Abd-Elgawad Radi

Label-free impedimetric immunosensor for sensitive detection of ochratoxin A

A novel label-free electrochemical impedimetric immunosensor for sensitive detection of ochratoxin A (OTA) was reported. A two-step reaction protocol was elaborated to modify the gold electrode. The electrode was first derivatized by electrochemical reduction of in situ generated 4-carboxyphenyl diazonium salt (4-CPDS) in acidic aqueous solution yielded stable 4-carboxyphenyl (4-CP) monolayer. The ochratoxin A antibody was then immobilized making use of the carbodiimide chemistry. The steps of the immunosensor elaboration and the immunochemical reaction between ochratoxin A and the surface-bound antibody were interrogated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The impedance change, due to the specific immuno-interaction at the immunosensor surface was utilized to detect ochratoxin A. The increase in electron-transfer resistance (DeltaR(et)) values was linearly proportional to the concentration of OTA in the range of 1-20ngmL(-1), with a detection limit of 0.5ngmL(-1).

Determination of levofloxacin in human urine by adsorptive square-wave anodic stripping voltammetry on a glassy carbon electrode

The adsorption behavior of levofloxacin on a glassy carbon electrode was explored by cyclic and square-wave voltammetry. The drug was accumulated on a glassy carbon electrode and a well-defined oxidation peak was obtained in acetate buffer pH 5.0. Using square-wave anodic stripping voltammetry and accumulation at +0.4 V versus Ag/AgCl (saturated KCl) for 300 s, linear calibration graph was obtained from 6.0x10(-9) to 5.0x10(-7) M levofloxacin. The detection limit was calculated to be 5.0x10(-9) M. The R.S.D. determined from ten determinations at the 1.0x10(-7) M level was 1.7%. The method was applied for the direct determination of levofloxacin in diluted urine samples. It was validated using high-performance liquid chromatography (HPLC) as a reference method.

Determination of Norfloxacin by square-wave adsorptive voltammetry on a glassy carbon electrode

The adsorptive and electrochemical behavior of norfloxacin on a glassy carbon electrode were investigated by cyclic and square-wave voltammetry. Cyclic voltammetric studies indicated that the process was irreversible and fundamentally controlled by adsorption. To obtain a good sensitivity, the solution conditions and instrumental parameters were studied using square-wave voltammetry. In acetate buffer of pH 5.0, norfloxacin gave a sensitive adsorptive oxidative peak at 0.920 V (versus Ag-AgCl). Applicability to measurement of norfloxacin at submicromolar levels in urine samples was illustrated. The peak current was linear with the norfloxacin concentration in the range 5-50 microg ml(-1) urine. The detection limit was 1.1 microg ml(-1) urine.

Highly sensitive ochratoxin A impedimetric aptasensor based on the immobilization of azido-aptamer onto electrografted binary film via click chemistry.

The aptamer immobilization onto organized mixed layers of diazonium salts via click chemistry was explored. The immobilized aptamer was employed in the fabrication of a highly sensitive and reusable electrochemical impedimetric aptasensor for the detection of ochratoxin A (OTA). The screen-printed carbon electrodes (SPCEs) were first modified by electrografting of a protected 4-((trimethylsilyl)ethynyl) benzene (TMSi-Eth-Ar) layer followed by a second one of p-nitrobenzene (p-NO(2)-Ar) by means of electrochemical reduction of their corresponding diazonium salts, (TMSi-Eth-Ar-N(2)(+)) and (p-NO(2)-ArN(2)(+)). After deprotection, a layer with active ethynyl groups was obtained. In the presence of copper (I) catalyst, the ethynyl groups reacted efficiently with aptamer bearing an azide function, thus forming a covalent 1,2,3-triazole linkage. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in the presence of ferri/ferrocyanide redox probe [Fe(CN)(6)](4-/3-) were used to characterize each step in the aptasensor development. The increase in electron-transfer resistance (R(et)) values due to the specific aptamer-OTA interaction was proportional to the concentration of OTA in a range between 1.25 ng/L and 500 ng/L, with a detection limit of 0.25 ng/L.

Anodic voltammetric assay of lansoprazole and omeprazole on a carbon paste electrode

The electrochemical oxidations of lansoprazole and omeprazole have been studied at a carbon paste electrode by cyclic and differential-pulse voltammetry in Britton-Robinson buffer solutions (0.04 M; pH 6.0-10.0). The drug produced a single oxidation step. By differential-pulse voltammetry, a linear response was obtained in B-R buffer pH 6.0 in a concentration range from 2.0 x 10(-7) to 5.0 x 10(-5) M for lansoprazole or omeprazole. The detection limits were 1.0 x 10(-8) and 2.5 x 10(-8) M for lansoprazole and omeprazole, respectively. The method was successfully applied for the analysis of omeprazole and lansoprazole in capsules. The results were comparable to those obtained by spectrophotometry.

Accumulation and trace measurement of chloroquine drug at DNA-modified carbon paste electrode

The voltammetric behaviour of chloroquine was investigated at carbon paste and dsDNA-modified carbon paste electrodes in different buffer systems over a wide pH range using cyclic and differential pulse voltammetry. Chloroquine was oxidized in the pH range 2.0-11.0 yielding one irreversible main oxidation peak. A second peak was also observed only in the pH range 5.0-7.0. The modification of the carbon paste surface with dsDNA allowed a preconcentration process to take place for chloroquine such that higher sensitivity was achieved as compared with the bare surface. The response was characterized with respect to solution pH, ionic strength, accumulation time and potential, chloroquine concentration, and other variables. Stripping voltammetric response showed a linear calibration curve in the range 1.0 x 10(-7) to 1.0 x 10(-5)moll(-1) with a detection limit of 3.0 x 10(-8)moll(-1) at the dsDNA-modified electrode. Application of the modified electrode to serum, without sample pretreatment, resulted in good recovery higher than 95% and the higher standard deviation was 3.0%.

Electrochemical reduction of meloxicam at mercury electrode and its determination in tablets.

The electrochemical reduction of meloxicam has been studied at a mercury electrode using various electrochemical methods in aqueous solutions over a wide pH range. The reduction of the drug produced a single reduction step in acidic media, whereas in slightly acidic and neutral media two reduction steps were observed. In alkaline media meloxicam shows a single pH-independent reduction step. The irreversibility of the electrode process was verified by different criteria. At all pH values, reactant adsorption at mercury electrode was observed. The mechanism of reduction was discussed. Using differential-pulse voltammetry, the drug yielded a well-defined voltammetric response in Britton-Robinson buffer solution, pH 4.0 at -1.286 V (vs. Ag/AgCl). This process could be used to determine meloxicam concentration in the range 1.0 x 10(-8)-5.0 x 10(-6) M. The method was successfully applied for the analysis of meloxicam in the tablet dosage form.

Electrochemical Oxidation of the Hypoglycaemic Drug Gliclazide

ABSTRACT The electrochemical oxidation of gliclazide has been studied in Britton-Robinson buffers in the pH range of 2.0-12.0 by cyclic voltammetry and differential pulse polarography at carbon paste electrode. Gliclazide gave rise to two voltammetric peaks, corresponding to the oxidation of hydrazide –CO-NH-NR2 moiety (where R2 are cycloalkylamino ring). The oxidation process has been shown to be irreversible and diffusion-controlled with adsorption characteristics over the entire pH range. Analytical methods with adequate precision and accuracy have been developed for the voltammetric determination of gliclazide in tablets.

Stripping voltammetric determination of indapamide in serum at castor oil-based carbon paste electrodes

The diuretic drug indapamide has been characterized voltammetrically at carbon paste electrodes by means of cyclic and differential pulse voltammetry. An adsorptive stripping method at carbon paste electrode modified with castor oil for trace determination of indapamide was described. A study of the variation of the peak current with solution variables such as pH, ionic strength, concentration of indapamide, possible interference, and instrumental variables such as scan rate, pulse amplitude, preconcentration time, accumulation potential, paste composition has resulted in the optimization of the oxidation signal for analytical purposes. By anodic stripping differential pulse voltammetry, the calibration plot was linear in the range 5 x 10(-8) x 10(-7) M with a detection limit of 5 x 10(-9) M at carbon paste electrode modified with castor oil in pH 4.0. The preconcentration medium-exchange approach was utilized for selective determination of indapamide in spiked serum. A detection limit of 15 ng ml(-1) was obtained for dilute serum sample after 3 min accumulation and medium-exchange procedure.

Electrochemistry of Cyclodextrin Inclusion Complexes of Pharmaceutical Compounds

Electrochemistry of cyclodextrins (CDs) and cyclodextrin inclusion complexes with different pharmaceutical compounds is reviewed. The article highlights some electrochemical investigations of the CD-drugs interactions in solution. Cyclodextrin modified electrodes and their applications as electrochemical sensors in pharmaceutical analysis based on the self-assembly of CD derivatives on metal electrodes and nanoparticles, and cyclodextrin-modified multiwalled carbon nanotubes are also discussed.