Rational materials design of sorbent coatings for explosives: applications with chemical sensors

A series of chemoselective polymers had been designed and synthesized to enhance the sorption properties of polymer coated chemical sensors for polynitroaromatic analytes. To evaluate the effectiveness of the chemoselective coatings, a polynitroaromatic vapor test bed was utilized to challenge polymer coated surface acoustic wave (SAW) devices with different explosive vapors. Dinitrotoluene detection limits were determined to be in the <100 parts per trillion ranges. ATR-FTIR studies were used to determine the nature of the polymer-polynitroaromatic analyte interactions, and confirm the presence of hydrogen-bonding between polymer pendant groups and the nitro functional groups of polynitroaromatic explosive materials.     

Modification of micro-cantilever sensors with sol-gels to enhance performance and immobilize chemically selective phases

A chemical sensor based on the deflection of a surface modified silicon micro-cantilever is presented. A thin film of sol-gel was applied to one side of the micro-cantilever surface using a spin coating procedure. The sensor has been shown to give different responses to vapor phase analytes of varying chemical composition, as well as to varying concentrations of a given analyte. Ethanol, a highly polar molecule, exhibits a strong affinity for the polar sol-gel coating resulting in a large response; pentane, a non-polar hydrocarbon, shows very little response. The sol-gel coating has also been shown to function as a backbone for the immobilization of chemically selective phases on the cantilever surface. Reaction of the sol-gel film with chlorotriethoxysilane and subsequent capping of the remaining reactive surface silanols with hexamethyldisilizane increases the non-polar nature of the film. This results in an increase in the response of the sensor to non-polar analytes. The effects of film thickness and cantilever structure thickness on response were also investigated.     

Rapid, sensitive, and multiplexed on-chip optical sensors for micro-gas chromatography

We developed and characterized a rapid, sensitive and integrated optical vapor sensor array for micro-gas chromatography (μGC) applications. The sensor is based on the Fabry-Pérot (FP) interferometer formed by a micrometre-thin vapor-sensitive polymer layer coated on a silicon wafer. The thickness and the refractive index of the polymer vary in response to the vapor analyte, resulting in a change in the reflected intensity of the laser impinged on the sensor. In our study, four different polymers were coated on four wells pre-etched on a silicon wafer to form a spatially separated sensor array. A CMOS imager was employed to simultaneously monitor the polymers' response, thus enabling multiplexed detection of a vapor analyte passing through the GC column. A sub-second detection time was demonstrated. In addition, a sub-picogram detection limit was achieved, representing orders of magnitude improvement over the on-chip vapor sensors previously reported.     

Ultrasensitive detection of aliphatic nitro-organics based on “turn-on” fluorescent sensor array

The broad class of explosives includes nitro aromatics as well as challenging aliphatic nitro-organics whose detection is important from counter-terrorism and national security perspectives. Here we report a turn-on fluorescent sensor array based on aggregation-induced emission (AIE) fluorophores as receptors. To achieve a good sensing system with fast response, good sensitivity and low detection limit, three receptors with abundant chemical diversities for target analytes were synthesized. The turn-on response of the individual receptor showed highly variable and cross-reactive analyte-dependent changes in fluorescence. The excellent ability to identify a variety of explosives, especially the challenging aliphatic nitro-organics (2,3-dimethyl-2,3-dinitrobutane (DMNB), 1,3,5-trinitro-1,3,5-triazinane (RDX), cyclotetramethylene tetranitramine (HMX) and entaerythritol tetranitrate (PETN)), was demonstrated in qualitative and quantitative analyses with 100% accuracy. The fluorescence signal amplification in the presence of explosives allows for application of these receptors in a sensor microarray suitable for high-throughput screening. These results suggested that the cross-reactive sensor array based on AIE fluorophores could find a wide range of applications for sensing various analytes or complex mixtures.     

Molecularly Imprinted Photonic Polymers as Sensing Elements for the Creation of Cross‐Reactive Sensor Arrays

By combining molecular imprinting and colloidal crystal templating, molecularly imprinted inverse‐opal photonic polymers (MIPPs) acting as sensing elements have been exploited to create sensor arrays for the first time. With this new strategy, abundant sensing elements with differential sensing abilities were easily accessible. Because of the unique hierarchical porous structure integrated in each sensing element, high sensitivity and selectivity, fast response and self‐reporting (label‐free) detection could be simultaneously achieved. All these fascinating features indicate that MIPPs are ideal sensing elements for creating sensor arrays. By integrating the individual sensing elements on a substrate, the formed array chip delivers better portability and high‐throughput capability. As a demonstration, six kinds of contaminants were selected as analytes. The detection and discrimination of these analytes and even their mixtures in a wide range of concentrations, particularly trace amounts of analyte against a high background of other components, could be achieved, indicating the powerful capability of MIPPs‐based sensor array for sensing. These results suggest that the described strategy opens a new route for sensor array creation and should find important applications in a wide range of areas.     

Investigation of QCM Sensors with Azobenzene Functionalized Coatings for the Detection of Nitroaromatics

Quartz Crystal Microbalance (QCM) based sensors have been used extensively to detect trace amounts of organic chemical vapors. These devices typically incorporate a polymer coating as an active layer that can bind the analytes of interest. Analyte adsorption causes a shift in the resonant frequency of the device proportional to the amount of adsorbed material. Currently some of the polymer coatings used in these sensors utilize hydrogen bonding to adsorb analytes. Dipole-dipole type interactions can also be utilized to promote interaction of the analytes with the polymer coating. Polymer coating containing segments that have a permanent molecular dipole can interact with explosive taggants. In this study, novel polypropylene glycol based polymers that incorporate both hydrogen bonding moieties and segments having large permanent dipole moment (p-nitroazobenzene functional groups) were synthesized and tested. The precursor polymer was prepared by the polymerization of the diglycidyl ether end functionalized polypropylene glycol macromer and aniline. The precursor polymers were post functionalized by an azo-coupling reaction. The sensor response to saturated vapors of o-nitrotoluene, nitrobenzene and 2,4 dinitrotoluene (DNT) saturated vapors was evaluated. Incorporation of p-nitroazobenzene moieties in the polymer increased the sensitivity of detection of the analytes. These studies offer new possibilities for using combination of interactions to improve the sensitivity of the QCM based sensors in the detection of nitroaromatic materials.     

Stress-induced chemical detection using flexible metal-organic frameworks

In this work we demonstrate the concept of stress-induced chemical detection using metal-organic frameworks (MOFs) by integrating a thin film of the MOF HKUST-1 with a microcantilever surface. The results show that the energy of molecular adsorption, which causes slight distortions in the MOF crystal structure, can be converted to mechanical energy to create a highly responsive, reversible, and selective sensor. This sensor responds to water, methanol, and ethanol vapors, but yields no response to either N2 or O2. The magnitude of the signal, which is measured by a built-in piezoresistor, is correlated with the concentration and can be fitted to a Langmuir isotherm. Furthermore, we show that the hydration state of the MOF layer can be used to impart selectivity to CO2. Finally, we report the first use of surface-enhanced Raman spectroscopy to characterize the structure of a MOF film. We conclude that the synthetic versatility of these nanoporous materials holds great promise for creating recognition chemistries to enable selective detection of a wide range of analytes.     

基于室温离子液体的电导型气体传感器

本文利用室温离子液体对水或有机蒸气吸收后其离子导电性的改变,研制了以离子液体BmimPF6为敏感材料的电导型气体传感器.考查了BmimPF6用量对传感器响应的影响,测定了传感器对不同浓度的水蒸汽及乙醇、二氯甲烷等饱和有机蒸气的响应.实验结果显示,该传感器具有制作方便、结构简单、稳定性高及线性范围宽等优点,可被用于不同浓度的水或有机蒸气/氮气混合气氛中,水蒸汽或有机蒸气浓度的测定.此外,还针对该传感器对乙醇等不同饱和有机蒸气响应信号与这些有机溶剂的理化性质参数间的定量关系,采用化学计量学方法进行了建模分析.     

Colorimetric molecularly imprinted polymer sensor array using dye displacement

A colorimetric sensor array composed of seven molecularly imprinted polymers was shown to accurately identify seven different aromatic amines. The response patterns were systematically classified using linear discriminant analysis with 94% classification accuracy. Analyses of the response patterns of the analytes to the imprinted polymer array suggest that the different selectivity patterns, although subtle, appear to arise from the imprinting process. The molecular imprinting process enabled the rapid preparation of the polymers in the array from ethylene glycol dimethacrylate and methacrylic acid (80:20) in the presence of six different template molecules plus a blank nonimprinted polymer. The response of the imprinted polymer array was coupled to a colorimetric response, using a dye displacement strategy. A benzofurazan dye was selected and shown to give an accurate measure of the binding properties of the imprinted polymer array to all seven analytes. The colorimetric response also enabled the inclusion of analytes that are not spectroscopically active and were not among the original analytes that were used as template molecules. This broadens the potential utility of the imprinted polymer sensor array strategy to a wider range of analytes and applications.     

A Rapid and Efficient Way to Dynamic Creation of Cross‐Reactive Sensor Arrays Based on Ionic Liquids

Based on the simple counterion exchange of ionic liquids, a rapid, facile, and efficient strategy to create a cross‐reactive sensor array with a dynamic tunable feature was developed, and exemplified by the construction of a sensor array for the identification and classification of nitroaromatics and explosives mimics. To achieve a good sensing system with fast response, good sensitivity, and low detection limit, the synthesized ionic liquid receptors were tethered onto a silica matrix with a macro‐mesoporous hierarchical structure. Through the facile anion exchange approach, abundant ionic‐liquid‐based individual receptors with diversiform properties, such as different micro‐environments, diverse molecular interactions, and distinctive physico‐chemical properties, were easily and quickly synthesized to generate a distinct fingerprint of explosives for pattern recognition. The reversible anion exchange ability further endowed the sensor array with a dynamic tunable feature as well as good controllability and practicality for real‐world application. With the assistance of statistical analysis, such as principal component analysis (PCA) and linear discrimination analysis (LDA), an optimized‐size array with a good resolution was rationally established from a large number of IL‐based receptors. The performed experiments suggested that the ionic‐liquid‐based sensing protocol is a general and powerful strategy for creating a cross‐reactive sensor array that could find a wide range of applications for sensing various analytes or complex mixtures.     

Temperature and humidity compensation in the determination of solvent vapors with a microsensor system

Accounting for changes in temperature and ambient humidity is critical to the development of practical field vapor-monitoring instrumentation employing microfabricated sensor arrays. In this study, responses to six organic vapors were collected from two prototype field instruments over a range of ambient temperatures and relative humidities (RH). Each instrument contains an array of three unthermostated polymer-coated surface acoustic wave (SAW) resonators, a thermally desorbed adsorbent preconcentrator bed, a reversible pump and a small scrubber cartridge. Negligible changes in the vapor sensitivities with atmospheric RH were observed owing, in large part, to the temporal separation of co-adsorbed water from the organic vapor analytes upon thermal desorption of preconcentrated air samples. As a result, calibrations performed at one RH level could be used to determine vapors at any other RH without corrections using standard pattern recognition methods. Negative exponential temperature dependences that agreed reasonably well with those predicted from theory were observed for many of the vapor-sensor combinations. It was possible to select a subset of sensors with structurally diverse polymer coatings whose sensitivities to all six test vapors and selected binary vapor mixtures had similar temperature dependences. Thus, vapor recognition could be rendered independent of temperature and vapor quantification could be corrected for temperature with sufficient accuracy for most applications. The results indicate that active temperature control is not necessary and that temperature and RH compensation is achievable with a relatively simple microsensor system.     

Gas chromatographic sensing on an optical fiber by mode-filtered light detection

A chemical sensor for gas phase measurements is reported which combines the principles of chemical separation and fiber optic detection. The analyzer incorporates an annular column Chromatographic sensor, constructed by inserting a polymer-clad optical fiber into a silica capillary. Light from a helium-neon laser is launched down the fiber, producing a steady intensity distribution within the fiber, but a low background of scattered light. When sample vapor is introduced to the sensor, and an analyte-rich volume interacts with the polymer cladding, Chromatographic retention is observed simultaneously with a change in the local refractive index of the cladding. An increase in cladding refractive index (RI) causes light to be coupled out of the fiber, with detection at a right-angle to the annular column length to provide optimum S/N ratio. This detection mechanism is called mode-filtered light detection. We report a gas Chromatographic separation on a 3.1 m annular column (320 microm i.d. silica tube, 228 microm o.d. fiber with a 12 microm fluorinated silicone clad) of methane, benzene, butanone and chlorobenzene in 6 min. The annular column length was reduced to 22 cm to function as a sensor, with selected organic vapors exhibiting unique retention times and detection selectivity. The detection selectivity is determined by the analyte RI and the partition coefficient into the cladding. The calculated limit of detection (LOD) for benzene vapor is 0.03% by volume in nitrogen, and several chlorinated species had LOD values less than 1%. For binary mixtures of organic vapors, the detected response appears to be the linear combination of the two organic standards, suggesting that the annular column may be useful as a general approach for designing chemical sensors that incorporate separation and optical detection principles simultaneously.     

Use of spatiotemporal response information from sorption-based sensor arrays to identify and quantify the composition of analyte mixtures

Linear sensor arrays made from small molecule/carbon black composite chemiresistors placed in a low-headspace volume chamber, with vapor delivered at low flow rates, allowed for the extraction of new chemical information that significantly increased the ability of the sensor arrays to identify vapor mixture components and to quantify their concentrations. Each sensor sorbed vapors from the gas stream and, thereby, as in gas chromatography, separated species having high vapor pressures from species having low vapor pressures. Instead of producing only equilibrium-based sensor responses that were representative of the thermodynamic equilibrium partitioning of analyte between each sensor and the initial vapor phase, the sensor responses varied depending on the position of the sensor in the chamber and the time since the beginning of the analyte exposure. The concomitant spatiotemporal (ST) sensor array response therefore provided information that was a function of time, as well as of the position of the sensor in the chamber. The responses to pure analytes and to multicomponent analyte mixtures comprised of hexane, decane, ethyl acetate, chlorobenzene, ethanol, and/or butanol were recorded along each of the sensor arrays. Use of a non-negative least-squares (NNLS) method for analysis of the ST data enabled the correct identification and quantification of the composition of two-, three-, four-, and five-component mixtures from arrays using only four chemically different sorbent films. In contrast, when traditional time- and position-independent sensor response information was used, these same mixtures could not be identified or quantified robustly. The work has also demonstrated that, for ST data, NNLS yielded significantly better results than analyses using extended disjoint principal components modeling. The ability to correctly identify and quantify constituent components of vapor mixtures through the use of such ST information significantly expands the capabilities of such broadly cross-reactive arrays of sensors.     

Colorimetric Recognition of Aldehydes and Ketones

A colorimetric sensor array has been designed for the identification of and discrimination among aldehydes and ketones in vapor phase. Due to rapid chemical reactions between the solid‐state sensor elements and gaseous analytes, distinct color difference patterns were produced and digitally imaged for chemometric analysis. The sensor array was developed from classical spot tests using aniline and phenylhydrazine dyes that enable molecular recognition of a wide variety of aliphatic or aromatic aldehydes and ketones, as demonstrated by hierarchical cluster, principal component, and support vector machine analyses. The aldehyde/ketone‐specific sensors were further employed for differentiation among and identification of ten liquor samples (whiskies, brandy, vodka) and ethanol controls, showing its potential applications in the beverage industry.     

Analytical application of poly{methyl[3-(2-hydroxy-3,4-difluoro)phenyl]propyl siloxane} as a QCM coating for DMMP detection

A new material-poly{methyl[3-(2-hydroxy-3,4-difluoro)phenyl]propyl siloxane}(PMDFPS) sensitive to toxic organophosphate vapor was synthesized with 2,3-difluorophenol, allyl bromide and poly (methyl hydrosiloxane) as raw materials, via O-alkylation, Claisen rearrange reaction and hydrosilylation reaction. This novel material was then coated on a quartz crystal microbalance (QCM) to investigate its gas sensitive properties to the nerve agent simulant dimethyl methylphosphonate (DMMP) vapor, as well as known interfering vapors. When tested with competing vapors, the sensor was more than 10 times sensitive to DMMP than to other interfering vapors. Thus, high selectivity of poly{methyl[3-(2-hydroxy-3,4-difluoro)phenyl]propyl siloxane} to DMMP was demonstrated. The poly{methyl[3-(2-hydroxy-3,4-difluoro)phenyl]propyl siloxane}-QCM sensor responded linearly to DMMP vapor with a slope of 14 Hz/ppm in the 1-50 ppm range with a detection limit of 0.21 ppm (S/N=3).     

Multifunctional chemical vapor sensors of aligned carbon nanotube and polymer composites

Partially coating perpendicularly aligned carbon nanotube arrays with an appropriate polymer thin film along their tube length provides a novel concept for developing new sensors of high sensitivity, good selectivity, and excellent environmental stability for the detection of a broad class of chemical vapors with low power consumption. The absorption and desorption of chemical vapors by the polymer matrix cause changes in the inter-tube distance and, hence, the surface resistance across the nanotube film. Simple measurements of the resistance change, therefore, constitute the nanotube-polymer chemical vapor sensors. These rationally designed, aligned carbon nanotube-polymer composite films are flexible and can be effectively integrated into many systems for a wide range of potential applications, including their use as multifunctional sensors for sensing chemical vapors, mechanical deformations, thermal and optical exposures.     

Vapor detection enabled by self-assembled colloidal photonic crystals

Here we report the sensitive and reversible detection of vapors by using self-assembled colloidal photonic crystals. The condensation of various vapors in the interstitials of silica colloidal photonic crystals leads to red-shift and amplitude reduction of optical stop bands. A linear relationship between wavelength shift and vapor partial pressure has been observed for a variety of vapors including ethanol, water, and toluene. Importantly, the sensitivity of colloidal photonic crystal-based vapor detectors can be improved by nearly two orders of magnitude by using a new full-peak analysis technique that takes advantage of the manifest amplitude reduction of optical stop bands during vapor condensation. Optical simulation based on a scalar-wave approximation model shows that the predicted optical responses during vapor condensation in colloidal photonic crystals agree well with experimental results. The condensation of vapors between submicrometer-scale microspheres, a topic that has received little examination, has also been investigated by both experiments and theoretical calculations. Predictions based on a modified Kelvin equation match with the experiments for a wide range of vapor partial pressures.     

A vapor selectivity study of microsensor arrays employing various functionalized ligand protected gold nanoclusters

We synthesized and tested four different monolayer protected gold nanoclusters (MPCs) as chemically selective interfaces for an organic vapor sensor array. The ligands chosen for capping the nano-Au particles and for selective organic vapor sorption were octanethiol, 2-naphthalenethiol, 2-benzothiazolethiol and 4-methoxythiolphenol. The same set of gold nanoclusters were tested on two different types of sensor platforms, a chemiresistor (CR) and a quartz crystal microbalance (QCM). The sensing properties of both sensor arrays were investigated with 10 organic vapors of various functional groups. Vapor sensing selectivity, dominated by the shell ligand structure of MPC, was demonstrated. The sensitivities of MPC coated CR are better than those of QCM sensors coated with the same material. The average CR/QCM amplification factors are range from 1.9 for 4-methoxythiolphenol MPC to 16.9 for octanethiol MPC. These differences in amplification factors indicate the functional group specific mechanisms for each vapor-MPC pair. The shell penetration mechanism of hydrogen-bonding vapor molecules into the 2-benzothiazolethiol capped MPC reduced the CR/QCM amplification factors. Strong attraction between MPC shell ligands can also reduce the magnitude of resistance changes during vapor sorption.     

Vapor sensing using polymer/carbon black composites in the percolative conduction regime

To investigate the behavior of chemiresistive vapor sensors operating below or around the percolation threshold, chemiresistors have been formed from composites of insulating organic polymers and low mass fractions of conductive carbon black (CB, 1-12% w/w). Such sensors produced extremely large relative differential resistance changes above certain threshold vapor concentrations. At high analyte partial pressures, these sensors exhibited better signal/noise characteristics and were typically less mutually correlated in their vapor response properties than composites formed using higher mass fractions of CB in the same set of polymer sorption layers. The responses of the low-mass-fraction CB sensors were, however, less repeatable, and their nonlinear response as a function of analyte concentration required more complicated calibration schemes to identify and quantify analyte vapors to compensate for drift of a sensor array and to compensate for variability in response between sensor arrays. Because of their much larger response signals, the low-mass-fraction CB sensors might be especially well suited for use with low-precision analog-to-digital signal readout electronics. These sensors serve well as a complement to composites formed from higher mass fractions of CB and have yielded insight into the tradeoffs of signal-to-noise improvements vs complexity of signal processing algorithms necessitated by the use of nonlinearly responding detectors in array-based sensing schemes.     

Static deflection measurements of cantilever arrays reveal polymer film expansion and contraction

An optical static method of detection is used to interpret surface stress induced bending related to cantilevers coated on one side with poly(vinyl alcohol), poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate), and poly(vinyl chloride-co-vinyl acetate-co-2-hydroxypropyl acrylate), or respectively, PVA, PVB, and PVC, and exposed to various solvent vapors. Results indicate that the adsorption and surface interactions of the different solvent vapors that cause polymer swelling and shrinking lead to rearrangements, which have been shown to change the elastic properties of the polymer film, and subsequently, the spring constant of the polymer coated cantilever. Static deflection measurements allow the direction of cantilever bending to be determined, which adds a new dimension of usefulness for surface functionalized cantilevers as transducers in the development of novel microelectromechanical systems (MEMS).