Jyh-Myng Zen

Simultaneous determination of caffeine and acetaminophen in drug formulations by square-wave voltammetry using a chemically modified electrode

A Nafion®/ruthenium oxide pyrochlore chemically modified electrode was used for the simultaneous determination of caffeine and acetaminophen in drug formulations by square-wave voltammetry. Compared to a bare glassy carbon electrode, the chemically modified electrode exhibits an apparent shift of the oxidation potentials in the cathodic direction and a marked enhancement in the current responses for both caffeine and acetaminophen. Linear calibration curves are obtained in 0.05 M perchloric acid, the supporting electrolyte, over 10–250 and 5–250 μM for caffeine and acetaminophen, respectively. The detection limits (3σ) are 2.2 and 1.2μM for caffeine and acetaminophen, respectively. The practical analytical utility is illustrated by selective measurements of caffeine and acetaminophen in several commercially available drugs, without any preliminary treatment.

Selective voltammetric method for uric acid detection using pre-anodized Nafion-coated glassy carbon electrodes

Electrochemically pre-anodized Nafion-coated glassy carbon electrodes (NCGCEs) were used for the selective determination of uric acid (UA) by square-wave voltammetry. The major interference from ascorbic acid (AA) was overcome owing to the distinct ability of the pre-anodized NCGCE to yield its best performance at around pH 5. At pH 5, AA exists in the anionic form whereas UA is in the cationic form, since their pKa values are 4.1 and 5.4, respectively, at 25 °C. Consequently, the Nafion film repels the negatively charged AA and selective sensing of UA can be achieved. Moreover, the anodized surface has a high affinity towards UA through hydrogen bonding. Hence the combination of pre-anodization and the Nafion film modification works excellently in the determination of UA. Under optimized conditions, the detection limit for UA has been improved to 10 nM (S/N = 3). The practical analytical utility of the method is demonstrated by the measurement of UA in human urine without any preliminary treatment.

Electrocatalytic oxidation and trace detection of amitrole using a Nafion/lead-ruthenium oxide pyrochlore chemically modified electrode

Abstract The Nafion/lead–ruthenium oxide pyrochlore (Pb 2 Ru 1.75 Pb 0.25 O 7− y ) chemically modified electrode (CME) exhibits an excellent electrocatalytic response towards the amitrole oxidation. It is proposed that the electrochemically active Ru VI/IV present in the octahedral site of the pyrochlore network mediates the oxidation of amitrole. Rotating disk electrode (RDE) experiments were carried out to evaluate the basic catalytic mechanism on the CME. The occurrence of Michaelis–Menten type kinetics on the CME for the amitrole oxidation reaction was demonstrated and the Michaelis–Menten constant ( K m ), electrochemical rate constant ( k ′ e ) and the catalytic rate constant ( k c ) were evaluated. The electrocatalytic behavior is further developed as a sensitive detection scheme for amitrole using both square-wave voltammetry and flow-injection analysis. Under optimized conditions, the calibration curve is linear up to 250 μM with a detection limit of 0.38 μM (signal/noise=3) in square-wave voltammetry. For flow-injection analysis, the linear range is 0–100 μM with a detection limit of 0.15 ng in 20 μl of sample.

A glucose sensor made of an enzymatic clay-modified electrode and methyl viologen mediator.

A novel glucose sensor has been contrived by immobilizing glucose oxidase between two nontronite clay coatings on glassy carbon electrode with methyl viologen as mediator. The sandwich configuration proved to be very effective in the determination of glucose. The response of the glucose sensor was determined by measuring cyclic voltammetric peak current values under aerobic solution conditions. The effects of the amount of enzyme immobilized, the operating pH, and the common interferences on the response of the glucose sensor were studied. The detection limit was 5 μM, with a linear range extending to about 6 mM, giving a dynamic range of over 3 orders of magnitude for 0.1 mM methyl viologen. When stored in pH 7 phosphate buffer at 4 °C, the sensor shows almost no change in performance after operating for at least 2 months. A mechanism for the operation of the glucose sensor is also proposed.

Flow injection analysis of hydrogen peroxide on copper-plated screen-printed carbon electrodes

A disposable copper-plated screen-printed carbon electrode (CuSPE) was developed for the determination of hydrogen peroxide by flow injection analysis (FIA) at ambient temperature without deoxygenation. Cyclic voltammetry on the CuSPE in pH 7.4 phosphate buffer solution showed the growth of CuO and Cu2O. A well-defined reduction signal corresponding to the mediation of CuO and Cu2O occurred in the presence of hydrogen peroxide. The mechanistic study revealed that the reduction is a coupled-chemical reaction mechanism with the operation of pseudo-first order kinetics with respect to the concentration of Cu2O. The calculated electrochemical rate constant (ke) from Laviron’s model is 13.7 s−1. Systematic investigations were made to optimize the experimental parameters for hydrogen peroxide detection by FIA. With a poised potential of −0.3 V versus Ag/AgCl and a flow rate of 2 ml min−1, the calibration curve was found to be linear up to 200 μM hydrogen peroxide with a detection limit of 0.97 μM (S/N = 3). The CuSPE is fairly stable for repetitive measurements.

A sensitive voltammetric method for the determination of parathion insecticide

A sensitive voltammetric method is developed for the determination of parathion (fNO2) using a Nafion 1 -coated glassy carbon electrode. In this method, parathion is first irreversibly reduced from fNO2 to fNHOH. The reversible peaks at around a0.33 V (versus Ag/AgCl) corresponding to a two-electron oxidation/reduction of hydroxylamine (fNHOH) to the nitroso (fNO) derivative were used for detection with square-wave voltammetry. The experimental parameters, such as, pH, film thickness, preconcentration potential, preconcentration time, and square-wave voltammetric parameters were optimized. Using this method, a linear calibration curve for parathion was obtained up to 15 mM range in pH 1.1 citrate buffer solution with a detection limit (S/Na 3) of 50 nM. # 1999 Elsevier Science B.V. All rights reserved.

A selective voltammetric method for uric acid detection at Nafion®-coated carbon paste electrodes

A square-wave voltammetric method together with Nafion(R)-coated carbon paste electrodes were used for the selective determination of uric acid in the presence of a high concentration of ascorbic acid. Since the oxidation potential of uric acid is about 200 mV more positive than that of ascorbic acid at the Nafion(R)-coated carbon paste electrode, the selectivity can be greatly improved simply by applying an electrolysis potential of +0.4 V vs. Ag/AgCl where only ascorbic acid is oxidised. The acceptable tolerance of ascorbic acid concentration for the determination of uric acid is as high as 1.5 mM. With 30 s of electrolysis time, a linear calibration curve is obtained over the 0-50 muM range in 0.05 M citrate buffer solution, pH 4.0, with slope (muA/muM) and correlation coefficient of 0.34 and 0.9984, respectively. The detection limit (3sigma) is 0.25 muM. The practical analytical utility is illustrated by selective measurements of uric acid in human urine without any preliminary treatment.

Activated Nickel Platform for Electrochemical Sensing of Phosphate

We report here a highly selective enzymeless approach for the determination of phosphate (PO(4)(3-)) by flow injection analysis (FIA). In this system, the activation of barrel plated nickel electrode (Ni-BPE) in alkaline media to form a Ni(OH)(2)/NiO(OH) film was found to trigger the adsorption of phosphate at the electrode surface. Based on the suppressed current of the electrocatalytic oxidation of glucose at the activated Ni-BPE in 0.1 M NaOH solution caused by adsorption of phosphate, we develop an FIA detection scheme for the determination of phosphate. Under the optimized conditions of flow rate = 300 microL/min and detection potential = 0.55 V vs Ag/AgCl with 25 microM glucose in 0.1 M NaOH as carrier solution, the calibration curve showed a linear range up to 1 mM. Possible interferences from the coexisting ions were also investigated. The results demonstrated that sensor could be used for the determination of phosphate in the presence of nitrate, chloride, sulfate, acetate, oxalate, carbonate, and some anionic species of toxicological and environmental interest, such as chlorate, chromate, and arsenate ions. The electrode can be effectively regenerated without extra treatment under the hydrodynamic condition. For eight continuous injections of 40 microM PO(4)(3-), a relative standard deviation of 0.28% was obtained, indicating good reproducibility of the proposed method. The detection limit (S/N = 3) was calculated as 0.3 microM.

Determination of paraquat by square-wave voltammetry at a perfluorosulfonated ionomer/clay-modified electrode.

A novel Nafion/clay-modified electrode (NCME) was developed for the determination of paraquat by square-wave cathodic stripping voltammetry. The clay that showed the best performance for the fabrication of the NCME is nontronite (SWa-1, ferruginous smectite). The electrochemical behavior of paraquat showed that the cathodic peak at -0.70 V vs Ag/AgCl permits adequate quantification of the analyte. Linear calibration curves are obtained over the 0-80 ppb range, with a detection limit of 0.5 ppb in pH 8 phosphate buffer solution for 4 min preconcentration time. Various factors influencing the determination of paraquat were thoroughly investigated in this study. The practical analytical utility is illustrated by selective measurements of paraquat in real water samples.

Simultaneous determination of guanine and adenine contents in DNA, RNA and synthetic oligonucleotides using a chemically modified electrode

Simultaneous determination of guanine and adenine contents in DNA, RNA and synthetic oligonucleotides on a Nafion–ruthenium oxide pyrochlore modified electrode is described. This chemically modified electrode shows very pronounced electrocatalytic effects towards the oxidation of guanine and adenine. The experimental parameters for individual determinations of guanine and adenine on the modified electrode were first optimised. Excellent detection limits (S/N = 3) of 0.86 ng ml–1 (5.7 nM) and 2.7 ng ml–1 (20 nM) for guanine and adenine, respectively, were obtained. Simultaneous determination of guanine and adenine in a mixture was also optimised. The denatured calf thymus DNA and yeast RNA showed oxidation peaks corresponding to guanine and adenine with a linear calibration line up to 5 µg ml–1 in pH 4.0 phosphate buffer. The detection limit estimated for these nucleic acids were 11.5 ng ml–1 and 37.7 ng ml–1 for guanine and adenine, respectively. The new method provides an alternative to estimate the guanine and adenine contents more selectively and sensitively than currently applied techniques.