In the present work, nanocomposite of bentonite clay with MgFe$_2$O$_4$ nanoparticles (NPs) was synthesized by sol–gel route. It was studied for the sequestration of Pb(II) and Ni(II) ions from the aqueous solution. The nanocomposite was analysed using X-ray diffraction, vibrating sample magnetometry, scanning electron microscopy equipped with energydispersiveX-ray spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy and Brunauer–Emmett–Teller (BET) as analytical tools. The lower value of saturation magnetization (Ms) of nanocomposite (5.70 emu g$^{−1}$) as compared with pristine MgFe$_2$O$_4$ NPs (12.32 emu g$^{−1}$) is due to the presence of non-magnetic bentonite clay. BET studies further revealed higher surface area for nanocomposite (75.43 m$^2$ g$^{−1}$) than MgFe$_2$O$_4$ NPs (62.51 m$^2$ g$^{−1}$). The presence of bentonite clay during sol–gel synthesis of MgFe$_2$O$_4$ NPs prevented particle growth. The adsorption data were modelled using Temkin, Freundlich, Dubinin–Radushkevitch and Langmuir adsorption isotherms. Comparative evaluation of adsorptionpotential of nanocomposite for Pb(II) and Ni(II) ions confirmed higher affinity of Pb(II) ions ($q_{\rm max} = 90.90$ mg g$^{−1}$) towards the nanocomposite as compared with Ni(II) ions ($q_{\rm max} = 76.92$ mg g$^{−1}$). The results were explained on the basis of their hydration enthalpy. Thermodynamic analysis confirmed endothermic and spontaneous nature of adsorption processwith $\Delta H^o$ values of 48.67 and 21.54 kJ mol$^{−1}$ for Pb(II) and Ni(II) ions, respectively. Kinetic studies confirmed that a pseudo-second-order kinetic model was followed. The obtained results suggested that adsorption capacity of nanofabricated composite for Pb(II) and Ni(II) ions was higher than that of pristine MgFe$_2$O$_4$ NPs and bentonite clay. The saturated adsorbentwas magnetically retrievable and easily regenerated with 0.1 MHCl solutions. It can serve as a potential composite adsorbent for the remediation of heavy metal ions.

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.

Ferrite nanoparticles (NPs) with composition MFe$_2$O$_4$ (M $=$ Mg/Co) were synthesized by a facile combustion method. NPs were characterized employing various physico-chemical techniques. X-ray diffraction patterns confirmed the phase purity, transmission electron micrographs indicated that NPs are spherical and average diameter of maximum fraction of NPs was in the range of 20–30 nm. Magnetic studies revealed that the saturation magnetization values for MgFe$_2$O$_4$ and CoFe$_2$O$_4$ NPs were 13.17 and 41.12 emu g$^{−1}$, respectively. The Brunauer–Emmett–Teller surface area of CoFe$_2$O$_4$ and MgFe$_2$O$_4$ NPs was 22.98 and 34.39vm$^2$ g$^{−1}$, respectively. Synthesized ferrite NPs and activated charcoal were comparatively analysed as adsorbents for removal of Pb(II) ions. The factors influencing uptake behaviour of Pb(II) ions viz. adsorbent dose,pH, concentration, temperature and contact time were quantified.The adsorption data showed good correlationwith Langmuir and Freundlich models as compared to Dubinin–Radushkevich model. The maximum adsorption capacity displayed a twofold increase for NPs as compared to activated charcoal. The easy magnetic separation of ferrite NPs from the solution and their regeneration with 0.1 N NaOH for reuse without any loss make them potential adsorbents. The trend in ascending order for the elimination of Pb(II) ions from the solution was activated charcoal < CoFe$_2$O$_4$ NPs < MgFe$_2$O$_4$ NPs. The observed differences in the adsorption potential of NPs are explained on the basis of structural and magnetic properties and the surface area of NPs.

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.

In the present study, nanocomposite (NC) of cetyl trimethyl ammonium bromide (CTAB) coated cobalt oxide nanoparticles (Co$_3$O$_4$ NPs) with graphene oxide (GO), i.e., CTAB@Co$_3$O$_4$@GO, was synthesized for superoxide dismutase mimic (SOD) activity. The NPs and NC were characterized using various analytical tools. X-ray diffraction patterns, Fourier transform infrared and scanning electron microscope–energy dispersive spectrum confirmed the presence of both GO and Co$_3$O$_4$ NPs in NC. Transmission electron microscope micrographs of NC showed GO nanosheets having CTAB-coated Co$_3$O$_4$ NPs on their surface. The NC was evaluated for SOD mimic activity using pyrogallol as a substrate. NC displayed maximum activity as compared to pristine GO and Co$_3$O$_4$ NPs. The results signified that the surfactant coating and embedding the NPs in the GO matrix helped in increasing the interaction of NC with the substrate molecules. Kinetics data was modelled using Michaelis–Menton equation. The calculated $K_m$ and $V_{max}$ values of NC were 0.0675 mM and 0.146 mol s$^{–1}$, respectively. Lower value of Michaelis constant $K_m$ as compared to the reported values, confirmits edge over other SOD mimics. Thus CTAB@Co$_3$O$_4$@GO NC holds potential for replacing natural enzyme in SOD based enzymatic assay.