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Miniaturization of analytical instrumentation is paramount to enabling convenient in-field sensing. The recent thrust in potentiostat miniaturization for electrochemical sensing and general use has led to the development of commercial application specific integrated circuits (ASICs) that pack all the power of a benchtop instrument into one 5 mm × 5 mm chip. While the capabilities of these integrated circuits far exceed those of open-source potentiostats in the literature, the activation barrier for their implementation requires extensive electrical and software engineering expertise to overcome. In order to more rapidly bring the utility of ASIC potentiostats to researchers, we present a low size, weight, power, and cost (Low SWaP-C) Army Corps of Engineers potentiostat (ACEstat) based on the widely available ADuCM355 offered by Analog Devices. This potentiostat is a streamlined and fully programmable device that leverages industry-leading integrated hardware to perform electrochemical measurements such as cyclic voltammetry, pulse voltammetry, and electrochemical impedance spectroscopy. The ACEstat enables control over a wide range of test parameters and displays results through an intuitive, open-source graphical user interface available on mobile devices and computers. In this report, we present an approachable, do-it-yourself guide to unlocking the capabilities of this integrated circuit potentiostat by outlining the fabrication and programming details necessary to facilitate electroanalysis. Furthermore, we demonstrate the practicality of this device by detecting 2,4,6-trinitrotoluene (TNT) in water at sub-mg/L detection limits, highlighting its potential for in-field use.

Precise determination of boundaries in co-culture systems is difficult to achieve with scanning electrochemical microscopy alone. Thus, biological scanning electrochemical microscope platforms generally consist of a scanning electrochemical microscope positioner mounted on the stage of an inverted microscope for correlated electrochemical and optical imaging. Use of a fluorescence microscope allows for site-specific fluorescence labeling to obtain more clearly resolved spatial and electrochemical data. Here, we construct a unique hyperspectral assisted-biological scanning electrochemical microscope platform to widen the scope of biological imaging. Specifically, we incorporate a variable fluorescence bandpass source into a biological scanning electrochemical microscope platform for simultaneous optical, spectral, and electrochemical imaging. Not only does this platform serve as a cost-effective alternative to white light laser imaging, but additionally it provides multi-functional analysis of biological samples. Here, we demonstrate the efficacy of our platform to discern the electrochemical contribution of site-specific cells by optically and spectroscopically resolving boundaries as well as cell types within a complex biological system.

Over 200 years ago, Alessandro Volta published his observations of a steady voltage when a piece of electrolyte-soaked cardboard was sandwiched between two dissimilar metals. This observation initiated a century of argument as to the origin of voltaic electricity (contact vs chemical) and catalyzed practical advances, such as the first demonstration of water electrolysis by William Nicholson and Anthony Carlisle within a few months of Volta’s announcement. A deep dive into the literature demonstrates that practitioners at the time were disadvantaged in their quest for a detailed understanding of the pile. For instance, the idea of salt dissociation into charged ions and the discovery of the electron did not come to light until nearly an entire century after the pile made its first debut. A thorough survey of the literature also indicates that original piles, which were composed of silver or copper as the cathode and zinc as the anode, operated by the dissolution of zinc at the anode and the reduction of some species at the cathode. A cathodic reaction commonly implicated is the hydrogen evolution reaction (HER). Here, we demonstrate that the oxygen reduction reaction (ORR) contributes to the cathodic half-reaction from pH 0 to 14, suggesting that the true mechanism of the voltaic pile depends on the mixed potential set by the HER/ORR and zinc oxidation. While the main point of this article is to implicate dissolved oxygen, we end the article with suggestions for experimental modifications to promote hands-on learning.

The ubiquitous Faraday cage, an experimental component particularly essential for nanoelectrochemical measurements, is responsible for neutralizing noise introduced by electromagnetic interference (EMI). Faraday cage designs abound in the literature, often exhibiting varying thicknesses, mesh sizes, and base materials. The fact that the Faraday cage composition most often goes unreported underscores the fact that many electrochemical researchers assume a 100% EMI reduction for any given design. In this work, this assumption is challenged from a theoretical and empirical perspective by highlighting the physical principles producing the Faraday effect. A brief history of the Faraday cage and a simplified theoretical approach introduce fundamental considerations regarding optimal design properties. In practice, time-domain noise profiles and corresponding Fourier transform frequency domain information for custom-built Faraday cages reveal that maximally conductive cages provide more optimal EMI exclusion.

The purpose of this study was to synthesize a novel and tunable organogel system capable of stand-alone use with integration via electrochemical tools for the detection of aerosol particles.

The interfacial properties of multiphase systems are often difficult to quantify. We describe the observation and quantification of immiscible solvent entrapment on a carbonaceous electrode surface using microscopy-coupled electrogenerated chemiluminescence (ECL). As aqueous microdroplets suspended in 1,2-dichloroethane collide with a glassy carbon electrode surface, small volumes of the solvent become entrapped between the electrode and aqueous phase, resulting in an overestimation of the true microdroplet/electrode contact area. To quantify the contribution of solvent entrapment decreasing the microdroplet contact area, we drive an ECL reaction within the microdroplet phase using tris(bipyridine)ruthenium(II) chloride ([Ru(bpy)3]Cl2) as the ECL luminophore and sodium oxalate (Na2C2O4) as the co-reactant. Importantly, the hydrophilicity of sodium oxalate ensures that the reaction proceeds in the aqueous phase, permitting a clear contrast between the aqueous and 1,2-dichloroethane present at the electrode interface. With the contrast provided by ECL imaging, we quantify the microdroplet radius, apparent microdroplet contact area (aqueous + entrapped 1,2-dichloroethane), entrapped solvent contact area, and the number of entrapped solvent pockets per droplet. These data permit the extraction of the true microdroplet/electrode contact area for a given droplet, as well as a statistical assessment regarding the probability of solvent entrapment based on microdroplet size.

Per- and polyfluoroalkyl substances (PFAS) are emerging as a hazardous class of environmental micropollutant, and robust, sensitive, and inexpensive sensing modalities are needed to detect the earl...

Reactivity at phase boundaries is central to many areas of chemistry, from synthesis to heterogeneous catalysis. New tools are necessary to gain a more detailed understanding of processes occurring at these boundaries. We describe a series of experiments to visualize phase boundaries using electrogenerated chemiluminescence (ECL) on glassy carbon electrodes. By taking advantage of the solubilities of the ECL luminophore and the coreactant in different liquid phases, we demonstrate that the interface of various phases (i.e., the boundaries formed between a water microdroplet, 1,2-dichloroethane, and a glassy carbon electrode and the boundaries formed between an oxygen bubble, water, and a glassy carbon electrode) can be evaluated. We measured microdroplet contact radii, the three-phase boundary thickness, and growth dynamics of electrogenerated O2 bubbles. These experimental tools and the fundamental knowledge they yield will find applications in biology, nanoscience, synthesis, and energy storage and conversion, where understanding phase boundary chemistry is essential.

The progression of nanoscience necessitates quantitative tools to understand reactivity at the nanoscale. Here, we report a quantitative model that describes both the electrokinetic and diffusion-l...

We demonstrate the application of open circuit potentiometry (OCP) to measure enzyme turnover kinetics, kturn. The electrode surface will become poised by the addition of a well-behaved redox pair, such as ferrocenemethanol/ferrocenium methanol (FcMeOH/FcMeOH+), which acts as the cosubstrate for the enzymatic process. A measurable change in potential results when an enzyme consumes the one-electron transfer mediator. Glucose oxidase was studied as a test-case, but the method is generalizable across oxidoreductase enzymes that rely on electron transfer mediators. In the presence of glucose and FcMeOH+, glucose oxidase delivers electrons to FcMeOH+, and the potential changes with respect to the Nernst equation. A theoretical model incorporating enzymatic rate expressions into the Nernst equation was derived to explain the observed potential transients, and experimental data fit theory well. A similar experiment was performed using amperometry on ultramicroelectrodes (UMEs). Here, the same enzymatic rate expression may be incorporated into the equation for steady-state flux to an UME to obtain kturn. While similar kinetic information was obtained from the potentiometric and amperometric responses, potentiometry is independent of electrode size and mass transfer effects. Finally, we show how kturn changes as a function of one-electron mediator. Our results may eventually find applications to biosensors, where electrode fouling plagues long-term sensor performance.

Experimental electrochemistry offers unique opportunities for interactive instruction at all levels of education; however, widespread adoption in curricula is hindered by high costs associated with...

We present a robust and facile method to produce metal nanoparticle (NP) alloys in a one-step synthesis using direct electrodeposition onto highly oriented pyrolytic graphite (HOPG). Precursor salts, H2PtCl6 and Pd(NO3)2, were dissolved in a 1 mM sodium dodecylsulfate (SDS) water droplet with 1× phosphate buffered saline solution and suspended in a dichloroethane (DCE) continuous phase. Tetrabutylammonium perchlorate was added to the DCE continuous phase to maintain charge balance during electrodeposition. NP fabrication via electrodeposition was driven by droplet collisions onto HOPG, which was biased at a potential where the metal precursor salts would reduce to their respective zero-valent atoms. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) were used to study the size, coverage, and morphology of the NPs as well as the atomic stoichiometry. EDX mapping indicated homogeneous distribution of Pd and Pt at the single NP level. Homogeneously alloyed PdPt NPs were realized from this study with demonstrated control over metal composition, surface coverage, and NP size. Keywords: Alloy, Bimetallic, Electrodeposition, Emulsion droplet, Nanoparticle

We demonstrate quantification of porosity, nanopore tortuosity, and electrical connectivity at the single-nanoparticle (NP) level for NPs synthesized by nanodroplet-mediated electrodeposition. Focused ion beam nanoslice tomography was used to slice NPs with ca. 10 nm slice resolution followed by imaging using scanning electron microscopy (SEM), allowing measurement of these parameters on NPs not amenable to transmission electron microscopy. Slices were reconstructed in three dimensions and revealed pores with an average size of 3 ± 2 nm and relative nanopore tortuosity of 46.8 ± 24.5. We also demonstrate a new technique to evaluate electrical connectivity at the single-NP level by taking advantage of material-selective electrodeposition. The rate of Cu electrodeposition differs significantly on Pt compared to carbon, implying Cu can be selectively electrodeposited onto Pt NPs adsorbed onto a carbon support. Following the Cu electrodeposition step, NP connectivity was determined by the presence of Cu on Pt...

Electrochemical measurements over an array of electrodes may be accomplished with one of three potentiostat architectures: a single-channel device which averages the signal from a number of interconnected electrodes, a multichannel device with dedicated circuits for each electrode, or a single-channel device with a multiplexer interface to isolate the signal from specific electrodes. Of these three architectures, the use of a multiplexer interface is best suited to facilitate measurements over individual electrodes without the need for large numbers of dedicated potentiostat channels. We present a versatile strategy for the development of flexible printed circuit (FPC) electrode arrays with accompanying multiplexing hardware to interface with single-channel potentiostats. The FPC array was fabricated with 78 individually addressable 0.3 mm diameter gold working electrodes and characterized using optical and scanning electron microscopy, energy dispersive spectroscopy, profilometry, impedance spectroscopy, and cyclic voltammetry to investigate the morphology, elemental composition, height profile, impedance characteristics, and electrochemical response, respectively. Interfacing the FPC array via a simple connector with three 32-channel ADG731 multiplexers permitted electrochemical measurements using single-channel commercial potentiostats. Voltammetric experiments were conducted to demonstrate the reliability, stability, and reproducibility of the FPC array and interfacing hardware. The combination of these devices represents an accessible hardware platform with robust, functionalizable electrodes, a simple connection interface with commercial potentiostats, and a low cost through the use of off-the-shelf components. Our reported strategy holds great promise to facilitate multiplexed electroanalysis in next-generation sensors to increase statistical sample size and multianalyte detection capabilities.

Harmful algal blooms (HABs) negatively impact numerous natural waterways worldwide and have significant socioeconomic and health-related ramifications for local populations. In order to better detect, characterize, and mitigate bloom events, novel field deployable analytical technologies capable of quantifying common HAB toxins (

Trace analysis of heavy metals in complex, environmentally relevant matrices remains a significant challenge for electrochemical sensors employing stripping voltammetry-based detection schemes. We present an alternative method capable of selectively preconcentrating Cu2+ ions at the electrode surface using chelating polymer-wrapped multiwalled carbon nanotubes (MWCNTs). An electrochemical sensor consisting of poly-4-vinyl pyridine (P4VP)-wrapped MWCNTs anchored to a poly(ethylene terephthalate) (PET)-modified gold electrode (r = 1.5 mm) was designed, produced, and evaluated. The P4VP is shown to form a strong association with Cu2+ ions, permitting preconcentration adjacent to the electrode surface for interrogation via cyclic voltammetry. The sensor exhibited a detection limit of 0.5 ppm with a linear range of 1.1–13.8 ppm (16.6–216 μM) and a relative standard deviation (RSD) of 4.9% at the Environmental Protection Agency (EPA) limit of 1.3 ppm. Evaluation in tap water, lake water, ocean water, and deionized water rendered similar results, highlighting the generalizability of the presented preconcentration strategy. The advantages of electrochemical analysis paired with polymeric chelation represent an effective platform for the design and deployment of heavy metal sensors for continuous monitoring of natural waters.

We report a set of voltammetric experiments for studying redox reactions and ion transfer in water microdroplets emulsified in 1,2-dichloroethane (DCE). The electrochemistry of microdroplets (rdrop ∼ 700 nm) loaded with either ferrocyanide ([Fe(CN)6]4-) or ferricyanide ([Fe(CN)6]3-), chosen due to their hydrophilic nature, was tracked using cyclic voltammetry. These heterogeneous reactions necessitated ion transfer at the droplet interface to maintain charge balance in the two liquid phases during oxidation or reduction, which was facilitated by the tetrabutylammonium perchlorate ([TBA][ClO4]) salt in the DCE phase. Experiments were performed with (1) a single macrodroplet (10-7 L) on a macroelectrode (r ∼ 1.5 mm), (2) millions of microdroplets (10-15 L) adsorbed on to a macroelectrode (r ∼ 1.5 mm), and (3) at the single microdroplet level via observing individual microdroplet collisions at an ultramicroelectrode (r ∼ 5 μm). We demonstrate that when millions of microdroplets are adsorbed onto a macroelectrode, there are two surprising observations: (1) the half-wave potential (E1/2) for the [Fe(CN)6]3-/4- redox couple shifts by +100 mV, which is shown to depend on the number of droplets on the electrode surface. (2) The reduction of [Fe(CN)6]3-, which is assisted by the transfer of TBA+ into the water droplet, displays two waves in the voltammogram. This dual-wave behavior can be explained by the formation of TBAxK3-xFe(CN)6, which is soluble in DCE. Additionally, we demonstrate that the adsorption of microdroplets onto an electrode surface offers significant amplification (×103) of the water/oil/electrode three-phase boundary when compared to the adsorption of larger macrodroplets, permitting a rigorous evaluation of heterogeneous chemistry at this distinct interface. In combination, these experiments provide new energetic and mechanistic insights for droplet systems, as well as reactivity differences between microscale and bulk multiphase systems.

Correction for ‘Enhancing scanning electrochemical microscopy's potential to probe dynamic co-culture systems via hyperspectral assisted-imaging’ by Sondrica Goines et al., Analyst, 2022, 147, 2396–2404, https://doi.org/10.1039/D2AN00319H.

Obtaining mechanistic insight into interfacial electron and ion transfer processes remains imperative to developing a comprehensive understanding of synthetic and biological processes. There is fundamental interest in the coupled electron-ion transfer processes characteristic of heterogeneous reactions at three-phase boundaries comprised of two immiscible liquids and a solid electrode surface. To probe reactivity at the three-phase boundary, we report a multifaceted analysis of ferrocene (Fc) oxidation in toluene droplets of diverse sizes. In one experiment, we study Fc oxidation in a large toluene macrodroplet. In a second experiment, we study Fc oxidation in an array of toluene microdroplets. We performed each experiment with and without an ionic liquid, trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide. Upon Fc oxidation in the oil phase without supporting electrolyte, the ferrocenium cation (Fc+), which is soluble in water, is expelled from the oil phase to maintain charge neutrality. Because Fc is only slightly soluble in water (∼40 µM), reversing the scan to reduce Fc+ in the aqueous phase generates a Fc precipitate adjacent to the three-phase boundary. This observation was confirmed by correlated optical microscopy. Increasing the voltammetric scan rate elucidated sequential versus concerted ion transfer mechanisms, which are shown to be highly dependent on the droplet size and the presence of trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide in the toluene phase. Differences in voltammetric responses are supported by finite element simulations.

Prodigious resources are currently being devoted to control the size and morphology of metal nanoparticles (NPs). Several homogeneous chemical and photochemical techniques exist for the synthesis o...

We demonstrate a method to electrodeposit and observe the electrocatalysis of small platinum clusters and nanoparticles (NPs) in real time as they form on an ultramicroelectrode (UME). Water droplets (rdrop ∼ 700 nm), stabilized by sodium dodecyl sulfate (SDS), were suspended in a solution of dichloromethane (DCM) and tetrabutylammonium perchlorate ([TBA][ClO4]), which was used to mitigate charge balance during droplet electrolysis. When droplets collided with an UME biased sufficiently negative to drive water reduction, large blips of current were observed. Droplets were synthesized with varying concentrations of H2PtCl6 (from 24.4 mM to 32 nM), which can be reduced to Pt0 at 0.8 V more positive than water reduction on a Au or C UME. The observation of current blips synthesized with mM amounts of H2PtCl6 indicated water droplets deliver H2PtCl6 to the electrode surface, where a cathodic potential caused Pt NPs to form. The formation of clusters was observed by biasing the electrode potential more negativ...

Porous metal nanoparticles (NPs) are important to a variety of applications; however, robust control over NP porosity is difficult to achieve. Here, we demonstrate control over NP porosity using nanodroplet-mediated electrodeposition by introducing glycerol into water droplets. Porosity approached 0 under viscous conditions (6 cP), and intermediate viscosities allowed the fine-tuning of NP porosity between 0 and 15%. This method also allowed for control over average pore radius (1 to 5 nm) and pore density (2 to 6 × 10

The undetected discharge of solubilized micropollutants into natural waterways constitutes a major public health concern. Thus, generalizable strategies to detect various micropollutant species coupled with deployable sensing platforms are paramount for indicating the presence of contamination at its earliest onset. Benchmark chromatographic methods paired with mass spectrometry to quantify micropollutants are expensive and difficult to deploy into waterways as passive environmental probes. In contrast, electrochemical sensors are inexpensive and easily miniaturized. We report the electrochemical detection of two compounds, 2,4,6-trinitrotoluene (TNT), a prevalent munition, and 2,4-dichlorophenoxyacetic acid (2,4-D), a common fertilizer, using molecularly imprinted polymers (MIPs). MIPs were rationally designed using DFT-level molecular simulations to determine an ideal functional monomer with maximal affinity for each micropollutant species. NMR titration methods were used to verify the simulation results and optimize the monomer/micropollutant ratio. Following optimization, MIPs were synthesized by anodic electropolymerization onto gold substrates in the presence of the micropollutant, which was subsequently stripped away via solvent washing to reveal micropollutant-specific binding sites. Upon micropollutant association with the MIP when placed in contaminated water, surface sites were blocked, which was tracked electrochemically. Each sensor exhibited a limit of detection in the sub-parts-per-billion regime, demonstrating the amenability of this sensing method for trace analysis. Combining the sensitivity of electroanalysis with the selectivity provided by the MIP recognition element constitutes a powerful platform for the detection of micropollutants in environmental matrices. FIGURE CAPTION: Electroanalytical signal obtained following the adsorption of TNT or 2,4-D molecules to an electropolymerized molecularly imprinted polymer (MIP) recognition element. MIPs may be rationally designed using density functional theory (DFT, inset) to maximize the monomer/toxin binding affinity, enhancing the analytical response and permitting the sub-ppb quantification of these toxins in environmental matrices. Figure 1

One frontier of measurement science is pushing the limit of what is measurable. Nanoelectrochemistry has transformed what is measurable at the nanoscale, elucidating reactivity of single atoms, molecules, and nanoparticles, one by one. The ability to interrogate physicochemical properties of single entities has elucidated new truths of nature that are otherwise averaged out during measurements over many entities (ensemble experiments). Single-entity experiments also give access to the ultimate sensitivity in measurement science: the specific detection of one single entity (not nanomolar quantities, not picomolar quantities—one single unit). One exciting subset of single-entity electrochemistry, and the topic of this review, is the study of reactions in nanoreactors of subfemtoliter (10−15 L) volumes with a particular focus on nanoparticle synthesis.

The opioid crisis has proliferated at an unprecedented rate worldwide, posing significant public health challenges. This review presents the current state of electrochemical methods to positively identify and quantify fentanyl, a potent and popular opioid, and its analogs in liquid and solid matrices. Specific emphasis is placed on point-of-use sensors, which are urgently needed to provide first responders with tools to identify unknown powders and treat victims of an opioid overdose. Electroanalytical techniques are uniquely poised to generate such sensors based on their portability, low cost, and ease of use. However, significant challenges remain, including enhancing sensor sensitivity for quantification of fentanyl at biologically relevant concentrations (

Creative approaches to the design of catalytic nanomaterials are necessary in achieving environmentally sustainable energy sources. Integrating dissimilar metals into a single nanoparticle (NP) offers a unique avenue for customizing catalytic activity and maximizing surface area. Alloys containing five or more equimolar components with a disordered, amorphous microstructure, referred to as High-Entropy Metallic Glasses (HEMGs), provide tunable catalytic performance based on the individual properties of incorporated metals. Here, we present a generalized strategy to electrosynthesize HEMG-NPs with up to eight equimolar components by confining multiple metal salt precursors to water nanodroplets emulsified in dichloroethane. Upon collision with an electrode, alloy NPs are electrodeposited into a disordered microstructure, where dissimilar metal atoms are proximally arranged. We also demonstrate precise control over metal stoichiometry by tuning the concentration of metal salt dissolved in the nanodroplet. The application of HEMG-NPs to energy conversion is highlighted with electrocatalytic water splitting on CoFeLaNiPt HEMG-NPs.