Composite of graphene quantum dots and Fe₃O₄ nanoparticles: Peroxidase activity and application in phenolic compound removal

Graphene quantum dots (GQDs) are graphene sheets with lateral sizes less than 100 nm, and have a higher electron conjugate state and a better dispersion ability in aqueous solution compared to micrometer-sized graphene oxide (GO) sheets. Therefore they can overcome the drawbacks of GO and are an ideal candidate for nano-composites. In this work, composites of GQDs and Fe ₃O₄ nanoparticles (NPs) (GQDs/Fe₃O₄) were prepared via a one-step co-precipitation approach. The as-prepared GQDs/Fe₃O₄ composites showed superb peroxidase-like activities, which were much higher than composites of micrometer sized GO and Fe₃O₄ NPs (GO/Fe₃O₄), individual GQDs, and individual Fe₃O₄ NPs. The excellent peroxidase activities of the GQDs/Fe₃O₄ composites can be attributed to the unique properties of GQDs and the synergistic interactions between the GQDs and Fe₃O₄ NPs. The GQDs/Fe₃O₄ composites also exhibited a higher stability and reusability than natural peroxidases. The application of a GQDs/Fe₃O₄ composite as a catalyst for the removal of phenolic compounds from aqueous solutions was explored with nine phenolic compounds, and showed better or comparable removal efficiencies for some phenolic compounds compared to native horseradish peroxidase (HRP) under the same conditions. The extraordinary catalytic performance and physical properties of the as-prepared GQDs/Fe₃O ₄ composite render it practically useful for industrial wastewater treatment.