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PASSIVE CHEM/BIO TAGS 15 TOPIC 4: PASSIVE CHEM/BIO TAGS Two papers were presented on this topic, one by Todd Allen, Isotag, Inc., and the other by Jill Trewhella, Los Alamos National Laboratory. ISOTOPIC AND FLUORESCENT TAGGANTS Todd Allen noted that isotopic tags, which are based on isotopic ratios not found in nature, were first used in the oil and gas industry out of concern for product tampering, product dilution, and liability issues. They are now widely used in consumer products and government applications (e.g., tax compliance and explosives marking). Active markers involve the addition of a unique compound to the product/package, while passive markers exploit compounds that are already present. Markers may be overt (e.g., a hologram visible to the general public) or covert (requiring special instruments or readers to decode). Isotag, Inc. uses multiple taggants to provide added security. The concentration of active tags is usually in the low parts-per-billion range. Isotag, Inc. uses stable isotope markers involving a non-naturally occurring distribution of mass-enhanced markers (e.g., deuterated benzene in place of natural benzene). Using isotope ratio mass spectrometry, one can track the isotope ratios of the compound of interest. Natural geographic variations in isotope ratios must be accounted for. Fluorescence tags may exploit various regions of the spectrum, both visible and invisible. An example might be a fluorescent bar code on a product package that can only be read when the illuminated product is viewed through a special filter. These tags can be read at a distance using handheld optical devices. Allen believes nanotechnology, including quantum dots, nanobarcodes, and manipulation of various nanoparticles, will have widespread application in the tagging industry. A key concern, however, is the chemical, structural, and thermal stability of nanomaterials. BIOSCIENCE IN THE NANOTECHNOLOGY OF TRACKING AND LOCATING Jill Trewhella discussed recent developments at the nano-bio interface, primarily for tagging and tracking biological entities, including viruses, bacteria, and toxins. One could use this tagging, for instance, to find out where people had been. The emerging nanotechnologies she discussed include new molecular recognition elements, called âfluorobodies,â that are more robust than DNA and antibody