Magnetic Fe–N–C nanoparticles as a dual nanozyme for label-free colorimetric detection of antibiotics

In this study, we report a simple pyrolytic strategy for N–C co-doped Fe-based nanoparticles (Fe–N–C) and exhibit their oxidase-mimicking and peroxidase-mimicking activity in a chromogenic reaction with 3,3′,5,5′-tetramethylbenzidine dihydrochloride (TMB) as a substrate. Structural analysis revealed that Fe–NX and Fe3C structures were formed with Fe-Imace coordination compounds as precursors and melamine as the nitrogen source, which provided abundant active sites for the Fe–N–C nanozyme. Interestingly, owing to the blocking effect triggered by π–π stacking between the tetracycline antibiotics (TCs) tetraphenyl skeleton and Fe–N–C, the substrate affinity of the Fe–N–C was significantly blocked, resulting in the solution fading. Under the optimum conditions, the UV absorption intensity versus the concentration of TCs was found to be linear over the range of 0.08–90 μM for tetracycline (TC), 0.1–80 μM for chlortetracycline (CTC) and 0.09–100 μM for oxytetracycline (OTC). The detection limits were 62 nM for TC, 85 nM for CTC and 88 nM for OTC. The Fe–N–C displayed good storage and long-term stability when stored at 4 °C for 30 days. In addition, based on its superparamagnetism, Fe–N–C can be effectively recycled and reused through an external magnetic field, avoiding secondary pollution.