Comparative evaluation of manganese oxide and its graphene oxide nanocomposite as polyphenol oxidase mimics

Abstract

Manganese oxide nanoparticles (Mn3O4 NPs) are widely used in catalytic reactions due to their electron transfer activity. In the present study, Mn3O4 NPs, graphene oxide (GO) and their nanocomposite (NC) were synthesized by sol-gel, modified Hummer’s and ultra-sonication methods respectively. Their polyphenol oxidase (PPO) mimic activity was comparatively analyzed. The synthesized NPs and NC were characterized by Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Scanning electron microscopy-Energy dispersive spectroscopy (SEM-EDS), Transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), Vibrating sample magnetometer (VSM) and X-ray photoelectron spectroscopy (XPS) techniques. FT-IR spectra and elemental analysis of NC revealed the existence of oxygen containing functional groups of pristine GO and Mn-O bonds of Mn3O4 NPs. Saturation magnetization value of NC was lower than pristine Mn3O4 due to the presence of non-magnetic GO. XRD analysis of synthesized materials depicted crystalline nature. TEM micrographs of NC showed Mn3O4 NPs with average particle size of 10 nm on the surface of GO with low aggregation and better dispersion. NC possessed higher BET surface area (45.12 m2/g) than pristine Mn3O4 NPs (25.81 m2/g) which could be attributed to inhibited aggregation of Mn3O4 NPs in the presence of GO. The PPO-like activity of synthesized compounds were analysed using catechol as a substrate. Pristine Mn3O4 NPs were observed to be the best PPO mimic followed by NC. Presence of GO in NC, hindered the electron transfer process. Kinetic analysis under optimized conditions indicated higher binding affinity of NC as compared to Mn3O4 with Michaelis constant values of 0.93 mM and 0.7 mM, respectively. The results of present studies have demonstrated potential of Mn3O4 NPs and NC as PPO mimics.