Covalently Modified Chitin with Quercetin as Fluorescent and Sustainable Chemosensor for Zirconium(IV) Ion Detection

This study reports the development of a sustainable solid-state fluorescent chemosensor for selective detection of zirconium ions (Zr4+) by covalently grafting quercetin onto chitin via a Mannich reaction. The chitin-quercetin (Ch-Q) film exhibits a distinct “turn-on” yellow-green fluorescence response at 540 nm upon Zr4+ binding, attributed to chelation-enhanced fluorescence (CHEF) and the suppression of excited-state intramolecular proton transfer (ESIPT). The sensor exhibits exceptional selectivity for Zr4+ over competing metal ions, along with a low detection limit (1.157 μM), rapid response (<60 s), and broad operational pH range (2–8). Density functional theory (DFT) calculations confirm the optimal Zr4+ binding site and elucidate the underlying ligand-to-metal charge transfer (LMCT) mechanism. This biomass-derived platform offers a simple, sustainable, and eco-friendly solution for real-time Zr4+ monitoring in environmental applications.