U-shaped fiber-optic biosensor with gold nanoparticle-assisted signal amplification for allergy-related biomarker detection.

Achieving high analytical sensitivity in fiber-optic biosensors frequently involves a trade-off between signal intensity and structural integrity, particularly when relying on cladding-removal strategies or complex grating inscription processes. Here, we present a cladding-preserved U-shaped fiber-optic biosensor designed to address this long-standing trade-off through simulation-guided geometry optimization and a gold nanoparticle (GNP)-assisted attenuation-based amplification strategy. Optical simulations were first used to identify an optimal bend radius of 5 mm predicted to maximize evanescent-field interaction while fully retaining the fiber's protective cladding. The sensor surface was then functionalized using optimized poly(ethyleneimine) (PEI) and glutaraldehyde (GA) chemistry, followed by a multilayer immunoassay with GNP-streptavidin conjugates for signal amplification. The fabricated 5 mm sensor demonstrated a refractive-index resolution of 10⁻4 RIU while fully retaining the protective cladding. For detection of a surrogate allergy-related target (anti-NACAD IgG) in a serum-containing surrogate matrix (5% horse serum in PBS), the biosensor achieved a competitive limit of detection (LOD) of 16 pg/mL. This work establishes a proof-of-concept cladding-preserved fiber-optic biosensing platform that integrates practical structural design with sensitive optical readout, and provides a basis for future development toward robust and deployable diagnostic systems.