The long standoff detection or sensing of chemical analytes is important in many applications. In the policing of borders and reduction or elimination of illegal drug trafficking, the detection of chemical constituents from a large distance associated with illegal drugs can help identify smuggling routes and perpetrators. The detection of these chemicals could drastically improve the border patrol's effectiveness. In biochemical warfare, the detection or sensing of hazardous bioagents from afar is also of high importance in order to provide early warning and identify proper countermeasures. In today's war zones, the detection or sensing of vapor-phase explosives with high-sensitivity is of utmost importance, particularly, because in many war zones, the use of improvised explosive devices (IEDs) have resulted in a large number of deaths, e.g., more than 50% of all coalition deaths in Iraq and Afghanistan. These deaths could be significantly reduced if it was possible to accurately detect the presence of these IEDs from a distance without jeopardizing the lives of human investigators.
Most commercially-available chemical sensing devices or systems have several drawbacks. First, these devices and systems generally require skilled human operators, making it difficult to deploy these devices or systems remotely and/or autonomously. Second, these systems and devices generally only provide chemical detection capability over a limited geographical area and are not amenable to wide area scanning. Third, these chemical detection devices are generally too large and/or heavy to be used for covert operations.
Vapor-phase explosives detection technology has emerged as one of the most sensitive tools in the military's explosives detecting arsenal. In these so-called “sniffing” explosives detection devices, e.g., the FIDO™ XT portable explosives detector (available from ICx Technologies, Mass.), the detection of the chemicals/explosives is based on the attachment of specific, known chemicals/explosives to fluorescent polymers. The fluorescence of these polymers is dependent on the binding of specific chemical analytes to the polymers. For example, an explosive may cause a reduction in fluorescence of the polymers. The polymer is excited by light from a light emitting diode (LED) or laser. These fluorescent polymers have the property that their photoluminescence becomes quenched as vapor-phase chemicals bind to the surface of the fluorescent polymer. The reduced fluorescence of the polymer film is detected by a photodetector to generate an electrical current signal.
Despite their demonstrable success, portable explosives detection devices such as the Fido™ XT also have several drawbacks. First, in devices such as the FIDO™ XT, there are significant optical losses in the transmission of photons from the fluorescent polymer to the photodetector, yielding inefficient chemical to electrical signal transduction. These detection devices are difficult to miniaturize because efficient isolation of optical signals with similar wavelengths is difficult if one wishes to have a small micrometer-scale device. In particular, the micro-fabrication of closely packed fluorescent polymer layers with similar fluorescence spectra can be problematic due to the difficulty in isolating the optical signal from each element. Of particular relevance, commercially-available chemical detectors cannot be deployed remotely as they do not have any remote interrogation capabilities, and due to their size, typically are deployed in the field with a skilled user, thereby putting lives at risk. In particular, commercially-available chemical detection systems and devices are not capable of being interrogated from large distances, e.g., a few km, making them unusable for covert detection applications, and greatly limiting the area over and speed with which chemical analytes can be found.