S. J. Yao

Potentiometric Measurement of Glucose Concentration with an Immobilized Glucose Oxidase/Catalase Electrode

A series of enzyme electrodes for measurement of glucose have been constructed. The electrodes contain glucose oxidase immobilized on platinum, either with or without co-immobilization of catalase. When placed in buffered glucose, the enzyme electrodes show a potentiometric response to glucose with respect to a Ag/AgCl reference electrode. This response is reproducible in the physiologic range of glucose concentrations. The immobilization technique, some of the environmental variables such as oxygen concentration and pH, and several compounds that might interfere with the selectivity of the enzyme electrodes for glucose have received preliminary study. This direct potentiometric approach is undergoing further evaluation to determine the basic electrochemical mechanism responsible for the potentiometric signal and whether it can be adapted for continuous in vivo monitoring of the glucose concentration in body fluids.

Immobilized glucose oxidase in the potentiometric detection of glucose

Previous work has shown that glucose oxidase can be immobilized on platinum to give an electrode that responds potentiometrically to glucose over the clinically useful range of about 10–250 mg glucose/100 mL. The present studies were carried out with electrochemically pretreated platinum and with gold or porous graphite substituted for the platinum support. The presence of the enzyme gave a significantly enhanced potentiometric response over that obtained with the bare support for both the pretreated platinum and the porous graphite, but not with gold. However, with platinum the potentiometric response became more negative with increasing glucose concentration. With porous graphite, the potential changed in the positive direction as the glucose concentration was increased. Hysteresis was demonstrated for the platinum-enzyme electrode. Mass transfer measurements with a rotating ring-disc electrode (RRDE) showed measurable diffusional resistances to the transport of a model electroactive compound (potassium ferrocyanide) through a matrix of immobilized enzyme attached to the disc of the RRDE. These results are part of a larger study to define the source of the potentiometric response by examining the roles of the support and the mass transfer resistances through the immobilized enzyme matrix.

The Low-Potential Approach of Glucose Sensing

Study of glucose sensing using a smooth Pt electrode and cyclic voltammetry (CV) at low potentials revealed two well-defined, redox current peaks of adsorbed species. At 370 C a reduction peak occurs at ¿0.80 V versus Ag/AgCl and an oxidation peak at ¿0.72 V. Furthermore, the redox couple has been shown to be reversible, involving a simple, direct, electron-transfer process under diffusion control. The reaction is not complicated by secondary chemical (nonelectro-chemical) reactions. Based on this approach, a square wave voltammetry (SWV) technique has been adopted to obtain pure faradaic currents (noncapacitance current), fast response times, and enhanced sensitivity-for the designated reversible redox peaks. The SWV technique also permits the employment of much smaller-sized Pt wire electrodes (0.5 mm diameter) for glucose sensing. Plots of current versus glucose concentration in Krebs-Ringer solutions, for both redox peaks, are linear for glucose levels in the 70-350 mg/dl range. Studies of glucose concentration variations in the ultrafiltrate of human serum have also demonstrated a linear relationship between current and glucose level. Thus far, the linearity is limited to a narrow glucose range (80¿180 mg/dl), but this is within the critical domain for diabetic control. The advantages of the low-potential approach using SWV is an improvement over CV and could be a promising method for implantable microelectrode glucose sensing.

Direct coupling of glucose oxidase to platinum and possible use forin vivo glucose determination

Glucose oxidase was attached to platinum-platinum oxide screens via alkylamine silaneglutaraldehyde coupling. The amount of immobilized enzyme was equivalent to 0.0031 µg of soluble glucose oxidase per cm2 of screen surface. The platinum-silane-glutaraldehydeenzyme screens were tested potentiometrically in buffered glucose solutions, with respect to a Ag/AgCl reference electrode. The results were expressed as the difference in potential for the enzyme screens placed in buffer containing glucose and placed in plain buffer. This difference in potential was related linearily to the logarithm of the glucose concentration over the range 5–150 mg glucose/100 ml. The source of the potential may be due to the decomposition of hydrogen peroxide produced by the glucose oxidase catalyzed oxidation of glucose. The approach is being studied for possible development of an implantable sensor for continuousin vivo monitoring of glucose levels.

262 - Quantitation of glucose concentration using a glucose oxidase-catalase electrode by potentiometric measurement

Abstract Glucose oxidase and catalase were immobilized in polyacrylamide gel around a platinum screen and used to measure the concentration of glucose in 0.1 M phosphate buffer, pH 7.3, at 37°C. This enzyme electrode produced a direct potentiometric signal with reference to a standard Ag|AgCl electrode. The potential difference was linearly related to the logarithm of the concentration of glucose over the range of 3–40 cg/dm3 (mg/dl). The electrode was stable for at least 32 hours at 37°C. Hydrogen peroxide, generated by the oxidation of glucose and depleted by decomposition and diffusion, was the apparent source of the potentiometric signal. Direct potentiometric measurement has the advantages of simple readout, continuous detection, and small electrode size, and does not require external polarising potential. This approach should be suitable for the continuous in vivo monitoring of blood glucose levels if the upper limit of specificity for glucose (presently 40 mg/dl) can be extended.

496—Cyclic voltammetric studies on the application of carbon elctrodes in the determination of oxalic acid

Abstract The application of carbon paste and glassy carbon electrodes in the analysis of oxalic acid was investigated by comparing the characteristics of cyclic voltammograms of oxalic acid obtained in various supporting electrolytes (acetate, borate, citrate, phosphate, etc.). when a semi-micro carbon paste electrode (area 0.49 cm2) was used, the oxalic acid was oxidized at +1.0 to +1.2 V vs Ag|AgCl yielding current in the μA range (scan rate 50 mV/s) for oxalic acid concentration of approximately 10−4 M. Oxalic acid oxidation was observed under both acidic and alkaline conditions. The presence of chloride ions or oxygen did not have any deleterious effect on the electrode response. The peak current was reproducible for repeated scans obtained with the same electrode after brief stirring. The glassy carbon electrode was found to be less suitable for oxalic acid oxidation studies because the peaks in the voltammograms were poorly defined and the current response was markedly reduced. These investigations suggest that carbon paste electrodes are sensitive and stable for oxalic acid oxidation studies. A plot of the peak currents obtained with carbon paste electrode for different concentrations of oxalic acid between 1×10−4 M to 1×10−3 M was linear and reproducible. It is suggested that a flow through carbon paste electrode coupled to a chromatographic column can be used in the development of a sensitive method for oxalic acid analysis in biological samples.

Potentiometric Enzymatic Measurement of Glucose: Possible in vivo Use

The development of a sensor for continuous in vivo measurement of glucose concentrations in body fluids, without the necessity for withdrawal of fluid from the body, is an important clinical and research need for improved understanding and treatment of diabetes. Several approaches for the development of such a sensor are discussed elsewhere (1). During our studies with glucose oxidase, we observed that a glucose oxidase-catalase-platinum electrode gave a direct Potentiometric response, with respect to a reference electrode, when both electrodes were placed in the same beaker of buffered glucose solutions. We have pursued these observations further to assess the applicability that this approach may have for the development of the needed in vivo glucose sensor. The purpose of this paper is to summarize our results to date.