Preparation Of Carbon Dots From Biomass And Application Analytical For Elemental Speciation Analysis

As a new type of carbon nanomaterials,carbon dots（CDs）have the advantages of excellent biocompatibility,good water solubility,high chemical stability,low toxicity,controllable photoluminescence and high fluorescence quantum yield.As an important carbon source for CDs preparation,biomass is not only naturally rich in a large number of carbon atoms and heteroatoms（such as N,S,P）,but also has a wide range of sources,low cost,and green environmental protection.It has become one of the best carbon sources for the preparation of CDs.Therefore,this thesis focuses on exploring the rapid and efficient preparation of heteroatom-doped CDs by hydrothermal or self-exothermic reaction using biomass as the carbon source,and developing new methods for the analysis of Mn O₄^-,Cr（III）/Cr（Ⅵ）and I^-/IO₃^-using the redox effect on the fluorescence quenching of the prepared CDs.The main research of this thesis is as follows:（1）N,P-doped CDs（N,P-CDs）were successfully prepared by a one-step hydrothermal method using soybean phospholipids as the carbon source.The reduced N,P-CDs（R-CDs）were obtained by reduction treatment with KBH₄.R-CDs can emit blue fluorescence（Ex 340nm,Em 425 nm）with a fluorescence quantum yield of 10.0%.It was found that the oxidation of permanganate（Mn O₄^-）had a significant fluorescence quenching effect on R-CDs in acidic medium,and based on this fluorescence quenching effect,a fluorescence sensing method for Mn O₄^-was developed.The method was successfully applied to the fluorescence determination of Mn O₄^-in environmental water samples.（2）N-doped CDs（P-CDs）were successfully prepared by a one-step hydrothermal method using Andrographis paniculata as the carbon source.The reduced state P-CDs（R-CDs）were obtained by reduction treatment of the prepared P-CDs using KBH₄.The prepared R-CDs showed good blue fluorescence（428 nm）with a fluorescence quantum yield of 9.5%at an excitation wavelength of 350 nm.It was found that Cr（VI）had a significant oxidative quenching effect on the fluorescence of R-CDs under strongly acidic conditions,and a highly sensitive and selective fluorescence assay for Cr（VI）was established based on this fluorescence quenching effect.The detection limit（3σ）was 10.0 n M,the relative standard deviation（RSD）was 0.5%（c=1.0μM,n=15）,and the linear ranges were 0.1-5μM and 5-50μM,respectively.Cr（III）was converted to Cr（VI）by the oxidation of H₂O₂,and the speciation analysis of Cr（III）/Cr（VI）was realized by subtraction method,which was successfully applied to the determination of Cr（III）/Cr（VI）in environmental water samples.（3）N-CDs were prepared simply and rapidly by using tea water-soluble substances as carbon source,polyethyleneimine（PEI,1800 M.W.）as passivation agent,and the heat released by the decomposition exothermic reaction of H₂O₂ under alkaline conditions as heat source.The whole preparation process takes only 5 min.The prepared N-CDs have obvious excitation wavelength dependence（the optimal excitation of N-CDs is 330 nm and the emission wavelength is 417 nm）.It was found that under acidic conditions,I^-can undergo a redox reaction with the carbonyl group on the surface of N-CDs,resulting in a significant decrease in the fluorescence of N-CDs.Based on this fluorescence quenching effect,an analytical method for I^-was developed.The detection limit（3σ）of the method was 3.0μM,the relative standard deviation（RSD）was 0.9%,and the linear range was 0.2-1.0 m M.The established analysis method was successfully applied to the fluorescence determination of iodine content in kelp samples.The determination of potassium iodate in salt was also realized by using the reaction of potassium iodate decomposition at high temperature to generate potassium iodide.