Design of a portable guided SH-SAW chemical sensor system for liquid environments

Following successful application in gas sensing, acoustic wave liquid sensors attracted considerable attention due to the need for real-time, rapid and direct detection where the device is in direct contact with the solution. More importantly, there is a need for field measurement capability with portable devices. Challenges include a physical layout of the RF circuitry to minimize parasitic and spurious noise, continuous and realtime measurements capability, and obtaining vector network analyzer (VNA) performance in a portable RF unit, especially since the sensor signal noise dictates the limit of detection (LOD). Polymer-guided shear horizontal surface acoustic wave (SH-SAW) sensor platforms operating around 105 MHz on 36deg rotated Y-cut LiTaO
₃
are investigated as portable detectors in liquid environments. The described system is self-contained including RF signal source, sensor input/output signal conditioning, and sensor signal amplitude and phase measuring capabilities. Amplitude and phase signals from the sensor are differentially compared with concomitant signals available directly from the RF signal source. The two primary outputs from the system are voltages related to the detected amplitude and phase changes that are caused by the sensor's response to analyte sorption by the coated device. Several devices, coated with chemically sensitive polymers, are investigated in the detection of low concentrations (10-60 ppm) of ethylbenzene and xylenes in water using the RF portable unit. The units were tested for both reproducibility and repeatability, and the results matched very well with VNA measurements