Revealing aggregation-induced emission effect of imidazolium derivatives and application for detection of Hg2+

Despite being highly emissive in solution, aggregation of 4-(4,5-bis(4-methoxyphenyl)-1H- imidazole-2-yl) benzaldehyde (BMI) molecules typically results in the quenching of fluorescence. To overcome the shortcomings of aggregation-caused quenching (ACQ), the substituent of imidazole in the nitrogen atom of the BMI have been found as a conformation function group (CFG) to turn the aggregation-induced emission (AIE) effect. The introduction of CFG not only causes the restriction of intramolecular rotations (RIR) effect, but also attenuates the coplanarity of the molecule. As a result, the BMI with ACQ effect is transformed into BMIs with AIE effect. As the steric hindrance of the CFG increases, the AIE characteristic of the derivative also becomes apparent. With the assistance of the thioacetal unit, BMIBD can act as an outstanding probe for the detection of Hg2+ with high sensitivity and selectivity. A series of characterizations were implemented to prove the unique response mechanism of BMIBD toward mercury ions, including optical behavior investigation, mass analysis and 1H NMR studies. Further, the detection limit is low up to 36 nM. Taking advantages of excellent optical properties of this AIE probe BMIBD, point-of-care testing (POCT) for Hg2+ detection was further investigated. Meanwhile, BMIBD presented the desirable analytical property for the real water samples. Additionally, cellular imaging experiment revealed that the probe has an excellent biocompatibility that could be applied for tracking Hg2+ in living cells.