Voltammetric study of nifuroxazide at unmodified and Sephadex-modified carbon paste electrodes

The electrooxidation of nifuroxazide was investigated by cyclic and differential-pulse voltammetry at carbon paste and Sephadex-modified carbon paste electrodes. Nifuroxazide is irreversibly oxidized at all pH values and gives rise to a well-defined oxidation peak. The modification of the carbon paste surface with Sephadex allowed a preconcentration process to take place for nifuroxazide such that higher sensitivity was achieved compared with the bare surface. The influence of the scan rate, time of accumulation, modifier loading, solution conditions and pH on the adsorptive step at the modified carbon paste electrodes was investigated. The direct determination of the drug in urine is also discussed. Received: 2 June 1998 / Revised: 29 January 1999 / Accepted: 2 February 1999     

Voltammetric study of nifuroxazide at unmodified and Sephadex-modified carbon paste electrodes

The electrooxidation of nifuroxazide was investigated by cyclic and differential-pulse voltammetry at carbon paste and Sephadex-modified carbon paste electrodes. Nifuroxazide is irreversibly oxidized at all pH values and gives rise to a well-defined oxidation peak. The modification of the carbon paste surface with Sephadex allowed a preconcentration process to take place for nifuroxazide such that higher sensitivity was achieved compared with the bare surface. The influence of the scan rate, time of accumulation, modifier loading, solution conditions and pH on the adsorptive step at the modified carbon paste electrodes was investigated. The direct determination of the drug in urine is also discussed.     

Voltammetric determination of Celiptium with carbon paste and lipid-modified carbon paste electrodes

The electrooxidation of the antitumour drug 2-methyl-9-hydroxyellipticinium (Celiptium) was investigated by cyclic, differential-pulse and adsorptive voltammetry at carbon paste (CPE) and lipid-modified carbon paste electrodes (LM-CPE). The influence of the paste composition, i.e., the ratio of graphite to binder, was studied in order to elucidate the nature of the accumulation process at the surface of the CPE. The electrode surface coverage at saturation was calculated. A.c. measurements at the CPE and at the LM-CPE during the accumulation of Celiptium demonstrated an increased differential double layer capacity of the LM-CPE. The influence of several parameters that affect the adsorptive step at the CPE was investigated, such as pH, ionic strength and interfering ions. Improved signals were obtained at the CPE and the detection limit in 0.1 M sodium perchlorate (t_acc.=3 min) was found to be 2 × 10^?10 M. Measurements of the drug in dilute standard serum samples were done using the medium-exchange technique.     

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 determination of azithromycin at the carbon paste electrode

Azithromycin (AZ) is the first member of a class of macrolide azalides antibiotics called azolides. A simple and selective square-wave voltammetric (SWV) method has been developed for the determination of azithromycin in pure form, in pharmaceutical preparation and in biological samples. Determination of azithromycin was accomplished with hand-make carbon paste electrode (CPE) in oxidative screen mode. The counter and reference electrodes were a Pt wire and a Ag/AgCl, respectively. Various parameters that can influence the peak signal (effect of buffer, ionic strength, accumulation time, pH and the composition of the paste) have been scrutinized. The best results were obtained in acetonitrile—aqueous 1 M sodium acetate-acetic acid buffer (pH 4.6) containing 0.1 M KCl (1:9; v/v) using a 15% paraffin oil CPE. The limits of detection and quantification of the pure drug are 0.463 and 1.544 ppb (with the correlation coefficient, r=0.9785and the standard deviation, S.D.=0.1 (n=5), for the accumulation time of 60 s), respectively. The method was successfully applied to the determination of the drug in urine and two forms of pharmaceutical formulations. Recoveries were 99.2—100.5% with S.D.=0.1—and 0.8% (n=5).     

Voltammetric study of 2-methyl-4,6-dinitrophenol at a modified carbon paste electrode

The voltammetric behavior of 2-methyl-4,6-dinitrophenol at a modified carbon paste electrode has been studied. Among the modifiers tested, hidepowder was found to give the best results. Cyclic voltammetry and differential pulse voltammetry were used to study the nature of the reaction; the possibility of accumulation of the analyte onto the electrode was studied by differential pulse voltammetry. An irreversible behavior and the adsorption of 2-methyl-4,6-dinitrophenol on the electrode were confirmed. The reduction signal shows two peaks. A linear relationship between the first peak height and the concentration of 2-methyl-4,6-dinitrophenol was obtained in the range 0.001–5 mg l⁻¹, with a detection limit of 3.2 μg l⁻¹ and a relative standard deviation of 3.20%. The interferences with the reduction peak of 2-methyl-4,6-dinitrophenol of several nitro- and chloro-phenols and inorganic species were tested.     

Detection of homocysteine at carbon nanotube paste electrodes

The use of a carbon-nanotube paste (CNTP) electrode provides an effective means for the determination of homocysteine. A decrease of ca. 120 mV in the overpotential for the oxidation of homocysteine compared to a traditional carbon paste electrode, is reported along with greatly enhanced signal-to-noise characteristics. The analytical parameters have been assessed with a linear range from 5 to 200 μM and a detection limit of 4.6 μM. Furthermore, the generic nature of this increased reactivity of the CNTP surface towards thiol moieties has been demonstrated with cysteine, glutathione and n-acetylcysteine, providing a greatly enhanced electrochemical response compared to the carbon paste electrode.     

Voltammetric determination of Rutin at Screen-Printed carbon disposable electrodes

Rutin is a flavonoid commonly employed for many therapeutic purposes. Although the electroactive phenolic groups of rutin might be oxidized at low applied potential, the adsorption of oxidized species changes the electrode surface. As a consequence, the repeatability and reproducibility of the method decreases, which limits electroanalytical applications. This paper describes the use of disposable screen-printed electrodes as an alternative to improve the electrochemical quantification of rutin in commercial and standard samples. The electrochemical behavior was consistent to what is observed using other carbon electrodes: an adsorption-involved step and a pH-dependent oxidation process. The replacement of the electrodes between the analyses ensured rapid analysis, good intermediate precision and repeatability. The proposed method was successfully applied to rutin determination in pharmaceutical samples of capsules, with the limit of quantification being 0.30 μM.      

Voltammetric determination of mifepristone at a DNA-modified carbon paste electrode

A new strategy for the preparation of a DNA-modified carbon paste electrode is developed. It is found that the anodic response of mifepristone is greatly enhanced at the dsDNA-modified carbon paste electrode comparing with that obtained at the bare electrode, while the response at a ssDNA-modified electrode is similar to bare electrode. So the dsDNA-modified electrode is employed as a sensitive biosensor for the detection of mifepristone. A linear dependence of the peak currents on the concentration is observed in the range 2.0 x 10(-7) approximately 2.0 x 10(6) mol/L, with a detection limit of 1.0 x 10(-7) mol/L. The relative standard deviation is 4.3% for six successive determinations of 1.0 x 10(6) mol/L mifepristone. The determination of mifepristone tablets is carried out and satisfactory results are obtained.     

Bismuth-film-plated carbon paste electrodes

In this preliminary note, carbon paste electrodes (CPEs) plated with a bismuth film are presented. The bismuth film can be generated onto the carbon paste surface either from an external plating solution or in situ; the latter being performed in two ways: (i) as a spike of the Bi³⁺ ions to the solution or (ii) via modifying the carbon paste with solid bismuth oxide (5% m/m). As shown on selected examples, bismuth-film-plated CPEs exhibit a good performance in voltammetric stripping analysis of some heavy metals such as Pb, Cd, and Zn.     

Glassy carbon paste electrodes

A new carbon composite electrode material, based on mixing glassy carbon (GC) microparticles with an organic pasting liquid is described. The resulting glassy carbon paste electrode (GCPE) combines the electrochemical properties of GC with the various advantages of composite electrodes. Glassy carbon pastes (GCPs) offer high electrochemical reactivity, a wide accessible potential window, a low background current, and are inexpensive, easy to prepare, modify, and renew. The new material has a lower double-layer capacitance and a higher heterogeneous rate constant (for ferricyanide) compared to conventional carbon pastes (CPs). Scanning electron microscopy (SEM) images indicate significant differences in the structure of GCPE and carbon paste electrode (CPE). Factors influencing the electrode kinetics of GCPE surfaces are discussed. The electrochemical properties and advantages of GCPE should be of broad utility in electroanalysis.     

Voltammetric determination of quercetin at a multi-walled carbon nanotubes paste electrode

Quercetin can effectively accumulate at multi-walled carbon nanotubes-paraffin oil paste electrodes (CNTPE) and cause a sensitive anodic peak at around 0.32 V (vs. SCE) in a 0.10 M phosphate buffer solution (pH = 4.0). Under optimized conditions, the anodic peak current is linear to quercetin concentration in the ranges of 2.0 × 10^− 9−1.0 × 10^− 7 M and 1.0 × 10^− 7−2.0 × 10^− 5 M, and the regression equations are i_p (μA) = 0.0017 + 0.928c (μM, r = 0.999) and i_p (μA) = 0.183 + 0.0731c (μM, r = 0.995), respectively. This paste electrode can be regenerated by repetitively cycling in a blank solution for about 2 min. A 1.0 × 10^− 6 M quercetin solution is measured for 10 times using the same electrode regenerated after every determination, and the relative standard deviation of the peak current is 1.7%. The method has been applied to the determination of quercetin in hydrolysate product of rutin and the recovery is 99.2–102.6%. In comparison with graphite paste electrode, carbon nanotubes-nujol paste electrode and carbon nanotubes casting film modified glassy carbon electrode, the CNTPE gives higher ratio of signal to background current and better defined voltammetric peak.     

Voltammetric determination of 2-nitrophenol at a bentonite-modified carbon paste electrode

A bentonite-modified carbon paste electrode has been applied to the determination of 2-nitrophenol by differential pulse voltammmetry. The electrochemical reduction of 2-nitrophenol at –0.8 V is carried out in an artificial sea water-formic acid/sodium formate medium at pH 4. The peak height was found to be dependent on the pH over the range 2–11; the presence of a secondary process was observed in the pH range 8–11. The peak potential showed a dependence on pH, with two linear regions with different slopes. A linear relationship between peak intensity and concentration was obtained in the range 0.07–10 mgl^–1, with a detection limit of 0.03 mg 1^–1 and a coefficient of variation of 1.3% at 5 mg 1^–1. The effects of organic and inorganic species on the 2-nitrophenol determination were studied with a view to testing the resolution of the voltammetric technique. The proposed method has been applied to sea water samples with good results.     

Voltammetric determination of Ioxynil and 2-methyl-3-nitroaniline using C18 modified carbon paste electrodes

The stripping voltammetric behaviour of Ioxynil and 2-methyl-3-nitroaniline has been studied by means of a C₁₈ modified carbon paste electrode. The analytes are preconcentrated under open-circuit conditions (Ioxynil at pH 3.9; 2-methyl-3-nitroaniline at pH 10). For the determination of Ioxynil and 2-methyl-3-nitroaniline 0.01 mol/l HCl and 1 mol/l KOH, respectively, have been used as supporting electrolyte. Under optimized conditions detection limits up to 0.1 g/ml Ioxynil and 0.3 g/ml 2-methyl-3-nitroaniline have been obtained. The methods have been applied to the determination of Ioxynil and 2-methyl-3-nitroaniline in drinking water.     

Zeolite-modified solid carbon paste electrodes

New zeolite-modified carbon-based electrodes are described. They are based either on the physical anchoring of zeolite particles on the surface of solid carbon paste (the viscosity of which can be tuned by temperature change or controlled dissolution by an organic solvent), or on the dispersion of zeolite particles in the bulk of a carbon paste matrix containing solid paraffin as a binder. Both these systems display superior electrochemical performance in comparison to corresponding "classical" zeolite-modified carbon paste electrodes using mineral oil as binder. These well-described composites usually suffer from poor mechanical stability in stirred media as well as memory effects due to significant ingress of the external solution into the bulk electrode. Advantages of the zeolite-modified solid carbon paste electrodes are reported mostly on the basis of two electroanalytical applications: the voltammetric detection of Cu²⁺ ions after accumulation by ion exchange at open circuit, and the indirect amperometric detection of non-electroactive species (i.e. Na⁺) in flow injection analysis.     

Lead dioxide-coated carbon paste electrodes

Lead dioxide electrodeposited under optimized conditions on carbon paste provides an electroanalytically useful electrode, with an anodic working range extending to 1.9 V. Voltammograms for sulphite are used to study electrode lifetime and reproducibility; calibration graphs are linear up to 10^-3M sulphite in 0.5 M sulphuric acid.     

Electrochemical behaviour of drugs at lipid modified carbon paste electrodes

Mixing carbon paste with a charged lipid, in an appropriate ratio, permitted us to prepare a modified electrode whose surface exhibits interesting characteristics of the constituting lipid. Two negatively charged phospholipids have been selected, namely: asolectin and cardiolipin. By investigating the electrooxidation of several drugs which are known to interact strongly with the lipid component of cellular membranes (adriamycin, epirubicin and promethazine) it was possible to demonstrate markedly different behaviour at the lipid modified carbon paste electrode (LMPCE) depending on the pH of the solution. By analysing the results of linear scan, ac and adsorptive voltammetry and by taking into account literature data on drug-membrane interactions, it was possible to postulate a model of the LMCPE interface with thin layers of lipids covering the graphite particles. The differential capacity measurements as a function of time (fixed potential) and in the presence of increasing amounts of drugs enabled us to point out profound increases in the capacity of the LMCPE in the presence of adriamycin and epirubicin. In addition, the more negatively charged the lipids were, the higher was the capacity increase.     

Voltammetric behavior of urapidil and its determination at multi-wall carbon nanotube paste electrode

The voltammetric behavior of urapidil was investigated. In pH 6.8 Britton-Robinson buffer, an irreversible oxidation peak of urapidil at 0.62 V (versus SCE) at a multi-wall carbon nanotube paste electrode (MWNT-PE) was observed, which was more sensitive with lower potential than that at the carbon paste electrode (CPE). The oxidation of urapidil was a two-electron and two-proton process with adsorption character. A differential pulse voltammetric method was proposed for the determination of urapidil. The peak current of the oxidation peak of urapidil was linearly with its concentration in a range from 5.0 × 10⁻⁸ to 2.0 × 10⁻⁶ mol/L at open-circuit accumulation for 60 s, with a detection limit of 3.8 × 10⁻⁸ mol/L. The proposed method was employed to determine urapidil in urapidil tablets.