Enzyme Mimetic Activity of ZnO-Pd Nanosheets Synthesized via a Green Route

Recent developments in the area of nanotechnology have focused on the development of nanomaterials with catalytic activities. The enzyme mimics, nanozymes, work efficiently in extreme pH and temperature conditions, and exhibit resistance to protease digestion, in contrast to enzymes. We developed an environment-friendly, cost-effective, and facile biological method for the synthesis of ZnO-Pd nanosheets. This is the first biosynthesis of ZnO-Pd nanosheets. The synthesized nanosheets were characterized by UV-visible spectroscopy, X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray. The d-spacing (inter-atomic spacing) of the palladium nanoparticles in the ZnO sheets was found to be 0.22 nm, which corresponds to the (111) plane. The XRD pattern revealed that the 2θ values of 21.8°, 33.3°, 47.7°, and 56.2° corresponded with the crystal planes of (100), (002), (112), and (201), respectively. The nanosheets were validated to possess peroxidase mimetic activity, which oxidized the 3,3',5,5'-tetramethylbenzidine (TMB) substrate in the presence of H₂O₂. After 20 min of incubation time, the colorless TMB substrate oxidized into a dark-blue-colored one and a strong peak was observed at 650 nm. The initial velocities of Pd-ZnO-catalyzed TMB oxidation by H₂O₂ were analyzed by Michaelis-Menten and Lineweaver-Burk plots, resulting in 64 × 10^-6 M, 8.72 × 10^-9 Msec^-1, and 8.72 × 10^-4 sec^-1 of K_M, V_max, and k_cat, respectively.

Keywords: Erigeron annuus leaf extract; ZnO-Pd nanosheets; green synthesis; nanozyme; peroxidase mimetic activity.