The Applicaiton Of Graphene Oxide Based Nanocomposites In Sample Preparation Of Multi-pesticide Residues Analysis In Tea

Pesticide residues in tea are the main focus in respect of tea quality and safety. However, tea matrix is very complex containing tea polyphenols, caffeine and pigment, which is seriously interfered with the trace pesticides analysis. Meanwhile, the cost of maintainment of analysis instrument is increased as well. Therefore, it is still a great challenge to accurate analysis of trace pesticide residues in tea. It is the key important to develop high efficient, green, quickly tea sample preparation method.Currently, it is attracted more and more interests of novle sample treatment techniques based on nanomaterials. The new nanomaterial provides new ideas of the development of new sample preparation method for complex matrix. Graphene oxide(GO) has inspired increasing interest in various fields owing to its unique physiochemical properties, such as large surface areas, abundant of oxygen-containg surface groups, easily modification with other organic molecules, etc. The thesis is mainly related to the synthesis of GO and chiston functionalized graphene nanocomposites. They can be used as efficient adsorbents for the pesticide residues analysis in tea infusion and tea. The thesis is devided into five chapters: the first charpter is the literature review for the research of this thesis; the second charpter is the synthesis of GO and amino functionalized mesoprous silica(SBA-15-NH2), and they are used as solid phase purification and extraction material for the analysis of four neoniconoids in tea infusion; the third charpter is about the controllable synthesis of chiston functionalized reduced graphene oxide nanocomposites and analyze its adsorption capacity of the high content of tea matrix co-extracts; the four charpter is about the three-dimensional chitosan-reduced graphene oxide(3D CS-r GO) as efficient tea matrix cleanup adsorbent for the multi-pesticide residues analysis and the last charpter is the summary of the thesis. The main conclusions are listing as follows.(1) The successful synthesis of GO with high oxidation degree. The structure of GO is characterized by XRD, Raman, UV-vis and TEM techniques.(2) The analysis method of neoniconoids in tea infusion based on GO and SBA-15-NH2 was successfully established. SBA-15-NH2 has been proved to be an efficient adsorbent for removal of polyphenols especially catechins from tea infusion. GO gave the best adsorption performance of four neoniconoids compared to other traditional adsorbents. The adsorption parameters like the amount of adsorbent, extraction time, p H, elution solvent, etc. are optimized. Adsorption isotherms and kinetics analyses were applied to illustrate the adsorption mechanisms of neonicotinoid pesticides on GO. It was demonstrated that the adsorption data fitted Freundlich isothermal model and the kinetic data were fitted well using a pseudo-second-order kinetics model. The mechanism for the adsorption of neonicotinoid pesticides on GO was deduced to be by π-π electron donor-acceptor and electrostatic interactions. The recoveries of the four neonicotinoid pesticides range from 72.2 to 95.0% with the RSD between 1.8 and 9.8%. The LOD and LOQ of this method range from 0.46 to 1.4 μg L-1 and 1.5 to 4.7 μg L-1, respectively. The method consists of a purification on amino-functionalized mesoporous silica SBA-15 followed by a solid-phase extraction based on SBA-15-NH2 and graphene oxide before ultra high performance liquid chromatography with tandem mass s pectrometry analysis was established.(3) 3D CS-r GO with porous structures was synthesized via one step solvothermal reduction method. The XRD, FT-IR, XPS spectroscopy, SEM and element analysis were utilized to identify the structure and morphology of 3D CS-r GO. The adsorption performance of high concentration of co-extracts(e.g. catechins, caffeine and chlorophylls) on 3D CS-r GO nanocomposites were conducted in detail. The results showed that 3D CS-r GO nanocomposites exhibited high removal ability compared to other commonly used adsorbents. The adsorption mechanism was studied with the aid of various characterization techniques. The electrostatic interaction, π-π interaction, hydrophobic effect and H-bonding and were the main adsorption force of polyphenols, caffeine and pigments on 3D CS-r GO nanocomposites.(4) A method using 3D CS-r GO nanocomposite as efficient matrix cleanup adsorbent followed by rapid UPLC-MS/MS analysis of multi-residual pesticides in tea was successfully established. The water addition of Qu ECh ERS method and adsorbent dosage were optimized and the influences of 3D CS-r GO on matrix effect on each pesticide were also studied. The method of analysis of fourteen pesticide residues in tea samples based on 3D CS-r GO nanocomposite as efficient clean-up adsorbent has been developed and was applied to analyze in real tea samples. With the neonicotinoid pesticides spiked at low, medium, and high concentration levels(0.05, 0.1 and 0.5 mg kg-1), the recoveries of these fourteen pesticides are ranging from 70.0% to 115.6% with the RSD lower than 13.9%, and the LOD and LOQ of this method ranges from 0.2 to 7.1 μg Kg-1 and 0.7 to 23.8 μg Kg-1.