Stability Analysis of a Vertical Curved Channel Flow with a Radial Temperature Gradient

The linear stability analysis of a Newtonian incompressible fluid in a vertical curved channel formed by two coaxial cylindrical surfaces with a radial temperature gradient and an azimuthal pressure gradient shows that critical modes are oscillatory and non-axisymmetric. We have derived a generalized Rayleigh discriminant which includes both the curvature and buoyancy effects. Centrifugal buoyancy induces weak asymmetry of the dependence of the control parameter critical values on the sign of the temperature gradient. The critical parameters depend on the temperature gradient, the radius ratio and the nature of the fluid. For a wide curvature channel flow, there are two critical modes: oscillatory Dean modes for small temperature gradients and oscillatory centrifugal-thermal modes for relatively large temperature gradients. Received 14 November 2001 and accepted 29 March 2002 Published online: 2 October 2002 Communicated by H.J.S. Fernando     

Variable Selection and Regularization in Quantile Regression via Minimum Covariance Determinant Based Weights

The importance of variable selection and regularization procedures in multiple regression analysis cannot be overemphasized. These procedures are adversely affected by predictor space data aberrations as well as outliers in the response space. To counter the latter, robust statistical procedures such as quantile regression which generalizes the well-known least absolute deviation procedure to all quantile levels have been proposed in the literature. Quantile regression is robust to response variable outliers but very susceptible to outliers in the predictor space (high leverage points) which may alter the eigen-structure of the predictor matrix. High leverage points that alter the eigen-structure of the predictor matrix by creating or hiding collinearity are referred to as collinearity influential points. In this paper, we suggest generalizing the penalized weighted least absolute deviation to all quantile levels, i.e., to penalized weighted quantile regression using the RIDGE, LASSO, and elastic net penalties as a remedy against collinearity influential points and high leverage points in general. To maintain robustness, we make use of very robust weights based on the computationally intensive high breakdown minimum covariance determinant. Simulations and applications to well-known data sets from the literature show an improvement in variable selection and regularization due to the robust weighting formulation.     

Zanthoxylum Species: A Comprehensive Review of Traditional Uses,Phytochemistry, Pharmacological and Nutraceutical Applications

Zanthoxylum species (Syn. Fagara species) of the Rutaceae family are widely used in many countries as food and in trado-medicinal practice due to their wide geographical distribution and medicinal properties. Peer reviewed journal articles and ethnobotanical records that reported the traditional knowledge, phytoconstituents, biological activities and toxicological profiles of Z. species with a focus on metabolic and neuronal health were reviewed. It was observed that many of the plant species are used as food ingredients and in treating inflammation, pain, hypertension and brain diseases. Over 500 compounds have been isolated from Z. species, and the biological activities of both the plant extracts and their phytoconstituents, including their mechanisms of action, are discussed. The phytochemicals responsible for the biological activities of some of the species are yet to be identified. Similarly, biological activities of some isolated compounds remain unknown. Taken together, the Z. species extracts and compounds possess promising biological activities and should be further explored as potential sources of new nutraceuticals and drugs.     

Concentric glucose/O2 biofuel cell

A concentric glucose/O₂ biofuel cell has been developed. The device is constituted of two carbon tubular electrodes, one in the other, and combines glucose electrooxidation at the anode and oxygen electroreduction at the cathode. The anodic catalyst is glucose oxidase co-immobilized with the mediator 8-hydroxyquinoline-5-sulfonic acid hydrate, and the cathodic catalyst is bilirubin oxidase co-immobilized with the mediator 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonate) diammonium salt. Both enzymes and mediators are entrapped at the surface of the tubular electrodes by an electrogenerated polypyrrole polymer. The originality of the concentric configuration is to compartmentalize the anode and cathode electrodes and to supply dissolved oxygen separate from the electrolyte in order to avoid secondary reactions. The dissolved oxygen circulates through the inside of the cathode tube and diffuses from the inner to the external surface of the tube to react directly with the immobilized bilirubin oxidase. The assembled biofuel cell is studied at 37 °C in phosphate buffer pH 7.4. We show the influence of the thickness of the polypyrrole polymer on the electrochemical activity of the biocathodes. We also demonstrate the effect of the chemical reticulation of the enzymes by glutaraldehyde within the polymer on the performances of the bioelectrodes. The maximum power density delivered by the assembled glucose/O₂ biofuel cell reaches 42 μW cm⁻², evaluated from the geometric area of the electrodes, at a cell voltage of 0.30 V with 10 mM glucose. The results demonstrate that the concentric design of the BFC based on compartmented electrodes is a promising architecture for further development of micro electronic devices.     

Nanofibrous Membranes Containing Carbon Nanotubes: Electrospun for Redox Enzyme Immobilization

Summary: Nanofibrous membranes that possess reactive groups are fabricated by the electrospinning process from PANCAA solutions that contain MWCNTs. Field emission scanning electron microscopy is used to evaluate the morphology and diameter of the nanofibers. Potentials for applying these nanofibrous membranes to immobilize redox enzymes by covalent bonding are explored. It is envisaged that the electrospun nanofibrous membranes could provide a large specific area and the MWCNTs could donate/accept electrons for the immobilized redox enzymes. Results indicate that, after blending with MWCNTs, the diameter of the PANCAA nanofiber increases slightly. The PANCAA/MWCNT nanofibrous membranes immobilize more enzymes than that without MWCNTs. Moreover, as the concentration of the MWCNTs increases, the activity of the immobilized catalase is enhanced by about 42%, which is mainly attributed to the promoted electron transfer through charge‐transfer complexes and the π system of MWCNTs.

The covalent immobilization of redox enzymes, such as catalase, on a PANCAA/MWCNTs nanofiber.     

A microfluidic glucose biofuel cell to generate micropower from enzymes at ambient temperature

A Y-shaped microfluidic channel is applied for the first time to the construction of a glucose/O₂ biofuel cell, based on both laminar flow and biological enzyme strategies. During operation, the fuel and oxidant streams flow parallel at gold electrode surfaces without convective mixing. At the anode, the glucose oxidation is performed by the enzyme glucose oxidase whereas at the cathode, the oxygen is reduced by the enzyme laccase, in the presence of specific redox mediators. Such cell design protects the anode from an interfering parasite reaction of O₂ at the anode and offers the advantage of using different streams of oxidant and fuel for optimal performance of the enzymes. Electrochemical characterizations of the device show the influence of the flow rate on the output potential and current density. The maximum power density delivered by the assembled biofuel cell reached 110 μW cm^?2 at 0.3 V with 10 mM glucose at 23 °C. The microfluidic approach reported here demonstrates the feasibility of advanced microfabrication techniques to build an efficient microfluidic glucose/O₂ biofuel cell device.     

Performance comparison with different methods for ethanol/O2 biofuel cell based on NAD+ cofactor immobilized and activated by two types of carbon nanoparticles

Journal of Solid State Electrochemistry - Biofuel cells are an attractive alternative to conventional fuel cells, because they use biological catalysts. We report in this article the construction...     

5(6)-Carboxyfluorescein diacetate as an indicator of Caenorhabditis elegans viability for the development of an in vitro anthelmintic drug assay

A new in vitro assay for anthelmintic activity using Caenorhabditis elegans is based on the ability of 5(6)-carboxyfluorescein diacetate (CFDA) to indicate the worm's viability. It is shown for the first time that the treatment of a suspension of worms with a solution of 5(6)-carboxyfluorescein diacetate (4.2%) for 30 min transiently induces fluorescence in dead worms only, allowing a fast and efficient determination of the proportion of dead worms by fluorescence microscopy.

The proposed test has been validated using mixtures of populations of living and killed C. elegans and proved to be selective, linear in the range 0–100%, accurate and precise.

The suitability of the assay to detect anthelmintic activity was then evaluated by studying the toxicity against C. elegans of a series of known anthelmintic compounds (mebendazole, levamisole, niclosamide, pyrantel, piperazine, and thiabendazole) with various modes of action.

The worms were exposed to each drug at two concentrations, 50 and 100 μg/ml for piperazine, niclosamide, pyrantel and 5 and 10 μg/ml for the others. We observed that, in the tested range of doses, piperazine and niclosamide were only moderately toxic, yielding 13.1 and 17.5% of dead worms; due to their mode of action and/or specificity, the low toxicity of these compounds was as expected. The marked activities of all the other compound fully agree with those described in the literature and obtained by other more laborious techniques.

These validation data indicate that the proposed in vitro anthelmintic assay using 5(6)-carboxyfluorescein diacetate allows for sensitive measurement of worm viability.     

Synthetic models for the zinc sites in the methionine synthases

The syntheses and molecular structures of a series of tetrahedral zinc complexes designed to model the active sites in Escherichia coli methionine synthases are reported. [PhTttBu]ZnBr (PhTttBu = phenyltris((tert-butylthio)-methyl)borate) was prepared and characterized crystallographically to provide entry into [S3]ZnX complexes. Metathesis with KSPh yielded the phenylthiolato complex, [PhTttBu]Zn(SPh), which represents a structural mimic of the homocysteine ligated form of the enzyme. Alternatively, [S2N]ZnX (X = Br, CH3, SPh) species were prepared using the new mixed-donor ligands, [Ph(pz)BttBu] (phenyl(pyrazolyl)bis((tert-butylthio)methyl)borate) and [Ph(pztBu)BttBu] (phenyl(3-tert-butylpyrazolyl)bis((tert- butylthio)methyl)borate). Protonolysis of [Ph(pztBu)-BttBu]Zn(CH3) by PhSH in toluene yielded [Ph(pztBu)BttBu]Zn(SPh), a synthetic analogue of the homocysteine ligated form of cobalamin-independent methionine synthase (Met E). The average Zn-S bond distance in [Ph-(pztBu)BttBu]Zn(SPh) of 2.37 A compares well with the EXAFS-derived distance of 2.31 A found in the homocysteine-bound form of Met E.