Acoustic Wave Sensor Research Articles

A Perspective on Recent Advances in Piezoelectric Chemical Sensors for Environmental Monitoring and Foodstuffs Analysis

This paper provides a selection of the last two decades publications on the development and application of chemical sensors based on piezoelectric quartz resonators for a wide range of analytical tasks. Most of the attention is devoted to an analysis of gas and liquid media and to industrial processes controls utilizing single quartz crystal microbalance (QCM) sensors, bulk acoustic wave (BAW) sensors, and their arrays in e-nose systems. The unique opportunity to estimate several heavy metals in natural and wastewater samples from the output of a QCM sensor array highly sensitive to changes in metal ion activity in water vapor is shown. The high potential of QCM multisensor systems for fast and cost-effective water contamination assessments “in situ” without sample pretreatment is demonstrated.

Wireless Interrogation of Implantable SAW Sensors.

Implantable sensors provide long-term, accurate physiological measurements after a minimally invasive procedure, particularly when designed as transponders. Wireless interrogation of deeply implanted transponders with RF remains a challenge due to the high loss at the skin-air interface and large tissue RF absorption. This paper presents a system for wirelessly interrogating surface acoustic wave (SAW) sensors implanted in the main pulmonary artery (PA), where the pressure (PAP) is a very important parameter in the management of heart failure patients. The proposed PAP monitoring system consists of an implantable SAW pressure sensor integrated with an antenna and anchor in a housing, an external antenna and an electronic interrogator. The PAP is determined by measuring the frequency of the echo signal from SAW sensor accurately. An asymmetric antenna was designed and integrated with the sensor. The combination of simulation, theoretical calculation and phantom measurement indicates that the path loss to the implant location, about 6cm below the skin, is around 25dB. A portable interrogator was designed based on a dual conversion receiver and single echo high frequency sampling approach to assess achievable frequency estimation accuracy predicted by Cramer-Rao Lower Bound (CRLB) analysis. The system was characterized using a high quality (Q) factor SAW sensor, fabricated at wafer level, wire-connected to the interrogator via an attenuator to simulate path loss. The signal-to-noise ratio (SNR) of captured echo signals was calculated and used in CRLB analysis. The analysis indicates that without using signal post processing, the sensor sensitivity has to be at least 440Hz/mmHg in order to achieve a target 1mmHg accuracy. Although the current sensor sensitivity is only 200Hz/mmHg, the in vivo measurement showed that acceptable accuracy can be obtained by signal post processing. The results from an invasive catheter tip transducer measured simultaneously with the SAW sensor showed that the differences in pulse pressure and relative mean pressure are 0.8mmHg and 1.4mmHg, respectively. The accuracy could be further improved by increasing the sensor Q factor and sensitivity and reducing path loss.

Mo-La2O3 Multilayer Metallization Systems for High Temperature Surface Acoustic Wave Sensor Devices.

Developing advanced thin film materials is the key challenge in high-temperature applications of surface acoustic wave sensor devices. One hundred nanometer thick (Mo-LaO) multilayer systems were fabricated at room temperature on thermally oxidized (100) Si substrates (SiO/Si) to study the effect of lanthanum oxide on the electrical resistivity of molybdenum thin films and their high-temperature stability. The multilayer systems were deposited by the magnetron sputter deposition of extremely thin (≤1 nm) La interlayers in between adjacent Mo layers. After deposition of each La layer the process was interrupted for 25 to 60 min to oxidize the La using the residual oxygen in the high vacuum of the deposition chamber. The samples were annealed at 800 C in high vacuum for up to 120 h. In case of a 1 nm thick La interlayer in-between the Mo a continuous layer of LaO is formed. For thinner La layers an interlayer between adjacent Mo layers is observed consisting of a (LaO-Mo) mixed structure of molybdenum and nm-sized lanthanum oxide particles. Measurements show that the (Mo-LaO) multilayer systems on SiO/Si substrates are stable at least up to 800 C for 120 h in high vacuum conditions.

Non-leaky longitudinal acoustic modes in ScxAl1-xN/sapphire structure for high-temperature sensor applications

Multilayered structures based on wide bandgap nitride piezoelectric thin films are very attractive for high-temperature surface acoustic wave (SAW) sensor applications. In this respect, scandium aluminum nitride (ScAlN) films are of particular interest as they combine enhanced piezoelectric properties and slower acoustic wave velocities when the Sc content steadily increases up to 40%. This property offers the possibility to combine slow ScAlN films on fast substrates like sapphire, to generate higher-order SAW modes which often show a better electromechanical coupling coefficient k2 compared to zero-order modes. In this letter, we show that low-attenuated longitudinal SAW can be generated in the ScxAl1-xN/sapphire structure, for the x parameter varying in a large range. This theoretical result is then confirmed by the experimental investigation of SAW resonators based on highly textured (002) Sc0.09Al0.91N films sputtered on c-cut sapphire substrates. It is finally shown that the use of electrodes based on metals with high density can lead to SAW structures offering a unique combination between a large bandgap over 5 eV, a k2 value beyond 1%, and a high SAW velocity near 10 000 m/s.

Polymer-Free 2-D Heterostructure Transfer Onto Quartz Crystal Microbalance Electrode Surface: Method and Sensing Characteristics

There has been tremendous interest to employ 2-D materials as sensing medium in acoustic wave sensors such as quartz crystal microbalance (QCM). We report a method to transfer polymer-free 2-D materials onto QCM through a tailor-made automated process system and resultant enhancement in the sensing characteristics of QCM. A customized polytetrafluoroethylene (PTFE) structure was designed for the automated transfer of a 2-D heterostructure material made of hexagonal boron nitride (hBN)/graphene onto the QCM. The design ensured that the 2-D material adhered to the most sensitive region of the QCM, at the center of the electrode surface. The transferred 2-D material was characterized using Raman spectroscopy and scanning electron microscopy. Also, the $S_{11}$ characteristics of the QCM were measured before and after the transfer process using a network analyzer. We observed increment in return loss and loaded $Q$ -factor ( $Q_{L}$ ) in the presence of the 2-D material. In order to investigate the sensing characteristics of the 2-D material integrated QCM, relative humidity (RH) was taken as an example measurand, and experiments were carried out to study the adsorption and desorption of moisture. The 2-D material integrated QCM showed good sensitivity with excellent signal-to-noise ratio for RH variations as evidenced from the significant changes in return loss and conductance measurements. On the other hand, the bare QCM exhibited poor signal-to-noise ratio and inconsistent response to RH changes.

Acoustic guided wave techniques for detecting corrosion damage of electrical grounding rods

Determining the integrity of grounding electrodes is challenging and there is need to augment expensive and time consuming excavation based methods. In this work pulse-echo methods based on acoustic guided waves were tested on model ground rods with machined model corrosion pits. The generation and propagation of the guided acoustic waves in the model grounding rods using different types of acoustic wave emitters and sensors was studied. The existence of three principle wave modes, longitudinal, flexural and torsional modes, was experimentally observed. The experimentally obtained group velocities versus frequency are consistent with numerical simulations. The principal longitudinal mode at low frequencies has been identified as the best candidate for damage detection on the grounding rods. The pulse-echo signals from the simulated corrosion pits are well correlated with the positions and cross-sectional areas of the pits. Wet-clay wrapping of the rods was used to simulate the influence of soil on the wave propagation. In the wet clay wrapped rods pulse-echo signals from the simulated corrosion pits are well correlated with the positions and cross-sectional areas of the pits, despite the significant attenuation of the echo signals. Acoustic pulse-echo methods based on acoustic guided waves have potential practical application for detecting corrosion-damage of the grounding rods.

One-dimensional surface phononic crystal ring resonator and its application in gas sensing

We introduce a ring resonator, which employs a one-dimensional phononic crystal on its inner surface, and investigate its performance as a gas sensor both numerically and experimentally. Having periodic equilateral trapezoidal protrusions, the ring resonator with 207 periods is optimized through band structure calculations via the finite-element method. A surface band linear around 58 kHz is observed. The resonator exhibits sharp transmission peaks with a broad free-spectral range of 0.54 kHz. Accordingly, a peak at 58.49 kHz with a high-quality factor of 8196 appears. Application in detection of the carbon dioxide level in air with high sensitivity is demonstrated. The 58.49 kHz peak red shifts linearly at 17.3 mHz/ppm and 17.8 mHz/ppm rates, as obtained from numerical calculations and experiments, respectively. Besides, the peak shape and maximum intensity are preserved. Due to the linear shift of the resonance peak with respect to the carbon dioxide concentration, acoustic intensity at initial peak frequency can be utilized as an auxiliary means for concentrations up to 1000 ppm. The proposed ring resonator can be adapted to a variety of acoustic devices such as liquid concentration sensors based on phononic crystals, surface acoustic wave sensors, and micromechanical resonators.

Fabrication of Three-Dimensional Piezoelectric Ceramics Using the Dispenser System

Abstract We fabricated lead-free (K,Na,Li)NbO3 (KNLN) and BaTiO3 (BT) piezoelectric ceramics using a dispenser system. After dissolving polyvinyl butyral in ethanol, a paste for extrusion was fabricated by mixing KNLN or BT ceramic powders. After several times of paste extrusion with a dispenser system, degreasing was performed at 650°C. The KNLN moldings were sintered, at 1,100–1,170°C, and sintering of BT moldings were performed by two-step sintering at 1,320°C and 1,150°C. From the x-ray diffraction pattern, the fabricated KNLN ceramics had a perovskite structure. The density of the KNLN and BT moldings fabricated by the dispenser system was 3.94 and 5.42 g/cm3, respectively. In addition, the BT moldings had the dielectric constant εr = 1.95 × 103. The Curie temperature of the BT moldings was confirmed at about 125°C, as with the BT piezoelectric ceramics fabricated by uniaxial pressure molding. The BT moldings had the piezoelectric constant d33 = 60 pC/N. The fabrication methods studied in this work provided the potential to fabricate simple three-dimensional piezoelectric ceramic devices for applications in acoustic wave sensors.

Palladium thin films for plasmonic hydrogen gas sensing

In this study, I prepared BK7 glass slides coated by palladium (Pd) layer by PVD technique. These samples have been employed as plasmon active structures in classic Kretschmann-based SPR set-up. The application of H2 sensing structures based on palladium plasmonic active thin films have been tested and investigated. Hydrogen sensing properties of Pd films were investigated at room temperature The reflectances of p-polarized light from Pd thin films as a function of angle of incidence and wavelength were measured in synthetic air (or nitrogen) and in gas mixtures including hydrogen. Variations of the reflectance in the presence of hydrogen gas at room temperature revealed that the samples can sense hydrogen in a wide range of concentration (0–2% vol/vol) without saturation behavior. The dynamic properties with various concentration of H2 at low temperature and dry gas mixtures was investigated and the effects of these factors on the hydrogen sensing properties were analyzed. Full Text: PDF ReferencesG. Korotcenkov, Handbook of Gas Sensor Materials: Properties, Advantages, and Shortcomings for Applications (Springer, New York 2013). CrossRef W. Jakubik, M. Urbanczyk, E. Maciak, "SAW hydrogen gas sensor based on WO3 and Pd nanostructures", Procedia Chemistry 1 (1), 200 (2009). CrossRef W. Jakubik, M. Urbanczyk, E. Maciak, T. Pustelny, "Bilayer Structures of NiOx and Pd in Surface Acoustic Wave and Electrical Gas Sensor Systems", Acta Physica Polonica A 116(3), 315 (2009). CrossRef E. Maciak, Z. Opilski, "Pd/V2O5 fiber optic hydrogen gas sensor", J. Phys. France IV 129, 137 (2005). CrossRef E. Maciak,. "Fiber optic sensor for H2 gas detection in the presence of methane based on Pd/WO3 low-coherence interferometric structure", Proc. SPIE 10455, UNSP 104550W (2017). CrossRef X. Bevenot, A. Truillet, C. Veillas, H. Gagnaire, M. Clement, "Hydrogen leak detection using an optical fibre sensor for aerospace applications", Sens. Actuators B 67, 57 (2000). CrossRef J. Homola, S.S. Yee, G. Gauglitz, "Surface plasmon resonance sensors: review", Sensors and Actuators B 54, 3 (1999). CrossRef H. Raether, Surface plasmons on smooth and rough surfaces and on gratings (Springer-Verlag, Berlin-Heidelberg 1988). CrossRef P. Tobiska, O. Hugon, A. Trouillet, H.Gagnarie, "An integrated optic hydrogen sensor based on SPR on palladium", Sensors and Actuators, B 74, 168 (2001). CrossRef Z. Opilski, E. Maciak, "Optical hydrogen sensor employing the phenomenon of the surface plasmons resonance in the palladium layer", Proc. SPIE 5576, 202 (2004). CrossRef T. Pustelny, E. Maciak, Z. Opilski, A. Piotrowska, E. Papis, K. Golaszewska, "Investigation of the ZnO sensing structure on NH3 action by means of the surface plasmon resonance method", European Physical Journal-Special Topics 154, 165 (2008). CrossRef E. Maciak, M. Procek, K. Kępska, A. Stolarczyk, "Study of optical and electrical properties of thin films of the conducting comb-like graft copolymer of polymethylsiloxane with poly(3-hexyltiophene) and poly(ethylene) glycol side chains for low temperature NO2 sensing", Thin Solid Films 618, 277 (2016). CrossRef

Background gas analysis with leaky attenuation in a trace moisture analyzer using a ball surface acoustic wave sensor

A method is described for background gas (BG) analysis in surface acoustic wave (SAW) gas sensors using two frequencies. The gas parameter G is defined as the square root of molecular weight times the ratio of specific heat at constant pressure to specific heat at constant volume, which is a representative property of BGs. In an experiment using varying BGs, the measured G values agreed with the true values. The concentrations of mixed BGs were successfully measured. This method will be useful in a trace moisture analyzer in natural gases and in the optimization of the purge process of glove boxes.

Detection of Volatile Organic Compounds Using Surface Acoustic Wave Sensor Based on Nanoparticles Incorporated in Polymer

In this work, surface acoustic wave (SAW) sensors with two types of polymer sensing films, polyethyleneimine (PEI) and polydimethylsiloxane (PDMS), containing embedded ZnO, TiO2, and WO3 nanoparticles (NPs) for detecting volatile organic compounds (VOCs) were produced and studied. The NPs were obtained using the pulsed laser ablation method, with the same deposition conditions used for all three materials studied. After incorporation of the NPs into the polymer, the suspension obtained was deposited using the airbrush method onto the quartz substrate of the sensor. Sensors were tested for four types of VOCs: Ethanol, toluene, acetone, and dichloroethane. Those based on PEI-sensitive films showed a superior sensitivity to those with PDMS. It was also found that the sensors with WO3 NPs had the best results for ethanol, acetone, and dichloroethane. The limit of detection (LOD) of the PEI/WO3 sensor was 6 ppm for ethanol, 15 ppm for acetone, and 9 ppm for dichloroethane. For toluene, the best response was obtained using the PEI/ZnO sensor, which produced a LOD of 9 ppm.

Investigation on the mass sensitivity of quartz crystal microbalance gas sensor using finite element simulation

The increasing global trends in healthcare priorities towards improving the effectiveness of diagnostic procedure by utilizing a non-invasive method which is breath analysis. This will benefit in increasing treatment efficiency and also reducing healthcare costs. Breath is a simple technique where the sample are easily obtained and can be provided immediately. The most popular method that had been used in hospital are urine and blood. Contradict with breath, urine and blood take too much time to analyse the disease and a painful process. The detection technique of breath analysis is done by using electroacoustic wave sensor from piezoelectric substrate. This acoustic wave sensor has been used to detect the changes in the frequency where it will be used to detect the disease. Breath analysis is a technique where it uses an electronic nose (E-nose) as a device. E-nose consist of Quartz Crystal Microbalance (QCM) sensor in order to differentiate odor in human breath. QCM is a sensitive weighing device which have a high efficiency. AT-cut quartz was chosen as the piezoelectric material and aluminum as the electrode. The objective of this paper is to design and simulate a QCM sensor for breath analysis application. Other than that, it also to determine the important key parameters that influence the performance of breath analysis which is sensitivity and resonant frequency. QCM sensor is being simulate by using COMSOL Multiphysics software. This is to evaluate the behavior of QCM sensor in terms of Eigen frequency analysis. The simulated QCM sensor with quartz radius of 166 um resonates at 8.871 MHz. The sensitivity of the sensor is 0.167 MHz/ng after exposed to acetone gas which act as the breath marker for detection of diseases in exhaled breath. Hence, the proposed design can be used as a non-invasive approach for early detection of disease through breath analysis.

A generalized dynamic model of nanoscale surface acoustic wave sensors and its applications in Love wave propagation and shear-horizontal vibration

• A generalized dynamic model based on the Hamilton's principle and variational approach is established. • The model is applied to depict the wave propagation in SAW nano-devices or vibration of piezoelectric nanoplates. • The surface effect is included with the aid of the surface elasticity , surface piezoelectricity and surface permittivity. • A critical thickness value of surface acoustic wave nano-devices or piezoelectric nanoplates can be obtained. • Below the critical thickness, the size-dependent properties will become evident and should be taken into account. A generalized dynamic model to depict the wave propagation properties in surface acoustic wave nano-devices is established based on the Hamilton's principle and variational approach. The surface effect, equivalent to additional thin films, is included with the aid of the surface elasticity, surface piezoelectricity and surface permittivity. It is demonstrated that this generalized dynamic model can be reduced into some classical cases, suitable for macro-scale and nano-scale, if some specific assumptions are utilized. In numerical simulations, Love wave propagation in a typical surface acoustic wave device composed of a piezoelectric ceramic transducer film and an aluminum substrate, as well as the shear-horizontal vibration of a piezoelectric plate, is investigated consequently to qualitatively and quantitatively analyze the surface effect. Correspondingly, a critical thickness that distinguishes surface effect from macro-mechanical behaviors is proposed, below which the size-dependent properties must be considered. Not limited as Love waves, the theoretical model will provide us a useful mathematical tool to analyze surface effect in nano-devices, which can be easily extended to other type of waves, such as Bleustein-Gulyaev waves and general Rayleigh waves.

QCM formaldehyde sensing materials: Design and sensing mechanism

Gaseous formaldehyde (HCHO) sensing is indispensable because HCHO is hazardous to health and widely exists in newly decorated house and newly painted furniture. Multitudinous HCHO sensors based on various platforms, such as semiconductor sensor, electrochemical sensor, surface acoustic wave sensor (SAW), fluorescent sensor, have been fabricated over the last few years. Except these sensors, quartz crystal microbalance (QCM) sensors have received increasing attention because of their high sensitivity, convenient operation, low energy consumption, and excellent modifiable property. Recently, based on QCM, many efforts have been made to design chemical sensing materials to detect HCHO gas sensitively and selectively. In this review, we summary the scientific researches of gaseous HCHO detection based on QCM platform, and introduce the related works of sensing HCHO carried out by our group as well as the exploration of sensing mechanism.

Analysis and test of a wireless impedance‐loaded SAW sensor

Instead of a conventional wireless transceiver system that requires batteries and complex circuits, the surface acoustic wave (SAW) sensor enables wireless passive measurement. A sensor consisting of an SAW device and external impedance sensing element is analysed. Since the reflection coefficient of the reflective grating on the SAW device depends on the load impedance, the echo characteristics are influenced by the change in the impedance of external sensing element. A resistance sensor or a capacitive sensor is selected as the external sensing element. The two different types of sensors are simulated using coupling-of-mode (COM) modelling, and the relationships between amplitude and phase with load impedance are analysed. On the basis of COM theory, a wireless impedance-loaded SAW sensor is fabricated by the lift-off process and tested by a network analyser to verify the simulation results. It is observed that the test results agree well with the simulation results. The phase change is more sensitive than the amplitude based on the results obtained. The sensitivity is 0.274°/Ω for the sensor with resistance and the sensitivity of the sensor with capacitance is 1.096°/pF. These results can guide the design of the high sensitivity impedance-loaded SAW sensors in the future.

Embedded Ceramic-Based Passive Wireless Sensors for Harsh-Environment Applications

Recent advancements in high temperature systems such as gasifiers, molten slag furnaces, super-heated steam turbines, and energy harvesting systems require in-situ temperature monitoring. Conventional temperature sensors such as thermocouples, thermistors, and resistance temperature devices (RTDs) are limited to such applications due to poor reliability. Additionally, these sensors require one end of the device to be maintained in a relatively cold junction which limits the integration with the sophisticated modern high temperature systems. To mitigate these issues, research has been focused on adapting wireless technology for measuring temperature in harsh environments where a conventional temperature sensor cannot be integrated with the system. Initially, research was focused heavily on fiber optics sensor systems, surface acoustic wave (SAW) sensors, laser interferometers, and infra-red sensors. However, these sensor systems have limited lifespan, poor reliability due to reactions taking place at the hot junction, and integration of the sensor with the high temperature system is challenging due to sophisticated design of the furnaces, gasifiers, or turbine blades. In order to mitigate these issues, research has been focused on adapting radio-frequency (RF) technology for high temperature systems. The objective of this work was to design and fabricate inductor-capacitor (LC) resonator based wireless sensors to monitor temperature systems above 500oC.The two-dimensional (planar) LC resonator sensors were designed using the ANSYS Maxwell simulation package. The inductor design was based upon a fifteen-turn coil. The coil geometry was based on a width of 0.15 mm and a spacing of 0.15 mm spacing between the turns. The outer diameter was ~20 mm and the inner diameter was near ~10 mm. The capacitor was based on an interdigitated capacitor (IDC) which could be easily printed onto a ceramic substrate, which would act as the dielectric. The common substrate used in this work was alumina (Al2O3) or yttrium-stabilized zirconia (YSZ). The LC circuit patterns were printed onto alumina or zirconia substrates using inks containing semi-conducting ceramic particles. A few different semi-conducting ceramic compositions were evaluated in this work. One example composition was the La2NiO4 (LN) composition, which displays relatively good high-temperature stability and high electrical conductivity in the range from >40 S/cm at temperatures above 450⁰C. The ceramic powder compositions were synthesized using a conventional solid-state reaction route by mixed the stoichiometric constituents by attrition-milling using zirconia media in ethanol. The resultant slurry was dried and calcined >1000°C for 4 h. X-ray diffraction (XRD) analysis was performed using Panalytical X-ray diffractometer to confirm single phase formation. The circuit patterns were fabricated by using a micro-casting process. Conventional photolithography was used to pattern the photoresist onto the ceramic substrates. The patterned photoresist acted as the molds to deposit the ceramic inks to form the desired LC patterns. The micro-casted patterns were dried and then fired in a muffle furnace to ~1200oC to densify the patterns and bond them to the ceramic substrates. During the firing process, the photoresist would be removed in situ through a basic pyrolysis mechanism. For initial testing in this work, platinum (Pt) interconnects connected to the contact pads were made using Pt ink and further connected to Pt wires to connect to the electrical analysis equipment. Figure 1 shows a magnified optical microscopy picture of a section of the inductor pattern, which shows the level of resolution of the ceramic pattern. The sensors were tested within a high temperature muffle furnace in air. Initial tests used matching sensors placed parallel to each other within the furnace. One sensor was attached using the Pt wires to a signal generator (HP E4433B, USA) to act as the interrogator. The second sensor was hard-wired to a signal analyzer (Tektronix RSA306B, USA). The sensors were characterized at 500 – 1000oC in an ambient atmosphere with an RF signal ranging from 10 – 80 MHz at 2 MHz/sec sweep rate. The sensors showed a sensitivity of 0.2 kHz/oC. Two signal processing methods were implemented in algorithm form (within MATLAB) to estimate the temperature given the frequency response of the sensor. The methods evaluated were the cross-correlation and minimization of absolute error methods. In addition to these two signal processing methods, a split window technique for analyzing the temperature/frequency correlation was used. The cross-correlation signal processing method showed the best fitting for the initial sensor testing. The first set of sensors based on the LN compositions demonstrated sensitivities ~0.2 kHz/oC up to 1000°C. The work will further discuss the results from all sensor compositions and geometries. In addition, the investigation of cyclic stability and repeatability analysis will also be discussed in the presentation. Figure 1

Microfluidic Bulk-Modulus Measurement by a Nanowavelength Longitudinal-Acoustic-Wave Microsensor in the Nonreflective Regime

Investigating the physical mechanisms of interaction between acoustic waves and fluids is significant for the detection of the properties of liquids. Researchers in this field have mainly used shear-mode acoustic wave sensors to measure fluid density and viscosity, but not the bulk modulus. Using an immersed gigahertz-range microsensor, one can obtain compressional information about a liquid from the observed propagation of longitudinal acoustic waves in the nonreflective regime. This work fills a gap by enabling measurement of a liquid's bulk modulus in a microfluidic setting, with an eye toward physiological fluids in particular.

High humidity enhanced surface acoustic wave (SAW) H2S sensors based on sol–gel CuO films

Surface acoustic wave (SAW) sensors based on sol–gel processed porous and nanoparticulate CuO films were explored for H2S detection operated at room temperature. The SAW sensor showed a negative frequency shift when exposed to H2S gas, due to the mass loading effect on the sensitive CuO film. It has a high sensitivity of H2S, a detection limit of 500 ppb and a good selectivity to H2S gas compared with H2, C2H5OH, acetone, NH3, CO, NO, NO2 gases. Relative humidity (RH) was identified to have a critical influence on the sensing performance of the sensor. The responses became much faster and stronger with the increase of RH. The significant enhancement in the sensing performance is attributed to that the chemical reaction between H2S and CuO is hindered and the physical adsorption of CuO to H2S is enhanced.

3D-printed Point-of-Care Platform for Genetic Testing of Infectious Diseases Directly in Human Samples Using Acoustic Sensors and a Smartphone.

The objective of this work is to develop a methodology and associated platform for nucleic acid detection at the point-of-care (POC) that is sensitive, user-friendly, affordable, rapid, and robust. The heart of this system is an acoustic wave sensor, based on a Surface Acoustic Wave (SAW) or Quartz Crystal Microbalance (QCM) device, which is employed for the label-free detection of isothermally amplified target DNA. Nucleic acids amplification and detection is demonstrated inside three crude human samples, i.e., whole blood, saliva, and nasal swab, spiked in with 10-100 Salmonella cells. To qualify for POC applications, a portable platform was developed based on 3D printing, integrating inside a single box: (i) simple fluidics based on plastic tubing and a mini peristaltic pump, (ii) a heating plate combined with disposable reaction tubes for isothermal amplification; (iii) a mini antenna analyzer operated through a tablet; and (iv) an acoustic wave device housing unit. The simplicity of the method combined with smartphone operation and detection, rapid sample-to-answer analysis time (30 min), and high performance (detection limit 4 × 103 CFU/ml) in three of the most important human samples in diagnostics suggest that the methodology could become a tool of choice for nucleic acid detection at the POC. In addition, the low cost of the platform and assay holds promise for its adoption in resource limited areas. The acoustic detection method is shown to give similar results with a standard colorimetric assay carried out in saliva and nasal swab but can also be used to detect nucleic acids inside whole blood, where a colorimetric assay failed to perform.

Stoichiometric Lithium Niobate Crystals: Towards Identifiable Wireless Surface Acoustic Wave Sensors Operable up to 600 °C

Wireless surface acoustic wave (SAW) sensors constitute a promising solution to some unsolved industrial sensing issues taking place at high temperatures. Currently, this technology enables wireless measurements up to 600-700$^\circ$C at best. However, the applicability of such sensors remains incomplete since they do not allow identification above 400$^\circ$C. The latter would require the use of a piezoelectric substrate providing a large electromechanical coupling coefficient K 2 , while being stable at high temperature. In this letter, we investigate the potentiality of stoichiometric lithium niobate (sLN) crystals for such purpose. Raman spectroscopy and X-ray diffraction attest that sLN crystals withstand high temperatures up to 800$^\circ$C, at least for several days. In situ measurements of sLN-based SAW resonators conducted up to 600$^\circ$C show that the K 2 of these crystals remains high and stable throughout the whole experiment, which is very promising for the future achievement of identifiable wireless high-temperature SAW sensors.