Preparation Of Doped Graphene Quantum Dots, Fluorescence Emission Mechanism And Its Application In Cell Imaging

Graphene quantum dots(GQDs),especially doped graphene quantum dots,have been widely used as fluorescent probes due to their broad band absorption,good chemical stability,high photobleachability,and excellent bioconpatibility.Applied to bioimaging,optical sensors and optoelectronic devices.Current studies have found that doping can effectively modulate the fluorescence properties of graphene quantum dots.However,how dopants modulate the luminescence mechanism of GQDs still needs further elucidation In this paper,the effect of doping elements on the fluorescence properties of graphene quantum dots was studied,and the mechanism of fluorescence emission of doped graphene quantum dots was revealed.These results provide a theoretical basis for the regulation of fluorescence properties of GQDs and fluorescence imaging of cells through doping.The main research contents are as follows:(1)The graphene quantum dots(UGQDs),nitrogen-doped graphene quantum dots(NGQDs)and boron-doped graphene quantum dots(BGQDs)were prepared by electrolysis method.The transmission electron microscope(TEM)and X-ray energy spectrum(XPS)were used.Raman spectroscopy(Raman),infared spectroscopy(FTUR)and other characterization methods,found that the prepared UGQDs,NGQDs and BGQDs have the same size,chemical composition and defects.UV-vis absorption spectroscopy(UV-Vis)studies have shown that UGQDs,NGQDs,and BGQDs have three absorption bands,which are the ?-?*electron transitions corresponding to the graphene sp2 hybridized carbon,and the ?-?*charge of the boundary carbon atom.The no2p-?*electron transitions of the transfer and surface oxygen-containing functional groups conpared to the absorption bands of UGQDs resulted in a blue shift in the absorption band of NGQDs and a red shift in the absorption band of BGQDs.Fluorescence excitation spectroscopy(PLE)studies have further found that nN2p-?*and nB2p-?*electron transitions are additionally present in NGQDs and BGQDs,and the absorption bands corresponding to no2p-?*electron transitions occur blue in comparison with UGQDs,respectively.Move and redshift.We confirmed by fluorescence emission spectroscopy(PL)and time-resolved fluorescence spectroscopy that there are two luminescent centers(labeled as band I and band ?)present in GQDs,where band I is the carbon nucleus sp2 hybridized by ?-?*The electronic transitions are generated by the excitation of independent fluorescence characteristics,while the band II fluorescence originates from the n-?*charge transfer of N/O/B groups and the carbon nuclei of GQDs and the ?-?*charge transfer of the boundary carbon atoms.Expressed as excitation-dependent fluorescence.By investigating the PL behavior of UGQDs,NGQDs and BGQDs in different protonated solvents,it was found that the fluorescence emission peak caused by the ?-?*charge transfer of the carbon and boundary carbon atoms dominates and the position of the peak is doped Influence of the elements,NGQDs are blue-shifted,and BGQDs are red-shifted.This chapter also combines density functional theory(DFT)and electrochemical methods to further study the influence of doping atoms on the electronic structure of GQDs.The results show that N doping causes the NGQDselectron cloud distribution to be more delocalized and the bandwidth becomes wider,while B doping Miscellaneous causes the BGQDs electron cloud distribution to become more localized and the bandwidth becomes narrower,so that the fluorescence emission peaks of NGQDs are blue-shifted,and the fluorescence emission peaks of BGQDs.are red-shifted.(2)NGQDs.with different N contents were prepared by electrols is,and the effects of N doping content on the fluorescence properties of NGQDs were studied.We prepared a series of NGQDs under diferent preparation conditions(such as ionic strength,voltage and N-containing electrolyte concentration,etc.).UV-Vis,Raman,XPS,PL,PLE and other spectral characterization results show that at the ionic strength of 0.01-4 mol/Kg,as the ionic strength increases,the concentration of the prepared NGQDs also increases with the voltage;For 3-7 V,as the voltage increases,the oxygen content of the prepared NGQDs also increases correspondingly;and changing the concentration of the N-containing electrolyte(tetramethylammonium hydroxide,0.05-1 mol/L)can effectively The N content of NGQDs was controlled,and the fluorescence quantum yield of the obtained NGQDs was affected.It was found that the NGQDs prepared with N-electrolyte concentration of 0.1 mol/L had a N content of 4.9%and the highest fluorescence quantum yield(12.7%).We further studied the stability of the NGQDs.The results showed that NGQDs have strong fluorescence stability,photobleachability,and salt resistance.Compared with other preparation methods,the method is very simple,and in the aqueous phase,the obtained NGQDs have high dispersibility in the aqueous phase,and the cytotoxicity experiment shows that the NGQDs have good bioconpatibility and low cytotoxicity.Therefore,we applied the NGQDs to HeLa(human cervical cancer)cells for fluorescence imaging.The imaging results were good,and multi-color fluorescence imaging of the cells could be obtained by selecting different fluorescence excitation wavelengths.