In the present work, pristine and cetyl trimethyl ammonium bromide (CTAB)-coated ferric oxide nanoparticles (CTAB@Fe$_2$O$_3$ NPs) were synthesized and studied as enzyme mimics. The w/w ratio of Fe$_2$O$_3$ to CTAB was varied as 1:1 and 1:2. Transmission electron microscopic analysis revealed that pristine NPs had an average size of 50 nm, whereas the presence of CTAB resulted in the formation of nanorods with length of 130 nm. BET studies confirmed enhancement of surface area on CTAB coating, which was maximum for w/w ratio 1:1. The synthesized pristine NPs and CTAB-coatedNPs were evaluated for their peroxidase mimic activity using o-dianisidine dihydrochloride as substrate. Optimum pH, temperature, substrate and NPs concentration for the reaction were 1, 25$^{\circ}$C, 0.16 mg ml$^{−1}$ and 1 mg ml$^{−1}$, respectively. Peroxidase mimic activity of CTAB@Fe$_2$O$_3$ NPs (w/w 1:1) was higher than that of pristine NPs. However, further increasein CTAB coating (w/w 1:2) resulted in lowering of peroxidase mimic activity. Kinetic analysis was carried out at optimized conditions; maximum velocity ($V_{\rm max}$) and Michaelis constant ($K_{\rm m}$) value of CTAB@Fe$_2$O$_3$ NPs at 1:1 w/w ratio were 7.69 mM and 1.12 $\mu$mol s$^{−1}$, respectively.

In the present study, superoxide dismutase (SOD) mimic activity of ferrite nanoparticles (NPs), having a formula MFe$_2$O$_4$ (M $=$ Mn, Co and Cu) was investigated. Spinel ferrite NPs were synthesized by employing sol–gelmethodology and characterized using scanning electron microscopy, X-ray diffraction, BET analysis and Fourier transform infrared spectroscopy techniques. BET analysis revealed that the surface area of ferrite NPs ranged from 0.43−23.49 m$2^$ g$^{−1}$. Enzyme mimic activity was compared using SOD as a model enzyme. CuFe$_2$O$_4$ NPs exhibited a maximum activity followedby CoFe$_2$O$_4$ and MnFe$_2$O$_4$ NPs. The results were correlated with a facile interconversion of the oxidation state leading to a stable electronic configuration in CuFe$_2$O$_4$ NPs. Optimum pH and contact time was 1 and 3 min respectively. Kinetic studies were performed under optimum conditions and data were analysed using the Michaelis Menten equation. The valuesof $V_{\rm max}$ (0.77 s$^{−1}$) and Km (4.20 mM) proved CuFe$_2$O$_4$ NPs as potential SOD mimic for a wide range of applications.