Glutaraldehyde-Cross-Linked Polyethylenimine Functionalized Tapered Fiber Interferometer for Ultrasensitive (Sub-nM) Detection of Copper Ions

This work presents a Mach–Zehnder interferometer (MZI) fiber-optic sensor functionalized with a glutaraldehyde (GA) cross-linked polyethylenimine (PEI) film for the ultrasensitive detection of copper ions (Cu2+) in aqueous environments. The sensing structure, based on a single-mode─graded-index multimode─tapered no-core─single-mode (SGTS) fiber configuration, leverages enhanced intermodal interference and evanescent field coupling to amplify the refractive index response induced by Cu2+-PEI coordination. Cross-linking PEI with GA not only stabilizes the polymeric layer but also improves its selectivity and adsorption affinity for Cu2+. Experimental results show a linear sensitivity of 0.0517 nm/nM in the low-concentration regime (0–100 nM), with a detection limit of 0.696 nM and a maximum detection concentration of 50 μM. The sensing mechanism is further validated by an extended Langmuir adsorption model, providing quantitative insight into the binding dynamics between Cu2+ and the functionalized surface. The sensor exhibits rapid response (20–110 s), excellent reproducibility (<5% variation across 10 devices), long-term stability, and strong specificity against interfering ions. Real wastewater analysis demonstrates close agreement with inductively coupled plasma mass spectrometry (ICP-MS), underscoring its practical utility. Owing to its simple fabrication, compact design, and high sensitivity, the proposed GA-PEI functionalized SGTS sensor offers a promising platform for real-time heavy-metal monitoring in environmental and biomedical applications.