Design And Application Of Nitrogen Doped Carbon Quantum Dots

In recent years,carbon quantum dots?CQDs?have been rapidly developed in the fields of bioimaging,environmental detection and electrochemistry due to their excellent physical and chemical properties.However,disadvantages of CQDs,such as low fluorescence quantum yield and weak fluorescence stability,limit their further application in these fields.Nitrogen?N?doping is an effective means to improve the fluorescence efficiency of CQDs.Here,N doped CQDs?N-CQDs?were designed to investigate the influence mechanism of N doping on the fluorescence properties of CQDs,from aspects of doping amount and doping bond,and to illuminate the most effective nitrogen-doping structure.The prepared N-CQDs were applied to iron ion(Fe³⁺)detection,cell imaging and polyaniline?PANI?modification.1.In order to illuminate the most effective nitrogen-doping structure that is favorable for improving the fluorescence efficiency,two kinds of N-CQDs were designed,and the influences of chemical bonds between nitrogen atoms and their adjacent carbon atoms on the fluorescence properties of CQDs were systematically studied.N-CQDs-1 and N-CQDs-2were obtained by employing glucose?GLU?as carbon source,metformin hydrochloride?MH?and diethylenetriamine pentaacetic acid?DTPA?as nitrogen sources,respectively.N-CQDs-1 and N-CQDs-2 that show differences in chemical bonds of doped nitrogen atoms but are common in the nitrogen/carbon?N/C?molar ratio,the content of sp² hybridized carbon atoms and particle size.We can concluded that pyridine N is beneficial to increase the fluorescence quantum yield?QY?of CQDs based on the experimental results,probably caused by the enhancement of electron exchange of complex luminescent centers,and the formation and introduction of electron traps.The effect of N/C molar ratio on the fluorescence performances of N-CQDs-1 was studied by changing the ratio of nitrogen source?MH?and carbon source?GLU?in the precursor.The QY of N-CQDs-1 does increase significantly due to the incorporation of N atoms and increases with the amount of doped N atoms.2.Pyridine N-rich N-CQDs-1 were prepared.The average size of prepared pyridine N-rich N-CQDs-1 is about 3.5 nm and the N/C molar ratio is as high as 40.1%.It is the high nitrogen content which provides pyridine N-rich N-CQDs-1 high QY?at excitation wavelength of 365 nm,400 nm,440 nm:34.2%,27.6%,22.3%?.And the pyridine N-rich N-CQDs-1 showed better fluorescence stability than fluorescent dyes under changed temperature?5⁵5??,wide pH values?1¹1?and high salt concentration?1 mol/L?.Even after UV irradiation for 120 min,the fluorescence intensity of N-CQDs-1 is almost unchanged,it has great photobleaching resistance.In virtue of above mentioned excellent fluorescence properties,the pyridine N-rich N-CQDs-1 perform very well as a multifunctional probe:On the one hand,N-CQDs-1 can be used for the detection of iron ions(Fe³⁺)with low detection limit,high selectivity and precision.It shows a very low detection limit?7.6 nM?and specific selectivity for Fe³⁺even in the presence of other 16 cations(Zr⁴⁺?K⁺?Zn²⁺?Co²⁺?Sr²⁺?Sn²⁺?Ca²⁺?Al³⁺?Fe²⁺?Li⁺?Ni²⁺?Ba²⁺?Mg²⁺?Cu²⁺?Na⁺and Pb²⁺).Furthermore,the N-CQDs-1 present high reproducibility and ccuracy when used to determine the concentration of Fe³⁺in tap water.On the other hand,N-CQDs-1 showed good biocompatibility to 3T3 and HepG2 cells,which could enter the cytoplasm of CT26.WT cells to emit blue,green and red fluorescence.3.To improve the processibility of polyaniline?PANI?by manipulating their growth using N-CQDs-1 as nucleation center.PANI which is not only simple in synthesis but also excellent in electrical conductivity has become a research hotspot in the field of conductive polymers.Its synthesis,structure,properties and applications have been extensively researched,and important research results have been obtained.However,there is still no breakthrough in the solubility of doped polyaniline,which makes it difficult to pave for film and be processed.The N-rich N-CQDs-1 is only about 3.5 nm in size,which make it is enable to induce the formation of small-sized CQDs/PANI nanocomposites?N-CQDs-1/PANI?as a nucleation center.Here,when the HCl is used as the dopant,ammonium persulfate is used as the initiator,N-CQDs-1/PANI was prepared by in-situ polymerization of aniline monomer on the surface of the N-rich N-CQDs-1.Compared with the pure PANI prepared under the same conditions,the water dispersion of the N-CQDs-1/PANI is outstanding.The dispersion with a concentration of 1 mg/mL is still stable after centrifuged by high speed?11000 r/min?for 15 min,no precipitation is appeared,and the zeta potential value is-1.82 mv.The conductivity of N-CQDs-1/PANI is about two times higher than that of pure PANI.In this paper,the most effective nitrogen-doped bond was clear which is pyridine nitrogen.Based on this,pyridine N-rich N-CQDs-1 with high fluorescence performance was prepared.The prepared N-rich N-CQDs-1 perform very well when used for Fe³⁺detection,tumor cell imaging,and dispersion-improvement of polyaniline.