Highly sensitive fluorescence detection of mercury (II) ions based on WS2 nanosheets and T7 exonuclease assisted cyclic enzymatic amplification

In this work, a simple and signal-on assay has been developed for highly sensitive and selective detection of mercury ion (Hg2+) based on the WS2 nanosheets and T7 exonuclease-assisted cyclic signal amplification. WS2 nanosheet exhibits differential affinity toward short oligonucleotide fragment versus single-stranded DNA (ssDNA) probe, which is used as an adsorption material for capturing ssDNA and an efficient fluorescence quencher for reducing the background signal. T7 exonuclease, a sequence independent nuclease, catalyzes the removal of 5' mononucleotides from the 5' termini of double stranded DNA, while its activity on ssDNA is limited. Without Hg2+, FAM-labelled ssDNA probe and target probe are adsorbed by the WS2 nanosheet and the fluorescence of FAM-labelled signal probe is quenched. In the presence of Hg2+, a FAM-labelled ssDNA probe could hybridize with the target probe to form duplex structures with a blunt 5'-terminal of signal probe through the formation of T-Hg2+-T base pairing. The FAM-labelled signal probe with a blunt 5'-terminal in the formed duplex can be digested by T7 exonuclease in the direction from 5' to 3', liberating the FAM fluorophore and releasing the Hg2+. The released target Hg2+ and the remaining probe then bind another FAM-labelled ssDNA, and initiate the next round of cleavage, resulting in the release of numerous FAM labels back into the solution and significantly amplified fluorescent signal. This approach can warrant the detection limit for Hg2+ down to 0.1 nM (S/N =3) with high selectivity against other metal ions. Moreover, the application of the sensor for lake water shows that the proposed method works well for real samples. This research demonstrates an alternative approach to detect targets of interest that holds high prospects for detecting other biomolecules or metal ions in the near future. (C) 2017 Elsevier B.V. All rights reserved.