Fluorescent Probe Based on Multiresponsive Amphiphilic Copolymers for Mercury Ion Detection in Real Water Samples and Bioimaging in Living Cells

A novel multistimuli-responsive fluorescence sensor for Hg2+, P(NIPAM-co-ATU), is synthesized through copolymerization of thermoresponsive N-isopropylacrylamide (NIPAM) as the actuator unit and aggregation-induced emission (AIE)-active tetraphenylethene-5-methyluridine (ATU) as the Hg2+ recognition sensor. Owing to the formation of ATU-Hg2+-ATU metallo-base pairs, the copolymer exhibits high selectivity and affinity toward Hg2+. Through the synergistic effect of ATU-Hg2+-ATU complexation and the phase transition of PNIPAM chains, P(NIPAM-co-ATU) switches from a weak fluorescence state to a significantly enhanced fluorescence emission upon binding of Hg2+ in the environment, thus enabling specific recognition and detection. The chemical structure and stimuli-responsive properties of P(NIPAM-co-ATU) are thoroughly investigated. The copolymer containing 2.2 mol % ATU (PNA-2) exhibits good solubility in both organic solvents and aqueous solutions. As a fluorescence-enhanced probe, PNA-2 exhibits high selectivity and sensitivity toward Hg2+ over other metal ions, even when the other ions are present at 5-fold higher concentrations. It also achieves a low detection limit for trace Hg2+ (LOD = 0.87 nM). The fluorescence enhancement mechanism, attributed to the formation of ATU-Hg2+-ATU metallo-base pairs, is further investigated by using dynamic light scattering (DLS) and theoretical calculations. Moreover, due to its good biocompatibility and low cytotoxicity, PNA-2 demonstrates the ability for detecting Hg2+ in real water samples and tracking Hg2+ in living cells by bioimaging.