▎ 摘　　要

The development of sensitive and selective methods for the monitoring of toxic heavy metal ions is highly demanded because of their threats to the environment and human health. Based on a new exonuclease III (Exo III)-assisted target recycling amplification strategy, a highly sensitive fluorescence turn-on nanosensor for Hg2+ detection using graphene oxide (GO)-quenched, thymine-rich FAM-ssDNA nanoprobes is developed. The target Hg2+ ions bind and fold the GO-adsorbed FAM-ssDNA into duplex structures through the formation of T-Hg2+-T base pairing, leading to the release of the FAM-ssDNA from the surface of GO and recovery of the fluorescent signal. Besides, the released and folded duplex can be digested by Exo III to liberate the bound Hg2+ ions, which can again associate with the GO-quenched FAM-ssDNA nanoprobes and trigger the target recycling process to cause cyclic cleavage of the GO-adsorbed FAM-ssDNA. This target recycling process therefore results in the release of numerous FAM labels back into the solution and significantly amplified fluorescent signal is obtained for highly sensitive detection of Hg2+ down to the sub-nanomolar level. The developed nanosensor also exhibits high selectivity against non-specific ions and can be potentially employed to monitor other toxic heavy metal ions at ultralow levels.