Electrochemical behavior of redox species at carbon fibre microdisk array electrode modified with mixed-valent molybdenum(VI, V) oxide

In this study, electrode responses to a large number of electroactive species with different standard potentials at the molybdenum oxide-modified carbon fibre microdisk array (CFMA) electrode were investigated. The results demonstrated that the electrochemical behavior for those redox species with formal potentials more positive than similar to 0.0 V at the molybdenum oxide-modified CFMA electrode were affected by the range and direction of the potential scan, which were different from that at a bare CFMA electrode. If the lower limit of the potential scan was more positive than the reduction potential of the molybdenum oxide film, neither the oxidation nor the reduction peaks of the redox species tested could be observed. This indicates that electron transfer between the molybdenum oxide film on the electrode and the electroactive species in solution is blocked due to the existence of a high resistance between the film and electrolyte in these potential ranges. If the lower limit of the potential scan was more negative than the reduction potential of the molybdenum oxide film (similar to - 0.6 V), the oxidation peaks of these species occurred at the potentials near their formal potentials. In addition, the electrochemical behavior of these redox species at the molybdenum oxide-modified CFMA electrode showed a diffusionless electron transfer process. On the other hand, the redox species with formal potentials more negative than similar to - 0.2 V showed similar reversible voltammetric behaviors at both the molybdenum oxide-modified CFMA electrode and the bare electrode. This can be explained by the structure changes of the film before and after reduction of the film. In addition we also observed that the peak currents of some redox species at the modified electrode were much larger than those at a bare electrode under the same conditions, which has been explained by the interaction between these redox species and the reduction state of the molybdenum oxide film. (C) 2000 Elsevier Science Ltd. All rights reserved.

Construction of a heteropolyanion-modified electrode by a two-step sol-gel method and its electro catalytic applications

The sol-gel technique was used here to construct heteropolyanion-containing modified electrodes. This involves two steps, i.e. the first forming a functionalized sol-gel thin film on the surface of the glassy carbon electrode and then immersing the electrode into a heteropolyanion solution to incorporate the heteropolyanion into the sol-gel film. Here a Dawson-type heteropolyanion, K6P2W18O62 (P2W18), was used as a representative to illuminate the behavior of the as-prepared composite film. The electrochemical performance of the P2W18-modified electrode was studied with respect to the pH effect and long-term stability. The modified electrode exhibited a high electrocatalytic response for the reduction of BrO3- and NO2-. Steady-state amperometry was applied to characterize the electrode as an amperometric sensor for the determination of NO2-. The sensor had a linear range from 0.02 to 34 mM and a detection limit of 5 x 10(-6) M. (C) 2001 Elsevier Science B.V. All rights reserved.

Amperometric tyrosinase biosensor based on a sol-gel-derived titanium oxide-copolymer composite matrix for detection of phenolic compounds

A new type of tyrosinase biosensor was developed for the detection of phenolic compounds, based on the immobilization of tyrosinase in a sol-gel-derived composite matrix that is composed of titanium oxide sol and a grafting copolymer of poly(vinyl alcohol) with 4-vinylpyridine. Tyrosinase entrapped in the composite matrix can retain its activity to a large extent owing to the good biocompatibility of the matrix. The parameters of the fabrication process and the variables of the experimental conditions for the enzyme electrode were optimized. The resulting sensor exhibited a fast response (20 s), high sensitivity (145.5 muA mmol(-1) 1) and good storage stability. A detection limit of 0.5 muM catechol was obtained at a signal-to-noise ratio of 3.

Electrochemistry and electrochemiluminescence of stable tris(2,2 '-bipyridyl)ruthenium(II) monolayer assembled on benzene sulfonic acid modified glassy carbon electrode

An electrochemically stable monolayer of tris(2,2'-bipyridyl)ruthenium(II) was obtained for the first time. It was based on the electrostatic attachment of Ru(bpy)(3)(2+) to the benzene sulfonic acid monolayer film, which was covalently bound onto glassy carbon electrode by the electrochemical reduction of diazobenzene sulfonic acid. The surface-confined Ru(bpy)(3)(2+) underwent reversible surface process, and reacted with the coreactant, tripropylamine, to produce electrochemiluminescence. In view of the stability of the electrode, the results strongly suggested that light was emitted from the surface-confined Ru(bpy)(3)(2+), not from the detached Ru(bpy)(3)(2+). The Ru(bpy)(3)(2+) modified electrode was used to the determination of tripropylamine. It showed good linearity in the concentration range from 5 muM to 1 muM with a detection limit of 1 muM (S/N = 4). The good stability of the Ru(bpy)(3)(2+) modified electrode also showed that the benzene sulfonic acid monolayer film prepared can be served as an excellent support to construct multilayers. (C) 2001 Elsevier Science B.V. All rights reserved.

Determination of p-toluene sulfonic acid in phenol matrix by capillary zone electrophoresis with ultraviolet detection

The p-toluene sulfonic acid (MA) in phenol matrix was separated and determined by capillary electrophoresis with ultraviolet detector. the effect of the concentration and pH of the buffer on separation was investigated. Cinnamic acid has been chosen as the internal standard from four compounds, the calibration curves of PTSA in 50 mg/L phenol matrix were obtained with and without the internal standard. The linear range was from 1.25 to 12.5 mg/L and the correlation coefficient was 0.9999 for both curves. The limit of detection of PISA was 0.75 mg/L at 3 times of SIN. Finally, the concentration of PTSA in four synthesized samples was determined with method of standard additions, and the effect of matrix was discussed. The values of MA in these samples were 1.01, 0.94, 1.56 and 0.00 mg/L respectively.

Acid-stable amperometric soybean peroxidase biosensor based on a self-gelatinizable grafting copolymer of polyvinyl alcohol and 4-vinylpyridine

A peroxidase was extracted from Chinese soybean seed coat, and its thermostability and acid-stability were characterized. This peroxidase was immobilized into a self-gelatinizable grafting copolymer of polyvinyl alcohol with 4-vinylpyridine(PVA-g-PVP) to construct an acid-stable hydrogen peroxide biosensor. The effect of pH was studied for optimum analytical performances by amperometric and spectro-photometric methods, also the K-m(app) and the stability of the soybean peroxidase-based biosensor are discussed. At pH 3.0, the soybean peroxidase maintained its bioactivity and the enzyme electrode had a linear range from 0.01 to 6.2 mM with a detection limit of 1.0 x 10(-7) M. In addition, the main characteristics of different hydrogen peroxide sensors were compared.

New microsystem of capillary electrophoresis with amperometric detection

We have made a cheap microsystem of capillary electrophoresis with a new method, integrating the electrodes, injection channel, separation channel, buffer reservoirs and detection cell on a polymethylmethacrylate (PMMA) chip. Using an integrated micro carbon fiber disk electrode as the working electrode in three electrodes system, 1 x 10(-4) mol/L dopamine(DA) could be detected with end-column amperometric detection. The reproducibility was good. Peak current was 6.73 nA,theoretical plate number was 71300/m and height equivalent of one theoretical plate height was 14.0 mum for 1 x 10(-4) mol/L DA. The limit of detection was 3.6 x 10(-8) mol/L and the linear range was extended from 5 x 10(-7) mol/L to 1 x 10(-4) mol/L for DA. 1 x 10(-4) mol/L catechol (CA) and 5 x 10(-5) mol/L DA were also separated completely with R-s = 10.1.

Determination of mercury in biological samples using organic compounds as matrix modifiers by inductively coupled plasma mass spectrometry

A method was developed for the determination of total mercury in biological samples. The effects of aqueous ammonia, ethylenediamine and triethanolamine on Hg signal intensity by inductively coupled plasma mass spectrometry has been evaluated and the possible mechanisms discussed. It has been proved that the signal intensity of Hg significantly increases with adding, in the presence of small amounts of aqueous ammonia, ethylenediamine or triethanolamine. The normalized intensity (the signal intensity ratio with amine and without amine) of Hg increases as the concentration of aqueous ammonia, ethylenediamine or triethanolamine increases, but the degree of enhancement of aqueous ammonia was smaller than that of ethylenediamine and triethanolamine. The normalized intensity of Hg with aqueous ammonia, ethylenediamine and triethanolamine decreases as the nebulizer flow rate increases, but decreasing degree of aqueous ammonia was smaller than that of ethylenediamine and triethanolamine. The higher the RF powers the higher the normalized intensity of Hg at the same nebulizer flow rate. The addition of aqueous ammonia, ethylenediamine and triethanolamine into analytical solutions significantly improved the transport efficiency of Hg. The detection limit of Hg is improved about ten times by the addition of ethylenediamine or triethanolamine under the optimum experimental parameters. The method has been used to determine mercury in biological standard reference materials (SRM). The analytical results are very close to the certified values and the determined values for similar samples.

Functionalized inorganic-organic composite material derivated by sol-gel for construction of mediated amperometric hydrogen peroxide biosensor

A novel functionalized inorganic-organic hybrid material with cation exchange property was prepared by sol-gel method. The H2O2 biosensor was fabricated by simply dipping the horseradish peroxidase-containing functionalized membrane modified electrode into Meldola's blue (MDB) solution. MDB was adsorbed and firmly immobilized within the membrane. The electrochemical behavior of MDB incorporated in the membrane was more reversible compared with that of the solution species and suitable as mediator for the horseradish peroxidase. The response time was less than 25 s. Linear range is up to 0.6 mM (COH. coeff. 0.9998) with detection Limit of 9 x 10(-7) M. High sensitivity of 75 nA mu M cm(-2) was obtained due to high MDB-loading. The biosensor exhibited a good stability. (C) 1999 Elsevier Science B.V. All rights reserved.

Evaluation of luminol-H2O2-KIO4 chemiluminescence system and its application to hydrogen peroxide, glucose and ascorbic acid assays

(A) novel chemiluminescence (CL) system was evaluated for the determination of hydrogen peroxide, glucose and ascorbic acid based on hydrogen peroxide, which has a catalytic-cooxidative effect on the oxidation of luminol by KIO4. Hydrogen peroxide can be directly determined by luminol-KIO4 -H2O2 CL system. The detection limit was 3.0 x 10(-8) mol l(-1) and the calibration graph was linear over the range of 2.0 x 10(-7)-6.0 x 10(-4) mol l(-1). The relative standard deviation of H2O2 was 1.1% for 2.0 x 10(-6) mol l(-1) (N = 11). Glucose was indirectly determined through measuring the H2O2 generated by the oxidation of glucose in the presence of glucose oxidase at pH 7.6. The present method provides a source for H2O2, which, in turn, coupled with the luminol-KIO4-H2O2 CL reaction system. The CL was linearly correlated with glucose concentration of 0.6-110 mu g ml(-1). The relative standard deviation was 2.1% for 10 mu g ml(-1) (N = 11). Detection limit of glucose was 0.08 mu g ml(-1). Ascorbic acid was also indirectly determined by the suppression of luminol-KIO4-H2O2 CL system. The calibration curve was linear over the range of 1.0 x 10(-7)-1.0 x 10(-5) mol l(-1) of ascorbic acid. The relative standard deviation was 1.0% for 8.0 x 10(-7) mol l(-1) (N = 11). Detection limit of ascorbic acid was 6.0 x 10(=8) mol l(-1). These proposed methods have been applied to determine glucose, ascorbic acid in tablets and injection. (C) 1999 Elsevier Science B.V. All rights reserved.

Unidirectional current flow of reversible redox couples on a MoO3 film-modified microelectrode

In this paper, we have investigated the reactivity of the molybdenum oxide film toward some standard redox systems (e.g., ferrocene (Fc) and its derivatives) and observed a few interesting phenomena. The results demonstrate that the electrochemical behaviour of Fc and its derivatives at the oxide-modified carbon fiber (CF) microelectrode differs from that at a bare CF microelectrode, The conductivity of the molybdenum oxide film is seriously affected by the range and the direction of the potential scan, which influences the electrochemical behaviour of these redox systems at the film electrode. If the cycling potential is more positive than the reduction potential of the molybdenum oxide film, the reduction and oxidation peak currents of Fc and its derivatives could not be observed. The result indicates that the molybdenum oxide film on a microelectrode surface cannot transfer electrons between the surface of the electrode and Fc or its derivatives due to the existence of a high resistance between the interface in these potential ranges. On the other hand, if the lower limit of the scan potential was extended to a potential more negative than the reduction peak potential of the film, the oxidation peak of Fc or its derivatives appeared at about the potential relative to E-0 of Fc or its derivatives on the bare electrode, and the peak current is proportional to the concentration of these couples in the electrolyte. To our surprise, the peak height on the modified electrode is much larger than that on the bare CF microelectrode under the same conditions in the range of low concentration of these couples, and the oxidation peak potential of these couples is more negative than that on the bare CF microelectrode. On the basis of the experimental observation, we propose that these redox couples may undergo an interaction with the reduction state of the molybdenum oxide film. The new phenomena that we observed have been explained by using this interaction. (C) 1997 Elsevier Science S.A.

Electrocatalytic oxidation and flow amperometric detection of hydrazine at an electropolymerized 4-vinylpyridine/palladium film electrode

A poly(4-vinyl)pyridine (PVP)/Pd film electrode was constructed for the electrocatalytic detection of hydrazine. The preparation of the PVP/GC electrode was performed by electropolymerization of the monomer 4-vinylpyridine onto the surface of a glassy carbon electrode. Subsequently, palladium is electrodeposited onto the polymer modified electrode surface. The ion-exchange function of PVP polymer is helpful to this process in view of the tetrachlorapalladate anion. Compared with the Pd/GC electrode, the modified electrode displays a better mechanical stability in a flowing stream. The PVP/Pd film electrode exhibits higher sensitivity when detecting hydrazine with a detection limit of 0.026 ng (S/N=3).

Liquid chromatography amperometric detection of nitrite using a polypyrrole modified glassy carbon electrode doped with tungstodiphosphate anion

Chromatography-amperometric detection of nitrite with a polypyrrole modified glassy carbon electrode doped with tungstodiphosphate anion (Dawson-type P2W18O626-/PPy/GC electrode) based on its electrocatalytic reduction of nitrite is described. The cyclic and hydrodynamic voltammetry of nitrite at the P2W18O626-/PPy/GC electrode was studied. The factors affecting the detection of nitrite and the analytical performance of the modified electrode in flowing stream were investigated. The results show that the modified electrode has a good sensitivity (the limit of detection is 1 mu mol dm(-3)) and a satisfactory reproducibility (RSD = 3.78%, N = 21). The modified electrode was used in the chromatographic detection of nitrite spiked in the liquid from a tin of mushrooms and the mineralized spring water. It was found that the modified electrode exhibited good selectivity for nitrite.

Detection of menadione sodium bisulfite (vitamin K-3) by reversed-phase high performance liquid chromatography with series dual-electrode amperometric detector

A reversed-phase high-performance liquid chromatography with series dual glassy carbon electrodes for the amperometric detection of water-soluble menadione is described. The complex post-column derivatization reaction and the high background currents were avoided. The menadione sodium bisulfite was reduced at -0.3 V vs. SCE at the upstream (generator) electrode and oxidized at +0.2V vs. SCE at the downstream (collector) electrode. The mobile phase was 0.2moll(-1) HAc-NaAc aqueous buffer (pH 5.50) and 40% (v/v) methanol. The linear response was in the range of 35 ng to 15 mu g, with a detection Limit of 15 ng (S/N=3). The correlation coefficient was 0.9997 (n=6). The electrochemical detection with series dual electrodes has a higher selectivity for menadione (vitamin K-3) compound than with UV detection.

The ion selectivity of monensin incorporated phospholipid/alkanethiol bilayers

Monensin was incorporated into phospholipid/alkanethiol bilayers on the gold electrode surface by a new, paint-freeze method to deposit a lipid monolayer on the self-assembled monolayers (SAMs) of alkanethiol. The advantages of this assembly system with a suitable function for investigating the ion selective transfer across the mimetic biomembrane are based on the characteristics of SAMs of alkanethiols and monensin. On the one hand, the SAMs of alkanethiols bring out their efficiency of packing and coverage of the metal substrate and relatively long-term stability; on the other hand, monensin improves the ion selectivity noticeably. The selectivity coefficients K-Na+,K-K+, K-Na+,K-Rb+ and K-Na+,K-Ag+ are 6 x 10(-2), 7.2 x 10(-3) and 30 respectively. However, the selectivity coefficient K-Na+,K-Li+ could not be obtained by a potentiometric method due to the specific interaction between Li+ and phospholipid and the lower degree of complexion between Li+ and monensin. The potential response of this bilayer system to monovalent ions is fairly good. For example, the slope of the response to Na+ is close to 60 mV per decade and its linearity range is from 10(-1) to 10(-5) M with a detection limit of 2 x 10(-6) M, The bilayer is stable for at least two months without changing its properties. This monensin incorporated lipid/alkanethiol bilayer is a good mimetic biomembrane system, which provides great promise for investigating the ion transfer mechanism across the biomembrane and developing a practical biosensor.