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 a TiO2-CS/PAA composite functionalized film on the LPFG surface via the layer-by-layer (LbL) self-assembly method. The Chitosan (CS)/PAA layer acts as a general sensing film, with the abundant amino (-NH2) and hydroxyl (-OH) groups on CS 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/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>M, a limit of detection (LOD) of 9.3 nM (0.00193 mg/L), and a rapid response time of approximately 10 min, along with high selectivity toward Pb2+. The proposed sensor features simple fabrication, excellent selectivity, and robust stability, indicating significant potential for Pb2+ monitoring applications.