Table of contents

A number of bioelectronic applications work with oxidase enzymes and many of them can operate with small molecule or polymer redox mediators. However, for some oxidases, there are no known redox mediators able to mediate electron transfer. Therefore, electron transfer must occur through peroxide production and oxidation at the electrode surface. Organic redox catalysts such as oxoammonium cations, are able to oxidize H₂O₂ to form nitroxyl radicals, which can be electro-oxidized and regenerate the oxoammonium cation form. In this study, we investigate the ability to use TEMPO as a platform for the electrocatalytic oxidation of H₂O₂ at different pHs. The results have shown that TEMPO can be used to monitor H₂O₂ in broad pH range (≥4) at 530 mV (vs SCE). Combining TEMPO with cholesterol oxidase, we have shown the possibility to monitor the cholesterol oxidation with a linear range between 20 μM and 2.5 mM with a sensitivity of 54.86 mA cm⁻² M⁻¹. Furthermore, we have studied the electrocatalytic oxidation of oxalate by oxalate oxidase for biofuel cell applications. These combined results demonstrate TEMPO as a promising electrocatalyst applied for the development of electrochemical biosensors or enzymatic biofuel cells.

We recently showed that viologen-based redox active polymers (RAPs) with molecular weights between 21 and 318 kDa are attractive charge storage materials as anolytes for size-selective non-aqueous redox flow batteries. Here, we characterize the electron transfer mechanisms of these RAPs, as well as a ferrocene based catholyte RAP, in acetonitrile/Li⁺ electrolyte. We utilized scanning electrochemical microscopy (SECM) and rotating disk electrode (RDE) voltammetry to measure the rate of electron transfer and the rate of charge hopping between neighboring pendants along the insulating backbone of RAPs. The electron transfer kinetics of a 271 kDa ferrocene RAP mimic the facile kinetics of its monomer repeating unit. In contrast, viologen RAPs displayed RDE and SECM signatures that suggest a preceding chemical step to electron transfer. Viologen RAPs adsorb strongly to the electrode surface and create a redox active film that controls the rate of electron transfer via self-exchange. In addition, finite element simulations including a preceding chemical step demonstrated that a purely mass-transfer limited model is insufficient to recreate the viologen RAP feedback SECM response. The mechanistic insight obtained by combining SECM and RDE provided a powerful toolset for understanding and enhancing RAP reactivity for size-selective flow battery applications.

Exocytosis from dense core vesicles is an important ubiquitous process by which neurotransmitters are released from many cells. This has stimulated strong efforts to monitor and understand vesicular release events in order to infer their biological and physicochemical mechanisms. Though much has been achieved so far, many experimental observations remain unexplained, even puzzling, essentially because they envision that the vesicle matrixes consist of a homogeneous polyelectrolytic condensed phase encapsulated by the vesicles membranes. This work discloses a new model of dense core matrixes based on the physics of polyelectrolytes involving long chains of anionic moieties (here, chromogranins) condensed by a mixture of bulky monocations and small dications (here, catecholamines and calcium ions). It follows that matrixes cannot be homogeneous but necessarily consist of a dispersion of tightly compacted nano-grains immersed in a less condensed phase involving loosely folded chromogranin strands in which catecholamine cations may diffuse at significant rates. Even if such pomegranate-like description has to remain essentially theoretical up to when direct experimental means of testing is available, it leads to a whole set of predictions that are fully coherent with all experimental observations based on amperometric monitoring of vesicular exocytosis including some recent extremely puzzling ones.

We present the study of dye-sensitized nanoparticles and their agglomerates with stochastic electrochemistry. We use a fluorine doped tin oxide (FTO) ultramicroelectrode (UME) with diameter in the range of 40 to 80 micrometers. To prepare the UME a sheet of perfluoroalkoxy alkane (PFA) is perforated to produce a recessed micro disk of the transparent conductive oxide. We demonstrate that the detection of the colloidal nanoparticles (NPs) sensitized with N719 is achieved via the interactions between agglomerates of the NPs that collide with the FTO. The NPs photooxidize MeOH and we detect the charge transfer from the NPs to the FTO ultramicroelectrode. The interactions between the dye sensitized NPs and the FTO yields oscillations that increase with exposure time. This increment in oscillation amplitude is assigned to changes in the aggregation of the NPs during the illumination even in the presence of 0.1 M supporting electrolyte. The formation of agglomerates is verified by dynamic light scattering and indicates that the agglomerates are able to separate photo-generated electrons and inject the carriers to the UME.

Nanoscale scanning electrochemical microscopy (SECM) has emerged as a powerful electrochemical method that enables the study of interfacial reactions with unprecedentedly high spatial and kinetic resolution. In this work, we develop carbon nanoprobes with high electrochemical reactivity and well-controlled size and geometry based on chemical vapor deposition of carbon in quartz nanopipets. Carbon-filled nanopipets are milled by focused ion beam (FIB) technology to yield flat disk tips with a thin quartz sheath as confirmed by transmission electron microscopy. The extremely high electroactivity of FIB-milled carbon nanotips is quantified by very high standard electron-transfer rate constants of ≥10 cm/s for Ru(NH₃)₆³⁺. The tip size and geometry are characterized in electrolyte solutions by SECM approach curve measurements not only to determine inner and outer tip radii of down to ∼27 and ∼38 nm, respectively, but also to ensure the absence of a conductive carbon layer on the outer wall. In addition, FIB-milled carbon nanotips reveal the limited conductivity of ∼100 nm-thick gold films under nanoscale mass-transport conditions. Importantly, carbon nanotips must be protected from electrostatic damage to enable reliable and quantitative nanoelectrochemical measurements.

We have designed and built a rotating dual-electrode infrared cell for in situ electrochemical (EC) attenuated-total-reflection infrared spectroscopic (ATR-IRS) investigations of fuel cells and batteries under operating conditions. Our design is unique in that it for the first time allows for ATR-IRS measurements to be made on both anode and cathode while a fuel cell is operational and electrochemical measurements are on-going. We demonstrate the device's capabilities by making sequential, iterative ATR-IRS measurements of direct methanol fuel cell (DMFC) reactions at the PtRu anode and Pt cathode as a function of cell driving current (thus potential) while controlling the electrochemical parameters. Different chemical species involved in the anodic methanol oxidation reaction (MOR) versus those involved in the cathodic oxygen reduction reaction (ORR) can be identified and quantified during the operation of the fuel cell. The same setup can be applied directly to study anodic and cathodic chemistry in batteries.

The redox processes, nucleation-growth mechanisms and magnesium (Mg) speciation associated with the room temperature electrodeposition of metallic Mg on an aluminum (Al) electrode are described from mixed deposition media composed of 0.095 M ethylmagnesium bromide (EtMgBr)/tetrahydrofuran (THF) and 0.54 M ionic liquid including: 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([Py₁₄][FAP]), 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Py₁₄][TFSI]), 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide ([Py₁₄][FSI]), 1-butyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide ([Pip][TFSI]), ethyldimethylpropylammonium bis(trifluoromethylsulfonyl)imide ([N₁₁₂₃][TFSI]) and diethylmethyl (2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl)imide ([DEME][TFSI]). Cyclic voltammetric results show a chemically reversible Mg/Mg²⁺ process in the case of [FAP]⁻ and [TFSI]⁻ based ionic liquids, but not for the [FSI]⁻ based one, ascribed to ionic liquid dependent Mg speciation. Analysis of chronoamperometric data reveal that in the presence of [Py₁₄][FAP] and [Pip][TFSI], Mg nucleation follows a 3D instantaneous nucleation and diffusion controlled semi-spherical growth model. However, in the case of [Py₁₄][TFSI], [DEME][TFSI] and [N₁₁₂₃][TFSI], deviations from this model are evident. The influence of deposition parameters, such as current density and deposition time, on appearance, crystallite size and preferred orientation of Mg are also reported in a deposition medium containing 90% 1 M EtMgBr/THF and 10% ionic liquid (v/v ratio). [Py₁₄][FAP] was found to be the most favourable ionic liquid for electrodepositon of Mg.

This study concentrates on the development of catalytic materials capable of enhancing oxidation of ethanol particularly at low potentials, i.e. at conditions resembling operation of anodes in low temperature fuel cells. Indeed, the ethanol oxidation currents measured under voltammetric and chronoamperometric conditions have been significantly increased following dispersing bimetallic PtRu nanoparticle (∼4 nm) catalysts over TiO₂ (80% anatase, 20% rutile) nanostructures (∼20–30 nm) supported onto multi-walled carbon nanotubes. The improvement in activity is attributed not only to better dispersion of PtRu centers (some increase of active surface area) or improved (carbon-nanotube-assisted) propagation of electrons at the catalytic interface but also to the presence of hydroxyl groups and high mobility of protons at neighboring (to PtRu) TiO₂. Further, specific interactions between TiO₂ and Pt or Ru metallic components are feasible. Importance of addition of TiO₂ is also evident from kinetic analysis based on measurements of steady-state currents at different concentrations.

We present a combined experimental and theoretical study of CO_ads on Pt₁₄₇ dendrimer-encapsulated nanoparticles (DENs). In-situ electrochemical IR spectroscopy reveals an 8 cm⁻¹ redshift of the CO_ads stretching frequency on Pt₁₄₇ DENs relative to a Pt(111) crystal. This value is in good agreement with the shift calculated by density functional theory. Importantly, the wavenumber shift observed in this study is significantly smaller than has been found previously. We attribute this primarily to the absence of support effects and the narrow size distribution of DENs. The agreement between experiment and theory validates the model nanoparticle system used for the calculations, and this will make it possible to use the CO_ads frequency as a probe to study more complex DEN structures and as a descriptor of the catalytic activity of DENs toward reactions such as formic acid oxidation and methanol oxidation.

Porous films are ubiquitous in electrochemistry. They frequently form on active electrodes due to the precipitation of insoluble reaction products. They can have beneficial effects, like the protection from electrochemical corrosion, or be of parasitic nature, as in the poisoning of fuel cell air cathodes. The effects of such layers on the electrochemical response of the substrate can be probed by Scanning Electrochemical Microscopy (SECM). Herein, we present modifications to the conventional analytical expressions for SECM microelectrode approach curves, to account for the effects of a porous layer. The modified expressions can be used to fit experimental approach curves and obtain film thickness and porosity parameters. Their performance is demonstrated through comparison to results obtained by finite element modeling, and by fitting experimental approach curves over well-defined filter membranes.

We present here the detection of dopamine (DA) at nanopipet electrodes with radii of hundreds of nanometers ranging from 160 nm to 480 nm. Dibenzo-18-crown-6 (DB18C6) was employed as an ionophore to facilitate DA transfer, resulting in a half-wave transfer potential, E_{1/2, DA}, of −0.322 (±0.020) V vs. E_{1/2, TBA}. Well-defined steady-state sigmoidal cyclic voltammograms were observed for the transfer of DA. High resolution scanning electron microscopy was used to measure the size and taper angle of the nanopipet electrodes. The detection is linear with concentration of DA ranging from 0.25 mM to 2 mM; calculated diffusion coefficient at nanopipet electrodes with above mentioned sizes is 4.87 (±0.28) × 10⁻¹⁰ m²/s. The effect of the common interferent ascorbic acid on DA detection with nanopipet electrodes was evaluated, where DA detection still shows linear behavior with well-defined sigmoidal CVs with E_{1/2, DA} being −0.328 (±0.029) V vs. E_{1/2, TBA}. The diffusion coefficient for DA transfer in MgCl₂ with the presence of 2 mM AA was measured to be 1.93 (±0.59) × 10⁻¹⁰ m²/s on nanoelectrodes with radii from 161 nm to 263 nm, but the physiological concentration of 0.1 mM AA had no effect on DA's diffusion coefficient.

Scanning electrochemical microscopy (SECM), a non-invasive variant of scanning probe microscopy, can be used to measure oxygen uptake or insulin secretion of isolated murine pancreatic islets in the presence of low and high physiological glucose concentrations levels. SECM imaging of islet topography and islet oxygen uptake is accomplished by the use of a redox mediator that cannot penetrate into the cell membranes of the islet coupled to direct reduction of oxygen at the SECM tip. An insulin sensing ultramicroelectrode (UME), adapted from a macro-scale insulin sensor incorporating a multiwalled carbon nanotube/dihydropyran film developed in our lab previously, was used for the direct electrochemical detection of insulin levels within extracellular media in real-time. We were able to stimulate and record increased insulin production in a single pancreatic islet by raising the concentration of extracellular glucose to the physiological 'high level.' These experiments generated probe scan curves (PSCs) for insulin production in the extracellular media surrounding single pancreatic islets extracted from male mice. This research is a part of an ongoing attempt to increase the scope of application for Bio-SECM, both through the adaptation of specialized biosensors and application of existing techniques for biological analysis of single cells and micro-organs.

Potentiometric redox measurements were made in solutions of increasing biological complexity starting with buffer solutions containing either potassium ferri/ferrocyanide or ascorbic acid and finishing with plasma and blood. When the concentration of ferri/ferrocyanide was high (∼0.2 mM), both biofouled planar and nanoporous gold electrodes gave Nernstian slopes of 55–59 mV. However, at or below a critical concentration (≤ 0.1 mM), ∼20% loss in sensitivity was observed at planar gold in contrast to nanoporous gold where Nernstian behavior was retained. For ascorbic acid, a Nernst slope of −41 mV was observed at biofouled nanoporous gold electrodes. In contrast, biofouled planar gold electrodes failed to give any potentiometric redox response. At all concentrations studied, cyclic voltammetric measurements on biofouled electrodes revealed significant impairment of faradaic electroactivity at planar gold electrodes while no impairment was shown at nanoporous gold. These results indicate that nanoporous gold is an ideal electrode material to use when making both potentiometric and cyclic voltammetric measurements, particularly in complex solutions containing relatively low concentrations of redox molecules. As proof of concept, the redox potential of plasma and blood has been measured using nanoporous and planar gold electrodes and their values compared.

Scanning electrochemical microscopy (SECM) is unique among scanning probe methods in its quantitative rigor and in its ability to study samples in liquid environments with ease. SECM has become a popular and mature technique with a wide range of applications in electrochemical imaging, chemical kinetics, biological redox processes, and electrocatalytic reactions, among others. A major development in recent years is the ongoing shift from micrometer-scale experiments to the nanoscale. Recent advances in methodology have greatly increased the capacity of SECM to characterize interfaces at the nanoscale and to obtain molecular-level chemical information. The principles of SECM will be briefly introduced, and recent advances using this technique will be discussed.

Theory for cyclic square wave voltammetry of an irreversible first-order chemical reaction coupling two electron transfers, i.e. an ECE mechanism, is presented. Theoretical voltammograms were calculated following systematic variation of empirical parameters to assess their impact on the shape of the voltammogram. Note that the results presented herein are applicable only to ECE processes where E⁰ for the second electron transfer step is negative of that for the first. Under this condition, disproportionation reactions do not occur. From the trends obtained, diagnostic criteria for this mechanism were deduced. When properly applied, these criteria will enable non-experts in voltammetry to assign the electrode reaction mechanism and accurately measure reaction kinetics over the range −2 ≤ log k_f ≤ 6.

For the first time, kesterite Cu₂ZnSnS₄ (CZTS) thin films were efficiently obtained by electrophoretic deposition (EPD) of CZTS nanocrystal (NC) dispersion in an environmentally-friendly solvent, isopropanol. By regulating the applied potential, deposition time, and NC suspension concentration, the EPD process was successfully optimized. Photoelectrochemical measurements (PECMs) were carried out to test the EPD film performance, where the photocurrent density generated from the reduction of methyl viologen (MV²⁺) was evaluated. EPD films showed a photocurrent three times higher than those obtained by dropcasting at the same concentration. X-ray diffraction and Raman spectroscopy confirmed the formation of a single phase Cu₂ZnSnS₄ kesterite structure, without impurities. Intensity modulated photocurrent spectroscopy was used to determine the hole-electron recombination and product separation kinetics in the photoprocesses. Lastly, UV-visible spectroscopy determined the bandgap as 1.45 eV, which is an ideal value for single junction solar cell devices. The EPD approach can be combined with our optimized one-pot synthesis of CZTS NCs, which avoids annealing steps after the deposition. Based on these findings, EPD is anticipated to become one of the most promising routes toward fabrication of solar cell devices.

An overview is given on great achievements in electrogenerated chemiluminescence or electrochemiluminescence (ECL) by Allen J. Bard. The established ECL theories and fundamentals are first described. The developed analytical instrumentation and methodology for ECL are then revealed. Studies of various types of ECL luminophores are summarized. The discovered ECL applications especially in bioanalysis and bioimaging are finally depicted. Bard's presentations and publications on ECL are indispensable resources of inspiration and guides for ECL researchers worldwide, who will further advance ECL research and applications already in a prosperous stage.

Water electrolysis using a catholyte and anolyte at different pH values requires a bipolar membrane to sustain the pH difference and 1.23 V to electrolyze water. Bipolar membranes that separated concentrated aqueous acid and base exhibited an open-circuit potential consistent with the Nernst equation and rapid transport of protons and hydroxide ions. When excess supporting electrolyte was added to both solutions the membrane potential was measured to be ∼0 V, which suggested that water electrolysis occurred at ≪1.23 V and therefore, protons and hydroxide ions were not the majority ionic charge carriers.

Natural convection actuated by electrochemically-generated density gradients at microelectrodes was investigated under different conditions by simultaneously visualizing fluid flow with the electrochemical response. The studies elucidate deviations of electrochemical behavior from theoretical expectations and parameters that control natural convection, which can be exploited in electroanalysis, microfluidics, and electrodeposition. Experiments involved an enclosed, small volume containing 0.00475-0.095 M each of K₃Fe(CN)₆ and K₄Fe(CN)₆ in 0.095 M KCl, over concentric gold disk (radius: 16–156 μm) and ring (inner radius: 200–1600 μm, outer radius: 250–2000 μm) microelectrodes. Fluid velocities were obtained with video microscopy by tracking 10-μm beads added to the solution. Flow radiates near the disk either inwardly or outwardly at the bottom of the cell and reverses direction at the top, producing a vertical circulation. Maximum velocities of ∼10 μm/s were measured for the 156-μm disk in 0.095 M. After application of potential or current, the onset of natural convection occurred at shorter times (6 s) than measurable affects in electrochemical current/voltage responses (tens of seconds). Convection from density gradients occurred without corresponding changes in electrochemical responses for the 78-μm disk at the lowest concentration (0.00475 M) and for the smallest, 16-μm disk at the highest concentration (0.095 M).

We report a new detection scheme to detect and study discrete collisional events of single metal nanoparticles on a carbon ultramicroelectrode (UME). Detection of single nanoparticle collision is based on rapid electrocatalytic reduction of silver chloride (AgCl) on metal nanoparticle surfaces when they diffuse and make electrical contacts with a AgCl-modified carbon UME substrate. Single collisional events can be recorded as individual millisecond current pulses on a carbon UME by a simple square wave voltage waveform. Detections of both 80-nm silver and 4-nm gold nanoparticles are demonstrated in this work. This method is simple yet powerful and allows continuous recording of nanoparticle collision events for hours. Moreover, this method uses surface-supported solid AgCl and does not involve added redox species in the bulk solution. Therefore, nanoparticle stability can be less affected by the presence of other redox species, such as hydrazine.

Operando Raman micro-spectroscopy of the membrane electrode assembly (MEA) of a fully operating hydrogen/oxygen Nafion electrolyte fuel cell is described. Coarse depth profiling of the fuel cell system enabled appropriate positioning of the micro-spectroscopy laser focal point for MEA catalytic layer spectroscopy. An increase in the ionomer state-of-hydration, from oxygen reduction at the cathode, transitions ion exchange sites from the sulfonic acid to the dissociated sulfonate form. Visualization of density functional theory calculated normal mode eigenvector animations enabled assignments of Nafion side-chain vibrational bands in terms of the exchange site local symmetry: C₁ and C_3V modes correlate to the sulfonic acid and sulfonate forms respectively. The gradual transition of the MEA spectra from C₁ to C_3V modes, from the fuel cell open circuit voltage to the short circuit current respectively, demonstrate the utility of vibrational group mode assignments in terms of exchange site local symmetry.

The lifetimes of individual H₂ and N₂ nanobubbles, electrochemically generated at Pt nanoelectrodes (7–85 nm-radius), have been measured using a fast-scan electrochemical technique. To measure lifetime, a stable single H₂ or N₂ bubble is first generated by reducing protons or oxidizing hydrazine, respectively, at the Pt nanoelectrode. The electrode potential is then rapidly stepped (<100 μs) to a value where the bubble is unstable and begins to dissolve by gas molecule transfer across the gas/water interface and diffusion. The electrode potential is immediately scanned back to values where the bubble was initially stable. Depending on the rate of this second voltammetric scan, the initial bubble may or may not have time to dissolve, as is readily determined by the characteristic voltammetric signature corresponding to the nucleation of a new bubble. The transition between these regimes is used to determine the bubble's lifetime. The results indicate that dissolution of a H₂ or N₂ nanobubble is, in part, limited by the transfer of molecules across the gas/water interface. A theoretical expression describing mixed diffusion/kinetic control is presented and fit to the experimental data to obtain an interfacial gas transfer rate of ∼10⁻⁹ mol N⁻¹ s⁻¹.

The principle of unchanging total concentration as described by Oldham and Feldberg [J. Phys. Chem. B, 103, 1699 (1999)] is invoked to analyze systems comprising a redox pair (Xz11 and Xz22) plus one or more non-electroactive species () where Xzjj is the jth species with charge zj and concentration; cj. The principle states that if the diffusion coefficients for all species are identical and mass transport is governed by the Nernst-Planck expression, the total concentration does not change during any electrochemical perturbation, i.e.: With this principle we deduce the electrochemically induced difference between the surface and bulk concentrations for each species. Those concentration differences are translated into density differences which are a function of the density of the solvent and of the concentration differences, molecular masses and the standard partial molar volumes of all species. Those density differences in turn can induce convection that will ultimately modify the observed current. However, we did not attempt to quantify details of the natural convection and current modification produced by those density differences. The principle of unchanging total concentration also allows us to suggest experimental ploys that might minimize, if not eliminate, density differences; if there are no density differences there should be no convection save for the possibility of spontaneous convection which Amatore, Szunerits, Thouin and Warkocz [J. Electroanal. Chem., 500 62 (2001)] have identified as a mode of convection that does not depend upon "macroscopic flow or density gradient". Following the lead of Ngamchuea, Eloul, Tschulik and Compton [Anal. Chem.,87, 7226 (2015)] we did not consider spontaneous convection in the present work.

Steady state rotating disk voltammetry provides excellent measurement of permeability for films and layers on electrodes because the hydrodynamic control of rotating disks establishes well defined boundary layers normal to the electrode. For a redox probe present in solution and pre-equilibrated in the layer, voltammetry measures electrolysis current for the probe as the probe transports from solution, through the film, and to the electrode where the probe is electrolyzed. Diagnostic equations relate the steady state, mass transport limited current i_ss to the rotation rate ω. The incompressible layer is electroinactive about the probe formal potential. Diagnostics are available for a single layer, uniform film (Gough and Leypoldt). Uniform films are homogeneous with no structural features. Here diagnostics are provided for bilayer and multilayer uniform films, where multilayers include discretely and continuously graded films. Consideration of serial mass transport resistances normal to the electrode allows diagnostics for uniform films. Heterogeneous, micro- and nano-structured layers are structured in the plane of the electrode. Consideration of parallel mass transport resistances as part of the layer mass transport resistance yields diagnostics for heterogeneous films. Diagnostics are vetted with literature data. Theoretical and practical advantages and limitations are noted.