Ag Nanoparticle/CeMnO ₃ Nanozyme for Colorimetric Detection of Pb ²⁺ Ions

Heavy metal contamination is a serious threat to both human health and the environment; thus, there is an urgent need for sensitive methods of detection. Herein, we demonstrate that an Ag/CeMnO3 nanozyme with its remarkable oxidase-like activity can be used as a colorimetric nanosensor for sensitive and selective detection of Pb2+ ions in aqueous solutions. In oxidase-like activity, the nanozyme catalyzes the oxidation of colorless 3,3′,5,5′-tetramethylbenzidine (TMB), producing a blue oxidized TMB. Extensive studies on CeMnO3 and Ag/CeMnO3 nanozymes were carried out, which show that the incorporation of Ag nanoparticles significantly enhanced the oxidase-like activity of CeMnO3. The nanozymes were optimized for better performance, which includes a pH of 4, a time period of 15 min, and the radical mediator was identified as superoxide radicals. Further, the oxidase-mimic activity is selectively inhibited by Pb2+ ions with a mixed inhibition type, which bind to the active site of both Ag/CeMnO3 and the Ag/CeMnO3–TMB–enzyme–substrate complex. This shows a high sensitivity of 7.6 nM over a range of 0 to 1 μM. Finally, the method was applied to the spiked real water samples, showing the recovery % as >90% and low RSD % as <2%.