Oxidase-Like Catalytic Performance of Nano-MnO2 and Its Potential Application for Metal Ions Detection in Water

Abstract

Certain nano-scale metal oxides exhibiting the intrinsic enzyme-like reactivity had been used for environment monitoring. Herein, we evaluated the oxidase-mimicking activity of environmentally relevant nano-MnO₂ and its sensitivity to the presence of metal ions, and particularly, the use of MnO₂ nanozyme to potentially detect Cu²⁺, Zn²⁺, Mn²⁺, and Fe²⁺ in water. The results indicated the oxidase-like activity of nano-MnO₂ at acidic pH-driven oxidation of 2,6-dimethoxyphenol (2,6-DMP) via a single-electron transfer process, leading to the formation of a yellow product. Notably, the presence of Cu²⁺ and Mn²⁺ heightened the oxidase-mimicking activity of nano-MnO₂ at 25°C and pH 3.8, showing that Cu²⁺ and Mn²⁺ could modify the reactive sites of nano-MnO₂ surface to ameliorate its catalytic activity, while the activity of MnO₂ nanozyme in systems with Zn²⁺ and Fe²⁺ was impeded probably because of the strong affinity of Zn²⁺ and Fe²⁺ toward nano-MnO₂ surface. Based on these effects, we designed a procedure to use MnO₂ nanozyme to, respectively, detect Cu²⁺, Zn²⁺, Mn²⁺, and Fe²⁺ in the real water samples. MnO₂ nanozyme-based detecting systems achieved high accuracy (relative errors: 2.2-26.1%) and recovery (93.0-124.0%) for detection of the four metal ions, respectively. Such cost-effective detecting systems may provide a potential application for quantitative determination of metal ions in real water environmental samples.