Active sensing

Three different ways in which a zeolite membrane can contribute to a better sensor performance can be distinguished (i) the add-on selective adsorption or molecular sieving layer to the sensor improves selectivity and sensitivity, (ii) the zeolite layer acts as active sensing material and adds the selective adsorption and molecular sieving properties to this, and (iii) the zeohte membrane adds a catalytically active layer to the sensor, improving the selectivity by specific reactions. 

A very recent example of the first case is presented by Vilaseca et al. [71] where an LTA coating on a micromachined sensor made the sensor much more selective to ethanol than methane. Moos et al. [72, 73] report H-ZSM5 NH3 sensor based on impedance spectroscopy using the zeolite as active sensing material. At elevated temperatures (>673 K) NH3 still adsorbs significantly in contrast to CO2, NO,... 

Receptors sense the signal and are activated. Sensing the signal causes a change in the structure of the receptor. This structural change activates the pathway. 

Two main groups of indicator electrodes are considered here. In one case, metal indicator electrodes that exhibit a potential difference as a consequence of a redox process occurring at the metal surface are examined. Later, ISEs that can respond to ionic species based on the principles of ion extraction across an active sensing membrane will be studied in detail. 

Many of the molecules described have been clearly prepared without any immediate purpose, but this is often the case, if a new class of compounds is to be explored. During the first stages it is just important to establish what is possible. However, after about 20 years of modern calixarene chemistry and after the accumulation of a huge body of knowledge concerning their properties and reactivities, the time seems ripe to use the potential of their chirality in a more active sense. 

A challenge in active sensing is the design of reversible sensing chemistries. The available fiterature is full of chemical reactions that lead to an end-product. There are fewer examples of reactions known to be reversible. Reversibifity may not be an issue if the compound one wishes to sense is not likely to be encountered, and if it was detected, evacuation of the vicinity would be required—such as in sensing for... 

Operando spectroscopy spectroscopic characterization of an active sensing element in real time and under operating conditions with the simultaneous read-out of the sensor activity and simultaneous monitoring of gas composition. 

More controversialJy, the process of technical change may be similarly directed, leading to a rational preference for some types of innovation over others (3.3-2)- Capitalist entrepreneurs are agents in the genuinely active sense. They cannot be reduced to mere place-holders in the capitalist system of production. 

Consider a thermopile infrared detector having an active surface area A and initially at the ambient temperature Tea (Fig. 3P-5). The active sensing element (or, active junction, equivalently) of the detector consists of two very tbmlayers (Si+52 10 5 cm) of bismuth and tellurium formed by vacuum deposition onto a film of Mylar. The upper surface is suddenly subjected to a heat flux of q" [W/m2]. The heat transferred from the detector to a sink at a temperature rsittk via a connection may be approximated as Q = Qo(T — T ) [W], where Qq is a known quantity and T is the temperature of the detector active junction-... 

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