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Design and optimization of polymeric silole and boronate photoluminescent probes for improved explosives detection applications

  • Author(s): Sanchez, Jason C.
  • et al.
Abstract

Hydrosilylation of silole and silafluorene fluorophores is an effective polymerization tool for the synthesis of [Sigma]*-[Pi]/[Pi]* delocalized structures. Improvements in molecular weights (4000-20 000), reaction conditions (0 -50 ⁰C), and functional group variety were obtained for 1, 1-polysilole and 1,1-polysilafluorene materials. The polymers were screened for their ability to detect organic based explosives by a fluorescence quenching mechanism. The explosives screened include the nitroaromatic-based explosives TNT, DNT, PA, and Tetryl, the nitramine-based explosives RDX and HMX, the nitrate ester-based explosives PETN, TNG, and EGDN, and the organic peroxide-based explosives TATP and HMTD. Explosive particulates were targeted to increase the sensitivity of detection for low volatility explosives. Detection limits as low as 1 pg cm⁻² were obtained by the optimization of the polymers properties. Sensitivity is gained by tuning the frontier molecular orbital band gap to better match the LUMO energies of the explosive analytes. The excited state electron transfer process is also improved by Lewis acid/ base interactions between the nitro groups of the explosives and the silacycle core of the silole and silafluorene fluorophores. Sensors target the specific chemical functionalities of nitramine and nitrate ester based explosives for fluorescence imaging. Nitramine and nitrate ester based explosives can be detected selectively through the release of nitrite by an [Alpha]-hydride abstraction and a subsequent acid-catalyzed triazotization with 2,3-diaminaphthalene. The naphthotriazole product is visibly fluorescent as a thin-film providing a turn-on fluorescence signal. Nitrate ester-based explosives can be specifically targeted using a turn-on fluorescence response from the oxo-anion specific oxidation of unprotected fluorene units in polysilafluorene- divinylenefluorene. Both turn-on fluorescence sensors can be used in tandem with the fluorescence quenching sensors to provide selective explosives detection assays based on a fluorescence signal response. Organic peroxide based explosives are targeted through a selective oxidative depolymerization of a boronate-based polymer. The polymer is synthesized by a double transesterification polymerization driven by formation of a stable six-member bis-boronate polymer with fluoran repeat units in the backbone. After exposure to hydrogen peroxide, which can be formed from the UV decomposition of TATP and HMTD, the polymer undergoes a selective oxidative deprotection of the boronate functionalities forming a highly luminescent fluorescein. Detection limits of 3 ppb over an 8 h period are achieved through observation of the turn-on fluorescence response

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