Preparation And Application Of Molecularly Imprinted Polymers Based On Covalent Organic Framework For Specifically Recognizing Norfloxacin

Norfloxacin（NFX）,as a common quinolone veterinary drug,is widely used in animal husbandry and aquaculture.However,many kinds of animals have low bioavailability,slow metabolism and elimination of NFX,and will inevitably be retained in the tissues and organs of animals.These NFX residues in animal-derived foods can eventually move through the food chain and cause harm to human health.Nowadays,many countries and organizations have strict restrictions on the amount of NFX residues in animal-derived foods.The maximum amount residues（MRL）of NFX in meat food should not exceed 0.1 mg/kg in the regulations of EU.Although a number of detection methods have been developed,such as chromatography coupled with/without MS,sample preprocessing is always required to remove interference from complex substrates prior to determination.Molecularly imprinted polymers（MIPs）,as a kind of synthetic‘antibody’materials,have the advantages of high affinity,good specificity and strong anti-interference to target molecules.Among which,the carrier is one of the key factors affecting the performance of MIPs.Covalent organic frameworks（COFs）,due to their ordered crystal structure and easy functionalization,indicate great potential as good carriers.However,the traditional for COFs preparing is complicated and time-consuming.In the second chapter,we developed one facile method.By the way,the prepared COF was used it as a carrier to prepare the covalent organic framework molecularly imprinted polymer（DP-COF@MIP）for the selective separation and enrichment of NFX in complex samples.And in the third chapter,the method was used to prepare chemically modified electrode（CME）with glassy carbon electrode（GCE）as support.And the CME was applied for fast detecting NFX.In the fourth chapter,three different metal cations were doped in the COF to further improve the sensitivity of the sensor.The details are as follows:1)Molecularly imprinted polymers based on covalent organic framework for specifically recognizing trace NorfloxacinIn this chapter,one kind of MIPs based on COFs was prepared.First,the"Schiff base"covalent organic framework（DP-COF）was rapidly synthesized using metal trifluorate as catalyst,p-benzaldehyde and 3,3’-diaminobiphenyl as raw materials at room temperature.Then NFX,methacrylic acid（MAA）,ethylene glycol dimethacrylate（EDMA）and DP-COF were mixed,and the polymerization reaction was initiated by azo diisobutyronitrile（AIBN）to obtain DP-COF@MIPs.The whole preparation process takes only 5 hours under mild conditions.Field emission scanning electron microscopy（SEM）,Fourier infrared spectroscopy（FTIR）,X-ray diffractometer（XRD）,BET specific surface area tester（BET）,and nano-particle size potential analyzer were used to characterize the samples.The results confirmed that DP-COF@MIPs was successfully prepared.The material has a coarse surface and a mesoporous pore diameter（17.79 nm）.The adsorption test and reusability test were used to evaluate the properties of the material.The results showed that the apparent adsorption capacity of the material was as high as 41.57 mg/g,and the material had good specificity and selectivity for NFX recognition,and the reusability rate was satisfactory.Combined with HPLC-UV-Vis,trace NFX in chicken breast and milk samples was detected.The average recoveries were in the range of 88.8%～92.9%with the relative standard deviations（RSD）less than 1.7%（milk）.The results showed that the method could achieve high selectivity,high sensitivity and high accuracy in the detection of animal residues in complex matrix.2)Norfloxacin imprinted electrochemical sensor based on covalent organic frameworkFirstly,the surface of GCE was ammoniated by p-phenylenediamine.The rich amino group on the surface of GCE was used to conduct a"Schiff base"reaction with the aldehyde group on the surface of DP-COF,and DP-COF was fixed on the surface of GCE.DP-COF@GCE was immersed in a mixture containing template molecule NFX,functional monomer MAA,cross-linking agent EDMA and initiator AIBN,and CME was prepared by in-situ polymerization.After polymerization,the template molecules were eluted with an eluent（methanol:acetic acid,9:1,v/v）to obtain an electrochemical sensor DP-COF@MIP@GCE.The surface roughness of the electrochemical sensor was observed by SEM.The selectivity of DP-COF@MIP@GCE was investigated by choosing ciprofloxacin,sulfadiazine and chloramphenicol as the interferences.The results showed that the electrochemical sensor has good selectivity,stability and reproducibility.After that,milk was selected as the actual sample,and it was found that the pretreatment step of the sample could be ignored by using the electrochemical sensor.The average recovery rate was still in the range of 85.1～92.3%and the RSD was less than 1.3%.The reliability of the method was verified by HPLC-UV-Vis.The results showed that this method could realize rapid,sensitive and selective detection of veterinary drug residues in complex substrates.3)Norfloxacin imprinted electrochemical sensor based on metal-doped covalent organic frameworkFirstly,DP-COF was modified onto the surface of GCE,and then Cu²⁺,Ag⁺or Ni²⁺was doped into COF by metal-post doping method,respectively,and then M-COF@MIP@GCE was prepared by in-situ polymerization.According to CV,EIS and SWV,the effect of doping with metal Cu was the best.After the characterization of Cu-COF@MIP@GCE,the results show that the surface of the electrochemical sensor is rough,and the Cu element ratio on the surface of the electrochemical sensor was 2.8%as measured by EDS.Moreover,Cu-COF@MIP@GCE has good stability and reproducibility,as well as good selectivity to NFX.The recovery of the sensor was in the range of 92.5～97.4%with RSDs less than 1.4%when it is applied to detect NFX in milk.The reliability of the method was verified by HPLC-UV-Vis.The results show that the electrical conductivity of the electrochemical sensor can be improved by metal doping,and the doping effect of Cu is the best,and the electrical conductivity and sensitivity of the electrochemical sensor are improved after doping.