Showing papers on "Amperometry published in 2002"

Preparation of porous titania sol-gel matrix for immobilization of horseradish peroxidase by a vapor deposition method.

Abstract: A new and facile vapor deposition method has been developed for the preparation of sol-gel matrix. This method was used to form a titania sol-gel thin film and to immobilize horseradish peroxidase (HRP) on a glassy carbon electrode surface for the production of an amperometric hydrogen peroxide biosensor. This process prevented the cracking of conventional sol-gel-derived glasses. The morphologies of both titania sol-gel and the enzyme membranes were characterized using scanning electron microscopy and proved to be chemically clean, porous, and homogeneous and to have a very narrow particle size distribution. The sol-gel-derived titania-modified electrode retained the enzyme bioactivity and provided for long-term stability of the enzyme in storage. In the presence of catechol as a mediator, the sensor exhibited a rapid electrocatalytic response (less than 5 s), a linear calibration range from 0.08 to 0.56 mM with a detection limit of 1.5 microM and a high sensitivity (61.5 microA mM(-1)) for monitoring of H2O2. Effects of pH and operating potential were also explored for optimum analytical performance by using the amperometric method. The apparent Michaelis-Menten constant of the encapsulated HRP was 1.89 +/- 0.21 mM.

Determination of thiols by capillary electrophoresis with amperometric detection at a coenzyme pyrroloquinoline quinone modified electrode.

Abstract: A chemically modified electrode has been developed as a detector for the sensitive and selective determination of thiol-containing compounds following capillary electrophoresis separation. Electrodes were constructed by entrapment of the coenzyme pyrroloquinoline quinone (PQQ) into a polypyrrole (PPy) matrix on a 245-μm graphite electrode during electropolymerization of pyrrole in the presence of PQQ. PQQ serves as an efficient biocatalyst to mediate the oxidation of thiols at a substantially reduced overpotential relative to an unmodified electrode. Furthermore, this design takes advantage of the pH-dependent reversible electrochemical properties of PQQ, which facilitates optimization of separation and detection conditions. The PQQ/PPy-modified electrode was incorporated as an end-column detector, and a separation of homocysteine, cysteine, N-acetylcysteine, and glutathione was developed. Detection limits for these four thiols were determined to be 11, 23, 104, and 134 nM, respectively, with mass detecti...

Response Times of Carbon Fiber Microelectrodes to Dynamic Changes in Catecholamine Concentration

Abstract: The electrode response time and the measured concentrations during dynamic catecholamine changes were compared using constant potential amperometry and fast-scan cyclic voltammetry. The amperometric response to a rectangular pulse of catecholamine is more rectangular than the cyclic voltammetric response; however, the response times are very similar when, during cyclic voltammetry, the temporal lag due to adsorption and desorption of catecholamine to the electrode is removed by deconvolution. Deconvolution of cyclic voltammetry data was applied to stimulated dopamine release in vivo, allowing for modeling of release and uptake kinetics and to measure catecholamine release from single cells, resulting in better resolution of peaks from single vesicles. In vitro postcalibrations were performed to calculate concentrations of catecholamine measured with cyclic voltammetry and amperometry. The addition of 600 microM ascorbic acid to the postcalibration buffer, allowing a catalytic reaction to regenerate dopamine, resulted in similar calculated concentrations for stimulated release of dopamine using amperometry and cyclic voltammetry. Using deconvoluted cyclic voltammetry to remove the response time lag and adding ascorbic acid to the calibration buffer, the shape and concentration of dynamic catecholamine changes are very similar when measured with constant potential amperometry and cyclic voltammetry.

Electrochemical detection of carbamate pesticides at conductive diamond electrodes.

Abstract: Conductive boron-doped diamond thin-film electrodes were used for the electrochemical detection of selected N-methylcarbamate pesticides (carbaryl, carbofuran, methyl 2-benzimidazolecarbamate, bendiocarb) after liquid chromatographic separation. Two kinds of detection methods were adopted in this study. In the first method, a direct detection of underivatized pesticides was carried out at an operating potential of 1.45 V versus Ag/AgCl, which resulted in the detection limits of 5−20 ng/mL (or 5−20 ppb) with S/N = 2 due to the low background current and wide potential window of the diamond electrode. In the second method, the detection limits were improved by subjecting the pesticide samples to alkaline hydrolysis in a separate step prior to injection. The phenolic derivatives obtained by alkaline hydrolysis oxidize at a relatively lower potential (0.9 V vs Ag/AgCl), which increases the sensitivity drastically. The advantage of the diamond electrode for the detection of phenolic derivatives is that it offe...

A dynamically modified microfluidic poly(dimethylsiloxane) chip with electrochemical detection for biological analysis.

Abstract: Separation and direct detection of amino acids, glucose and peptide in a 3.1 cm separation channel made of poly(dimethylsiloxane) (PDMS) with end-column amperometric detection at a copper microdisk electrode was developed. This system is the integration of a normal sized working electrode with electrochemical detection on a PDMS microfabricated device. The PDMS channels dynamically modified by 2-morpholinoethanesulfonic acid (MES) show less adsorption and more enhanced efficiency than that of unmodified ones when applied to separations of these biological molecules. The migration time is less than 100 s and the reproducibility of migration time is satisfactory with relative standard deviation (RSD) of 2.8% in 19 successive injections. The limits of detection of arginine (Arg), glucose, and methionine-glycine (Met-Gly) are estimated to be 2.0, 8.5, and 64.0 microM at S/N = 3, approximately 0.5-16.0 fmol, respectively. Variances influencing the separation efficiency and amperometric response, including injection, separation voltage, detection potential, or concentration of buffer and additive, are assessed and optimized.

Continuous on-line measurement of cerebral hydrogen peroxide using enzyme-modified ring-disk plastic carbon film electrode.

Abstract: An amperometric method suitable for the continuous on-line measurement of cerebral hydrogen peroxide from a microdialysate has been successfully performed for the first time by using an enzyme-modified ring−disk plastic carbon film electrode (PCFE) in a thin-layer radial flow cell. PCFE consists of a ring electrode modified with horseradish peroxidase to detect H2O2 at 0.0 V (vs Ag/AgCl) and a disk electrode coated with ascorbate oxidase (AOx) to preoxidize ascorbic acid (AA) and thus suppress interference via direct oxidation. Analytes in solution (brain dialysates or standards) are mixed on-line with a phosphate-buffered solution containing dissolved oxygen and chelating agent, EDTA. The buffered solution is used to provide the O2 necessary for the AOx catalytic reaction, stabilize the changes in dialysate pH that are associated with the in vivo formation of H2O2, and remove heavy metal ion impurities and thus suppress reactions between AA and H2O2. This procedure enables trace levels of H2O2 to be read...

Amperometric microcells for alkaline phosphatase assay

Abstract: To develop simple electrochemical immunoassays, a screen printed amperometric microcell with graphite working and Ag/AgCl reference electrodes was tested for the determination of alkaline phosphatase enzyme (ALP) and anti-humanIgG conjugated ALP (α-hIgG-ALP) activity in 5–10 μl samples. To ensure reproducible, steady state conditions, the working electrode surface was coated with mass-transport controlling hydrogel layer. The kinetic response curves of the hydrogel coated electrodes were linear. In addition, the hydrogel layer reduced the nonspecific adsorption of the α-hIgG-ALP conjugate on the working electrode surface. The measurements were made in the range of 2 ÷ 4000 mU ml−1 enzyme activities using ascorbic acid 2-phosphate (AAP) as the enzyme substrate. AAP is commercially available, non-toxic and has excellent stability. The sensitivity of the determinations was about 71% of the sensitivity which could be achieved using p-aminophenylphosphate (PAPP), a not easily accessible and unstable enzyme substrate. The experimentally determined kinetic parameters of the ALP enzyme catalyzed reactions were the same with the bare and hydrogel layer coated electrodes.

Amperometric Detection of Sulfide at a Boron Doped Diamond Electrode: The Electrocatalytic Reaction of Sulfide with Ferricyanide in Aqueous Solution.

Abstract: The oxidation of sulfide at a boron doped diamond (BDD) electrode has been examined and its analytical utility appraised and compared with other common electrode substrates (Pt, Au, glassy carbon). The suitability of using BDD as a sulfide sensing substrate has been assessed through examining the electrocatalytic oxidation of ferrocyanide to ferricyanide in the presence of sulfide using both cyclic voltammetry and chronoamperometry. The high oxidation potential for the direct oxidation of sulfide at a BDD electrode allows clear resolution of both the ferrocyanide wave and sulfide oxidation wave producing a distinct analytical signal at the oxidation peak potential of ferrocyanide. The procedure was applied to the recovery of a sulfide spike in sewage effluents with a recovery of 102±4.5%.

Amperometric determination of glutathione and cysteine on a Pd-IrO(2) modified electrode with high performance liquid chromatography in rat brain microdialysate.

Abstract: A Pd/IrO(2) co-electrodeposited glassy carbon electrode was prepared and the electrochemical behavior of glutathione (GSH) at this chemically modified electrode (CME) has been studied by cyclic voltammetry (CV). The results indicated that the modified electrode efficiently exhibited electrocatalytic oxidation for GSH with relatively high sensitivity, stability, and long-life. Coupled with high-performance liquid chromatography (HPLC), the Pd/IrO(2) modified electrode was utilized for the electrochemical detection (ECD) of the thiocompounds, glutathione and cysteine (Cys). The peak currents were linear with the substance concentrations in the range of 1.0 x 10(-5) mol L(-1) to 8.0 x 10(-4) mol L(-1) for GSH and 4.0 x 10(-6) mol L(-1) to 2.0 x 10(-4) mol L(-1) for Cys. The detection limits were 2.0 x 10(-6) mol L(-1) for GSH and 5.0 x 10(-7) mol L(-1) for Cys with S/N of 3. The method has been successfully applied to assess the contents of GSH and Cys in rat brain microdialysates.

Properties of glucose biosensors based on dendrimer layers. Effect of enzyme immobilization.

Abstract: The properties of glucose biosensors based on dendrimer layers on a gold support, which depend on the method of immobilization of glucose oxidase (GOX), were studied by amperometry The kinetic parameters of enzymatic reactions, response time, sensitivity, detection limit, linear range, and enzyme turnover were determined We showed that a more stable and sensitive sensor was obtained when GOX was immobilized on the dendrimer by crosslinking with glutaraldehyde in vacuum This biosensor was stable for at least eight weeks The response time was approximately 13 min, the detection limit of glucose was 25 µM, and the apparent Michaelis-Menten constant was relative low (Km=11±01 mM) in comparison with that for GOX in solution The reason for these differences is discussed The example of the application of the developed biosensors for the detection of mercury is also presented The inhibitory effect of mercury on GOX activity was observed at mercury concentration of 100 nM

Pulsed amperometric detection of carbohydrates on an electrophoretic microchip

Abstract: The first report of pulsed amperometric detection (PAD) on an electrophoretic microchip is presented. A hybrid poly(dimethylsiloxane)/glass device was coupled with a platinum working electrode for the electrochemical detection of glucose, maltose, and xylose. Under optimized detection conditions, glucose was found to respond linearly from 20 to 500 μM with a measured detection limit of 20 μM. The coupling of PAD with a microchip provides a straightforward approach to the analysis of a wide range of carbohydrates using microfluidics.

Electrochemistry and Electrocatalytic Activity of Pyrocatechol Violet (PCV) Film on a Glassy Carbon Electrode Towards the Oxidation of Reduced Nicotinamide Adenine Dinucleotide(NADH)

Abstract: Electrocatalytic oxidation of nicotinamide adenine dinucleotied (NADH) at a glassy carbon electrode modified with electrodeposited film derived from pyrocatechol violet (PCV) has been studied. The PCV modified glassy carbon electrode shows excellent electrocatalytic activity toward NADH oxidation in phosphate buffer solution (pH 7.0) with an overpotential of about 447 mV lower than that of the bare electrode. The apparent charge transfer rate constant and charge transfer coefficient for electron transfer between the electrode surface and immobilized PCV were calculated as 5.3 s−1 and 0.49, respectively. The catalytic rate constant of the modified glassy carbon electrode for the oxidation of NADH is determined by cyclic voltammetry and rotating disk electrode measurements. Also, it has been shown that using the PCV modified electrode, NADH can be estimated by amperometry with high sensitivity and good limit of detection.

Electrochemical investigation of glucose sensor fabricated at copper-plated screen-printed carbon electrodes

Abstract: Copper-plated screen-printed carbon electrode (CuSPE) provides a suitable catalytic surface for the amperometric detection of hydrogen peroxide. Glucose oxidase (GOD) is immobilized on the top of the CuSPE to form a glucose sensor. The interaction of copper oxide with GOD was found to be an important factor in the glucose detection. Preliminary investigation under hydrodynamic conditions showed a linear calibration plot up to 26.7 mM glucose with a slope and regression coefficient of 4.5 μA/mM and 0.9902, respectively. The Michaelis-Menten kinetics by nonlinear curve fitting yielded a Km value close to that in solution indicating the ideality and suitability of the present system. Classical mixed potential mechanism is for the first time applied to the enzyme-coated CuSPE to further understand the system.