| Summary: | Long-term use of norfloxacin (NOR) will cause NOR residues in foods and harm human bodies. The determination of NOR residues is important for guaranteeing food safety. In this study, a simple, selective, and label-free colorimetric chemosensor for in situ NOR detection was developed based on Fe<sub>3</sub>O<sub>4</sub> magnetic molecularly imprinted nanoparticles (Fe<sub>3</sub>O<sub>4</sub> MMIP NPs). The Fe<sub>3</sub>O<sub>4</sub> MMIP NPs showed good peroxidase-like catalytic activity to 3,3′,5,5′-tetramethylbenzidine (TMB) and selective adsorption ability to NOR. The colorimetric chemosensor was constructed based on the Fe<sub>3</sub>O<sub>4</sub> MMIP NPs-H<sub>2</sub>O<sub>2</sub>-TMB reaction system. The absorbance differences were proportional to the concentrations of NOR in the range of 10–300 ng/mL with a limit of detection at 9 ng/mL. The colorimetric chemosensor was successfully applied to detect NOR residue in milk. The recovery range was 78.2–95.81%, with a relative standard deviation of 2.1–9.88%. Together, the proposed colorimetric chemosensor provides a reliable strategy for the detection of NOR residues in foods.
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