Gold nanorod vacancy-filling based optical fiber sensor for sensitive folic acid detection.

Folic acid is an essential micronutrient for human growth, and either its excess or deficiency can provoke adverse physiological effects. Therefore, the development of a rapid and highly sensitive detection platform for folic acid is of substantial practical importance. In this study, a D-shaped optical fiber sensor is functionalized with a folate receptor protein to capture target analytes through specific ligand-receptor interactions. Vacant binding sites are subsequently occupied by functionalized gold nanorods for amplifying the fiber's surface plasmon resonance (SPR) response. The incorporation of gold nanorods yields an enhanced SPR wavelength shift, leading to a significant improvement in detection sensitivity. The tunable nanoparticle occupancy time ensures the stability of detection sensitivity at various folic acid concentrations. Stability and specificity tests corroborated the sensor's robustness and selectivity. The proposed device achieved a sensitivity of 6.5 nm/(ng· mL-1) and a limit of detection of 0.012 ng/mL toward folic acid. This label-free, high-sensitivity approach offers considerable potential for biomedical applications.