As the standard for heavy metal sensing systems increases, it becomes more important to find out new self-indicating materials as alternatives. The traditional design of optical probe contains receptor and fluorophore as two moieties. Here a novel sulfur quantum dot as the intrinsic fluorescent sensor has been established and the signal transduction involves interaction of mercury ions with sulfur atoms which belongs to the internal system of the fluorophore. Its analytical performances can be assessed by fluorometric and colorimetric titrations simultaneously and the detection limits for Hg2+ ions are determined to be 65 nM and 1.86 mu M respectively. In the presence of sulfur dots, very low cytotoxicity has been verified in two cell lines (HeLa and K562) via both cell counting kit-8 method and flow cytometry. For the first time, four sorts of chemical inhibitors have been employed to clarify the endocytosis mechanism based on sulfur quantum dots. The collected results indicate that clathrin and lipid raft mediated endocytosis play as primary roles in the internalization processes. It has been found that sulfur dots are located in the cytoplasm with intense blue emissions and the cellular tracking of mercury ions has been realized.