Detection Of Tetracycline And Its Degradation Products Based On Asymmetric Resonator Combined With THz Spectroscopy

Tetracycline is widely used to prevent and treat animal disease in animal breeding industry due to its good curative effect and low cost.However,tetracycline is seriously instable and can produce harmful degradation products including epitetracycline,anhydrotetracycline,and epianhydrotetracycline during its producing and reserving process.In order to reduce the risk of animal diseases and costs as much as possible,powder made from residual fermented liquid derived from tetracycline production process is used as feed additive for daily animals feeding by some illegal businessmen,which leads to the residual of tetracycline,epitetracycline,anhydrotetracycline,and epianhydrotetracycline in animal body and does great harm to human health.Therefore,it is very important to realize the fast detection of tetracycline and its degradation products.Terahertz spectroscopy,especially terahertz time-domain spectroscopy(THz-TDS),has been a newly developed detection method in recent years.Compared with traditional techniques,THz-TDS is safe,non-destructive and fast.It has been widely used in detection of biology and chemistry molecule.However,the sensitivity of THz-TDS is not very high due to the technical immaturity.Besides,water has strong absorption which causes the big difficulty of highly absorptive liquid detection using THz-TDS.Metamaterial is an artificial composite structure or material with unique electromagnetic response characteristics,which can significantly improve the sensitivity of terahertz spectroscopy detection and has been widely used in terahertz sensing.Nevertheless,the detection of highly absorptive solution remains a big challenge due to the strong absorption of water.In this study,common tetracyclines in food including tetracycline hydrochloride(TCH)and its degradation products including epitetracycline hydrochloride(ETCH),anhydrotetracycline hydrochloride(ATCH)and epianhydrotetracycline hydrochloride(EATCH)were used as research objects,we aim to prove the feasibility of qualitative identification of them using THz-TDS by studying their terahertz absorption spectra,and build a method system for their aqueous solutions detection based on asymmetric dual-wire resonator(ADWR)and THz-TDS,providing a new idea for the detection of tetracycline and its degradation product and other antibiotics residues in liquid foods.The main contents,results and conclusions are summarized as follows:(1)The terahertz spectra of TCH and its degradation products at room temperature were analyzed.The vibration modes of their characteristic absorption peaks,mainly the intramolecular vibration modes,were analyzed using density function theory(DFT)method.The results indicated:1)in 0.2-1.8 THz,TCH had two strong characteristic absorption peaks at 0.77 and 1.40 THz,respectively.ETCH had five weak characteristic absorption peaks at 0.61 THz,1.01 THz,1.17 THz,1.32 THz,and 1.59 THz,respectively.ATCH had two strong and one weak characteristic absorption peaks,which were located at 0.88 THz,1.17 THz and 1.30 THz,respectively.EATCH had no characteristic absorption peaks at 0.2-1.8 THz.And overall,the absorption coefficient of EATCH was the highest,followed by TCH and ATCH,and then ETCH.Besides,EATCH had the highest refractive index,followed by TCH,ATCH,and then ETCH.2)DFT results indicated the characteristic absorption peak of TCH at 0.77 THz was from intramolecular vibration,while the characteristic absorption peak at 1.40 THz was possible from intermolecular vibration modes.All the characteristic absorption peaks of ETCH were from intramolecular vibration modes.The characteristic absorption peak of ATCH at 0.88 THz is from intramolecular vibration,those of 1.17 THz and 1.30 THz are possible from the intermolecular vibration.EATCH has a lot of intramolecular vibration modes with close resonance intensity,and does not show characteristic absorption peaks.The results indicated that TCH and its degradation products had distinctly different terahertz spectra signatures,which allowed qualitative identification of them,and DFT could explain origin of terahertz characteristic absorption peaks in part,which built theoretical basis for follow-up studies.(2)The terahertz spectra of TCH and its degradation products under different temperatures were analyzed.Combined with DFT results,the vibrational modes of their characteristic absorption peaks,especially the intermolecular vibration modes,were further analyzed.The results indicated:1)with the decrease of temperature,the two characteristic absorption peaks of TCH in 0.2-1.8 THz moved,and the relationship between the frequency of characteristic absorption peak at 1.40 THz and temperature accorded with the empirical formula of resonant frequency and temperature of phonons.Combined with the result of the DFT analysis,the characteristic absorption peak at 0.77 THz of TCH was from intramolecular vibrational modes,and that at 1.40 THz was from the phonon-phonon scatting caused by the anharmonicity vibration potential of intermolecular hydrogen bond.2)with the decrease of temperature,the five characteristic absorption peaks of ETCH in 0.2-1.8 THz moved.And the relationship between the frequency of the five characteristic absorption peaks and temperature were not compl:iant with the empirical formula of frequency and temperature of phonons.Combined with the result of the DFT,the five characteristic absorption peaks of ETCH were all from intramolecular vibration modes.3)The characteristic absorption peaks of ATCH in 0.2-1.8 THz moved with decreasing temperature,and the relationship between the frequency of the characteristic absorption peak at 1.17 THz and temperature accorded with the phonon frequency-temperature empirical formula.Combined with the DFT results,the characteristic absorption peak of ATCH at 0.88 THz came from the intramolecular vibrational mode.The characteristic absorption peak at 1.17 THz resulted from the phonon-phonon scattering caused by anharmonicity vibration potential of intermolecular hydrogen bond.The vibrational mode at 1.3 THz came from the common mode of intermolecular vibration;4)No discernible characteristic absorption peak of EATCH is found in 0.2-1.8 THz under different temperature,which validated the DFT results.The results indicated that the origin of each characteristic absorption peak of TCH and its degradation products could be further analyzed and clarified by studying the temperature dependence of characteristic absorption peak,which provided theoretical foundation for follow-up studies.(3)Using simple metal-wire array as basic research structure and Mylar as the substrate,structure of metal-wire arry,especially asymmetry between adjacent wires on its electromagnetic characteristics was analyzed using CST simulaition software.The structure parameters of the metal-wire structure was optimized to obtain metamaterial which had multiple resonance modes,high resonance intensity,high sensitivity,and could be used for detection of highly absorptive solution in terahertz region where TCH and its degradation products could be qualitatively identified.And the optimal structure was used to detect the highly absorptive water-methanol mixed solution to verify the feasibility of terahertz detection of highly absorptive liquid using it.The results indicated:1)asymmetric dual-wire resonator(ADWR)with multiple resonances,including a Fano resonance dip,a Fano transmittance peak,and a Dipole resonance dip in 0.5-2.0 THz was obtained once the structural symmetry was breaking.With the asymmetry increased,the sensitivity of Fano resonance dip and Fano transmittance peak were decreased,that of Dipole resonance dip was increased,and the amplitude transmission of Fano transmittance peak was increased.After comprehensive consideration of resonance intensity and sensitivity,the ADWR with the length of the two wires to be 60 ?m and 106?m were selected to be the final used structure.And the sensitivity of Dipole resonance dip of the selected ADWR was the highest,followed by Fano transmittance peak,and Dipole resonance dip.2)With the increase of water concentration in water-methanol mixture,the Fano resonance dip,Fano transmittance peak,and Dipole resonance of ADWR exhibited red shifts,and the transmittance of Fano transmittance peak decreased.And sensitivities of the Fano resonance dip,Fano transmittance peak,and dipole resonance dip for detection of TCH and its degradation products aqueous solution turned out to be 93 GHz/RIU,160 GHz/RIU,and 305 GHz/RIU.The results indicated that the designed ADWR had multiple resonances,higher resonance intensity,higher sensitivity,and could be effectively used for terahertz detection of highly absorptive liquid,which provided a method for following terahertz detection of TCH and its degradation products aqueous solution with high sensitivity and accuracy.(4)Using dropping-drying method,the common used sample preparation method in terahertz metamaterial detection to prepare sample,the indirect detection efficiency of aqueous solutions of TCH and its degradation products using ADWR combined with THz-TDS were analyzed.The results indicated:1)all resonance modes showed red shifts and the amplitude transmission of Fano transmittance peak decreased in terahertz absorption spectra with the increase of TCH and its degradation products concentration.And dipole resonance dip exhibited the largest frequency shift,followed by Fano transmittance peak and Fano resonance dip,which was in accord with their simulation results in Chapter 4 of this paper.Besides,EATCH caused the largest frequency shifts for the three resonance modes of ADWR and biggest transmittance change for the Fano transmittance peak,followed by TCH and ATCH,then ETCH,which related to the size of the refractive indices and absorption coefficients of these four substances in Chapter 2 of this paper.2)The PCA results indicated that the lowest detectable concentration of TCH aqueous solution using Fano resonance dip,Fano transmittance peak and dipole resonance dip combined with THz-TDS were 0.1 mg/L,0.01 mg/L and 0.01 mg/L,respectively,that of ATCH and EATCH was the same with TCH,and the lowest detectable concentration of ETCH aqueous were 0.1 mg/L,0.1 mg/L,and 0.1 mg/L,respectively.These results were smaller than the maximum residue limit(MRL)of tetracyclines in animal-origin food from international regulations(0.1-0.6 mg/L).Furthermore,the detection accuracy could be improved using all resonance modes of ADWR to detect TCH and its degradation products at the same time.The results indicated that dropping-drying method-based ADWR combined with THz-TDS had a certain application prospect in indirect teraherz detection of TCH and its degradation products aqueous solutions with high sensitivity and accuracy.(5)Using liquid cuvette with small pathlength as the sample cell,the direct detection efficiency of aqueous solutions of TCH and its degradation products using ADWR combined with THz-TDS were analyzed.The results indicated:1)the resonance intensity of the three resonance modes of ADWR decreased obviously,especially the Dipole resonance dip in detection of solutions.Besides,the three resonance modes,especially the Fano Resonance and Fano transmission peak in lower frequency region exhibited blue shifts,and the amplitude transmission of Fano transmittance peak increased with the increase of the concentration of sample solution,which was contrary to the results of the solid film.For TCH and ATCH aqueous solution detection,the frequency shift of Fano resonance dip was the most,followed by the Fano transmission peak and Dipole resonance dip.For ETCH and EATCH aqueous solution detection,the frequency shift of Fano transmittance peak was the most,followed by the Fano resonance dip and Dipole resonance dip.2)The PCA results indicated that the lowest detectable concentrations of TCH,ETCH,ATCH and EATCH aqueous solution using Fano resonance dip,Fano transmission peak and Dipole resonance dip of ADWR combined with THz-TDS were 10 mg/L,10 mg/L and 100 mg/L,respectively.Although it did not reach the MRL(0.1-0.6 mg/L)of the tetracycline antibiotic in animal-origin food from international regulations,it was significantly lower than reported limit of detection of tetracycline antibiotic solution by conventional liquid cuvette method(730 mg/L)and the attenuated total reflection method(450 mg/L).Besides,the detection accuracy could be improved using all resonance modes of ADWR to detect TCH and its degradation products at the same time.The results indicated that liquid cuvette-based ADWR combined with THz-TDS had a certain application prospect in direct,fast,higher sensitive and accurate teraherz detection of TCH and its degradation products aqueous solutions.