Surface-modified polyoxometalate-complexed metal oxide nanocrystals and use thereof

Abstract

Complexes of polyoxometalate (POM) cluster anions and metal oxide nanocrystal (NC) cores (POM-NC) with tunable NC surface protonation are disclosed. These protonated POM-NC complexes enable precise control over catalytic properties, enhancing catalytic activity under near-neutral pH conditions while improving stability and solubility across various applications. The protonated POM- metal oxide complexes facilitate the activation of airborne O2 under ambient conditions and the production of reactive oxygen species (ROS), including hydroxyl radicals, superoxide, and peroxides. In certain catalytic processes, these complexes mimic enzymatic functions, such as oxidase, peroxidase, catalase, and superoxide dismutase activity, making them valuable for various biological, therapeutic, chemical and environmental applications such as applications related to oxidative catalysis and ROS scavenging.

Description

[0001] SURFACE-MODIFIED POLYOXOMETALATE-COMPLEXED METAL OXIDE NANOCRYSTALS AND USE THEREOF

[0002] FIELD OF THE INVENTION

[0003] The present disclosure relates to complexes comprising polyoxometalate (POM) ligands on modified surfaces of crystalline metal-oxide and use thereof in catalytic reactions.

[0004] BACKGROUND

[0005] Polyoxometalates (POMs) are a diverse class of polynuclear metal-oxo anions formed, e.g., by early transition metals in high oxidation states. These inorganic clusters exhibit a wide range of structures and properties, making them valuable in catalysis, nanotechnology, medicine, materials science, and sensing applications. Notably, POMs can act as robust, polydentate ligands, facilitating the stabilization and functionalization of various electrophilic species, including transition-metal cations and rare-earth elements.

[0006] Metal hydroxide and colloidal metal-oxide nanocrystals (NCs) are highly reactive species that serve as catalysts and precursors for functional materials. However, their instability in both organic and aqueous solutions, particularly their tendency to aggregate and precipitate, presents a major challenge in their practical application. Traditional solvothermal methods address this by preparing NCs in organic solvents at elevated temperatures in the presence of organic ligands for stabilizing the NCs. These methods provide impressive control over crystal phase, size and shape; however, the densely bound organic protecting ligands block access of substrates to the reactive NC surfaces, hence creating a barrier to catalytic activity. This limits the efficiency and versatility of NCs in catalytic and electrochemical processes.

[0007] Recent advances demonstrate that POM ligands offer a unique solution by stabilizing metal- oxide and metal hydroxide NCs while maintaining their catalytic functionality. Unlike organic ligands, which densely coat the NC surface, bulky POMs cluster anions selectively bind to a limited fraction of surface sites on NCs, leaving active sites exposed. Furthermore, the large negative charges of POM ligands and their numerous alkali-metal counter-cations, render POM-complexed NCs remarkably soluble and stable in water, while solubility in organic solvents is imparted by simply exchanging alkali-metal counter-cations by organic ones. SUMMARY

[0008] It has been discovered by the present inventors that polyoxometalate (POM) cluster anion ligation onto metal oxide and metal hydroxide nanocrystals (NCs) enables rational control over NC surface protonation, thereby allowing fine-tuning of catalytic properties without altering bulk pH. This discovery enables catalytic reactions that traditionally require strongly acidic or basic conditions (i.e., extreme pH conditions) to be conducted under near-neutral pH, significantly expanding the utility of metal hydroxide and metal-oxide NCs in aqueous and biological environments, for example, expanding the applicability of metal-oxide NCs in diverse chemical processes.

[0009] The present disclosure relates to innovative methods for strategically controlling the protonation states of NCs through the binding of POM ligands. This approach offers a versatile and effective means of enhancing catalytic activity, improving solubility, and expanding the applicability of POM-complexed metal oxide and metal hydroxide NCs across various catalytic systems.

[0010] As a specific example, the disclosure highlights the use of POMs on Ce02 NCs to enhance stability while preserving access to reactive surface sites.

[0011] In one aspect, the present disclosure relates to a protonated polyoxometalate-nanocrystal complex of the Formula (I):

[0012] [ / -POM]m[H / (NC)].

[0013] POM is a polyoxometalate anion presented by the Formula (la):

[0014] Co[XzMpDdOy]n- wherein

[0015] X is H, at least one positively charged heteroatom (herein also referred to as "heterocation") and / or at least one positively charged metal atom (metal cation), wherein the metal atom is selected from main-group metals, transition metals, or lanthanides or any combination thereof.

[0016] In some embodiments, X is one, two, three or more heterocations and / or metal cations selected from Be, P, As, Sb, S, Si, Ge, or F, or at least one metallic heteroatom selected from Na, Al, Ga, Ti, Zr, Hf, V, Cu, Fe, Mn, Co, Sn, Pb, or Ce. M is at least one transition and / or main-group metal cation, optionally, in a high oxidation state. For example, M may be one, two three or more metal cations selected from Al(lll) (Al+3), Ga(lll) In(lll), Tl(lll), Sn(lll), Pb(IV), Bi(lll), Bi(V), Sc(lll), Tl(lll), (Ti(IV), V(III),V(IV), V(V), Cr(lll), Cr(VI), Mn(ll), Mn(lll), Mn(IV), Mn(VII), Fe(ll), Fe(lll), Co(ll), Ni(ll), Ni(lll), Cu(ll), Cu(l), Zn(ll), Y(lll), Zr(IV), Nb(V), Mo(V), Mo(VI), Tc(VII), Ru(ll), Ru(lll), Ru(IV), Ru(VI), Pd(ll), Pd(IV), Hf(IV), Ta(V) or W(VI), and any combination thereof.

[0017] In some embodiments, M is one or more cations selected from V+5, V+4, Nb+5, Ta+5, Mo+5, Mo+6and / or W+6.

[0018] D is H, OH, OH2, a lower alkyl, lower hydroxyalkyl, lower silylalkyl, lower silylalkoxy and / or carboxylate. D may be absent in the complex. If present, in some embodiments, it may be OH, a lower alkyl of 1 to 3 carbon atoms (C1-C3 alkyl), a lower hydroxyalkyl selected from HO-Ci alkyl, HO- C2 alkyl or HO-C3, or any combination thereof

[0019] C, representing herein the complex countercation, may be H+, an inorganic cation, or organic cation. When C is H+, then H+is the only counteraction present, and the complexes is referred herein as a "fully protonated complex", or "H+form complex". Additionally, or alternatively, at least some of C may be Li+, Na+, K+, Cs+or RXH4-XN+, where R is alkyl and / or thioalkyl. Such complexes are referred to herein as "partially protonated complexes". n is the overall charge of the POM cluster anion, and it may be an integer of 1 to 75. a, the amount of countercations that balances the negative charge of the anion, is an integer of 1 to 75.

[0020] The indices z, p, d, and y, represent the relative amounts of X, M, D and oxygen (O), respectively, wherein z is 0 to 100; p is 6 to 250; d is 0 to 100; and y is 15 to 800. i indicates a specific POM isomer, if present. m, the number of POMs bound to an individual nanocrystal (NC), may be 1 to 50,000, for example, 100 to 6000 or 2 to 1000. f, the relative amounts of protons in the complex, is equal or less than m multiplied by n

[0021] (n-m). When / = n-m, the complex is considered as a fully protonated complex, and when / < n-m, the complex is considered as partially protonated. The core nanocrystal (NC) of the protonated complex comprises metal oxide unit cells, each represented by Formula (lb):

[0022] [M'iOfc]; wherein

[0023] M' is one or more cations of metals from the main-group elements, transition metals and / or lanthanide elements.

[0024] I and k represent the relative amounts of M' and oxygen, respectively, in a cell unit, j may be an integer or a fraction in the range of from 1 to 20, for example 1 to 8, or 1 to 3; and k may be an integer or a fraction in the range of from 1 to 20, preferably 1 to 8.

[0025] M' may be a combination of main-group, transition metal and / or lanthanide elements that in combination with oxide, hydroxide, or oxyhydroxide, forms nanocrystals that contain oxygen vacancies and / or after being reduced, can be re-oxidized by O2, preferably using air under ambient conditions. Thus, M' may be a cation of at least one atom selected from Mg, Ca, Al, Ga, In, Sn, Bi, Ti, Ce, Gd, La, Li, Rb, Cs, Sr, Ba, Al, Si, Ge, Pb, Bi, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mo, La, Ce, Pr, W, Nd, Sm, Eu, Gs, Dy and / or Tb. In some embodiments, M' is a lanthanide cation selected from Ce3+, Eu3+, Tb3+, Dy3+, Gd3+, La3+and / or Nd3+.

[0026] The unit cell M' / O^ may be, for example, MgO, CaO, SrO, BaO, AI2O3, Ga2O3, ln2O3, SiC>2, GeCh, SnC>2, PbO, PbC>2, Bi2O3, TiO2, TiO2, V2O5, Cr2O3, Fe3O4, Fe2O3, Co3O4, NiO, CuO, ZnO, MoO3La2O3, CeO2, Pr6On, Pr2O3, WO3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Dy2O3, Tb2O3, transition-metal doped epsilon- MnO2, transition-metal doped CeO2, and / or transition-metal doped SnO2. In some embodiments, the nanocrystal core comprises CeO2.

[0027] The POM may be an isopolytungstate, isopolyvanadate, isopolymolybdate, isopolyniobate, heteropolytungstate, heteropolyvanadate, heteropolymolybdate, polyoxoniobate, and modified form thereof selected from defect POMs, substituted POMs and "out of pocket" metal binding POMs.

[0028] In some embodiments, the complex is the fully protonated POM-NC complex [a-

[0029] PWnO39Ce] 22[H88(Ce02)gio], herein designated "Complex 1". In some embodiments, the complex is the partially protonated complex Cs8-x[a-

[0030] PWnO39Ce] 22[Hx(Ce02)9io], wherein C is an inorganic or organic cation, for example K+, and x is an integer or a fraction of 1 to 87.

[0031] In a further aspect, the present disclosure relates to methods for catalytic oxidation of a substance, operated with a protonated polyoxometalate-nanocrystal complex as defined herein. In some embodiments, the substance is dioxygen (O2), for example air-borne O2, and the catalytic oxidation thereof produces reactive oxygen species (ROS) such as hydroxyl radicals (*01-1), superoxide (*02 ), peroxides and / or ozone.

[0032] In another aspect, the present disclosure relates to an enzyme-mimicking catalytic process, comprising contacting a protonated polyoxometalate-nanocrystal complex disclosed herein with a substrate. The enzyme-mimicking catalytic process may mimic oxidase, peroxidase, catalase and / or superoxide dismutase biochemical reactions. In some embodiments, the catalytic process features dual-oxidase activity.

[0033] In some embodiments, the enzyme-mimicking catalytic process is utilized for conversion of superoxide to hydrogen peroxide.

[0034] The catalytic oxidation methods and / or the enzyme-mimicking catalytic processes disclosed herein may be carried out at near-neutral pH values and / or at ambient conditions, optionally in absence of added oxidant such as H2O2. The catalytic oxidation methods disclosed herein are highly efficient, rapid and display indefinitely long and continuous operation compared to catalytic oxidation with corresponding colloidal metal oxide nanoparticles not associated with ROMs ligands.

[0035] The efficacy of a contemplated catalytic oxidation and / or enzyme-mimicking catalytic process is tunable by adjusting the protonation degree of the POM-NC complex. In some embodiments, the protonation degree is adjusted by the addition of countercations and / or OH" anions.

[0036] Any of the catalytic methods or enzyme-like processes disclosed herein may be applied in a wide range of applications and uses such as, but not limited to: (i) antimicrobial therapy; (ii) treatment of cancer, optionally in precision medicine and / or targeted therapy; (iii) environmental remediation, industrial decontamination, sewage treatment; (iv) wound healing; (v) anti-fungal treatment; (vi) air purification and decontamination, odor control and deodorization; (vii) self- cleaning and anti-fouling surface coatings; (viii) sterilization of medical devices and surfaces; (ix) biosensing and diagnostic applications; (x) fuel cell and battery catalysis; (xi) food packaging and preservation; (xii) photocatalytic applications; and / or (xiii) catalysis in organic synthesis.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Fig. 1 is a graph showing zeta potential values as a function of pH for colloidal Ce02 nanoparticles;

[0039] Fig. 2 is a scheme showing exchange of Na+countercations by H+cations via dialysis in water, providing the protonated POM-CeO2complex, [a-PWn039Ce]22[H88(Ce02)9io] (Complex 1);

[0040] Fig. 3 is a graph showing the zeta-potential distribution of the protonated complex [ot- PWn039Ce]22[H88(Ce02)9io] (Complex 1), obtained from Electrophoretic Light Scattering (ELS) measurements;

[0041] Fig. 4 presents the results of the elemental analysis of Complex 1. The spectrum on the left shows the energy-dispersive X-ray spectroscopy (EDX) elemental distribution of Complex 1, and the elemental mapping on the right is of a region containing numerous particles of Complex 1, showing the distribution of oxygen (O), cerium (Ce) and tungsten (W);

[0042] Figs. 5A-5B are cryogenic transmission electron microscopy (cryo-TEM) images of Complex 1. 5A: two individual particles, approx. 6-7 nm in diameter, with visible variations in contrast due to presence of POM ligands. 5B: a closeup of a particle, with numerous visible POMs on the surface, denoted by red arrows;

[0043] Figs. 6A-6B show results of phase analysis of Ce02 nanocores of Complex 1. 6A: two overlapping powder X-ray diffraction (PXRD) spectra: an experimental spectrum (upper thick peaks), and a theoretical (simulated) XRD of Fluorite type Ce02 (lower peaks). 6B: Scherrer analysis of the (1 1 1) peak in the experimental PXRD spectrum;

[0044] Figs. 7A-7B characterize Complex 1 with respect to the Ce3+content. 7A: a deconvoluted high-resolution XPS 3d spectrum of Complex 1, showing the 10 peaks associated with the complex nature of the multiple d-splitting, which were assigned according to previous studies. 7B: a schematic demonstration of oxygen vacancies location on the surface of the CeO2 nanocrystal (the (1 1 1) planes); Fig. 8 is a graph showing the catalytic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) measured as absorbance at Amax- 652 nm, under air and under inert environment, as a function of time;

[0045] Figs. 9A-9C are graphs illustrating Michaelis Menten kinetics, showing dioxygen activation rate as a function of TMB concentration for Complex 1 in different protonation states. 9A: reaction kinetics of fully protonated Complex 1 (Squares: reaction rate vs TMB concentration; solid line: Michaelis Menten model fit). 9B: Comparison of Complex 1 (squares) and its surface-modified form K88-x[a-PWii039Ce]22[Hx(Ce02)9io] (circles), where a portion of 88-x H+are replaced with K+; solid lines represent model fitting. 9C: reaction kinetics of three systems: Complex 1 (squares), H+ / K+surface-modified Complex 1 (circles), and further modified Complex 1 with partial H+neutralization using 10 pM OH- (triangles); solid lines show model fitting;

[0046] Figs. 10A-10B Electron paramagnetic resonance study (EPR) analysis of radical species formed shortly following addition of 5,5-dimethyl-l-pyrroline N-oxide (DMPO) to a solution of Complex 1. 10A: overlapping EPR spectra of experimental (black) and simulated (red) signals. 10B: deconvolution of the simulated spectrum into four components: two superoxide radicals (»DMPO- O2 ), a hydroxyl form of the DMPO radical (•DMPO-OH), and a decomposition product of DMPO (oxidized DMPO (DMPOX));

[0047] Fig. 11 EPR spectra recorded 24 hours after DMPO addition to Complex 1: experimental (black) and simulated (red) spectra. Radical species detected: »DMPO-OH (hydroxyl radical, circles) and DMPOX (oxidized DMPO, triangles);

[0048] Figs. 12A-12B are graphs showing optical density measurements in antibacterial tests. 12A: absorbance vs. time plot of Streptococcus cultures treated or untreated with the protonated POM- CeO2complex [a-PWn039Ce]22[H88(Ce02)9io] (Complex 1), following 24 hours of incubation. 12B: ODeoo absorbance as a function of Complex 1 concentrations in E. coli culture, measured at 0, 24 and 48 hours post incubation;

[0049] Figs. 13A-13B are images of bread slices after 11 and 30 days of incubation at 37°C under moist conditions. 13A: slice treated with Complex 1; 13B: untreated control slice; and

[0050] Fig. 14 is a bar graph showing antioxidant activity of two protonated form of Complex 1: the fully protonated form ([a-PWn039Ce]22[H88(Ce02)9io]) and the partially protonated form (Ks8-x[a- PWii039Ce]22[Hx(Ce02)9io]), each at different concentrations, based on 2,2-Diphenyl-l- picrylhydrazyl (DPPH) assay results.

[0051] DETAILED DESCRIPTION

[0052] The present disclosure relates to complexes of various polyoxometalates (POMs) and metal oxide, metal oxy-hydroxide and / or metal hydroxide nanocrystals (NCs) featuring variable degrees of core protonation and use thereof in various biologic and chemical environments.

[0053] Colloidal nanoparticles are 1-100 nm particles dispersed in a medium, where surface interactions, small size prevent their precipitation out of solution. Their high surface-to-volume ratio enhances reactivity, while surface charges help maintain dispersion. However, metal-oxide colloidal nanoparticles (NPs) or NCs face challenges such as stability issues and aggregation, limiting development of their solution-state chemistry for catalytic and related applications.

[0054] To address these challenges, the inventors previously demonstrated that polyoxometalate (POM) ligands can covalently bind to metal-oxide NCs, introducing new structural and electronic features (WO 2023 / 175512). POM ligands are polyanions (-3 to -8 charge), and with multiple POMs attached to each NC, they create highly charged anionic complexes (ca. -20 to -200). These negative charges are balanced by countercations (e.g., Na+, K+).

[0055] Unlike bare colloidal metal-oxide NCs, which suffer from instability and uncontrolled reactivity, POM-complexed NCs offer enhanced stability, controlled surface reactivity, and improved dispersion. POMs stabilize NC surfaces and regulate electronic properties, enabling better redox activity, catalytic control, and electron transfer. Notably, POMs can act as electron donors / acceptors, enhancing oxidation and photocatalysis.

[0056] The present inventors discovered that extensive dialysis in water of POM-metal oxide complexes, for example, POM-CeO2, led to the complete stoichiometric exchange of alkali-metal countercations with protons. As a result, the metal oxide NC surfaces became extensively protonated, and the positively charged NCs themselves acted as a new type of countercations for the multiple negatively charged POM ligands. Remarkably, this transformation, also referred to herein as "nanocrystal surface modification", occurred without a significant decrease in pH and was fully reversible upon the addition of alkali-metal hydroxides (MOH), while the complexes retained high solubility regardless of their surface protonation state. Such surface-modified NC core exhibits unique characteristics which may affect, inter alia, its reactivity.

[0057] These recent findings by the present inventors demonstrate that POM ligation to protonated metal-oxide NCs dramatically affects pH-dependent iso-electric point precipitation: POMs change / shift the isoelectric point (due to their very negative charge) to such an extent that it is orders of magnitude more positive (i.e., a substantially high concentration of counteraction would have to be added to effect iso-electric point precipitation). This breakthrough allows precise tuning of NC surface protonation, optimizing catalytic reactions that typically require acidic or basic conditions but can now proceed near neutral pH. The findings highlight how POM complexation enables controlled surface protonation, directly influencing both catalytic activity and assembly. For example, the size- and shape-dependent catalytic properties of CeO2NCs make them highly tunable for various applications. This discovery opens new possibilities for developing therapeutic strategies against oxidative stress-related diseases and expands the potential of cerium oxide in environmental remediation.

[0058] Protonated POM-nanocrystal complexes

[0059] In one aspect, the present disclosure relates to a protonated polyoxometalate- nanocrystal (NC) complex of the Formula (I):

[0060] [ / -POM]m[H / (NC)], wherein the indicator m is the number of POMs bound to an individual nanocrystal (NC). M may be any number between 1 to 50,000, for example, between 1 to 10,000 or 1 to 6000. / is the number of amounts of H+associated with the complex. / reflects the number of H+that counterbalance the negative charge of the complex and is equal or less than the overall negative charge of all POMs in the complex. If each POM bares a net negative charge n-, than / < n-m. The complex is said to be fully protonated if f - n-m, and partially protonated if f < n-m.

[0061] In some embodiments, m is any number from 2 to 1000. A polyoxometalate anion consists of one, two, three or more transition metal and / or main- group metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks or cages. A negatively charged POM is also referred to herein as "cluster-anion". The metal atoms are usually group 6 (Mo, W) or, less commonly, group 5 (V, Nb, Ta) transition metals in their high oxidation states. In general, POMs comprise MOt units, where M is a metal ion and b indicates the coordination number of M. Usually, b - 6, although it can be 4, 5 or 7, or higher. A basic POM framework is designated herein "MpOy", wherein p and y are the relative amounts of metal and oxygen ions, respectively. Apart from M and O, other elements, herein labelled as X, can be part of the POM framework. As a rule, the X elements are 4-fold- or 6-fold-coordinated and lie in the center of the MpOyshell or cage. Depending on whether X is present or not, two types of POM species may be distinguished, based on a purely structural criterion:

[0062] Isopolyanions (IPAs; also referred to as iso-polyoxometalates), designated herein as [MpOy]n“; and heteropolyanions (HPAs; also referred to as heteropolymetalates), designated herein as [XzMpOy]n“, wherein z < p denoting the relative amount of X in the POM. Many exceptions to these two types of POMs exist.

[0063] X is also referred to herein, interchangeably, as a "primary heteroatom", "core heteroatom" or "central heteroatom". In general, any element can be X in a POM cluster since there are no strict physical requirements for this position. Exemplary X includes, but is not limited to, phosphate, silicate, metals in various oxidation states such as Fe(ll / lll), Co(l / ll), Ni(ll / IV), Zn(ll) and the like, and even two H+.

[0064] The metal atom M is referred to herein, interchangeably, as "secondary", "peripheral" or "addenda" atom. Herein, M represents one type of metal or 2 or more different metals. Usually, only certain metals are typically found in such compounds. In cluster anions, in which more than one type of addenda atom (M) is present in the framework, the molecule is known as a mixed- addenda cluster. Despite the simplicity of the IPAs and HPAs formulas above, the chemical structure of a cluster anion can be highly complex, with various metals taking part in the structure.

[0065] The typical framework building blocks (MOt) are polyhedral units, with typically 6-coordinate metal centers (MOe) forming an approximate octahedron (i.e., pseudo-octahedral symmetry). These octahedra are usually packed to form countless shaped cages. They are joined to each other in accordance with a few simple rules, such that the cluster (or cage) is built by edge and / or corner sharing MOe octahedra. The most stable POMs are formed by corner and edge sharing octahedra, in which the Mm+ions (m is the oxidation state of M) are far enough from each other, and their mutual repulsion is modest. The coordination number of the oxide ligands varies according to their location in the cage. Surface oxides tend to be terminal or doubly bridging (two coordinate) oxo ligands. Interior oxides are typically triply bridging (three coordinate) or even six coordinated.

[0066] Recurring structural motifs determine the ROM's classification: isopolyanions (IPAs; [MpOy]n“ ) feature octahedral metal centers, whereas heteropolyanions ([XzMpOy]n“) form distinct structures because the core heteroatom at the center is usually tetrahedrally coordinated by four oxide ligands. The Lindqvist structure exemplifies an IPA, whereas Keggin, Anderson [XM6O24]n“ and Wells-Dawson (WD) [X2Mi8O62]n“ structures are common motifs for heteropolyanions, with the coordination numbers of the central X cations varying between these three general classes.

[0067] The term "Keggin structure", is the structural form of ot-Keggin anions, which are represented herein by the general formula [XMi204o]n“, where the heteroatom X is actually a heterocation Xm+(e.g., P5+, Si4+, or B3+, while many other cations are optional), and M, the addendum atom is, e.g., molybdenum (Mo), Vanadium (V) or tungsten (W). M can denote 2 or more different metal atoms within the same POM, e.g., W and Ti. The Keggin structure self-assembles in acidic aqueous solution and is noted for its stability in catalysis at suitable pH values. The structure has a full tetrahedral symmetry and is composed of one core heteroatom, Xm+, surrounded by four oxygen atoms to form a tetrahedron. The core heteroatom and it's four oxide ligands, i.e., [XmO4](8 m)_is located centrally and caged by 12 pseudo-octahedral MOe units linked to one another by the neighboring oxygen atoms. There are a total of 24 bridging oxygen atoms that link the 12 addenda atoms. The metal centers in the 12 pseudo-octahedra are arranged on a sphere as four distinct triads (i.e., M3O13 units) almost equidistant from each other, which account for the overall tetrahedral symmetry of the complete structure. The bond length between atoms varies depending on the core heteroatom and the addenda metal atoms. This structure allows the molecule to be hydrated and then dehydrated and to be reversibly reduced and reoxidized without significant structural changes.

[0068] Including the original Keggin structure, there are 5 isomers, designated by the prefixes a, (3, y, 6 and s. The generally most-stable Keggin structure for anionic species is designated a. These isomers arise due to different rotational orientations of the M3O13 units, which lower the symmetry of the overall structure.

[0069] In the heteropolyanion Anderson [XM6O24]n-, the core heteroatom X is surrounded by six oxo ligands in a pseudo-octahedral symmetry, whereas in Wells-Dawson [X2Mi8O62]n“ two heteroatoms X are tetrahedrally coordinated. In the case of Wells-Dawson structures [X2Mi7O62]n-, in which a single M atom (e.g., W) is removed to generate a metal binding site, the location of the vacancy is indicated by the designations "02" if the defect lies in one of the two triads at the "top" or "bottom" of the cluster, or "ai" if located in one of the two central belts of originally six M cations each.

[0070] Lindqvist metal oxide clusters contain two structurally different atoms: M and O, and are represented as [MeOw]0-. However, the central oxygen atom is chemically very different from the external ones since it is surrounded by six cations.

[0071] (POMs with p-block elements (X - P, Si, Al, Ga, Ge...), transition metal elements (X - Fe(l l / l 11 ), Co(l / I I ), Ni(l l / l V), Zn(ll)...), and even two H+have been synthesized. The heteroatom position can be either tetrahedrally coordinated (as in Keggin and Wells-Dawson anions) or octahedrally coordinated (as in the Anderson structure).

[0072] A bridging ligand is a ligand that connects two or more atoms. In the context of the present disclosure, the ligand in a POM is an atom or group of atoms which connect two or more metal ions. The bridging ligand is labeled by the Greek letter p with a subscript number, which is the number of metals bound to the bridging ligand. The symbol P2 (i.e., a ligand binding two Ms) is often denoted simply as p. In the context of POMs described herein, the bridging ligand may be oxygen, also referred to herein as a "p-oxo" ligand, which can bridge two Ms (p2-0 or -O) or more than two metal cations (e.g., ps-O, P4-O). The bridging ligand may also be hydroxy group, also referred to herein as a "p-OH" ligand, that bridges, e.g., two metal cations.

[0073] The bridged metal cations may belong to the POM and / or to the complexed or ligated nanocrystal (e.g., a metal oxide nanocrystal).

[0074] Polyoxometalates typically exhibit coordinate metal-oxo bonds of different multiplicity and strength. In a typical POM such as the Keggin structure [PWi204o]3“ (i.e., X - P, M - W), each addendum atom connects to a single terminal oxo ligand, four bridging P2-O ligands and one bridging ps-0 which connects also to the central heteroatom (P). In some embodiments, POM is presented by the Formula (la):

[0075] Co[XzMpDdOy]n- wherein

[0076] X is H, at least one positively charged heteroatom (also referred to herein as "heterocation") and / or at least one positively charged metal atom (metal cation), wherein the metal atom is selected from main-group metals, transition metals, or lanthanides or any combination thereof.

[0077] In some embodiments, X is one, two, three or more heterocations of a non-metal element selected from P (phosphorus), S (sulfur) and / or F (fluorine) and / or of a metalloid (semimetal) selected from As (arsenic), Sb (antimony), Si (silicon) and / or Ge (germanium).

[0078] Transition metals are elements found in Groups 3-12 of the periodic table, characterized by their ability to form multiple oxidation states and by their partially filled d-orbitals, and include scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), palladium (Pd), hafnium (Hf) and silver (Ag).

[0079] Main-group metals are metallic elements found in Groups 1, 2, and 13-15 of the periodic table. These elements primarily follow s- and p-block electron configurations and exhibit characteristics typical of metals, such as high electrical conductivity, malleability, and a tendency to form cations. Metal included in these groups include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), aluminum (Al), gallium (Ga), indium (In), thallium (TI), tin (Sn), lead (Pb) and bismuth (Bi).

[0080] Lanthanides do not form classical POMs by themselves, but they can be incorporated into hybrid POM structures, leading to enhanced catalytic, optical, and magnetic properties. For example, lanthanide-substituted POMs (Ln-POMs) can be formed, wherein lanthanide ions (Ln3+, such as La3+, Ce3+, Nd3+, Eu3+, etc.) replace one or more metal centers in classical POM frameworks. These substituted POMs often retain the redox and catalytic properties of traditional POMs while incorporating the unique luminescent or magnetic properties of lanthanides. Exemplary Ln-POMs include Ln-substituted Keggin-type POMs such as [PWnO39Ln]n_and Ln-substituted Dawson-type POMs such as [P2W17O6iLn]n-. Lanthanides can act as countercations in POM structures, interacting electrostatically with negatively charged POM clusters to form lanthanide-POM coordination complexes. They can also bind directly to oxygen atoms in POM frameworks, modifying their electronic properties.

[0081] Some POMs form lanthanide-bridged clusters, where lanthanide ions act as linkers between multiple POM units. This is common in high-nuclearity POM-based clusters with magnetic and catalytic applications.

[0082] In some embodiments, X is one, two, three or more metal cations selected from Be, Na, Al, Ga, Ti, Zr, Hf, V, Cu, Fe, Mn, Co Sn, Pb, or Ce.

[0083] M, the addendum metal, is at least one, i.e., one, two, three or more transition and / or main- group metal cations, optionally, in a high oxidation state. Main group metals that can exist in high oxidation states and have potential to form POM complexes include:

[0084] (i) p-Block heavy metals (post-transition metals), Group 13: Al, Ga, In, TI, having high oxidation states: Al3+, Ga3+, ln3+, Tl3+(but TI often prefers Tl1+);

[0085] (ii) Group 14: Sn, Pb, having high oxidation states: Sn4+, Pb4+(though Pb2+is more stable). Sn4+and Pb4+can be incorporated into POMs, influencing redox and catalytic properties; and

[0086] (iii) Group 15: Bi, having high oxidation states: Bi3+, Bi5+(though Bi3+is more common). Bi3+can interact with POM structures and influence their electronic properties.

[0087] In some embodiments, M is one or more of Al(lll), Ga(lll), In(lll), Tl(lll), Sn(lll), Pb(IV), Bi(lll) and / or Bi(V).

[0088] Polyoxometalates are typically built from early transition metals in their high oxidation states (e.g., W6+, Mo6+, V5+), but other transition metals can be incorporated into POM structures or act as modifiers. Transition metals which can be part of the POM structure are listed in Table 1.

[0089] Table 1. Transition metals that can form POMs

[0090] In some embodiments, M is one of the transition metal cations listed in Table 1, or any combination of two, tree, four or more of the listed cations, preferably in a higher oxidation sate.

[0091] In some embodiments, M is at least one of V(V), V(IV), Nb(V), Ta(V), Mo(V), Mo(VI) and / or W(VI).

[0092] D is H, OH, H2O (also denoted herein as OH2), or an organic moiety such as a lower alkyl, lower hydroxyalkyl, lower silylalkyl, lower silylalkoxy and / or carboxylate. Optionally, D is linked directly to M or O of POM.

[0093] C is H+, an inorganic cation, or organic cation, also referred to herein as "countercation" such as, but not limited to, H+, Li+, Na+, K+, Cs+, RxH4-xN+, where R is alkyl or thioalkyl, and any combination thereof and x is 1 to 4. The amount of cation that balances the negative charge of the anion, a, is an integer of 1 to 75;

[0094] In some protonated POM complexes disclosed herein C is H+.

[0095] The indicators z, p, d and y represent the relative amount of X, M, D and O, respectively, z is 0 to 100; p is 6 to 250; d is 0 to 100; and y is 15 to 800. Any of these indicators may be any integer or a fraction in the indicated ranges.

[0096] The indicator n, the overall charge of the anion, is an integer of 1 to 75.

[0097] "i" designates a specific POM isomer, if known. Otherwise, i is absent.

[0098] In some embodiments, D is OH and the POM is referred to herein as a "hydroxylated POM" and presented as [XzMpOyOHd]n’, wherein d is the relative amount of OH (hydroxy) groups.

[0099] The term "lower alkyl", as used herein, refers to a saturated, branched or unbranched (straight chain) hydrocarbyl group with 1 to 6 carbon (C) atoms (herein also denoted as Ci-Ce alkyl) such as, but not limited methyl (Ci alkyl), ethyl (C2 alkyl), C3 alkyl such as n-propyl or iso-propyl, C4 alkyl such as butyl, iso-butyl, sec-butyl or tert-butyl, Cs alkyl such as n-pentyl, iso-pentyl, neo-pentyl or tert-pentyl, Ce alkyl such as n-hexyl, and iso-hexyl. In some embodiments, the lower alkyl is a Ci- C3 alkyl.

[0100] The terms "lower hydroxyalkyl" and "alkoxy", as used herein, are interchangeable and refer to a lower alkyl, as defined herein, substituted with one or two hydroxy groups, provided that if two hydroxy groups are present, they are not both on the same carbon atom. Representative examples include, but are not limited to, hydroxymethyl (also denoted herein as "HO-Ci alkyl"), 2- hydroxyethyl (HO-C2 alkyl), 2-hydroxypropyl (HO-C3 alkyl), 3-hydroxypropyl (HO-C3 alkyl), I- (hydroxymethyl)-2- methylpropyl ((HO -Cs alkyl), 2-hydroxybutyl (HO-C4 alkyl), 3-hydroxybutyl (HO- C4 alkyl), 4-hydroxybutyl, 2,3-dihydroxypropyl, l-(hydroxymethyl)-2-hydroxyethyl, 2,3- dihydroxybutyl, 3,4-dihydroxybutyl ((HO)2-C4 alkyl) and 2-(hydroxymethyl)-3-hydroxypropyl ((HO)2- Cs alkyl). In some embodiments, the lower hydroxyalkyl is HO-Ci alkyl, HO-C2 alkyl and / or HO-C3 alkyl.

[0101] The term "lower silylalkyl", as used herein, refers to the radical SiRiR2Rs derived from silane (SiHs), wherein the silicon atom Si is covalently bonded to one, two or three lower alkyl groups. Namely, Ri, R2 and R3, each independently is H or a lower alkyl a defined above. When one or more of Ri, R2 and R3 is an alkoxy, then the radical is referred to herein as "silylalkoxy", wherein alkoxy is as defined herein.

[0102] The term "carboxylate", herein interchangeable with "carboxylate ion", is the conjugate base (RCO2 ) of a carboxylic acid (RCOOH or RCO2H), wherein R is H or a lower alkyl as defined herein.

[0103] In some embodiments, D is absent (d - 0).

[0104] In some embodiments, D is a lower alkyl of 1 to 3 carbon atoms (C1-C3 alkyl), a lower hydroxyalkyl selected from HO-Ci alkyl, HO-C2 alkyl or HO-C3, or any combination thereof.

[0105] The term "unit cell" refers to the smallest repeating structural unit of a crystal that, when repeated in three dimensions, creates the entire crystal lattice. It defines the crystal's symmetry, dimensions, and atomic arrangement.

[0106] The lattice parameters are defined by three edge lengths (a, b, c) and three angles (a, 6, y). The symmetry of a unit cell determines how atoms, ions, or molecules are arranged in the structure. Stacking unit cells in all directions forms the complete crystal. Types of unit cells in crystals include: (i) simple (primitive) unit cell , wherein the atoms only at the corners; (ii) body-centered unit cell (BCC) , wherein there is additional atom at the center (iii) face-centered unit cell (FCC), wherein the atoms are at corners and center of each face; (iv) and base-centered unit cell, wherein the atoms are at corners and centers of two opposite faces.

[0107] For example, CeO2(cerium dioxide) has a face-centered cubic (FCC) unit cell, meaning cerium atoms are at the corners and oxygen atoms at the face centers.

[0108] The core nanocrystal (NC) of a disclosed protonated POM complex comprises unit cells represented by Formula (lb):

[0109] [M'iOfc]; wherein the indicators i and k refer to the relative amounts or relative quantities of M' and O, respectively, and can have any value in the range of from 0 to 20, for example, a value in the range of 1 to 8, such as, for example, 0.1, 0.5, 0.75, 1, 1.5, 2, 2.6, 3, 3.5, 4, 4.75, 5, 6, 7 or 8.

[0110] The terms "relative amount" and "relative quantity", as used herein, are interchangeable and indicate how much there is of a given element, e.g., H, M, M', O, relative to the other elements in the complex. These terms also refer to the proportion, ratio, fraction, relative occurrence or abundance of each element in the empirical chemical formula of a disclosed complex.

[0111] In a broad sense, M', represents one or more cations of metals from the main-group elements, transition metals and / or lanthanide elements that in combination with oxide, hydroxide, or oxyhydroxide, form nanocrystals that contain oxygen vacancies. Alternatively, or additionally, M' is one or more cations of metals from the main-group elements, transition metals and / or lanthanide elements that, after being reduced, can be re-oxidized by dioxygen, O2, preferably using air under ambient conditions.

[0112] The main group metals are the metals found in the s-block and p-block of the periodic table. These elements are typically characterized by their metallic properties, such as conductivity, malleability, and luster. Exemplary s-block metals (Group 1 and Group 2), which, when combined with oxygen, form stable metal oxides that can exhibit oxygen vacancies, include Magnesium (Mg) that forms MgO (magnesium oxide), which can have oxygen vacancies, especially at high temperatures; Calcium (Ca), forms CaO (calcium oxide), which has been shown to have oxygen vacancies that play a role in catalytic processes; Strontium (Sr), forms SrO (strontium oxide), which can also exhibit oxygen vacancies under reducing conditions; Barium (Ba), Forms BaO (barium oxide), which can contain oxygen vacancies. Lithium (Li), Rubidium (Rb) and Cesium (Cs) can form metal oxides that can exhibit oxygen vacancies, e.g., Li2O, Li2C>2, Rb2O and CS2O, however these metal oxides are not stable in water. They react rapidly with water to give corresponding hydroxides. The hydroxides are quite soluble, so they will not likely form POM-complexed hydroxide NCs. p-block metals (Groups 13-15) that in combination with oxygen often form metal oxides that can exhibit oxygen vacancies, particularly under high-temperature conditions or in the presence of reducing agents include, for example, Aluminum (Al), forms AI2O3 (aluminum oxide), which can have oxygen vacancies, especially in the form of non-stoichiometric or amorphous phases; Gallium (Ga), forms Ga2C>3 (gallium oxide), which can have oxygen vacancies, especially under reducing conditions; Indium (In), forms ln2Os (indium oxide), which can also exhibit oxygen vacancies, important for catalytic and electronic applications; Silicon (Si) and Germanium (Ge), while primarily semiconductors, in reduced forms, they can form oxygen-deficient oxides such as SiC>2 and GeCh, which can show some oxygen vacancy behavior under certain conditions; Tin (Sn), forms SnC>2 (tin oxide), which can exhibit oxygen vacancies, especially in reduced environments; Lead (Pb), forms PbO (lead oxide) and PbC>2 (lead dioxide), both of which can have oxygen vacancies under specific conditions; Bismuth (Bi), forms Bi2O3 (bismuth oxide), which can exhibit oxygen vacancies, particularly in its non-stoichiometric forms.

[0113] The metals of blocks s and p listed above often form metal oxides that can exhibit oxygen vacancies in nanocrystals are especially important for enhancing the materials' reactivity, making them valuable for applications in catalysis, environmental remediation, and energy storage.

[0114] In some embodiments, M' is a cation of an alkali earth metals, referred to here as main group metals, selected from Mg. Ca, Al, Ga, In, Sn and / or Bi.

[0115] Transition metals are elements of Groups 3-12 of the periodic table exhibit multiple oxidation states, can form colored compounds, and participate in complex chemical bonding. Nanocrystals of transition metal oxides often exhibit oxygen vacancies which can enhance electronic, catalytic, and optical properties. Key transition metals that form oxygen-deficient nanocrystals include, for example Titanium (Ti), oxidizes to titanium dioxide (TiO2) can form oxygen-deficient TiO2-x, which is widely used in photocatalysis (e.g., solar cells, environmental remediation). It this particular context, x denotes missing oxygens; Vanadium (V), oxidizes to vanadium pentoxide (V2O5) that forms oxygen-deficient phases with enhanced catalytic and electrochemical properties; Chromium (Cr) oxidizes to Cr2O3.x, exhibiting oxygen vacancies that affect its conductivity and catalytic performance; Manganese (Mn), which oxidizes to manganese dioxide (MnO2) with oxygen vacancies (MnO2.x) is widely used in batteries and catalysis; Iron (Fe), which oxidizes to magnetite (Fe3O4) and hematite (Fe2O3.x), exhibit oxygen deficiencies, enhancing their catalytic and magnetic properties; Cobalt (Co), forms cobalt oxide nanocrystals Co3O4often with oxygen vacancies, Co3O4.x, that show improved activity in energy storage and catalysis; Nickel (Ni), forms nickel oxide (NiO) nanocrystals form oxygen-deficient NiO-%, which improves electronic and catalytic performance; Copper (Cu), forms CuO nanocrystals with oxygen vacancies, CuO.x, that are highly active in oxidation and reduction reactions; Zinc (Zn), forms zinc oxide nanocrystals with oxygen vacancies, ZnO.x, that are used in sensors, catalysis, and optoelectronics; Molybdenum (Mo), forms molybdenum trioxide (MOO3) that can form oxygen-deficient MoO3.x, affecting its catalytic and electronic behavior; and Tungsten (W), forms tungsten trioxide (WO3) that with oxygen vacancies (WO3-X) enhances photocatalytic and electrochromic properties.

[0116] The lanthanides (or lanthanoids) are a group of 15 metallic elements with atomic numbers 57 to 71, spanningfrom lanthanum (La) to lutetium (Lu). They are characterized by their partially filled 4f orbitals, which give them unique optical, magnetic, and catalytic properties. Lanthanides are also known as rare-earth elements (REEs) (excluding Sc and Y, which are sometimes grouped with them). Many lanthanide oxides can form oxygen-deficient nanocrystals, where oxygen vacancies play a critical role in their catalytic, electronic, and optical properties. The most well-known is cerium (Ce) that forms oxygen-deficient cerium dioxide (CeCh-x) NCs widely known for their oxygen storage and catalytic applications. Lanthanides which exhibit oxygen vacancy formation that fins use in various applications are listed in Table 2.

[0117] Table 2. Lanthanides oxides NPs and optional application thereof In some embodiments, M' is a lanthanide cation selected from Ce3+, Eu3+, Tb3+, Dy3+, Gd3+,

[0118] La3+and / or Nd3+.

[0119] Many lanthanides can form stable POM complexes, particularly those that exhibit strong Lewis acidity and can accommodate oxygen vacancies in their oxides. Lanthanides have a strong affinity for oxygen-rich ligands like POMs. Their large ionic radii allow for coordination with POM clusters via multiple coordination sites. Lanthanides, especially Ce, Pr and Eu have variable oxidation states enabling redox-active complexes. POM-lanthanide complexes improve redox activity, charge transfer, and stability in catalytic reactions. Eu3+, Tb3+, Dy3+and Gd3+that have optical and magnetic properties, readily complex with POMs and are widely used in luminescent and magnetic materials. The most studied lanthanide-pom complexes include, but are not limited to, cerium-POM complexes, e.g., Ce(IV) in Keggin-type POMs which are used in oxidation catalysis and redox reactions; Europium- POM complexes, which are employed in luminescent and sensing applications; lanthanum and neodymium-POM complexes (La / Nd-POMs), applied in catalysis and energy storage.

[0120] Further metal oxide NCs encompassed by the present disclosure include any combination of main-group, transition metal or lanthanide elements that in combination with oxide forms nanocrystals that contain oxygen vacancies such as, but not limited to, transition-metal doped epsilon-MnO2, transition-metal doped CeCh, and transition-metal doped SnC>2.

[0121] The core NC may comprise a combination of two different metals or of three different metal cations, termed herein "binary NC" or ternary NP", respectively. Non-limiting examples of binary oxides include BaTiOs, SrTiOs, KNbOs, LiNbOs and LiTaOs.

[0122] Binary or ternary NCs may be doped by addition of dopants or "impurities" to alter their properties, e.g., their electric resistivity or conductivity. In some embodiment one of the metal cations in the binary or ternary NCs act as a dopant in which case its amount is small relative to the other cationic metal(s).

[0123] Any of the one or more metal cations represented by M' may sometimes be shared with the polyoxometalate. POMs complexed to NCs disclosed herein include complexes in which the metals of the NC are confined to the NC, and complexes in which at least one metal of the NC is shared with the POM.

[0124] The number of unit cells in NC may range from 50 to 100,000,000, depending on the symmetry, composition and dimensions, including nanocrystals that are relatively large, but commonly prepared.

[0125] In some embodiments, a defect form of the POM, also referred to interchangeably as a "substituted POM," "defect POM," or "vacant POM," in which at least one of bound metals is missing (e.g., has been removed) or is further coordinated by oxide (jU-O) or hydroxide Gu-OH) bridges, linking them to metal sites on the surfaces of complexed nanocrystals.

[0126] A monodefect POM is a POM in which a single addendum atom, such as tungsten (W), has been removed, creating a vacancy that can be occupied by a different metal cation. For instance, a monovacant POM may incorporate ln3+in place of a removed W6+.

[0127] Non-limiting examples of monovacant heteropolytungstate POMs include [a-XznWnO39](12 n)“, where X / 1can be P5+, Si4+, or Al3+, and z is an indicator as defined herein, for example, monovacant Wells-Dawson (WD) POMs such as [a2-P2Wi7O6i]10“.

[0128] In some embodiments, POM substituted by I n(lll)OH, binds to In(lll) cations of complexed ln(OH3) NCs, optionally, via the OH ligand on the POM-complexed I n(l II )OH moiety.

[0129] In some embodiments, the substituted POM is I n ( 111 )OH-su bstituted monodefect WD cluster- anion, for example, [{ot2-P2Wi706i}(ln3+OH)]8’.

[0130] In defect WD POMs, the substituted In(lll) ions are bound in a pentacoordinate "in-pocket" fashion within the defect site of the WD anions, and at neutral or slightly basic pH values, the OH- anions occupying their sixth coordination site are available to serve as donor ligands for In(lll) cations, e.g., at the I n (OH )3NP surface.

[0131] The POM complexes themselves, each with a specific NC core, can aggregate into amorphous or crystalline supra-assemblies comprising numerous POM-complexed NCs, for example, up to 100,000 complexes.

[0132] Diversified combinations of POMs and NCs featuring various protonation states are encompassed by the present disclosure, for example, POM comprising Nb or W complexed to NCs such as, but not limited to, Mn02, SnO2, CeO2, ZrO2, CuO, TiO2, NiNbO3H, HfO2, CrFe2O3, Co(OH)2 or Ti / Sn / 'O , wherein i, i' and k, representing the relative amounts of Ti, Sn and O, respectively, each independently may vary from 1 to 20.

[0133] In some embodiments, the complex is a heteropolytungstate POM-complexed Mn02.

[0134] In some embodiments, the complex is a heteropolytungstate POM-complexed SnO2.

[0135] In some embodiments, the complex is a hexaniobate POM-complexed SnO2.

[0136] In some embodiments, the complex is a heteropolytungstate POM-complexed CeO2.

[0137] In some embodiments, the complex is a heteropolytungstate POM-complexed ZrO2. In some embodiments, the complex is a heteropolytungstate POM-complexed HfCh.

[0138] In some embodiments, the complex is a hexaniobate POM-complexed CuO.

[0139] In some embodiments, the complex is a hexaniobate POM-complexed TiO2.

[0140] In some embodiments, the complex is a hexaniobate POM-complexed Co(OH)2.

[0141] In some embodiments, the complex is a hexaniobate POM-complexed NiNbOsH.

[0142] In some embodiments, the complex is a heteropolytungstate POM-complexed CrFeOs.

[0143] In some embodiments, the complex is a hexaniobate POM-complexed TijSn / 'Ok.

[0144] Overall, the relative amount of each of Ti and Sn in POM-complexed TijSnrOk can vary from 5 to 100% of the metal-cation composition. The tuning of the different metals affords the obtention of different physical properties and has advantages over single metal oxide NCs. For example, changing the relative amounts of Ti and Sn in the NCs improves optical properties of the NCs. It also improves the charge separation and transfer, which could be helpful for photocatalysis, advanced oxidation processes, and gas sensing.

[0145] In some embodiments, the complex is a heteropolytungstate-POM complexed Fe20s with Cr(lll) as dopant.

[0146] In some embodiments, the metal cations in the NCs are complexed by the POM ligands in alternative ways. For example, larger metal cations of the POM ligand are bound to the POM cluster and then to the NCs via a weaker, tetra-coordinated "out-of-pocket" fashion. Such larger metal cation include, e.g., Zr(IV), Hf(IV), Ag(l), Ag(lll), Pd(ll), Pd(IV), Pt(ll), Pt(IV), Pb(ll), Pb(IV), Cd(ll) and Ce(IV). This mode of complexing is substantially different from known complexes in which the metal cations in the POM ligands are bound via a very thermodynamically favorable (strong and stable) pentacoordinated, "in-pocket" fashion.

[0147] Cations larger than Ti(IV) and Fe(lll), are too large to fit into the pentacoordinate "pocket" such as in known POM ligands, thus, they remain farther from the center of the POM cluster and are bound by four oxygen atoms in a tetra-coordinated "out-of-pocket" fashion. In addition, the "out-of-pocket" coordinated metal cations can bind to one NC core metal cation via two / J.-0 linkages or bind via one / J.-0 linkage to more than one metal cation at the metal oxide surface. Advantages and applications of protonated POM-NC complexes (i) Catalytic Oxidation reactions

[0148] While colloidal metal-oxide nanoparticles may offer more accessible surface sites, their inconsistent crystallinity and surface chemistry often result in instability and unpredictable reactivity. In contrast, POM-complexed metal-oxide NCs exhibit well-defined structures with precisely tuned electronic and catalytic properties, enhanced by their strong interaction with POM clusters. For example, the inventors have previously shown that hexaniobate POMs, when complexed with CuO NCs, dramatically improved visible-light-driven water oxidation, without additional photosensitizers. These quantum-confined CuO NCs, stabilized by hexaniobate, maintained activity at high pH, showcasing their potential for advanced photocatalytic applications.

[0149] Lanthanides represent a remarkably versatile class of elements with a broad and continually expanding range of applications, including automotive catalytic converters, recyclable Lewis acid catalysts, reducing agents, high-performance magnets, superconductors, and biomedical applications.

[0150] Cerium oxide (CeO2) colloidal nanoparticles, in particular, are widely utilized in catalysis due to the Lewis acidity of Ce3+(Ce(lll)), which plays a crucial role in reactivity. For example, cerium oxides have been widely demonstrated to support various catalytic processes such as electrophilic cyclization, chemoselective reductions, oxidation and asymmetric hydrogenation. The catalytic performance of CeO2NPs is closely linked to their oxidation state, surface defects, and crystallinity.

[0151] Colloidal CeO2nanoparticles can adopt various morphologies and often exhibit less ordered crystalline structures with higher defect densities. In contrast, CeO2nanocrystals are highly crystalline, with well-defined size and shape, leading to lower defect densities. The surfaces of both forms contain a mix of Ce3+and Ce4+oxidation states, with abundant oxygen vacancies that strongly impact their catalytic and redox behavior. Studies indicate that as CeO2particle size decreases, the fraction of Ce3+increases due to a higher density of oxygen vacancies, enhancing reactivity. Therefore, precise control over particle size and oxidation state is crucial for optimizing catalytic performance.

[0152] When complexed with POM cluster anions, the interaction between the POM and CeO2surface modifies the Ce3+ / Ce4+ratio, alters charge transfer dynamics, and enhances electron storage capabilities. These modifications are driven by strong electrostatic and / or covalent interactions between the POM ligands and the CeO2surface, which profoundly impact its electronic and catalytic behavior.

[0153] In colloid science, nanoparticle suspensions are typically stabilized electrostatically, preventing aggregation. At low pH, nanoparticle surfaces become protonated, acquiring a positive charge, and stability arises from electrostatic repulsion between positively charged particles. Conversely, at high pH, surfaces are negatively charged, usually due to the presence of hydroxyl groups, and stability is maintained through repulsion between negatively charged particles. Between these two extremes, as the pH increases, particle charge transitions from positive (low pH) to negative (high pH). At the point of zero charge (or isoelectric point), the particles have no net surface charge, eliminating electrostatic repulsion. In this state, the lack of stabilization leads to particle aggregation and rapid precipitation at the isoelectric point. The zeta potential values as a function of pH for colloidal CeO2NCs in various protonation levels is schematically demonstrated in Fig. 1.

[0154] The present inventors discovered that protonated POM-complexed cerium oxide NCs exhibit exceptional properties in dioxygen activation, demonstrating that activation of air-born O2could proceed at near natural pH, at ambient conditions and for a long period. One key feature of the protonated POM-metal oxides disclosed in Examples 3-5 herein is their ability to activate molecular oxygen and generate reactive oxygen species (ROS), such as superoxide radicals and hydrogen peroxide, devoid of in tandem use of added oxidants such as H2O2. These ROS play crucial roles in oxidative stress, with applications in antimicrobial treatments and cancer therapy.

[0155] POM ligation completely frees metal-oxide NCs from iso-electric point precipitation at near natural pH, which typically arises from pH-dependent protonation or deprotonation in electrostatically stabilized colloids. This breakthrough allows precise tuning of NC surface states, enabling reactions that traditionally require strongly acidic or basic conditions to proceed at nearneutral pH. For example, protonated CeO2NCs can now activate dioxygen without acidic environments, while alkali-metal countercation forms of POM-complexed NCs can quench reduced oxygen species without the need for strong bases. This ability to perform such reactions at pH 7 was entirely unanticipated and represents a transformative discovery. The rapid and continuous dioxygen activation, brough about by replacement or exchange of at least part of the POM countercations by proton, enables long-term catalytic performance, making protonated POM-complexed CeO2NCs highly effective in various applications such as, but not limited to biomedical and environmental applications.

[0156] The present disclosure leverages catalytic dioxygen activation by protonated polyoxometalate-complexed CeO2nanocrystals, setting them apart from all other CeO2materials.

[0157] (ii) Enzyme mimicking activity

[0158] A particularly interesting aspect of cerium oxide is its enzyme mimicking activity, such as superoxide dismutase, catalase, and peroxidase (Liu, X. et al., Chemical Communications, 48(26): 3155-3157, 2012; Singh R. et al., Colloids and Surfaces B: Biointerfaces, 132:78-84, 2015; Zhao H. et al., ACS Applied Materials & Interfaces, 7(12):6451-6461, 2015). The distinctive redox properties of ceria are attributed to the co-existence of Ce3+and Ce4+oxidation states on its surface, which facilitate the scavenging of reactive oxygen species, thus making them potent antioxidants.

[0159] Examples 3 and 4 herein demonstrate the enzyme-mimicking traits of a protonated POM- complexed CeO2(Complex 1) and its role in the formation and interaction with reactive oxygen species. Key findings show the dominance of oxygen vacancies and the in-tandem appearance of Ce3+centers, integral to the unparalleled dual-oxidase activity displayed.

[0160] The protonated POM-NC complexes disclosed herein exhibit a range of unprecedented advantages due to their unique reactivity at ambient conditions. This breakthrough chemistry enables at least the following:

[0161] (1) Dioxygen activation at near-neutral pH. Unlike conventional methods that require strongly acidic conditions, these complexes facilitate dioxygen activation under near-neutral conditions, reducing toxicity concerns and lowering operational costs.

[0162] (2) Efficient quenching of reactive oxygen species (ROS). The complexes effectively neutralize reduced oxygen species (radicals) without requiring strongly basic conditions, enhancing safety and applicability. (3) Redox reactions at physiological pH. Traditionally, many redox reactions necessitate extreme pH conditions. The protonated POM-NC complexes disclosed herein enable redox activity at pH 7, significantly broadening their applicability, including use in biological and medical contexts.

[0163] (4) Enhanced reactivity and effectiveness. The complexes exhibit superior catalytic performance, improving efficiency across various chemical and biological applications.

[0164] (5) Increased stability and longevity. Improved stability ensures consistent performance over time, extending shelf life in medical, industrial, and environmental applications.

[0165] (6) Improved solubility and bioavailability. Enhanced solubility contributes to greater bioavailability, making these complexes particularly beneficial for pharmaceutical and biomedical applications.

[0166] The unique properties of protonated POM-NC complexes disclosed herein position them as highly versatile materials with wide-ranging applications, including but not limited to:

[0167] (I) Medical & therapeutic uses: antimicrobial therapy; cancer treatment, including precision medicine and targeted therapy; wound healing and anti-fungal treatments; sterilization of medical devices and surfaces; and / or Biosensing and diagnostic applications.

[0168] (ii) Environmental & industrial applications: environmental remediation and industrial decontamination; sewage treatment and water purification; air purification, odor control and deodorization; and / or self-cleaning and anti-fouling surface coatings.

[0169] (Hi) Catalysis & energy applications: fuel cell and battery catalysis; photocata lytic applications; and / or Organic synthesis catalysis

[0170] (iv) Consumer & food industry applications: food packaging and preservation.

[0171] It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination or as suitable in any other described embodiment of the present disclosure.

[0172] As used herein the term "about" refers to ± 10 %. The terms "comprises", "comprising", "includes", "including", "having" and their conjugates mean "including but not limited to".

[0173] As used herein, the singular form "a", "an" and "the" include plural references unless the context clearly dictates otherwise. For example, the term "a compound" or "at least one compound" may include a plurality of compounds, including mixtures thereof.

[0174] Throughout this application, various embodiments described may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosure. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

[0175] It is appreciated that certain features of the present disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the present disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other embodiment described herein.

[0176] Various embodiments and aspects of the present disclosure as delineated hereinabove and as claimed in the claims section below find experimental support in the following examples.

[0177] EXAMPLES

[0178] Materials

[0179] All the reagents were of analytical grade and used as received without further purification. Deionized water (18.2 Q cm resistivity) was used in all aqueous solutions from a Millipore Direct-Q water-purification system.

[0180] Materials for POM synthesis: sodium tungstate dihydrate (Na2WO3-2H2O, Sigma Aldrich, ACS reagent, >99% purity), disodium hydrogen phosphate dihydrate (Na2HPO4-2H2O, Merck, analytical grade), sodium bicarbonate (NaHCOs, Sigma Aldrich, ACS reagent, >99.7% purity). The polyoxometalate salt, Na7[PWnO39]-12H2O was prepared by a known procedure for synthesis of Na7[a-PWiiO39] (Haraguchi et al., Inorg. Chem. 2002, 33(6): 1015-1020. https: / / doi.org / 10.1021 / IC00084A008). The purity of Na7[PWnO39]-12H2O was confirmed, as appropriate, by31P NMR and FTIR characterization techniques.

[0181] Materials for POM-complexed CeC>2 synthesis: ammonium cerium nitrate (NI-khCefNOs 99%, Acros Organics), sodium hydroxide (NaOH pellets, 98%, Alfa-Aesar), 3, 3', 5,5'- tetramethylbenzidine (TMB, 99+%, Sigma-Aldrich), sodium chloride (NaCI, >99%, Sigma-Aldrich), potassium chloride (KCI, >99.5%, Sigma-Aldrich).

[0182] Luria-Bertani (LB) broth (Lennox, Formedium) and agar (Formedium) were used to prepare liquid broth for bacterial culture. 2,2-Diphenyl-l-picrylhydrazyl (DPPH; Sigma-Aldrich), methanol (C2H5OH, ACS reagent, Sigma-Aldrich) and ascorbic acid (Sigma-Aldrich) were used for antioxidant studies.

[0183] Instruments and methods

[0184] Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)

[0185] Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) is an analytical technique used to determine the elemental composition of a sample. ICP-OES operates in several key steps. First, the sample, usually in liquid form (dissolved in acid or another solvent), is nebulized into tiny droplets. Then, plasma is created using argon gas flow and an RF (radio frequency) coil, which ionizes the gas to extremely high temperatures. Then, the sample droplets are introduced into the high-temperature plasma (around 6, 000-10, 000K) where they undergo complete vaporization, atomization, and ionization. The next step is optical emission spectroscopy. The atoms and ions in the sample at these high temperatures emit light at characteristic wavelengths when they return to lower energy states. Each element emits light at unique wavelengths, allowing for its identification. Finaly, a monochromator or spectrometer detects the emitted light and determines which elements are present (based on the wavelength) and their concentration (based on the intensity of the emission). ICP-OES can detect many elements at once and with high sensitivity as it detects trace elements in parts per billion (ppb) or lower. Data was acquired using Spectro Arcos FHM22 Instrument (AMETEK®) equipped with vertical plasma torch box (SOP) and analyzed using Smart Analyzer Vision software. Samples were diluted to suit the instrument concentration range and were measured without farther treatment.

[0186] Electrophoretic Light Scattering (ELS)

[0187] Electrophoretic Light Scattering (ELS) is a technique used to measure zeta potential ( ), which represents the electrical potential at the slipping plane of a particle in a liquid. ELS measures the movement (electrophoretic mobility) of charged particles in a liquid when an electric field is applied. The main steps include, first, applying an electric field. When an external electric field is introduced, charged particles in the liquid move toward the oppositely charged electrode. Positive particles move toward the negative electrode, and negative particles move toward the positive electrode. Next, electrophoretic mobility is measured wherein a laser beam is directed at the particle suspension. Moving particles scatter the laser light, and this scattered light undergoes Doppler shift due to the motion of the particles. The frequency shift in the scattered light is detected and analyzed to determine the velocity of the particles. Lastly the measured velocity is converted into electrophoretic mobility (p), from which the zeta potential is calculated using the Henry equation: where: q is the viscosity of the liquid; E is the dielectric constant of the medium; and f(ica) is Henry's function (depends on particle size and medium conductivity).

[0188] Cryogenic transmission electron microscopy (cryo-TEM)

[0189] Cryo-TEM is a high-resolution imaging technique used to study the structure of nanoparticles, biomolecules, and soft materials in their native state. Unlike conventional TEM, which requires samples to be dried or embedded in resin, Cryo-TEM preserves the sample in a nearnatural hydrated state by flash-freezing it at ultra-low temperatures (~ -180°C or lower).

[0190] Cryo-TEM is operated by, first, sample preparation. A thin liquid film of the sample (e.g., nanoparticle suspension) is placed on a carbon-coated TEM grid. The sample is rapidly frozen in liquid ethane at cryogenic temperatures (~-196°C, liquid nitrogen conditions). Rapid freezing prevents the formation of ice crystals, ensuring the sample remains intact and well-preserved. Next, the frozen sample is inserted into a Cryo-TEM holder, which keeps it at ultra-low temperatures to prevent melting or damage. A high-energy electron beam (typically 80-300 keV) is passed through the thin sample. Some electrons are scattered, while others pass through, forming a high-resolution 2D projection of the sample on a detector.

[0191] The last step of the technique relates to image processing and reconstruction. The images can be captured directly or processed using computational techniques like single-particle analysis or tomography to reconstruct 3D structures.

[0192] In the case of nanoparticles, Cryo-TEM helps visualize size, shape, surface coating, and ligand distribution. It Can resolve individual nanoparticles down to the atomic level. Furthermore, it provides insights into core-shell structures, ligand distribution, and interactions.

[0193] Cryogenically frozen samples were prepared using Leica, a fully automated vitrification device. Three pL of the sample solution were placed onto a glow discharged 300 Mesh Cu grid covered with a lacey-carbon film, held inside a 100% humidity chamber. The grid was then mechanically "blotted" and immediately plunged into liquid ethane cooled by liquid nitrogen. Data were collected on the FEI Tecnai 12 G2 instrument (120 kV) and the Gatan slow-scan camera, using a low-dose regime). All images from both dry- and cryo-TEM (including electron diffraction patterns) were analyzed using Digital Micrograph Gatan Inc. software.

[0194] Energy Dispersive X-ray Spectroscopy (EDX or EDS)

[0195] Energy-Dispersive X-ray Spectroscopy (EDX or EDS) is an analytical technique used to determine the elemental composition of a material. It is commonly used in combination with Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy (TEM) to analyze the surface chemistry and distribution of elements in a sample.

[0196] EDX operates by analyzing the X-rays emitted from a sample when it is exposed to a high- energy electron beam. The first step in the technique involves electron beam excitation wherein following directing a high-energy electron beam (from SEM or TEM) onto the sample, the electrons interact with atoms in the material, knocking out inner-shell electrons (typically from the K, L, or M shells). When an inner-shell electron is ejected, the atom becomes unstable. An electron from a higher energy level (outer shell) moves down to fill the vacancy. This transition releases energy in the form of an X-ray, and the energy of this emitted X-ray is characteristic of the specific element. In the final step elements are detected and identified. A detector (usually a silicon drift detector, SDD) collects these emitted X-rays, and measures the energy (keV) of the X-ray, thereby identifying the element, and the intensity of the X-ray signal, thereby determining the concentration of the element. A spectrum is generated, showing peaks at different characteristic X-ray energies, each corresponding to a specific element.

[0197] Measurement was acquired using a JEOL JEM- 2100F TEM analytical electron microscope operating at 200 kV equipped with a JED2300T energy dispersive X-ray spectrometer.

[0198] Powder X-ray Diffraction (PXRD)

[0199] Powder X-ray Diffraction (PXRD) is a technique used to analyze the crystalline structure of powdered or polycrystalline materials. It is based on the principle of X-ray diffraction (XRD), where X-rays interact with the atomic planes of a material, producing a diffraction pattern unique to its structure.

[0200] PXRD works by first generating a beam of monochromatic X-rays (e.g., Cu Karadiation, A - 5418 A), directed at a powdered sample. Diffraction by crystalline planes is determined according to Bragg's Law: n ■ A = 2d ■ sin(0) (2) where n is and integer (order of diffraction); A is the X-ray wavelength; d is interplanar spacing in the crystal; and 0 is the diffraction angle.

[0201] The diffracted X-rays form a unique fingerprint of the crystal structure, recorded as intensity vs. 20 (diffraction angle).

[0202] PXRD identifies the crystalline phases present in a sample by comparing patterns to standard databases. This technique also provides crystallite size estimation using Scherrer's equation to estimate particle size: where D is crystallite size; K is the shape factor (~0.9); (3 is peak broadening (FWHM), and 0 is the diffraction angle. Furthermore, PXRD provides lattice parameter determination as it allows the refinement of unit cell dimensions (and angles, when relevant).

[0203] The X-ray diffraction (XRD) equipment and conditions used for data collection included two different diffractometers:

[0204] 1. Rigaku XtaLAB Synergy-S Diffractometer, a single-crystal X-ray diffractometer used for detailed structural analysis at the atomic level of small-molecule crystals. It provides high-resolution diffraction data for determining precise atomic positions. Radiation Source: Cu Kot radiation (Copper anode X-ray source), wavelength: 5418 A (Cu Ka). Operating Conditions: 50 kV (voltage) and 1 mA (current) for a total of 50 W (power). These settings control the X-ray intensity and energy. This diffractometer was equipped with the detector Hy-Pix-6000HE, a hybrid photon-counting detector with high sensitivity and resolution.

[0205] 2. PANalytical Empyrean Multi-Purpose Diffractometer, a powder X-ray diffractometer used for analyzing powdered or polycrystalline materials. This diffractometer measures diffraction patterns for phase identification, crystallinity, and structural refinement. Radiation Source: Cu Ka radiation. Operating Conditions: 40 kV and 30 mA. These settings produce a stronger X-ray beam (higher current than the Rigaku system), improving diffraction intensity. This diffractometer was accompanied by two detectors: (i) PSD X'Celerator ID Detector, a position-sensitive detector (PSD) that captures diffraction data faster than traditional detectors, and (ii) PIXcel-3D Detector, a high- performance hybrid detector with 3D photon counting, allowing precise diffraction pattern collection.

[0206] X-ray Photoelectron Spectroscopy (XPS)

[0207] X-ray photoelectron spectroscopy is an analytical technique used to determine the elemental composition and oxidation states of materials by measuring the energy of emitted electrons when a surface is exposed to X-rays works by directing a beam of monochromatic X-rays (usually from Al Kaat 1486.6 eV or Mg Kaat 1253.6 eV) at the sample placed in a high-vacuum chamber. X-ray photons have enough energy to ionize atoms by ejecting core electrons from inner atomic orbitals (e.g., Is, 2p, 3d). The kinetic energy of the emitted electrons is measured, sorted and converted to binding energy. The binding energy is unique for each element and oxidation state, allowing identification of the material's composition and chemical states. The XPS spectrum plots intensity (number of detected electrons) vs. binding energy (BE). Each element has characteristic peaks corresponding to its core-level binding energies (e.g., Ce 3d, O Is, C Is).

[0208] A high-resolution XPS spectrum focuses on specific core-level peaks of elements (like Ce 3d for cerium) to get detailed chemical information. In the case of CeO2, the Ce 3d spectrum is particularly important because it reveals the ratio of Ce3+(reduced form) to Ce4+(oxidized form).

[0209] The XPS spectrum of cerium is complex because the Ce 3d peaks are not simple single peaks - instead, they are overlapping multiple peaks due to spin-orbit coupling and satellite features. Deconvolution is a mathematical process used to separate these overlapping peaks and assign them to their correct chemical states (Ce3+or Ce4+). After deconvolution, the spectrum clearly distinguishes between Ce3+and Ce4+peaks.

[0210] By analyzing the peak intensities, the Ce3+: Ce4+atomic ratio can be determined, which is crucial for understanding the oxygen vacancy concentration and redox activity of the material.

[0211] Electron Paramagnetic Resonance (EPR)

[0212] Electron paramagnetic resonance, also known as electron spin resonance (ESR), is a spectroscopic technique used to study materials with unpaired electrons. It is widely applied to analyze free radicals, transition metal complexes, and defects in solids.

[0213] EPR is based on the interaction between unpaired electrons and an external magnetic field. Electrons have spin and behave like tiny magnets. In most atoms, electrons are paired, and their spins cancel each other out. However, in species like free radicals or transition metals, unpaired electrons exist, making them paramagnetic (i.e., responding to a magnetic field). When a sample with unpaired electrons is placed in an external magnetic field, the electrons' spin states split into two energy levels (Zeeman effect). These two levels correspond to spin-up (ms- +1 / 2) and spindown (ms- -1 / 2) states. The sample is then exposed to microwave radiation at a fixed frequency. When the microwave energy matches the energy difference between the two spin states, electrons absorb the radiation and transition from the lower to the higher energy state. This resonance condition occurs at a specific magnetic field strength. The instrument detects the absorption of microwaves and generates an EPR spectrum, which provides valuable information about the number and type of unpaired electrons, the chemical environment of the paramagnetic species, and interactions with nearby atoms, especially hydrogen and other magnetic nuclei (via hyperfine splitting).

[0214] The g-value (or g-factor) is a dimensionless quantity that describes the magnetic behavior of an electron in an external magnetic field. It is a fundamental parameter in EPR that determines the resonance condition (when microwave absorption occurs). The free electron has a g-value of 2.0023. In real systems, the g-value deviates due to interactions with the surrounding chemical environment. Metals, radicals, and different materials have characteristic g-values that help identify them.

[0215] Hyperfine coupling constant (a-value) is the interaction between the unpaired electron and nearby atomic nuclei with a magnetic moment (such as1H,14N,19F or31P). This interaction causes the EPR signal to split into multiple peaks. If an unpaired electron interacts with a nucleus with spin / , the resonance peak splits into 21 +1 lines. The separation between these lines is the hyperfine coupling constant a, measured in gauss (G) or MHz. Hyperfine coupling provides information about the number and type of nearby nuclei, the distance between the electron and the nucleus and the structure of radicals and metal complexes.

[0216] A spin-adduct is a stable complex formed when a spin-trapping agent reacts with a transient free radical. These adducts make short-lived radicals detectable in EPR. When a spin trap (such as 5,5-dimethyl-l-pyrroline N-oxide (DMPO)) reacts with a radical (e.g., *02“ or »OH), it may form a stable adduct that produces a unique EPR spectrum. By analyzing the hyperfine splitting, the identity of the original radical can be determined. DMPO forms distinct spin-adducts with superoxide and hydroxyl radicals, which have different EPR signatures. Spin-adduct formation detects short-lived radicals that would otherwise disappear too quickly.

[0217] Synthesis of POM-complexed metal oxide nanocrystals

[0218] Usually, an aqueous solution of a salt of the desired metal is used as a precursor (also referred to herein as "precursor solution"), and the pH is adjusted forming a hydrated form of the metal ion. Afterward, the POM is introduced to the solution. The sample can be heated hydrothermally in an autoclave to facilitate condensation of the hydrated metal into metal-oxide nanocrystals (NCs) and attachment of the POM ligands to the surface of the metal oxide nanocrystals. The reaction mixture is then separated from the reaction by-products. Using alkali- metal salt, the POM-complexed NCs are selectively and reversibly precipitated, and the soluble byproducts are discarded using a centrifuge. The purified complexes are redissolved, and the solution may be concentrated using air evaporation or centrifuge.

[0219] Synthesis of POM-complexed binary, doped and ternary metal oxide nanocrystals

[0220] For the preparation of POM-complexes of NCs comprising two or three different metal cations, two or more precursor solutions are prepared by separately dissolving the salt forms of the metals to be joined or mixed (or structurally combined) in the NCs in either water or water-miscible solvent. The pH of each solution is adjusted if needed. Then, the precursor solutions are combined, while vigorously stirring one solution and, optionally, dropwise adding the other solution. Afterward, POM is introduced, the sample is heated hydrothermally, and the reaction mixture is treated in a similar manner as described above for POM-complexed metal oxide NCs.

[0221] Antibacterial studies

[0222] Liquid Luria-Bertani (LB) broth was prepared by dissolving 5 g of LB powder in 200 mL deionized water in a media bottle. The medium was sterilized by autoclaving it at 121°C for 2 hours. After cooling to room temperature, the broth was either used directly or stored at 4°C for later use.

[0223] For Escherichia coli (E. coli) bacterial culture, 5 g of agar was added to 200 mL of LB broth before autoclaving. After sterilization, the medium was allowed to cool to approximately 50°C and poured into sterile Petri dishes. Plates were allowed to solidify at room temperature and stored at 4°C until use. An inoculation loop was used to streak a bacterial colony onto the agar plate. Following overnight incubation, distinct E. coli colonies were obtained for subsequent experiments.

[0224] A single E. coli colony from an LB agar plate was inoculated into 5 mL of sterile LB broth for use in antibacterial studies. To evaluate the antibacterial properties of POM-complexed CeO2nanocrystals, 100 pL, 200 pL, and 300 pL of 1 pM POM-CeO2nanocrystal solutions were added to liquid broth, followed by the addition of 10 pL of bacterial inoculum to each solution. A control sample containing the same concentrations of liquid broth and bacterial inoculum, but without POM-complexed nanocrystals, was prepared for comparison. Similar procedures were followed for both Streptococcus and E. coli bacterial inoculums.

[0225] Optical density (O.D.) measurements were performed using a UV-Visible (UV-vis) spectrophotometer (HP 8452A spectrophotometer equipped with a diode-array detector (190-1100 nm range)). Sterile LB broth was used as a blank to zero the spectrophotometer before sample measurements. The prepared bacterial inoculum solutions were transferred to clean quartz cuvettes with a 1 cm path length. The optical density at 600 nm (ODeoo) was recorded to monitor bacterial growth, as this wavelength minimizes interference from media components while effectively measuring light scattering caused by bacterial cells.

[0226] Antifungal studies

[0227] Antifungal tests were performed following the procedure described in Fauzia et al. (Fauzia et al., Scientific Reports, 14: 6606, 2024). Briefly, regular hamburger bread was cut into uniformly sized pieces, and 500 pL of 100 pM POM-complexed CeO2solution was evenly spread across the surface. For the control experiment, no solution was added to the bread. All samples were sealed in zip-lock plastic bags and incubated at 37°C under moist conditions under careful observations.

[0228] Antioxidant studies

[0229] A stock solution of DPPH (0.1 mM, 100 mL) was prepared in methanol for use in the antioxidant experiments. Samples of POM-complexed CeO2 nanocrystals (H+and K+countercation forms) were prepared by dissolving the complex either in ethanol (positive control) or ascorbic acid (a standard antioxidant) to obtain various concentrations (5, 10, 20, 40, 80 ppm).

[0230] The antioxidant activity of the samples was determined using the DPPH radical scavenging assay. Briefly, 100 pL of the 0.01 mM DPPH solution was added to each of the samples, followed by incubation in the dark at room temperature for 30 minutes to allow sufficient time for the reaction to take place. After the incubation period, the absorbance at 517 nm was measured using a microplate reader to determine the residual DPPH concentration. The DPPH radical absorbs light at this wavelength, and reduction in absorbance is proportional to the antioxidant activity. The blank was prepared using 100 pL of methanol and 100 pL of DPPH solution, and the control was prepared by adding 100 pL of methanol to 100 pL of DPPH solution.

[0231] The percentage of radical scavenging activity was calculated using the following equation: 100 (4) where ACOntroi is the absorbance of the control ample (DPPH without antioxidant), and Asampie is the absorbance of the DPPH solution with the antioxidant sample.

[0232] Enzyme mimicking studies

[0233] A standard reaction was conducted in a glass quartz cuvette using UV-vis monitored at wavelength A - 652 nm, with a total solution volume of 1 mL. The reactions utilized a freshly prepared stock solution containing 10 mM 3,3',5,5'-tetramethylbenzidine (TMB) in ethanol (EtOH) as the substrate. Varying amounts of the stock solution were added to an aqueous solution (10% EtOH v / v), used to verify the solubility of TMB and the resulting charge transfer complex. For the kinetic assay, the amounts of TMB (pmol) used were 0, 0.02, 0.05, 0.07, 0.09, 0.15, 0.30, 0.50 and 0.80. Finally, 0.1 mL of a solution containing [a-PWiiOsgCe^EHssfCeChhio] (Complex 1) was added and the reaction was monitored using UV-vis. An exemplary 0.15 pmol TMB solution comprised (all volumes in mL):

[0234] VH2O VTMB SOI. VEIOH V Complex 1 Vfjnal

[0235] 0.600 0.015 0.285 0.100 1.000

[0236] Debye length and effective charge calculation

[0237] The Debye length ( D) represents the characteristic distance over which electrostatic interactions are screened due to the presence of mobile charge carriers (e.g., ions in solution). It determines how far electrostatic forces extend in ionic medium before they are neutralized by surrounding counterions. In other words, the Debye length is the distance over which electrostatic interactions are significant in a solution. Beyond this distance, the charge effects of a particle are screened by the surrounding ions in the medium. A short Debye length (AD small) means strong screening, where charges are quickly neutralized by the surrounding ions, whereas a long Debye length (POM-complexed TijSn / 'Ok large) means weaker screening, and electrostatic interactions which persist over longer distances. The Debye length helps predict whether charged colloidal particles will aggregate or remain stable.

[0238] The Debye-Huckel parameter, denoted as K (kappa), is a measure of the electrostatic screening in an electrolyte solution. It represents the inverse of the Debye length and determines how rapidly the electrostatic potential of an ion is screened by surrounding ions in the solution. The Debye-Huckel parameter, K (nm-1) is calculated using equation (5): where F is Faraday's constant (96480 Cmol-1), s is the permittivity of water (7.12-1010Fm1at 298K), R is the universal gas constant (8.314 Jmo K1), and q and z\ are concentration and charges of all significant ions in solution.

[0239] The charge, z, of the particles is calculated using equation (6):

[0240] Z = 477 where E is the permittivity of water (7.12-1010Fm1at 298K); r is the radius of the NCs including the ligands shell; and is the measured zeta potential value.

[0241] The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory describes the stability of colloidal dispersions by considering the balance between electrostatic repulsion and van der Waals attraction between particles. It helps predict whether particles will remain dispersed or aggregate in a solution.

[0242] Electrostatic repulsion (l / eiec) originates from the electrical double layer surrounding each charged particle. It is modeled using the Debye-Huckel parameter (K) and zeta potential «): where h is the particle separation distance; A is a repulsion factor, which depends on the surface potential or zeta potential; and K is the Debye-Huckel parameter.

[0243] A more detailed model that includes the zeta potential is:

[0244] Veiec(h) = 71 - Er- EQ - a - (, ■ 2e~Kh(8) where sris the relative permittivity of the medium; so is permittivity of free space; a is the particle radius; and is zeta potential (surface potential at the slipping plane).

[0245] The electrical double layer (EDL) is a structure that forms around a charged particle in a liquid medium due to the attraction of oppositely charged ions and the repulsion of similarly charged ions. The EDL consists of two main regions: inner Stern layer (compact layer) and outer diffuse layer. The inner Stern layer is closest to the particle surface and contains specifically adsorbed counterions. Ions in this layer are strongly bound and do not move freely. The outer diffuse layer contains a mix of counterions and co-ions (same charge as the particle). Ions in this layer are more loosely associated and diffuse into the bulk solution. The potential in this layer gradually decreases toward the bulk solution.

[0246] The zeta potential (<") is the electrostatic potential at the shear plane of a particle's electrical double layer. It is a key factor in DLVO theory because it determines the magnitude of electrostatic repulsion. The higher the zeta potential, the stronger the repulsion, leading to greater colloidal stability.

[0247] EXAMPLE 1

[0248] Preparation of protonated [a-PWn039Ce]7-complexed CeO2 nanocrystals (Complex 1)

[0249] The synthesis of POM-complexed CeC>2 nanocrystals (NCs) initiated by dropwise addition of 3.7 eq. of NaOH (0.2 M, 1.66 mL) to Ce(IV) salt solution (ammonium cerium nitrate ,(NH4)2Ce(NOs)6 (9 mM, 10 mL) until the pH 3.45 was obtained. Then, 0.3 eq. of Na?[PWnO39] lacunary POM (40 mM, 0.75 mL) was added dropwise to the reaction mixture, resulting in a white cloudy slurry. The reaction mixture was left to stir overnight, followed by further dropwise addition of 0.2 eq. of Na?[PWnO39] (40 mM, 0.50 mL) to make a molar ratio POM : Ce(IV) of 1:2. The obtained slurry was then kept at 180°C for 24 hours, which yielded an optically clear light-yellow solution at final pH 6.72. The isolation and purification of the synthesized material was done by precipitating and re-dissolving the product using a salt followed by dialysis to remove unreacted species and excess salts. Briefly, the product was precipitated with 2M NaCI and the supernatant was separated from the solid using centrifugation, after which the supernatant was decanted and the solid, yellow, pellet was redissolved in deionized water. This process was repeated a total of 3 times and, once UV-vis has shown no signs of POM in the supernatant, was followed by a 5-day dialysis period against water, resulting in a transparent yellow solution of the fully protonated POM-complexed CeO2 nanocrystals. [ot-PWn039Ce]22[H88(Ce02)9io] is designated herein "Complex 1". At completion of the synthesis, the POM ligand complexed to CeO2 contained one Ce atom as a heteroatom. Notably, the amount of H+in the complex is an average amount. The number of POM ligands (22 in the instant complex) is an average value, because NC sizes vary slightly. Smaller NCs have fewer POM ligands, a smaller charge, and fewer H+ions, and vice versa. In addition, the calculated number of H+per NC depends on the accuracy of the methods used to determine the average NC size and the ratio of W to Ce, which is used to calculate the number of POM ligands per NC.

[0250] The exchange of Na+countercations by H+cations by dialysis, shown schematically in Fig. 2, provided a form of complex that would typically require strongly acidic pH values to generate. However, unexpectedly, this exchange occurred at pH values above 6, with the slight decrease in pH upon dialysis attributed to partial dissociation of the H+into solution.

[0251] A partially protonated form of Complex 1, in which an amount x of a countercations such as K+, Na+, or any other countercation (C in formula 1(a)), is replaced by H+, is referred herein as "partially protonated Complex 1" and designated, e.g., as (K88-x[a-PWn039Ce]22[Hx(Ce02)9io]. Such partially protonated forms were obtained by dialyzing the K+from of the complex (x - 0) against water for 2 to 4 days, by which time at least some of the K+are replaced by H+.

[0252] Characterization of Complex 1

[0253] (I) Dynamic Light Scattering (DLS) measurements

[0254] Dynamic Light Scattering (DLS) measures the size of particles in suspension by analyzing the fluctuations in scattered light caused by their Brownian motion. The hydrodynamic radius (Rh) refers to the apparent size of a particle, including its hydration shell in solution. DLS measurement of Complex 1 solution indicated that the solution contains particles with an approximate size of 28 nm (when considering number-weighted distribution, meaning the most frequent particle size in the sample).

[0255] (ii) Zeta Potential ( analysis

[0256] Electrophoretic Light Scattering (ELS) was used to measure zeta potential of Complex 1. ELS is useful for surface charge analysis as it helps assessing functionalization of nanoparticles (e.g., ligand attachment). A highly negative zeta potential of =-92.9 mV was measured for Complex 1, as seen in Fig. 3. This very negative zeta potential suggests a highly charged surface attributed to the POM ligands, each carrying a -4 charge. The extremely negative zeta potential indicates that the POM ligands are effectively bound to the nanoparticle surfaces, enhancing stability and preventing aggregation. This supports the fact that surface modification with POMs was successful, influencing both the size and charge of the nanocrystals.

[0257] (Hi) Elemental analysis

[0258] Elemental analysis of Complex 1 was done using two techniques: Energy-Dispersive X-ray Spectroscopy (EDX) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES).

[0259] Energy-Dispersive X-ray Spectroscopy (EDX) is a technique used in electron microscopy (SEM / TEM) to determine the elemental composition of a sample. EDX provides (i) elemental composition, namely, identifies which elements are present in the sample; (ii) relative elemental ratios, i.e., estimates the proportion of elements in a given area; and (iii) elemental mapping, i.e., visualizes how elements are distributed across the sample.

[0260] The EDX spectrum of Complex 1 shown in Fig. 4 shows the relative amounts of tungsten (W) and cerium (Ce) in the sample. The measured atomic ratio was 19.7% W : 80.3% Ce, which means that CeO2is the major component (80.3%). POM ligands contributed tungsten (W) at 19.7%, suggesting their presence on the surface of CeO2NCs.

[0261] ICP-OES is a highly sensitive technique for detecting elemental composition in solutions. The EDX result matched the ICP-OES result (not shown), reinforcing confidence in the accuracy of the composition. Thus, the EDX spectrum confirmed the expected composition of the sample, and the agreement with ICP-OES validated the accuracy of the measurement.

[0262] Elemental mapping confirmed that oxygen (O), cerium (Ce), and tungsten (W) are evenly spread, indicating that the CeO2nanocrystals were well-formed and the POM ligands (containing W) were uniformly bound to the CeO2surface, rather than aggregating in isolated regions, meaning the functionalization of CeO2NCs with POMs was successful and homogeneous.

[0263] Calculation of the surface coverage of the POMs using the elemental distribution data from EDX shows that each particle (NC) is covered by an average of 22 POMs. The calculated POM coverage, the amount of elements constituting Complex 1 and the dimensions of the core NC and POM footprint of the core NC surface are listed in Table 3.

[0264] Table 3. Calculation of POM coverage of Complex 1, considering an ideal spherical CeO24.3 (diameter) NC core and 1.8 nm2POM footprint on the surface

[0265] (iv) Cryogenic Transmission Electron Microscopy (Cryo-TEM) imaging

[0266] Cryo-TEM (Cryogenic Transmission Electron Microscopy) is a specialized TEM technique where samples are rapidly frozen to preserve their native state in solution. It allows visualization of materials without structural damage and is useful for imaging nanoparticles (NPs) and their surface coatings with high resolution.

[0267] Particle size observations of Complex 1 revealed a total particle size of ~6.7 nm with NC cerium oxide size of ~4.3 nm and POM ligand layer thickness of ~1.2 nm (on each side), indicating that POM molecules were successfully attached to the NC surface. The total size of ~6.7 nm matches expectations based on the core-shell structure, supporting the successful functionalization of CeO2with POM ligands. Thus, Cryo-TEM imaging (Figs. 5A-5B) confirmed that the POM ligands were effectively coating the CeO2nanocrystals, ensuring stability in solution, and that the POM layer is not aggregated but evenly distributed around each nanoparticle.

[0268] To conclude, surface-modified [PWnO39]Ce-jU-O -complexed CeO2 NCs, prepared at pH 6-7, were bound by ca. 22 POM ligands readily seen by cryo-TEM. After dialysis of the initially isolated Na+form, elemental analysis (ICP-OES) revealed the complete absence of Na+. The explanation, confirmed by XPS, is that the CeO2 NC surface was protonated to give a weakly-acidic multiply protonated polycationic core. This multiply protonated core serves as a new type of POM "countercation".

[0269] (v) Powder X-ray Diffraction (PXRD) analysis

[0270] Powder X-ray Diffraction (PXRD) was used to analyze the core structure of the POM- complexed NCs and determine their crystal structure and phase composition. The obtained PXRD pattern was compared with a reference pattern (from the ICDD PDF database and / or literature). The results confirmed that the nanocrystals have a face-centered cubic (FCC) fluorite structure, which is characteristic of ceria (CeO2), as shown in Fig. 6A. Fluorite structure means that cerium (Ce) atoms are arranged in an FCC lattice, with oxygen (O) atoms filling tetrahedral voids. The Scherrer equation was used to estimate the crystallite size of the NCs (see Materials and Methods), taking the (111) peak (the most intense diffraction peak in the PXRD spectrum), for Scherrer analysis. The broadening of this peak in the XRD pattern was analyzed to estimate the crystallite size. The Scherrer analysis is shown in Fig. 6B.

[0271] Using this method, the calculated average nanocrystallite core size (diameter) was 4.3 nm, with an uncertainty of ± 0.2 nm.

[0272] (vi) Debye length and effective charge calculation

[0273] The charge of a particle is not always uniformly felt at all distances. Beyond the Debye length, the surrounding medium screens much of the charge, so the "effective" or "formal average charge" is what remains perceptible at that distance. The concentration of Complex 1 in solution was evaluated as 330 nM based on particle size obtained from PXRD, the density of CeC>2 and the concentration of Ce in solution obtained from ICP-

[0274] OES.

[0275] The radius of the complex particles, including the ligands shell (r), was calculated based on PXRD. The PXRD radius refers to the CeC>2 NC alone, i.e., NC "naked" or free of POM ligands. The radius of 3.35 gives a CeO2 diameter of 6.7 nm. Adding POMs increases the radius by 1.2 nm, corresponding to r - 4.47 (and increase in diameter of 2.4 nm).

[0276] Given zeta potential of -92.9 mV, the charge per particle (z), calculated using equation (6), was -2.78-1018C, translating to an average of -17.4 elementary charges (units of electrons, or e ) per particle.

[0277] The Debye length was calculated as K~r- 13.8 nm, using equation (5) (see Materials and Methods section).

[0278] Even though the particles may have a higher total charge, due to screening effects, only a charge of -17.4 e- per particle was effectively influencing interactions at distances beyond the Debye length. This net charge per particle suggests that the particles carry a significant surface charge, leading to a strong electrostatic repulsion. This high surface charge contributes to a larger diffuse double layer, increasing the hydrodynamic diameter in DLS measurements, resulting in ca 28 nm.

[0279] The larger Debye length obtained (radius of ca. 14 nm) confirms that the electrical double layer (EDL) was thick, significantly increasing the DLS-measured size compared to the core size calculated based on PXRD analysis.

[0280] The large Debye length indicates that electrostatic repulsion is significant, preventing particles from aggregating. DLVO theory explains this effect by considering the balance between electrostatic repulsion (due to the surface charge and zeta potential) and van der Waals attraction.

[0281] As demonstrated in Fig. 1, the formation of protonated CeO2NCs in traditional colloidal systems typically requires strongly acidic conditions. However, a distinct feature of POM-complexed NCs is that their solubility and stability are not solely dependent on particle charge. Instead, POM ligands provide steric stabilization, preventing aggregation in a manner similar to how thiolate ligands stabilize gold nanoparticles. Additionally, POM ligands are highly negatively charged, contributing to electrostatic stabilization that is independent of the NCs' protonation state. This inherent negative charge necessitates the presence of countercations, which precisely balance the multiple POM ligands. Upon dialysis in pure water, the initially present Na+countercations are replaced by protons.

[0282] Notably, as shown in Example 2 below, most of these protons localize at or near the CeO2NC surface, mirroring the effect of lowering the pH of traditional colloidal CeO2suspensions. However, in POM-complexed CeO2NCs, the presence of POM-associated countercations allows access to the highly protonated NC form without requiring strongly acidic conditions.

[0283] EXAMPLE 2

[0284] Location of the proton countercations within the protonated form of Complex 1

[0285] Interpreting the zeta potential data using DLVO theory, the highly negative value (-92.9 mV) means that the POM-CeO2nanoclusters (complexes) are strongly negatively charged, which implies that a significant number of counterions (H+) are nearby to maintain charge neutrality.

[0286] Thus, the zeta potential measurement suggests that each POM-CeO2nanocluster has an approximate formal charge of -17.4 e- (see Example 1 above). This charge of ca. -17 e- implies that an equivalent number of positive charges (H+protons) must be present to balance the charge in solution. Given the charge contribution per proton is +1, approximately 71 protons are calculated to be closely associated with each POM-CO2 nanocluster or complex to maintain electrostatic balance.

[0287] The significant number of protons closely associated with each complexed CeO2NC, combined with the near-neutral bulk pH of 6 (Fig. 2), suggests that replacing Na+with H+leads to a system where the CeO2surface is either directly protonated or surrounded by a highly concentrated H+environment. This situation is comparable to immersing traditional colloidal CeO2in strongly acidic solutions.

[0288] More specifically, within the volume of the complex (radius - 3.2 x 10"9m), excluding the volume occupied by the NC itself, the presence of 71 protons would have resulted in an H+concentration of 1.05 M, corresponding to a pH just below 0. However, assuming that these protons are homogeneously distributed between the NC surface and the Stern layer, the estimated pH rises to 2.0.

[0289] The Stern layer is the innermost region of the electrical double layer (EDL) that forms around a charged particle in a solution. It consists of counterions that are tightly bound to the particle's surface. In this case, the CeO2NC is negatively charged, meaning H+(protons) accumulate around it. These protons can either be directly attached to the NC surface or reside in the Stern layer. Initially, if all 71 protons were concentrated only in the free volume of the complex, the local H+concentration would have been very high (1.05 M), giving a pH below 0. However, if these protons are spread out more evenly between the NC surface and the Stern layer, the effective H+concentration decreases.

[0290] Elemental analysis indicates that each complex comprises 22 POMs attached onto each ceria NC (see Example 1). As each POM carries a charge of -4, the total negative charge of each complex is -88, meaning the 88 H+counterbalancing this charge resides in close association with the core NC.

[0291] In summary, the Na+to H+exchange effectively creates a local pH between 0 and 2 at the NC surface, meaning that the NCs experience an environment equivalent to strongly acidic solutions ranging from 1 M to 0.01 M H+.

[0292] EXAMPLE 3

[0293] Enzyme mimetic activity of Complex 1

[0294] The presence of oxygen vacancies in CeO2nanoparticles creates Ce3+centers, which can act as catalytically active sites. These vacancies arise when oxygen atoms are missing from the crystal lattice, causing some Ce4+ions to be reduced to Ce3+to maintain charge balance. The dynamic redox cycle between Ce3+(reduced form) and Ce4+(oxidized form) plays a critical role in the enzyme-like activity of CeO2nanoparticles.

[0295] This self-regenerating Ce3+ / Ce4+redox cycle, enables CeO2nanoparticles to mimic the function of several key enzymes involved in protecting cells from oxidative damage. Specifically, CeO2nanoparticles have demonstrated catalytic activities similar to known enzymes including: (i) peroxidase, which breaks down hydrogen peroxide (H2O2) into water and oxygen; (ii) superoxide dismutase (SOD), which converts harmful superoxide radicals into oxygen and hydrogen peroxide; (iii) catalase, which further decomposes hydrogen peroxide into water and oxygen, and (iv) oxidase, which catalyzes oxidation reactions involving oxygen.

[0296] Together, these enzyme-like activities allow CeO2nanoparticles to scavenge and neutralize various reactive oxygen species (ROS), making them promising materials for applications in biomedicine, environmental protection, and antioxidant therapies.

[0297] For Complex 1, analysis of the deconvoluted high-resolution X-ray photoelectron spectroscopy (XPS) 3d spectrum (namely, the processed XPS spectrum where overlapping Ce 3d peaks are separated to accurately determine the oxidation state of cerium), revealed a Ce3+to Ce4+atomic ratio of 25.2:74.8 percent (Fig. 7A), namely, 25% of the Ce atoms are Ce(lll). Given the NC diameter of 4.3 ± 0.2 nm determined via Scherrer analysis (see Example 1), and considering the density and molar mass of CeO2, the surface distribution of Ce3+could be estimated.

[0298] Cerium dioxide unit cells have a fluorite crystal structure, meaning its atoms are arranged in a highly symmetric cubic pattern. The (111) surface refers to a specific atomic plane in the crystal that is oriented in a particular direction. The (111) surface is thermodynamically the most stable because it has the lowest surface energy, meaning it is less prone to reconstruction or degradation. Oxygen vacancies tend to move towards a surface, which is energetically favorable site for Ce3+formation, therefore, it is reasonable to assume that the Ce3+species are predominantly located at the (111) surface. The surface layer dimensions in Fig. 7B were used to calculate the percentage of surface Ce atoms present as Ce(lll). The result is that 55-60 % of the surface atoms are Ce(lll). This is a relatively large number and supports the high activity of these NCs resulting from Ce(lll) cations being at the surface are active sites. Based on these calculations, ca. 57% of the surface cerium atoms exist as Ce3+, corresponding to an average of 274 Ce3+atoms per NC.

[0299] The enzyme-mimetic activity of Complex 1 was assessed by way of reaction with 3, 3', 5,5'- tetramethylbenzidine (TMB), a known chromogenic substrate, as described in Materials and Methods above. Usually, enzymes or nanoparticles are used in tandem with H2O2 to oxidize TMB forming a cation radical (TMB-OX), which reacts with TMB itself to form a blue charge transfer complex [TMB-OX, TMB] that can be easily monitored by UV-Visible spectroscopy at Amax- 652 nm. The chemical reaction is shown in Scheme 1:

[0300] Scheme 1

[0301] Unexpectedly, the addition of Complex 1 to a solution containing TMB resulted in an immediate color change, even in the absence of H2O2. This suggests the involvement of a different reactive oxygen species (ROS) in the reaction. To investigate further, the catalytic activity of Complex 1 was evaluated under both inert (N2atmosphere) and ambient air conditions. The striking results are shown in Fig. 8.

[0302] Thus, under air exposure, the reaction proceeded without interruption, indicating a sustained catalytic process. Under a nitrogen (N2) atmosphere, the reaction almost completely stopped after an initial increase in activity, which is attributed to pre-adsorbed ROS on the CeO2NC surface (the slight residual activity observed after this initial spike is likely due to minor air leakage into the reaction vessel).

[0303] These findings suggest that Complex 1 exhibits unique enzyme-mimicking behavior as dual oxidase, facilitating multiple oxidation steps:

[0304] (i) reduction of molecular oxygen (O2) to superoxide radicals («O2);

[0305] (ii) dismutation of »O2into hydrogen peroxide (H2O2); and

[0306] (iii) generation of hydroxyl radicals (»HO) in the final oxidation step.

[0307] This dual oxidase activity highlights an unprecedented catalytic mechanism, making Complex 1 a promising candidate for oxidation-based biochemical applications.

[0308] As shown in Fig. 8, upper curve (reaction under air), the reaction proceeded in two distinct phases: an initial, very rapid oxidation of TMB, which was completed in less than 5 seconds. This phase was driven by pre-existing reactive oxygen species (ROS) adsorbed on the nanocluster (NC) surface. A subsequent slower, sustained reactivation of the NC surface, which occurred through dioxygen (O2) activation and continued over time. The first phase follows Michaelis-Menten kinetics, where the reaction rate is influenced by the protonation state of the POM-complexed NCs.

[0309] EXAMPLE 4

[0310] Michaelis-Menten Kinetics of dioxygen activation by Complex 1

[0311] Michaelis-Menten Kinetics is a model describing how enzyme-like catalysts behave, where the reaction rate depends on substrate concentration.

[0312] Following the enzyme-mimetic studies described in Example 3, a Michaelis-Menten kinetic assay was performed to investigate dioxygen activation by Complex 1. To achieve this, the reaction was studied by varying the concentration of the colorimetric probe TMB (3, 3', 5,5'- tetramethylbenzidine) used as the substrate. The initial reaction velocity was measured, specifically the fast initial phase, which completes within less than 5 seconds.

[0313] The reaction rates from this initial phase were then plotted (Fig. 9A) and fitted into the Michaelis-Menten equation, allowing extraction of key kinetic parameters that describe the catalytic efficiency of a system:

[0314] (i) Vmax (maximum reaction velocity): the highest possible reaction rate when Complex 1 is fully saturated with the substrate.

[0315] (ii) / CM (Michaelis Constant): represents the substrate concentration at which the reaction reaches half of l / max. A higher / M indicates a loweraffinity for the substrate, meaning more substrate is needed to reach a given reaction rate.

[0316] (iii) kcat (Turnover Number): a measure of how many substrate molecules an enzyme (or Complex 1) can convert per second. It is calculated as:

[0317] , Vmax kccaatt- 7 [ -Complex 17] 9

[0318] (iv) Catalytic Efficiency (CE): a measure of how efficiently the catalyst works, defined as the ratio:

[0319] Calculated values for these parameters are shown in Table 4. The concentration of Complex

[0320] 1 was taken 330 nM as determined by ICP-OES (see Example 1 above). Table 4. Michaelis-Menton parameters for initial rates of TMB oxidation by Complex 1

[0321] This analysis provided deeper insights into the catalytic efficiency of Complex 1 and its role in dioxygen activation.

[0322] Next, the protonation state of Complex 1 was modified by replacing H+with K+, and the Michaelis Menten kinetic assay was performed. The K+countercation forms of the POM-complexed CeO2 NCs (K88-x[a-PWii039Ce]22[Hx(Ce02)9io]), were obtained by the addition of 2M KCI. The very large concentration of K+was simply added to the solution of the H+form of the complex, with no further dialysis. So, the complex has both H+(which provides faster rate) and on top of that, K+. Together, the rate is faster than with H+alone as seen in Fig. 9B. However, when H+is replaced by K+by adding KOH, the rate decreases dramatically (Fig. 9C) as further discussed below.

[0323] Notably, the maximum reaction velocity ( l / max) doubled upon K+addition (Fig. 9B). However, despite this increase in reaction rate, the catalytic efficiency (CE) decreased slightly due to a higher / CM value, changing from 2.6 to 2.3 M-1s-1. A higher / M suggests a lower substrate (TMB) affinity, which may be due to the larger size of K+cations, interfering with substrate access to the nanoparticle surface. Thus, the decrease affinity may be explained by steric hindrance that may have slowed down the interaction between TMB and catalytic sites.

[0324] To further explore this effect, a control experiment was performed using both KOH and KNO3, ensuring a final concentration of 1 mM K+counter-cations to retain charge balance, and 10 pM OH“ to neutralize proton countercations. The results are shown in Fig. 9C.

[0325] As seen in the figure, a clear decrease in both reaction rate and substrate affinity were observed. This led to a 10-fold reduction (an order of magnitude) in catalytic efficiency (Table 5), suggesting that hydroxide ions and potassium counterions disrupt the optimal catalytic environment.

[0326] Thus, while K+enhanced reaction velocity, it also reduced substrate affinity, likely due to steric interference. Additionally, OH“ neutralization further disrupted catalytic efficiency, emphasizing the importance of protonation in maintaining optimal enzyme-mimetic activity. Table 5. Michaelis-Menton parameters for initial rates of TMB oxidation by Complex 1 after additions of K+and KOH° a Catalytic criterion same as in the study conducted by Baldim et al. (Baldim, et al., ACS

[0327] Applied Materials & Interfaces, 12 (37): 42056-42066, 2020).

[0328] EXAMPLE 5

[0329] Identifying the reactive oxygen species (ROS) associated with Complex 1 using electron paramagnetic resonance (EPR)

[0330] The initial jump in absorbance seen in the enzyme-mimetic activity experiment described in Example 3, even under an inert atmosphere, indicates the presence of pre-adsorbed reactive oxygen species (ROS) on the surface of the protonated CeO2nanoclusters (NCs). This aligns with the high concentration of oxygen vacancies determined from X-ray photoelectron spectroscopy (XPS) data.

[0331] To identify the types of ROS present, electron paramagnetic resonance (EPR) was performed using 5,5-dimethyl-l-pyrroline N-oxide (DMPO), a spin-trapping agent that distinguishes between superoxide (O2*“) and hydroxyl (HO’) radicals. A solution of Complex 1 was analyzed without any additional additives. The results are shown in Figs. 10A-10B.

[0332] Upon immediate addition of DMPO, a complex EPR spectrum was observed (Fig. IDA), requiring deconvolution into four distinct components shown in Fig. 10B:

[0333] (1) two superoxide radical species (71% total), differentiated by slight variations in hyperfine splitting. These radical species are situated in separate chemical environments, one possibly more localized on the surface, the other further from it;

[0334] (2) hydroxyl radicals (15%); and (3) a DMPO decomposition product (DMPOX) (14%).

[0335] The EPR parameters used to simulate the spectrum are listed in Table 6.

[0336] Table 6. Spin Hamiltonian parameters used in simulating the various components of the

[0337] EPR signal obtained upon addition of DMPO to a solution containing Complex 1

[0338] To assess the stability of the ROS, the same solution was re-examined 24 hours after DMPO addition. The resulting spectrum showed signals only from hydroxyl radicals and DMPOX (Fig. 11), suggesting that superoxide radicals had fully reacted or decomposed.

[0339] The key conclusions from EPR Data and Figs. 8-11 are:

[0340] (i) Pre-existing ROS on CeO2NCs. Even before the addition of a substrate (DMPO or TMB), a mixture of ROS is present in the solution containing Complex 1. These ROS likely form due to O2insertion into oxygen vacancies, where Ce3+sites donate electrons, generating superoxide radicals.

[0341] (ii) Rapid ROS reaction upon substrate addition. When a substrate like TMB or DMPO is introduced, superoxide and hydroxyl radicals react rapidly. This explains the initial jump in absorbance in TMB oxidation, as TMB is quickly oxidized.

[0342] (iii) ROS depletion and O2activation. Once the surface-bound superoxide "reservoir" is exhausted, the reaction shifts to an O2activation mechanism, consistent with inert atmosphere experiments (Fig. 8).

[0343] (iv) catalytic stability over time. Even 24 hours after DMPO addition, short-lived radicals are still detectable, confirming that Complex 1 remains catalytically active for extended periods. EXAMPLE 6

[0344] Bacterial growth inhibition by Complex 1

[0345] Streptococcus and E. coli bacteria cultures were separately grown and treated with Complex 1, as described in the Materials and Methods section. The results are presented in Figs. 12A-12B.

[0346] Streptococcus: UV-Vis measurements taken after 24 hours revealed a significant increase in light scattering for the untreated control compared to the Complex 1-treated sample, indicating greater bacterial growth in the control (Fig. 12A).

[0347] E. coli: OD600 absorbance was measured at 0, 24, and 48 hours to track bacterial growth at varying Complex 1 concentrations. The data show a clear reduction in bacterial growth with increasing concentrations of Complex 1 (Fig. 12B).

[0348] EXAMPLE 7

[0349] Fungal growth inhibition by Complex 1

[0350] Fungal growth inhibition evaluated by measuring the zone of inhibition in the presence and absence of Complex 1, as described in the Materials and Methods section. The results are presented in Figs. 13A-13B.

[0351] After approximately two weeks, fluffy white to greyish mold growth (likely Mucor species) appeared on the untreated bread. Over time, the mold proliferated, and black spore formation was observed, characteristic of Rhizopus stolonifera (Fig. 13B).

[0352] In contrast, bread treated with Complex 1 showed no visible mold growth after two weeks and remained largely mold-free, even after 30 days of incubation (Fig. 13A).

[0353] EXAMPLE 8

[0354] Antioxidant activity of Complex 1 in various protonated forms

[0355] Antioxidants neutralize reactive oxygen species (ROS), preventing oxidative damage to lipids, proteins, and DNA. By reducing oxidative stress, antioxidants can regulate inflammatory responses, enhance cell regeneration, and promote wound healing.

[0356] To evaluate antioxidant potential of various protonation forms of Complex 1, the fully protonated form, Complex 1 [a-PWiiOsgCe^HssfCeChbio], and a partially protonated from thereof K88-x[a-PWn039Ce]22[Hx(Ce02)9io], in which an amount x of H+was replace by K+countercation (following dialysis as described in the Materials and Methods section) were assessed. The 2,2- Diphenyl-l-picrylhydrazyl (DPPH) assay was used (see Materials and Methods section), which measures DPPH radical scavenging activity. The antioxidant activity of each sample was expressed as the percentage inhibition of DPPH radicals, indicating the concentration required for this effect.

[0357] Fig. 14 presents the antioxidant activity of Complex 1 and the partially protonated form thereof at varying concentrations. The samples exhibited strong antioxidant activity, ranging between 75% and 80% inhibition. These findings suggest that both H+and K+forms of Complex 1 have significant potential for wound healing applications.

Claims

WHAT IS CLAIMED IS:

1. A protonated polyoxometalate-nanocrystal complex of the Formula (I):[ / -POM]m[H / (NC)], whereinPOM is a polyoxometalate anion presented by the Formula (la):Co[XzMpDdOy]n- whereinX is H, at least one positively charged heteroatom and / or at least one positively charged metal atom, wherein the metal atom is selected from main-group metals, transition metals, or lanthanides or any combination thereof;M is at least one transition and / or main-group metal cation, optionally, in a high oxidation state;D is H, OH, OH2, a lower alkyl, lower hydroxyalkyl, lower silylalkyl, lower silylalkoxy and / or carboxylate;C is H+, an inorganic cation, or organic cation; z is 0 to 100; p is 6 to 250; d is 0 to 100; y is 15 to 800; n, the overall charge of the anion, is an integer of 1 to 75; a, the amount of cation that balances the negative charge of the anion, is an integer of 1 to 75; i is absent or a specific POM isomer; core nanocrystal (NC) comprises unit cell represented by Formula (lb):[M'iOk]; whereinM' is one or more cations of metals from the main-group elements, transition metals and / or lanthanide elements; m, the number of POMs bound to an individual nanocrystal (NC), is 1 to 50,000, preferably, 100 to 6000 or 2 to 1000; and f, the amounts of protons, is equal or less than m multiplied by n (n-m).

2. The complex of claim 1 wherein X is H or one, two, three or more heterocations and / or metal cation selected from Be, P, As, Sb, S , Si, Ge, or F or at least one metallic heteroatom selected from Na, Al, Ga, Ti, Zr, Hf, V, Cu, Fe, Mn, Co, Sn, Pb, or Ce.

3. The complex of claim 1 or 2, wherein M is selected from Al(lll), Ga(lll) In(lll), Tl(lll), Sn(lll), Pb(IV), Bi(lll), Bi(V), Sc(lll), Tl(lll), (Ti(IV), V(III),V(IV), V(V), Cr(lll), Cr(VI), Mn(ll), Mn(lll), Mn(IV), Mn(VII), Fe(ll), Fe(lll), Co(ll), Ni(ll), Ni(lll), Cu(ll), Cu(l), Zn(ll), Y(lll), Zr(IV), Nb(V), Mo(V), Mo(VI), Tc(VII), Ru(ll), Ru(lll), Ru(IV), Ru(VI), Pd(ll), Pd(IV), Hf(IV), Ta(V) or W(VI), and any combination thereof.

4. The complex of claim 3, wherein M is at least one of V(V), V(IV), Nb(V), Ta(V), Mo(V), Mo(VI) and / or W(VI).

5. The complex of any one of claim 1 to 4, wherein D is absent.

6. The complex of any one of claim 1 to 4, wherein D is OH, a lower alkyl of 1 to 3 carbon atoms (C1-C3 alkyl), a lower hydroxyalkyl selected from HO-Ci alkyl, HO-C2 alkyl or HO-C3, or any combination thereof.

7. The complex of any one of claim 1 to 6, wherein C is H+.

8. The complex of any one of claim 1 to 6, wherein C is selected from Li+, Na+, K+, Cs+or RXH4-XN+, where R is alkyl and / or thioalkyl.

9. The complex of any one of claims 1 to 8, wherein M' is a combination of main-group, transition metal or lanthanide elements that in combination with oxide, hydroxide, or oxyhydroxide, forms nanocrystals that contain oxygen vacancies.

10. The complex of any one of claims 1 to 9, wherein M', is a combination of main-group, transition metal or lanthanide elements that after being reduced, can be re-oxidized by O2, preferably using air under ambient conditions.

11. The complex of any one of claims 1 to 10, wherein M' is a cation of an atom selected from Mg, Ca, Al, Ga, In, Sn, Bi, Ti, Ce, Gd, La, Li, Rb, Cs, Sr, Ba, Al, Si, Ge, Pb, Bi, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mo, La, Ce, Pr, W, Nd, Sm, Eu, Gs, Dy and / or Tb.

12. The complex of claim 11, wherein M' is a lanthanide cation selected from Ce3+, Eu3+, Tb3+, Dy3+, Gd3+, La3+and / or Nd3+.

13. The complex of any one of claims 1 to 12, wherein the unit cell is selected from MgO, CaO, SrO, BaO, AI2O3, Ga2O3, ln2O3, SiC>2, GeO2, SnO2, PbO, PbO2, Bi2O3, TiO2, TiO2, V2O5, Cr2O3, Fe3O4, Fe2O3, Co3O4, NiO, CuO, ZnO, MoO3La2O3, CeO2, PrgOn, Pr2O3, WO3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Dy2O3, Tb2O3, transition-metal doped epsilon-MnCh, transition-metal doped CeO2, and / or transition-metal doped SnO2.

14. The complex of any one of claims 1 to 13, wherein POM is selected from an isopolytungstate, isopolyvanadate, isopolymolybdate, isopolyniobate, heteropolytungstate, heteropolyvanadate, heteropolymolybdate, polyoxoniobate, and modified form thereof selected from defect POMs, substituted POMs and "out of pocket" metal binding POMs.

15. The complex of any one of claims 1 to 14, which if fully protonated (f- n-m).

16. The complex of any one of claims 1 to 14, which if partially protonated (f< n-m).

17. The complex of claim 15 or 16, wherein the nanocrystal core comprises CeC>2.

18. The complex of claim 15 or 17, which is [a-PWiiOsgCe^CHaafCeChhio] (Complex 1).

19. The complex of claim 16 or 17, which is or C88-x[a-PWn039Ce]22[Hx(Ce02)9io], wherein C is an inorganic or organic cation, preferably, K+, and x is an integer or a fraction of 1 to 87.

20. A method for catalytic oxidation of a substance, comprising contacting the substance with a protonated polyoxometalate-nanocrystal complex as defined in any one of claims 1 to 19.

21. The method of claim 20, wherein the substance is dioxygen (O2), and the catalytic oxidation produces reactive oxygen species (ROS) selected from hydroxyl radicals (»OH), superoxide (•O2), peroxides and / or ozone.

22. The method of claim 21, wherein the dioxygen is air-borne O2.

23. The method of claim 21 or 22, wherein the catalytic oxidation is carried out at nearneutral pH values.

24. The method of any one of claims 20 to 23, wherein the catalytic oxidation is devoid of in tandem use of added oxidant, preferably H2O2.

25. The method of any one of claims 20 to 24, wherein the catalytic oxidation is carried out at ambient conditions.

26. The method of any one of claims 20 to 25, wherein the catalytic oxidation is highly efficient, rapid and displays indefinitely long and continuous operation compared to catalytic oxidation with corresponding colloidal metal oxide nanoparticles.

27. The method of any one of claims 20 to 20, wherein the efficacy of the catalytic oxidation is tunable by adjusting the protonation degree of the complex.

28. The method of claim 27, wherein protonation degree is adjusted by the addition of countercations and / or OH- anions.

29. An enzyme-mimicking catalytic process, comprising contacting a protonated polyoxometalate-nanocrystal complex as defined in any one of claims 1 to 19 with a substrate.

30. The enzyme-mimicking catalytic process of claim 29, which mimics oxidase, peroxidase, catalase and / or superoxide dismutase biochemical reactions.

31. The enzyme-mimicking catalytic process of claim 30, for conversion of superoxide to hydrogen peroxide.

32. The enzyme-mimicking catalytic process of claim 30 or 31, featuring dual-oxidase activity.

33. The enzyme-mimicking catalytic process of any one of claims 29 to 32, wherein the efficacy of the catalytic process is tunable by adjusting the protonation degree of the complex, preferably wherein protonation degree is adjusted by the addition of countercations and / or OH’ anions.

34. The method of any one of claims 20 to 28, or the enzyme-mimicking catalytic process of any one of claims 29 to 33, applied for one or more of:(i) antimicrobial therapy;(ii) treatment of cancer, optionally in precision medicine and / or targeted therapy;(iii) environmental remediation, industrial decontamination, sewage treatment;(iv) wound healing;(v) anti-fungal treatment;(vi) air purification and decontamination, odor control and deodorization;(vii) self-cleaning and anti-fouling surface coatings;(viii) sterilization of medical devices and surfaces;(ix) biosensing and diagnostic applications;(x) fuel cell and battery catalysis;(xi) food packaging and preservation;(xii) photocatalytic applications; and / or(xiii) catalysis in organic synthesis.

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