The ion selectivity of monensin incorporated phospholipid/alkanethiol bilayers

Monensin was incorporated into phospholipid/alkanethiol bilayers on the gold electrode surface by a new, paint-freeze method to deposit a lipid monolayer on the self-assembled monolayers (SAMs) of alkanethiol. The advantages of this assembly system with a suitable function for investigating the ion selective transfer across the mimetic biomembrane are based on the characteristics of SAMs of alkanethiols and monensin. On the one hand, the SAMs of alkanethiols bring out their efficiency of packing and coverage of the metal substrate and relatively long-term stability; on the other hand, monensin improves the ion selectivity noticeably. The selectivity coefficients K_Na+,K+, K_Na+,Rd+, and K_Na+,Ag+ are 6×10^?2, 7.2 × 10^?3 and 30 respectively. However, the selectivity coefficient K_Na+,Li+ could not be obtained by a potentiometric method due to the specific interaction between Li⁺ and phospholipid and the lower degree of complexion between Li⁺ and monensin. The potential response of this bilayer system to monovalent ions is fairly good. For example, the slope of the response to Na⁺ is close to 60 mV per decade and its linearity range is from 10^?1 to 10^?5m with a detection limit of 2 × 10^?6m. The bilayer is stable for at least two months without changing its properties. This monensin incorporated lipid/alkanethiol bilayer is a good mimetic biomembrane system, which provides great promise for investigating the ion transfer mechanism across the biomembrane and developing a practical biosensor.     

Temperature dependence of the electric properties of molten ternary chlorides by MD simulation

The electric properties are simulated in the temperature region of 800–1100 K for molten ternary mixtures (Li, Na, Cs)Cl of various compositions by molecular dynamics simulation in order to investigate the temperature dependence of the Chemla crossing points and the limit of the degree of enrichment of Cs. In addition, the self-diffusion coefficient, self-exchange velocity and the electric conductivities were calculated in molten LiCl–NaCl eutectic mixtures containing CsCl. From the results, it is concluded that the self-diffusion coefficients and self-exchange velocities of Li⁺, Na⁺, and Cs⁺ with reference to Cl^? show very similar tendencies with increasing temperature for each composition and the most appropriate temperature of Cs is 800–900 K for enrichment up to x_Cs=0.5–0.6 in molten LiCl–NaCl eutectic.     

Corrigendum to: “Use of dynamically adaptive grid techniques for the solution of electrochemical kinetic equations. Part 13. Patch-adaptive simulation of wave propagation along ring electrodes: one-dimensional approximation” [J. Electroanal. Chem. 529 (2002) 51–58]

A new electroactive polynuclear inorganic compound of a rare earth metal hexacyanoferrate, samarium hexacyanoferrate (SmHCF), was prepared by electrochemical deposition on the surface of a glassy carbon electrode with a potential cycling procedure. The cyclic voltammogram of SmHCF exhibits a pair of well-defined redox peaks with a formal potential of 191 mV (vs. SCE) at a scan rate of 100 mV/s in 0.2 M NaCl solution and the redox peak currents increase linearly with the scan rate up to 100 mV/s. The effects of the concentration of supporting electrolyte on the electrochemical characteristics of SmHCF and the transport behavior of K⁺, Na⁺ and Li⁺ counter ions through the ion channels of SmHCF were studied by voltammetry. The different electrochemical behavior of SmHCF in various cation-containing supporting electrolytes was investigated by cyclic voltammetry and scanning electron microscopy. The SmHCF was also characterized by FTIR techniques.     

Preparation,characterization, and electrocatalytic properties of copper hexacyanoferrate film and bilayer film modified electrodes

Copper(II)hexacyanoferrate films have been prepared from various electrolyte aqueous solutions using consecutive cyclic voltammetry. The cyclic voltammograms recorded the direct deposition of copper(II)hexacyanoferrate films from the mixing of Cu²⁺ and Fe(CN)₆^3? ions from solutions of ten cations: Li⁺, Na⁺, K⁺, Rb⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, H⁺ and Al³⁺. An electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth of the copper(II)hexacyanoferrate films. The copper(II)hexacyanoferrate film showed a single redox couple that exhibited a cation effect (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) and an anion effect (F^?, Cl^? and Br^?) in the cyclic voltammograms and formal potential of the redox couple. The electrochemical and EQCM properties of the film indicate that the redox process was confined to the immobilized copper(II)hexacyanoferrate, and the interaction between the copper(II)hexacyanoferrate film with K⁺ (monovalent cation) and Ca²⁺ (divalent cation). The electrocatalytic oxidation properties of NADH, NH₂OH, N₂H₄, SO₃^2? and S₂O₃^2? were also determined. The electrocatalytic reduction properties of SO₅^2? and S₂O₈^2? by monolayered iron, nickel, and cobalt hexacyanoferrate films, and by bilayered metal–copper hexacyanoferrate films were determined. Two-layered modified electrodes and hybrid films composed of a copper(II)hexacyanoferrate film with iron(II)hexacyanoferrate, cobalt(II)hexacyanoferrate, or nickel(II)hexacyanoferrate film were prepared, and these films caused the electrocatalytic reduction of SO₅^2? and S₂O₈^2?.     

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.     

Photoelectrochemical behaviour of C60 films in various oxidation states

The photoelectrochemical behaviour of C₆₀ films, deposited on gold and glassy carbon surfaces, was studied in various oxidation states, and with various compounds in the solution phase. It was established that neutral C₆₀ films behave as porous intrinsic semiconductors, and they can be converted to porous n-type semiconductors upon reduction in the presence of Li⁺, or Na⁺. If the reduction was carried out in K⁺-, Rb⁺-, or Cs⁺-containing solutions, the reduced films were pure conductors. Depending on the size of the electroactive species in the electrolyte, both photo-oxidation and photoreduction could be observed on the semiconducting reduced films. The spectral dependence of the photocurrents was also determined, and some basic parameters of the films were established.     

Preparation,characterization and electrocatalytic properties of polynuclear mixed-valent ruthenium oxide/hexacyanoruthenate film modified electrodes

Polynuclear mixed-valent ruthenium oxide/ruthenocyanide (ruthenium oxide/hexacyanoruthenate or mvRuO/RuCN) films were prepared using consecutive cyclic voltammetry directly from the mixing of Ru³⁺ and Ru(CN)₆^4? ions from solutions of two divalent cations (Ba²⁺ and Ca²⁺), and seven monovalent cations (H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and Ga⁺). The films exhibited three redox couples with Ba(NO₃)₂ or BaCl₂ aqueous solutions, and the formal potentials of the redox couples showed a cation and pH effect. An electrochemical quartz crystal microbalance (EQCM), cyclic voltammetry, UV–visible spectroscopy, and the stopped-flow method (SFM) were used to study the growth mechanism of the mvRuO/RuCN films. The results indicated that the redox process was confined to the immobilized ruthenium oxide/ruthenocyanide. The EQCM results showed a Ba²⁺ ion exchange reaction for the two most negative redox couples. The electrocatalytic reduction properties of SO₅^2?, and S₂O₈^2? by the ruthenium oxide/ruthenocyanide films were determined. The electrocatalytic oxidation of NADH and dopamine were also determined, and revealed two different types of properties. The electrocatalytic oxidations of SO₃^2?, S₂O₃^2?, and N₂H₄ were also investigated. The electrocatalytic reactions of the ruthenium oxide/ruthenocyanide films were investigated using the rotating ring-disk electrode method.     

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.     

Nature of ion sensitivity of polypyrrole electrodes

The ion sensitivity of polypyrrole (PPy) films deposited electrochemically onto Pt, glassy carbon and SnO₂ substrate was studied by potentiometric measurements. The influence of electrochemical (overoxidation, reduction and potentiodynamic cycling) and chemical (alkaline solution) pre-treatment on the potentiometric behaviour of PPy electrodes was determined. The dependence of the potential of the PPy electrode after long-term conditioning on the log of electrolyte activity of conditioning solution is characterised by low negative (solutions of NaSCN, NaNO₃, KCl, or NaF) and low positive (Cs₂SO₄, Na₂SO₄ or Li₂SO₄) slopes. The exceptionally low exchange rate of anions between sulphate-doped PPy films and solution was established by electron probe microanalysis.Short-term (2–5 min) potential responses of PPy electrodes to the change of composition and concentration of electrolyte were studied. PPy electrodes conditioned in NaSCN, NaNO₃, KCl, and NaF solutions possess anion sensitivity, and electrodes conditioned in Cs₂SO₄, Na₂SO₄ and Li₂SO₄ solutions, cation sensitivity, independent of the composition of the solution used for the preparation of the PPy film. The short-term response is generally not selective and depends on the arrangement of the ions in the Hofmeister lyophilic series. This potential response is interpreted as the result of asymmetric movement of anions or cations through the PPylsolution boundary.     

Characterization of a microsomal fraction from rabbit parotid gland: Partial separation of alkaline phosphatase from Na,K-ATPase

Subfractionation of the 140,000 g_max pellet from the rabbit parotid gland revealed 3 fractions. Both linear and discontinuous sucrose-density gradients were used to separate a region of high-density particles with considerable mitochondria, a middle region rich in Na⁺,K⁺-ATPase (sodium-potassium activated adenosine triphosphatase) and a low-density (top) region with elevated alkaline phosphatase. The top and middle fractions appeared to represent subfractions of plasma-membrane enriched membranes. Treatment of these 2 gradient fractions with low concentrations of sodium dodecyl sulphate confirmed that the middle fraction was preferentially enriched in Na⁺,K⁺-ATPase. Thus, alkaline phosphatase-enriched membranes are at least partially separable from Na⁺,K⁺-ATPase-enriched membranes. Separation of alkaline phosphatase from Na⁺,K⁺-ATPase may be an important purification step in future work involving isolation of plasma membranes of secretory cells, as both alkaline phosphatase and Na⁺,K⁺-ATPase have been used as plasma-membrane markers in the parotid gland. The present findings demonstrate that considerable caution should be exercised in defining the purity of plasma-membrane fractions from the parotid gland in view of the heterogeneity of the plasma-membrane enriched fraction.     

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.     

Characterization and electrocatalytic properties of cobalt hexacyanoferrate films immobilized on Au-colloid modified gold electrodes

An electroactive cobalt hexacyanoferrate (CoHCF) film was electrodeposited from a solution containing Co²⁺ and Fe(CN)₆^3? ions on the bare gold or the Au-colloid modified electrode. The cation (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) and the anion (F^?, Cl^? and Br^?) effects on the redox peak of the CoHCF film were investigated in detail. On the other hand, the electrocatalytic oxidations of thiosulfate at the CoHCF/gold and CoHCF/Au-colloid/gold electrodes were compared. At the CoHCF/Au-colloid/gold electrode, we obtained a response current larger by a factor of 2 and a three times lower detection limit than those at a CoHCF/gold electrode. The linear ranges were 1.0 × 10^?4 to 2.8 × 10^?3 M for the CoHCF/gold electrode and 7.5 × 10^?5 to 4.8 × 10^?3 M for the CoHCF/Au-colloid/gold electrode. These results showed that the immobilized CoHCF at the Au-colloid modified electrode exhibited a higher catalytic activity and a wider linear range toward thiosulfate. Additionally, the effects of the applied potential and the solution pH were studied.     

Current–potential curves for liquid ∣ liquid microinterfaces with no added supporting electrolyte in the water phase

Equations describing steady-state voltammograms have been derived for ion transfer at liquid ∣ liquid microinterfaces with no supporting electrolyte in the aqueous phase. The effect of the absence of any added supporting electrolyte on limiting current, half-wave potential and shape of the current–potential curves is derived theoretically and verified experimentally. Ion-transfer voltammetry across a water ∣ nitrobenzene microinterface with no supporting electrolyte in the aqueous phase has been employed for the determination of standard Gibbs energies of ion partition (in kJ mol^?1) of relatively hydrophilic ions such as Tl⁺ (19.7), K⁺ (22.4), Ag⁺ (26.0), Na⁺ (31.5), Li⁺ (35.5) and H⁺ (34.4). The data obtained agree well with those obtained previously by voltammetry and extraction measurements. With respect to the paper of Quinn et al. (J. Electroanal. Chem. 460 (1999) 149), it is assumed that measurements without a supporting electrolyte in the aqueous phase are useful in systems with a relatively high polarity of the organic phase, e.g. the system water ∣ nitrobenzene. The decisive prerequisite is that the ions of interest in the aqueous phase are transferred sufficiently before the hydrophobic ion of the reverse sign of the charge number in the organic phase.     

Polynuclear nickel hexacyanoferrates: monitoring of film growth and hydrated counter-cation flux/storage during redox reactions

An electrochemical quartz crystal microbalance (EQCM) was employed to monitor directly the growth of nickel(II) hexacyanoferrate(III) (NiHCNFe) films on gold substrates during electrodeposition as well as a result of sol-gel aggregation in colloidal nickel ferricyanide solutions used for modification. Frequency changes due to mass changes of the gold/crystal working electrode were correlated with cyclic voltammetric (CV) data. Evidence is also provided for the sorption of counter-cations (Li⁺, Na⁺ and K⁺), and associated water molecules, during redox reactions of the film. There is a strict relationship between the amount of alkali metal ions incorporated into the film during reduction, or excluded from the film during oxidation, and the frequency changes during EQCM measurements. The amount of solvent (H₂O) transferred and sorbed in the NiHCNFe film reflects the degree of hydration of the investigated counter-ions. Anions also seem to participate in NiHCNFe electrochemistry, but their role is much less pronounced.     

Solvent effects on the solid-state electrochemistry of samarium (III) hexacyanoferrate (II)

An electroactive polynuclear inorganic compound of a rare earth metal hexacyanoferrate, samarium (III) hexacyanoferrate (II) (SmHCF), was prepared chemically. The electrochemical characteristics of SmHCF mechanically attached to the surface of a graphite electrode were studied in organic solvents, such as tetrahydrofuran, acetone and N,N-dimethylformamide, etc., using cyclic voltammetry and ac impedance. The cyclic voltammograms of SmHCF in organic solvent were well defined and exhibited a pair of redox peaks, which corresponded to the redox reaction of the Fe(II)/Fe(III) couple in SmHCF, and the redox reaction was accompanied by insertion of a counter-cation into the SmHCF during the reduction and its exclusion upon oxidation. The redox potentials increased linearly with the increase of the dielectric constant of solvents. The transport behavior of K⁺, Na⁺ and Li⁺ counter-cations through the ion channel of SmHCF were also studied by voltammetry.     

Effect of extracellular K+ on saliva secretion by isolated,perfused rat submandibular glands

Changes in perfusate K⁺ concentration altered the secretory response of the glands to 10^?6 M acetylcholine. Lack of extracellular K⁺ caused a transient fluid secretory response lasting less than 10 min, and a 91 per cent reduction in the overall volume of saliva secreted in 60 min; it inhibited the response to acetylcholine even when the perfusate was changed to K⁺-containing solutions after 30 min. Absence of K⁺ in the perfusate resulted in increased Na⁺ and decreased K⁺ and Cl^? concentrations in saliva. An increase in the perfusate K⁺ concentration to 50 mM/l caused a reduced but more sustained secretory response, although the volume of saliva secreted in 60 min was still reduced by 76 per cent compared to that obtained when the perfusate contained 4.6 m-equiv./l K⁺. Acetylcholine release induced by the high K seemed mostly responsible as the response was inhibited by atropine. However, in the presence of excess of exogenous acetylcholine, perfusion with high K⁺ medium resulted in reduced Na⁺ and elevated K⁺ and Cl^? concentrations in saliva. It seems that a physiological (4–5 m-equiv./l) extracellular K⁺ concentration is required for acinar fluid secretion and for transductal electrolyte transport in the rat glands; lack of external K⁺ hyperpolarizes salivary cells and, although allowing an initial increase in Na⁺ conductance capable of causing secretion, prevents the further influx of this ion required to sustain saliva secretion. It also inhibits K⁺secretion and Na⁺ re-absorption in salivary ducts, probably by inhibiting the Na⁺, K⁺ pump in duct cells; high external K⁺ depolarizes acinar cells and may reduce Na⁺ conductance. It also enhances K⁺ secretion and Na⁺ re-absorption in salivary ducts; the effects of K⁺ omission and of high K⁺ on fluid secretion are likely to be the result of changes in the transmembrane K⁺ gradient in acinar cells and of the way they affect the opening of Na⁺ conductance pathways; perfusion with K⁺-free or high K⁺ solutions reveals a dissociation in ductal reabsorption of Na⁺ and Cl^? and unmasks the presence of independent transport mechanisms for these two ions in salivary ducts.     

Electrochemical detection of silver ions and the study of metal–polymer interactions on a polybenzidine film electrode

Silver ions (Ag(I)) were preconcentrated efficiently at open circuit from aqueous silver nitrate solution into polybenzidine (poly-Bz) film electrodes. The poly-Bz films were electrodeposited by means of cyclic voltammetry from benzidine (Bz) in acidic aqueous solutions. It was found that preconcentration of Ag(I) ions into the film is highly dependent on the pH, being more favored at neutral pH, that is, when the nitrogen in imine–amine groups in the polymer chain are not protonated. Under these conditions the Ag(I) ions do not compete with the H⁺ ions for the same site within the film. A simple diffusion model is assumed to explain the incorporation of Ag(I) ion into the poly-Bz electrodes. The distribution of the Ag(I) ion between the solution and film was established and a probable analytical application given.     

Application of a direct pulp capping cement containing S-PRG filler

Objectives

To evaluate new pulp capping cements containing surface pre-reacted glass ionomer (S-PRG) filler and to investigate ion release kinetics and pH shift of eluates from the cement.

Materials and methods

Molars of Wistar rats were directly pulp capped using three kinds of cement containing S-PRG filler and mineral tri-oxide aggregate (MTA) was used as a control. After 1, 2, or 4 weeks, histological evaluation was performed and differences of tertiary dentin formation were analyzed. Release of Sr²⁺, BO₃³⁻, SiO₃²⁻, Na⁺, and Al³⁺ ions was determined by inductively coupled plasma-atomic emission spectrometry, and F⁻ ion release was measured using a fluoride ion selective electrode. The pH of the eluate from each cement after mixing was measured with a pH electrode.

Results

One of S-PRG cements promoted tertiary dentin formation to the same extent as the control (p > 0.05) and it showed a tendency of less inflammatory response. This cement released more BO₃³⁻ and SiO₃²⁻, but less Sr²⁺, Na⁺, and F⁻ than other S-PRG specimens. Each cement recovered nearly neutral compared with glass ionomer cement.

Conclusions

S-PRG cement induced tertiary dentin formation based on multiple ion releases, suggesting that it is suitable as a pulp capping material.

Clinical relevance

This new material can be an alternative pulp capping agent to MTA.

     

On the facilitating effect of neutral macrocyclic ligands on the ion transfer across the interface between aqueous and organic solutions: Part III. Competitive facilitated ion-transfer

A theoretical equation of the reversible current–potential curves is derived for the competitive ion-transfer of two kinds of cations present in an aqueous (w) phase simultaneously facilitated by a macrocyclic ligand (L) present in an organic (o) phase. Especially, for the two limiting cases, i.e. case (A): c*_Mj (j=1, 2)?c*_L and case (B): c*_L?c*_Mj (where c*_Mj and c*_L denote the bulk concentrations of cation M_j in the w-phase and of L in the o-phase, respectively), simple explicit expressions are derived and a method analyzing the facilitated waves is presented. Furthermore, the main part of the theoretical predictions obtained is verified experimentally at 25 °C by using the ion-transfer-polarographic method with the electrolyte dropping electrode, for the following four combinations: (i) competitive cations: protonated alanine and H⁺, L: benzo-18-crown-6 ether; (ii) Na⁺ and Li⁺, benzo-15-crown-5 ether; (iii) K⁺ and Na⁺, dibenzo-18-crown-6 ether; and (iv) Na⁺ and Ba²⁺, dibenzo-24-crown-8 ether.     

Ion transport in nanocrystalline materials: a computer simulation study

The results of molecular dynamics simulations of ionic transport in nanostructured materials are presented. Two types of materials are considered, the single component nanostructured oxide, sodium β-alumina and the mixed phase composite LiI/Al₂O₃. In nanostructured β-alumina, the conductivity is observed to be considerably lower than in crystalline β-alumina. Blocking grain boundaries between the nanosized grains cause the reduction. In the case of the composite, a crystallite of LiI with (0 0 1) surface planes was sandwiched between α and γ-Al₂O₃, respectively. In both cases Li⁺-ions were observed to transfer from LiI to tetrahedral sites in the Al₂O₃ surface, α-alumina having the higher surface charge density. As a result, a defective region develops near the interface in the LiI and lithium transport along the surface can occur. The ions migrate through vacancy hopping in the defective regions in LiI. The defective region is only two Li⁺-layers thick. When the thickness of the LiI slab becomes comparable with the thickness of the defective region, the LiI becomes more disordered. The activation energy for Li⁺-diffusion decreases, but this is more than compensated for by a reduction in charge carriers.