Aggregation induced emission enhancement behavior of conformationally rigid pyreneamide-based probe for ultra-trace detection of picric acid (PA)

Abstract

In this study, we designed and prepared pyrene-based dipodal probes with a range of molecular flexibility for picric acid (PA) detection. These probes commonly contain a central rigid core, but probe 1 contains relatively short linkers/spacers compared to probes 2 and 3. The dipodal probes produced dominant pyrene excimer fluorescence in an aqueous solution, and aggregation-induced emission enhancement (AIEE) was observed. The fluorescence emission of probe 1 was significantly quenched only by the presence of PA, while probes 2 and 3 were responsive to 4-nitrophenol or 2,4-dinitrophenol as well as PA. Of the three dipodal probes, the highest binding strength was observed for probe 1 with PA, which gave an association constant of Ka = 3.96 × 107 M−1.1H NMR measurements and DFT calculations indicate that, upon addition of PA, these dipodal probes formed individual probe-PA complexes via π–π interactions between the pyrene ring and PA in an aqueous solution. This complex formation enabled energy transfer from electron-rich pyrene to electron-poor PA upon excitation, leading to fluorescence quenching. Probe 1 provided ultra-trace detection of PA in water, with observed and calculated limit of detections (LOD) of 1 nM and 0.13 pM, respectively, one of the lowest values reported. Effective fluorescence quenching was also obtained using probe 1-coated filter paper or TLC plate with PA solution/vapor, suggesting the significant potential of this sensing system in real-time applications. These studies were supported by FE-SEM, TEM, and UV-visible, fluorescence, NMR and DFT calculations.