A facile one-step approach was proposed to make hydrophilic and DNA-functionalizable upconversion nanoparticles through ligand exchange at the liquid–liquid interface, and an ultrasensitive and selective biosensor for nuclease assay and its inhibitors assay based on FRET from the DNA-functionalizable UCNPs to graphene oxide was designed. A high sensitivity exhibited with a detectable minimum concentration of 1 × 10−4 units per mL S1 nuclease, which was more sensitive than the developed approaches.
