Design considerations for high sensitivity guided SH-SAW chemical sensor for detection in aqueous environments

Guided shear horizontal surface acoustic wave (guided SH-SAW) devices coated with a polymer waveguiding layer and/or chemically sensitive layer have been investigated for the detection of analytes in liquid environments in our previous work. Design considerations for optimizing these devices for liquid phase detection is the focus of the current work. Using dual delay line geometry on LiTaO₃, guided SH-SAW sensors are designed and analyzed. The reference line, used to correct for changes in environmental conditions such as temperature fluctuations, is coated with a waveguiding layer of poly(methylmethacrylate) (PMMA). The sensing line is coated either with a polymer that functions as both the waveguiding layer and the chemically sensitive layer (3-layer model) or with a PMMA waveguiding layer below the chemically sensitive layer (4-layer model). Experimental measurements show the 3-layer model provides higher sensitivity than the 4-layer model. Increased sensitivity when using the 4-layer model can only be achieved through rigorous selection of the guiding polymer layer and chemically sensitive layer, considering both mass loading and viscoelastic effects. Appropriate selection of the partially selective chemical layer to optimize sensitivity is also a critical design factor, particularly in sensing polar analytes in aqueous sensing applications. A methodology based on attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR) for screening the potential effectiveness of new polymer coatings for these devices has been developed and used in our work. The ATR-FTIR methodology provides an accurate determination of trends for partitioning of analytes from water into polymer coatings.