Cortisol detection via a fiber optic biosensor based on localized surface plasmon resonance of gold nanostars

In this work, we report innovative fiber optic biosensors exploiting localized surface plasmon resonance (LSPR) generated by gold nanostructures, specifically spherical nanoparticles (NPs) and nanostars (NSs), immobilized on uncladded silica multi-mode fibers to form a compact reflective probe architecture. The transition from NPs to NSs produced a redshift of the LSPR peak from 545 nm to 665 nm and markedly enhanced bulk refractive index sensitivity: 16 nm diameter NPs reached 156 nm/RIU, while 78 nm diameter NSs achieved 402 nm/RIU. To demonstrate biosensing capability, the nanostructure coated fibers were biofunctionalized with anti-cortisol antibodies, enabling selective detection of cortisol, a key biomarker for stress and endocrine disorders. The resulting sensors exhibited a wide dynamic range (1 pg/mL–100 μg/mL), with the NS-based configuration achieving an outstanding limit of detection of 0.1 pg/mL. This broad range encompasses clinically relevant cortisol levels, underscoring the potential of LSPR-based fiber probes for non-invasive stress monitoring and personalized healthcare, where the synergy between nanostructure design and fiber integration paves the way for compact, label-free, highly sensitive point-of-care diagnostics.