Mark Fisher

Using novel fluorescent polymers as sensory materials for above-ground sensing of chemical signature compounds emanating from buried landmines

Chemical vapors originating from the explosive charge within landmines and unexploded ordnance (UXO) form a chemical signature unique to these devices. The fact that canines can detect this signature was a primary motivation for the Defense Advanced Research Projects Agencys (DARPA) Dogs Nose Program. One goal of this program was to develop electronic chemical sensors that mimic the canines ability to detect landmines. The sensor described here, developed under this program, utilizes novel fluorescent polymers to detect landmine signature vapors in air at ultratrace concentration levels (parts-per-trillion or less). Thin films of the polymers are highly emissive but undergo a dramatic reduction in emission intensity when molecules of target analytes bind to the polymer. Binding of a single explosive molecule can quench the fluorescence from hundreds of polymer repeat units, resulting in an amplification of the quenching response. The polymer structure contains receptor sites designed to interact specifically with nitroaromatic explosives, enhancing the selectivity of the polymers for target analytes. A man-portable sensor prototype, similar in size and configuration to metal detectors currently used for mine detection, has demonstrated performance comparable to that of canines during field tests monitored by DARPA at Fort Leonard Wood, MO.

Detection of land mines by amplified fluorescence quenching of polymer films: a man-portable chemical sniffer for detection of ultratrace concentrations of explosives emanating from land mines

The explosive charge within a landmine is the source for a mixture of chemical vapors that form a distinctive chemical signature indicative of a landmine. The concentration of these compounds in the air over landmines is extremely low, well below the minimum detection limits of most field- portable chemical sensors. Described in this paper is a man- portable landmine detection system that has for the first time demonstrated the ability to detect landmines by direct sensing of the vapors of signature compounds in the air over landmines. The system utilizes fluorescent polymers developed by collaborators at the MIT. The sensor can detect ultra-trace concentrations of TNT vapor and other nitroaromatic compounds found in many landmine explosives. Thin films of the polymers exhibit intense fluorescence, but when exposed to vapors of nitroaromatic explosives the intensity of the light emitted from the films decreases. A single molecule of TNT binding to a receptor site quenches the fluorescence from many polymer repeat units, increasing the sensitivity by orders of magnitude. A sensor prototype has been develop that response in near real-time to low femtogram quantities of nitroaromatic explosives. The prototype is portable, lightweight, has low power consumption, is simple to operate, and is relatively inexpensive. Simultaneous field testing of the sensor and experienced canine landmine detection teams was recently completed. Although the testing was limited in scope, the performance of the senor met or exceeded that of the canines against buried landmines.

Implementation of serial amplifying fluorescent polymer arrays for enhanced chemical vapor sensing of landmines

A sensor (known as Fido) that utilizes amplification of fluorescence quenching as the transduction mechanism for ultra-trace detection of nitroaromatic compounds associated with landmines has been described previously. Previous sensor prototypes utilized a single band of amplifying polymer deployed inside a capillary waveguide to form the sensing element of the detector. A new prototype has been developed that incorporates multiple, discrete bands of different amplifying polymers deployed in a linear array inside the capillary. Vapor-phase samples are introduced into the sensor as a sharp pulse via a gated inlet. As the vapor pulse is swept through the capillary by flow of a carrier gas, the pulse of analyte encounters the bands of polymer sequentially. If the sample contains nitroaromatic explosives, the bands of polymer will respond with a reduction in emission intensity proportional to the mass of analyte in the sample. Because the polymer bands are deployed serially, the analyte pulse does not reach the bands of polymer simultaneously. Hence, a temporal response pattern will be observed as the analyte pulse traverses the length of the capillary. In addition, the intensity of response for each band will vary, producing a ratiometric response. The temporal and ratiometric responses are characteristic of a given analyte, enhancing discrimination of target analytes from potential interferents. This should translate into a reduction in sensor false alarm rates.

Landmine detection by chemical signature: detection of vapors of nitroaromatic compounds by fluorescence quenching of novel polymer materials

The concentration of the chemical vapors emanating form landmines is very low. The equilibrium vapor concentration above pure, crystalline TNT at room temperature is approximately 70 ng/liter. It has been estimated that the TNT concentration in the air over a buried TNT-containing landmine is three to six orders of magnitude less than this value. TNT vapor concentrations three orders of magnitude less than equilibrium are difficult to detect with research quality laboratory instruments and are beyond the capabilities of most commercially available field-portable instruments. Hence, new ultra-sensitive detection technologies for explosives are needed. Collaborators at the MIT have synthesized novel fluorescent polymers that have been implemented as sensory materials in a landmine detection system. When vapors of nitroaromatic compounds of the type found in most landmines bind to thin films of the polymers, the fluorescence of the films decreases. A single molecular binding even quenches the fluorescence of many polymer repeat units, resulting in an amplification of the quenching. Analyte binding to the films is reversible, so the films can be reused. A prototype sensor package has been developed that response almost immediately to sub-picogram quantities of target nitroaromatics. The prototype is portable, is lightweight, has low power consumption, is simple to operate, and is relatively inexpensive. Improvements in the sensitivity of the package are expected. A sample preconcentrator is also being developed for use when the concentration of target analytes is to low to be sensed directly.

Explosive detection using high-volume vapor sampling and analysis by trained canines and ultra-trace detection equipment

The dogs nose is an effective, highly-mobile sampling system, while the canine olfactory organs are an extremely sensitive detector. Having been trained to detect a wide variety of substances with exceptional results, canines are widely regarded as the gold standard in chemical vapor detection. Historically, attempts to mimic the ability of dogs to detect vapors of explosives using electronic dogs noses has proven difficult. However, recent advances in technology have resulted in development of detection (i.e., sampling and sensor) systems with performance that is rapidly approaching that of trained canines. The Nomadics Fido was the first sensor to demonstrate under field conditions the detection of landmines with performance approaching that of canines. More recently, comparative testing of Fido against canines has revealed that electronic vapor detection, when coupled with effective sampling methods, can produce results comparable to that of highly-trained canines. The results of these comparative tests will be presented, as will recent test results in which explosives hidden in cargo were detected using Fido with a high-volume sampling technique. Finally, the use of canines along with electronic sensors will be discussed as a means of improving the performance and expanding the capabilities of both methods.

Minefield edge detection using a novel chemical vapor sensing technique

Nomadics has developed a novel sensing technology that detects the chemical signature of explosives emanating from buried landmines. Canines have demonstrated the ability to detect these signatures, but use of canines for this task presents a number of logistical and physical limitations that can be overcome by use of chemical sensors. Nomadics is the exclusive licensee of novel amplifying fluorescent polymer materials developed by the Massachusetts Institute of Technology (MIT). These materials enable detection of ultra-trace concentrations of nitroaromatic compounds such as TNT, the most commonly utilized explosive in the production of landmines. When vapors of nitroaromatics are presented to the sensor, the fluorescent polymers emit light at a greatly reduced intensity, a property that enables rapid detection of trace quantities of explosives using relatively low-cost electronics and optics. Studies performed by Jenkins et al suggest that the chemical signature of a landmine is heterogeneous and can be dispersed a significant distance from the location of the mine. Because the signature is not highly localized and is not characterized by a well-defined concentration gradient, the sensor may have difficulty indicating the exact position of a mine, especially in high-density minefields. Conversely, if the chemical signature extends some distance from the mine position, the sensor may have utility in detecting the edges of minefields. In combat scenarios, this will allow commanders to select safe paths for personnel and vehicles. This paper will present the latest findings related to minefield edge detection at several test sites.

Detection of vehicle-based improvised explosives using ultra-trace detection equipment

Vehicle-borne improvised explosive devices (VBIEDs) have become the weapon of choice for insurgents in Iraq. At the same time, these devices are becoming increasingly sophisticated and effective. VBIEDs can be difficult to detect during visual inspection of vehicles. This is especially true when explosives have been hidden behind a vehicle’s panels, inside seat cushions, under floorboards, or behind cargo. Even though the explosive may not be visible, vapors of explosive emanating from the device are often present in the vehicle, but the current generation of trace detection equipment has not been sensitive enough to detect these low concentrations of vapor. This paper presents initial test results using the Nomadics Fido sensor for detection of VBIEDs. The sensor is a small, explosives detector with unprecedented levels of sensitivity for detection of nitroaromatic explosives. Fido utilizes fluorescence quenching of novel polymer materials to detect traces of explosive vapor emanating from targets containing explosives. These materials, developed by collaborators at the Massachusetts Institute of Technology (MIT), amplify the quenching response that occurs when molecules of explosive bind to films of the polymer. These materials have enabled development of sensors with performance approaching that of canines trained to detect explosives. The ability of the sensor to detect explosives in vehicles and on persons who have recently been in close proximity to explosives has recently been demonstrated. In these tests, simulated targets were quickly and easily detected using a Fido sensor in conjunction with both direct vapor and swipe sampling methods. The results of these tests suggest that chemical vapor sensing has utility as a means of screening vehicles for explosives at checkpoints and on patrols.

Amplifying Fluorescent Polymer Arrays for Chemical Detection of Explosives

Amplifying fluorescent polymers developed by collaborators at the Massachusetts Institute of Technology (MIT) have been integrated into a handheld sensor platform capable of detecting single femtogram masses of vapour-phase nitroaromatic explosives in real-time. This sensor, known as Fido, was originally developed under the Defense Advanced Research Projects Agency (DARPA) Dog’s Nose program. During field tests against buried landmines, the sensor has demonstrated the ability to detect trace levels of vapours of nitroaromatic explosives emanating from landmines. The sensor utilizes amplification of fluorescence quenching as a transduction mechanism for detection of nitroaromatic explosives and other closely related compounds. Earlier Fido prototypes utilized a single band of amplifying polymer deployed inside a capillary waveguide to form the sensing element of the detector. A new prototype has been developed that incorporates multiple, discrete bands of different amplifying polymers deployed in a linear array inside the capillary. Vapour phase samples are introduced into the sensor as a sharp pulse via a gated inlet. As the vapour pulse is swept through the capillary by flow of a carrier gas (air), the pulse of analyte encounters the bands of polymer sequentially. If the sample contains nitroaromatic explosives, the bands of polymer will respond with a reduction in emission intensity proportional to the mass of analyte in the sample.

Detection of Landmines and Other Explosives with an Ultra-Trace Chemical Detector

The Nomadics Fido sensor was developed to meet the challenges of detecting modern plastic-cased landmines with low metal content. The search for these mines using metal detectors often results in an unmanageable number of false alarms due to detection of shrapnel, debris, and mineralized soil. Trained dogs have been used for many years as an effective means for finding landmines. However, dogs are expensive to purchase and maintain, are inconsistent, difficult to train, tire within a few hours, have problems in areas with many strong competing scents, are unacceptable for use in certain cultures, cannot work under all environmental conditions, and are prone to health problems. In tests to date, the Fido sensor has demonstrated canine-comparable performance.

Investigation of an area reduction method for suspected minefields using an ultrasensitive chemical vapor detector

Detection of landmines by vapor-phase sensing of key chemical signature compounds was first demonstrated in 1998 using a sensor we developed as part of the DARPA Dogs Nose Program. This sensor utilizes novel fluorescent polymers to detect ultra-trace concentrations of nitroaromatic compounds emanating from explosives contained in landmines. Much has been learned about the chemical signature of landmines in recent years. For example, it has been shown that the landmine chemical signature tends to be heterogeneous and can be dispersed in the environment near the mine location. This makes it difficult to pinpoint the exact location of the mine using trace chemical detection methods. However, evidence currently available indicates that it may be possible to isolate a mine location to within a small, well-defined area. Data supporting this conclusion have been obtained using our sensor, and the conclusions drawn have been supported using other accepted laboratory analysis methods. Often, minefields contain relatively few mines. Methods of sampling suitable for rapidly isolating the mined areas from large, mine-free areas are being pursued. High-volume vapor sampling and soil particle sampling strategies are being refined for this application. Preliminary data from field tests using prototype samplers and sensors will be presented.