Nizam Diab

Electropolymerized manganese porphyrin/polypyrrole films as catalytic surfaces for the oxidation of nitric oxide

A porphyrin-based NO sensor has been prepared using Mn(II)-meso-tetracarboxyphenylporphyrin linked via a spacer chain to a pyrrole unit. By means of electrochemically induced formation of a porphyrin/polypyrrole film on the microelectrode a suitable catalytic surface for NO oxidation could be obtained. The modified electrode displays excellent electrocatalytic activity for the oxidation of NO at ∼720 mV versus Ag/AgCl. A Nafion membrane coating was used as charge-selective layer in order to diminish interferences by negatively charged ions, especially NO2−. The response of the microsensor for NO-oxidation was monitored in phosphate buffer using differential pulse voltammetry and constant-potential amperometry. The detection limit in the latter case has been determined to be 1×10−7 M NO.

Pyrrole functionalised metalloporphyrins as electrocatalysts for the oxidation of nitric oxide

Pyrrole-functionalised tetracarboxyphenyl porphyrin and trimethoxyphenylcarboxy-phenyl porphyrin containing Ni, Mn and Pd as the central metal ion were used to modify Pt-disk microelectrodes (slashed circle 50 mum) (by repetitive cyclic voltammetry, dip-dry and pulse-amperometry methods) for the detection of nitric oxide (NO). Electrodes modified with Mn(II) trimethoxyphenylcarboxyphenyl porphyrin using the pulse amperomery approach, were found to be sensitive, stable and fast in response towards the oxidation of NO. Thus, they were used for the detection of NO release from a population of transformed human umbilical vein endothelial cells (T-HUVEC) into a droplet of electrolyte solution following stimulation with vascular endothelial growth factor (VEGF). The electrode surface was covered with an additional layer of Nafion(R) to prevent interference from anionic molecules such as nitrite.

Visualization of Oxygen Consumption of Single Living Cells by Scanning Electrochemical Microscopy: The Influence of the Faradaic Tip Reaction†

The respiration activity of an individual living cell is an indicator of its metabolic vitality. Closely positioned microelectrodes have been suggested for determination of the respiration activity by monitoring the local oxygen concentration. Although first attempts for visualization of the oxygen consumption rate of single living cells by means of scanning electrochemical microscopy (SECM) were already described in 1998, evaluation of the respiratory activity of individual cells remains challenging and the complexity is often underestimated. In particular, the dimensions of the cell itself lead to limitations of conventionally used constant-height mode SECM investigations. Apart from convolution of the oxygen reduction current at the SECM tip with topographic effects, constant-height mode experiments require working distances comparable or below the height of the cell body, thus increasing the risk of tip crash. Attempts to overcome these restrictions include among others positioning of the tip to distances outside the feedback range, embedding of the cells into cavities, or efforts to subtract topographic contributions after cell death. Moreover, as living cells are irregular in dimension, the tip-to-cell distance varies with the tip position. Therefore, constant-distance mode (cd-mode) SECM techniques are inherently advantageous for this purpose. In particular, coupling SECM with scanning probe techniques, such as atomic force microscopy (AFM) and scanning ion conductance microscopy (SICM) as well as shearforce and impedance-based techniques, led to efficient strategies to control the tip-to-sample separation. Recently, we described a shearforce-based cd method (4D SF/ CD-SECM) that is able to work at various tip-to-sample separations. It can hence detect complete diffusion profiles in the surroundings of sources or sinks of redox-active species. Although SECM distance control systems are available, the detection of the respiration activity of single living cells remains challenging. Owing to the small rate of oxygen consumption by a single cell, only small current variations ontop of a high background current are measured. Even more importantly, a biological cell acts as an immiscible liquid– liquid interface in a SECM experiment. Lipophilic redox mediators are known to undergo transmembrane diffusion processes and can be utilized to investigate intracellular redox activity. However, concentration changes in the vicinity of the cellular membrane, for example by the tip reaction, may induce local concentration gradients and cause a diffusional exchange of redox species over the lipid bilayer in a socalled SECM-induced transfer (SECM-IT) mode. The high solubility of oxygen in lipids promotes this transmembrane diffusion and oxygen can easily cross the cell membrane. This diffusion process superimposes the detection of cell respiration. As a result, in most reports addressing detection of cell metabolism based on the detection of variations in the local oxygen concentration, the positioned microelectrode does not act as a passive observer but actively influences the oxygen concentration inside the gap between tip and cell, resulting in imaging artifacts that have not previously been addressed. Even though mentioned occasionally, this effect was neglected in SECM investigations of respiration activity at living cells. Herein, we address the influence of the oxygen reduction rate at the SECM tip on imaging the respiration activity at living cells. We provide strategies to avoid limitations resulting from a strong tip reaction using a potential pulse profile at the tip with a time dependent data acquisition in the shearforce-based cd-mode of SECM. Commonly, the detection of the local oxygen concentration in close proximity to the cell body is performed by means of a variation of the generator-collector mode of SECM with the tip being continuously polarized at oxygen reduction potential. The tip competes with the respiring living cell for the available oxygen inside the gap between SECM tip and cell surface. Crossing the cell body during a SECM line scan should therefore lead to a decrease of the tip current owing to a locally lowered oxygen concentration caused by cell [*] Dr. M. Nebel, S. Gr tzke, Prof. Dr. W. Schuhmann Lehrstuhl f r Analytische Chemie, Elektroanalytik & Sensorik and Center for Electrochemical Sciences, CES Ruhr-Universit t Bochum Universit tsstrasse 150, 44780 Bochum (Germany) E-mail: wolfgang.schuhmann@rub.de

Selenium status of healthy Turkish children

Selenium concentrations, in blood plasma, red blood cells hair of 61 healthy children, ages 0–14 yr, were determined using hydride generation atomic absorption spectrometry. Starting from late lactation period, selenium concentrations in all these matrices were found to be increasing with age. A good correlation was found between erythrocyte and plasma, hair and plasma, and hair and erythrocyte selenium concentrations. Although plasma, erythrocyte, and hair selenium concentrations of girls seemed to be higher than that of boys (only in erythrocytes), selenium concentrations of girls were found to be significantly (p<0.002) higher than that of boys, 71±9 ng/mL, vs 65±10 ng/mL, respectively. Selenium status of Turkish children is found to be lower than that found in the literature; marginal selenium deficiency could be important in the development of some selenium deficiency could be important in the development of some selenium deficiency related diseases. There is a need for extension of this study to healthy children from different regions in Turkey and to different disease states.

Detection of NO release from endothelial cells using Pt micro electrodes modified with a pyrrole-functionalised Mn(II) porphyrin

Pt-disk electrodes (50 μm diameter) were modified with pyrrole-functionalised Mn(II) trimethoxycarboxyphenylporphyrin by means of an electrochemical pulse technique leading to stable and sensitive microsensors that displayed electrocatalytic activitiy suitable for the detection and quantification of nitric oxide (NO). In order to improve selectivity, a thin layer of Nafion® was deposited on top of the porphyrin-modified surface of the microsensor by means of a dip-coating procedure. These microelectrodes were successfully applied for the detection of the secretion of NO from a population of human umbilical vein endothelial cells (HUVEC) into a small volume of electrolyte.

Microelectrochemical visualization of oxygen consumption of single living cells.

The detection of cellular respiration activity is important for the assessment of the status of a biological cell. Due to its non-invasive character and high spatial resolution scanning electrochemical microscopy (SECM) is a powerful tool for single cell measurements. Common limitations of respiration studies performed by SECM are discussed and strategies provided to further adapt SECM detection schemes to the specific requirements for the investigation of single cell respiration. In particular the combination of a potential pulse technique in the redox competition mode of SECM with a shearforce-based constant-distance positioning of the SECM tip is proposed for characterising the impact of the tip reaction during SECM imaging. The adjustment of the driving force of the tip reaction and the selection of the time for data acquisition after applying the potential pulse allowed a successful visualization of cell respiration activity.

Sequential-injection stripping analysis of nifuroxime using DNA-modified glassy carbon electrodes.

The voltammetric behavior of nifuroxime was investigated comparing stationary voltammetric methods with the recently proposed sequential-injection stripping analysis (SISA), by using cyclic voltammetry (CV) and differential-pulse voltammetry at bare and DNA-modified glassy carbon (GC) electrodes. In cyclic voltammetry, reduction of nifuroxime at DNA-modified electrodes gives rise to a well-defined peak, and in contrast to bare GC surfaces, a re-oxidation peak could be observed. Optimization of the pre-concentration process at the DNA-modified surface led to a significant enhancement of the voltammetric current response, a better defined peak shape and an improved dynamic range. Based on this optimized voltammetric procedure, SISA has been evaluated for the determination of nifuroxime. The flow-system significantly facilitates the regeneration of the DNA-modified electrode surface, hence diminishing problems related to accumulation and memory effects. The linear detection range could be extended to 65 microM with a detection limit (3 s) of 0.68 microM, which corresponds to an absolute amount of 21 ng nifuroxime.

Selenium status of normal Turkish children

Selenium is an essential trace e lement with great impor tance in h u m a n nutr i t ion and health. It plays a major role in some diseases in animals and h u m a n beings. Because of the significance of se len ium intake to h u m a n health, it is impor tan t to assess se len ium status for establishing deficient states. The data about se lenium status of heal thy chi ldren are l imited in the literature. We a imed to obtain informat ion about the se len ium level of blood and hair of heal thy Turkish Chi ldren in the p resen t s tudy. We measured plasma, red blood cell, and hair se len ium concentrat ions wi th an atomic absorpt ion spec t rophotometer . Sixty-two heal thy children living in Ankara were included in this s tudy. The med ian values of plasma, red blood cell, and hair se len ium levels of different age groups are as follows:

Kinetics and Mechanism of Oxidation of L-Cysteine by Bis-3-di-2-pyridylketone-2-thiophenylhydrazone- iron(III) Complex in Acidic Medium

The kinetics of oxidation of L-cysteine by 3-di-2-pyridylketone-2- thiophenylhydrazone-iron(III), (Fe(DPKTH) 2) 3+ complex in acidic medium was studied spectrophotometrically at 36 o C temperature. The molar ratios of DPKTH to iron(III) and iron(II) individually, were found to be (2:1) (DPKTH : iron(III)/(II)). The reaction was stroked to be first-order with respect to iron(III) and L-cysteine, second-order with respected to DPKTH ligand and reversed second-order with respected to hydrogen ion concentration. Added salts did not affect the rate and no free radical was detected when radical detector was placed in the reaction mixture. Ethanol solvent ratio was found to effect both the initial rate and the maximum absorbance ( λmax ) of (Fe(DPKTH) 2) 2+ complex. The initial rate rose when the temperature was increased which empowered to calculate the activation parameters. A suitable reaction mechanism was proposed.