Bipyridine-Functionalized Covalent Organic Polymer for Highly Sensitive Turn-On Fluorescent Detection of Zn2+ in Pure Water
Pornthip Piyanuch, Mekin Chuethaisong, Mintada Kongnondang, Anyanee Kamkaew, Supinya Nijpanich, Bunyarat Rungtaweevoranit, Weerakanya Maneeprakorn, Watchara SrimontreeAbstract
A bipyridine-functionalized covalent organic polymer (Bpy-COP) was synthesized as a fluorescent sensor for Zn2+ detection in pure water. Structural characterization confirmed the formation of an imine-linked amorphous polymer incorporating accessible bipyridine coordination sites within a conjugated framework. Upon Zn2+ coordination, Bpy-COP exhibited a pronounced fluorescence turn-on response, consistent with the combined effects of photoinduced electron transfer (PET) suppression and chelation-enhanced fluorescence (CHEF). DFT and TD-DFT calculations further supported the proposed PET suppression mechanism by indicating the thermodynamic feasibility of PET in the metal-free sensor and its suppression upon Zn2+ coordination. The sensor displayed a detection limit of 10 nM (0.67 ppb), a response-time of 30 s, and high selectivity toward Zn2+ over a broad range of competing metal ions. Unlike many previously reported fluorescent probes, Bpy-COP operates directly in pure water without the need for organic cosolvents. The sensor also exhibited reversible fluorescence switching over five Zn2+/EDTA cycles and accurately determined Zn2+ in drinking water, tap water, and river water, with recoveries of 97–105% and relative standard deviations below 2%. These results demonstrate that Bpy-COP provides a recyclable platform for selective Zn2+ sensing under fully aqueous conditions.