Functionalized terahertz plasmonic metasensors: Femtomolar-level detection of SARS-CoV-2 spike proteins

Abstract

Effective and efficient management of human betacoronavirus severe acute respiratory syndrome (SARS)-CoV-2 virus infection i.e., COVID-19 pandemic, required sensitive and selective sensors with short sample-to-result durations for performing desired diagnostics. In this direction, one appropriate alternative approach to detect SARS-CoV-2 virus protein at low level i.e., femtomolar (fM) is exploring plasmonic metasensor technology for COVID-19 diagnostics, which offers exquisite opportunities in advanced healthcare programs, and modern clinical diagnostics. The intrinsic merits of plasmonic metasensors stem from their capability to squeeze electromagnetic fields, simultaneously in frequency, time, and space. However, the detection of low-molecular weight biomolecules at low densities is a typical drawback of conventional metasensors that has recently been addressed using toroidal metasurface technology. This research is focused on the fabrication of a miniaturized plasmonic immunosensor based on toroidal electrodynamics concept that can sustain robustly confined plasmonic modes with ultranarrow lineshapes in the terahertz (THz) frequencies. By exciting toroidal dipole mode using our quasi-infinite metasurface and a judiciously optimized protocol based on functionalized gold nanoparticles (AuNPs) conjugated with the specific monoclonal antibody specific to spike protein (S1) of SARS-CoV-2 virus onto the metasurface, the resonance shifts for diverse concentrations of the spike protein are monitored. Possessing molecular weight around ~76 kDa allowed to detect the presence of SARS-CoV-2 virus protein with significantly low as limit of detection (LoD) was achieved as ~4.2 fM. We envisage that outcomes of this research will pave the way toward the use of toroidal metasensors as practical technologies for rapid and precise screening of SARS-CoV-2 virus carriers, symptomatic or asymptomatic, and spike proteins in hospitals, clinics, laboratories, and site of infection.

Keywords: COVID-19 pandemic; Femtomole-level detection; SARS-CoV-2 spike protein; Terahertz plasmonic biosensors; Toroidal metasurfaces.

Fig. 1

Functionalization of colloidal AuNPs. a, A schematic for the workflow of the developed functionalized AuNPs conjugated with the respective SARS-CoV-2 antibody and spike proteins. b, A TEM image of AuNPs conjugated with the respective antibody of SARS-CoV-2.

Fig. 2

Simulation, fabrication, and characterization of toroidal THz plasmonic metasurface. a, An artistic rendering of the designed multipixel toroidal unit cell, including the dispersed functionalized AuNPs on top. The polarization direction of the incidence is demonstrated inside the panel. b, Numerically calculated (solid) and experimentally probed (dot) transmission spectra of the toroidal metasurface under transverse polarized THz light. The inset is the SEM image of the fabricated metasurface. Scale bar: 100 μm e, Theoretically calculated scattered power from individual electromagnetic multipoles induced in the spectral response of metasurface. p: electric dipole, m: magnetic dipole, Q^(e): electric quadrupole, Q^(m): magnetic quadrupole, T: toroidal dipole.

Fig. 3

E-field confinement in the toroidal metamolecule. a, Cross-cut electric field enhancement (|E|²) as a function of position along x-axis, simulated for a unit cell at the toroidal dipole frequency, showing significant intensity of the confined fields at the capacitive openings in each resonator. b, 3D E-field enhancement map at the frequency of toroidal dipole, in which the strong field confinement was spotted at capacitive gap regions. The polarization and incident light direction are indicated inside the panel.

Fig. 4

Evaluation of metasensor performance using functionalized AuNPs and captured spike proteins. a, Measured transmission spectra of the THz metasensor device for different concentrations (4–12 fM) of SARS-CoV-2 spike protein. For 0 fM of spike protein, the spectral response was defined merely for antibody conjugated AuNPs plus PBS. b, Variations of the toroidal mode frequency shift for different concentrations of injected spike proteins captured by relevant antibody ranging from 2 to 20 fm c, The Q-factor and dephasing time (τ_d) of the toroidal lineshape as a function of protein concentration.

Fig. 5

Evaluation of the THz metasensor in diluted solution. a, b, Measured transmission spectra of the THz metasensor device for different concentrations (from 4 to 30 fM) of BSA and SARS-CoV-2 spike protein + BSA, respectively. For 0 fM of spike protein, the spectral response was defined merely for antibody conjugated AuNPs plus PBS. c, Variations of the toroidal dipole mode for different concentrations of BSA, SARS-CoV-2 spike protein, and BSA + SARS-CoV-2 spike protein. The spike proteins in all experiments were captured by functionalized AuNPs conjugated with the respective antibody.