Model analysis of enzyme monolayer- and bilayer-modified electrodes: the transient response.

The transient responses of enzyme monolayer- and bilayer-modified electrodes were theoretically analyzed. The response was numerically simulated with parameters used in the foregoing paper. The transient process consists of three elementary steps: enzymatic reaction step, substrate and mediator consumption step, and charge-carrying mediator accumulation step. Contributions of the parameters to these elementary steps and the overall response were examined. Experimental observations are successfully interpreted in terms of the theoretical results.     

可同步旋转双转子稳态响应数值分析

介绍可同步微速差双转子动平衡实验装置,建立该转子系统的动力学模型。应用有限元方法分析其临界转速,不平衡响应等动力学特性。比较双转子系统在单个转子运行(内转子或者外转子)、内外转子同步运行和内外转子微速差运行三种不同工作状态下的动力学特性。并对装置的动力学特性进行了实测,结果表明该实验装置满足了可同步微速差双转子动平衡原理研究的需要。     

Acid stability of carbon paste enzyme electrodes

This note reports on the unusual protection of several enzymes against harsh pH conditions provided by carbon paste electrodes. Both glucose oxidase and polyphenol oxidase carbon paste amperometric biosensors display a remarkable resistance to acid deactivation compared to conventional biosensors prepared by electropolymeric entrapment of enzymes. For example, the carbon paste enzyme electrodes fully retain their activity upon stressing in strongly acidic conditions (pH approximately 2.0-2.5) for prolonged periods, where conventional (polymer-based) biosensors rapidly lose most of their response. Such unusual acid stability of carbon paste enzyme electrodes is attributed to the "pH memory" of enzymes in the hydrophobic paste environment, to the barrier to hydronium ions provided by the pasting liquid and to decreased conformational mobility.     

Modified electrodes prepared from ferrocene functionalized silicate xerogels: analysis of the electrochemical response in isotropic and fractal structural systems

New xerogels functionalized by ferrocenes as redox probes have been prepared using various types of catalysis. The analysis of the electrochemical response in the case of modified electrodes allows us to get an insight into the gel nanostructure which has been found to be isotropic in the case of acidic catalysis, and fractal in the case of fluoride catalysis, provided that the amount of functionalized silanes is not too high. Indeed the results of electrochemical measurement prove that the fractality of functionalized hybrid silica is general in the case of basic catalysis, except in the case of very high functional moieties loading, where the electrode become essentially organic and isotropic. On the other hand, in the case of acidic catalysis, whatever the amount of organic material, the hybrid gel remains isotropic, and a classical electron pseudo-diffusive behaviour is observed. The diffusion coefficient values extracted from the current-time slopes do not vary monotonically with the amount of redox probe in the case of fractal xerogel structures, which is indicative that not all the functional moieties are electroactive in that case.     

求解圆柱壳稳态谐响应的微分求积单元法

本文以Donnell经典壳体振动微分方程为基础,研究微分求积单元法(DQEM)在圆柱壳稳态谐响应计算中的应用。研究结果表明：微分求积单元法可较为方便的处理多种边界条件;与有限元法相比,微分求积单元法直接面向问题的微分方程,可用较少的节点获得较高的计算精度,计算效率较高。本文结果可为微分求积单元法在结构动力响应问题求解中的应用提供参考。     

Fuzzy analysis for steady-state availability: a mathematical programming approach

Steady-state availability has been widely applied as a measure to evaluate the reliability characteristics of a repairable system. However, it is generally not realistic to make assumptions concerning failure time and repair time distributions. Thus, this article has developed a procedure to construct the membership function for fuzzy steady-state availability. Based on Zadeh’s extension principle, a pair of mathematical programs is formulated to find α-cuts of fuzzy steady-state availability. An explicit closed-form expression for the membership function is derived by taking the inverse function of the $\alpha$-cut. To illustrate the interpretation and practical value of fuzzy availability in real-world applications, several numerical examples are provided and discussed.     

A qualitative and quantitative study of steady-state response of towed floating bodies

The problem of multiple equilibria in steady towing of a floating body is considered. The two coordinates of the towing point are the main bifurcation parameters. An approach to bifurcation of steady-state equilibria using singularity theory reveals all qualitatively different bifurcation diagrams that occur locally. It is shown that these bifurcation problems may be viewed as paths in the universal unfolding space of the cusp catastrophe. The organizing centre for the towing problem is the pitchfork singularity. Numerical calculations suggest that results obtained by singularity-theory techniques are valid globally.     

Time-dependent phenomena in the potential response of ion-selective electrodes treated by the Nernst-Planck-Poisson model. 1. Intramembrane processes and selectivity

The variability of selectivity coefficients, resulting from potential changes over time and the concentration ratio of primary to interfering ions, impedes many practical applications of ion-selective electrodes (ISEs). Existing theoretical interpretations of ISE selectivity are restricted by severe assumptions, such as steady state and electroneutrality, which hinder theorizing on this problem. For this reason, for the first time, the Nernst-Planck-Poisson equations are used to predict and visualize the selectivity variability over time and the concentration ratio. Special emphasis is placed on the non-Nernstian response in the measurements with liquid-ion-exchanger- and neutral-carrier-based ISEs. The conditions under which measured selectivity coefficients are true (unbiased) are demonstrated.     

A qualitative and quantitative study of steady-state response of towed floating bodies

The problem of multiple equilibria in steady towing of a floating body is considered. The two coordinates of the towing point are the main bifurcation parameters. An approach to bifurcation of steady-state equilibria using singularity theory reveals all qualitatively different bifurcation diagrams that occur locally. It is shown that these bifurcation problems may be viewed as paths in the universal unfolding space of the cusp catastrophe. The organizing centre for the towing problem is the pitchfork singularity. Numerical calculations suggest that results obtained by singularity-theory techniques are valid globally.     

Cyclic voltammetry response of an undoped CVD diamond electrodes

The polycrystalline undoped diamond layers were deposited on tungsten wire substrates by using hot filament chemical vapor deposition (HFCVD) technique. As a working gas the mixture of methanol in excess of hydrogen was used. The morphologies and quality of as-deposited films were monitored by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy respectively. The electrochemical activity of the obtained diamond layers was monitored by using cyclic voltammetry measurements. Analysis of the ferrocyanide–ferricyanide couple at undoped diamond electrode suggests that electrochemical reaction at diamond electrode has a quasireversibile character. The ratio of the anodic and cathodic peak currents was always close to unity. In this work we showed that the amorphous carbon admixture in the CVD diamond layer has a crucial influence on its electrochemical performance.     

How to assess the limits of ion-selective electrodes: method for the determination of the ultimate span, response range, and selectivity coefficients of neutral carrier-based cation selective electrodes

The span and range of an ion-selective electrode (ISE) has been identified by IUPAC as a potential or activity difference between the upper and lower detection limits of the electrode. Once the span is known, the ultimately attainable detection limit of the ISE can be calculated using its theoretical response slope. In this paper, we propose an original method for the determination of the ultimate span and response range of ISEs. The simple measurement of span is recommended to aid the fast screening of novel ionophores and help to focus optimization processes to the most promising candidates. The measurement of span is combined with a generally applicable procedure for the determination of the three seminal parameters of ISEs: the response slope, the ultimate selectivity coefficients, and detection limit. In the proposed procedure, following the span measurement, two subsequent exponential dilution experiments are completed in which the responses of the electrode for the primary and the interfering ions are tested using a solution of a discriminated ion and deionized water as diluting electrolytes in consecution. The advantages and the practical usefulness of the proposed methods and procedures are demonstrated through the evaluation of the performance characteristics of novel and well-characterized ionophore-based potassium and calcium sensors.     

Effect of ionic strength on the behavior of amperometric enzyme electrodes mediated by redox hydrogels

The electron-transfer behavior of electroactive hydrogels formed by cross-linking ferrocene poly(allylamine) (Fc-PAA) and glucose oxidase is investigated as a function of electrolyte ionic strength using several techniques. Cyclic voltammetry and electrochemical impedance spectroscopy show that the quantity cD(e)(1/2) increases with electrolyte concentration. Enhancement of enzyme catalysis for the oxidation of glucose mediated by Fc-PAA is also apparent at higher KNO(3) concentration. The electroactive redox center concentration, c, and the diffusion coefficient due to electron hopping in the gel, D(e), are independently measured by chronoamperometry at ultramicroelectrodes. Larger electrolyte ionic strength induces an increase in electroactive redox center concentration while D(e) slightly decreases. These results are rationalized in terms of the electrostatic interactions within the redox gel backbone due to water and ion exchange with the external electrolyte, producing swelling and shrinking of the hydrogel.     

Numerical analysis of the steady-state behaviour of a MOS field effect transistor

A numerical scheme is developed to simulate the non-isothermal steady-state behaviour of a MOS field effect transistor. In a desire to develop a fast, stable numerical scheme, physical instabilities were eliminated by using a simplified device model. The numerical technique developed permits a computer solution of the majority carrier transport equation, the nonlinear heat conduction equation, in which the heat generation term is obtained from the solution of the transport equation, and a number of auxiliary differential equations. The simplified model of the MOS transistor adopted will not, of course, produce any information on the actual operation of the short channel MOS transistor of practical interest today, but the numerical scheme can be extended to simulate short channel models that are of great practical interest.     

Conductivity response of porous electrodes supported on perfluorosulfonic acid membranes to acidic gas mixtures.

Gold or platinum films deposited on a Nafion membrane were used to measure surface conductance of the membrane. Acidic gases such as HCI or SO2, introduced as mixtures with an inert gas, were found to affect the conductance of the membrane surface facing this mixture while the other side of the membrane was constantly supplied with a moist inert gas. The relative conductance (G/Go) is most affected within the 0-0.5 and 3-4.5 vol % ranges both for HCI and SO2. The effect is higher for the Pt-deposited than it is for the gold-deposited membrane: for an HCI concentration of 1.5 vol %, G/Go is 5 for Pt deposited on Nafion while it is only 1.3 for gold deposited on Nafion. This is attributed to different geometries and porosity of the Pt and Au electrodes. No conductance response was observed for CO2 which yields with water a much weaker acid than those formed by HCI and SO2.     

Load modelling for steady-state and transient analysis of low-voltage DC systems

The modelling of loads for low-voltage (LV) DC system studies is treated. Results from measurements on 63 different loads supplied with DC are presented. The measurement results are used to derive simplified load models, which can be used for steady-state and transient analysis of LV DC systems. These load models extend existing standards for load flow and short circuit current calculations.