The Fido explosives detector is a battery-powered, handheld sensory device that uses amplifying fluorescent polymer (AFP) materials to detect trace levels of high explosives like trinitrotoluene (TNT). It was developed by Nomadics, a subsidiary of ICX Technologies (now owned by FLIR Systems), in the early 2000s as part of the Defense Advanced Research Projects Agency's (DARPA) Dog's Nose program. The Fido explosives detector is considered the first artificial nose capable of detecting landmines in the real world. The device was named after its ability to detect explosive vapors at concentrations of parts per quadrillion (1 in 10^15), which is comparable to the sensitivity of a bomb-sniffing dog’s nose, i.e. the historical “gold standard” for finding concealed explosives.
Overview
The Fido explosives detector functions as a trace explosives detector through its use of a specially-made fluorescent polymer film that is extremely sensitive to molecules of TNT, which can be found in more than 85 percent of deployed landmines.
The thin film consists of many repeating chains of amplifying fluorescent polymers that naturally emit visible light when exposed to ultraviolet rays. The fluorescence is the result of excited state electrons (i.e. excitons) travelling down the polymer backbone and between adjacent polymer chains upon absorbing a photon of light. However, the fluorescence reactions become quenched the moment an electron-deficient molecule such as TNT binds to the polymer and traps the migrating exciton at the binding site. A single molecule of TNT is capable of diminishing the fluorescence of entire polymer chains in the thin film, thereby amplifying the effect of a single TNT binding event that may bypass the detection of less-sensitive molecular sensors. The design of the thin film allows the TNT molecules to bind anywhere along the polymer chain, drastically increasing the number of opportunities for the TNT binding event to occur. The amplifying fluorescent polymers used in the Fido explosives detector were engineered to be preferentially responsive to nitroaromatic explosives. The binding of the TNT molecules is believed to be caused by an electrostatic-type interaction between the polymer and the target analyte. Selectivity can also be improved by synthesizing structures into the polymer that are electrostatic mirror images of the desired target analytes. According to reports, there is evidence that the polymers can amplify the quenching of the fluorescence reactions between 100- and 1000-fold compared to conventional quenching mechanisms.
The fluorescent polymer film is coated on the interior surface of the tiny glass tubes that the Fido explosives detector use to draw in air. A blue light-emitting diode (LED) inside the detector serves to excite fluorescent polymer electrons, and the detector's photomultiplier tubes amplifies and reads the wavelength of the emitted light in order to discern whether the light produced by the polymer film had dimmed. The Fido explosives detector provides near real-time, almost instantaneous analysis of the sampled air by registering the intensity of the photomultiplier tubes, which is inversely proportional to the mass of analyte that binds to the polymer films. The polymer films can be exposed repeatedly to samples due to the reversible nature of the binding of the analytes to the film. The Fido explosives detector can return the fluorescence intensity of the polymer films to near the initial baseline reading by drawing in a new flow of clean air to sweep over the polymer film and desorb the analyte.
History
The invention of the Fido explosives detector relied heavily on the invention of the amplifying fluorescent polymer (AFP) in the late 1990s. At the time, the use of fluorescent polymers in their solid state was difficult due to their significantly decreased sensitivity and fluorescence compared to polymers in solution. In 1995, chemist Timothy Swager at the Massachusetts Institute of Technology (MIT) was the first to demonstrate the possibility of sensory signal amplification using the molecular wire approach. This technique was later used by Swager to develop AFPs for the first time. Soon afterwards, Swager was awarded a U.S. military research grant to incorporate AFPs into anti-mine technology as part of DARPA’s Unexploded Ordnance Detection Program, which was also more informally known as the Dog's Nose program. He licensed the AFP technology to Nomadics and worked with the company to create a prototype of the fluorescent polymers that could be used for explosives detection.