Layered WS2 nanosheet is a kind of twodimensional (2D) covalent-network solid material with remarkable structural and electronic properties that has attracted increasing interest in recent years. In this work, we propose a one-step sonication-assisted exfoliation method to prepare water-soluble WS2 nanosheet and demonstrate its application as a biosensing platform. The synthesis route is simple and straightforward. We reveal that single-strand DNA (ssDNA) chains can readily be adsorbed on the nanosheet,leading to complete and fast quenching of a fluorescent dye
tagged to the DNA chain. The adsorbed ssDNA is detachable from the nanosheet upon the interaction with other biomolecules,resulting in the restoration of the fluorescence. The 2D WS2 nanosheet thus acts as an efficient platform for assembling of bioprobes. Because of the extraordinarily high quenching efficiency, which is the synergic result of both excited-state energy transfer and static quenching, the WS2 platform affords minimal background and high sensitivity. Our attempt will extend the application of this material to biosensing and probing areas.