Signal catalytic sensors

Inorganic ions, drugs, nucleic acids, proteins, and even cells are successful examples of imprinting. In this way, affinity sensors, receptor sensors, and catalytic sensors based on MIPs have been explored. For affinity sensors, immunosensor-like devices were prepared by a 2D MIP technique with molecular imprinting on chemisorbed alkanethiol SAMs then after necessary procedures, vitamins Ki, K2, E, cholesterol, and adamantine could be detected by the strong electrochemical signals yielded. The sensors for nucleic acids, cholesterol, and catechol derivatives can be fabricated first by their adsorption as a template on the ITO surface and then by the treatment of the electrode with adsorbed template using trimethyl chlorosilane from the gas phase. 

When NOj levels are measured electrochemicaUy, NO and NO2 can lead to opposing signals because NO is oxidized and NO2 tends to be reduced. Moreover, it is preferred to obtain a total NO, measurement instead of only one of the constituents. The latter can be achieved by catalytically equilibrating the feed with oxygen before contact with the sensor by coating an active zeolite layer on top or placing a active catalyst bed in front of the sensor. Both approaches have been demonstrated successfully with a Pt-Y zeohte as active catalyst [74, 75]. The additional advantage of the filter bed is a reduction in the cross-sensitivity with CO due to CO oxidation above 673 K. 

Schalwig et al. have tested the feasibility of using a SiC MOS capacitor sensor containing a contact metal of 40-nm TaSi plus 45-nm Pt to detect NO and HC after the catalytic converter. This was carried out by simulating lean burn engine exhausts [116]. It was observed that the sensor signal increased for NO detection and decreased for HC detection. This could permit this sensor to be used in a sensor array to differentiate these two gases. 

In spite of all their advantages, sensitivity and selectivity, bio-sensors, however, do possess disadvantages connected with thermal and timely instability, high cost of bio-receptors and the need to add substrates in the solution under analysis as signal-generating substances. Some attempts to synthesize and use as receptors chemical organic catalytic systems, which will ensure the required selectivity and response rate, have become the basis for developing enzyme-free sensors [11], or biomimetic sensors. 

For monitoring catalytic (enzymatic) products, various techniques, such as spectrophotometry [32], potentiometry [33,34], coulometry [35,36] and amperometry [37,38], have been proposed. An advantage of these sensors is their high selectivity. However, time and thermal instability of the enzyme, the need of a substrate use and indirect determination of urea (logarithmic dependence of a signal upon concentration while measuring pH) cause difficulties in the use and storage of sensors. 

Because the spatial area with higher temperature on the catalyst surface of one of the samples of the library is very small the detection of catalytic activities through temperature measurement cannot be carried out by direct temperature measurements but only by non-contact methods such as pyrometry or IRT. The IR video camera used here measures the emission at every point of the library in parallel. The detector consists of a 256x256 pixel array of Pt-silicide-IR-sensors. Each pixel delivers a voltage-signal that depends on the infrared radiation and the sensitivity of that pixel (fixed pattern noise). 

Fig. 8.2 Output signal changes of five parallel CO sensors with increasing catalytic reaction temperature. The catalysts whose effluents were analyzed by CO gas sensors were Rh/Sn02, Rh/W03, Rh/Si02, Rh/Ti02, Rh/Ce02 (reproduced by permission of Elsevier from [19]).

Fixed-point sensors, such as the catalytic, electrochemical, or infrared types, will respond rapidly to gas exposure and generate an output signal within 10 to 30 seconds of exposure to the gas.13... 

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