Factors controlling sensor response

Metal/metal oxides are the materials of choice for construction of all-solid-state pH microelectrodes. A further understanding of pH sensing mechanisms for metal/metal oxide electrodes will have a significant impact on sensor development. This will help in understanding which factors control Nemstian responses and how to reduce interference of the potentiometric detection of pH by redox reactions at the metal-metal oxide interface. While glass pH electrodes will remain as a gold standard for many applications, all-solid-state pH sensors, especially those that are metal/metal oxide-based microelectrodes, will continue to make potentiometric in-vivo pH determination an attractive analytical method in the future. 

The results indicate that sensor response behavior is not only related to the thickness of the auxiliary phase, but is also controlled by other working electrode geometric factors - most likely the aspect ratio of the working electrode surface. Because of the high aspect ratio, the response and recovery times of the sensor are... 

A wide range of sensor types are based on polymeric media. The sensor response may be based on the electrical or optical properties of the polymer, or may be controlled by a secondary factor such as permeability by a chemical entity to be detected. 

The non-invasive requirement for the hot leg temperature sensor must therefore be evaluated against impacts on reactor control response time. Other factors affecting sensor placement include sensor accuracy and resolution, temperature tolerance, sensor size, and feasible attachment methodologies. The sensor technologies under consideration for measuring the hot gas temperature were ultrasonics, thermocouples, resistance temperature deviees (RTDs), optical pyrometry, and fiber Bragg grating. 

The experimental studies of the surface properties of monocrystals of oxides of various metals recently conducted at well-controlled conditions [32, 210] enable one to proceed with detailed analysis of separate effects of various factors on characteristics of semiconductor gas sensors. In this direction numerous interesting results have been obtained regarding the fact of various electrophysical characteristics of monocrystalline adsorbents on the value of adsorption-related response. Among these characteristics there are crystallographic orientation of facets [211], availability of structural defects, the disorder in stoichiometry [32], application of metal additives, etc. These results are very useful while manufacturing sensors for specific gases with required characteristics. 

The factors that influence the chemical resolution of sensors are well understood and are not discussed here. This section reviews the factors that control the temporal resolution of sensors to be used for eddy correlation. In the analysis of the design of chemical sensors to be used for eddy correlation it is instructive to consider the different components of chemical sensor systems separately to determine the influences that they have on the temporal response to variations in the atmospheric concentration of a trace constituent. Of course this analysis is an oversimplification because the total systems operate in a more complex fashion, but it is a useful exercise. 

A bioelectrode functioning optimally has a short response time, which is often controlled by the thickness of the immmobilized enzyme layer rather than by the sensor as well as many other factors (see Table 7). The biosensor response time depends on (1) how rapidly the substrate diffuses through the solution to the membrane surface, (2) how rapidly the substrate diffuses through the membrane cmd reacts with the biocatalyst at the active site, and (3) how rapidly the products formed diffuse to the electrode surface where they are measured. Mathematical models describing this effea are thoroughly presented in the biosensor literature (5, 68). 

In the context of this chapter, the sorbent phase is a coating on an AW sensor surface, where sorption can refer to adsorption (onto a surface or sorption site) and/or absorption (dissolution in the bulk). In the discussion following Section 5.4.1, adsorption and absorption are treated separately, and each of these interactions is discussed in terms of its energetics, or thermodynamics, which control the amount of analyte in/on the coating under equilibrium conditions. Kinetic factors, which determine the rate of response and also bear upon the reversibility of the sensor, are then considered. The kinetics of adsorption are described in Section 5.4.3 details of absorption kinetics, which are essentially diffusional in nature, can be found in Chapter 4. With this groundwork in place, a number of instances where these effects have been utilized in AW chemical sensors are described. Section 5.4 concludes with a discussion of biochemical/biological AW sensors. 

Laval etal. (1984) bound LDH covalently to electrochemically pretreated carbon. The enzyme was fixed by carbodiimide coupling simultaneously with anodic oxidation of the electrode surface. The total amount of immobilized LDH was determined fluorimetrically after removal from the electrode and hydrolysis. The authors found that at a maximal enzyme loading of 13 pmol/cm2 six enzyme layers are formed. The immobilization yield was about 15%. The kinetic constants, pmax and. Km, were not affected by the immobilization. The obtained enzyme loading factor of 10-3 indicates that diffusion in the enzyme layer was of minor influence on the response of the sensor. The layer behaved like a kinetically controlled enzyme membrane, i.e., the NADH oxidation current was proportional to the substrate concentration only far below Km- With increasing enzyme loading the sensitivity for NADH decreased due to masking of the electrode surface. 

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