Terahertz determination of imidacloprid in soil based on a metasurface sensor.

Pesticides in soil are continuously one of the most studied analytes due to their environmental and human health effects. Thus the detection of pesticides in soil is an important means to control and assess soil quality. Here, we theoretically and experimentally present a novel method for the determination of imidacloprid in soil by using a metasurface sensor operating at terahertz frequencies. The metasurface shows a resonance peak at 880 GHz and the electric field at the peak is strongly localized and concentrated in the gap of split I-shaped resonator. The detection of complex refractive index shows that the position and the transmittance of resonance peak are depend on the change in the complex refractive index. The measurement of imidacloprid concentration in soil demonstrates that both the frequency shift and the transmittance change at peak increase almost linearly with the increasing of imidacloprid concentration ranging from 0.25% to 2%. In this case, the frequency shift reaches 97 GHz and the transmittance change at peak is as high as 30.9%. Our work enables the determination of imidacloprid in soil at terahertz frequencies with good reliability and high sensitivity, showing the potential application of terahertz spectroscopy in environmental monitoring.