Electrochemical properties of mono-particle-layer films of polyaniline-coated latex microspheres

Latex particles with the structure of a polystyrene core and a polyaniline shell were synthesized. They were nearly mono-dispersed spheres 1.62 μm in diameter. The aqueous suspension of the latex particles was spread on a glass substrate and dried in a mono-particle layer. The arrangement of the spread particles depended on the concentrations of the latex, those of the surfactant, and drying speed. The particles were also arranged on a platinum electrode in a mono-particle layer at a very slow drying speed although the particles were likely to gather at the boundary between the platinum and the insulating wall. The cyclic voltammograms of the mono-particle layer were similar to those of electrochemically polymerized films and of the film prepared by spreading and drying the polyaniline-dissolved pyrrolidone. The peak currents at high scan rates deviated downward from a linear relation with the scan rate. The mono-particle layer was inserted into tetrahydrofuran to remove the core-polystyrene. The resulting film covered the electrode surface smoothly. Its voltammetric peak current was almost proportional to the scan rate, as was observed at electropolymerized films.     

In vivo electrochemical detection of 5-hydroxyindole within the trigeminal nucleus caudalis of freely moving rats: the effect of morphine

The trigeminal nucleus caudalis is considered the equivalent of the orofacial nociceptive system of the dorsal horn of the spinal cord. At the level of this trigeminal area (i.e. medullary dorsal horn) the transmission of noxious inputs is strongly modulated by a descending, serotonergic system mainly originating from the nucleus raphe magnus (NRM). The present study in freely moving animals reports the effect of morphine on the 5-hydroxyindole oxidation current recorded in the medullary dorsal horn. Complementary data from recordings in spinal dorsal horn in acutely anesthetized rats are also presented. A current recorded at 270–290 mV (peak ‘3’), characteristic of 5-hydroxyindoleacetic acid (5-HIAA), was measured with treated multi-fiber carbon electrodes, using differential pulse voltammetry (DPV) or differential normal pulse voltammetry (DNPV). In control rats, the amplitude of the peak remained constant for many hours. Morphine (10 mg/kg i.p.) caused a significant increase which plateaued between 35 and 80 min (mean increase: 127 ± 5 % of control values); recovery was complete by about 3 h. Simultaneous injection of naloxone (1 mg/kg i.p.) totally abolished the effect of morphine. By contrast, morphine was without effect on peak 3 recorded in the spinal dorsal horn of chloral hydrate (450 mg/kg i.p.) anesthetized rats. It is concluded that in non-anesthetized freely moving animals morphine clearly increases the metabolism of serotonin (5-HT) in the medullary dorsal horn. This finding confirms previous neurochemical data showing an increased synthesis or release of 5-HT in the spinal cord after systemic morphine or its microinjection into either the periaqueductal gray matter or the NRM, and underlines the value of in vivo electrochemistry in monitoring changes in 5-HT metabolism directly and continuously during various physiological and pharmacological procedures.     

Ascorbic acid in the brain

Ascorbic acid is highly concentrated in the central nervous system. Measurement of the extracellular concentration of ascorbate in animals, mainly by the technique of voltammetry in vivo, has demonstrated fluctuation in release from neuropil, both spontaneously and in response to physical stimulation of the animal and to certain drugs. Although in the adrenal medulla ascorbate is co-released with catecholamines, release of ascorbate from brain cells is associated principally with the activity of glutamatergic neurones, mainly by glutamate-ascorbate heteroexchange across cell membranes of neurones or glia. This phenomenon is discussed in relation to a possible role of ascorbate as a neuromodulator or neuroprotective agent in the brain.     

The role of dissolution in the voltammetry of microdroplets and microparticles adhered to electrode surfaces in contact with aqueous electrolytes or ionic liquids

Detailed voltammetric studies have been undertaken on the redox chemistry associated with arrays of liquid microdroplets (ethylferrocene) or surface adhered microparticles (1,3,5-tris(3-pyridiniumferrocenylmethylamine)-2,4,6-triethyl benzene hexafluorophosphate and 7,7,8,8-teracyanoquinodimethane) in contact with aqueous electrolyte solutions, hydrophobic ionic liquids or a layer of the ionic liquid in contact with aqueous electrolyte media. The results support the hypothesis that dissolution processes play an important role in the voltammetric response detected under each set of conditions examined.     

Simultaneous electrochemical and quartz crystal microbalance studies of non-conducting microcrystalline particles of trans-Cr(CO)2(dpe)2 and trans-[Cr(CO)2(dpe)2]+ (dpe = Ph2PCH2CH2PPh2) attached to gold electrodes

Simultaneous cyclic voltammetric double potential step and electrochemical quartz crystal microbalance (EQCM) experiments on water insoluble trans-Cr(CO)₂(dpe)₂ and trans-[Cr(CO)₂(dpe)₂]X (dpe = Ph₂PCH₂CH₂PPh₂, X = Cl^?, Br^? and I^?), attached as an array of microcrystals, have been employed to probe mechanistic aspects of the redox chemistry of the [trans-Cr(CO)₂(dpe)₂]^+/0 process at the electrode |solid| solvent (electrolyte) interface in a variety of aqueous electrolytes. EQCM experiments show that the oxidation of solid trans-Cr(CO)₂(dpe)₂ involves the slow incorporation of non-solvated anions from the electrolyte solution into the solid. Interestingly, on the reverse scan of cyclic voltammetric experiments, EQCM data reveal that some but not all the anions are rapidly expelled from the crystal lattice. Double potential step experiments with the neutral chromium compound confirm that the oxidation reaction is a relatively slow process. The conclusion reached from all experiments is that the reduction process predominantly expels the anions that are relatively close to the solid|solution interface. EQCM investigations of trans-[Cr(CO)₂(dpe)₂]X compounds in electrolytes containing a different anion to that in the compound show that the anion originally in the salt is rapidly replaced by the anion in the aqueous electrolyte at open circuit potential, presumably via a rapid ion exchange process. The anion from the electrolyte is then expelled and incorporated into the solid during the reduction and oxidation steps respectively.     

Hydrodynamic voltammetry at membrane-covered electrodes

The modification of platinum electrodes with commercial filtration membranes is used to study the interplay between solution and membrane mass-transport in non-aqueous solution. Two types of commercial membrane, “track-etched” polyethylene terephthalate and γ-Al₂O₃, are employed. Voltammetric and amperometric data is obtained under both hydrodynamic (rotating-disc) and stationary conditions, which is interpreted in terms of a pinhole model. Contrasting behaviour is seen for the different membranes, which can be understood by consideration of the membrane thickness relative to the diffusion layer established in solution. In particular, overlapping diffusion behaviour is observed for the much thinner polymer membranes under hydrodynamic conditions.     

Simultaneous Voltammetry Determination of Dihydroxybenzene Isomers by Poly-bromophenol Blue/Carbon Nanotubes Composite Modified Electrode

A novel modified electrode was constructed by electropolymerization of bromophenol blue at a multi-walled carbon nanotubes modified glassy carbon electrode. The electrode developed was used for the simultaneous determination of the isomers of dihydroxybenzene in environmental samples using a voltammetry method. There was a linear relationship over the range 10⁻⁶–10⁻⁴ mol L⁻¹ of hydroquinone, catechol and resorcinol; the detection limits was 3 × 10⁻⁷ mol L⁻¹. The constructed electrode showed excellent reproducibility and stability. Actual water samples were analyzed and satisfactory result was obtained.     

Voltammetric determination of concentrations of ferrocene-included nitrobenzene droplets in water

When a nitrobenzene (NB) phase came in quiescent contact with a water phase, water droplets were formed spontaneously near the oil|water interfaces (K. Aoki, M. Li, J. Chen, T. Nishiumi, Electrochem. Commn. 11 (2009) 239). We reported here quantitative data of the NB-droplets by use of UV-absorbance, dynamic light scattering (DLS) and voltammetry with a help of ferrocene. The supernatant separated spontaneously from the oil–water mixture contained 15.5 mmol dm⁻³ evaluated by UV spectra, whereas the centrifuged supernatant did 9.3 mM. The difference suggested the presence of NB-droplets, the diameter of which ranged 0.15–0.5 μm obtained by DLS. Ferrocene was dissolved deliberately in the aqueous solution and the NB solution up to saturation. The voltammograms in the ultrasonicated supernatant exhibited diffusion-controlled redox peaks of ferrocene, which should be supplied from both dissolved ferrocene and ferrocene-dissolved NB droplets. The former was 1/3.6 times of the latter by the comparison with the current of ferrocene-saturated aqueous solution. Applying the expression for the diffusion-controlled peak current of a big particle, we estimated the number concentration of NB droplet to be 1.1 × 10¹⁴ dm⁻³. This value is equivalent to the average distance, L = 2.1 μm, between neighboring two droplets, corresponding to the diffusional traveling time, L²/D = 1.3 s, which would be long enough for collision of droplets to stabilize the emulsions.     

Alteration of dopamine transport in the striatum and nucleus accumbens of ovariectomized and estrogen-primed rats following N-(p-isothiocyanatophenethyl) spiperone (NIPS) treatment

The ability of N-(p-isothiocyanatophenethyl) spiperone (NIPS, 10 mg/kg, 24 h), a selective, irreversible alkylating agent of the dopamine D(2) receptor, to alter properties of dopamine uptake and clearance in the striatum and nucleus accumbens of ovariectomized and estrogen-primed (estradiol benzoate, 10 microg, 48 h, 24 h) rats was examined using voltammetry. The effectiveness of NIPS was evaluated independently by measuring agonist mediated potentiation of [35S]-guanosine 5'-(gamma-thiotriphosphate) ([35S]-GTPgammaS) binding and [3H]-dopamine uptake. A decrease in E(max) for ligand potentiated [35S]-GTPgammaS binding and a loss of quinpirole potentiated [3H]-dopamine uptake was observed consistent with a NIPS mediated alkylation and functional down-regulation of the dopamine D(2) receptor. This down-regulation was associated with an attenuation of the dose dependent uptake of dopamine in both the striatum and the accumbens. Co-administration of estrogen and NIPS resulted in a further attenuation of dopamine potentiated [35S]-GTPgammaS binding measured in vitro and dopamine uptake measured in vivo. Analysis of the voltammetric profile revealed that clearance and T(50) times were significantly prolonged in animals treated with estrogen and NIPS compared with those treated with NIPS alone. These data are consistent with both a steroid mediated impairment in dopamine autoreceptor/dopamine transporter coupling and an independent action of estrogen at the level of the dopamine transporter.     

Voltammetric assay of anti-anginal drug nicorandil in different solvents

Voltammetric behaviour and assay of nicorandil were investigated using square-wave and cyclic voltammetry. Voltammograms in Britton-Robinson (BR) buffer exhibited one well-defined and two merged reduction peaks. The influence of different buffers, electrolytes, pH, scan rates, and concentration of the drug on cathodic peak current was studied. On the basis of electrochemical behaviour of nicorandil, a direct square-wave voltammetric procedure for quantitation of nicorandil has been developed and validated. The proposed square-wave voltammetric method allows quantitation over the range 12.5-62.5 μg mL(-1) with correlation coefficient of 0.992. The limit of quantification and limit of detection were 21.95 μg mL(-1) and 6.58 μg mL(-1) , respectively. Precision and accuracy were also checked and found within limits. The developed square-wave method has also been successfully applied for the determination of nicorandil in pharmaceutical formulations.