Abstract
The illegal use of explosives by terrorists and other criminals is an increasing issue in public spaces, such as airports, railway stations, highways, sports venues, theaters, and other large buildings. Security in these environments can be achieved by different means, including the installation of scanners and other analytical devices to detect ultra-small traces of explosives in a very short time-frame to be able to take action as early as possible to prevent the detonation of such devices. Unfortunately, an ideal explosive detection system still does not exist, which means that a compromise is needed in practice. Most detection devices lack the extreme analytical sensitivity, which is nevertheless necessary due to the low vapor pressure of nearly all explosives. In addition, the rate of false positives needs to be virtually zero, which is also very difficult to achieve. Here we present an immunosensor system based on kinetic competition, which is known to be very fast and may even overcome affinity limitation, which impairs the performance of many traditional competitive assays. This immunosensor consists of a monolithic glass column with a vast excess of immobilized hapten, which traps the fluorescently labeled antibody as long as no explosive is present. In the case of the explosive 2,4,6-trinitrotoluene (TNT), some binding sites of the antibody will be blocked, which leads to an immediate breakthrough of the labeled protein, detectable by highly sensitive laser-induced fluorescence with the help of a Peltier-cooled complementary metal-oxide-semiconductor (CMOS) camera. Liquid handling is performed with high-precision syringe pumps and chip-based mixing-devices and flow-cells. The system achieved limits of detection of 1 pM (1 ppt) of the fluorescent label and around 100 pM (20 ppt) of TNT. The total assay time is less than 8 min. A cross-reactivity test with 5000 pM solutions showed no signal by pentaerythritol tetranitrate (PETN), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). This immunosensor belongs to the most sensitive and fastest detectors for TNT with no significant cross-reactivity by non-related compounds. The consumption of the labeled antibody is surprisingly low: 1 mg of the reagent would be sufficient for more than one year of continuous biosensor operation.
Highlights
The fast and extremely sensitive detection of explosives [1,2,3] is one of the most relevant tasks to guarantee security in areas of public access
Considering the high cost of a trained animal with its handler and the inability to use them for extended missions, it becomes clear that an automated sensor system would be highly desirable
The excitation laser has a center wavelength of 638 nm and an fullwas width half maximum (FWHM) of has proven to be3 beneficial
Summary
The fast and extremely sensitive detection of explosives [1,2,3] is one of the most relevant tasks to guarantee security in areas of public access. X-ray-based scanners are used in most airports, which may detect larger amounts of explosives. For trace analysis, additional wipe tests may be necessary. The ultimate explosive detector is still the dog, which is requested in most critical situations. Considering the high cost of a trained animal with its handler and the inability to use them for extended missions, it becomes clear that an automated sensor system would be highly desirable. No sensor system can compete with dogs and other animals, yet. The most powerful instrumental analysis systems are not mobile and are limited to a laboratory environment
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