Color Adjust Mechanism Of Multicolor Fluorescent Carbon Quantum Dots Made From Doping Citric Acid

Carbon quantum dots(CQDs)are novel carbon nanomaterials with strong fluorescence,low toxicity,good photostability and biocompatibility.The diversity of structure and properties of carbon quantum dots is determined by the choice and processing of materials and modifiers.There are many factors affecting on the fluorescence performance of carbon quantum dots,such as particle sizes,degrees of oxidation,pH value of dispersant,types of surface and edge groups,etc.The effects of these factors on the fluorescence wavelength and intensity of carbon quantum dots have been reported.However,it is particularly important to find a simple and green method to prepare multi-color fluorescent carbon quantum dots to be tuned color.This thesis focuses on the study of the law of material ratio of carbon source,through simple regulation of the material ratio of carbon source,looking for the correlation between nitrogen concentration of N doping CQDs(N-CQDs)and the fluorescence wavelength and fluorescence intensity of N-CQDs.Research shows that:(1)When the ratio of citric acid to L-glutamate was 1:1,1:2 and 1:3,the N-CQDs prepared by microwave pyrolysis emit blue,green and red color fluorescence,at peak wavelengths of 444 nm,546 nm and 621 nm,respectively.Moreover,the fluorescence wavelength is redshifted with the increase of N concentration.(2)When the ratio of citric acid to L-glutamate was 6:2,5:2,4:2,3:2,2:2,2:3,2:3,2:4,2:5,2:6,the emission fluorescence of N-CQDs prepared by hydrothermal method gradually shifted from green to orange red,and its corresponding wavelength was 536nm,545 nm,554 nm,567 nm,571 nm,581 nm,601 nm,596 nm,597 nm,respectively.When the ratio of 4:2,N-CQDs emitted yellow fluorescence,and the fluorescence was the strongest.When the ratio was 3:2,N-CQDs emitted orange fluorescence,and the fluorescence was the weakest.When the ratio was 2:4,N-CQDs emitted red fluorescence,and the fluorescence was stronger(more C-O groups,less C=O groups).(3)When the ratio of citric acid to L-glutamate was the same,the fluorescence emission wavelength of N-CQDs prepared using DI water as a dispersant was the shortest and the fluorescence intensity was the weakest.The fluorescence emission wavelength of N-CQDs prepared with C₂H₅OH as a dispersant was the longest wavelength and the strongest fluorescence intensity compared with other N-CQDs.The fluorescence intensity of N-CQDs is also affected by C-O and C-O-C groups,and increases with the increasing of C-OH groups on the edge and the decreasing of C-O-C groups on the surface of N-CQDs.(4)when the ratios of methionine to L-glutamate was 2:2,4:4,6:6,8:8 and 10:10,the emission fluorescence of N-CQDs prepared by hydrothermal method gradually shifted from yellow to red,and their corresponding wavelengths of emission peaks were571 nm,585 nm,625 nm,635 nm and 650 nm,respectively.(5)When the ratio of citric acid and glutathione as 2:2,8:8,16:16,respectively,N-CQDs were made by using hydrothermal method,their corresponding fluorescence wavelength of 440 nm 585 nm 441 nm,respectively.Among of them,N-CQDs fabricated with the ratio of citric acid and glutathione as 16:16 has the fluorescence quenching phenomenon caused by the together of N-CQDs,thus its fluorescence is abated,and the fluorescence wavelength of N-CQDs redshifts with increasing concentration if no aggregate structures.(6)No matter how the ratio of carbon source changes,with the increase of nitrogen content in N-CQDs,the bandwidth of N impurity level in the band gap of N-CQDs increases,resulting in the red shift of fluorescence emission of N-CQDs,and blue shift on the contrary.(7)No matter what ratio of carbon source or modifier is used to prepare N-CQDs,its fluorescence intensity is closely related to the contents of C-O and C=O groups on the surface or edge of N-CQDs.The fluorescence intensity of N-CQDs increases with the increase of C-O groups,and increases with the decrease of C=O groups.