Ultrasensitive Label-Free Biosensor Based on the Graphene-Oxide-Coated-U-Bent Long-Period Fiber Grating Inscribed in a Two-Mode Fiber

In this article, we theoretically and experimentally report a newly-designed biosensor, which consists of a particular graphene-oxide (GO)-coated-U-bent long-period fiber grating inscribed in a two-mode fiber (LPFG-TMF). Due to the enhancement of evanescent field, the bent LPFG-TMF is more sensitive to the surrounding refractive index and it allows an extremely high sensitivity. The impact of bent radius on the sensing performance is discussed. The experimental result demonstrates, the sensitivity increases nonlinearly with the surrounding refractive index and the highest sensitivity reaches 6632.32 nm/RIU in the refractive index region from 1.3731 to 1.3830. To optimize the immune-sensing performance, the goat anti-human IgG is immobilized on the sensor via covalent bond through EDC/NHS cross-linker for human IgG recognition. The GO-coated-U-bent LPFG-TMF with bent radius of 15 mm has gained a low limit of detection (23 ng/ml) in the 3-20 μg/ml. In addition, the repeatability of the sensor is also investigated. With its premium ability of easy fabrication, high sensitivity, and compactness, the proposed sensor is adapted for detection of human IgG in clinical diagnostics, pharmaceutical researches, and bioengineering applications.