Balloon-Shaped Mach-Zehnder fiber sensor functionalized with CS-PMAA for trace Cr(VI) detection in water

A highly sensitive, selective, and reproducible optical fiber sensor based on a balloon-shaped multimode fiber (MMF)-dual-core fiber (DCF)-multimode fiber (MMF) (balloon-shaped MDM) structure was proposed for the detection of trace hexavalent chromium ions (Cr(VI), in the form of dichromate (Cr 2 O 7 2- )) in water. A chitosan-polymethacrylic acid (CS-PMAA) composite film was applied to the sensing region of the fiber, serving as the functional layer of the sensor, which enables strong and specific adsorption of Cr(VI) ions. The refractive index (RI) of the sensing region would be changed by environmental variation during adsorption and desorption processes of the film, resulting in a measurable shift in the interference spectra. Experimental results demonstrate a detection range of 0–50 ppb, a high sensitivity of 0.116 nm/ppb and a low limit of detection (LOD) of 0.59 ppb. Furthermore, the sensor also exhibits excellent selectivity, reproducibility, and stability in complex water environment, and its performance was successfully validated in real water samples (tap and river water) with recovery rates ranging from 95.10% to 104.87%. This work proposed a compact and effective optical fiber sensing method for real-time, in-situ, and ultra-trace monitoring of Cr(VI) ions in environmental water samples.