Ultra-Sensitive LPFG Optical Sensor Based on TiO2-CS/PAA Hybrid Film for Lead Ion Detection

A long-period fiber grating (LPFG) sensor functionalized with titanium dioxide-chitosan/poly(acrylic acid) (TiO₂-CS/PAA) composite film was proposed for detecting lead ion (Pb²⁺) concentration. The sensor was fabricated by sequentially coating TiO₂-CS/PAA composite functionalized film on the LPFG surface via the Layer-by-Layer self-assembly (LbL) method. The CS/PAA layer acts as a general sensing film, with the abundant amino (-NH₂) and hydroxyl (-OH) groups on chitosan providing sufficient chelation sites for Pb²⁺. Incorporation of PAA significantly improves both adsorption efficiency and mechanical stability. The sensitivity of the sensor is further enhanced by inserting a TiO₂ layer with a high refractive index (RI), which can reduce the RI difference between the fiber and the external environment. Experimental results demonstrated that the resonance wavelength of LPFG exhibited a blue shift with the increase of Pb²⁺ concentration, achieving a maximum sensitivity of 36.5 nm/μM, a limit of detection (LOD) of 9.3 nM (0.00193 mg/L), and a rapid response time of approximately 10 minutes, along with high selectivity toward Pb²⁺. The proposed sensor features simple fabrication, excellent selectivity, and robust stability, indicating significant potential for Pb²⁺ monitoring applications.