Red Fluorescent Carbon Dots For Selective Recognition Of NO₂^- And Photothermal Treatment Of Tumors

Carbon dots（CDs）are mainly a kind of zero-dimensional nanomaterial with size less than 10 nm and carbon as the main element.As a fluorescent nanomaterial,CDs offer many advantages.,such as wide range of materials,simple fabrication,low toxicity,good biocompatibility and easy modification.It is widely used in fluorescence imaging,ion detection,drug delivery and tumor therapy.However,most CDs emit blue or green fluorescence,and their low tissue penetration ability limits their application in the biomedical field,so the preparation of CDs with long wavelength emission has become a major research.The present studies show that the band gap and energy level of CDs can be controlled by doping heteroatoms,resulting in redshift of emission wavelength.In this work,red fluorescent CDs responsive to NO₂^-with photothermal effect was synthesized by one-step solvothermal method using 2-diaminodiphenylamine as carbon and nitrogen sources and 0.3 M sulfuric acid as solvent.The as-prepared CDs are spherical with an average size of about 3.33±0.4 nm.The quantum yield was 26%at p H=2.The prepared CDs have been successfully used for sensing assays,cell imaging,and photothermal treatment of tumor-bearing mice.The details are as follows:（1）Under the acidic environment,the addition of NO₂^-caused the fluorescence quench of CDs,and a linear relationship was displayed by the NO₂^-concentration and the fluorescence intensity of CDs at 10-100μM and 10-350μM,respectively,with a limit of detection（LOD）of 54 n M.In addition,the addition of different concentrations of NO₂^-also lead to local linear changes in the UV-vis absorbance of CDs and the color of aqueous solution,according to which the colorimetric method and the intelligent image recognition method were used to detect NO₂^-,respectively.（2）The biocompatibility of CDs was verified using human normal hepatocytes（HL-7702）and cervical cancer cells（He La）.When CDs were incubated with HL-7702 and He La cells for 24 h,respectively,a survival rate of 93%for HL-7702 cells and 98%for He La cells was obtained by the CCK-8 method,indicating that CDs have good biocompatibility.On this basis,they were successfully used for cell imaging of HL-7702 and He La cells.（3）The photothermal performance of CDs was investigated.The photothermal conversion efficiency of CDs was calculated to be 26.4%,The photothermal therapeutic effect of CDs was verified with cellular experiments and in vivo experiments.The photothermal therapeutic effect of CDs on cells was carried out by CCK-8 method and live-dead double-staining method,respectively,and the results demonstrated that CDs had strong killing effect on cells under laser 660 nm irradiation.In vivo experiments demonstrated that CDs can be successfully used for in vivo imaging and photothermal treatment of tumor-bearing mice.In addition,the wrapping of CDs with sodium alginate（SA）was found to allow CDs to stay longer in the tumor site.