pH-Switchable Multienzymatic Activities of an Fe Single-Atom Nanozyme Enable Dual-Cascade Catalysis for Robust Dual-Mode Biosensing

Nanozymes with multienzymatic activities provide synergistic effects for diverse biosensing applications. However, precise manipulation of their functionalities without cross-reactivity for advanced multimode sensing remains challenging. Herein, we employed a cytochrome <i>c</i> (Cyt <i>c</i>)-templated pyrolysis strategy to construct Fe single-atom nanozymes (FeSAN) featuring Fe-N₅ moieties with pH-switchable multienzymatic activities. Notably, the oxidase and peroxidase-like activities of FeSAN exhibit acid-dependent catalytic behavior, enabling a self-supplying oxidative catalytic cascade in acidic conditions. Conversely, in alkaline media, FeSAN demonstrates superoxide dismutase and catalase-like activity, forming a complementary antioxidant pathway for superoxide anion scavenging. The enzyme-mimicking mechanism and potential cascade pathways were investigated through comprehensive experiments and theoretical calculations. Capitalizing on the divergent pH requirements of colorimetric (acidic) and electrochemiluminescence (alkaline) systems, this pH-switchable dual-cascade catalytic platform enables amplified colorimetric response via TMB oxidation in acidic media while suppressing electrochemiluminescence intensity in alkaline circumstances. Using an organophosphorus pesticide (e.g., trichlorfon) as a proof-of-concept target, this platform with inverse signal correction achieved at least 10-fold higher sensitivity and improved accuracy compared to conventional methods. This work establishes a novel paradigm for advanced biosensing by fully utilizing the multienzymatic functionalities of single-atom nanozymes to construct dual-cascade catalysis in dual-mode platforms.