Nanoparticle Taggants for Explosive Precursors
Aggressive Homeland Defense initiatives may force foreign or domestic terrorists to construct explosive devices from commercial sources such as ammonium nitrate fuel oil (ANFO). As seen in the case of the Oklahoma City bombing, such low technology munitions have the potential for inflicting severe damage to unprotected assets. Exacerbating this problem, a large number of chemicals and commercial sources are available to formulate improvised explosives. Once devices are detonated, sophisticated forensic techniques are needed to rapidly trace the source of the raw materials from explosive residues, i.e., domestic versus foreign, geographic source, production date, supplier, and ideally the person that purchased the material. This can be a formidable challenge since source databases do not exist and material compositions vary widely with time, even from the same industrial source. High fidelity taggant systems that are inexpensive, compatible with commercial manufacturing practices, and can survive intact explosive events are needed.
Researchers at The Johns Hopkins University Applied Physics Laboratory are fabricating a sophisticated taggant system, including a hierarchy of nanostructures. The nanostructures will be coded by size, shape, and chemical composition, providing a highly individualized taggant useful for distinguishing explosives and batches forensically, and potentially allowing detection at a distance. This technology utilizes nanoparticle synthesis techniques to produce highly specific "taggant concentrate formulations" that can be added to explosive precursor chemicals prior to distribution. In the context of this discussion, the term concentrates implies nanoparticle mixtures with greater than 50% nanoparticles in a suitable solvent matrix. For example, sol gel techniques have been devised to produce nanoparticle concentrates of many metal oxide nanoparticles. Already in the oxidized form, such nanoparticles should survive explosive events essentailly unaffected. Nanoparticle concentrates are produced in varying chemistry, size, and shape (spheres, rods, etc.) that can be combined in unique ways to provide highly specific taggant signatures of commercial products when incorporated at very low concentrations. At the nanoscale, the particle number is enormous compared to the microscale, e.g., the particle number of spherical aluminum oxide (A12O3) particles would be ~ 10^9 particles per gram of material
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