Au nanoparticle-loaded mixed-valence MnOₓ nanosheets as an oxidase-like nanozyme for rapid colorimetric determination of leucomalachite green in fish and water

Au nanoparticle-loaded mixed-valence MnOₓ nanosheets were prepared as an oxidase-like nanozyme for colorimetric determination of leucomalachite green (LMG). The nanozyme oxidized colorless LMG to the strongly absorbing malachite green (MG) chromophore using dissolved molecular oxygen without externally added H 2 O 2 . The resulting MG produced a characteristic absorption band at 620 nm. Electron microscopy, elemental mapping, X-ray diffraction, and X-ray photoelectron spectroscopy confirmed crystalline Au nanodomains dispersed on a low-crystallinity mixed-valence MnO x matrix. The catalytic response was maximal at pH 3.0 and reached a stable value within approximately 60 s. Initial-rate measurements showed apparent Michaelis–Menten behavior, with an apparent K m of 39.2 µM and V max of 0.0345 µM s⁻¹. Radical-scavenging and electron paramagnetic resonance experiments supported a multistep reactive-oxygen-species pathway involving hydroxyl radicals and singlet oxygen. The analytical response was linear over 30–1000 µg L⁻¹, with a detection limit of 2.1 µg L⁻¹. This corresponds to a theoretical fish-sample equivalent of 0.42 µg kg⁻¹ after sample concentration. Recoveries of 81.6–110.2% were obtained for fish and water samples. A positive fish sample gave 25.4 µg kg⁻¹ by the Au–MnO x assay, in reasonable agreement with the HPLC result of 23.4 µg kg⁻¹. Graphical abstract