Highly selective and sensitive turn-on fluorescent probes for sensing Hg2+ ions in mixed aqueous solution

A highly selective and sensitive probe 1b was successfully developed for detecting Hg2+ ions in mixed aqueous medium with a turn on fluorescence mode, via a series of structural optimization of probes. These probes could be easily synthesized by one-step condensation reactions between o-phenylenediamine or 2, 3-diaminonaphthalene and o (salicylaldehyde), m, or p-hydroxybenzaldehyde. Introducing a certain amount of Hg2+ ions the fluorescence of probes bearing one or two salicylaldehyde moieties including probes 1a, 1b, and 4 could be turned on. The fluorescent response is likely due to the well matching between Hg2+ ions size and the geometric structure of probes, resulting in that probes bearing salicylaldehyde moiety are mostly favorable for binding with Hg2+ ions by way of 1:1 molar ratio. These bindings likely facilitate excited state internal proton transfer (ESIPT) or ESIPT coupled aggregation-induced enhancement of emission (AIEE) process. Considering the sensitivity and the potential of practical application, probe 1b was chosen to establish a fluorescent enhancement sensing system towards Hg2+ ions over other competing metal ions, with excellent limit of detection of 12.5 ppb.