Hydrophilic modification enables a performance breakthrough in the conventional metallic metasurface

Metasurfaces, as advanced photonic platforms capable of precise electromagnetic manipulation, confine optical energy within meta-atoms, generating intense near-field hotspots that greatly enhance light–matter interactions and enable ultrasensitive biosensing. However, the superhydrophobicity of gold leads to a pronounced coffee-ring effect during the evaporation of liquid samples, resulting in uneven distribution of biomolecules and weakened light–matter interactions. Herein, we present a metallic terahertz (THz) metasurface sensor whose metal array region is modified with ionic liquids (ILs), resulting in a reduction of the droplet contact angle from 105.7° to 39.4°. This indicates excellent hydrophilicity, which facilitates the uniform deposition of protein molecules on the surface. This strategy circumvents the conventional dependence on high quality ( Q ) factor resonances and delivers outstanding sensing performance. Specifically, under y -polarized THz excitation, the two resonance modes excited by the metasurface enable reliable qualitative and quantitative detection of various proteins. Among these modes, Mode II exhibited a direct limit of detection (LoD) of 0.03125 mg/mL for insulin. This work establishes a new paradigm for trace biomolecule sensing and provides valuable insights for future sensor innovations.