Highly and blue light emitting fluorescent carbon dots(C-Dots)were synthesized by a facile and environmentally friendly hydrothermal process using citric acid and polyethylene polyamine.The C-Dots showed excellent water solubility,good biocompatibility and had been successfully applied to cellular imaging.Large amount of amino group on the surface of C-Dots was convenient to be biotinylated.The competition between the biotinylated C-Dots and Vitamin B7(D-biotin,Coenzyme R)for the limited streptavidin on the magnetic bead enabled the determination of Vitamin B7.After incubation and magnetic separation,the fluorescence intensities of immobilized biotinylated C-Dots releasing from beads was inversely proportional to the concentration of Vitamin B7 ranging from 0.5 ng/mL to 100 ng/mL.The detection limit is 0.1 ng/mL.The proposed method could be further extended to the determination of biotinylated biomolecules.