Preparation Of Carbon Quantum Dots From Graphitized Carbon And Its Application

In this paper,the selective oxidant HNO₃/HClO₄ was used to selective oxidation on graphitized carbon materials to prepare carbon quantum dots?CQDs?with different photoluminescence colors,and we further improved its fluorescence performance through surface chemical modification.High-resolution transmission electron microscopy,solid nuclear magnetic resonance spectrometry,Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,X-ray diffraction and Raman spectroscopy were employed to characterize the microstructure and surface chemistry of the CQDs.UV-vis spectrophotometer,fluorescence spectrophotometer and FM-4P-TCSPC fluorescence spectrofluorometerfluorescent were employed to characterize the fluorescent properties of CQDs.In addition,we also explored it's application in fluorescence probes and biological imaging.Activated carbon is a kind of carbon material composed of large amount of graphite-like microcrystalline units connected by amorphous carbon.The amorphous carbon can be gradually transformed into microcrystalline carbon by heating treatment at 1500-2500 ?,and the size of graphite-like microcrystalline becomes larger with the increasing temperature.The different structures of CQDs can be obtained by selective oxidation of graphitized carbon with HNO₃/HClO₄ and the CQDs prepared from graphitized carbon with higher degree of graphitization have high ordered carbon layer planes dominated by sp2 hybrid carbon.Meanwhile,the degree of graphitization of activated carbon has an significant influence on the microstructure and surface chemical structure of the as-prepared CQDs.In addition,the CQDs with the same kind of surface functional groups obtained from higher degree of graphitization of activated carbon have larger size and clearer lattice stripe.The CQDs prepared from activated carbon with higher degree of graphitization emits blue shift of fluorescence emission spectrum peak and has a higher quantum yield,but the fluorescence lifetime is reduced.The CQDs obtained from the activated carbon with a higher degree of graphitization have a larger size,so that they have a more complete conjugated system,which can promote the radiation-recombination of electrons on the surface of the CQDs,thus reducing the fluorescence self-absorption quenching phenomenon.In addition,the fluorescence intensity of CQDs prepared from raw material activated carbon is highly sensitive to pH value,it can be stable in acidic solution,but the high alkaline solution can lead to the fluorescence quenching.The CQDs prepared by chemical oxidation method has many surface defects.The surface chemical structure of CQDs can be changed through reduction of NaBH₄,surface modification with adenine and surface modification with 3-mercapto propionic acid.All of these method can reduce the surface defects,and thus improved the fluorescence performance of CQDs.At the same time,the tolerance of U-CQDs to pH value increased after surface reduction,so that it still has a certain fluorescence intensity in the solution with higher alkalinity.In addition,the fluorescence self-absorption quenching can be further reduced by the surface reduction of2000-CQDs which has a highly ordered carbon layer plane.The CQDs prepared from raw carbon can be used as fluorescent probes for copper ions and KMnO₄.Moreover,the as-prepared CQDs have no cytotoxicity and can be successfully applied to cell imaging experiments.