Virus quantification by nano-metamaterial based THz detection. (a–d) Normalized THz spectra for various concentrations (0, 1, 0.14, 0.28 mg/ml) of H9N2 virus in the buffer solution. (e) The changes in the maximum values of the normalized transmittances (ΔT, magenta closed circle) and shifted resonance frequency (Δf, green closed triangle) are plotted for H9N2 virus in different concentrations as a function of concentration level. The red bar is error bar of buffer solution measurement. Black line and gray dashed line are linear fitting of the transmittance change and frequency shift data, respectively. (f) FDTD simulation results of transmittances for three different model samples with various composition of dielectric constants (n and κ) are shown.