Ultrasensitive Terahertz Label-Free Metasensors Enabled by Quasi-Bound States in the Continuum

Abstract

Advanced sensing devices based on metasurfaces have emerged as a revolutionary platform for innovative label-free biosensors, holding promise for early diagnostics and the detection of low-concentration analytes. Here, we developed a chip-based ultrasensitive terahertz (THz) metasensor, leveraging a quasi-bound state in the continuum (q-BIC) to address the challenges associated with intricate operations in trace biochemical detection. The metasensor design features an open-ring resonator metasurface, which supports magnetic dipole q-BIC combining functionalized gold nanoparticles (AuNPs) bound with a specific antibody. The substantial enhancement in THz-analyte interactions, facilitated by the potent near-field enhancement enabled by the q-BICs, results in a substantial boost in biosensor sensitivity by up to 560 GHz/refractive index units. This methodology allows for the detection of conjugated antibody-AuNPs for cardiac troponin I at concentrations as low as 0.5 pg/ml. These discoveries deliver valuable insight for AuNP-based trace biomolecule sensing and pave the path for the development of chip-scale biosensors with profound light-matter interactions.

Fig. 1.

Schematic of the THz plasmonic ultrasensitive biosensor. (A) THz q-BIC metasensor for trace troponin detection based on biological functionalization. (B) Geometrical configuration of the gold meta-atom on the polymethyl pentene (TPX) substrate within one unit cell: l = 40 μm, w = 5 μm, g₁ = 14 μm, and g₂ is variable for asymmetry. (C) Micrograph of the fabricated metasurface sensor at the lower right with the unit cell: p = 50 μm.

Fig. 2.

The electromagnetic resonance mode analysis of THz metasurface. (A and B) Simulated reflectance spectra at representative asymmetry degrees with different gap g₂. (C) The simulated current density distribution with g₂ = 4 μm. (D) The nonlinear fitting Q-factor of the simulated q-BIC resonance as a function of g₂ with the coefficient of determination R² > 0.99.

Fig. 3.

Normalized electromagnetic near-field distribution. (A) Calculated reflection spectrum of the unit cell with a structure parameter g₂ = 4 μm. (B) Spherical multipolar decomposition result when g₂ = 4 μm. The q-BIC mode is mainly contributed by the MD, while another resonance is the ED. The inset shows the fabricated samples corresponding to the condition of g₂ = 4 μm. (C and E) Comparison of electric near-field intensity enhancement |E/E₀|² between the ED mode (1.18 THz) and q-BIC mode (1.89 THz), where |E₀| represents the incident field amplitude. (D and F) Simulated magnetic near-field intensity enhancement |H/H₀|² for g₂ = 4 μm, where |H₀| denotes the incident magnetic field amplitude. These results indicate that the q-BIC mode has a stronger electromagnetic near field and therefore higher surface sensitivity.

Fig. 4.

Comparison of sensing performance between the 2 resonant modes. (A and B) Frequency shift and linear fitting of q-BIC resonant mode with different refractive index. The sensitivity of q-BIC resonance reaches 560 GHz/RIU. (C and D) Reflection spectra and linear fitting of ED resonant mode with the refractive index n varies from 1.0 to 2.0. The dotted arrows denote the frequency shift as the refractive index increases.

Fig. 5.

THz sensor evaluation of trace cardiac troponin measurements. (A and B) Simulation calculations and experimental measurements of reflection spectra with g₂ = 4, 8, 14, 20, 24, and 28 μm, respectively. (C) Q-factor of q-BIC mode from the simulation and the experiment. (D) Comparison of reflection spectra between the simulation and the experiment when g₂ = 4 μm.

Fig. 6.

Evaluation of trace cardiac troponin measurements. (A) Reflection spectra of the ED mode (left) and q-BIC mode (right) at different concentrations of cardiac troponin I (cTnI) solution. (B) Full display of 2 peak frequency shifts, showing a smaller insert intuitively demonstrating the relative position of the q-BIC mode and ED mode. (C) Fitting curve of the q-BIC frequency shift.