Redox potential combination sensor

L. B. Kriksunov, C. Liu and D.D. Macdonald. Oxygen, Hydrogen and Redox Potential Combination Sensors for Supercritical Aqueous Systems, Proc. 1 IntT. Workshop Supercritical Water Oxidation, Amelia Island Plantation, FL, February, 1995. 

Future trends are expected to focus on the development of multirespon-sive polymers that combine a LCST phase transition with another response, such as redox, pH or the presence of certain analytes. As such, the phase transition can be induced isothermally by the second response parameter to further broaden the application potential for sensors and biomedical applications. Furthermore, the development of novel polymers with UCST behavior will be an important research topic for the coming years as well as the development of applications of UCST polymers. Finally, I am convinced that the application potential of both LCST and UCST polymers for smart materials will be significantly broadened in the near future. 

This sensor was developed for monitoring oxygen, hydrogen, and redox potential in aqueous solutions at temperatures extending above the critical temperature of water. The sensor is based on a combination of two electrodes that are structurally combined into a single unit a redox insensitive yttria stabilized zirconia (YSZ) membrane pH electrode, of the type described in the previous section, and a Pt electrode. The potential of this latter electrode is, of course, sensitive to oxygen and hydrogen concentration and to pH. The potential of the YSZ membrane can be represented as... 

The potentiostatic multi-pulse potentiometry described here allows the dynamic measurement of potentials. The advantages of this method are the short time required for the analysis and the low noise of the signal. The "ancestor" of this technique, enzyme chronopotentiometry [7 ], posed problems of reproducibility when it was applied to the immobilized redox polymer. The excellent reproducibility of our method is clearly shown in fig. 3b. These techniques were the fundamental developments to conceive redox-FETs for the first time. After immobilization of NAD -dependent dehydrogenases covalently on the surface of the transducer the enzymatically produced NADH would be catalytically oxidized in situ by the polymeric mediator. To this very compact combination the substrate and NAD+ as cosubstrate have to be applied externally. The coimmobilization of the coenzyme NAD+ would lead to reagentless sensors. This is a subject of forthcoming investigations... 

Figure 38. Potential of the combination redox sensor in 0.01 m HCl at a temperature of 400°C and at a pressure of3000psi (204 atm) upon cycling the concentrations of hydrogen and oxygen in the feed. Reprinted from Ref. 5, Copyright (1997) with permission by Elsevier.

Interference from water was eliminated by calibrating with a redox peak that was not proton related. Gas phase analysis showed strong redox signals for TNT and DNT and demonstrated that ILs serve as a preconcentrator to improve the sensitivity of very low vapor pressure analytes. These examples demonstrate that the combination of IL materials and electrochemical transducers overcomes many obstacles in forming an effective sensor system. IL-electrochemical sensors are also well suited for miniaturization and can be fabricated with very low cost. In contrast to nonspecific transducers such as a surface acoustic wave device, arrays of amperomertic and EIS transducers allow a secondary perturbation (e.g., potential) that enhances selee-tivity and increases analytical information content without increasing the number of physical sensor elements. 

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