Mechanism And Arrangement Optimization Of Carbon Nanotube Terahertz Metamaterials For Quality And Safety Detection Of Agricultural Products

Terahertz(THz)spectroscopy has demonstrated its attractive prospects in the quality and safety inspection of agricultural products due to its safe,transmission,and fingerprint properties.However,the interaction between THz waves and samples is relatively poor,which limits the further improvement of detection sensitivity of THz spectroscopy.By using metamaterials to excite surface plasmons to enhance the interaction between the THz waves and the sample,the signal acquisition ability can be effectively improved.Carbon nanotubes(CNTs)have excellent adsorption and fast electron transfer properties,and the plasma generated by CNTs is low-loss and tunable.Therefore,metamaterials based on carbon nanotubes have considerable potential for THz sensing applications.In this thesis,the research was conducted to use disordered and aligned CNT terahertz metamaterials(THz-MMs)fabricated by a two-step method respectively,which aimed at testing glucose and chlorpyrifos-methyl(CM)samples based on terahertz time-domain spectroscopy(THz-TDS)technique.The detection method based on THz-MMs applied to quality and safety of agricultural products was also explored and proposed.This study could provide a new solution to the common problems in THz sensing and a novel idea for the rapid detection of the quality and safety of agricultural products.The main research contents and results in this thesis are illustrated as follows:(1)A two-step method was proposed to fabricate disordered CNT THz-MMs used to test glucose and CM samples.In terahertz reflection mode,the resonant frequency of THz-MMs was 0.4 THz by experimental test and simulation calculation.On this basis,THz-MMs can achieve the rapid determination of the minimum concentration of 30 ng/m L glucose and 10ng/m L CM samples.Besides,the detection mechanism of THz-MMs was then analyzed by simulating the distribution of electric fields.(2)Highly filled and aligned CNT membranes were prepared by the slow vacuum filtration method.The relationship between the direction of the stripe structure and the orientation of the CNT alignment during THz-MM preparation was investigated.THz-MMs were fabricated by engraving stripe structures along the different orientations of aligned CNTs.The results showed that the terahertz transmission range of parallel THz-MMs was close to 0-1.Therefore,it was determined that the direction parallel to the CNT alignment was the best to prepare THz-MMs.(3)The layer-by-layer method was adopted to prepare multi-layer aligned CNT membranes which were used to fabricate parallel aligned CNT THz-MMs for the highsensitivity detection of glucose and CM samples.Through experimental tests,the relationship between the transmission characteristics of THz-MMs and the incidence direction of THz waves was investigated,and the resonance peak of THz-MMs in the transmission mode was obtained.On this basis,the dropping-and-drying method was used to test the transmission of glucose and CM samples of different concentrations,with the limit of detection(LOD)of 20ng/m L and 10 ng/m L,respectively.Compared with disordered THz-MMs,the detection sensitivity of parallel aligned CNT THz-MMs was increased by 10 ng/m L.The detection mechanism was also explained based on simulation analysis.(4)The adsorption behavior of disordered and aligned CNT THz-MMs was explored and compared,which could interpret adsorption's influence on the detection sensitivity of THzMMs.By carrying out elemental analysis,adsorption,molecular dynamic(MD)simulation,and contact angle experiments,the adsorption capacity of disordered and aligned CNT membranes was tested and simulated.The results indicated that the adsorption performance of the aligned CNT membranes was twice that of the disordered,meaning that the molecules were more easily captured and enriched on the aligned CNT membranes.Therefore,the detection sensitivity of aligned CNT THz-MMs can be further improved.