Anodic oxidation of free nitric oxide at gold electrodes modified by a film of trans-[Ru(III)(NH3)4(SO4)4pic]+ and molybdenum oxide

Gold surfaces were modified with an electrochemically deposited layer of non-stoichiometric molybdenum oxides. At these surfaces, trans-[Ru(III)(NH₃)₄(SO₄)4pic]⁺ complex was incorporated in a controlled way by cycling the potential consecutively in the range +0.50 to ?0.25 V at pH 2.6. Very reproducible voltammetric curves corresponding to the electrochemical process of the ruthenium complex were obtained, confirming the immobilisation of the material into the molybdenum oxide film. The anodic oxidation of nitric oxide (NO) at pH 7.4 was investigated at the modified electrode containing the molybdenum oxide+trans-[Ru(III)(NH₃)₄(SO₄)4pic]⁺ complex and an enhancement in the current response was observed compared with the signal at a bare electrode. The rate for NO electrochemical oxidation was dependent on the amount of catalytic ruthenium sites dispersed into the molybdenum oxide film. A linear relationship between current signals measured by square wave voltammetry and NO concentration was obtained in the 0–10 μM range. The applicability of the modified electrode as a sensor for real-time NO monitoring was also demonstrated.     

Alkaline-earth cation transfer assisted by monensin across the water ∣ 1,2-dichloroethane interface in the presence of alkali cations

The electrochemical transfer of alkaline-earth cations (Me²⁺) assisted by monensin (HX) across the water ∣ 1,2-dichloroethane (DCE) in the presence of alkali cations (M⁺) is reported. A shift in the peak potential for Ba²⁺ transfer and an increase in ΔE_p from 0.030 to 0.060 V are the principal effects produced by the presence of M⁺. These findings depend on pH and the relative concentrations of both cations. The following chemical exchange reaction M_(w)⁺+HX_(c)+H₂O?MX_(o)+H₃O⁺ coupled to the electrochemical transfer of Ba²⁺ accounts for ΔE_p=0.060 V of the peak observed as a consequence of the net charge transfer of +1 at the interface. These results are in agreement with the hyper-Nernstian slope of 60 mV found for Ca²⁺ and Ba²⁺ ion selective electrodes based on antibiotics with carboxylic groups at intermediate pH values. A theoretical equation for the dependence of Δ_o^wφ_1/2 for Me²⁺ with pH and M⁺ activity was developed.     

Electrochemical characterisation of bis(1,10-phenanthroline) gadolinium(III) trichloride formed in situ

The in situ formation and electrochemical behaviour of a complex between gadolinium(III) and 1,10-phenanthroline (phen) are discussed. The electron withdrawing ability of phen decreases the redox potential of Gd(III). A cathodic peak at ∼−1.7 V vs. Ag/Ag⁺ 10 mM, corresponds to an irreversible electron transfer reaction involving the exchange of two electrons, where the first charge transfer reaction is the rate-determining step. The diffusion coefficient and the total number of electrons were derived using the Shoup–Zsabo equation. Kinetic parameters such us k⁰, α and i⁰ are calculated. Voltammetric simulations are carried out to evaluate the accuracy of the results. A fast chemical reaction consumes the reduced species formed at ∼−1.7 V, which is then oxidised at ∼−0.1 V. The stoichiometry of the complex is determined by the Yoe and Jones’ method. An average value for the stability constant of the complex of (2.3 ± 0.8) 10⁵ M⁻² is obtained.     

Electrode reactions and accumulation of hydrogen at carbon paste electrodes in the presence of tetrachloropalladate

The electrochemical behaviour of the unmodified carbon paste electrode (CPE) in the presence of [PdCl₄]^2? in chloride solutions of different pH (in the range from 3 to 7) is studied by cyclic voltammetry (CV). During cathodic treatment or cycling, Pd(0) is deposited in situ on the electrode surface. Results obtained at negative potentials as a function of the concentrations of Pd(II) and protons in the solution are presented. It is concluded that hydrogen ions are reduced in different ways: first, the reduction at deposited Pd(0) or the simultaneous reduction of H⁺ and [PdCl₄]^2?, respectively, which occurs during the whole cathodic cycle; second, the reduction in the potential range from about ?0.5 to ?1.0 V versus SCE, where surface groups are reduced at the unmodified CPE. The second pathway is the prevailing reduction at lower pH, which occurs at the carbon surface probably via protonated chloropalladate complexes adsorbed at the electrode surface. Potentiostatic pretreatment in the voltammetric range between 0 and +0.2 V leads in solutions of pH 5 to a pronounced increase of the H⁺ reduction current caused by an increase of the sorption capacity of the electrode or by the accumulation of protons at the electrode surface. Protons could be accumulated at the electrode surface by formation of protonated chloropalladate complexes at the CPE.     

Peroxidase mimicking: Fe(Salen)Cl modified electrodes,fundamental properties and applications for biosensing

Iron(III) N,N′-bis(salicylidene)ethylenediamine (denoted as Fe(Salen)⁺) was prepared and characterized for its application in chemical analysis. From the stability constant of Fe(III)(Salen)⁺ (7.1×10²⁵ M^?1) and the formal potentials of Fe^3+/2+ and Fe(Salen)^+/0, the stability constant of Fe(II)(Salen) was calculated to be 3×10¹⁷ M^?1. This relatively weaker stability constant, compared with that of Fe(III)(Salen)⁺, led to the occurrence of the electron transfer reactions between Fe(Salen) and electron acceptors, like oxygen and H₂O₂. Experimental results supported this hypothesis, showing that the pseudo-first order rate constants for the reactions of Fe(II)(Salen) with O₂ (DMSO–H₂O, v/v 4:1) and H₂O₂ (pH 7) are 330 and 4400 M^?1 s^?1, respectively. Because of this catalytic effect, a sensing electrode for glucose or uric acid was constructed on the basis of Fe(Salen)⁺ and glucose oxidase (GOx) or uricase (UOx). According to the flow injection analysis (FIA), the detection limits were 1 μM for glucose at pH 7 and 0.1 μM for uric acid at pH 8.5, respectively. The linear response to each substrate covered a region of 1 μM–10 mM for glucose and 5–40 μM for uric acid. Fe(Salen)⁺ might form a 1:1 adduct with β-cyclodextrin (β-CDx); the equilibrium constant was determined to be about 6 M^?1. Although this chemical equilibrium, in terms of the numerical value, was not significant, the formation of {Fe(Salen)}₂O was effectively limited as β-CDx was incorporated.     

Symmetric current oscillations at tip and substrate electrodes of scanning electrochemical microscope during silver deposition/stripping

Symmetric current oscillations at adjacent tip and substrate electrodes of the scanning electrochemical microscope during silver deposition and dissolution were described. Effects of the tip–substrate distance, the applied potential, the radius of the glass around the tip, diameters of the tip and substrate electrodes as well as the solution ionic strength (I) on the silver growth were examined and discussed. Electrochemical quartz crystal microbalance (EQCM) experiments were also conducted to examine the solution density–viscosity properties and the diffusion coefficients of Ag⁺ (D) in solutions of different ionic strength, and the obtained results are discussed according to the theoretical D?I relationship with the diffusion coefficient of Ag⁺ at infinite dilution and solvent factor. It is concluded that the lateral transport of silver ions via diffusion from the bulk solution to the thin-layer space enclosed by the substrate electrode and the tip remarkably affect the oscillations. An equivalent circuit is proposed to evaluate the resistance for the localized nanoscale silver contact between tip and substrate electrodes. When the tip was located far away from the substrate, instead, a current–time waveform at the tip similar to that at the dropping mercury electrode and periodic current pulses at the substrate electrode were observed.     

Scanning electrochemical microscopy (SECM) of facilitated ion transfer at the liquid|liquid interface1

A new mode of the scanning electrochemical microscope (SECM) operation is presented. A micropipette-based electrode is used as an amperometric microprobe (tip). The tip current is produced by facilitated transfer of ions from the aqueous solution inside the pipette into the organic solvent, e.g., the transfer of K⁺ into 1,2-dichloroethane (DCE) assisted by dibenzo-18-crown-6 (DB18C6). A positive feedback current is observed when the tip approaches the water|DCE interface due to the transfer of potassium into the aqueous layer and regeneration of DB18C6 species. Good agreement with the SECM theory can be demonstrated. Negative feedback is observed when the tip approaches a glass insulator. The dependence of the SECM response on the shape of the liquid meniscus formed at the micropipette tip is discussed. The technique developed can be used for studies of ion transfer processes at the liquid|liquid and liquid|membrane interfaces and for high resolution electrochemical imaging.     

Comparative analysis of alkali and alkaline-earth cation transfer assisted by monensin across the water ∣ 1,2-dichloroethane interface

In the first part of this paper the electrochemical transfer of alkali cations (M⁺) assisted by monensin (HX) across the water ∣ 1,2-dichloroethane (DCE) interface at pH<5, combined with a chemical exchange reaction at 5<pH<9, is proposed as the only mechanism responsible for the transfer of these cations. At pH>9 the current is voltammetrically negligible. An equation for the dependence of Δ_o^wφ_1/2 on pH and Na⁺ concentration is developed. In the second part of the paper the transfer of alkaline earth cations assisted by the same ionophore is studied. The electrochemical reactions (Me_(w)²⁺+HX_(o)?MeHX²⁺_(o)) and (Me_(w)²⁺+X^?_(o)?MeX⁺_(o)) are responsible for the peaks observed at pH<5.0 and at pH>9.0, respectively. As expected, in both cases ΔE_p=0.030 V while at intermediate pH the electrochemical exchange reaction (Me²⁺_(w)+HX_(o)?MeX⁺_(o)+H⁺_(w)) is proposed. The net charge transfer of +1 at the interface accounts for ΔE_p=0.060 V for the peak observed ΔE_p in agreement with the hyper-Nernstian slope of 60 mV found for Ca²⁺ and Ba²⁺ ISE based on antibiotics with carboxylic groups at intermediate pH values. The higher selectivity for Ba²⁺ and the tendency in selectivity at alkaline pH found in the ISEs are also observed and thus explained according to the mechanism proposed.     

Nitrite reduction in acidic solution at a GC/Eastman-AQ-Os(bpy)32+-PVP composite modified electrode

The negatively charged polymer polyester sulfonic acid (Eastman-AQ, abbreviated to AQ), positively charged polymer polyvinyl pyridine (PVP) and mediator Os(bpy)₃²⁺ were used to construct composite modified glassy carbon (GC) electrodes (abbreviated to GC/AQ-Os(bpy)₃²⁺-PVP). The reduction of NO₂^? in acidic solution was taken as a model reaction to explore the properties of the modified electrode. On the steady state polarization curve of NO₂^? reduction there were two well-developed waves with much enhanced plateau current densities and positively shifted half-wave potentials compared with bare GC electrodes. In 0.05 mol/l H₂SO₄ + 5 mmol/l NO₂^? the modified electrode exhibited plateau current densities of 723 and 1153 μA cm^?2 and half-wave potential shifts of 0.29 and 0.56 V for the first and second current wave, respectively, showing promising potential for nitrite detection. The catalytic activity for NO₂^? reduction did not decrease appreciably over 4 months. A number of relevant kinetic and thermodynamic parameters are estimated experimentally. NO₂^? reduction at the composite modified electrodes is suggested to follow the SR mechanism (pure kinetic conditions involving mutual compensation between a catalytic reaction and substrate diffusion in the film in addition to diffusion in the solution phase) according to the Savéant–Andrieux theory.     

Effect of pH on direct electron transfer between graphite and horseradish peroxidase

The effect of pH on the kinetics of the electroreduction of H₂O₂ catalysed by horseradish peroxidase (HRP) has been studied with LSV in the potential range from 700 to ?50 mV versus SCE (under steady-state conditions and with an RDE system) and at ?50 mV versus Ag/AgCl on HRP-modified graphite electrodes placed in a wall-jet flow-through electrochemical cell. Increasing [H₃O⁺] was shown to enhance significantly the current of the bioelectroreduction of H₂O₂ due to direct electron transfer (ET) between graphite and the enzyme over the potential range involved. It is demonstrated that at high overvoltages (E<0.2 V) H₃O⁺ does not affect the rate of the enzymatic reduction of H₂O₂, but it increases the rate of direct ET between graphite and HRP. The values of the apparent rate constant of heterogeneous ET between HRP and graphite, k_s, changed from a value of 0.54±0.05 s^?1 in phosphate buffer solution (PBS) at pH 7.9, to a value of 11.0±1.7 s^?1 in PBS at pH 6.0. Analysing the pH rate profile and the variation of the k_s with increasing [H₃O⁺] made it possible to consider the reaction mechanism as implying the participation of a proton in the limiting step of charge transfer.     

Solvent-effect on the interconversion among one-, two- and four-electron reduction waves for the 18-molybdodiphosphate complex, [(P2O7)Mo18O54]4−

For the [(P₂O₇)Mo₁₈O₅₄]^4? complex, the presence of small cations such as H⁺, Li⁺ and Na⁺ caused one-electron waves to be converted into four- and two-electron waves in a complex manner. With the addition of a trace amount of H⁺, a four-electron reduction wave was obtained in solvents of weaker basicity like acetone, acetonitrile and propylene carbonate (PC); the relative permittivity did not affect the appearance of the four-electron wave. On the other hand, two-electron waves were obtained in solvents of stronger basicity like N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), and N-methylpyrrolidinone (NMP). With the addition of Li⁺ or Na⁺, the one-electron waves were converted into two-electron waves only in acetone, indicating that the conversion can occur in solvents of both weak basicity and low relative permittivity.     

A DFT study of electric field effects on proton transfer reactions at H+(H2O)2/Pt(111) and Ag(111)

The electric field (EF) effects on the proton transfer reaction that takes place on the surfaces of Pt(111) and Ag(111) electrodes, are studied based on density functional theory (DFT) at the level of B3LYP/LanL2DZ. According to calculations based on a ‘one-dimensional’ proton transfer model, the potential energy curve of proton transfer between two water molecules on the surfaces, depends substantially on the direction of the EF. An EF shifts the equilibrium of proton transfer in the direction of electrostatic attraction. The potential energy surface for a ‘two-dimensional’ proton transfer model, in which a proton transfers between two water molecules and the surface, revealed that an EF effectively accelerates the proton transfer from H₃O⁺ to the surface. The rate-determining step of the proton transfer is discussed in the light of the results.     

Effects of sodium sulfamate on electrodeposition of Cu(In,Ga)Se2 thin film

Cu(In,Ga)Se₂ thin films have been prepared by electrodeposition from chloride electrolytes using sodium sulfamate as a complexing agent. Cyclic voltammograms indicate that sodium sulfamate can inhibit the reduction of Cu²⁺, Cu⁺ and H₂SeO₃, and accordingly hinder the formation of copper selenides. EDS analysis reveals that with the increase of sodium sulfamate concentration in the bath, atom ratio (Cu + Se)/(In + Ga) decreases while gallium content increases, and the film composition transforms from Cu-rich to Cu-poor. SEM and Raman spectra also show that copper selenides phases in electrodeposited films diminish with the increase of sodium sulfamate concentration.     

Scanning electrochemical microscopy: theory and experiment for the positive feedback mode with unequal diffusion coefficients of the redox mediator couple1

Theory for the chronoamperometric positive feedback mode of the scanning electrochemical microscope (SECM) is extended to include the situation where the oxidised and reduced forms of the redox mediator couple have arbitrary diffusion coefficients. Under typical positive feedback conditions, the solution initially contains a redox-active species, R, along with excess supporting electrolyte. The potential of the tip ultramicroelectrode (UME), positioned close to an interface of interest, is adjusted to a value where R is electrolysed to produce species O at a diffusion-controlled rate. O diffuses away from the tip towards the interface, where the reverse redox reaction occurs leading to the production, and diffusional feedback of R for electrolysis at the tip electrode. When positive feedback measurements are carried out under diffusion-controlled chronoamperometric conditions, the form of the normalised current–time behaviour, at a particular tip to interface distance, is found to be sensitive to the ratio of the diffusion coefficients of the O/R couple. As a steady-state is established, the normalised current becomes independent of the diffusion coefficient ratio and depends only on the tip to interface distance. Experimental measurements on the chronoamperometric oxidation of ferrocene (Fc) in acetonitrile solution at a Pt tip UME positioned close to a Pt substrate electrode, provide support for the theoretical predictions and demonstrate that the diffusion coefficient ratio can readily be determined from SECM chronoamperometry. Although Fc and Fc⁺ are often assumed to have the same diffusion coefficients, steady-state and chronoamperometric measurements at a conventional UME yield a value of 2.0×10^?5 cm² s^?1 for the diffusion coefficient of Fc, while SECM chronoamperometry indicates that the diffusion coefficient of Fc⁺ is 1.6×10^?5 cm² s^?1.     

Analysis of the temperature dependence of the α and β coefficients of the electrode reactions in the Cr(III)/Cr(II) and Cr(II)/Cr(Hg) systems

The temperature dependences of the four transfer coefficients for cathodic and anodic electrode reactions of two different types of systems, namely Cr(III)/Cr(II) and Cr(II)/Cr(Hg), were determined in 5.0 M NaClO₄ solution. The results were obtained for a wide range of temperatures from ?20 to 90°C. The α parameter determined from the log k_fh on E dependence corresponding to the Cr³⁺+e^?→Cr²⁺ electrode process shows the highest dependence on temperature (?α/?T=1.1±0.3×10^?3 K^?1), whereas the β parameter corresponding to the Cr²⁺→Cr³⁺+e^? electrode process is virtually independent of temperature (?β/?T=0). The same results were obtained when the transfer coefficients were determined from dependence of the enthalpy and entropy of activation on overpotential. The observed temperature dependence of α is attributed to the changes of the ?₂ potential. The transfer coefficients for the Cr^2+/0 electrode reaction also show some dependence on temperature (?α/?T=8.7±2.0×10^?4 K^?1 and ?β/?T=3.2±1.4×10^?4 K^?1), which probably results from the influence of the ?₂ potential.     

Spectroscopic investigations on the adsorption of trifluoracetate at Pt(100), Pt(110) and Pt(111)

The adsorption of trifluoracetate anions on Pt(100), Pt(110) and Pt(111) has been studied by means of cyclic voltammetry and FTIR spectroscopy. For all three surfaces, adsorbed trifluoracetate shows the same characteristic band around 1215 cm^?1 assigned to the stretching vibration of the CF₃ group. The potential dependence of the integrated band intensity and band center frequency varies with the single crystal plane used. Lateral interactions within the adsorbed layer are the main origin of the band frequency shift observed. Trifluoracetate is less strongly bonded to the metal surface than its parent acetate. This effect may be due to the presence of the ?CF₃ group, which is known to cause a withdrawal of electrons from the COO^? group.     

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.     

Histamine H1 receptor-stimulated Ca2+ signaling pathway in human periodontal ligament cells

We studied histamine-induced Ca²⁺ mobilization in human periodontal ligament (HPDL) cells. Histamine induced a transient rise in intracellular Ca²⁺ ([Ca²⁺]_i) and maintained a sustained phase in the presence of extracellular Ca²⁺. In the absence of extracellular Ca²⁺, the transient peak was slightly reduced and the sustained phase was decreased to the basal level. The initial rise in [Ca²⁺]_i was attributed to two components: intracellular Ca²⁺ release and Ca²⁺ influx, whereas the sustained phase was due to Ca²⁺ influx. After depletion of intracellular Ca²⁺ stores with thapsigargin, a known Ca²⁺-ATPase inhibitor, histamine-induced increase in [Ca²⁺]_i was significantly reduced, suggesting histamine induces Ca²⁺ release from inositol 1,4,5-trisphosphate [Ins(l,4,5)P₃]- and thapsigargin-sensitive Ca²⁺ stores. Histamine-induced peak in [Ca²⁺]_i was increased dose-dependently in the presence and absence of extracellular Ca²⁺. The histamine-mediated response in [Ca²⁺]_i was specifically attenuated by chlorpheniramine (H₁ antagonist) but not by cimetidine (H₂ antagonist), clearly indicating that activation of H₁ receptor mediates histamine-induced Ca²⁺ mobilization. We next examined the effect of histamine on inositol phosphates formation. Histamine stimulated the formation of inositol phosphates which changed time-dependently. In particular, the formation of Ins(1,4,5)P₃ was increased significantly for 10 s. The histamine-induced Ca²⁺ mobilization caused an increase of prostaglandin E₂ (PGE₂) release which was reduced in excluding extracellular Ca²⁺. These results indicate that activation of histamine H₁ receptor induces the accumulation of Ins(l,4,5)P₃ and the following transient increase in [Ca²⁺]_i, and elicits the release of PGE₂ which may be coupled with Ca²⁺ influx.     

Voltammetric studies of the interaction of the lithium cation with reduced forms of the Dawson [S2Mo18O62]4− polyoxometalate anion

Multiple electron transfer steps observed in the reduction of [S₂Mo₁₈O₆₂]^4?, when LiClO₄ is used as electrolyte, are accounted for by assuming that Li⁺ acts as a moderately strong Lewis acid. For example, in the 95:5 CH₃CN+H₂O solvent mixture (0.1 M NBu₄ClO₄), the voltammetric behavior obtained on addition of Li⁺ can be simulated according a reaction scheme involving an extensive series of reversible potentials and equilibrium constants. Precipitation of highly reduced polyoxometalate salts occurring at the electrode surface complicates the voltammetry at very negative potentials when 0.1 M LiClO₄ is used as the electrolyte.