Coating-analyte interactions

Perturbation mechanisms for the various acoustic devices were discussed in general terms in Chapter 3. In this chapter, these mechanisms are reviewed specifically in the context of chemical and biochemical analysis. Performance criteria are discussed, and the fundamental coating-analyte interactions giving rise to sensor responses are presented as a basis for classification. Relevant physical and chemical models of these interactions are described, and examples of analytical applications employing each type of interaction are given to illustrate their advantages and limitations. While references have been included to illustrate specific points, this chapter is not intended to comprise an exhaustive review of the literature, particularly for TSM resonators, for which the number of references is far too great to be fully reviewed here. For more detailed information on the diversity of sensor applications, the reader is referred to the many review articles that have been published on these topics [2-8,13-15]. 

The relative importance of the mass-loading and viscoelastic contributions to the observed acoustic sensor response is an issue that has yet to be resolved. Capitalizing on these effects to improve chemical selectivity and detection sensitivity requires further characterization of sensor response, in terms of both velocity and attenuation changes, in addition to more accurate models describing how coating-analyte interactions affect relevant film properties. 

The issue of selectivity continues to represent a most active area of research for this class of sensors and will be discussed further in the context of coating-analyte interactions. 

Like sensitivity, LOD depends on the inherent sensitivity of the device itself, as well as the kinetics and thermodynamics of the coating-analyte interaction and the quantity (thickness and/or surface area) of coating available. Unlike sensitivity, however, LOD also depends on the system noise level. The LOD is expressed in terms of the ratio [response when analyte is present]/[noise level when there is no analyte present]. Commonly, LODs are denned as signal-to-noise (S/N) ratios of two or three, corresponding roughly to situations where the signal exceeds the noise at statistical confidence levels of 95% and 99%, respectively [91]. Thus, in the latter case, the LOD can be defined as 3N/sensitivity. The LOD is expressed in units of concentration (e.g., M, fig/L, or iqnn). 

For irreversible (non-equilibrium) sensors that utilize activated reaction ter-action mechanisms, sensitivity usually increases as a result of increased reaction rates at higher temperatures. This type of response behavior is illustrated in Ing-ure 5.5 for the reaction of a Pt-olefin-complex with ethyl acrylate [92d]. In contrast to the steady decrease in sensitivity for the reversible sensor, die SAW response rate (in Hz/min) increases with temperature. Thus, depending on the type of coating-analyte interactions being utilized, the sensor sensitivity can be improved by selecting an operating temperature consistent with the predominant response mechanism. 

Coating-Analyte Interactions and Acoustic-Wave Chemical Sensors... 

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