Bacterial biofilm infection is perplexing people's life. Systemic administration of antibiotics usually causes adverse effects and bacterial resistance. How to make antibacterial agents quickly penetrate and eradicate the biofilm without adverse effects is an urgent problem. Here, polyamine-functionalized carbon quantum dots (CQDs), abbreviated as CQD(600), CQD(1w) and CQD(2.5w), are synthesized by a simple hydrothermal treatment of citric acid and branched polyethyleneimine (bPEI) with different molecular weight (MW). It is found that the antibacterial activity is promoted with high MW of bPEI, which is due to the protonated amines on the surface of the CQDs. Moreover, in terms of eradicating matureStaphylococcus aureusbiofilm, CQD(2.5w) possesses significantly higher activity than CQD(600) and CQD(1w) because of stronger electrostatic interactions and longer surface coronas. Additionally, the cytotoxicity test exhibits the L929 cells can keep at high survival rates even treated with 500 mu g mL(-1) CQDs for 3 days. In this study, the surface chemistry related to antibacterial activity and low cytotoxicity give the chance in designing and developing better CQDs for biofilm infection. [GRAPHICS] .