Sensitivity catalytic sensors

Li, G.J., Zhang, X.H., and Kawi, S., 1999. Relationships between sensitivity, catalytic activity and surface areas of SnO2 gas sensors. Sensors Actuators B., 60, pp. 64-70. 

Also for CO sensing, the present sensors are available only for the field of security not for environmental use because of the insufficient sensitivity and selectivity to monitor CO in the atmosphere. Examples of CO sensor which have been improved their sensitivity and selectivity are, for example, SnO semiconductor sensors operated under periodic temperature cycle[85-87], a electrochemical sensor using nafion membrane[88], a catalytic combustion sensor composed of catalysts and hydrophobic pol uner[89], a SnOj diode sensor doped with Pd[90] and an optical fiber catalytic sensor with Au/CogO as combustion catalyst[91]. 

The DNA machines described in this work, particularly those that start a catalytic process upon exposme to their target molecules, may offer new approaches to the design of clever biosensorial strategies which due to their catalytic nature may improve sensitivities for sensors based on the interaction of aptamers with their respective targets. 

The catalytic sensor is less sensitive to temperature and humidity effects, offers repeatable performance, and is relatively stable. However, it is susceptible to poisoning or inhibition from some gases, which may decrease its sensitivity or damage the sensor beyond recovery. 

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. 

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. 

Chapter 4 deals with several physical and chemical processes featuring various types of active particles to be detected by semiconductor sensors. The most important of them are recombination of atoms and radicals, pyrolysis of simple molecules on hot filaments, photolysis in gaseous phase and in absorbed layer as well as separate stages of several catalytic heterogeneous processes developing on oxides. In this case semiconductor adsorbents play a two-fold role they are acting botii as catalysts and as sensitive elements, i.e. sensors in respect to intermediate active particles appearing on the surface of catalyst in the course of development of catal rtic process. 

Liu J, Lu Y (2007) A DNAzyme catalytic beacon sensor for paramagnetic Cu2+ ions in aqueous solution with high sensitivity and selectivity. J Am Chem Soc 129 9838-9839... 

Liu J, Brown AK, Meng X et al (2007) A catalytic beacon sensor for uranium with parts-per-trillion sensitivity and millionfold selectivity. Proc Natl Acad Sci USA 104 2056-2061... 

Layer doping with catalytic metals can be done either during the powder preparation or later in the deposition and annealing process. Doping is an important procedure in tuning the gas-sensor characteristics (selectivity pattern of the sensitive layers) [67,68]. 

The organic conductor properties of tetrathiaflulvalenetetracyanoquino-dimethane (TTF-TCNQ) as a material for constructing electrodes, viz. its catalytic response and resistance to passivation, are of special interest for the determination of biological compounds, which usually have slow electrode kinetics and a low sensitivity, and tend to foul electrode surfaces. The response of a TTF-TCNQ microarray sensor inserted in a flow system for... 

The hydrogen sensitivity of palladinm-oxide-semiconductor (Pd-MOS) strnctnres was first reported hy Lnndstrom et al. in 1975 [61]. A variety of devices can he nsed as field-effect chemical sensor devices (Fignre 2.6) and these are introdnced in this section. The simplest electronic devices are capacitors and Schottky diodes. SiC chemical gas sensors based on these devices have been under development for several years. Capacitor devices with a platinum catalytic layer were presented in 1992 [62], and Schottky diodes with palladium gates the same year [63]. In 1999 gas sensors based on FET devices were presented [64, 65]. There are also a few publications where p-n junctions have been tested as gas sensor devices [66, 67]. 

SCR is a process by which NO gases in diesel exhausts can be reduced to levels that will meet future legislation. SCR is based on the reduction of NO in the catalytic converter by the injection of ammonia or urea into the exhaust gases before they enter the catalytic converter, where NO and NHj react to form and H O. The ammonia injection process may be controlled by measuring either the ammonia or nitric oxide slip after the catalytic converter. Such an ammonia sensor should be able to tolerate contaminants such as particles in the exhaust gases and should show very low cross sensitivity to NO and HC. Typical diesel exhaust contains 3-9% CO 50-250 ppm CO, 6-12% O, 200-1,000 ppm NO, and 130-260 ppm HC. Furthermore, the response to NH3 should have a time constant in the order of 1 second. 

Because of this type of behavior, a sharp transition at stoichiometry but low sensitivity and temperature effects either rich or lean of this point, the oxygen sensor is most useful in controlling at the stoichiometric point. It is of limited usefulness at other exhaust compositions. However, as shown in Figure 5, this is exactly the point at which a three-way or dual bed catalytic converter is most efficient. Only when the exhaust composition is near the stoichiometric point will both the oxidation of the HC and CO and the reduction of the NO occur satisfactorily. 

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