Pyrene-appended imidazolium probe for 2,4,6-trinitrophenol in water

Pyrene-appended imidazolium salts (1-3) were synthesized for sensing 2,4,6-trinitrophenol (TNP); the salts selectively and sensitively detected TNP in a pH 7.4 PBS-DMSO (99.5:0.5 v/v) solution. The binding of TNP to probe 3 completely quenched its green excimer emission because of the loss of π-π interactions between the pyrene rings and energy transfer from electron-rich pyrene to electron-deficient TNP. Probe 3 showed the highest association constant with TNP (K_a = 8.34 × 10⁶ M^-1) in a 1:2 stoichiometry; the limit of detection (LOD) of TNP in a pH 7.4 PBS-DMSO (99.5:0.5 v/v) solution was 10 nM. Probe 3 also detected TNP in tap water and river water with an LOD of 10 nM and a good association constant; the calculated LOD was 0.067 ppb. Filter paper strips coated with 3 detected TNP in an aqueous solution. The interactions of TNP with probes 1-3 were investigated by UV-vis, fluorescence, and ¹H NMR spectroscopies as well as density functional theory calculations.