Ni²⁺-Imprinted Chitosan Coated STMS Microfiber Sensor for High-Sensitivity Detection of Ni²⁺ Ions

Ni²⁺ is a toxic heavy metal pollutant with cumulative and irreversible toxicity, requiring reliable monitoring methods. This study developed a single-mode-tapered multimode-single-mode (STMS) microfiber sensor with a Ni²⁺ ion-imprinted chitosan (Ni²⁺-II-CS) thin film on its surface for specific detection. The sensor demonstrated excellent stability in deionised water, with wavelength fluctuations of only 0.0155 nm within 20 minutes. Its detection limit is as low as 1.01 nM (far below the WHO drinking water standard of 1.19 μM), with a response time of less than 8 minutes, during which approximately 90% of the total wavelength shift is completed.In pond water sample tests, the sensor demonstrated strong selectivity, with wavelength shifts induced by interfering ions (e.g., Cu²⁺, Cd²⁺, Cr³⁺, K⁺, Na⁺, Pb²⁺) being less than 20% of that for Ni²⁺ under the same concentration (100 nM). It also maintained reliability over a 10-day testing period, with a test RSD = 8.9%. Spiked recovery rates obtained from pond water validation experiments ranged from 93.22% to 115.12% (relative standard deviation RSD ≤ 4.17%), validating its accuracy in practical applications. This cost-effective, miniaturised platform provides a rapid and reliable solution for ultra-sensitive Ni²⁺ detection in environmental monitoring.