The Intrinsic Enzyme Activities of the Classic Polyoxometalates

Toward Distinguishing between the Superchaotropic and Hydrophobic Characters of Nanometric-Sized Ions in Interaction with PEGylated Surfaces

In this study, we explore the superchaotropic effect of various polyoxometalate or boron cluster nano-ions on hydrophilic neutral surfaces. Nano-ions, characterized by low charge densities, exhibit strong adsorption on non-ionic hydrophilic surfaces like PEGylated micelles. This adsorption phenomenon was attributed to the enthalpically favorable dehydration of nano-ions, the so-called superchaotropic effect. Here, we investigate the adsorption of three nano-ions, α-SiW12O404-, α-PW12O403-, and B12I122-, with decreasing charge density or increasing superchaotropicity (or hydrophobicity), on hydrophilic solid surfaces, PEGylated gold nanoparticles, and PEGylated gold-coated quartz crystal. Solid surfaces are devoid of hydrophobic regions, enabling the study of the subtle nuance between hydrophobic and superchaotropic effects. Unlike adsorption on PEGylated micelles, the adsorption constant decreases with a reduced charge density, aligning with the well-established principle that hydrophobic ions do not adsorb on hydrophilic surfaces. This research improves our understanding of the subtle difference between superchaotropic and hydrophobic effects in nano-ion adsorption phenomena.

Polyoxometalate-Nanozyme-Integrated Nanomotors (POMotors) for Self-Propulsion-Promoted Synergistic Photothermal-Catalytic Tumor Therapy

Enzyme-powered nanomotors have demonstrated promising potential in biomedical applications, especially for catalytic tumor therapy, owing to their ability of self-propulsion and bio-catalysis. However, the fragility of natural enzymes limits their environmental adaptability and also therapeutic efficacy in catalysis-enabled tumor therapy. Herein, polyoxometalate-nanozyme-based light-driven nanomotors were designed and synthesized for targeted synergistic photothermal-catalytic tumor therapy. In this construct, the peroxidase-like activity of the P2 W18 Fe4 polyoxometalates-based nanomotors can provide self-propulsion and facilitate their production of reactive oxygen species thus killing tumor cells, even in the weakly acidic tumor microenvironment. Conjugated polydopamine endows the nanomotors with the capability of light-driven self-propulsion behavior. After 10 min of NIR (808 nm) irradiation, along with the help of epidermal growth factor receptor antibody, the targeted accumulation and penetration of nanomotors in the tumor enabled highly efficient synergistic photothermal-catalytic therapy. This approach overcomes the disadvantages of the intrinsically fragile nature of enzyme-powered nanomotors in physiological environments and, more importantly, provides a motility-behavior promoted synergistic anti-tumor strategy.

Polyoxometalates as Potential Artificial Enzymes toward Biological Applications

Artificial enzymes, as alternatives to natural enzymes, have attracted enormous attention in the fields of catalysis, biosensing, diagnostics, and therapeutics because of their high stability and low cost. Polyoxometalates (POMs), a class of inorganic metal oxides, have recently shown great potential in mimicking enzyme activity due to their well-defined structure, tunable composition, high catalytic efficiency, and easy storage properties. This review focuses on the recent advances in POM-based artificial enzymes. Different types of POMs and their derivatives-based mimetic enzyme functions are covered, as well as the corresponding catalytic mechanisms (where available). An overview of the broad applications of representative POM-based artificial enzymes from biosensing to theragnostic is provided. Insight into the current challenges and the future directions for POMs-based artificial enzymes is discussed.

Exploring the Reactivity of Polyoxometalates toward Proteins: From Interactions to Mechanistic Insights

The latest advances in the study of the reactivity of metal-oxo clusters toward proteins showcase how fundamental insights obtained so far open new opportunities in biotechnology and medicine. In this Perspective, these studies are discussed through the lens of the reactivity of a family of soluble anionic metal-oxo nanoclusters known as polyoxometalates (POMs). POMs act as catalysts in a wide range of reactions with several different types of biomolecules and have promising therapeutic applications due to their antiviral, antibacterial, and antitumor activities. However, the lack of a detailed understanding of the mechanisms behind biochemically relevant reactions-particularly with complex biological systems such as proteins-still hinders further developments. Hence, in this Perspective, special attention is given to reactions of POMs with peptides and proteins showcasing a molecular-level understanding of the reaction mechanism. In doing so, we aim to highlight both existing limitations and promising directions of future research on the reactivity of metal-oxo clusters toward proteins and beyond.

Recent Advances in Polyoxometalates with Enzyme-like Characteristics for Analytical Applications

Artificial enzymes based on inorganic solids with both enzyme-mimetic activities and the special material features has been a promising candidate to overcome many deleterious effects of native enzymes in analytical applications. Polyoxometalates (POMs) are an importance class of molecular metal-oxygen anionic clusters. Their outstanding physicochemical properties, versatility and potential applications in energy conversion, magnetism, catalysis, molecular electronics and biomedicine have long been studied. However, the analytical applications of them is limited. Recently, the intrinsic enzymatic activities of POMs have also been found and become an area of growing interest. In this review, along with other reports, we aimed to classify the enzymatic activity of POMs, summarize the construction of POMs-based enzymes, and survey their recent advances in analytical fields. Finally, the current challenges and trends of the polyoxometalates with enzymatic activity in future chemo-/bio-sensing applications are briefly discussed.

A Multifunctional Janus Electrospun Nanofiber Dressing with Biofluid Draining, Monitoring, and Antibacterial Properties for Wound Healing

Wound healing greatly affects patients' health and produces medical burden. Therefore, we developed a multifunctional electrospun nanofiber dressing, which can inhibit methicillin-resistant Staphylococcus aureus (MRSA), drain excessive biofluid to promote wound healing, and simultaneously monitor wound pH level. The polyoxometalate (α-K₆P₂W₁₈O₆₂·14H₂O, P₂W₁₈) and oxacillin (OXA) are encapsulated in hydrophobic polylactide (PLA) nanofiber to synergistically inhibit MRSA. The phenol red (PSP) is encapsulated in hydrophilic polyacrylonitrile (PAN) nanofiber to sensitively indicate wound pH in situ. The PSP/PAN nanofiber is directly electrospun on the patterning OXA/P₂W₁₈/PLA nanofiber layer to form a Janus dressing. By taking advantage of the wettability difference between the two layers, the excess biofluid can be drained away from the wound. In addition, the Janus dressing exhibits good biocompatibility and accelerates wound healing via its antimicrobial activity and skin repairing function. This multifunctional Janus electrospun nanofiber dressing would be beneficial for wound management and treatment.

Thermotolerance and Cellulolytic Activity of Fungi Isolated from Soils/Waste Materials in the Industrial Region of Nigeria

The current study aimed on isolating thermotolerant, cellulolytic fungi from different tropical soil/waste materials samples such as wood waste, sawmill, decomposing straw and compost pit sites in Abraka, Southern Nigeria and assessing their applications in diverse cellulolytic processes. Fungal isolates were identified based on cultural, morphological, ITS-5.8S barcoding, reproductive structures and thereafter screened for thermotolerance and cellulolytic activities [carboxy methyl cellulase (CMC-ase) and filter paperase (FP-ase)] by cultivating at 45, 50, 60, 70, 80° and 45 °C, respectively. The highest fungal abundance (44.4%) was observed in the compost pit while the lowest (11.1%) was recorded for sawmill. Nine thermotolerant fungal isolates were identified: Aspergillus flavus (4), Blakeslea sp. (3), and Trichoderma asperellum (2). Among them only five, including three A. flavus, one Blakeslea sp. and one T. asperellum, exhibited cellulolytic activity ranging from 12.11 ± 0.01 to 18.42 ± 5.39 µg/mL and 0.36 ± 0.01-9.21 ± 2.52 µg/mL for CMC-ase and filter paperase FP-ase assay, respectively. The low Michaelis-Menten constants of 1.137 for CMC-ase and 1.195 for FP-ase were obtained, indicated a strong affinity for the substrate. The thermotolerance coupled with cellulolytic activity of these isolates make them attractive for potential application in industries where they can be of economic and environmental benefits as against the use of chemicals.

Anderson polyoxometalates with intrinsic oxidase-mimic activity for "turn on" fluorescence sensing of dopamine

Anderson-type polyoxometalate containing Fe³⁺ and Mo⁶⁺, (NH₄)₃[H₆Fe(III)Mo₆O₂₄] (FeMo₆), was found to work as an oxidase-mimicking nanoenzyme for the first time, exhibiting the ability of catalytic oxidation of o-phenylenediamine (OPD), 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTs), and 3,3',5,5'-tetramethylbenzidine (TMB), which features easy synthesis, low cost, simple operation, and low consumption. Attributed to the nature of FeMo₆ and Fenton-like effect, a novel sensor based on two consecutive "turn on" fluorescence was developed for detecting dopamine (DA) by employing the FeMo₆-OPD system, and the linear range was from 1 to 100 μM with the detection limit 0.0227 μM (3σ/s). Moreover, to increase oxidase-mimic activity of FeMo₆, reduced graphene oxide (rGO) loading FeMo₆ composites (FeMo₆@rGO (n), n = 5%, 10%, 15%) was fabricated, and results show that oxidase-like activities of FeMo₆@rGO (n) are dependent on the mass ratio of FeMo₆/rGO, and FeMo₆@rGO (10%) exhibits the highest oxidase-mimic activity and the fastest respond time (4 min) among all reported oxidase mimic of DA to date. Graphical abstract Anderson-type Mo-POMs FeMo₆ was found to work as an oxidase-mimicking nanoenzyme for the first time and was used to detect DA for two consecutive "turn on" fluorescence sensor modes.

Polyoxometalate-Based Nanomaterials Toward Efficient Cancer Diagnosis and Therapy

As an emerging class of inorganic metal oxides, organically functionalized polyoxometalates (POMs) or POM-based nanohybrids have been demonstrated promising potential for the inhibition of various cancer types by the virtue of their diversity in structures and significantly reduced toxicity. This contribution summarizes the latest achievement of POM-based nanomaterials in cancer diagnosis and various therapeutics to put forward our fundamental viewpoints on the design principles of modified POMs based on their application. In addition, major challenges and perspectives in this field are also discussed. We expect that this review will provide a valuable and systematic reference for the further development of POM-based nanomaterials.

Fluorometric Detection of Thiamine Based on Hemoglobin-Cu3(PO4)2 Nanoflowers (NFs) with Peroxidase Mimetic Activity

Component analysis plays an important role in food production, pharmaceutics and agriculture. Nanozymes have attracted wide attention in analytical applications for their enzyme-like properties. In this work, a fluorometric method is described for the determination of thiamine (TH) (vitamin B1) based on hemoglobin-Cu3(PO4)2 nanoflowers (Hb-Cu3(PO4)2 NFs) with peroxidase-like properties. The Hb-Cu3(PO4)2 NFs catalyzed the decomposition of H2O2 into ·OH radicals in an alkaline solution that could efficiently react with nonfluorescent thiamine to fluoresce thiochrome. The fluorescence of thiochrome was further enhanced with a nonionic surfactant, Tween 80. Under optimal reaction conditions, the linear range for thiamine was from 5 × 10-8 to 5 × 10-5 mol/L. The correlation coefficient for the calibration curve and the limit of detection (LOD) were 0.9972 and 4.8 × 10-8 mol/L, respectively. The other vitamins did not bring about any obvious changes in fluorescence. The developed method based on hybrid nanoflowers is specific, pragmatically simple and sensitive, and has potential for application in thiamine detection.

AuNPs@PMo12 nanozyme: highly oxidase mimetic activity for sensitive and specific colorimetric detection of acetaminophen

The design of a highly specific and sensitive approach for the quantitative and qualitative determination of acetaminophen (AP) is crucial from a human health point of view. In this study, AuNPs@PMo12, as a nanozyme, has been developed for the highly sensitive and selective detection of AP with 3,3',5,5'-tetramethylbenzidine (TMB) within a few seconds without adding oxidizing reagents (e.g. H2O2). Synthesized nanosensors are able to oxidize TMB to yellow-brown oxidized TMB (oxTMB). The maximum peak wavelength of oxTMB was observed at 450 nm. The addition of AP and then increasing its concentration led to the production of different products in blue color. In experimental measurements, the limit of detection was obtained as 14.52 mg L-1. The quantitative determination of AP concentrations can be carried out using UV-vis spectroscopy. The design of nanosensors is cost-effective and application of them in H2O2-free and enzyme-free conditions provides a rapid sensing approach for practical use in disease monitoring and diagnosis.

Nanozymes in electrochemical affinity biosensing

Over the past decade, artificial nanomaterials that exhibit properties similar to those of enzymes are gaining attraction in electrochemical biosensing as highly stable and low-cost alternatives to enzymes. This review article discusses the main features of the various nanomaterials (metal oxide, metal, and carbon-based materials) explored so far to mimic different kinds of enzymes. The unprecedented opportunities imparted by these functional nanomaterials or their nanohybrids, mostly providing peroxidase-like activity, in electrochemical affinity biosensing are critically discussed mainly in connection with their use as catalytic labels or electrode surface modifiers by highlighting representative strategies reported in the past 5 years with application in the food, environmental, and biomedical fields. Apart from outlining the pros and cons of nanomaterial-based enzyme mimetics arising from the impressive development they have experienced over the last few years, current challenges and future directions for achieving their widespread use and exploiting their full potential in the development of electrochemical biosensors are discussed. Graphical abstract.