Reversibility, chemical sensors

Mohr G. Citterio D., Demuth C., Fehlmann M., Luzi J., Lohse C., Moradian A., Nezel T., Rothmaier M. Spichiger U.E., Reversible chemical reactions as the basis for optical sensors used to detect amines, alcohols and humidity, Journal of Materials Chemistry 1999 9 2259. 

Horvath, R. Skivesen, N. Larsen, N. B. Pedersen, H. C., Reverse symmetry waveguide for optical biosensing, In Frontiers in Chemical Sensors. Novel Principles and Techniques Orellana, G. Moreno Bondi, M. C., Eds. Springer Series on Chemical Sensors and Biosen sors Springer, Berlin, 2005, Vol. 3, 279 301... 

L. Chen, D.W. McBranch, H.-L. Wang, R. Helgeson, F. Wudl, and D.G. Whitten, Highly sensitive biological and chemical sensors based on reversible fluorescence quenching in a conjugated polymer, Proc. Natl. Acad. Sci. USA, 96 12287-12292, 1999. 

In another study, Darrach et al. [2] reported that samples collected near intact UUXO targets contained traces of explosives at up to parts per billion (ppb) concentration levels. The samples were analyzed in the laboratory, using solid-phase microextraction (SPME) to extract target analytes from the samples. The samples were then processed using a reversal electron attachment detection (READ) technique. If the levels of contamination found in these studies are representative of that emanating from most UUXO, the implication is that sensitive chemical sensors such as the SeaDog may be useful for detecting UUXO. 

The ideal (bio)chemical sensor should operate reversibly and respond like a physical sensor (e.g. a thermometer), i.e. it should be responsive to both high and low analyte concentrations and provide a nil response in its absence. One typical example is the pH electrode. In short, a reversible (bio)chemical sensor provides a response consistent with the actual variation in the analyte concentration in the sample and is not limited by any change or disruption in practical terms, responsiveness is inherent in reversibility. An irreversible-non-regenerable (bio)chemical sensor only responds to increases in the analyte concentration and can readily become saturated only those (bio)chemical sensors of this type intended for a single service (disposable or single-use sensors) are of practical interest. On the other hand, an irreversible-reusable sensor produces a response similar to that from an irreversible sensor but does not work in a continuous fashion as it requires two steps (measurement and renewal) to be rendered reusable. Figures 1.12 and 1.13 show the typical responses provided by this type of sensor. Note... 

A sensor is a device able to respond to the presence of one or many given substances in a more-or-less selective way, by means of a reversible chemical interaction it may be employed for qualitative or quantitative determinations (Cattrall, 1997). All sensors are composed of two parts the responsive region and the transducer. The responsive region is responsible for sensitivity and selectivity of the sensor, while the transducer converts energy from one to another form, providing a signal which is informative about the system analyzed. Usually, basic signalprocessing electronics, and control and display units complete the device. 

Chemical sensors for mercury vapour, based on QMB, provide sensitivity of 0.7-5 ng/1 [24]. Reversibility is reached by heating the sensor a microheater can be integrated on the sensor surface. This detection principle was also used by several other groups, for example [7,9,25]. Another realization of this transducing principle is based on exploitation of gold-coated microcantilevers for mercury detection [21,22]. 

The chemical sensor area has proved to be one of the most dynamic fields in analytical chemistry. During the last few years, an enormous effort has been made in order to design sensors that show appropriate features such as selectivity, low detection limits, reversibility, robustness, portability and easy handling. However, this continuously evolving field is improving to achieve the goals of the conceptual term of sensor and most of the existing ones still present clear limitations. 

In the last decade, fiber-optic chemical sensors (FOCS), also known as optrodes, have emerged as alternatives to conventional methods of analysis. FOCS development for a particular analyte depends on the availability of reversible indicating schemes to detect the analyte of interest. Typically, the indicating schemes use commercially available colorimetric or fluorometric indicators (e.g. fluorescein to measure pH (1)). However, the utility of these indicators is limited. Furthermore, indicators may not exist for many analytes. Several reviews discuss the scope of this approach (2,3,4). 

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. 

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