A Fe ₃ O ₄ @MXene Nanocomposite-Decorated Mach–Zehnder Interferometer Optical Fiber Sensor for Rapid and Ultratrace Cu ²⁺ Detection

Even exposure to trace amounts of copper ions in the environment can have serious effects on the human body. Building highly efficient sensors for copper ion detection are urgently needed. In this work, a Fe 3 O 4 nanoparticles@MXene composite was synthesized via a one-step solvothermal method and integrated with a Mach–Zehnder interferometer (MZI) optical fiber sensor for ultratrace Cu 2+ detection. In situ self-assembly of Fe 3 O 4 nanoparticles on MXene facilitated Cu 2+ adsorption via charge transfer. The optimized Fe 3 O 4 @Ti 3 C 2 -5 mg (FT5) material-based MZI optical fiber sensor with a sensitivity of 114.7 nm/μM exhibits a rapid response time of 5 s and a lower limit of detection 0.023 nM. The wavelength shift capacity of the FT5 sample exhibits approximately 6 and 2.4 times improvement compared to the MXene and pure Fe 3 O 4 for the Cu 2+ detection at a concentration of 1 nM, respectively. Furthermore, density functional theory and X-ray photoelectron spectroscopy were exploited to explain the underlying sensing mechanism for the enhanced Cu 2+ binding. In summary, our work presents a promising sensing material for detecting ultratrace Cu 2+ in environmental applications.