Flexible Surface Acoustic Wave Sensor Based on an SXFA Sensitive Film for DMMP Detection

Rapid and reliable detection of organophosphorus nerve agents is essential for public safety and individual protection. In this work, a flexible surface acoustic wave (SAW) gas sensor based on a hexafluoroisopropyl (HFIP)-functionalized polysiloxane (SXFA) sensitive film was developed for the detection of dimethyl methylphosphonate (DMMP), a widely used simulant of sarin (GB). SXFA was synthesized through a two-step route and systematically characterized by Fourier-transform infrared spectroscopy (FTIR), 1H/19F/13C nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM/EDX). The results confirm the successful incorporation of HFIP groups and reveal a low glass-transition temperature (Tg = −23.5 °C, ΔCp = 0.295 J·g−1·K−1), a weight-average molecular weight of ~3200 g·mol−1, a thermal-stability limit of ~200 °C, and a characteristic HFIP side-chain cleavage at 343.4 °C with a final residue of 18.37%. The flexible device consisted of a polyimide (PI) substrate, a magnetron-sputtered AlN piezoelectric layer and silver interdigital transducers (IDTs) and operated at a center frequency of ~200 MHz; a rigid quartz-based delay line was used as a reference. The dependence of the insertion loss (IL) on film thickness was described by a third-order polynomial (R2 = 0.960), from which an optimal thickness window of 150–450 nm was identified, and 450 nm gave the best gas-sensing performance (response 23.5 mV at 3 mg·m−3 DMMP, IL −16.0 dB; mean ± SD, n = 3 independently prepared devices per thickness). The optimized sensor exhibited a linear response to DMMP from 0.4 to 6 mg·m−3 (y = 30.88x + 8.988, R2 = 0.9985) at 28.1 °C, fast response and recovery (~75 s each), excellent reproducibility (relative standard deviation, RSD = 0.50%, n = 7), pronounced selectivity towards DMMP and DFP over common interferents, and a 60-day response retention of 96.0%. The sensing mechanism is attributed to hydrogen-bonding adsorption between the acidic HFIP groups and the basic P=O moiety of DMMP, combined with mass-loading and film-viscoelastic effects. These results demonstrate that SXFA-functionalized flexible SAW devices are promising candidates for wearable and low-power detection of organophosphorus nerve agents.