A2.2 - SAWs for Chemical Sensing - Optimizing Receptor Layers and Transducers in Respect to Analytes of Interest
- SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
- Proceedings SENSOR 2011
- A2 - Surface Acoustic Wave Sensors
- F. Dickert, P. Lieberzeit - University of Vienna (Austria)
- 53 - 54
Mass sensitive detection is the most universal strategy to combine sensitive layers with transducer principles. Coating SAWs with rigid materials will increase the resonance frequency due to mass load in a quadratic manner. The SAW resonance frequency can be enhanced up to approximately two GHz leading to superior applications.
Well known piezoelectrical materials are quartz and lithiumtantalate. Gas phase measurements can be performed with quartz devices in typical resonance frequencies from 430 MHz to 1 GHz with Rayleigh-waves leading to a mass-sensitivity below one p-gram. In liquid phase measurements high damping avoids resonance. This problem can be solved in going to shear waves. Quartz shear wave devices will work in media of low dielectric constant, otherwise lithiumtantalate is a superior material.
The high sensitivity of SAW transducers makes possible application of mono-layers as sensitive coating. Thus, diffusion processes are eliminated and instantaneous responses are obtained. The quartz surfaces of SAWs can be hydrophobized by silylchloride spacers converting their polarity. Thus, humidity cross sensitivities are eliminated and all types of hydrocarbons are detected. Furthermore, patterning the surface with silyl spacers in a different elongation will create molecular cavities for engulfing efficiently aromatic and halogenated hydrocarbons. This type of host-guest chemistry allows to differentiate between isomer xylenes, which are based only by their geometry. These ideas are further developed by linking molecular cavities such as cyclodextrines and calixarines to the SAW-surfaces. This can be performed by electrostatic strategies, improved stability is obtained, however, by covalent anchoring to the surface.
Supramolecular strategies according to hostguest chemistry can be combined with technological progress in sensor fabricating via molecular imprinting. The analytes to be investigated are added to the polymers as templates. After evaporation or dissolving the template suitable cavities are generated for re-incorporation the analyte. Typical layer heights of the sensitive coatings on the piezoelectrical substrates are e.g. 40 nm to guarantee adequate response times. Thus, the production of the transducer is combined with the sensitive layer. Additional to detect distinct analytes, complex mixtures can be characterized, too. Composting processes, automotive oil degradation and food quality can be analysed by these synthetic antibodies. Bioanalytes are also accessible by this technique, even a single yeast or cancer cell can be detected.