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Modern directions for potentiometric sensors

Bakker,Eric; Chumbimuni-Torres,Karin
Fonte: Sociedade Brasileira de Química Publicador: Sociedade Brasileira de Química
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2008 EN
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This paper gives an overview of the newest developments of polymeric membrane ion-selective electrodes. A short essence of the underlying theory is given, emphasizing how the electromotive force may be used to assess binding constants of the ionophore, and how the selectivity and detection limit are related to the basic membrane processes. The recent developments in lowering the detection limits of ISEs are described, including recent approaches of developing all solid state ISEs, and breakthroughs in detecting ultra-small quantities of ions at low concentrations. These developments have paved the way to use potentiometric sensors as in ultra-sensitive affinity bioanalysis in conjunction with nanoparticle labels. Recent results establish that potentiometry compares favorably to electrochemical stripping analysis. Other new developments with ion-selective electrodes are also described, including the concept of backside calibration potentiometry, controlled current coulometry, pulsed chronopotentiometry, and localized flash titration with ion-selective membranes to design sensors for the direct detection of total acidity without net sample perturbation. These developments have further opened the field for exciting new possibilities and applications.

A kinetic and equilibrium study of ligand binding to a Root-effect haemoglobin.

Brittain, T
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 01/06/1985 EN
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17.329766%
The blood of the striped marlin (Tetrapturus audax) contains one major Root-effect haemoglobin. Titrations of this haemoglobin with CO show that at high pH the molecule is highly co-operative (Hill coefficient 2.8) whereas at low pH the titration data can best be described as the sum of contributions from non-co-operating subunits of different affinity. In terms of the two-state model the R-state affinity constant is much more sensitive to pH than is that of the T state. Flash-photolysis studies were used to characterize the kinetics of ligand binding to this haemoglobin. Both T and R states show kinetic heterogeneity in their recombination time courses, associated with the alpha- and beta-chains of the molecule. The rate constants for ligand binding to each chain, in each quaternary state, were determined, and in conjunction with the allosteric equilibrium parameters determined at pH8.0 were used in the two-state analysis of reaction curves, over a range of ligand concentration. The two-state model, extended to take account of chain difference, adequately fits the homotrophic effects observed for this haemoglobin. The two-state model is, however, inadequate in its description of the heterotropic effects produced by protons.

Studies on carbon monoxide binding by shark haemoglobin.

Dickinson, F M; Gibson, Q H
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 01/08/1981 EN
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17.329766%
The kinetics of the reactions of Pacific-porbeagle haemoglobin with CO were studied by flash-photolysis and stopped-flow methods, and the equilibrium binding curves for CO were measured in spectrophotometric titrations. Measurements were made in the pH range 6-8 and in the temperature range 0-40 degrees C. The results are discussed in terms of the allosteric model proposed by Monod, Wyman & Changeux [(1965) J. Mol. Biol. 12, 88-118]. Within this framework the results indicate that in the R-state the haem groups fall into two classes of different reactivity with different spectral characteristics, but that in the T-state the groups may be essentially equivalent. The physiological importance of the temperature-insensitivity of the equilibrium ligand-binding curves for porbeagle haemoglobin is discussed.

Ligand binding to ferrocytochrome c at high pH.

Moore, T A; Greenwood, C; Wilson, M T
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /05/1975 EN
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17.329766%
Ferrocytochrome c has been shown to bind two molecules of CO at pH 14. The second CO is thought to be bound only when the cytochrome c molecule is denatured, and once bound appears to be spectrally silent. Insolubilization of native cytochrome c prevents the binding of the second CO molecule. A scheme is proposed to explain these observations based on evidence from static titrations and flash-photolysis experiments, use of carboxymethyl cytochrome c and insoluble cytochrome c, and use of cyanide instead of CO as a ligand.

Direct Sensing of Total Acidity by Chronopotentiometric Flash Titrations at Polymer Membrane Ion-Selective Electrodes

Gemene, Kebede L.; Bakker, Eric
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
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38.621836%
Polymer membrane ion-selective electrodes containing lipophilic ionophores are traditionally interrogated by zero current potentiometry, which, ideally, gives information on the sample activity of ionic species. It is shown here that a discrete cathodic current pulse across an H+-selective polymeric membrane doped with the ionophore ETH 5294 may be used for the chronopotentiometric detection of pH in well buffered samples. However, a reduction in the buffer capacity leads to large deviations from the expected Nernstian response slope. This is explained by the local depletion of hydrogen ions at the sample-membrane interface as a result of the galvanostatically imposed ion flux in direction of the membrane. This depletion is found to be a function of the total acidity of the sample and can be directly monitored chronopotentiometrically in a flash titration experiment. The subsequent application of a baseline potential pulse reverses the extraction process of the current pulse, allowing one to interrogate the sample with minimal perturbation. In one protocol, total acidity is found to be proportional to the magnitude of applied current at the flash titration endpoint. More conveniently, the square root of the flash titration endpoint time observed at a fixed applied current is a linear function of the total acid concentration. This suggests that it is possible to perform rapid localized pH titrations at ion-selective electrodes without the need for volumetric titrimetry. The technique is explored here for acetic acid...

CO and O2 Binding to Pseudo-Tetradentate Ligand-Copper(I)-Complexes with a Variable N-Donor Moiety: Kinetic/Thermodynamic Investigation Reveals Ligand Induced Changes in Reaction Mechanism

Lucas, Heather R.; Meyer, Gerald J.; Karlin, Kenneth D.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 22/09/2010 EN
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17.722816%
The kinetics, thermodynamics, and coordination dynamics for O2 and CO 1:1 binding to a series of pseudo-tetradentate ligand-copper(I)-complexes (DLCuI) to give CuI/O2 and CuI/CO product species are reported. The DLCuI series possess an identical tridentate core structure where the cuprous ion binds to the bispicolylamine (L) fragment. DL also contains a fourth variable N-donor moiety {D = benzyl (Bz); pyridyl (Py); imidazolyl (Im); dimethylamino (NMe2-); tert-butylphenyl pyridyl (TBP); quinolyl (Q)}. The structural characteristics of DLCuI-CO and DLCuI are detailed, with X-ray crystal structures reported for TBPLCuI-CO, BzLCuI-CO, and QLCuI. Infrared studies (solution and solid-state) confirm that DLCuI-CO possess the same four-coordinate core structure in solution with the variable D moiety ‘dangling’, i.e. not coordinated to the copper(I) ion. Other trends observed for the present series appear to derive from the degree to which the D-group interacts with the cuprous ion center. Electrochemical studies reveal close similarities of behavior for ImLCuI and NMe2LCuI (as well as for TBPLCuI and QLCuI), which relate to the O2-binding kinetics and thermodynamics. Equilibrium CO binding data (KCO, ΔH°, ΔS°) were obtained by conducting UV-visible spectrophotometric CO titrations...