Spectral Considerations for Standoff Infrared Detection of RDX on Reflective Aluminum.

This paper examines infrared spectroscopic effects for the standoff detection of an explosive material, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), inkjet printed on an aluminum surface. Results of a spectroscopic study are described, using multiple optical setups. These setups were selected to explore how variations in the angles of incidence and collection from the surface of the material result in corresponding variations in the spectral signatures. The goal of these studies is to provide an understanding of these spectral changes since it affects standoff detection of hazardous materials on a reflective substrate. We demonstrate that variations in spectral effects are dependent on the relative surface concentration of the deposited RDX. We also show that it is reasonable to use spectroscopic data collected in a standard laboratory infrared spectrometer outfitted with a variable angle reflectometer set at 0° as reference spectra for data collected in a standoff configuration. These results are important to provide a systematic approach to understanding infrared (IR) spectra collection using standoff systems in the field, and to allow for comparison between such data, and data collected in the laboratory. Although the precise results are constrained to a specific material system (thin layers on a reflective substrate), the approach and general discussion provided are applicable to a broad range of IR standoff sensing techniques and applications.

Acoustic Steering Using Thermally Induced Optical Reflection of Sound (THORS).

Using the recently discovered THermally induced Optical Reflection of Sound (THORS) phenomenon, it is possible to generate optically induced, local density barriers in air by the absorption of intense, modulated laser light (the THORS phenomenon), which results in abrupt differences in compressibility of the air at these barriers that can efficiently reflect incident acoustic waves. In this note, we demonstrate the ability to optically manipulate and reflect acoustic waves in air as well as optimize the functional parameters (optical modulation and acoustic frequency) and characterize the effects of common physical parameters, including localized thermal gradients and incident angle of reflection on the efficiency of the resulting acoustic reflection. Finally, the ability to efficiently steer acoustic waves around a physical obstruction using THORS is also demonstrated.

Demonstration of a Human Color Vision Mimic in the Infrared.

Color vision results from the interaction of retinal photopigments with reflected or transmitted visible light. The International Commission on Illumination (CIE) developed the CIE color-matching chart, which separates colors on the basis of the interaction of their spectral profiles with three retinal photopigments in the human eye. We report the development of an infrared chromaticity (CIE-IR) chart, which mimics the CIE chart, in order to discriminate between different chemicals on the basis of the interactions of their IR signatures with three different IR optical filters, instead of the retinal photopigments in the human eye. Our results demonstrate that the CIE-IR chart enables separation of different classes of chemicals, as the visible CIE chart does with color, except for those in the IR spectral region. Such results clearly show that the biomimetic sensing method based on human color vision is in fact a true analogue to color vision and that the proposed CIE-IR chart can be used as a classification method unique to this biomimetic sensing modality.

Infrared Reflectance Spectroscopic Evaluation of Inkjet Printed Standards of Cyclotrimethylenetrinitramine (RDX) on Aluminum Substrates.

The Department of Defense (DOD) and first responder communities are evaluating and developing optical systems for the detection and identification of explosives and components used for assembling homemade explosives (HMEs). Emerging detection technologies must be evaluated with authentic hazard material concentrations to ensure their accurate and reliable use in the field. In this work, infrared (IR) reflectance spectra over the spectral rage of 1000-1700 cm-1 were collected for different concentrations of inkjet-printed RDX (cyclotrimethylenetrinitramine) samples deposited onto aluminum substrates. A plot of the integrated area of both the symmetric and asymmetric NO2 vibrational bands for RDX on aluminum exhibited good linearity over the concentration range 20-500 µg/cm2. Detection limits for RDX on an aluminum surface were calculated to be 10.7 µg/cm2 for the symmetric NO2 vibrational band and 1.4 µg/cm2 for the asymmetric NO2 vibrational band. Evaluation of the NO2 vibrational band areas at different locations of the RDX array demonstrated that the samples exhibited good homogeneity across the surface. The concentration of an unknown sample of RDX on aluminum was determined using the fitted equations; results showed good agreement between the calculated and actual RDX surface concentration. The lot-to-lot variation of RDX on the aluminum surface was compared using the long wavelength infrared (LWIR) spectral band areas for two different lots of standards printed at the same RDX surface concentration. Results showed excellent lot-to-lot agreement indicating good reproducibility of the standards for RDX.

Optical reflection and waveguiding of sound by photo-thermally induced barriers.

Control and manipulation of sound is of critical importance to many different scientific and engineering fields, requiring the design of rigid physical structures with precise geometries and material properties for the desired acoustics. In this work, we demonstrate the ability to manipulate the direction and magnitude of sound waves traveling in air using laser light, without the need for physical interfaces associated with different materials. Efficient reflection of sound waves off of transient, optically generated, abrupt air density barriers is demonstrated, with acoustic reflections greater than 25% of the incident acoustic wave amplitude. Implementation of multiple barriers, can result in complete suppress the transmission of incident acoustic signals as great as 70 dB. Additionally, shaping the laser beam acoustic waveguides can be generated with dramatically reduced transmission losses.

Development of photoacoustic sensing platforms at the Army Research Laboratory.

Traditionally, chemical sensing platforms have been hampered by the opposing concerns of increasing sensor capability while maintaining a minimal package size. Current sensors, although reasonably sized, are geared to more classical chemical threats, and the ability to expand their capabilities to a broader range of emerging threats is uncertain. Recently, photoacoustic spectroscopy, employed in a sensor format, has shown enormous potential to address these ever-changing threats. Photoacoustic spectroscopy is one of the more flexible infrared spectroscopy variants, and that flexibility allows for the construction of sensors that are designed for specific tasks. The Army Research Laboratory has, for the past 14 years, engaged in research into the development of photoacoustic sensing platforms with the goal of sensor miniaturization and the detection of a variety of chemical targets both proximally and at range. This paper reviews this work.

Standoff Photoacoustic Spectroscopy of Explosives.

Detection and identification of unknown and possibly hazardous materials is a vital area of research to which infrared (IR) spectroscopy is ideally suited. Infrared absorption spectra can be measured with many sensing paradigms of which photoacoustic spectroscopy (PAS) is a sensitive and flexible variant. The flexibility of PAS allows for the construction of narrowly tailored spectroscopic sensors that are designed for specific tasks. We discuss the evaluation of an interferometric PAS sensor by the measurement of common explosive hazards from a standoff distance of 1 m. Reproduction of IR absorption spectra for 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), pentaerythritol tetranitrate (PETN), and 2,4,6-trinitrotoluene (TNT) demonstrate the capabilities of the interferometric sensor for standoff explosives detection.

Surface-enhanced Raman scattering detection of ammonium nitrate samples fabricated using drop-on-demand inkjet technology.

The United States Army and the first responder community are increasingly focusing efforts on energetic materials detection and identification. Main hazards encountered in theater include homemade explosives and improvised explosive devices, in part fabricated from simple components like ammonium nitrate (AN). In order to accurately detect and identify these unknowns (energetic or benign), fielded detection systems must be accurately trained using well-understood universal testing substrates. These training substrates must contain target species at known concentrations and recognized polymorphic phases. Ammonium nitrate is an explosive precursor material that demonstrates several different polymorphic phases dependent upon how the material is deposited onto testing substrates. In this paper, known concentrations of AN were uniformly deposited onto commercially available surface-enhanced Raman scattering (SERS) substrates using a drop-on-demand inkjet printing system. The phase changes observed after the deposition of AN under several solvent conditions are investigated. Characteristics of the collected SERS spectra of AN are discussed, and it is demonstrated that an understanding of the exact nature of the AN samples deposited will result in an increased ability to accurately and reliably "train" hazard detection systems.

Standardized sample preparation using a drop-on-demand printing platform.

Hazard detection systems must be evaluated with appropriate test material concentrations under controlled conditions in order to accurately identify and quantify unknown residues commonly utilized in theater. The existing assortment of hazard reference sample preparation methods/techniques presents a range of variability and reproducibility concerns, making it increasingly difficult to accurately assess optically- based detection technologies. To overcome these challenges, we examined the optimization, characterization, and calibration of microdroplets from a drop-on-demand microdispenser that has a proven capability for the preparation of energetic reference materials. Research presented herein focuses on the development of a simplistic instrument calibration technique and sample preparation protocol for explosive materials testing based on drop-on-demand technology. Droplet mass and reproducibility were measured using ultraviolet-visible (UV-Vis) absorption spectroscopy. The results presented here demonstrate the operational factors that influence droplet dispensing for specific materials (e.g., energetic and interferents). Understanding these parameters permits the determination of droplet and sample uniformity and reproducibility (typical R2 values of 0.991, relative standard deviation or RSD ≤ 5%), and thus the demonstrated maturation of a successful and robust methodology for energetic sample preparation.

Characterization of polymorphic states in energetic samples of 1,3,5-trinitro-1,3,5-triazine (RDX) fabricated using drop-on-demand inkjet technology.

The United States Army and the first responder community are evaluating optical detection systems for the trace detection of hazardous energetic materials. Fielded detection systems must be evaluated with the appropriate material concentrations to accurately identify the residue in theater. Trace levels of energetic materials have been observed in mutable polymorphic phases and, therefore, the systems being evaluated must be able to detect and accurately identify variant sample phases observed in spectral data. In this work, we report on the novel application of drop-on-demand technology for the fabrication of standardized trace 1,3,5-trinitro-1,3,5-triazine (RDX) samples. The drop-on-demand sample fabrication technique is compared both visually and spectrally to the more commonly used drop-and-dry technique. As the drop-on-demand technique allows for the fabrication of trace level hazard materials, concerted efforts focused on characterization of the polymorphic phase changes observed with low concentrations of RDX commonly used in drop-on-demand processing. This information is important when evaluating optical detection technologies using samples prepared with a drop-on-demand inkjet system, as the technology may be "trained" to detect the common bulk α phase of the explosive based on its spectral features but fall short in positively detecting a trace quantity of RDX (ß-phase). We report the polymorphic shifts observed between α- and ß-phases of this energetic material and discuss the conditions leading to the favoring of one phase over the other.

A nanosensor for TNT detection based on molecularly imprinted polymers and surface enhanced Raman scattering.

We report on a new sensor strategy that integrates molecularly imprinted polymers (MIPs) with surface enhanced Raman scattering (SERS). The sensor was developed to detect the explosive, 2,4,6-trinitrotoluene (TNT). Micron thick films of sol gel-derived xerogels were deposited on a SERS-active surface as the sensing layer. Xerogels were molecularly imprinted for TNT using non-covalent interactions with the polymer matrix. Binding of the TNT within the polymer matrix results in unique SERS bands, which allow for detection and identification of the molecule in the MIP. This MIP-SERS sensor exhibits an apparent dissociation constant of (2.3 ± 0.3) × 10(-5) M for TNT and a 3 µM detection limit. The response to TNT is reversible and the sensor is stable for at least 6 months. Key challenges, including developing a MIP formulation that is stable and integrated with the SERS substrate, and ensuring the MIP does not mask the spectral features of the target analyte through SERS polymer background, were successfully met. The results also suggest the MIP-SERS protocol can be extended to other target analytes of interest.

Dynamics within site selectively templated and tagged xerogel sensor platforms.

In a nitrobenzo-2-oxa-1,3-diazole (NBD) -based, 9-anthrol-responsive site selectively templated and tagged xerogel (SSTTX) sensor platform, there are two reporter molecule site types (responsive and non-responsive) that are responsible for the observed fluorescence signals. These NBD sites function independently. Site 1 alone binds the target analyte and yields an analyte-dependent signal. This signal arises from analyte binding decreasing the photo-induced electron transfer (PET) efficiency between a strategically placed amine residue and the excited NBD reporter molecule within the template site. Site 2 does not respond to analyte, it is not fully formed, and it manifests itself as a background signal. In an n-octyl residue-free SSTTX, the local microviscosity sensed by the site 1 NBD reporter molecules in the absence and presence of target analyte is ∼260 cP and ∼540 cP, respectively. These local microviscosity values are substantially greater in comparison to free NBD dissolved in THF (η = 0.46 cP at 298 K, ϕ ∼25 ps). As the SSTTX n-octyl content is increased, the local microviscosity sensed by the site 1 NBD reporter molecules in the absence and presence of target analyte is ∼360 cP and ∼760 cP, respectively. This behavior is consistent with the n-octyl chains crowding the cybotactic region surrounding the site 1 NBD reporter molecules. This n-octyl-induced site 1 "crowding" is also associated with improved analyte binding to site 1 and better overall SSTTX analytical performance.

Photophysics associated with site selectively templated and tagged xerogel sensor platforms.

The analytical signal produced from a site selectively templated and tagged xerogel (SSTTX) based sensor is more completely elucidated. In an SSTTX, target analyte (TA) binding sites are created within a xerogel and a fluorescent reporter molecule (e.g., nitrobenzo-2-oxa-1,3-diazole, NBD) is covalently and selectively positioned within the template site. TA binding modulates the NBD cybotactic region, yielding a TA concentration-dependent change in the NBD reporter fluorescence. Our results reveal that there are two NBD sub-populations within the SSTTX. One sub-population is associated with misformed template sites that bind TA poorly and the NBD reporter molecules are heavily quenched by an adjacent amine residue that is intentionally positioned within close proximity to the NBD reporter molecule. The second NBD sub-population is associated with a properly formed template site that binds TA; the adjacent amine is still present but it is not as close to the NBD in comparison to the misformed site. When TA is present, the TA-responsive site binds TA, the TA disrupts the adjacent amine-NBD interaction (dequenching), and the NBD fluorescence increases. All NBD quenching occurs via photo-induced electron transfer (PET) from the adjacent amines to the excited NBD reporter molecules. As the mole fraction ratio of n-octyltrimethoxysilane (C(8)-TMOS) to tetramethoxysilane (TMOS) within an SSTTX increases, the misformed site mole fraction decreases and the dissociation constant (K(d)) for TA binding to the TA-responsive site decreases by 13-fold.

Quantum cascade laser-based photoacoustic spectroscopy for trace vapor detection and molecular discrimination.

We report on the development of a microelectromechanical systems (MEMS)-scale photoacoustic sensor for the detection of trace gases. A mid-infrared quantum cascade laser (QCL) was used to determine detection limits for acetic acid, acetone, 1,4-dioxane, and vinyl acetate. The source was continuously tunable from 1015 cm(-1) to 1240 cm(-1), allowing for the collection of photoacoustic vibrational spectra for these gases. Exceptional agreement between the measured photoacoustic spectra and the infrared spectra for acetic acid, acetone, 1,4-dioxane, and vinyl acetate was observed. Partial least-squares (PLS) regression was used to develop an algorithm for classification of these compounds based solely on photoacoustic spectra.

Photophysics of 9,10-anthracenediol and a bifunctional sacrificial template in solution and xerogels.

Site selectively templated and tagged xerogels (SSTTX) represent a new sensing platform. Although this platform has several attractive features, the template formation process is not fully understood. To address this issue we have explored the photophysics of two model compounds (9,10-anthracenediol and a bifunctional sacrificial template (BST)) when dissolved in solution and when sequestered within a xerogel. The solution experiments show that the carbamate tethers on the BST (which are eventually cleaved to form the analyte responsive sites that make up the SSTTX) do not alter the anthracene residue's intrinsic photophysics. In contrast, 9,10-anthracenediol and BST molecules sequestered within a xerogel sense and report from a distribution of microenvironments. The distribution mean values are very similar, but the variance is statistically greater for the BST-doped xerogel in comparison to the 9,10-anthracenediol-doped xerogel. The most likely causes of this behavior are heterogeneity and electron and energy transfer processes that are controlled by differences in the position/orientation of the anthracene moiety at the pore surface in the Class I (9,10-anthracenediol) and Class II (BST) xerogels. These results also suggest that the initial template sites produced during the SSTTX formation process are not discrete; they are intrinsically more diverse (maybe 30%) in comparison to the types of template sites created by traditional molecular imprinting strategies. However, our previously reported SSTTX binding studies do not reveal any evidence for a distribution of analyte-to-SSTTX binding. This apparently anomalous behavior may result because the relative standard deviation of the binding process is intrinsically small and/or one or more of the steps that follow template site formation attenuate the final template site distribution within the SSTTX.

Molecularly imprinted xerogels as platforms for sensing.

Detection and quantification of analytes in clinical settings (e.g., routine blood testing), at home (e.g., glucose monitoring), in the field (e.g., environmental monitoring, war fighter protection, homeland security), and in the factory (e.g., worker health, beverage and food safety) is exceedingly challenging. Chemical sensors and biosensors have attracted considerable attention because of their perceived ability to meet these challenges. Chemical sensors exploit a recognition element in concert with a transduction strategy. When the recognition element is biological (e.g., antibody, aptamer, enzyme), the sensor is termed a biosensor. There is substantial literature on biosensing; however, there are compelling reasons for developing inexpensive, robust, and reusable alternatives for the expensive or unstable biorecognition elements. This Account summarizes recent research on designing and producing analyte-responsive materials based on molecularly imprinted xerogels.

Molecularly templated materials in chemical sensing.

Biologically-based recognition elements (e.g., antibodies, aptamers, enzymes, etc.) are used as the recognition element within a wide variety of assays and sensor systems. There are, however, compelling reasons for researchers to develop inexpensive, robust, and reusable alternatives for these expensive and unstable biorecognition elements. This review summarizes recent research efforts on the development of molecularly templated (sometimes called molecularly imprinted) organic and inorganic polymers as possible replacements for expensive/labile biorecognition elements. The review begins with a briefing on biosensing and the pertinent issues and limitations. The focus then swings toward molecularly templating within organic and inorganic (xerogels) polymers to create materials with analyte binding characteristics akin to a biorecognition element. The review then describes several recent developments wherein analyte recognition and an analyte-dependent transduction methodology are simultaneously incorporated directly within the templated materials. The review ends by outlining the current state-of-the-art and the remaining issues and impediments.