Guided shear-horizontal surface acoustic wave (SH-SAW) chemical sensors for detection of organic contaminants in aqueous environments

With the growing concern of the threat of the use of (bio) chemical weapons, and the ever-present environmental pollution, acoustic wave sensors attract considerable attention for the detection of various (bio) chemical compounds due to the need for real-time, stable and direct detection in aqueous environments. In this work, direct chemical sensing in liquid environments using polymer-guided shear horizontal surface acoustic wave (SH-SAW) sensor platforms on 36° rotated Y-cut LiTaO3 is investigated. For implementing high sensitivity guided SH-SAW sensors, design considerations are explored theoretically and experimentally, which includes sensor geometries comparison, the analysis of the coating properties and their effects on the sensor responses, and appropriate selection of chemically sensitive coating for liquids. Dual delay line devices are used with a reference line coated with poly (methyl methacrylate) (PMMA) and a sensing line coated with a chemically sensitive polymer (three-layer geometry), which acts as both a guiding layer and a sensing layer or with a PMMA waveguide and a chemically sensitive polymer (four-layer geometry). These two sensor geometries are analyzed theoretically by simulating the sensor responses as a function of mass loading and polymer shear moduli. The three-layer geometry shows higher sensitivity than the four-layer geometry of all the configurations tested experimentally while the latter shows better stability. Increased sensitivity when using the four-layer geometry can only be achieved through rigorous selection of the guiding polymer layer and the chemically sensitive layer, considering both mass loading and viscoelastic effects. The polymer coating effects including the thickness effect and the curing condition effect on stability, sensitivity and partial selectivity are experimentally and/or theoretically studied in order to optimize the sensor design. Contributions from mass loading and coating viscoelasticity changes to the sensor response are evaluated, taking into account the added mass, swelling, and plasticization. Chemically sensitive polymers are investigated in the detection of low concentrations (1-60 ppm) of toluene, xylenes and ethylbenzene in water. A low ppb level detection limit is estimated from the present experimental measurements. Partition coefficients for polymer-aqueous analyte pairs are used to explain the observed trend in sensitivity for the chemically sensitive polymers used in this work