Chemical Sensing with Surface Acoustic Wave Devices

There is much current interest in developing new techniques for the continuous monitoring of chemical environments. However, as there are many applications where the excessive size and power demands of conventional sensors have precluded their use, interest in microsensors such as the surface acoustic wave (SAW) device has been spurred by their small size, ruggedness, sensitivity, and low power consumption. In this work, 158 MHz SAW oscillators coated with organic polymers and inorganic crystallites were tested as model systems for vapor sensors that can monitor the level of humidity and carbon dioxide in a continuous stream of nitrogen. All coatings exhibited responses to water vapor on the order of kilohertz, while the responses to carbon dioxide were significantly (generally one order of magnitude) smaller. Interferences by the presence of both water and CO2 on the SAW device sensitivity were observed with poly-(ethylenimine) (PEI) coatings as evidenced by the large extent of hysteresis. P El coatings were also found to exhibit anomalous directions in frequency shifts when exposed to pulses of humidified CO2, as did poly-(4-vinylpyridine) (PVP) coatings for lower humidity levels. ZSM-5 coatings, however, showed no evidence of such interference between water and CO2.