Plasmonic-enhanced annular fiber grating for label-free carcinoembryonic antigen detection with synchronous temperature monitoring.

The accurate and sensitive detection of oncological biomarkers carries paramount clinical significance, serving as a critical gateway for early disease diagnosis and precision therapeutic strategies. Here, we demonstrate a plasmonic-enhanced annular small-period long-period fiber grating biosensor, where localized electric fields from gold nanoparticles (AuNPs) amplify the sensitivity for label-free carcinoembryonic antigen (CEA) detection. The innovative annular grating design simultaneously excites both Bragg resonance and cladding mode resonance, enabling synchronous temperature and molecular sensing, which is a crucial feature for precise measurements as fiber-optic biosensors are inherently susceptible to temperature cross-talk. By leveraging the localized surface plasmon-effect of AuNPs, the sensor achieves significantly enhanced sensitivity toward target molecules. This nanoscale field confinement effectively prevents nonspecific amplification of the whole fiber surface, thereby eliminating false-positive signals. The optimized sensor demonstrates a detection limit of 1 ng/mL in pure CEA solutions (1-100 ng/mL concentration range) while maintaining comparable performance in mouse serum and exhibiting excellent specificity against potential interferents. This work establishes a reliable and highly sensitive platform that addresses both multifunctional sensing requirements and clinical needs for early cancer biomarker detection.