ELECTROANALYTICAL FLOW MEASUREMENTS

A review based on 94 cited original papers describes recent achievements in application of different electrochemical detection in flow analysis, injection techniques of flow analysis, liquid chromatography and capillary electrophoresis.

Validating MODIS-derived land surface evapotranspiration with in situ measurements at two AmeriFlux sites in a semiarid region

Reducing uncertainties in the estimation of land surface evapotranspiration (ET) from remote-sensing data is essential to better understand earth-atmosphere interactions. This paper demonstrates the applicability of temperature-vegetation index triangle (T-s-VI) method in estimating regional ET and evaporative fraction (EF, defined as the ratio of latent heat flux to surface available energy) from MODIS/Terra and MODIS/Aqua products in a semiarid region. We have compared the satellite-based estimates of ET and EF with eddy covariance measurements made over 4 years at two semiarid grassland sites: Audubon Ranch (AR) and Kendall Grassland (KG). The lack of closure in the eddy covariance measured surface energy components is shown to be more serious at MODIS/Aqua overpass time than that at MODIS/Terra overpass time for both AR and KG sites. The T-s-VI-derived EF could reproduce in situ EF reasonably well with BIAS and root-mean-square difference (RMSD) of less than 0.07 and 0.13, respectively. Surface net radiation has been shown to be systematically overestimated by as large as about 60 W/m(2). Satisfactory validation results of the T-s-VI-derived sensible and latent heat fluxes have been obtained with RMSD within 54 W/m(2). The simplicity and yet easy use of the T-s-VI triangle method show a great potential in estimating regional ET with highly acceptable accuracy that is of critical significance in better understanding water and energy budgets on the Earth. Nevertheless, more validation work should be carried out over various climatic regions and under other different land use/land cover conditions in the future.

The time delay in electrochemical measurements of a finite-volume system

For a sphere electrode enclosed in finite-volume electrolyte, the measured current will deviate from the result predicted by the semi-infinite diffusion theory after some time. By random-walk simulation, we compared this time to the one needed for diffusion layer to reach electrolyte boundary, and revealed a clear signal delay of electrochemical current. Further we presented a quantitative description of this delay time. The simulation results suggested that the semi-infinite diffusion theory can even be applied when the theoretical diffusion layer grows to 1.28 electrolyte thicknesses, with an accuracy better than 0.5%. We attributed this time delay to the molecules' finite propagation velocity. Finally, we discussed how this delay can influence and facilitate the following electrochemical detection towards the nanometer and single-cell scale.

Structural diversity of lanthanide-amino acid complexes under near physiological pH conditions and their recognition of single-stranded DNA

We reported here four structures of lanthanide-amino acid complexes obtained under near physiological pH conditions and their individual formula can be described as [Tb-2(DL-Cys)(4)(H2O)(8)]Cl-2 (1), [Eu-4(mu(3)-OH)(4)(L-Asp)(2)(L-HAsp)(3)(H2O)(7)] Cl center dot 11.5H(2)O (2), [Eu-8-(L-HVal) (16)(H2O)(32)]Cl-24 center dot 12.5H(2)O (3), and [Tb-2(DL-HVal)(4)(H2O)(8)]Cl-6 center dot 2H(2)O (4). These complexes showed diverse structures and have shown potential application in DNA detection. We studied the interactions of the complexes with five single-stranded DNA and found different fluorescence enhancement, binding affinity and binding stoichiometry when the complexes are bound to DNA.

Evaluation of coverage of self-assembled polyaniline-analogue monolayer by capacitance measurements

A method is developed to estimate the coverage of an electropolymerizable aniline-analogue monolayer (mixture of 2- and 3-aminophenols, 2/3-ATP) by measuring the charge capacitance of the electrode (theta = 81%). The technique of filling the uncovered area (defect sites) of the aniline-analogue monolayer with alkanethiols with long alkane chains (1-decanethiol, 1-DT) has been used to determine the coverage. The dielectric constant (permittivity) of the PANI-analogue monolayer was determined to 8.4. Adsorption kinetics of 1-DT was also studied, and the value of the rate constant of the secondary adsorption was measured to 0.9 mol(-1) dm(3) s(-1).

Effect of La3+ on structure and electrochemical reaction of microperoxidase-11 in imitated physiological solution

In this paper, the interaction mechanism between La3+ and microperoxidase-11 (MP-11) in the imitated physiological solution was investigated with the electrochemical and spectroscopic methods. It was found that when the molar ratio of La3+, and MP-11 is low, such as 2, La3+ can coordinate with oxygen in the propionic acid group of the heme group in the MP-11 molecule, forming the La-MP-11 complexes and leading to the increase in the non-planarity of the porphyrin cycle in the heme group and then the increase in the extent of exposure of the electrochemically active center, Fe(I I I) in the porphyrin cycle of the heme group. The increase in the extent of exposure of the electrochemically active center, Fe(III) in the porphyrin cycle of the heme group would increase the reversibility of the electrochemical reaction of the La-MP-11 complexes and its electrocatalytic activity for the reduction of H2O2. The results of the chromatographic analysis demonstrated that the average molar ratio of La3+ and MP-11 in the La-MP-11 complexes is 1.62.When the molar ratio of La3+ and MP-11 is high, such as 3, La3+ would shear some amino acid residues of the peptide of MP-11. Therefore, many La3+ ions can bind to the oxygen- and/or nitrogen-containing groups in the sheared amino acid residues except coordinating with the sheared and non-sheared MP-11 molecules.

Interaction between La3+ and MP-11 in the physiological solution

In this paper, the interaction between La3+ and microperoxidase-11 (MP-11) in the imitated physiological solution was investigated with the electrochemical method, circular dichroism (CD) and ultraviolet-visible (UV-vis) absorption spectroscopy. It was found that the interaction ways between La3+ and MP-11 are different with increasing the molar ratio of La3+ and MP-11. When the molar ratio of La3+ and MP-11 is less than 2, La3+ mainly interacts with the metacetonic acid group of the heme group in the MP-11 molecules, causing the increase in the non-planarity of the porphyrin cycle in the heme group and the decrease in the content of the random coil conformation of MP-11. These structural changes would increase the exposure extent of the electrochemical active center of MP-11 and thus, La3+ can promote the electrochemical reaction of MP-11 and its electrocatalytic activity for the reduction of H2O2 at the glassy carbon (GC) electrode. However, when the molar ratio of La3+ and MP-11 is larger than 3, except binding to the carbonyl oxygen of the metacetonic acid group in the heme group, La3+ interacts also with the oxygen-containing groups of the amides in the polypeptide chains of the MP-11 molecules, leading to the increase in the contents of the random coil conformation in the peptide of the MP-11 molecule, comparing with that for the molar ratio of less than 2.

Determination of electron mobility in a blue-emitting alternating block copolymer by space-charge-limited current measurements

In this communication we analyse current versus voltage data obtained using one carrier injection at metal/polymer/metal structures, The used polymer is a soluble blue-emitting alternating block copolymer, Our experimental results demonstrate that the electron current is limited by a large amount of traps with exponential energy distribution in the copolymer. The electron ;mobility of 5.1 x 10(-10) cm(2)/V s is directly determined by space-charge-limited current measurements. The electron mobility is at least three orders of magnitude smaller than that for holes in the copolymer. (C) 1999 Elsevier Science Ltd. All rights reserved.

SAXS measurements of the interface in polyacrylate and epoxy interpenetrating networks with fractal geometry

The interface thickness in two-component interpenetrating polymer networks (IPN) system based on polyacrylate and epoxy were determined using small-angle X-ray scattering (SAXS) in terms of the theory proposed by Ruland. The thickness was found to be nonexistent for the samples at various compositions and synthesized at variable conditions-temperature and initiator concentration. By viewing the system as a two-phase system with a sharp boundary, the roughness of the interface was described by fractal dimension, D, which slightly varies with composition and synthesis condition. Length scales in which surface fractals are proved to be correct exist for each sample and range from 0.02 to 0.4 Angstrom(-1). The interface in the present IPN system was treated as fractal, which reasonably explained the differences between Pored's law and experimental data, and gained an insight into the interaction between different segments on the interface. (C) 1997 Elsevier Science Ltd.

Direct electron transfer to cytochrome c oxidase in self-assembled monolayers on gold electrodes

The monolayer of cytochrome c oxidase maintaining physiological activity and attached covalently to the self-assembled monolayers of 3-mercaptopropionic acid (MPA) on a gold electrode was obtained. The results of cyclic voltammetry show that direct electron transfer between cytochrome c oxidase and the electrode surface is a fast and diffusionless process. MPA has a dual role as both electrode modifier and the bridging molecule which: keeps cytochrome c oxidase at an appropriate orientation without denaturation and enables direct electron transfer between the protein and the modified electrode. Immobilized cytochrome c oxidase exhibits biphasic phenomena between the concentration of the electrolyte and the normal potentials; meanwhile its electrochemical behavior is also influenced by the buffer components. The quasi-reversible electron transfer process of cytochrome c oxidase with formal potential 385 mV vs. SHE in 5mM phosphate buffer solution (pH 6.4) corresponds to the redox reaction of cyt a(3) in cytochrome c oxidase, and the heterogeneous electron transfer rate constant obtained is 1.56 s(-1). By cyclic voltammetry measurements, it was observed that oxidation and reduction of cytochrome c in solution were catalyzed by the immobilized cytochrome c oxidase. This cytochrome c oxidase/MPA/Au system provides a good mimetic model to study the physiological functions of membrane-associated enzymes and hopefully to build a third-generation biosensor without using a mediator.

STUDIES ON THE COORDINATION OF TB(III) AND CA(III) WITH AMINO-ACID UNDER THE PHYSIOLOGICAL CONDITION

Tb(Ca)-glycine, Tb(Ca)-alanine, Tb(Ca)-glycine-alanine systems were studied by potentiometry (37%, I = 0.15 mol/L NaCl). The stability constants of complexes and distribution of species in ternary system were obtained. The results show CaTb for the ternary complexes. The species convert each other with changing pH values.

LIFETIME MEASUREMENTS OF SOME TMI ODD PARITY LEVELS

Eleven new lifetimes of odd parity excited energy levels in four configurations: 4f12 5d 6s 6p, 4f12 6S2 6p, 4f13 5d 6s and 4f13 6s 7s of atomic thulium have been mesured with atomic-beam laser spectrocopy. Two pulsed dye lasers are used for stepwise excitation and the time-resolved fluorescence decay was used to determine lifetime values. The accuracy of the measurements is about 10%.