Surface acoustic wave devices reversibility

The quartz-crystal microbalance (QCM) piezoelectric sensor operating system is based on interactions between thin organic layers, coated on the surface of a quartz crystal, and analytes. The ability of a QCM sensor to selectively recognize some molecules in a pomplex mixture depends on how selective and sensitive is the coated receptor. In order to obtain selective responses the coating of the quartz must be stable and capable of specific interactions with the desired analyte. Reversibility of the responses is another essential feature which requires to resort to weak interactions, since the formation of covalent or ionic bonds would lead to irreversible saturation of the sensitive layer. On the other hand pure dispersion forces are unsuitable due to their aspecificity. Sensitivity in mass sensors depends mainly on the transduction mechanism employed. Surface acoustic wave devices (SAW) are usually at least two order of magnitude more sensitive than QCM ones with the same coating. 

A chemical microsensor can be defined as an extremely small device that detects components in gases or Hquids (52—55). Ideally, such a sensor generates a response which either varies with the nature or concentration of the material or is reversible for repeated cycles of exposure. Of the many types of microsensors that have been described (56), three are the most prominent the chemiresistor, the bulk-wave piezoelectric quartz crystal sensor, and the surface acoustic wave (saw) device (57). 

Piezoelectric phenomena are related to the reversible electric polarization generated by mechanical strain in crystals that do not display a centre of S5mimetry. The signal produced by acoustic wave devices is generated by bulk or surface acoustic waves launched by metal transducers at ultrasonic fi-equencies. Such waves are propagated through piezoelectric materials. 

The key feature of all acoustic wave sensors for detecting vapors is that measurable characteristics of the acoustic wave is altered as a result of adsorption on the surface of a receptive layer or absorption into the bulk of a thin layer (Figure 3). After sorption of the vapor by a thin film on top of the acoustic resonator equilibrium conditions are established and as a consequence of the increased mass or more accurately the change in the phase velocity of the acoustic wave a signal is created. Surface coatings generally enhance the sorption of vapors with the key properties of selectivity and sensitivity while affording reversibility. Typically, rubbery polymers were used on SAW devices such as polyisobutylene or substituted polysiloxanes but also self-assembled... 

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