Rational design of a water-soluble TICT-AIEE-active fluorescent probe for mercury ion detection

Considering that mercury ions (Hg²⁺) have long been a threat to human health and the environment due to their persistence, mobility and bioenrichment, the detection and removal of Hg²⁺ is of great significance. Therefore, a simple water-soluble naphthalimide derived fluorescent dye with AIEE characteristics was reasonably constructed based on twisted intramolecular charge transfer (TICT) mechanism, a series of probes were synthesized to demonstrate this mechanism. The probe NIDEA (naphthalimide-diethanolamine) bonding Hg²⁺ through the specific combination of the N-unsubstituted naphthalimide group and Hg²⁺ to form a classic "imide-Hg-imide" structure. Moreover, the introduction of diethanolamine moiety enhanced the water-solubility of the probe, and also made the dye molecule possess the feature of AIEE. The fluorescence titration experiment showed that there were two good linear relationships between the fluorescence intensity of the probe NIDEA and the concentration of Hg²⁺ in the range of 0-2.5 μM and 2.5-7.5 μM, and the limit of detection was 46.7 nM. Also, the probe could detect Hg²⁺ in aqueous solution sensitively, ensuring its application in the actual water sample in the environment and living cells. At the same time, NIDEA can be used to detect Hg²⁺ by Tyndall effect (TE) without limitation of instrument and technology, the limit of detection was 20.9 nM. Furthermore, this paradigm by introduction of the highly effective TICT structure supports a promising methodology for the construction of simple water-soluble AIE/AIEE-active probes.