Controllable self-assembly of organic-inorganic amphiphiles containing Dawson polyoxometalate clusters

Bioorthogonal chemistry of polyoxometalates - challenges and prospects

Bioorthogonal chemistry has enabled scientists to carry out controlled chemical processes in high yields in vivo while minimizing hazardous effects. Its extension to the field of polyoxometalates (POMs) could open up new possibilities and new applications in molecular electronics, sensing and catalysis, including inside living cells. However, this comes with many challenges that need to be addressed to effectively implement and exploit bioorthogonal reactions in the chemistry of POMs. In particular, how to protect POMs from the biological environment but make their reactivity selective towards specific bioorthogonal tags (and thereby reduce their toxicity), as well as which bioorthogonal chemistry protocols are suitable for POMs and how reactions can be carried out are questions that we are exploring herein. This perspective conceptualizes and discusses advances in the supramolecular chemistry of POMs, their click chemistry, and POM-based surface engineering to develop innovative bioorthogonal approaches tailored to POMs and to improve POM biological tolerance.

Molecular Geometry-Directed Self-Recognition in the Self-Assembly of Giant Amphiphiles

Three sets of polyoxometalate (POM)-based amphiphilic hybrid macromolecules with different rigidity in their organic tails are used as models to understand the effect of molecular rigidity on their possible self-recognition feature during self-assembly processes. Self-recognition is achieved in the mixed solution of two structurally similar, sphere-rigid T-shape-linked oligofluorene(TOF₄ ) rod amphiphiles, with the hydrophilic clusters being Anderson (Anderson-TOF₄ ) and Dawson (Dawson-TOF₄ ), respectively. Anderson-TOF₄ is observed to self-assemble into onion-like multilayer structures and Dawson-TOF₄ forms multilayer vesicles. The self-assembly is controlled by the interdigitation of hydrophobic rods and the counterion-mediated attraction among charged hydrophilic inorganic clusters. When the hydrophobic blocks are less rigid, e.g., partially rigid polystyrene and fully flexible alkyl chains, self-recognition is not observed, attributing to the flexible conformation of hydrophobic molecules in the solvophobic domain. This study reveals that the self-recognition among amphiphiles can be achieved by the geometrical limitation of the supramolecular structure due to the rigidity of solvophobic domains.

Side Group of Hydrophobic Amino Acids Controls Chiral Discrimination among Chiral Counterions and Metal-Organic Cages

The self-assembly of chiral Pd₁₂L₂₄ metal-organic cages (MOCs) based on hydrophobic amino acids, including alanine (Ala), valine (Val), and leucine (Leu), into single-layered hollow spherical blackberry-type structures is triggered by nitrates through counterion-mediated attraction. In addition to nitrates, anionic N-(tert-butoxycarbonyl) (Boc)-protected Ala, Val, and Leu were used as chiral counterions during the self-assembly of d-MOCs. Previously, we showed that l-Ala suppresses the self-assembly process of d-Pd₁₂Ala₂₄ but has no effect on l-Pd₁₂Ala₂₄, i.e., chiral discrimination. Here, we indicate when the amino acid used as the chiral counterion has a bulkier side group than the amino acid in the MOC structure, no chiral discrimination exists; otherwise, chiral discrimination exists. For example, Ala can induce chiral discrimination in all chiral MOCs, whereas Leu can induce chiral discrimination only in Pd₁₂Leu₂₄. Moreover, chiral anionic d- and l-alanine-based surfactants have no chiral discrimination, indicating that bulkier chiral counterions with more hydropohobic side groups can erase chiral discrimination.

Development of PAMAM dendrimer-modified magnetic polyoxometalate: A novel platform to reinforce mechanical and thermal properties of diglycidyl ether of bisphenol A/isophorone diamine hardener epoxy

The present study explores the mechanical and thermal properties of DGEBA/IPD epoxy reinforced with dendrimer-functionalized magnetitepolyoxometalate nanoparticles. Magnetic iron oxide nanoparticles (MNP’s) were stabilized and functionalized by the poly (amido-amine) dendrimer via encapsulation within dendrimer; afterwards, H9 [α-P2V3W15O62] polyoxometalate (POM) was modified with dendrimer-functionalized magnetic iron oxide nanoparticles (DMNP’s). The polyoxometalate can be complexed with DMNP’s via protonation of dendrimer amino groups. In the next step, dendrimer-functionalized magnetitepolyoxometalate nanoparticles (DMNP’s-POM) were loaded into diglycidyl ether of bisphenol A (DGEBA) epoxy resin. The DMNP’s-POM nanoparticles can initiate polymerizations of epoxy resin with isophorone diamine hardener (IPD); on the other hand, the terminal amino groups of the dendrimer in the DMNP’s-POM nanoparticles allow them to be covalently linked to the polymer matrix alongside the main amine hardener. The resulting epoxy/magnetitepolyoxometalate nanocomposites (DMNP’s-POM@EN 5%) are thoroughly characterized by FT-IR, FE-SEM, and XRD analysis. Probing thermal behaviors of epoxy/magnetitepolyoxometalate nanocomposites by TGA reveals that the resulting composites are degraded thermally through a simple one-step process with an initial degradation close to 340°C, and show significant stability toward heat. Dynamic Mechanical Thermal Analysis indicates that no considerable agglomerate is formed during the synthesis process, and the incorporated nanoparticles somewhat limit the segmental motions of the epoxy macromolecular chains.

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.

Polyoxometalates as chemically and structurally versatile components in self-assembled materials

Polyoxometalates (POMs) are anionic molecular metal oxides with expansive diversity in terms of their composition, structure, nuclearity and charge. Within this vast collection of compounds are dominant structural motifs (POM platforms), that are amenable to significant chemical tuning with minimal perturbation of the inorganic oxide molecular structure. Consequently, this enables the systematic investigation of these compounds as inorganic additives within materials whereby structure and charge can be tuned independently i.e. [PW12O40]3- vs. [SiW12O40]4- while also investigating the impact of varying the charge balancing cations on self-assembly. The rich surface chemistry of POMs also supports their functionalisation by organic components to yield so-called inorganic-organic hybrids which will be the key focus of this perspective. We will introduce the modifications possible for each POM platform, as well as discussing the range of nanoparticles, microparticles and surfaces that have been developed using both surfactant and polymer building blocks. We will also illustrate important examples of POM-hybrids alongside their potential utility in applications such as imaging, therapeutic delivery and energy storage.

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.

An inorganic-organic hybrid MnIII{Mn}2 cluster consisting of rare Lindqvist-like Mn6 subunits with high proton conductivity

A new inorganic-organic hybrid Mn^III{Mn}₂ with rare Lindqvist-like Mn₆ subunits was synthesized. Our studies reveal the distinct advantages of the Mn^III{Mn}₂ compound as a potential material of proton exchange membranes, including its facile and cost-effective synthesis, insolubility in water, and most importantly, its maximum proton conductivity of 4.69 × 10^-3 S cm^-1 at 368 K and 97% RH.

Competition and Cooperation among Different Attractive Forces in Solutions of Inorganic-Organic Hybrids Containing Macroionic Clusters

Hybrids composed of nanoscale inorganic clusters and organic ligands are ideal models for understanding the different attractive forces during the self-assembly processes of complex macromolecules in solution. The counterion-mediated attraction induced by electrostatic interaction from the large, hydrophilic macroionic clusters can compete or cooperate with other types of attractive forces such as hydrophobic interactions, hydrogen bonding, π-π stacking, and cation-π interactions from the organic ligands, consequently determining the solution behaviors of the hybrid molecules including their self-assembly process and the final supramolecular structures. The incorporation of organic ligands also leads to interesting responsive behaviors to external stimuli. Through the manipulation of the hybrid composition, architecture, topology, and solution conditions (e.g., solvent polarity, pH, and temperature), versatile self-assembled morphologies can be achieved, providing new scientific opportunities and potential applications.

Nanoscale Tungsten-Microbial Interface of the Metal Immobilizing Thermoacidophilic Archaeon Metallosphaera sedula Cultivated With Tungsten Polyoxometalate

Inorganic systems based upon polyoxometalate (POM) clusters provide an experimental approach to develop artificial life. These artificial symmetric anionic macromolecules with oxidometalate polyhedra as building blocks were shown to be well suited as inorganic frameworks for complex self-assembling and organizing systems with emergent properties. Analogously to mineral cells based on iron sulfides, POMs are considered as inorganic cells in facilitating prelife chemical processes and displaying "life-like" characteristics. However, the relevance of POMs to life-sustaining processes (e.g., microbial respiration) has not yet been addressed, while iron sulfides are very well known as ubiquitous mineral precursors and energy sources for chemolithotrophic metabolism. Metallosphaera sedula is an extreme metallophilic and thermoacidophilic archaeon, which flourishes in hot acid and respires by metal oxidation. In the present study we provide our observations on M. sedula cultivated on tungsten polyoxometalate (W-POM). The decomposition of W-POM macromolecular clusters and the appearance of low molecular weight W species (e.g., WO) in the presence of M. sedula have been detected by electrospray ionization mass spectrometry (ESI-MS) analysis. Here, we document the presence of metalloorganic assemblages at the interface between M. sedula and W-POM resolved down to the nanometer scale using scanning and transmission electron microscopy (SEM and TEM) coupled to electron energy loss spectroscopy (EELS). High-resolution TEM (HR-TEM) and selected-area electron diffraction (SAED) patterns indicated the deposition of redox heterogeneous tungsten species on the S-layer of M. sedula along with the accumulation of intracellular tungsten-bearing nanoparticles, i.e., clusters of tungsten atoms. These results reveal the effectiveness of the analytical spectroscopy coupled to the wet chemistry approach as a tool in the analysis of metal-microbial interactions and microbial cultivation on supramolecular self-assemblages based on inorganic metal clusters. We discuss the possible mechanism of W-POM decomposition by M. sedula in light of unique electrochemical properties of POMs. The findings presented herein highlight unique metallophilicity in hostile environments, extending our knowledge of the relevance of POMs to life-sustaining processes, understanding of the transition of POMs as inorganic prebiotic model to life-sustainable material precursors and revealing biogenic signatures obtained after the decomposition of an artificial inorganic compound, which previously was not associated with any living matter.

Light-Triggered Boost of Activity of Catalytic Bola-Type Surfactants by a Plasmonic Metal-Support Interaction Effect

The maximization of activity is a general aim in catalysis research. The possibility for light-triggered enhancement of a catalytic process, even if the process is not photochemical in nature, represents an intriguing concept. Here, we present a novel system for the exploration of the latter idea. A surfactant with a catalytically active head group, a protonated polyoxometalate (POM) cluster, is attached to the surface of a gold nanoparticle (Au NP) using thiol coupling chemistry. The distance of the catalytically active center to the gold surface could be adjusted precisely using surfactants containing hydrocarbon chains (C n) of different lengths ( n = 4-10). Radiation with VIS-light has no effect on the catalytic activity of micellar aggregates of the surfactant. The situation changes, as soon as the surfactants have been attached to the Au NPs. The catalytic activity could almost be doubled. It was proven that the effect is caused by coupling the surface plasmon resonance of the Au NPs with the properties of the POM head group. The improvement of activity could only be observed if the excitation wavelength matches the absorption band of the used Au NPs. Furthermore, the shorter the distance between the POM group and the surface of the NP, the stronger is the effect. This phenomenon was explained by lowering the activation energy of the transition state relevant to the catalytic process by the strong electric fields in the vicinity of the surfaces of plasmonic nanoparticles. Because the catalytic enhancement is wavelength-selective, one can imagine the creation of complex systems in the future, a system of differently sized NPs, each responsible for a different catalytic step and activated by light of different colors.

Assembling [M(P4Mo6)2] (M = Na, Mn, Na/Cu) dimeric clusters via transition metal/sodium ions into 0D to 3D phosphomolybdates

Six novel molybdenum(v) phosphates with structures ranging from a zero-dimensional cluster to three-dimensional frameworks have been solvothermally synthesized.

Synthesis and spectroscopic properties of silver-fluorescein co-doped phosphotungstate hollow spheres

Herein, silver-fluorescein co-doped phosphotungstate hollow spheres have been successfully synthesized using a chemical precipitation method by introducing silver ions and fluorescein at room temperature. This hybrid nanomaterial possesses a uniform structure, exhibiting a strong emission spectrum with a peak centered at 517 nm. It is anticipated that silver and fluorescein as functionalized dopants for the phosphotungstate hollow sphere structure would provide this material with multiple properties. The control experiments indicate that fluorescein plays a key role in the formation of the hollow sphere structure. Hence, the co-doping strategy is proposed to be a general method to endow polyoxometalate nanomaterials with new structures, new functions and potential applications in bioimaging, fluorescent chemical sensors, and antibacterial or optoelectronic devices.

Self-Assembly of Polyoxometalate-Peptide Hybrids in Solution: Elucidating the Contributions of Multiple Possible Driving Forces

Incorporating the building blocks of nature (e.g., peptides and DNA) into inorganic polyoxometalate (POM) clusters is a promising approach to improve the compatibilities of POMs in biological fields. To extend their biological applications, it is necessary to understand the importance of different non‐covalent interactions during self‐organization. A series of Anderson POM–peptide hybrids have been used as a simple model to demonstrate the role of different interactions in POM–peptide (biomolecules) systems. Regardless of peptide chain length, these hybrids follow similar solution behaviors, forming hollow, spherical supramolecular structures in acetonitrile/water mixed solvents. The incorporation of peptide tails introduces interesting stimuli‐responsive properties to temperature, hybrid concentration, solvent polarity and ionic strength. Unlike the typical bilayer amphiphilic vesicles, they are found to follow the blackberry‐type assemblies of hydrophilic macroions, which are regulated by electrostatic interaction and hydrogen bonding. The formation of electrostatic assemblies before the supramolecular formation is confirmed by ion‐mobility mass spectrometry (IMS‐MS).

Recent advances in RAFT-mediated surfactant-free emulsion polymerization

We summarized the RAFT-mediated surfactant-free emulsion polymerization using various RAFT agents and the polymerization types for the preparation of organic/inorganic hybrid materials.

Long rigid ligand induced basket-type phosphomolybdate photo-/electro-catalytic materials

Two long rigid ligands are introduced into {P₆Mo₁₈O₇₃} systems, leading to two unique supramolecular hybrid assemblies, which show highly efficient catalytic activity for the degradation of refractory dye AP under UV light and bifunctional electrocatalytic behavior for the oxidation of dopamine (DA) and reduction of H₂O₂.

Effective photocatalytic and bifunctional electrocatalytic materials based on Keggin arsenomolybdates and different transition metal capped assemblies

V, Co, and Cu were introduced into {AsMo₁₂} system to build three Keggin derivatives with different caps, which show excellent photocatalytic activity and bifunctional electrocatalytic behavior.

Efficient and robust photocatalysts based on {P2W18} modified by an Ag complex

Ag⁺ and the flexible ligand bimb were introduced into Dawson phosphomolybdate systems as linkage units to induce two fascinating 3-D inorganic–organic networks, which exhibit excellent electro- and photo-catalytic behavior.

Ionic Complexes of Metal Oxide Clusters for Versatile Self-Assemblies

The combination of rational design of building components and suitable utilization of driving force affords spontaneous molecular assemblies with well-defined nanostructure and morphology over multiple length scales. The serious challenges in constructing assemblies with structural advantages for the realization of functions programmed into the building components usually lie ahead since the process that occurs does not always follow the expected roadmap in the absence of external intervention. Thus, prefabricated intermediates that help in governing the target self-assemblies are developed into a type of unique building blocks. Metal oxide cluster polyanions are considered as a type of molecular nanoclusters with size scale and structural morphology similar to those of many known inorganic particles and clusters but possess distinctive characteristics. Following the understanding of these clusters in self-assembly and the rationalization of their most efficient design strategy and approach, the obtained fundamental principles can also be applied in common nanoparticle- and cluster-based systems. On the other hand, the deliberate synergy offered by organic countercations that support the self-assembly of these clusters greatly expands the opportunity for the functionalization of complex building units via control of multiple interactions. The ionic combination of the inorganic clusters with hydrophilicity and the cationic organic component with hydrophobicity leads to discrete properties of the complexes. Significantly, the core-shell structure with rigid-flexible features and amphiphilicity will pave the way for hierarchical self-assemblies of the obtained complexes, while the intrinsic characteristics of the metal oxide clusters can be modulated through external physicochemical stimuli. Within this context, over the past decade we have extensively explored the ionic combination of inorganic polyanionic clusters with cationic organic amphiphiles and devoted our efforts to establishing the general rules and structure-property relationships of the formed complexes for constructing self-assemblies at the interface, in solution, and in solid matrixes. Specific interest has been focused on the functional synergy deriving from the incompatible components in highly organized self-assemblies. In this Account, we describe the recent progress on the ionic complexation of polyoxometalate clusters with cationic amphiphiles and the construction of diverse self-assembled nanostructures. First, the fundamental structural characteristics and molecular geometries of the prepared complexes are analyzed. The construction principle and diversity of the self-assembly based on the complexes and the smart stimuli response are then discussed, subject to the adjustment of various non-covalent interactions occurring in the assemblies. Subsequently, we enumerate the functional applications of the ionic complexes assembling into organic, inorganic, and even biological matrixes. The inspiration from the construction of ionic complexation and self-assembly in this Account provides vivid profiles for the design of hybrid materials involving nanoclusters and/or nanoparticles with rich potentials in addition to polyoxometalate chemistry.

A mild post-functionalization method for the vanadium substituted P2W15V3 Wells–Dawson polyoxometalate based on a copper catalyzed azide–alkyne cycloaddition

A novel post-functionalization method for the vanadium substituted P₂W₁₅V₃ Wells–Dawson polyoxometalate has been developed using a copper catalyzed azide–alkyne cycloaddition.

The basket-type dimer layers based on tetra-electron reduced heteropoly blue directed by copper/nickel and strontium linkers

Two basket-type dimer layers have been prepared, which exhibit highly efficient catalytic ability for the degradation of typical dyes and bifunctional electrocatalytic behaviors.

Multinuclear diffusion NMR spectroscopy and DFT modeling: a powerful combination for unraveling the mechanism of phosphoester bond hydrolysis catalyzed by metal-substituted polyoxometalates

A detailed reaction mechanism is proposed for the hydrolysis of the phosphoester bonds in the DNA model substrate bis(4-nitrophenyl) phosphate (BNPP) in the presence of the Zr(IV)-substituted Keggin type polyoxometalate (Et2NH2)8[{α-PW11O39Zr(μ-OH)(H2O)}2]⋅7 H2O (ZrK 2:2) at pD 6.4. Low-temperature (31)P DOSY spectra at pD 6.4 gave the first experimental evidence for the presence of ZrK 1:1 in fast equilibrium with ZrK 2:2 in purely aqueous solution. Moreover, theoretical calculations identified the ZrK 1:1 form as the potentially active species in solution. The reaction intermediates involved in the hydrolysis were identified by means of (1)H/(31)P NMR studies, including EXSY and DOSY NMR spectroscopy, which were supported by DFT calculations. This experimental/theoretical approach enabled the determination of the structures of four intermediate species in which the starting compound BNPP, nitrophenyl phosphate (NPP), or the end product phosphate (P) is coordinated to ZrK 1:1. In the proposed reaction mechanism, BNPP initially coordinates to ZrK 1:1 in a monodentate fashion, which results in hydrolysis of the first phosphoester bond in BNPP and formation of NPP. EXSY NMR studies showed that the bidentate complex between NPP and ZrK 1:1 is in equilibrium with monobound and free NPP. Subsequently, hydrolysis of NPP results in P, which is in equilibrium with its monobound form.

Synthesis, structure, self-assembly and genotoxicity evaluation of a series of Mn-Anderson cluster based polyoxometalate–organic hybrids

A new series of POM–organic hybrids have been developed which show less genotoxicity compared to the parent polyoxometalate cluster.

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.

A novel organic–inorganic hybrid polyoxometalate for the selective adsorption/isolation of β-lactoglobulin

A novel polyoxometalate (POM)-based organic–inorganic hybrid is preparedviaa one-pot hydrothermal reaction between Keggin POM and a star-like N-donor ligand. The hybrid exhibits excellent adsorption performance towards β-lactoglobulin with a favorable sorption capacity.

Design of polystyrene latex particles covered with polyoxometalate clusters via multiple covalent bonding

Polyoxometalate clusters can be chemically grafted onto the surface of polymer latex via simple emulsion polymerization reaction. Such hierarchical nano-structures could serve as highly efficient quasi-homogeneous catalysts.

Assembly of a basket-like {Sr ⊂ P6Mo18O73} cage from 0D dimmer to 2D network and its photo-/electro-catalytic properties

Five semiconductor materials based on basket-like POM have been hydrothermally synthesized, which show high-efficient degradation ability for organic dyes RhB, MB, and AP in short time and bifunctional electrocatalytic behavior for oxidation of AA and reduction of NO₂⁻.

High-efficiency photo- and electro-catalytic material based on a basket-like {Sr⊂P6Mo18O73} cage

The first 8-connected 2D layer based on basket-liked {Sr⊂P₆Mo₁₈O₇₃} cage was prepared, which exhibits high-efficient photodegradation ability for MB, RhB, and AP under visible and UV light, as well as bifunctional electrocatalytic behavior.