Ellen L. Holthoff

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 (β-phase). We report the polymorphic shifts observed between α- and β-phases of this energetic material and discuss the conditions leading to the favoring of one phase over the other.

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.

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 residues 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.

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 (C8-TMOS) to tetramethoxysilane (TMOS) within an SSTTX increases, the misformed site mole fraction decreases and the dissociation constant (Kd) for TA binding to the TA-responsive site decreases by 13-fold.

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.

High-confidence discrimination of explosive materials on surfaces using a non-spectroscopic optical biomimetic sensing method

Field detection of chemical, biological, radiological, nuclear and explosive (CBRNE) threats requires the development of highly selective sensors with low size, weight, power and cost (SWaP-c). Recent developments have demonstrated that an optical biomimetic sensing approach, based on human-eye color detection can provide high-confidence discrimination of target chemicals while rejecting potential interferents with similar chemical structures. This biomimetic sensing method operates by identifying differences in the overlap between target and interferent chemical infrared absorption bands utilizing three, overlapping, optical bandpass filters. This method is non-spectroscopic and requires only the use of commercially available, off-the-shelf optical components. This approach has been demonstrated for volatile chemical vapors in the mid-wave-infrared (3 – 5 μm). Based on this success, experimental studies of this biomimetic sensing approach have been expanded further into the long wave infrared spectral region (6 – 12 μm) and for detection of explosives on surfaces, including aluminum and plastics. We present discrimination results using this biomimetic sensing approach for explosive samples on surfaces in both the mid- and long- wave infrared. Numerical data, along with experimentally collected data, are discussed. We demonstrate that this method is capable of discriminating between similar explosives on surfaces as well as between these explosives and potential environmental interferents. We present the results of these experiments and discuss potential transition of this approach to future field-ready stand-off devices and applications.

Standoff photoacoustic spectroscopy for hazard detection

Photoacoustic spectroscopy (PAS) is a versatile and sensitive chemical sensing method. This versatility allows for the construction of a variety of sensors that are optimized for specific sensing tasks. Current research at the U.S. Army Research Laboratory (ARL) is focused on the development of a standoff hazardous materials detection technique based on an interferometric sensor. The standoff detection paradigm increases operator safety and reduces sample preparation requirements as compared to traditional photoacoustic cell-based sensors. We demonstrate the collection of photoacoustic spectra of layered solid samples at a standoff distance of one meter. The layered samples are constructed via deposition of a thin layer of energetic or other hazardous substance upon a thick substrate. We will also discuss excitation source selection as it relates to the operating mode of the source (i.e., pulsed or continuous wave (CW) modulated).

Characterizing hazardous inkjet samples using Raman scattering

The US Army and first response units are increasingly focused on the detection and identification of energetic materials, such as homemade and improvised explosive devices. To accurately detect and identify these unknowns (energetic or benign), we must accurately train fielded detection systems for trace and bulk quantities, using well-understood universal testing substrates. Here, we discuss characterizing various phases of ammonium nitrate (AN) using surface-enhanced Raman scattering (SERS) fabricated using drop-on-demand inkjet technologies. AN, which has practical uses in agriculture and industry, is of interest to the Army as an analyte because it is commonly used in improvised explosive devices. To accurately detect and identify this material, hazard evaluation systems require known ANcontaining training materials. This can be challenging, as AN demonstrates different polymorphic phases (generally phase III and phase IV being most common at standard conditions) dependent on the material handling history and even the concentration of materials deposited. Typically, under standard conditions, phase IV is considered the most stable format. However, when catalyzing solvents are present, AN can shift from phase IV to phase III. Under normal conditions, phase IV is orthorhombic (having three unequal axes intersecting at right angles) and has two formula units per cell. Phase III is also orthorhombic, but has four formula units per cell.1 Phase transition from IV to III results in swelling of the AN due to a 4% increase in unit cell volume,2 and therefore an increase in porosity and explosive potential of fertilizer granules. At room temperatures, AN phase IV to III transition can undergo an intermediary stage very similar to phase II. When phase transitions for AN are observed with Raman, the bands typically associated with symmetric stretch mode of nitrate (NO 3 / shift from phase II at 1050cm 1, phase III at 1048cm 1, and phase IV at 1044cm 1.3 Figure 1. (A) An example image of a MicroFab 4 jetting system with print head highlighted in red. (B) Image of typical print head as compared to a dime

Substrate interaction in ranged photoacoustic spectroscopy of layered samples

Photoacoustic spectroscopy (PAS) is a useful monitoring technique that is well suited for ranged detection of condensed materials. Ranged PAS has been demonstrated using an interferometer as the sensor. Interferometric measurement of photoacoustic phenomena focuses on the measurement of changes in path length of a probe laser beam. That probe beam measures, without discrimination, the acoustic, thermal, and physical changes to the excited sample and the layer of gas adjacent to the surface of the solid sample. For layered samples, the photoacoustic response of the system is influenced by the physical properties of the substrate as well as the sample under investigation. We will discuss the affect that substrate absorption of the excitation source has on the spectra collected in PAS. We also discuss the role that the vibrational modes of the substrate have in photoacoustic signal generation.

High Throughput Production and Screening Strategies for Creating Advanced Biomaterials and Chemical Sensors

Development of new materials is needed for numerous applications in engineering, medical, and scientific arenas. In this chapter, we describe some of our research efforts that focus on developing strategies and tools for high throughput production and screening to create advanced biomaterials and chemical sensors. Using our developed tools, we are able to produce and screen a wide array of materials in a short period of time. In several current embodiments, the system can readily produce and fully screen 100–1,000 samples/day. Our developed automated systems can provide results with minimal user input, yet with better precision and accuracy in comparison to traditional manual methods.