Ultrasensitive metasurface biosensor based on quasi-bound states in the continuum for human serum albumin detection

Accurate and sensitive detection of trace-level biomarkers is of vital importance for early disease diagnosis and timely intervention. In this work, we theoretically and experimentally present a metasurface biosensor based on quasi-bound states in the continuum (<i>q</i>-BIC) with a high <i>Q</i>-factor for ultrasensitive detection of human serum albumin (HSA). Rigorous multipole decomposition and near-field distribution analysis reveal that the <i>q</i>-BIC resonance is dominated by the magnetic dipole (MD) mode. The strong near-field enhancement generated by this <i>q</i>-BIC resonance significantly enhances the light-analyte interaction, enabling the excellent sensing performance with a simulated refractive index sensitivity of 510 nm/RIU and a simulated FOM of 1637 RIU^-1 for the proposed metasurface. The further experiment demonstrates that the limit of detection (LOD) for HSA reaches 2.3 ng/mL. This metasurface biosensor is highly sensitive, label-free, and cost-effective, showing great potential for practical biomedical detection.