Investigation of the electrochemical behavior of metallo-tetraazaporphyrin modified silver and pyrolytic graphite electrodes in aqueous nitrite solution

Five new complexes of the type [M(OBTTAP)] (where M = Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺; OBTTAP = 2,3,7,8,12,13,17,18-octakis(benzylthio)-5,10,15,20-tetraazaporphyrin(2?)) were used to prepare modified silver and pyrolytic graphite electrodes and their electrochemical behavior towards the reduction of aqueous nitrite solution was investigated in acidic, neutral and alkaline medium. Of these electrodes, [Co(OBTTAP)] and [Cu(OBTTAP)] modified electrodes showed affinity for ${\mathrm{NO}}_2^{-}$ ions and thereby exhibited significantly altered cathodic responses. The [Co(OBTTAP)]/Ag electrode in acidic, neutral and also in alkaline nitrite solutions, exhibited a new one electron irreversible reduction wave at E_p,c ～ ?0.2 V vs. Ag|AgCl. The [Cu(OBTTAP)]/Ag electrode in acidic and neutral nitrite solutions also behaved in a similar manner. However, in alkaline aqueous nitrite solution it showed two successive one electron reductions, the first reversible reduction at E_1/2 = ?0.26 V and the second irreversible reduction at E_p,c = ?0.73 V vs. Ag|AgCl. Controlled potential electrolysis of alkaline nitrite solutions using a [Cu(OBTTAP)]/Ag cathode led to isolation of a nitrosyl complex. The peak current, particularly with [Cu(OBTTAP)]/Ag microelectrode in alkaline medium, showed a linear response to $[{\mathrm{NO}}_2^{-}]$ and could be exploited in the electrochemical sensing and determination of ${\mathrm{NO}}_2^{-}$.     

Potentiometric determination of activity coefficients for NH4Cl in the ternary NH4Cl/LiCl/H2O mixed electrolyte system

This work reports the results of the determination of mean activity coefficients for NH₄Cl in ternary (NH₄Cl(m₁) + LiCl(m₂) + H₂O) electrolyte system by potentiometric method at 25 °C. The experimental potentiometric data were obtained by combining a solvent polymeric (PVC) ammonium-selective membrane electrode (${\mathrm{NH}}_4^{+}$ ISE), containing a mixture of nonactin/monactin as ionophore, and an Ag|AgCl electrode. The measurements were performed, at constant ionic strength, in different series of mixed salt solutions characterized by a fixed salt mole ratio r (where r = m₁/m₂ = 25, 50, 75, 100). The non-ideal behavior of the system was described on the base of the Pitzer ion-interaction model for mixed salts, over the molality ranging from 0.01 to 3 mol/kg.     

Electrochemical reduction of l-(meso-tetraphenylporphyrin)-pyridinium cations

1-(M-meso-tetraphenylporphyrin)-4-R-pyridinium cations MTPP-β-(4-R-Py⁺) (M = H₂, Zn^II, Ni^II, Cu^II or Pd^II, R = H, phenyl or pyridine), formed by a pyridinium cation bearing a porphyrin, metalated or not, at the N-position, were synthesised electrochemically and studied by polarography, stationary and cyclic voltammetry, UV–Vis spectroelectrochemistry and exhaustive coulometry. The pyridinium and the porphyrin rings are both electroactive species in the range of potentials investigated. The results allowed us to discriminate the distinct sites of the different charge transfers and to propose a mechanism for the primary electrochemical reduction step which corresponds to a one-electron transfer to the pyridinium cation. The redox behaviour of the pyridinium in these systems depends on the nature of the core of the porphyrin (free-base or metalated). The differences observed between the free-base and the metalated porphyrin systems were explained by differences in the distribution of the electronic density on the porphyrin ligand in the two types of molecules. In the metalated porphyrin–pyridinium systems, the measured reduction potentials of the pyridinium cation were correlated with the electronegativity ${\chi }_{\mathrm{M}}^{\mathrm{P}}$ of the metal in the porphyrin core. The obtained correlation revealed the occurrence of strong coulombic interactions between the metal in the porphyrin and the pyridinium. The chemical reactivity of the electrochemically generated pyridinium radical depends on the substitution at the 4-position of the ring. The generation of a dimeric species following the first electron transfer to the pyridinium cation was detected by cyclic voltammetry.     

Glucose sensing based on an effective conversion of O2 and H2O2 into superoxide anion radical with clay minerals

An effective promotion for the generation of superoxide anion radical from H₂O₂ and O₂ was accomplished with montmorillonite K10 clay mineral (denoted Mont. K10). This catalytic event was characterized by a fluorescence assay using amplex red and superoxide dismutase (SOD) as probes. The iron species (Fe^II and Fe^III) contained in Mont. K10 are thought to contribute to this catalytic event. Due to this property, a glucose sensor was fabricated on the basis of Mont. K10, amplex red, and glucose oxidase (GO_x). When glucose is injected with amplex red over the clay|GO_x electrode, H₂O₂ is produced in the enzyme zone, transduced to superoxide anion radical by the clay, and quenched by amplex red. Since the resulting product, resorufin, is emission-active under the irradiation of visible light (λ_ex: 563 nm; λ_em: 583 nm, pH 8), the analyte can thus be recognized and reported as optical signals with a luminescence spectrometer via a Y-type optical fiber atop of the electrode. The optical signal showed a linear response to glucose up to 150 μM at pH 8. The lower and upper limits were estimated to be 1.0 and 150 μM, respectively, at this pH. These results suggest that Mont. K10 is a promising ${\mathrm{O}}_2^{-\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ transducer.     

The electrochemical behaviour of the Pr(III)/Pr redox system at Bi and Cd liquid electrodes in molten eutectic LiCl–KCl

The electrode reactions of LiCl–KCl–PrCl₃ solutions at the surface of liquid electrodes, i.e. Cd and Bi, were investigated by electrochemical techniques. The redox potential of the Pr(III)/Pr couple at the liquid electrodes were observed at more positive potential values than those at one inert electrode. This potential shift was thermodynamically analysed by a lowering of activity of Pr in the metal phase. Cyclic voltammetry, using Cd and Bi pools as working electrodes, was conducted in order to study the reaction mechanism. The results suggest a quasi-reversible behaviour of the Pr(III)/Pr electrochemical system at the liquid electrodes, and the values of the kinetic parameters, k⁰ and α as well as the reversible half wave potential, $E_{1/2}^{\mathrm{r}}$, have been obtained. The interdiffusion coefficients of Pr in the metal phases seem to be similar to the diffusion coefficient of Pr(III) in solution. From comparison between the equilibrium potential adopted by a praseodymium electrode immersed in a solution containing Pr(III) ions, and the calculated $E_{1/2}^{\mathrm{r}}$ of the system Pr(III)/Pr–M (being M = Cd or Bi) it has been possible to estimate the activity coefficients of Pr in the Cd and Bi phases. Open circuit chronopotentiometry using Cd or Bi film electrodes was also conducted. Electromotive force, emf, measurements for various intermetallic compounds in two-phase coexisting states were carried out in the temperature range of 673–823 K. The activities and relative partial molar Gibbs energies of Pr were obtained from the measured emf for various Pr–Cd and Pr–Bi intermetallic compounds, PrCd₁₁, PrCd₆, PrCd_4.46, PrBi₂ and PrBi. The relative partial molar entropies and enthalpies of Pr were also calculated from the temperature dependence of the emf. The standard Gibbs energies of formation for the Pr–Cd and Pr–Bi intermetallic compounds were calculated.     

Scanning electrochemical microscopy (SECM) study of superoxide generation and its reactivity with 1,4-dihydropyridines

A scanning electrochemical microscope (SECM), in the tip generation substrate collection and feedback modes, was used in a method to characterize the electrode mechanism of the ${\mathrm{O}}_2/{\mathrm{O}}_2^{\text{<mglyph src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ couple in dimethylsulphoxide (DMSO) containing 0.1 M tetrabutylammonium perchlorate (TBAP) as the supporting electrolyte. Also the quantification of the interaction between ${\mathrm{O}}_2^{\text{<mglyph src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ and different 1,4-dihydropyridine compounds is reported.The SECM results demonstrated that the ${\mathrm{O}}_2/{\mathrm{O}}_2^{\text{<mglyph src="https://sdfestaticassets-eu-west-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}-}$ couple follows an E mechanism, in contrast with the EC₂ mechanism (DISP 2) that was previously reported by cyclic voltammetry. This result implies that in the time scale of SECM measurements there is no time for a homogeneous chemical reaction to be coupled to the electron transfer, i.e., the superoxide is a stable radical. Also we have determined the heterogeneous standard rate constant, k⁰ of the quasi-reversible reduction of oxygen to superoxide anion.Taking advantage of the fact that the superoxide suffers no chemical decay during the SECM experiment, a method to obtain the direct interaction of different 1,4-dihydropyridine molecules with superoxide was developed. The study revealed that all the 1,4-DHPs scavenged superoxide with sufficiently high interaction constants (～10⁵ M^?1 s^?1). No significant difference between the different molecules was found.This paper shows that the SECM feedback mode is a sensitive technique, giving an accurate determination of the homogeneous interaction constant and allowing the determination of faster rate constants than those found from cyclic voltammetry.     

The adsorption of thiourea on the mercury from neutral and acidic solutions of perchlorates

The adsorption of thiourea at the interface the mercury/1 mol dm^?3 NaClO₄ and the mercury/1 mol dm^?3 HClO₄ was studied in the function of the electrode potential and the adsorbate concentration. In the study was used, the experimental data obtained from the measurement of differential capacity of double layer, the measurement of zero charge potential, and surface tension at the zero charge potential. The information concerning thiourea adsorption was obtained from values of relative surface excess, free adsorption energy, interactions constants and the electrostatic parameters of the inner layer. It was found out that protonized thiourea molecules from HClO₄ solutions adsorb better at the interface the mercury/${\mathrm{ClO}}_4^{-}$. They are also characterized by stronger intermolecular interactions in comparison with thiourea molecules from NaClO₄ solutions adsorbing on the mercury.     

Electrodeposition of gold from cyanide solutions on different n-GaAs crystal faces

The initial stages of the electrodeposition of gold from $\mathrm{Au}(\mathrm{CN}{)}_2^{?}$-solutions on n-GaAs were investigated by means of chronoamperometry and ex situ atomic force microscopy (AFM). Three different crystal orientations were examined, i.e. (1 1 1), $(\overline{1}\overline{1}\overline{1})$ and (1 0 0) n-GaAs. Analysis of the experimental current transients showed that the nucleation mechanism of Au on n-GaAs depends strongly on the surface orientation and the deposition potential, which was confirmed by AFM measurements. This can be explained by differences in chemical composition of the surface. Further, it was observed that the Au nuclei grow laterally, which results in a flat morphology.     

On the parameters affecting the characteristics of the “wired” glucose oxidase anode

We recently described a redox polymer with an $\mathrm{Os}(N,N^{\prime }\text{-}\mathrm{dialkylated}\mathrm{biimidazole}{)}_3^{2+/3+}$ redox center with a redox potential of ?195 mV vs. Ag|AgCl, 800 mV reducing relative to its 2,2′-bipyridine analog. The tethering of the centers to a poly(4-vinylpyridine) backbone through a 13 atom long spacer arm, provided for an apparent electron diffusion coefficient as large as 5 × 8.10^?6 cm² s^?1. The glucose electro-oxidation catalyst formed by “wiring” glucose oxidase with this polymer allowed the electro-oxidation of glucose already at ?360 mV vs. Ag|AgCl at pH 7.2, near the reversible potential of glucose oxidase, at a current density of 1.3 mA cm^?2 in the presence of 32 mM glucose concentration. Here we consider the parameters affecting the characteristics of this anode, compare the characteristics with those of an anode made with a short-tethered “wire” and describe the optimization of the composition of the “wired” glucose oxidase.     

Electropreparation and characterization of polyNiTSPc films. An EQCM study

Thin films of polyNi(II)tetrasulfophthalocyanine (polyNiTSPc) electrodeposited on a gold electrode by potential cycling in 0.1 M NaOH solutions containing 1 mM Ni(II) tetrasulfophthalocyanine were studied by cyclic voltammetry, electrochemical quartz crystal microbalance and impedance techniques. Both the mass increase due to the electrodeposited film, and the current of the Ni(II)/Ni(III) process in the film, increase monotonically with the number of electrodeposition cycles, a linear correlation existing between these two parameters. AFM micrographs show that the film is rugose and porous, this porosity being confirmed by the fact that the voltammetric features of gold oxide formation and reduction are unaffected by the film. The cyclic voltammogram of the film in a pH 11 carbonate–hydrogencarbonate buffer shows the characteristic anodic and cathodic peaks of the $\mathrm{Ni}(\mathrm{II})?\mathrm{Ni}(\mathrm{III})$ process, with a large mass decrease upon Ni(II) electrooxidation and an equally large mass increase upon Ni(III) electroreduction. The sign of the mass change is compatible with a β-Ni(OH)₂-character of the film. The large slopes of the plots of mass change vs. charge in the Ni(II)/Ni(III) region clearly show that large fluxes of water and/or ions accompany the electron transfer. The fairly low activation energy of Ni(II) electrooxidation, 38 kJ mol^?1, is in agreement with the moderately non-Nernstian behaviour of the Ni(II)/Ni(III) process. The super-Nernstian dependence on pH of the peak potentials of the Ni(II)/Ni(III) process indicates that the species involved are non-stoichiometric, similarly to the Ni_1 ? xH_x(OH)₂ species in nickel hydroxide.     

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.     

Kinetic studies of reduction of nitrate ions at Sn-modified Pt electrodes using a quartz crystal microbalance

An electrochemical quartz crystal microbalance (EQCM) was used to determine the surface coverage of adsorbed nitrate during the reduction of nitrate on Sn-modified Pt electrodes. The rate equation was derived as a function of the surface coverage of nitrate, the concentration of hydronium ion and the electrode potential for an electrode with a tin coverage of ca. 0.35, which showed the highest activity of the electrodes reported so far. N₂ (30%) and ${\mathrm{NH}}_4^{+}(62\%)$ were produced. From the Tafel slope at a constant surface coverage (?0.115 V decade^?1) and the reaction orders (1.1 and 0.94 with respect to the surface coverage of the adsorbed nitrate and the concentration of hydronium ion, respectively), it is concluded that the rate-determining step is the reaction of the adsorbed nitrate with the hydronium ion and an electron.     

Hydrofluoric acid concentrations: Effect on the cyclic load-to-failure of machined lithium disilicate restorations

Objectives

To evaluate the effects of the etching with different hydrofluoric acid (HF) concentrations on the cyclic load-to-failure $(C_{Lf})$ of machined lithium disilicate crowns cemented to dentin analogue material.

Novel photocathodic characteristics of organic bilayer composed of a phthalocyanine and a perylene derivative in a water phase containing a redox molecule

An organic bilayer composed of metal-free phthalocyanine (H₂Pc, p-type semiconductor) and perylene derivative (PTCBI, n-type semiconductor) was found to involve novel characteristics of photoelectrode working in the water phase. When the photoelectrode characteristics were investigated when the PTCBI has contact with water containing the redox molecule (${\mathrm{Fe}}^{\mathrm{III}}(\mathrm{CN}{)}_6^{3-}$ (electron acceptor)), it was shown by voltammetry that the photocathodic current due to the ${\mathrm{Fe}}^{\mathrm{III}}(\mathrm{CN}{)}_6^{3-}$ reduction occurs at the present electrode, which is different from the ordinary characteristics at the n-type semiconductor/water interface of the Schttoky junction. Separate voltammetric studies showed that the photocathodic characteristics of the H₂Pc/PTCBI bilayer are consistent with those of the H₂Pc single layer, indicating that there are pin-holes in the PTCBI layer of the bilayer; that is, the H₂Pc has direct contact with ${\mathrm{Fe}}^{\mathrm{III}}(\mathrm{CN}{)}_6^{3-}$ dissolved in water. Thus, it is inferred that the photocathodic current occurs at the H₂Pc surface. However, the action spectrum for the photocathodic current indicated that a broad visible light absorption (400–750 nm) by only PTCBI can also induce photocurrent generation especially at wavelengths shorter than 500 nm where absorption of the H₂Pc is absent or relatively weak. This is supported by the previous knowledge that the PTCBI exciton alone can contribute to carrier generation through charge separation at the H₂Pc/PTCBI interface. This study showed the novel photocathodic characteristics at the organic solid/water interface coupled with electron conduction through the n-type semiconductor, in addition to the ordinary characteristics at the p-type semiconductor/water interface.     

Effect of niobium on the electronic properties of passive films on zirconium alloys

The effects of niobium on the structure and electronic properties of passive films formed on zirconium alloys in pH 8.5 buffer solution were examined by photo-electrochemical analysis. For Zr–xNb alloys (x = 0, 0.45, 1.5, 2.5 wt%), the photocurrent began to increase at an incident energy of 3.5–3.7 eV and exhibited the first peak at 4.3 eV and the second peak at 5.7 eV. From the (I_ph hν)^1/2 vs. hν plot, indirect band gap energies of $E_{\mathrm{g}}^1=3.01''–''3.47\mathrm{eV}\mathrm{and}{E}_{\mathrm{g}}^2=4.44''–''4.91\mathrm{eV}$ were obtained. With increasing Nb content, the relative photocurrent intensity of the first peak increased significantly. Compared with the photocurrent spectrum of the thermal oxide of Zr–2.5Nb, it was revealed that the first peak in the photocurrent spectrum for the passive film formed on Zr–Nb alloy was generated by two types of electron transitions; one caused by hydrous ZrO₂ and the other created by Nb. Two electron transition sources were overlapped over the same range of incident photon energy. In the photocurrent spectrum for passive film formed on Zr–2.5Nb alloy in which Nb is dissolved into the matrix by quenching, the relative photocurrent intensity of the first peak increased, implying that dissolved Nb acts as another electron transition source.     

Electrochemical behavior of quercetin: Experimental and theoretical studies

Electrochemical oxidation of quercetin, as important biological molecule, has been studied in 0.1 M phosphate buffer solution, using cyclic voltammetry, chronoamperometry, rotating disk electrode voltammetry as well as quantum mechanical calculations. The heterogeneous charge transfer rate constant, k′, transfer coefficient, α, and exchange current density, j₀, for oxidation of quercetin at the glassy carbon electrode are determined as 4.84 × 10^?2 cm s^?1, 0.65 ± 0.01 and (1.17 ± 0.39) × 10^?7 A cm^?2, respectively. The formal potential, E^0′, of quercetin is pH dependent with a slope of ?60.1 mV per unit of pH which is close to the anticipated Nernstian value of ?59 mV for a two electrons and two protons process. The standard formal potential, E⁰, of quercetin was found to be equal with 558 mV versus saturated calomel electrode (SCE). The mechanism of oxidation was deduced from voltammetric data in various pHs and also in different concentrations of quercetin. The diffusion coefficient of quercetin was calculated as 3.18 × 10^?6 cm² s^?1 for the experimental condition, using chronoamperometric results. The results of density functional theory (DFT) calculations for the oxidation of quercetin in aqueous solution, are also presented. The theoretical standard electrode potential of quercetin is obtained to be 568 mV versus SCE, which is in good agreement with the experimental value. The discrepancy between theoretical and experimental values is only 10 mV. The agreement verifies the accuracy of experimental method and the validity of mathematical model.