Conditions Of Square-wave Voltammetry Research Articles

Dopamine modified electrodes for indirect voltammetric determination of magnesium ions

This article outlines the fabrication process of an electrochemical sensor designed for the innovative determination of magnesium ions based on the electrochemistry of dopamine. The sensor operates under voltammetric conditions, accounting for variations in the electrochemical reversibility of immobilized dopamine in the presence of magnesium ions under conditions of square-wave voltammetry. The immobilization of dopamine on the glassy carbon electrode is achieved through the electrochemical oxidation of its side amino group, leading to covalent grafting onto the electrode surface. All stages of the proposed electrode surface modification procedure were carefully optimized. The dopamine sensor exhibited a linear response in the concentration range of magnesium ions from 0.1 to 10 mmol L−1, with a limit of detection (LOD) value equal to 1.3 × 10−5 mol L−1. To validate the electroanalytical significance of the developed methodology, real food supplement samples were quantitatively analyzed, demonstrating a highly satisfactory rate of recovery. The proposed voltammetric method serves as a simple and cost-effective procedure for the indirect determination of magnesium ions. Additionally, this approach allows for the analysis of real samples without the need for time-consuming preparation steps, as the complex matrices of food supplement samples did not adversely affect the registered currents.

The net peak splitting phenomenon in square-wave voltammetry – A simple diagnostic tool to distinguish between surface electrode mechanisms associated with different chemical reactions

Utilizing pulse voltammetric techniques for the electrochemical analysis of lipophilic redox systems has proven to be an effective method for studying a diverse range of redox compounds, from simple molecules to intricate proteins. To extract relevant thermodynamic and kinetic data from electrochemical analysis of redox systems exhibiting strong surface activity, there's a crucial need to decipher the underlying electrochemical mechanism in the system being examined. The "split net peak" phenomenon, a defining characteristic observed in all surface-active redox systems featuring fast electron transfer under square-wave voltammetry conditions, has been investigated in this study to establish diagnostic criteria for identifying surface electrochemical mechanisms associated with preceding, subsequent, regenerative, and reactant-inactivating chemical reactions. This understanding can be achieved by tracking the influence of the chemical reaction rate on the split square-wave voltammetric peaks in a particular electrochemical mechanism. The approach reported in the current work enables a very simple and precise differentiation between the elaborated mechanisms frequently encountered in protein-film voltammetry methodologies.

Distinguishing Heterogeneous and Homogeneous CE Mechanisms: Theoretical Insights into Square-Wave Voltammetry

A theoretical study of an electrode mechanism where the electrode reaction (E) is preceded by a chemical reaction that takes place solely at the electrode surface (Chet) is presented under conditions of square-wave voltammetry (SWV). Rigorous mathematical solutions in the form of integral equations derived by means of Laplace transforms are presented for the surface concentration of all species involved in the electrode mechanism, yielding explicit recurrent formulas for the simulation of the voltammetric response. The theory approximately predicts that the chemical reaction starts at the beginning of the voltammetric experiment, disregarding its occurrence in the short time period between inserting the electrode in the solution until starting the voltammetric experiment. It is demonstrated that SWV can differentiate between the common CE mechanism, where C is a homogeneous chemical reaction taking place in the vicinity of the electrode, and the current ChetE mechanism, which is relevant for plethora of electrocatalytic processes at electrodes modified with catalytically active enzymes and/or noble-metal nanoparticles, as well as for electrocatalytic processes of fundamental importance such as CO2, N2, O2, and H+ reductions.

Development of a multianalyte electrochemical sensor for depression biomarkers based on a waste of the steel industry for a sustainable and one-step electrode modification

This study aimed to reuse waste from the steel industry as a sustainable electrode modifier material in a simple and one-step modification approach for the electrochemical monitoring of the neurotransmitters dopamine (DA), serotonin (5-HT), and tryptophan (TRP), biomarkers related to depression diagnostics. Iron oxides constitute the mill scale (MS), composed of wüstite (FeO), magnetite (Fe3O4), and hematite (Fe2O3). This material was generated from the hot rolling process, and later it was ball milled using a high-energy shaker mill. X-ray powder diffraction (XRPD) was used to investigate the structural and microstructural evolution of the MS with milling time. Effects of mechanical force on solids include a reduction in the crystalline size by controlling the milling time, with consequently peculiar properties and chemical reactivity. The magnetic measurements show superparamagnetic characteristics in the samples, governed mainly by small crystallites of iron oxides. Two samples (milled for 2 and 30 min) were applied in the construction of new sensors. The MS carbon paste electrode (MS/CPE) with 30 min-sample allowed more defined oxidation peaks through a simple and one-step electrode modification step. Selectivity for the detection of the multianalytes was achieved after the modification of the carbonaceous substrate with the mixture of metal oxides with magnetic properties; overlapping peaks were observed for DA and 5-HT using the unmodified electrode. Under optimum conditions of square wave voltammetry, a linear relationship was observed between the peak current intensities and the concentrations of the analytes. Linear responses were obtained for DA, 5-HT, and TRP in the 0.49–21, 0.49–13.6, and 2.9–25 µmol L−1 concentration ranges, with detection limits of 0.21, 0.13, and 2.18 µmol L−1, respectively. The reuse of waste as a sustainable electrode modifier material for clinical and biomedical applications, with improved selectivity and sensibility, is hereby present.

Hybrid chitosan-coated manganese ferrite nanoparticles for electrochemical sensing of bifenox herbicide

A novel glassy carbon electrode (GCE) modified with hybrid chitosan-coated manganese ferrite nanoparticles (MnFe2O4@CTS) was applied for the electrochemical determination of bifenox, which is a nitrodiphenyl ether herbicide. The core-shell MnFe2O4@CTS nanoparticles were synthesized via ionic gelation and characterized by different techniques including transmission electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, vibrating sample magnetometry, and thermogravimetric analysis. The modification of the GCE was performed by drop-casting method using MnFe2O4@CTS, previously dispersed by ultrasound. For the first time, the synthesized material was tested in the electroanalysis of environmental pollutants. The innovative modified MnFe2O4@CTS/GCE provided a significant increase of peak currents in the presence of bifenox. Electrochemical and interfacial characteristics of the electrodes were assessed by cyclic voltammetry and electrochemical impedance spectroscopy. Under optimized conditions of square wave voltammetry and supporting electrolyte, it was possible to observe a linear dependence between the anodic peak current and the concentration of bifenox in the range from 0.3 to 4.4 μmol L−1. The limit of detection was estimated as 0.09 µmol L−1. The selectivity of the sensor was verified in the presence of other pesticides and the river water matrix; the interference response was found to be lower than 5%. The sensor showed excellent intra-day and inter-day repeatability presenting a relative standard deviation of 3.99% and 5.79%, respectively. Electrochemical determination of bifenox was performed in river and tap water samples. The recovery values were in the range of 82–97%. The novel analytical platform MnFe2O4@CTS/GCE presented promising features for application in environmental analysis of bifenox.

Analysis of enzyme-substrate interactions from square-wave protein-film voltammetry of complex electrochemical-catalytic mechanism associated with reversible regenerative reaction

The surface catalytic EC′ mechanism is considered as one of the most relevant models to describe electrochemical behavior of many enzyme-substrate systems under voltammetric conditions. In this work, we elaborate theoretically a specific surface catalytic EC′ mechanism that features reversible regenerative reaction under conditions of square-wave voltammetry. The features of calculated voltammograms strongly depend on the parameters related to the thermodynamics and kinetics of regenerative chemical step involved in this mechanism. We present a series of simulated square-wave voltammograms that can be used to establish diagnostic criteria for recognizing surface EC′rev mechanism. In addition, we propose simple methodologies that can be explored to evaluate the kinetics and thermodynamics parameters relevant to chemical catalytic reaction. In a comprehensive study, we elaborate the effect of parameters of regenerative reaction to the features of phenomena known as “quasireversible maximum” and “split net peaks of square-wave voltammograms”. The results reported in this work can be of significance to many experimentalists working in the field of enzymatic voltammetry and drug-drug interactions.

Electrode mechanisms with coupled chemical reaction — Amplitude effect in square-wave voltammetry

This article presents the theoretical prediction of square-wave amplitude effect on electrode reaction mechanisms coupled with chemical reaction under conditions of square-wave voltammetry. A systematic change in simulated voltammograms occurs following a change in the applied square-wave amplitude parameter. The obtained results help to assess the generated impact on the peak potential separation of square-wave forward and backward components from conventional and potential corrected voltammograms as well as on the so-called ‘amplitude-based quasireversible maximum’ acquired during the analysis of the net response peak height. In most cases, these features are independent of the parameters of coupled chemical reaction, and electrode kinetics can be estimated without their prior knowledge. Some characteristic criteria can be deduced from the observed trends, which can serve as a means to elucidate electron transfer kinetics. Under these conditions, even nonexperts in the square-wave voltammetry technique can characterize the studied experimental systems.

Square-wave protein-film voltammetry: new insights in the enzymatic electrode processes coupled with chemical reactions

Redox mechanisms in which the redox transformation is coupled to other chemical reactions are of significant interest since they are regarded as relevant models for many physiological systems. Protein-film voltammetry, based on surface confined electrochemical processes, is a methodology of exceptional importance, which is designed to provide information on enzyme redox chemistry. In this work, we address some theoretical aspects of surface confined electrode mechanisms under conditions of square-wave voltammetry (SWV). Attention is paid to a collection of specific voltammetric features of a surface electrode reaction coupled with a follow-up (ECrev), preceding (CrevE) and regenerative (EC’) chemical reaction. While presenting a collection of numerically calculated square-wave voltammograms, several intriguing and simple features enabling kinetic characterization of studied mechanisms in time-independent experiments (i.e., voltammetric experiments at a constant scan rate) are addressed. The aim of the work is to help in designing a suitable experimental set-up for studying surface electrode processes, as well as to provide a means for determination of kinetic and/or thermodynamic parameters of both electrode and chemical steps.

Electrode kinetics from a single square-wave voltammogram

A novel methodological approach for processing electrochemical data measured under conditions of square-wave voltammetry (SWV) is proposed. The methodology takes advantage of the chronoamperometric characteristics of the electrochemical system observed in the course of the voltammetric experiment, which are commonly not considered in conventional SWV. The method requires processing of a single voltammogram only, measured under given set of the potential modulation parameters (i.e. SW frequency and height of the potential pulses). The core idea is to analyze the variation of the current with time, considering the last quarter time period of each potential pulse of the potential modulation, instead of measuring the single current value from the very end of the pulses. Hence, a single point current measurement is replaced with a multisampling current procedure, leading to a series of new SW voltammograms measured at different time increments of each potential pulse. The novel method is illustrated by the theoretical analysis of a quasireversible electrode reaction of a dissolved and surface confined redox couple, as well as a reversible electrode reaction preceded by a chemical equilibrium reaction (C<sub>r</sub>E<sub>r</sub> mechanism).

Electrode kinetic measurements with square-wave voltammetry at a constant scan rate

A simple methodology for kinetic measurements of electrode processes is proposed under conditions of square-wave voltammetry (swv) at a constant scan rate of the voltammetric experiment. The method requires measurement of the peak potential separation between the forward and backward components of the voltammetric response by varying the sw amplitude, i.e., the height of the potential pulses. The methodology is equally valid for electrode reactions of an immobilized and dissolved redox couple. The estimation of the standard rate constant can be carried out either by direct fitting between the simulated and experimentally measured data, or by using the working curves derived by simulations. The proposed methodology was confirmed by kinetic measurements of the electrode reactions of azobenzene, immobilized on mercury electrode, and dissolved hexacyanoferrate at glassy carbon electrode.

New Approach to Electrode Kinetic Measurements in Square-Wave Voltammetry: Amplitude-Based Quasireversible Maximum

The influence of the potential pulse height of square-wave voltammetry (SWV) (i.e., the SW amplitude) is studied for a variety of quasireversible electrode mechanisms, including a simple solution-phase electrode reaction at a planar or spherical electrode, a solution phase electrode reaction coupled with a reversible follow-up chemical reaction, and a diffusionless surface confined electrode reaction. The electrode kinetics of all the electrode mechanisms depends critically on the SW amplitude, and the quasireversible kinetic region is a function of both frequency-related electrode kinetic parameters and the SW amplitude. Thus, a novel methodology for electrode kinetics measurements is proposed by altering the SW amplitude only, at a fixed frequency of the SW potential modulation, that is, at a constant scan rate of the voltammetric experiment. Electrode kinetic measurements at a constant SW frequency are of exceptional importance especially when complex electrode mechanisms are studied, which depend on several frequency-related kinetic parameters. The electrode kinetic measurements are based on a novel feature termed the "amplitude-based quasireversible maximum", manifested as a parabolic dependence of the amplitude-normalized net SW peak current versus the SW amplitude. The position of the amplitude-based quasireversible maximum depends on the standard rate constant of the electrode reaction, enabling estimation of this important kinetic parameter in a simple and fast procedure. The novel quasireversible maximum is attributed to all studied electrode mechanisms, implying that it is a general feature of most electrode mechanisms under conditions of SWV.

Assisted Ion Transfer at Organic Film-Modified Electrodes

An experimental and theoretical study of a complex electrochemical mechanism at three-phase and thin organic film-modified electrodes, where the coupled electron–ion transfer reaction is complicated by complexation reaction of the transferring ion, is reported. The transfer of monovalent and divalent cations across water|nitrobenzene interface, coupled with the complexation reactions with the ionophore valinomycin, is studied. Both types of electrodes are assembled of an edge plane pyrolytic graphite electrode modified with a nitrobenzene solution of lutetium bis(tetra-tert-butylphthalocyaninato) as a redox mediator and valinomycin as an ionophore. The reversible redox transformations of the redox mediator to either a monovalent hydrophobic anion or cation serve to drive the ion transfer across the liquid|liquid interface. In contact of the modified electrode with an aqueous electrolyte containing alkali or earth alkaline metal cations, significant partition of the aqueous electrolyte is taking place, due to the interfacial complexation of the cation with valinomycin. Thus, the thermodynamics and kinetics of the interfacial complexation–partition reaction at the liquid|liquid interface affect markedly the overall electron–ion transfer reaction at the modified electrodes under voltammetric conditions. Experiments are qualitatively compared with theoretical data collected by simulation of two different electrochemical mechanisms coupled with chemical reactions under conditions of square-wave voltammetry. It has been concluded that the overall electrochemical mechanism at three-phase electrodes can be described as a specific CrE reaction scheme, where Cr represents the reversible interfacial complexation–partition reaction of the transferring ion with valinomycin at the liquid|liquid interface.

Diagnostics of Anodic Stripping Mechanisms under Square-Wave Voltammetry Conditions Using Bismuth Film Substrates

A mechanistic study to provide diagnostics of anodic stripping electrode processes at bismuth-film electrodes is presented from both theoretical and experimental points of view. Theoretical models for three types of electrode mechanisms are developed under conditions of square-wave voltammetry, combining rigorous modeling based on integral equations and the step function method, resulting in derivation of a single numerical recurrent formula to predict the outcome of the voltammetric experiment. In the course of the deposition step, it has been assumed that a uniform film of the metal analyte is formed on the bismuth substrate, in situ deposited onto a glassy carbon electrode surface, without considering mass transfer within either the bismuth or the metal analyte film. Theoretical data are analyzed in terms of dimensionless critical parameters related with electrode kinetics, mass transfer, adsorption equilibria, and possible lateral interactions within the deposited metal particles. Theoretical analysis enables definition of simple criteria for differentiation and characterization of electrode processes. Comparing theoretical and experimental data, anodic stripping processes of zinc(II), cadmium(II), and lead(II) are successfully characterized, revealing significant differences in their reaction pathways. The proposed easy-to-perform diagnostic route is considered to be of a general use while the bismuth film exploited in this study served as a convenient nonmercury model substrate surface.

Protein film voltammetry: electrochemical enzymatic spectroscopy. A review on recent progress

This review is focused on the basic principles, the main applications, and the theoretical models developed for various redox mechanisms in protein film voltammetry, with a special emphasis to square-wave voltammetry as a working technique. Special attention is paid to the thermodynamic and kinetic parameters of relevant enzymes studied in the last decade at various modified electrodes, and their use as a platform for the detection of reactive oxygen species is also discussed. A set of recurrent formulas for simulations of different redox mechanisms of lipophilic enzymes is supplied together with representative simulated voltammograms that illustrate the most relevant voltammetric features of proteins studied under conditions of square-wave voltammetry.

Theoretical Treatment of a Cathodic Stripping Mechanism of an Insoluble Salt Coupled with a Chemical Reaction in Conditions of Square Wave Voltammetry. Application to 6‐Mercaptopurine‐9‐D‐Riboside in the Presence of Ni(II)

AbstractCathodic stripping mechanism of an insoluble salt coupled with a homogenous chemical reaction is considered both theoretically and experimentally under conditions of square‐wave voltammetry. For the mercury electrode in aqueous solution, the electrode reaction is described as L(aq)+Hg(l)=HgL(s)+2e−, where L(aq) is the reactive ligand that forms a sparingly soluble compound HgL(s). The electrode reaction is coupled with a homogenous, first‐order chemical reaction, A(aq)=L(aq). Theoretical predictions are confirmed by experiments with 6‐mercaptopurine‐9‐D‐riboside in the presence of nickel(II) ions.

Catalytic mechanism in successive two-step protein-film voltammetry—Theoretical study in square-wave voltammetry

Protein-film voltammetry is established as an effective tool that provides insight to the redox features of various lipophilic proteins by using a simple methodology. Although the protein-film experimental set up is relatively simple, the redox mechanisms of many proteins are quite complicated, and very often they cannot be resolved without having support from adequate mathematical models. In this work we continue our contribution to modeling relevant redox mechanisms in protein-film voltammetry. We present results from the theoretical simulations of catalytic mechanism at the two-step successive surface redox reaction under conditions of square-wave voltammetry. This mechanism is assigned as a surface EEC′, and it can be presented by the following simplified scheme: A(ads) + ne− ⇄ B(ads) + ne− ⇄ C(ads) + Substrate → k cat B(ads). Our attention is focused on several phenomena of this complex protein-film mechanism, while we give set of qualitative criteria to distinguish this mechanism from similar ones studied under voltammetric conditions. Moreover, we also provide hints to use methodologies for the determination of thermodynamic and kinetic parameters relevant to the protein-film EEC′ mechanism. The considered protein-film EEC′ mechanism is applicable to all lipophilic redox proteins that undergo electrochemical transformations in more than one successive electron steps. Such examples exist by proteins containing quinone moiety and some polyvalent ions of transition metals as redox active sites.

Electrocatalysis of the first and second kind: Theoretical and experimental study in conditions of square-wave voltammetry

A comparative analysis of two common electrocatalytic mechanisms in conditions of square-wave voltammetry (SWV) is presented from both theoretical and experimental points of view. The catalytic mechanism of the first kind is represented by a simple regenerative EC′ reaction scheme, where C′ is a follow-up irreversible chemical reaction through which the substrate is transformed and the catalyst regenerated. The second mechanism, known as electrocatalysis of the second kind, assumes the formation of an electroactive complex YS between the catalyst Y and the electroinactive substrate S in a chemically reversible reaction, followed by an irreversible electrochemical reaction of YS that yields the catalyst Y and the final electroinactive product P. The latter mechanism is mathematically modelled based on the semi-infinite diffusion model, deriving a general solution in the form of an integral equation that may be used for any voltammetric technique. A specific recurrent formula is derived based on the step-function method, enabling the simulation of SW voltammetric experiments with a wide range of experimental parameters. The properties of the response are thoroughly analyzed in terms of the kinetics and thermodynamics of the preceding chemical reaction, as well as the kinetics of the electrode reaction. The comparative analysis of the two reaction schemes reveals that both mechanisms are rather similar, exhibiting comparable sensitivity toward the kinetics of the catalytic reaction. Theoretical analysis is experimentally illustrated with the electrocatalytic reduction of bromate and nitrite ions catalyzed by Fe(II) (catalysis of the first kind) and Co(II) ions (catalysis of the second kind), respectively.

Electrochemical studies of ganciclovir as the adsorbed catalyst on mercury electrode

Although ganciclovir (gan) as a purine analogue is a compound of biological interest (antiviral drug), it has been rarely electrochemically studied. In this paper surface catalytic electrode mechanism based on the hydrogen evolution reaction is analyzed under conditions of square-wave voltammetry and differential capacity curves of double layer measurements. The electrode mechanism is assumed to involve a preceding chemical reaction in which the adsorbed catalyst (ganads) is protonated at the electrode surface, i.e., ganads + H+aq → ganH+ads. The protonated form of the catalyst (ganH+ads) is irreversibly reduced at potential about –1.35 V vs Ag|AgCl, yielding the initial form of the catalyst and atomic hydrogen, i.e., ganH+ads + e → ganads + Haq. Changes of zero charge potential and surface tension point to the adsorption of ganciclovir molecule directed with guanine group to the mercury surface and suggests that ganciclovir molecules are not placed flat on the mercury surface. The effect of adsorption on mercury electrode was studied in detail in respect to analytical usefulness of the obtained results. A new catalytic method for voltammetric determination of ganciclovir was developed. The detection and quantification limits were 1.3 × 10–7 and 4.3 × 10–7 mol l–1 for square-wave voltammetry, and 1.4 × 10–7 and 4.7 × 10–7 mol l–1 for linear-sweep voltammetry.

Surface ECE mechanism in protein film voltammetry—a theoretical study under conditions of square-wave voltammetry

For the first time, the features of a surface electron transfer-chemical reaction-electron transfer (ECE) mecha- nism, relevant to protein-film set-up, have been studied theoretically under conditions of square-wave voltammetry. The considered surface ECE mechanism is presented by following reaction scheme:A adsorbed ðÞ þ ne � ! B adsorbed ðÞ þ Y Y kf C adsorbed ðÞ þ ne � ! D adsorbed ðÞ . The mathematical sol- utions of this complex redox mechanism are given in form of integral equations, and they can be applied to any chro- noamperometric technique. Attention is given to two fre- quently met situations: (a) case where the energy for the reduction in the second electron transfer step is lower or equal to that of the first reduction step and (b) case where the energy for the reduction of the second electron transfer step is much higher than that of the first reduction step. The theoretical square-wave voltammograms feature various shapes, depending mainly on the energy difference between the two electron transfer steps, but they also depend on the kinetics of the first and the second electron transfer, as well as on the rate of the chemical reaction. Hints are given for qualitative recognition of the surface ECE mechanism and for its distinguishing from similar surface redox systems. Reliable methods are proposed for the estimation of kinetic parameters of the electron transfer steps and that of the chemical reaction. Since many biological compounds un- dergo this redox mechanism, the theoretical results presented in this work can be of help for the people dealing with organic electrochemistry or protein-film voltammetry.

Homogeneous versus Heterogeneous Catalysis at Electrodes Modified with a Thin Organic Layer:  Theoretical and Experimental Study under Conditions of Square-Wave Voltammetry

A catalytic mechanism at thin film-modified electrodes is studied both theoretically and experimentally under conditions of square-wave voltammetry (SWV). The electrode system considered consists of a solid electrode covered with an electrochemically inactive thin film (e.g., a layer of a water-immiscible organic solvent) containing a neutral redox probe (R1). The modified electrode is immersed in an electrolyte solution (typically an aqueous electrolyte solution) that contains a second redox probe (R2) present in large excess compared to R1. The product of the assumed one-electron electrode reaction O1+, confined within the film, is transformed back to initial reactant R1 by means of a catalytic redox reaction with R2. If the permeation of R2 in the thin film is significant, then the catalytic reaction is a homogeneous process confined within the boundaries of the thin film. Otherwise, the catalytic reaction is a heterogeneous process involving electron transfer across the interface between the thin film...