Oxidase-Like Catalytic Performance of Nano-MnO(2) and Its Potential Application for Metal Ions Detection in Water

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-MnO(2) and its sensitivity to the presence of metal ions, and particularly, the use of MnO(2) nanozyme to potentially detect Cu(2+), Zn(2+), Mn(2+), and Fe(2+) in water. The results indicated the oxidase-like activity of nano-MnO(2) 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 Cu(2+) and Mn(2+) heightened the oxidase-mimicking activity of nano-MnO(2) at 25°C and pH 3.8, showing that Cu(2+) and Mn(2+) could modify the reactive sites of nano-MnO(2) surface to ameliorate its catalytic activity, while the activity of MnO(2) nanozyme in systems with Zn(2+) and Fe(2+) was impeded probably because of the strong affinity of Zn(2+) and Fe(2+) toward nano-MnO(2) surface. Based on these effects, we designed a procedure to use MnO(2) nanozyme to, respectively, detect Cu(2+), Zn(2+), Mn(2+), and Fe(2+) in the real water samples. MnO(2) 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.