Long-term monitoring of environmental warfare agengts is a challenge for chemical gas sensors. To address this issue, we developed a 433 MHz passive wireless surface acoustic wave (WSAW) gas sensor for dimethyl methylphosphonate (DMMP) detection. This WSAW gas sensor includes a YZ lithium niobate (LiNbO₃) substrate with metallic interdigital transducers (IDTs) etched on it, and an antenna was placed near the IDT. A DMMP-sensitive viscoelastic polymer fluoroalcoholpolysiloxane (SXFA) film was prepared on a LiNbO₃ substrate, and mode modeling coupling was used to optimize the design parameters. The sensor can function properly in an environments between −30 °C and 100 °C with humidity less than 60% RH. When the wireless transmission distance was within the range of 0–90 cm, the sensor noise increased with distance, and the stability was less than 32°/h. While optimizing the film thickness of SXFA, a relationship was observed between sensor sensitivity and film thickness. When the film thickness of SXFA reached 450 nm, the optimal value was reached. At a distance of 20 cm between the transmitting and receiving antennas, DMMP was detected at different concentrations with the developed WSAW gas sensor. The lower detection limit of DMMP was 0.48 mg/m³, the sensitivity of the sensor was 4.63°/(mg/m³), and repeatable performance of the sensor was confirmed.

Hydrogen sulfide (H₂S) detection remains a significant concern and the sensitivity, selectivity, and detection limit must be balanced at low temperatures. Herein, we utilized a facile solvothermal method to prepare Cu-doped SnO₂/rGO nanocomposites that have emerged as promising candidate materials for H₂S sensors. Characterization of the Cu-SnO₂/rGO was carried out to determine its surface morphology, chemical composition, and crystal defects. The optimal sensor response for 10 ppm H₂S was ~1415.7 at 120 °C, which was over 320 times higher than that seen for pristine SnO₂ CQDs (R_a/R_g = 4.4) at 280 °C. Moreover, the sensor material exhibited excellent selectivity, a superior linear working range (R² = 0.991, 1–150 ppm), a fast response time (31 s to 2 ppm), and ppb-level H₂S detection (R_a/R_g = 1.26 to 50 ppb) at 120 °C. In addition, the sensor maintained a high performance even at extremely high humidity (90%) and showed outstanding long-term stability. These superb H₂S sensing properties were attributed to catalytic sensitization by the Cu dopant and a synergistic effect of the Cu-SnO₂ and rGO, which offered abundant active sites for O₂ and H₂S absorption and accelerated the transfer of electrons/holes.

A new room temperature supra-molecular cryptophane A (CrypA)-coated surface acoustic wave (SAW) sensor for sensing methane gas is presented. The sensor is composed of differential resonator-oscillators, a supra-molecular CrypA coated along the acoustic propagation path, and a frequency signal acquisition module (FSAM). A two-port SAW resonator configuration with low insertion loss, single resonation mode, and high quality factor was designed on a temperature-compensated ST-X quartz substrate, and as the feedback of the differntial oscillators. Prior to development, the coupling of modes (COM) simulation was conducted to predict the device performance. The supramolecular CrypA was synthesized from vanillyl alcohol using a double trimerisation method and deposited onto the SAW propagation path of the sensing resonators via different film deposition methods. Experiential results indicate the CrypA-coated sensor made using a dropping method exhibits higher sensor response compared to the unit prepared by the spinning approach because of the obviously larger surface roughness. Fast response and excellent repeatability were observed in gas sensing experiments, and the estimated detection limit and measured sensitivity are ~0.05% and ~204 Hz/%, respectively.

A polymer-coated surface acoustic wave (SAW)-based chemical sensor for organophosphorous compound sensing at extremely low concentrations was developed, in which a dual-delay-line oscillator coated with fluoroalcoholpolysiloxane (SXFA) acted as the sensor element. Response mechanism analysis was performed on the SXFA-coated chemical sensor, resulting in the optimal design parameters. The shear modulus of the SXFA, which is the key parameter for theoretical simulation, was extracted experimentally. New designs were done on the SAW devices to decrease the insertion loss. Referring to the new phase modulation approach, superior short-term frequency stability (±2 Hz in seconds) was achieved from the SAW oscillator using the fabricated 300 MHz delay line as the feedback element. In the sensor experiment on dimethylmethylphosphonate (DMMP) detection, the fabricated SXFA-coated chemical sensor exhibited an excellent threshold detection limit up to 0.004 mg/m³ (0.7 ppb) and good sensitivity (~485 Hz/mg/m³ for a DMMP concentration of 2~14 mg/m³).

The wireless and passive surface acoustic wave (SAW) sensing system comprises SAW sensors and a SAW reader. In dynamic application scenarios such as strain, torque, and vibration measurements, there is a critical need for high sensing speeds. However, existing high-speed readers encounter challenges related to limited sensing range and reduced demodulation accuracy, which impede the broader adoption of SAW wireless passive sensing technology. This paper presents a time-domain simulation model for SAW echo signals and explores the amplitude-frequency response characteristics of the reader to optimize its excitation time. Additionally, a noise model is developed based on actual data to assess the sensor bandwidth coverage provided by a single reader excitation, thereby optimizing the reader’s interrogation frequency. Experimental platforms for wireless temperature and strain sensing are also constructed. Experimental results indicate that the developed high-speed reader achieves a measurement speed of 13 kHz while maintaining practical measurement accuracy and distance for the SAW sensor.

A surface-acoustic-wave (SAW) gas sensor with a low detection limit and fast response for volatile organic compounds (VOCs) based on the condensate-adsorption effect detection is developed. In this sensor a gas chromatography (GC) column acts as the separator element and a dual-resonator oscillator acts as the detector element. Regarding the surface effective permittivity method, the response mechanism analysis, which relates the condensate-adsorption effect, is performed, leading to the sensor performance prediction prior to fabrication. New designs of SAW resonators, which act as feedback of the oscillator, are devised in order to decrease the insertion loss and to achieve single-mode control, resulting in superior frequency stability of the oscillator. Based on the new phase modulation approach, excellent short-term frequency stability (±3 Hz/s) is achieved with the SAW oscillator by using the 500 MHz dual-port resonator as feedback element. In a sensor experiment investigating formaldehyde detection, the implemented SAW gas sensor exhibits an excellent threshold detection limit as low as 0.38 pg.