In this paper, a sensitive time-gated fluorescent sensing strategy for mercury ions (Hg 2+ ) monitoring is developed based on Hg 2+ -mediated thymine (T)–Hg 2+ –T structure and the mechanism of fluorescence resonance energy transfer from Mn-doped CdS/ZnS quantum dots to graphene oxide. The authors employ two T-rich single-stranded DNA (ssDNA) as the capture probes for Hg 2+ , and one of them is modified with Mn-doped CdS/ZnS quantum dots. The addition of Hg 2+ makes the two T-rich ssDNA hybrids with each other to form stable T-Hg 2+ -T coordination chemistry, which makes Mn-doped CdS/ZnS quantum dots far away from the surface of graphene oxide. As a result, the fluorescence signal is increased obviously compared with that without Hg 2+ . The time-gated fluorescence intensities are linear with the concentrations of Hg 2+ in the range from 0.20 to 10nM with a limit of detection of 0.11nM. The detection limit is much lower than the U.S. Environmental Protection Agency limit of the concentration of Hg 2+ for drinking water. The time-gated fluorescent sensing strategy is specific for Hg 2+ even with interference by other metal ions based on the results of selectivity experiments. Importantly, the proposed sensing strategy is applied successfully to the determination of Hg 2+ in environmental water samples.