Nanocomposite antimicrobials prevent bacterial growth through the enzyme-like activity of Bi-doped cerium dioxide (Ce1-xBixO2-δ)

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.

Harnessing cerium-based biomaterials for the treatment of bone diseases

Bone, an actively metabolic organ, undergoes constant remodeling throughout life. Disturbances in the bone microenvironment can be responsible for pathologically bone diseases such as periodontitis, osteoarthritis, rheumatoid arthritis and osteoporosis. Conventional bone tissue biomaterials are not adequately adapted to complex bone microenvironment. Therefore, there is an urgent clinical need to find an effective strategy to improve the status quo. In recent years, nanotechnology has caused a revolution in biomedicine. Cerium(III, IV) oxide, as an important member of metal oxide nanomaterials, has dual redox properties through reversible binding with oxygen atoms, which continuously cycle between Ce(III) and Ce(IV). Due to its special physicochemical properties, cerium(III, IV) oxide has received widespread attention as a versatile nanomaterial, especially in bone diseases. This review describes the characteristics of bone microenvironment. The enzyme-like properties and biosafety of cerium(III, IV) oxide are also emphasized. Meanwhile, we summarizes controllable synthesis of cerium(III, IV) oxide with different nanostructural morphologies. Following resolution of synthetic principles of cerium(III, IV) oxide, a variety of tailored cerium-based biomaterials have been widely developed, including bioactive glasses, scaffolds, nanomembranes, coatings, and nanocomposites. Furthermore, we highlight the latest advances in cerium-based biomaterials for inflammatory and metabolic bone diseases and bone-related tumors. Tailored cerium-based biomaterials have already demonstrated their value in disease prevention, diagnosis (imaging and biosensors) and treatment. Therefore, it is important to assist in bone disease management by clarifying tailored properties of cerium(III, IV) oxide in order to promote the use of cerium-based biomaterials in the future clinical setting. STATEMENT OF SIGNIFICANCE: In this review, we focused on the promising of cerium-based biomaterials for bone diseases. We reviewed the key role of bone microenvironment in bone diseases and the main biological activities of cerium(III, IV) oxide. By setting different synthesis conditions, cerium(III, IV) oxide nanostructures with different morphologies can be controlled. Meanwhile, tailored cerium-based biomaterials can serve as a versatile toolbox (e.g., bioactive glasses, scaffolds, nanofibrous membranes, coatings, and nanocomposites). Then, the latest research advances based on cerium-based biomaterials for the treatment of bone diseases were also highlighted. Most importantly, we analyzed the perspectives and challenges of cerium-based biomaterials. In future perspectives, this insight has given rise to a cascade of cerium-based biomaterial strategies, including disease prevention, diagnosis (imaging and biosensors) and treatment.

Nanozymes for biomedical applications: Multi-metallic systems may improve activity but at the cost of higher toxicity?

Nanozymes are nanomaterials with intrinsic enzyme-like activity with selected advantages over native enzymes such as simple synthesis, controllable activity, high stability, and low cost. These materials have been explored as surrogates to natural enzymes in biosensing, therapeutics, environmental protection, and many other fields. Among different nanozymes classes, metal- and metal oxide-based nanozymes are the most widely studied. In recent years, bi- and tri-metallic nanomaterials have emerged often showing improved nanozyme activity, some of which even possess multifunctional enzyme-like activity. Taking this concept even further, high-entropy nanomaterials, that is, complex multicomponent alloys and ceramics like oxides, may potentially enhance activity even further. However, the addition of various elements to increase catalytic activity may come at the cost of increased toxicity. Since many nanozyme compositions are currently being explored for in vivo biomedical applications, such as cancer therapeutics, toxicity considerations in relation to nanozyme application in biomedicine are of vital importance for translation. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Toxicology and Regulatory Issues in Nanomedicine > Toxicology of Nanomaterials Diagnostic Tools > Diagnostic Nanodevices.

Defect Engineered Bi2Te3 Nanosheets with Enhanced Haloperoxidase Activity for Marine Antibiofouling

Defective bismuth telluride (Bi2Te3) nanosheets, an artificial nanozyme mimicking haloperoxidase activity (hPOD), show promise as eco-friendly, bactericidal, and antimicrofouling materials by enhancing cytotoxic hypohalous acid production from halides and H2O2. Microscopic and spectroscopic characterization reveals that controlled NaOH (upto X = 250 µL) etching of the nearly inactive non-transition metal chalcogenide Bi2Te3 nanosheets creates controlled defects (d), such as Bi3+species, in d-Bi2Te3-X that induces enhanced hPOD activity. d-Bi2Te3-250 exhibits approximately eight-fold improved hPOD than the as-grown Bi2Te3 nanosheets. The antibacterial activity of d-Bi2Te3-250 nanozymes, studied by bacterial viability, show 1, and 45% viability for Staphylococcus aureus and Pseudomonas aeruginosa, respectively, prevalent in marine environments. The hPOD mechanism is confirmed using scavengers, implicating HOBr and singlet oxygen for the effect. The antimicrofouling property of the d-Bi2Te3-250 nanozyme has been studied on Pseudomonas aeruginosa biofilm in a lab setting by multiple assays, and also on titanium (Ti) plates coated with the nanozyme mixed commercial paint, exposed to seawater in a real setting. All studies, including direct microscopic evidence, exhibit inhibition of microfouling, up to ≈73%, in the presence of nanozymes. This approach showcases that defect engineering can induce antibacterial, and antimicrofouling activity in non-transition metal chalcogenides, offering an inexpensive alternative to noble metals.

Facet-dependent haloperoxidase-like activities of CeO2 nanoparticles contribute to their excellent biofilm formation suppression abilities

Biofilms adhering to different surfaces have significant negative impacts in various fields. Cerium oxide nanoparticles can serve as mimics of haloperoxidase for biological biofilm inhibition applications. The regulation of the exposed facet of CeO2 nanoparticles influences their efficiency in various catalytic processes. However, there is still a lack of systematic studies on the facet-dependent haloperoxidase-like activity of CeO2. In the present study, the facet-dependent haloperoxidase activities and antibiofilm performance of CeO2 nanoparticles were elucidated through experiment analysis and density function theory calculation. The as-prepared CeO2 nanoparticles inhibited bacterial survival and catalyzed the oxidative bromination of quorum sensing signaling molecules, achieving biofilm inhibition performance. The antibacterial and biofilm formation suppression abilities were consistent with their haloperoxidase activities. The {111}- and {110}-facet CeO2 nanopolyhedra, as well as the {110}- and {100}-facet CeO2 nanorods, which had higher haloperoxidase activity showed better antibiofilm performance than the {100}-facet CeO2 cubes. The present findings provide a comprehensive understanding of the facet-dependent haloperoxidase-like activity of CeO2. Furthermore, engineering CeO2 morphologies with different crystal facets may represent a novel method for significantly adjusting their haloperoxidase-like activity.

Antimicrobial Nanoparticles Mediated Prevention and Control of Membrane Biofouling in Water and Wastewater Treatment: Current Trends and Future Perspectives

Global water scarcity and water pollution necessitate wastewater reclamation for further use. As an alternative to conventional techniques, membrane technology is extensively used as an advanced method for water purification and wastewater treatment due to its selectivity, permeability, and efficient removal of pollutants. However, microbial biofouling is a major threat that deteriorates membrane performance and imparts operational challenges. It is a natural phenomenon caused by the undesirable colonization of microbes on membrane surfaces. The economic penalties associated with this menace are enormous. The traditional preventive measures are dominated by biocides, toxic chemicals, cleaners and antifouling surfaces, which are costly and often cause secondary pollution. Recent focus is thus being directed to promote inputs from nanotechnology to control and mitigate this major threat. Different anti-microbial nanomaterials can be effectively used to prevent the adhesion of microbes onto the membrane surfaces and eliminate microbial biofilms, to provide an economical and eco-friendly solution to biofouling. This review addresses the formation of microbial biofilms and biofouling in membrane operations. The potential of nanocomposite membranes in alleviating this problem and the challenges in commercialization are discussed. The antifouling mechanisms are also highlighted, which are not widely elucidated.

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.

Communication Breakdown: Into the Molecular Mechanism of Biofilm Inhibition by CeO2 Nanocrystal Enzyme Mimics and How It Can Be Exploited

Bacterial biofilm formation is a huge problem in industry and medicine. Therefore, the discovery of anti-biofilm agents may hold great promise. Biofilm formation is usually a consequence of bacterial cell-cell communication, a process called quorum sensing (QS). CeO₂ nanocrystals (NCs) have been established as haloperoxidase (HPO) mimics and ecologically beneficial biofilm inhibitors. They were suggested to interfere with QS, a mechanism termed quorum quenching (QQ), but their molecular mechanism remained elusive. We show that CeO₂ NCs are effective QQ agents, inactivating QS signals by bromination. Catalytic bromination of 3-oxo-C₁₂-AHL a QS signaling compound used by Pseudomonas aeruginosa, was detected in the presence of CeO₂ NCs, bromide ions, and hydrogen peroxide. Brominated acyl-homoserine lactones (AHLs) no longer act as QS signals but were not detected in the bacterial cultures. Externally added brominated AHLs also disappeared in P. aeruginosa cultures within minutes of their addition, indicating that they are rapidly degraded by the bacteria. Moreover, we detected the catalytic bromination of 2-heptyl-1-hydroxyquinolin-4(1H)-one (HQNO), a multifunctional non-AHL QS signal from P. aeruginosa with antibacterial and algicidal properties controlling the expression of many virulence genes. Brominated HQNO was not degraded by the bacteria in vivo. The repression of the Pseudomonas quinolone signal (PQS) production and biofilm formation in P. aeruginosa through the catalytic formation of Br-HQNO on surfaces with coatings containing CeO₂ enzyme mimics validates the non-toxic strategy for the development of anti-infectives.

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

Designing the shape and size of catalyst particles, and their interfacial charge, at the nanometer scale can radically change their performance. We demonstrate this with ceria nanoparticles. In aqueous media, nanoceria is a functional mimic of haloperoxidases, a group of enzymes that oxidize organic substrates, or of peroxidases that can degrade reactive oxygen species (ROS) such as H₂O₂ by oxidizing an organic substrate. We show that the chemical activity of CeO_2-x nanoparticles in haloperoxidase- and peroxidase-like reactions scales with their active surface area, their surface charge, given by the ζ-potential, and their surface defects (via the Ce³⁺/Ce⁴⁺ ratio). Haloperoxidase-like reactions are controlled through the ζ-potential as they involve the adsorption of charged halide anions to the CeO₂ surface, whereas peroxidase-like reactions without charged substrates are controlled through the specific surface area S_BET. Mesoporous CeO_2-x particles, with large surface areas, were prepared via template-free hydrothermal reactions and characterized by small-angle X-ray scattering. Surface area, ζ-potential and the Ce³⁺/Ce⁴⁺ ratio are controlled in a simple and predictable manner by the synthesis time of the hydrothermal reaction as demonstrated by X-ray photoelectron spectroscopy, sorption and ζ-potential measurements. The surface area increased with synthesis time, whilst the Ce³⁺/Ce⁴⁺ ratio scales inversely with decreasing ζ-potential. In this way the catalytic activity of mesoporous CeO_2-x particles could be tailored selectively for haloperoxidase- and peroxidase-like reactions. The ease of tuning the surface properties of mesoporous CeO_2x particles by varying the synthesis time makes the synthesis a powerful general tool for the preparation of nanocatalysts according to individual needs.

Bacillus-derived probiotics: metabolites and mechanisms involved in bacteria-host interactions

Bacillus probiotics have a sporulation capacity that makes them more suitable for processing and storage and for surviving passage through the gastrointestinal tract. The probiotic functions and regulatory mechanisms of different Bacillus have been exploited in many reports, but little is known about how various Bacillus probiotics perform different functions. This knowledge gap results in a lack of specificity in the selection and application of Bacillus. The probiotic properties are strain-specific and cell-type-specific, and are related to the germination potential and to the diversity of metabolites produced following intestinal germination, as this causes the variation in probiotic function and mechanisms. In this review, we discuss the Bacillus metabolites produced during germination and sporulation in the GI tract, as well as possible processes affecting intestinal homeostasis. We conclude that the oxygen-capturing capability and the production of antimicrobials, exoenzymes, competence and sporulation factors (CSF), exopolysaccharides, lactic acid, and cell components are specifically associated with the functional mechanisms of probiotic Bacillus. The aim of this review is to guide the screening of potential Bacillus strains for probiotics and their application in nutrition research. The information provided will also promote further research on Bacillus-derived functional metabolites in human nutrition.

Nanozybiotics: Nanozyme-Based Antibacterials against Bacterial Resistance

Infectious diseases caused by bacteria represent a global threat to human health. However, due to the abuse of antibiotics, drug-resistant bacteria have evolved rapidly and led to the failure of antibiotics treatment. Alternative antimicrobial strategies different to traditional antibiotics are urgently needed. Enzyme-based antibacterials (Enzybiotics) have gradually attracted interest owing to their advantages including high specificity, rapid mode-of-action, no resistance development, etc. However, due to their low stability, potential immunogenicity, and high cost of natural enzymes, enzybiotics have limitations in practical antibacterial therapy. In recent years, many nanomaterials with enzyme-like activities (Nanozymes) have been discovered as a new generation of artificial enzymes and perform catalytic antibacterial effects against bacterial resistance. To highlight the progress in this field of nanozyme-based antibacterials (Nanozybiotics), this review discussed the antibacterial mechanism of action of nanozybiotics with a comparison with enzybiotics. We propose that nanozybiotics may bear promising applications in antibacterial therapy, due to their high stability, rapid bacterial killing, biofilm elimination, and low cost.

Haloperoxidase-mimicking CeO2-x nanorods for the deactivation of human coronavirus OC43

Despite the excellent antibacterial and antifouling effects of haloperoxidase (HPO)-mimicking CeO_2-x nanorods, their antiviral efficiency has not been explored. Herein, we designed and synthesized CeO_2-x nanorods with varying aspect ratios via the hydrothermal method. CeO_2-x nanorods catalysed the oxidative bromination of Br^- and H₂O₂ to HOBr, the kinetics of which were studied systematically using a phenol red assay. The CeO_2-x nanorods with the optimized aspect ratio (i.e., 4.5) demonstrated strong antiviral efficacies against the human coronavirus OC43, with no visible toxicity to the HCT-8 host cells.

High-throughput synthesis of CeO2 nanoparticles for transparent nanocomposites repelling Pseudomonas aeruginosa biofilms

Preventing bacteria from adhering to material surfaces is an important technical problem and a major cause of infection. One of nature's defense strategies against bacterial colonization is based on the biohalogenation of signal substances that interfere with bacterial communication. Biohalogenation is catalyzed by haloperoxidases, a class of metal-dependent enzymes whose activity can be mimicked by ceria nanoparticles. Transparent CeO2/polycarbonate surfaces that prevent adhesion, proliferation, and spread of Pseudomonas aeruginosa PA14 were manufactured. Large amounts of monodisperse CeO2 nanoparticles were synthesized in segmented flow using a high-throughput microfluidic benchtop system using water/benzyl alcohol mixtures and oleylamine as capping agent. This reduced the reaction time for nanoceria by more than one order of magnitude compared to conventional batch methods. Ceria nanoparticles prepared by segmented flow showed high catalytic activity in halogenation reactions, which makes them highly efficient functional mimics of haloperoxidase enzymes. Haloperoxidases are used in nature by macroalgae to prevent formation of biofilms via halogenation of signaling compounds that interfere with bacterial cell-cell communication ("quorum sensing"). CeO2/polycarbonate nanocomposites were prepared by dip-coating plasma-treated polycarbonate panels in CeO2 dispersions. These showed a reduction in bacterial biofilm formation of up to 85% using P. aeruginosa PA14 as model organism. Besides biofilm formation, also the production of the virulence factor pyocyanin in is under control of the entire quorum sensing systems P. aeruginosa. CeO2/PC showed a decrease of up to 55% in pyocyanin production, whereas no effect on bacterial growth in liquid culture was observed. This indicates that CeO2 nanoparticles affect quorum sensing and inhibit biofilm formation in a non-biocidal manner.

Insights on catalytic mechanism of CeO2 as multiple nanozymes

CeO₂ with the reversible Ce³⁺/Ce⁴⁺ redox pair exhibits multiple enzyme-like catalytic performance, which has been recognized as a promising nanozyme with potentials for disease diagnosis and treatments. Tailorable surface physicochemical properties of various CeO₂ catalysts with controllable sizes, morphologies, and surface states enable a rich surface chemistry for their interactions with various molecules and species, thus delivering a wide variety of catalytic behaviors under different conditions. Despite the significant progress made in developing CeO₂-based nanozymes and their explorations for practical applications, their catalytic activity and specificity are still uncompetitive to their counterparts of natural enzymes under physiological environments. With the attempt to provide the insights on the rational design of highly performed CeO₂ nanozymes, this review focuses on the recent explorations on the catalytic mechanisms of CeO₂ with multiple enzyme-like performance. Given the detailed discussion and proposed perspectives, we hope this review can raise more interest and stimulate more efforts on this multi-disciplinary field.

Nanostructured Ceria: Biomolecular Templates and (Bio)applications

Ceria (CeO2) nanostructures are well-known in catalysis for energy and environmental preservation and remediation. Recently, they have also been gaining momentum for biological applications in virtue of their unique redox properties that make them antioxidant or pro-oxidant, depending on the experimental conditions and ceria nanomorphology. In particular, interest has grown in the use of biotemplates to exert control over ceria morphology and reactivity. However, only a handful of reports exist on the use of specific biomolecules to template ceria nucleation and growth into defined nanostructures. This review focusses on the latest advancements in the area of biomolecular templates for ceria nanostructures and existing opportunities for their (bio)applications.

Ce-MOF with Intrinsic Haloperoxidase-Like Activity for Ratiometric Colorimetric Detection of Hydrogen Peroxide

Metal–organic framework (MOF) nanozymes, as emerging members of the nanozymes, have received more and more attention due to their composition and structural characteristics. In this work, we report that mixed-valence state Ce-MOF (MVCM) has intrinsic haloperoxidase-mimicking activity. MVCM was synthesized by partial oxidation method using Ce-MOF as a precursor. In the presence of H₂O₂ and Br⁻, MVCM can catalyze oxidative bromination of chromogenic substrate phenol red (PR) to produce the blue product bromophenol blue (Br₄PR), showing good haloperoxidase-like activity. Because of the special chromogenic substrate, we constructed a ratiometric colorimetric-sensing platform by detecting the absorbance of the MVCM-(PR, Br⁻) system at wavelengths of 590 and 430, for quantifying H₂O₂, where the detection limit of the H₂O₂ is 3.25 μM. In addition, the haloperoxidase-mimicking mechanism of the MVCM is proposed. Moreover, through enzyme kinetics monitoring, the K_m (H₂O₂ and NH₄Br) of the MVCM is lower than that of cerium oxide nanomaterials, indicating that the MVCM has a stronger binding affinity for H₂O₂ and NH₄Br than other materials. This work provides more application prospects for the development of nanozymes in the field of biosensors in the future.

Synthesis-temperature-regulated multi-enzyme-mimicking activities of ceria nanozymes

Ceria (CeO2) nanozymes have drawn much attention in recent years due to their unique physiochemical properties and excellent biocompatibility. It is therefore very important to establish a simple and robust guideline to regulate CeO2 with desired multi-enzyme-mimicking activities that are ideal for practical bioapplications. In this work, the multi-enzyme-mimicking activities of CeO2 were regulated in a facile manner by a wet-chemical method with different synthesis temperatures. Interestingly, a distinct response in multi-enzyme-mimicking activities of CeO2 was observed towards different synthesis temperatures. And the regulation was ascribed to the comprehensive effect of the oxygen species, size, and self-restoring abilities of CeO2. This study demonstrates that high-performance CeO2 can be rationally designed by a specific synthesis temperature, and the guidelines from radar chart analysis established here can advance the biomedical applications of ceria-based nanozymes.

Solvothermal Synthesis Routes to Substituted Cerium Dioxide Materials

We review the solution-based synthesis routes to cerium oxide materials where one or more elements are included in place of a proportion of the cerium, i.e., substitution of cerium is performed. The focus is on the solvothermal method, where reagents are heated above the boiling point of the solvent to induce crystallisation directly from the solution. This yields unusual compositions with crystal morphology often on the nanoscale. Chemical elements from all parts of the periodic table are considered, from transition metals to main group elements and the rare earths, including isovalent and aliovalent cations, and surveyed using the literature published in the past ten years. We illustrate the versatility of this synthesis method to allow the formation of functional materials with applications in contemporary applications such as heterogeneous catalysis, electrodes for solid oxide fuel cells, photocatalysis, luminescence and biomedicine. We pick out emerging trends towards control of crystal habit by use of non-aqueous solvents and solution additives and identify challenges still remaining, including in detailed structural characterisation, the understanding of crystallisation mechanisms and the scale-up of synthesis.