Selective Fluorescent Sensor Based on Coumarin–Cysteine-Polynorbornene for Sequential Cd ²⁺ and Anion Detection

We report fluorescent, amino acid–based polymeric chemosensors, Poly-Cou-Cys, which were synthesized via ROMP using coumarin (Cou)-cysteine (Cys)-functionalized norbornene monomers. The resulting materials exhibit a selective Cd2+ turn-off-turn-on response and subsequent anion recognition. Spectroscopic measurements were performed in DMSO solution (excitation = 350 nm, emission = 411 nm). The Cd2+ turn-off mechanism relies on the Cd2+ ion preferentially coordinating with the polymer’s soft sulfur donor atoms, along with other heteroatoms of coumarin and cysteine, leading to structural rigidification and fluorescence quenching. This mechanism was supported by 1H NMR and ultraviolet–visible (UV–vis) spectroscopy. The limit of detection (LOD) for Cd2+ was 55.10 nM (Polymer) and 41.94 nM (MWCNTs composite). The signal was restored (turn-on response) by biologically relevant anions: ATP, HS–, and H2PO4–. This anion recognition mechanism operates by displacing the bound Cd2+ ion. The most effective anions showed LOD values of 12.26 nM for ATP, 18.16 nM for HS–, and 13.41 nM for H2PO4–. The probes demonstrated clear on–off-on fluorescence cycles in solution and on test strips. The systems also enabled Cd2+ detection in artificial saliva. These results highlight the potential of this sensor for environmental Cd2+ monitoring and for detecting clinically relevant anion biomarkers.