Subsecond-response SAW humidity sensor with porphyrin nanostructure deposited on bare and metallised piezoelectric substrate

The surface acoustic wave (SAW) humidity sensor based on the Meso-tetra (4 sulfonatophenyl) porphyrin (TPPS4) nanostrip structure deposited on a bare or metallised surface of the piezoelectric YZ LiNbO₃ substrate has been investigated. The SAW attenuation, which greatly increases with humidity in sensors with TPPS4 layers on bare substrates, is almost humidity-independent in devices with TPPS4 layers on metallised substrates. For both types of sensors, the SAW velocity exhibits close-to-linear decrease with growing humidity. The sensor exhibited fast response and recovery times of from 0.8 to 0.5 s in both amplitude and phase of the output signal. The real-time monitoring of human breath with the help of the sensor was demonstrated.