Magnetic materials grafted with carboxylic acid (Fe3O4@SiO2@COOH MNPs) were successfully prepared via the incorporation of maleic anhydride as a functional group on the surface of ferrite–silica nanoparticles. The properties of the magnetite nanoparticles prepared were characterized using transmission electron microscopy (TEM), FT-infrared spectroscopy (FT-IR), vibrating sample magnetometer (VSM), and X-ray diffraction (XRD) analysis. The prepared magnetic catalyst (Fe3O4@SiO2@COOH) showed a high catalytic activity for the synthesis of diindolyl oxindoles via the coupling of indole and isatin compounds. The products were produced with good to high yield in an aqueous medium. The catalyst could be readily separated from the reaction mixture using a permanent magnet and showed high stability in recycling experiments.
