Stable photochromism and controllable reduction properties of surfactant-encapsulated polyoxometalate/silica hybrid films

In this paper, we present a novel strategy for fabricating polyoxometalate (POM)-based photochromic silica hybrid films. To combine metal nanoparticles (NPs) into the POMs embedded silica matrix, furthermore, we realized the controllable in situ synthesis of metal NPs in the film by utilizing the reduction property of POMs existing in the reduced state. Through electrostatic encapsulation with hydroxyl-terminated surfactants, the POMs with good redox property can be covalently grafted onto a silica matrix by means of a sol-gel approach, and stable silica sol-gel thin films containing surfactant-encapsulated POMs can be obtained. The functional hybrid film exhibits both the transparent and easily processible properties of silica matrix and the stable and reversible photochromism of POMs. In addition, well-dispersed POMs in a hydrophobic microenvironment within the hybrid film can be used as reductants for the in situ synthesis of metal NPs. More significantly, the size and location of NPs can be tuned by controlling the adsorption time of metal ions and mask blocking the surface. The hybrid film containing both POMs and metal NPs with patterned morphology can be obtained, which has potential applications in optical display, memory, catalysis, microelectronic devices and antibacterial materials.     

Layer-by-layer assembly of polyoxometalates into microcapsules

The polyoxometalate (POM) chemistry world has been experiencing an unparalleled development of rapid synthesis of new compounds and slow development of POM-based functional materials and devices. Meanwhile, researchers in the microcapsule world, encouraged by the introduction of the layer-by-layer method, are pursuing good components for constructing functional capsule devices. Here, in view of the versatile properties that POM-based microcapsules may possess, various types of POM-polyelectrolyte composite microcapsules were constructed using the layer-by-layer method. Microscopy reveals that polyoxometalates form nanoparticles on the shell in the presence of cationic polyelectrolytes. These nanoparticles connected with polyelectrolytes constitute the shell and support the microcapsule from collapse after drying, and this is an interesting characteristic different from those of common composite and polyelectrolyte capsules. Fourier transform infrared (FTIR), UV-vis absorption, and X-ray photoelectron spectroscopy (XPS) were used to examine the properties of the POMs in the microcapsules. The obtained microcapsules exhibit higher thermal stability than polyelectrolyte microcapsules. Furthermore, the functions of POMs were maintained when they were assembled into microcapsules. It is proved that microcapsules bearing POMs with redox activity can provide a reduction environment, which can lead to the realization of in situ synthesis of materials, and that microcapsules with photoluminescent POMs as a component can also have a photoluminescent property, providing a way to develop functional capsule devices. This work may provide an opportunity to enrich both the polyoxometalate chemistry and the capsule field.     

Soft Matter Approaches for Enhancing the Catalytic Capabilities of Polyoxometalate Clusters

In this minireview, we discuss the recent efforts on expanding the catalytic capabilities of polyoxometalates (POM) through emulsion catalysis approaches with novel catalytic-active POM–organic hybrid clusters as emulsifiers. The hybrid emulsifiers include surfactant encapsulated POM complexes, molecular POMs–organic hybrids, and POM-based solid nanoparticles. With such novel approaches the catalytic efficiency of the POMs can be significantly improved by enhancing the compatibility of the POMs with organic media, providing catalytic interface for biphasic reactions, as well as easier preparation, and better recyclability. Particularly, a simple, green chemistry method to prepare metal nanoparticle materials with POMs as both reducing and capping agents in aqueous is reviewed.     

Recent Progress in the Application of Polyoxometalates for Dye‐sensitized/Organic Solar Cells

At present, high efficiency and low fabrication cost are still the main goal that people pursuit for next‐generation solar cells such as dye‐sensitized solar cells (DSSCs) and organic solar cells (OPVs). Polyoxometalates (POMs), as an environmentally friendly material, are a type of stable, low cost and soluble oxide clusters with desirable features, including highly tunable structural properties, peculiar optoelectronic properties and excellent redox properties. Thus, during the recent years, POMs have been increasingly recognized as important building blocks for DSSCs and OPVs. In this review, the development of various molecular and hybrid materials derived from POMs is discussed with regard to the function in solar cells.     

Polyoxometalates as Potential Next‐Generation Metallodrugs in the Combat Against Cancer

Polyoxometalates (POMs) are an emerging class of inorganic metal oxides, which over the last decades demonstrated promising biological activities by the virtue of their great diversity in structures and properties. They possess high potential for the inhibition of various tumor types; however, their unspecific interactions with biomolecules and toxicity impede their clinical usage. The current focus of the field of biologically active POMs lies on organically functionalized and POM‐based nanocomposite structures as these hybrids show enhanced anticancer activity and significantly reduced toxicity towards normal cells in comparison to unmodified POMs. Although the antitumor activity of POMs is well documented, their mechanisms of action are still not well understood. In this Review, an overview is given of the cytotoxic effects of POMs with a special focus on POM‐based hybrid and nanocomposite structures. Furthermore, we aim to provide proposed mode of actions and to identify molecular targets. POMs are expected to develop into the next generation of anticancer drugs that selectively target cancer cells while sparing healthy cells.     

Encapsulating Metal Nanoparticles into a Layered Zeolite Precursor with Surface Silanol Nests Enhances Sintering Resistance**

Supported metal nanoparticles are used as heterogeneous catalysts but often deactivated due to sintering at high temperatures. Confining metal species into a porous matrix reduces sintering, yet supports rarely provide additional stabilization. Here, we used the silanol-rich layered zeolite IPC-1P to stabilize ultra-small Rh nanoparticles. By adjusting the IPC-1P interlayer space through swelling, we prepared various architectures, including microporous and disordered mesoporous. In situ scanning transmission electron microscopy confirmed that Rh nanoparticles are resistant to sintering at high temperature (750 °C, 6 hrs). Rh clusters strongly bind to surface silanol quadruplets at IPC-1P layers by hydrogen transfer to clusters, while high silanol density hinders their migration based on density functional theory calculations. Ultimately, combining swelling with long-chain surfactant and utilizing metal-silanol interactions resulted in a novel, catalytically active material—Rh@IPC_C22.     

Hybrid Sub-1 nm Nanosheets of Co-assembled MnZnCuOx and Polyoxometalate Clusters as Anodes for Li-ion Batteries

Transition metal oxide (TMO) anode materials in lithium-ion batteries (LIBs) usually suffer from serious volume expansion leading to the pulverization of structures, further giving rise to lower specific capacity and worse cycling stability. Herein, by introducing polyoxometalate (POM) clusters into TMOs and precisely controlling the amount of POMs, the MnZnCuO_x-phosphomolybdic acid hybrid sub-1 nm nanosheets (MZC-PMA HSNSs) anode is successfully fabricated, where the special electron rich structure of POMs is conducive to accelerating the migration of lithium ions on the anode to obtain higher specific capacity, and the non-covalent interactions between POMs and TMOs make the HSNSs possess excellent structural and chemical stability, thus exhibiting outstanding electrochemical performance in LIBs, achieving a high reversible capacity (1157 mAh g⁻¹ at 100 mA g⁻¹) and an admirable long-term cycling stability at low and high current densities.     

The Old Polyoxometalates in New Application as Molecular Resistive Switching Memristors

The coming big-data era has created a huge demand for next-generation memory technologies with characters of higher data-storage densities, faster access speeds, lower power consumption and better environmental compatibility. In this field, the design of resistive switching active materials is pivotal but challengeable. Polyoxometalates (POMs) are promising candidates for next-generation molecular memristors due to their versatile redox characters, excellent electron reservoirs and good compatibility/convenience in microelectronics processing. In this review, five kinds of POM-based active materials in nonvolatile memories (inorganic POMs, crystalline organic-inorganic hybrid POMOFs, polymer modified POMs, POM/transition metal oxides composites and the deposition of POM on metal surfaces) were described. The components of POMs active materials, device fabrications, device parameters, and resistive switching mechanisms relative to their structures were summarized. Finally, challenges and future perspectives of POMs-based memristors were also presented.     

Polyoxometalate-Based Nanomaterials Toward Efficient Cancer Diagnosis and Therapy

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

Photochromic hybrid organic-inorganic liquid-crystalline materials built from nonionic surfactants and polyoxometalates: elaboration and structural study

This work reports the elaboration and structural study of new hybrid organic-inorganic materials constructed via the coupling of liquid-crystalline nonionic surfactants and polyoxometalates (POMs). X-ray scattering and polarized light microscopy demonstrate that these hybrid materials, highly loaded with POMs (up to 18 wt %), are nanocomposites of liquid-crystalline lamellar structure (Lalpha), with viscoelastic properties close to those of gels. The interpretation of X-ray scattering data strongly suggests that the POMs are located close to the terminal -OH groups of the nonionic surfactants, within the aqueous sublayers. Moreover, these materials exhibit a reversible photochromism associated to the photoreduction of the polyanion. The photoinduced mixed-valence behavior has been characterized through ESR and UV-visible-near-IR spectroscopies that demonstrate the presence of W(V) metal cations and of the characteristic intervalence charge transfer band in the near-IR region, respectively. These hybrid nanocomposites exhibit optical properties that may be useful for applications involving UV-light-sensitive coatings or liquid-crystal-based photochromic switches. From a more fundamental point of view, these hybrid materials should be very helpful models for the study of both the static and dynamic properties of nano-objects confined within soft lamellar structures.     

New Anderson-Based Polyoxometalate Covalent Organic Frameworks as Electrodes for Energy Storage Boosted Through Keto-Enol Tautomerization

The unique electrochemical properties of polyoxometalates (POMs) render them ideal components for the fabrication of next-generation high-performance energy storage systems. However, their practical applications have been hindered by their high solubility in common electrolytes. This problem can be overcome by the effective hybridization of POMs with other materials. Here we present the design and synthesis of two novel polyoxometalate-covalent organic frameworks (POCOF) via one-pot solvothermal strategy between an amino-functionalized Anderson-type POM and a trialdehyde-based building unit. We show that structural and functional complexity can be enriched by adding hydroxyl groups in the 2,4,6 position to the benzene-1,3,5-tricarbaldehyde allowing to exploit for the first time in POCOFs the keto-enol tautomerization as additional feature to impart greater chemical stability to the COFs and enhanced properties leading to large specific surface area (347 m²/g) and superior electrochemical performance of the POCOF-1 electrodes, when compared with POCOF-2 electrodes that possess only imine-type linkage and with pristine POM electrodes. Specifically, POCOF-1 electrodes display remarkable specific, areal, and volumetric capacitance (125 F/g, 248 mF/cm² and 41.9 mF/cm³, respectively) at a current density of 0.5 A/g, a maximum energy density (56.2 Wh/kg), a maximum power density (3.7 kW/kg) and an outstanding cyclability (90 % capacitance retention after 5000 cycles).     

Polyoxometalates-Based Semi-flexible Metal-Semiconductor Triboelectric Nanogenerators for Low Frequency and Small Amplitude Mechanical Energy Harvesting

The development of high-performance and low-cost durable triboelectric nanogenerators (TENGs) is essential for converting mechanical energy into electrical energy. Many organic polymer friction materials used widely have thermal stability problems, which makes TENGs with semiconductors as friction materials stand out. Here, we report a semi-flexible TENG based on metal and TiO₂ modified by polyoxometalates (POMs) as pure inorganic friction materials. Six different POMs are firstly selected to modify the friction materials of TENGs, and the output performance of TENGs with different POMs-modified semiconductors and different metals as friction materials are tested. Compared with the unmodified TENGs, the open-circuit voltage (V_OC) of the optimal Ag-K₆P₂Mo₁₈O₆₂(P₂Mo₁₈)/TiO₂ TENG device is increased by more than 4 times, which is mainly attributed to the strong electron-accepting and storage capabilities of POMs. This study has demonstrated that TENGs modified by POMs have potential application prospects and provided a new method for increasing the electrical output of TENGs.     

Beyond Charge Balance: Counter‐Cations in Polyoxometalate Chemistry

Polyoxometalates (POMs) are molecular metal‐oxide anions applied in energy conversion and storage, manipulation of biomolecules, catalysis, as well as materials design and assembly. Although often overlooked, the interplay of intrinsically anionic POMs with organic and inorganic cations is crucial to control POM self‐assembly, stabilization, solubility, and function. Beyond simple alkali metals and ammonium, chemically diverse cations including dendrimers, polyvalent metals, metal complexes, amphiphiles, and alkaloids allow tailoring properties for known applications, and those yet to be discovered. This review provides an overview of fundamental POM–cation interactions in solution, the resulting solid‐state compounds, and behavior and properties that emerge from these POM–cation interactions. We will explore how application‐inspired research has exploited cation‐controlled design to discover new POM materials, which in turn has led to the quest for fundamental understanding of POM–cation interactions.     

The use of nanoparticles in anti-microbial materials and their characterization

Anti-microbial materials have multiple applications in medicine, industry and commercial products. Recent research has proposed the use of nanoparticles in a range of materials, as some metal nanoparticles are known to possess antibacterial properties. The development of such materials presents both the chemist and the biologist with the challenge to effectively choose analytical methods that provide relevant information regarding these materials. Herein, we describe techniques for the characterization of the nanoparticle-doped materials and methods for the determination of their efficacy against biofilm formation.     

Self-Growing Organic Materials

The growth of living systems is ubiquitous. Living organisms can continually update their sizes, shapes, and properties to meet various environmental challenges. Such a capability is also demonstrated by emerging self-growing materials that can incorporate externally provided compounds to grow as living organisms. In this Minireview, we summarize these materials in terms of six aspects. First, we discuss their essential characteristics, then describe the strategies for enabling crosslinked organic materials to self-grow from nutrient solutions containing polymerizable compounds. The developed examples are grouped into five categories based on their molecular mechanisms. We then explain the mechanism of mass transport within polymer networks during growth, which is critical for controlling the shape and morphology of the grown products. Afterwards, simulation models built to explain the interesting phenomena observed in self-growing materials are discussed. The development of self-growing materials is accompanied by various applications, including tuning bulk properties, creating textured surfaces, growth-induced self-healing, 4D printing, self-growing implants, actuation, self-growing structural coloration, and others. These examples are then summed up. Finally, we discuss the opportunities brought by self-growing materials and their facing challenges.     

Optical spectroscopy following the incorporation of a rare-earth containing (Eu) polyoxometalate into a sol-gel derived media

Polyoxometalates (POMs) are an emerging class of materials which can be considered as inorganic complexes with distinct structural and optical characteristics. To be suitable in biomedical applications such as imaging, the materials may need to be embedded in a suitable host material, which may affect the optical properties of the emitting polyoxometalate. Here, we demonstrate that POMs can successfully be included into a sol-gel derived silica matrix. We report on the effects of one such potential host on the luminescence excitation and emission spectra, as well as the POM luminescence decay times. It appears that the POMs do not interact with the bulk oxide of the matrix, but are retained within the hosts' internal pore structure.     

Exploiting Interactions between Polyoxometalates and Proteins for Applications in (Bio)chemistry and Medicine

The specific interactions of anionic metal-oxo clusters, known as polyoxometalates (POMs), with proteins can be leveraged for a wide range of analytical and biomedical applications. For example, POMs have been developed as selective catalysts that can induce protein modifications and have also been shown to facilitate protein crystallization, both of which are instrumental in the structural characterization of proteins. POMs can also be used for selective protein separation and enzyme inhibition, which makes them promising therapeutic agents. Hence, understanding POM-protein interactions is essential for the development of POM-based materials and their implementation in several fields. In this Review we summarize in detail the key insights that have been gained so far on POM-protein interactions. Emphasis is also given to hybrid POMs functionalized with organic ligands to prompt further research in this direction owing to the promising recent results on tuning POM-protein interactions through POM functionalization.     

Strategies for Organizing Nanoparticles into Aggregate Structures and Functional Materials

In this review, we focus on some of the most successful approaches to date for organizing nanoparticles into macroscopic aggregates and functional materials. The preparation and resulting properties of two- and three-dimensional arrays of nanoparticles are detailed, and some potential uses for these materials are discussed. Although many types of nanoparticles can be organized into such structures, this review focuses specifically on Au and CdE (E=S, Se) nanoparticles. Gold nanoparticles are easily prepared, highly stable, well-studied, and excellent models for other metal colloids. CdE colloids are the most extensively studied semiconductor nanoparticles and hold much promise in the optoelectronics field.