The surface catalytic mechanism: a comparative study with square-wave and staircase cyclic voltammetry

A pseudo-first-order catalytic mechanism in which both reactant and product of a redox reaction are strongly immobilized on an electrode surface is theoretically analysed under conditions of square-wave (SWV) and staircase cyclic voltammetry (SCV). A mathematical procedure is developed under diffusionless conditions. The relationships between the properties of the voltammetric response and both the kinetic parameters of the redox reaction and the parameters of the excitation signal are studied. The phenomenon of the quasi-reversible maximum is discussed. A comparative study between SWV and SCV is presented and the limitations and advantages of both techniques, from analytical and kinetic points of view, are discussed. The theoretical predictions are experimentally confirmed by the redox reaction of azobenzene in the presence of hydrogen peroxide as an oxidizing agent. Electronic Publication     

Theoretical Aspects of a Surface Electrode Reaction Coupled with Preceding and Regenerative Chemical Steps: Square‐wave Voltammetry of a Surface CEC’ Mechanism

Large number of lipophilic substances, whose electrochemical transformation takes place from adsorbed state, belong to the class of so‐called “surface‐redox reactions”. Of these, especially important are the enzymatic redox reactions. With the technique named “protein‐film voltammetry” we can get insight into the chemical features of many lipophilic redox enzymes. Electrochemical processes of many redox adsorbates, occurring at a surface of working electrode, are very often coupled with chemical reactions. In this work, we focus on the application of square‐wave voltammetry (SWV) to study the theoretical features of a surface electrode reaction coupled with two chemical steps. The starting electroactive form Ox_(ads) in this mechanism gets initially generated via preceding chemical reaction. After undergoing redox transformation at the working electrode, Ox_(ads) species got additionally regenerated via chemical reaction of electrochemically generated product Red_(ads) with a given substrate Y. The theory of this so‐called surface CEC’ mechanism is presented for the first time under conditions of square‐wave voltammetry. While we present plenty of calculated voltammograms of this complex electrode mechanism, we focus on the effect of rate of regenerative (catalytic) step to simulated voltammograms. We consider both, electrochemical reactions featuring moderate and fast electron transfer. The obtained voltammetric patterns are very specific, having sometime hybrid‐like features of voltammograms as typical for CE, EC and EC’ mechanisms. We give diagnostic criteria to recognize this complex mechanism in SWV, but we also present hints to access the kinetic and thermodynamic parameters relevant to both chemical steps, and the electrochemical reaction, too. Indeed, the results presented in this work can help experimentalists to design proper experiments to study chemical features of important lipophilic systems.     

Determining the Gibbs energy of ion transfer across water-organic liquid interfaces with three-phase electrodes.

Ions can be transferred between immiscible liquid phases across a common interface, with the help of a three-electrode potentiostat, when one phase is an organic droplet attached to a solid electrode and containing a redox probe. This novel approach has been used in studies to determine the Gibbs energy of anion and cation transfer, ranging from simple inorganic and organic ions to the ionic forms of drugs and small peptides. This method of studying ion transfer has the following advantages: (1) no base electrolytes are necessary in the organic phase; (2) the aqueous phase contains only the salt to be studied; (3) a three-electrode potentiostat is used; (4) organic solvents such as n-octanol and chiral liquids such as D- and L-2-octanol can be used; (5) the range of accessible Gibbs energies of transfer is wider than in the classic 4-electrode experiments; (6) the volume of the organic phase can be very small, for example, 1 microL or less; (7) the experiments can be performed routinely and fast. Herein, the basic 5 principle is outlined, as well as a summary of the results obtained to date, and a discussion on the theoretical treatments concerning the kinetic regime of the three-phase electrodes with immobilized droplets.     

Theoretical Contribution towards Understanding Specific Behaviour of “Simple” Protein‐film Reactions in Square‐wave Voltammetry

Surface reactions of uniformly adsorbed redox molecules at working electrode surface are seen as adequate models to studying chemical reactivity of many lipophilic enzymes. When considered under pulse voltammetric techniques, these systems show several uncommon features, whose origin is still not completely clear. The phenomena of “quasireverible maximum”, “splitting” of the net peak in square‐wave voltammetry, and the very steep descent of Faradaic currents of simple surface redox reactions exhibiting fast electron transfer are just some of the features that make these systems quite interesting for further elaborations. In this work, we present a set of theoretical calculations under conditions of square‐wave voltammetry in order try to explain some of aforementioned phenomena. The major goal of our work is to get insight to some voltammetric and chrono‐amperometric features of two considered surface reactions, i. e. (1) the “simple” surface redox reaction, and (2) surface redox reaction coupled to follow‐up irreversible chemical reaction of electrochemically generated redox species (or surface ECirr). We focus on the role of created Red(ads) (here in the reduction pulses only) to the current components of calculated square‐wave voltammograms exhibiting fast electrode reaction. We show that the irreversible chemical removal of electrochemically generated Red(ads) species, created in the potential pulses where half‐reaction of reduction Ox(ads)+ne‐?→Red(ads) is “defined” to take place, causes significant increase of all square‐wave current components. The results presented in this work show how complex the chrono‐amperometric features of surface redox reactions under pulse voltammetric conditions might be. In addition, we point out that both half reactions of a given simple surface redox process can occur, at both, “only reduction” and “only oxidation” potential pulses in square‐wave voltammetry. This, in turn, contributes to the occurrence of many phenomena observed in simple protein‐film voltammetry reactions. The effects of chemical reaction rate to the features of calculated square‐wave voltammograms of surface ECirr systems with fast electrode reaction are reported for the first time in this work.     

Square‐wave Voltammetry of Two‐step Surface Electrode Mechanisms Coupled with Chemical Reactions – A Theoretical Overview

Square‐wave voltammetry (SWV) of so‐called “surface redox reactions” is seen as a simple and efficient tool to quantify large number of drugs, physiologically active substances and other important chemicals. It also provides elegant methods to get access to relevant kinetic and thermodynamic parameters related to many lipophilic compounds. Moreover, with this technique we can study activity of various enzymes by exploring the “protein‐film voltammetry” set up. In this work, we focus on theoretical SWV features of four complex surface electrode mechanisms, in which the electron exchange between the working electrode and the studied redox substrate takes place in two successive steps. While we present large number of calculated square‐wave voltammograms, we give hints to recognize particular two‐step surface mechanism, but also to distinguish it from other similar mechanisms. We present plenty of relevant aspects of surface two‐step surface EE, two‐step surface ECE and surface catalytic EEC’ mechanisms. Moreover, we present for the first time a series of theoretical results related to two‐step surface EECrev mechanism (i. e. two‐step surface reaction coupled to follow‐up reversible chemical step). The simulated voltammetric patterns presented in this work can bring relevant aspects to resolve some experimental situations met in voltammetry of many redox enzymes and other important substances whose electrochemical transformation occurs in two‐steps.     

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

Journal of Solid State Electrochemistry - Redox mechanisms in which the redox transformation is coupled to other chemical reactions are of significant interest since they are regarded as relevant...     

Enzymatic formation of ions and their detection at a three-phase electrode

An electrochemical method for the detection of enzymatically created anions is described that uses a thin-film electrode with decamethylferrocene as an electroactive redox probe. The enzymatic oxidation of glucose with enzyme glucose oxidase produces gluconic acid as a final product. The oxidation of decamethylferrocene dissolved in the thin-nitrobenzene film, that is spread on the working graphite electrode and submerged in the aqueous solution containing glucose and glucose oxidase, is followed by the up-take of gluconate anions from the aqueous phase to nitrobenzene. The peak currents of the square-wave voltammetric responses of that system are a linear function of the glucose concentration in the milimolar range from 0.1 mmol/L to 0.7 mmol/L (R²=0.994).     

Simple electrochemical method for deposition and voltammetric inspection of silver particles at the liquid-liquid interface of a thin-film electrode

A novel experimental methodology for depositing and voltammetric study of Ag nanoparticles at the water-nitrobenzene (W-NB) interface is proposed by means of thin-film electrodes. The electrode assembly consists of a graphite electrode modified with a thin NB film containing decamethylferrocene (DMFC) as a redox probe. In contact with an aqueous electrolyte containing Ag(+) ions, a heterogeneous electron-transfer reaction between DMFC((NB)) and Ag(+)((W)) takes place to form DMFC(+)((NB)) and Ag deposit at the W-NB interface. Based on this interfacial reaction, two different deposition strategies have been applied. In the uncontrolled potential deposition protocol, the electrode is immersed into an AgNO(3) aqueous solution for a certain period under open circuit conditions. Following the deposition step, the Ag-modified thin-film electrode is transferred into an aqueous electrolyte free of Ag(+) ions and voltammetrically inspected. In the second protocol the deposition was carried out under controlled potential conditions, i.e., in an aqueous electrolyte solution containing Ag(+) ions by permanent cycling of the electrode potential. In this procedure, DMFC((NB)) is electrochemically regenerated at the electrode surface, hence enabling continuation and voltammetric control of the Ag deposition. Hence, the overall electrochemical process can be regarded as an electrochemical reduction of Ag(+)((W)) at the W-NB interface, where the redox couple DMFC(+)/DMFC acts as a mediator for shuttling electrons from the electrode to the W-NB interface. Ag-particles deposited at the W-NB interface affect the ion transfer across the interface, which provides the basis for voltammetric inspection of the metal deposit at the liquid-liquid interface with thin-film electrodes. Voltammetric properties of thin-film electrodes are particularly sensitive to the deposition procedure, reflecting differences in the properties of the Ag deposit. Moreover, this methodology is particularly suited to inspect catalytic activities of metal particles deposited at the liquid-liquid interface toward heterogeneous electron-transfer reactions occurring at the at the liquid-liquid interface.     

Calcium binding and transport by coenzyme Q

Coenzyme Q10 (CoQ10) is one of the essential components of the mitochondrial electron-transport chain (ETC) with the primary function to transfer electrons along and protons across the inner mitochondrial membrane (IMM). The concomitant proton gradient across the IMM is essential for the process of oxidative phosphorylation and consequently ATP production. Cytochrome P450 (CYP450) monoxygenase enzymes are known to induce structural changes in a variety of compounds and are expressed in the IMM. However, it is unknown if CYP450 interacts with CoQ10 and how such an interaction would affect mitochondrial function. Using voltammetry, UV-vis spectrometry, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), fluorescence microscopy and high performance liquid chromatography-mass spectrometry (HPLC-MS), we show that both CoQ10 and its analogue CoQ1, when exposed to CYP450 or alkaline media, undergo structural changes through a complex reaction pathway and form quinone structures with distinct properties. Hereby, one or both methoxy groups at positions 2 and 3 on the quinone ring are replaced by hydroxyl groups in a time-dependent manner. In comparison with the native forms, the electrochemically reduced forms of the new hydroxylated CoQs have higher antioxidative potential and are also now able to bind and transport Ca(2+) across artificial biomimetic membranes. Our results open new perspectives on the physiological importance of CoQ10 and its analogues, not only as electron and proton transporters, but also as potential regulators of mitochondrial Ca(2+) and redox homeostasis.     

Electrochemical characterization of polyelectrolyte/gold nanoparticle multilayers self-assembled on gold electrodes

Polyelectrolyte/gold nanoparticle multilayers composed of poly(l-lysine) (pLys) and mercaptosuccinic acid (MSA) stabilized gold nanoparticles (Au NPs) were built up using the electrostatic layer-by-layer self-assembly technique upon a gold electrode modified with a first layer of MSA. The assemblies were characterized using UV-vis absorption spectroscopy, cyclic and square-wave voltammetry, electrochemical impedance spectroscopy, and atomic force microscopy. Charge transport through the multilayer was studied experimentally as well as theoretically by using two different redox pairs [Fe(CN)(6)](3-/4-) and [Ru(NH(3))(6)](3+/2+). This paper reports a large sensitivity to the charge of the outermost layer for the permeability of these assemblies to the probe ions. With the former redox pair, dramatic changes in the impedance response were obtained for thin multilayers each time a new layer was deposited. In the latter case, the multilayer behaves as a conductor exhibiting a strikingly lower impedance response, the electric current being enhanced as more layers are added for Au NP terminated multilayers. These results are interpreted quite satisfactorily by means of a capillary membrane model that encompasses the wide variety of behaviors observed. It is concluded that nonlinear slow diffusion through defects (pinholes) in the multilayer is the governing mechanism for the [Fe(CN)(6)](3-/4-) species, whereas electron transfer through the Au NPs is the dominant mechanism in the case of the [Ru(NH(3))(6)](3+/2+) pair.