The effect of ionic versus covalent functionalization of polyoxometalate hybrid materials with coordinating subunits on their stability and interaction with DNA

Inorganic-organic hybrid materials that combine both Polyoxometalates (POMs) and metal ion coordinating subunits (CSUs) represent promising multifunctional materials. Though their individual components are often biologically active, utilization of hybrid materials in bioassays significantly depends on the functionalization method and thus resulting stability of the system. Quite intriguingly, these aspects were very scarcely studied in hybrid materials based on the Wells-Dawson POM (WD POM) scaffold and remain unknown. We chose two model WD POM hybrid systems to establish how the functionalization mode (ionic vs. covalent) affects their stability in biological medium and interaction with nucleic acids. The synthetic scope and limitations of the covalent POM-terpyridine hybrids were demonstrated and compared with the ionic Complex-Decorated Surfactant Encapsulated-Clusters (CD-SECs) hybrids. The nature of POM and CSU binding can be utilized to modulate the stability of the hybrid and the extent of DNA binding. The above systems show potential to behave as model cargo-platforms for potential utilization in medicine and pharmacy.

Design and applications of metallo-vesicular structures using inorganic-organic hybrids

In advanced biomedical diagnosis, various supramolecular assemblies based on inorganic-organic hybrids have found great interest as functional materials. These assemblies describe a new field of metallovesicles where the introduction of metal ions enables the chemical manipulation of assemblies in terms of their structural stability, redox activity, and pH stability. Additionally, they mimic the elaborative architecture of natural liposomal assemblies and exhibit hierarchical morphologies, and promise novel functions. With the constant developments in this field, various supramolecular assemblies such as MCsomes, Polymersomes, and Metallosomes, etc. came into existence. These hybrid assemblies have been utilized for several applications such as drug delivery, MRI contrasting, DNA delivery, and catalytic activity. The key advantage of these assemblies is their ability to deliver therapeutics to specific locations due to their biomimetic properties and release their contents at the desired time. Hence, they provide a valuable platform for the treatment of a variety of diseases. Through the present article, we intend to provide insights into the latest developments made in this field. This modularity underscores the tremendous promise of supramolecular assemblies as an emerging interdisciplinary research branch at the interface of chemistry and biological sciences.

Supramolecular assemblies of organo-functionalised hybrid polyoxometalates: from functional building blocks to hierarchical nanomaterials

This review provides a comprehensive overview of recent advances in the supramolecular organisation and hierarchical self-assembly of organo-functionalised hybrid polyoxometalates (hereafter referred to as hybrid POMs), and their emerging role as multi-functional building blocks in the construction of new nanomaterials. Polyoxometalates have long been studied as a fascinating outgrowth of traditional metal-oxide chemistry, where the unusual position they occupy between individual metal oxoanions and solid-state bulk oxides imbues them with a range of attractive properties (e.g. solubility, high structural modularity and tuneable properties/reactivity). Specifically, the capacity for POMs to be covalently coupled to an effectively limitless range of organic moieties has opened exciting new avenues in their rational design, while the combination of distinct organic and inorganic components facilitates the formation of complex molecular architectures and the emergence of new, unique functionalities. Here, we present a detailed discussion of the design opportunities afforded by hybrid POMs, where fine control over their size, topology and their covalent and non-covalent interactions with a range of other species and/or substrates makes them ideal building blocks in the assembly of a broad range of supramolecular hybrid nanomaterials. We review both direct self-assembly approaches (encompassing both solution and solid-state approaches) and the non-covalent interactions of hybrid POMs with a range of suitable substrates (including cavitands, carbon nanotubes and biological systems), while giving key consideration to the underlying driving forces in each case. Ultimately, this review aims to demonstrate the enormous potential that the rational assembly of hybrid POM clusters shows for the development of next-generation nanomaterials with applications in areas as diverse as catalysis, energy-storage and molecular biology, while providing our perspective on where the next major developments in the field may emerge.

Self-Assembly of Photoresponsive Molecular Amphiphiles in Aqueous Media

Amphiphilic molecules, comprising hydrophobic and hydrophilic moieties and the intrinsic propensity to self-assemble in aqueous environment, sustain a fascinating spectrum of structures and functions ranging from biological membranes to ordinary soap. Facing the challenge to design responsive, adaptive, and out-of-equilibrium systems in water, the incorporation of photoresponsive motifs in amphiphilic molecular structures offers ample opportunity to design supramolecular systems that enables functional responses in water in a non-invasive way using light. Here, we discuss the design of photoresponsive molecular amphiphiles, their self-assembled structures in aqueous media and at air-water interfaces, and various approaches to arrive at adaptive and dynamic functions in isotropic and anisotropic systems, including motion at the air-water interface, foam formation, reversible nanoscale assembly, and artificial muscle function. Controlling the delicate interplay of structural design, self-assembling conditions and external stimuli, these responsive amphiphiles open several avenues towards application such as soft adaptive materials, controlled delivery or soft actuators, bridging a gap between artificial and natural dynamic systems.

Polyoxotungstate ([PW11O39]7-) immobilized on mesoporous polymer for selective liquid-phase oxidation of alcohols using H2O2

Selective oxidation of alcohols is an attractive organic transformation and has received tremendous attention from the scientific community over the years. Herein, a mesoporous polymer (MP) was synthesized by a template-free solvothermal approach. The surface of the MP was functionalized with quaternary ammonium groups and polyoxotungstate anion (PW11O39 7-) was subsequently supported on the MP as a counter anion to the ammonium cation by a simple ion-exchange procedure. The structure of PW11 and PW4 complexes was confirmed by 31P NMR and FTIR analysis. The surface properties of all the catalysts synthesized were explored by various characterization techniques such as nitrogen sorption, TGA, contact angle measurement, and ICP-OES analysis. The synthesized PW11/MP catalysts were employed for selective oxidation of alcohols. Among the various PW11 supported catalysts, PW11/MP (80 : 20) demonstrated excellent catalytic activity for the oxidation of alcohols using aqueous H2O2. The PW11/MP (80 : 20) catalyst showed good catalytic activity for oxidation of a wide range of alcohols including substituted, heterocyclic and secondary alcohols. The superior catalytic activity of PW11/MP (80 : 20) is attributed to an optimum balance in the hydrophilicity/hydrophobicity in the mesoporous environment, better catalyst wettability, and enrichment of reactants in the catalytic active sites.

Transition metal decorated soft nanomaterials through modular self-assembly of an asymmetric hybrid polyoxometalate

An asymmetrically functionalised Wells-Dawson organic-inorganic hybrid polyoxometalate has been post-functionalised by Pt²⁺ coordination, and demonstrates self-assembly into surface-decorated micellar nanostructures. This multifunctional hybrid material is found to be a redox-active soft nanomaterial and demonstrates a new molecular design strategy with potential for applications in photo- or electro-catalysis.

Frontiers and progress in cation-uptake and exchange chemistry of polyoxometalate-based compounds

Cation-uptake and exchange has been an important topic in both basic and applied chemistry relevant to life and materials science. For example, living cells contain appreciable amounts of Na+ and K+, and their concentrations are regulated by the sodium-potassium pump. Solid-state cation-exchangers such as clays and zeolites both natural and synthetic have been used widely in water softening and purification, separation of metal ions and biomolecules, etc. Polyoxometalates (POMs) are robust, discrete, and structurally well-defined metal-oxide cluster anions, and have stimulated research in broad fields of sciences. In this perspective, cation-uptake and exchange in POM and POM-based compounds are categorized and reviewed in three groups: (i) POMs as inorganic crown ethers and cryptands, (ii) POM-based ionic solids as cation-exchangers, and (iii) reduction-induced cation-uptake in POM-based ionic solids, which is based on a feature of POMs that they are redox-active and multi-electron transfer occurs reversibly in multiple steps. This method can be utilized to synthesize mixed-valence metal clusters in metal ion-exchanged POM-based ionic solids.

Immobilization of Polyoxometalates on Tailored Polymeric Surfaces

Herein we describe the preparation of hybrid polymer-inorganic interfaces by the immobilization of polyoxometalate nanoclusters on functionalized polymer surfaces. The polymeric surfaces were made of polystyrene-b-poly(acrylic acid)/polystyrene (PS-b-PAA/PS) blends by spin coating on a silicon wafer. The functionalization of the polymer film was obtained by interfacial migration of the amphiphilic block copolymer toward the interface upon water vapor annealing. The carboxylic acid functional groups contained in the PAA block were then employed to anchor the [LnIII(α-SiW11O39)]5- polyoxometalates (Ln: Ce, Er). This purpose was achieved by immersing the films in aqueous solutions of the in situ-formed inorganic nanoclusters. X-ray photoelectron and confocal Raman spectroscopies, together with atomic force microscopy, confirmed the immobilization of the inorganic species at the interface.

Heterogeneous catalysis by tungsten-based heteropoly compounds

In this review, the recent works on heterogeneous catalytic applications of polyoxotungstates in liquid-phase organic reactions are reviewed.

Manipulation of Self-Assembled Nanostructure Dimensions in Molecular Janus Particles

The ability to manipulate self-assembly of molecular building blocks is the key to achieving precise "bottom-up" fabrications of desired nanostructures. Herein, we report a rational design, facile synthesis, and self-assembly of a series of molecular Janus particles (MJPs) constructed by chemically linking α-Keggin-type polyoxometalate (POM) nanoclusters with functionalized polyhedral oligomeric silsesquioxane (POSS) cages. Diverse nanostructures were obtained by tuning secondary interactions among the building blocks and solvents via three factors: solvent polarity, surface functionality of POSS derivatives, and molecular topology. Self-assembled morphologies of KPOM-BPOSS (B denotes isobutyl groups) were found dependent on solvent polarity. In acetonitrile/water mixtures with a high dielectric constant, colloidal nanoparticles with nanophase-separated internal lamellar structures quickly formed, which gradually turned into one-dimensional nanobelt crystals upon aging, while stacked crystalline lamellae were dominantly observed in less polar methanol/chloroform solutions. When the crystallizable BPOSS was replaced with noncrystallizable cyclohexyl-functionalized CPOSS, the resulting KPOM-CPOSS also formed colloidal spheres; however, it failed to further evolve into crystalline nanobelt structures. In less polar solvents, KPOM-CPOSS crystallized into isolated two-dimensional nanosheets, which were composed of two inner crystalline layers of Keggin POM covered by two monolayers of amorphous CPOSS. In contrast, self-assembly of KPOM-2BPOSS was dominated by crystallization of the BPOSS cages, which was hardly sensitive to solvent polarity. The BPOSS cages formed the crystalline inner bilayer, sandwiched by two outer layers of Keggin POM clusters. These results illustrate a rational strategy to purposely fabricate self-assembled nanostructures with diverse dimensionality from MJPs with controlled molecular composition and topology.

Solution behaviour of a polymer with polyoxometalate inorganic molecular clusters in its main chain

A polymer containing polyoxometalates in its main chain exists as a thin rod in a good solvent. The addition of a poor solvent induces the formation of hollow cylinders through counterion mediated attraction.

Synthesis of silsesquioxane-based element-block amphiphiles and their self-assembly in water

The self-assembly of silsesquioxane-based amphiphiles in water was investigated.

Polyoxometalate-Based Organic-Inorganic Hybrids as Antitumor Drugs

Polyoxometalates (POMs) have shown encouraging antitumor activity. However, their cytotoxicity in normal cells and unspecific interactions with biomolecules are two major obstacles that impede the practical applications of POMs in clinical cancer treatment. Derivatization of POMs with more biocompatible organic ligands is expected to cause a synergetic effect and achieve improved bioactivity and biospecificity. Herein, the synthesis of an amphiphilic organic-inorganic hybrid is reported by grafting a long-chain organoalkoxysilane lipid onto a POM. The amphiphilic POM hybrid could spontaneously assemble into the vesicles and exhibits enhanced antitumor activity for human colorectal cancer cell lines (HT29) compared to that of parent POMs. This detailed study reveals that the amphiphilic nature of POM hybrids enables the as-formed vesicles to easily bind to the cell membranes and then be uptaken by the cells, thus leading to a substantial increase in antitumor activity. Such prominent antitumor action is mostly accomplished via cell apoptosis, which ultimately results in cell death. Our finding demonstrates that novel POM hybrids-based drugs with increased bioactivity could be obtained by decorating POMs with selective organic ligands.

Self-assembly of triangular polyoxometalate–organic hybrid macroions in mixed solvents

A cartoon illustrates the self-assembly process of the “triangular” shaped hybrids with TBA as countercations in acetone–water mixed solvents.

Synthesis and structural characterization of BINOL-modified chiral polyoxometalates

Chiral ligand-modified polyoxometalates (POMs) R-I and S-I were successfully synthesized by the introduction of BINOL into the dititanium-substituted POM.

Chiral self-assembly and reversible light modulation of a polyoxometalate complex via host-guest recognition

Chiral self-assembly of an azobenzene-grafted POM complex has been constructed through self-crosslinking by both the electrostatic and host-guest interactions and reversibly modulated by dynamic controlling of the isomerization of photoactive groups in the complex.

Mesoporous polyoxometalate-based ionic hybrid as a triphasic catalyst for oxidation of benzyl alcohol with H(2)O(2) on water

A self-assembled mesoporous polyoxometalate-based ionic hybrid catalyst [TMGHA]2.4H0.6PW was prepared by combination of alcohol amino-tethered guanidinium ionic liquid [TMGHA]Cl with Keggin phosphotungstic acid H3PW12O40 (PW). Nitrogen sorption experiment validated the formation of mesostructure with moderate BET surface area, and scanning and transmission electron microscopy (SEM and TEM) showed a fluffy coral-shaped morphology for the hybrid. The contact angle test displayed that the hybrid owned hydrophilic-hydrophobic balanced surface that exhibited well wettability for both water and organic substrate like benzyl alcohol. Therefore, the hybrid can efficiently catalyze the water-mediated triphasic oxidation of benzyl alcohol with H2O2. During the reaction, the triphase catalytic system showed a special "on water" effect mainly due to the suitable mesostructure and surface wettability, thus providing some clues for the preparation of green heterogeneous catalyst.

Self-assembled structures of amphiphiles regulated via implanting external stimuli

This review article has summarized recent achievements of manipulating amphiphilic molecules and their self-assembled structures via different external stimuli.

Langmuir and Langmuir-Blodgett films of hybrid amphiphiles with a polyoxometalate headgroup

A hybrid was at first synthesized by a postfunctionalization of an aminomethane trisalkoxo-functionalized Anderson-type polyoxometalate (POM) encapsulated by three tetrabutylammonium ions using a 3,5-bis(tetradecyloxy)benzoic acid by amidation. Then the three TBA(+) counter cations were programmatically replaced by protons (H(+)) following a molecule-to-amphiphile conversion. In this way one hybrid and three POM-containing hybrid amphiphiles (PCHAs) were acquired by adjusting the number (n) of TBA(+) ions and number (3 - n) of H(+) ions (n = 3, 2, 1, and 0). These compounds can be spread onto a water surface to form a Langmuir monolayer film at the air-water interface. Surface pressure-molecular area measurements exhibit the TBA(+) (H(+)) number playing an important role in the forming ability and stability of Langmuir monolayer films. Also, the Langmuir-Blodgett (LB) technique has been used to transfer the monolayer film onto solid supports to fabricate solid multilayer films. It was found that the PCHA with three H(+) ions had the best Langmuir film-forming ability and thus formed stable LB films with a two-dimensional ordered structure. Our findings are instructive in fabricating and using solid films of the amphiphiles with POM headgroups.

Solution behaviors and self-assembly of polyoxometalates as models of macroions and amphiphilic polyoxometalate-organic hybrids as novel surfactants

Large, hydrophilic polyoxoanions with high solubility in water and/or other polar solvents demonstrate unique solution behavior by self-assembling into single layer, hollow, spherical "blackberry" structures, which is obviously different from small, simple ions. These macroions cannot be treated as insoluble colloidal suspensions because they form stable "real solutions". Counterion-mediated attraction is considered as the main driving force for the self-assembly behavior. The size disparity between the macroions and their counterions results in macroion-counterion pairing which leads to the inter-macroanionic attraction. The blackberries, with robust membranes semi-permeable to cations, can adjust their size accurately and reversibly in response to the change of solvent polarity and charge density of individual macroions. The inorganic macroions with well-defined size, shape, mass, charge density, but no intramolecular interactions, are ideal model systems to study the intermolecular interactions in polyelectrolyte and bio-macromolecular solutions. The blackberry structures show certain similarities to spherical viral capsids, from the overall structure to the formation kinetics. More amazingly, these inorganic macroions demonstrate some features usually believed to belong only to complex biological molecules, such as the self-recognition in dilute solutions. Meanwhile, polyoxometalates-based organic-inorganic hybrid materials demonstrate amphiphilic properties by self-assembling into vesicles and reverse vesicles in polar and non-polar solvents, respectively, and form monolayer at the water/air interface. Different from conventional amphiphiles, these hybrids show pH-dependent and counterion-dependent self-assembly behaviors with controllable functionality, e.g. fluorescence and catalytic activity, due to the high and tunable charges and the functionalities of POM polar head groups.

Functionalization and post-functionalization: a step towards polyoxometalate-based materials

Polyoxometalates (POMs) have remarkable properties and a great deal of potential to meet contemporary societal demands regarding health, environment, energy and information technologies. However, implementation of POMs in various functional architectures, devices or materials requires a processing step. Most developments have considered the exchange of POM counterions in an electrostatically driven approach: immobilization of POMs on electrodes and other surfaces including oxides, embedding in polymers, incorporation into Layer-by-Layer assemblies or Langmuir-Blodgett films and hierarchical self-assembly of surfactant-encapsulated POMs have thus been thoroughly investigated. Meanwhile, the field of organic-inorganic POM hybrids has expanded and offers the opportunity to explore the covalent approach for the organization or immobilization of POMs. In this critical review, we focus on the use of POM hybrids in selected fields of applications such as catalysis, energy conversion and molecular nanosciences and we endeavor to discuss the impact of the covalent approach compared to the electrostatic one. The synthesis of organic-inorganic POM hybrids starting from bare POMs, that is the direct functionalization of POMs, is well documented and reliable and efficient synthetic procedures are available. However, as the complexity of the targeted functional system increases a multi-step strategy relying on the post-functionalization of preformed hybrid POM platforms could prove more appealing. In the second part of this review, we thus survey the synthetic methodologies of post-functionalization of POMs and critically discuss the opportunities it offers compared to direct functionalization.