Design and characterization of surface acoustic wave (SAW) sensor for detection of Lactobacillus in liquid medium.

Surface Acoustic Wave (SAW) sensors are pivotal Micro-Electrical-Mechanical Systems (MEMS) devices for micro-particle detection, offering compact design, high throughput, and low fabrication cost. This work presents the design, fabrication, and characterization of a SAW sensor employing a Polydimethylsiloxane (PDMS) microfluidic channel as a dual-function waveguide to effectively localize Love Wave (LW) confinement and convert Rayleigh waves to LW. Utilizing a comprehensive approach integrating multi-parametric Finite Element Analysis (FEA), analytical modeling, and experimental validation, two SAW devices with distinct interdigitated transducer (IDT) electrode configurations (12 μm and 38 μm width and spacing) have been developed. FEA and experimental results consistently confirm the superior performance of the 12 μm electrode configuration. This device achieved significant BAW suppression, evidenced by a low insertion loss (S21) of -57 dB (FEA) and a narrow admittance peak (Δf = 0.6 MHz at FWHM), yielding a high Q-factor at its center frequency (fc = 82.5 MHz). Performance metrics for the 12 μm electrode configuration include a reflection coefficient (S11) of -85 × 10⁻⁷ dB (vs. -40 × 10⁻⁸ dB for 38 μm), experimental insertion losses of -64.86 dB, -67.05 dB, and - 69.27 dB for 50, 40, and 30 finger pairs respectively, and low limit of detection (LoD) with higher number of finger pairs. The PDMS waveguide maximized acoustic energy confinement at the surface, enabling efficient Love wave propagation, which minimizes dissipative losses in Liquids. Moreover, the dominant y-direction surface displacement of 0.026 μm, and a higher admittance peak (80 × 10⁻⁷), indicating high sensitivity in liquid medium and high quality (Q) factor, respectively. The sensor's micro-particle detection capability, based on monitoring IL changes - established as an effective metric for quantifying particle-induced perturbations in flow-through configurations - across varying particle concentrations, has been experimentally validated using 10 μm diameter Polystyrene (PS) particles as Lactobacillus analogs. The strong agreement between analytical, FEA, and experimental results validates this high-fidelity SAW device with integrated microfluidics as a promising, cost-effective, and highly sensitive platform for micro-particle detection in liquid media, with potential extension to gas sensing applications, if used without any waveguide.