Merid Tessema

Indirect voltammetric determination of caffeine content in coffee using 1,4-benzoquinone modified carbon paste electrode.

In this paper a simple and highly sensitive electroanalytical method for the determination of caffeine content using 1,4-benzoquinone modified carbon paste electrode is presented. The method is based on suppression of 1,4-benzoquinone peak current on addition of caffeine. Square-wave and cyclic voltammetric techniques were utilised for the investigation. The 1,4-benzoquinone modified electrode exhibited a well-defined peak with reproducible peak current values for repetitive measurements; and showed a decrease in peak current value with an increase in caffeine content. The result revealed two linear range regions between 0 mmol L(-1) and 0.5 mmol L(-1) and 0.5 mmol L(-1) and 8.0 mmol L(-1), with detection limits of 0.3 micromol L(-1) and 5.1 micromol L(-1), respectively. The method was then successfully applied to the determination of caffeine content in coffee samples. The effects of pH, electrode composition, step potential, pulse amplitude and square-wave frequency on the voltammetric responses were also investigated.

Electron transfer between cellobiose dehydrogenase and graphite electrodes

Abstract Electron transfer between the enzyme cellobiose dehydrogenase (CDH) and a flavin adenine dinucleotide (FAD) containing a catalytic active fragment of CDH (FAD-fragment) and a graphite electrode, respectively, was established. The current response in the presence of the enzyme substrate for graphite electrodes with CDH or the FAD-fragment adsorbed on the freshly polished graphite surface were compared with that of electrodes where CDH or the FAD-fragment were crosslinked in a redox polymer at the electrode surface. The initial slope, dj d[S] s = 0 , where j is the current density and [S]s = 0 the zero substrate concentration, was taken as a measure of the substrate response. For the electrodes with enzymes adsorbed directly on the surface, dj d[S] s = 0 , was a factor of 3 lower than for electrodes prepared with the polymer mixture. The redox polymer based electrodes, with CDH and with FAD-fragment, both showed a high and close to equal substrate response. In contrast the surface adsorbed CDH gave a much higher substrate response than the FAD-fragment.

Simultaneous determination of N-acetyl-p-aminophenol and p-aminophenol with poly(3,4-ethylenedioxythiophene) modified glassy carbon electrode

A sensitive and selective method was developed for the determination of N-acetyl-p-aminophenol (APAP) and p-aminophenol (PAP) using poly(3,4-ethylenedioxythiophene) (PEDOT)-modified glassy carbon electrode (GCE). Cyclic voltammetry and differential pulse voltammetry were used to investigate the electrochemical reaction of APAP and PAP at the modified electrode. Both APAP and PAP showed quasireversible redox reactions with formal potentials of 367 mV and 101 mV (vs. Ag/AgCl), respectively, in phosphate buffer solution of pH 7.0. The significant peak potential difference (266 mV) between APAP and PAP enabled the simultaneous determination both species based on differential pulse voltammetry. The voltammetric responses gave linear ranges of 1.0×10(-6)-1.0×10(-4) mol L(-1) and 4.0×10(-6)-3.2×10(-4) mol L(-1), with detection limits of 4.0×10(-7) mol L(-1) and 1.2×10(-6) mol L(-1) for APAP and PAP, respectively. The method was successfully applied for the determination of APAP and PAP in pharmaceutical formulations and biological samples.

Simultaneous determination of caffeine and paracetamol by square wave voltammetry at poly(4-amino-3-hydroxynaphthalene sulfonic acid)-modified glassy carbon electrode

Poly(4-amino-3-hydroxynaphthalene sulfonic acid)-modified glassy carbon electrode (poly(AHNSA)/GCE) was prepared for simultaneous determination of caffeine and paracetamol using square-wave voltammetry. The method was used to study the effects of pH and scan rate on the voltammetric response of caffeine and paracetamol. Linear calibration curves in the range of 10-125μM were obtained for both caffeine and paracetamol in acetate buffer solution of pH 4.5 with a correlation coefficient of 0.9989 and 0.9986, respectively. The calculated detection limits (S/N=3) were 0.79μM for caffeine and 0.45μM for paracetamol. The effects of some interfering substances in the determination of caffeine and paracetamol were also studied and their interferences were found to be negligible which proved the selectivity of the modified electrode. The method was successfully applied for the quantitative determination of caffeine and paracetamol in Coca-Cola, Pepsi-Cola and tea samples.

Simultaneous amperometric determination of some mono-, di-, and oligosaccharides in flow injection and liquid chromatography using two working enzyme electrodes with different selectivity

Abstract A selective dual enzyme electrode system for the monitoring of sugars in flow injection was developed. The working electrodes were based on cellobiose dehydrogenase (CDH) and oligosaccharide dehydrogenase (ODH) ‘wired’ with an osmium-based redox polymer on solid graphite electrodes. In each case the respective enzyme and poly(1-vinylimidazole) (PVI) in which every tenth mer is complexed with osmium (4,4′-dimethylbpy)2Cl, (denoted PVI10dmeOs) were crosslinked with poly(ethylene glycol) diglycidyl ether (PEGDGE). The two enzyme electrodes had distinct but different selectivities and sensitivities towards a number of investigated sugars. The CDH-modified electrodes responded, with relative activities, for cellobiose (100%), lactose (62.5%), maltose (1.2%), glucose (1%) and for higher cellodextrines (Glu3–6) while the ODH electrodes responded to all investigated sugars except d -fructose and the higher cellodextrins with relative activities for d -glucose (100%), cellobiose (60.4%), lactose (47.3%), maltose (40.6%), l -arabinose (26.1%), maltotriose (24.0%), d -galactose (21.9%), d -xylose (13.1%) and d -mannose (8.9%), 1 mM concentrations each. Linear calibration curves for cellobiose were obtained between 25 μM and 3 mM for the CDH- and ODH-modified electrodes with a sensitivity of 7.9 and 1.4 μA mM−1 cm−2, respectively. The dual electrode system was also used as an end column detector for detection of various sugars after their separation in a size-exclusion Chromatographic system.

Flow injection amperometric determination of glucose and some other low molecular weight saccharides based on oligosaccharide dehydrogenase mediated by benzoquinone systems

Abstract A comparison has been made on the performance of different oligosaccharide dehydrogenase (ODH) modified electrodes as sensors for the detection of a series of mono-, di-, and oligosaccharides. Monomeric benzoquinone (BQ) or poly(ether amine benzoquinone), PEAQ, as electron transfer mediators were used for the preparation of the electrodes. Four sensor configurations operating in the flow injection mode were studied: (i) ODH adsorbed on the surface of solid graphite with BQ in the flow carrier solution; (ii) ODH doped into the bulk of carbon paste electrode with BQ in the flow carrier solution; (iii) ODH and BQ doped into the bulk of carbon paste electrode, and (iv) ODH and PEAQ doped into the bulk of carbon paste electrode. All four systems responded to glucose and equal responses were obtained for the α- and the β-anomeric forms. The best sensitivity for glucose was obtained with the solid ODH-modified electrode, which was equal to 1.8 μA mM −1 cm −2 . The lowest noise was inherent for carbon paste electrodes with BQ in the paste. For this electrode configuration the responses for 16 sugars were observed. The highest currents were obtained for glucose (100%), lactose (52%), cellobiose (50%), and maltose (49%). For all four sensor systems selectivity profiles were almost the same.

LC−Biosensor System for the Determination of the Neurotoxin β-N-Oxalyl-l-α,β-diaminopropionic Acid

An analysis system is described for the determination of the neurotoxin β-N-oxalyl-l-α,β-diaminopropionic acid (β-ODAP). The system is based on liquid chromatographic separation of β-ODAP from interfering amino acids on an anion exchange column coupled with an amperometric enzyme electrode for the registration of β-ODAP. The electrode is based on a graphite rod modified with an Os(2+/3+) redox polymer cross-linked with l-glutamate oxidase and horseradish peroxidase. This LC-bienzyme electrode analytical system enabled monitoring of as low as 4 μM β-ODAP (injection volume 100 μL). The β-ODAP content in real grass pea samples was measured to range between 3.3 and 5.2 g kg(-)(1) in dry grass pea.

Stripping voltammetric determination of pyridine-2-aldoxime methochloride at the iron(III) doped zeolite modified glassy carbon electrode

An iron(III) doped zeolite modified glassy carbon electrode was constructed for the determination of pyridine-2-aldoxime methochloride. X-ray diffraction and chemical analysis were utilized to determine the optimum pH and chemical content for doping zeolite. Cyclic voltammetry was used to characterize the modified electrode and study the kinetics of the acid treated and untreated modified electrode. Acid treatment of the modified electrode showed a better electrochemical behavior compared to the untreated iron(III) doped zeolite modified electrode. Square wave anodic stripping voltammetry was employed to investigate the working pH and preconcentration time. The analytical performance of the modified electrode was evaluated, and a linear anodic stripping response for pyridine-2-aldoxime methochloride in the concentration range of 0.5-100.0 μM with a detection limit of 1.61 × 10(-7) M was obtained. Finally, the developed method was successfully applied for the determination of pyridine-2-aldoxime methochloride in a biological sample.

Determination of Phenol and Chlorophenols at Single-Wall Carbon Nanotubes/Poly(3,4-ethylenedioxythiophene) Modified Glassy Carbon Electrode Using Flow Injection Amperometry

Phenol and chlorophenols were investigated using single-wall carbon nanotubes (SWCNT) and poly(3,4-ethylenedioxythiophene) (PEDOT) composite modified glassy carbon electrode (SWCNT/PEDOT/GCE) as a detector in flow injection system. Optimization of experimental variables such as the detection potential, flow rate, and pH of the carrier solution (0.1 M sodium acetate) for the determination of phenol (P), 4-chlorophenol (CP), 2,4-dichlorophenol (DCP), 2,4,6-trichlorophenol (TCP), and pentachlorophenol (PCP) were performed. Under these conditions, analytical parameters were calculated from the calibration curve of measured amperometric responses as a function of concentrations of phenol and chlorophenols. The designed electrode exhibited very good analytical performance. The designed electrode was tested with 20 repetitive injections of each analyte and showed good operational stability. The analytical performance of the SWCNT/PEDOT/GCE electrode under flow through conditions was tested and was found to be impressive. The electrode showed a wider dynamic range for the detection of phenol and chlorophenols with low limits of detection compared with other enzymatic and nonenzymatic sensors. These results suggest that the method is quite useful for the analysis and monitoring of phenols and chlorophenols.

Synthesis, characterization and catalytic application of zeolite based heterogeneous catalyst of iron(III), nickel(II) and copper(II) salen complexes for oxidation of organic pollutants

In this study heterogeneous catalysts have been synthesized based on encapsulation of iron(III), nickel(II) and copper(II) complex of N,N′-Ethylenebis(salicylimine) or salen ligand into zeolite Y cavities by ship in a bottle method. The catalysts have been characterized by X-ray powder diffraction, BET surface area and pore volume analysis, FT-IR spectroscopy, thermo-gravimetric analysis, elemental analysis and electron paramagnetic resonance. The results show that ship in a bottle method is an efficient and simple approach for encapsulation of metal complexes into zeolite Y supercage. The prepared catalysts have been found to be very effective in the removal of azo dye from wastewater. The extent of its reusability for the catalytic oxidation of azo dye and the possible deactivation mechanism after ten successive reuse of the catalyst has also been studied.