Kinetic response of an oxygen-selective electrode evaluated for the quantification of oxygen

An evaluation of the transient response of a membrane-based amperometric electrode for the quantification of oxygen in aqueous and gaseous phases is described. Data collected during the early part of the responses are used to compute the steady-state signals that would be measured if the responses were monitored to steady state. Both first-order and variable-order models were used successfully to fit data for aqueous samples; only the variable-order model worked successfully with data for gaseous samples. Because the shapes of response curves did not permit steady-state signals to be measured, comparisons are based on measured values of current at fixed times in each response. The slopes of computed vs. measured signals varied from 0.90 to 1.03 for fits of data over 3–5 half-lives with both models. Calibration graphs of computed current vs. concentrations were linear for similar fitting ranges and the least-squares parameters were similar to those for current measured at different fixed points in time.     

Extended detection range for an optical enzymatic glucose sensor coupling with a novel data-processing method

A new data-processing method was established and applied for optical enzymatic glucose sensing, in which oxygen and glucose were simultaneously consumed. The oxygen level remaining in the detection system, which was equal to the difference between the initial and consumed oxygen concentrations, could be measured using fluorescent oxygen indicators immobilized in the sensing layer. It was deduced that the ratio of I ₀ and I was inversely proportional to glucose concentration, where I ₀ is the maximum fluorescence intensity in various glucose solutions, and I is the fluorescence intensity at various concentrations of glucose. Using the new data-processing method, the detection range of the calibration curve method was extended from 0 to 1.2 mmol L^?1, which was enlarged about 2–3 folds over that in ordinary approaches. The prepared glucose sensor could be directly applied to detect high concentrations of glucose.     

Differences between signal currents for both polarities of applied voltages on cavity ionization chambers

A difference between the surface potential of the charge collecting electrode and that of the guard electrode of an ionization chamber changes the charge collecting volume depending on the applied voltage. If the difference is large, the saturation curve of the signal current shows a maximum at a low applied voltage. Even when there is no electrical or mechanical defect, the signal current from a parallel plate ionization chamber irradiated with ⁶⁰Co γ-rays increases or decreases with the applied voltage beyond the recombination region depending on the polarity of the applied voltage. The variation in the signal current is explained as a result of the change in the stopping power of air due to the acceleration or deceleration of secondary electrons. These electrons are emitted from the polarizing electrode towards the collector as a result of Compton scattering. In a range of low applied voltages, the signal current from a cylindrical ionization chamber is expected to be smaller for a negative applied voltage than for a positive applied voltage. This is because epithermal electrons are expected to have a higher probability of being lost by back diffusion than positive ions which are originally produced in a thermal equilibrium condition. An experimental result, however, showed no difference in the polarities of the applied voltage. The result may be explained as a consequence of the fact that epithemal electrons do not drift for long distances and maintain their energies.     

Improved ruggedness for flow-based kinetic methods of analysis

This paper introduces a new approach to improve the ruggedness of flow-based kinetic determinations. After reagents and sample are mixed, the flow is stopped, signal vs. time data are recorded by an on-line computer, and curve-fitting methods are used to compute the signal that would be measured if the reaction were monitored to equilibrium. The new approach is evaluated using the Mo(VI) catalyzed oxidation of iodide to triiodide by hydrogen peroxide. Time-dependent concentrations of triiodide are monitored by changes in absorbance at 360 nm. Effects of changes in pH as well as the concentrations of iodide and Mo(VI) are evaluated using four different data-processing options, two of which were expected to exhibit poor ruggedness to changes in these variables and two of which were expected to exhibit significantly improved ruggedness. All data-processing options yielded linear calibration plots and had similar degrees of precision. As expected, the principal difference among the options was the ability to reject effects of changes in experimental variables. The new options are 10- to 70-fold less dependent on changes in pH and concentrations of iodide and molybdate than the more conventional options.     

Improved performances of oxygen potentiometric sensor by electrochemical activation

It is shown that the performances of the oxygen potentiometric sensor can be remarkably improved by electrochemical activation. The electrochemical activation were carried out by firstly employing the oxygen potentiometric sensor as a fuel cell (H₂ vs. O₂), and then running the fuel cell at a low discharge voltage for some time. This activation effect was valid for sensors domestically prepared under different conditions. On the basis of the literature reports and the experimental observations, the possible reasons for the electrochemical activation have been analyzed. The decrease of the concentration of the oxygen ion (oxide) species at the electrode and the decrease of the electrolyte contribution to the total impedance seemed to be the main reasons that have led to the remarkable improvement of the performances of the oxygen potentiometric sensor after the electrochemical activation.     

Photopolarization and depolarization characteristics of poly(N-vinyl carbazole)

The photoelectret properties of poly(N-vinyl carbazole) sandwiched between a conducting glass electrode and an aluminum electrode have been studied. Photoelectret polarization was carried out by simultaneous application of an electric field and ultraviolet illumination. The polarization exhibits a linear dependence on the polarizing voltage, polarization time, and illumination intensity, with saturation effects for large values of these parameters. The depolarization current decay is quite fast, which indicates the absence of deep-trap levels. The ratio of photoelectret charge to the dark polarization charge is found to be 100, which determines the image contrast if the polymer film is used in persistent internal polarization (PIP) electrophotography.     

The current voltage plot of PEM fuel cell with long feed channels

The formula for voltage loss on the cathode side of the polymer electrolyte membrane fuel cell (PEMFC) is derived, taking into account oxygen consumption in the feed channel. Limiting current density due to the oxygen exhaustion along the channel is obtained. Theory predictions are in line with experiments that were performed to test the theory. The results reveal new reserves for the optimization of the cell performance. They also show new in situ electroanalytical options: the study of electrochemical reaction in the catalyst layer via the cell voltage–current plots and via detecting of the feed gas consumption or local current distribution along channel.     

Direct optical sensors: principles and selected applications

In the field of bio and chemosensors a large number of detection principles has been published within the last decade. These detection principles are based either on the observation of fluorescence-labelled systems or on direct optical detection in the heterogeneous phase. Direct optical detection can be measured by remission (absorption of reflected radiation, opt(r)odes), by measuring micro-refractivity, or measuring interference. In the last case either Mach–Zehnder interferometers or measurement of changes in the physical thickness of the layer (measuring micro-reflectivity) caused, e.g., by swelling effects in polymers (due to interaction with analytes) or in bioassays (due to affinity reactions) also play an important role. Here, an overview of methods of microrefractometric and microreflectometric principles is given and benefits and drawbacks of the various approaches are demonstrated using samples from the chemo and biosensor field. The quality of sensors does not just depend on transduction principles but on the total sensor system defined by this transduction, the sensitive layer, data acquisition electronics, and evaluation software. The intention of this article is, therefore, to demonstrate the essentials of the interaction of these parts within the system, and the focus is on optical sensing using planar transducers, because fibre optical sensors have been reviewed in this journal only recently. Lack of selectivity of chemosensors can be compensated either by the use of sensor arrays or by evaluating time-resolved measurements of analyte/sensitive layer interaction. In both cases chemometrics enables the quantification of analyte mixtures. These data-processing methods have also been successfully applied to antibody/antigen interactions even using cross-reactive antibodies. Because miniaturisation and parallelisation are essential approaches in recent years, some aspects and current trends, especially for bio-applications, will be discussed. Miniaturisation is especially well covered in the literature.     

Immobilization of uricase to gas diffusion carbon felt by electropolymerization of aniline and its application as an enzyme reactor for uric acid sensor

Electropolymerizations of aniline and pyrrole solutions containing uricase at a neutral pH was performed to immobilize the enzyme on the surface of the gas diffusion carbon felt, and a selective uric acid sensor was fabricated by combining immobilized enzyme carbon felt and an oxygen electrode with oxygen gas permeable membrane. This carbon felt has a desirable feature for uricase sensing because an extremely efficient supply of oxygen for the enzymatic reaction can be realized due to its porosity permitting a transfer of oxygen. The current response time required from when sample is added until when current reaches the steady-state value is <5 min. The calibration graph of uric acid showed a linear line in a concentration range from 1x10(-5) to 4x10(-4) M with a detection limit of 4x10(-6) M.     

Sensor System for Simultaneous Measurement of Oxygen and Hydrogen Peroxide Concentration During Glucose Oxidase Activity

An amperometric sensor system, based on a repetitive double step potential method at a glassy carbon electrode, has been developed for the simultaneous measurement of hydrogen peroxide and oxygen concentrations. The current measured at a potential of –1 V (vs. Ag/AgCl/saturated Cl^–) corresponds to the sum of the reduction currents of hydrogen peroxide and dissolved oxygen. The current measured at –0.55 V (vs. Ag/AgCl/saturated Cl^–) is due to the reduction of dissolved oxygen to hydrogen peroxide. Alternatively, the concentration of dissolved oxygen can also be determined using a Clark electrode. The concentration of hydrogen peroxide and dissolved oxygen during enzymatic conversion of glucose can be followed on line and be used to control the process.     

Thin layer gas electrode and its application to measurement of dissolved oxygen

A novel thin layer cell equipped with thin layer gas electrode (TLGE) was studied as electrochemical gas sensor for the measurement of dissolved oxygen in water or aqueous solutions. The working electrode (TLGE) is a hydrophobic gas diffusing electrode placed between the cell electrolyte and the solution to be tested. The hydrophobic pores in TLGE serve as a gas chamber. After the sampling period, in which the partial pressure of dissolved oxygen in test solution becomes in equilibrium with that in the gas chamber, the TLGE is polarized with square wave or linear potential signal. Then the Faradaic charge (Q) consumed in depletion of the oxygen contained in pores of TLGE is measured. The main merits of this system are good linearity between the partial pressure of dissolved oxygen in test solution and Q, low zero-reading, negligible liquid-gas difference, convenient calibration and very low temperature coefficient (ca. 0.5%/°C). This technique can also be applied to the measurement of oxygen partial pressure in gas phases.     

Oxygen electroreduction on a granulated layer of a copper-containing electron-ion exchanger

The reduction of dissolved oxygen from a flowing aqueous solution of sodium sulfite on a cathodically polarized granulated layer of a copper-containing electron-ion exchanger is studied. It is established that the polarizing current is distributed over the layer height nonuniformly. A peak current corresponding to the oxygen electroreduction is discovered. The peak shifts from the inlet into the granulated layer to the exit out of it, which is connected with the advance of the concentration front and with an increase in ohmic resistance due to partial oxidation of copper centers. The distribution of the polarizing current is analogous to the distribution of the limiting current of the oxygen reduction, which is determined from polarization curves. The reaching of a stationary position of the peak of the polarizing current and the oxygen reduction degree with time testifies to the onset of a stationary state, at which the current turns limiting and the balance between the arrival and electroreduction of oxygen is fulfilled.     

Modeling and simulation of chemo-electro-mechanical behavior of pH-electric-sensitive hydrogel

A chemo-electro-mechanical multi-field model, termed the multi-effect-coupling pH-electric-stimuli (MECpHe) model, has been developed to simulate the response behavior of smart hydrogels subject to pH and electric voltage coupled stimuli when the hydrogels are immersed in a pH buffer solution subject to an externally applied electric field. The MECpHe model developed considers multiphysics effects and formulates the fixed charge density with the coupled buffer solution pH and electric voltage effects, expressed by a set of nonlinear partial differential governing equations. The model can be used to predict the hydrogel displacement and the distributive profiles of the concentrations of diffusive ionic species and the electric potential and the fixed charge density in both the hydrogels and surrounding solution. After validation of the model by comparison of current numerical results with experiment data extracted from the literature, one-dimensional steady-state simulations were carried out for equilibrium of the smart hydrogels subject to pH and electric coupled stimuli. The effects of several important physical conditions, including the externally applied electric voltage, on the distributions of the concentrations of diffusive ionic species, the electric potential, the fixed charge density, and the displacement of the hydrogel strip were studied in detail. The effects of the ionic strength on the bending deformation of the hydrogels under the solution pH and electric voltage coupled stimuli are also discussed.     

Kinetic aspects of analytical chemistry

A broad view of the role of kinetics in analytical chemistry is presented. Two principal themes are that the role of kinetic methods is much greater than is generally acknowledged and that the most commonly used kinetic approaches make inadequate use of information that is available. After a brief discussion of historical developments that date back to the latter part of the 1800s, primary attention is focused on a classification of measurement and data-processing approaches that have been developed. Approaches are grouped into methods for single components without and with error compensation and methods for resolving two or more components in mixtures. They are further grouped into direct-computation and curve-fitting approaches and into integral and derivative methods within these more general categories. Correlations are then drawn between these approaches as applied to chemical processes and to physicochemical and purely physical processes such as luminescence, electrode responses, and flow systems, including chromatographic processes. It is argued that all analytical methods are either kinetic or equilibrium in character and that kinetic-based methods represent a much larger fraction of the total than is generally recognized.     

Methods of realisation of short rise-times of the cell voltage in pulse polarography

Three methods for producing a voltage step on the dropping mercury electrode cell, as required in pulse polarography, are considered: the double pulse shape of the potentiostat reference voltage, iR-compensation, and charge injection. On an analogue model two methods of iR-compensation are simulated and compared, with reference to their convenience for reducing the rise-time of the polarizing voltage. A charge injection technique is proposed in which the potentiostat reference voltage is derived from the cell voltage after charge injection by taking a sample of it and applying it to the reference input of the potentiostat.     

Enzyme Electrode Sensor for Hydrogen Peroxide

Abstract

An enzymatic sensor was constructed from an oxygen amperometric sensor on whose surface a dialysis membrane containing covalently bonded catalase was set. Immobilization of the enzyme on the dialysis membrane surface was achieved with 2,4-dichloro-6-methoxy-s-triazine, a derivative of cyanuric chloride, which unlike others of its monosubstituted derivatives ensures some advantages. The time of measurement is less than 12 sec, for the kinetic method of the initial slope and one minute for the steady-state method. The sensor responds linearly to hydrogen peroxide in the concentration range 10^?3 - 10^?5 moles/1. The utilization of this sensor in intermittent and continuous operation in a laboratory enzymatic minireactor is discussed.     

Amperometric biosensor for rapid measurement of 3-hydroxybutyrate in undiluted whole blood and plasma

A simple, rapid (< 30 s) electrochemical method for the determination of 3-hydroxybutyrate in whole blood or plasma is described, which uses NAD⁺-dependent d-3-hydroxybutyrate dehydrogenase immobilized at novel platinized carbon electrodes. The steady-state oxidation current produced by enzymatically generated NADH is measured at + 150 mV vs. Ag/AgCl. Enzyme electrodes produced by direct adsorption were stable for at least 3 months. Undiluted whole blood measurement with the sensor was compared with routine spectrophotometric analysis of plasma and perchloric acid extracts of whole blood.     

Numeric Simulation of Natural Convection in Horizontal Layer of Binary Electrolyte at Constant Cell Voltage

Space dissipative structures resulting from convective instability of a binary electrolyte solution between two horizontal plane electrodes at a constant cell voltage are discussed. A system of hydrodynamic nonlinear equations is numerically integrated on a computer by the finite-difference method with spatial grid of 20 × 20. A relation between the applied voltage and Raleigh number is presented. The time dependences of current and electrode polarization during system relaxation towards steady state are obtained. The dependence of the electrode process acceleration due to natural convection on the applied voltage is found. The steady-state isolines of functions of current and concentration are constructed for some voltages. At too high Raleigh numbers, transition is observed to a turbulent mode.     

The response of a colloidal suspension to an alternating electric field

This paper is concerned with the calculation of the complex conductivity K* of a suspension, a quantity which may be determined experimentally from the measurement of the alternating current which flows between a pair of electrodes in the suspension due to an alternating voltage difference. A semi-analytic formula is derived for the complex conductivity of a dilute suspension of spherical particles with small dielectric constant which is reasonably accurate for ?-potentials of less than 50 mV. For such suspensions this formula represents a very economical alternative to the exact computer calculation of K* described by DeLacey and White (ref. 2). Although the formula for K* is derived for particles with fixed surface charge, it is shown that the formula can also be applied to a more general class of suspensions, in which the surface charge arises from the dissociation of a single type of surface group.