Electric-dipole resonance in a graphene-coated double-D terahertz metamaterial sensor for trace-level glucose...

Terahertz metamaterial sensors provide an efficient platform for highly sensitive biochemical detection. Herein, a graphene-enhanced terahertz metamaterial sensor was developed and coupled with a terahertz frequency-domain spectroscopy (THz-FDS) system for trace-level detection of glucose. A metamaterial sensor comprising a double-D-shaped split-ring aluminum resonator was fabricated, which exhibited a pronounced resonance at 0.951 THz under transverse-electric (TE)-polarized terahertz incidence. Multipole decomposition, together with electric-field, magnetic-field, and surface-current analyses, showed that the resonance is dominated by the electric dipole (ED). The ED channel accounts for approximately 98.59% of the summed isolated multipolar scattering power, whereas the toroidal dipole (TD) and magnetic quadrupole (MQ) provide only minor contributions. After structural optimization, CST simulations of bulk refractive-index variation yielded a sensitivity of 204 GHz/RIU, characterizing the simulated index-sensing performance of the resonator. To achieve high-precision detection of trace-level glucose, monolayer graphene was transferred onto the metamaterial surface using a wet transfer method. Experimental measurements showed a measurable response at the lowest tested concentration of 1 μM for the graphene-coated sensor, whereas the bare sensor showed a clear concentration-dependent response primarily in the millimolar range. Although the resonance amplitude varied nonmonotonically with concentration, the resonance-frequency shift increased monotonically and yielded a log-concentration calibration with R² = 0.932. Controlled paired simulations separated the contributions of graphene Fermi-level modulation and conventional dielectric loading. At an analyte thickness of 40 μm, the graphene-associated shift was +182.609 GHz, whereas dielectric loading produced redshifts of -245.081 GHz at E_f = 0 and -247.997 GHz at E_f = -120 meV. The interaction residual was -2.916 GHz, or 1.60% of the graphene contribution, indicating that the two effects are approximately additive under full loading. This work demonstrates that graphene-enhanced terahertz metamaterials can serve as highly sensitive sensing platforms for trace substance detection and provides both a physical strategy and experimental basis for developing high-performance terahertz biosensors involving interactions with low-dimensional materials.