Tuning ceria catalysts in aqueous media at the nanoscale: how do surface charge and surface defects determine peroxidase- and haloperoxidase-like reactivity

Spatially Decoupled H2O2 Activation Pathways and Multi-Enzyme Activities in Rod-Shaped CeO2 with Implications for Facet Distribution

CeO2, particularly in the shape of rod, has recently gained considerable attention for its ability to mimic peroxidase (POD) and haloperoxidase (HPO). However, this multi-enzyme activities unavoidably compete for H2O2 affecting its performance in relevant applications. The lack of consensus on facet distribution in rod-shaped CeO2 further complicates the establishment of structure-activity correlations, presenting challenges for progress in the field. In this study, the HPO-like activity of rod-shaped CeO2 is successfully enhanced while maintaining its POD-like activity through a facile post-calcination method. By studying the spatial distribution of these two activities and their exclusive H2O2 activation pathways on CeO2 surfaces, this study finds that the increased HPO-like activity originated from the newly exposed (111) surface at the tip of the shortened rods after calcination, while the unchanged POD-like activity is attributed to the retained (110) surface in their lateral area. These findings not only address facet distribution discrepancies commonly reported in the literature for rod-shaped CeO2 but also offer a simple approach to enhance its antibacterial performance. This work is expected to provide atomic insights into catalytic correlations and guide the design of nanozymes with improved activity and reaction specificity.

Isomorphic Insertion of Ce(III)/Ce(IV) Centers into Layered Double Hydroxide as a Heterogeneous Multifunctional Catalyst for Efficient Meerwein-Ponndorf-Verley Reduction

The development of highly active acid-base catalysts for transfer hydrogenations of biomass derived carbonyl compounds is a pressing challenge. Solid frustrated Lewis pairs (FLP) catalysis is possibly a solution, but the development of this concept is still at a very early stage. Herein, stable, phase-pure, crystalline hydrotalcite-like compounds were synthesized by incorporating cerium cations into layered double hydroxide (MgAlCe-LDH). Besides the insertion of well-isolated cerium centers surrounded by hydroxyl groups, the formation of hydroxyl vacancies near the aluminum centers, which were formed by the insertion of cerium centers into the layered double hydroxides (LDH) lattice, was also identified. Depending on the initial cerium concentration, LDHs with different Ce(III)/Ce(IV) ratios were produced, which had Lewis acidic and basic characters, respectively. However, the acid-base character of these LDHs was related to the actual Ce(III)/Ce(IV) molar ratios, resulting in significant differences in their catalytic performance. The as-prepared structures enabled varying degrees of transfer hydrogenation (Meerwein-Ponndorf-Verley MPV reduction) of biomass-derived carbonyl compounds to the corresponding alcohols without the collapse of the original lamellar structure of the LDH. The catalytic markers through the test reactions were changed as a function of the amount of Ce(III) centers, indicating the active role of Ce(III)-OH units. However, the cooperative interplay between the active sites of Ce(III)-containing specimens and the hydroxyl vacancies was necessary to maximize catalytic efficiency, pointing out that Ce-containing LDH is a potentially commercial solid FLP catalysts. Furthermore, the crucial role of the surface hydroxyl groups in the MPV reactions and the negative impact of the interlamellar water molecules on the catalytic activity of MgAlCe-LDH were demonstrated. These solid FLP-like catalysts exhibited excellent catalytic performance (cyclohexanol yield of 45%; furfuryl alcohol yield of 51%), which is competitive to the benchmark Sn- and Zr-containing zeolite catalysts, under mild reaction conditions, especially at low temperature (T = 65 °C).

Ceria-Based Therapeutic Antioxidants for Biomedical Applications

The growing interest in nanomedicine over the last 20 years has carved out a research field called "nanocatalytic therapy," where catalytic reactions mediated by nanomaterials are employed to intervene in disease-critical biomolecular processes. Among many kinds of catalytic/enzyme-mimetic nanomaterials investigated thus far, ceria nanoparticles stand out from others owing to their unique scavenging properties against biologically noxious free radicals, including reactive oxygen species (ROS) and reactive nitrogen species (RNS), by exerting enzyme mimicry and nonenzymatic activities. Much effort has been made to utilize ceria nanoparticles as self-regenerating antioxidative and anti-inflammatory agents for various kinds of diseases, given the detrimental effects of ROS and RNS therein that need alleviation. In this context, this review is intended to provide an overview as to what makes ceria nanoparticles merit attention in disease therapy. The introductory part describes the characteristics of ceria nanoparticles as an oxygen-deficient metal oxide. The pathophysiological roles of ROS and RNS are then presented, as well as their scavenging mechanisms by ceria nanoparticles. Representative examples of recent ceria-nanoparticle-based therapeutics are summarized by categorization into organ and disease types, followed by the discussion on the remaining challenges and future research directions.

Regulating the H2O2 Activation Pathway on a Well-Defined CeO2 Nanozyme Allows the Entire Steering of Its Specificity between Associated Enzymatic Reactions

Nanozymes are promising alternatives to natural enzymes, but their use remains limited owing to poor specificity. For example, CeO2 activates H2O2 and displays peroxidase (POD)-like, catalase (CAT)-like, and haloperoxidase (HPO)-like activities. Since they unavoidably compete for H2O2, affecting its utilization in the target application, the precise manipulation of reaction specificity is thus imperative. Herein, we showed that one can simply achieve this by manipulating the H2O2 activation pathway on pristine CeO2 in well-defined shapes. This is because the coordination and electronic structures of Ce sites vary with CeO2 surfaces, wherein the (100) and (111) surfaces display nearly 100% specificity toward POD-/CAT-like and HPO-like activities, respectively. The antibacterial results suggest that the latter surface can well-utilize H2O2 to kill bacteria (cf., the former), which is promising for anti-biofouling applications. This work provides atomic insights into the synthesis of nanozymes with improved activity, reaction specificity, and H2O2 utilization.

Peroxidase-like Activity of CeO2 Nanozymes: Particle Size and Chemical Environment Matter

The enzyme-like activity of metal oxide nanoparticles is governed by a number of factors, including their size, shape, surface chemistry and substrate affinity. For CeO₂ nanoparticles, one of the most prominent inorganic nanozymes that have diverse enzymatic activities, the size effect remains poorly understood. The low-temperature hydrothermal treatment of ceric ammonium nitrate aqueous solutions made it possible to obtain CeO₂ aqueous sols with different particle sizes (2.5, 2.8, 3.9 and 5.1 nm). The peroxidase-like activity of ceria nanoparticles was assessed using the chemiluminescent method in different biologically relevant buffer solutions with an identical pH value (phosphate buffer and Tris-HCl buffer, pH of 7.4). In the phosphate buffer, doubling CeO₂ nanoparticles' size resulted in a two-fold increase in their peroxidase-like activity. The opposite effect was observed for the enzymatic activity of CeO₂ nanoparticles in the phosphate-free Tris-HCl buffer. The possible reasons for the differences in CeO₂ enzyme-like activity are discussed.

In situ generation of H2O2 using CaO2 as peroxide storage depot for haloperoxidase mimicry with surface-tailored Bi-doped mesoporous CeO2 nanozymes

Designing the size, morphology and interfacial charge of catalyst particles at the nanometer scale can enhance their performance. We demonstrate this with nanoceria which is a functional mimic of haloperoxidases, a group of enzymes that halogenates organic substrates in the presence of hydrogen peroxide. These reactions in aqueous solution require the presence of H₂O₂. We demonstrate in situ generation of H₂O₂ from a CaO₂ reservoir in polyether sulfone (PES) and poly(vinylidene fluoride) (PVDF) polymer beads, which circumvents the external addition of H₂O₂ and expands the scope of applications for haloperoxidase reactions. The catalytic activity of nanoceria was enhanced significantly by Bi³⁺ substitution. Bi-doped mesoporous ceria nanoparticles with tunable surface properties were prepared by changing the reaction time. Increasing reaction time increases the surface area S_BET of the mesoporous Bi_0.2Ce_0.8O_1.9 nanoparticles and the Ce³⁺/Ce⁴⁺ ratio, which is associated with the ζ-potential. In this way, the catalytic activity of nanoceria could be tuned in a straightforward manner. H₂O₂ required for the reaction was released steadily over a long period of time from a CaO₂ storage depot incorporated in polyether sulfone (PES) and poly(vinylidene fluoride) (PVDF) beads together with Bi_0.2Ce_0.8O_1.9 particles, which may be used as precision fillers and templates for biological applications. The spheres are prepared as a dry powder with no surface functionalization or coatings. They are inert, chemically stable, and safe for handling. The feasibility of this approach was demonstrated using a haloperoxidase assay.

A simple method for the preparation of CeO2with high antioxidant activity and wide application range

Cerium oxide (CeO₂) is a well-known antioxidant with the ability to scavenge reactive oxygen species due to its unique electronic structure and chemical properties. Although many methods to enhance the antioxidant activity of CeO₂have been reported, its antioxidant activity is still not high enough, and some enhancement effects are limited by the material concentration. There are also some CeO₂obtained with high antioxidant activity at high concentrations, which is not conducive to the application of biomedicine. Therefore, it is urgent to obtain CeO₂material with low cell cytotoxicity, high antioxidant activity and wide application range. In this work, rod-like metal organic framework derived CeO₂(CeO₂-MOF) was prepared by a simple method. Compared with the CeO₂nanorods prepared by hydrothermal method, it shows better antioxidant activity compared with the CeO₂nanorods prepared by hydrothermal method. Moreover, the advantage of CeO₂-MOF's antioxidant activity is not affected by the hydroxyl radical and material concentrations The reason why CeO₂-MOF has higher antioxidant activity should be attributed to its higher Ce³⁺content and larger specific surface area. In addition, CeO₂-MOF also exhibits low cytotoxicity to HeLa cells and PC12 cellsin vitro. The strategy of using MOF as a structural and compositional material to create CeO₂provides a new method to explore highly efficient and biocompatible CeO₂for practical applications.