A new fluorescence "turn-on" nanosensor for carbonate determination was reported based on CdSe quantum dots (QDs) modified with thio ligands containing urea groups: N-(5-mercapto-1,3,4-thiadiazol-2-ylcarbamoyl)-2-(o-tolyloxy)acetamide (AASH-CdSe QDs). The AASH-CdSe QDs were prepared through a ligand exchange process and characterized by transmission electron microscopy (TEM), fluorescence spectroscopy, UV-vis spectroscopy and FT-IR spectroscopy. The synthesized AASH-CdSe QDs allowed a selective fluorescence "turn-on" response towards carbonate. Under optimal conditions, the relative fluorescence intensity increases linearly with carbonate concentration in the range 1 x 10(-7)-1 x 10(-4) M with a detection limit of 2.3 x 10(-8) M. A Langmuir-type binding model was highly effective in describing the carbonate concentration dependence of the luminescence intensity of the AASH-CdSe QDs. The possible mechanism was discussed.