Nitrogen-doped carbon quantum dots(CQDs) with thermal sensitivity were rapidly prepared using microwave-assisted method with oxalic acid and urea as carbon source,and ultrapure water as solvent.The morphology,structure,fluorescence properties of the as-prepared CQDs,and influencing factors of fluorescence intensity,as well as cytotoxicity of CQDs with MTT assay were discussed in the article.Finally,the CQDs applied in CT26.WT cell imaging.The results showed that the as-prepared CQDs,possessing a quasispherical morphology with an average diameter of approximately 3.8 nm,exhibited a good temperature-sensitive photoluminescence property.The temperature-dependent fluorescence was from 10 ℃ to 80 ℃.With the increase of temperature,the fluorescence intensity of CQDs decreased,while the relative fluorescence intensity(I/I0) of CQDs decreased by 25%with the increase of temperature from 10 ℃ to 80 ℃,which could recover to the initial value with the decrease of temperature.There was an accurate linear relationship between fluorescence intensity and temperature of CQDs,which demonstrated one of its potential applications in the field of temperature--dependent photoluminescence sensing.The nitrogen doping could effectively improve the fluorescence quantum yield(QY) of CQDs.When the mass ratio of oxalic acid and urea was 3∶1,the fluorescence quantum yield reached the maximum value of 9.5%.FTIR analysis showed that the CQDs surfaces were rich in hydroxyl and carboxyl groups,which were helpful for water-solubility and cell imaging application.The as-prepared CQDs exhibited an excitation-dependent photoluminescence performance,excellent salt tolerance ability and good stability.Moreover,the luminescent CQDs could not only be efficiently taken up by CT26.WT cells,but also exhibited a low cytotoxicity and a favorable biocompatibility.MTT assay results showed that cell viability remained 85%for CT26.WT in the presence of CQDs in the concentration of 1 000 μg·mL-1.All these excellent properties make the thermo-sensitive CQDs become a promising candidate in the applications of cell research.