The Synthesis Of Nitrogen-doped Graphene Quantum Dots By Microwave Method And Its Application In Fluorescence Sensing Analysis

Graphene quantum dots?GQDs?is monolayer graphene with lateral size less than 10 nm,which not only inherits the excellent properties of graphene,but also possesses quantum confinement effect and edge effect because of its small size.And it has become competing fluorescence properties.Compared with the traditional quantum dots,GQDs also has good biocompatibility and its synthesis processes are environmental freindlly.It has broad application prospects in biological imaging,analysis of detection,catalytic and other fields.However,the quantum yield of innocent GQDs is low,which limits its application in fluorescence sensing analysis.The internal element-doped and functionalization is an important means to improve its quantum yield and reactivity.Currently,people can use different methods to prepare GQDs,in which microwave hydrothermal assisted method attracts widespread attention because of its simple,fast,efficient and environmentally friendly features.In this paper,microwave hydrothermal assisted method was used: AC-GQDs were synthesized by taking ammonium citrate?AC?as precursors and double-distilled water as the medium;N-GQDs were synthesized by taking citric acid?CA?and urea?Ur?as precursors,double-distilled water as the medium.Both can be applied in the fluorescence sensing analysis of Cr???,hypochlorite?MClO?,and Hg²⁺.The details are as follows:?1?The synthesis of AC-GQDs and determination of Cr???in water samples by fluorescence sensingUsing the Bottom-up synthesis strategy,we take AC as precursor and double-distilled water as medium.The microwave power was set as 800 W by adjusting microwave heating time to synthesize AC-GQDs.The UV-Vis absorption spectra?UV-Vis?,fluorescence spectra?FL?,infrared spectroscopy?IR?and transmission electron microscopy?TEM?were used to characterize the synthesized AC-GQDs.When the excitation wavelength is 360 nm,the strong fluorescence emission of AC-GQDs was at 400-500 nm.There is great degree overlap between the excitation and emission spectra of AC-GQDs and the absorption of potassium dichromate between 340-420 nm.The internal filtering effect can occur between the two to make the fluorescence of AC-GQDs quench Based on the above principle,the fluorescence sensing analysis method of Cr???was established.Under the optimum experimental conditions,the concentration of potassium dichromate and the fluorescence quenching efficiency of the system were linear in the range of 0.10-6.0 ?mol/L,and the detection limit was 0.03 ?mol/L.Common ion does not interfere with the system,the method can be used for the determination of Cr???in tap water and lake water.?2?The synthesis of N-GQDs by microwave hydrothermal assisted methed,characterization and its properties researchDue to the nitrogen and carbon ratio in the above synthesis process of GQDs is definnite,which can not be adjusted.So in this chapter we take CA as the carbon source,Ur as the nitrogen source and double-distilled water as medium,and use the Bottom-up synthesis strategy by adjusting the molar ratio of CA and Ur,microwave power,microwave heating time to control the synthesis of N-GQDs.The UV-Vis absorption spectra?UV-Vis?,fluorescence spectra?FL?,infrared spectroscopy?IR?and transmission electron microscopy?TEM?and X-ray photoelectron spectroscopy?XPS?were used to characterize the synthesized N-GQDs.The results show: when the ratio of CA and Ur are different,the emission wavelengths of N-GQDs red shifted from 440 nm to 525 nm;Microwave power and microwave heating time play an important role in the shift of the emission wavelengths,we initially realized the fluorescence emission wavelength controllable synthesis of N-GQDs.?3?The construction and application of hypochlorite fluorescence sensor based on N-GQDsHypochlorite has strong oxidizing ability,can react with the amino groups on the surface of N-GQDs to form chloramine.The intramolecular charge-transfer?ICT?between N-GQDs skeleton and chloramine could induce the fluorescence quenching.Under the optimal experimental conditions,the concentration of hypochlorite and the fluorescence quenching efficiency of the system have linear relationship in the range of 0.20-13 ?mol/L,and the detection limit was 0.07 ?mol/L.The common ions do not interfere with the fluorescence of the system.The analytical method has high selectivity and considerable sensitivity to hypochlorite,can be used for the determination of hypochlorite in water samples such as tap water and lake water.?4?The construction and application of Hg²⁺ fluorescence sensor based on N-GQDsThe surface of N-GQDs has many different functional groups,such as-COOH,-NH₂,-OH,which can interact with Hg²⁺ to quench the fluorescence of system.Under the optimal experimental conditions,the concentration of Hg²⁺ and the fluorescence quenching efficiency of the system have linear relationship in the range of 0.03-24 ?mol/L,and the detection limit was 0.01 ?mol/L.The common ions do not interfere with the fluorescence of system.The analytical method has high selectivity and considerable sensitivity to Hg²⁺ and can be used for the determination of Hg²⁺ in water samples such as tap water and lake water.