Hydrogel binding sodium alginate based optical fiber surface plasmon resonance for calcium ion trace detection

A plasmonic tilted fiber Bragg grating (TFBG)-based sensor for the detection of calcium ion (Ca²⁺) was proposed and demonstrated experimentally. Hydrogel material was synthesized by utilizing hydrogen bond recombination between cellulose nanocrystals (CNC) and polyvinyl alcohol (PVA). Sodium alginate (SA) was incorporated into this hydrogel material, resulting in a composite membrane with specific binding properties for Ca²⁺. The membrane was applied as a coating on the surface of a gold-coated TFBG. The CNC/PVA-SA modified gold on the TFBG surface enhanced the localized refractive index changes caused by variations of Ca²⁺ concentrations. The experimental results demonstrated an impressive limit of detection (LOD) of approximately 0.025 fM, which is five orders of magnitude better than the current LODs of similar Ca²⁺ sensors. And the proposed Ca²⁺ sensor exhibited a wide dynamic range of 10^-16 M to 10^-6 M.