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In this work, fluorescent carbon quantum dots (CQDs) was prepared using natural on ions as carbon source with hydrothermal method and it was modified with polyethyleneimine (PEI). The properties of PEI modified CQDs (PEI-CQDs) were characterized by fluorescence, infrared spectroscopy and ultraviolet method, the morphology characteristics of PEI-CQDs was observed by transmission electron microscope. The results shown that the fluorescence excitation and emission wavelength were at 340 nm/462 nm, respectively. The fluorescence quantum yield