Organic-Inorganic Hybrid Mesoporous Materials as Regenerable Sensing Systems for the Recognition of Nitroaromatic Explosives

Abstract

Fluorescent organic-inorganic mesoporous hybrid materials have been prepared and characterised, and their behaviour against nitroaromatic explosives have been tested. MCM-41 silica was used as an inorganic scaffold and pyrene (P derivative containing trialkoxysilane moieties), dansyl and fluorescein (D and F derivatives also containing trialkoxysilane groups, respectively) fluorophores have been anchored on hybrid materials by a co-condensation method to obtain a homogenous distribution of dyes into the pores of the support. Six sensing materials have been prepared, of which SP, SD, SF were hydrophilic and SPh, SDh, SFh were hydrophobic. Template-free hydrophilic materials (SP, SD, SF) were obtained after repeated NH₄ NO₃ /ethanol extractions under temperature from as-synthesised (MP, MD and MF supports, respectively) solids. Hydrophobic materials (SPh, SDh, SFh) were prepared by using excess 1,1,1,3,3,3-hexamethyldisilazane with template-free hydrophilic (SP, SD and SF) materials. The six final materials displayed the typical emission bands of the grafted fluorophores. In particular, SP and SPh show the typical pyrene monomer (370-420 nm) and excimer (430-600 nm) emissions. SD and SDh exhibit the broad dansyl fluorescence band in the 450-600 nm range, whereas solids SF and SFh present sharp fluorescein emission centred at 525 nm. The fluorescent behaviour of the six final materials was tested in the presence of explosives (pentaerythritol tetranitrate (PETN), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), nitrobenzene (NB), 1,3,5-trinitrobenzene (TNB), 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrophenylmethylnitramine (Tetryl) and picric acid (PA)). Only nitroaromatic compounds were able to induce emission quenching. As a general trend, the quenching degree depended on the nature of the final material. The best response was obtained with explosives PA and Tetryl, which were able to significantly quench the emission of the sensing supports. The observed quenching was ascribed to the π-π stacking interactions between the electron-donor fluorophores and the electron-withdrawing nitroaromatic explosives. When using SPh for Tetryl and PA, the limits of detection were 8.5 and 1.4 ppm, respectively, whereas they were 14.4 and 1.2 ppm for SDh. Principal component analysis algorithms were applied to the fluorescence measurements taken with the six hybrid materials and the seven explosives. The obtained score plot showed well-defined clusters for the seven explosives tested. Finally, solid SDh was applied to detect trace amounts of Tetryl in soil samples with good results.

Keywords: fluorogenic recognition; fluorophores; hybrid materials; mesoporous materials; nitroaromatic explosives.