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Transfer Rate Constant (transfer + rate_constant)

Distribution by Scientific Domains
Chemistry81%
Medical Sciences18%

Kinds of Transfer Rate Constant

  • electron transfer rate constant
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    Selected Abstracts

    Preparation, Electrochemistry, and Electrocatalytic Activity of Lead Pentacyanonitrosylferrate Film Immobilized on Carbon Ceramic Electrode

    ELECTROANALYSIS, Issue 21 2008
    H. Razmi
    Abstract Lead pentacyanonitrosylferrate (PbPCNF), a new Prussian blue analog, was immobilized on the surface of a carbon ceramic electrode (CCE) prepared by sol-gel method. The immobilization process consists of adding a certain amount of metallic lead to the electrode matrix before gelation, and chemical derivatization of Pb on the electrode surface to a PbPCNF solid film by immersing the electrode in a solution of sodium pentacyanonitrosylferrate (PCNF). The composition of the synthesized PbPCNF was characterized by FTIR, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) techniques. The resulting modified electrode showed electroactivity at two redox centers. The electrochemical behavior of the PbPCNF modified carbon ceramic electrode (PbPCNF|CCE) was studied by cyclic voltammetry. Under optimized conditions the peak-to-peak separation is only 39,mV, indicative of a surface reaction. Ion effects of the supporting electrolyte suggest that cations have a considerable effect on the electrochemical behavior of the modified electrode. The transfer coefficient (,) and the charge transfer rate constant at the modifying film|electrode interface (ks) were calculated. The electrocatalytic activity of the modified electrode toward the electro-reduction of peroxodisulfate was studied in details. [source]

    Xanthine Sensors Based on Anodic and Cathodic Detection of Enzymatically Generated Hydrogen Peroxide

    ELECTROANALYSIS, Issue 6 2007
    Aminur Rahman
    Abstract A xanthine biosensor was fabricated by the covalent immobilization of xanthine oxidase (XO) onto a functionalized conducting polymer (Poly-5, 2,: 5,, 2,-terthiophine-3-carboxylic acid), poly-TTCA through the formation of amide bond between carboxylic acid groups of poly-TTCA and amine groups of enzyme. The immobilization of XO onto the conducting polymer (XO/poly-TTCA) was characterized using cyclic voltammetry, quartz crystal microbalance (QCM), and X-ray photoelectron spectroscopy (XPS) techniques. The direct electron transfer of the immobilized XO at poly-TTCA was found to be quasireversible and the electron transfer rate constant was determined to be 0.73,s,1. The biosensor efficiently detected xanthine through oxidation at +0.35,V and reduction at ,0.25,V (versus Ag/AgCl) of enzymatically generated hydrogen peroxide. Various experimental parameters, such as pH, temperature, and applied potential were optimized. The linear dynamic ranges of anodic and cathodic detections of xanthine were between 5.0×10,6,1.0×10,4 M and 5.0×10,7 to 1.0×10,4,M, respectively. The detection limits were determined to be of 1.0×10,6,M and 9.0×10,8,M with anodic and cathodic processes, respectively. The applicability of the biosensor was tested by detecting xanthine in blood serum and urine real samples. [source]

    Immobilization and Electrochemistry of Negatively Charged Proteins on Modified Nanocrystalline Metal Oxide Electrodes

    ELECTROANALYSIS, Issue 12 2005
    Emmanuel Topoglidis
    Abstract The immobilization of two acidic, low isoelectric point proteins, green fluorescence protein and ferredoxin (FRD) is investigated on nanocrystalline, mesoporous TiO2 and SnO2 electrodes. Modification of these electrodes with a cationic polypeptide (poly- L -lysine) or an aminosilane prior to protein immobilization is found to enhance protein binding at least ten fold, attributed to more favorable protein/electrode electrostatic interactions. Cyclic voltammetry studies of FRD-modified SnO2 electrodes indicate reversible protein electrochemistry with a midpoint potential of ,0.59,V (vs. Ag/AgCl) and an interfacial electron transfer rate constant of 0.45,s,1. [source]

    Immobilized Cytochrome c Sensor in Organic/Aqueous Media for the Characterization of Hydrophilic and Hydrophobic Antioxidants

    ELECTROANALYSIS, Issue 18 2003
    Moritz Beissenhirtz
    Abstract A method for the characterization of antioxidants is introduced, which allows the measurement of pure hydrophilic and hydrophobic substances as well as complex cosmetic creams. The sensor is based on cytochrome c covalently immobilized on a gold wire electrode working in mixtures of phosphate buffer and organic solvents. It is combined with a superoxide generating enzyme system. The decrease of the superoxide concentration in the test solution by the added antioxidants is detected and used for the quantification of their antioxidative efficiency. Electrochemical properties of immobilized cytochrome c, such as formal potential and heterogeneous electron transfer rate constant, have been investigated in mixtures of aqueous buffer and DMSO, methanol, butanediol, and THF. The maximum organic solvent content for quasi-reversible electrode behavior was correlated to spectroscopic measurements. The activity of the radical producing enzyme in such media was determined and the radical generation characterized. The antioxidative properties of pure substance such as ascorbic acid and Biochanin A as well as of five anti-ageing cosmetic creams were studied. This showed also the influence of matrix composition on the efficiency of antioxidative supplements. [source]

    Noncompartmental kinetic analysis of DCE-MRI data from malignant tumors: Application to glioblastoma treated with bevacizumab

    MAGNETIC RESONANCE IN MEDICINE, Issue 2 2010
    Ruediger E. Port
    Abstract Dynamic contrast enhanced MRI contrast agent kinetics in malignant tumors are typically complex, requiring multicompartment tumor models for adequate description. For consistent comparisons among tumors or among successive studies of the same tumor, we propose to estimate the total contrast agent,accessible volume fraction of tumor, including blood plasma, vpe, and an average transfer rate constant across all tumor compartments, Ktrans.av, by fitting a three-compartment tumor model and then calculating the area under the tumor impulse-response function (= vpe) and the ratio area under the tumor impulse response function over mean residence time in tumor (= Ktrans.av). If the duration of dynamic contrast enhanced MRI was too short to extrapolate the tumor impulse-response function to infinity with any confidence, then conditional parameters v and Ktrans.av* should be calculated from the available incomplete impulse response function. Median decreases of 33% were found for both v and Ktrans.av* in glioblastoma patients (n = 16) 24 hours after the administration of bevacizumab (P < 0.001). Median total contrast-enhancing tumor volume was reduced by 18% (P < 0.0001). The combined changes of tumor volume, v, and Ktrans.av* suggest a reduction of true vpe, possibly accompanied by a reduction of true Ktrans.av. The proposed method provides estimates of a scale and a shape parameter to describe contrast agent kinetics of varying complexity in a uniform way. Magn Reson Med, 2010. © 2010 Wiley-Liss, Inc. [source]

    Evidence for shutter-speed variation in CR bolus-tracking studies of human pathology

    NMR IN BIOMEDICINE, Issue 3 2005
    Thomas E. Yankeelov
    Abstract The standard pharmacokinetic model for the analysis of MRI contrast reagent (CR) bolus-tracking (B-T) data assumes that the mean intracellular water molecule lifetime (,i) is effectively zero. This assertion is inconsistent with a considerable body of physiological measurements. Furthermore, theory and simulation show the B-T time-course shape to be very sensitive to the ,i magnitude in the physiological range (hundreds of milliseconds to several seconds). Consequently, this standard model aspect can cause significant underestimations (factors of 2 or 3) of the two parameters usually determined: Ktrans, the vascular wall CR transfer rate constant, and ve, the CR distribution volume (the extracellular, extravascular space fraction). Analyses of animal model data confirmed two predicted behaviors indicative of this standard model inadequacy: (1) a specific temporal pattern for the mismatch between the best-fitted curve and data; and (2) an inverse dependence of the curve's Ktrans and ve magnitudes on the CR dose. These parameters should be CR dose-independent. The most parsimonious analysis allowing for realistic ,i values is the ,shutter-speed' model. Its application to the experimental animal data essentially eliminated the two standard model signature inadequacies. This paper reports the first survey for the extent of this ,shutter-speed effect' in human data. Retrospective analyses are made of clinical data chosen from a range of pathology (the active multiple sclerosis lesion, the invasive ductal carcinoma breast tumor, and osteosarcoma in the leg) that provides a wide variation, particularly of Ktrans. The signature temporal mismatch of the standard model is observed in all cases, and is essentially eliminated by use of the shutter-speed model. Pixel-by-pixel maps show that parameter values from the shutter-speed analysis are increased by more than a factor of 3 for some lesion regions. This endows the lesions with very high contrast, and reveals heterogeneities that are often not seen in the standard model maps. Normal muscle regions in the leg allow validation of the shutter-speed model Ktrans, ve, and ,i magnitudes, by comparison with results of previous careful rat leg studies not possible for human subjects. Copyright © 2004 John Wiley & Sons, Ltd. [source]

    Some Issues on Core-Annulus and Cluster Models of Circulating Fluidized Bed Reactors

    THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2002
    Hsiaotao T. BiArticle first published online: 19 MAY 200
    Abstract The one-dimensional cluster model and the core-annulus model are examined based on existing correlations. The core-annulus model is found to give reasonable agreement with ozone decomposition data when the effective interphase mass transfer rate constant is equal to about 0.1 1/s, which is one order smaller than the reported values based on gas tracer tests. The prediction from the core-annulus model that the reactor performance decreases with increasing the riser diameter is found to be inconsistent with limited experimental data The one-dimensional cluster model predicts that a riser reactor performs very close to the pseudo-homogeneous plug flow reactor because of the high mass transfer rate between the cluster and the dilute phase. The improvement of model predictions lies in the better characterization of the cluster shape, size and the mass transfer rate between the cluster and the dilute phases. Le modèle de grappes unidimensionnel et le modèle c,ur-espace annulaire sont examinés d'après des corrélations existantes. On a trouvé que le modèle c,ur-espace annulaire décrivait raisonnablement bien les données de décomposition de l'ozone lorsque la constante de taux de transfert de matière entre phases réelle est égale à environ 0,1 s-1, ce qui est d'un ordre de grandeur plus petit que les valeurs obtenues d'après des tests par gaz traceurs. La prédiction du modèle c,ur-espace annulaire selon laquelle la performance du réacteur diminue avec l'augmentation du diamètre de la colonne montante s'avère non cohérente avec les quelques données expérimentales. Le modèle de grappes unidimensionnel prédit que la performance d'un réacteur à colonne montante est très proche de celle d'un réacteur à écoulement piston pseudo-homogène du fait du taux de transfert de matière élevé entre la grappe et la phase diluée. L'amélioration des prédictions du modèle repose sur une meilleure caractérisation de la forme des grappes, de leur taille et du taux de transfert de matière entre les grappes et les phases diluées. [source]

    Electrocatalytic Oxidation of Nitrite at Gold Nanoparticle- polypyrrole Nanowire Modified Glassy Carbon Electrode

    CHINESE JOURNAL OF CHEMISTRY, Issue 12 2009
    Jing Li
    Abstract A novel chemically modified electrode based on the dispersion of gold nanoparticles on polypyrrole nanowires has been developed to investigate the oxidation behavior of nitrite using cyclic voltammetry, differential pulse voltammetry and chronoamperometry techniques. The diffusion coefficient (D), electron transfer coefficient (,) and charge transfer rate constant (k) for the oxidation of nitrite were determined. The modified electrode exhibited high electrocatalytic activity toward the oxidation of nitrite. The catalytic peak current was found to be linear with nitrite concentrations in the range of 8.0×10,7,2.5×10,3 mol·L,1 with a detection limit of 1.0×10,7 mol·L,1 (s/n=3). The proposed method was successfully applied to the detection of nitrite in water samples with obtained satisfactory results. Additionally, the sensor also showed excellent sensitivity, anti-interference ability, reproducibility and stability properties. [source]

    Electron Transfer Kinetics of Ferrocene at Microcrystalline Boron-Doped Diamond Electrodes: Effect of Solvent and Electrolyte

    ELECTROANALYSIS, Issue 4 2003
    Shannon Haymond
    Abstract Cyclic voltammetric measurements were made using well-characterized microcrystalline boron-doped diamond thin-film electrodes to test the material's responsiveness for ferrocene as a function of scan rate, solvent, and electrolyte composition. Apparent heterogeneous electron transfer rate constants, k°app, of 0.042±0.015, 0.048±0.015, and 0.008±0.002,cm/s were observed in 0.1,M NaClO4/CH3CN, 0.1,M TBAClO4/CH3CN, and 0.1M TBAClO4/CH2Cl2, respectively. These rate constants, obtained using electrodes without any type of pretreatment, are similar to those observed for freshly polished glassy carbon. The results demonstrate that boron-doped diamond is a viable material for the electrochemical analysis of nonaqueous analytes. [source]

    Distance dependence of long-range electron transfer through helical peptides,

    JOURNAL OF PEPTIDE SCIENCE, Issue 2 2008
    Minako Kai
    Abstract Helical peptides of 8mer, 16mer, and 24mer carrying a disulfide group at the N -terminal and a ferrocene moiety at the C -terminal were synthesized, and they were self-assembled on gold by a sulfur,gold linkage. Infrared reflection,absorption spectroscopy and ellipsometry confirmed that they formed a monolayer with upright orientation. Cyclic voltammetry showed that the electron transfer from the ferrocene moiety to gold occurred even with the longest 24mer peptide. Chronoamperometry and electrochemical impedance spectroscopy were carried out to determine the standard electron transfer rate constants. It was found that the dependence of the electron-transfer rates on the distance was significantly weak with the extension of the chain from 16mer to 24mer (decay constant , = 0.02,0.04). This dependence on distance cannot be explained by an electron tunneling mechanism even if increased hydrogen-bonding cooperativity or molecular dynamics is considered. It is thus concluded that this long-range electron transfer is operated by an electron hopping mechanism. Copyright © 2007 European Peptide Society and John Wiley & Sons, Ltd. [source]

    Photoinduced Electron Transfer Reactions by SmI2 in THF: Luminescence Quenching Studies and Mechanistic Investigations

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2005
    Edamana Prasad Dr.
    Abstract Photoluminescence quenching studies of SmI2 in dry THF were carried out in the presence of five different classes of compounds: ketone, alkyl chloride, nitrile, alkene and imine. The free energy change (,G,0) of the photoinduced electron transfer (PET) reactions was calculated from the redox potentials of the donor (SmI2) and acceptors. The bimolecular quenching constants (kq) derived from the Stern,Volmer experiments parallel the free energy changes of the PET processes. The observed quenching constants were compared with the theoretically derived electron transfer rate constants (ket) from Marcus theory and found to be in good agreement when a value of ,=167 kJ,mol,1 (40 kcal,mol,1) was used for the reorganization energy of the system. A careful comparison of the excited state dynamics of SmII in the solid state to the results obtained in solution (THF) provides new insight in to the excited states of SmII in THF. The activation parameters determined for the PET reactions in SmI2/1-chlorobutane system are consistent with a less ordered transition state and high degree of bond reorganization in the activated complex compared to similar ground state reactions. Irradiation studies clearly show that SmI2 acts as a better reductant in the excited state and provides an alternative pathway for rate enhancement in known and novel functional group reductions. [source]