Microwall‐Integrated Terahertz Metasurfaces for Spatially Guided Label‐Free Sensing of Micrometer‐Scale Plastic Particles

Microplastic pollution is a critical environmental problem owing to its effects on ecosystems and human health. Therefore, technologies capable of accurately detecting micrometer‐scale polymer particles and converting their presence into measurable device‐level signals are indispensable for mitigating their impact. This study proposes a microwall‐integrated terahertz metasurface structure for spatially guided, label‐free sensing of micrometer‐scale plastic particles. The device integrates a metal–dielectric–metal resonator with recessed regions defined by microwall structures, enabling solid particles to be deposited and concentrated near localized terahertz electric‐field regions following solvent evaporation. An electromagnetic simulation‐based design strategy for both the resonant metasurface and microwall geometry maintains a clear resonance, while the microwall geometry provides particle placement control. The as‐fabricated devices exhibit distinct terahertz reflection resonances. Optical microscopy confirms that the presence of microwalls increases the number of 3.1‐µm polystyrene particles located in the high‐field region by a factor of up to approximately 12 relative to devices without microwalls. Terahertz frequency‐domain spectroscopy further verifies concentration‐dependent resonance shifts in the 0–0.081 mg mL −1 range, with an apparent concentration sensitivity of 102 GHz (mg mL −1 ) −1 . These results establish that the microstructure‐engineered spatial overlap between solid microparticles and localized terahertz fields effectively enhances metasurface‐based particle sensing.