Voltammetric studies on the reduction of polyoxometalate anions in ionic liquids

The electrochemical reduction of tetrabutylammonium salts of isostructural pairs of polyoxometalates [Bu4N]2[M6O19], [Bu4N]4[alpha-SiM12O40], and [Bu4N]4[alpha-S2M18O62] (M = Mo or W) has been investigated at glassy carbon electrodes in dissolved and surface-confined states in ionic liquids and other media. In the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF(6)], between two and six reversible one-electron-transfer processes were detected. Detailed studies on the process [alpha-S2W18O62](4-/5-) in a range of ionic liquids, water, and conventional organic solvents (containing 0.1 M electrolyte) suggest that the polarity of the medium plays a key role in the determination of the reversible potential. Reduction processes involving very highly charged [alpha-S2W18O62](8-/9-/10-) species are strongly influenced by the purity of the medium.     

Voltammetric studies of the interaction of quinacrine with DNA

The binding interaction of the antimalarial drug quinacrine with herring sperm deoxyribonucleic acid (DNA) has been studied by square wave voltammetry. The binding parameters, the binding constant K and the binding site size s, were obtained simultaneously by the analysis of bound and free quinacrine concentration corresponding to the limits of slow and fast binding kinetics compared to the experimental timescale. The binding constant and the binding site size for the interaction of quinacrine with DNA were K=1.59 (±0.18)×10⁵ M^–1 and s=7.1 (±0.15) base pairs and K=7.35 (±0.83)×10⁵ M^–1, s=6.2 (±0.02) base pairs for the limiting conditions of static and mobile binding equilibrium respectively. The standard Gibbs free energy change (G⁰=–RT ln K) is approximately –29.67 kJ/mol at 25 °C, which highlights the spontaneity of the binding of quinacrine with DNA. The binding of quinacrine to herring sperm DNA results in peak potential shifts in voltammetric and a red shift in UV-absorption measurements. The ionic strength dependence of the binding constant is not large. Furthermore, the relative viscosity of DNA increases in the presence of quinacrine. These characteristics strongly support the intercalation of quinacrine into DNA. The results also show that the intercalation of quinacrine into DNA may occur at approximately every seventh base pair.     

Voltammetric studies of indigo adsorbed on pre-treated carbon paste electrodes

The adsorptive behaviour of indigo on the surface of pre-treated carbon paste electrodes (CPE) has been studied in various aqueous media at different pH, by cyclic (CV) and alternating current voltammetry (ACV). Cyclic voltammograms of this molecule are characterised by two main electrodic processes in the potential range from −0.7 V to +0.9 V (vs. Ag/AgCl/sat. KCl), which are reversible in the pH range of 3–11, these being irreversible when both are recorded at pH 1. The first one at pH 1 becomes purely reversible when the applied potential range is from −0.2 V to +0.2 V and constitutes the best analytical signal. Due to these electrodic features a significant increase in sensitivity was achieved when the signal was recorded by alternating current voltammetry. Indigo was quantified by CV and ACV, in 0.1 M HClO₄ pH 1 and 0.1 M Tris-HCl pH 7.2 solutions. Limits of detection in the sub-nanomolar range were achieved for a 5-min accumulation time by ACV. These data provides useful information about the suitability of this electrodic process as a detection scheme in the development of alkaline phosphatase (AP) based voltammetric affinity devices, in which indigo is generated by the enzymatic hydrolysis of the 3-indoxyl phosphate substrate.     

Voltammetric studies with silicone rubber-based graphite electrodes

The behaviour of a new type of graphite electrode prepared by mixing spectral graphite with silicone rubber before cold vulcanisation, is described. Electrodes can be prepared as plates or rods. The residual current is reproducibly small or negligible in the potential range -0.3 V to +0.7V. The height of the peak current is proportional to the concentration for a wide range of substances. The reproducibility of i-E curves at the same electrode surface is excellent. The electrode requires no preliminary treatment and the surface can be easily cleaned or renewed. The temperature coefficient measured for hexacyanoferrate(II) was 1.1%/°C.     

Simultaneous optimization of extraction and antioxidant activity from Blumea laciniata and the protective effect on Hela cells against oxidative damage

Blumea was the resource for medicine such as l-borneol showing a great economic value. Interestingly, Blumea laciniata was widely used as a folk medicine in south of China and Asia. But the chemical compounds and specific pharmacological activity were rarely reported. Therefore, in this present work, the chemical components from B. laciniata were determined and the antioxidant ability on scavenging radicals and the protection on Hela cells against H₂O₂ were evaluated. The results showed the antioxidant ability was associated with the presence of polyphenols via response surface method. Additionally, chlorogenic acid, isoorientin, rutin, luteolin-4′-O-glucoside and cinnamic acid were firstly identified in B. laciniata. Moreover, the extract from B. laciniata (EBL) showed a strong antioxidant on clearing DPPH and ABTS free radicals with a lower EC₅₀. Besides, EBL showed no toxicity on Hela cells and even could protect cells from H₂O₂ induced damage by sharply reducing the excessive reactive oxygen species, improving the mitochondrial membrane potential and then decreasing the generation of cell apoptosis. These outcomes could provide a promising understanding on the potential antioxidant.     

Voltammetric studies on the electrochemical determination of methylmercury in chloride medium at carbon microelectrodes

Electroanalytical techniques have been used to determine methylmercury at low levels in environmental matrices. The electrochemical behaviour of methylmercury at carbon microelectrodes in a hydrochloric acid medium using cyclic, square wave and fast-scan linear-sweep voltammetric techniques has been investigated. The analytical utility of the methylmercury reoxidation peak has been explored, but the recorded peak currents were found to be poorly reproducible. This is ascribed to two factors: the adsorption of insoluble chloromercury compounds on the electrode surface, which appears to be an important contribution to hinder the voltammetric signal of methylmercury; and the competition between the reoxidation of the methylmercury radical and its dimerization reaction, which limits the reproducibility of the methylmercury peak. These problems were successfully overcome by adopting the appropriate experimental conditions. Fast-scan rates were employed and an efficient electrochemical regeneration procedure of the electrode surface was achieved, under potentiostatic conditions in a mercury-free solution containing potassium thiocyanate—a strong complexing agent. The influence of chloride ion concentration was analysed. Interference by metals, such as lead and cadmium, was considered. Calibration plots were obtained in the micromolar and submicromolar concentration ranges, allowing the electrochemical determination of methylmercury in trace amounts. An estuarine water sample was analysed using the new method with a glassy carbon microelectrode.     

Voltammetric studies on the interaction of amikacin with methyl blue and its analytical application

A new method to determine the concentration of amikacin(AMK)using methyl blue(MB)as electrochemical probe was developed in this paper.In pH 4.5 Britton-Robinson(B-R)buffer solution,the MB reacted with AMK to form ion association complexes,which led to the reductive peak current of MB at-0.275 V(versus SCE)to decrease,and the decreases were linear with the concentration of AMK in the range of 1.0-60.0 mg/L,the regression of equation isΔIp(nA)=-8.48 102.36c(mg/L), correlation coefficientγis 0.997.The conditions for determining the concentration of AMK using linear sweep voltammetry(SLV) were optimized.The method was used to determine the content of amikacin commercially available with satisfactory results.     

Voltammetric and impedance studies of inosine-5'-monophosphate and hypoxanthine

The oxidation mechanism and adsorption of inosine 5'-monophosphate and hypoxanthine were investigated in solutions of different pH using voltammetric and impedance methods at glassy carbon electrodes. For both compounds, the pH dependence from differential pulse voltammetry showed that the same number of electrons and protons are involved in the rate-determining step of the electrochemical reaction. In the case of hypoxanthine, it was also possible to study the effect of different concentrations. At high concentrations of hypoxanthine, two oxidation peaks were observed, the first due to hypoxanthine oxidation with formation of oligomers and the second due to hypoxanthine oligomer oxidation, both compounds adsorbing strongly. Impedance measurements corroborated the voltammetric results and enabled the study of the adsorption of hypoxanthine on glassy carbon.     

Voltammetric studies of purine bases and purine nucleosides with copper

Anodic stripping voltammetry with a hanging mercury drop electrode has been used to investigate the interaction of cooper with the purines adenine, hypoxanthine, xanthine and purine nucleosides adenosine, guanosine and inosine at an ionic strength of 0.1 M in KNO₃ and in the pH range 3.5–5.5. In all cases stabilisation of copper(I) occurs suggesting that the oxidation of copper(0) in the presence of excess ligand proceeds in two one-electron steps.Adsorption onto the electrode has been analysed and conditions where this is negligible were chosen for complexation studies. From the shift of the peak potential corresponding to Cu(0)/Cu(I) oxidation with increasing ligand concentration the stoichiometry of the complexes and their formation constants have been determined. The values obtained are discussed in terms of the ligand structure.     

Voltammetric studies in phosphorous oxychloride solutions containing lithium tetrachloroborate

The solvent reduction/oxidation in 0.1, 0.5 and 1 molar LiBCl₄?POCl₃ solutions was studied at pyrolytic graphite electrodes using the technique of cyclic voltammetry. The solvent reduction was found to occur at ～ 2.25 V versus the lithium reference and resulted in the passivation of the pyrolytic graphite electrodes. The passivating film mainly consisted of polymeric phosphorous monoxide and lithium chloride. The cathodic potential limit in these solutions was, however, caused by the deposition of lithium metal. The anodic potential limit was caused by the formation of chlorine at ～ 4.5 V versus the lithium reference due to the oxidation of the solvent. The chlorine formed in the anodic process was retained in the solution and could be reduced at less positive potentials (～ 3 V versus the lithium reference).     

Voltammetric studies of the interaction between lead metal ion and the methyl parathion pesticide

Journal of Solid State Electrochemistry - In this work, the interaction of the pesticide methyl parathion (MP) with the lead metal ion was evaluated using a carbon electrode reused from a zinc...     

Voltammetric and spectroscopic studies of the interaction between copper (II) ions with the pesticide carbendazim and its effect in the soil

Journal of Solid State Electrochemistry - The pesticide carbendazim (MBC) has nitrogen and oxygen in their structure which can interact with metal ions. This work evaluated the interaction between...     

Research of the relationship of intracellular acidification and apoptosis in Hela cells based on pH nanosensors

In this paper,the relationship of intracellular acidification and apoptosis in Hela cells induced by vin-cristine sulfate has been studied by use of the ratiometric pH nanosensors that have been developed by our group,employing fluorescein isothiocyanate(FITC) doped as the pH-sensitive dye and Tris(2,2'-bipyidyl) dichlororuthenium(II) hexahydrate(RuBpy) doped as reference dye. The pH change of the Hela cells induced by vincristine sulfate has been monitored in vivo,in situ and real time by use of the ratiometric pH nanosensors. The experimental results show that the pH of the apoptotic Hela cells induced by vincristine sulfate has been acidified from 7.11 to 6.51,and the percentage of intra-cellular acidification is correlated with the induced concentration and incubation time of the vincristine sulfate. The further study of the percentage of intracellular acidification and the percentage of apop-tosis of Hela cells at the same time reveals that apoptosis of Hela cells induced by vincristine sulfate is preceded by intracellular acidification. These results would provide theoretical foundation for the therapy of cancer through interfering the pH of cells by use of vincristine sulfate or other anti-cancer drugs.     

Voltammetric determination of hypoxanthine based on the enhancement effect of mesoporous TiO2-modified electrode

A mesoporous TiO₂ was synthesized according to the reported method, and then used to modify the carbon paste electrode (CPE). The electrochemical behavior of hypoxanthine was investigated with great detail. Compared with the unmodified CPE, the mesoporous TiO₂-modified CPE greatly enhances the oxidation signal of hypoxanthine. Due to huge surface area, well-defined and special mesopores, the mesoporous TiO₂-modified CPE shows considerable enhancement effect toward hypoxanthine. Based on this, a sensitive, rapid and convenient electrochemical method was developed for the determination of hypoxanthine. The linear range is over the range from 2.0 × 10⁻⁷ to 5.0 × 10⁻⁵ mol L⁻¹, and the limit of detection is estimated to be 5.0 × 10⁻⁸ mol L⁻¹. The relative standard deviation (RSD) for 10 mesoporous TiO₂-modified CPEs is 5.7%. Finally, this sensing method was successfully used to determine hypoxanthine in human blood serum samples.     

Voltammetric studies on some substituted 5-arylidene-2-thiohydantoin in non aqueous medium

Summary Differently substituted 5-arylidene-2-thiohydantoins (2a–f) were studied electrochemically in benzonitrile with 0.1M tetra-n-butylammonium-perchlorate as supporting electrolyte using DC-, cyclic voltammetry (CV), coulometry and controlled potential electrolysis (CPE). These compounds are oxidized in a one two-electron transfer process or in irreversible two successive one-electron processes to the corresponding diradical, which simaltaneously reacts with the solvent to regenerate the starting species. On the other hand the reduction products are the 5-arylidene-4-imidazolidinone-2-thiols.

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Voltammetrische Untersuchungen einiger substituierter 5-Aryliden-2-thiohydantoine in nichtwäßrigem Medium

Zusammenfassung Es wurden verschieden substituierte 5-Aryliden-2-thiohydantoine (2a–f) in Benzonitril mit 0.1M Tetra-n-butylammoniumperchlorat als Leitsalz elektrochemisch untersucht, wobei DC-, Cyclische Voltammetry (CV), Coulometrie und kontrollierte-Potential-Elektrolyse (CPE) eingesetzt wurden. Die Verbindungen wurden entweder in einem Zweielektronen-Transferprozeß oder in zwei irreversiblen Prozessen aufeinanderfolgender Einelektronen-Stufen zum entsprechenden Diradikal oxidiert, welches simultan mit dem Solvens unter Regenerierung der Startspezies reagiert. Andererseits sind die Reduktionsprodukte die 5-Aryliden-4-imidazolidinon-2-thiole.

     

Voltammetric and spectroscopic studies on the interaction of anti-cancer herbal drug rutin with an anti-tuberculosis agent rifampicin

In this paper, the interaction of rutin (Rt) with rifampicin (RIF) has been investigated using voltammetric and spectroscopic techniques. With the addition of RIF into the Rt solution, both the reduction and oxidation currents of rutin decrease with few changes in the peak potentials and no new peaks appeared. The binding of Rt with RIF forms a kind of supramolecular complex Rt-RIF, which is electrochemically non-active. The experimental data showed that the formation of supramolecular complex is due to the electrostatic attraction of Rt with RIF. The binding reaction resulted in the decrease of the concentration of free Rt in the solution, and the decrease of the cathodic peak current of Rt. The stoichiometry of the Rt-RIF biocomplex was determinated to be 1: 1 by means of voltammetric data. The effect of pH on the binding reaction has been also studied. Based on the peak current values of Rt in the presence and absence of RIF, the binding constants of Rt-RIF at pHs 4, 7 and 9 were calculated as 0.72 × 10⁴ M⁻¹, 1.28 × 10⁴ M⁻¹ and 0.19 × 10⁴ M⁻¹, respectively. The experimental results indicate that there is a strength interaction between Rt and RIF in neutral pH. This binding reaction was supported by UV-Vis. and IR spectroscopy techniques.     

Voltammetric,potentiometric and amperometric studies with a rotated aluminum wire electrode: Voltammetric behavior of the r.al.e.

A rotated aluminum wire electrode (R.Al.E.) is described for the determination of voltammograms and potentials. An aluminum electrode is highly polarizable both cathodically and anodically. Of all ions tested only hydroxyl and fluoride ions depolarize it anodically at highly negative potentials. In the absence of fluoride it is not a pH electrode, but it is a pOH electrode in the presence of an excess of hydroxyl ions. Fluoride in acid medium and hydroxyl ions yield well defined anodic diffusion currents. In the absence of fluoride or of an excess of hydroxyl ions the potential is ill defined. In acid medium a trace of fluoride (2.10^-5M or 0.4 p.p.m.) causes the potential to become approximately 600 mV more negative than in the absence of fluoride. At a pH greater than j l the potential varies 66 mV per unit change of pH.     

Voltammetric studies of the Kolbe synthesis with perfluorocarboxylic acids prepared from hexafluoropropene oxide oligomers

Carboxylic acids prepared from hexafluoropropene oxide CF₃CF₂CF₂O[CF(CF₃)CF₂O]_nCF (CF₃)COOH [n = 0, 2,5-bis(trifluoromethyl)-3,6-dioxaperfluorononanoic acid; n = 1, 2,5,8- tris(trifluoromethyl)-3,6,9-trioxaperfluorododecanoic acid], according to voltammetric data, enter the Kolbe reaction both at the Pt anode and at anodes from carbon materials in H₂O-CH₃CN and CH₃OH-CH₃CN solutions. The critical potential appreciably depends both on the anode material and on the solvent composition. Favorable effect of pyridine additions in H₂O-CH₃CN solutions is due to replacement of water molecules from the electrical double layer. The sodium ions exert a negative effect on the Kolbe synthesis.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 11, 2004, pp. 1842–1846.Original Russian Text Copyright © 2004 by Chechina, Sokolov, Tomilov.This revised version was published online in April 2005 with a corrected cover date.     

罗红霉素的伏安法测定研究

本文报道了新一代大环内酯类抗生素罗红霉素的伏安法测定。研究中发现 ,罗红霉素除本身发生不可逆电化学氧化外 ,且可与溶解氧结合并发生电荷转移生成结合态的超氧阴离子 ,因而具有更高的电化学氧化活性。采用伏安法测定 ,检测下限可达 6× 1 0 - 6mol/L,对罗红霉素制剂可不经分离直接进行测定。相对标准偏差及回收率均符合要求。