Electrochemical evaluation of the reaction rate between methyl viologen mediator and diaphorase enzyme for the electrocatalytic reduction of NAD+ and digital simulation for its voltammetric responses

The electrocatalytic reduction of NAD⁺ using diaphorase enzyme was studied. Methyl viologen was used as an electron transfer mediator between an electrode and the enzyme. A catalytic wave for the reduction of NAD⁺ when all the species were in the solution was measured with cyclic voltammetry at a gold-amalgam electrode which showed low background currents at negative potentials. Steady-state currents could be obtained under the conditions of slow scan rate, low methyl viologen concentration, and high NAD⁺ concentration as the electrode reaction was converted to an electrochemical-catalytic (EC′) reaction. The bimolecular rate constant for the reaction of the reduced methyl viologen with the oxidized diaphorase was estimated as 7.5×10³ M^?1 s^?1 from the slope of the current versus [MV²⁺] plot. Another slope of the current against the square root of the enzyme concentration also gave a close value of 6.7×10³ M^?1 s^?1. In the calculation of the rate constant, the stoichiometric factor when it is not one-to-one was considered. With the evaluated rate constant, digital simulation using the suggested reaction mechanism was compared with the experimentally obtained voltammograms. Satisfactory agreement indicates that the evaluation methods of the rate constant and the suggested mechanism are appropriate for the mediated enzyme-catalyzed electrochemical reactions.     

Electrocatalysis of carbon dioxide with hemin and hemin-coated latex

Carbon dioxide was reduced catalytically with hemin in 2-propanol to reveal a cathodic wave at ?1.1 V. The catalytic activity was demonstrated with the UV spectra in which Fe²⁺ (hemin) was oxidized with CO₂. Hemin was immobilized on polyallylamine–polystyrene latex as a quasi solid catalyst. The hemin-coated latex in a suspension of 2-propanol showed the catalytic activity for CO₂ similar to the hemin molecule. The rate constant of the catalytic reaction of dissolved hemin was evaluated on the assumption of the pseudo-first order reaction to give 11.5 M^?1 s^?1, whereas the hemin-coated latex suspension did not obey the simple combination of the pseudo-first order reaction and diffusion because of the reaction with CO₂ accumulated in the particles.     

Potential-dependent electrochemiluminescence of luminol in alkaline solution at a gold electrode

The behavior of luminol electrochemiluminescence (ECL) at a polycrystalline gold electrode was studied under conventional cyclic voltammetric (CV) conditions. At least six ECL peaks were observed at 0.28 (ECL-1), 0.56 (ECL-2), 0.95 (ECL-3), 1.37 (ECL-4), ?0.43 (ECL-5) and 1.00 (ECL-6, a broad wave after the reverse scan from +1.66) V (vs. SCE), respectively, on the curve of ECL intensity versus the potential. These ECL peaks were found to depend on the presence of O₂ and N₂, the pH of the solution, KCl concentration, scan rate, and potential scan ranges. The emitter of all ECL peaks was identified as 3-aminophthalate by analyzing the CL spectra. It is believed that ECL-1 at 0.28 V was correlated to luminol radicals produced by the electro-oxidation of luminol anion and ECL-2 at 0.56 V was caused by the reaction of luminol radical anions with gold oxide formed on the electrode surface. ECL-1 and ECL-2 could be strongly enhanced by O₂ and O₂^?. ECL-3 at 0.95 V was likely to be due to the reaction of luminol radical anions with O₂ oxidized by OH^?. ECL-4 at 1.37 V suggested that OH^? was electro-oxidized to HO₂^? at this potential and then to O₂^?, which reacted with luminol radical anions to produce light emission. ECL-5 at ?0.43 V seems to be due to the reaction of luminol with ClO^? electrogenerated at higher positive potential and HO₂^? electrogenerated at negative potential. ECL-6 was attributed to the reaction of luminol radical anions and ClO^? electrogenerated at higher positive potential. The results indicated that luminol ECL can be readily initiated by various oxygen-containing species electrogenerated at different potentials, leading to multi-channel light emissions. Furthermore, the present work also reveals that ECL-2 is a predominant ECL reaction route at a gold electrode with higher potential scan rates under CV conditions.     

Spectroelectrochemical investigations on the reduction of thin films of hexadecafluorophthalocyaninatozinc (F16PcZn)

Redox potentials were determined for solutions and thin films of hexadecafluorophthalocyaninatozinc (F₁₆PcZn). A value of ?0.6 V versus SCE was determined for the first reduction to the radical anion of F₁₆PcZn^? in N,N′-dimethylformamide (DMF) and ?0.9 V versus SCE for the second reduction to F₁₆PcZn^2?. Both potentials were shifted about 0.4 V towards positive potentials when compared to the unsubstituted phthalocyaninatozinc (PcZn) caused by the stabilization of the π-system by the electron-withdrawing fluorine atoms in the ligand. Vapor-deposited thin films of F₁₆PcZn on indiumtinoxide (ITO) were reduced in contact with aqueous electrolytes of pH 5.5 to avoid H⁺-reduction in acidic regimes. A chemically reversible reduction accompanied by cation intercalation was found. The kinetics of the reaction were studied in detail by cyclic voltammetry under variation of the intercalating ionic species, the film thickness and sweep rate. From the charge uptake as well as from the dependence of peak current densities on the square root of the sweep rate it was found that the reaction rate is limited by the diffusion of intercalating cations. Optical absorption spectra were collected in situ. An irreversible structural change was observed during initial reduction, also in accordance with a strongly irreversible initial CV before the reversible behavior was obtained. The presence of the dianionic F₁₆PcZn^2? was detected even under conditions of an average charge uptake of less than 1 electron/molecule. From the analysis of peak current densities according to the Randles–Sev?ik equation as well as the observed charge flow dependent on film thickness and chronoamperometric characterization of the reaction a diffusion constant D_i for K⁺ in F₁₆PcZn in the range from 1.6×10^?12 to 8.0×10^?12 cm² V^?1 s^?1 was calculated.     

Photoelectrochemical reduction of CO2 mediated with methylviologen at roughened silver electrodes

The photoelectrocatalytic effect for the reduction of CO₂ mediated with methylviologen (MV) was studied at mercury, polished silver and roughened silver electrodes using electrochemical and surface-enhanced Raman scattering (SERS) techniques. A large photoelectrocatalytic effect for the reduction of CO₂ in the presence of MV was observed at the roughened silver electrode, whereas there was only a very small photoelectrocatalytic current at a more negative potential on mercury and polished silver electrodes. The SERS spectra of MV in the presence and absence of CO₂, along with the electrochemical results, demonstrate that the surface adsorbed complexes, MV⁺–Ag and MV⁰–Ag, played a role as the mediator for photoinduced electron transfer to CO₂ in the solution. The results also suggest that the surface plasmon resonance of the nanoscale silver particle contributes to the overall photoelectrocatalytic effect on a roughened silver electrode.     

An in situ UV-vis and IR spectroelectrochemical study of the deposition of a hydroquinone anion salt on platinum electrodes from dichloromethane solutions

Cyclic voltammetry with a platinum electrode of hydroquinone (BQH₂) solutions in dichloromethane, containing tetrabutylammonium perchlorate supporting electrolyte, shows a sharp asymmetric irreversible oxidation peak at about ?0.3 V (SCE). This feature is seen, in addition to the expected features in this system, when the cycle is extended to potentials more negative than ?0.6 V (SCE). Cyclic voltammetry, in situ UV-vis and infrared spectroelectrochemistry have shown that hydroquinone anion (BQH^?) is formed at negative potentials and this appears to arise via surface decomposition of hydroquinone to p-benzosemiquinone (BQH) followed by reaction of BQH with the p-benzoquinone radical anion (BQ^?). The sharp asymmetric peak in the cyclic voltammograms is due to oxidation of the hydroquinone anion in the insoluble tetrabutylammonium salt on the electrode surface. The oxidation of BQH^? appears to occur via disproportionation of (BQH) and leads to BQH₂ and p-benzoquinone (BQ) as the products.     

Temperature-dependence of oxygen reduction activity at a platinum electrode in an acidic electrolyte solution investigated with a channel flow double electrode

The temperature dependence of the oxygen reduction reaction (orr) activity at a platinum electrode in 0.1 M HClO₄ electrolyte solution was investigated with a channel flow double electrode at 20–90 °C. We demonstrate that an apparent rate constant (k_app) for the orr can be evaluated from the hydrodynamic voltammograms by correcting the oxygen concentration in the electrolyte solution. The apparent activation energy for the k_app was found to be 38 kJ/mol at ?0.525 V vs. E⁰ (0.760 V vs. RHE at 30 °C). Production of H₂O₂ was not detected at potentials more positive than 0.3 V in the temperature region examined. One capacitive semicircle was observed in electrochemical impedance spectroscopy for the orr, suggesting the fast complete reduction of intermediates to H₂O following the rate determining step of the first electron-transfer.     

Electrochemical characteristics of a cobalt pentacyanonitrosylferrate film on a modified glassy carbon electrode and its catalytic effect on the electrooxidation of hydrazine

Electrochemically prepared thin films of cobalt pentacyanonitrosylferrate (CoPCNF) were used as surface modifiers for glassy carbon electrodes. The electrochemical behavior of a CoPCNF-modified glassy carbon electrode was studied by cyclic voltammetry; the modified electrode shows one pair of peaks with a surface-confined characteristic in 0.5 M KNO₃ as supporting electrolyte. The effect of different alkali metal cations in the supporting electrolyte on the behavior of the modified electrode was studied and the transfer coefficient (α) and charge transfer rate constant (k_s) for the electron transfer between the electrode and modifier layer were calculated. The experimental results show that the peak potential and peak current vary with different alkali metal cations, but anions such as Cl^?, NO₃^?, CH₃COO^?, H₂PO₄^?/HPO₄^2? and SO₄^2? at 0.5 M concentration have no effect on the peak potential and peak current. An extensive study showed that the response of the modified electrode is not affected within a pH range of 2–8. The CoPCNF films on glassy carbon electrodes show excellent electrocatalytic activity toward the oxidation of hydrazine in 0.5 M KNO₃. The kinetics of the catalytic reaction were investigated by using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and chronoamperometry. The average value of the rate constant for the catalytic reaction and the diffusion coefficient were evaluated by different approaches for hydrazine.     

The effect of triphenylmethyl cations on the electroreduction of O2 in nitrobenzene or acetonitrile

The electroreduction of O₂ in two aprotic solvents (nitrobenzene and acetonitrile) was examined in the absence and in the presence of the very strong Lewis acid, triphenylmethyl cation, φ₃C⁺ (φ=phenyl). The addition of φ₃C⁺ causes the electroreduction of O₂ to change from the one-electron reduction to O₂^? into the two-electron reduction to φ₃CO₂Cφ₃ at a potential that is 1 V more positive than that where O₂ is reduced to O₂^?. This large positive shift in potential is used to estimate a value of the equilibrium constant for the association between O₂^2? and two φ₃C⁺ cations (K=3×10²⁶ M^?2). The mechanism of the reduction of O₂ to φ₃CO₂Cφ₃ can be regarded as an inner-sphere electron-transfer reaction between φ₃C and O₂ or, equivalently, as a radical addition to O₂.     

Electrochemical properties of a tetrabromo-p-benzoquinone modified carbon paste electrode. Application to the simultaneous determination of ascorbic acid,dopamine and uric acid

The electrochemical study of a tetrabromo-p-benzoquinone modified carbon paste electrode (TBQ-MCPE), as well as its efficiency for electrocatalytic oxidation of ascorbic acid, dopamine and uric acid, is described. Cyclic voltammetry was used to investigate the redox properties of this modified electrode at various solution pH values and at various scan rates. Three linear segments were found with slope values of ?58.4 mV/pH, ?28.1 mV/pH and 0.0 mV/pH in the pH range 2.0–7.1, pH 7.1–9.0 and pH 9.0–11.0, respectively. The apparent charge transfer rate constant, k_s, and transfer coefficient, α, for electron transfer between TBQ and CPE were calculated as 3.79 ± 0.10 s^?1 and 0.55, respectively. The electrode was also employed to study the electrocatalytic oxidation of AA, using cyclic voltammetry, chronoamperometry and differential pulse voltammetry as diagnostic techniques. It has been found that the oxidation of AA at the surface of TBQ-MCPE occurs at a potential of about 430 mV less positive than that of an unmodified CPE. The diffusion coefficient of AA was also estimated using chronoamperometry. The kinetic parameters such as the electron transfer coefficient, α, and heterogeneous rate constant, $k_{\mathrm{h}}^{\prime }$, for oxidation of AA at the TBQ-MCPE surface was determined using cyclic voltammetry. Differential pulse voltammetry (DPV) exhibits two linear dynamic ranges and a detection limit of 0.62 μM for AA. In DPV, the TBQ-MCPE could separate the oxidation peak potentials of AA, DA and UA present in the same solution, though at the unmodified CPE the peak potentials were indistinguishable. This modified electrode was quite effective not only to detect AA, DA and UA, but also in simultaneous determination of each component concentration in the mixture.     

Formation of ferryl porphyrin by electrochemical reduction of iron porphyrin in aqueous solution

The formation of ferryl porphyrin, induced by electrochemical reduction of Fe^IIITMPyP, was examined by measuring luminol chemiluminescence and recording the absorption spectrum using a flow-injection method. Both the anodic oxidation and the cathodic reduction of iron(III) porphyrin induced the formation of a ferryl species, which is widely accepted to be responsible for stimulating luminol chemiluminescence. Emission was observed both below the reduction potential of Fe^IIITMPyP (?0.08 V at pH 11, ?0.02 V at pH 7 and 0.15 V at pH 3) and above the oxidation potential (0.6 V at pH 11, 0.75 V at pH 7 and 1.1 V at pH 3). The emission was relatively greater below the reduction potential than in the oxidative region. It was significantly higher especially at lower pH (<pH 4). The cathodic emission was largely dependent on the concentration of dissolved molecular oxygen while the anodic emission was not. Emission on both sides was inhibited significantly by the addition of alkene solutions downstream of the working electrode. The spectra on both the positive and negative potential sides were shifted to longer wavelengths (>424 nm) compared to the original spectrum of Fe^IIITMPyP (422 nm). Computer analysis also showed that the spectra on both sides were perfectly reproduced by a combination of two spectra, those of Fe^IIITMPyP and OFe^IVTMPyP (436 nm in pH 11). We therefore believed that the ferryl species, formed by the autoxidation reaction of electro-generated ferrous porphyrin, was responsible for the emission. Although, ferryl formation by anodic oxidation has been reported, this is the first time, to our knowledge, that the cathodic formation has been reported.     

Oxygen reduction on copper in neutral NaCl solution

The reduction of oxygen on copper in neutral unbuffered 1 mol dm^?3 NaCl has been studied using rotating ring-disc electrodes at six oxygen concentrations equivalent to atmospheres of 2% O₂ + N₂ to 100% O₂. Steady-state potentiostatic measurements show that the reaction is first order with respect to [O₂] and that, following adsorption of O₂, the first electron transfer is rate determining. In 50% O₂ + N₂ and 100% O₂, a cathodic oxygen reduction peak is observed in both potentiodynamic and potentiostatic experiments at a disc potential of ?0.3 to ?0.4 V/SCE. The reaction is dominated by the overall four-electron reduction to OH^?, with only small amounts of peroxide detected by the ring electrode at disc potentials corresponding to the formation of the cathodic oxygen reduction peak. Tafel slopes increase with [O₂] and vary from ?0.13₅ V in 2% O₂ + N₂ to a limiting value of ?0.16 V to ?0.18 V in air, 50% O₂ + N₂ and 100% O₂.The results are explained by a mechanism involving oxygen reduction on two types of surface site with different reactivities. The most catalytic surface is believed to comprise Cu(0) and Cu(I) sites, where the Cu(I) species is stabilized as Cu(OH)_ads and/or submonolayer Cu₂O. The less catalytic site consists of Cu(0) only. Oxygen reduction is believed to proceed by a series pathway involving an adsorbed peroxide intermediate on both sites. Peroxide is reduced to OH^? prior to desorption at Cu(0) sites, but some is released before being reduced at Cu(0)/Cu(I) sites. Surface coverage by catalytic Cu(0)/Cu(I) species is favoured by a higher interfacial pH and more positive disc potentials.     

Electrocatalytic oxidation of styrene by a high valent ruthenium porphyrin cation radical

A sterically-hindered carbonylruthenium(II) porphyrin Ru^II(CO)(TMP) (where TMP=meso-tetramesitylporphyrinato dianion) has been synthesized. Chemical oxidation of Ru^II(CO)(TMP) by m-chloroperbenzoic acid (m-CPBA) gives the dioxoruthenium(VI) porphyrin (Ru^VI(O)₂TMP). Cyclic voltammograms show that Ru^VI(O)₂TMP is reversibly oxidized at E_1/2=+1.24 V in CH₂Cl₂. Thin-layer absorption spectra for oxidation of Ru^VI(O)₂TMP at +1.32 V indicates that the product is a porphyrin cation radical (Ru^VI(O)₂TMP⁺). Electrogenerated Ru^VI(O)₂TMP⁺ reacts selectively with styrene to give phenylacetaldehyde (96%) and benzaldehyde (4%). We report the first case of styrene oxidation by high valent ruthenium porphyrin under electrochemical conditions. An electrocatalytic oxidation reaction scheme 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.     

Study of the electrocatalytic reduction of nitrite with silicotungstic heteropolyanion

Three couples of reversible redox waves of the SiW₁₂O^4?₄₀ anion which are composed of two one-electron and one two-electron processes occur in the potential range +0.1 to ?0.7 V in aqueous solutions. The first (most positive) and third (most negative) redox waves exhibit good electrocatalytic activities for nitrite reduction in acid solutions with pH < 2 for the former and with $\text{pH}\text{? 4}$ for the latter. The behavior of the third catalytic wave, which is quite unusual, was studied in detail. The rate constant governing the reduction of NO^?₂ by the first wave of SiW₁₂O^4?₄₀ was measured by an ultramicrodisk electrode as 3.73 × 10³ M^?1 s^?1.     

EFFECT OF FLUORIDATED VARNISH AND SILVER DIAMINE FLUORIDE SOLUTION ON ENAMEL DEMINERALIZATION: pH-CYCLING STUDY

Objective:

In the present investigation, the anticariogenic effect of fluoride released by two products commonly applied in infants was evaluated.

Methods:

Bovine sound enamel blocks were randomly allocated to each one of the treatment groups: control (C), varnish (V) and diamine silver fluoride solution (D). The blocks were submitted to pH cycles in an oven at 37°C. Next, surface and cross-sectional microhardness were assessed to calculate the percentage loss of surface microhardness (%SML) and the mineral loss (∆Z). The fluoride present in enamel was also determined.

Results:

F/Px10^-3(ANOVA, p<0.05) in the 1^stlayer of enamel before pH-cycling were (C, V and D): 1.61^a; 21.59^band 3.98^c. The %SMH (Kruskal-Wallis, p<0.05) were: - 64.0^a, -45.2^band -53.1^c. %∆Z values (ANOVA, p<0.05) were: -18.7^a, -7.7^band -17.3^a.

Conclusion:

The data suggested that the fluoride released by varnish showed greater interaction with sound enamel and provided less mineral loss when compared with silver diamine solution.     

Electrochemical behavior of Ni particles modified polypyrrole films studied by EQCN technique

Polypyrrole films modified with Ni Particles were eletrodeposited using a constant potential on a Pt/quartz crystal electrode with the aim to investigate the changes in the voltammetric and mass profiles compared with pure polypyrrole. The electrochemical behavior of the polypyrrole with Ni showed a new cathodic peak at ?0.73 V associated to Ni²⁺ reduction. Besides, it was characterized that there is a redox-pair of the polymer and Ni and a model was proposed to explain the phenomenon considering the eletroneutrality principle. The transport of the ${\mathrm{ClO}}_4^{-}$ ion into (or outside) the polymer is opposite to the expected direction. This occurs due to the redox-pair between the PPy chain and Ni species which can be explained by a loss of anions during Ni oxidation and the gain of anions during the Ni reduction.     

Determination of reaction rate between cathodically formed FeII-triethanolamine-complex and FeIII-hepta-d-gluconate complex by cyclic voltammetry

Electron transfer reactions between iron^II/III-complexes are of general interest for corrosion processes, biological processes and technical applications. The cathodic reduction of Fe^III-triethanolamine (TEA) complex to form the Fe^IITEA-form of the complex has been examined by cyclic voltammetry. In the presence of Fe^III-d-heptagluconate (HDGL), complex catalytic currents are observed due to homogenous chemical reaction between Fe^IITEA and Fe^III(HDGL)₂ which regenerates Fe^IIITEA. This method can be used to determine the rate constant of the redox reaction between Fe^IITEA and Fe^III(HDGL)₂ with $k_{\mathrm{f}}^{\prime }=88\pm 12{\mathrm{dm}}^3{\mathrm{mol}}^{-1}{\mathrm{s}}^{-1}$.     

Slow crack growth and reliability of dental ceramics

Objective

To determine the slow crack growth (SCG) and Weibull parameters of five dental ceramics: a vitreous porcelain (V), a leucite-based porcelain (D), a leucite-based glass-ceramic (E1), a lithium disilicate glass-ceramic (E2) and a glass-infiltrated alumina composite (IC).

Methods

Eighty disks (Ø 12 mm × 1.1 mm thick) of each material were constructed according to manufacturers’ recommendations and polished. The stress corrosion susceptibility coefficient (n) was obtained by dynamic fatigue test, and specimens were tested in biaxial flexure at five stress rates immersed in artificial saliva at 37 °C. Weibull parameters were calculated for the 30 specimens tested at 1 MPa/s in artificial saliva at 37 °C. The 80 specimens were distributed as follows: 10 for each stress rate (10⁻², 10⁻¹, 10¹, 10² MPa/s), 10 for inert strength (10² MPa/s, silicon oil) and 30 for 10⁰ MPa/s. Fractographic analysis was also performed to investigate the fracture origin.

Results

E2 showed the lowest slow crack growth susceptibility coefficient (17.2), followed by D (20.4) and V (26.3). E1 and IC presented the highest n values (30.1 and 31.1, respectively). Porcelain V presented the lowest Weibull modulus (5.2). All other materials showed similar Weibull modulus values, ranging from 9.4 to 11.7. Fractographic analysis indicated that for porcelain D, glass-ceramics E1 and E2, and composite IC crack deflection was the main toughening mechanism.

Significance

This study provides a detailed microstructural and slow crack growth characterization of widely used dental ceramics. This is important from a clinical standpoint to assist the clinician in choosing the best ceramic material for each situation as well as predicting its clinical longevity. It also can be helpful in developing new materials for dental prostheses.