Surface acoustic wave sensors SAWs

The methods and means for ecological diagnostics make rapid strides among all the NDT and TD developing areas. To provide the atmosphere monitoring recently the good results were achieved in the development of surface-acoustics wave sensors (SAW), laser measuring systems, infrared detectors and systems based on other physical principles. 

In recent years there has been an interest in the development of polymer-supported supramolecules for sensor arrays. Such arrays include electronic noses to sense vapour analytes. There are many polymer supports that have been used to create sensor arrays, for example, conductive polymers, polymer composites, dye-doped polymer matrixes and surface acoustic wave sensors (SAWs). 

Microcantilever sensors offer many orders of magnitude better sensitivity compared to other sensors such as quartz crystal microbalances (QCM), flexural plate wave oscillators (FPW), and surface acoustic wave devices (SAW). There are several distinct advantages of the microcantilever sensors compared to the above mentioned and other MEMS sensors ... 

FIGURE 4.9 Permittivity-conductivity chart to derive electrical properties from SH-SAW responses. Kondoh J. and Shiokawa S Shear Horizontal Surface Acoustic Wave Sensors. Sensors Update. 2001. 6. Copyright Wiley-VCH Verlag GmhH Co. KGaA. Reproduced with permission. 

An electronic tongue based on dnal shear horizontal surface acoustic wave (SH-SAW) devices was developed to discriminate between the basic tastes of sour, salt, bitter, and sweet [57]. Sixty MHz SH-SAW delay line sensors were fabricated and placed below a miniature PTFE housing containing the test liquid. All the tastes were correctly classified without the need for a selective biological or chemical coating. 

Mass sensitive sensors (surface acoustic wave devices, SAW s)... 

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. 

The surface acoustic wave device (SAW) is an example of a transducer that is batch fabricated using IC technology and provides improved performance. SAW devices operate at much higher frequencies than the quartz crystal oscillator (or microbalance, QCM), and this results in improved detection limitsl, 29, 30 This can make measurements of absorption into films coating the SAW device possible, under circumstances where the QCM is insufficiently sensitive. On the other hand the ( M can be used in aqueous systems, while the SAW device is essentially restricted to gas phase measurements. Here too, IC techniques have provided means to fabricate thin membranes that can be made to oscillate at frequencies similar to the SAW device, but in a mode that is not over-damped in aqueous solutions. Nevertheless, regardless of the specific oscillator involved, it is the coating films and interfaces that provide the chemical specificity required of the sensor. 

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). 

Bulk and surface imprinting strategies are straightforward tools to generate artificial antibodies. Combined with transducers such as QCM (quartz crystal microbalance), SAW (surface acoustic wave resonator), IDC (interdigital capacitor) or SPR (surface plasmon resonator) they yield powerful chemical sensors for a very broad range of analytes. 

A wide variety of solid-state sensors based on hydrogen-specific palladium, metal oxide semiconductor (MOS), CB, electrochemical, and surface acoustic wave (SAW) technology are used in the industry for several years. Microelectromechanical systems (MEMS), and nanotechnology-based devices for the measurement of hydrogen are the recent developments. These developments are mainly driven by the demands of the fuel cell industry. Solid-state approaches are gaining rapid popularity within the industry due to their low cost, low maintenance, replacements, and flexibility of multiple installations with minimal labor. 

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