Surface Acoustic Wave Vapor Sensors Research Articles

Surface Acoustic Wave Vapor Sensor with Graphene Interdigital Transducer for TNT Detection

The performance of the surface acoustic wave (SAW) vapor sensor with graphene interdigital transducer (IDT) is investigated to study the improvement of sensitivity and decrease of secondary effects by the finite element method (FEM). Unlike conventional Al metal electrodes, the observed increase in the SAW phase velocity confirms the existence of elastic loading for the graphene electrode to the contact surface. Accordingly, the mass sensitivity of the SAW device with graphene electrode shows one order of magnitude higher. In the presence of TNT, the resonance frequency shift of the SAW sensor with graphene IDT is approximately 10 times larger than that with Al. Also, graphene electrodes offer the less secondary effects (BAW generations). So the SAW sensor with graphene IDT possesses superior sensing performance.

FEM modeling of SAW organic vapor sensors

A finite element method (FEM) simulation approach of surface acoustic wave (SAW) organic vapor sensors was investigated. The effects of the polymer coating on the propagation characteristics of the SAW were studied by FEM modeling in the absence and presence of vapor. The sensitivities for each vapor-coating combination can be obtained from the simulation frequency shift in different vapor concentration. Comparisons of the simulation sensitivities of SAW vapor sensors with the experimental values reported by Edward T. Zellers and co-workers confirm the general validity of the approach. The approach is expected to select polymer, which offers the best sensitivity for particular vapor.

Implementation of Surface Acoustic Wave Vapor Sensor Using Complementary Metal–Oxide–Semiconductor Amplifiers

A surface acoustic wave (SAW) vapor sensor is presented in this work. A SAW delay line oscillator on quartz substrate with the high gain complementary metal–oxide–semiconductor (CMOS) amplifier using a two-poly–two-metal (2P2M) 0.35 µm process was designed. The gain of the CMOS amplifier and its total power consumption are 20 dB and 70 mW, respectively. The achieved phase noise of this SAW oscillator is -150 dBc/Hz at 100 kHz offset. The sensing is successfully demonstrated by a thin poly(epichlorohydrin) (PECH) polymer film on a SAW oscillator with alcohol vapor. This two-in-one sensor unit includes the SAW device and the CMOS amplifier provides designers with comprehensive model for using these components for sensor circuit fabrication. Furthermore it will be promising for future chemical and biological sensing applications.

Development of handheld SAW vapor sensors for explosives and CW agents

This article reports about the development of various discrete (single sensor) surface acoustic wave (SAW) handheld sensor systems, for detection and quantification of explosives and chemical warfare agents (CWA). The sensing element is a SAW device of delay line, resonator or dispersive delay line type with frequency of operation ranging from 36 MHz to 434 MHz. The SAW devices are coated with various polymers having good selectivity to explosives and CWA. Oscillator with the SAW device in the feedback loop as a frequency determining element is used. Dual oscillator configuration with one coated and one uncoated SAW device is used. The outputs of the oscillators are mixed and signal conditioned before frequency measurement. The frequency measurement is carried out by a high-speed high-resolution reciprocal counting method using microcontroller-based readout circuitry. Data acquisition software written in Visual Basic performs online display and storage. The performance of various parts and the sensor systems is analyzed for their stability, resolution, accuracy and sensitivity.

Design of ST-cut quartz surface acoustic wave chemical sensors

This research investigates the design rules polymer-based ST-cut quartz surface acoustic wave (SAW) sensors used in detecting organophosphorous compounds. A complex shear modulus is used to represent express different types of polymer (glassy, glassy–rubbery, and rubbery). Different propagation directions lead to different wave propagation properties in SAW devices, such as attenuation and velocity. Hence, the choice of polymer type and wave propagation direction is a very important aspect of sensor design. Calculations indicate that the glassy–rubbery and rubbery film coated on SAW sensors very sensitively detect organophosphorous vapor. However, the glassy–rubbery film is most suitable in sensing applications, because it makes signal attenuation significantly. Although sensitivity is high when an acoustic wave propagates in the Y-direction, the X-direction is preferred at the optimum path on account of the excellent temperature stability in the X-propagation direction. Therefore, SAW vapor sensors with an appropriate chemical interface and wave propagation direction are designed to yield superior performance.

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.

Inverse Least-Squares Modeling of Vapor Descriptors Using Polymer-Coated Surface Acoustic Wave Sensor Array Responses

In previous work, it was shown that, in principle, vapor descriptors could be derived from the responses of an array of polymer-coated acoustic wave devices. This new chemometric classification approach was based on polymer/vapor interactions following the well-established linear solvation energy relationships (LSERs) and the surface acoustic wave (SAW) transducers being mass sensitive. Mathematical derivations were included and were supported by simulations. In this work, an experimental data set of polymer-coated SAW vapor sensors is investigated. The data set includes 20 diverse polymers tested against 18 diverse organic vapors. It is shown that interfacial adsorption can influence the response behavior of sensors with nonpolar polymers in response to hydrogen-bonding vapors; however, in general, most sensor responses are related to vapor interactions with the polymers. It is also shown that polymer-coated SAW sensor responses can be empirically modeled with LSERs, deriving an LSER for each individual sensor based on its responses to the 18 vapors. Inverse least-squares methods are used to develop models that correlate and predict vapor descriptors from sensor array responses. Successful correlations can be developed by multiple linear regression (MLR), principal components regression (PCR), and partial least-squares (PLS) regression. MLR yields the best fits to the training data, however cross-validation shows that prediction of vapor descriptors for vapors not in the training set is significantly more successful using PCR or PLS. In addition, the optimal dimension of the PCR and PLS models supports the dimensionality of the LSER formulation and SAW response models.

Use of linear solvation energy relationships for modeling responses from polymer-coated acoustic-wave vapor sensors.

The applicability and performance of linear solvation energy relationships (LSERs) as models of responses from polymer-coated acoustic-wave vapor sensors are critically examined. Criteria for the use of these thermodynamic models with thickness-shear-mode resonator (TSMR) and surface-acoustic-wave (SAW) vapor sensors are clarified. Published partition coefficient values derived from gas-liquid chromatography (GLC) are found to be consistently lower than those obtained gravimetrically, in accordance with previous reports, suggesting that LSERs based on GLC-derived partition coefficients will not provide accurate estimates of acoustic-wave sensor responses. The development of LSER models directly from polymer-coated TSMR vapor sensor response data is demonstrated and a revised model developed from SAW vapor sensor response data, which takes account of viscoelastic changes in polymeric coating films, is presented and compared to those developed by other methods.

The fractional free volume of the sorbed vapor in modeling the viscoelastic contribution to polymer-coated surface acoustic wave vapor sensor responses.

Surface acoustic wave (SAW) vapor sensors with polymeric sorbent layers can respond to vapors on the basis of mass loading and modulus decreases of the polymer film. The modulus changes are associated with volume changes that occur as vapor is sorbed by the film. A factor based on the fractional free volume of the vapor as a liquid has been incorporated into a model for the contribution of swelling-induced modulus changes to observed SAW vapor sensor responses. In this model, it is not the entire volume added to the film by the vapor that contributes to the modulus effect; it is the fractional free volume associated with the vapor molecules that causes the modulus to decrease in a manner that is equivalent to free volume changes from thermal expansion. The amplification of the SAW vapor sensor response due to modulus effects that are predicted by this model has been compared to amplification factors determined by comparing the responses of polymer-coated SAW vapor sensors with the responses of similarly coated thickness shear mode (TSM) vapor sensors, the latter being gravimetric. Results for six to eight vapors on each of two polymers, poly(isobutylene) and poly(epichlorohydrin), were examined. The model predicts amplification factors of the order of about 1.5-3, and vapor-dependent variations in the amplification factors are related to the specific volume of the vapor as a liquid. The fractional free volume factor provides a physically meaningful addition to the model and is consistent with conventional polymer physics treatments of the effects of temperature and plasticization on polymer modulus.

Establishing a limit of recognition for a vapor sensor array.

Organic vapor analysis with microsensor arrays relies principally on two output parameters: the response pattern, which provides qualitative information, and the response sensitivity, which determines the limit of detection (LOD). The latter is used to define the operating limit in the low-concentration range, under the implicit assumption that, if a vapor can be detected, it can be identified and differentiated from other vapors on the basis of its response pattern. In this study, the performance of an array of four polymer-coated surface acoustic wave vapor sensors was explored using calibrated response data from 16 solvent vapors in Monte Carlo simulations coupled with pattern recognition analysis. The statistical modeling revealed that the ability to recognize a vapor from its response pattern decreases with decreasing vapor concentration, as expected, but also that the concentration at which errors in vapor recognition become excessive is well above the calculated LOD in most cases, despite the LOD being based on the least sensitive sensor in the array. These results suggest the adoption of a limit of recognition (LOR), defined as the concentration below which a vapor can no longer be reliably recognized from its response pattern, as an additional criterion for evaluating the performance of multisensor arrays. A generalized method for estimating the LOR is presented, as well as a means for improving the LOR via residual error analysis.

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.

Effects of temperature and humidity on the performance of polymer-coated surface acoustic wave vapor sensor arrays.

The influences of temperature and atmospheric humidity on the performance of an array of eight polymer-coated 158-MHz surface acoustic wave vapor sensors were investigated. Sensitivities to the seven organic vapors examined all exhibited negative Arrhenius temperature dependencies, with responses increasing by factors of 1.5-4.4 on going from 38 to 18 degrees C. The magnitudes of the temperature effects, while generally similar, differed sufficiently among certain sensor-vapor combinations to cause marked changes in vapor response patterns. In addition, it was found that operating identically coated sensors at different temperatures could provide a means for discriminating certain vapors. The changes in sensor responses with temperature agreed reasonably well with those expected assuming ideal vapor sorption behavior and indicated that changes in the moduli of the sensor coatings were not important mediating factors. Responses to relative humidity (RH) from 0 to 85% RH were important even for the nonpolar sensor coatings. Significant changes in the sensitivities to the organic vapors were observed as a function of atmospheric humidity for several sensor-vapor combinations, which, in turn, affected the patterns of responses obtained from the sensor array. Results indicate that small changes in temperature or humidity have a larger effect on baseline stabilities than on the responses to the vapors. Monte Carlo simulations of sensor responses show that the ability to discriminate vapors in binary and ternary mixtures using a four-sensor array remains high regardless of the operating temperature and ambient humidity, provided that temperature-or humidity-induced changes in the response patterns are taken into account.

Method for Estimating Polymer-Coated Acoustic Wave Vapor Sensor Responses

A method for estimating the responses of polymer-coated acoustic wave vapor sensors has been developed. Polymer/gas partition coefficients, determined experimentally for at least 20-30 solute vapors by a simple gas chromatographic method, are used to construct linear solvation energy relationships that correlate partition coefficients on a given polymer with various solute solvation parameters. Since these parameters are known for over 2000 compounds, it is possible to estimate polymer/gas partition coefficient values for thousands of vapor/polymer pairs. It is shown how these partition coefficients can, in turn, be used to estimate acoustic wave vapor sensor responses. Comparisons of predicted surface acoustic wave vapor sensor sensitivities with observed responses confirm the general validity of the approach. The approach can be used to select polymers offering the best sensitivities for particular vapors. Limits of detection have been calculated and compared with permissible exposure limits and threshold limit values for a variety of vapors of interest in environmental remediation and occupational safety. These results indicate that polymer-coated surface acoustic wave vapor sensors are capable of detecting the majority of the vapors considered at concentrations of interest. 52 refs., 2 figs., 3 tabs.

Examination of Vapor Sorption by Fullerene, Fullerene-Coated Surface Acoustic Wave Sensors, Graphite, and Low-Polarity Polymers Using Linear Solvation Energy Relationships

The sorption of vapors by fullerene is compared with the sorption of vapors by an assembled fullerene thin film on a surface acoustic wave vapor sensor. A linear solvation energy relationship derived for solid fullerene at 298 K was used to calculate gas/solid partition coefficients for the same vapors as those examined using the vapor sensor. This relationship correctly predicted the relative vapor sensitivities observed with the vapor sensor. A new linear solvation energy relationship for vapor adsorption by graphite at 298 K has been determined, and solid fullerene and solid graphite are found to be quite similar in their vapor sorption properties. Comparisons have also been made with linear organic and inorganic polymers, including poly(isobutylene), poly(epichorophydrin), OV25, and OV202. In all cases, sorption is driven primarily by dispersion interactions. The assembled fullerene material is generally similar in vapor selectivity to the other nonpolar sorbent materials considered but yields less sensitive vapor sensors than linear organic polymers. 39 refs., 2 figs., 2 tabs.

The predominant role of swelling-induced modulus changes of the sorbent phase in determining the responses of polymer-coated surface acoustic wave vapor sensors

The sorption of vapors by fluoropolyol, poly(epichlorohydrin), and poly(Isobutylen) is examined by gas-liquid chromatography (GLC), and these results are compared with the responses of surface acoustic wave (SAW) vapor sensors coated with the same polymers. The sensor responses exceed those which can be attributed to gravimetric effects, indicating that the SAW devices are responding to some other change in the coating properties

Surface acoustic wave vapor sensors based on resonator devices

Abstract : Surface acoustic wave (SAW) devices fabricated in the resonator configuration have been used as organic vapor sensors and compared with delay line devices more commonly used. The experimentally determined mass sensitivities of 200, 300, and 400 MHz resonators and 158 MHz delay lines coated with Langmuir-Blodgett films of poly(vinyl tetradecanal) are in excellent agreement with theoretical predictions. The response of LB- and spray-coated sensors to various organic vapors were determined, and scaling laws for mass sensitivities, vapor sensitivities, and detection limits are discussed. The 200 MHz resonators provide the lowest noise levels and detection limits of all the devices examined.

Determination of partition coefficients from surface acoustic wave vapor sensor responses and correlation with gas-liquid chromatographic partition coefficients

Determination of partition coefficients from surface acoustic wave vapor sensor responses and correlation with gas-liquid chromatographic partition coefficients