Peroxidase-like activity of Fe3O4 nanoparticles and Fe3O4-graphene oxide nanohybrids: Effect of the amino- and carboxyl-surface modifications on H2O2 sensing

Magnetic, iron oxide nanoparticles and graphene-iron oxide nanohybrids have been introduced as artificial peroxidase-mimics. In the present paper, the impact of coating with different functional groups on the peroxidase activity of both nanoparticles and nanohybrids, was studied. Magnetite nanoparticles and graphene-iron oxide nanohybrids, with -amino or -carboxyl terminal groups were synthesized and fully characterized by FTIR, TGA, TEM, XRD, VSM and zeta potential. All particle formulations were evaluated for peroxidase activity, by studying the oxidation reaction of the peroxidase substrate 3,3,5,5-tetramethylbenzidine (TMB) by hydrogen peroxide (H2O2). It was found that all tested formulations retained their catalytic ability (>70%) at 95 °C, at wide pH range, and were reusable with 60% of their catalytic activity after 10 uses. All nanomaterials were found to follow Michaelis–Menten kinetics and peroxidase activity varied with respect to electrostatic affinity between nanoparticles or nanohybrids and the peroxidase substrates, evidenced by differences in determined kinetic parameters. The Km values of all tested particles with H2O2 substrate, were higher than those reported for HRP (0.214-6.36 mM), while the calculated values of reaction rates (Vmax) for the studied nanoparticles and nanohybrids with TMB and H2O2 as substrates were comparable with those reported for HRP. Our findings confirm that iron oxide nanoparticles and graphene based metallic nanohybrids can be tailor-made to possess improved peroxidase-like activity. Such enhancements could further widen nanohybrids application in a wide range of biomedical applications, including H2O2 detection in a proof-of-concept assay, which confirmed low detection limits of the tested formulations (5.3 – 11.3 μM).