Carbon quantum dots(CQDs) are widely used in fluorescence imaging due to their negligible toxicity,low-cost and excellent fluorescence properties.The fluorescence properties of CQDs are greatly affected by the synthesis method,passivation agent and reaction media.In this study,the nitrogen-doped CQDs(N-CQDs) were synthesized by a facile microwave-assisted method with citric acid(CA) as a carbon source and deep eutectic solvents as a N-dopant as well as the reaction solvent.After detailed characterizations,the as-prepared N-CQDs exhibited near spherical morphology with an average size of (2.64±0.55) nm.Besides,the abundant hydrophilic functional groups including hydroxyl,carboxyl,amine and quatemary ammonium were present on the surface of the N-CQDs.The N-CQDs displayed the excitation-dependent fluorescence feature under the excitation wavelength of 350-480 nm.Moreover,the N-CQDs could be efficiently uptaken by RAW264.7 cells and emitted bright green light.Meanwhile,the resultant N-CQDs showed the low cytotoxicity in living cells,suggesting the great potential in bio-imaging applications.