Label-Free Detection of Biological Cells Using Quasi-Bound States in the Continuum With Terahertz Metasurface

Terahertz (THz) biosensors as an advanced approach for nondestructive, label-free, and long-term monitoring of cellular states, hold tremendous potential in early disease screening and cellular drug sensitivity testing. This article presents a THz metasurface composed of two semicircular split-ring resonators based on quasi-bound states in the continuum (Q-BIC) mechanism. The localized field enhancement of the Q-BIC structure can significantly boost strong light-matter interactions, enabling highly sensitive detection of biological analytes. The sensitivity of the presented biosensor reaches as high as 517 GHz/RIU. Via resonance frequency shifts, we achieve ultrasensitive distinguishment between different concentrations of cancer cells (HepG2) and normal cells (293T). The results demonstrate that, without the need for additional factors, the frequency shift of the Q-BIC resonance can successfully identify different types of biological cells of varying cell concentrations. The experimental sensitivity reaches up to 456.64 kHz/(cell mL $^{-{1}}$ ), with a detection limit of $5 \times \; 10^{{3}}$ cells/mL, confirming the feasibility of the designed structure. The QBIC-based THz biosensor exhibits significant potential in distinguishing different biological cells and detecting low-concentration cells, paving a new path for ultrasensitive biomedical detection.