O2 sensors

By changing the enzyme and mediator, the amperometric sensor in Figure 11.39 is easily extended to the analysis of other substrates. Other bioselective materials may be incorporated into amperometric sensors. For example, a CO2 sensor has been developed using an amperometric O2 sensor with a two-layer membrane, one of which contains an immobilized preparation of autotrophic bacteria. As CO2 diffuses through the membranes, it is converted to O2 by the bacteria, increasing the concentration of O2 at the Pt cathode. 

The success of the O2 sensor has made the auto manufacturers, regulators, and environmentalists anxious to extend chemical sensing to a variety of tailpipe gases, notably CO, NO, and short-chain hydrocarbons. Considerable research and development is needed for these molecules to be monitored in the hostile exhaust system environment (36). 

One system for measuring catalyst failure is based on two oxygen sensors, one located in the normal control location, the other downstream of the catalyst (102,103). The second O2 sensor indicates relative catalyst performance by measuring the abiUty to respond to a change in air/fuel mixture. Other techniques using temperatures sensors have also been described (104—107). Whereas the dual O2 sensor method is likely to be used initially, a criticism of the two O2 sensors system has been reported (44) showing that properly functioning catalysts would be detected as a failure by the method. 

Ru(dpp)3]2+) sequestered within the xerogels. The results of SEM and luminescence measurements shown that certain ([Ru(dpp)3]2) doped Octyl-triEOS/TEOS composites form uniform, crack-free xerogel films that can be used to construct high-sensitivity O2 sensors that have linear calibration curves and excellent long-term stability (over a period of 11 months). 

In contrast to the 02 generator, the polymer used in the O2 sensor must be highly gas permeable but solvent impenetrable. Solvent penetration will alter the probe properties and make calibration dependent on environment. Furthermore, good solvents for the probe will leach the probe and destroy the sensor. Again, it is important that the support dissolve the probe well and not greatly quench the luminescence. 

These results show that the complexes in silicone rubber, as well as silica supports (discussed below), show considerable promise as O2 sensors. Further, they serve to demonstrate the complexity present in persistent microheterogeneous systems. 

The first requirement in developing the ACC was to control air-fuel mixtures, and in contemporary cars this is maintained at precisely the 19 1 stoichiometric ratio for aU engine loads, using an exhaust O2 sensor that controls the fuel injection system. The composition and location of the catalyst within the reactor are also crucial to proper design. 

Direct measurement of oxygen uptake can be performed with O2 sensors in the headspace of a closed system and organic deposition by measuring the weight changes with a quartz microbalance. This method was applied to investigate the dependence of jet fuel autoxidation on temperature and the presence of antioxidants . 

Wang and coworkers [28] reported a similar gaseous O2 sensor using [C4CiIm][PFJ which is more hydrophobic than [C4QIm][BF4]. The sensor was tested with both dry and water-saturated O2, which produces a two-electron reduction instead of a one-electron reduction, and gave similar performance to [C4CiIm][BF4]. 

Another type of high temperature solid state O2 sensor that has been developed is based on the principle of electrochemical pumping of oxygen with Zr02 electrolytes. These sensors have higher sensitivity (generally, a first power dependence on Pq) than the Nernst cell and the resistive device and possess a number of other characteristics that make them very promising for many new applications. 

Exhaust O2 Sensor and Carburetor Feed-back Control... 

O2 sensors Op signal transducers K+ channels Na+ channels Ca++ channels Pathways to and within CNS ... 

Several sensors for L-alanine are obtained by immobilizing L-alanine dehydrogenase over an ammonia gas-sensing electrode (288) or over an O2 sensor (280). The enzyme catalyzes the specific deamination of alanine in the presence of the coenzyme NAD ... 

Perhaps the most interesting recent development in the area of Hrs is the discovery that Hr-like proteins are present in bacteria and archaea, where they may fimction as O2 sensors. This discovery potentially opens an entire new chapter in Hr biochemistry. 

Continuous measurement of OSC of the catalyst inserted in the converter is one of the simplest tool to monitor catalyst efficiency. This is known as the On-board diagnostic" (OBD). Sideris has recently reviewed the OBD technologies on the basis of the corresponding patent literature [103]. Even though other techniques are available (temperature sensors, specific sensors for CO, HC and NOx), OBD determined by means of O2 sensors remains the technique the most widely employed in car industry. This technique is based on the generation of periodic rich/lean perturbations monitored by two lambda sensors, one located upstream and the second downstream the converter. Catalysts that have good OSC considerably attenuate the upstream perturbation (with a significant time lag) while virtually dead... 

It is generally assumed that the lung neuroepithelial cells as well as carotid cells are O2 sensors [8]. From this point of view, however, it is difficult to understand why the O2 sensors of animals are organized in such a complex manner. It is known that the O2 sensors are already inherent in bacteria, where they are much simpler than in animals and are competent in [O2] monitoring by means of a direct O2 binding. ... 

X.W. Fu et al., NADPH oxidase is an O2 sensor in airway chemoreceptors evidence from current modulation in wild-type and oxidase-deficient mice, Proc. Natl. Acad. Sci. USA 97 (2000) 4374-4379. 

Maintaining the continued efficiency of all three reactions in a three-way catalytic converter is a delicate matter. It requires control of such factors as the O2 supply pressure and the order in which the reactants reach the catalyst Modern automobile engines use microcomputer chips, based on an O2 sensor in the exhaust stream, to control air valves. 

Potentiometric sensors for oxygen measurement are typically formed by a layer of stabilized zirconia pressed between two porous Pt electrodes, as schematized in Figure 9.1 (Bard et al., 2008 Pasierb and Rekas, 2009). Such electrodes are in contact with the gas sample and a reference gas, respectively. A typical example is constituted by O2 sensors based on yttrium- or scandium-doped zirconias. The... 

Archer SL, Weir EK, Reeve HE, MichelaJds E. 2000, Molecular identification of O2 sensors and O2-sensitive potassium channels in the pulmonary circulation. Adv Exp Med Biol 475 219-240,... 

Therefore, the following system of equations is represented as the mathematical model of the YSZ-based O2 sensor with the Pt SE and Me-MeO RE ... 

Development of ionic conductors based on stabilized zirconia has reached a level of maturity, where most of the research on such materials concentrates mainly on obtaining incremental empirical improvements in conductivity by better processing control and refinement of the microstructure of the solid electrolyte and SE. Further increases in the conductivity are important in terms of enhancing the efficiency of systems such as O2 sensors, zirconia-based mixed-potential gas sensors, electrochemical oxygen pumps, heating elements, and fuel cells [4-7]. The systematic errors, as have been considered before, are errors with a known determined functional connection with the source of their cause, and the conformity of their appearance can be definitely described. 

Figure 13.5 The response of the amperometric O2 sensor to a step change from 0 to 100% O2. Applied potential, —0.6 V against Ag/AgjO.

In addition to fast response, the O2 sensors demonstrated good reproducibility and reliability. This sensor design also allows incorporation of a water reservoir on the front side (the electrode side) of the sensor to increase sensor lifetime without degrading the sensor s response time. This reservoir provides suflScient moisture to the Nation electrolyte which requires water to be functional. 

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