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Critical evaluation of the coated wire glucose sensor

University of British Columbia

Previous literature indicated that a coated wire glucose sensor consisting of a liquid ion exchange material and an ionic glucose complex coated on a platinum wire responded to changes in glucose concentration because of a shift in the equilibrium between the associated and dissociated forms of the Ba-glucose complex, which could be monitored electrochemically. The purpose of this work originally was to investigate experimentally the implantation of a coated wire glucose sensor in subcutaneous tissue for monitoring glucose concentration. A glucose sensor is needed to continuously monitor blood glucose levels in conjunction with an insulin delivery system for the control of type one diabetes. From the data gathered in this research, it was observed that glucose addition does not cause any change in the potentiometric response of the coated wire electrodes either transiently or at steady state. Despite the rather poor reproducibility of some of the measurements reported in the literature, this work has been able to explain how the misinterpretation of the reported results could have occurred. The objective of the thesis as research progressed changed to the explanation of the transient behavior of the glucose sensor and to the determination of what is actually causing the potentiometric response. Experiments were conducted to determine the role played by each component which was used in the preparation of the electrode. The effect of each individual component was observed potentiometrically and it was found that by removing the quaternary ammonium salt (Ali-quat) the potentiometric drift stopped. By replicating identical electrodes, variability of the potentiometric signals was recognized. It was suspected that the variability could be caused by reactions at the platinum surface. Potentiometric measurements were made using platinum wires which had been used in constructing the previous electrodes. The levels and types of contamination and oxidation of the surfaces were studied with X-ray photoelectron spectroscopy and the results were compared to their individual potentiometric responses. Irreproducibility of platinum surfaces was found to be effected by the degree of contamination and the type and amount of oxidation to which the platinum surfaces were exposed. By placing a coating around the platinum wire, the surface saturates with oxygen causing blockage of active sites of the platinum, resulting in a different morphology of the surface which can result in a different equilibrium potential. It was observed that : 1. Using platinum wires potentiometrically can cause irreproducible signals. 2. Coated wire sensors are not suitable for glucose sensing. It is suggested that the observed changes of potential which had been reported in the literature were actually due to the exchange of Cl⁻ and (PO₄)³⁻ ions in the ion-exchange material as well as to a platinum surface potential. This thesis confirms that coated wire glucose sensors as described in the literature do not work according to the proposed mechanism. This electrode is not in principle a reliable working electrode. The operation of the electrode is independent of any changes in glucose concentration.

Text

2011-02-18

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http://hdl.handle.net/2429/31512

Chemical and Biological Engineering

University of British Columbia

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