Carbon dots (C-dots) are superior in the aspects of excellent water solubility, good biocompatibility, environmentally friendliness and non-blinking fluorescence. In this work, highly photoluminescent small size C-dots (QY = 18.8%, quinine sulfate as standard) with narrow size distribution (1.70 +/- 0.21 nm) have been synthesized by using citric acid and triethylamine through hydrothermal method. The optimal excitation and emission wavelength of C-dots are 350 nm and 437 nm, respectively. And the prepared C-dots display excitation-independent behavior due to less surface defects and uniform size. Interestingly, the fluorescence of C-dots could be rapidly and selectively quenched by Hg2+ within 200 s at room temperature without further modification. Under optimal conditions, the limit of detection (LOD) was measured to be nanomolar level (2.8 nM) with a linear range of 0.05-7 mu M, lower than the previous published reports. Furthermore, our results reveal that static quenching mechanism was dominated in the process in which Hg2+ coordinate with the oxygen-containing groups of C-dots to form nonfluorescent complexes. And only the addition of Hg2+ destroyed the surface defects of C-dots resulting in the fluorescent quenching. The presence of other common interfering metal ions reported in previous literature (Ag+, Cu2+, Fe3+) do not affect the surface defects, which has rarely been reported before. Besides, this sensing platform has been further successfully applied to the label free detection of Hg2+ in tap water and living cells. These conclusions demonstrate the great potential of our C-dots in selective detection of environmental and cellular Hg2+ which may achieve a lot of achievements in clinical diagnosis and other biological researches. (C) 2019 Elsevier Inc. All rights reserved.