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.     

Organic vapor sensors based on SAW resonator and organic films

The majority of investigations of SAW devices used as chemical sensors are based on delay line oscillators. However, SAW resonator oscillator offers some advantages over the SAW delay line oscillator for its higher stability. In the incipient stage of fabricating gas sensors based upon SAW resonator, taking detection of organic vapor as an example, the analysis method that combines the SAW theory with organic film technology has been adopted to give an intensive insight into the responses of two-port SAW resonator coated with LB-film and cast-film after exposure to organic vapors.     

Differentiation of chemical components in a binary solvent vapor mixture using carbon/polymer composite-based chemiresistors

We demonstrate a "universal solvent sensor" constructed from a small array of carbon/polymer composite chemiresistors that respond to solvents spanning a wide range of Hildebrand solubility parameters. Conductive carbon particles provide electrical continuity in these composite films. When the polymer matrix absorbs solvent vapors, the composite film swells, the average separation between carbon particles increases, and an increase in film resistance results, as some of the conduction pathways are broken. The adverse effects of contact resistance at high solvent concentrations are reported. Solvent vapors including isooctane, ethanol, diisopropylmethylphosphonate (DIMP), and water are correctly identified ("classified") using three chemiresistors, their composite coatings chosen to span the full range of solubility parameters. With the same three sensors, binary mixtures of solvent vapor and water vapor are correctly classified; following classification, two sensors suffice to determine the concentrations of both vapor components. Poly(ethylenevinyl acetate) and poly(vinyl alcohol) (PVA) are two such polymers that are used to classify binary mixtures of DIMP with water vapor; the PVA/carbon particle composite films are sensitive to less than 0.25% relative humidity. The Sandia-developed visual-empirical region of influence (VERI) technique is used as a method of pattern recognition to classify the solvents and mixtures and to distinguish them from water vapor. In many cases, the response of a given composite sensing film to a binary mixture deviates significantly from the sum of the responses to the isolated vapor components at the same concentrations. While these nonlinearities pose significant difficulty for (primarily) linear methods such as principal component analysis, VERI handles both linear and nonlinear data with equal ease. In the present study, the maximum speciation accuracy is achieved by an array containing three or four sensor elements, with the addition of more sensors resulting in a measurable accuracy decrease.     

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.     

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.     

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.     

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.     

Highly Sorbent Films Derived from Ni(SCN)(2)(4-picoline)(4) for the Detection of Chlorinated and Aromatic Hydrocarbons with Quartz Crystal Microbalance Sensors

A quartz crystal microbalance (QCM) spray-coated with a Ni(SCN)(2)(4-picoline)(4) film is a sensitive detector for small aromatic (benzene, toluene) and chlorinated (trichloroethylene, perchloroethylene) vapors with a planar molecular geometry. Frequency changes during transient exposures to these vapors are rapid and reversible. In contrast, frequency changes during transient exposures to carbon tetrachloride vapor exhibit a very slow rise and decay. Impedance studies demonstrate that the QCMs are responding only to mass changes in the film. Calibration curves exhibit both linear and near-saturation responses, depending on the vapor and vapor concentration. Partition coefficients obtained from the linear response regimes of the calibration curves are in the 10?000-100?000 range, more than an order of magnitude larger than the partition coefficients for a prototypical soft polymer, poly(isobutylene). Despite the absence of evidence for crystallinity by optical or X-ray diffraction methods, the spray-coated films appear to be forming clathrates with the organic vapors. The Ni(SCN)(2)(4-picoline)(4) film is promising for the development of very sensitive and partially selective piezoelectric sensors for nonpolar or weakly polar organic vapors in air.     

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.     

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.     

Improved vapor selectivity and stability of localized surface plasmon resonance with a surfactant-coated Au nanoparticles film

Here, we report the use of tetraoctylammonium bromide (TOABr)-coated Au nanoparticles (NPs) for the optical sensing of volatile organic compounds (VOCs). We find that the film responded selectively to the presence of polar and nonpolar vapors by changes in the maximum wavelength (λ(max)) toward higher and lower wavelengths, respectively, as determined by UV-visible spectroscopy. We also observed that the organic coating reorganizes when vapors partition into the film indicated by FT-IR and the film contracts in the presence of water indicated by scanning electron microscopy (SEM). In the present sensor, the metallic Au core serves as the plasmonic signal while the organic coating acts as the receptor material providing vapor selectivity and sensor stability. Correlating changes in (λ(max)) with changes in the refractive index (RI) and nanoparticle-to-nanoparticle separation in the film is important both fundamentally and for improving selectivity in localized surface plasmon resonance (LSPR) sensors.     

Organic vapor discrimination with chemiresistor arrays of temperature modulated tin-oxide nanowires and thiolate-monolayer-protected gold nanoparticles

This paper explores the discrimination of organic vapors with arrays of chemiresistors (CRs) employing interface layers of tin-oxide nanowires (NWs) and thiolate-monolayer-protected gold nanoparticles (MPNs). The former devices use contact-printed mats of NWs on micro-hotplate membranes to bridge a pair of metal electrodes. Oxidation at the NW surface causes changes in charge transport, the temperature dependence of which differs among different vapors, permitting vapor discrimination. The latter devices use solvent cast films of MPNs on interdigital electrodes operated at room temperature. Sorption into the organic monolayers causes changes in film tunneling resistance that differ among different vapors and MPN structures, permitting vapor discrimination. Here, we compare the performance and assess the 'complementarity' of these two types of sensors. Calibrated responses from an NW CR operated at two different temperatures and from a set of four different MPN CRs were generated for three test vapors: n-hexane, toluene, and nitromethane. This pooled data set was then analyzed using principal components regression classification models with varying degrees of random error superimposed on the responses via Monte Carlo simulation in order to estimate the rates of recognition/discrimination for arrays comprising different combinations of sensors. Results indicate that the diversity of most of the dual MPN-CR arrays exceeds that of the dual NW-CR array. Additionally, in assessing all possible arrays of 4-6 CR sensors, the recognition rates of the hybrid arrays (i.e. MPN + NW) were no better than that of the 4-sensor array containing only MPN CRs.     

Use of a surface-acoustic-wave sensor to characterize the reaction of styrene vapor with a square-planar organoplatinum complex

A coated surface-acoustic-wave (SAW) sensor is used to probe the reaction of styrene vapor with the square-planar platinum-ethylene pi-complex, trans-PtCl2(ethylene)(pyridine). A dual-SAW delay-line oscillator configuration is employed: one oscillator is coated with a solvent-cast film of the solid platinum-ethylene complex dispersed in a poly(isobutylene) matrix, and the second oscillator is coated only with polymer. Absorbed styrene vapor displaces ethylene to form the stable styrene-substituted complex, trans-PtCl2(styrene)(pyridine), causing a decrease in the oscillator frequency from the increase of mass on the surface of the sensor. For short-term exposures, there is a linear relationship between the logarithm of the rate of frequency change and the logarithm of the styrene vapor concentration, consistent with a power-law kinetic model for the heterogeneous trapping reaction. Deviation from this relationship above 300 ppm at 25 degrees C is attributed to the onset of multilayer adsorption of styrene at the surface of the trapping reagent. The sensor response exhibits an Arrhenius temperature dependence permitting estimation of the thermal activation energy for the olefin-substitution reaction. Calculated detection limits of 3 and 0.6 ppm of styrene vapor are achieved for operation at 25 and 40 degrees C, respectively.     

Quartz crystal microbalance sensor for organic vapor detection based on molecularly imprinted polymers

Molecularly imprinted polymers on quartz crystal microbalances (QCM) are examined for their ability to detect vapors of small organic molecules with greater sensitivity and selectivity than the traditional amorphous polymer coatings. Hydroquinone and phenol serve as noncovalently bound templates that generate shape-selective cavities in a poly(acrylic) or poly(methacrylic) polymer matrix. The imprinted polymers are immobilized on the piezoelectric crystal surface via a precoated poly(isobutylene) layer. The behavior of the imprinted polymer films is characterized by the dynamic and steady-state response of the QCM frequency to pulses of organic vapors in dry air. The apparent partition coefficients are determined for imprinted and nonimprinted polymers prepared by two synthetic methods and for varying mole ratios of template to monomer. The hydroquinone-imprinted polymers and, to a lesser extent, the phenol-imprinted polymers exhibit greater sensitivity and higher selectivity than the nonimprinted polymers toward organic vapors that are structurally related to the templates. These results indicate that molecularly imprinted polymers are promising for the development of selective piezoelectric sensors for organic vapor detection.     

Gas sensitivity comparison of polymer coated SAW and STW resonators operating at the same acoustic wave length

Results from systematic gas sensing experiments on polymer coated surface-transverse-wave (STW) and surface-acoustic-wave (SAW) based two-port resonators on rotated Y-cut quartz, operating at the same acoustic wavelength of 7.22 /spl mu/m, are presented. The acoustic devices are coated with chemosensitive films of different viscoelastic properties and thicknesses, such as solid hexamethyldisiloxane (HMDSO), semisolid styrene (ST), and soft allyl alcohol (AA). The sensor sensitivities to vapors of different chemical analytes are automatically measured in a sensor head, evaluated, and compared. It is shown that thin HMDSO- and ST-coated STW sensors are up to 3.8 times more sensitive than their SAW counterparts, while SAW devices coated with thick soft AA-films are up to 3.6 times more sensitive than the STW ones. This implies that SAWs are more suitable for operation with soft coatings while STWs perform better with solid and semisolid films. A close-to-carrier phase noise evaluation shows that the vapor flow homogeneity, the analyte concentration, its sorption dynamics, and the sensor oscillator design are the major limiting factors for the sensor noise and its resolution. A well designed ST-coated 700 MHz STW sensor provides a 178 kHz sensor signal at a 630 ppm concentration of tetra-chloroethylene and demonstrates short-term stability of 3/spl times/10/sup -9//s which results in a sensor resolution of about 7 parts per billion (ppb).     

Sorptive behavior of monolayer-protected gold nanoparticle films: implications for chemical vapor sensing

Monolayer-protected gold nanoparticle materials were synthesized and characterized for use as sorptive layers on chemical sensors. Thiols investigated as monolayer-forming molecules included dodecanethiol, benzenethiol, 4-chlorobenzenethiol, 4-bromobenzenethiol, 4-(trifluoromethyl)benzenethiol, 4-hydroxybenzenethiol, and 4-aminobenzenethiol. Films of selected monolayer-protected nanoparticle (MPN) materials were deposited on thickness shear mode devices and vapor uptake properties were measured at 298 K. Many, but not all, MPN-based sensing layers demonstrated rapid and reversible uptake of vapors, and sorptive selectivity varies with the monolayer structure. The mass of vapor sorbed per mass of sorptive material was determined and compared with poly(isobutylene) and poly(epichlorohydrin) as examples of simple sorptive polymers that have been used on vapor sensors. The nanoparticle-based films considered here were less sorptive than the selected polymers on a per-mass basis. Partition coefficients, which measure the mass of vapor sorbed per volume of the sorptive phase, were estimated for these MPN materials and found to be comparable to or less than those of the polymer layers. Implications for the roles of sorption and transduction in determining the performance of chemical sensors coated with nanoparticle-based films are discussed.     

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.     

Multianalyte chemical identification and quantitation using a single radio frequency identification sensor

We demonstrate an approach for multianalyte chemical identification and quantitation using a single conventional radio frequency identification (RFID) tag that has been adapted for chemical sensing. Unlike other approaches of using RFID sensors, where a special tag should be designed at a much higher cost, we utilize a conventional RFID tag and coat it with a chemically sensitive film. As an example, we demonstrate detection of several vapors of industrial, health, law enforcement, and security interest (ethanol, methanol, acetonitrile, water vapors) with a single 13.56-MHz RFID tag coated with a solid polymer electrolyte sensing film. By measuring simultaneously several parameters of the complex impedance from such an RFID sensor and applying multivariate statistical analysis methods, we were able to identify and quantify several vapors of interest. With a careful selection of the sensing film and measurement conditions, we achieved parts-per-billion vapor detection limits in air. These RFID sensors are very attractive as ubiquitous multianalyte distributed sensor networks.