Terahertz Metamaterial Absorber in Pesticide/Herbicide Detection

Terahertz (THz) technology has been attracted great interest in many research areas, especially THz plasmonics for sensing applications since intra- and inter-molecular vibrations are within the THz range. The sensitivity of free-space THz detection can be boosted up by use of metamaterials. Here, metamaterial absorber (MA) which are artificial structures in subwavelength scale of the incident light is introduced as a sensing device in THz regime. The operational principle of the MA sensor system is based on trapping the incident electromagnetic field at the interface of metallic antennas and the dielectric layer. These sensors are very sensitive to the change in their surrounding medium; therefore, highly sensitive, label-free, real-time sensing, and non-invasive detection are possible.<br/>This work focuses on the detection of paraquat [(C6H7N)2Cl2 and glyphosate [N-(phosphonomethly)glycine] herbicide/pesticide based on the polarization-insensitive THz MA. The MA sensor consisting of a metal-dielectric-metal structure is numerically designed by a commercial electromagnetic solver (CST microwave studio, 2018) software. The spectral response of the MA sensor with different geometrical parameters, such as metallic antenna structure and dielectric thickness is analyzed to optimize the resonance frequency and absorption efficiency. In the sensor fabrication process, general C-MOS technology is applied and characterized. Subsequently, the sensing capacity of the THz MA systems is analyzed numerically and measured experimentally for different concentrations of paraquat and glyphosate (5 ppM to 500 ppM). The Fourier Transform Infrared Spectroscopy (FT-IR) is used for measurement from the chip surface with/without paraquat and glyphosate. The experimental results show that the presence of paraquat and glyphosate with concentrations ranging from 5 ppM to 500 ppM in the THz MA sensor surface shifts the spectral position of the absorption peak to the lower frequency. The numerical and experimental results are in good agreement for different concentrations of pesticide molecules. The experimental sensitivity of this THz MA sensor is around 52 GHz/ppM and 89 GHz/ppM for paraquat and glyphosate, respectively These results indicate that the proposed MA sensor platform has potential use for highly sensitive THz applications in food quality and safety control [1].<br/><br/>Reference<br/>[1] K. Tantiwanichapan and H. Durmaz, “Herbicide/pesticide sensing with metamaterial absorber in THz regime,” <i>Sensors and Actuators A</i> <b>331</b>, 112960 (2021).