Mesfin Redi-Abshiro

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.

Excitation Wavelength Dependence of Dual Fluorescence of DMABN in Polar Solvents

Steady-state absorption, fluorescence excitation and emission spectra of 4-(N,N-dimethylamino)benzonitrile (DMABN) have been measured at room temperature in cyclohexane, 1,4-dioxane, dichloromethane, and acetonitrile solutions. The fluorescence spectra of DMABN are found to exhibit dual emission in 1,4-dioxane, dichloromethane, and acetonitrile solutions and single emission in cyclohexane solution. The effect of solvent polarity and excitation wavelength on the emission spectra has also been studied. The fluorescence excitation spectra of DMABN monitored at the emission bands are different. The presence of two different conformations of the same molecule in the ground state has lead to two close lying excited states; local excited (LE) and charge transfer (CT), and thereby results in the dual fluorescence of the compound. The experimental studies were supported by ab initio density functional theory (DFT) calculations performed at the B3LYP/6-31Gd level of theory. On the basis of the experimental results and our theoretical calculations, we suggest that there are two conformers of DMABN, which are stable in the ground state, equilibrated in solution at room temperature that give rise dual fluorescence upon excitation.

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.

Solvent Polarity and Excitation Wavelength Dependence of the Dual Fluorescence in N,N-Diethyl-4-Nitrosoaniline

Dual fluorescence in N,N-Diethyl-4-nitrosoaniline (DENA) has been studied employing absorption, excitation and emission spectroscopic techniques and computational methods. The absorption and fluorescence spectra of DENA were measured in solvents of various polarities at room temperature. The emission spectra of DENA were found to exhibit a single emission band in non polar solvent (cyclohexane) and in a highly polar solvent (acetonitrile). In the contrary, two emission bands were observed in medium polar solvents (tetrahydrofuran, 1,2-dichloroethane and dichloromethane) whereby the short (local excited; LE) and long (charge transfer; CT) emission maxima correspond to the emission maxima of the compound observed in cyclohexane and acetonitrile solutions, respectively. Moreover, the two emission bands have shown strong excitation wavelength dependence, and area normalization resulted in an iso-emissive point. The two emission maxima were in addition found to correspond to two excitation maxima in 3D fluorescence spectra. Further, two minima were obtained in potential energy surface calculation of DENA. From the experimental and computational results it was concluded that the dual fluorescence may be attributed to the presence of two different ground state structural conformers of DENA in equilibrium that are stabilized through solute-solvent interaction.

A (-)-norephedrine-based molecularly imprinted polymer for the solid-phase extraction of psychoactive phenylpropylamino alkaloids from Khat (Catha edulis Vahl. Endl.) chewing leaves.

A molecularly imprinted polymer (MIP) was prepared using (-)-norephedrine as the template, methacrylic acid as the functional monomer, ethylene glycol dimethacrylate as the cross-linker and chloroform as the porogen. The MIP was used as a selective sorbent in the molecularly imprinted solid-phase extraction (MIP-SPE) of the psychoactive phenylpropylamino alkaloids, norephedrine and its analogs, cathinone and cathine, from Khat (Catha edulis Vahl. Endl.) leaf extracts prior to HPLC-DAD analysis. The MIP was able to selectively extract the alkaloids from the aqueous extracts of Khat. Loading, washing and elution of the alkaloids bound to the MIP were evaluated under different conditions. The clean baseline of the Khat extract obtained after MIP-SPE confirmed that a selective and efficient sample clean-up was achieved. Good recoveries (90.0-107%) and precision (RSDs 2.3-3.2%) were obtained in the validation of the MIP-SPE-HPLC procedure. The content of the three alkaloids in Khat samples determined after treatment with MIP-SPE and a commercial Isolute C18 (EC) SPE cartridge were in good agreement. These findings indicate that MIP-SPE is a reliable method that can be used for sample pre-treatment for the determination of Khat alkaloids in plant extracts or similar matrices and could be applicable in pharmaceutical, forensic and biomedical laboratories. Copyright

Experimental and computational studies on zeolite-Y encapsulated iron(III) and nickel(II) complexes containing mixed-ligands of 2,2′-bipyridine and 1,10-phenanthroline

Mixed ligand complexes of 2,2′-bipyridine and 1,10-phenanthroline with iron(III) and nickel(II) have been encapsulated into a zeolite cage by the reaction of zeolite exchanged metal ion with flexible ligands. The synthesized catalyst has been characterized by X-ray powder diffraction, scanning electron microscopy, BET surface area and pore volume analysis, FT-IR spectroscopy, thermo-gravimetric analysis and elemental analysis. Density functional theory calculations have been carried out on both neat complexes as well as metal complexes encapsulated into NaY zeolite to investigate changes in structural parameters, energies of the HOMO and LUMO, and global hardness and softness of the two metal complexes upon encapsulation into zeolite. Experimental results confirm successful formation of a mixed ligand complex of Fe(III) and Ni(II) inside the zeolite cage. Density functional theory calculations predict a higher reactivity of the zeolite encapsulated metal complexes compared to respective metal complexes. The zeolite encapsulated Fe(III) and Ni(II) complexes are found to be catalytically active toward the oxidation of 2-phenyl phenol (OPP).

Oxidation of 4–chloro–3–methylphenol using zeolite Y–encapsulated iron(III)–, nickel(II)–, and copper(II)–N,N‘-disalicylidene-1, 2-phenylenediamine complexes

The degradation of 4-chloro-3-methylphenol (PCMC) using zeolite-encapsulated iron(III), nickel(II), and copper(II) complexes of N,N‘-disalicylidene-1,2-phenylenediamine as catalysts, in a heterogeneous Fenton-like advanced oxidation process, was studied. The physicochemical properties of the catalysts were determined using powder X-ray diffraction, thermogravimetric analysis, Brunauer-Emmett-Teller surface area analysis, Fourier-transform infrared spectroscopy, elemental analysis, and scanning electron microscopy. The effects of four factors, namely initial H2O2 concentration, catalyst dosage, temperature, and pH, on the degradation of a model organic pollutant were determined. The results show that at low acidic pH, almost complete removal of PCMC was achieved with the iron(III), nickel(II), and copper(II) catalysts after 120 min under the optimum reaction conditions: catalyst dosage 0.1 g, H2O2 concentration 75 mmol/L, initial PCMC concentration 0.35 mmol/L, and 50 °C. The reusability of the prepared catalysts in PCMC degradation was also studied and a possible catalyst deactivation mechanism is proposed. The possible intermediate products, degradation pathway, and kinetics of PCMC oxidation were also studied.

Occurrence, distribution, and ecological risk assessment of potentially toxic elements in surface sediments of Lake Awassa and Lake Ziway, Ethiopia

Microwave-assisted acid digestion and modified aqua regia (HNO3:HCl:HF:H3BO3) leaching techniques were used for the determination of 15 potentially toxic elements (V, Cr, Fe, Mn, Co, Ni, Cu, Zn, As, Se, Ag, Cd, Sn, Hg and Pb) in sediment samples from Lake Awassa and Lake Ziway, Ethiopia. The digests were subsequently analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-optical emission spectrometry (ICP-OES). Mercury was directly determined in the solid samples using an elemental mercury analyzer. The precision and accuracy of the digestion procedures were verified using certified reference materials. The experimental results were in good agreement with the certified values (P < 0.05) and the recoveries were quantitative (>90%). The average relative standard deviations were below 10%. There is significant correlation between the two lakes at the 0.01 level (2-tailed). Using the sediment quality guidelines, both lakes are heavily polluted with Zn and some of the sites are heavily polluted with Cu, Ni and Pb. Based on effect range low (ERL) – effect range medium (ERM), in both lakes for Ag were greater than the ERM, indicating that the areas could be toxic to aquatic organisms, while for Cr, Cu, As and Hg the values were less than ERL.

Assessment of potentially toxic elements in Swiss chard and sediments of Akaki River, Ethiopia

For the determination of 15 potentially toxic elements (V, Cr, Fe, Mn, Co, Ni, Cu, Zn, As, Se, Ag, Cd, Sn, Hg, and Pb) in plant and sediment samples of the Akaki River, Ethiopia, inductively coupled plasma–mass spectrometry, inductively coupled plasma–optical emission spectrometry, and a RA-915+ mercury analyzer were applied. Sediment and plant samples were mineralized using a closed-vessel microwave-assisted digestion system. The elemental concentrations varied considerably in plant and sediment samples. The minimum concentration was observed for known toxic elements (As, Hg, and Cd) while the highest concentration was observed for the elements of relatively low toxicity Zn, Mn, and Fe. The concentration of Cr, Fe, Pb, Zn, and As in Swiss chard surpassed the maximum permissible levels at specific sites. At some of the sites, the sediment quality guidelines are surpassed for Cu, Zn, and Pb.

Dual fluorescence of (E)-N-(4-(dimethylamino)benzylidene)-2H-1,2,4-triazol-3-amine (DMABA-Amtr): A ground state perspective

We present the absorption and fluorescence spectra of (E)-N-(4-(dimethylamino)benzylidene)-2H-1,2,4-triazol-3-amine (DMABA-Amtr), an electron donor-π-acceptor system. The molecule shows a single fluorescence emission band in non-polar solvents while dual emissions were observed in polar aprotic solvents. Although several researchers over the years provide different explanations for the mechanism of the phenomena, based on solvent assisted excited state geometry changes of such systems, it is still a matter of controversy since such systems are unique as they contradict Kashas rule. The emission spectrum of the molecule shows strong dependence on solvent polarity and excitation wavelength. This observation together with a single iso-emissive point found in the area normalize emission spectra indicates the presence of two ground state equilibrium structures of the compound which are both fluorescent. Density functional theory (DFT) and time-dependent (TD-DFT) calculations also support the experimental findings.