Study On Application And Sensing Mechanism Of Carbon Dots-molecular Imprinting Sensors In Detection Of Chloramphenicol

Chloramphenicol（CAP）is a broad-spectrum antibiotic that has been banned firstly for adding into animal-derived foods in China.Its harm is self-evident.However,it is occasionally abused in animal-derived foods due to its low cost and high efficiency.In this paper,a molecularly imprinted（MIP）fluorescence sensor based on carbon dots（CDs）was designed for detecting CAP in food systems.The CDs@MIP fluorescence sensor was utilized CAP as the target compound,CDs as fluorescent probes,and MIP as selective enrichment container.The CDs@MIP has combined the excellent fluorescence properties of CDs and the functions of MIP for customizing specific recognition sites for target compounds.The main contents of this thesis are as follows:Combined with a reversed microemulsion method and surface molecular imprinting method,the CDs@MIP fluorescence sensor with a good water dispersibility was synthesized with CAP as the template molecule.The conditions of synthesis and elution were optimized to obtain the optimum content of reagents and single elution scheme.The optimized ratio of template molecule,functional monomer,crosslinking agent and CDs was determined to be1:5:20:1.The single elution scheme was prepared through eluting with 30 mL of methanol-acetic acid（8:2,v/v）mixed solvent for 20 min.In addition,the structure and morphology of the CDs@MIP were characterized:the prepared CDs@MIP were fluorescent microspheres with a diameter around 10 nm and have excitation wavelength dependence,and its interior was wrapped with CDs cores.Secondly,a rapid molecular imprinting fluorescent detection method for CAP was established.The parameters,including types of solvent,concentration of CDs@MIP,pH and detection time were studied.Under the optimized detection conditions,the sensing performance of CDs@MIP on CAP was evaluated.The quenching of CDs@MIP showed two different linear relationships with CAP concentration of 1.50×10^-3-125μmol/L due to the uneven size of CDs@MIP particles and imprinted cavities.There was a good linear correlation（R²=0.9969）between the concentration of CAP and the quenching of CDs@MIP when the CAP concentration was ranging from 1.50×10^-3-1.50×10^-2μmol/L,and the detection limit was calculated to be 12.83 nmol/L.Subsequently,this established method was applied to real food sample（i.e.,crucian）.The recovery rate in the crucian was between90.02%-102.53%,and the reaction would reach to equilibrium within 5 min,which was suitable for the rapid detection of CAP in real food samples.Thirdly,the quenching responses of CAP to CDs@MIP,and that to carbon dots-non-imprinted polymers（CDs@NIP）as well as CDs were compared.The results showed that CDs@MIP had an excellent imprinting effect and quantitative detection ability for detecting CAP at a relatively low concentration.The imprinting factor was calculated to be 8.22.Furthermore,the CDs@MIP also showed an excellent specific selectivity and anti-interference ability.The fluorescence intensity would still keep at 79.86%after repeated use for 4 times,and the quenching effect of the same concentration of CAP remained stable for each time.The fluorescence intensity was also stable in the range of pH 5.0-10.0.The sensing mechanism of CDs@MIP to CAP was studied lastly.The sensing processes would be divided into two steps,i.e.,adsorption and fluorescence quenching.Through the fitting analysis of several models commonly used in the study of adsorption mechanism,it was found that the adsorption of CAP by CDs@MIP was a relatively complex heterogeneous multi-molecular layer adsorption process,and there were identification sites with uneven affinity on the surface of CDs@MIP.The adsorption rate was mainly controlled by liquid film diffusion at the initial stage of adsorption,and then gradually changed to the particle diffusion.When the CAP molecules were adsorbed and transferred into the interior of CDs@MIP particles,fluorescence quenching of the fluorescent element CDs occurred.It could be concluded that the quenching type of CAP to CDs@MIP was a dynamic quenching processing according to a shortening of fluorescence lifetime of CDs@MIP after adding CAP.Moreover,it can be inferred that the possible mechanism of fluorescence quenching was the charge transfer based on the charge properties of the groups contained in CDs@MIP and CAP.In addition,the charge transfer quenching mechanism of CAP on CDs@MIP was explained based on the molecular orbital theory.