Self-Cascade System Based on Cupric Oxide Nanoparticles as Dual-Functional Enzyme Mimics for Ultrasensitive Detection of Silver Ions

Artificial enzyme mimics based on nanomaterials have attracted sustained attention owing to their multiple advantages compared with natural enzymes. However, there are a few enzyme self-cascade systems for the highly sensitive detection of analytical targets. Herein, we have described a self-cascade catalytic system based on single-component cupric oxide nanoparticles (CuO NPs) for an ultrasensitive fluorescent detection toward glutathione (GSH) and Ag+ ions. The limit of detection is lower for nanomolar (nM) and picomolar (pM) levels for GSH and Ag+, respectively. To the best of our knowledge, for the first time, we find that CuO NPs possess the intrinsic GSH-oxidase and peroxidase-like activity as a dual-functional nanozyme, coupling with terephthalic acid (TA) and GSH to construct a self-cascade fluorescent system. The turn-on fluorescence signal of oxidation hydroxyterephthalate (TAOH) is generated in the presence of GSH. Then, the fluorescence of a reaction mixture is quenched after the addition of Ag+ ions, operating as a turn-off switch. The turn-on–off switch allows the analysis of GSH and Ag+ ions by a change of fluorescence status. The detection limits are 32 nM and 37 pM for GSH and Ag+, respectively. To the best of our knowledge, the approach presented in this work shows the highest sensitivity for Ag+ detection among all reported fluorescent/colorimetric methods. Moreover, there is no obvious interference with the addition of other interferences without a masking agent. Our study opens a new avenue for the use of a single nanomaterial as an artificial enzyme self-cascade catalytic system for highly sensitive target analysis in biosensor, diagnosis, and environmental fields.