| Graphene quantum dots(GQDs)are novel carbon dots with a size of less than 10 nm and a thickness of several graphene sheets.They have some properties of fluorescent carbon nano quantum dots such as small size,good light stability,fluorescence tunability,functionalization,strong fluorescence,anti-photobleaching and excellent biocompatibility.In addition to,they have large carrier mobility,excellent mechanical flexibility,good thermal/chemical stability,and environmental friendliness like graphene.Therefore,its have broad application prospects in biological imaging,drug delivery,photoelectric detector,photocatalysis as well as sensing and other fields,has received the widespread attention.However,low fluorescence quantum yield and the difficulty of controlling the wavelength,especially longer wavelength,poses a huge challenge to their development and applications,and element doping is a common method that can chang chemistry or physics properties effectively.In this work,GQDs and nitrogen-doped graphene quantum dots(N-GQDs)were prepared using citric acid as the main raw material.GQDs were studied from the structure,morphology and optical properties.The details are as follows:1.GQDs were prepared by pyrolysis method using citric acid as carbon source material.X-ray diffraction(XRD)and atomic force microscopy(AFM)characterization results showed that the prepared GQDs had a similar structure to graphene and consist of several layers of graphene sheets.Fourier transform infrared spectroscopy(FT-IR)analysis results showed that there were a lot of oxygen-containing functional groups on the surface of GQDs;optical properties analysis results showed that GQDs emitted blue fluorescence,and have excitation wavelength independence.the fluorescence quantum yield of GQDs was 0.93%.2.Using the prepared GQDs as precursors,urea,ammonia and ethylenediamine as doping nitrogen sources,three N-GQDs(GQDs-urea,GQDs-ammonia and GQDs-EDA)were prepared by hydrothermal method.The results of XRD and AFM measurements showed that the three N-GQDs had a similar structure to graphene and consist of several layers of graphene sheets.FT-IR analysis results showed that the graphene quantum dots were successfully doped with nitrogen.Nitrogen existed in the form of C?N and N?H.Optical properties analysis results showed that the three N-GQDs emitted blue fluorescence and do not have excitation wave dependence.The fluorescence quantum yields were 19.25%,9.79%,and 20.39%.The nitrogen source played the role doping and passivating the surface of the graphene quantum dots to increase the fluorescence quantum yield.3.nitrogen-doped graphene quantum dots N-GQDs were prepared using citric acid and urea as raw materials,DMF and water as solvent.The results of XRD and AFM measurements showed that the prepared product had a similar structure to graphene and consists of several layers of graphene sheets.X-ray photoelectron spectroscopy(XPS)and FT-IR analysis results showed that the graphene quantum dots were successfully doped with nitrogen and nitrogen was present as C?N.Compared with hydrothermal method,N-GQDs prepared by solvothermal method had excitation wavelength dependence,and the maximum emission wavelength of its could reach 523 nm.Their longer emission makes it a potential candidate for bio-labelling and bio-imaging.The characterization results indicated that the solvent N,N-Dimethyl formamide could also participate in the N-doping and surface passivation processes,confirming its importance in regulating the properties of GQDs.In this work,we have improved the fluorescence quantum yield of graphene quantum dots by the method of nitrogen doping;N-GQDs with the higher wavelength were prepared by one-step solvothermal method,which helps to improve imaging resolution in autoimaging.and the autofluorescence of N-GQDs is relatively small.It provide theoretical support for the preparation of graphene quantum dots with high wavelength and its application in areas such as bio-imaging and bio-labelling. |
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