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.     

Vapor recognition with small arrays of polymer-coated microsensors. A comprehensive analysis.

A comprehensive analysis of vapor recognition as a function of the number of sensors in a vapor-sensor array is presented. Responses to 16 organic vapors collected from six polymer-coated surface acoustic wave (SAW) sensors were used in Monte Carlo simulations coupled with pattern recognition analyses to derive statistical estimates of vapor recognition rates as a function of the number of sensors in the array (< or = 6), the polymer sensor coatings employed, and the number and concentration of vapors being analyzed. Results indicate that as few as two sensors can recognize individual vapors from a set of 16 possibilities with < 6% average recognition error, as long as the vapor concentrations are > 5 x LOD for the array. At lower concentrations, a minimum of three sensors is required, but arrays of 3-6 sensors provide comparable results. Analyses also revealed that individual-vapor recognition hinges more on the similarity of the vapor response patterns than on the total number of possible vapors considered. Vapor mixtures were also analyzed for specific 2-, 3-, 4-, 5-, and 6-vapor subsets where all possible combinations of vapors within each subset were considered simultaneously. Excellent recognition rates were obtainable for mixtures of up to four vapors using the same number of sensors as vapors in the subset. Lower recognition rates were generally observed for mixtures that included structurally homologous vapors. Acceptable recognition rates could not be obtained for the 5- and 6-vapor subsets examined, due, apparently, to the large number of vapor combinations considered (i.e., 31 and 63, respectively). Importantly, increasing the number of sensors in the array did not improve performance significantly for any of the mixture analyses, suggesting that for SAW sensors and other sensors whose responses rely on equilibrium vapor-polymer partitioning, large arrays are not necessary for accurate vapor recognition and quantification.     

Effective use of molecular recognition in gas sensing: results from acoustic wave and in situ FT-IR measurements.

To probe directly the analyte/film interactions that characterize molecular recognition in gas sensors, we recorded changes to the in situ surface vibrational spectra of specifically functionalized surface acoustic wave (SAW) devices concurrently with analyte exposure and SAW measurement of the extent of sorption. Fourier transform infrared external-reflectance spectra (FT-IR-ERS) were collected from operating 97-MHz SAW delay lines during exposure to a range of analytes as they interacted with thin-film coatings previously shown to be selective: cyclodextrins for chiral recognition, nickel camphorates for Lewis bases such as pyridine or organophosphonates, and phthalocyanines for aromatic compounds. In most cases where specific chemical interactions--metal coordination, "cage" compound inclusion, or pi-stacking--were expected, analyte dosing caused distinctive changes in the IR spectra, together with anomalously large SAW sensor responses. In contrast, control experiments involving the physisorption of the same analytes by conventional organic polymers did not cause similar changes in the IR spectra, and the SAW responses were smaller. For a given conventional polymer, the partition coefficients (or SAW sensor signals) roughly followed the analyte fraction of saturation vapor pressure. These SAW/FT-IR results support earlier conclusions derived from thickness-shear mode resonator data.     

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.     

Acoustic wave-based sensors using mode conversion in periodic gratings

Using periodic gratings etched into the surface of a piezoelectric plate, surface acoustic waves (SAW) can be converted into bulk waves and vice versa with high efficiency. If parallel grating structures are fabricated on opposite surfaces of a piezoelectric plate, a SAW also can be directed from one surface to the other. Using such structures, acoustic wave-based sensors can be designed that utilize SAW for the detection of chemical analytes on an electrode-free surface, i.e., the back surface. As a result, spurious sensor response and electrode aging that may occur when a chemical analyte comes in contact with the transducers are minimized. The design principles of these grating-based SAW sensors are explained, and the mass sensitivity is investigated using chemical vapor deposited thin polymer films, a type of material used in many practical chemical sensor applications. Experimental results are presented for the detection of nitrogen dioxide (NO(2 )) in sub-ppm concentrations.     

Characterization of dense arrays of chemiresistor vapor sensors with submicrometer features and patterned nanoparticle interface layers

The performance of arrays of small, densely integrated chemiresistor (CR) vapor sensors with electron-beam patterned interface layers of thiolate-monolayer-protected gold nanoparticles (MPNs) is explored. Each CR in the array consists of a 100-μm(2) interdigital electrode separated from adjacent devices by 4 μm. Initial studies involved four separate arrays, each containing four CRs coated with one of four different MPNs, which were calibrated with five vapors before and after MPN-film patterning. MPNs derived from n-octanethiol (C8), 4-(phenylethynyl)-benzenethiol (DPA), 6-phenoxyhexane-1-thiol (OPH), and methyl-6-mercaptohexanoate (HME) were tested. Parallel calibrations of MPN-coated thickness-shear-mode resonators (TSMR) were used to derive partition coefficients of unpatterned films and to assess transducer-dependent factors affecting responses. A 600-μm(2) 4-CR array with four different patterned MPN interface layers, in which the MPN derived from 7-hydroxy-7,7-bis(trifluoro-methyl)heptane-1-thiol (HFA) was substituted for HME, was then characterized. This is the smallest multi-MPN array yet reported. Reductions in the diversity of the collective response patterns are observed with the patterned films, but projected vapor discrimination rates remain high. The use of such arrays as ultralow-dead-volume detectors in microscale gas chromatographic analyzers is discussed.     

An interrogation unit for passive wireless SAW sensors based on fourier transform

The application of surface acoustic wave (SAW) resonators as sensor elements for different physical parameters such as temperature, pressure, and force has been well-known for several years. The energy storage in the SAW and the direct conversion from physical parameter to a parameter of the wave, such as frequency or phase, enables the construction of a passive sensor that can be interrogated wireless. This paper presents a temperature-measurement system based on passive wireless SAW sensors. The principle of SAW sensors and SAW sensor interrogation is discussed briefly. A new measurement device developed for analyzing the sensor signals is introduced. Compared to former interrogation units that detect resonance frequency of the SAW resonator by comparing amplitudes of sensor response signals related to different stimulating frequencies, the new equipment is able to measure the resonance frequency directly by calculating a Fourier transformation of the resonator response signal. Measurement results of an experimental setup and field tests are presented and discussed.     

Limits of recognition for simple vapor mixtures determined with a microsensor array

The "limit of recognition" (LOR) has been defined as the minimum concentration at which reliable individual vapor recognition can be achieved with a multisensor array, and methodology for determining the LORs of individual vapors probabilistically on the basis of sensor array response patterns has been reported. This article explores the problems of defining and evaluating LORs for vapor mixtures in terms of the absolute and relative component vapor concentrations, where the mixture must be discriminated from those component vapors and from the subset of possible lower-order component mixtures. Monte Carlo simulations and principal components regression analyses of an extant database of calibrated responses to a set of 16 vapors from an array of 6 diverse polymer-coated surface acoustic wave sensors are used to illustrate the approach and to examine trends in LOR values among the 120 possible binary mixtures and 560 possible ternary mixtures in the data set. At concentrations exceeding the LOD, 89% of the binary mixtures could be reliably recognized (<5% error) over some composite concentration range, while only 3% of the ternary mixtures could be recognized. Most binary mixtures could be recognized only if the constituent vapor relative concentration ratio, defined in terms of multiples of the LOD for each vapor, was < or =20. Correlations with the Euclidean distance(s) separating the normalized constituent vapor response vectors allow reasonably accurate predictions of the limiting recognizable mixture composition ranges for binary and ternary cases. Results are considered in the context of using microsensor arrays for vapor detection and recognition in microanalytical systems.     

Ultralow-Power Alcohol Vapor Sensors Using Chemically Functionalized Multiwalled Carbon Nanotubes

Alcohol sensors, batch fabricated by forming bundles of chemically functionalized multiwalled carbon nanotubes (f-CNTs) across Au electrodes on SiO₂/Si substrates using an AC electrophoretic technique, were developed for alcohol vapor detection using an ultralow input power of ~ 0.01 - 1 muW, which is lower than the power required for most commercially available alcohol sensors by more than four orders of magnitude. The multiwalled carbon nanotubes (MWCNTs) have been chemically functionalized with the COOH groups by oxidation. We found that the sensors are selective with respect to flow from air, water vapor, and alcohol vapor. The sensor response is linear for alcohol vapor concentrations from 1 to 21 ppm with a detection limit of 0.9 ppm. The transient response of these sensors is experimentally shown to be ~1 s and the variation of the responses at each concentration is within 10% for all of the tested sensors. The sensors could also easily be reset to their initial states by annealing the f-CNTs sensing elements at a current of 100-200 muA within ~ 100-200 s. We demonstrated that the response of the sensors can be increased by one order of magnitude after adding the functional group COOH onto the nanotubes, i.e., from ~0.9% of a bare MWCNTs sensor to ~9.6% of an f-CNTs sensor with a dose of 21 ppm alcohol vapor.     

A portable, high-speed, vacuum-outlet GC vapor analyzer employing air as carrier gas and surface acoustic wave detection

Vacuum-outlet GC with atmospheric-pressure air as the carrier gas is implemented at outlet pressures up to 0.8 atm using a low-dead-volume polymer-coated surface acoustic wave (SAW) detector. Increases in the system outlet pressure from 0.1 to 0.8 atm lead to proportional increases in detector sensitivity and significant increases in column efficiency. The latter effect arises from the fact that optimal carrier gas velocities are lower in air than in more conventional carrier gases such as helium or hydrogen due to the smaller binary diffusion coefficients of vapors in air. A 12-m-long, 0.25-mm-i.d. tandem column ensemble consisting of 4.5-m dimethyl polysiloxane and 7.5-m trifluoropropylmethyl polysiloxane operated at an outlet pressure of 0.5 atm provides up to 4 x 10(4) theoretical plates and a peak capacity of 65 (resolution, 1.5) for a 3-min isothermal analysis. At 30 degrees C, mixtures of vapors ranging in vapor pressure from 8.6 to 76 Torr are separated in this time frame. The SAW detector cell has an internal volume of < 2 microL, which allows the use of higher column outlet pressures with minimal dead time. The sensor response is linear with solute mass over at least 2-3 decades and provides detection limits of 20-50 ng for the vapors tested. The combination of atmospheric-pressure air as carrier gas, modest operating pressures, and SAW sensor detection is well-suited for field instrumentation since it eliminates the need for support gases, permits smaller, low-power pumps to be used, and provides sensitivity to a wide range of vapor analytes.     

TiO(2)/LiCl-based nanostructured thin film for humidity sensor applications

A simple and straightforward method of depositing nanostructured thin films, based on LiCl-doped TiO(2), on glass and LiNbO(3) sensor substrates is demonstrated. A spin-coating technique is employed to transfer a polymer-assisted precursor solution onto substrate surfaces, followed by annealing at 520°C to remove organic components and drive nanostructure formation. The sensor material obtained consists of coin-shaped nanoparticles several hundred nanometers in diameter and less than 50 nm thick. The average thickness of the film was estimated by atomic force microscopy (AFM) to be 140 nm. Humidity sensing properties of the nanostructured material and sensor response times were studied using conductometric and surface acoustic wave (SAW) sensor techniques, revealing reversible signals with good reproducibility and fast response times of about 0.75 s. The applicability of this nanostructured film for construction of rapid humidity sensors was demonstrated. Compared with known complex and expensive methods of synthesizing sophisticated nanostructures for sensor applications, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), this work presents a relatively simple and inexpensive technique to produce SAW humidity sensor devices with competitive performance characteristics.     

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.     

Dual-chemiresistor GC detector employing monolayer-protected metal nanocluster interfaces

The synthesis and testing of two gold-thiolate monolayer-protected (nano)clusters as interfacial layers on a dual-chemiresistor vapor sensor array are described. Responses (changes in dc resistance) to each of 11 organic solvent vapors are rapid, reversible, and linear with concentration at low vapor concentrations, becoming sublinear at higher concentrations. Limits of detection (LODs) range from 0.1 to 24 parts per million and vary inversely with solvent vapor pressure. When configured as a GC detector and used to analyze 0.5-L preconcentrated samples of the 11-vapor mixture, the array provides LODs of < or = 700 parts per trillion for most vapors, comparing favorably with those from an integrated array of polymer-coated surface acoustic wave sensors configured and tested similarly. This first report on the use of such an array as a GC detector shows that the combination of response patterns and GC retention times improves capabilities for vapor recognition compared to the sensor array alone or to single-detector GC systems. Spray-coated nanocluster thin films can be deposited reproducibly and exhibit response stability in air that ranges from fair to excellent for up to several months. Scaling the active device area down by a factor of 16 has no significant effect on sensitivity. Implications of these results for portable vapor sensing systems are discussed.     

Surface acoustic wave gas sensor of the sorption type sensitive to the thermal properties of gases

The basic principles of a new surface acoustic wave (SAW) gas sensor are described. Being essentially a sensor of the sorption type, the proposed device possesses certain features of the thermometric SAW sensors and is not only sensitive to the vapors of volatile substances, but capable of detecting gases by their thermal properties as well. In contrast to the known thermometric SAW sensors, the proposed sensor is characterized by high temperature stability and fast response. A variant of the sensor based on a LiNbO₃ SAW delay line is described and some results of the test for detecting propane-butane mixtures are presented.     

Model-based optimal design of polymer-coated chemical sensors

A model-based methodology for optimal design of polymer-coated chemical sensors is developed and is illustrated for the example of infrared evanescent field chemical sensors. The methodology is based on rigorous and computationally efficient modeling of combined fluid mechanics and mass transfer, including transport of multiple analytes. A simple algebraic equation for the optimal size of the sensor flow cell is developed to guide sensor design and validated by extensive CFD simulations. Based upon these calculations, optimized geometries of the sensor flow cell are proposed to further improve the response time of chemical sensors.     

Possible design procedure for low-loss 180 degrees reflecting arrays in SAW devices

A procedure for designing highly reflecting arrays to specified responses in surface acoustic-wave (SAW) devices is proposed. Multiple reflections in the low-loss arrays are fully taken into account. The problem is made tractable by using a 180 degrees reflecting geometry rather than reflecting the SAW twice through 90 degrees.     

Ultrasensitive humidity detection using metal-organic framework-coated microsensors

The use of metal-organic framework (MOF) thin films to detect water vapor across a wide concentration range is demonstrated using MOF-functionalized quartz surface acoustic wave (SAW) sensors. A range of 3-14?800 ppmv was obtained with thin films of the MOF Cu(3)(benzenetricarboxylate)(2) (Cu-BTC) deposited by an automated layer-by-layer method. Devices coated by a manual technique demonstrated sensitivity from 0.28 to 14?800 ppmv, the limit of our test system. This exceeds the sensitivity of many commercially available sensors. Cu-BTC layers were covalently bonded directly to the silicon oxide surface, allowing devices to be heated beyond 100 °C to desorb water adsorbed in the pores without decomposition, thereby regenerating the sensors. Sensor response as a function of coating thickness was evaluated, showing that the SAW sensor response is bounded by maximum and minimum layer thicknesses. Computer simulation of H(2)O uptake shows a multistep adsorption isotherm defined by initial adsorption at open Cu-sites, followed by pore-filling and finally full saturation. Modeling and experimental results are consistent. Calculated uptake values suggest an efficient adsorption of H(2)O by Cu-BTC. These results provide the first convincing evidence that MOF functionalization of compact sensing technologies such as SAW devices and microcantilevers can compete with state-of-the art devices.     

Pore-bridging poly(dimethylsiloxane) membranes as selective interfaces for vapor-phase chemical sensing

A new kind of polymer-based sensor is described in which the experimental parameters controlling selectivity and sensitivity are decoupled. The sensor is based on a surface acoustic wave (SAW) gravimetric transducer modified with a high-surface-area, nanoporous alumina coating. A very thin ( approximately 40 nm) poly(dimethylsiloxane) (PDMS) coating resides atop the porous alumina structure. In this configuration, the total surface area of the nanoporous alumina coating controls the sensitivity of the device, while the chemical properties of the PDMS membrane control selectivity. In conventional polymer-based sensors, the polymer plays the dual role of controlling both selectivity (via the chemical composition of the coating) and sensitivity (via the volume of the film). In this paper, we show that PDMS acts as a chemically selective gate that absorbs polar and nonpolar VOCs but does not transport these analytes to the underlying pore volume. In contrast, water vapor is absorbed by the PDMS to a very minor extent but is easily transported through it to the underlying walls of the porous substructure. Specifically, there was little difference in the mass-loading response arising from polar and nonpolar VOCs dosed onto planar and nanoporous SAW devices modified with PDMS. In contrast, SAW devices having nanoporous coatings responded up to 24 times more selectively to water than planar sensors modified identically.