Generation of Reactive Oxygen Species via Electroprotic Interaction of H₂O₂with ZrO₂Gel: Ionic Sponge Effect and pH-Switchable Peroxidase- and Catalase-Like Activity

Abstract

Formation of reactive oxygen species (ROS) is of vital importance in catalytic oxidation chemistry. In this paper we have shown that a nonredox system such as amorphous zirconium dioxide (a-ZrO2) is highly active in ROS formation via H2O2 decomposition. Interaction between a-ZrO2 and H2O2 in aqueous solution was investigated by means of EPR, HYSCORE, Raman, and UV–vis, along with auxiliary FTIR, TG-MS, and XPS techniques, in a broad range of pH values and H2O2 concentrations. Various reaction intermediates such as superoxide (O2 •–) and hydroxyl (•OH) radicals as well as peroxide (O2 2–) species were identified. At pH <5.3 the superoxide and hydroxyl radicals were generated simultaneously in large amounts with the peak concentration being reached around the isoelectric point of the gel catalyst. In this pH region, the ZrO2 gel exhibited peroxidase-type activity, quantified by an o-phenylenediamine assay. At pH >5.3 formation of O2 2– is accompanied by a substantial release of O2 due to the pronounced catalase-like activity of a-ZrO2. The role of electroprotic processes (an interfacial proton transfer coupled with an intermolecular electron transfer) in H2O2 decomposition and ROS formation was elucidated, and a plausible mechanism of this reaction, Zr+–HO2 – (surf) + H2O2(aq) → •OH(aq) + Zr+–O2 •– (surf) + H2O, was proposed. The surface of a-ZrO2 covered with hydroxyl groups plays a role of an ionic sponge, which controls the electroprotic equilibrium by capturing the charged reaction intermediates. Unlike the amorphous gel, crystalline zirconia exhibits only weak activity in the production of the O2 •– and •OH radicals, and a different mechanism is involved. It is worth mentioning that the activity of the zirconia gel catalyst in ROS generation, as gauged by the Michaelis–Menten constant, is comparable (ca. 40%) to that of the Fenton-type oxides (Fe3O4, Co3O4).

Comparison of Enhanced Photocatalytic Degradation Efficiency and Toxicity Evaluations of CeO2 Nanoparticles Synthesized Through Double-Modulation

“…Versatile metal oxide nanoparticles (MO NPs) have been extensively applied in various scientific and engineering fields such as electrochemical reactions, photocatalytic degradation, fuel cells, and photoreactions [1][2][3][4][5][6][7][8][9]. However, nanometer-sized MO NPs can easily penetrate the human body through the skin, and, thus, the biocompatibility of these nanoparticles (NPs) should be seriously considered as a safety measure [10][11][12][13].…”

Progress and Trend on the Regulation Methods for Nanozyme Activity and Its Application

“…The pH factor plays an important role in the reactions catalyzed by nanozymes. It was found that some nanozymes exhibited different enzyme activities at different pH values [70][71][72][73]; for example, if the pH of α-ZrO 2 is lower than 5.3, it shows the catalytic activity of peroxidase, and when the pH is higher than 5.3, it shows the catalytic activity of catalase [71]. Additionally, the temperature factor can also regulate the catalytic efficiency of nanozymes.…”

Large Breathing Effect Induced by Water Sorption in the Exceptionally Stable Nonporous Non-MOF Crystalline Material

“…58,[66][67][68] Noteworthy, EPR spectroscopy is commonly applied to study the state of the paramagnetic active sites doped into the mesoporous hosts such as zeolites in various catalytic processes. [69][70][71][72] In the case of 1Mn, presented herein, the state of the Mn II sites was successfully monitored in the real time by employing in situ EPR spectroscopy during the sorption/desorption of water molecules. Structural and electronic changes within the coordination sphere of Mn II affects also the low-temperature photo-induced magnetization of the compound which is evidenced by detailed photomagnetic studies.…”