High‐Sensitivity Label‐Free Biosensing Enabled by a Fabrication‐Tolerant Dual‐Ribbon Quasi‐BIC Metasurface

ABSTRACT Plasmonic metasurfaces that host bound states in the continuum (BICs) enable high sensitivity optical biosensing, yet most implementations rely on angle resolved excitation or deliberate symmetry breaking. Here, we present a dual‐ribbon metal‐insulator‐metal metasurface that hosts robust quasi‐BIC (qBIC) resonances at normal incidence without angular tuning. By combining numerical simulations and experiments, we have identified a highly sensitive qBIC mode with a strong analyte overlap, a refractive index sensitivity of more than 400 nm , and robustness to fabrication tolerances. Integrated into a microfluidic chip, this metasurface enables real‐time, stepwise immunoassay monitoring of anti‐insulin antibodies under continuous flow (surface sensitivity 0.065 nm ; estimated limit of detection (LOD) 1.71 nM) and demonstrates label‐free monitoring of refractive index variations in UV‐stressed HeLa cell supernatants, with resonance shifts consistent with changes in cell viability. The alignment‐free, robust platform provides a practical route toward high‐performance, label‐free biosensing.