Thin-layer Electrochemical Cell Research Articles

FullThrOTTLE-TrIR: Time-Resolved IR Spectroscopy of Electrochemically Generated Species Using a Full Throughput Optically Transparent Thin-Layer Electrochemical Cell.

An optically transparent thin-layer electrochemical cell with stopped-flow sample transport has been developed for optical-pump infrared-probe transient absorption spectroscopy of prereduced or preoxidized molecules. Time-resolved IR-spectra of Re(bpy)(CO)3X (X = Cl, Br) complexes in different oxidation states are presented as a proof-of-principle application for this combined electrochemical and spectroscopic tool. The excited-state lifetimes and IR-spectroscopic signatures of various oxidation states of the molecule, including follow-up reaction intermediates, are disentangled by kinetic sorting, using lifetime density analysis. The method can be applied to assign and differentiate molecular intermediates in photo- and electrochemical reactions, adding new analytic coordinates to classical FTIR- and UV-vis-spectroelectrochemistry.

Oscillations and bistability of complex electrochemical reactions in 3D printed microfluidic devices

We investigate the kinetic features of complex charge transfer chemical reactions using a microelectrode integrated in a flow cell made with 3D printing technology. Thin layer electrochemical cells with 350 μm (height) × 1000 μm (width) channels were designed. Ferrocyanide oxidation on a 100 μm diameter Pt electrode using the printed cell showed mass transfer limiting current at large overpotentials. The variation of the limiting current with the flow rate (1 μL/min ≤ Q ≤ 100 μL/min) was interpreted with the superposition of a constant current due to radial diffusion and a Levich-type square-root relationship, indicating a mixed radial diffusion/thin layer mass transport condition. These findings were further supported by numerical simulation of a diffusion-convection equation in the flow channel. Experiments with copper electrodissolution in phosphoric acid electrolyte reproduced oscillatory behavior that was previously seen in macrocells and polydimethylsiloxane (PDMS) microchips. The 3D printed cell can accommodate multiple electrodes – to demonstrate this application we showed that the current oscillations with copper electrodissolution can exhibit frequency synhcronization with a dual electrode configuration. Finally, hydrogen oxidation on a Pt wire in the presence of Cu2+ and Cl− inhibitors displayed bistability at relatively large external resistance and oscillations at high volumetric flow rates. The results show that the 3D printed thin layer design is an alternative to studying nonlinear phenomena in electrochemical cells with distinct advantages to the traditional approaches of using macrocells and PDMS microchip flow cells.

Current density profiles at thin layer electrochemical cells under laminar and turbulent regimes

The transition of laminar to turbulent regimes on smooth electrocatalysts in thin-film electrochemical cells is a subject of study with the aim of predicting current density profiles. Analytical solutions are obtained for velocity profiles under steady state conditions for both hydrodynamic conditions applying a proper velocity contour condition on the catalyst surface. This state is necessary to describe electrochemical cell performances since in these cases, electrode reactions occur at the surface and not in the bulk of the electrolyte. The obtained equations for linear momentum under asymptotic conditions reduce to the solutions obtained by classical Power Series methods years ago. Concentration and current density profiles are also solved accordingly and compared with experimental data of fast redox and dissolved gas reactions on smooth noble metal electrocatalysts.

67 Fipronil In Vitro Metabolism Revisited: Microsomal Incubations, Electrochemistry, and Their Relevance to Human Biomonitoring

Abstract Fipronil is a phenylpyrazole insecticide widely used in indoor pest control and treatment of flea and tick infestations in household pets. Its applications may act as a source of exposure and cause negative health effects to humans and environment. Biomonitoring is a crucial tool for assessment of actual chemical exposure. However, this approach requires a biomarker which reflects exposure to parent compound and, preferably, can be detected in urine. Despite the fact a few studies on metabolism and biomonitoring of fipronil were published, there is no consensus on optimal biomarker of fipronil exposure in humans. In this work we attempt to address this issue by investigating the metabolism of fipronil and its selected biotransformation products using microsomal incubations and a purely instrumental approach utilizing thin-layer electrochemical cell equipped with a glassy carbon or boron-doped diamond working electrode. Liquid chromatography-tandem mass spectrometry experiments were conducted to elucidate the structure of generated metabolites. Moreover, samples from humans probably exposed to fipronil were analyzed as well. A comparison of products obtained using the two aforementioned in vitro approaches was drawn and relevance to in vivo results was discussed. The results presented here shed light on complexity of fipronil metabolism and highlight the possibilities associated with electrochemical studies. However, a large-scale human biomonitoring study is needed to confirm the results obtained in our work.

Higher-order derivative electronic absorption spectroscopy of ferricyanide electrogenerated in situ by optically transparent thin layer electrochemical cell

• Higher-order derivative UV-vis spectra obtained in spectroelectrochemical measurements. • Separated spectra of ferricyanide obtained by electrolysis in OTTLE. • The maxima of the potential-dependent fourth-derivative spectra satisfy Nernst’s relation. • Shoulder peaks in spectra of electrolytic products could also be used for reaction analysis. Higher-order differential transformations of electronic absorption spectra obtained by in situ thin-layer spectroelectrochemical cell were performed. The characteristics of the derivative spectra obtained in the electro-oxidation of ferrocyanide were in good agreement with those of ferricyanide in homogeneous solution systems. Several common crossing points (CCP) appearing in the derivative spectra depending on the applied potential of the reversible system made it clear that the reaction was unitary. The maximum peaks of the derivative spectra corresponding to the shoulders of the original spectra and the applied potential satisfy the Nernstian relationship, which allows us to evaluate the n and E o ′ values in the electrode reaction.

Electrochemistry Under Microscope: Measuring the Thickness of Diffusion Layer in Thin Layer Electrochemical Cells with Optical Microscope

Electrochemical reactions are heterogeneous in nature, and an electrochemical reaction involves mass transfer from the bulk solution to the electrode surface, heterogeneous electron transfer at the electrode surface, and mass transfer back to the bulk solution. There are three modes of mass transport to the working electrode surface: diffusion, migration, and convection. Among these modes, diffusion is the one that involved in all electrochemical processes and sometimes is the sole mode of mass transfer. Thus, the key concept underpinning electrochemical science are that of the diffusion and diffusing layer, the region in the vicinity of an electrode layer that affected by electrode reaction. The thickness of diffusion layer is an important parameter in understanding chronoamperometry, cyclic and hydrodynamic voltammetry. The thickness of this zone can be measured either by simulation or analysis of the electrochemical responses. But interpretation of the concentration profiles and related current time responses require understanding the Fick’s law and its related mathematical representations. Although these basic are crucial in understanding electrochemistry but their mathematical representation is challenging specially for the students or scientists who are new to the field. The thickness of diffusion layer for a typical electroanalytical experiment varies between 20 to 200 micrometers, depends on the time of experiment. The size is not visible to the human eye easily but doesn’t required expensive research-grade microscopes and can be seen using even kids grade microscopes with 100 times magnification. Here we have developed a thin-layer electrochemical cell compatible with low-cost microscopes to demonstrate a microscopy activity in which students measure the thickness of micrometer-sized diffusion layer. The electrode processes involve the reactions of redox indicators with sharp color changes or acid base indicators with sharp color change in response to electrochemically generated proton or hydroxide, from water electrolysis. This Imaging tools advance electrochemistry education by enabling students to see the electrode processes that are not visible to the human eye without any research grade instrumentation. This device also aids in the explicit visualization and interpretation of a typical concentration profiles near the working electrode surface for chronoamperometry, cyclic and hydrodynamic (e.g., rotated disk) voltammetry experiments.

A Model for Sonochemistry in a Thin Layer Sonochemical Cell: What Constructive Interference Yields

Traditional sonochemistry (TS) in bulk solution is violent and expensive. Typically, a high power (> 60 W) ultrasonic horn is directed at a planar working electrode. Compression and rarefaction oscillations elicit cavitation bubble formation. Upon bubble collapse, high temperature (> 5000 K) and pressure (> 1000 atm) are produced locally, which enhances rates of chemical reactions. Bubble collapse near the electrode causes solvent jets directed at the surface to effect the surface cleaning and mass transport enhancements attributed to TS.While productive, TS obscures quantitative measurement of electron transfer kinetics and mass transport and requires a large laboratory footprint. An alternative and efficient sonochemistry is needed. Thin Layer Sonochemistry (TLS) exploits natural acoustics in a thin layer electrochemical cell. Low power transducers (e.g., quartz crystal oscillators) are appropriate because TLS achieves resonance in the cell to amplify input sound to the limit set by the solvent. TLS is more efficient and allows finer control of experimental and acoustic parameters. The footprint of TLS is minimal, and so TLS is appropriate for energy production applications (e.g., fuel cells) in portable devices.Experience suggests a subtle relationship between acoustic parameters and resonance. Here, a model is presented for calculation and explanation of resonance in a thin layer sonochemical cell. Background physics are explained and a step-by-step assembly guide is presented.

(Invited) Spectroelectrochemical Sensors: The Quest for Selectivity

Introduction Exceptional selectivity is needed for sensors to accurately measure analytes in complex samples such as nuclear waste and natural water that contain many potentially interfering substances. Spectroelectrochemistry offers a unique means for providing selectivity by electrochemically modulating the optical signal used for quantitation. The modulated signal can be distinguished from the constant signals of potential interferences. An additional level of selectivity can be provided by adding a very thin film to the transparent electrode to first preconcentrated the analyte while rejecting interferences. Sensors using both visible absorption and fluorescence modes of detection have been developed for detection of organic and inorganic species including ferrocyanide [1] and technetium [2] complexes in nuclear waste and natural water. We have recently extended this research to the spectroelectrochemical investigation of molten salt systems, [3] including lanthanide and actinide measurements related to pyroprocessing [4] and molten salt reactor applications. Thin layer fluorescence-based spectroelectrochemical sensor A planar optically transparent electrode chip was fabricated and characterized for multimode sensing of target species. The chip was comprised of an indium tin oxide (ITO) working electrode, a platinum auxiliary electrode, and a Ag/AgCl reference electrode. Fluorescence was used for detection because limits of detection are typically several magnitudes lower than for absorption spectroscopy and selectivity improved by having adjustable wavelengths for both excitation and emission. A fluorescence spectroelectrochemical measurement using this chip incorporated into an optically transparent thin layer electrochemical (OTTLE) cell was demonstrated using the modulated emission of [Ru(bpy)3]2+. Molten salt absorbance-based spectroelectrochemistry Molten salt systems are of growing interest for a variety of applications, including meeting future energy needs such as molten salt reactors (MSRs). These reactors are of interest for several factors, including the ability to be operated at atmospheric pressures even at extremely high temperatures. The molten salts serve several functions, including heat transfer fluid, reactor coolant, and ability to contain high concentrations of dissolved fuel (plutonium, uranium, thorium). While the benefits of molten salt reactors have been identified, the chemistry of the molten salts is still not understood. Spectroelectrochemistry can provide the tools to characterize, monitor, and control the chemical behavior of fuel species (U, Pu), fission, and corrosion products throughout the molten salt reactor loop. Several of these species are electroactive and have a unique spectroscopic fingerprint that can be optically detected and modulated using electrochemistry. Spectroelectrochemical characterization can provide fundamental information such as concentration, oxidation state, and speciation at any point in the molten salt reactor system. The absorbance-based spectroelectrochemical modulation of uranium was demonstrated in the molten LiCl-KCl eutectic. The unique spectroscopic signatures for U(III) and U(IV) were identified and monitored as it was electrochemically modulated between the U(III)/U(IV) redox states. Based on variable scan-rate cyclic voltammetry, the diffusion coefficients of U(III) and U(IV) were measured.

Asymptotic Velocity Profiles for Mass and Current Density Profiles in a 2-D Thin Layer Electrochemical Cell

Asymptotic Velocity Profiles for Mass and Current Density Profiles in a 2-D Thin Layer Electrochemical Cell

Windowless thin layer electrochemical Raman spectroscopy of Ni-Fe oxide electrocatalysts during oxygen evolution reaction

In-situ Raman spectroelectrochemistry is a powerful tool to understand the structures and reaction mechanisms of electrochemical interfaces yet usually suffers from a low sensitivity and detection efficiency. Herein, we develop a windowless thin layer electrochemical cell design to improve the detection efficiency of in-situ Raman spectroscopy for practical nanoscale electrocatalysts. This was achieved by perforating a hole in the optical window of the cell, which not only eliminates the refraction of light by the window but also induces a meniscus thin electrolyte layer on the working electrode, thus significantly improving the detection efficiency of Raman spectra. We successfully applied this method to conduct in-situ Raman spectroscopy of structural evolutions of two representative Ni-Fe oxide catalysts (layered double hydroxides and spinel oxides) during oxygen evolution reaction (OER) electrocatalysis. It was shown that both catalysts underwent surface transformation to oxyhydroxides and formed active oxygen species at OER relevant potentials. However, the extent of the surface transformation was much lower on the Ni-Fe spinel catalyst, accounting for its lower OER activity. We further show that adding a carbon support can promote the surface transformation of Ni-Fe spinel to oxyhydroxides, thus significantly increasing the electrocatalytic activities. The established windowless thin layer method provides a new way to conduct in-situ electrochemical Raman spectroscopy of a wide range of materials and may be also combined with surface-enhanced Raman spectroscopic methods.

(Invited) Spectroelectrochemical Sensors: The Quest for Selectivity

Introduction Exceptional selectivity is needed for sensors to accurately measure analytes in complex samples such as nuclear waste and natural water that contain many potentially interfering substances. Spectroelectrochemistry offers a unique means for providing selectivity by electrochemically modulating the optical signal used for quantitation. The modulated signal can be distinguished from the constant signals of potential interferences. An additional level of selectivity can be provided by adding a very thin film to the transparent electrode to first preconcentrated the analyte while rejecting interferences. Sensors using both visible absorption and fluorescence modes of detection have been developed for detection of organic and inorganic species including ferrocyanide [1] and technetium [2] complexes in nuclear waste and natural water. We have recently extended this research to the spectroelectrochemical investigation of molten salt systems, [3] including lanthanide and actinide measurements related to pyroprocessing [4] and molten salt reactor applications. Thin layer fluorescence-based spectroelectrochemical sensor A planar optically transparent electrode chip was fabricated and characterized for multimode sensing of target species. The chip was comprised of an indium tin oxide (ITO) working electrode, a platinum auxiliary electrode, and a Ag/AgCl reference electrode (Figure 1A,B). Fluorescence was used for detection because limits of detection are typically several magnitudes lower than for absorption spectroscopy and selectivity improved by having adjustable wavelengths for both excitation and emission. A fluorescence spectroelectrochemical measurement using this chip incorporated into an optically transparent thin layer electrochemical (OTTLE) cell was demonstrated using the modulated emission of [Ru(bpy)3]2+. Molten salt absorbance-based spectroelectrochemistry Molten salt systems are of growing interest for a variety of applications, including meeting future energy needs such as molten salt reactors (MSRs). These reactors are of interest for several factors, including the ability to be operated at atmospheric pressures even at extremely high temperatures. The molten salts serve several functions, including heat transfer fluid, reactor coolant, and ability to contain high concentrations of dissolved fuel (plutonium, uranium, thorium). While the benefits of molten salt reactors have been identified, the chemistry of the molten salts is still not understood. Spectroelectrochemistry can provide the tools to characterize, monitor, and control the chemical behavior of fuel species (U, Pu), fission, and corrosion products throughout the molten salt reactor loop. Several of these species are electroactive and have a unique spectroscopic fingerprint that can be optically detected and modulated using electrochemistry. Spectroelectrochemical characterization can provide fundamental information such as concentration, oxidation state, and speciation at any point in the molten salt reactor system. Absorbance-based spectroelectrochemical modulation of uranium was demonstrated in the molten LiCl-KCl eutectic. The unique spectroscopic signatures for U(III) and U(IV) were identified and monitored as it was electrochemically modulated between the U(III)/U(IV) redox states. Based on variable scan-rate cyclic voltammetry, the diffusion coefficients of U(III) and U(IV) were measured. The spectroelectrochemistry of lanthanides and other actinides in chloride melts is being explored for sensor development.

In-situ monitoring of electropolymerization processes at boron-doped diamond electrodes by Mach-Zehnder interferometer

In this work, the Mach-Zehnder interferometer was designed to monitor the electrochemical processes conducted at boron-doped diamond electrode surface. The diamond electrodes were synthesized via Microwave Plasma-Assisted Chemical Vapor Deposition on optical grade quartz glass. The achieved transmittance in working are of diamond electrodes reached 55 %. A cage system-based Mach-Zehnder interferometer was used which allowed the insertion of thin-layer electrochemical cells. Electrochemical studies were carried out in a thin-layer working cell. The application of such setup, allows to combine optical monitoring of surface of the working electrode during electrochemical measurements, electropolymerization or surface modification. The conducted investigation shows that during surface modification by melamine the phase shift is up to 0.0328 μm−1. The aforementioned set up can be applied for in situ monitoring of surface modifications with various compounds, and to detect organic substances whose oxidation or reduction products absorb onto the electrode surface.

Novel Electrochemical Flow Injection Procedure for Albendazole Determination using Boron-doped Diamond thin Film Electrode

The homemade electrochemical cell for electrochemical determination of albendazole using boron-doped diamond (BDD) thin film electrode is developed. Electrochemical activity of albendazole was investigated by cyclic voltammetry in 0.10 M phosphate buffer containing 20% (v/v) methanol at pH 2.5. Using the BDD electrode well-defined and highly reproducible cyclic voltammogram was obtained with current signal-to-background ratio of higher than those obtained from the glassy carbon electrode. Flow injection analysis system with amperometric detection using the homemade thin-layer electrochemical flow-through cell with BDD electrode was studied. The hydrodynamic voltammogram of the drug exhibited single well defined maximum oxidation peak at 0.95 V versus Ag/AgCl. A low detection limit of 2 ppm was obtained. The current–concentration relationship was rectilinear over the concentration range of 3.3‒150 ppm. The proposed method was applied to albendazole tablets with satisfactory results.

Electrochemical simulation of metabolism for antitumor-active imidazoacridinone C-1311 and in silico prediction of drug metabolic reactions

The metabolism of antitumor-active 5-diethylaminoethylamino-8-hydroxyimidazoacridinone (C-1311) has been investigated widely over the last decade but some aspects of molecular mechanisms of its metabolic transformation are still not explained. In the current work, we have reported a direct and rapid analytical tool for better prediction of C-1311 metabolism which is based on electrochemistry (EC) coupled on-line with electrospray ionization mass spectrometry (ESI-MS). Simulation of the oxidative phase I metabolism of the compound was achieved in a simple electrochemical thin-layer cell consisting of three electrodes (ROXY™, Antec Leyden, the Netherlands). We demonstrated that the formation of the products of N-dealkylation reactions can be easily simulated using purely instrumental approach. Newly reported products of oxidative transformations like hydroxylated or oxygenated derivatives become accessible. Structures of the electrochemically generated metabolites were elucidated on the basis of accurate mass ion data and tandem mass spectrometry experiments. In silico prediction of main sites of C-1311 metabolism was performed using MetaSite software. The compound was evaluated for cytochrome P450 1A2-, 3A4-, and 2D6-mediated reactions. The results obtained by EC were also compared and correlated with those of reported earlier for conventional in vitro enzymatic studies in the presence of liver microsomes and in the model peroxidase system. The in vitro experimental approach and the in silico metabolism findings showed a quite good agreement with the data from EC/ESI-MS analysis. Thus, we conclude here that the electrochemical technique provides the promising platform for the simple evaluation of drug metabolism and the reaction mechanism studies, giving first clues to the metabolic transformation of pharmaceuticals in the human body.

Resonance Raman Spectro-Electrochemistry to Illuminate Photo-Induced Molecular Reaction Pathways.

Electron transfer reactions play a key role for artificial solar energy conversion, however, the underlying reaction mechanisms and the interplay with the molecular structure are still poorly understood due to the complexity of the reaction pathways and ultrafast timescales. In order to investigate such light-induced reaction pathways, a new spectroscopic tool has been applied, which combines UV-vis and resonance Raman spectroscopy at multiple excitation wavelengths with electrochemistry in a thin-layer electrochemical cell to study [RuII(tbtpy)2]2+ (tbtpy = tri-tert-butyl-2,2′:6′,2′′-terpyridine) as a model compound for the photo-activated electron donor in structurally related molecular and supramolecular assemblies. The new spectroscopic method substantiates previous suggestions regarding the reduction mechanism of this complex by localizing photo-electrons and identifying structural changes of metastable intermediates along the reaction cascade. This has been realized by monitoring selective enhancement of Raman-active vibrations associated with structural changes upon electronic absorption when tuning the excitation wavelength into new UV-vis absorption bands of intermediate structures. Additional interpretation of shifts in Raman band positions upon reduction with the help of quantum chemical calculations provides a consistent picture of the sequential reduction of the individual terpyridine ligands, i.e., the first reduction results in the monocation [(tbtpy)Ru(tbtpy•)]+, while the second reduction generates [(tbtpy•)Ru(tbtpy•)]0 of triplet multiplicity. Therefore, the combination of this versatile spectro-electrochemical tool allows us to deepen the fundamental understanding of light-induced charge transfer processes in more relevant and complex systems.

Phase I and phase II metabolism simulation of antitumor-active 2-hydroxyacridinone with electrochemistry coupled on-line with mass spectrometry

Here, we report the metabolic profile and the results of associated metabolic studies of 2-hydroxy-acridinone (2-OH-AC), the reference compound for antitumor-active imidazo- and triazoloacridinones.Electrochemistry coupled with mass spectrometry was applied to simulate the general oxidative metabolism of 2-OH-AC for the first time. The reactivity of 2-OH-AC products to biomolecules was also examined. The usefulness of the electrochemistry for studying the reactive drug metabolite trapping (conjugation reactions) was evaluated by the comparison with conventional electrochemical (controlled-potential electrolysis) and enzymatic (microsomal incubation) approaches.2-OH-AC oxidation products were generated in an electrochemical thin-layer cell. Their tentative structures were assigned based on tandem mass spectrometry in combination with accurate mass measurements. Moreover, the electrochemical conversion of 2-OH-AC in the presence of reduced glutathione and/or N-acetylcysteine unveiled the formation of reactive metabolite-nucleophilic trapping agent conjugates (m/z 517 and m/z 373, respectively) through the thiol group. This glutathione S-conjugate was also identified after electrolysis experiment as well as was detected in liver microsomes.Summing up, the present work illustrates that the electrochemical simulation of metabolic reactions successfully supports the results of classical electrochemical and enzymatic studies. Therefore, it can be a useful tool for synthesis of drug metabolites, including reactive metabolites.

Cathodic Kinetics on Stainless Steels in Conditions Relevant to Atmospheric Corrosion

The objective of this work is to develop an understanding of oxygen reduction kinetics on 304 and 316 stainless steels in concentrated electrolytes relevant to saline atmospheric conditions. Atmospheric corrosion in marine environments typically occurs under concentrated electrolyte films of micron-scaled thickness formed by deposited sea salt aerosols. Under actively corroding conditions, the corrosion rate is ultimately controlled by the available current provided by the cathode, which is predominately the result of oxygen reduction on stainless steel.1 The rate of oxygen reduction under thin electrolyte films is often diffusion controlled, and therefore, is dependent on the physicochemical state of the electrolyte, such as film thickness and oxygen solubility. Relative humidity, temperature and salt load are all environmental parameters that control the state of the electrolyte. Therefore, understanding the impact that environmental parameters have in controlling the electrolyte layer chemistry and, in turn, the rate of oxygen reduction is paramount. Relevant atmospheric electrolytes may comprise up to 6M NaCl, 10M NaOH, and 5M MgCl2 and be as thin as a few monolayers of water. Existing work studying oxygen reduction on stainless steels is, for the most part, confined to NaCl or NaOH solutions of low concentrations (<1M) and low to moderate rotation speeds (<3600rpm), corresponding to diffusion layer thicknesses greater than 10μm.2,3,4 It has been shown, within this range of thicknesses, that the oxide film can cause a reduction in the limiting current predicted by Levich.5 It is expected that this effect will be even more prevalent in thinner diffusion layers. In solutions of high concentrations, the precipitation of salts may also hinder cathodic kinetics. In this work, an understanding of the influence temperature, relative humidity and salt load have on the cathodic kinetics will be developed. This will be determined by relating experimental parameters to environmental conditions through knowledge of electrolyte layer chemistry and thermodynamics. Many researchers have studied corrosion kinetics under thin electrolyte films by manufacturing thin layer electrochemical cells in which the electrolyte level over the substrate was controlled within the micron to millimeter range.6 In this work, a rotating disk electrode is used as an alternative. By controlling the rotation speed of the disk, the thickness of the diffusion layer, which is analogous to thin electrolyte layers, is also controlled. The advantages and limitations of the rotating disk system to replicate thin layer atmospheric conditions will be discussed. References Z. Y. Chen and R. G. Kelly, J. Electrochem. Soc., 157, C69–C78 (2010).Le Bozec, N., C. Compere, M. L’her, A. Laouenan, D. Costa, and P. Marcus. Corrosion Science, 43, 765-786 (2001).Babić, Ranko, and Mirjana Metikoš-Huković. Journal of applied electrochemistry 23, 352-357 (1993).Gojković, S. LJ, S. K. Zečević, M. D. Obradović, and D. M. Dražić. Corrosion science 40, 849-860 (1998).C. Liu, J. Srinivasan and R. G. Kelly J. Electrochem. Soc., 164, C845-C855 (2017).Remita, E., E. Sutter, B. Tribollet, F. Ropital, X. Longaygue, C. Taravel-Condat, and N. Desamais. Electrochimica Acta 52, 7715-7723 (2007).

Direct and Mediated Spectro-Electrochemistry of Highly Oxidized Heme Species in Horseradish Peroxidase

Highly oxidized iron species play key role in the catalysis of many heme and non-heme enzymes due to their ability to activate substrates via electron or hydrogen atom transfer. Their high reactivity also makes them unstable and typically transient in nature, which hinders their structural and mechanistic investigation. Furthermore, generation of ferryloxo states typically requires addition of either oxygen and reductants or partially reduced oxygen equivalents, followed by irreversible oxygen activation. This unidirectional reaction effectively excludes investigation of associated structural changes in the protein moiety by such sensitive techniques as infrared spectroelectrochemistry, for example. Here we report on our effort to develop novel approaches toward reversible generation of ferryloxo species in horseradish peroxidase and other heme and non-heme iron proteins. We demonstrate reversible, sequential formation and quenching of Compounds I and II of HRP using several mediators in optically transparent thin layer electrochemical cell and a novel bulk optical transmission electrochemical cell. We also show that pre-conditioning of carbon-based electrodes enables generation of highly oxidized species without the use of mediators or oxygen. Following predictions of kinetically limited pre-equilibrium computational modelling and taking advantage of our chemical-free approach, we investigated the mechanisms of spontaneous quenching of highly oxidized species in HRP, the role of solvent, and the effect of pH, which yielded an unexpected result. General rules and potential pitfalls in electrochemistry of highly oxidized species in metalloenzymes will be outlined. Figure 1

Structural characterization of electrochemically and in vivo generated potential metabolites of selected cardiovascular drugs by EC-UHPLC/ESI-MS using an experimental design approach

In the last few years, a number of studies were conducted which aimed at understanding the mechanisms of cardiovascular drug, metabolism, and there is still the need to determine the metabolites of cardiac drugs for the purpose of metabolism control. In this study, we employ a direct combination of electrochemical oxidation and mass spectrometric (EC-MS) identification for monitoring the oxidation pathway of ten cardiovascular drugs (metoprolol, propranolol, propafenone, mexiletine, oxprenolol, pirbuterol, pindolol, cicloprolol, acebutolol and atenolol). Oxidation was accomplished in an electrochemical thin-layer cell coupled on-line to electrospray ionization mass spectrometry (EC/ESI-MS). For further characterization of electrochemical products, the approach involving liquid chromatography linked to tandem mass spectrometry was used. Appropriate conditions for oxidation and identification processes with such parameters as the potential value, mobile phase (type and pH) and working electrode were optimized. Optimization was performed with the use of central composite design (CCD). Besides electrochemical oxidation of analytes (phase I of metabolic transformation), addition of glutathione (GSH) for follow-up reactions (phase II conjunction) was also investigated. The electrochemical results were compared to in-vivo experiments by analyzing plasma and urine samples from patients who had been administered selected cardiovascular drugs. These results show that electrochemistry coupled to mass spectrometry turned out to be an analytical tool suitable to procure a feasible analytical base for the envisioned in vivo experiments.

Ruthenium Complexes with Strongly Electron-Donating Terpyridine Ligands: Effect of the Working Electrode on Electrochemical and Spectroelectrochemical Properties.

The combination of 2,2':6',2''-terpyridines (tpy) and RuII is known to deliver molecular and supramolecular assemblies with remarkable properties. Here new RuII complexes, with modified tpy ligands substituted with varying numbers of dimethlyamino groups, are presented. Electrochemistry shows that the incorporation of the strongly electron-donating groups on the tpy ligands leads to a negative shift of the RuII oxidation potential by close to 1 V. The reductive electrochemical responses are strongly dependent on the nature of the working electrode, with glassy carbon and gold working electrodes showing the best results. These observations led to the development of a modified Optically Transparent Thin Layer Electrochemical (OTTLE) cell, based on a gold working electrode. The use of UV/Vis/NIR spectroelectrochemical methods with that OTTLE cell, together with simulations of the cyclic voltammograms, allowed the characterization of four reduction steps in these complexes, the final two of which lead to bond activations at the ruthenium center. This observation is to the best of our knowledge unprecedented in coordinatively saturated complexes of type [Ru(tpy)2 ]2+ . The various redox states of the complexes were characterized by EPR spectroelectrochemistry and through DFT calculations. The results presented here establish these substituted tpy ligands as highly attractive ligands in coordination chemistry, and display the utility of a gold-based OTTLE cell for spectroelectrochemical measurements.