these environments play a dramatic role in interfacial reactivity. Water droplets, loaded with the ECL luminophore [Ru(bpy) 3 ] 2+ and the coreactant [C 2 O 4 ] 2- were allowed to stochastically collide and adsorb at the surface of a glassy carbon macroelectrode. When water droplets coalesce on the surface, they leave behind femtoliter droplets of the DCE phase (inclusions). We report the surprising finding that the addition of multiple interfaces, due to the presence of continuous phase's femtoliter inclusions, allows sustained ECL over time after successive potential applications at the triple-phase boundary between water droplet|electrode|DCE inclusion. When femtoliter droplets of DCE form on the electrode surface, bright rings of ECL are observed during the simultaneous oxidation of [Ru(bpy) 3 ] 2+ and [C 2 O 4 ] 2- . Control experiments and finite element modelling allowed us to propose that these rings arise because CO 2 that is generated near the 1,2-dichloroethane droplet partitions in due to relative solubility of CO 2 in 1,2-dichloroethane and builds up and/or is expelled at the top of the droplet. The small droplets of the DCE phase act as micropumps, pumping away carbon dioxide from the interface. These results highlight the unexpected point that confined microenvironments and their geometry can tune chemical reactions of industrial importance and fundamental interest., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Electrodeposition has been used for centuries to create new materials. However, synthetic platforms are still necessary to enrich a variety of nanomaterials that can be electrodeposited. For instance, IrO x is a popular material for the water oxidation reaction, but electrodeposition strategies for the controlled growth of IrO x nanoparticles are lacking. Here, we demonstrate the anodic electrodeposition of IrO x nanoparticles from aqueous nanodroplets. Field emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy (STEM) images confirm the macro- and microstructure of the resulting nanoparticles. IrO x nanoparticles of 43 ± 10 nm in diameter were achieved. X-ray photoelectron spectroscopy (XPS) showed the presence of Ir(III) and Ir(IV) hydrated oxyhydroxide species. The synthesis of IrO x nanoparticles under anodic conditions using water nanodroplets expands the capabilities of our technique and provides a tunable platform for IrO x nanoparticle electrodeposition., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

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 (Cp 2 *(Fe) II ) in the droplet. We demonstrate robust electrochemical detection down to sub-nM (800 pM) concentrations of Cp 2 *(Fe) II . For this droplet, 800 pM constitutes only about 10 6 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.

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.

Recent reports have detailed the striking observation that electroactive molecules, such as hydrogen peroxide (H 2 O 2 ) and radical water species (H 2 O .+ /H 2 O .- ), 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 (H 2 O 2 ) in single aqueous microdroplets suspended in 1,2-dichloroethane. The oxidation of H 2 O 2 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 H 2 O 2 . Further, we show that the stochastic electrochemical signal is highest at the mass transfer limitation of the H 2 O 2 couple and is dampened when the potential nears the formal potential. We validate these results by showing that the addition of a H 2 O 2 selective probe, luminol, decreases the stochastic electrochemical response at alkaline pH (11.5). Our results support the observation that H 2 O 2 is generated in water microdroplets at concentrations of ~100 s of µM., Competing Interests: Competing interests statement:The authors declare no competing interest.

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(SO 4 ) x (OH) y , Zn(ClO 4 ) 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/cm 2 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 H 2 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., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Competing Interests: Competing interests statement:The authors declare no competing interest.

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.

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 C 2 O 4 2- in an aqueous phase adjacent to a 1,2-dichlorethane droplet, CO 2 accumulates in the 1,2-dichloroethane droplet. Upon buildup, we demonstrate that a CO 2 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.

Per- and polyfluoroalkyl substances (PFAS), nicknamed "forever chemicals" due to the strength of their carbon-fluorine bonds, are a class of potent micropollutants that cause deleterious health effects in mammals. The current state-of-the-art detection method requires the collection and transport of water samples to a centralized facility where chromatography and mass spectrometry are performed for the separation, identification, and quantification of PFAS. However, for efficient remediation efforts to be properly informed, a more rapid in-field testing method is required. We previously demonstrated the development and use of dioxygen as the mediator molecule. The use of dioxygen is predicated on the assumption that there will be consistent ambient dioxygen levels in natural waters. This is not always the case in hypoxic groundwater and at high altitudes. To overcome this challenge and further advance the strategies that will enable in-field electroanalysis of PFAS, we demonstrate, as a proof of concept, that dioxygen can be generated in solution through the hydrolysis of water. The electrogenerated dioxygen can then be used as a mediator molecule for the indirect detection of PFOS via molecularly imprinted polymer (MIP)-based electroanalysis. We demonstrate that calibration curves can be constructed with high precision and sensitivity (LOD < 1 ppt or 1 ng/L). Our results provide a foundation for enabling in-field hypoxic PFAS 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 cm 2 /s and (5.2 ± 1.1) × 10 -6 cm 2 /s, respectively. This work establishes a reliable electrochemical approach for quantifying diffusion coefficients based on droplet lifetime analysis.