Thin-layer Voltammetry Research Articles

Ion–ionophore interactions in polymeric membranes studied by thin layer voltammetry

In ion sensing applications, selective ion-receptor complexation is the molecular basis for endowing the sensing material with selectivity. In this work, thin polymeric membrane-based ion transfer voltammetry is used to investigate ion-receptor complexation, using a range of electrically neutral ionophores and surfactants as examples. Previous studies lacked a convincing approach to eliminate the influence from transducing layer, resulting in deviations of the observed binding constants compared to potentiometric methods. A recently developed method allows for subtracting the potential changes of the transducing layer, thereby overcoming this challenge. Using this approach, a range of ionophores are assessed. Valinomycin for the detection of potassium gave a logarithmic complex formation constant in the membrane of 9.69 ± 0.25 with a 1:1 stoichiometry. Lithium ionophore VI for lithium gave a logarithmic stability constant of 5.97 ± 0.06 with 1:2 complexes; while sodium ionophore X for sodium (7.57 ± 0.03, 1:1) and calcium ionophore IV for calcium (21.57 ± 0.25, 1:3) were also characterized, in addition to their complexes with potential interfering ions. The complex formation of three surfactants with potassium are also explored in membranes containing valinomycin, with Brij-35 (4.88 ± 0.08, 1:1), Triton X-100 (5.63 ± 0.10, 1:1), F-127 (4.63 ± 0.49, 1:1). Limitations of the approach are discussed, which includes the need for electrochemical reversibility and a sufficiently high lipophilicity to adequately retain the components in the membrane

Stochastic Analysis of Electron Transfer and Mass Transport in Confined Solid/Liquid Interfaces

Molecular-level understanding of electrified solid/liquid interfaces has recently been enabled thanks to the development of novel in situ/operando spectroscopic tools. Among those, ambient pressure photoelectron spectroscopy performed in the tender/hard X-ray region and coupled with the “dip and pull” method makes it possible to simultaneously interrogate the chemical composition of the interface and built-in electrical potentials. On the other hand, only thin liquid films (on the order of tens of nanometers at most) can be investigated, since the photo-emitted electrons must travel through the electrolyte layer to reach the photoelectron analyzer. Due to the challenging control and stability of nm-thick liquid films, a detailed experimental electrochemical investigation of such thin electrolyte layers is still lacking. This work therefore aims at characterizing the electrochemical behavior of solid/liquid interfaces when confined in nanometer-sized regions using a stochastic simulation approach. The investigation was performed by modeling (i) the electron transfer between a solid surface and a one-electron redox couple and (ii) its diffusion in solution. Our findings show that the well-known thin-layer voltammetry theory elaborated by Hubbard can be successfully applied to describe the voltammetric behavior of such nanometer-sized interfaces. We also provide an estimation of the current densities developed in these confined interfaces, resulting in values on the order of few hundreds of nA·cm−2. We believe that our results can contribute to the comprehension of the physical/chemical properties of nano-interfaces, thereby aiding to a better understanding of the capabilities and limitations of the “dip and pull” method.

Electrocatalysis of ferricyanide reduction mediated by electron transfer through the DNA duplex: Kinetic analysis by thin layer voltammetry

DNA-mediated electron transfer (ET) underlies a variety of bioelectronics applications, and, thus, simple approaches for its kinetic analysis are required. Here, electrocatalysis of a ferricyanide reduction catalyzed by methylene blue (MB) bound to either (dGdC)20 or (dAdT)25 duplexes tethered to gold via an alkanethiol linker was voltammetrically studied at stationary electrodes, and its kinetics was analyzed by two theoretical models. A thin layer voltammetry analysis, assuming that the reaction occurs within the l-nm thick layer formed by the DNA duplexes of the length l, yielded the electrocatalytic rate constant k1 of (2.1 ± 0.4)105 and (2.4 ± 0.5)106 M−1 s−1, for (dGdC)20 and (dAdT)25. The sequence-specific k1 values were consistent with structural differences of (dGdC)20 or (dAdT)25 at the negatively polarized interface and approached the k1 previously reported for the rotating disk electrode system (Angew. Chemie Int. Ed. 58 (2019) 3048). These results offer new tools for immediate kinetic analysis of electrocatalytic reactions involving long-range ET along the DNA duplexes, the tools that allow differentiating electrical properties of nucleic acids in the most convenient and simple way.

SU24NEW COMMON GENETIC VARIANTS ASSOCIATED WITH CHRONIC PAIN: CONDITIONAL FALSE DISCOVERY RATE ANALYSIS WITH MAJOR DEPRESSIVE DISORDER

We report here on paper-supported thin sample layer voltammetry for the determination of ions. To achieve this goal, a simple setup for the coupling of a commercially available electrode to a silver rod electrode was designed and evaluated for paper-supported thin-layer voltammetry. Linear scan ion transfer voltammetry was explored here for ion-selective membranes doped with an ionophore. The ion-transfer processes and electrochemical behaviors of the system are here evaluated and confirmed by numerical simulation. In the proof-of-concept experiments described, the ions tetrabutylammonium chloride (TBA+) and potassium (K+) were studied as model analytes at membranes without and with ionophore, respectively. A linear relationship from 0.1 mM to 1.0 mM K+ was obtained between the charge and ion concentration. The coexistence of background sodium ions did not give appreciable interference, but the background wave was not completely isolated from the analyte wave, as also confirmed by the model. The methodology was successfully demonstrated for determination of K+ in mineral water. It is anticipated that this paper-supported thin-layer detection approach may provide an attractive readout protocol for disposable paper-based analytical devices as the methodology does not place strict demands on reference electrode performance.

Paper-supported thin-layer ion transfer voltammetry for ion detection

We report here on paper-supported thin sample layer voltammetry for the determination of ions. To achieve this goal, a simple setup for the coupling of a commercially available electrode to a silver rod electrode was designed and evaluated for paper-supported thin-layer voltammetry. Linear scan ion transfer voltammetry was explored here for ion-selective membranes doped with an ionophore. The ion-transfer processes and electrochemical behaviors of the system are here evaluated and confirmed by numerical simulation. In the proof-of-concept experiments described, the ions tetrabutylammonium chloride (TBA+) and potassium (K+) were studied as model analytes at membranes without and with ionophore, respectively. A linear relationship from 0.1 mM to 1.0 mM K+ was obtained between the charge and ion concentration. The coexistence of background sodium ions did not give appreciable interference, but the background wave was not completely isolated from the analyte wave, as also confirmed by the model. The methodology was successfully demonstrated for determination of K+ in mineral water. It is anticipated that this paper-supported thin-layer detection approach may provide an attractive readout protocol for disposable paper-based analytical devices as the methodology does not place strict demands on reference electrode performance.

Thin-layer voltammetry of soluble species on screen-printed electrodes: proof of concept.

Thin-layer diffusion conditions were accomplished on screen-printed electrodes by placing a controlled-weight onto the cast solution and allowing for its natural spreading. The restricted diffusive conditions were assessed by cyclic voltammetry at low voltage scan rates and electrochemical impedance spectroscopy. The relationship between the weight exerted over the drop and the thin-layer thickness achieved was determined, in such a way that the simple experimental set-up designed for this work could be developed into a commercial device with variable control of the thin-layer conditions. The experimental results obtained resemble those reported for the voltammetric features of electroactive soluble species employing electrodes modified with carbon nanotubes or graphene layers, suggesting that the attainment of the benefits reported for these nanomaterials could be done simply by forcing the solution to spread over the screen-printed electrodic system to form a thin layer solution. The advantages of thin-layer voltammetry in the kinetic characterization of quasi-reversible and irreversible processes are highlighted.

Redox Spectra Of Wines

AbstractThe method of thin layer voltammetry on pyrolytic graphite electrodes was applied for analysis of Crimean wines on the content of electrochemically active components. The cyclic voltammetric curves were resolved into a set of peaks corresponding to the electrochemical conversion of quinones and phenolics. The final results of such analysis are represented as diagrams, which we call the “Redox Spectra of Wines”. The Redox Spectra of Wines were found to be their specific characteristics. The method may be regarded as promising for the evaluation of authenticity of wines.

Analysis of the diffusion layer thickness, equivalent circuit and conductance behaviour for reversible electron transfer processes in linear sweep voltammetry

The parametric dependence of the diffusion layer thickness upon electrode potential and scan rate has been derived for reversible electron transfer processes in linear sweep voltammetry (LSV) using a simple analytical expression for the dimensionless current function. The quantitative variation of the conductance with potential and scan rate has been illustrated and the equivalent circuit pertaining to LSV has been proposed. Several earlier results pertaining to potential step experiments, thin layer voltammetry, surface bound LSV, radial flow at micro-ring electrodes, etc. are shown to arise from the general expression for the current function obtained here, thus indicating an isomorphism among various potential perturbation techniques.

Optical and electrochemical properties of optically transparent, boron-doped diamond thin films deposited on quartz.

The optical and electrochemical properties of transparent, boron-doped diamond thin film, deposited on quartz, are discussed. The films were deposited by microwave-assisted chemical vapor deposition, for 1-2 h, using a 0.5% CH4/H2 source gas mixture at 45 Torr and 600 W of power. A high rate of diamond nucleation was achieved by mechanically scratching the quartz. This pretreatment leads to the formation of a continuous film, in a short period of time, which consists of nanometer-sized grains of diamond. The thin-film electrode was characterized by cyclic voltammetry, atomic force microscopy, and UV-visible absorption spectrophotometry. The film's electrochemical response was evaluated using Ru(NH3)6(3+/2+) in 1 M KCl, Fe(CN)6(3-/4-) in 1 M KCl, and chlorpromazine (CPZ) in 10 mM HClO4. The film exhibited a low voltammetric background current and a stable and active voltammetric response for all three redox systems. The optical transparency of the polycrystalline film in the visible region was near 50% and fairly constant between 300 and 800 nm. The optical and electrical properties were extremely stable during 48-h exposure tests in various aqueous (HNO3, NaOH) solutions and nonaqueous (e.g., chlorinated) solvents. The properties were also extremely stable during anodic and cathodic potential cycling in harsh aqueous environments. This stability is in stark contrast to what was observed for an indium-doped tin oxide thin film coated on quartz. The spectroelectrochemical response (transmission mode) for CPZ was studied in detail, using a thin-layer spectroelectrochemical cell. Thin-layer voltammetry, potential step/ absorption measurements, and detection analytical figures of merit are presented. The results demonstrate that durable, stable, and optically transparent diamond thin films, with low electrical resistivity (approximately 0.026 omega x cm) laterally through the film, can be deposited on quartz.

Redox properties of naturally occurring naphthoquinones: Vitamin K2(20) and lapachol

Redox properties of two naturally occurring naphthoquinones, vitamin K2(20) and lapochol, are examined by thin-layer voltammetry on a pyrolytic graphite electrode. The pH-potential diagrams of these compounds are plotted. In aqueous solutions, the quinones are shown to exist each in five forms. Thermodynamic parameters for all redox and acid-base equilibria between conjugated forms are determined. In contrast to lapachol, vitamin K2(20) forms, during its reduction, a large amount of stable semiquinone in a wide pH interval

Electrochemical investigations of mono- and multilayers of phenoxazine dyes on graphite electrodes

For the determination of heterogeneous charge transfer rate constants kct°, the working curve data from peak-to-peak splitting referring to surface and thin layer voltammetry given by Laviron were supplemented by an explicit equation, where the electron number n is replaced by the "‘surface redox valency’' n′. The pH dependent peak potentials of Pyocyanine and Coelestin Blue were measured at decreasing surface concentrations and extrapolated to zero coverage. The plateau behaviour of the catalytical steady-state currents with increasing numbers of multilayers of β-Naphthoyl Nile Blue at graphite electrodes immersed in NADH solution suggest good plane-to-plane electronic coupling.

Production and Identification of N-Glucosylrubropunctamine and N-Glucosylmonascorubramine from Monascus ruber and Occurrence of Electron Donor-Acceptor Complexes in These Red Pigments

The filamentous fungus Monascus ruber produces water-soluble red pigments in a submerged culture when grown in a chemically defined medium containing glucose as a carbon source and monosodium glutamate as a nitrogen source. Two new molecules with polyketide structures, N-glucosylrubropunctamine and N-glucosylmonascorubramine, constituting under some conditions 10% of the total extracellular coloring matter when glucose as a carbon source was in excess (20 g/liter), were isolated and structurally characterized by high-pressure liquid chromatography, Dionex methods, (sup1)H and (sup13)C nuclear magnetic resonance spectroscopy, and mass spectrometry. The occurrence of the electron donor-acceptor complex effect was demonstrated by UV spectroscopy, polarography, and thin-layer voltammetry. The use of n-butanol as an extraction solvent stabilized the pigments against the effects of daylight for several months, promoting the stability of this type of complex.

A Mediated Thin-Layer Voltammetry Method for the Study of Redox Protein Electrochemistry

A novel mediated thin-layer voltammetry technique that allows the rapid determination of midpoint potentials and electron transfer rate constants for small quantities of redox active proteins is described. Thin-layer voltammograms simulated for an electrolyte containing a redox active protein and an electron transfer mediator show that the rapid homogeneous electron exchange reaction between the protein and the mediator serves to mediate the charge transfer of the protein at the electrode, which does not take place in the absence of the mediator, and results in the observation of an apparently reversible redox couple. Both theoretical and experimental data are presented which suggest that the thin-layer voltammetry method will be generally applicable for the determination of protein redox potentials with the proper selection of mediators. Rate constants for the electron transfer between metalloproteins and mediators can be evaluated by comparing experimental voltammograms with theoretical data from simulations. The technique is demonstrated for the metalloproteins cytochrome c, ferredoxin, and the iron protein of nitrogenase.

Electrochemical and spectroscopic characterization of anodically formed nickel salen polymer films on glassy carbon, platinum, and optically transparent tin oxide electrodes in acetonitrile containing tetramethylammonium tetrafluoroborate

Anodically polymerized films of nickel salen formed on glassy carbon, optically transparent tin oxide, and platinum electrodes in acetonitrile containing tetramethylammonium tetrafluoroborate have been examined by means of cyclic voltammetry, thin-layer voltammetry, spectroelectrochemistry, and scanning electron microscopy. With the aid of thin-layer voltammetry, it has been confirmed that the global oxidative polymerization of nickel(II) salen involves three electrons per monomer. Polymerization proceeds through two distinct phases, the formation of which depend on the potential. Once the polymer film has been formed, the anodic process consists of the reversible one-electron nickel(III)/nickel(II) redox couple. Cyclic voltammetry along with spectroelectrochemistry has been employed to probe the roles of the nickel(III)/nickel(II) and nickel(II)/nickel(I) redox couples in the electrochemical response of the polymer film as well as the interconversion of the different oxidation states of nickel.

Redox properties of echinochrome A

Abstract The electrochemical properties of polyhydroxynaphthoquinone from the shells of sea-urchins Strongylocentrotus intermedius (echinochrome A) and related compounds were investigated using thin layer voltammetry on pyrolytic graphite electrodes. On the basis of experimental data pH-potential diagrams for echinochrome A and its trimethyl ether are plotted. Five different forms of echinochrome A and six forms of the trimethyl ether of echinochrome A are shown to exist over the whole range of pH studied. Equilibrium constants and formal redox potentials for all conjugated forms are obtained. Electrochemical oxidation of echinochrome A and its trimethyl ether lead to the formation of different products. Tetraketotetralin is formed in the former case and naphthodiquinone in the latter.

Electrochemical properties of some naturally occurring quinones

The electrochemical properties of some hydroxy-1,4-naphthoquinones of plant origin, namely juglon, lawson and plumbagin, have been investigated using thin layer voltammetry on pyrolytic graphite electrodes. The pH-potential diagrams for all three quinones are plotted on the basis of the experimental data. Five different forms of lawson and plumbagin and four forms of juglon are shown to exist in the whole range of pH. Equilibrium constants and formal redox potentials for all conjugated forms are obtained.

Probing the surface electrochemical properties of bimetallic alloys by chemisorption of redox-active species: Iodine at smooth Au 90Pt 10

The interfacial electrochemical properties of a smooth polycrystalline Au + Pt alloy consisting of 90% Au and 10% Pt (Au 90Pt 10) were investigated by comparison of the redox-activated adsorption-desorption behavior of iodine at the bimetallic surface with that at pure Au and Pt. This approach is possible since the electrochemical properties of iodine absorbed on pure Au are different from those of iodine chemisorbed on pure Pt. Experimental measurements were based upon thin-layer voltammetry and coulometry. The primary findings are: (i) In terms of its influence on the electrochemical properties of chemisorbed iodine, the Au 90Pt 10 electrode behaves virtually identically to pure Au. (ii) The hydrogen evolution reaction, which is retarded at Au, is enhanced at the bimetallic electrode whether iodine is chemisorbed or not. (iii) Oxidized Pt atoms at the alloy surface facilitate the reduction of unadsorbed IO − 3 to I 2 relative to that at Au. (iv) The surface of the Au 90Pt 10 electrode is homogeneous; that is, domains of aggregated Pt and Au atoms do not exist.

Thin-layer steady-state voltammetry under control by diffusion and migration

Measurements of steady-state currents in thin-layer cells provide an opportunity to apply a stringent test of theories of electrochemical transport by simultaneous diffusion and migration. We have applied such a test and find the theory to be vindicated.

Oxidative voltammetry of some tetraphenylporphyrins in the presence of nucleophiles leading to isoporphyrins

The oxidation of tetraphenylporphinatozinc has been examined in dichloroethane or dichloromethane solutions in the presence of nitrate, trifluoroacetate, or methanol by dc, ac, and thin layer voltammetry, and by thin layer visible and FTIR spectroelectrochemistry. Data is also presented for oxidation of tetraphenylporphine in dichloroethane saturated with water, and of chloro(tetraphenylporphinato)manganese(III) in dry dichloroethane. In all of these cases, isoporphyrin formation is indicated after the removal of two electrons from the porphyrin ring. The isoporphyrin undergoes one-electron oxidation at more positive potentials, and can be reduced with two electrons back to starting material. Two competing mechanisms are indicated for the isoporphyrin reduction.

Versatile thin layer spectroelectrochemical cell employing specular reflectance

A versatile thin layer spectroelectrochemical cell employing specular reflection of the incident light beam from the electrode surface is described. Its application to in-situ uv–vis and FTIR characterization of the products of electrochemical reactions and to thin layer voltammetry and coulometry as well as conventional cyclic voltammetry is demonstrated for the oxidation of tetraphenylporphinatozinc in dichloroethane/tetrabutylammonium perchlorate solution. The advantages and disadvantages of this type of cell as compared to more conventional sandwich type optically transparent thin layer electrodes are discussed.