Eu-containing Polyoxometalate Research Articles

Eu-containing polyoxometalate composed of mixed trivacant Keggin fragments: Synthesis, structure, and catalysis for Knoevenagel condensation

A novel Eu-containing antimono-seleno-tungstate [H2N(CH3)2]8Na8Cs4H9[Eu2SeW4O11(OH)(H2O)4(SbW9O33) (SeW9O33)(Se1/2Sb1/2W9O33)]2·32H2O (1) has been synthesized under the aqueous solution conditions. The polyoxoanion is a hexameric architecture constituted by mixed trivacant Keggin fragments, namely 2 {SbW9}, 2 {SeW9} and 2 {Se1/2Sb1/2W9}, enclosing a heterometallic central Eu4Se2W8O22(OH)2(H2O)8 cluster core. Compound 1 was characterized by single-crystal X-ray diffraction, powder X-ray diffraction, IR spectrometry and thermogravimetric analysis. Furthermore, catalytic study shows that compound 1 can efficiently accelerate the Knoevenagel condensation reactions of various aldehydes with malononitrile under mild reaction conditions.

Adaptive Responses of a Peroxidase-like Polyoxometalate-Based Tri-Assembly to Bacterial Microenvironment (BME) Significantly Improved the Anti-Bacterial Effects.

The present study presents the tertiary assembly of a POM, peptide, and biogenic amine, which is a concept to construct new hybrid bio-inorganic materials for antibacterial applications and will help to promote the development of antivirus agents in the future. To achieve this, a Eu-containing polyoxometalate (EuW10) was first co-assembled with a biogenic amine of spermine (Spm), which improved both the luminescence and antibacterial effect of EuW10. Further introduction of a basic peptide from HPV E6, GL-22, induced more extensive enhancements, both of them being attributed to the cooperation and synergistic effects between the constituents, particularly the adaptive responses of assembly to the bacterial microenvironment (BME). Further intrinsic mechanism investigations revealed in detail that the encapsulation of EuW10 in Spm and further GL-22 enhanced the uptake abilities of EuW10 in bacteria, which further improved the ROS generation in BME via the abundant H2O2 involved there and significantly promoted the antibacterial effects.

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.

Photoluminescent polymer hydrogels with stimuli-responsiveness constructed from Eu-containing polyoxometalate and imidazolium zwitterions.

Inorganic-organic co-assembly of anionic polyoxometalates (POMs) with zwitterions provides a facile way to fabricate functional soft materials. In this paper, a translucent, photoluminescent polymer hydrogel was fabricated from Weakley-type POM Na9EuW10O36 (EuW10) and polymerizable imidazole-type zwitterion 3-(1-vinyl-3-imidazolio)propanesulfonate (VIPS) via a one-step synthesis method. Detailed characterization indicated that the polymerization of double bonds in VIPS and electrostatic interactions between EuW10 and VIPS play important roles in the formation of the hydrogels. Additionally, the introduction of non-polymerizable zwitterions 3-(1-methyl-3-imidazolio)propanesulfonate (MIPS) or 3-(1-decyl-3-imidazolio)propanesulfonate (C10IPS) can improve the mechanical and luminous performances of the hydrogels. Especially, C10IPS with a long alkyl chain would more significantly alter the coordination environment of EuW10, and consequently resulted in a more efficient energy transfer process. Further investigations revealed that the chemical environment around the Eu3+ can be highly influenced by organic solvents with stronger coordination abilities than water molecules, such as acetone. The translucency and luminescence intensity of the hydrogels can be reversibly transformed after alternately immersing in acetone or H2O for several minutes. Our results provided a useful strategy for the fabrication of luminescent hydrogels by regulating the noncovalent interactions between POMs and zwitterions.

A Luminescent Thermometer Based on Linearly Thermo-responsive Copolymer and Polyoxometalates

A novel switchable luminescent thermometer based on thermo-responsive triblock copolymer poly(ethylene glycol)-b-poly(acrylamide-co-acrylonitrile-co-dimethylaminoethylmethacrylate) (PEO113-b-P(AAm264-co-AN112-co-DMA8)) and Eu-containing polyoxometalate (Eu-POM) was successfully constructed. The copolymer synthesized by RAFT exhibited a linear response to temperature variations in aqueous media, which was quite different from the uncharged copolymer P(AAm-co-AN) having a specific upper critical solution temperature (UCST). Eu-POM was surrounded around thermo-responsive blocks through electrostatic interactions, and its luminescence could be finely tuned due to the sensitivity of copolymer to the temperature variation. In cold water, POMs were trapped in highly hydrophobic cores, exhibiting an intense emission. With the upraising of temperature, the emission intensity presented a gradual decrease and showed a linear correlation with temperature. When the complex solution cooled down, the luminescence could also be perfectly restored. This temperature-luminescence correlation could be held for numerous trials, showing a potential application in thermometer.

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.

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.

Study of Bi-directional detection for ascorbic acid and sodium nitrite based on Eu-containing luminescent polyoxometalate

Eu-containing polyoxometalate K13Eu(SiMoW10O39)2·28H2O (Eu-SiMoW) owns the stimu-chromic and photoluminescence properties. An ingenious test of ascorbic acid (AA) and sodium nitrite (NaNO2) was carried out based on the dual properties of Eu-SiMoW in solutions. First, the redox reaction of Eu-SiMoW and AA generated the blue reduced Eu-SiMoW, accompanied by fluorescence quenching; then the redox reaction of the reduced Eu-SiMoW and NaNO2 made Eu-SiMoW back to its original pale yellow state with red luminescence. Accordingly, the content of AA and NaNO2 could be measured by the reversible change of color and luminescence of Eu-SiMoW. This bi-directional detection method is first discovered and proven to be a simple and effective method for the detection of AA and NaNO2. The proposed method exhibited a linear response range (LRR) from 0.1 to 0.9 mmol L−1 with a limit of detection (LOD) of 0.53 µmol L−1 in UV–vis spectra and 4.67 µmol L−1 in luminescence spectra for AA as well as a LRR from 0.05 to 0.4 mmol L−1 with a LOD of 1.16 µmol L−1 in UV–vis spectra and 5.39 µmol L−1 in luminescence spectra for NaNO2. Moreover, the fluorescence switching of Eu-SiMoW could be realized by reacting with reductant and oxidant through the redox reaction. The detection mechanism is considered as a fluorescence resonance energy transfer process between discolor component SiMoW and luminescence component Eu in Eu-SiMoW.

Self-healing hydrogel containing Eu-polyoxometalate as acid-base vapor modulated luminescent switch

The integration of self-healing ability into luminescent switches have attracted increasing interest in designing smart optical devices. In this paper, an acid-base vapor modulated luminescent switch of hydrogels based on Eu-containing polyoxometalate (EuPOM) and poly(2-acrylamido-2-methyl-1-propanesulfonicacid) (PAMPSA) was readily prepared, which can self-heal within only about few minutes under ambient atmosphere. Besides, the hydrogels can easily be shaped into any desired form by pouring into molds. These remarkable features enable the resulting hydrogel as a promising “smart” material with use in smart devices, sensors, etc.

Unusual Aggregation Arrangement of Eu-Containing Polyoxometalate Hybrid in a Protic Ionic Liquid with Improved Luminescence Property.

Hybridization of polyoxometalates (POMs) with cationic surfactants offers the opportunity to greatly improve their functionalities as well as processabilities. Here, a surfactant-encapsulated Eu-containing POM complex (SEP) was formed via electrostatic interaction between 1-octadecyl-3-methylimidazolium bromide (OB) and Na9(EuW10O36)·32H2O (EuW10). SEP was first self-assembled in a protic ionic liquid to prepare the soft aggregates to fundamentally avoid the fluorescence quenching by water molecules. The structures and photophysical properties of SEP or aggregates were investigated thoroughly by NMR and FTIR spectroscopy, optical and electron microscopy, small-angle X-ray scattering, and fluorescence measurements. The formed gel-like aggregates were found to compose of three-dimensional networks of microribbons with an interdigitated layered molecular packing of SEP, which was different from the usual inverse bilayer model of POM hybrids in common organic solvents. Compared to EuW10 solid or its aqueous solution, both SEP and its aggregates exhibited intense red luminescence with much improved lifetime and quantum efficiency. In addition, the soft aggregates exhibited an efficient energy transfer and an obviously enhanced monochromaticity, owning to the organized arrangement of EuW10 units and a confined microenvironment to isolate them from each other between adjacent layers. The obtained results will not only present a useful reference to the aggregation behavior of POM hybrids in ionic liquids, but also provide an easy way to design EuW10 luminescent soft materials based on the nonaqueous media.

Tunable Aggregation-Induced Emission of Polyoxometalates via Amino Acid-Directed Self-Assembly and Their Application in Detecting Dopamine

In this work, through the aqueous phase self-assembly of an Eu-containing polyoxometalate (POM), Na9[EuW10O36]·32H2O (EuW10) and different amino acids, we obtained spontaneously formed vesicles that showed luminescence enhancement for EuW10 and arginine (Arg), lysine (Lys), or histidine (His) complexes, but luminescence quenching for EuW10 and glutamic acid (Glu) or aspartic acid (Asp) complexes. The binding mechanisms between them have been explored at the molecular level by using different characterization techniques. It was found that EuW10 acted as polar head groups interact with the positively charged residues for alkaline amino acids, protonated amide groups for acidic amino and nonpolar acid aminos through electrostatic interactions, and the remaining segments of amino acids served as relatively hydrophobic parts aggregated together forming bilayer membrane structures. Moreover, the different influences of amino acids on the fluorescence property of EuW10 revealed that the electrostatic interaction between the positive charged group of amino acid and the polyanionic cluster dominates the fluorescence properties of assemblies. Furthermore, a turn-off sensing application of the EuW10/Arg platform to probe dopamine (DA) against various other biological molecules such as neurotransmitters or amino acids was also established. The concept of combining POMs with amino acids extends the research category of POM-based functional materials and devices.

A two-step binding process of Eu-containing polyoxometalates to bovine serum albumin.

Polyoxometalates (POMs) represent a type of typical polyanionic nanoclusters that can be utilized as inorganic bioactive materials; however, the detailed interactions of them with many target biomolecules such as peptides and proteins were not well clarified due to the complexity of the binding process. In the present study, the binding-induced physiochemical phenomena of a highly charged Eu-containing polyoxometalate, K13[Eu(SiW9Mo2O39)2] (EuSiWMo), with a model protein, bovine serum albumin (BSA), was identified upon the examination of luminescence of both the components during the titration. The large emission enhancement and subsequent quenching of the EuSiWMo were found in close relation to the amount of added BSA. Being different from the known binding type of less charged POMs, a distinct two-step binding process was concluded, and the possible mechanism was proposed through the analysis on the time-resolved fluorescence spectra, isothermal titration calorimetry (ITC), transmission electron microscope (TEM), and two-dimensional correlation spectroscopy (2D COS). The present results directed a new understanding for the charge numbers and existing state of POMs affecting the interaction with proteins, which is important to exploit the biological functionalities of POMs in related systems and the development of POMs as potential inorganic drugs.

Self-Assembly of an Europium-Containing Polyoxometalate and the Arginine/Lysine-Rich Peptides from Human Papillomavirus Capsid Protein L1 in Forming Luminescence-Enhanced Hybrid Nanoparticles

Through a self-assembly of arginine/lysine-rich peptide from human papillomavirus (HPV) capsid protein and an Eu-containing polyoxometalate (POM), Na9[EuW10O36]·32H2O (EuW10), the formation of well-defined hybrid nanospheres in aqueous solution is presented, showing large luminescence enhancement of POM and use as a potential “turn-on” fluorescence probe in biology. The binding mechanisms between them have been explored at the molecular level by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), fluorescence spectra, isothermal titration calorimetry (ITC), ζ-potential, and nuclear magnetic resonance (1H NMR) titration spectra. ITC study confirmed the assembly was completely enthalpy driven, and ζ-potential proved that the driving force was governed mainly by the electrostatic interaction. 1H NMR spectroscopy indicated changes in hydrogen bond of EuW10 and the peptide segment, and the binding model was clarified. Our design constructed the self-assembly fabrication of well-de...

Synthesis and photoluminescent property of Eu-containing organic–inorganic hybrid polyoxometalate

Abstract Sodium salt of photoluminescent Eu-containing polyoxometalate (Eu-POM) has been prepared and its chemical structure and physical properties characterized. The Eu-POM was unstable outside the pH range 5.5–8.5, and a pH just over 7 gave the best yield. We investigated the photoluminescent properties of Eu-POM with changing the reactants ratio of heteropolytungstate and EuCl3 · 6H2O. The Eu-POM was characterized by UV/Vis, FT-IR, 1H-NMR, PL and XPS. Eu-POM emits orange light of 590 nm when excited with the light of 280 nm and we made dodecyltrimethylammonium (DDTA) salt of Eu-POM (Eu-POM/DDTA) by exchange of the counter cation. The Eu-POM/DDTA is soluble in common organic solvents, while Eu-POM is only soluble in water and it emits reddish orange light of 625 nm when excited with light of 250 nm.