Abstract: Objective:To investigate the transport pathwayand intracellular distribution of the of fluorescent carbon dots(CDs) synthesized by folic acid and polyethyleneimine(PEI) through the membrane of MC3T3-E1 cells and its effect on the cells, and to clarify the mechanism. Methods: The fluorescent CDs with the function of cell imaging were synthesized by hydrothermal method using folic acid and PEI as the raw materials; MTT assay was applied to screen the best concentration of CDs. The MC3T3-E1 cells were divided into blank control group, folic acid group and CDs group. The biocompatibility of CDs was evaluated by the detection of cell cycle, apoptosis and cellular reactive oxygen species(ROS) level. Nystatin as a kind of caveolae inhibitor and nocodazole as a kind of macropinocytosis inhibitor were used to find out the pathway through which the cells took in the CDs. Using the charcteristic of CDs with blue fluorescence stimulated by ultraviolet ray,the organelle probes were used to observe the distribution of CDs. Results: Compared with blank control group, the cells in different concentrations(100-450 mg·L^-1) of CDs groups showed no cytotoxicity at 24 h (P>0.05);at 48 h, the cell proliferation rate was reduced to 68.4% of blank control group when the concentration of CDs reached 350 mg · L ^-1(P<0.05).Compared with blank control group, the percentages of cells in G₀ phase and G₁ phase in CDs group were decreased (P<0.05), and the percentage of cells in S phase was increased (P<0.05); the percentages of cells in G₂ phase and M phase were increased, but there no was significant differences (P>0.05). Compared with blank control group, the apoptotic rates of the cells in folic acid group and CDs group had no significant differences (P>0.05). Compared with blank control group,the intracellular ROS levels in folic acid group and CDs group were significantly decreased(P<0.05). Compared with blank control group, the uptake amount of CDs in the cells was decreased in nystatin group(P<0.05). The blue fluorescence of CDs overlapped with the red fluorescence of mitochondria under an inverted fluorescence microscope, the blue fluorescence of CDs overlapped with the red fluorescence of lysosomes;they didn't overlap completely with the red fluorescence of the endoplasmic reticulum;the blue fluorescence of CDs overlapped poorly with the red fluorescence of Golgi apparatus. Conclusion: CDs perform well in biocompatibility and they can be distributed to different organelles after taken in by the cells. They can be used as a kind of gene carrier in transgenic therapy.

Key words: cellular uptake, polyethyleneimine, transgenic therapy, fluorescent carbon dots, folic acid