Double-layered Metasurface-based Dual-function Sensor for Detecting Ionic Liquids and Moisture

Ionic liquids， as a new type of molten salts with high thermal stability， low vapor pressure， and strong designability， have important application value in cutting-edge fields such as electrochemistry， catalytic synthesis， and biomedicine. However， the accuracy and efficiency of ionic liquid type identification still need to be continuously explored and optimized. Traditional characterization methods， such as infrared spectroscopy and nuclear magnetic resonance， usually rely on large precision instruments， which have limitations such as complex operating procedures， high equipment costs， and long analysis cycles. At the same time， the inherent strong hygroscopicity of ionic liquids makes the control of sample purity more stringent， and existing moisture detection methods find it difficult to achieve rapid and precise measurement. Against this background， metasurfaces， as two-dimensional artificial materials that can flexibly control the amplitude， phase， and polarization of electromagnetic waves， provide a new technical path for the development of high-performance sensing technologies. This type of structure has both high sensitivity and non-contact detection advantages， and has shown significant potential in dielectric property analysis and substance identification. However， existing metasurface sensors still have certain limitations in practical applications， such as being easily affected by external electromagnetic interference or requiring direct contact with samples. These issues not only reduce measurement accuracy but may also cause sensor contamination， thereby limiting their widespread application in practical scenarios.Aiming at the technical bottlenecks of ionic liquid type identification and moisture content detection mentioned above， this paper proposes a metasurface microwave sensor based on a dielectric substrate integrated with a double-layer metal resonant structure. The sensor converts the dielectric properties of ionic liquids into significant changes in resonant frequency shift and resonance linearity through its unique electromagnetic field localization and enhancement mechanism， providing a new way to achieve rapid and precise sensing analysis. To deeply study the working mechanism of the sensor， this study adopted a multi-level research method： from the physical mechanism level， the formation and energy distribution of electromagnetic resonance were intuitively presented through surface current distribution analysis； from the model construction level， the physical structure was transformed into circuit parameters through equivalent circuit analysis， and the regulation law of resonant frequency was quantitatively revealed； from the performance prediction level， the electromagnetic response characteristics of the sensor were accurately obtained through full-wave numerical simulation， and the structure was optimized； from the final verification level， experimental tests confirmed that the sensor can not only accurately distinguish six different types of ionic liquids but also achieve high-precision detection of trace moisture content.Before conducting experimental characterization， this study first compared and analyzed the reflection characteristic changes of the sensor before and after loading ionic liquids in the 7~10 GHz frequency band based on electromagnetic simulation software. The simulation results show that the introduction of ionic liquids not only changed the original resonant mode of the sensor but also induced a stronger electromagnetic response through the dielectric regulation mechanism， thereby verifying that the metasurface sensor has excellent dielectric sensitivity and electromagnetic regulation capabilities. Subsequently， six representative ionic liquids were selected as the measured samples for actual measurement analysis. The experimental measurement results show that when the sample to be tested in the sample holder is changed from ［EMIm］［BF₄］ to ［OMIm］［BF₄］， the resonant peak position of the reflection spectrum shifts from 8.37 GHz to 8.86 GHz， resulting in a significant frequency shift of 490 MHz. Calculations show that the sensor's average sensitivity to dielectric constant changes can reach 185.42 MHz/ε′. In addition， this study systematically evaluated the sensor's ability to detect trace moisture content. Experimental data show that when the water content increases from 0% to 10%， the resonant peak of the reflection spectrum shows a significant frequency shift of 180 MHz （8.22 GHz→8.40 GHz）， proving that the sensor can effectively detect moisture changes in ionic liquids at the ppm level.In summary， this paper successfully designed and verified a metasurface microwave sensor based on a double-layer metal resonant unit. By using a three-layer structure constructed with an FR4 epoxy resin substrate， combined with an open sample holder and a fully copper-plated bottom plate design， the sensor exhibits excellent detection performance in the 7~10 GHz working frequency band， achieving non-contact and high-precision detection of ionic liquid types and their moisture content. Simulation and experimentalresults show that the sensor has different characteristic frequency responses for six typical ionic liquids （including ［EMIm］［BF4］， ［BMIm］［SCN］， ［BMIm］［OTf］， ［BMIm］［BF4］， ［HMIm］［NTf₂］， and ［OMIm］［BF4］）， with a maximum frequency shift of 490 MHz and a detection sensitivity of 185.42 MHz/ε′. In terms of moisture detection， the sensor shows a good resonant response to 0~10% moisture content changes in ［EMIm］［BF4］ solutions， with a resonant frequency shift of 180 MHz. The metasurface sensor designed in this study has high sensitivity and good measurement repeatability， providing an effective technical means for ionic liquid purity detection and deterioration monitoring， and has important engineering application prospects in fields such as chemical process control and real-time monitoring in hazardous environments.