A U-shaped microfiber ring laser sensor for ultrasensitive detection of Ni²⁺ ions

As a common heavy metal pollutant, Ni²⁺ necessitates a rapid, sensitive, and accurate detection method to mitigate its impact on human health and the environment. This study developed a U-shaped single mode-tapered four core-single mode (STFS) based fiber ring laser (FRL) sensor coated with Ni²⁺ ion-imprinted chitosan (Ni²⁺-II-CS) for Ni²⁺ detection. Experimental results demonstrate that the average spectral stability in deionized water is 0.016 nm over 60 min. The detection time for Ni²⁺ is less than 8 min, with a limit of detection (LoD) as low as 1 nM. The proposed U-shaped STFS-based FRL sensors also show good selectivity, displaying limited cross-reactivity to other common heavy metal ions, including Cu²⁺, Cr²⁺, Pb²⁺, and Cd²⁺. Additionally, the analytical findings from pond and tap water specimens demonstrate wavelength shifts of -23.30 % and -27.06 %, respectively, when compared with measurements in deionized water. These findings indicate that the proposed microfiber sensor may have potential applications in heavy metal ion detection and environmental monitoring.