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In this study, polyacrylic acid (PAA) films were deposited onto a quartz surface acoustic wave (SAW) resonator using a spin-coating technique for ammonia sensing operated at room temperature, and the sensing mechanisms and performance were systematically studied. The oxygen-containing functional groups on the surfaces of the PAA film make it sensitive and selective to ammonia molecules, even when tested at room temperature. The ammonia molecules adsorbed by the oxygen-containing functional groups of PAA (e.g., hydroxyl and epoxy groups) increase the membrane's stiffness, which was identified as the primary mechanism leading to the positive frequency shifts. However, mass loading due to adsorption of ammonia molecules is not a major reason as it will result in a negative frequency shifts. When the PAA coated SAW sensor was exposed to ammonia with a low concentration of 500 ppb, it showed a positive frequency shift of 225 Hz, with both good repeatability and stability, as well as a good selectivity to ammonia compared with those to C2H5OH, H2, HCl, H2S, CO, NO2, NO, and CH3COCH3. [ABSTRACT FROM AUTHOR]

Mucosal-like ZnS nanostructures were synthesized on surface of ST-cut quartz surface acoustic wave (SAW) device using a chemical bath deposition method for ammonia sensing applications. Results showed the SAW device with ZnS mucosal nanostructures achieved a good sensitivity, e.g., with a frequency shift of ~190 Hz upon exposure to 1 ppm ammonia gas. This is mainly attributed to the large specific surface areas and more active sites on the surfaces of the ZnS mucosal nanostructures. Selectivity of the SAW device with the ZnS mucosal nanostructure for ammonia sensing was excellent in comparisons with those for other types of gases such as hydrogen sulfide, hydrogen, nitrogen dioxide, carbon monoxide and ethanol. Unlabelled Image • Mucosal-like ZnS nanostructures were deposited on surface of SAW device for sensing ammonia. • The SAW device with ZnS mucosal nanostructures achieved a good sensitivity and selectivity to ammonia. • The negative frequency shift of SAW sensor is associated to the effects of mass loading and electrical loading. [ABSTRACT FROM AUTHOR]

A surface acoustic wave (SAW) H 2 S gas sensor based on CuO-TiO 2 p-n heterojunction film was designed and fabricated. The sensor exhibits high sensitivity to H 2 S due to the significant current response of CuO-TiO 2 film to H 2 S gas and the appropriate work point of SAW sensor based on CuO-TiO 2 composite film. The linearity change for sensor frequency shift is strongly associated with the location of work point when the H 2 S concentration increases from 1 to 300 ppm. In addition, the porosity of CuO-TiO 2 film is beneficial to the efficient adsorption for H 2 S, leading to the higher frequency shift of the sensor. The SAW sensor also shows good selectivity and stability to H 2 S with stable frequency shift under different humidity levels. • A H 2 S gas sensor is designed and fabricated using CuO-TiO 2 composite film. • The establishment of CuO-TiO 2 p-n junction significantly enhances the sensitivity of SAW sensor to H 2 S. • The linearity variation of frequency shift is closely related to the position of work point at different H 2 S concentrations. • The key sensing mechanism of SAW sensor based on CuO-TiO 2 film is electric loading. • The SAW sensor shows fast response in high humidity level, and good gas selectivity and repeatability. [ABSTRACT FROM AUTHOR]

Surface acoustic wave (SAW) technology has been widely used in the field of harmful gas detection, but there remain a lot of challenges, such as long response/recovery time as well as poor selectivity and repeatability. Herein, we activate graphene oxide (GO) using MXene with excellent adsorption capacity (MXene/GO) to realize efficient NO 2 gas detection. The SAW sensor assembled with the multilayered MXene/GO composite showcases an enhanced gas sensing performance to the NO 2 gas (1–200 ppm) enabling high sensitivity (Δf = 260 Hz/ppm), fast response/recovery time, excellent selectivity, and good repeatability at room temperature. The enhanced high sensitivity and selectivity detection of NO 2 gas is attributed to the large pore size, hydrophilicity, and abundant surface functional groups of the multilayered MXene/GO composite. This elaborate design is expected to propel the development of next-generation acoustic gas phase sensors. [Display omitted] • The MXene is investigated in surface acoustic wave sensors for the first time and filles the material gap. • The facile synthetic method for the MXene/GO composite is expected to realize rapid and large-scale synthesis of sensitive layers for surface acoustic wave sensors. • The working mechanism of MXene-based sensitive layers is proposed and the factors to influence sensing performance are also considered. • The excellent sensing performance of MXene/GO-based surface acoustic wave sensors is obtained under different conditions. • A novel MXene/GO composite is proposed for the first time to evaluate its working mechanism and sensing performance in SAW gas sensors. [ABSTRACT FROM AUTHOR]

• The sensor can detect H 2 S gas with a concentration as low as 0.5 ppm. • The frequency shift of sensor is derived from the change of mass loading on films. • The responses became much faster and stronger with the increase of RH. Surface acoustic wave (SAW) sensors based on sol–gel processed porous and nanoparticulate CuO films were explored for H 2 S detection operated at room temperature. The SAW sensor showed a negative frequency shift when exposed to H 2 S gas, due to the mass loading effect on the sensitive CuO film. It has a high sensitivity of H 2 S, a detection limit of 500 ppb and a good selectivity to H 2 S gas compared with H 2 , C 2 H 5 OH, acetone, NH 3 , CO, NO, NO 2 gases. Relative humidity (RH) was identified to have a critical influence on the sensing performance of the sensor. The responses became much faster and stronger with the increase of RH. The significant enhancement in the sensing performance is attributed to that the chemical reaction between H 2 S and CuO is hindered and the physical adsorption of CuO to H 2 S is enhanced. [ABSTRACT FROM AUTHOR]

• CuO in mesoporous film can selectively adsorb and react with H 2 S gas molecules. • The adsorbed H 2 S changes both the film's mass and conductivity. • The SAW sensor has better performance because of the mass sensitive mechanism. • The SAW sensor can precisely detect H 2 S gas ranging from 5 ppb to100 ppm. Surface acoustic wave and chemiresistor based gas sensors integrated with a sensing layer of sol-gel CuO-Al 2 O 3 composite film were fabricated and their performance and mechanisms for H 2 S sensing were characterized and compared. In the composite film, CuO nanoparticles provide active sites for adsorption and reaction of H 2 S molecules while Al 2 O 3 nanoparticles help to form a uniform and mesoporous film structure, both of which enhance the sensitivity of the sensors by providing numerous active CuO surfaces. Through the comparative studies, the SAW based H 2 S sensor operated at room temperature showed a lower detection limit, higher sensitivity, better linearity and good selectivity to H 2 S gas with its concentration ranging from 5 ppb to 100 ppm, compared with those of the chemiresistor sensor, which are mainly attributed to the effective mass sensing properties of the SAW sensor, because a minor change in the mass of the film caused by adsorbed H 2 S molecules would lead to a significant and monotonous change of the resonant frequency of the SAW devices. [ABSTRACT FROM AUTHOR]

Gas sensors detecting low HCl gas concentrations used for early detection and continuous monitoring of leakage is of great concern because HCl gas is extremely hazardous. In order to tackle the potential hazardous threats, surface acoustic wave HCl gas sensors based on hydroxyl-rich sol-gel AlO x OH y films are reported. The fabricated SAW sensors show negative frequency shifts with exposure to HCl gas. The negative frequency shift is found to be caused by the increasing weight of sensitive films induced by the adsorption of HCl assisted by the hydroxyl groups and H 2 O captured on the films. The sensing performance of sensors is highly dependent on the heat-treatment temperature and thickness of films, as well as the ambient humidity because these parameters are highly influential on the amount of hydroxyl groups, H 2 O on films and the pore volume of films. For a sensor with a 198-nm-thick film annealed at 300 °C, it shows a response of ~ -1.4 KHz to HCl at a low concentration level (0.05 ppm) in an environment with a temperature of 25 °C and a RH value of 40%. Image 10822 • The sensor can detect HCl gas with a concentration as low as 0.05 ppm. • The hydroxyl groups and H 2 O on the sol-gel AlO x (OH) y films assist the response. • The frequency shift of sensor is derived from the change of weight of the films. [ABSTRACT FROM AUTHOR]

• The sensor can detect HCl gas with a concentration as low as 1 ppm. • H 2 O on CNCs layers assists the response. • Changes of mass of the CNCs layers cause the response. We report that cellulose nano-crystals (CNCs) can be used as a sensitive and selective layer in surface acoustic wave (SAW) sensors for in-situ HCl gas detection. CNCs were prepared through directly hydrolysis of cotton fiber and were spin-coated on quartz SAW resonators to form the sensitive layer. The CNCs have been identified to have abundant hydroxyl groups on their surfaces, which can act as the perfect adsorption sites for H 2 O, which can further act as the active sites for HCl gas adsorption. The absorption of HCl on the CNCs layer, thus leads to an increase of its mass, causing negative responses of the SAW sensors. Ambient humidity and thickness of CNCs layer are found to have significant influences on the responses of the SAW sensor. With an 80-nm-thick CNCs layer, the sensor shows a response of −2 kHz to 1 ppm HCl at 25 °C and relative humidity of 50 % with an excellent selectivity and recovery characteristics. [ABSTRACT FROM AUTHOR]