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Flexible surface acoustic wave technology has garnered significant attention for wearable electronics and sensing applications. However, the mechanical strains induced by random deformation of these flexible SAWs during sensing often significantly alter the specific sensing signals, causing critical issues such as inconsistency of the sensing results on a curved/flexible surface. To address this challenge, we first developed high-performance AlScN piezoelectric film-based flexible SAW sensors, investigated their response characteristics both theoretically and experimentally under various bending strains and UV illumination conditions, and achieved a high UV sensitivity of 1.71 KHz/(mW/cm²). To ensure reliable and consistent UV detection and eliminate the interference of bending strain on SAW sensors, we proposed using key features within the response signals of a single flexible SAW device to establish a regression model based on machine learning algorithms for precise UV detection under dynamic strain disturbances, successfully decoupling the interference of bending strain from target UV detection. The results indicate that under strain interferences from 0 to 1160 με the model based on the extreme gradient boosting algorithm exhibits optimal UV prediction performance. As a demonstration for practical applications, flexible SAW sensors were adhered to four different locations on spacecraft model surfaces, including flat and three curved surfaces with radii of curvature of 14.5, 11.5, and 5.8 cm. These flexible SAW sensors demonstrated high reliability and consistency in terms of UV sensing performance under random bending conditions, with results consistent with those on a flat surface. [ABSTRACT FROM AUTHOR]

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]

In this study, AlO(OH) (boehmite) film was deposited onto a surface acoustic wave (SAW) resonator using a combined sol-gel and spin-coating technology, and prepared and used as a sensitive layer for a high-performance ammonia sensor. The prepared AlO(OH) film has a mesoporous structure and a good affinity to NH3 (ammonia gas) molecules, and thus can selectively adsorb and react with NH3. When exposed to ammonia gases, the SAW sensor shows an initial positive response of the frequency shift, and then a slight decrease of the frequency responses. The sensing mechanism of the NH3 sensor is based on the competition between mass-loading and elastic-loading effects. The sensor operated at room temperature shows a positive response of 1540 Hz to 10 ppm NH3, with excellent sensitivity, selectivity and stability. [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]

Highlights • Small-size and high-crystallinity PbS CQDs were synthesized via a simple cation exchange method. • The PbS CQDs were successfully integrated into the SAW delay lines by spin-coating at room temperature. • The CQD-coated SAW sensor exhibited high sensor response to low-concentration of NO 2 gas at room temperature. Abstract Surface acoustic wave (SAW) sensors have great advantages in real-time and in-situ gas detection due to their wireless and passive characteristics. Using nanostructured sensing materials to enhance the SAW sensor's responses has become a research focus in recent years. In this paper, solution-processed PbS colloidal quantum dots (CQDs) were integrated into quartz SAW devices for enhancing the performance of NO 2 detection operated at room temperature. The PbS CQDs were directly spin-coated onto ST-cut quartz SAW delay lines, followed by a ligand exchange treatment using Pb(NO 3) 2. Upon exposure to 10 ppm of NO 2 gas, the sensor coated with untreated PbS CQDs showed response and recovery times of 487 s and 302 s, and a negative frequency shift of −2.2 kHz, mainly due to the mass loading effect caused by the absorption of NO 2 gas on the surface of the dense CQD film. Whereas the Pb(NO 3) 2 -treated sensor showed fast response and recovery times of 45 s and 58 s, and a large positive frequency shift of 9.8 kHz, which might be attributed to the trapping of NO 2 molecules in the porous structure and thus making the film stiffer. Moreover, the Pb(NO 3) 2 -treated sensor showed good stability and selectivity at room temperature. [ABSTRACT FROM AUTHOR]

Surface acoustic wave (SAW) technology is promising for humidity monitoring due to its digital output, small size, large-scale production and wireless passive capability, but there are major challenges to achieve ultra-high sensitivity and fast responses using the conventional SAW devices. Herein, ultrahigh frequency (4.7 GHz and 5.9 GHz) shear-horizontal (SH) SAW devices were developed and a ternary nanocomposite strategy of graphene quantum dots/polyethyleneimine/silicon dioxide nanoparticles (GQDs-PEI-SiO 2 NPs) was proposed as a sensitive layer to achieve ultrahigh sensitivity and fast response. This ternary material system was constructed by modifying the surface of SiO 2 NPs with the PEI through an electrostatic force, and then adsorbing the GQDs onto the PEI through hydrogen bonds. Compared with the conventional low frequency SAW devices, the ultrahigh frequency SH-SAW devices showed exceptionally ultra-high sensitivity (2.4 MHz/%RH, 1000 times as high as a 202 MHz SAW device), fast response (20 s/5 s), excellent linearity, and good repeatability in the range of 20–80% RH. These superior performances are attributed to ultrahigh frequency of SAW devices, large specific surface areas of the nanocomposite (which exposed multiple hydrophilic groups in PEI and GQDs), and high vapor pressure of convex spherical curved liquid surface (which accelerated the adsorption and desorption of water molecules). [Display omitted] • 5.9 GHz shear-horizontal SAW humidity sensor with ternary GQDs-PEI-SiO 2 NPs was proposed. • Bonding and humidity sensing mechanisms of the GQDs-PEI-SiO 2 NPs were investigated. • Ultra-high humidity sensitivity of 2.4 MHz/% RH (1000 times as high as a 200 MHz SAW) was achieved. [ABSTRACT FROM AUTHOR]

With extensive and widespread uses of miniaturized and intelligent wearable devices, continuously monitoring subtle spatial and temporal changes in human physiological states becomes crucial for daily healthcare and professional medical diagnosis. Wearable acoustical sensors and related monitoring systems can be comfortably applied onto human body with a distinctive function of non-invasive detection. This paper reviews recent advances in wearable acoustical sensors for medical applications. Structural designs and characteristics of the structural components of wearable electronics, including piezoelectric and capacitive micromachined ultrasonic transducer (i.e., pMUT and cMUT), surface acoustic wave sensors (SAW) and triboelectric nanogenerators (TENGs) are discussed, along with their fabrication techniques and manufacturing processes. Diagnostic applications of these wearable sensors for detection of biomarkers or bioreceptors and diagnostic imaging have further been discussed. Finally, main challenges and future research directions in these fields are highlighted. [ABSTRACT FROM AUTHOR]

It has been a critical challenge to develop highly sensitive H 2 S gas sensors, due to its wide-range application in industry and frequent leakage, which endangers people's lives. In this work, a highly sensitive room-temperature SAW H 2 S gas sensor based on Si-Cu nanocomposite was developed. The Cu content in this composite layer plays a key role for H 2 S response of the sensor because CuO, an excellent adsorption site for H 2 S, presents on the surface of Cu. The amorphous Si content results in the highly porous structure enhancing the interaction between CuO and H 2 S. The interaction leads to the formation of CuS and a negative frequency response of the sensor. H 2 O molecules effectively participate the reactions between CuO and H 2 S, and the responses of H 2 S were significantly enhanced in a moister environment. With the relative humidity of 60 %, the sensor can detect 50 ppb H 2 S with a response of − 225 Hz. [Display omitted] • The surface CuO in the composite adsorbs and reacts with H 2 S and then form CuS. • The formation of CuS lead to the negative frequency responses of the SAW sensors. • The sensing performance of the senor toward H 2 S is critically influenced by H 2 O. • The SAW sensor can precisely detect H 2 S gas ranging from 50 ppb to16 ppm. [ABSTRACT FROM AUTHOR]

[Display omitted] • The SAW sensor has a sensitivity of -490 mm2 ng−1Hz toward AMC. • Antireflection silica coatings are susceptible to AMC. • Changes of weight of the silica coatings cause the response. Real time monitoring of contamination on antireflection (AR) silica coatings in high peak power laser systems (HPLs) is critically needed in order to avoid reductions of transmission and laser damage to optical surfaces. Herein we proposed to apply a surface acoustic wave (SAW) sensor to real-time monitor trace amounts of airborne molecular contaminants (AMCs) adsorbed on the AR silica coatings. The silica coating is found to be susceptible to AMCs because of its mesoporous structure, huge surface area and polar nature. The adsorbed AMCs caused the increased mass on the silica coating of the SAW sensor, which resulted in a significant increase of its frequency shift. The fabricated sensor showed a high sensitivity of ∼-490 mm2 ng−1Hz and an excellent linearity vs. the areal density of adsorbed AMCs since the frequency shift of the sensor is linearly related to the change of mass of the silica coating. [ABSTRACT FROM AUTHOR]

• N doped DLC film has a porous structure and polar groups. • Polar groups in the film can selectively adsorb NH 3 gas molecules. • The adsorbed NH 3 increases film's elastic modulus and the sensing performance. • The SAW sensor can detect 100 ppb NH 3 gas with a response of 700 Hz. We proposed a surface acoustic wave (SAW) NH 3 gas sensor based on nitrogen doped diamond like carbon (N-DLC) film. The N-DLC film, prepared using a microwave electron cyclotron resonance plasma chemical vapor deposition (ECR-PECVD) method, is highly porous and physically and chemically stable, and have active polar groups on its surface, which can selectively absorb polar NH 3 gas molecules. These features of the film lead to the high sensitivity, low noise and excellent stability of the sensor. The sensor can achieve capabilities of in-situ monitoring NH 3 in a concentration range from 100 ppb to 100 ppm with fast response (∼5 s) and recovery (∼29 s) at room temperature. The NH 3 sensing mechanism is attributed to the decreased porosity of the N-DLC film caused by adsorbed NH 3 molecules on its polar groups, which leads an increase of the elastic modulus of the film. [ABSTRACT FROM AUTHOR]

• We manufacture transparent surface acoustic wave ultraviolet photodetectors made on ultra-thin glass. This transmission signal is ∼160 % higher than that the previous polymer based flexible SAW devices. • We apply Graphene quantum dots decorated zinc oxide nanowires (GQDs@ZnO-NWs) to enhance sensing performance of highly flexible SAW sensor. The UV sensitivity of sensors was enhanced by three times and the response time was shorten by four times. • The flexible SAW device maintains good performance under a bending angle of ∼30° for 200 times without visible degradation, showing the excellent flexibility. Graphene quantum dots decorated zinc oxide nanowires (GQDs@ZnO-NWs) were applied to enhance sensing performance of highly flexible and transparent surface acoustic wave (SAW) ultraviolet (UV) photodetectors made on ultra-thin flexible glass. The developed flexible SAW sensors possess better performance than that of the previously developed polymer based flexible SAW devices, due to insignificant acoustic loss of flexible glass substrate. UV sensitivity of the flexible glass based SAW sensors was enhanced by three times, and the response time was shorten by four times after the sensor was coated with the GQDs@ZnO-NWs hybrid nanomaterials. These improvements are mainly attributed to: (1) large specific surface areas of ZnO NWs which can generate a large number of photon-generated carriers; (2) introduction of GQDs which can reduce the carrier recombination rate. The resonant frequency of flexible glass SAW UV photodetectors exhibited a good repeatability and stability in responses to cyclic changes of the UV lights at different wavelengths. They also maintained a good performance under a bending angle of ∼30° for 200 times without apparent degradation, showing the excellent flexibility and stability of the UV photodetector. [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]

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]

• ZnO-Al 2 O 3 layer has a mesoporous structure and good affinity to H 2 S gas. • ZnO in layer can selectively adsorb and react with H 2 S gas molecules to form ZnS. • The formation of ZnS increases layer's elastic modulus and the sensing performance. • The SAW sensor can detect 10 ppb H 2 S gas with a response of 500 Hz. ZnO-Al 2 O 3 nanocomposite was synthesized and developed as a high performance sensitive and selective layer for surface acoustic wave (SAW) sensor, aiming for in-situ detection of H 2 S gas in ppb level operated at room temperature. ZnO-Al 2 O 3 nanocomposite, synthesized though a sol-gel method, was spin-coated onto a quartz based SAW resonator. This composite layer inherits the mesoporous structure of the Al 2 O 3 layer and good affinity to H 2 S gas molecules of the ZnO layer, and thus can selectively adsorb and react with H 2 S gas molecules to form ZnS compounds on its surface. This reaction leads to significant decreases of both pore sizes and total pore volume of the layer, an increase of layer's elastic modulus, thus causing a large positive shift of the frequency responses of the SAW sensor. The sensor operated at room temperature shows a frequency response of ∼500 Hz to 10 ppb H 2 S, with an excellent selectivity and good recovery property. [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]