Novel Gemini Surfactant for Binding Eu(III)-Polyoxometalate into Hydrogels and Polymer Latexes

The incorporation of rare-earth ions into polymer matrices can lead to useful materials in various fields such as biomarkers, lasers, luminescent devices, optical storage materials, and so on. Methods of incorporation are either extremely simple, such as mixing the polymer and the ion of interest in adequate solvents, or more sophisticated such as synthesizing predesigned monomers that contain the rare-earth ion or binding the ion on an already formed polymer chain. Cationic gemini surfactants represent a class of surfactants that can be used to incorporate metal-oxygen cluster compounds by means of strong electrostatic interactions. In this study, first, a novel cationic gemini surfactant having double bonds on both side chains was designed and prepared. After characterization, the surfactant was used to synthesize hydrogels with different degrees of crosslinking and also as a surfmer in emulsion polymerization of methyl methacrylate. The resulted polymer matrices were able to bind europium-polyoxometalate Na₉[EuW₁₀O₃₆]^.32H₂O. In case of luminescent lanthanide ions, changing the microenvironment around the metal ion also changes the intensity of some emission peaks as well as other luminescent parameters. Investigation of emission spectra of Eu³⁺ indicates a decrease in the symmetry of the microenvironment, when the polyanions pass from water to latex, to surfactant solution, and to hydrogel.

Detection of organophosphorus pesticides with liquid crystals supported on the surface deposited with polyoxometalate-based acetylcholinesterase-responsive supramolecular spheres

Here, we demonstrate use of acetylcholinesterase (AChE)-responsive polyoxometalate (POM)/surfactant supramolecular spheres to build a liquid crystal (LC)-based sensing platform for detection of organophosphorus pesticides. The self-assembled spheres are composed of hybrid materials of a POM, sodium dodecatungstophosphate (PW₁₂), and a surfactant, myristoylcholine (Myr). It displays dark appearance when the aqueous solution is in contact with LCs supported on the octadecyltrichlorosilane-treated glass deposited with the supramolecular spheres, suggesting perpendicular orientation of LCs at the aqueous/LC interface. In contrast, LCs show bright appearance when the surface-deposited supramolecular spheres are enzymatically hydrolyzed by AChE, corresponding to planar orientation of LCs at the aqueous/LC interface. Detection of organophosphates are successfully achieved as they are potent inhibitors of AChE. The detection limit of the sensing platform reached 0.9 ng/mL for dimethoate. This method can avoid disturbance of external interference with excellent specificity and sensitivity, which makes it very promise in detection of organophosphorus pesticides.

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.

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.

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.

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.

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.

Hierarchical Nanostructures Self-Assembled by Polyoxometalate and Alkylamine for Photocatalytic Degradation of Dye

A novel simple strategy for alkylamine-directed self-assembly of Weakley-type polyoxometalate (POM, Na₉[EuW₁₀O₃₆]·32H₂O, abbreviated to EuW₁₀) to form three-dimensional nanoflowers has been successfully developed through the ionic self-assembly (ISA) method. For comparison, different molecular weights of alkylamines including diethylenetriamine, triethylenetetramine, and tetraethylenepentamine (TEPA) were selected to construct hierarchical nanostructures. Our results revealed that the morphologies and sizes of the nanostructures could be simply controlled by varying the molecular weights and concentrations of alkylamines. The fluorescent color of EuW₁₀/TEPA nanoflowers changed compared with that of EuW₁₀ owing to the varied symmetry degree of europium coordination in EuW₁₀/TEPA nanoflowers. It is demonstrated that this effective self-assembly occurs mainly though the hydrogen bond and electrostatic interaction between EuW₁₀ and TEPA. What's more, the EuW₁₀/TEPA nanoflowers after calcining showed excellent decomposition efficiency toward methylene blue dyes. Our results further confirmed that ISA method between small molecules and POM can provide a unique "bottom-up" strategy to construct novel structures with functional properties.

Multi-stimuli Responsive Supramolecular Structures Based on Azobenzene Surfactant-Encapsulated Polyoxometalate

Multi-stimuli responsive materials have attracted intense attention as extensive application prospect in many fields, yet achievement of multi-stimuli responsiveness remains a challenge. Herein, we report a tri-stimuli responsive supramolecular structure fabricated by a cationic surfactant, 4-ethyl-4'-(trimethylaminohexyloxy) azobenzene bromide (ETAB), and anionic Eu-containing polyoxometalates (Eu-POM), based on an ionic self-assembly (ISA) strategy. Following different responsive mechanisms, the resultant ETAB/Eu-POM supramolecular materials are responsive to UV light, pH, and Cu(2+), respectively. The response to UV irradiation is based on the configuration change of azobenzene molecules. The response to H(+) can be attributed to the formation of a hydrogen bond W-O···H···O-H among Eu-POM, H(+), and H2O, which blocks the energy transfer pathway from O → W, while the coordination interaction between Cu(2+) and Oc (bridged oxygen of two octahedra sharing an edge in the Eu-POM molecule) causes the response to Cu(2+). The multi-stimuli responsive characteristics for the ETAB/Eu-POM supramolecular structures maybe provide a potential application in ultraviolet detection, optical storage devices, and chemical substance sensors, etc.