N doped graphene quantum dots(N-GQDs)with extremely high quantum yield(0.74)was synthesized via cutting graphene oxide and in situ doping progress.Typicalcharacterization results of N-GQDs are shown in Figure 1.Figure 1a presents homogeneous dots with a lateral size distribution of 1-3 nm and crystallinity with lattices of 0.21 nm.[1]The atomic force microscopy(AFM)image confirms that the topographic height is from 0.5 to 1.0 nm,which indicates the 1-2 layer graphene nature of these N-GQDs(figure 1b).[2] The fluorescent characteristics are shown in figure 1c-f.In view of the excellent properties,we designed a series of comparison experiments and found that the key factor for the eminent PL properties was the n-π* transition between the Nar and conjugate structure of GQD.[3] Furthermore,The N-GQDs produced by our method were not toxic in vitro,and could be used as a practical bioimaging material for HeLa cells as well as traditional fluorescent dyes.This work will be helpful for synthesis new bioimaging materials with low cost,high anti-jamming performance,good stability,well biocompatibility and high quantum yield.Figure 1.a)TEM image and size distribution of N-GQDs,inset shows HR-TEM image;b)AFM images of N-GQDs on a mica in tapping mode with the Bruker Dimension Icon AFM microscope,inset is the heights analysis along the line in the image;c)Photographs of N-GQDs DMF solution and RhB ethanol solution with the same mass concentration(10μg/mL)under visible light(left)and 365 nm UV light(right),and the light of N-GQDs is almost as bright as RhB; d)UV-vis absorption(The peak at ca.235 nm can be ascribed to the π-π* transition of benzene,the significant absorption peak at ca.370 nm reveals the n-π* transition of p-π orbit between the Nar and conjugate structure.),PL(on a 400nm excitation),and PLE(the detection wavelength of 500 nm)spectra of N-GQDs in DMF;e)PLand PLEspectra of N-GQDs in solid state,inset: Photo of bamboo drawing with N-GQDs ink under visible light and(left)365 nm UV light(right); f)PL decay curves of N-GQDs measured at room temperature excitations at 400 nm,the PL lifetime of N-GQDs is about 11.5 ns fitted with a bi-exponential decay.