Fluorescent nitroaromatic

Key words TNT, explosives, sUole, germole, polysilane, pofygermole, polysilole, sensors, luminescence, fluorescence, nitroaromatics, quenching, inorganic polymers... 

Under the chosen conditions aromatic compounds are nitrated to nitroaromatics [1]. The detection of rotenone [1] (see below) depends on the reduction of silver ions, incorporated into the layer, to metallic silver in the presence of ammonia [4]. The mechanism of the reaction of many substances leading to fluorescent derivatives has not yet been elucidated [2],... 

Brief exposure to nitrous fumes (up to 3 min) leaves the fluorescent power of the acid-instable fluorescence indicator 254. incorporated into most TLC layers, largely unaffected, so that the nitroaromatics so formed can be detected as dark zones on a green fluorescent background [1]. For purposes of in situ quantitation it is recommended that the fluorescence indicator be destroyed by 10 min exposure to nitrous fumes in order to avoid difficulties in the subsequent evaluation [1]. 

An important demonstrated application of this artificial nose system is the high-speed detection of low levels of explosives and explosive-like vapors. Several sensors, based on Nile Red attached to silica microspheres, show high sensitivity to nitroaromatic compounds (NAC) within a mixture12. Different fluorescence response profiles were observed for several NAC s, such as 1,3,5-trinitrotoluene (TNT) and 1,3-dinitrobenzene (DNB), despite their similar structures. These responses were monitored at low concentrations of the NAC vapors (ca. 5 ppb) and at short vapor exposure... 

Although many light-induced reactions of nitro compounds are known, attempts to elucidate the multiplicity of the reacting excited state have been made in only a fraction of cases. Conclusive information is not easily obtained since in general nitroaromatics suffer from the disadvantage that they show very weak or even no fluorescence and have very short triplet lifetimes. 

The selectivity inherent to TNT detection by amplified fluorescent polymers, as described in Section 4, helps to rriinirriize false-positives in land mine detection. These sensor devices respond only to nitroaromatics and similarly small, electron-deficient analytes, which are found typically only in or close to explosives and explosive devices. Field-tests to date have demonstrated that these devices are at least as reliable as trained dogs in detecting explosives that contain nitroaromatics. There is still uncertainty concerning what chemical that dogs actually detect when searching for explosives [17]. This... 

The SeaDog sensor utilized in this work is capable of near real-time detection of low concentrations of explosives in water. The sensor utilizes novel sensing materials originally developed by collaborators at MIT. These materials are fluorescent polymers that are highly emissive when deployed as solid-state thin films. When the polymers interact with nitroaromatic explosives such as TNT, the fluorescence is quenched [3-5], The response of these materials to target analytes... 

Fisher, M. and C. Cumming. Detection of trace concentrations of vapor phase nitroaromatic explosives by fluorescence quenching of novel polymer materials, in Proceedings of 7th International Symposium on the Analysis and Detection of Explosives, Defense Evaluation and Research Agency, Edinburgh, Scotland, UK, June, 2001. 

Figure 9.1 Two vials containing the amplifying fluorescent polymer (AFP) that has been developed to detect nitroaromatic explosives. They fluoresce when excited by light of a given wavelength as shown by the vial on the right. The vial on the left contains trace amounts of TNT, which has quenched the fluorescence.

Fisher, M. and C. Cumming. Utilization of novel fluorescent polymer materials for trace level vapor-phase detection of nitroaromatic explosives, in Proceedings of the U.S. Federal Aviation Administration s Third International Aviation Security Technology Symposium, Atlantic City, NJ, November 27-30, 2001c. 

Shinkai s 38 is also a PET system whose fluorescence is controlled by Na" binding to a coordinatively active spacer which is a calix[4]aiene tetraester in this case. However, the through-space distance between the photoactive termini is expanded by Na complexation, thus reducing the PET efficiency. Kuhn s 39 is not dissimilar in that a PET-type quencher (a nitroaromatic unit) is held away from the lumophore by Ca binding. However a conventional lumophore-spacer-receptor is also contained within 39 as found in 24. At this point it would not be out of place to mention several important studies on the control of PET/EET by ion binding to a coordinatively active spacer between photoactive terminii." " System 36 is structurally related to Verhoewen s 40 since they both contain an aromatic lumophore and an aromatic amine with one or more interposed aliphatic amines. System 40 also displays the functional similarities that PET processes were... 

In another approach, a fluorescent conjugated polymer was used as the material for the preparation of a chemosensor to detect 2,4,6-trinitrotoluene (TNT) and its related nitroaromatic compounds. To this end, microparticles, made of three-dimensionally cross-linked poly(l,4-phenylene vinylene) (PPV) via emulsion polymerization, were synthesized [61]. This material was chosen due to its high fluorescence intensity and sensitivity to changes in its microenvironment. The chemosensor was exposed to vapour containing different amounts of TNT and quenching of the polymer luminescence at 560 nm was observed after excitation at 430 nm. The dependence of the fluorescence signal in response to the analyte was described by a modified Stem-Volmer equation that assumes the existence of two different cavity types. The authors proposed the modified Stem-Volmer equation as follows ... 

Simonson et al. [148] demonstrated remote detection of explosives in soil by combining distributed sensor particles with UV/vis fluorescence LIDAR technology. The key to this approach is that the fluorescence emission spectrum of the distributed particles is strongly affected by absorption of nitroaromatic explosives from the surrounding environment. Remote sensing of the fluorescence quenching by TNT or DNT is achieved by fluorescence LIDAR - the emission spectra were excited in field LIDAR measurements by a frequency-tripled Nd YAG laser at 355 nm and the fluorescence collected with a telescope and various detector systems housed in a 10 x 50 trailer. TNT has been detected in the ppm range at a standoff distance of 0.5 km with this system (Fig. 16). An important limitation to this technique is the pre-concentration of the explosives on the sensor particles, which requires the presence of water to facilitate the transport of the explosive from the surface of the soil particles to the sensor particles. 

Figure 2. Highly stable sensor polymer designed to bind and detect TNT and DNT. The rigid three dimensional structure provides for interchain spacing that prevents fluorescence quenching and allows for the formation of galleries capable of binding nitroaromatics.

Organic polymers and optical fibres [17] have been previously used to detect vapours of explosive analytes [18,19]. The transduction methods include absorption, fluorescence, conductivity, etc [16]. Such simple techniques are promising, because they can be incorporated into inexpensive and portable microelectronic devices. For example, a chemically selective silicone polymer layer on a SAW (surface acoustic wave) device has been shown to provide efficient detection for the nitroaromatic compounds [20]. The fluorescence of pentiptycene conjugated polymers [21,22] and... 

FLUORESCENCE QUENCHING STUDIES WITH NITROAROMATIC ANALYTES... 

M. laGrone, C. Cumming, M. Fisher, D. Reust and R. Taylor, Landmine detection by chemical signature detection of vapors of nitroaromatic compounds by fluorescence quenching of novel polymer materials, Proc. SPIE, Detection and Remediation Technologies for Mines and Minelike Targets IV, 3710 (1999) 409-420. 

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