Polydopamine-Coated Fe3O4 Nanoparticles as Synergistic Redox Mediators for Catalytic Reduction of Azo Dyes

Polydopamine-coated Fe3O4 (Fe3O4@ PDA) nanoparticles (NPs) were prepared as synergistic redox mediators for the catalytic reduction, by NaBH4, of azo dyes such as methyl orange (MO) and methyl red (MR). Transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were applied to determine their surface morphology, surface chemistry and detailed chemical composition, respectively. The latter technique confirmed the presence of quinone moieties. Moreover, a vibrating sample magnetometer (VSM) was used to confirm the superparamagnetic properties of these NPs. The characteristic optical absorption maximum of MO at 462 nm was used to monitor the decolorization process. This was employed to determine the catalytic activity in the reaction. An enhancement of the catalytic activity of the magnetic-separable Fe3O4@ PDA nanocatalyst over that of PDA microspheres (MPs) was observed. Moreover, their reusability and stability were also investigated. A synergistic electron transfer mechanism involving both Fe3O4 and PDA moieties was proposed as follows: the quinone moieties and Fe (III) species in Fe3O4@ PDA NPs served as systematic redox mediators, with quinone receiving an electron from NaBH4. The reduced quinone next transfers an electron to the Fe (III) moiety, generating an Fe (II) species that in turn transfers an electron to the azo dye. We determined that this process resulted in enhanced reductive degradation of azo dyes when compared with PDA MPs. Moreover, Fe3O4@ PDA NPs could be magnetically separated and recycled. We therefore concluded that these NPs show great potential in the immobilization of homogeneous catalysts in the chemical reduction processes of azo dyes.