Synthesis And Properties Of Novel Imide-based Fluorescent Probes For Mercury Ions

Mercury（Hg）,as a highly toxic metal recognized worldwide,can exist in nature in three forms:elemental mercury,inorganic mercury and organic mercury.Due to its persistence,mobility and bioenrichment characteristics,it is easy to enter the human body through the food chain,and gradually accumulate in the human body,causing irreversible damage to the nervous system,endocrine system,liver,kidney and other organs.Thus,the detection and removal of mercury ion(Hg²⁺)is absolutely necessary.Fluorescent probes,as a new type of detection method,have received widespread attention due to their high sensitivity,low cost,simple operation,and the ability to achieve in-site detection.Among the developed mercury ion fluorescence probes,coordination probes have unique advantages over reactive probes in constructing new multifunctional materials for detecting and removing mercury ions.However,the reported coordination probes still have some shortcomings such as low sensitivity,poor selectivity,and complex synthesis.Therefore,it is of theoretical significance and practical application value to design and synthesize coordination fluorescence probes with simple structure and specific recognition of mercury ions.In this paper,the unique coordination structure of Hg²⁺and thymine was simulated,and“imide-Hg²⁺-imide”was used as the recognition strategy of mercury ion.The 1,8-naphthalene imide,which is simple in structure,excellent in spectral performance,stable and easy to modify,was used as the recognition group and fluorescence signal group.Through different structural modifications,three new mercury ion fluorescence probes were successfully constructed.（1）A simple and highly sensitive mercury-ion fluorescent probe NIPBA was synthesized by modifying the benzene ring at 4 sites of 1,8-naphthylimide.The probe is capable of transient,highly selective recognition of mercury ions,and can quantitatively detect mercury ions at concentrations of 0-2.5μM,with limits of detection as low as 6.16 n M.In addition,the probe can be used to detect mercury ions in actual water samples and He La cells.（2）A mercury-ion fluorescence probe NIDEA with good water solubility was synthesized by modifying diethanolamine at 4 sites of 1,8-naphthalimide.Surprisingly,it’s a“turn-on”fluorescence probe of mercury ion.Further experiments confirmed that the introduction of the diethanolamine group not only increased the water solubility of the probe,but also gave the probe the structural property of twisted intramolecular charge transfer（TICT）.The probe molecules,after coordinating with mercury ions,would induce aggregation and exhibit enhanced signal of fluorescence by inhibiting TICT.Therefore,we developed a water-soluble “turn-on” fluorescence probe through a simple structure modification of 1,8-naphthalene imide dye,which can inhibit TICT by mercury ion induction,resulting in aggregation induced emission enhancement（AIEE）.In addition,using this probe can not only achieve quantitative detection of mercury ions through the Tyndall effect,but also detect mercury ions in actual water samples and cells.（3）Based on the excellent performance of the second probe,we modified morpholine at position 4 of 1,8-naphthalene imide,optimized the structure of the second probe,and synthesized a highly sensitive “turn-on” probe NIML for mercury ions.Compared with the second probe NIDEA,probe NIML has a faster and more sensitive response to mercury ions,with a detection limit as low as 8.8 n M.This probe can not only detect mercury ions in actual water samples,but also detect mercury ions in cells and zebrafish.In addition,the successful design of the probe further verifies the recognition mechanism of the second probe.In a nutshell,we successfully designed and synthesized three highly selective coordination mercury ion fluorescence probes.Although the three probes all adopted the recognition strategy of “imide-Hg2+-imide”,the fluorescence signal after coordination with mercury ion can be regulated by different structural modifications on the 4 position of1,8-naphthalene imide.To the best of our knowledge,this is the first time that naphthalimide with TICT structure has been found to exhibit AIEE phenomenon,which lays a theoretical foundation for the construction of a new water-soluble AIEE fluorescent dye.