Telemetric surface acoustic wave chemical sensors

The present paper deals with surface acoustic wave (SAW) chemical sensors that operate without power supply and can be read wirelessly by electromagnetic interrogation signals. These novel chemical sensors are based on the well-known SAW identification (ID) tag principle with a sensitive coating applied to the device surface. It is shown that a gas detection limit of a few ppm to some hundred ppm requires a reflector time distance of several microseconds, corresponding to a spatial distance of typically 10 mm. This relatively large device geometry places an upper limit on the additional insertion attenuation caused by the sensitive layer and restricts the possible coating materials. As a practical demonstration, we have designed and investigated a humidity sensor at 433.92 MHz a frequency band allotted to industrial, scientific, and medical apparatus in Germany. The measured return loss without sensitive layer was between 20 and 25 dB. A coating of cross-linked polyvinyl alcohol served as sensitive layer, the thickness of which was kept at 3 to 10 nm. With these devices, relative humidity changes of a few percent are detected quite easily. This illustrates the fact that telemetric SAW chemical sensors ran actually be realized.