  Effective and efficient management of human betacoronavirus severe acute
respiratory syndrome (SARS)-CoV-2 infection i.e., COVID-19 pandemic, required
sensitive sensors with short sample-to-result durations for performing
diagnostics. In this direction, one of appropriate alternative approach to
detect SARS-CoV-2 at low level (fmol) 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 reports 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 (NPs) conjugated with the specific monoclonal
antibody of SARS-CoV-2 onto the metasurface, the resonance shifts for diverse
concentrations of the spike protein is monitored. Possessing molecular weight
around ~76 kDa allowed us to detect the presence of spike protein with
significantly low LoD ~4.2 fmol.
