Synthesis Of Carbon Quantum Dots(CDs) And Its Application In Analysis Of Trace Elements

With the development of society,heavy metal pollution is becoming more and more serious,which has characteristics of wide distribution,diverse forms,difficult degradation and high toxicity.Heavy metals can enter the soil,air and water through various forms,and eventually were accumulated through the food chain,resulting in irreversible damage to human health.Therefore,developing a fast,sensitive and accurate method for heavy metal analysis is of great significance for protecting the environment and improving the quality of life of human beings.Carbon dots?CDs?is a new type of carbon material composed of sp² hybrid carbon or amorphous carbon.It has a quasi spherical structure and can stable the luminescence.Carbon dots has characteristics of excellent water solubility,chemical stability,low toxicity and good biocompatibility.It has potential applications in many fields,such as bioimaging,organic analysis and photocatalysis.The application of CDs in the field of metal ion recognition and detection is one of the hot fields of CDs research.The fluorescence analysis based on carbon dots for metal ions has the advantages of wide linear range,high sensitivity and selectivity,and the sample preparation is simple without complex preprocessing,and the sample needs less.At present,there are some problems in the application of CDs to the analysis of metal ions,such as the low fluorescence quantum yield of unpassivated CDs,and the detection mechanism is mostly based on the complexation or electrostatic force between metal ions and CDs,which leads to the fluorescence quenching of CDs.In view of the above problems,the main contents of this research are as follows:?1?CDs was prepared by hydrothermal method using citric acid as a carbon source and deionized water as a solvent.By coupling reaction between-NH₂ and-COOH,the o-phenylenediamine?OPD?was grafted to the surface of CDs,and the phthalic two amine modified fluorescent carbon dots?CDs-OPD?was obtained.The optical properties of the prepared CDs-OPD were characterized by UV-vis,FT-IR and fluorescence spectroscopy.The results show that CDs-OPD has a three fluorescence emission peaks?295 nm,440 nm and 580nm?with a excitation wavelength of 260 nm,compared with the CDs without OPD modification.The effects of metal ions on the fluorescence properties of CDs-OPD were investigated.The results showed that Cu²⁺could selectively quencher the fluorescence of CDs-OPD,and other metal ions did not produce obvious fluorescence quenching.Cu²⁺can interact with the groups on the surface of CDs-OPD,which makes the fluorescence peak?295 nm?of CDs-OPD obviously quenched,and the degree of fluorescence quenching is marked as?F(?F=F₀-F,F0 is the fluorescence emission intensity of the blank solution,F is the fluorescence emission intensity of the test solution with Cu²⁺),and the?F is linear with the concentration of Cu²⁺.A new analytical method for Cu²⁺based on CDs-OPD was established.The detection limit?3??was 1.4 ng mL^-1,the relative standard deviation?RSD?was 5%(C=100 ng mL^-1,n=11),and the linear range was 10⁵00 ng mL^-1.The developed method was successfully applied to the determination of trace amounts in tea samples.The results were satisfactory.?2?CDs was prepared by hydrothermal method with glucose,ethylene glycol in deionized water as raw material,and CDs-BPEI was obtained by surface modification of CDs by polyethyleneimine?BPEI?.The optical properties of the prepared CDs-BPEI were characterized by UV-vis,FT-IR and fluorescence spectroscopy.The results showed that the fluorescence emission peak?Em 470 nm?of BPEI has an obvious red shift compared with that bare CDs without modification.The effects of metal ions on the fluorescence intensity of CDs-BPEI was investigated.It was found that,CDs-BPEI had strong binding ability to Cu²⁺because of the rich-NH₂ group on the surface of CDs-BPEI,and had better selectivity.The selective combination of CDs-BPEI and Cu²⁺leads to a great fluorescence quenching,and the degree of fluorescence quenching??F?has a linear relationship with the concentration of Cu²⁺.A new analytical method for Cu²⁺based on CDs-BPEI was established.The detection limit?3??was 1 ng mL^-1,the relative standard deviation?RSD?was 2.3%(C=100 ng mL^-1,n=11),and the linear range was 10²40ng mL^-1.The developed method was successfully applied to the determination of trace Cu²⁺in environmental water samples with a satisfactory result.The effects of reduced glutathione?GSH?on the fluorescence recovery of CDs-BPEI+Cu²⁺system was further studied.Due to the strong binding ability of-SH to Cu²⁺,GSH is added to the CDs-BPEI+Cu²⁺system.Because of the stronger binding ability between GSH and Cu²⁺,the Cu²⁺is was desorbed from the CDs-BPEI surface,and the fluorescence of CDs-BPEI system is restored.There is a linear relationship between the?F and the concentration of GSH??F=F-F₀,F is the fluorescence intensity of the sample solution at the 470 nm emission peak,and F₀ is the fluorescence intensity of the blank solution at the 470 nm emission peak?.Based on the above new analytical method for GSH was established.The detection limit?3??was 4×10^-6 mol L^-1,the relative standard deviation?RSD?was 1.5%(C=7×10^-5 mol L^-1,n=11),and the linear range was 4×10^-514×10^-5 mol L^-1,The proposed method is successfully applied to the determination of GSH in the wheat embryo samples with a satisfactory result.?3?Anhydrous citric acid and n-??-aminoethyl-?-Aminopropyl?methyl-methoxy-silane as the carbon source,CDs was prepared by thermal cracking.Afterwards,silica nanoparticles as interconnect medium,links CDs to RhB which is another organic fluorescent dye,to be synthesis of CDs coated dual-emission silica nanoparticles.This dual-emission nanoparticles combining tow different fluorophores,which are CDs and RhB,CDs's fluorophore as reference and RhB's fluorophore as a signal report unit.,use of ratiometric fluorescence method for quantitative analysis.A new catalytic fluorimetric method for determination of V?V?is proposed.In acetic acid media,the oxidation of CDs coated dual-emission silica nanoparticles by potassium bromate?KBrO₃?is catalyzed by V?V?.Resulting in the ratiometric fluorescence quenching of the dual-emission silica nanoparticles.Several important parameters affecting the performance of the nanosensor were investigated and optimized.The detection limit?3??of this nanosensor was1.1 ng·mL^-1 with a linear range from 10 to 800 ng·mL^-1.The proposed method is successfully to determine V?V?in the mineral water samples with a satisfactory result.