Colorimetric/fluorescent dual-mode detection of norfloxacin in food samples using Fe/Mn-MOF/CeO₂ nanozyme with high...

A dual-metal metal-organic framework (MOF)-based nanozyme, Fe/Mn-MOF/CeO₂, with high peroxidase-like activity was successfully synthesized via a stepwise solvothermal-hydrothermal combined liquid-phase composite strategy. A colorimetric/fluorescence dual-mode sensing platform was fabricated for the detection of norfloxacin (NOR) in milk and honey. The material takes advantage of the synergistic electronic effect of Fe/Mn bimetallic active sites and the reversible Ce⁴⁺/Ce³⁺ valence cycle to remarkably enhance its peroxidase-like activity. The nanozyme can trigger the decomposition of hydrogen peroxide (H₂O₂) to generate reactive oxygen species (ROS), which oxidizes colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue oxidized TMB (oxTMB). In the established sensing system, NOR can elevate the absorbance signal at 652 nm and quench the fluorescence emission at 435 nm (λ_ex = 360 nm), thereby enabling dual-signal quantitative detection of NOR. Under optimized experimental conditions, the colorimetric mode afforded a linear detection range of 0.100-225 µM and a limit of detection (LOD) of 78.0 nM, while the fluorescence mode delivered a linear range of 0.100-225 µM and an LOD of 46.0 nM. The freshly prepared nanozyme exhibits selectivity against both structural analogs and non-specific interferents, as well as outstanding long-term stability for up to 90 days. In spike-and-recovery experiments with real milk and honey samples, the established sensor presented satisfactory recoveries of 98.6%-103% and relative standard deviation (RSD) values less than 5.00%. This work proposes an innovative sensing strategy for the quantitative determination of antibiotic residues in milk, honey, and other dairy products, which holds great potential for improving the on-site monitoring capability of food safety.