Sezawa wave acoustic humidity sensor based on graphene oxide sensitive film with enhanced sensitivity

Hyodo T., Urata K., Kamada K., Ueda T., Shimizu Y.

NO2-sensing properties of typical oxide (SnO2, In2O3, or WO3)-based semiconductor gas sensors were measured at 30 °C with and without UV-light irradiation (main wavelength: 365 nm), and effects of noble-metal (Pd or Pt) loading, UV-light intensity (0–134 mW cm−2) and relative humidity in target gas (0–80%RH) on their NO2-sensing properties were investigated in this study. The UV-light irradiation effectively reduced the resistances of all sensors, enhanced their NO2 responses in some cases, and tended to accelerate their response and recovery speeds in dry air, because the UV-light irradiation promoted the adsorption and desorption of NO2-species on the surface. The SnO2 sensor showed the largest NO2 response in dry air, among all the pristine oxide sensors, especially under weak UV-light irradiation (≤35 mW cm−2), together with relatively fast response and recovery speeds. The Pd or Pt loading onto SnO2 enhanced the NO2 response of the SnO2 sensor and accelerated their response and recovery speeds in dry air, while XPS analysis indicated that most of the Pd and Pt nanoparticles loaded on the surface were oxidized after heat treatment at 500 °C. Among all the sensors, the 0.05 wt% Pd-loaded SnO2 sensor showed the largest NO2 response under weak UV-light irradiation (≤35 mW cm−2), together with relatively fast response and recovery speeds. The addition of moisture to the target gas had adverse effects on the NO2 responses and the response speeds of the SnO2 and 0.05 wt% Pd-loaded SnO2 sensors, but the weak UV-light irradiation (7 mW cm−2) largely reduced the dependence of the NO2 response of the 0.05Pd/SnO2 sensor on relative humidity while maintaining the large NO2 response, probably because the weak UV-light irradiation promotes the desorption of physisorbed water molecules and then the effective adsorption of NO2 on the 0.05Pd/SnO2 surface.

Zang Z., Zeng X., Wang M., Hu W., Liu C., Tang X.

Water-soluble quantum dots (QDs) with biocompatibility and photostability show a potential application in biomedical optical imaging. Herein, we reported biocompatible, photostable and eco-friendly AgInZnS–graphene oxide (AIZS–GO) nanocomposites with tunable emissions by transferring hydrophobic AIZS QDs on water-soluble GO via a mini-emulsion method. The as-prepared hydrophilic AIZS–GO nanocomposites still exhibited bright and stable photoluminescence (PL) even after phase-transfer by using GO, moreover, their PL emission could be well tuned widely in the range of 530 nm–680 nm by controlling the composition of Zn. In addition, AIZS–GO nanocomposites could be used for imaging of SK-BR-3 breast cancer cells via intratumoral administration. MTT assay result proved that these nanocomposites demonstrated comparable low cytotoxicity due to the absence of highly toxic cadmium. The in vivo imaging experiments of mice indicated that the prepared red emitted AIZS–GO nanocomposites could be used for the cancer cell and organ of mice labeling, which shows potential applications in bioimaging and related fields such as phototherapy and imaging.

Kuznetsova I.E., Anisimkin V.I., Gubin S.P., Tkachev S.V., Kolesov V.V., Kashin V.V., Zaitsev B.D., Shikhabudinov A.M., Verona E., Sun S.

The changes of density and elastic modules due to water vapor adsorption are measured for graphene oxide film at room temperature. Dominant mechanism for acoustic wave humidity sensing by the film is shown to be related with variation of its electric conductivity. Basing on the data, super high sensitive humidity sensor employing high-order Lamb wave with large coupling constant, standard lithium niobate plate, and graphene oxide sorbent film is developed. The minimal detectable level of the sensor is as low 0.03% RH, response times are 60/120s, and reproducibility is ±2.5%. The sensor is completely selective towards H2, CO, CH4, NO, O2.

Kuznetsova I., Kolesov V., Zaitsev B., Tkachev S., Kashin V., Shikhabudinov A., Fionov A., Gubin S., Sun S.

Graphene oxide (GO) possesses some unique properties, which allow its technological applications in many fields (energy storage, microelectronic devices, and chemical sensors). It can also be used as a sensitive layer in the development of the chemical acoustoelectronic sensors. We characterized the produced GO films by measuring their structural properties, permittivity, and longitudinal/shear elastic moduli. For the first time, we determined the longitudinal and shear elastic moduli of a GO film under study to be 12.3 ± 0.1 MPa and 3.0 ± 0.1 MPa, respectively. The density of the GO film is equal to ∼850 kg m−3. The permittivity of the GO film was found to be 21 for a frequency f = 20 Hz. The obtained results may be used in further application of GO films in acoustoelectronic sensor devices.

Xuan W., Cole M., Gardner J.W., Thomas S., Villa-López F., Wang X., Dong S., Luo J.

A film bulk acoustic wave resonator (FBAR) is a type of resonator with high frequency and small dimensions, particularly suitable for use as a sensor for physical and biochemical sensing with high sensitivity. FBAR-based sensors have been extensively studied, however they commonly use discrete devices and network analyzers for evaluation, and therefore are far from being able to be used in practical applications. This paper reports the design and analysis of an FBAR-based Pierce oscillator and a field-programmable gate array (FPGA)-based frequency counter, and the use of the oscillator as a humidity sensor with the frequency counter as the measuring circuit. Graphene oxide (GO) is used as the sensitive film to improve the sensitivity. The resonant frequency of the oscillator with a GO film shows a linear decrease with an increase in relative humidity, with a sensitivity of ca. 5 kHz per %RH (relative humidity) in the range of 3%RH to 70%RH, and a higher frequency shift is induced above 70%RH. The FBAR oscillator sensor shows excellent stability and repeatability, demonstrating the feasibility and potential sensing application using the integrated FBAR chip and simple frequency counter, particularly suitable for portable electronics.

Devkota J., Ohodnicki P., Greve D.

Surface acoustic wave (SAW) technology provides a sensitive platform for sensing chemicals in gaseous and fluidic states with the inherent advantages of passive and wireless operation. In this review, we provide a general overview on the fundamental aspects and some major advances of Rayleigh wave-based SAW sensors in sensing chemicals in a gaseous phase. In particular, we review the progress in general understanding of the SAW chemical sensing mechanism, optimization of the sensor characteristics, and the development of the sensors operational at different conditions. Based on previous publications, we suggest some appropriate sensing approaches for particular applications and identify new opportunities and needs for additional research in this area moving into the future.

Liu W., Qu H., Hu J., Pang W., Zhang H., Duan X.

We developed a highly sensitive humidity sensor based on the combination of ultrahigh-frequency film bulk acoustic resonator (FBAR) and nano-assembled polyelectrolyte (PET) thin films. The water molecule absorption efficiency was optimized by forming loosely-packed PET nanostructures. Then, the humidity sensing characteristics were analyzed in terms of sensitivity, linearity, reversibility, stability and detection limit. As a result, PET-coated FBAR exhibits excellent humidity sensitivity of 2202.20 Hz/ppm, which is five orders of magnitude higher than quartz crystal microbalance (QCM). Additionally, temperature dependence was investigated with the result that PET-coated FBAR possessed a higher sensitivity at low temperature. Furthermore, we realized the selective detection of water vapor from volatile organic compounds (VOCs) with respect to the polarity property. Owing to the high sensitivity, miniaturized size and ultrahigh operating frequency, PET-coated FBAR is uniquely favorable as a wireless humidity sensor node to integrate into wireless sensor networks (WSNs).

Wei J., Zang Z., Zhang Y., Wang M., Du J., Tang X.

A significant enhancement of photoresponse from the light-controlled conductive switching based on Cu2O/rGO nanocomposites was experimentally demonstrated. Cu2O/rGO nanocomposites were synthesized via a facile wet-reduced method. The crystalline structure, morphologies, and photoluminescence of the Cu2O/rGO nanocomposites were characterized and analyzed. The fabricated conductive switching was measured under the irradiation of a continuous laser. When the laser was turned on and off alternately, the photoconductive switching obviously displayed a state conversion between "on" and "off" reversibly. Furthermore, the typical current-voltage (I-V) and current-time (I-t) curves exhibited a relatively high switching ratio (Ion/Ioff) of 3.25 and a fast response time of 0.45 s. The excellent "on-off" characteristics of the device show promising applications in memory storage and logic circuits.

Rimeika R., Čiplys D., Poderys V., Rotomskis R., Shur M.S.

The impact of ambient air humidity on surface acoustic wave (SAW) propagation in structures consisting of bovine serum albumin (BSA) and bovine serum albumin - gold nanoclusters (BSA-Au NCs) films deposited on YZ lithium niobate (LiNbO3) substrate has been investigated. The BSA-Au NCs film deposited on the surface of LiNbO3 significantly reduced the low-humidity value of the SAW transmission loss and considerably improved its long-term stability as compared to the BSA film. The amplitude of the output signal of the BSA-Au NCs on LiNbO3 SAW delay line was measured as a function of relative humidity (RH) at the steady-state conditions and as a function of time upon the step-like RH variation. An observed decrease in the transmitted SAW amplitude with increasing RH was explained in terms of the acoustoelectric attenuation. The response and recovery times of the structure with BSA-Au NCs on LiNbO3 SAW delay line upon abrupt humidity changes were on the order of 0.2 s. No impact of RH variation on the SAW propagation was observed in the structures with BSA-Au NCs films deposited on metalized LiNbO3 substrate surface. The time-stable, fast-response and low-loss BSA-Au NCs on LiNbO3 SAW humidity sensor has been applied for monitoring the human respiration by detecting moisture contained in the exhaled air.

Raj V.B., Singh H., Nimal A.T., Sharma M.U., Tomar M., Gupta V.

ZnO/SAW sensor was reported to give distinct response towards liquor ammonia. To study the complete mechanism, ZnO thin films (40 nm) were deposited using rf sputtering in different reactive gas composition of argon and oxygen. The increase in oxygen content (30–100%) during film growth leads to decrease in the value of stress and bond energy. The individual contribution of different SAW sensing mechanisms such as mass loading, elastic effects and acousto-electric interaction, was evaluated and analyzed to understand the distinct response for liquor ammonia. It was found that mass loading seems to get affected by the presence of stress whereas elastic loading was affected by the crystallite size and bond energy (Zn O) in ZnO thin films.

Sayar Irani F., Tunaboylu B.

In this research, we investigated the influence of the surface coatings of silver nanowires on the sensitivity of surface acoustic wave (SAW) humidity sensors. Silver nanowires, with poly(vinylpyrrolidone) (PVP), which is a hydrophilic capping agent, were chemically synthesized, with an average length of 15 µm and an average diameter of 60 nm. Humidity sensors, with 433 MHz frequency dual-port resonator Rayleigh-SAW devices, were coated by silver nanowires (AgNWs) using the electrospray coating method. It was demonstrated that increasing thickness of coated AgNW on the surfaces of SAW devices results in increased sensitivity. The highest frequency shift (262 kHz) in these SAW devices was obtained with an injection of 0.5 mL of the AgNW solution with a concentration of 0.5 mg/mL at an injection rate of 1 mL/h. It also showed the highest humidity sensitivity among the other prepared SAW devices.

Nikolaou I., Hallil H., Conedera V., Deligeorgis G., Dejous C., Rebiere D.

We report inkjet printing as an alternative deposition method for low-cost gas or humidity sensors based on graphene oxide (GO)-coated Love wave devices. Our inkjet printing method paves the way toward massive, large-area industrial production of multi-layered GO chemical sensing films for volatile organic compounds and relative humidity (RH) detection applications. The adsorption of vapor compounds on GO led to the sensitivities of 30 Hz/ppm, 24 Hz/ppm, and 2.4 kHz/1% of ethanol (C2H6O), toluene (C7H8), and RH, respectively. Electrical, gaseous, and RH characterization analyses showed that this GO-based inkjet printing method is one of the most promising features for inexpensive and high-speed patterning devices for high demanding gas trace or RH sensing applications.

Yuan Z., Tai H., Ye Z., Liu C., Xie G., Du X., Jiang Y.

In this work, a novel graphene oxide (GO)/Poly(ethyleneimine) (PEI) layered film was designed and prepared on quartz crystal microbalance (QCM) for humidity detection through a feasible spray process at room temperature. The morphological and chemical properties of GO/PEI layered film were examined by means of field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Fourier Transform infrared spectroscopy (FTIR), and Raman spectroscopy. The effect of GO solution concentration on humidity-sensing characteristics was investigated and an optimized QCM humidity sensor was obtained. An intriguing phenomenon was observed that a three-dimension structure of GO sheets was formed due to the redissolution effect at the interface between PEI and GO layers. Results showed that a layered film QCM sensor exhibited superior humidity sensing properties than a pure GO film or a PEI one including high response, short response/recovery time, small humidity hysteresis (less than 1%RH), excellent repeatability, selectivity and long-term stability. Moreover, the humidity features of sensors were also researched via the method of impedance analysis. Lastly, a convincing sensing mechanism model was established to interpret the enhanced humidity-sensing performance of layered film sensors. The results demonstrated the potential application of GO/PEI layered films to humidity sensors.

Liu Y., Huang H., Wang L., Cai D., Liu B., Wang D., Li Q., Wang T.

Nowadays the working frequencies of surface acoustic wave (SAW) sensors are usually not higher than 500 MHz, typically in the range of a few dozen to hundred megahertz, while an increase of working frequency should be beneficial to the sensor performance. Thus, a high frequency SAW resonator operating at 1.56 GHz was fabricated for relative humidity (RH) detection. The CeO 2 nanoparticles (NPs)/polyvinylpyrrolidone (PVP) nanofibers were prepared by electrospinning of PVP solution with CeO 2 NPs working as the sensitive layer. The inorganic CeO 2 NPs were synthesized previously using a hydrothermal method. In contrast with SAW sensor working at lower resonant frequency (879 MHz), the resonant frequency shift of the sensor based on 1.56 GHz was about −2.5 MHz in the RH range of 11% to 95%, which was approximately 8 times of the former one. Further analysis demonstrated that the additional acoustoelectric loading effect arising from increased electrical conductivity of CeO 2 /PVP nanofibers in high RH improved the frequency response compared with pure PVP nanofibers based SAW sensor. Moreover, the SAW sensor based on inorganic/organic nanohybrid also showed high stability under humid environment and negligible cross-sensitivity effects ensuring further wireless humidity detection.

Yao Y., Xue Y.

In this work, functionalized graphene films with different oxygen contents were prepared and their humidity sensing properties, such as humidity response, humidity hysteresis, dynamic response and recovery, were studied by combining them with a bulk acoustic wave sensor. The experimental results revealed that the humidity sensing properties of the functionalized graphene films were strongly associated with their oxygen content. The humidity response of the sensor could be improved by increasing the oxygen content of the functionalized graphene films. The stability of the sensor was also investigated using an impedance analysis method. The results indicated that the stability of the sensor was also influenced by the oxygen content of the functionalized graphene films during humidity detection. An increase in the oxygen content of the functionalized graphene film reduced the sensor stability. Thus, our work suggests that the oxygen content of functionalized graphene films is a key parameter for the design of graphene-based bulk acoustic wave humidity sensors.

Tang Z., Wu W., Yang P., Luo J., Fu C., Han J., Zhou Y., Wang L., Wu Y., Huang Y.

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.

Zhao L., Ouyang P., Yi X., Li G.

All-inorganic perovskites have gained extensive attention due to their exceptional performance in the realm of optoelectronics. However, there are few reports about the applications of all-inorganic perovskites in humidity detection....

Liu Y., Zhou J., Wen S., Chen Y., Fu Y., Duan H.

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-SiO2 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 SiO2 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).

Tian S., Wen F., Qian L., Li C., Wang L., Li D., Xu S., Li H., Li M.

Zhu Y., Dong X., Cheng J., Wang L., Zhao C., Deng Y., Xie S., Pan Y., Zhao Y., Sun G., Ni T.

To achieve real-time monitoring of humidity in various applications, we prepared facile and ultra-thin CoAl layered double hydroxide (CoAl LDH) nanosheets to engineer quartz crystal microbalances (QCM). The characteristics of CoAl LDH were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectric spectroscopy (XPS), Brunauer–Emmett–Telle (BET), atomic force microscopy (AFM) and zeta potential. Due to their large specific surface area and abundant hydroxyl groups, CoAl LDH nanosheets exhibit good humidity sensing performance. In a range of 11.3% and 97.6% relative humidity (RH), the sensor behaved an ultrahigh sensitivity (127.8 Hz/%RH), fast response (9.1 s) and recovery time (3.1 s), low hysteresis (3.1%RH), good linearity ( R 2 = 0.9993), stability and selectivity. Besides, the sensor can recover the initial response frequency after being wetted by deionized water, revealing superior self-recovery ability under high humidity. Based on in-situ Fourier transform infrared spectroscopy (FT-IR), the adsorption mechanism of CoAl LDH toward water molecules was explored. The QCM sensor can distinguish different respiratory states of people and wetting degree of fingers, as well as monitor the humidity in vegetable packaging, suggesting excellent properties and a promising application in humidity sensing. CoAl LDH nanosheets with interlayer structure were prepared and modified on QCM sensor, which can selectively capture water molecules on their surface due to their high specific surface area and abundant hydroxyl groups. The humidity of human respiration and fruits and vegetables in storage can be sensitively and rapidly monitored by the QCM sensor.

Liu Y., Zhou J., Ji Z., Zhuo F., Wen S., Chen Y., Fu Y., Duan H.

Yang Y., Yin Z., Zhang W.

Recently, graphene has emerged as a promising electrode material for various flexible electronics considering its excellent electrical conductivity, high surface area, thermal conductivity, flexibility and mechanical strength. Based on these excellent properties, many researchers have fabricated various graphene-based temperature, humidity, and strain sensors, which have made important contributions in agricultural/industrial production, daily life, and consumer electronics. This paper first summarizes the sensing principles and methods to optimize the performance of these graphene sensors, then classifies their main application scenarios. Finally, a summary of the sensitive elements, manufacturing processes and sensing performance of this type of sensors in recent years is presented, with the aim of promoting the development of graphene-based physical sensors.

Gouda M., Ghazzawy H.S., Alqahtani N., Li X.

One of the most significant developed technologies is the use of acoustic waves to determine the chemical structures of biological tissues and their bioactivities. In addition, the use of new acoustic techniques for in vivo visualizing and imaging of animal and plant cellular chemical compositions could significantly help pave the way toward advanced analytical technologies. For instance, acoustic wave sensors (AWSs) based on quartz crystal microbalance (QCM) were used to identify the aromas of fermenting tea such as linalool, geraniol, and trans-2-hexenal. Therefore, this review focuses on the use of advanced acoustic technologies for tracking the composition changes in plant and animal tissues. In addition, a few key configurations of the AWS sensors and their different wave pattern applications in biomedical and microfluidic media progress are discussed.

Chen C., Chen L., Yao Y., Peng Y.

Abstract In this paper, flexible substrate integrated waveguide (SIW) resonators have been designed and fabricated on polyimide substrates for humidity sensing applications. The proposed SIW resonant cavity allows the resonator to obtain the maximum humidity sensitivity and meet the demand for flexible microwave sensing detection. Meanwhile, the humidity response performance can be further significantly enhanced by introducing nanodiamond (ND) sensing material. Three prototypes of ND-coated SIW sensors with different bending radii are measured to analyze their humidity sensing performance. The experimental results demonstrate that the proposed ND-coated SIW sensor with the minimum bending radius can achieve a maximum humidity sensitivity of 1.09 MHz/% relative humidity (RH) in the high RH region (>75.3% RH) and a low humidity hysteresis of 1.8% in the range of 11.3–97.3% RH. This study provides a promising candidate to realize flexible microwave sensors with excellent sensing performance.

Ahmed I., Rawat U., Chen J., Weinstein D.

Sett A., Biswas K., Majumder S., Datta A., Kanti Bhattacharyya T.

Humidity sensors are of utmost importance in certain areas of life, in processing industries, in fabrication laboratories and in agriculture. Precise evaluation of humidity percentage in air is the need of various applications. Graphene and its composites have shown great potential in performing as humidity sensors owing to enormous surface area, very low electrical noise, high electrical conductivity, mechanical and thermal stability and high room temperature mobility. There is no such extensive review on graphene-based devices for humidity sensing applications. This review extensively discusses graphene-based devices intended towards sensing humidity, starting from the methods of synthesizing graphene, its electronic and mechanical properties favoring sensing behavior and different types of sensing mechanisms. The review also studies the performance and recent trends in humidity sensor based on graphene, graphene quantum dots, graphene oxide, reduced graphene oxide and various composite materials based on graphene such as graphene/polymer, graphene/metal oxide or graphene/metal. Discussions on the limitations and challenges of the graphene-based humidity sensors along with its future trends are made.

Zhao C., Li C., Li M., Qian L., Wang L., Li H.

A delay line configuration Rayleigh SAW immunosensor with an Au-nanoparticles-decorated graphene (AuNPs-Gr) fluidic channel is proposed for the detection of CA125. The tube composed of Gr sheets is prepared by chemical vapor deposition (CVD) and Au-nanoparticles are decorated on the inner wall of the Gr tube by electrodeposition method. The AuNPs-Gr tube with a volume of 4.8 μL is placed on the bus bar of the SAW device and acts as sensing element as well as a micro-fluidic chamber. To estimate the sensing performances of the immunosensor comprehensively, besides the amplitude-frequency characteristics of the insert loss (S 21 ), the phase-frequency characteristics of S 21 , the equivalent conductance-frequency and susceptance-frequency characteristics are also analyzed. The deposition time of the Gr tubes, antibody concentration, antibody incubation time, and antigen incubation time are optimized to enhance the response of the SAW biosensor. Although the response sensitivity extracted from the amplitude-frequency curves (8.57 Hz/log(mU/mL)) is a bit lower than that from phase-frequency data (11.88 Hz/log(mU/mL)) and that from susceptance-frequency data (12.14 Hz/log(mU/mL)), a wider linear response range of 0.01–300 mU/mL is obtained from the amplitude-frequency curves and the limit of detection is calculated to be 0.00371 mU/mL. In addition, the proposed AuNPs-Gr fluidic channel SAW immunosensor exhibits good specificity and long-time stability and can be used to detect the CA125 in human serum. • SAW CA125 immunosensor with AuNPs-Gr fluidic channel is proposed. • S 21 amplitude and phase, complex impedance vs. frequency are analyzed. • Wide linear range of 0.01–300 mU/mL is obtained from the amplitude-frequency data. • The immunosensor exhibits good specificity, long-time stability and recovery in serum.

Yang J., Wang T., Gao W., Zhu C., Yin X., Dong P., Wu X.

Memon M.M., Hongyuan Y., Pan S., Wang T., Zhang W.

This work presents the experimental results for a surface acoustic wave (SAW) humidity sensor based on fluorinated polyimide (PI) as the sensing layer. The SAW sensor was configured as one port resonator onto an aluminium nitride (AlN)/silicon substrate. The interdigital transducers (IDTs) and reflectors were fully coated with PI film. With the hydrophobic sensing film, the total frequency shift was obtained as 332 kHz in the 10–90% relative humidity (RH) range, and the corresponding sensitivity was calculated as 4.15 kHz/%RH. In contrast to existing SAW humidity sensors, the fluorinated PI-based sensor showed a positive frequency shift in the whole detection range. Based on perturbation theory, the positive shift can be observed due to the stiffening effect during the sorption of vapor molecules occupying polymer-free volume. Furthermore, the sensor was tested in both very low (< 10%RH) and very high (> 90%RH) humidity ranges to verify the reverse trend of increasing frequency. The hydrophobic film was observed to be sensitive in both ranges and the frequency shift was found large in low ranges compared to high ranges, mainly due to the stiffening effect. At high RH levels, two contrary effects were contributing, mass loading, which shifts frequency negatively, and stiffening, which shifts positively. The cancellation between these effects results in low-frequency change at high ranges. Moreover, the sensor exhibits a fast response time, low hysteresis and good repeatability. These findings demonstrate that the SAW sensor with hydrophobic PI sensing film is a good choice for humidity sensing applications.

Golovanov E., Kolesov V., Anisimkin V., Osipenko V., Kuznetsova I.

Zinc oxide is one of the most popular materials for acoustoelectronic sensors and vibro-piezo-transducers used in nano-piezo-generators. In the present paper, thick piezoelectric ZnO films are fabricated on both sides of various substrates using magnetron sputtering technique. It is shown that the main problem for double film deposition is the difference in thermal expansion coefficients of the ZnO films and the substrate materials. The problem is solved by decreasing the plate temperature up to 140 °C, reducing the growing rate up to 0.8 ± 0.05 μm/h, and diminishing the oxygen content in Ar mixture up to 40%. Using the modified sputtering conditions, the ZnO films with thickness up to 15 μm, grain size 0.3 μm, and piezoelectric module as large as 7.5 × 10−12 C/N are fabricated on both faces of quartz and lithium niobate plates as well as on flexible polyimide flexible film known as Kapton. The films are characterized by chemical composition, crystallographic orientation, piezoelectric effect, and acoustic wave generation. They are applied for vibro-piezo-transducer based on flexible ZnO/Kapton/Al/ZnO/Al structure. When the structure is mechanical excited, the variable electric voltage of about 35 mV is generated. The value of the voltage is sufficient for an unstable energy source used in autonomic micro-energetic energy-store systems.