“On–Off” electrochemical sensing toward mercury pollution in tea via exonuclease III triggered target recycling amplification

Mercury ion (Hg²⁺) pollution in tea poses significant risks to human health due to its high toxicity, bioaccumulation, and potential to cause neurological and kidney damage. Sensitive monitoring of Hg²⁺ in tea is therefore critical for food safety and public health. Here, we developed an electrochemical biosensor for ultrasensitive Hg²⁺ assay by integrating thymine–Hg²⁺–thymine (T–Hg²⁺–T) mismatch recognition with exonuclease III (EXO III)-assisted dual signal amplification. The sensing mechanism relies on Hg²⁺-triggered formation of T–Hg²⁺–T structures, which activate EXO III to release Hg²⁺ for cyclic reuse while generating reporter DNA (RDNA). Signal amplification is further enhanced by MOF(Zr)/Th/AuPt nanocomposites, where the Zr-based metal–organic framework (MOF(Zr)) and AuPt nanoparticles synergistically catalyze thionine (Th) oxidation, producing a strong electrochemical response. This dual-amplification strategy achieves a detection limit of 4.45 pM, surpassing that of conventional methods. The biosensor demonstrates high specificity against interfering metal ions (e.g., Cd²⁺ and Cu²⁺) and reliable performance in real tea samples (93.7%∼103.4% recovery), offering a promising tool for monitoring Hg²⁺ contamination in food products.