Coupled electron and proton transfers: compared behaviour of oxometalates in aqueous solution or after entrapment in polymer matrices

The anion PW₁₂O^3?₄₀ is very sparingly soluble in concentrated mineral acid solutions. This shortcoming has impeded the kind of study that demonstrated the merging of the first two waves of SiW₁₂O^4?₄₀ and P₂W₁₈O^6?₆₂ by an EEC mechanism in aqueous solutions. Entrapment of PW₁₂O^3?₄₀ in protonated slightly quaternized poly(4-vinylpyridine) films (QPVP) or in polyaniline films (PANI) has now permitted such a study. As also observed previously, a negative potential shift of the first redox wave of the heteropolyanion is obtained when the acid concentration in the bathing solution increases. All the results are compared with those obtained for QPVP–SiW₁₂O^4?₄₀, QPVP–P₂W₁₈O^6?₆₂, PANI–SiW₁₂O^4?₄₀ and PANI–P₂W₁₈O^6?₆₂ composites. In each case, the same EEC mechanism is operative. EQCM experiments were also very useful in this issue. Except for the case of PW₁₂^O3?₄₀ which could not be studied in concentrated acid solutions, all other systems show that the merging of waves necessitates higher acid concentrations in solution than when the heteropolyanions are entrapped in polymer matrices. This observation was termed the ‘microenvironment effect’. In the present work, confocal microscopy measurements of fluorescence intensity variation between bathing solutions and appropriate composites show directly the existence of differences of pH between the two phases. The results concord with predictions from Donnan effects, and reinforce such an explanation for the ‘microenvironment effect’.     

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.     

Electrochemical response of α-H4SiW12O40 on Ag and Au electrodes

Electrochemical measurements are presented for acidic solutions containing α-H₄SiW₁₂O₄₀ adsorbed on Ag(111) and Au(111) electrodes. These measurements show that this molecule passivates the Ag surface towards solution redox events. This passivation is unique to Ag, as it is not observed on Au or carbon electrodes. Depassivation can be accomplished by moving the potential of the Ag electrode into the hydrogen evolution region. These results are discussed in terms of formation of a reduced AgSiW₁₂O₄₀ complex that is stable only at negative potentials.     

Quantitative studies of the inhibitory effect of perchlorate on the concentration of 36ClO4−125I− and 99mTcO4− in salivary glands of male and female mice

[³⁶Cl]-Perchlorate, [¹²⁵I]-iodide and [^99mTc]-pertechnetate were concentrated in the mouse submandibular salivary gland in the order ${}^{36}\text{ClO}{}_4{}^{\mathrm{?}}\text{=}{}^{\mathrm{99m}}\text{TcO}{}_4{}^{\mathrm{?}}\text{>}{}^{125}\text{I}{}_{\mathrm{?}}$. Gland plasma ($\text{G}\text{P}$) concentration ratios for ³⁶ClO₄^? and ^99mTcO₄^? rose to a maximum of 12 compared to a value of five for ¹²⁵I^?. In the case of all the anions studied, a significantly higher concentration was observed in male compared to female mice. This anion concentration difference between the sexes was of the same order for ³⁶ClO₄^? and ^99mTcO₄^? and was smaller in the case of ¹²⁵I^?. Results suggest that ³⁶ClO₄^? is concentrated in the duct cells of the salivary gland. The serum perchlorate levels required to decrease the $\text{(G}\text{P)}$ ratio to 1 varied for the different anions and was in the order ${}^{\mathrm{99m}}\text{TcO}{}_4{}^{\mathrm{?}}\text{>}{}^{36}\text{ClO}{}_4{}^{\mathrm{?}}\text{>}{}^{125}\text{I}{}^{\mathrm{?}}$. The concentration of perchlorate required to inhibit the concentrating mechanisms for the three anions was greater in males than females. The lowest dose of ClO₄^? used (0.28 μ-equiv./kg) was such that it did not inhibit ¹²⁵I^?or ^99mTcO₄^? uptake in either sex. With a dose of 1.43 μ-equiv./kg, the ClO₄^? concentrating mechanisms for ¹²⁵I^? and ^99mTcO₄^? were inhibited in the female but not in the male.     

Some remarks on a method suggested for the distinction between SO42− and HSO42− ions adsorbed on an electrode

Contradictions between views on the nature of adsorbed species in $\text{HSO}{}_4{}^{\mathrm{?}}\text{SO}{}_4{}^{\mathrm{2?}}$ systems are once again discussed. It is demonstrated that adsorbed HSO₄^?and SO₄^2?species cannot be distinguished on the basis of a method proposed earlier.     

Electrochemical scanning tunneling microscope study of irreversibly adsorbed Te on a Pt(111) single crystal electrode surface

Atomic STM images of the irreversibly adsorbed oxygenated Te layer and its reduced layer on Pt(111) are presented. On the oxygenated Te layer, an atomic image with an apparent $\text{(}\text{2}\text{×}\text{2}\text{)}$(α=80°) unit cell was observed, which was interpreted to be a $\text{(2×}\text{3}\text{)}$rectangular structure containing one Te atom and one O atom, i.e. θ_Te=0.25 and θ_O=0.25. This structural information, combined with the voltammetric charge (244±2.4 μC cm^?2) concerning the redox process of the adsorbed Te layer, led to the conclusion that the chemical unit of the oxygenated Te layer is TeO²⁺. The reduced layer of Te on Pt(111) was observed to show two different structures of elemental Te: the (2×2) and (11×8) structures. The observed elemental Te structures differed from those anticipated from the $\text{(2×}\text{3}\text{)}$rectangular structure of the oxygenated Te layer, implying the movement of the Te atom during reduction of the oxygenated layer. It is concluded that the elemental Te atoms move laterally to form the (2×2) (θ_Te=0.25) and (11×8) (θ_Te=0.284) structures.     

Monomolecular films of a nickel(II) pentaazamacrocyclic complex for the electrocatalytic oxidation of hydrogen peroxide at gold electrodes

Self-assembled monolayers (SAMs) of a redox active nickel(II) pentaazamacrocyclic complex 1 and mixed monolayers of 1 with ethyl disulfide have been fabricated on a gold electrode, and their electrochemical behavior has been studied by cyclic voltammetry in aqueous solutions of Na₂SO₄ and NaNO₃. The results demonstrate that the redox behavior as well as the electrocatalytic activity towards the oxidation of H₂O₂ of the SAM and mixed monolayers of 1 largely depend on the electrolyte anions, i.e. SO₄^2? and NO₃^?: the formal potential in the SO₄^2? electrolyte is about 220 mV less positive than that in the NO₃^? electrolyte, and the SAM and mixed monolayers of 1 possess an efficient electrocatalysis for the oxidation of H₂O₂ in the NO₃^? electrolyte, but not in the SO₄^2? electrolyte. In addition, a unique cyclic voltammogram with a sharp peak of inverted ‘V’ shape has been observed in the cathodic scan for the electrocatalytic oxidation of H₂O₂, largely depending on the concentration of the NO₃^? electrolyte anion and the solution pH. These electrolyte anion-dependent redox behaviors have been discussed on the basis of different coordinating tendencies of SO₄^2? and NO₃^? to the nickel(III) centre of the complex and a possible reaction mechanism for the observed electrocatalytic reaction.     

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?.     

Evidence of self-protonation on the electrodic reduction mechanism of an anti-Helicobacter pylori metronidazole isotere

The electrochemical reduction mechanism of 1-(2-ammoniumethyl)-2-methyl-5-nitroimidazole bromide (2) in DMSO + 0.1 mol l^?1 TBAP has been investigated by cyclic voltammetry and macroscale electrolysis, on a glassy carbon electrode, in comparison with metronidazole (1). The cyclic voltammogram of 2 is represented by three reduction waves, one of them at less negative potential, when compared to the first wave of metronidazole which indicates that it undergoes easier reduction. There is evidence for a self-protonation mechanism in the electroreduction of 2, represented by the absence of the first wave in the successive cyclic voltammogram, by the disappearance of the first reduction wave upon addition of base and increase of the same wave in the presence of exogenous proton donors. The stoichiometry of the reaction, at the first reduction wave, involves 0.8 mol electron mol^?1 and yields 0.2 mol of 4e^?/4H⁺ reduced derivative (probably an unstable hydroxylamine) and 0.8 mol of the amine derivative, the conjugated base of 2. The second and third waves are typical for nitroaromatic reduction and are related to the reduction of the nitro group in this aminoderivative.     

A monomer–dimer equilibrium in self-assembled monolayers of N-ethyl-N′-octadecylviologen on the electrode surface. The influence of water content and hexafluorophosphate ion

This paper describes the dimerization of self-assembled monolayers (SAMs) of N-ethyl-N′-octadecylviologen (1) on GC and Au electrode surfaces in the presence of 0.1 M NH₄PF₆ aqueous solutions. The ‘wet’ and ‘dry’ SAMs of 1 showed multiple redox peaks for the first reduction of 1 in the presence of NH₄PF₆, in contrast to the case of other supporting electrolytes (typically KCl, NaNO₃, Na₂SO₄ and NaClO₄) where both wet and dry SAMs of 1 exhibited a single redox wave for the first reduction. The dry SAM showed a well defined reduction peak at ?0.57 V along with a shoulder reduction peak at ?0.50 V and two oxidation peaks at ?0.50 and ?0.42 V. On the contrary, the wet SAM gave a very sharp reduction peak at ?0.50 V and a small shoulder peak at ?0.57 V in addition to two oxidation peaks like those observed for the dry SAM. The reduction peak of ?0.50 V was ascribed to the reduction of strongly hydrated dications of 1, while the reduction peak at more negative potential (?0.57 V) was attributed to the reduction of the dehydrated dications of 1. The two oxidation peaks at ?0.50 and ?0.42 V were ascribable to the oxidation of the usual radical cation monomer and the radical cation dimer, respectively. In the case of the wet SAM, upon continuous potential cycling, the sharp reduction peak of ?0.50 V clearly decreased, whereas the more negative reduction peak of ?0.57 V was highly stable. In this case, in the oxidation process, the monomer peak of ?0.50 V increased, while the dimer peak of ?0.42 V decreased. Thus it is reasonably assumed that in the wet SAM, initially the radical cations of 1 feel an aqueous environment in the monolayer where the dimerization is highly favored and at subsequent potential cycles, due to the entry of hydrophobic anions of PF₆^? into the monolayer, the pre-existent water molecules are expelled from the monolayer and under this circumstance the radical cations of 1 may feel the environment very similar to non-aqueous media where the dimerization is totally suppressed. The adsorption tendency of 1 on the electrode surface was also studied using the SAMs prepared by dissolving 1 in water+ethanol mixtures of different ratios. The appearance of multiple peaks was found to depend significantly on the alkyl chain length of asymmetric viologen. The inclusion/expulsion of solvents and anions into/from the SAM during the redox reaction were studied by the electrochemical quartz crystal microbalance (EQCM). It was found that in the presence of SO₄^2? ions ca. 17 water molecules per one SO₄^2? ion were transported to the SAM of 1 during the oxidation, whereas ca. five water molecules were transported in the presence of PF₆^? ions.     

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.     

Oxygen reduction mechanism on corroded zinc

The mechanism of oxygen reduction on the as-polished and corroded zinc specimens has been studied using a rotating ring disc electrode (RRDE) system. On the as-polished surface, oxygen was reduced into two distinct steps. In the first step, about 44% of O₂ was reduced to H₂O₂ in a 2-electron reaction with the rest being reduced to OH^? in a 4-electron reaction. On the other hand, in the second step, with the increase of overpotential O₂ was almost exclusively reduced to OH^? in a 4-electron reaction. The first step reduction occurred on an air-formed oxide-covered surface at more positive potential than ?1.2 V vs. Ag/AgCl and the second step reduction (E < ?1.2 V) took place on a semi-uniformly active surface. On the corroded surface, the second step was not distinctly observed on the polarization curve, because reduction of the zinc corrosion products simultaneously took place around ?1.2 V. The O₂ reduction in the first step was inhibited by deposition of the corrosion products, though the ratio of amount of O₂ reduced to OH^? in a 4-electron reaction was larger than that on the as-polished surface. The mechanism of oxygen reduction is discussed on the basis of results obtained from the RRDE experiment.     

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.     

Diffusion of sugars and carboxylic acids through human dental plaque in vitro

Diffusion through plaque was measured with an apparatus consisting of 3 identical diffusion cells contained in an aluminium alloy block thermostatically controlled at 35 °C. In two of the cells, measurements were made of diffusion through a composite membrane consisting of 18 h dental plaque contained between bacteriological grade filters and support screens. In the third cell, diffusion occurred through a reference membrane from which the bulk of the plaque was absent. Subtraction of the diffusion resistance of this reference membrane allowed calculation of apparent tracer diffusion coefficients in the bulk of the plaque alone. Diffusion of ¹⁴C-labelled sugars and carboxylic acids was compared with that of tritiated water using dual-channel liquid scintillation counting. The diffusion coefficient of tritiated water in the plaque samples varied from 0.81 to 1.02 × 10^?9 m²s^?1 depending on the wet wt used, with coefficients of variation of ~ 15 per cent. This is between $\text{1}\text{4}$ and $\text{1}\text{3}$ of the value in aqueous solution. Diffusion coefficients relative to that of tritiated water in plaque ranged from 0.154 ± 0.014 for sucrose to 0.363 ± 0.029 for acetate. Permselectivity was low, and diffusion coefficients of I he test molecules were all between $\text{1}\text{4}$ and $\text{1}\text{5}$ of their values in free aqueous solution.     

The intrinsic pKa values for phosphatidic acid in monolayers deposited on mercury electrodes

The intrinsic K_a values of the phosphate group of phosphatidic acid (PA) in self-organized monolayers deposited on a hanging mercury drop electrode were determined by a novel procedure based on measurements of the differential capacity C of this lipid-coated electrode. In line with the Gouy-Chapman theory, plots of $\text{1}\text{C}$ at constant bulk pH and variable KCl concentration against the reciprocal of the calculated diffuse-layer capacity C_d,0 at zero charge exibit slopes that decrease from an almost unit value to zero as the absolute value of the charge density on the lipid increases from zero to ≈ 2 μC cm^?2. The values so determined are K₁ = 10⁸ M^?1 and K₂ = 10⁴ M^?1. The plots of $\text{1}\text{C}$ against $\text{1}\text{C}{}_{\mathrm{<mtext>d</mtext><mtext>,0</mtext>}}$ for PA exhibit slopes that pass from zero to a maximum value and then again to zero as pH is varied from 7.5 to 1.5, indicating that the charge density of the lipid film passes from slightly negative to slightly positive values over this pH range. An explanation for this anomalous behavior is provided.     

Oxygen reduction on copper in neutral NaCl solution

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

Electroactive endohedral metallofullerene film electrodes in aqueous solutions

The aqueous electrochemistry of Dy@C₈₂ films and membrane films of didodecyldimethylammonium bromide (DDAB) encapsulated with Dy@C₈₂ has been studied systematically. The redox responses of the Dy@C₈₂ films depend strongly on the supporting electrolytes. In the aqueous solution of tetrabutylammonium bromide, we obtained four cathodic waves at 0.040, ?0.40, ?1.04, and ?1.35 V, and one oxidation wave at +0.31 V. For the membrane films of Dy@C₈₂–DDAB film, however, the well-defined redox responses in water are relatively independent of the supporting electrolytes because of the encapsulation and fine dispersion of Dy@C₈₂ in the DDAB membrane. Reversible reduction and oxidation waves were obtained even in the pH 7.0 buffer solution containing KCl, which is associated with the charge balance of the DDAB ions with the ions of Dy@C₈₂. It was found that Dy@C₈₂ in the DDAB membrane promotes the encapsulation of hemoglobin (Hb) and Hb can catalyze the reduction of Dy@C₈₂. Significantly enhanced electrocatalytic reduction of O₂ by Hb in the presence of Dy@C₈₂ has also been demonstrated. Here Dy@C₈₂ acts an electron-transfer mediator, which points to the potential application of metallofullerenes in biochemical sensing.     

Anion recognition and electrochemical characteristics of the self-assembled monolayer of nickel(II) azamacrocyclic complex

Anionic recognition of the self-assembled monolayer of dinickel(II) (2,2-bis(1,3,5,8,12-pentaazacyclotetradec-3-yl)-diethyl disulfide) perchlorate (1) was studied electrochemically. The dinickel(II) complex 1 adsorbs on gold electrodes from methanol solutions and yields stable, self-assembled electroactive monolayers (SEMs); the SEM of 1 shows a reversible redox wave at 0.82 V in aqueous 0.1 M NaNO₃ corresponding to the Ni^3+/2+ redox reaction. The surface coverage, Γ, of the self-assembly of 1 determined by cyclic voltammetry is constant (Γ=(1.4±0.08)×10^?10 mol cm^?2) with change in the deposition time (2–36 h) and the concentration of 1 in methanol solution (0.2–5 mM) and is equivalent to a monolayer coverage of the nickel macrocyclic complex. The capacitance of the monolayer of 1 was determined from the double-layer capacitance measurements by chronoamperometry; the monolayer of 1 is assigned to be well-solvated by observing that the dielectric constant of the self-assembly domain (?_film=74) is nearly equal to that of water (?_water=78). Electrochemical investigations reveal that the monolayer of 1 can sense electrochemically various non-electroactive anions, NO₃^?, CF₃COO^?, SO₄^2?, H₂PO₄^?, HPO₄^2?, ClO₄^?, PF₆^? and SCN^?, from the variation of the formal potential, E°′, in aqueous solutions of different anions. The E°′ of the monolayer of 1 is 0.82 V in aqueous 0.1 M NaNO₃ and shifts to a less positive potential, 0.55 V, in aqueous 0.1 M Na₂SO₄; the shift in the E°′ was reversible on exchanging the monolayer of 1 between 0.1 M NaNO₃ and 0.1 M Na₂SO₄. The shift in the E°′ of the monolayer has been explained by an axial coordination of electrolyte anions with the trivalent nickel ion. The redox reaction of the SEM of 1 is not observed in aqueous solutions of 0.1 M NaClO₄ and 0.1 M NaSCN; but the redox activity was retained on changing the monolayer electrode to an aqueous solution of 0.1 M Na₂SO₄ or 0.1 M NaNO₃. The monolayer of 1 could detect electrochemically the biologically important phosphate anion, adenosine triphosphate (ATP), at submillimolar concentrations; on addition of 1 mM ATP, the formal potential of the monolayer shifts towards the less positive potential region by about 250 mV. The CVs of the SEM of 1 were recorded in aqueous solutions containing different concentrations of NaH₂PO₄ or Na₂SO₄, keeping the ionic strength of the electrolyte solution constant with added NaNO₃. The E°′ of the monolayer shifts to the less positive potential region with an increase in the concentration of H₂PO₄^? or SO₄^2? anion in solution phase, and the analysis of cyclic voltammetric results reveals that the nickel(III) complex forms a 1:1 complex with SO₄^2? anion but a 1:2 complex with H₂PO₄^? anion.     

The growth and ductal epithelial cell response of mouse submandibular salivary glands to selected neurotransmitters in vitro

Dopa, noradrenaline, isoprenaline, acetylcholine, carbachol and serotonin do not increase submandibular salivary gland growth in vitro when used at a dose of 10^?4M. Noradrenaline, isoprenaline, acetylcholine and serotonin in a dose of 10^?4 M significantly inhibits growth at 7 days to below that of control cultures. Dopa and carbachol at 10^?4 M produce growth equivalent to that of control cultures. The inhibitory effect on growth produced by isoprenaline is dose-dependent and is not present at 10^?8M concentration but is present at 10^?6 M. Morphologically the cell populations present in the cellular outgrowth are altered by noradrenaline, acetylcholine, carbachol and serotonin at 10^?4 M but not by dopa or isoprenaline at this dose. The inability of isoprenaline to alter the ratio of epithelioid to fibroblast-like cells ($\text{E}\text{F}$) is maintained throughout the dose range 10^?4 to 10^?10 M. Isoprenaline at 10^?3 M appeared to have the unusual property of maintaining the explants in an organotypic structure. Using previously described histochemical and immunological criteria for identifying ductal epithelial cells, it is concluded that the change in cell populations present as partly evidenced morphologically by the $\text{E}\text{F}$ ratio, reflects a true change in the proportion of ductal epithelial cells in the cellular outgrowth.