Electrochemical reduction of the synthetic pyrethroid insecticides tralomethrin and tralocythrin at glassy carbon and mercury electrodes

Tralomethrin and tralocythrin may be electrochemically reduced in an initial irreversible two-electron process in the solvents acetonitrile and methanol at glassy carbon and mercury electrodes. The reduction potentials at mercury electrodes are between 600–900 mV less negative than at glassy carbon electrodes. Despite this difference, under both voltammetric and controlled potential electrolysis (CPE) conditions at either electrode surface, the overall reduction process for both compounds involves the elimination of two molecules of bromide ion to yield quantitatively the synthetic pyrethroid compounds deltamethrin (from tralomethrin) and cypermethrin (from tralocythrin) as products. The mechanism of bromide elimination therefore is highly dependent upon the electrode surface. At a glassy carbon electrode, reduction of the pyrethroid compounds is a concerted process involving bromide elimination. In contrast, the presence of the mercury electrode introduces the possibility of chemically reducing tralomethrin (or tralocythrin) to give deltamethrin (or cypermethrin) and a mercury bromide species at the electrode surface. The reaction provides a catalytic pathway for reduction and under voltammetric or CPE conditions the reduction process at the mercury electrode, therefore, is actually associated with the reduction of the mercury bromide species, present at the electrode surface, to elemental mercury and free bromide ion.     

An electrochemical and spectroelectrochemical study of pentacoordinated CoTPP on glassy carbon electrode

A glassy carbon electrode (GCE) modified with a monolayer of 4-aminothiophenol (4-ATP) and 5,10,15,20-meso-tetraphenylporphyrin cobalt (CoTPP) was prepared and used to examine the electroreduction of oxygen. Cyclic voltammetry showed that CoTPP chemisorbed on carbon via a 4-ATP monolayer (GCE/4-ATP–CoTPP) displayed a catalytic effect similar to that of physisorbed CoTPP on a GCE (GCE/CoTPP). UV–visible spectra recorded in solution and on chemisorbed layers showed that pentacoordination of the cobalt and a cobalt oxidation state +II were required to facilitate the complexation with oxygen. A stable oxygenated intermediate (4-ATP–Co^(II)TPP–O₂) was identified both in solution and on a carbon substrate. Finally, UV–visible reflectance (UVDRS) spectra recorded at different potentials revealed the relation between this oxygenated intermediate and the production of peroxide. Based on these data a catalytic mechanism is proposed.     

Mechanism of the catalysis by Alcaligenes eutrophus H16 hydrogenase of direct electrochemical reduction of NAD+

The mechanism of direct electrochemical reduction of NAD⁺ into NADH catalysed by Alcaligenes eutrophus H16 hydrogenase was analysed in thin layer electrochemical cells with platinum and carbon electrodes. Two phases can be distinguished in the catalytic reaction occurring on platinum electrodes. In the potential range from approximately ?0.620 to ?0.675 V (SCE) direct electron transfer occurred via the diaphorase-like dimer of the hydrogenase. Below ?0.69 V versus SCE the hydrogenase used hydrogen species adsorbed onto the platinum electrode, but no molecular hydrogen was required for this second catalytic phase. The mechanism was quite similar to those which had been previously determined for Rhodococcus opacus hydrogenase. This confirmed the very great similarity of the two enzymes, even if the maximum NAD⁺ reduction rate of 0.36 mM min^?1 obtained here remains lower than those reached with R. opacus hydrogenase in a previous study. Careful analysis of the experimental data obtained on a carbon electrode led to the conclusion that no direct electron transfer was observed on this material under the operating conditions used. On the other hand, the voltammetric experiments performed on a carbon electrode in a thin layer cell showed clearly the occurrence of a catalytic current due to the hydrogenase-catalysed reduction of NAD⁺ by molecular hydrogen. This may be a useful tool for further analysis of the hydrogenase kinetics.     

Evidence of the withdrawing effect of a pyridinium N-cation on the electrochemical reduction of gold (III) tetraphenylporphyrin

1-(Au^III-meso-tetraphenylporphyrin)-4-pyridinium dication Au^IIITPP⁺-β-Py⁺, formed by a pyridinium cation bearing a charged gold porphyrin at the N-position, has been synthetised and studied by stationary voltammetry, cyclic voltammetry, UV–Vis spectroelectrochemistry and exhaustive coulometry. The pyridinium cation and the porphyrin ring are both electroactive species on the investigated range of potentials. The obtained results allowed to discriminate the sites of the different charge transfers and to propose a mechanism for the three first electrochemical reduction processes involved. The gold porphyrin reduces before the reduction of the pyridinium cation whose signal is intercalated between two reduction steps of the porphyrin. A transient dimeric gold porphyrin is detected. The withdrawing effect of the pyridinium cation on the reduction potentials of the porphyrin is discussed and quantified.     

Pd–P electroless deposition on carbon steel: An electrochemical impedance spectroscopy study

By the combination of two techniques, namely, polarization curves and electrochemical impedance spectroscopy (EIS) it is possible to obtain information about solution/metal surface interactions occurring during Pd–P electroless deposition at the equilibrium potential, on carbon steel. The polarization curves indicate that the deposition reaction does not obey the mixed potential theory. The values of the charge transfer resistance, when compared with those obtained by the equivalent circuit proposed to model the EIS data, indicate that there are changes in the rate-determining step of the reaction over the course of the deposition. The values obtained for the elements of the equivalent circuit that correspond to the film, permit the characterization of the film as a semiconductor. This conduction feature seems to be due to the presence of incorporated phosphorus (≈7wt.%) in the deposit at the end of the deposition process.     

Iron(III) octaethylporphyrin chloride supported on glassy carbon as an electrocatalyst for oxygen reduction

Oxygen reduction was investigated at iron(III) octaethylporphyrin chloride adsorbed on a glassy carbon electrode. The title porphyrin was adsorbed irreversibly and strongly on the surface of a glassy carbon electrode. The electrochemical behavior and stability of the modified electrode were investigated using cyclic voltammetry, chronoamperometry and rotating disk electrode methods. The modified electrode showed clear but modest electrocatalytic activity for the reduction of oxygen to a mixture of water and hydrogen peroxide in buffered solutions on both the acid and basic sides of neutral with the domination of an overpotential of about 690 mV and an increase in peak current. The heterogeneous rate constant for the reduction of O₂ at the surface of the modified electrode and the diffusion coefficient of oxygen were determined by rotation disk electrode voltammetry using the Koutecký–Levich plots. In addition, iron(III) octaethylporphyrin chloride exhibited strong catalytic activity toward the reduction of H₂O₂.     

Effect of multi-walled carbon nanotubes as second phase on the copper electrochemical reduction behavior for fabricating their nanostructured composite films

Nanostructured copper/multi-walled carbon nanotube (Cu/MWCNT) films have been fabricated by means of pulse electrodeposition in an acidic plating bath containing copper sulfate and purified MWCNTs. The influence of MWCNTs on copper electrochemical reduction behavior is studied by cathodic polarization and electrochemical impedance spectrums (EIS) analysis. Results show that the existence of the hydrochloric acid purified MWCNTs in the electrolytes shows the accelerating action toward Cu electrochemical reduction during the electrodeposition process, making the Cu films finely grow with the crystal growth toward the (1 1 1) orientation compared to the pure Cu electrodeposited film. The resulting Cu/MWCNT composite film is comprised of Cu matrix and the spatial network of MWCNTs with a considerable high MWCNT content.     

Identification of reaction mechanism for anodic dissolution of metals using Electrochemical Impedance Spectroscopy

Electrochemical Impedance Spectroscopy (EIS) is a sensitive technique for determining the mechanistic pathway of an electrochemical reaction. In particular, the low frequency behavior of the impedance at various over-potentials can help differentiate between very similar mechanisms. Nine different mechanisms related to anodic dissolution of metals are simulated and the EIS patterns that are expected for these mechanisms are identified. This library of patterns reported here will help in eliminating certain mechanisms and identifying mechanisms with the least number of parameters that can be applied for modeling experimental EIS data.     

An electrochemical study on the interaction of surfactin with a supported bilayer lipid membrane on a glassy carbon electrode

Surfactin is an acidic lipopeptide with amphiphilic character, which can interact with a biomembrane. The interaction of surfactin with a supported bilayer lipid membrane on a glassy carbon electrode (GCE) was investigated by cyclic voltammetry and ac impedance spectroscopy in this paper. Surfactin could induce pores in the bilayer lipid membrane and even cause the destruction of the membrane. The mechanism of the interaction of surfactin with the supported bilayer lipid membrane was studied. The insertion of surfactin into the lipid membrane was the first step in the formation of pores. Subsequently, surfactin aggregated in the lipid membrane to form the pores. Along with the transition of surfactin molecules from the outer to the inner layer, which was connected to the surface of the GCE, the lipid membrane was destroyed.     

On the application of the logistic differential equation in electrochemical dynamics

The relevance of the logistic differential equation to the dynamics of the electrochemical intercalation and adsorption/desorption processes postulated recently in this Journal by Gonzales et al. (461 (1999) 161) and confirmed and elaborated soon after by Montella et al. (475 (1999) 190), is challenged. The equations postulated there are not evolution equations in terms of system dynamics but rather limiting forms valid in the special case of linear potential scan and a fully reversible or irreversible process conforming to the Langmuir isotherm. No reaction mechanism requiring replacement of the conventional kinetic equations by the logistic equation was reported. It is recalled here that peculiarities of the logistic equation such as bifurcation, period doubling and chaos are manifested only by its numerical approximation i.e. by the corresponding difference equation used as the real transformation (map) of the interval (0, 1) in the dynamics of discrete systems. These peculiarities do not appear for the continuous form of the logistic differential equation having closed form solution and describing continuous evolution towards the single stable equilibrium point at 1.0.     

Chiral resolution of R and S 1-phenylethanol on glassy carbon electrodes

The electrochemical oxidation of the chiral alcohol 1-phenyl ethanol (1PE), on carbon electrodes, using TEMPO and an achiral (luitidine) and chiral ((–)-sparteine) base was reinvestigated in order to resolve conflicting reports in literature and to ascertain whether electrochemical chiral resolution is indeed possible with this system. Investigations using electrochemical and FTIR techniques reveal that chiral oxidation occurs due to modification of the working electrode material by the chiral base. This important observation has not been reported previously, in relation to the mechanism discussed here.     

Electrooxidation of quercetin at glassy carbon electrode studied by a.c. impedance spectroscopy

The present paper reports cyclic voltammetry and a.c. impedance spectroscopy studies on adsorption and electrooxidation of quercetin (3,3′,4′,5,7-pentahydroxyflavone) compound at glassy carbon electrode surface in 0.1 M sodium acetate–acetic acid buffer in 90% methanol solution. The resulted information provided support for a cascade electrooxidation mechanism, which process commences with oxidation of catechol hydroxyl groups and involves strongly adsorbed reaction intermediate. The significance of each oxidation step is explained through associated charge-transfer resistance (derived for all individual oxidation steps and electrosorption of quercetin) and capacitance parameters. This work also presents an original way to regenerate the surface of glassy carbon electrode (after being blocked by quercetin oxidation products) through voltammetric cycling over the potential range negative to the hydrogen reversible potential. The above is realized by means of in situ evolved hydrogen, which species is capable of electrochemically reducing products formed during the cascade electrooxidation reaction of quercetin.     

Nonaqueous synthesis and reduction of diazonium ions (without isolation) to modify glassy carbon electrodes using mild electrografting conditions

Synthesis and reduction of diazonium cations in acetonitrile – without isolation of the solid – was carried out to modify glassy carbon (GC) electrodes with p-nitrophenyl or p-carboxyphenyl groups. Reduction of diazonium ions was investigated under a variety of conditions to further understand the process. Cyclic voltammetry at GC consistently yields two distinct peaks for the reduction of nitrobenzenediazonium ions; the first peak is particularly dependent upon the GC surface treatment and history. Subsequent reduction of the surface nitrophenyl groups was used to assess the extent of modification, and results were compared for different electrografting conditions. Electrolysis of diazonium ions, using a more positive reduction potential than typical, results in modified surfaces, and the electrolysis charge can be varied to affect the surface coverage of modifying groups, though with %RSD > 25%. Electrochemistry of common redox systems (dopamine, ruthenium hexammine, ferrocyanide) was studied as a function of modification parameters. Under mild electrografting conditions, it appears that inhomogeneously covered surfaces are produced, with blocking properties similar to spontaneously grafted GC. Additionally, electron transfer for Fe^2+/3+ is suppressed at carboxyphenyl-modified GC rather than accelerated as it is at oxidized carbon electrodes containing carbonyl groups.     

Electrochemical reduction of CO2 with high current density in a CO2 + methanol medium at various metal electrodes

Various metals were used as working electrodes in the electrochemical reduction of highly concentrated CO₂ in a CO₂ + methanol medium. Reduction products were CO, CH₄, C₂H₄ and methyl formate (HCOOCH₃). Methyl formate was formed by the reaction between solvent, methanol, and a CO₂ reduction product, formic acid, which corresponds to formic acid formation in aqueous systems. Basically, most electrodes gave the same principal product both in the present system and in the aqueous system. At W, Ti and Pt electrodes, CO₂ reduction was inefficient. Sn and Pb electrodes were active in formate production. However, CO was formed much more efficiently in the present system than in the aqueous system. It was indicated that Sn and Pb served as electrodes catalyzing formate production, while the supporting electrolyte, tetrabutylammonium cation, promoted CO formation. Electrolysis at Ag, Zn and Pd electrodes yielded CO mainly. Hydrocarbon formation at a Cu electrode was less efficient than in aqueous systems. However, hydrocarbon was formed efficiently at an Ni electrode. These differences in hydrocarbon formation in the present system, in comparison with aqueous systems, could be explained by the balance between hydrogen atom and CO₂ reduction intermediates on the electrode surface.     

Electrochemical reduction of copper(II) salen at carbon cathodes in dimethylformamide

In dimethylformamide containing tetramethylammonium tetrafluoroborate, copper(II) salen undergoes a three-step reduction at a glassy carbon cathode; the first voltammetric wave is attributed to reduction of copper(II) to copper(I), the second is due to reduction of copper(I) to copper metal, and the third voltammetric wave results from reduction of the salen-dianion. In the presence of adventitious water which coordinates loosely to square-planar copper(II) salen and to distorted tetrahedral copper(I) salen, the first voltammetric wave exhibits only semi-reversible behavior, which differs from that reported for other metal–salen complexes. However, removing the water causes reversibility to be achieved. Upon addition of acetic acid, a new complex is formed which exhibits an irreversible one-electron reduction; a mechanism for the formation and electrochemical behavior of this species has been formulated.     

Impedance voltammetry of electro-dimerization mechanisms: Application to the reduction of the methyl viologen di-cation at mercury electrodes and aqueous solutions

The faradaic impedance for an electrode mechanism with a reversible homogeneous dimerization reaction following the electron transfer step is derived. The chemical reaction shows up in the frequency dependence of the faradaic impedance and admittance in a similar way as deduced by Sluyters-Rehbach and Sluyters (J. Electroanal. Chem. 23 (1989) 457; J. Electroanal. Chem. 26 (1990) 237) for a homogeneous first-order chemical reaction. Two limiting cases can be distinguished in which the general expression reduces to the simpler Randles or pseudo-Randles expression. Under those conditions, the presence of the dimerization reaction can be inferred from the potential dependence of the impedance parameters. The theory is applied to the reduction of the methyl viologen di-cation at mercury electrodes in aqueous solution. The rate and the equilibrium constants for the dimerization reaction and the standard potential for the electron transfer step are obtained from the Warburg coefficient, while the potential dependence of the irreversibility coefficient allows the calculation of the standard rate constant and the transfer coefficient for the electron transfer step.     

Voltammetric studies of electrochemical pretreatment of rotating-disc glassy carbon electrodes in phosphate buffer

Cyclic voltammetry was used to study the formation of graphite oxide film on the surface of rotating-disc glassy carbon electrodes. The film was formed by continuous potential cycling in phosphate buffer electrolyte. It was found that when cycling between the totally oxidized and totally reduced potential ranges (between + 2.3 and ?1.2V vs. SCE), well-shaped redox peaks were obtained. Typical voltammograms comprised one cathodic and two anodic waves, where the anodic waves were directly related to the oxidation of surface-bound products associated with the cathodic process. All three processes were pH dependent. Freshly formed surface graphite oxide films were not stable and would undergo transformation to give the stable state upon potential cycling between potentials ± 1.0V. The ultimate stable voltammograms were characterized by two electrode processes. The quasi-reversible process involved surface quinone-like functional groups, while the other irreversible process might be related to the uptake and release of hydrogen ions at the porous graphite oxide film.     

电热电化学治疗牙龈瘤的初步研究

目的:探讨电热电化学方法治疗牙龈瘤的可行性及疗效。方法:用电热电化学治疗仪,把电热针当作负极,插入牙龈瘤内,把正极电化学针插入牙龈瘤波及的邻牙间隙内,进行治疗,观察其疗效。结果:81例牙龈瘤患者中,可评估的66例随访48个月,66例均经电热电化学治疗(ETECT)一次治愈,不拔邻牙、未见复发,4年治愈率达100%(66/66)。结论:用电热电化学治疗牙龈瘤,操作简便,安全可靠,为治疗牙龈瘤提供了一种新的方法。