NO 2 Gas-Concentration Measurement with a SAW-Chemosensor

A highly stable and selective SAW-chemosensor has been realized. The sensor operation is based upon the interaction of SAW fields with the simultaneous changes of mass and electrical conductivity of a chemical interface originating from changes in the gas concentra- tion. The sensor system is based upon a dual delay-line oscillator. The difference between the two oscillator frequencies is a measure of the gas concentration. The delay lines are implemented in an STX-quartz substrate. One delay line is provided with the chemical interface ob- tained by vacuum sublimation of an organic semiconductor, metal-free phthalocyanine. The oscillator is stabilized by means of automatic gain control. The oscillator operates without tuning inductors and with symmetrical driving of the transducers, aiming at a (total) monolithic integration of this sensor in a ZnO-SiOz-Si layered structure. A spe- cial feature of the sensor is that the delay lines and the electronic sys- tem are constructed in separate modules. This simplifies the inter- change of different delay lines and allows measurements to be performed when only the delay lines are at an elevated temperature (e.g., environmental monitoring and automobile emission control). The operating frequencies of the sensors are between 30 and 110 MHz. To date one sensor has a sensitivity of about 100 Hz/ppm NOZ at an op- erating frequency of 39.48 MHz with a threshold sensitivity of about 0.5 ppm.