Hybrid Assemblies of Eu-Containing Polyoxometalates and Hydrophilic Block Copolymers with Enhanced Emission in Aqueous Solution

Modular co-assembly of peptides and polyoxometalates into underwater adhesives with photoluminescence and adjustable adhesion

Supramolecular polymerization between cationic peptides and anionic polyoxometalates has emerged as a promising strategy for the creation of peptide-based biomimetic underwater adhesives. However, the extremely rigorous requirements for peptide design are an important obstacle to the fabrication of available peptide adhesives with controlled adhesion and versatile functionality. Inspired by marine sessile organisms in nature, here we reported a modular co-assembly method to easily produce peptide/polyoxometalate underwater adhesive materials through mixing two complementary cationic peptides (Pep1 and Pep2) with a single anionic polyoxometalate K6H[SiW9V3O40] in aqueous solution, which are not possible to be obtained from an individual peptide module. We demonstrated that the relatively hydrophobic Pep1 contributes to the bulk cohesion of the resulting adhesive, while the relatively hydrophilic Pep2 not only enables the interfacial adhesion but also regulates the bulk cohesion of the Pep1/Pep2/SiW9V3 adhesive. Rheological and shear adhesion tests showed that the macroscopic adhesion performance of the resulting adhesive materials could be conveniently adjusted by simply changing the molar ratio of the complementary peptide modules without any complicated peptide design. Interestingly, the luminescence properties of K11[Eu(PW11O39)2] (labelled as EuPW11) could be maintained within the Pep1/Pep2/EuPW11 adhesive even in a water environment. The lifetime of the Pep1/Pep2/EuPW11 adhesive was 2.19 ms. The fluorescence quantum yield of the Pep1/Pep2/EuPW11 adhesive was measured to be 27.46%. This study unveils that the modular co-assembly method can effectively simplify the material design of peptide/polyoxometalate underwater adhesives, which will significantly broaden the horizon of material pools and extend their availability space.

Double-hydrophilic block copolymer-metal ion associations: Structures, properties and applications

Hybrid polyionic complexes (HPICs), constructed from double-hydrophilic block copolymers and metal ions, have been largely developed with increasing interest in the past decade in the fields of catalysis, materials science and biological applications. The chemical natures of both blocks are very versatile, but one block should be able to interact with ions, and the second one should be neutral. Many metals have been used to form HPICs, which have, in their simplest architectural form, a core-shell structure of a few tens of nanometers in radius with an external shell made of the neutral block of the copolymer. In this review, we focus our discussion on the stability, shape, size and inner structure of these hybrid micelles. We then describe the most recent applications of HPICs, as reported in the literature, and point out the current challenges, missing structural information and future perspectives for this class of organized structures.

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.

Lanthanide Luminescence Supramolecular Switch Based on Photoreactive Ammonium Molybdate

Lanthanide supramolecular assemblies as photoswitches have attracted much attention in the fields of cellular imaging and light-emitting materials. However, the regulation of lanthanide luminescence behavior by redox of metal ions is rare. Herein, we constructed a lanthanide luminescence supramolecular switch, that is, a binary assembly constructed by mono-(6-ethylenediamine-6-deoxy)-β-cyclodextrin (ECD) and ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O, Mo₇), and further assembled into ternary assemblies with polyoxometalate Na₉[XW₁₀O₃₆]·32H₂O (X-POM, X = Eu and Dy), which was comprehensively characterized by UV-vis, fluorescence, NMR, Fourier transform infrared, dynamic light scattering, scanning electron microscopy, and ζ potential. Thanks to the oxygen-shielding effect of secondary supramolecular assembly, the photoreduction process of Mo₇ (VI) could occur rapidly and efficiently. Due to the high Förster resonance energy transfer (FRET) efficiency of X-POM and Mo₇ (V) in supramolecular assembly, the photoreduction process is accompanied by fluorescence quenching. In addition, the oxidation process of the Mo₇ (V) could be rapidly promoted by heating, which allowed the X-POM fluorescence to recover. Interestingly, ECD-mediated ternary supramolecular assemblies not only tune the lanthanide luminescence but also strongly increase the lanthanide luminescence behavior, leading to the emission of strong narrow red light at ⁵D₀-⁷F₄, which can be successfully applied to two-dimensional code anticounterfeiting. In this study, a new approach is provided for the construction of lanthanide luminescence supramolecular switches tuned by photoreactive polyoxometalate.

On Complex Coacervate Core Micelles: Structure-Function Perspectives

The co-assembly of ionic-neutral block copolymers with oppositely charged species produces nanometric colloidal complexes, known, among other names, as complex coacervates core micelles (C3Ms). C3Ms are of widespread interest in nanomedicine for controlled delivery and release, whilst research activity into other application areas, such as gelation, catalysis, nanoparticle synthesis, and sensing, is increasing. In this review, we discuss recent studies on the functional roles that C3Ms can fulfil in these and other fields, focusing on emerging structure-function relations and remaining knowledge gaps.

Cucurbit[7]uril-Mediated 2D Single-Layer Hybrid Frameworks Assembled by Tetraphenylethene and Polyoxometalate toward Modulation of the α-Chymotrypsin Activity

Construction of large-scale single-layer two-dimensional (2D) frameworks in water is significant due to their utilities in various fields. Utilizing macrocycle-mediated supramolecular self-assembly represents a promising approach; however, challenges still remain in their practical preparation. Here, we exploited a two-step supramolecular strategy to build 2D organic-inorganic hybrid frameworks at a micrometer scale in water. Taking advantage of the high binding affinity to cucurbit[7]uril (CB[7]), mono-quaternary ammonium tetraphenylethene (MQATPE) derivatives were first included with CB[7] to form a 1:1 complex (MQATPE@CB[7]). Then, just mixing the complex with anionic polyoxometalate Na₉[EuW₁₀O₃₆]·32H₂O (denoted as Eu-POM) in a 3:1 molar ratio leads to the formation of single-layer 2D films with tens of micrometers via electrostatic and π-π stacking interactions. The most unique feature of this strategy is that the steric effect imposed by CB[7] would not only lead the modules to adopt a periodic hexagonal assembly but also forbid stacking between layers through comparison with the merely multilayered 2D nanosheets self-assembled by MQATPE/Eu-POM. Interestingly, the charge interactions between MQATPE and Eu-POM would lead to the aggregation-induced emission (AIE) fluorescence of MQATPE, and white light emission could be obtained through the simple regulation of the contents of Eu-POM and MQATPE. Furthermore, due to the high surface areas and more accessible active sites, the single-layer films can act as an effective enzyme inhibitor to modulate the activity of α-chymotrypsin (ChT). These findings suggest a simple but universal approach for single-layer hybrid materials, which may hold promise for practical applications in photophysical and biomedical fields.

Ionic Self-Assembled Luminescent Nanospheres from Cationic Polyelectrolyte and Eu-Containing Polyoxometalate

Using the ionic self-assembly (ISA) strategy to combine Eu-containing polyoxometalates (Eu-POMs) and organic molecules mainly through noncovalent electrostatic interactions can protect Eu-POMs from solvent quenching of luminescence and enhance their processability. For this reason, a cationic polyelectrolyte, branched polyethyleneimine (PEI), and a Eu-POM, Na₉(EuW₁₀O₃₆)·32H₂O (EuW₁₀), were used here to construct luminescence-enhanced spherical aggregates with diameters ranging from 50 to 200 nm. At a fixed concentration of EuW₁₀, the phase behavior and luminescence properties of the mixture could be modulated by the PEI concentration. Such ISA-induced aggregates could effectively shield water molecules and result in better photophysical properties. Compared to bare EuW₁₀, the absolute quantum yield and lifetime of luminescence for aggregates increased 10 and 5 times, respectively. Meanwhile, the sensitivity of the EuW₁₀ coordination structure to the environment made it possible for obtained aggregates being used to detect either copper cations or permanganate anions due to their strong specific quenching effects to luminescence. Such a new type of luminescent soft material not only provided a reference for exploring the luminescence enhancement mechanism of lanthanide through self-assembly in aqueous solution but also exhibited potential in detection by luminescence analysis.

Recent advances in amphiphilic block copolymer templated mesoporous metal-based materials: assembly engineering and applications

Mesoporous metal-based materials (MMBMs) have received unprecedented attention in catalysis, sensing, and energy storage and conversion owing to their unique electronic structures, uniform mesopore size and high specific surface area. In the last decade, great progress has been made in the design and application of MMBMs; in particular, many novel assembly engineering methods and strategies based on amphiphilic block copolymers as structure-directing agents have also been developed for the "bottom-up" construction of a variety of MMBMs. Development of MMBMs is therefore of significant importance from both academic and practical points of view. In this review, we provide a systematic elaboration of the molecular assembly methods and strategies for MMBMs, such as tuning the driving force between amphiphilic block copolymers and various precursors (i.e., metal salts, nanoparticles/clusters and polyoxometalates) for pore characteristics and physicochemical properties. The structure-performance relationship of MMBMs (e.g., pore size, surface area, crystallinity and crystal structure) based on various spectroscopy analysis techniques and density functional theory (DFT) calculation is discussed and the influence of the surface/interfacial properties of MMBMs (e.g., active surfaces, heterojunctions, binding sites and acid-base properties) in various applications is also included. The prospect of accurately designing functional mesoporous materials and future research directions in the field of MMBMs is pointed out in this review, and it will open a new avenue for the inorganic-organic assembly in various fields.

Recent advances of polyoxometalates in multi-functional imaging and photothermal therapy

The recent advances of polyoxometalate clusters in terms of near infrared photothermal properties for targeted tumor therapy have been summarized while the combined applications with various bio-imaging techniques and chemotherapies are reviewed.

Dual stimuli-responsive lanthanide-based phosphors for an advanced full-color anti-counterfeiting system

Dual stimuli-responsive lanthanide (Ln)-based phosphors were prepared for constructing a multi-level full color anti-counterfeiting system. The encrypted information can be visually read out under UV irradiation. Further triggered by water, not only are the fluorescence colors changed, but so are the patterns of the encrypted information. The Ln-based phosphors can be cost-effectively prepared in a simple way, presenting great potential application in constructing an advanced anti-counterfeiting system with a convenient authentication assay.

Dual stimuli-responsive lanthanide-based phosphors were prepared for constructing a multilevel full color anti-counterfeiting system, augmenting the security level for information protection.

Nanocomposites of ionic copolymer integrating Gd-containing polyoxometalate as a multiple platform for enhanced MRI and pH-response chemotherapy

Nanocomposites from the co-assemblies of block copolymers and a gadolinium-grafting inorganic cluster were constructed as a multifunctional platform for MRI enhancement, drug loading, and environment-response release at local positions.

Twining Poly(polyoxometalate) Chains into Nanoropes

Organic polymers and inorganic clusters belong to two different disciplines and have completely different properties and structures. When a cluster is attached to the backbone of a polymer as a pendant, the resultant hybrid polymers (polyclusters) exhibit unique behaviours totally different from those of conventional polymers owing to the nanoscale size of the cluster and its particular interactions. Herein, the aggregation of a poly(polyoxometalate)-a polynorbornene backbone with inorganic polyoxometalate cluster pendants-upon addition of a non-solvent to its dilute solution is reported. A three-dimensional network of tangled and snake-like nanothreads was observed. Direct visualisation of individual nanoscale clusters enabled identification of single chains within the nanothreads. These observations suggest that during the process of aggregation, the hybrid polymer forms curved or extended chains as a consequence of an armouring effect in which the collapsed cluster pendants wrap around the backbone. The collapse occurs because they become less soluble in the solvent/non-solvent mixture. The extended chains then become entwined and form nanoropes consisting of multiple chains wound around each other. This study provides a deeper understanding of the nature of polyclusters and should also prove useful for their future development and application.

Modulation of thermodynamic and kinetic inverted phase behavior of block copolymers by inorganic polyoxometalates

The Keggin polyoxometalates (POM) H₃PW₁₂O₄₀ (PW) electrostatically complexed with poly(styrene-block-2-vinyl pyridine) (PS-b-P2VP) in DMF, and ordered microphase separation occurred through solvent evaporation. The phase behaviors of PS-b-P2VP/PW in bulk were systematically investigated by using SAXS and TEM to discover the effect of POM content and molecular weight of the block copolymers. Computational simulation was also performed to reveal the same phase transition sequence as the experimental results. As the POM content increases, the PS-b-P2VP/PW complex with a low molecular weight changed from lamellar phase (LAM) to hexagonal cylindrical phase (HEX), and finally transited into spherical phase (SPH). Unexpectedly, PS-b-P2VP/PW complexes with a high molecular weight were inclined to form a kinetic-trapped intermediate phase (inverted HEX). The mechanism of formation of inverted phases was proposed based on simulation that asymmetric swelling in the concentrated DMF solution would result in the ultimate kinetic-trapped nanostructure in the bulk.

Multiscale Self-Assembly of Mobile-Ligand Molecular Nanoparticles for Hierarchical Nanocomposites

Multiscale hierarchical morphologies are greatly desired for fabricating nanocomposites with tunable macroscopic properties, but challenges remain in precisely manipulating the spatial arrangement of nanoparticles in polymer matrices across multiple length scales. Here, we demonstrate a class of mobile-ligand nanoparticle system built upon 1 nm anionic polyoxometalate molecular nanoparticles and cationic terminated polymer chains by electrostatic interaction. The highly rearrangeable polymer chains can serve as mobile ligands to direct the polyoxometalates to align into sub-10 nm anisotropic superlattice-like nanoarrays in the bulk state. Moreover, these nanoarrays can further serve as structural units to assemble into hierarchically ordered morphologies in polymer matrices, e.g., percolated networks over hundreds of micrometers which are comprised of cylindrically packed polyoxometalate superlattices down to sub-10 nm scale. These hierarchical morphologies enable the nanocomposites with reinforced mechanical performance. The presented mobile-ligand approach can provide a paradigm to design functional polymer nanocomposites with improved properties such as mechanical reinforcement and collective optical and electronic functions.

Zwitterionic Surfactant Micelle-Directed Self-Assembly of Eu-Containing Polyoxometalate into Organized Nanobelts with Improved Emission and pH Responsiveness

Recently, hybrid coassembly between polyoxometalates (POMs) and cationic building blocks provides an efficient strategy to greatly optimize POMs' functionality as well as their aggregate structural diversity. Adaptive hybrid supramolecular materials with enhanced luminescence have then been obtained from lanthanide-containing POMs. In this work, a commercially available and pH-switchable zwitterionic surfactant, tetradecyldimethylamine oxide (C₁₄DMAO), was chosen to coassemble with a lanthanide-containing anionic POM [Na₉(EuW₁₀O₃₆)·32H₂O, abbreviated as EuW₁₀] in water. The much improved red-emitting luminescent nanobelts at a C₁₄DMAO/EuW₁₀ molar ratio ( R) of 20 were obtained, which exhibited longer luminescence lifetime and higher quantum yield compared with EuW₁₀ aqueous solution. After careful characterization of morphology and structure of nanobelts, an unusual axial lamellar aggregation arrangement mechanism was proposed. It was the partial protonation of C₁₄DMAO at the solution pH of about 6.5 that led to positively charged micelles, being bridged by anionic EuW₁₀ clusters to aggregate into such novel nanobelts under the synergetic effects of appropriate electrostatic, hydrogen-bonding, and hydrophobic interactions. The resulted pH-responsive luminescent nanobelts and their aggregation model should offer attractive references for preparing smart optical supramolecular materials.

Guest Controlled Pillar[5]arene and Polyoxometalate Based Two-Dimensional Nanostructures toward Reversible Iodine Capture

The two-dimensional (2D) nanostructures comprised of polyoxometalate based building blocks are of great value in nanoarchitectures, which have unique properties and widespread potential applications, but it is still challenging in mature preparation. Herein a new strategy to build Cr(III) centered Anderson type polyoxometalate 2D nanostructures based on the modulation of host-guest interaction between cationic pillar[5]arenes and sodium dodecyl sulfonate (SDS) in aqueous media was exploited in this work. Through regulating stoichiometry of SDS, the morphology of assemblies vary from nanobones to 2D nanosheets. The fine assembled structure was discovered by combined ¹H NMR, SAXS, and element analyses. The nanomaterials can be used as adsorbents for I₂ in various solutions, including n-hexane, cyclohexane, water, and chloroform, where the polyoxometalates play a key role in the effective adsorption of iodine since they can expand the interspace between pillar[5]arenes in the as-prepared nanostructure. Furthermore, such adsorbents are easily regenerated and reused as iodine can be released spontaneously from nanobones@I₂ and nanosheets@I₂ solids when being immersed in dimethyl sulfoxide.

Co-assemblies of polyoxometalate {Mo72Fe30}/double-tailed magnetic-surfactant for magnetic-driven anchorage and enrichment of protein

Covalent grafting and electrostatic-driven assembly have been two strategies in constructing well-defined polyoxometalate (POM) assemblies to produce specific morphologies and desirable properties. The modification of anionic counter-ions of amphiphilic compounds in POM-surfactant hybrid systems is still unexploited. Herein, we report the co-assembly of a synthetic double-tailed magnetic surfactant (MagSurf), (C₁₈)₂C₂N⁺[FeCl₄]^-, and POM, {Mo₇₂Fe₃₀}. The magnetic aggregate (POM/MagSurf) results from the building up hierarchical structures at a time-dependent interface. In this construct, both the MagSurfs and {Mo₇₂Fe₃₀} POMs contribute to and mutually strengthen the magnetization of the designed magnetic assembles. Interestingly, the POM/MagSurf aggregates are compatible with aqueous mixtures and successfully employed to serve as magnetic transporting vehicles to anchor and deliver a protein molecule, myoglobin (Mb). Upon applying a magnetic field (0.3 T), the magnetic aggregates induced a directional migration and enrichment of the Mb protein (71-90%). During this process, the protein/POM/MagSurf complexes exhibited strong interactions facilitating stable anchoring and efficient enrichment of the Mb.

ZnII Complexes for Bioimaging and Correlated Applications

Zinc is a biocompatible element that exists as the second most abundant transition metal ion and an indispensable trace element in the human body. Compared to traditional metal-organic complexes systems, d¹⁰ metal Zn^II complexes not only exhibit a large Stokes shift and good photon stability but also possess strong emission and low cytotoxicity with a relatively small molecular weight. The use of Zn^II complexes has emerged in the last decade as a versatile and convenient tool for numerous biological applications, including bioimaging, molecular and protein recognition, as well as photodynamic therapy. Herein, we review recent developments involving Zn^II metal complexes applied as specific subcellular compartment imaging probes and their correlated utilizations.

Highly luminescent and multi-sensing aggregates co-assembled from Eu-containing polyoxometalate and an enzyme-responsive surfactant in water

Hybrid co-assembly of polyoxometalates (POMs) with cationic organic matrices offers a preferable way to greatly enhance POM functionality as well as processability. Thus, multi-stimulus responsive supramolecular materials based on lanthanide-containing POMs with improved luminescence may be fabricated from appropriate components through this convenient strategy. Herein, we reported that the co-assembly of Na₉(EuW₁₀O₃₆)·32H₂O (EuW₁₀) and a commercially available cationic surfactant, myristoylcholine chloride (Myr), in water could produce enhanced red-emitting luminescent aggregates, with their photophysical properties highly dependent on the molar ratio (R) between Myr and EuW₁₀. The R of 36 was finally selected owing to the displayed superior luminescence intensity and good aggregate stability. The Myr/EuW₁₀ hybrids induced by electrostatic and hydrophobic forces presented practically as multilamellar spheres with diameters varying from 80 to 300 nm. Compared to an aqueous solution of EuW₁₀ nanoclusters, a 12-fold increase in absolute luminescence quantum yield (∼23.3%) was observed for the hybrid spheres, which was ascribed to the efficient shielding of water molecules. An unusual aggregation arrangement mechanism and the excellent photophysical properties of these aggregates were thoroughly investigated. Both the enzyme substrate character of Myr and the sensitive coordination structure of EuW₁₀ to the surrounding environment made Myr/EuW₁₀ aggregates exhibit multi-stimulus responsiveness to enzymes, pH, and transition metal ions, thus providing potential applications in fluorescence sensing, targeted-release, and optoelectronics.

Surface plasmon resonance-assisted circularly polarized luminescent hybrid assemblies of Eu-containing polyoxometalates

Circularly polarized luminescence of achiral Eu-containing polyoxometalates was induced by chiral cationic polymers and enhanced by Ag nanoparticles.

Controlled introduction of metal cations into polymerizable ionic liquid-polyoxomolybdate hybrid crystals

The first syntheses of polyoxomolybdate hybrid crystals were achieved by using polymerizable ionic-liquid.

Self-Assembly of Europium-Containing Polyoxometalates/Tetra-n-alkyl Ammonium with Enhanced Emission for Cu2+ Detection

Lanthanide-containing polyoxometalates (POMs) can be used to detect various materials, but their luminescence in water has suffered enormous limitations due to the strong fluorescence quenching. Herein, to resolve this problem, three-dimensional nanoparticles built by mixed Weakley-type europium-containing POMs (Na9[EuW10O36]·32H2O, abbreviated to EuW10) and tetra-n-alkyl ammonium (TA) with enhanced fluorescent properties have been designed in aqueous solution using an ionic self-assembly (ISA) technique, which is mainly driven by the electrostatic interaction between EuW10 and TA. The morphology and fluorescent properties of the system as well as some influencing factors (alkyl chain length, amino group, and inorganic salt concentration) were systematically investigated. The results indicated that the fluorescent intensity of EuW10/tetramethylammonium bromide (TMAB) composite increased about 14 times, whereas the extent of increase of fluorescence for EuW10/tetraethylammonium bromide (TEAB) and EuW10/tetrabutylammonium bromide (TMAB) composites gradually decrease due to the bulkier steric hindrance of the longer alkyl chain. Besides, the luminescence of EuW10/TMAB nanoparticles is pH responsive, and the reversibility of their structures and luminescence can be realized upon the addition of NaOH/HCl. Moreover, the EuW10/TMAB system also shows great fluorescence-sensing behavior, which could detect Cu2+ with a detection limit of 0.15 μM. Our work provides a facile construction strategy for a functional fluorescent complex via POMs-based supramolecular self-assembly in aqueous solution, which will be further used in biomarkers and sensors.

Flexible and enhanced multicolor-emitting films co-assembled by lanthanide complexes and a polymerizable surfactant in aqueous solution

Lanthanide complex doped lyotropic liquid crystals (LLCs) are soft materials which are impressive due to their excellent luminescence efficiencies and stabilities. The introduction of lanthanide complexes into polymerizable LLCs, however, may produce organized films with better optical and mechanical properties through in situ photopolymerization. An environmentally friendly strategy to fabricate flexible multicolor-emitting films has been developed through co-assembling red-/green-emitting trisdipicolinate lanthanide complexes [choline]3[Ln(DPA)3] (Ln-DPA, Ln = Eu, Tb) into LLC matrices mainly via electrostatic interaction and further photopolymerization. The LLCs were constructed from a polymerizable surfactant, 3-dodecyl-1-vinylimidazolium bromide (C12VIMBr), in aqueous solution. The maintenance of the well-defined LLC nanostructures in the luminescent films was validated by small-angle X-ray scattering (SAXS) as well as scanning electron microscopy (SEM) measurements. Remarkably, the lifetime and absolute luminescence quantum efficiency of such films have been improved significantly compared with those of the corresponding solid Eu-DPA complex or in aqueous solution and LLC matrices. Through tuning the molar ratio of Eu-DPA to Tb-DPA complexes, the emission color of the films could be finely-tailored between red and green in the CIE chromaticity diagram. Furthermore, the films also possessed certain mechanical strength and stability against pH, metal ions, and temperature, indicating their potential application as robust luminescent materials.

Luminescent, stabilized and environmentally friendly [EuW10O36]9--Chitosan films for sensitive detection of hydrogen peroxide

Fabrications and applications of luminescent films have been an interesting and important challenge within the realm of academia and industry. Herein, a novel fluorescence-based strategy for the H₂O₂ detection has been developed by fabrication of stabilized, thin, transparent, and luminescent films composed of europium-containing polyoxometalates (Eu-POM) and environmentally friendly chitosan (CS) via a facile solution casting approach. In comparison with pure Eu-POM, enhanced fluorescent properties are obtained from the as-prepared Eu-POM/CS films in terms of prolonged fluorescence lifetime and a remarkable fluorescent quenching effect in the presence of hydrogen peroxide (H₂O₂). The fluorescence intensity of Eu-POM/CS films exhibits a linear correlation in response to the H₂O₂ concentration over a wide range of 1.1-66 μM, with a detection limit of 0.11 μM. Furthermore, the fluorescent films display a high detection selectivity which are capable of differentiating hydrogen peroxide (H₂O₂) from the interfering species, such as sugars, l-amino acids, and other metabolites. All these advances towards the development of Eu-POM/CS films open up new applications for luminescent films, but most importantly, they can help in the far-reaching technological implementations of a simple, cost-effective method for the detection of H₂O₂ in many fields.

Realizing enhanced luminescence of silver nanocluster-peptide soft hydrogels by PEI reinforcement

Metal nanoclusters (NCs) are a new type of fluorescent nanomaterial composed of several to several tens of metal ions or atoms with a wide range of applications in the fields of catalysis, optics, and biomedicine. However, fluorescence quenching when existing as individuals in aqueous solutions greatly limits their applications. In this study, six-core Ag(i) NCs (Ag₆-NCs) were interacted with peptides (DD-5) in water to form soft hydrogels with the aggregation-induced emission (AIE) of Ag₆-NCs. The introduction of polyethyleneimine (PEI) into the Ag₆-NCs/DD-5 hydrogel succeeded in further enhancing the fluorescence intensity. This dual-AIE behavior of the Ag₆-NCs/DD-5/PEI hydrogels is mainly ascribed to the strong hydrogen bonding among the carboxyl groups of Ag₆, those of DD-5 and the amino groups of PEI, which effectively restricted intramolecular vibration of the capping ligands on the Ag₆-NCs. Moreover, the addition of PEI can effectively promote the gelation speed of Ag₆-NCs/DD-5 and act as a physical cross-linker, leading to an increase of the mechanical strength of the hydrogel. This work opens a new pathway for the fabrication of smart composite materials with multiple functions, which show a variety of applications such as chemical/biosensing and bioimaging.

Quench ionic flash nano precipitation as a simple and tunable approach to decouple growth and functionalization for the one-step synthesis of functional LnPO4-based nanoparticles in water

A novel, one-step method for the synthesis of functional, organic-inorganic hybrid nanoparticles is reported. The quench ionic Flash NanoPrecipitation (qiFNP) method enables the straightforward synthesis of nanoparticles by decoupling the formation of the inorganic core and surface functionalization. As a proof-of-concept, the qiFNP method was successfully applied for the tunable and highly controlled synthesis of various LnPO4-based nanomaterials for bioimaging applications.

Fabrication of Biocompatible, Luminescent Supramolecular Structures and Their Applications in the Detection of Dopamine

Supramolecular materials assembled by amide-functionalized surface active ionic liquid, N-dodecyl- N'-acetamido imidazolium bromide ([C₁₂ImCONH₂]Br), and europium-containing polyoxometalates (Eu-POM) were fabricated in aqueous solution by a one-step method via ionic self-assembly strategy. The [C₁₂ImCONH₂]Br/Eu-POM supramolecular structures exhibit favorable fluorescence properties and represent a 15-fold increase in quantum yield (∼13.68%) compared to Eu-POM. Besides, more fluorescence was quenched obviously with the increasing concentration of dopamine (DA) (within the range of 0-100 μM), based on which DA monitoring could be achieved. The detection limit was identified to be 0.1 μM. The supramolecular nanoparticles are highly specific for the detection of DA. In addition, the hybrid assemblies display not only low cytotoxicity but also excellent biocompatibility to MC3T3-E1 cells. As a result, as-prepared supramolecular materials with these superior properties show the promising application in some fields such as biochemistry and biomedical science.

Morphology Evolution of Stimuli-Responsive Triblock Copolymer Modulated by Polyoxometalates

Polyoxometalate (POM) H₃PMo₁₂O₄₀ was coassembled with stimuli-responsive triblock copolymer poly(ethylene oxide)- block-polystyrene- block-poly(2-(dimethylamino)ethyl methacrylate) (PEO- b-PS- b-PDMAEMA) by electrostatic interactions. Depending on the POM contents, the hybrid complexes can self-assemble into a series of morphologies: micelles, rods, toroids, and vesicles. Unlike traditional morphology transition of amphiphilic block copolymer derived from a broad range of hydrophobic volume fractions, POM-induced morphology transitions just occurred in a narrow range of volume fractions. The length of rod micelles exponentially decreased with solvent compositions (tetrahydrofuran/H₂O). The hybrid assemblies showed acid-base responsibility due to the PDMAEMA block. Rod micelles could further assemble and disassemble reversibly upon adding acid/base. Fluorescent polyoxometalate Na₉EuW₁₀O₃₆ was also complexed with PEO- b-PS- b-PDMAEMA to prepare fluorescent vesicles. The vesicles showed off-on switchable fluorescence behavior accompanied with reversible vesicle-to-micelle transformation in response to pH stimuli.

Coassembly of Mixed Weakley-Type Polyoxometalates to Novel Nanoflowers with Tunable Fluorescence for the Detection of Toluene

In this work, three-dimensional nanoflowers with tunable fluorescent properties constructed with mixed Weakley-type polyoxometalates (POMs, Na₉[LnW₁₀O₃₆]·32H₂O, Ln = Eu, Tb, abbreviated to LnW₁₀) and tetraethylenepentamine (TEPA) have been successfully prepared through a facile ionic self-assembly (ISA) method. The shape and petal size of the nanoflower as well as its fluorescent behaviors can be tuned through varying the ratio of EuW₁₀/TbW₁₀. The varied-temperature emission behaviors at 80-260 K show that the fluorescent intensity of both Tb³⁺ and Eu³⁺ decreased with the increase in temperature, which makes them potential luminescent ratiometric thermometers. Moreover, after being mixed with polydimethylsiloxane (PDMS), the as-formed hybrid films showed stable fluorescence along with good transparency. The robustness of the hybrid films was also demonstrated by corrosion resistance upon treatment with strong acid and alkali and thus can be used as a sensor to detect toluene circularly. Our results provide a new avenue to the facile construction of fluorescent composites and demonstrate that the POM complexes can be further used in supramolecular chemistry and nanomaterials.

Nano-assemblies of cationic mPEG brush block copolymers with gadolinium polyoxotungstate [Gd(W5O18)2]9- form stable, high relaxivity MRI contrast agents

Polyoxometalates (POMs) incorporating paramagnetic ions, such as gadolinium, show promise as contrast agents for application in magnetic resonance imaging (MRI). Specifically, [Gd(W5O18)2]9- (denoted as GdWO) has been reported to have a higher relaxivity than commercially available contrast agents, but it's clinical utility has been limited by the intrinsic instability of POMs at physiological pH (7.4). In the current report we present a stability study on neat GdWO and nano-assemblies of block copolymers with GdWO in the pH range 5.0-7.4 to assess their suitability as MRI contrast agents. Neat GdWO only maintained structural stability between pH 5.4 and 6.4, and demonstrated poor MRI contrast at pH 7.4. To address this pH instability, GdWO was self-assembled with cationic mPEG brush block copolymers containing 20 or 40 units derived from the cationic monomer, 2-dimethylaminoethyl methacrylate (DMAEMA). Nano-assemblies with different charge ratios were synthesised and characterised according to their size, stability, contrasting properties and toxicity. The longitudinal relaxivity (r1) of the nano-assemblies was found to be dependent on the charge ratio, but not on the length of the cationic polymer block. Further investigation of PDMAEMA20 nano-assemblies demonstrated that they were stable over the pH range 5.0-7.4, exhibiting a higher r1 than either neat GdWO (2.77 s-1 mM-1) or clinical MRI contrast agent Gd-DTPA (4.1 s-1 mM-1) at pH 7.4. Importantly, the nano-assembly with the lowest charge ratio (0.2), showed the highest r1 (12.1 s-1 mM-1) whilst, stabilising GdWO over the pH range studied, eliciting low toxicity with MDA-MB231 cells.

Flexible and transparent films consisting of lanthanide complexes for ratiometric luminescence thermometry

Herein, a flexible and transparent film consisting Eu³⁺/Tb³⁺ lanthanide complexes and poly(methylmethacrylate) was constructed via solution casting method, and further developed as a ratiometric luminescent thermometer with an excellent linear response to temperature variation from 77 to 297 K. The thermometer displays higher photo- and thermostability than corresponding pure complexs. Based on that the emission intensity ratio of ⁵D₄ → ⁷F₅ transition (Tb³⁺) to ⁵D₀ → ⁷F₂ transition (Eu³⁺) can be linearly related to the temperature, the resulting thermometer is not only more reliable than single Eu³⁺(or Tb³⁺) material based on one emission, and but also has higher sensitivity than other types of luminescent thermometers. This work highlights the practical applications of luminescent films in temperature-sensing fields.

pH-Responsive Nanovesicles with Enhanced Emission Co-Assembled by Ag(I) Nanoclusters and Polyethyleneimine as a Superior Sensor for Al3

Metal nanoclusters (NCs) have been engineered as a new kind of luminescent material, whereas the application of metal NCs in aqueous solution was subjected to great limitations owing to their poor solubility, stability, and strong luminescence quenching in a single-molecule state. Herein, facile supramolecular self-assembly strategy was carried out to enhance the luminescence of Ag(I) NCs (Ag₆-NCs) through multiple electrostatic interactions with polyethyleneimine (PEI). Functional colloid aggregates of Ag₆-NCs such as nanospheres and nanovesicles were formed along with the enhanced emission because of the formation of compact-ordered self-assemblies, which effectively restricted intramolecular vibration of the capping ligands on Ag₆-NCs to diminish the nonradiative decay. All those could block energy loss and facilitated the radiative relaxation of excited states which ultimately induced an aggregation-induced emission (AIE) phenomenon. Furthermore, the luminescent Ag₆-NCs/PEI nanovesicles are pH-responsive and show a superior fluorescent sensing behavior for the detection of Al³⁺ with a limit of detection low to 3 μM. This is the first report about AIE of silver NCs with polymers in aqueous solution. This work sheds light on the controlled NCs-based supramolecular self-assembly and the NCs-based functional materials, which will be well-established candidates in controllable drug delivery, biomarkers, and sensors in aqueous solution.

Nanoparticle encapsulation in vesicles formed by amphiphilic diblock copolymers

We investigated the co-assembly of nanoparticles P and amphiphilic diblock copolymers AB in selective solvents using a dissipative particle dynamics (DPD) method. By controlling the nanoparticle concentration and the interaction parameter between the hydrophobic blocks and the solvents, we found that the aggregation morphology can be changed from rod-like micelles to disk-like micelles and further to vesicles. The ratio of the hydrophobic/hydrophilic block and the nanoparticle concentration largely affects the structural characteristics of vesicles and the dispersion of nanoparticles. Copolymers with a longer hydrophobic block length are more likely to form vesicles with a smaller aqueous cavity size and vesicle size as well as a thicker wall. At the same time, the nanoparticles in the hydrophobic membrane tend to locate closer to the center of the vesicle and they become more compactly organized. A significant discovery has found that the larger the nanoparticle concentration, the smaller the aqueous cavity and the larger the vesicle size. We can also locate the nanoparticles at the center of spherical micelles or the hydrophobic membranes of vesicles by varying the nanoparticle concentration. This provides an effective and simple method to prepare size-controlled vesicles containing nanoparticles, project the localization of nanoparticles within the vesicles, and even tune the distance between the nanoparticles.

Solvent Dielectricity-Modulated Helical Assembly and Morphologic Transformation of Achiral Surfactant-Inorganic Cluster Ionic Complexes

Ionic complexes comprising single/double chain cationic surfactant and Lindqvist-type polyoxomolybdate anionic cluster were used for controlled self-assembly in organic solutions. In the solvent with low dielectric constant the complexes self-assembled into flat ribbon like lamellar aggregations with an inverse bilayer substructure where the cluster located at the middle. Under the condition of increased dielectric constant, the solvent triggered the formation of helical self-assemblies, which finally transformed from helical ribbons to the flower-like assemblies due to the bilayer becoming excessively twisted. The self-assembled morphology and the substructure were characterized by SEM, TEM, and XRD. The solvent dielectricity-controlled morphologic transformations modulated by the variation of electrostatic interactions between organic cations and inorganic polyanions were demonstrated by ¹H NMR and IR spectra. The strategy in this work represents an effective route in targeting the chirality-directed functionalization of inorganic clusters by combining controllable and helical assemblies of achiral polyoxometalate complexes in one system.

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.