Sensitive Surface Acoustic Wave Research Articles

The development of sensitive graphene-based surface acoustic wave sensors for NO2 detection at room temperature

Bilayer graphene (Bl-Gr) and sulphur-doped graphene (S-Gr) have been integrated with LiTaO3 surface acustic wave (SAW) sensors to enhance the performance of NO2 detection at room temperature. The sensitivity of the Bl-Gr SAW sensors toward NO2, measured at room temperature, was 0.29º/ppm, with a limit of detection of 0.068 ppm. The S-Gr SAW sensors showed 0.19º/ppm sensitivity and a limit of detection of 0.140 ppm. The origin of these high sensitivities was attributed to the mass loading and elastic effects of the graphene-based sensing materials, with surface changes caused by the absorption of the NO2 molecules on the sensing films. Although there are no significant differences regarding the sensitivity and detection limit of the two types of sensors, the measurements in the presence of interferent gases and various humidity conditions outlined much better selectivity and sensing performances towards NO2 gas for the Bl-Gr SAW sensors.Graphical

Modeling and testing of a highly sensitive surface acoustic wave pressure sensor for liquid depth measurements

The fluid depth/level measurement application extends from industry to domestic contexts. This article explores the underutilized potential of Surface Acoustic Wave (SAW) sensor-based devices for liquid depth and pressure measurements. Experiments conducted at room temperature address design challenges using a quartz Y34° cut single-port resonator. Theoretically predicted longitudinal strain sensitivity of −1.15 kHz/μStrain (−1.32 ppm/μStrain) was reduced to −421 Hz/μStrain (-0.48 ppm/μStrain) due to adhesive layer thickness. Numerical simulations investigate the effect of adhesive thickness on strain transfer from a cantilever to the Surface Acoustic Wave (SAW) sensor's top surface. The sensor is tested on a cantilever and integrated into a 20 μm thick diaphragm-based depth gauge, achieving a depth sensitivity of 42 Hz/mm (0.048 ppm/mm). This sensitivity results from bending of the SAW sensor under hydrostatic pressure, causing compression at the top SAW surface, which was an interesting observation. Additional numerical investigations confirm this behavior, aligning well with experimental findings. The article summarizes the comprehensive study of the SAW resonator’s performance and effect of adhesive thickness and provides a guide for designing thin diaphragm-based SAW pressure and depth sensors. It also identifies the device’s high sensitivity compared to other sensors in the literature and outlines potential avenues for future research.

Highly sensitive surface acoustic wave magnetic field sensor based on the loss mechanism

Currently, the surface acoustic wave (SAW) magnetic field sensing technique utilises the SAW velocity/frequency mechanism of magnetoacoustic interaction as an indicator of the magnetic sensitivity mechanism. However, this method has low sensitivity and poor stability. To address this problem, a dynamic magnetoelastic coupling theoretical model is constructed to theoretically simulate the influence of the ΔE effect of magnetically sensitive thin films on SAW propagation attenuation. This study describes a high-sensitivity SAW magnetic field sensing mechanism based on magnetoacoustic attenuation. The simulation results show a clear relationship between the acoustic propagation loss and external magnetic field, indicating a structure-property relationship. An amorphous soft magnetic material (Fe90Co10)78Si12B10 was used as a magnetically sensitive thin film due to its high permeability, low coercivity (Hc), low hysteresis, ease of magnetisation and demagnetisation. SAW magnetosensitive device operating on a frequency of 200 MHz has been experimentally developed using a standard semiconductor photolithography process. A SiO2 layer was deposited on a 36° YX-LiTaO3 substrate as a waveguide, and a (Fe90Co10)78Si12B10 layer was on the top of the propagation area as a magnetosensitive film. The experimental results showed that the acoustic loss change due to the magnetic field variation was 4.63 dB within a magnetic field range of 0 Oe to ±10 Oe, which agreed with the theoretical results. The sensor had a sensitivity of 0.7546 dB Oe−1 within the range of 0–4 Oe and the lower detection limit of magnetic fields was 0.272 Oe, low hysteresis error of 0.54%, multiple repeatability error of 0.13%, excellent repeatability and stability were achieved in the experiments from the developed sensing device.

Development of highly sensitive and solar blind surface acoustic wave UV-C photodetector based on the In2O3/Ta2O5 nano-heterojunction and its interface electronics

A highly sensitive surface acoustic wave (SAW) ultraviolet-C (UV-C) sensor based on In2O3/Ta2O5 nano-heterojunctions and its interface electronics was developed to detect solar blind corona discharge near high voltage power lines. The SAW sensor, based on pristine In2O3 nanoparticles, exhibits a weaker and slower response to UV-C light. To improve the response to deep UV light, In2O3 nanoparticles/Ta2O5 heterojunctions were formed by depositing Ta2O5 films of different thicknesses (2–10 nm) around the In2O3 nanoparticles via atomic layer deposition (ALD) on the cavity between two interdigital transducers (IDTs) in the SAW delay line. The experimental results show a significantly enhanced sensor response towards UV-C light, which is attributed to a high surface-to-volume ratio, an inhibited recombination rate of the photogenerated electron-hole pairs on the surface of the sensing material owing to Ta2O5 conformal coverage, and well-matched energy bandgap engineering. An empirical mechanism was proposed to elucidate the enhanced sensing behavior of the heterostructure. To observe the frequency shifts in real-time on a PC display, portable interface electronics were developed on a single printed circuit board (PCB) for the SAW UV-C sensor, eliminating the effect of frequency shifts caused by ambient temperature and humidity changes. The sensor characteristics were evaluated based on the size and annealing conditions of the In2O3 nanoparticles and the thickness of the decorated Ta2O5, and the scientific and technical causes and analysis of the results were thoroughly investigated. In the selectivity test, the fabricated sensor exhibited a strong response to a UV-C light beam with a sensitivity of 368.3 (ppm (μW/cm2)−1 at 254 nm, and its spectral response to UV-C light was 3.3 times higher than that under UV-A 365 nm light.

Ultra-High Frequency Surface Acoustic Wave Sensors for Temperature Detection.

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.

Highly Sensitive Surface Acoustic Wave H2S Gas Sensor Using Electron-beam-evaporated CuO as Sensitive Layer

Highly Sensitive Surface Acoustic Wave H2S Gas Sensor Using Electron-beam-evaporated CuO as Sensitive Layer

Development of Highly Stable and Sensitive SAW UV Sensor Based on Ta2O5 Coated 1D ZnO/SnO2 Core–Shell Nanostructures

AbstractDespite extensive research on ultraviolet (UV) sensors based on core–shell heterostructures, most of them exhibit low sensitivities, slow response/recovery times, and unsatisfactory long‐term stabilities, thereby limiting their use in real‐time applications. To solve these problems, a highly sensitive and stable surface acoustic wave (SAW) UV sensor based on novel tantalum pentoxide (Ta2O5) coated 1D zinc oxide (ZnO)/tin dioxide (SnO2) core–shell nanostructures is introduced. The fabricated SAW‐based UV sensor system, including sensor interface electronics, exhibits a high UV response, fast response/recovery time, and long‐term stability. Prior to device fabrication, coupling of mode (COM) modeling is performed to calculate the optimal parameters for the SAW device. Photoluminescence and X‐ray photoelectron spectroscopy measurements reveal that the Ta2O5 layer coating effectively suppresses the recombination of photogenerated carriers, leading to an increase in the carrier density and improvement in the UV sensing properties. The evaluated sensitivity and linearity of the proposed sensor are 120.36 ppm (mW cm−2)−1 and 0.988, respectively, in the range of 1–60 µW cm−2. The sensing mechanism of the proposed UV sensor is discussed in detail. Using COMSOL simulations and photo‐corrosion experiments, it is found that the Ta2O5 coated ZnO/SnO2 core–shell heterostructure minimizes the effects of humidity and improves stability.

Highly sensitive surface acoustic wave biosensor for the detection of Hg2+ based on the thymine-Hg2+-thymine structure.

The detection of mercury ions (Hg2+) with an ultralow concentration is of great significance for the treatment of heavy metal pollution in industrial waste water and the monitoring of drinking water quality. In this study, a simple, sensitive, and portable surface acoustic wave (SAW) biosensor for the determination of Hg2+ concentration was developed. First, a DNA fragment with a specific binding capacity for Hg2+ was chemically adsorbed on to the sensitive region through forming Au-S bonds with the SAW biosensor. The DNA probe could then form a thymine-Hg2+-thymine (T-Hg2+-T) complex with strong affinity for Hg2+ after immersion in a test solution containing Hg2+, resulting in a significant change in the response frequency of the device. The linear detection range of the device was 10 pM to 1 nM, while the limit of detection (LOD) was as low as 6.3 pM. Furthermore, the SAW biosensor exhibited excellent selectivity to Hg2+ compared with that of interfering ions, e.g., Ag+, Ba2+, Cu2+, Zn2+, Mn2+, Fe3+, Ca2+, and Na+. The results provide a new strategy for the preparation of portable devices that can monitor toxic heavy metal ions with high sensitivity and selectivity.

Boron doped g-C3N4 quantum dots based highly sensitive surface acoustic wave NO2 sensor with faster gas kinetics under UV light illumination

The elegant two-dimensional (2D) graphitic carbon nitride (GCN) holds great promise in the detection of hazardous toxic gases especially NO2, for environmental protection and public health safety. However, the poor sensitivity and low surface area of bulk 2D GCN restrains its practical applicability of room temperature (RT) NO2 sensing. Herein, we present the fabrication of boron-doped GCN quantum dots (B@GCN QD) coated langasite (LGS) based surface acoustic wave (SAW) sensor for the RT NO2 gas detection, and applied UV activation energy (365 nm) as an effective strategy to further enhance the NO2 sensing. Fascinatingly, under UV light, the B@GCN QD/LGS SAW sensor showed an enhanced gas sensing performance to the NO2 gas (500 ppb - 50 ppm) with high sensitivity (Δf=35.97 kHz/50 ppm), good selectivity, ultra-low detection limit (DL∼7.9 ppb), fast response/recovery times (35 s/43 s), excellent long-term stability as compared to dark condition. This could be attributed to the generation of abundant photoinduced charge carriers under UV activation, which leads to the significant enhancement in conductivity and enlarged mass loading effect, also, the device showed robust sensitivity responses to NO2 gas at various temperatures (RT-200 °C) and relative humidity (20–80%) conditions. Moreover, the effective doping of B to the GCN QD exhibited a typical p-type behavior which is good in agreement with density functional theory predictions that can induce structural defects, vacancies, and amine functional groups leading to a high specific surface area, which can remarkably enhance the sensitivity and selectivity of NO2 gas. This work may provide insights into the design and development of 2D GCN nanostructure-based SAW RT gas sensors for environmental monitoring.

Highly Sensitive and Selective Surface Acoustic Wave Ammonia Sensor Operated at Room Temperature with a Polyacrylic Acid Sensing Layer.

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.

Highly Sensitive Love Mode Acoustic Wave Platform with SiO2 Wave-Guiding Layer and Gold Nanoparticles for Detection of Carcinoembryonic Antigens.

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.

Development of a highly sensitive and portable particulate matter SAW sensor and interface electronics

In this study, a highly sensitive and portable surface acoustic wave (SAW)-based PM2.5 sensor and its interface electronics were developed for real-time monitoring of air quality. Two 2-port SAW resonators were employed for the sensor system to eliminate any environmental disturbance (temperature and humidity variations) on the output of the sensor. The overall system comprised a 2-port SAW resonator for a particulate matter (PM) sensor, a reference sensor covered with polydimethylsiloxane (PDMS) on the top surface, two self-oscillators, an RF mixer, a low-pass filter, a comparator, a field-programmable gate array, and a personal computer. All electronic components were integrated into a single printed circuit board assembly. COMSOL simulations were performed to determine the optimal center frequency for the operation of the PM2.5-based SAW sensor. Based on COMSOL simulation, the sensors were fabricated and their characteristics categorized using a custom-built testbed. We experimentally observed the effect of the center frequency on the frequency response of PM2.5. The experimental data showed that the sensor operating at a center frequency of 180 MHz exhibited the largest frequency shift when exposed to PM2.5, suggesting that 180 MHz is the optimal center frequency for operating a PM2.5-based SAW sensor. Herein, a simple mechanism is proposed to understand the working principle of the PM2.5 sensor. Our results suggest that for the effective detection of PM2.5 particles, it is necessary to use different center frequencies of the SAW sensor with optimal penetration depth to match the size of PM2.5.

A highly sensitive surface acoustic wave sensor modified with molecularly imprinted hydrophilic PVDF for the selective amino acid detection

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.

Highly sensitive thick diaphragm-based surface acoustic wave pressure sensor

Diaphragm-based Surface acoustic wave (SAW) pressure sensors are of great interest to fulfill high-pressure sensing requirements in industrial and commercial applications. These sensors are developed with a thick diaphragm, can endure considerable pressure but adversely result in low sensitivity and affect device accuracy. The trade-off between diaphragm thickness and sensitivity is the major problem in designing pressure sensors for high-pressure sensing ranges. This work proposed a highly sensitive pressure sensor design based on one port SAW resonator with AlN/Mo/SOI multilayer structure. The sensor utilizes a 50 μm thick rectangular diaphragm and has been fabricated with simple microfabrication techniques, making it cost-effective and easy to implement in pressure sensing equipment. Finite element analysis (FEA) has been performed for analyzing diaphragm to locate IDTs based on bending, uniformity of stress distribution and induced strain type. The developed sensor exhibits good performance characteristics, including high sensitivity (up to 129.17 ppm/MPa), wide pressure sensing range (up to 2 MPa), and good repeatability with linear behavior for applied pressure range. Additionally, this work has presented a novel approach to enhance pressure sensitivity based on increased velocity without compromising sensing range. The obtained sensitivity has been enhanced up to 228.46 ppm/MPa by modifying the primarily developed sensor design such that the SAW propagation direction becomes parallel to the induced strain direction. With the proposed novel concept, the sensitivity has been significantly improved by 76.87 % compared to the primarily developed sensor. Besides improved sensitivity, the modified sensor also shows the best repeatability and linearity. All these performance characteristics pave the way to make this design a good choice for high-pressure sensing applications.

Design and Fabrication of ZnO-Based SAW Sensor Using Low Power Homo-Buffer Layer for Enhanced Humidity Sensing

Zinc oxide (ZnO) nanostructures were proven to enhance the sensitivity performance of nanoscale ZnO-based surface acoustic wave (SAW) sensor greatly. Here, we developed ZnO sensitive layer-based SAW sensors by low power homo-buffer layer (LPHBL) method and focused on the quality of preparation, their mass sensitivity and humidity sensing applications. ZnO films were prepared on the surface of the SAW devices by radio frequency magnetron sputte ring method by employing different sputtering power. Island-shape ZnO nanostructures were successfully prepared by the simple LPHBL method. The mass sensitivities of these sensor devices based on Rayleigh wave mode and Sezawa wave mode were increased dramatically by increasing ZnO top layer thickness. To evaluate the humidity sensing performance, the humidity testing experiments were conducted for a range of moisture concentrations between 1% and 95%. The ZnO/quartz sensor without LPHBL frequency response magnitudes varied between 0-19.25 kHz and insertion loss (IL) shifts changed between 0-0.815 dB for 1%-95% RH while the ZnO/ZnO/quartz SAW sensor with LPHBL showed 0-26.25 kHz response magnitudes and 0-1.878 dB IL shifts at the same conditions. The Love-SAW sensor with LPHBL exhibited more than 36% greater humidity response sensitivity compared to its no intermediate layer counterpart. In addition, the recovery of the ZnO/ZnO/quartz humidity sensing device could reach 91.5%. Overall, the ZnO-based Love-SAW sensor fabricated by the LPHBL method may hold promise for chemical and biological detection applications.

Highly Sensitive Surface Acoustic Wave NH3 Gas Sensor based on TiO2 Film

Highly Sensitive Surface Acoustic Wave NH3 Gas Sensor based on TiO2 Film

Highly Sensitive Surface Acoustic Wave NH3 Gas Sensor based on TiO2 Film

Highly Sensitive Surface Acoustic Wave NH3 Gas Sensor based on TiO2 Film

Simulation/Experiment Confrontation, an Efficient Approach for Sensitive SAW Sensors Design.

Sensitivity is one of the most important parameters to put in the foreground in all sensing applications. Its increase is therefore an ongoing challenge, particularly for surface acoustic wave (SAW) sensors. Herein, finite element method (FEM) simulation using COMSOL Multiphysics software is first used to simulate the physical and electrical properties of SAW delay line. Results indicate that 2D configuration permits to accurately obtain all pertinent parameters, as in 3D simulation, with very substantial time saving. A good agreement between calculation and experiment, in terms of transfer functions (S21 spectra), was also shown to evaluate the dependence of the SAW sensors sensitivity on the operating frequency; 2D simulations have been conducted on 104 MHz and 208 MHz delay lines, coated with a polyisobutylene (PIB) as sensitive layer to dichloromethane (DCM). A fourfold increase in sensitivity was obtained by doubling frequency. Both sensors were then realized and tested as chem-sensors to detect zinc ions in liquid media. 9-{[4-({[4-(9anthrylmethoxy)phenyl]sulfanyl} methyl)]methyl] anthracene (TDP-AN) was selected as the sensing layer. Results show a comparable response curves for both designed sensors, in terms of limit of detection and dissociation constants Kd values. On the other hand, experimental sensitivity values were of the order of [7.0 ± 2.8] × 108 [°/M] and [16.0 ± 7.6] × 108 [°/M] for 104 MHz and 208 MHz sensors, respectively, confirming that the sensitivity increases with frequency.

Highly sensitive surface acoustic wave HCl gas sensors based on hydroxyl-rich sol-gel AlOxOHy films

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 AlOxOHy 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 H2O 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, H2O 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%.

Enhanced Sensitive Surface Acoustic Wave Device Designed for Nitric Oxide Gas Detection

Enhanced Sensitive Surface Acoustic Wave Device Designed for Nitric Oxide Gas Detection