Assembly And Functionalization Of Polyoxometalate Supramolecular Complex In Silica Spheres

Supramolecular assembly has become research hotspot which is rapidly developing recently. Basing on the concept of self-assembly, which had break through the conventional methods of covalent synthesis, intermolecular interactions such as hydrogen bonding, coordinate bonding, electrostatic interaction and van de Waals force have built different individual molecules into novel supramolecular materials with ordered structures and integrated functionalities. Mild assembly condition, flexible building blocks, especially the favorable for realization of synergic effects between different building blocks make the supramolecular assembly be regarded as an important approach to create new materials in future. Polyoxometalates (POMs) are a family of nanosized inorganic metal oxide clusters with various constitutions and topologies, multinegative charges, which make them exhibit extensive potential applications in the field of medicine, catalysis and the functional materials with magnetic, electronic, optical and so forth. Even be considered as"the versatile inorganic building blocks", the research development of POM-based devices and practical materials has been seriously obstructed by their intrinsic drawbacks such as high crystalline energy, pH sensitivity, and the poor processability of pure POMs as the inorganic crystalline or powdered materials. Therefore, it is of great significance to incorporate POMs into other matrices such as silica nanoparticles which are easily to be processed. As inorganic polymer, silica nanoparticles with biocompatibility, easy modification, chemical inert, porosity, and size controllability are considered perfect carriers for POMs. But the intrinsic defects such as low pH stabilities make POMs easily be destroyed during the basic Sol–Gel process for preparing silica nanoparticles. So to realize the uniformly doping and functionalization of POMs in silica nanoparticles, the POMs need protective nanoenvironment and proper surface modification. Using cationic surfactants to encapsulate is a flexible and effective method of environment construction and surface modification for POMs. The resulted surfactant encapsulated POMs (SEPs) can be building blocks which are bearing functions of POMs and surfactants, and synergetic functions between POMs and surfactants. In this thesis, taking the strategy from supramolecular interaction, we used different cationic surfactants to encapsulate POMs for constructing protective nanoenvironment and modifying surface, the resulted SEPs were used as carriers of POMs and functional building blocks, and for the first time, the uniform assembly and functionalization of the SEPs in silica particles were realized. We carried out two important parts of research works concerning the assembly and functionalization of SEPs in silica particles. On one hand, the uniform covalent assembly and functionalization of–OH surfaced SEPs with different functional POMs in silica particles were achieved, basing on a basic in situ Sol–Gel process. On the other hand, the uniform assembly of hydrophobe surfaced SEPs in silica spheres were achieved, basing on a basic in situ Sol–Gel process and supramolecular interactions such as hydrophobic and electrostatic interaction in reaction system. Also, the functionalization of SEPs in silica spheres were achived, basing on the composition of SEP and structural property of uniform assembly of SEPs in hybrid silica spheres. The main research achievements are as follows:1,Uniform Covalent Assembly and Functionalization of Fluorescent SEP in Silica Nanoparticles: The hydroxyl group terminated surfactant, di(11-hydroxyundecyl) dimethylammonium bromide (DOHDA), was prepared and applied for the encapsulation of luminescent Na9[EuW10O₃6] (POM-1), yielding an organic-inorganic complex (SEP-1) with core-shell structure bearing a twelve-alkyl-chain composed hydrophobic nanoenvironment for POM-1 and hydroxyl groups locating at periphery. The grafted twelve hydroxyl groups on outside surface of the complex can make the obtained SEP-1 easily dissolve in the basic Sol–Gel system, and react with siloxane agents (TEOS), leading to hybrid silica nanoparticles (SEP-1/SiO₂) with SEP-1 chemically binding to silica matrix uniformly. The SEP-1s with well kept structure and function of POM-1 were successfully assembled into SEP-1/SiO₂ hybrid nanoparticles in well dispersed state. It was found that, the size of hybrid silica nanoparticles decreased with the concentration of SEP-1 in reaction system, due to the potential nucleation effect of SEP-1; the yield of SEP-1/SiO₂ was 100%; due to the protection of hydrophobic nanoenvironment in SEP-1 to POM-1, SEP-1/SiO₂ nanomaterials showed high fluorescent quantum yield (>44%), wider fluorescent pH range of 2–11, and were used to image living Hela cells after simple–NH2 surface modification. So, both the uniform covalent assembly and functionalization of POM-1 (SEP-1 as carrier) in silica nanoparticles were achieved. The whole procedure is general for incorporating POM into silica particles and realizing the function of POM in hybrid silica particles.2. Uniform Covalent Assembly and Functionalization of SEP with photochromism and photoredox properties in Silica Spheres: Based on above procedure, 11-hydroxylundecyldimethylamine hydrobromide (HUDAH), another hydroxyl terminated surfactant was prepared and used to encapsulate the [EuP₅W₃₀O₁₁₀]^12- (POM-2) with photo-induced redox property. The obtained complexes (SEP-2) with anticipated structure were covalently assembled into SiO₂ spheres (SEP-2/SiO₂) during the basic in situ Sol–Gel reaction. Based on the well kept photo-induced redox property of POM-1 and the porosity of silica matrix, the metal ions were reduced and the metal nanoparticles (MNP) were in situ obtained in SEP-1/SiO₂. Raman scattering points the location of MNPs in obtained MNP/SEP-2/SiO₂. Interestingly, it is probable that water is the electron donor to reduce metal ions in the reaction. So, the SEP-2/SiO₂ spheres can be used to in situ reduce metal ions repeatedly, if there is still water left or the reactive H atom in HUDAH has not been consumed in reaction system, to obtain multi-metal nanoparticels doped hybrid spheres. So, the uniform covalent assembly and functionalization of SEP with photochromism and photoredox in silica spheres were achieved. And the MNP/SEP-2/SiO₂ spheres, serving as the catalyst carrier, should be potentially useful for the oxidation and hydrolysis of some organic molecules through the synergetic catalysis of POMs and MNPs as well as the hydrophobic microenvironment or provide potential applications in antibacterial and negative refractive materials.3. Uniform Assembly and In Situ Preparation of Cavity Supported Metal Oxide Nanocrystals of SEP in Silica Spheres: Based on hydrophobic supramolecular interaction, the cetyltrimethylammonium bromide (CTAB) was used to phase transfer dioctadecyldimethylammonium cations (DODA+) encapsulated POM (SEP) into water, then to the basic Sol–Gel system, where the negative monomers or oligomers from hydrolyzed TEOS interacted with positive cations of CTAB around SEP-1, aggregate and cross-link together; the hydrophobic interaction between SEP-1 and CTAB tail, the electrostatic interaction between ionic head of CTAB and the hydrolyzed TEOS, as well as the condensability of hydrolyzed TEOS contributed to the final formation of SEP/SiO₂ with POM confined in a hydrophobic nanoenvironment of alkyl chains, immobilized and uniformly dispersed in silica matrix. The yield is near 100%. Due to the protection from alkyl chains of DODA+ and CTAB, the fluorescent SEP-3/SiO₂ (based on fluorescent [EuW₁₀O₃₆]^9-) show high pH stability (pH 1-12). Considering the combustibility and decomposition of organic composition, the ability of crystalline for inorganic composition in SEPs at high temperature, the porosity of rigid silica matrix, and the well dispersion of SEPs in SEP/SiO₂, it is not surprisingly that the Eu-doped WO₃ nanocrystallines locating inside cavities were formed in resulted WO₃/SiO₂ with well dispersed state, after a calcinations process in the presence of O₂. It was found that, there was luminescent from Eu3+ which is strong and WO₃ nanocrystallines which is faintish simultaneously; due to the size effect, comparing to WO₃ (W-SEP-3) obtained from sintered SEP-3 in nature state, the WO₃/SiO₂ nanomaterials showed blue shift in UV absorption and stronger photovoltaic property in the UV absorption region, even they have the same phase and composition; due to the SEPs performing the template for cavities in rigid hybrid silica spheres, the specific surface of WO₃/SiO₂ show controllability, and it increased with the loading of SEP in original SEP-3/SiO₂. Due to the universality of the procedure to polyoxotungates with various compositions, the WO₃/SiO₂ with tunable band gaps and specific element doping can be obtained by changing the source polyoxotungstates. Based on different polyoxotungstates, [PW₁₂O₄₀]^3-, [EuP₅W₃₀O₁₁₀]^12-, [EuW₁₀O₃₆]^9-, and [NaP₅W₃₀O₁₁₀]^14-, corresponding same structured WO₃/SiO₂ nanomaterials with different band gaps of 2.42, 2.63, 2.69, and 2.72 eV respectively were obtained. There is universality for the whole procedure to almost all POMs and even other inorganic nanoparticles, so based on different POMs, the uniform assembly of different SEPs and corresponding cavities supported metal oxide nanocrystallines in silica spheres can be achieved. So, by virtue of the composition character, the uniform assembly and functionalization of SEPs in silica spheres was realized again. The obtained nanomaterials have potential applications in photonic, gas sensor, catalysis and so on.