Ultrasensitive lead ion detection using polydopamine–maleic acid coated two-mode fiber with cascaded fiber Bragg grating compensation

Abstract

An ultrasensitive optical fiber sensor is developed for real-time and selective detection of lead (Pb²⁺) ions in aqueous environments. The device integrates a tapered two-mode fiber (T²MF) coated with a polydopamine–maleic acid (PDA–MA) film, which exhibits strong Pb²⁺ affinity through multidentate carboxyl coordination. The optimized taper geometry enables efficient excitation of the $H{E}_{\text{11}}$ and $H{E}_{\text{12}}$ modes, yeilding high refractive index sensitivity while retaining mechanical robustness. Upon Pb²⁺ binding, the local refractive index increases, including a redshift in the interference spectrum. Across concentrations from 2.07 to 207.2 ppb, the sensor demonstrates a linear spectral response (R²$>$0.99) with a peak sensitivity of 0.219 nm/ppb and a detection limit of 2.4 ppb—well below the WHO drinking water standard. Incorporating a cascaded fiber Bragg grating (FBG) enables dual-parameter demodulation for real-time temperature compensation, ensuring high measurement fidelity. This compact and low-cost platform demonstrates strong selectivity, excellent reproducibility, and scalable potential for in situ heavy metal monitoring in environmental and public health applications.