Fig. 1. (a) Microscopic image of the fabricated metal array (without graphene); (b) Schematic of the unit cell of the metal array; (c) Schematic of the working sensor; (d) Microscopic image of the fabricated sensor covered by glutamic acid

Fig. 2. (a) THz transmission spectra for the Dev.2, where the red line indicates the experimental result and the black one indicates the simulated result; the simulated electric field distribution for (b) f=0.35 THz (the resonant dip D_A) and (c) f=0.82 THz (the resonant dip D_B), where the red color represents the maximum electric field intensity, and the blue one represents the minimum electric field intensity

Fig. 3. Transmission spectra for (a) Dev.1, (b) Dev.2, and (c) Dev.3 covered by glutamaic acid solution with the concentration of C₀-C₆; \begin{document}$ \Delta T $\end{document} as a function of the solution concentration for (d) Dev.1, (e) Dev.2 at the resonant peak P, and (f) Dev.3 at f=0.58 THz

Fig. 4. The evolution of E_F in graphene energy band with the increasing solution concentration

Fig. 5. \begin{document}$ \Delta P $\end{document} spectra for (a) Dev.1, (b) Dev.2, and (c) Dev.3 covered by the glutamic acid solution with the concentration of C₀-C₆; the slope extracted from \begin{document}$ \Delta P $\end{document} spectra as a function of the solution concentration for (d) Dev.1, (e) Dev.2, and (f) Dev.3