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Purpose: Surface acoustic wave (SAW) sensors have attracted great attention worldwide for a variety of applications in measuring physical, chemical and biological parameters. However, stability has been one of the key issues which have limited their effective commercial applications. To fully understand this challenge of operation stability, this paper aims to systematically review mechanisms, stability issues and future challenges of SAW sensors for various applications. Design/methodology/approach: This review paper starts with different types of SAWs, advantages and disadvantages of different types of SAW sensors and then the stability issues of SAW sensors. Subsequently, recent efforts made by researchers for improving working stability of SAW sensors are reviewed. Finally, it discusses the existing challenges and future prospects of SAW sensors in the rapidly growing Internet of Things-enabled application market. Findings: A large number of scientific articles related to SAW technologies were found, and a number of opportunities for future researchers were identified. Over the past 20 years, SAW-related research has gained a growing interest of researchers. SAW sensors have attracted more and more researchers worldwide over the years, but the research topics of SAW sensor stability only own an extremely poor percentage in the total researc topics of SAWs or SAW sensors. Originality/value: Although SAW sensors have been attracting researchers worldwide for decades, researchers mainly focused on the new materials and design strategies for SAW sensors to achieve good sensitivity and selectivity, and little work can be found on the stability issues of SAW sensors, which are so important for SAW sensor industries and one of the key factors to be mature products. Therefore, this paper systematically reviewed the SAW sensors from their fundamental mechanisms to stability issues and indicated their future challenges for various applications. [ABSTRACT FROM AUTHOR]

Highly sensitive surface acoustic wave (SAW) sensors have recently been recognized as a promising tool for various industrial and medical applications. However, existing SAW sensors generally suffer from a complex design, large size, and poor robustness. In this paper, we develop a simple and stable delay line ultra-high frequency (UHF) SAW sensor for highly sensitive detection of temperature. A Z-shaped delay line is specially designed on the piezoelectric substrate to improve the sensitivity and reduce the substrate size. Herein, the optimum design parameters of extremely short-pitch interdigital transducers (IDTs) are given by numerical simulations. The extremely short pitch gives the SAW sensor ultra-high operating frequency and consequently ultra-high sensitivity. Several experiments are conducted to demonstrate that the sensitivity of the Z-shaped SAW delay line sensor can reach up to 116.685°/°C for temperature detection. The results show that the sensor is an attractive alternative to current SAW sensing platforms in many applications. [ABSTRACT FROM AUTHOR]

A highly sensitive and precise Love wave mode surface acoustic wave (SAW) immunosensor based on an ST-cut 90°X quartz substrate and an SiO2 wave-guiding layer was developed to detect cancer-related biomarkers of carcinoembryonic antigens (CEAs). A delay line structure of the SAW device with a resonant frequency of 196 MHz was designed/fabricated, and its surface was functionalized through CEA antibody immobilization. The CEA antibodies were bound with gold nanoparticles and CEA antibodies to form a sandwich structure, which significantly amplified the mass loading effect and enhanced the maximum responses by 30 times. The center frequency of the Love wave immunosensor showed a linear response as a function of the CEA concentration in the range of 0.2–5 ng/mL. It showed a limit of detection of 0.2 ng/mL, and its coefficient of determination was 0.983. The sensor also showed minimal interference from nonspecific adsorptions, thus demonstrating its promise for point-of-care applications for cancer biomarkers. [ABSTRACT FROM AUTHOR]

Acoustic wave devices have great potential for integration with lab-on-chip highly efficient microfluidics. This article investigates Lamb wave-based unidirectional transducers for application in acoustic wave-driven microfluidic devices with high efficiency. The simulation of the unidirectional transducer is performed via the finite element analysis. The optimal cell design of the transducer is suggested according to the Lamb wave uneven excitation. In particular, we propose a sophisticated double-side IDT pattern to enhance Lamb wave transduction. The anti-symmetric ${A}_{{0}}$ mode implemented with double-side unidirectional transducers is determined and optimized for the microfluidic device application. The optimum Lamb wave-based devices are fabricated on a wafer of 128° YX LiNbO3 with a thickness of 300 $\mu \text{m}$ using an elaborate two-side lithography technique. The amplitude of Lamb waves excited from the unidirectional transducers are measured and confirmed the unidirectionality, accordingly. Thorough atomization and jetting experiments driven by the unidirectional transducer are presented. The results agree with the simulation and verify the efficiency of the proposed double-side patterned unidirectional transducers in microfluidic applications. [ABSTRACT FROM AUTHOR]

The rapid development of the fifth-generation (5G) wireless system is driving strong demand for high-performance radio frequency filters. This work studies shear horizontal surface acoustic wave (SAW) devices using 15°-rotated Y-cut X-propagating (15°Y-X) LiNbO3/SiO2/SiC multilayered substrates. Single-crystalline 15°Y-X LiNbO3 films are bonded to SiO2/SiC handling substrates by the smart cut technology. On the basis of accurate finite-element-method simulations, LiNbO3/SiO2/SiC wafer configurations are optimized to suppress spurious resonance due to Rayleigh-mode and transverse-mode responses, and one-port resonators with a clean spectrum, a high electromechanical coupling coefficient of 22.00%, and an admittance ratio (impedance ratio) over 65 dB are successfully implemented. Based on the characteristics of the resonators, high-performance filters with a center frequency of 1.28 GHz, a large 3-dB fractional bandwidth of 16.65%, and a low minimum insertion loss of 1.02 dB are successfully designed and fabricated. Furthermore, no ripples in the passband of the filters are observed. Additionally, the filters exhibit a temperature coefficient of center frequency of −63.8 ppm/°C and a large power durability of 33.2 dBm. This work confirms the high performances of the SAW devices using the 15°Y-X LiNbO3/SiO2/SiC multilayered substrate, and this type of SAW device exhibits a prospect of commercial applications in the 5G wireless system. [ABSTRACT FROM AUTHOR]

Different acoustic wave modes are required for effective implementation of biosensing and liquid actuation functions in an acoustic wave-based lab-on-a-chip. For efficient sensing in liquids, shear waves (either a thickness-shear bulk wave or a shear-horizontal surface acoustic wave) can achieve a high sensitivity, without significant loss of acoustic wave energy. On the other hand, longitudinal bulk waves or out-of-plane displacement waves (such as Rayleigh waves) enable efficient sampling functions and liquid manipulation. However, there are significant challenges in developing a lab-on-a-chip to efficiently generate multiple wave modes and perform both these functions on a single piezoelectric substrate, especially when a single crystalline orientation is available. This paper highlights the latest progress in the theories and techniques to deliver both sensing and microfluidic manipulation functions using engineered inclined-angled piezoelectric films, allowing for the simultaneous generation of longitudinal (or Rayleigh) and thickness-shear bulk (or shear-horizontal surface acoustic) waves. Challenges and theoretical constraints for generating various wave modes in the inclined films and techniques to efficiently produce inclined columnar and inclined crystalline piezoelectric films using sputtering deposition methods are presented. Applications of different wave modes in the inclined film-based lab-on-chips with multiple sensing and acoustofluidic functions are also discussed. [ABSTRACT FROM AUTHOR]

Nozzleless jetting of droplets with different jetting angles is a crucial requirement for 2D and 3D printing/bioprinting applications, and Rayleigh mode surface acoustic waves (SAWs) could be a potential technique for achieving this purpose. Currently, it is critical to vary the jetting angles of liquid droplets induced by SAWs and control the liquid jet directions. Generally, the direction of the liquid jet induced by SAWs generated from a bulk piezoelectric substrate such as LiNbO3 is along the theoretical Rayleigh angle of ∼22°. In this study, we designed and manufactured thin-film SAW devices by depositing ZnO films on different substrates (including silicon and aluminium) to realize a wide range of jetting angles from ∼16° to 55° using propagating waves generated from one interdigital transducer. We then systematically investigated different factors affecting the jetting angles, including liquid properties, applied SAW power and SAW device resonant frequency. Finally, we proposed various methods using thin-film SAW devices together with different transducer designs for realizing a wide range of jetting angles within the 3D domain. A nozzleless jetting method is proposed using thin-film based surface acoustic wave devices to achieve a wide range of jetting angles for droplets. [ABSTRACT FROM AUTHOR]

Shear-horizontal (SH) surface acoustic wave (SAW) propagation characteristics in IDT/(110)ZnO/SiO 2 /Si multilayered structure were theoretically investigated using 3-dimensions (3D) finite element analysis (FEA). X-ray diffraction result shows that the prepared ZnO films have preferred (110) orientation with its c -axis parallel to the substrate. 3D FEA results and the out of plane vibration experiment confirmed that SH-SAW was successfully excited in the multilayered structure. The phase velocities V p , electromechanical coupling coefficients K 2 , and temperature coefficient of frequency (TCF) dispersion properties of mode 0 were studied considering various thicknesses of ZnO and SiO 2 thin films, which were verified by experiments. SiO 2 film with a positive TCF not only compensate the negative TCF of the (110) ZnO but also enhance the K 2 . A large K 2 of 3.37 and nearly zero TCF of −0.1 ppm/°C was achieved in SH-SAW device based on IDT/(110)ZnO(2 μm)/SiO 2 (1 μm)/Si multilayer structure, which is promising for high sensitive and temperature stable bio-sensing application. [ABSTRACT FROM AUTHOR]

While protein kinases play a pivotal role in various fundamental biological processes, their overexpression can lead to several human diseases. Herein, we propose a highly sensitive leaky surface acoustic wave (LSAW) biosensor for detecting protein kinase A (PKA) activity based on peptide–templated copper nanoclusters for signal enhancement. The substrate peptide P1, specific for PKA, was self–assembled on a gold-coated SAW surface. In the presence of PKA and adenosine-5′-triphosphate (ATP), the serine within P1 was phosphorylated. Another peptide, P2, which contained a cluster formation sequence CCY, was used to prepare the peptide-templated copper nanoclusters (P2-CuNCs) and a guanidine group within arginine residue was used to identify the phosphorylation site specifically. The P2-CuNCs were captured on phosphorylated peptide P1 through covalent-like the interaction of guanidine–phosphate, which resulted in amplified SAW signals. Under the optimal conditions, the SAW sensor showed a quantitative readout to PKA concentration in the range 0−50 U/mL with a detection limit of 0.02 U/mL for PKA. In addition, the biosensor was used to screen protein kinase inhibitors and detect protein kinase activity in drug–stimulated complex cell lysates, showing great potential use in drug discovery and disease diagnosis. [Display omitted] • We reported a novel leaky surface acoustic wave biosensor for detection of PKA activity. • Versatile peptide–templated copper nanoclusters (P2–CuNCs) were employed for highly sensitive detection of PKA. • The SAW sensor showed high sensitivity, with a detection limit of 0.02 U/mL for PKA. • The biosensor was used to detect protein kinase activity in drug-stimulated complex cell lysates. [ABSTRACT FROM AUTHOR]

Highly c-axis oriented aluminum nitride (AlN) films, which can be used in flexible surface acoustic wave (SAW) devices, were successfully deposited on polyimide (PI) substrates by direct current reactive magnetron sputtering without heating. The sputtering power, film thickness, and deposition pressure were optimized. The characterization studies show that at the optimized conditions, the deposited AlN films are composed of columnar grains, which penetrate through the entire film thickness (~2 μm) and exhibit an excellent (0002) texture with a full width at half maximum value of the rocking curve equal to 2.96°. The film surface is smooth with a root mean square value of roughness of 3.79 nm. SAW prototype devices with a center frequency of about 520 MHz and a phase velocity of Rayleigh wave of about 4160 m/s were successfully fabricated using the AlN/PI composite structure. The obtained results demonstrate that the highly c-axis oriented AlN films with a smooth surface and low stress can be produced on relatively rough, flexible substrates, and this composite structure can be possibly used in flexible SAW devices. [ABSTRACT FROM AUTHOR]

We propose an ultrasensitive leaky surface acoustic wave (LSAW) immunosensor based on molybdenum disulfide @ cuprous oxide—gold (MoS2@Cu2O-Au) nanoparticles and subsequent gold staining for the detection of alpha-fetoprotein (AFP). MoS2@Cu2O-Au nanoparticles, with their large specific surface area and good biocompatibility, not only capture the secondary antibodies (Ab2) but also amplify the mass loading effect of the acoustic wave sensor in the detection of AFP. The immunosensor signals are further amplified upon injection of gold staining solution. The developed immunosensor achieved a low detection limit of 5.5 and 25.0 pg/mL with and without gold staining, respectively. The immunosensor demonstrated its efficiency for the quantitative detection of AFP in complex biological fluids, including human serum and saliva samples, with excellent selectivity and long-term stability, showing great potential for the quantification of AFP in clinical diagnosis. [ABSTRACT FROM AUTHOR]

Abstract: A sensitive glucose biosensor without using glucose test strips was fabricated based on a ZnO/SiO2/Si surface acoustic wave (SAW) device. The SAW devices were functionalized by immobilizing glucose oxidase onto a pH-sensitive polymer which was coated on the sensitive area of the SAW device. The sensitivity of the SAW glucose biosensor was 1.589MHz/mM and the detection limit was lower than 6.29×10−4 mM, which was sensitive enough for glucose testing. The SAW glucose biosensor system showed good anti-interference to ascorbic acid, lactic acid and uric acid. The SAW glucose biosensor system kept a constant differential frequency for 10h, demonstrating a good degree of reversibility and stability. [Copyright &y& Elsevier]

• Real-time detection of CEA using SAW device. • Obtained a limit of detection of 0.31 ng/ml of CEA. • SAW devise for anti-CEA with minimum increase in insertion loss under liquid state analysis. • High selectivity towards CEA from a mixture of similar tumour marking proteins. • High stability over 30 days with only around 8% performance lose. • Validated using Langmuir and Freundlich sorption isotherms. A Love-mode surface acoustic wave (SAW) biosensor based on ST-cut quartz was developed for highly selective and label-free detection of carcinoembryonic antigen (CEA). The delay line area of an interdigital transducer (IDT) based SAW device was coated with gold and then chemically modified through thioglycolic acid–EDC/NHS reaction mechanism. A self-assembled monolayer of anti-CEA was further immobilized on the bioreceptors through the coupling layer. The biosensing capability of the SAW device was evaluated using solutions of CEA with various concentrations and limit of detection was obtained at 0.31 ng/ml of CEA, which is better than the results reported by the literatures available for CEA detection using SAW device. The real-time detection capability of the biosensor was evaluated using clinical serum samples and selectivity was evaluated using mixed solutions of CEA with other common tumor marking proteins. Long-term stability of the biosensor was also evaluated over a period of 30 days and the immunoassay response has shown only 8% decrease in performance within the whole period. The binding of CEA onto the bioreceptor was evaluated through Langmuir and Freundlich sorption isotherm kinetic studies as well. [ABSTRACT FROM AUTHOR]

Herein, presents a novel method for the preparation of a Love mode SAW biosensor for the selective detection of carcinoembryonic antigen (CEA) using a transuding polymer nanocomposite thin film based bioreceptor. Graphene oxide (GO) was synthesized using modified Hummers' method and flower-like MoS 2 nanoparticles were allowed to grow on the 2D layers GO. The rGO-MoS 2 was further used as a host for the synthesis of Au nanoparticles (AuNP) and the final three-component nano-cluster was introduced to the previously synthesized polyamic acid diethyl ethanolamine salt precursor. The uniform mixture was coated on the delay line area of SAW device and conducted thermal imidization process to obtain polyimide nanocomposite. The thickness of the thin film was optimized based on the insertion loss and centre frequency response of the SAW device. Further, anti-CEA self-assembled monolayer (SAM) based bioreceptor was prepared on the polyimide nanocomposite thin film through thioglycolic acid – EDC/NHS immobilization mechanism. The bioreceptor was tested for immunoassay analysis with CEA solution with varying concentrations. The LOD of the biosensor was estimated at 0.084 ng/ml. The real-time applicability of the biosensor was validated using clinical serum sample analysis and the selectivity was evaluated through the affinity test towards other common tumour marking proteins. The biosensor also showed excellent stability, only 10% reduction activity was observed till 80th day of storage. The antigen-antibody adsorption parameters were also evaluated through Langmuir and Freundlich adsorption isotherms. • A novel nanomaterial cluster of AuNP-MoS 2 -rGO was synthesised. • A thin film of the same with PI was used for biosensing of CEA. • Limit of detection of the biosensor was obtained at 0.084 ng/ml. • The biosensor has recorded excellent stability for 70-80 days. • The biosensor was validated through clinical serum sample analysis and compared with ELISA results. [ABSTRACT FROM AUTHOR]

Propagation of surface acoustic waves (SAWs) on bulk piezoelectric substrates such as LiNbO 3 and quartz, exhibits an in-plane anisotropic effect due to their crystal cut orientations. Thin film SAW devices, such as those based on ZnO or AlN, offer potential advantages, including isotropic wave velocities in all in-plane directions, higher power handling capability, and potentially lower failure rates. This paper reports experimental and simulation results of nebulization behaviour for water droplets using ZnO/Si surface acoustic wave devices with focused interdigital transducers (IDTs). Post-deposition annealing of the films at various temperatures was applied to improve the quality of the sputtering-deposited ZnO films, and 500 °C was found to be the optimal annealing temperature. Thin film ZnO/Si focused SAW devices were fabricated using the IDT designs with arc angles ranging from 30° to 90°. Nebulization was significantly enhanced with increasing the arc angles of the IDTs, e.g., increased nebulization rate, reduced critical powers required to initialise nebulization, and concentration of the nebulised plume into a narrower size of spray. Effects of applied RF power and droplet size have been systematically studied, and increased RF power and reduced droplet size significantly enhanced the nebulization phenomena. • Post-deposition annealing was applied to improve the quality of the ZnO film, and 500 °C was found to be the optimal annealing temperature. • Effects of arc central angle, RF power and droplet size on the nebulization performance of ZnO/Si focused SAW devices were investigated. • Acoustic heating effect for nebulization of thin film ZnO/Si focused SAW devices was investigated. [ABSTRACT FROM AUTHOR]

There are significant challenges in controlling uniformity of crystal inclination angles, growth orientations and film thicknesses to generate dual-mode surface acoustic waves (e.g., Rayleigh ones and shear-horizontal ones) for lab-on-a-chip applications. In this study, we demonstrate large area (up to three inches) and uniformly inclined piezoelectric ZnO films, sputtering-deposited on silicon using a glancing angle deposition method. Characterization using X-ray diffraction showed that the inclined ZnO films have an average crystal inclination angle of 29.0°, apart from the vertical (0002) orientation, at a substrate tilting angle of 30o. Reflection signals of ZnO/Si surface acoustic wave devices clearly show the generations of both shear horizontal surface acoustic waves and Rayleigh waves. The Rayleigh waves enable efficient acoustofluidic functions including streaming and transportation of sessile droplets. Excitation direction of Rayleigh waves on the acoustofluidics versus the inclined angle direction has apparent influences on the acoustofluidic performance due to the anisotropic microstructures of the inclined films. The same device has been used to demonstrate biosensing of biotin/streptavidin interactions in a liquid environment using the shear-horizontal surface acoustic waves, to demonstrate its potential for integration into a complete lab-on-a-chip device. [ABSTRACT FROM AUTHOR]

Molecularly imprinted templates for L -Tryptophan sensor using hydrophilic poly (vinylidene fluoride) (PVDF) was fabricated through in-situ polymerization of 2-hydroxyethyl methacrylate (HEMA) within PVDF matrix. The work proposes a novel strategy for the integration of antifouling layer modification for water treatment technique into molecular imprinting polymer for surface acoustic wave (SAW) sensor fabrication and demonstrates their effective utilization in direct biological applications. The L -Tryptophan templates were carefully prepared on the delay line area of a SAW device prepared through photolithographic method internally. The imprinted template was qualitatively analysed using Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and atomic force microscopy (AFM) to understand the bond formation and distribution of imprinted textures on the receptor surface. Quantitative measurements as the L -Tryptophan receptor was conducted through SAW analysis within range of concentration 0.5–150 ng/ml and the sensor responded linearly with the varying concentration. The limit of detection and limit of quantification calculated to be 0.2 and 0.6 ng/ml, respectively. The mechanism of adsorption was further studied using Langmuir, Freundlich and Redlich-Peterson adsorption isotherm. The MIP modified SAW sensor had high selectivity in the presence of structural analogues such as D- tryptophan, ascorbic acid, leucine and tyrosine. The hydrophilic PVDF coated MIP-SAW sensor exhibited relatively high imprinting factor towards L -tryptophan of about 9.34. Schematic diagram of SAW sensing setup [Display omitted] • Hydrophilic poly(vinylidene fluoride) (PVDF) film based MIP is developed via in situ polymerization technique. • The limit of detection and limit of quantification were determined as 0.2 ng/ml and 0.6 ng/ml respectively. • The experimental findings indexed with various theoretical adsorption models. • MIP system exhibited a good selectivity in the presence of structural analogues. [ABSTRACT FROM AUTHOR]

Shear horizontal surface acoustic wave (SH-SAW) sensors are regarded as a promising alternative for label-free, sensitive, real time and low-cost detection. Nevertheless, achieving high sensitivity with SH-SAW has approached its limit imposed by the mass transport and probe-target affinity. We present here an SH-SAW biosensor accompanied by a unique Rayleigh wave-based actuator. The platform assembled on an ST-quartz substrate consists of dual-channel SH-SAW delay lines fabricated along a 90°-rotated direction, whilst another interdigital electrode (IDT) is orthogonally placed to generate Rayleigh waves so as to induce favourable streaming in the bio-chamber, enhancing the binding efficiency of the bio-target. Theoretical foundation and simulation have shown that Rayleigh acoustic streaming generates a level of agitation that accelerates the mass transport of the biomolecules to the surface. A fourfold improvement in sensitivity is achieved compared with conventional SH-SAW biosensors by means of complementary DNA hybridization with the aid of the Rayleigh wave device, giving a sensitivity level up to 6.15 Hz/(ng/mL) and a limit of detection of 0.617 ng/mL. This suggests that the proposed scheme could improve the sensitivity of SAW biosensors in real-time detection. [ABSTRACT FROM AUTHOR]

Abstract Piezoelectric ZnO film-based multilayer structures with versatile surface acoustic wave (SAW) modes are promising for use in the cutting-edge lab-on-chip-level bio-sensing applications which are desired to integrate sensing and microfluidics functions on the same miniature chip. This paper investigates a multilayer structure of SiO 2/ ZnO 11 2 ¯ 0 /r-sapphire with the feasibility of Rayleigh and Love wave modes along the two orthogonal directions. Theoretical calculations are performed to analyze different SAW modes and to optimize the structure parameters accordingly. ZnO is prepared by magnetron sputtering and its optimal fabrication parameters of sputtering pressure of 0.2 Pa, pure oxygen ambient gas and substrate temperature (500 °C) are determined towards the preferred 11 2 ¯ 0 orientation. The crystallographic characteristics and surface morphology of the fabricated ZnO film are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface vibration trajectory of the two modes both are measured using a laser vibrometer. It is verified the SAW mode parallel to the c-axis (0002) is a Rayleigh wave while that along 1 1 ¯ 00 is an SH-SAW/Love wave. As the SAWs encounter a PDMS cell filled with water on the film surface, the Love wave shows an insignificant acoustic damping of less than 2 dB and a reliable transmission response is maintained. In contrast, the acoustic energy of the Rayleigh wave based devices mostly radiates into liquid accompanied by an insertion loss of 13 dB and leading to obvious streaming. Highlights • Rayleigh wave and Love wave modes in a 11 2 ¯ 0 ZnO film based multilayer structure haven been investigated and verified. • The fabrication of sputtering ZnO thin film with the orientation of 11 2 ¯ 0 have been researched. • An SAW devices prototype combining sensing and microfluidic has been demonstrated. [ABSTRACT FROM AUTHOR]

Humidity sensing is crucial for food production, environmental monitoring, microelectronic manufacture, and common living conditions. Thus, there is a high demand for development of high-performance humidity sensors with good stability, high precision and fast responses. In this study, Love wave mode humidity sensors were prepared using quartz-based SAW devices coated with composite sensing layers of accordion-like Ti 3 C 2 T x , MoS 2 nanoflowers and MoS 2 @Ti 3 C 2 T x hierarchical structure, respectively. The hydrophilic Ti 3 C 2 T x was used to provide abundant surface functional groups for adsorption of water molecules, and the hydrophobic MoS 2 enabled rapid adsorption and desorption of water molecules from the surface of MoS 2 @Ti 3 C 2 T x. The MoS 2 @Ti 3 C 2 T x coated Love wave humidity sensor presented an ultra-high sensitivity of 0.67 kHz/% RH in the wide humidity range from 11.3% RH to 97.3% RH, mainly attributed to the hierarchical structure of MoS 2 @Ti 3 C 2 T x with large specific surface areas, abundant adsorption sites and superhydrophilicity. Meanwhile, this Love wave humidity sensor showed fast response/recovery rates, great repeatability and long-term stability, indicating its great prospects in humidity detection applications. • The MoS 2 @Ti 3 C 2 T x hierarchical structure was successfully synthesized. • The proposed hierarchical structure promotes humidity sensing property. • The humidity sensor exhibits high sensitivity and fast response speed. • The sensing mechanism of the hierarchical structure is analyzed in detail. [ABSTRACT FROM AUTHOR]

Transparent microfluidic devices based on ZnO thin film/glass surface acoustic waves (SAWs) were explored for active surface cleaning based on its acoustofluidic performance. Acoustic waves generated from ZnO films on glass substrate were investigated and their acoustofluidic performance including transportation, jetting and nebulization were evaluated. Ash particles and starch solutions were used as model contaminants on the surface of the ZnO/glass SAW devices, and the mass loading of the contaminants on the device's surface was monitored using the SAW device with a high sensitivity of 280.0 ± 9.0 Hz/(μg/mm2). Active surface cleaning of the contaminants was demonstrated based on the transportation of water droplets, and optimized SAW powers were identified which caused strong interactions between water droplet and contaminants, thus effectively cleaning the surfaces. Studies of surface heating effects induced by SAWs showed that the cleaning efficiency was also influenced by the substrate temperature induced by SAW agitations. [Display omitted] • An active surface cleaning platform is developed based on acoustofluidics generated using ZnO/glass thin film acoustic waves. • Monitoring surface contaminants is achieved using the same surface acoustic wave (SAW) device. • Efficiency of active surface cleaning is based on various acoustofluidics functions and acousto-surface heating effect. • Integrated functions with feedback of contamination and active surface cleaning based on SAW device are proposed. [ABSTRACT FROM AUTHOR]

Surface acoustic wave (SAW) sensors have great advantages in real-time and in-situ gas detection due to the inherent advantages of passive and wireless operation. This inevitably requires the sensor to improve gas sensing performances at room temperature. The facet engineering of metal oxide (MOS) provided an effective way to obtain MOS gas-sensitive materials with superior performance by the facet-dependent properties. Here, we developed a novel strategy to prepare SnO 2 quantum wires with different (110) facets ratio by adjusting the synthesis time. The as-prepared SnO 2 quantum wires were integrated into SAW devices to optimize the performance of the gas sensor. When the synthesis time was 8 h, the response of the SAW gas sensor was enhanced with the frequency change of 17 kHz, the response and recovery times of 45 s and 96 s, respectively. Our experimental results revealed that the effect of mass loading was the responsible underlying mechanism for the superior NO 2 gas sensing performances. In addition, the adsorption and charge transfer properties between the SnO 2 surface and NO 2 molecules were further discussed by QCM and resistance measurement, respectively. The calculation of density functional theory (DFT) also proves the adsorption energy of NO 2 on the SnO 2 (110) facet is larger than the (101) facet and (211) facet, indicating that the (110) facet is more beneficial to the adsorption of NO 2 molecules. This indicated that the facet engineering of MOS by facet-dependent properties to enhance gas sensitivity may open new opportunities for the design of SAW sensor. [Display omitted] • The SnO 2 with different (110) facets ratio were prepared by adjusting the synthesis time. • The SnO 2 with more exposed (110) facets exhibited high sensor response and low detection limit. • Chemiresistor sensor and QCM sensor were used to understand the SAW sensing mechanism for the NO 2. [ABSTRACT FROM AUTHOR]

Schematic of microfluidic heating within a PDMS microchamber using the AlN/Si thin film SAW device. • Acoustothermal heating characteristics of different thin film (including AlN/Si, ZnO/Si and ZnO/Al) SAW devices have been investigated and compared. • The dynamic heating processes of AlN/Si SAW device for both the sessile droplet and liquid within a PDMS microchamber are characterized. • The liquid heating temperature within a PDMS microchamber can be digitally controlled by the input SAW power with good stability and repeatability. • An improved temperature uniformity using the standing SAW heating is demonstrated. Temperature control within a microreactor is critical for biochemical and biomedical applications. Recently acoustothermal heating using surface acoustic wave (SAW) devices made of bulk LiNbO 3 substrates have been demonstrated. However, these are generally fragile and difficult to be integrated into a single lab-on-a-chip. In this paper, we propose a rapid and controllable acoustothermal microheater using AlN/Si thin film SAWs. The device's acoustothermal heating characteristics have been investigated and are superior to other types of thin film SAW devices (e.g., ZnO/Al and ZnO/Si). The dynamic heating processes of the AlN/Si SAW device for both the sessile droplet and liquid within a polydimethylsiloxane (PDMS) microchamber were characterized. Results show that for the sessile droplet heating, the temperature at a high RF power is unstable due to significant droplet deformation and vibration, whereas for the liquid within the microchamber, the temperature can be precisely controlled by the input power with good stability and repeatability. In addition, an improved temperature uniformity using the standing SAW heating was demonstrated as compared to that of the travelling SAWs. Our work shows that the AlN/Si thin film SAWs have a great potential for applications in microfluidic heating such as accelerating biochemical reactions and DNA amplification. [ABSTRACT FROM AUTHOR]

• 3D manipulation of yeast cells inside a chamber with a height of 1 mm was realized. • ZnO/Si SAW devices could be seamlessly integrated into a lab-on-chip (LOC) device. • Factors influencing microparticle manipulation in both 2D and 3D were investigated. • A numerical model has been developed to investigate the 3D motions of yeast cells. Manipulating biological cells or microparticles in three dimensions (3D) is invaluable for many biomedical applications, and recently effective and rapid manipulations of microparticles in 2D and 3D within microchannels or chambers using surface acoustic waves (SAWs) with bulk piezoelectric materials have been reported. However, these are generally expensive, or brittle and cannot be easily integrated into a single lab-on-chip. In this paper, we realized microparticle/cell patterning and 3D manipulation of yeast cells inside a chamber with a height of 1 mm using thin film ZnO/Si SAW devices. Effects of SAW frequency, channel width and thickness on alignment of microparticles were firstly investigated, and positions of the microparticles in the direction of SAW propagation can be controlled precisely by changing the phase angle of the acoustic waves from the ZnO/Si SAW device. A numerical model has been developed to investigate the SAW acoustic field and the resulted 3D motions of microparticles under the acoustic radiation forces within the microchamber. Finally, we realized and observed the 3D patterning of yeast cells within the microchannel. Our work shows a great potential for acoustofluidic, neural network research and biomedical applications using the ZnO/Si SAW devices. [ABSTRACT FROM AUTHOR]