Chitosan-Ion Imprinted Polymer Based Modal Interferometer for Ultra-trace Cd²⁺ Detection

A low-cost, easily fabricated, and compact modal interferometric sensor coated with chitosan-cadmium ion imprinted polymer (CS-Cd²⁺IP) is proposed and experimentally demonstrated for ultra-trace cadmium detection. The interferometer is constructed by sequentially splicing fixed lengths of multimode fiber (MMF) and no-core fiber (NCF) between single-mode fibers (SMFs). The formation of the CS-Cd²⁺IP layer for specific cadmium binding is achieved by modifying the interferometer surface with a CS-Cd²⁺ compound and 0.2% epichlorohydrin (ECH) solution. The sensor exhibits a linear cadmium sensitivity of 1.1528 nm/parts per trillion (ppt) and a limit of detection (LoD) of 0.0695 ppt within a rapid and stable response range of 0 to 1 ppt. The sensor demonstrates significantly enhanced selectivity towards Cd²⁺ while minimizing responses to other heavy metal ions (HMIs). The effects of environmental factors such as pH and temperature are investigated to minimize measurement errors and improve accuracy. Two additional sensors are fabricated using the same process to demonstrate the reproducibility of the proposed cadmium sensing scheme.