A dual emission ratiometric fluorescent sensor for metronidazole (MNZ) detection is presented, constructed from a zinc metal-organic framework (Zn MOF) and a carbon dot doped molecularly imprinted polymer (CD@MIP) embedded within a polyvinyl alcohol (PVA) hydrogel. The sensor combines the molecular selectivity of imprinting with the intrinsic self-calibration of ratiometric fluorescence to enable robust quantification. Upon MNZ binding, the Zn MOF emission at 443 nm is quenched through an inner filter effect, while the CD emission at 668 nm is concomitantly enhanced. This reciprocal modulation yields a reliable ratiometric signal (I668/I443), providing a linear detection range of 10–500 µM and a limit of detection of 1 µM. The composite demonstrates high selectivity, excellent photostability, and strong analytical performance in river water and pharmaceutical samples, achieving recoveries of 99.9–108%. Cytotoxicity assays in HT 29 and HEK293 cells confirm favorable biocompatibility. Overall, this hydrogel based ratiometric platform represents a practical, eco-friendly, and scalable approach for MNZ screening in real world matrices.
