Single-Site Iridium Catalyst on Metal–Organic Framework as Light-Responsive Oxidase-Like Nanozyme with High Stability for Colorimetric Detection of Antioxidant Capacity

The design and synthesis of advanced artificial enzymes are essential for developing promising surrogates for natural enzymes. Herein, we reported an efficient and facile strategy for the synthesis of a single-site iridium catalyst on a metal-organic framework (UiO-67@Ir) as a light-responsive oxidase-like nanozyme, in which UiO-67 was utilized as a host template and the iridium(III) complex was utilized as a photosensitizer with a light-responsive property. A single-site iridium catalyst on UiO-67@Ir by the coordination of the Ir (III) complex with the nitrogen atom of UiO-67 is confirmed by X-ray photoelectron spectroscopy and aberration-corrected atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. The UiO-67@Ir possesses remarkable light-responsive oxidase-like activity and good cycle and storage stability. Excellent catalytic activity toward the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) was obtained with 0.204 mM of Michaelis-Menten constant (K_m) due to its large surface areas and abundant active sites. TMB was oxidized by UiO-67@Ir in the presence of O₂ under light irradiation through the formation of both ^•OH and O₂^•- by type I photosensitization processes (electron transfer) and the formation of ¹O₂ by type II photosensitization processes (energy transfer). Moreover, a sensitive colorimetric method was developed for the detection of antioxidants with a detection limit of 0.6, 0.5, and 0.3 μM for ascorbic acid, glutathione, and cysteine, respectively. The total antioxidant capacity in fruit and drink samples were analyzed with desirable results. This study not only enlightens the novel nanozyme designing strategies but also suggests its good analytical performance in colorimetric sensing.