Voltammetric analysis of Cs+-DB18C6 complex stoichiometry at the water ∣ 1,2-dichloroethane interface

The transfer of Cs⁺ across the water ∣ 1,2-dichloroethane interface assisted by dibenzo-18-crown-6 (DB18C6) was studied using cyclic voltammetry in a concentration range wide enough to fulfil the following experimental conditions: c_Cs(w)?c_DB18C6(o), c_Cs(w)?c_DB18C6(o) and c_Cs(w)?c_DB18C6(o). In all cases, the total voltammetric current comprises two transfer processes which were compared with the theoretical voltammograms for facilitated transfer by using different models. From the results obtained, a first electrochemical step of Cs⁺ transfer with interfacial formation of the complex (Cs: DB18C6) 1:2 was followed by a second electrochemical step with formation of the complex 1:1. The relative contribution of each process to the global current depends on the concentration ratio (λ=c_Cs/c_DB18C6) employed. At high λ values, the 1:1 complex is formed through the interfacial destruction of the 1: 2 complex, and at low λ values, through the facilitated transfer of M(w) by L(o). An important aspect which results from the treatment carried out is that not only the ratio but also the concentration values determine the appearance of one or two processes. Spectroscopic UV-visible experiments corroborate this postulate.     

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.     

Novel voltammetric responses of a 1,1′-ferrocene bis(methylene aza-18-crown-6) receptor to group 1 and 2 metal cations in acetonitrile

A 1,1′-ferrocene bis(methylene aza-18-crown-6) ligand (L) and its inclusion complexes with Na⁺, K⁺, Ba²⁺ and Mg²⁺ are studied in acetonitrile by cyclic and square wave voltammetry. The ligand-metal complex stoichiometry is found to vary from 1:1 to 1:2 with increasing concentration of the metal cation. There is also a binding selectivity for dications over monocations. Simultaneous addition of Mg²⁺ and Ba²⁺ gives rise to a novel voltammetric feature which is rationalised by the formation of three different species; the symmetrical homobimetallic complexes, L—2Ba²⁺ and L—2Mg²⁺, and the asymmetrical heterobimetallic complex, L—Ba²⁺—Mg²⁺. L is therefore a redox active receptor capable of the simultaneous electrochemical recognition of different metal cations. An electrostatic model is proposed to account for the observed changes in the mean potential of L upon cation complexation.     

DFT calculations of the interaction of alkali ions with copper and silver

The interaction of alkali ions with the Cu(100) and Ag(100) surfaces is studied using the DFT method. The results of the B3LYP calculations performed for five cations, Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺, adsorbed on the surface of the M₁₂(6,6) cluster (M=Cu, Ag) are presented. On both metals the interaction is found to be strongest for the Li⁺ ion and weakest for the Cs⁺ ion. Three sites were tested for the adsorption of ions on the (100) surface: top, bridge and hollow. For the two smaller ions, Li⁺ and Na⁺, the hollow site is found to be the most stable. For the three larger ions the top position is more attractive. Nevertheless, the energy value at the different sites for a given ion in most cases, differs by less than 5 kJ mol^?1. For all ions the interaction with silver is stronger than the interaction with copper.     

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.     

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.     

Cholinergic autoantibodies from primary Sjögren’s syndrome modulate submandibular gland Na+/K+‐ATPase activity via prostaglandin E2 and cyclic AMP

Passafaro D, Reina S, Sterin‐Borda L, Borda E. Cholinergic autoantibodies from primary Sjögren’s syndrome modulate submandibular gland Na ⁺ /K ⁺ ‐ATPase activity via prostaglandin E ₂ and cyclic AMP. Eur J Oral Sci 2010; 118: 131–138. © 2010 The Authors. Journal compilation © 2010 Eur J Oral Sci We demonstrate that patients with primary Sjögren’s syndrome (pSS) produce functional IgG autoantibodies that interact with the glandular M₃ muscarinic acetylcholine receptors (mAChRs). These autoantibodies act as a partial muscarinic agonist, increasing prostaglandin E₂ (PGE₂) and cyclic AMP production through modifying Na⁺/K⁺‐ATPase activity, but also interfere with the secretory effect of the parasympathetic neurotransmitter. The IgG from patients with pSS has two effects on the submandibular gland. On the one hand, it may act as an inducer of the proinflammatory molecule (PGE₂) that, in turn, inhibits Na⁺/K⁺‐ATPase activity. On the other hand, it plays a role in the pathogenesis of dry mouth, abolishing the Na⁺/K⁺‐ATPase inhibition and the net K⁺ efflux stimulation of the salivary gland in response to the authentic agonist pilocarpine, decreasing salivary fluid production.     

A sodium-specific condensed film of α-cyclodextrin at the mercury ∣ water interface

α-Cyclodextrin adsorption at the mercury ∣ water interface exhibits several condensed phases, depending on the anions and cations present. At low charge densities the interface can be covered by a condensed film of α-cyclodextrin. In the presence of sodium ions, a condensed film containing both α-cyclodextrin and Na⁺ can be observed at a negatively charged electrode. This highly sodium-specific film contains three α-cyclodextrin molecules for every two Na⁺ cations, and can be characterized by the interfacial solubility product [αCD]³[Na⁺]²=1.65×10^–6 M⁵ at 25°C. This film exhibits isomer-specific inhibition of reduction processes; its formation can be stochastic.     

Analysis of the hydrogen electrode reaction mechanism in thin-layer cells. 1. Theory

This work develops the equations that relate the kinetic parameters of the hydrogen electrode reaction (HER) with the current density (j) vs. potential (E) dependences of a thin-layer cell (TLC). Two operation modes of the TLC are analyzed. The first one proposes to examine the j(E) dependence of the hydrogen evolution reaction (her) on one electrode while the paired electrode oxidized the dissolved H₂ back to H⁺ under diffusion control. The second mode proposes to analyze the j(E) dependence of the hydrogen oxidation reaction (hor) on one electrode while the other generates dissolved H₂ from H⁺ under diffusion control. In both cases, as very high mass-transport rates are reached for distances in the micrometer range, the j(E) curves are sensitive to the complete set of kinetic parameters even for very large reaction rates. Possible ways to incorporate these equations in the theoretical formalisms of well-established TLC-based techniques such as scanning electrochemical microscopy are discussed.     

Modification of carbon electrodes with zinc hexacyanoferrate

Electrodes modified with metal hexacyanoferrates find a unique place in the contemporary electrochemistry in view of their multifarious applications. The modification of carbon substrates with thin films of zinc hexacyanoferrate (ZnHCF) is reported for the first time. Stable surface modification is favoured when the Zn²⁺/Fe(CN)₆^3? ratio is 1:1, while deviation from this ratio leads to interesting electrocrystallisation phenomena. K⁺ ion has a facile entry and exit into the lattice of ZnHCF compared to other cations such as Na⁺, Li⁺, NH₄⁺ etc.     

Electrochemistry of cis-diaquo ruthenium(II) complexes with substituted 2,2′-bipyridine ligands in a non-coordinating solvent. Application to the elaboration of corresponding functionalized polypyrrole films

New cis-diaquo ruthenium(II) complexes [Ru(L)₂(OH₂)2]²⁺ with L = 2,2′-bipyridine substituted by ramified or long chain alkyl groups have been prepared in order to study their electrochemical behaviour in a non-coordinating solvent like CH₂Cl₂. Slow mono and bis-aquo substitutions by counter anions (Y^?) from the electrolyte are observed leading to [Ru(L)₂(OH₂)(Y)]⁺ and [Ru(L)₂(Y)₂]. The kinetic of the process is time dependent on the electrolyte used (TBAP or TBATF). By using a 2,2′-bipyridine ligand functionalized by a pyrrole group (L₁), films of poly[Ru(L₁)₂(OH₂)₂]²⁺, poly[Ru(L₁)₂(OH₂(Y)]⁺ and poly[Ru(L₁)₂(Y)₂] are readily obtained by anodic polymerization. These modified electrodes were electrochemically and spectrochemically characterized and the electrocatalytical properties of the C/poly[Ru(L₁)₂(OH₂)₂]²⁺ modified electrode have been examined.     

Electrochemical recognition properties of 13- and 16-membered azo- and azoxycrowns in solution

The voltammetric reduction of 13- and 16-membered azo- and azoxycrowns is investigated in the presence of alkali metal cations (Na⁺ or K⁺ for 13- or 16-membered azocrowns, respectively). Two types of recognition behaviour have been observed. The first type consists in a positive shift of the redox potential (100–150 mV) upon addition of metal cation (single peak behaviour). The second type consists in the appearance of two separated redox peaks (150–320 mV more positive, two-wave situation). Better electrochemical recognition results were obtained using SW voltammetry with the peak potential for the complex up to 0.32 V (for the azoxycrown derivative) more positive of the original azocrown compound. For azo- and azoxycrowns exhibiting ‘two-wave’ behaviour, the binding enhancement (ratio of binding constants for reduced and neutral species) was 10²–10⁵ M^?1. For other azocrown compounds with single peak recognition behaviour the binding constants for reduced species were estimated to be 10⁴–10⁵ M^?1. In general, K⁺ binds to 16-membered neutral azocrowns more strongly than Na⁺ to 13-membered ones, whilst binding to the anion-radical has the opposite trend.     

Effect of the shades of background substructures on the overall color of zirconia-based all-ceramic crowns

PURPOSE

The objective of this study was to determine the effect of the color of a background substructure on the overall color of a zirconia-based all-ceramic crown.

MATERIALS AND METHODS

Twenty one posterior zirconia crowns were made for twenty subjects. Seven premolar crowns and six molar crowns were cemented onto abutments with metal post and core in the first and second group. In the third group, eight molar crowns were cemented onto abutments with a prefabricated post and composite core build-up. The color measurements of all-ceramic crowns were made before try-in, before and after cementation. A repeated measure ANOVA was used for a statistical analysis of a color change of all-ceramic crowns at α=.05. Twenty four zirconia specimens, with different core thicknesses (0.4-1 mm) were also prepared to obtain the contrast ratio of zirconia materials after veneering.

RESULTS

L^*, a^*, and b^* values of all-ceramic crowns cemented either on a metal cast post and core or on a prefabricated post did not show significant changes (P>.05). However, the slight color changes of zirconia crowns were detected and represented by ΔE^*_ab values, ranging from 1.2 to 3.1. The contrast ratios of zirconia specimens were 0.92-0.95 after veneering.

CONCLUSION

No significant differences were observed between the L^*, a^*, and b^* values of zirconia crowns cemented either on a metal cast post and core or a prefabricated post and composite core. However, the color of a background substructure could affect the overall color of posterior zirconia restorations with clinically recommended core thickness according to ΔE^*_ab values.     

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.     

The kinetics of neutral methyl viologen in acidic H2O+DMF mixed solutions studied by cyclic voltammetry

The chemistry of the two-electron reduction product of viologen (1,1′-dialkyl-4,4′-bipyridinium, V²⁺) neutral species, is important in understanding the electrochemical behavior of viologens and their utilization. The kinetics for the reactions of neutral methyl viologen (V⁰) in the presence of H⁺ (from HCl), CH₃COOH (pK_a=4.75), ClCH₂CH₂COOH (pK_a=4.00), HCOOH (pK_a=3.75) in aqueous media was examined by cyclic voltammetry according to the EEC_i mechanism. To avoid the electrodeposition of V⁰, we used a 9:1 (v/v%) H₂O+DMF mixture as the solvent medium. To evaluate the rate constants for the chemical reaction followed by the second electron transfer step of V²⁺, the ratio of the anodic and cathodic peak current (I_pa2/I_pc2) corresponding to V⁰–e^??V⁺ was plotted against log τ, where τ is the time between E_1/2 and the switching potential, at various scan rates of 0.02–3.5 V s^?1. The chemical reaction was found to be a parallel reaction consisting of H⁺-catalyzed and general-acid (HA) catalyzed reactions. The second-order rate constants are determined as k_H⁺=3.5×10³ M^?1 s^?1, k_CH3COOH=5.7 M^?1 s^?1, k_HCOOH=4.6×10¹ M^?1 s^?1, and k_ClCH2CH2COOH=3.2×10¹ M^?1 s^?1 using the Nicholson–Shain method and k_H2O was estimated as <3×10^?6 M^?1 s^?1. The CVs were digitally simulated under the assumption of a two-step reaction of V⁰ following the two-step electrode reactions of V²⁺ to V⁰. The simulated CVs show good agreement with those obtained experimentally, when the first-step reaction of V⁰ is a relatively fast reversible reaction and the second-step reaction is a slow irreversible one. Based on these results, we propose that V⁰ is in pseudo-equilibrium with H⁺ or HA to produce VH⁺ which undergoes a reaction with H₂O.     

Cation effects on the voltammetric behavior of α-Keggin-type [SiMo12O40]4− and [PMo12O40]3− complexes in CH3COCH3 and CH3CN

The effect of small cations such as H⁺, Li⁺ and Na⁺ on the voltammetric behavior of α-Keggin-type [SiMo₁₂O₄₀]^4? and [PMo₁₂O₄₀]^3? complexes was investigated in CH₃COCH₃ and CH₃CN. For the [SiMo₁₂O₄₀]^4? complex, the presence of Li⁺ or Na⁺ caused the one-electron waves to be converted into a two-electron wave at ca. 0.3 V more positive than the first one-electron wave. In the presence of Li⁺ or Na⁺, the [PMo₁₂O₄₀]^3? complex underwent a two-electron reduction at the same potential as the original first one-electron wave in CH₃COCH₃, whereas it exhibited only successive one-electron waves in CH₃CN. The addition of a trace amount of H⁺ produced new two-electron waves at more positive potentials. These findings give a clue to the understanding of the reactivity of polyoxometalates as redox catalysts.     

Adsorption of crown ethers on single and polycrystalline platinum-electrodes

For the functionalisation of electrode surfaces by organic molecules, platinum substrates offer the advantage of forming stable adsorption bonds to the molecules. The chemical stability and surface coverage of several adsorbed crown ethers, 18-crown-6 (18C6), benzo-18-crown-6 (B18C6) and 4′-amino-dibenzo-18-crown-6 (4′-ADB18C6) and for competitive purposes methoxy-benzene, have been studied at single- and polycrystalline platinum surfaces in 0.5 M sulphuric acid with differential electrochemical mass spectrometry (DEMS). The crown ethers adsorb chemically intact in a limited potential region between 200 and 800 mV vs. RHE. They undergo reductive cleaving of the ether ring moieties during cathodic potential shifts down to ?100 mV. By ¹³CO co-adsorption and subsequent oxidative desorption, the stability and reversibility of the adsorption of the crown ethers was investigated. STM images of 4′-ADB18C6 reveal a disordered adsorbate layer.     

Electrochemical oxidation in different media of bis(p-phenylene crown ether) as a symmetrical molecule bearing two identical redox centers

The oxidation of the long chain polyether p-disubstituted benzene derivative (BO5O5) and the related macrocyclic paracyclophane (BBO5O5) have been investigated by cyclic voltammetry at different concentrations (10⁻³–3×10⁻² M) and potential scan rates (0.05–250 V s⁻¹). At about 10⁻³ M in CH₃CN or CH₂Cl₂ containing a perchlorate salt as the supporting electrolyte, BO5O5 was oxidized reversibly into moderately stable radical cations. The formal potential E°′ corresponding to this first oxidation step was found to be shifted towards more positive values when the cation was changed from Bu₄N⁺ to Na⁺, and this variation may be ascribed to the coordination of the alkali cation by the polyether chains. At higher concentrations of the reactant and in a thoroughly dried medium, no deposit of a polymeric material onto the electrode surface was electrogenerated. At low scan rates and under the same conditions, the oxidation of BBO5O5 gave rise to two closely spaced, irreversible processes. The radical cations involved in each step were thought to be highly reactive towards nucleophilic species present in solution. At high scan rates, a single reversible one-electron transfer was observed, the formal potential of which varied with the nature of the electrolyte cation in the same way as for BO5O5. The heterogeneous charge-transfer rate constants estimated for BO5O5 and BBO5O5 were compared with the values predicted by the Marcus theory. As for BO5O5, attempts to electropolymerize BBO5O5 were unsuccessful.     

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.     

Kinetics and growth modes of quasi-2d silver branched electrodeposits produced in the presence of a supporting electrolyte

Quasi-2d silver electrodeposits were grown electrochemically at constant potential from aqueous Ag⁺ ion-containing solutions in the presence of a supporting electrolyte, at room temperature, using a three-electrode quasi-2d circular electrochemical cell. Open branching and dense radial branching patterns were distinguished on the centimetre scale, and growth mode transitions could be observed during the process. Branching patterns exhibited a mass fractal behaviour with a mass fractal dimension increasing from that expected for a DLA-like pattern to that of a dense branching pattern as either the cathodic overpotential (η_c) or the Ag⁺ ion concentration in the solution (c) was increased. The electrodeposition current increased with time exhibiting different regimes depending on whether an open branching or a dense radial branching growth mode prevailed. When the electrodeposition time exceeded a certain critical value, the radial growth rate of electrodeposits (v_r) approached a v_rαη_cc relationship. The experimental morphologies and growth kinetics were reproduced by Monte Carlo simulations of a growth model in which depositing particles follow a biased random walk.