The reliable monitoring of carbonate ions in water is critical for assessing water hardness, buffering capacity, and potential ecological risks, yet existing sensors suffer from limited sensitivity and matrix interference. Herein, we report a highly sensitive optical fiber sensor for carbonate ion detection, constructed by coating a polyaniline (PANI)/zinc oxide (ZnO)-doped polyvinyl alcohol (PVA)/cellulose nanocrystal (CNC) composite film onto a low-cost Mach-Zehnder interferometer (MZI) fabricated via three-segment single mode fiber offset fusion splicing. In this design, CNC serve as nano-reinforcers that crosslink with PVA through hydrogen bonding to form a PVA/CNC composite matrix, while the embedded PANI/ZnO nanocomposites enable enhanced carbonate response via ZnO coordination and the electrostatic adsorption via PANI. The sensor responds to carbonate ions over the range of 0.5-200 μg/L, with a low-concentration sensitivity of −0.21 dB/(μg/L) at 0.5-10 μg/L and a higher-concentration sensitivity of −0.008 dB/(μg/L) at 10-200 μg/L. The detection limit is 1.7 μg/L, which is about 6-fold lower than that of the reported fiber-optic SPR carbonate sensor. The response time is 10 min, and the sensor demonstrates good practical applicability, with recovery rates of 95.9-98% in pure water and 94.2-97% in tap water at spiked concentrations of 1, 10, and 100 μg/L. Experimental results demonstrate that the PANI/ZnO@PVA/CNC-coated MZI represents a promising platform for sensitive and reliable carbonate ions monitoring, which is highly applicable to environmental water quality evaluation.
