Temperature-compensated microfluidic terahertz metasurface for trace moisture quantification in insulating oil.

Quantitative detection of trace moisture in transformer insulation oil continues to pose a fundamental bottleneck for terahertz (THz) liquid-phase sensing due to severe electromagnetic dissipation in polar media and temperature-induced spectral instability. Here, an electromagnetically induced transparency (EIT)-like THz metasurface integrated with a 7 µm ultra-thin microfluidic channel is engineered to enable reliable moisture quantification under variable temperature conditions. By leveraging ultra-thin analyte confinement, the electromagnetic interaction between the dielectric medium and the confined resonant field is markedly intensified, effectively mitigating bulk absorption effects and converting subtle moisture perturbations into readily resolvable non-radiative damping responses. With an injection volume of only 10 µL, the sensor exhibits a pronounced amplitude response over the 0-700 ppm range, yielding an estimated limit of detection (LOD) of 48 ppm based on the system noise floor, over an experimentally evaluated non-zero concentration range of 50-700 ppm (R2=0.9976). Furthermore, temperature variation from 25°C to 80°C predominantly modulates the resonance amplitude while inducing minimal spectral displacement, thereby establishing a stable amplitude-dominant sensing regime with minimal spectral drift. To further suppress thermal interference, a second-order polynomial compensation model was developed, reducing the standard deviation of transmittance fluctuations from 1.25 to 0.08 after compensation. This work offers a promising strategy for THz sensing in lossy liquid media and provides useful insights for the future development of condition-monitoring systems for power equipment.