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 H2O2. 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 MnOx 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 Km of 39.2 µM and Vmax 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-MnOx assay, in reasonable agreement with the HPLC result of 23.4 µg kg⁻¹.