▎ 摘　　要

In this study, the fluorescence of Hoechst dyes is significantly lit up by addition of our designed MB probe, forming a complex of "molecular beacon"-hosted Hoechst dyes (HMB). Combined with this property, a novel graphene oxide (GO)-based fluorescent "on/off" switch was developed to visually follow bioassay utilizing HMB as signal indicators and GO as an excellent energy acceptor to efficiently quench the fluorescence of HMB in a label-easy format. We have demonstrated its application for label-easy fluorescence "turn on" detection of sequence-specific DNA and "turn off" detection of exonuclease with sensitivity and selectivity in a single step in homogeneous solution. Compared to traditional molecular beacons, the proposed design is cost-effective and simple to prepare without fluorescence labeling or chemical modification.