A Photoresponsive Azo-Functionalized 2,4-Dinitrophenylhydrazone Polymer for Selective Fluoride Sensing in Environmental and Biological Media

Fluoride (F-) plays an essential role in dental health and osteoporosis treatment; however, excessive intake can lead to severe disorders, including dental fluorosis and urolithiasis, necessitating sensitive monitoring in environmental and biological systems. Herein, we report a photoresponsive azo-2,4-dinitrophenylhydrazone (AZO-DNP) platform comprising monomeric and polymeric architectures for selective fluoride detection. Both systems enable rapid naked-eye recognition through an intense yellow-to-purple color transition, accompanied by a pronounced bathochromic shift and ratiometric UV-vis response upon fluoride binding. The polymeric analog, Poly(AZO-DNP), was synthesized via CuAAC click functionalization followed by ring-opening metathesis polymerization of a norbornene precursor using Grubbs third-generation catalyst. The monomeric and polymeric probes exhibit ultralow detection limits of 21.37 and 31.81 nM, respectively, surpassing most reported colorimetric fluoride sensors. Mechanistic studies using 1H NMR confirm fluoride-induced NH deprotonation with HF formation, while density functional theory calculations reveal a reduced HOMO-LUMO energy gap (0.32 to 0.12 a.u.), consistent with enhanced intramolecular charge transfer. The polymeric probe shows low cytotoxicity toward A549 cells at working concentrations in MTT assays and enables intracellular ratiometric fluoride imaging. Immobilization on filter paper affords a portable test strip for on-site fluoride detection. Notably, UV irradiation (370 nm, 30 min) converts the monomer into a cis-rich, fluoride-inactive OFF state, whereas Poly(AZO-DNP) retains >95% trans configuration and full sensing capability due to its rigid polymer backbone. These results establish AZO-DNP as a versatile photoresponsive platform for fluoride sensing across environmental and biological applications.