Sensing of chemical warfare agents is a topic of current and growing interest. Chemical agents can be detected or signaled by changes in photoinduced properties, such as absorption (color), emission intensity (brightness), and luminescence wavelength (color of emission). Quenching of photoluminescence intensity is of particular interest since sensitivity is inherently enhanced because of a distinct contrast between signaling events (i.e., luminescent and nonluminescent states). Researchers have utilized photoinduced electron transfer (PET), energy transfer, and other methods to produce on/off sensors based on aromatic and polycyclic aromatic hydrocarbons, aromatic heterocycles, and transition metal complexes. Each approach targets specific sensing applications and maintains distinct advantages and disadvantages that warrant continued investigations.
Representation of sensing behavior. Top: ABI-NH2 in neutral and acidic environments. PEG, polyethylene glycol. Bottom: Off-on luminescence sensing.
At the NASA Glenn Research Center, our approach to chemical sensors is based on creating new luminescent species that can be adapted to respond to generic or specific target molecules. Specifically, an aminofunctionalized anthracene bisimide (ABI-NH2) (ref. 1) was synthesized and evaluated as a sensory material for highly reactive chemical species. Anthracene was utilized as the luminescent core because of its relative abundance (i.e., low cost) and depth of synthetic manipulation. Anthracene also possesses favorable photophysical properties, such as strong absorption and high quantum yield. Limitations include photoinduced cycloaddition reactions, including dimerization under concentrated conditions and formation of peroxides in the presence of oxygen.
ABI-NH2 was prepared using standard and advanced synthetic methodology, and its chemosensory behavior was studied (ref. 2). Photophysical evaluation of ABI-NO2, the precursor and model for ABI-NH2, in N, N-dimethylformamide (DMF) revealed unique properties, in comparison to anthracene, including visible absorption, green fluorescence, and photostability in the presence of oxygen. Unlike the nitrosubstituted compound, emission from ABI-NH2 was quenched via efficient intramolecular photoinduced electron transfer (PET) from the amine substituents.
Titration data. The complete curve represents three independent thionyl chloride experiments with varying initial concentrations of thionyl chloride. Inset: Selected emission spectra recorded with 425-nm excitation.
Long description of figure 2.
Reaction of ABI-NH2 with common acid halides, such as thionyl chloride and acetyl chloride, resulted in dramatic fluorescence enhancement. Acids, such as hydrochloric acid or trifluoroacetic acid, also turned “on” the luminescence of ABI-NH2 by a reversible dequenching process. In addition, acid halides of organophosphates, such as methylphosphonic dichloride and dimethylphosphinic chloride, initiated the luminescence of ABI-NH2. These analytes were chosen to mimic the reactivity of organophosphate-based nerve gases (e.g., sarin gas). These data suggest that ABI-NH2 could be applied for the early detection of toxic chemicals.
Emission spectra. ABI–NH2 in the presence of organophosphate-based substances similar to nerve gases. PL, photoluminescence.
Long description of figure 3.
Last updated: July 19, 2005 3:15 PM
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