Simple and rapid voltammetric determination of morphine at electrochemically pretreated glassy carbon electrodes

A simple and rapid method for morphine detection has been described based on electrochemical pretreatment of glassy carbon electrode (GCE) which was treated by anodic oxidation at 1.75 V, following potential cycling in the potential range from 0 V to 1.0 V vs. Ag|AgCl reference electrode. The sensitivity for morphine detection was improved greatly and the detection limit was 0.2 μM. The reproducibility of the voltammetric measurements was usually less than 3% RSD for six replicate measurements. Moreover, this method could readily discriminate morphine from codeine. And an electrochemical detection of morphine in spiked urine sample was succeeded with satisfactory results.     

Adhesion of gold nanoparticles on an electrochemically pretreated glassy carbon electrode.

This study describes a simple new method for the adsorption of gold nanoparticles on a glassy carbon (GC) electrode. By electrochemically oxidizing the GC electrode and immersing it into a gold colloidal solution, one could induce the adhesion of the gold nanoparticles. The image of the field emission scanning electron microscopy confirmed that the nanoparticles were evenly distributed over the surface. A 2-aminoethanethiol self-assembled monolayer was formed on the surfaces of the adsorbed gold nanoparticles and a reductive desorption wave was observed during the reduction of the adsorbed 2-aminoethanethiol.     

Voltammetric behavior and determination of 17 beta-estradiol at multi-wall carbon nanotube-Nafion modified glassy carbon electrode.

For the purpose of low cost and sensitive electrochemical detection of 17 beta-estradiol (E2), a multi-wall carbon nanotube (MWNT)-Nafion modified electrode was fabricated. The MWNT modified electrode shows enhancement for the anodic peak current of E2 compared with the value obtained using a bare electrode. The anodic peak current measured by square wave voltammetry after 5 min open-circuit accumulation was proportional to the concentration of E2 over the range of 2.5 x 10(-7) to 10 10(-6) M, and a detection limit of 1 x 10(-8) M was obtained.     

Voltammetric study of the hydrolysis product of fenthion at the Nafion®-modified glassy carbon electrode

The differential pulse voltammetric (DPV) method is proposed for the determination of phosphorothioate insecticide (fenthion) based on the oxidation of its hydrolysis product. A single peak at +0.65 V (vs. Ag/AgCl) is observed at the Nafion®-modified glassy carbon electrode in 0.1 M Britton-Robinson buffer solution (pH 4.0) as a supporting electrolyte. The voltammetric behavior of fenthion was investigated over a wide range of pH (2.0–8.0). The effect of the solution and operational parameters on the sensitivity of the DPV peak was carefully examined in order to select the optimum conditions for the determination of fenthion. Under optimum conditions, the oxidation response gives a linear calibration plot over a concentration range of 8.41 × 10^?7?5.98 × 10^?6 M and the detection limit is found to be 7.6 × 10^?7 M. The effects of some diverse metal ions, anions, and some other organic molecules on the determination of fenthion were studied. The applicability of DPV for the determination of fenthion insecticide in a commercial sample as well as in some water samples was demonstrated.     

Voltammetric behavior and determination of estrogens at Nafion-modified glassy carbon electrode in the presence of cetyltrimethylammonium bromide

A method is described for the determination of estrogens (estradiol, estrone and estriol) by stripping voltammetry. These estrogens yield one well-defined oxidation peak at the Nafion-modified glassy carbon electrode in the presence of cetyltrimethylammonium bromide (CTAB). The peak current is proportional to the concentration of estradiol in the range from 2.5×10⁻⁸ to 1.5×10⁻⁶ mol/l, and the detection limit is 1×10⁻⁹ mol/l after accumulation of 6 min. The total amounts of estrogens in the blood serums were determined using the voltammetric method, and the average recovery was 106.04%. The mechanism of the oxidation of estradiol was investigated by electrochemical techniques and UV spectra.     

Voltammetric behavior of neptunium(vi) in glutamic acid media at the rotating glassy carbon electrode

The voltammetry of neptunium(VI) glutamate was investigated over the pH range 3.8–10.0. A reversible, one-electron wave was obtained for glutamate concentrations above 0.1 M in the pH range 3.8–6.1, or above 0.3 M in the pH range 6.1–10.0. At pH 3.8–6.1, the half-wave potential was independent of pH, but at pH 6.1–10.0, it was a function of pH. The metal-ligand ratio was found to be 1:2 by conductometric titration. The limiting current was proportional to the concentration of the neptunium(VI) from 7.83·10^-5 to 1.96·10^-3M. The diffusion coefficient was 0.35·10^-5 cm² sec^-1 at pH 4.5 and 0.30·10^-5 cm² sec^-1 at pH 9.4.     

Electrolytic nucleation of silver on a glassy carbon electrode: Part II. Steady-state nucleation rate

The dependence of the number of nuclei on time is investigated in a wide overvoltage interval using different glassy carbon electrodes. The data for the steady-state nucleation rate are interpreted on the basis of both classical and atomistic theoretical concepts. Conclusions are drawn for the applicability of the two models of nucleation.     

扁桃酸在玻碳电极上的电化学行为研究

扁桃酸(mandelicacid)具有较强的抑菌作用,可用于治疗泌尿系统疾病,同时也是合成许多抗生素药物的中间体.因此,它在医药合成中具有广泛的用途.对扁桃酸的测定,已有紫外、HPLC、气相色谱、毛细管电泳等方法报道;电化学法测定还未见报道.本文采用现代电化学分析方法系统研究了扁桃酸在玻碳电极上的电化学行为.实验表明扁桃酸在玻碳电极上的氧化是不可逆过程,在pH 2.0介质中,在玻碳电极上都有一定的吸附性.实验研究了在pH 2.0条件下扁桃酸的各种电化学性质,并分别寻找到在不同条件下的定量方法,对实际样品进行了测定,方法简便、快速.     

Electrochemical study of the oxidation of tin(II) at glassy carbon electrodes

The voltammetric characteristics of the Sn(IV)/Sn(II) system have been studied at a glassy carbon rotating disk electrode in 4M hydrochloric acid. Anodic oxidation of Sn(II) produces a single well-defined wave, which has been examined in the temperature range 20-50 degrees , and shows distinct irreversibility. The kinetic parameters of the oxidation reaction have been calculated. Kinetic features of the reduction of Sn(IV) to Sn(II) were not evaluated because the reduction proceeds directly to tin metal.     

Electrochemical properties of a two-component DNA-polyaniline film at the surface of glassy carbon electrode

Aniline electropolymerization on a DNA-modified glassy carbon electrode gives rise to a stable composite DNA-polyaniline film possessing redox activity over a wide range of pH values. The heights and potentials of the redox peaks linearly depend on pH in the pH 3.0–8.0 range. It was established that the inclusion of DNA into the polyaniline composition enhances considerably the film conductivity and capacitance in the weakly acid and weakly alkaline pH regions; this effect is most pronounced for the reduced polymer form. The properties of the prepared DNA-polyaniline film point to its promise for the use in electrochemical biosensors.     

Oxygen reduction at the surface of glassy carbon electrodes modified with anthraquinone derivatives and dyes

The preparation, electrochemical and catalytic behaviour of glassy carbon electrodes modified by anthra-9,10-quinone, its amino derivatives and dyes were investigated. The stability of the modified electrodes was studied by cyclic voltammetry in acidic and neutral media. The electrocatalytic ability of the modified electrodes for the reduction of dioxygen to hydrogen peroxide was examined by cyclic voltammetry, chronoamperometry and chronocoulometry techniques. The influence of pH on the electrochemical and catalytic behaviour was studied and pH 5.0–8.0 was chosen as the optimum working pH by comparing the shift in oxygen reduction potential. The anthraquinone-adsorbed glassy carbon electrodes possess excellent electrocatalytic abilities for dioxygen reduction with overpotential ranging from 280 to 560 mV lower than that at a plain glassy carbon electrode. Hydrodynamic voltammetric studies were performed to determine the heterogeneous rate constants for the reduction of O₂ at the surface of the modified electrodes, mass specific activity of the anthraquinones used and the apparent diffusion coefficient of O₂ in buffered aqueous O₂-saturated solutions. Studies showed the involvement of two electrons in dioxygen reduction.     

Linear sweep voltammetry at the tubular graphite electrode: Part II. Totally irreversible processes

The theory of linear sweep voltammetry at the tubular graphite electrode has been developed for irreversible processes. The convective diffusion differential equations have been transformed into an integral equation which is solved numerically. The current-potential curves have been calculated theoretically and verified experimentally. The dependence of the current-potential curves on velocity has been studied. A procedure for the determination of kinetic parameters, i.e. standard rate constant and transfer coefficient, is presented.     

Differential pulse anodic stripping voltammetry of copper(II) at the glassy carbon electrode

Optimum conditions for the use of the bare glassy carbon electrode (GCE) are reported. Linear calibration graphs are obtained in the range 5 × 10^-7–3.5 × 10^-5 M copper(II). The detection limit for copper(II) is 5.9 × 10^-9 M at pH 4.5 and 3.3 × 10^-8 M at pH 6.5.     

Automatic amperometric microtitration of barbiturates with mercury(II) at the glassy carbon electrode

Summary The amperometric, mercurimetric microtitration of barbiturates at the glassy carbon electrode with an improved model of an automatic recording titrator is described. The reduction of mercury at the glassy carbon electrode has been studied by automatic steady-state voltammetry. A new, two-phase titration (Na acetate/organic solvent) and two methods of which the medium (KNO₃ resp. Na acetate) of the titration has been described in literature, are compared with a macro reference titration. Phenobarbital, sodium phenobarbital, barbital, sodium barbital, sodium amobarbital and pentobarbital have been titrated. The two-phase titration is better than the other two methods for samples in the range of 10^–4–10^–5 M. The titration of 50 g of barbiturate is possible with a precision and accuracy of about 2%.

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Automatische amperometrische Mikrotitration von Barbituraten mit Quecksilber(II) an der glasartigen KohlenstoffelektrodeVergleich von Ein- und Zweiphasentitrationen

Zusammenfassung Für die Titrationen wurde ein verbesserter automatisch registrierender Titrator benutzt. Die Reduktion des Quecksilbers an der Kohlenstoffelektrode wurde mit Hilfe der automatischen steady-state Voltammetrie untersucht. Eine neuartige Zweiphasentitration (Naacetat/organ. Lösungsmittel) sowie zwei Verfahren in bekannten Medien (KNO₃ bzw. Na-acetat) wurden mit einer Makrotitration verglichen. Phenobarbital, Phenobarbitalnatrium, Barbital, Barbitalnatrium, Amobarbitalnatrium und Pentobarbital wurden titriert. Für den Konzentrationsbereich 10^–4 bis 10^–5 M ist das Zweiphasenverfahren am besten geeignet. Eine Titration von 50 g eines Barbiturats ist auf etwa 2% genau durchzuführen.

     

Voltammetric behaviour of salicylic acid at a glassy carbon electrode and its determination in serum using liquid chromatography with amperometric detection.

Cyclic voltammetry was used to investigate the oxidation of salicylic acid at a planar glassy carbon electrode. The electrode reaction was found to be dependent on the pH and ionic strength of the acetate buffer, which contained 35% methanol. Under these conditions the maximum electrochemical signal was obtained with a supporting electrolyte of 0.06 mol dm-3 acetate buffer in 35% methanol (pH 5.0). The peak current value (ip) increased by approximately 10% when the methanol concentration was decreased to 8%. The substance was found to undergo an irreversible reaction involving one electron and possibly two protons in the initial oxidation step with at least one possible quasi-reversible follow-up reaction. The optimum mobile phase for the liquid chromatography, with amperometric detection, of the serum extracts was found to be 0.06 mol dm-3 acetate buffer in 8% methanol (pH 5.0); the acidified serum was extracted with a mixture of chloroform and acetonitrile (60 + 40), prior to injection onto a reversed-phase column. The peak current was measured at +1.35 V and the calibration graph was found to be linear in the range 4-200 ng of sample injected. The average recovery from serum was found to be 60% with a relative standard deviation of 5.8%. A pharmacokinetic study was carried out and the results obtained were comparable to those found in the literature. It was concluded that the method developed had possible application for the measurement of trace levels of salicylic acid in clinical studies.     

Electrocatalytic reduction of dioxygen at the surface of glassy carbon electrodes modified by calix[6]arene-methyl viologen

The preparation and electrochemical characterization of glassy carbon electrodes modified with calixarene-methyl viologen (MV) were investigated. The intermolecular complex of calix[6]arene and MV has an electrocatalytic effect on oxygen reduction. The cyclic voltammograms of the electroreduction of oxygen showed an enhanced current peak at approximately −0.60 V in air-saturated phosphate buffer. The experimental parameters were optimized and the mechanism of the catalytic process was discussed.     

Voltammetric sensor for nitrite determination based on its electrocatalytic reduction at the surface of p-duroquinone modified carbon paste electrode

A p-duroquinone (tetramethyl-p-benzoquinone) modified carbon paste electrode (DMCPE) was employed to study the electrocatalytic reduction of nitrite in aqueous solutions using cyclic voltammetry (CV), double potential-step chronoamperometry, and differential pulse voltammetry (DPV). It has found that under an optimum condition (pH 1.00), the reduction of nitrite at the surface of DMCPE occurs at a potential of about 660 mV less negative than that of an unmodified carbon paste electrode (CPE). The catalytic rate constant, k′_h, based on Andrieux and Saveant theoretical model was calculated as for scan rate 10 mV s^-1. Also, the apparent diffusion coefficient, D _app, was found as 2.5 × 10^–10 and 3.61 × 10^–5 cm² s^-1 for p-duroquinone in carbon paste matrix and nitrite in aqueous buffered solution, respectively. The values for αn_α were estimated to be −0.65 and −0.19 for the reduction of nitrite at the surface of DMCPE and CPE, respectively. The electrocatalytic reduction peak currents showed a linear dependence on the nitrite concentration, and a linear analytical curve was obtained in the ranges of 5.0 × 10^–5 M to 8.0 × 10^–3 M and 6.0 × 10^–6 M to 8.0 × 10^–4 M of nitrite concentration with CV and DPV methods, respectively. The detection limits (2σ) were determined as 2.5 × 10^–5 M and 4.3 × 10^–6 M by CV and DPV methods. This method was also applied as a simple, selective and precise method for determination of nitrite in real samples (the weak liquor from the wood and paper factory of Mazandaran province in Iran) by using a standard addition method.     

Voltammetric behaviour of morphine at a glassy carbon electrode and its determination in human serum by liquid chromatography with electrochemical detection under basic conditions.

The electrochemical oxidation of morphine was studied at pH values of between 7.00 and 12.00 by cyclic voltammetry and chronoamperometry at a planar glassy carbon electrode. The peak potential was dependent on pH over the range 7.00-9.75; it was independent of pH above the latter value, indicating a pKa value of 9.75. The peak current was found to be independent of pH, ionic strength of phosphate buffer (0.02-0.1 mol dm-3) and percentage of acetonitrile (0-40% v/v). The oxidation was found to occur in three steps; these are considered to result from a one-electron oxidation of the phenoxide group, followed by a one-electron loss from the oxidation product, pseudomorphine, and finally a two-electron loss from a tertiary amine group. A simple method of analysis by high-performance liquid chromatography was developed which employed a column packed with a reversed-phase, pH-stable, octadecylsilane-modified silica. Separation was achieved with a mobile phase containing 20% v/v acetonitrile in 0.05 mol dm-3 phosphate buffer, pH 11.0. Amperometric detection was carried out with an applied potential of +0.45 V versus Ag-AgCl. The detection limit was 1.24 x 10(-13) mol of morphine injected. The detector gave a linear response from 1.2 x 10(-12) to 4.0 x 10(-10) mol of morphine injected. The extraction method required 0.5 ml of serum, and no solvent evaporation was needed. The recovery of morphine was 80.9%. The method gave a linear response to at least 15.0 x 10(-7) mol dm-3.(ABSTRACT TRUNCATED AT 250 WORDS)     

Silver based mercury film electrode: Part II. The influence of aging of electrode on the electrode processes of various metal ions

The cyclic voltammetric behaviour of 8 metal ions at solid silver amalgam electrodes prepared by aging of a thin silver based mercury film electrode (SBMFE) and by deposition of silver and mercury on platinum were investigated. It was established that such electrodes behave in relation to some metals (Pb, Bi, Sn) similarly as silver electrodes i.e. the cyclic curves obtained with these electrodes at concentration 10^?3M range show a prepeak-postpeak system corresponding to deposition and dissolution of the monolayer of deposit. On the other hand under the same conditions no prepeaks were observed for cadmium, zinc and thallium. In all cases investigated the heights of anodic stripping peaks were lower on curves obtained with aged SBMFE than on those obtained with fresh SBMFE having a mercury layer 1 μm thick.