Preparation, characterization and electrocatalytic behavior of zinc oxide/zinchexacyanoferrate and ruthenium oxide hexacyanoferrate hybrid film-modified electrodes

Polynuclear mixed-valent hybrid films of zinc oxide/zinchexacyanoferrate and ruthenium oxide hexacyanoferrate (ZnO/ZnHCF-RuOHCF) have been deposited on electrode surfaces from H₂SO₄ solution containing Zn(NO₃)₂, RuCl₃ and K₃[Fe(CN)₆] by potentiodynamic cycling method. Simultaneous cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM) measurements demonstrate the steady growth of hybrid film. Surface morphology of hybrid film was investigated using scanning electron microscopy (SEM). Energy dispersive spectrometer (EDS) data confirm existence of zinc oxide and ruthenium oxide hexacyanoferrate (RuOHCF) in the hybrid film. The effect of type of monovalent cations on the redox behavior of hybrid film was investigated. In pure supporting electrolyte, electrochemical responses of Ru^II/III redox transition occurring at negative potential region resemble with that of a surface immobilized redox couple. The electrocatalytic activity of ZnO/ZnHCF-RuOHCF hybrid film was investigated towards oxidation of epinephrine, dopamine and l-cysteine, and reduction of S₂O₈²⁻ and SO₅²⁻ as well as IO₃⁻ using cyclic voltammetry and rotating ring disc electrode (RRDE) techniques.     

EQCM study of polypyrrole modified electrodes doped with Keggin-type heteropolyanion for cation detection

The incorporation of a Keggin-type heteropolyanion, the phosphotungstate ([PW₁₂O₄₀]³⁻), into polypyrrole has been achieved during the electrochemical preparation of the polymer films in aqueous solution. The redox behaviour of these modified electrodes is described by using cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM). The data allowed to estimate the doping level that indicates the extent of the heteropolyanion insertion. It is found that the polymer films, in aqueous solution, exhibit sensibility to cations, namely to Na⁺.     

The characterization and bioelectrocatalytic properties of hemoglobin by direct electrochemistry of DDAB film modified electrodes

Direct electrochemistry of hemoglobin can be performed in acidic and basic aqueous solutions in the pH range 1-13, using stable, electrochemically active films deposited on a didodecyldimethylammonium bromide (DDAB) modified glassy carbon electrode. Films can also be produced on gold, platinum, and transparent semiconductor tin oxide electrodes. Hemoglobin/DDAB films exhibit one, two, and three redox couples when transferred to strong acidic, weak acidic and weak basic, and strong basic aqueous solutions, respectively. These redox couples, and their formal potentials, were found to be pH dependent. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ deposition of DDAB on gold disc electrodes and hemoglobin deposition on DDAB film modified electrodes. A hemoglobin/DDAB/GC modified electrode is electrocatalytically reduction active for oxygen and H₂O₂, and electrocatalytically oxidation active for S₂O₄²⁻ through the Fe(III)/Fe(II) redox couple. In the electrocatalytic reduction of S₄O₆²⁻, S₂O₄²⁻, and SO₃²⁻, and the dithio compounds of 2,2′-dithiosalicylic acid and 1,2-dithiolane-3-pentanoic acid, the electrocatalytic current develops from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in neutral and weakly basic aqueous solutions. Hemoglobin/DDAB/GC modified electrodes are electrocatalytically reduction active for trichloroacetic acid in strong acidic buffered aqueous solutions through the Fe(III)/Fe(II) redox couple. However, the electrocatalytic current developed from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in weak acidic and basic aqueous solutions. The electrocatalytic properties were investigated using the rotating ring-disk electrode method.     

Electrocatalytic properties of NDGA and NDGA/FAD hybrid film modified electrodes for NADH/NAD redox reaction

The fabrication of monolayers composed of nordihydroguaiaretic acid (NDGA), and hybrid films composed of NDGA-flavin adenine dinucleotide (FAD) adsorbed films was performed in neutral aqueous solutions to produce electrochemically active thin films exhibiting one and two redox couples, respectively. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ growth of the NDGA and hybrid NDGA/FAD film monolayers. The NDGA modified film electrocatalytically oxidized NADH, ascorbic acid, dopamine, and N₂H₄ in neutral aqueous solutions. Well-separated voltammetric peaks were observed for dopamine and uric acid mixtures, and also for ascorbic acid and uric acid mixtures using the NDGA/GC modified electrode. When transferred to various aqueous buffered solutions, the two redox couples of the NDGA/FAD hybrid film and their formal potentials were observed to be pH-dependent. The electrocatalytic oxidation and reduction of NADH and NAD⁺ by a NDGA/FAD hybrid film in neutral aqueous solutions was carried out, and the electrocatalytic oxidation of NADH was performed using a NDGA/FAD hybrid film.     

Electrochemical preparation of epinephrine/Nafion chemically modified electrodes and their electrocatalytic oxidation of ascorbic acid and dopamine

Two types of epinephrine and cyclized epinephrine quinone films have been prepared using cyclic voltammetry from the epinephrine in the strong acidic solutions and neutral aqueous solutions over different scanning potential ranges. The cyclic voltammogram of the epinephrine film is characterized by one redox couple at about +0.5 V (versus Ag|AgCl) and cyclized epinephrine quinone film exhibits one redox couples at about −0.15 V (versus Ag|AgCl) .In addition to cyclic voltammetry and an electrochemical quartz crystal microbalance (EQCM) were used to study the growth mechanism of the epinephrine and cyclized epinephrine quinone molecules. The electrocatalytic oxidation of catecholamines (dopamine and norepinephrine) and also ascorbic acid were investigated in acidic aqueous solutions using epinephrine films. The rotating ring-disk electrode technique was used to investigate the mechanism of electrochemical oxidation of dopamine and ascorbic acid.     

Sol-gel derived carbon ceramic electrode for the investigation of the electrochemical behavior and electrocatalytic activity of neodymium hexacyanoferrate

The electrochemical properties of an electroactive rare earth metal hexacyanoferrate, neodymium hexacyanoferrate (NdHCF) were studied by mechanically attaching NdHCF samples to the surface of carbon ceramic electrodes (CCEs) derived from sol-gel technique. The resulting modified electrodes exhibit well-defined redox responses with the formal potential of 0.241 V (versus SCE) at a scan rate of 20 mV s⁻¹ in 0.5 M KCl solution. The voltammetric characteristics of the NdHCF-modified CCEs in the presence of different alkali metal cations (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) were investigated by voltammetry. The NdHCF-modified CCEs presented a good electrocatalytic activity towards the reduction of hydrogen peroxide (H₂O₂), and was used for amperometric detection of H₂O₂. In addition, the NdHCF-CCEs exhibited a distinct advantage of simple preparation, surface renewal, good stability and reproducibility.     

Preparation, characterization and electrocatalytic properties of poly(luminol) and polyoxometalate hybrid film modified electrodes

Hybrid films composed of poly(luminol) and nanometer-sized clusters of polyoxometalate, SiMo₁₂O₄₀⁴⁻ and PMo₁₂O₄₀³⁻ have been prepared in acidic aqueous solutions. These films are stable and electrochemically active, and produced on glassy carbon, platinum, gold and transparent semiconductor tin oxide electrodes. The electrochemical quartz crystal microbalance and cyclic voltammetry were used to study in situ growth of the hybrid poly(luminol)/SiMo₁₂O₄₀⁴⁻ and poly(luminol)/PMo₁₂O₄₀³⁻. Both the poly(luminol)/SiMo₁₂O₄₀⁴⁻ and poly(luminol)/PMo₁₂O₄₀³⁻ hybrid films showed four redox couples and the electrochemical properties were compared to SiMo₁₂O₄₀⁴⁻ and PMo₁₂O₄₀³⁻. When transferred to various acidity aqueous solutions, the four redox couples and the formal potentials of two hybride film were observed to be pH-dependent. The electrocatalytic reduction of ClO₃⁻, BrO₃⁻, IO₃⁻, S₂O₈²⁻ and NO₂⁻ by a poly(luminol)/PMo₁₂O₄₀³⁻ hybrid film in an acidic aqueous solution showed an electrocatalytic reduction activity of IO₃⁻ > BrO₃⁻ and ClO₃⁻. The electrocatalytic oxidation of dopamine and epinephrine by a poly(luminol)/PMo₁₂O₄₀³⁻ hybrid film was also investigated.     

Electrochemical properties of modified copper-thallium hexacyanoferrate electrode in the presence of different univalent cations

The preparation of copper(II) hexacyanoferrate (CuHCF) films on the surface of gold electrodes as well as their characterization in solutions of various alkali metal and NH₄⁺ cations and in the presence of thallium(I) are described. The electrochemical quartz crystal microbalance and cyclic voltammetric techniques were used. In 0.50 M lithium nitrate, even at submillimolar concentration of Tl(I), the formal potential of CuHCF was shifted to more positive values. At higher Tl(I) concentrations, the formal potential of the CuHCF redox reaction changed linearly with the logarithm of Tl(I) concentration (in the 0.50 M solution of lithium or another alkali metal nitrate). From such dependencies, selectivity coefficients K_Tl/M were calculated, and they show that the CuHCF film on the gold electrode interacts preferentially with Tl(I). High affinity of Tl(I) to copper hexacyanoferrate, that was observed in the presence of alkali metal cations, was explained by relatively strong donor-acceptor interactions of Tl(I) ions with nitrogen in CN groups of the CuHCF film.It was also shown for simple M₄[Fe(CN)₆] metal ferrocyanate salts (where M = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and Tl⁺) that there is a preferential interaction of Tl⁺ with CN group consistent with formation of a Tl-NC-Fe bridge.     

Application of PEDOT layers for the electrogravimetric detection of sulphate and phosphate in aqueous media

Poly(3,4-ethylenedioxythiophene), PEDOT, films were electrochemically prepared from aqueous solutions containing different supporting electrolytes (ClO₄⁻, SO₄²⁻ and H₂PO₄⁻). Characterization of the resulting films was carried out using cyclic voltammetry and potential step experiments combined with the dissipative quartz crystal microbalance technique. The ion incorporation caused an increase of the film mass and below 10⁻² M depended on the concentration of the solutions. Potential step experiments showed that the pH of the solutions also has an influence on the mass change of the films, which can only be partially attributed to the pH dependence of the charge of the anions. In principle, the ratio of the mass and the passed electric charge (apparent molar mass) during the redox switching of the polymer can be used to identify the ionic species.     

Study by EQCM on the voltammetric electrogeneration of poly(neutral red). The effect of the pH and the nature of cations and anions on the electrochemistry of the films

Generation of poly(neutral red) films has been studied by means of the simultaneous measurements of current-potential and mass-potential curves during cyclic voltammetry (CV) experiments. It has been proved that the presence of molecular oxygen in the solution increases the amount of polymer deposited on the electrode. Otherwise, using the mass/charge ratio it is possible to obtain quantitative information about the electrodeposition by different procedures. It is observed that this ratio decreases when the amount of polymer electrogenerated increases, except when the polymer is not reduced and oxidised after its electrogeneration. The study of poly(neutral red) by CV and quartz crystal microbalance in solutions without monomer allows to discern between the role of different charged species which are present in the solution: salt cations (Cs⁺, Na⁺ and K⁺), salt anions (NO₃⁻, Cl⁻, I⁻ and Br⁻) and hydrated protons that can compensate electrical charge within the film during electrochemical processes.     

Interaction between myoglobin and hyaluronic acid in layer-by-layer structures—An electrochemical study

The layer-by-layer (LBL) self-assembled film construction of the biocompatible polymer hyaluronic acid (HA) and single heme redox protein, myoglobin (Mb) is described. The films were built upon gold electrode substrates, both gold quartz crystal electrodes and bulk gold (Au(bulk)) electrodes, and formation of the LBL films was gravimetrically monitored by an electrochemical quartz crystal microbalance. The electrochemical properties of the hyaluronic acid/myoglobin films ({HA/Mb}_n) were investigated after each deposition step using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV response presented an oxidation peak at +0.3 V vs. SCE, not characteristic of the redox protein myoglobin, and, the peak current decreased slightly with each additional bilayer. CV at Au(bulk) electrodes in pH 5.0 acetate buffer solution, containing Mb, presented the same oxidation peak as observed at {HA/Mb}_n modified electrodes, confirming the presence of the same electroactive species. The Mb oxidation peak current depends linearly on scan rate, characteristic of adsorbed thin-layer electrochemical systems, attributed to free adsorbed heme. Impedance spectra, recorded after deposition of each bilayer, were in agreement with the cyclic voltammetry observations.     

Voltammetric and electrochemical impedance spectroscopy characterization of a cathodic and anodic pre-treated boron doped diamond electrode

The effect of boron doped diamond (BDD) surface termination, immediately after cathodic and anodic electrochemical pre-treatments, on the electrochemical response of a BDD electrode in aqueous media and the influence of the different supporting electrolytes utilized in these pre-treatments on the final surface termination was investigated with [Fe(CN)₆]^4−/3−, as redox probe, by cyclic and differential pulse voltammetry and electrochemical impedance spectroscopy. The cyclic voltammetry results indicate that the electrochemical behavior for the redox couple [Fe(CN)₆]^4−/3− is very dependent on the state of the BDD surface, and a reversible response was observed after the cathodic electrochemical pre-treatment, whereas a quasi-reversible response occurred after anodic electrochemical pre-treatment. Differential pulse voltammetry in acetate buffer also showed that the potential window is very much influenced by the electrochemical pre-treatment of the BDD surface. Electroactivity of non-diamond carbon surface species (sp² inclusions) incorporated into the diamond structure was observed after cathodic and anodic pre-treatments. Electrochemical impedance spectroscopy confirmed the cyclic voltammetry results and indicates that the BDD surface resistance and capacitance vary significantly with the electrolyte and with the electrochemical pre-treatment, caused by different surface terminations of the BDD electrode surface.     

A voltammetric and nanogravimetric study of Te underpotential deposition on Pt in perchloric acid medium

This work describes the study of Te underpotential deposition on Pt in acid media using cyclic voltammetry, rotating ring-disc electrode and electrochemical quartz crystal microbalance techniques. The voltammetric results indicate the presence of two dissolution peaks in the positive scan with a total charge density of 420 μC cm⁻². These phenomena are attributed to the deposition of one Te monolayer with the occupancy of two active Pt sites by each ad-atom. This is confirmed by rotating ring-disc electrode results. The electrochemical quartz crystal microbalance (EQCM) experiments yielded the small mass variation of −32 ng cm⁻² (while the theoretical one is −140.4 ng cm⁻² for a complete Te monolayer). This low value can be attributed to the simultaneous adsorption of water, perchlorate anions and the formation of platinum oxide.     

Electrochemical quartz crystal microbalance studies of polymerization and redox process of poly(1,8-diaminocarbazole) in protic and aprotic solutions

Electropolymerization and doping/undoping process of poly(1,8-diaminocarbazole) (PDACz) were investigated by cyclic voltammetry combined with electrochemical quartz microbalance (EQCM), in aqueous solution of 0.1 M HClO₄ and in 0.1 M LiClO₄ in acetonitrile. The results demonstrated that protonation of the polymer in HClO₄ strongly influences the gravimetric response obtained during polymerization as well as the mechanism of redox process of the polymer in the monomer-free solution. The data are discussed in terms of the presence of protons and ClO₄⁻ counter ions in the reduced (neutral) polymer matrix. In aprotic acetonitrile containing LiClO₄ as the supporting electrolyte, the polymer doping/undoping occurs according to the simple mechanism with exchange of anions. The gravimetric results obtained in acidic aqueous solution allowed for some revision of the scheme of the redox process proposed by the authors in the previous paper.     

Effects of 1,3-dialkylimidazolium cations with different lengths of alkyl chains on the Pt electrode/electrolyte interface in dye-sensitized solar cells

The electrochemical behaviors of the tri-iodide (I₃⁻)/iodide (I⁻) redox couple of symmetric cells were investigated by cyclic voltammetry, steady-state polarization, chronocoulometry, and electrochemical impedance spectroscopy. 1,3-Dialkylimidazolium cations affected the characteristics of the Pt electrode/electrolyte interface by adsorbing on the Pt electrode, as a result of electrostatic interactions, and further affected the reduction of I₃⁻ on the Pt electrode. Capacitance of the double layers of the Pt electrode/electrolyte interface was chiefly determined by capacitance of the compact layer according to the Helmoholtz theory.     

Electropolymerization of a poly(3,4-ethylenedioxythiophene) and functionalized, multi-walled, carbon nanotubes counter electrode for dye-sensitized solar cells and characterization of its performance

Composite films of poly(3,4-ethylenedioxythiophene) and functionalized, multi-walled, carbon nanotubes (PEDOT–MWCNT) were fabricated by a simple oxidative electropolymerization method. These films were formed on fluorine-doped, tin oxide, glass substrates as counter electrodes (CEs) of platinum-free, dye-sensitized solar cells (DSSCs). The surface morphology, formation mechanism and electrochemical nature of PEDOT–MWCNT films were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), cyclic voltammetry (CV) and alternating current (AC) impedance spectroscopy. The SEM and AFM images showed that PEDOT–MWCNT films were more porous than PEDOT films. CV and AC impedance spectroscopy revealed that the PEDOT–MWCNT electrode had higher electrocatalytic activity for the I₃⁻/I⁻ redox reaction and a smaller charge transfer resistance than the PEDOT electrodes. The energy conversion efficiency of the DSSC with a PEDOT–MWCNT CE was 13.0% higher than with a PEDOT CE using the same conditions with a ruthenium sensitizer.     

The underpotential deposition of Sn on Pt in acid media. Cyclic voltammetric and electrochemical quartz crystal microbalance studies

This work describes the studies of tin underpotential deposition on Pt electrodes in acid media by using cyclic voltammetry and electrochemical quartz crystal microbalance techniques. The voltammetric results were analyzed and it was verified the same values of anodic charge densities for the Sn in the solutions of perchloric acid with tin sulfate or chloride. The charge density value found (380 μC cm⁻²), in the potential region from 0.4 to 0.8 V, was attributed to the oxidation of Sn to Sn⁴⁺ (0.9 monolayer). The electrochemical quartz crystal microbalance results showed a mass-charge ratio of 16 g mol⁻¹ (43 ng cm⁻²) and 8 g mol⁻¹ (50 ng cm⁻²) in the same potential region for tin oxidation in different electrolyte solutions. These relationships are due to the formation of Sn(OH)₄ and SnO₂, respectively. The differences observed in relation to the formation of Sn(OH)₄ in solutions containing SnSO₄ and SnCl₂ were due to the presence of HSO₄⁻ in these solutions. In this manner the OH^- can adsorb on Sn in perchloric acid media with SnSO₄. The tin hydroxide and oxide are reduced in the scanning to the potentials between 1.55 and 0.05 V.     

Surface properties of self-assembled monolayer films of tetra-substituted cobalt, iron and manganese alkylthio phthalocyanine complexes

Self-assembled monolayer (SAM) films of iron (SAM-1), cobalt (SAM-2) and manganese (SAM-3) phthalocyanine complexes, tetra-substituted with diethylaminoethanethio at the non-peripheral positions, were formed on gold electrode in dimethylformamide (DMF). Electrochemical, impedimentary and surface properties of the SAM films were investigated. Cyclic voltammetry was used to investigate the electrochemical properties of the films. Ability of the films to inhibit common faradaic processes on bare gold surface (gold oxidation, solution redox chemistry of [Fe(H₂O)₆]³⁺/[Fe(H₂O)₆]²⁺ and underpotential deposition (UDP) of copper) was investigated. Electrochemical impedance spectroscopy (EIS), using [Fe(CN)₆]^3−/4− redox process as a probe, offered insights into the electrical properties of the films/electrode interfaces. Surface properties of the films were probed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The films were employed for the electrocatalytic oxidation of the pesticide, carbofuran. Electrocatalysis was evidenced from enhanced current signal and less positive oxidation potential of the pesticide on each film, relative to that observed on the bare gold electrode. Mechanism of electrocatalytic oxidation of the pesticide was studied using rotating disc electrode voltammetry.     

Application of the electrochemical quartz crystal microbalance technique to copper sonoelectrochemistry: Part 1. Sulfate-based electrolytes

The applicability of the electrochemical quartz crystal microbalance technique in an ultrasonic field created by an ultrasound probe is demonstrated for the electrodeposition of copper. Cyclic voltammetry and potentiostatic depositions in acidic sulfate-based copper electrolytes were performed at different ultrasonic intensities. The electrochemical quartz crystal microbalance was operated in ultrasonic fields with intensities up to 30 W cm⁻². For cyclic voltammetry, potential resolved and averaged (apparent) current efficiencies were calculated from mass and charge data in function of the amplitude of the ultrasonic horn. Ultrasound slightly affected the current efficiencies during copper deposition in cyclic voltammetry, but did not change the efficiencies during dissolution. During potentiostatic depositions the current efficiency increased from 84% to almost 100% upon application of ultrasound. Morphology of deposits prepared by potentiostatic depositions was analyzed by scanning electron microscopy, and found to be different at high ultrasonic intensities.     

Electrochemical quartz crystal microbalance study of formation and redox transformations of poly(diphenylamine)

The electropolymerization of diphenylamine (DPA) dissolved in 2 mol dm⁻³ H₂SO₄ has been studied on a gold electrode by cyclic electrochemical quartz crystal microbalance measurements. Similar experiments have been accomplished with DPA microcrystals attached to a gold electrode, in the presence of aqueous acidic media. In both cases the redox transformations of the poly(diphenylamine) films formed on Au have been investigated in solutions of different pHs. The cyclic voltammograms and the mass changes refer to two closely spaced redox reactions resulting in the formation of cation radicals and dications, respectively. The irregular, broad shape of the voltammetric waves may be explained by presence of other electrochemically active sites beside the linear diphenylbenzidine structure. The results indicate a pH and potential-dependent relative contribution of anions and hydrogen ions to the overall ion exchange process. The expulsion of protons is the predominant ion transport process at the beginning of the oxidation and the incorporation of anions occurs at a later stage of oxidation. The sorption/desorption of solvent molecules also contributes to the mass change observed during the redox transformations of the polymer.