Comparison of the FT-IR spectra of PDA powders, Fe3O4 NPs and Fe3O4@PDA NPs is shown in Fig. 2a. In the spectrum of bare Fe3O4 NPs, the characteristic absorption peak at 590 cm−1 derives from the Fe─O stretching vibration. After the coating of PDA, the broader peaks at 3420 cm−1 and 1630 cm−1 are attributed to the O─H stretching vibration and C═C stretching vibration of catechol structures. The adsorption peaks at 3120 cm−1 is from N─H bond, and 1055 cm−1 is ascribed to C─N stretching vibration. These results demonstrate that the magnetic nanoparticles have been successfully coated by PDA polymer.