Surface Acoustic Wave Sensor System Research Articles

Wireless surface acoustic wave humidity sensor with chitosan/porous cyclodextrin–TiO2 composites for monitoring air and human respiration

Wireless surface acoustic wave (SAW) technology can be used for real-time and contactless humidity measurements, as well as for monitoring human respiration and environmental humidity. In this study, we developed a wireless humidity sensor based on a 433 MHz SAW resonator covered with chitosan (CS)/heat-treated cyclodextrin(α-, β-, γ-CD)–TiO2 sensitive films. The mesoporous distribution and hydrophilicity of the CD–TiO2 composite, which affect the sensitivity of the CS/CD–TiO2/SAW sensor response to humidity, can be controlled through the CD type and TiO2 content of the composite. The frequency shift of the optimized CS/β-CD–TiO2/SAW sensor varied linearly with the relative humidity (RH) in the RH ranges of 0%76% and 76%90% with the corresponding sensitivities of − 2.96 and − 99.71 kHz%−1, respectively. The response and recovery times at 70% RH were 13.3 and 28.8 s, respectively, with an insignificant hysteresis of 0.61%. The excellent humidity response was due to the hydrophobicity of the inner cavity in the β-CD template molecule and the hydrogen bond adsorption between the surface of the CS/β-CD–TiO2 and H2O molecules. The response rates of the wireless SAW sensor system to normal air humidity and human exhaled/inhaled humidity were 3.13% and 30% RH s−1, respectively, and the wireless transmission distance was 30 m. The wireless CS/β-CD–TiO2/SAW humidity sensor has considerable application potential in forest fire early warning and firefighter tracking systems that require the remote monitoring of air humidity and human respiration.

Selective Detection of Aromatic Compounds with a Re-Designed Surface Acoustic Wave Sensor System Using a Short Packed Column

A self-developed and newly re-designed chemical SAW sensor system composed of four polymer-coated and four differently modified nano-diamond-coated SAW sensors was applied to measure aromatic compounds in gasoline in a low-cost, fast, and easy way. An additional short packed column at the system inlet improve the selectivity for various possible fuel applications. The column allows the direct sampling of liquid fuels and pre-separates the different components in groups (aromatic and aliphatic compounds) from a fuel sample. Since the sensors employed show linearity towards concentration, an easy quantification of single fuel components was possible even within the group of aromatic compounds.

An Algorithm for Solving the Phase Ambiguity Problem of SAW Delay-Line Sensor Systems.

Solving the phase ambiguity problem is crucial to achieving a wide-range and high-precision measurement for the frequency-domain sampling (FDS)-based surface acoustic wave (SAW) delay-line sensor systems. This study proposes an improved phase estimation algorithm called dual-band phase estimation (DBPE) to solve the problem. By using DBPE, the SAW sensor system can obtain an extensive and alterable measuring range without further requirements for sensor design or transmitted signals. Thus, it can be widely used in various FDS-based SAW delay-line sensor systems. Monte Carlo simulations and temperature measuring experiments, based on a YZ-cut LiNbO3 SAW delay-line sensor and a switched frequency-stepped continuous wave (S-FSCW) reader, are performed to demonstrate the algorithm's validity. The Monte Carlo simulations show that DBPE can effectively solve the phase ambiguity problem and has better performance than frequency estimation in measuring precision at a low signal-to-noise ratio (SNR). The temperature-sensing experiments show that DBPE has a good performance in measuring range and precision, serving as a phase ambiguity solver in the temperature sensor system.

Design of a Portable Orthogonal Surface Acoustic Wave Sensor System for Simultaneous Sensing and Removal of Nonspecifically Bound Proteins.

One challenge for current surface acoustic wave (SAW) biosensors is reducing nonspecific adsorption. A device propagating Rayleigh and shear horizontal surface acoustic waves in orthogonal directions fabricated in ST quartz has the capability of achieving simultaneous detection and nonspecific binding (NSB) protein removal. Current measurement methods for a SAW sensor system based on this device require large-size and expensive equipment such as a vector network analyzer (VNA), signal generator, and frequency counter, which are not suitable for portable, especially point-of-care, applications. In this work, a portable platform based on a direct digital synthesizer (DDS) is investigated for the orthogonal SAW sensor, integrating signal synthesis, gain control, phase/amplitude measurement, and data processing in a small, portable electronic system. This prototype was verified for both stability and repeatability, and the results matched very well with VNA measurements. Finally, system performance in real-time sensing and NSB removal was evaluated.

Concrete temperature monitoring using passive wireless surface acoustic wave sensor system

In this paper, a novel nondestructive method for detecting temperature variations of concrete using a passive wireless surface acoustic wave (SAW) sensor system is described. Performance of the SAW orthogonal frequency coded (OFC) temperature sensors was evaluated in air, liquid, and concrete. To verify the response of the SAW sensor, temperature was also measured using commercially available thermocouples and compared with the extracted temperature in all environmental conditions. Analysis of the measured data showed that the SAW temperature sensor system was successful in detecting temperature over a wide range in all tested conditions with a high degree of accuracy and short interrogation time. When the sensors were embedded within a 2-in. thick concrete cover, the maximum read distance was approximately 4m.

Magnetic Sensor Based on Surface Acoustic Wave Resonators

In this paper, we describe a new type of magnetic surface acoustic wave (SAW) sensor that consists of interdigital transducers made of a magnetostrictive material, Ni, on a quartz substrate. This sensor can be used as an element of a resonator-type wireless SAW sensor system because it has a high Q. Moreover, it has the merits of being fabricated easily using the existing process for SAW devices because its structure is the same as that of general SAW resonators. When an external magnetic field of about 100 mT was applied perpendicularly to the direction of SAW propagation, the frequency change obtained was 200 ppm. In addition, the developed sensor was applied in a passive and wireless open/close sensing system and the reading distance obtained was 3 m.

Applications of wireless SAW sensing in the steel industry

Three novel applications of surface acoustic wave (SAW) sensor systems in the steel industry are presented: radio frequency identification (RFID) and temperature monitoring of slide gate plates, monitoring of the temperature inside a lining of a metallurgical vessel and RFID tagging of slag ladles. The motivation, the system requirements and constraints of the sensor design and the sensor integration are discussed. Results of field trials of the SAW sensor systems within the steel plants are presented.

Selection process generating peptide aptamers and analysis of their binding to the TiO2 surface of a surface acoustic wave sensor

The set-up presented in this article is intended for the selection of peptides which serve as specific binders to suitable materials. Additionally, the interaction of such binders with material surfaces can be characterized. Using this approach, a subset of peptides which adhere to the mineral TiO2 was generated by means of a cell surface display library. The peptides are constrained by a thioredoxin scaffold. Selection of proteins was carried out on a silicium wafer sputtered with TiO2 in anatase conformation. To verify binders and to analyze the binding kinetics of the diluted suspension of the purified proteins, the chip-based S-sens® K5 surface acoustic wave sensor system was used. The surface of the sensor chips was also TiO2, resembling the material of the Si wafer selection target retaining the peptides. Several peptides were identified. The respective binding behavior differed. The data derived from real-time interaction analysis were evaluated to select for strong and specific binders. For one of these peptides, the binding kinetics was analyzed. On- and off-rate binding constants were extracted from the fitted curves. With the resulting association rate constant kon and the dissociation constant koff, the affinity of the peptide for the TiO2 surface was calculated, represented by the equilibrium dissociation constant KD=81nM.

Development of a Shear Horizontal Surface Acoustic Wave Sensor System with a Floating Electrode Unidirectional Transducer

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Development of a Shear Horizontal Surface Acoustic Wave Sensor System for Liquids with a Floating Electrode Unidirectional Transducer

The development of sensors using surface acoustic wave (SAW) devices has been attracting attention. A shear horizontal SAW (SH-SAW) sensor can detect the properties and chemical reactions in liquids. To realize practical applications of the SAW sensor, it is necessary to discuss its properties. In this paper, SH-SAW sensors with a floating electrode unidirectional transducer (FEUDT) or an interdigital transducer (IDT) are compared and the insertion loss and phase characteristics are measured. Also, the phase shift between sample and reference liquids is evaluated. The results indicate that the SH-SAW sensor with the FEUDT is suitable for liquid measurements. The SH-SAW sensor with the FEUDT is then mounted in a newly developed SH-SAW sensing system. The system configuration is the same as that of a vector voltmeter measurement system. However, circuits have been developed that reduce its size. Using this system, several liquids are measured. The obtained results agree well with the theoretical values.

Investigations of thin film structures of WO 3 and WO 3 with Pd for hydrogen detection in a surface acoustic wave sensor system

A single thin film sensor structure of WO 3 (∼ 50 nm) and bilayer sensor structure of WO 3 (∼ 50 nm) with a very thin film of palladium (Pd ∼ 18 nm) on the top, have been studied for hydrogen gas-sensing application at ∼ 30 °C and ∼ 50 °C. The structures were obtained by vacuum deposition (first the WO 3 and than the Pd film) onto a LiNbO 3 Y-cut Z-propagating substrate making use of the surface acoustic wave method and additionally (in this same technological processes) onto a glass substrate with a planar microelectrode array for simultaneously monitoring of the planar resistance of the structure. In the case of a bilayer structure a very good correlation has been observed between these two methods — frequency changes in SAW method correlate very well with decreases of the bilayer structure resistance. These frequency changes are on the level of 2.4 kHz to 4% of hydrogen concentration in dry air, whereas in the case of a single WO 3 structure almost no frequency shift is observed.

Palladium and Metal-Free Phthalocyanine Bilayer Structures for Hydrogen Detection in the SAW Sensor System Based on Interaction Speed

Bilayer structures with various thicknesses of metal-free phthalocyanine (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Pc) (~80, ~120, and ~160 nm) but the same thickness of palladium (Pd) (~20 nm), have been studied for hydrogen gas-sensing application at temperatures of ~30degC and ~50degC with a method based on interaction speed. The structures were fabricated in two different vacuum deposition processes (first the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Pc film and than the Pd) onto an LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Y- cut Z-propagating substrate for the surface acoustic wave (SAW) method and additionally (in these same technological processes) onto a glass substrate with a planar microelectrode array for the simultaneous monitoring of the structure planar resistance. A very good correlation has been observed between these two methods (frequency changes for the SAW method coincide with the decrease of the bilayer structure resistance), especially for higher hydrogen concentrations. Although simultaneous measurements were not always feasible (too great resistance in the samples for the structure with the thinnest H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Pc), they can provide information about the acoustoelectric interactions between SAW and charge carriers in the bilayer structure. The interaction speed method is based on the great variance in interaction speeds at various hydrogen concentrations (from 2.5% to 4% in synthetic dry air), even though the amplitude signal reaches almost the same frequency level. For a particular chosen initial interaction time interval, a distinct interaction speed can be distinguished with great resolution (from 7.5 Hz/s for 2.5% H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in air to 29.1 Hz/s for 4% for the structure with 160-nm H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Pc and 20-nm Pd). These initial interaction fragments are linear versus time for the investigated medium hydrogen concentrations in synthetic dry air. In the case of the investigated bilayer structures, the interaction speed is higher for the structure with the thinnest H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Pc film (~80 nm)

Power consumption analysis of surface acoustic wave sensor systems using ANSYS and PSPICE

The power consumption of a surface acoustic wave (SAW) sensor system was investigated using ANSYS and PSPICE. Simulation results show that several design parameters of the surface acoustic wave sensor, such as the center distance between two interdigital transducers (IDT), the thickness of the piezoelectric substrate, the finger space of the IDT, etc., can greatly affect the power consumption of the whole system. The results of this study will be helpful to optimum design and fabricate the SAW sensor systems with very low-power consumption.

Surface acoustic wave immunosensors based on immobilized C60-proteins

Fullerene C60-proteins, C60-hemoglobin (C60-Hb) and C60-myoglobin (C60-Mb), were prepared and applied in surface acoustic wave (SAW) immunosensors to detect the anti-hemoglobin (anti-Hb) and anti-myoglobin (anti-Mb) antibodies, respectively, in aqueous solutions. The immobilizations of Hb and Mb onto fullerene were studied through a C60-coated SAW sensor system in liquid. The partially irreversible responses of these proteins were observed by the desorption study, which implied that fullerene could chemically react with these proteins. Both C60-Hb and C60-Mb coating materials were successfully prepared and identified with an FTIR spectrometer. The C60-protein-coated SAW immunosensors exhibited linear frequency responses to the concentrations of anti-Hb and anti-Mb antibodies with sensitivities of 0.14 and 1.27 kHz/(μg/mL), respectively. Both C60-protein-coated SAW immunosensors showed detection limits of 0.32 and 0.035 μg/mL for anti-Hb and anti-Mb antibodies, respectively. The optimum pH and temperature on the frequency response of the anti-Hb SAW immunosensor were also observed to be at 7.3 and 27 °C, respectively. The interference of various common bio-species in human blood, e.g. urea, ascorbic acid, tyrosine, and metal ions, to the SAW immunosensor coated with C60-Hb was investigated. These common bio-species interferences showed nearly no interference to the SAW immunosensors coated with C60-Hb. The reproducibility and lifetime of these SAW immunosensors immobilized with C60-proteins were also investigated and discussed. Dual-channel SAW immunosensors coated with C60-Hb and C60-Mb were applied to detect anti-Hb and anti-Mb simultaneously.

A fuzzy filter to study the selectivity and sensitivity of a SPME enhanced SAW sensor system characterizing virgin olive oil aroma

An electronic nose based on a solid phase microextration (SPME) unit coupled with a surface acoustic wave (SAW) sensor array has been optimized towards the analysis of virgin olive oil aroma. The parameters of the system, the SPME fibre coating and the kind of carrier gas, have been adjusted to get the optimum response with the minimum relative standard deviation. The selectivity and sensitivity of the sensors were studied by means of a fuzzy filter based on the membership grade functions of the sensor responses for the “non-lampante and “lampante” classes. A multilayer perceptron (MLP) was designed to classify non-lampante and lampante virgin olive oils. Only one sample out of 26 was erroneously classified in the training set, whereas a classification rate of 100% was obtained in a external validation set of 16 samples.

Functional Passive Sensor System Using Reflective SAW Delay Line

ABSTRACTA new passive and remote SAW sensor system having ID tag function is presented. Surface acoustic wave (SAW) devices have been widely used as a chemical and physical sensor for measuring the environment. SAW devices have been also used as identification tags (ID tags). The majority of the work reported on SAW sensors to date has used Rayleigh mode SAW and are concerning a delay line. The other hand, it has been shown that the sensors utilizing the shear horizontal mode SAW (SH-SAW) can be designed to sense the liquid properties. The SH-SAW has the unique characteristic of complete reflection at a 90 degree edge normal to the direction of the propagation. This characteristic makes it possible to construct a reflective delay line using the edge reflection. The reflective delay line can be used for as passive and remote sensing.In this paper, a passive and remote SAW sensor with a function of identification tags has been designed by using the reflective SH-SAW delay line. The reflective delay line was constructed on the 36 YX LiTaO3 substrate having a 90 degree edge normal to the direction of the SAW propagation. Passive and remote SAW sensor consists of several reflective delay lines having different propagation length. The passive sensor and ID tags were made by the multi-channel delay line. The responses of the sensor and ID tags using the edge reflection of SH-SAW are obtained in time domain. Several experiments were performed to verify the performances of the multi-channel edge reflection sensor. The basic characteristics of the reflective SAW sensor were obtained by measuring the amplitude and phase of reflected wave. The obtained results showed that the reflective SH-SAW sensor was very effective for sensing the physical and chemical properties.Wireless systems are also suggested for the effective operation of the passive SAW sensor. A novel and simple electronic circuit system for accurately measuring the phase characteristics of responses form multi-channel is proposed. In the application, the reflective delay line has been mounted on cantilever to monitor the strain experimentally. The variation of the SH-SAW velocity on the delay line caused by a strain was measured as the passive SAW sensor response. The velocity varied in proportion to the strain applied to the propagation surface. The results showed that the passive sensor was very effective to measure the strain in a wireless mode and it was found that the system is very suitable to health and safety monitoring of building or airplane wing.Finally it will be shown some applications of the wireless sensor with ID tags on the measurement of the conductivity and viscosity of same liquids.

Multimode and multifrequency gigahertz surface acoustic wave sensors

A simple surface acoustic wave sensor system is presented that combines the advantages of multifrequency/multimode operation and GHz frequencies in a single acoustic device structure. Analyzing multiple frequencies and modes is the basis for the multicomponent analysis of gases and liquids. The sensor system is based on floating electrode unidirectional transducers that allow efficient excitation of two modes and up to 48 harmonics (or 5 GHz). It can be fabricated without the need of costly nanofabrication techniques. Higher sensor frequencies are advantageous as the sensitivity increases with the frequency. We tested the basic operation of our sensor system by applying it to humidity sensing without a sensitive layer.

SAW-based redio sensor systems for short-range applications

Wireless autonomous surface acoustic wave (SAW) sensors offer high flexibility for modern sensor systems. Because no battery or wiring is required for power-supply and communication tasks, they can be advantageously employed for nearly all kinds of short-range identification and measurement applications, where the use of conventional sensors, e.g., on moving or rotating parts or in industrial process chambers. Here, the basics of SAW-based radio sensor systems are reviewed and different examples out of a manifold of possible applications are given. Wireless SAW identification and sensor systems operate stable and maintenance free over many years even in harsh industrial environments. With a fast readout of only a few microseconds, a readout-distance of up to several meters, and a high sensor stability, even at temperatures above 200 /spl deg/C, these highly flexible SAW-based radio systems are ideally suited for a multitude of measurement tasks in industrial, automotive, transportation, and domestic applications.

Palladium and phthalocyanine bilayer films for hydrogen detection in a surface acoustic wave sensor system

Presented here are the results concerning a hydrogen sensor based on two various bilayer structures in a surface acoustic wave dual-delay line system. The sensor material consists of two layers produced in two different vapour deposition processes. The first one is a 110 nm free phthalocyanine (H 2Pc) or 230 nm nickel phthalocyanine (NiPc) layer, whereas the second is a 20 nm thin palladium (Pd) film. These structures were formed in one of the dual delay line systems on a LiNbO 3 Y-cut Z-propagation substrate, while the other serves as a reference, permitting an easy detection of the arising differential frequency Δ f. In such a bilayer structure we can detect hydrogen in a medium concentration range (from 1 to 4% in nitrogen), even at room temperature. The achieved results are compared with a previous structure of 720 nm copper phthalocyanine covered by 20 nm Pd film. The best results from the point of view of interaction stability have been achieved for the “old” structure (CuPc+Pd), whereas the biggest response is observed in the case of the new structure H 2Pc+Pd. The response was almost five times bigger than in the case of the CuPc+Pd structure and ∼12 times bigger than in the NiPc+Pd structure of these same hydrogen concentrations (from 2 to 4% in nitrogen) at 30 °C. For all the structures the interaction response depends on temperature, and decreases with the increase of the interaction temperature. The comparison of responses for 1 and 3% of H 2 in N 2 has shown a difference of about one order of magnitude.

Bilayer structure for hydrogen detection in a surface acoustic wave sensor system

Presented here are the results concerning a hydrogen sensor based on a bilayer structure in a surface acoustic wave (SAW) dual delay line system. The sensor material consists of two layers performed in two different vapour deposition processes. The first one is a 720 nm copper phthalocyanine (CuPc) layer, the second is a 20 nm thin palladium (Pd) film. This structure was formed in one of the dual delay line systems on a LiNbO 3 Y-cut Z-propagation substrate, while the other serves as a reference, permitting easy detection of the arising differential frequency Δ f. This frequency, depending on the operating frequency modes, is in the range of 200–400 kHz, whereas the oscillator frequencies are in the range of 43.6 MHz. The wavelength is 80 μm. In such a bilayer structure we can detect hydrogen in a medium concentration range (from 0.5 to 3% in nitrogen), even at room temperature. The sensor is highly sensitive, very stable and is entirely reversible. The response and recovery times defined as 90% of the saturation level are quite good (from 100 s for 0.5% to 1000 s for 1.5%), which is very important from a practical point of view. The sensitivity depends on temperature, and decreases with the increase of the interaction temperature. In addition, the phase transition of the palladium hydride at 30 and 43 °C is distinct and repeatably visible as an “interaction jump”, which is something of a disadvantage in respect of the sensor. This undesirable phase transition shifts in the direction of higher hydrogen concentrations at higher temperatures, and consequently can be avoided; for instance, at 61 °C for the investigated concentration range (0.5–3% of hydrogen in nitrogen).