The enzymatic performance derived from the lattice planes of Ir nanoparticles

Abstract

Iridium nanoparticles (IrNPs) have shown intriguing enzyme-like activities; however, the mechanism behind their versatile enzymatic behaviors is still elusive. A new kind of IrNPs stabilized with β-cyclodextrin (CD) was prepared in this work, and these IrNPs show oxidase-like, catalase-like and peroxidase-like activities. Their catalytic performances were comprehensively studied both by experimental characterization and theoretical calculations. The obtained results were systematically compared with other previously reported IrNPs. It was found that the catalytic properties of the IrNPs are derived from the lattice planes. All the IrNPs could catalyze the degradation of H₂O₂ due to the presence of the Ir(111) plane, while only the IrNPs with the Ir(220) plane exhibit oxidase-like activity. First of all, a new mechanism for the catalase-like activity of IrNPs was proposed here. The adsorption of H₂O₂ on IrNPs induces the breakdown of its O–O bond to form ˙OH radicals. The produced ˙OH will then couple with another ˙OH or H₂O₂, forming the key intermediate species (˙O or ˙OOH) in the production of O₂. According to the energy diagrams of H₂O₂ decomposition on different Ir planes, we suggest that the catalase-like activity is intrinsic to the Ir(111) plane and is independent of other crystal faces or coating agents. In addition, the radicals (˙OH and ˙OOH) generated on different Ir crystal planes show different stabilities and reactivities, which is responsible for the diverse peroxidase-like activities of IrNPs. Finally, the oxidase-like properties are derived from the Ir(220) plane, where the chemisorbed O₂ could accept one electron to generate superoxide species. The mechanism revealed in the present work should be helpful for understanding the catalytic role of the lattice planes of IrNPs, which may also be applicable to other metal nanozymes.