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 Cd 2+ turn-off-turn-on response and subsequent anion recognition. Spectroscopic measurements were performed in DMSO solution (excitation = 350 nm, emission = 411 nm). The Cd 2+ turn-off mechanism relies on the Cd 2+ 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 1 H NMR and ultraviolet–visible (UV–vis) spectroscopy. The limit of detection (LOD) for Cd 2+ 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 H 2 PO 4 – . This anion recognition mechanism operates by displacing the bound Cd 2+ ion. The most effective anions showed LOD values of 12.26 nM for ATP, 18.16 nM for HS –, and 13.41 nM for H 2 PO 4 – . The probes demonstrated clear on–off-on fluorescence cycles in solution and on test strips. The systems also enabled Cd 2+ detection in artificial saliva. These results highlight the potential of this sensor for environmental Cd 2+ monitoring and for detecting clinically relevant anion biomarkers.