A new catalytic electrode for the amperometric determination of hydrogen peroxide

The electrode is based on the decomposition of H₂O₂ by an inorganic polymer catalyst. The steady -state decomposition currents of H₂0₂ obtained with the catalyst electrode are independent of ionic strength (0.1–1.5) and pH (2.5–10.5). A linear relationship is obtained from 0.02 to 2 mM H₂0₂ in 0.1 M KCl at 25°C.     

Mixed ferrocene-glucose oxidase-carbon-paste electrode for amperometric determination of glucose

The simultaneous incorporation of ferrocene derivatives and glucose oxidase into a carbon-paste matrix results in an effective microelectrode for sensing glucose. The close proximity of the enzymatic, redox mediating and sensing sites offers extremely short response times (t_95%=18 s) compared with early ferrocene-based glucose sensors. Several ferrocene derivatives are evaluated, with 1,1′-dimethylferrocene yielding the best results. The influence of the paste composition, operating potential, glucose concentration and other variables is described. The incorporation of stearic acid into the enzyme-containing paste greatly reduces the interference due to ascorbic acid. The microelectrode is easily fabricated, as time-consuming multi-step immobilization schemes are eliminated. Flow-injection measurements at a rate of 90 samples per hour and relative standard deviations of 3% are also reported.     

An enzyme electrode for the amperometric determination of glucose

An amperometric method utilizing a glucose electrode has been developed for the determination of blood glucose. The time of measurement is less than 12 s if a kinetic method is used and 1 min if a steady-state method is used. The long-term stability of the electrode is ca. 0.1% change from maximum response per day when stored at room temperature for over 10 months. The enzyme electrode determination of blood sugar compares favorably with commonly used methods with respect to accuracy, precision, and stability. The only reagent required for blood sugar determinations is a buffer solution. The electrode consists of a metallic sensing layer covered by a thin film of immobilized glucose oxidase held in place by means of cellophane. When poised at the correct potential, the current produced is proportional to the glucose concentration.     

A novel palygorskite-modified carbon paste amperometric sensor for catechol determination

A palygorskite-modified carbon paste electrode (CPE) was constructed using graphite powder mixed with palygorskite particles. Compared with the unmodified CPE, the resulting palygorskite-modified CPE remarkably increases the peak currents of catechol, and greatly lowers the peak potential separation. Therefore, the palygorskite exhibits catalytic activity to catechol and significantly improves the determining sensitivity. The electrocatalytic activity of palygorskite is attributed to its high adsorption capability and the –OH groups on its surface, which plays an important role in the electron transfer between the modified CPE and the catechol in the solution. The sensor shows a linear response range between 5 and 100 μM catechol with a correlation coefficient of 0.998. The detection limit was calculated as 0.57 μM (s/n = 3).     

An amperometric enzyme electrode for the determination of 3alpha-hydroxysteroids

An enzyme electrode for quantifying the total content of 3alpha-hydroxysteroids is described. 3alpha-Hydroxysteroid dehydrogenase (E.C. 1.1.1.50) was immobilized on the surface of glassy-carbon and low-temperature isotropic-carbon electrodes by intermolecular cross-linking with bovine serum albumin and glutaraldehyde. The effects of the following factors on the response to androsterone were studied: applied potential, the concentration and pH of the pyrophosphate buffer used, and the NAD concentration. The Michaelis-Menten constant for 3alpha-hydroxysteroid dehydrogenase in solution was determined amperometrically with androsterone as substrate (K(m) = 189muM). A preliminary report of the response of the system to serum samples containing a bile acid is presented.     

A novel amperometric sensor and chromatographic detector for determination of parathion

A novel amperometric sensor and chromatographic detector for determination of parathion has been fabricated from a multi-wall carbon nano-tube (MWCNT)/Nafion film-modified glassy-carbon electrode (GCE). The electrochemical response to parathion at the MWCNT/Nafion film electrode was investigated by cyclic voltammetry and linear sweep voltammetry. The redox current of parathion at the MWCNT/Nafion film electrode was significantly higher than that at the bare GCE, the MWCNT-modified GCE, and the Nafion-modified GCE. The results indicated that the MWCNT/Nafion film had an efficient electrocatalytic effect on the electrochemical response to parathion. The peak current was proportional to the concentration of parathion in the range 5.0×10^–9–2.0×10^–5 mol L^–1. The detection limit was 1.0×10^–9 mol L^–1 (after 120 s accumulation). In high-performance liquid chromatography with electrochemical detection (HPLC–ED) a stable and sensitive current response was obtained for parathion at the MWCNT/Nafion film electrode. The linear range for parathion was over four orders of magnitude and the detection limit was 6.0×10^–9 mol L^–1. Application of the method for determination of parathion in rice was satisfactory.     

A novel amperometric inhibition biosensor based on HRP and gold sononanoparticles immobilised onto Sonogel-Carbon electrode for the determination of sulphides

A novel inhibition biosensor used for the detection of sulphides (Na₂S) has been developed. The biosensor is based on the immobilisation of horseradish peroxidase (HRP) on the Sonogel-Carbon (SNGC) electrode using glutaraldehyde, Poly(4-vinylpyridine) and gold sononanoparticles (AuSNPs). The Poly(4-vinylpyridine) was used due to its high affinity for sulphide anions, while the presence of gold sononanoparticles enhances the electron transfer reaction and improves the analytical performance of the biosensor. The amperometric measurements were performed at an applied potential of ?0.15 V vs. Ag/AgCl in 50 mM sodium acetate buffer solution pH = 6.0. The apparent kinetic parameters (K_mapp, V_max) of immobilised HRP were calculated in the absence of inhibitor (sulphide) using caffeic acid as substrate. Under the optimal experimental conditions, the determination of sulphide can be achieved in a dynamic range of 0.4–2.8 µM with a low limit of detection of 0.15 µM. The electrochemical impedance spectroscopy (EIS) was also used to characterise the interactions of substrate and inhibitor with the enzyme-modified electrode. The developed biosensor exhibited high sensitivity, selectivity and stability, and can be successfully applied to the detection of sulphide in water.     

A novel amperometric method for antioxidant activity determination using DPPH free radical

A new method for the determination of antioxidant activity based on the amperometric reduction of 2,2-diphenyl-1-picrylhydrazyl (DPPH) at the glassy carbon electrode is proposed. All experiments were done in three-electrode electrochemical cell at 140 mV vs. Hg(2)Cl(2) | 3 M KCl using ethanolic solution (phi=40%) and 0.033 M KCl in 0.033 M phosphate buffer, pH=7.4. The linear range obtained for Trolox in 100 microM DPPH ethanol-water solution was up to 30 microM, with a limit of detection of 0.05 microM. The developed method was applied for the evaluation of antioxidant activity of some water or ethanol soluble pure compounds of antioxidants and of the samples of tea, wine and some other beverages. The good correlation of measurements (R(2)=0.9993) expressed as Trolox equivalent was obtained between the proposed amperometric method and classic spectroscopic method.     

Flow-injection amperometric determination of chlorine at a gold electrode

The applied potential is +0.2 V vs. SCE, flow rate is 1 ml min^?1 and sample volume is 30 μ1. The background electrolyte is 0.05 M phosphate, pH 7.4. Electrode pretreatment is +1.3 V vs. SCE for 40 s, followed by a pre-injection delay of 20 s. Peak current over a receding baseline is used. Linear range extends down to 0.4 μM for chloramine and 0.2 μM for hypochlorite. Sensitivities are 70 and 95 nA μM^? respectively. Time per determination is less than 1.5 min. Monochlorinated glycine is active whereas chlorinated cyanuratesshow no response. Chlorine and monochloramine in river water were determined.     

Sol-gel derived metal dispersed ceramic-graphite composite electrode for amperometric determination of dopamine

A new silver dispersed composite electrode made of sol-gel derived ceramic-graphite has been demonstrated for the determination of dopamine. Silver has been immobilized within the porous and rigid silicate network in the composite by the specific interaction with (3-mercaptopropyl)trimethoxysilane (MPS). The modified composite electrode coated with Nafion exhibited good catalytic activity for the oxidation of dopamine at a reduced potential of 0.35 V with good sensitivity and selectivity. The sensor showed a linear response to dopamine in the concentration range from 6.6×10⁻⁶ to 1.2×10⁻³ M with a correlation coefficient of 0.9987. The electrode surface can be easily renewed by a simple mechanical polishing and has the advantages of good reproducibility, rapid response and remarkable stability.     

Mathematical description of concentration profiles and anodic currents for amperometric two-enzyme electrodes

Formulae are presented for explicit solutions of partial differential equations describing the transient behaviour of concentration profiles and the anodic current in amperometric two-enzyme electrodes. The mathematical treatment is based on reaction-diffusion models with irreversible first-order catalytic reactions. Numerical results are presented to demonstrate the practical use of the derived formulae.     

A chromatographic determination of nitrate with amperometric detection at a copperized cadmium electrode

A copperized cadmium flow-through electrode is applied for cathodic detection of nitrate in the effluent stream of a liquid chromatograph. The nitrate is separated from dissolved oxygen in a small (10 cm) column of strong-base, anion-exchange resin (Dowex 1-X8) with 5 mM perchloric acid as the eluent. The effluent stream is buffered to pH 8 by mixing with a stream of buffer prior to detection. A large excess of chloride added to the sample dramatically improves the separation of nitrate from dissolved oxygen in the sample.     

A novel correlation for rapid lactose determination in milk by a cryoscopic technique

Residual lactose in special milk was systematically determined for people with lactose intolerance by means of a rapid on-line measurement of the cryoscopic point. A proposed cryoscopic procedure was compared to 2 conventional yet highly laborious methods: the enzymatic procedure with spectrophotometric control and the polarimetric method. Several experiments with different mixtures of both semi-skimmed and low-lactose milk were performed. A lineal relationship was found between lactose concentration and freezing point, the analytical equation for which shows a close relationship regarding the 3 methods used. The advantages of the cryoscopic procedure include speed in obtaining results and operational simplicity at a low cost, better monitoring of enzymatic hydrolysis kinetics, and greater control over the production process for delactosed milk. The equation obtained also enables prediction of the lactose percentage in commercial milk by a simple measurement of freezing point.     

beta-Cyclodextrin-ferrocene inclusion complex modified carbon paste electrode for amperometric determination of ascorbic acid

Inclusion complex of ferrocene (Fc) with beta-cyclodextrin (beta-CD) has been synthesized in ethylene glycol. It was unsolvable in water and had been successfully used to the preparation of beta-CD-Fc inclusion complex modified carbon paste electrode (CFCPE). Solid paraffin was used as the binder of the electrode. Ascorbic acid (AA) was electrocatalytically oxidized at the electrode in NH(3)-NH(4)Cl buffer (pH 10.0) with a anodic peak potential of +0.20 V (vs. SCE). The anodic current was proportional to the concentration of AA in the range 1.0x10(-3)-5.0x10(-7) mol l(-1) with the detection limit of 1.0x10(-7) mol l(-1). Using the inclusion complex as the electroactive substance greatly increased the stability and reproducibility of CFCPE than using Fc; the lifetime of the electrode can be over 1 year. Because CFCPE responds rapidly and sensitively, it has been successfully applied to the determination of AA in fruit juices.     

A disposable, screen-printed electrode for the amperometric determination of azide based on the immobilization with catalase or tyrosinase.

A disposable, screen-printed electrode based on the immobilization of catalase or tyrosinase was developed to construct biosensors for the amperometric determination of azide. The determination principles for azide by these two methods are based on inhibiting the enzymatic consumption of an electrode-detectable substance (hydrogen peroxide or catechol) on an enzyme-immobilized electrode. Both of these methods show a sensitive detection range and a short measuring time.     

Electrocatalytic amperometric determination of amitrole using a cobalt-phthalocyanine-modified carbon paste electrode

Cobalt-phthalocyanine-modified carbon paste electrodes are shown to be excellent indicators for electrocatalytic amperometric measurements of triazolic herbicides such as amitrole, at low oxidation potentials (+0.40 V). The detection and determination of amitrole in flow injection analysis with a modified carbon paste electrode with Co-phthalocyanine is described. The concentrations of amitrole in 0.1 M NaOH solutions were determined using the electrocatalytic oxidation signal corresponding to the Co(II)/Co(III) redox process. A detection limit of 0.04 microg mL(-1) (4 ng amitrole) was obtained for a sample loop of 100 microL at a fixed potential of +0.55 V (vs. Ag/AgCl) in 0.1 M NaOH and a flow rate of 4.0 mL min(-1). Furthermore, the modified carbon paste electrodes offers reproducible responses in such a system, and the relative standard deviation was 3.3% using the same surface, 5.1% using different surface, and 6.9% using different pastes. The performance of the cobalt-phthalocyanine-modified carbon paste electrodes is illustrated here for the determination of amitrole in commercial formulations. The response of the electrodes is stable, with more than 80% of the initial retained activity after 50 min of continuous use.     

Voltammetric and amperometric determination of metoclopramide on boron-doped diamond film electrode

New methods for the determination of metoclopramide, antiemetic and gastroprokinetic pharmaceutical, were developed, using differential pulse voltammetry (DPV) and flow injection analysis (FIA) with amperometric detection on a boron-doped diamond film electrode. Electrode pretreatment necessary to ensure the stable results was investigated and it was found, that while DPV requires frequent electrode cleaning, FIA with a sufficiently high flow rate can maintain a stable signal with no signs of electrode passivation. The calculated quantification limits of the DPV and FIA with amperometric detection were 0.13 μmol L⁻¹ and 0.015 mmol L⁻¹, respectively. The applicability of the new methods was verified by the determination of metoclopramide in a pharmaceutical preparation. FIA with amperometic detection proved to be sensitive, accurate and, due to the resistance of the electrode to the passivation, also simple to handle.      

An amperometric immunosensor based on a conducting immunocomposite electrode for the determination of Schistosoma japonicum antigen.

A renewable amperometric immunosensor based on a graphite-paraffin-Schistosoma japonicum antibody (SjAb) biocomposite electrode has been prepared for the detection of Schistosoma japonicum antigen (SjAg). Competitive ELISA was employed involving HRP-SjAg as a tracer and 3,3',5,5'-tetramethylbenzidine (TMB) as a substrate. The product of an enzyme catalytic reaction was detected at +0.1 V (vs. Ag/AgCl reference electrode) for measuring the amount of HRP-labeled SjAg binding to the electrode surface. The assay conditions were optimized, including the amount of SjAb loading in the electrode and HRP-SjAg in the incubation solution, the pH of the measuring solution and the incubation time. The measuring range was 0.5-30 microg/ml under the optimum conditions. Rabbit serum samples of different infection degree were measured, which demonstrated that the immunosensor meets the demands of clinical analysis. It exhibits some advantages, such as simplicity of fabrication, rapidity of measurement, and satisfactory sensitivity and reproducibility.     

Bienzyme amperometric lactate-specific electrode

The electrode, based on a lactate dehydrogenase and a diaphorase, permits the assay of l-lactate in the concentration range 0.2–8 mM with a response time of about 40 s. Both the enzymes are commercially available. The amperometric detection of hexacyanoferrate(II) at a platinum electrode is done at 0.3 V (vs. SCE) instead of 0.8 V as in the detection of NADH, improving the selectivity of the sensor.