Ion-imprinted no-core-fiber sensor for operando on-line monitoring of Cu²⁺ in environmental waters

A compact optical fiber sensor based on a no-core fiber (NCF) segment functionalized with a Cu²⁺-imprinted chitosan/polyvinyl alcohol (CS/PVA) film was developed to enable operando on-line monitoring of Cu²⁺ with high selectivity and sensitivity in complex environmental water matrices. The optimal length of the NCF segment was determined to be 1 cm based on simulation and experimental evaluation, under which the highest sensitivity was achieved. The pseudo-second-order kinetic behavior of Cu²⁺ adsorption was first validated by analyzing the time-dependent shift of the interference valley, providing direct spectral evidence of a chemisorption-driven mechanism. A sensitivity of 0.0768 nm/μM and a detection limit of 0.1245 μM were obtained. A rapid response time of less than 20 s and excellent measurement repeatability, with strong specificity toward the target ion and relative standard deviation below 5 percent over five regeneration cycles, were also demonstrated. The sensor was further applied to spiked water samples from the Yangtze River and tap water. By integrating ion-imprinting technology with an optimized NCF-based fiber structure, a portable, cost-effective, and interference-resistant platform has been established for real-time and on-site monitoring of heavy metal ions in complex aquatic environments.