Abdollah Salimi

Catalytic oxidation of thiols at preheated glassy carbon electrode modified with abrasive immobilization of multiwall carbon nanotubes : applications to amperometric detection of thiocytosine, L-cysteine and glutathione

The performance of preheated glassy carbon electrode modified with carbon nanotubes is described. First glassy carbon electrode is heated for 5min at 50 degrees C, then abrasive immobilization of multiwall carbon nanotubes on a preheated glassy carbon electrode was achieved by gentle rubbing of electrode surface on a filter paper supporting carbon nanotubes. Carbon nanotubes (CNTs)-modified glassy carbon electrodes exhibit strong and stable electrocatalytic response toward thiols oxidation in wide pH range. These properties permit an important decrease in over voltage for the oxidation of thiocytosine, glutathione and l-cysteine, as well as a dramatic increase in the peak currents in comparison with bare glassy carbon electrode. Furthermore, the thiols amperometric response of the coated electrodes is extremely stable, with more than 95% of the initial activity after 30min stirring of 0.1mM thiols. The electrocatalytic behavior is further exploited as a sensitive detection scheme for thiols detection by hydrodynamic amperometry. The substantial decrease in the overvoltage of the thiols oxidation associated with a stable amperometric response and antifouling properties of nanotubes films allow the development of highly sensitive thiols sensor without using any redox mediator. Such ability of carbon nanotubes to promote the thiols electron transfer reaction, short response time (5s) and long-term stability, low detection limit, extended linear concentration range, high sensitivity suggest great promise for thiols amperometric sensors and detector for chromatographic analysis of thiol derivatives.

Renewable sol-gel carbon ceramic electrodes modified with a Ru-complex for the amperometric detection of l-cysteine and glutathione.

A renewable three-dimensional chemically modified carbon ceramic electrode containing Ru [(tpy)(bpy)Cl] PF(6) was constructed by sol-gel technique. It exhibits an excellent electro-catalytic activity for oxidation of l-cysteine and glutathione at pH range 2-8. Cyclic voltammetry was employed to characterize the electrochemical behavior of the chemically modified electrode. The electrocatalytic behavior is further exploited as a sensitive detection scheme for l-cysteine and glutathione by hydrodynamic amperometry. Optimum pH value for detection is 2 for both l-cysteine and glutathione. The catalytic rate constants for l-cysteine and glutathione were determined, which were about 2.1x10(3) and 2.5x10(3) M(-1)s(-1), respectively. Under the optimized condition the calibration curves are linear in the concentration range 5-685 and 5-700 muM for l-cysteine and glutathione determination, respectively. The detection limit (S/N=3) and sensitivity is 1 muM, 5 nA/muM for l-cysteine and 1 muM, 7.8 nA/muM for glutathione. The relative standard deviation (RSD) for the amperograms currents with five injections of l-cysteine or glutathione at concentration range of linear calibration is <1.5%. The advantages of this amperometric detector are: high sensitivity, good catalytic effect, short response time (t<3 s), remarkable long-term stability, simplicity of preparation and reproducibility of surface fouling (RSD for six successive polishing is 3.31%). This sensor can be used as a chromatographic detector for analysis of l-cysteine and glutathione.

Simultaneous determination of ascorbic acid, uric acid and neurotransmitters with a carbon ceramic electrode prepared by sol-gel technique

A sol-gel carbon composite electrode (CCE) has been prepared by mixing a sol-gel precursor (e.g. methyltrimethoxysilane) and carbon powder without adding any electron transfer mediator or specific reagents. It was demonstrated that this sensor can be used for simultaneous determination ascorbic acid, neurotransmitters (dopamine and adrenaline) and uric acid. Direct electrochemical oxidation of ascorbic acid, uric acid and catecholamines at a carbon composite electrode was investigated. The experimental results were compared with other common carbon based electrodes, specifically, boron doped diamond, glassy carbon, graphite and carbon paste electrodes. It was found that the CCE shows a significantly higher of reversibility for dopamine. In addition, in comparison to the other electrodes used, for CCE the oxidation peaks of uric acid, ascorbic acid and catecholamines in cyclic and square wave voltammetry were well resolved at the low positive potential with good sensitivity. The advantages of this sensor were high sensitivity, inherent stability and simplicity and ability for simultaneous determination of uric acid, catecholamines and ascorbic acid without using any chromatography or separation systems. The analytical performance of this sensor has been evaluated for detection of biological molecules in urine and serum as real samples.

Enhancement of the analytical properties and catalytic activity of a nickel hexacyanoferrate modified carbon ceramic electrode prepared by two-step sol–gel technique: application to amperometric detection of hydrazine and hydroxyl amine

The electroless sol-gel technique was used for the construction of nickel hexacyanoferrat (NiHCF) modified carbon composite electrodes (CCEs).This involves two steps: formation of a carbon ceramic electrode fabricated by nickel powder and then immersing the electrode into a sodium- hexacyanoferate solution for the immobilization of NiHCF films. The cyclic voltammety of the resulting modified CCEs prepared under optimum conditions, shows a well defined surface redox couple due to the [Ni(II)Fe(III/II)(CN)(6)](-2/-1) system. The effect of different alkali metal cations in supporting electrolyte on the behavior of the modified electrode were studied. The charge transfer coefficient (alpha) and charge transfer rate constant (k(s)) for modified films were calculated. Hydrazine and hydroxylamine have been chosen as a model to elucidate the electocatalytic ability and analytical parameters of NiHCF modified CCE prepared by one and two-step sol-gel techniques and these compounds determined amperometically at the surface of modified electrodes. The latter shows a good electocatalytic activity towards the oxidation of hydrazine and hydroxylamine in the pH range 3-8 in comparison with CCEs modified by homogeneous mixture of graphite powder, Ni(NO(3))(2) and Na(2)[Fe(CN)(6)], (one-step sol-gel technique). Furthermore, the catalytic rate constant, linear dynamic range, limit of detection, and sensitivity for hydrazine and hydroxylamine detections were evaluated and compared with CCEs prepared with one-step sol-gel method. The modified CCEs containing NiHCF shows good repeatability, short response time, t 90%<3s, long term stability (3 months) and excellent catalytic activity. Furthermore, the method of preparation is rapid and simple and the modified electrodes are renewed by simple mechanical polishing and immersing in [Na(3)Fe(CN](6)] solution.

Ultrasonic effects on the electro-reduction of oxygen at a glassy carbon anthraquinone-modified electrode. The Koutecky–Levich equation applied to insonated electro-catalytic reactions

We report the electro-reduction of oxygen in aqueous solution at glassy carbon electrodes modified by the physical adsorption of 1,2-dihydroxyanthraquinone. The catalytic currents are significantly increased under insonation in a manner quantitatively consistent with the increased mass transport as predicted via the Koutecky–Levich equation using diffusion layer thicknesses calculated via studies on well-defined, simple one-electron systems.

Sol–gel derived carbon ceramic composite electrode containing a ruthenium complex for amperometric detection of insulin at physiological pH

Abstract A novel modified ceramic carbon electrode (CCE) containing [Ru(bpy)(tpy)Cl]PF 6 complex was fabricated by the sol–gel technique. The reversible redox couple of Ru(II)/Ru(III) was investigated both as a solute in acetonitrile solution and as a component of a carbon-based conducting composite electrode. The electrochemical behavior and stability of the modified CCE were investigated by cyclic voltammetry. Peak currents for the Ru-doped CCE were controlled by diffusion from a limited field at lower scan rates and by semi-infinite diffusion at higher scan rates. Peak potentials and peak currents were not changed with the increase of pH in the range 3–10. The apparent electron transfer rate constant ( k s ) and transfer coefficient ( α ) were determined by cyclic voltammetry and they were about 29 s −1 and 0.63. The modified electrode showed excellent electrocatalytic activity toward insulin electro-oxidation in acidic solution and at physiological pH values. Insulin was determined chronoamperometrically at the surface of the modified electrode at pH 7. Under the optimized conditions, the calibration curve is linear in the concentration range 0.5 nM–0.85 μM. The detection limit (signal to noise is 3) and sensitivity are 0.4 nM and 7.602 A/M, respectively. The Ru complex-doped CCE shows good repeatability. The response time, t (90%) was short and equal to 0.3 s or less. The long-term stability was remarkable (6 months) and, especially the surface renewal repeatability by simple mechanical polishing (RSD for seven successive polishing is 1.3%). The advantages of the insulin amperometric detector based on the ruthenium complex doped CCE are high sensitivity, inherent stability at physiological pH, excellent catalytic activity for insulin oxidation and a less expensive and simple preparation in comparison with recently published methods.

Boron doped diamond electrode modified with iridium oxide for amperometic detection of ultra trace amounts of arsenic(III)

Boron doped diamond (BDD) electrodes modified by electrodeposition from hydrous iridium oxide (IrOx) have been developed for the detection of arsenic(III). Potential cycling is used to deposit films of hydrous iridium oxide onto boron doped diamond electrode from a saturated solution of alkaline iridium(III) solution. A stable reversible redox couple was observed at the surface of modified electrode in both acidic and basic solutions. The properties of iridium oxide films, stability and its electrochemical properties were investigated by atomic force microscopy (AFM) and cyclic voltammetry. The modified electrodes showed excellent electrocatalytic activity toward oxidation arsenic(III) over a wide pH range (2–8); also they showed an excellent analytical performance for the amperometric detection of arsenic(III). The detection limit, sensitivity, response time and linearity are 2 nM, 4.2 nA nM−1, 60 ms and 20 nM–50 µM. The precision for 10 replicate determinations of 40 µM arsenic was 0.80% (RSD). These analytical parameters compare favourably with those obtained with modern analytical techniques such as inductively coupled plasma mass spectrometry and hydride generation atomic fluorescence spectrometry. The advantageous properties of this modified electrode for arsenic determination are its inherent stability, excellent catalytic activity over a wide pH range, high sensitivity and simplicity.

Renewable-surface sol–gel derived carbon ceramic electrode fabricated by [Ru(bpy)(tpy)Cl]PF6 and its application as an amperometric sensor for sulfide and sulfur oxoanions

A highly sensitive and fast responding sensor for the determination of thiosulfate, sulfite, sulfide and dithionite is described. It consists of a chemically modified carbon ceramic composite electrode (CCE) containing [Ru(bpy)(tpy)Cl]PF6 complex that was constructed by the sol-gel technique. A reversible redox couple of Ru(II)/Ru(III) was observed as a solute in acetonitrile solution and as a component of carbon based conducting composite electrode. Electrochemical behavior and stability of modified CCE were investigated by cyclic voltametry, the apparent electron transfer rate constant (kappa(S)) and transfer coefficient (a) were determined by cyclic voltametry which were about 28 s(-1) and 0.43 respectively. Electrocatalytic oxidation of S(2-), SO3(2-), S2O4(2-) and S2O3(2-) were effective at the modified electrode at significantly reduced overpotentials and in the pH range 1-11. Optimum pH values for amperometric detection of thiosulfate, dithionite, sulfide and sulfite are 7, 9, 2 and 2. Under the optimized conditions the calibration curves are linear in the concentration ranges 1-500, 3-80, 2-90 and 1-100 microM for S2O3(2-), SO3(2-), S2- and S2O4(2-) determination. The detection limit (signal to noise is 3) and sensitivity are 0.5 and 12, 2.8 and 6, 1.6 and 8, and 0.65 microM and 80 nA microM(-1) for thiosulfate, sulfite, sulfide and dithionite detection. The modified carbon ceramic electrode doped with Ru-complex shows good reproducibility, a short response time (t < 2 s), remarkable long term stability (> 6 month) and especially good surface renewability by simple mechanical polishing (RSD for eight successive polishing is 2%). The advantages of this sulfur compound amperometric detector based on ruthenium doped CCE are high sensitivity, inherent stability at a broader pH range, excellent catalytic activity, less expense and simplicity of preparation in comparison with recently published papers. This sensor can be used as a chromatographic detector for analysis of sulfur derivatives.

Modification of carbon ceramic electrode prepared with sol–gel technique by a thin film of chlorogenic acid: application to amperometric detection of NADH

The carbon ceramic electrode prepared with sol-gel technique is modified by a thin film of chlorogenic acid (CGA). By immersing the carbon ceramic electrode in aqueous solution of chlorogenic acid at less than 2s a thin film of chlorogenic acid adsorbed strongly and irreversibly on the surface of electrode. The cyclic voltammetry of the resulting modified CCE prepared at optimum conditions shows a well-defined stable reversible redox couple due to hydroquinone/quinone system in both acidic and basic solutions. The modified electrode showed excellent electrocatalytic activity toward NADH oxidation and it also showed a high analytical performance for amperometric detection of NADH. The catalytic rate constant of the modified carbon ceramic electrode for the oxidation of NADH is determined by cyclic voltammetry measurement. Under the optimised conditions the calibration curve is linear in the concentration range 1-120mum. The detection limit (S/N = 3) and sensitivity are 0.2muM and 25nAmuM(-1).The results of six successive measurement-regeneration cycles show relative standard deviations of 2.5% for electrolyte solution containing 1mM NADH, indicating that the electrode renewal gives a good reproducible and antifouling surface. The advantages of this amperometric detector are: high sensitivity, excellent catalytic activity, short response time t < 2s, remarkable long-term stability, simplicity of preparation at short time and good reproducibility.

Amperometric detection of ultra trace amounts of Hg(I) at the surface boron doped diamond electrode modified with iridium oxide

Iridium oxide (IrOx) films formed electrochemically on the surface boron doped diamond electrode by potential cycling in the range -0.2 to 1.2V from a saturated solution of alkaline iridium(III) solution. A strongly adherent deposit of iridium oxide is formed after 5-10 potential scans. The properties, stability and electrochemical behavior of iridium oxide layers were investigated by atomic force microscopy and cyclic voltammetry. The boron doped diamond (BDD) electrode modified with electrodeposition of a thin film, exhibited an excellent catalytic activity for oxidation of Hg(I) over a wide pH range. The modified electrode shows excellent analytical performance for Hg(I) amperometric detection. The detection limit, sensitivity, response time and dynamic concentration ranges are 3.2nM, 77nAmuM(-1), 100ms and 5nM-5muM. These analytical parameters compare favorably with those obtained with modern analytical techniques and recently published electrochemical methods.