In this paper,citric acid was used as carbon source for preparing fluorescent carbon quantum dots( CQDs). Due to the obtained CQDs had carboxyl groups on their surfaces,Man-CQDs were synthesized by immobilizing 4-aminopheny-l-α-D-mannopyanoside on the surfaces of CQDs via covalent bond. Furthermore, the binding constants of Man-CQDs and Escherichia coli JM109/Escherichia DH5 a/Salmonella typhimurium S. 123443 were obtained by fluorescent competition experiments. The results showed that the sizes of CQDs wer...