Ultra-sensitive graphene-based toroidal dipole meta-biosensor for trace detection of protein biomarkers in the terahertz band

Biosensors designed using metasurfaces have attracted considerable attention in biomedical diagnostics due to their rapid, label-free, and non-destructive detection capabilities, but it remains challenging to achieve ultra-sensitive detection for trace proteins in the terahertz (THz) range. Benefiting from the high sensitivity of toroidal dipole resonance, we proposed a graphene-based meta-biosensor to detect protein concentrations at picogram-level via utilizing the feature that the chemical vapor deposition graphene’s initial Fermi level shifts toward the Dirac point in response to weak external stimuli. With the Fermi level transitions from the valence band toward the Dirac point upon the increased protein concentrations, the toroidal dipole response is enhanced and reaches a maximum when the Fermi level approaches the Dirac point. Experimental validation confirmed that the proposed biosensor enables trace detection of midkine and cytokeratin fragment 21-1 with detection limits of 200 and 100 pg/ml, respectively. The results demonstrate the sensor’s potential for high-sensitivity molecular detection in the THz band, offering promising applications in biomedicine diagnostics and drug detection.