Research On Highly-sensitivity Terahertz Metamaterial Sensing Detection Technology For Grain Storage Contaminants

Grain concerns about the well-being of the country and people’s livelihood,and grain security is the guarantee of national security.Grain reserves are the main means of regulation and control to maintain the balance of supply and demand and ensure grain supply in China.Grain storage contaminants are important factors affecting grain quality and safety,and efficient and accurate detection methods are of great significance to ensure national food security.In this thesis,based on the study of traditional detection methods of grain storage contaminants,a highly-sensitivity terahertz metamaterial sensing detection method for grain storage contaminants is explored to address the need for rapid and accurate detection of grain storage quality.Terahertz（THz）technology is a promising emerging spectroscopic technology with unique advantages such as low energy,high penetration,and fingerprint,which shows great development potential and application value in the field of rapid detection of grain storage quality.However,there are still some problems in terahertz spectroscopy detection,such as insensitivity of terahertz waves to trace substances,low spectral discrimination and poor detection sensitivity,which cannot meet the demands for high-sensitivity detection of trace grain storage contaminants.Terahertz metamaterial,as a label-free affinity-type sensor,can break through the detection limit of terahertz spectroscopy,convert the detection of terahertz spectra of substances into the detection of changes in the resonance characteristics of terahertz metamaterials caused by substances,and enhance the detection sensitivity of grain storage contaminants,which has important research value and development prospects.This thesis combines metamaterials and terahertz spectroscopy detection technology and carries out a series of research work on highly-sensitivity characteristics of metamaterials,dual-band resonances characteristics of metamaterials,the construction of terahertz metamaterial sensing detection system,and the terahertz sensing detection of mycotoxins in grain storage.The purpose is to explore the relationship between the sensing characteristics of metamaterials and the ability to identify minute features of trace substances,while study the signal characterization of substances in terahertz sensing detection,and use the resonant frequency shift of metamaterials caused by trace substances to build regression models for different concentration samples,so as to realize the identification and quantitative analysis of mycotoxin contaminants in grain storage,as well as to provide the basis for the applied research in the field of rapid and accurate detection of grain storage quality.Major parts of the research in the thesis are as follows:1.To solve the problem of insufficient sensitivity of terahertz spectroscopy for the detection of trace substances,a new unit structure of Cross Rectangular Split-ring Resonator（CRSRR）is proposed to design a highly-sensitivity metamaterial sensor to detect small refractive index differences of trace substances.Based on the theory of equivalent circuit analysis,the relationship between resonant frequency and equivalent capacitance and inductance of metamaterial is explained,and the influence of unit structure parameters on the resonant characteristics of metamaterial is studied.The sensing performance of the CRSRR metamaterial is numerically calculated and analyzed to investigate the detection limit of the amount of refractive index change of the substance to be measured.The experimental results show that a new terahertz metamaterial of specific bands and resonant frequencies can be designed by changing the geometric layout and parameters of the unit structure,which can meet the detection requirements of different frequency bands and improve the detection sensitivity.The designed CRSRR metamaterial produces resonances at 1.04 and 2.95 THz with refractive index sensitivity of 309 and 730 GHz/RIU,respectively.The CRSRR metamaterials can sense the refractive index changes as low as 0.02 by using a terahertz time-domain spectroscopy system with a resolution of 4 GHz.It is verified that the sensing detection based on CRSRR is capable of measuring substances with refractive index change greater than 0.02.2.To solve the problem of insufficient detection stability caused by a single sensing frequency of metamaterial,a metamaterial design scheme based on the Split Metal Stacking Ring Resonator（SMSRR）structure is proposed to realize dual-band cooperative multi-point feature matching stable detection.Based on the electromagnetic response,the generation mechanism and characteristic difference of LC and dipole resonances of the SMSRR metamaterial are compared and explained,and the sensing performances of resonances are numerically analyzed.Meanwhile,the collaborative detection ability of LC and dipole resonances is studied.The experimental results show that the designed SMSRR metamaterial has highly-sensitivity LC and dipole resonances in two independent terahertz bands with refractive index sensitivities of 304 and 912GHz/RIU respectively.In the detection of the same sample at the same time,the calculated refractive indices of the sample are consistent according to the frequency shifts of the LC resonance and dipole resonances respectively,which verifies the multi-point feature matching detection ability.3.To solve the problem of the sensitivity of terahertz spectroscopic detection is difficult to accurately characterize trace substances,a terahertz metamaterial sensing detection system is built based on the designed metamaterial sensors,which can detect the effective characterization of trace substances.The metamaterial sensors are fabricated using a surface micromachining process and characterized in the terahertz time-domain spectroscopy（THz-TDS）system.The PPM（Part per Million）concentration trace substances are detected by both terahertz spectroscopic detection and metamaterial sensing detection to verify that the metamaterial sensing detection method can improve the characterization ability of trace substances.The experimental results show that the CRSRR and SMSRR metamaterial sensors have highly-sensitivity dual-band resonances in the effective detection band（0.2-3 THz）of the THz-TDS system,and can be used for sensing detection.In the terahertz sensor detection based on two sensors,the trace substances that can not be detected by free space terahertz wave are effectively identified according to the resonance frequency shift of the transmission spectrum.4.To meet the requirements for highly-sensitivity detection of mycotoxin contaminants in grain storage,based on the constructed terahertz metamaterial sensing detection system,experimental research is carried out on the sensing detection of mycotoxins with concentrations of PPB(（Part per Billion）,and a regression model of sample concentration is built to achieve highly-sensitivity detection of mycotoxin contaminants in grain storage.Two metamaterial sensors,CRSRR and SMSRR,are used to detect aflatoxin B1,aflatoxin B2,ochratoxin A,and zearalenone at different concentrations respectively.The sensing detection ability of the sensors on the concentration of PPB mycotoxins is studied,and the influences of resonance characteristics on the detection sensitivity are analyzed.According to the experimental results of zearalenone based on CRSRR sensor,the quantitative analysis model is established to explore the detection limit.The experimental results show that both metamaterial sensors can effectively detect mycotoxins at PPB concentration,and the detection sensitivity is greater than 14.84GHz·（μg/m L）^-1for different kinds of mycotoxins at 1000 ppb concentration.The quantitative regression model constructed by SVM method with dual-band fusion data has a high prediction accuracy（R²=0.989）,and can detect zearalenone with a concentration as low as 20 ppb,which can meet the national grain security standard for the minimum of ZEN in cereals（<60 ppb）.