Influence of surface conductivity on sensitivity of acoustic wave gas sensors based on multilayered structures

The influences of the surface conductivity on the velocity of an acoustic wave (AW) in a multilayered material are studied theoretically with the transfer matrix method and the conductivity sensitivity of the AW sensor is presented. It is found that the velocity of the AW increases with decreasing surface conductivity and vice versa. The result is used to explain the abnormal response of AW sensors, in which the central frequencies of AW sensors increase after they sorb the detected gases. Meanwhile, the conductivity sensitivity is found to be related to the dielectric constants of the multilayered material and the electromechanical coupling coefficient of the sensor. Finally, the sensitivities of AW sensors based on multilayered structures are optimized by considering the influences of the surface conductivities of the sensors with different initial conductivities and thicknesses of the sensitive layers.     

Scanning displacement sensors with surface acoustic wave modulation

Translated from Izmeritel'naya Tekhnika, No. 10, pp. 3–4, October, 1990.     

Investigation of cocaine plumes using surface acoustic wave immunoassay sensors

Vapor sensors, aka electronic noses, are becoming an increasingly popular analytical tool for detection and identification of small molecules in the gas phase. In this paper, we present the results of a series of experiments demonstrating real-time vapor phase detection of cocaine molecules. A distinctive response or signature was observed under laboratory conditions in which the cocaine vapors were presented using an INEL vapor generator and under "field" conditions facilitated by the Georgia Bureau of Investigation (GBI) Crime Lab. For these experiments, the sensor component was a two-port resonator on ST-X quartz with a center frequency of approximately 250 MHz. On this cut of quartz, a temperature-compensated surface acoustic wave is generated via an interdigital transducer. Antibenzoylecgonine (anti-BZE) antibodies are attached to the electrodes on the device surface via a protein-A cross linker. We observed a large transient frequency shift accompanied by baseline shift with the anti-BZE coated sensor. After repeated experiments and the use of numerous controls, we believe that we have achieved real time molecular recognition of cocaine molecules.     

Modeling of surface acoustic wave strain sensors using coupling-of-modes analysis

SAW devices may be configured as strain sensors, providing passive, wireless strain measurement in demanding conditions. A key consideration is the modeling of the sensors, enabling different device designs to be considered. This paper presents a simulation scheme using coupling-of-modes (COM) analysis which allows both the frequency response of a SAW strain sensor and its bias sensitivity to be evaluated. Example applications are presented to demonstrate the use of the model.     

Viscoelastic properties of polymer films on surface acoustic wave organophosphorous vapor sensors

This work investigates the viscoelastic properties of the fluoropolyol (FPOL) polymer on the surface acoustic wave (SAW) organophosphorous vapor sensors. A complex shear modulus is used to express different polymer types (glassy, glassy-rubbery, and rubbery). The different polymer types leads to different propagating properties of SAW, such as attenuation change and velocity shift. Calculation results indicate that the glassy-rubbery film exhibits the highest sensitivity for detecting organophosphorous vapor. The thicker the glassy and glassy-rubbery film implies a higher sensitivity. Moreover, the SAW vapor sensor based on the rubbery film represents the response of acoustically thick layers which has a peak in attenuation with an increasing vapor adsorption. The selectivity factor between DMMP (10 ppm) and H₂O (40%RH) is so low that the selectivity of FPOL film towards water is ineffecient. However, the selectivity factor between ethanol (10 ppm) and DMMP (10 ppm) is as high as 2512, thus confirming that the selectivity of FPOL film towards ethanol is good. Therefore, a precise and dry humidity control in the sensors system with FPOL coating is required.     

Bending mode effect on sensitivity of plate surface acoustic wave pressure sensors

The bending mode effect on sensitivity of pressure sensors operating with surface acoustic waves (SAW) propagating over the surface of a plate is analyzed using Tiersten's perturbation integral for frequency shifts in a piezoelectric resonator due to the presence of initial fields. For a plate subjected to nonpure bending, the SAW speed, whose shift in response to pressure determines the sensitivity, can be defined locally through the local variations of the effective material constants. Three common bending modes of pure bending, bending under a concentrate load, and bending under a uniformly load are analyzed and compared.     

Molecular recognition for electronic noses using surface acoustic wave immunoassay sensors

In this paper, we present the results of a series of experiments on vapor phase surface acoustic wave (SAW) sensors using a layer of antibodies as the chemically sensitive film. For these experiments, the sensor component was a ST-quartz resonator with a center frequency of approximately 250 MHz. Anti-FITC antibodies were attached to the electrodes on the device surface via a protein-A crosslinker. SAW resonator devices with various coatings were mounted in TO-8 packages, inserted into a sensor head module and subjected to various fluorescent analyte gases. Numerous controls were performed including the use of coated and uncoated devices along with devices coated with antibodies which were not specific for the target analyte. The SAW immunosensor response was monitored and a baseline frequency shift was observed when the analyte being presented was the antigen for the immobilized antibody. To provide an independent measure of antibody/antigen binding, the devices were removed from the sensor head, washed with a buffer solution to remove any unbound analyte, and then inspected using a confocal laser scanning microscope (CLSM). Since all the analytes being used in these experiments were fluorescent, this afforded us the opportunity to visualize the attachment of the analyte to the antibody film. Given the high resolution of the CLSM, we were able to identify the location of the attachment of the fluorescent analytes relative to the 1.5 /spl mu/m wide electrodes of the SAW device. We believe that these experiments demonstrate that we have achieved real time molecular recognition of these small molecules in the vapor phase.     

ZnO nanomaterials based surface acoustic wave ethanol gas sensor

ZnO nanomaterials based surface acoustic wave (SAW) gas sensor has been investigated in ethanol environment at room temperature. The ZnO nanomaterials have been prepared through thermal evaporation of high-purity zinc powder. The as-prepared ZnO nanomaterials have been characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray Diffraction (XRD) techniques. The results indicate that the obtained ZnO nanomaterials, including many types of nanostructures such as nanobelts, nanorods, nanowires as well as nanosheets, are wurtzite with hexagonal structure and well-crystallized. The SAW sensor coated with the nanostructured ZnO materials has been tested in ethanol gas of various concentrations at room temperature. A network analyzer is used to monitor the change of the insertion loss of the SAW sensor when exposed to ethanol gas. The insertion loss of the SAW sensor varies significantly with the change of ethanol concentration. The experimental results manifest that the ZnO nanomaterials based SAW ethanol gas sensor exhibits excellent sensitivity and good short-term reproducibility at room temperature.     

Vapor phase detection of a narcotic using surface acoustic wave immunoassay sensors

Currently, the narcotic sniffing dog remains the most accurate, reliable, and widely used sensing technology in the war on drugs. However, recent studies done at the Institute for Biological Detection Systems at Auburn University, Auburn, AL, have shown that in the presence of extraneous odors (nontarget odors), these animals show a higher propensity for so-called false alarms. For this reason, there have been an increasing demand for a portable, highly specific vapor-sensing device capable of distinguishing a target vapor signature in a complex odor. In this paper, we present the results of a series of experiments demonstrating real-time vapor phase detection of cocaine molecules. A distinctive response or signature was observed under laboratory conditions, where the cocaine vapors were presented using an INEL vapor generator and under "field" conditions facilitated by the Georgia Bureau of Investigation Crime Lab. For these experiments, the sensor component was an ST-X quartz resonator with a center frequency of approximately 250-MHz. Anti-benzoylecgonine (anti-BZE) antibodies are attached to the electrodes on the device surface via a protein-A cross linker. We observed a large transient frequency shift accompanied by baseline shift with the anti-BZE coated sensor. After repeated experiments and the use of numerous controls, we believe that we have achieved real-time molecular recognition of cocaine molecules.     

Surface acoustic wave-based gas sensors

A brief review of the Surface Acoustic Wave (SAW)-based gas sensors and their physical operation background in a dual-delay line oscillator system is presented together with some own achievements dealing with SAW layered sensor structures.     

Hydrogen bond acidic polymers for surface acoustic wave vapor sensors and arrays

Four hydrogen bond acidic polymers are examined as sorbent layers on acoustic wave devices for the detection of basic vapors. A polysiloxane polymer with pendant hexafluoro-2-propanol groups and polymers with hexafluorobisphenol groups linked by oligosiloxane spacers yield sensors that respond more rapidly and with greater sensitivity than fluoropolyol, a material used in previous SAW sensor studies. Sensors coated with the new materials all reach 90% of full response within 6 s of the first indication of a response. Unsupervised learning techniques applied to pattern-normalized sensor array data were used to examine the spread of vapor data in feature space when the array does or does not contain hydrogen bond acidic polymers. The radial distance in degrees between pattern-normalized data points was utilized to obtain quantifiable distances that could be compared as the number and chemical diversity of the polymers in the array were varied. The hydrogen bond acidic polymers significantly increase the distances between basic vapors and nonpolar vapors when included in the array.     

Coupling-of-modes analysis and modeling of polymer-coated surface acoustic wave resonators for chemical sensors

The analysis and modeling of SAW resonator devices based on the coupling-of-modes (COM) theory are described, integrating the effect of polymer coating so that the sensor effects can be accounted for in the device transfer function. Based on the perturbation method, the effects of film coating are included in determining the parameters for the model. The COM parameters are, therefore, modified and its simple analytical approaches are presented. The model is validated using the experimental data of a two-port SAW resonator device fabricated on ST-X quartz substrate. The experimental results for a device coated with Parylene C are compared with the simulation results of the proposed model. The comparative results of the electrical characteristics and the frequency sensitivity to film thickness show a good agreement which proves the validity of the model. This analysis and model will provide insight into the influence of the device design parameters on the sensor performance and help in practical design and optimization of SAW-based chemical sensor systems.     

Comparisons of polymer/gas partition coefficients calculated from responses of thickness shear mode and surface acoustic wave vapor sensors

Apparent partition coefficients, K, for the sorption of toluene by four different polymer thin films on thickness shear mode (TSM) and surface acoustic wave (SAW) devices are compared. The polymers examined were poly(isobutylene) (PIB), poly(epichlorohydrin) (PECH), poly(butadiene) (PBD), and poly(dimethylsiloxane) (PDMS). Independent data on partition coefficients for toluene in these polymers were compiled for comparison, and TSM sensor measurements were made using both oscillator and impedance analysis methods. K values from SAW sensor measurements were about twice those calculated from TSM sensor measurements when the polymers were PIB and PECH, and they were also at least twice the values of the independent partition coefficient data, which is interpreted as indicating that the SAW sensor responds to polymer modulus changes as well as to mass changes. K values from SAW and TSM measurements were in agreement with each other and with independent data when the polymer was PBD. Similarly, K values from the PDMS-coated SAW sensor were not much larger than values from independent measurements. These results indicate that modulus effects were not contributing to the SAW sensor responses in the cases of PBD and PDMS. However, K values from the PDMS-coated TSM device were larger than the values from the SAW device or independent measurements, and the impedance analyzer results indicated that this sensor using our sample of PDMS at the applied thickness did not behave as a simple mass sensor. Differences in behavior among the test polymers on SAW devices are interpreted in terms of their differing viscoelastic properties.     

Surface acoustic wave sensors incorporating Langmuir-Blodgett films

Acoustoelectric devices offer many attractive features for applications as physical and chemical sensors. Surface acoustic wave (SAW) oscillators are of particular importance owing to their high sensitivity. This paper describes the use of Langmuir-Blodgett (LB) films as gas absorbent layers on the surface of SAW devices.

Areal densities of standard LB film forming materials were measured and found to agree with those obtained from pressure-area isotherms. Sensors incorporating ω-tricosenoic acid and docosylamine overlayers were examined and their responses to alkanoic acids reported.

The room temperature chemiresponse of a SAW device coated with monolayers of tetra-4-tert butyl silicon phthalocyanine dichloride showed response and recovery times comparable with those reported for other phthalocyanine-based sensors operated at much higher temperatures. The detection limit of the LB film device was found to be 40 ppb NO₂ in dry air at an operating frequency of 98.6 MHz and an ambient temperature of 22°C. The frequency change was shown to be entirely due to the mass of gas absorbed by the film.     

Mass sensitivity of thin rod acoustic wave sensors

Theoretical and experimental investigations of mass sensitivities of thin rod acoustic wave sensor are presented. From the low-frequency approximation of the dispersion equations, explicit forms of the relation describing the mass sensitivity are derived with the consideration of the effects due to elasticity and inertia of the loading layer. The three lowest thin rod acoustic modes are presented. Mass sensing experiments are based on the electrodeposition of loading material on a thin metallic fiber (the thin rod). Copper has been used to load the propagation of acoustic waves in gold fibers. The mass of copper deposited and the phase shift of the acoustical thin rod delay line were monitored simultaneously by a computer. Mass response curves showing the variation in phase due to the mass deposited per unit surface area were then obtained in order to determine the mass sensitivity. Both flexural and extensional wave modes have been excited. Theoretical and experimental results were found to be consistent in both sign and magnitude     

Surface acoustic wave sensors of delay lines based on MEMS

A high frequency Surface Acoustic Wave (SAW) filter which is the key point of the SAW sensor is made by MEMS technology. The SAW filter used for sensors needs to have the factors of high frequency, low loss and high quality factor. In order to satisfy the performance of the factors, the SAW delay lines of Electrode Wave Control Single Phase Unidirectional Transducer (EWC/SPUDT) are used in the filter. This transducer can be considered as the networks of connection of some units of IDTs and the loss of device can be effectively reduced. Based on the Coupling of Modes (COM) theory and equivalent circuit modal, the mixed matrix (P matrix) of the transducers can be deduced, and further, the response of the delay lines can be obtained. According to the calculated results, a mask layout is made. Then the SAW delay line is completed and found to agree with the theoretical simulation. Its center frequency is 503 MHz and insertion loss is low enough for use of sensors.     

Emission of a surface acoustic wave by an internal acoustic wave propagating near the surface

It was observed that the propagation of an internal acoustic wave near a surface is accompanied by the excitation of a surface acoustic wave directed at an angle to the internal wave. Pis’ma Zh. Tekh. Fiz. 24, 57–61 (September 26, 1998)     

基于WO3薄膜的双声路声表面波型SO2气体传感器

以金属钨粉,H2O2,CH3OH和PVP(聚乙烯吡咯烷酮)为原料,利用热喷射方法在双声路声表面波器件的测量声路上制作了细微多网孔状WO3薄膜,提出并实现了一种在常温下可以实现对二氧化硫(SO2)气体进行物理吸附和解吸附的基于WO3薄膜的双声路声表面波型SO2气体传感器.声表面波器件的双声路结构消除了由于外界测量条件改变引起的测量误差,也进一步提高了传感器的可靠性和准确性.实验结果表明,该传感器具有好的重复性,在测量范围内对各种浓度的SO2气体具有好的响应特性;传感器在0.5ppm到20ppm浓度范围内的线性灵敏度大约为6.8KHz/ppm.