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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]

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]

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]

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]

Abstract Colloidal quantum dots (CQDs) have shown their advantages in gas-sensing applications due to their extremely small particle size and facile solution based processes. In this study, a high sensitivity of surface acoustic wave (SAW) NO 2 sensor was demonstrated using SnS CQDs as the sensing layer. The delay line based SAW device with a resonant frequency of 200 MHz were fabricated on ST-cut quartz substrate. The SnS CQDs with average sizes of 5.0 nm were synthesized and deposited onto SAW sensors using a spin-coating method. The fabricated SAW sensor was capable of detecting a low concentration of NO 2 gas at room temperature with a good efficiency and selectivity e.g., with a 1.8 kHz decrease of center frequency of the SAW delay line when exposed to 10 ppm NO 2 at room temperature. Highlights • Small-size and high-crystallinity SnS CQDs were synthesized via hot injection. • The SnS CQDs were successfully integrated onto the SAW delay lines by spin-coating. • The SnS CQD-coated SAW sensor exhibited high response to NO 2 at room temperature. [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]