Voltammetry of Neurotransmitters

In this work, the potential of voltammetry as a simple and cheap electrochemical tool to investigate the redox properties of neurotransmitters is presented. It is shown that the redox features of some neurotransmitters can be effectively studied by voltammetry in a simple way. In addition, the determination of the kinetics of interaction of a series of common neurotransmitters (adrenaline, dopamine, serotonin) with pro-oxidative molecules or ions brings insights into the antagonistic substances that hinder the neurotransmitters functions. A theory of studying the transfer of some neurotransmitters across phospholipid membranes is also considered

Square-wave Voltammetry of Surface Electrode Mechanisms with Non-Unity Stoichiometry-Simulation Protocol in MATHCAD

Many metal ions and other physiologically active systems (like quinones and Vitamin B2 complexes) undergo electrochemical transformation in a fashion that is different from 1:1 stoichiometry. Cu II ions, and H+ ions, for example, are typical systems whose reduction happens in non-unity stoichiometry. As we know, reduction of H+ ions at platinum electrode goes via redox scheme 2H+ + 2e- = H2(gas) (dissolved in the Pt) We present in this simulation model for the first time simulation procedure of 2:1 stoichiometry under conditions of square-wave voltammetry. As expected, the peak potential of SWV voltammetric patterns of such systems is sensitive to analyte concentration, shifting for -59 mV in negative direction per decadic increase of c(H+). In addition, other SW voltammetric parameters are also affected by the analyte concentrations of 2:1 stoichiometric systems. This model is suitable for studying metal-ligand complexes of many transient metal ions, but also of many drugs that undergo direct 2 electron transformation, as the quinones in aqueous media, for example. A general explicit solution of all non-stoichiometry mechanisms for diffusional systems is published in recent work in Journal of Electroanalytical Chemistry. Here we present model only for 2:1 specific surface stoichiometry

Diffusional "CE Mechanism" in Square Wave Voltammetry-MATHCAD Simulation Procedure

Reaction of disproportionation or degradation of given compound „A” leads to generation of reactive species “Ox” that can undergo electron transfer at the working electrode surface. The mechnism known as Diffusional CE or simply CE Mechanism is considered under conditions of square-wave voltammetry. The reaction scheme of this mechanism met in many physiological systems is as follow A --> Ox(water) + ne- = Red(water) SW voltammetric patterns are function of the electron transfer parameter related to the electrode reaction, but they also depend on the kinetic and thermodynamic parameters related to the chemical step. The entire interplay of all parameters leads to very specific voltammograms, whose feature can reveal important kinetic and thermodynamic parameters relevant to the physiological systems of interest. Electrode transformation of many transient metal ion, drugs, enzymes, neurotransmitters, vitamins and metal-Ligand complexes follow this pathway

New aspects of the electrochemical-catalytic (EC’) mechanism in square-wave voltammetry

Several new theoretical aspects of the electrocatalytic (regenerative) EC’ mechanism under conditions of square-wave (SWV) and staircase cyclic voltammetry (SCV) are presented. Elaborating the effect of the rate of the catalytic reaction in the diffusion-controlled catalytic mechanism (diffusional EC’ mechanism) and surface catalytic mechanism (surface EC’ mechanism), we refer to several phenomena related to the shift of the position and the half-peak width of the net peak in square-wave voltammetry (SWV). If the rate of the catalytic reaction is much higher than the kinetics of the electrode reaction, a linear dependence between the peak potential of the simulated net SWV peaks and the logarithm of the catalytic parameter can be observed. The intercept of that linear dependence is a function of the kinetics of the electrode reaction. Based on this finding, we propose a new methodology to determine the electrode kinetics rate constant. The proposed approach relies on the variation of the concentration of the regenerative reagent. To the best of our knowledge, this is one of very few voltammetric approaches for electrode kinetic measurements not based on the time or potential variation in the experimental analyzes. In addition, we present a brief analysis of the catalytic mechanism under conditions of staircase cyclic voltammetry in order to emphasize the main differences between SCV and SWV

MATHCAD – a Tool for Numerical Calculation of Square-Wave Voltammograms

An alternative approach for numerical calculation of the square-wave voltammograms using the mathematical programming package MATHCAD is presented. A quasi-reversible redox reaction is considered and a mathematical model is developed under conditions of the square-wave voltammetry (SWV). Application of the mathematical model in MATHCAD is discussed and the file used for numerical simulation is presented. The relationships between the properties of the SW response and the parameters of both the quasireversible redox reaction and the excitation signal are discussed

A Theoretical and Experimental Study of a Two-step Quasireversible Surface Redox Reaction by Square-wave Voltammetry

An extended theoretical treatment of a two-step surface redox reaction under the conditions of square-wave voltammetry (SWV) is presented. The theoretical model is applicable to any reversibility of both redox steps of the EE reaction. The integral equations representing this theoretical model were solved numerically. The apparent reversibility of the redox steps depends on two dimensionless kinetic parameters, κ1 and κ2, defined as κ1 = ks,1/f and κ2 = ks,2/f, where ks,1 and ks,2 are the standard rate constants of the first and second redox steps, respectively, and f is the excitation signal frequency. The response consists of either a single or two separate SW peaks, depending mainly on the difference between the formal potentials of the first and second redox steps as well as on the ratio of the kinetic parameters κ1/κ2. The effect of the electron transfer coefficients of both redox steps is also discussed. A part of the theoretical results are qualitatively illustrated by SW voltamograms of the azo-dye Sudan-III

A Theoretical and Experimental Study of a Two-step Quasireversible Surface Redox Reaction by Square-wave Voltammetry

An extended theoretical treatment of a two-step surface redox reaction under the conditions of square-wave voltammetry (SWV) is presented. The theoretical model is applicable to any reversibility of both redox steps of the EE reaction. The integral equations representing this theoretical model were solved numerically. The apparent reversibility of the redox steps depends on two dimensionless kinetic parameters, κ1 and κ2, defined as κ1 = ks,1/f and κ2 = ks,2/f, where ks,1 and ks,2 are the standard rate constants of the first and second redox steps, respectively, and f is the excitation signal frequency. The response consists of either a single or two separate SW peaks, depending mainly on the difference between the formal potentials of the first and second redox steps as well as on the ratio of the kinetic parameters κ1/κ2. The effect of the electron transfer coefficients of both redox steps is also discussed. A part of the theoretical results are qualitatively illustrated by SW voltamograms of the azo-dye Sudan-III

Diffusional Electrochemical Catalytic (EC’) Mechanism Featuring Chemical Reversibility of Regenerative Reaction-Theoretical Analysis in Cyclic Voltammetry

We consider theoretically a specific electrochemical-catalytic mechanism associated with reversible regenerative chemical reaction, under conditions of cyclic staircase voltammetry (CSV). We suppose scenario in which two electrochemically inactive substrates “S” and “Y”, together with initial electrochemically active reactant Ox are present in voltammetric cell from the beginning of the experiment. Substrate “S” selectively reacts with initial electroactive reactant Ox and creates electroactive “product” Red (+ Y) in a reversible chemical fashion. The initial chemical equilibrium determines the amounts of Ox and Red available for electrode transformation at the beginning of the electrochemical experiment. Under conditions of applied potential, the electrode reaction Ox(aq) + ne– ⇋ Red(aq) occurs, producing flow of electric current. Under such circumstances, the chemical reaction coupled to the electrochemical step causes a regeneration of initial electroactive species during the time-frame of current-measuring segment in CSV. The features of cyclic voltammograms get significantly affected by the kinetics and thermodynamics of reversible regenerative reaction. We elaborate several aspects of this specific electrode mechanism, and we focus on the role of parameters related to chemical step to the features of calculated voltammograms. While we provide a specific set of results of this particular mechanism, we propose methods to get access to relevant kinetic and thermodynamic parameters relevant to regenerative chemical reaction. The results elaborated in this work can be valuable in evaluating kinetics of many drug-drug interactions, but they can be relevant to study interactions of many enzyme-substrate systems, as well. This work is licensed under a Creative Commons Attribution 4.0 International License

Electrode mechanism considering reduction from adsorbed state and oxidation from dissolved state

Redox processes in which oxidation and reduction take place from different state are important systems in many physiological and industrial processes. Rechargeable batteries, for example, are system in which water soluble ions get created from metals in atomic state. Also, many physiological processes of amphiphilic enzymes can be regarded to take occur redox transformation from adsorbed and dissolved state, too. In this work, we give short overview on the model of simple redox mechanism in which reduction occurs from adsorbed state, while re-oxidation happens from dissolved state, with mass transfer occurring via diffusion. This hybrid model is quite relevant to understand the processes in rechargeable batteries and in enzymatic voltammetry