Silica Cage Research Articles

Oxygen Transport in Amino-Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS)

The transport properties of oxygen in three amino-functionalized cubic polyhedral oligomeric silsesquioxanes (POSS) have been studied using classical molecular dynamics (MD) simulations over a timescale long enough to reach the Fickian regime for diffusion. An amount of O2 corresponding to an applied pressure of 3 bars was inserted into molecular models of hybrid organic/inorganic POSS with the chemical composition (RSiO3/2)8, which differed by the end-groups of their organic pendant chains, that is, R = −(CH2)3−NH−CO−X with X = −C6H4OH, −C6H5 or −C6H11. The oxygen … POSS energies were found to be small with respect to the POSS… POSS interactions. The O2 molecules permeate the organic phase and move through combinations of oscillations within available free volumes in the matrices and occasional jumping events. Gas mobility was more restricted in the system with the salicylic end-group and the largest hydrogen-bond network, whereas it was enhanced in the system with the cyclohexyl end-group. The most energetically-favorable sites for O2 insertion were either in the vicinity of the silica cages or close to the rings of the chain end-groups. On the other hand, the amide and hydroxyls groups engaging in H-bonds were less energetically favorable. This confirms that H-bonding networks are a hindrance for O2 transport in such systems.

Low temperature DSC characterisation of water in opal

A low temperature (−60 to +105 °C) DSC characterisation of opal was carried out to determine the proportion of crystallisable water and to estimate the cavity size in which the crystallisable water is contained. Circa 10 % of the molecular water contained in the opals was found to be crystallisable suggesting that the remaining molecular water is present either trapped in silica cages or surface-adsorbed in micropores. For the opals derived from a sedimentary environment in Australia, the crystallisable water was found to melt in a manner consistent with the melting of bulk water, suggesting that the water is contained in cavities in the opal. The lack of depressed melting temperatures suggested little or no mesoporosity. A volcanic opal specimen of Mexican origin was found to contain both mesoporous and cavity water, while a Tintenbar opal, also of volcanic origin, was found to contain only mesoporous water due to the melting of the crystallisable water, with an estimated pore diameter size range 4–7 nm. The differences in mesoporosity observed between the volcanic and sedimentary opals are consistent with the demarcation in the physical properties observed between these types of opals in previous studies.

POSS fillers for modulating the thermal properties of ionic liquids

We report a polyhedral oligomeric silsesquioxane (POSS)-based filler for improving the thermal properties of ionic liquids (ILs). We prepared homogeneous mixtures containing the octa-substituted carboxy-POSS as a filler via a solution process. Initially, it was found that the melting temperatures of the ionic pairs were lowered by adding the POSS filler. Based on this effect, some of ionic salts were transformed into ILs by adding the POSS filler. Reinforcement of thermal stability was also observed from the salts which have an intrinsically low decomposition temperature. According to the thermodynamic parameters in melting, the introduction of the POSS filler significantly reduces both the fusion enthalpy and entropy. Furthermore, from experiments on the substituent effect on POSS, it was found that only octa-carboxy POSS had an effect on the modulation of the thermal properties of ion salts. Considering these results, we summarized that the highly symmetrical rigid structure of the silica cage could be responsible for the improvement of the thermal properties of the ion salt matrices via the strong interaction between the carboxyl groups and the ion salt matrices. Our concept is feasible not only for the conversion of conventional ionic salts into ILs, but also for the development of new series of ILs.

Construction of silica-enhanced S-layer protein cages

The work presented here shows for the first time that it is possible to silicify S-layer coated liposomes and to obtain stable functionalized hollow nano-containers. For this purpose, the S-layer protein of Geobacillus stearothermophilus PV72/p2 was recombinantly expressed and used for coating positively charged liposomes composed of dipalmitoylphosphatidylcholine, cholesterol and hexadecylamine in a molar ratio of 10:5:4. Subsequently, plain (uncoated) liposomes and S-layer coated liposomes were silicified. Determination of the charge of the constructs during silicification allowed the deposition process to be followed. After the particles had been silicified, lipids were dissolved by treatment with Triton X-100 with the release of previously entrapped fluorescent dyes being determined by fluorimetry. Both, ζ-potential and release experiments showed differences between silicified plain liposomes and silicified S-layer coated liposomes. The results of the individual preparation steps were examined by embedding the respective assemblies in resin, ultrathin sectioning and inspection by bright-field transmission electron microscopy (TEM). Energy filtered TEM confirmed the successful construction of S-layer based silica cages. It is anticipated that this approach will provide a key to enabling technology for the fabrication of nanoporous protein cages for applications ranging from nano medicine to materials science.

Mechanism of interaction between colloids and bacteria as evidenced by tailored silica–lysozyme composites

The ever increasing antibiotic resistance levels in pathogenic and non-pathogenic bacteria have boosted the search for effective controlling methods of bacterial infections. In this context, we prepared colloidal silica–lysozyme composites to evaluate the interaction between these structures and bacteria. Lysozyme was chosen since it is a protein that holds bactericidal properties and that, in the reaction process, acts as a catalyst which favors silica hydrolysis and condensation. The high entrapment yield of lysozyme in the silica cage (approximately 95%) does not change the secondary structure of the protein resulting in a material with superior bactericidal properties. The antimicrobial differences when silica–lysozyme composites are tested against Escherichia coli and Staphylococcus aureus are used to propose an interaction mechanism between colloids and bacteria. Bactericidal properties of the composites are attributed to the ultra-structural organization of the composite which, due to the positive surface charge, is attracted by the negative bacterial cell wall while lysozyme is delivered.

DFT investigation of NH3 physisorption on CuSO4 impregnated SiO2

In this quantum chemical investigation, NH(3) physisorption onto a model of copper sulfate impregnated silica is compared with pure silica and copper sulfate adsorbents. The physisorption process is modeled as direct binding of the NH(3) molecule to the adsorption site of the dry adsorbents and as displacement of a H(2)O molecule by NH(3) in the hydrated complexes. The surface of silica is represented by a hydroxyl group attached to a silsesquioxane cage, H(7)Si(8)O(12)(OH) and silica impregnated with CuSO(4) by the most stable configuration of the cluster containing a CuSO(4) ion pair placed adjacent to the silica cage. H(2)O is systematically added to the dehydrated adsorbents to investigate the role of water in NH(3) adsorption. Modeling hydrated environments of each type of adsorbent is focused on H(2)O molecules that directly coordinate with the active sites. The results indicate that the binding energy of adsorbing NH(3) onto the mixed adsorbent is greater than in pure silica. This enhanced binding in the mixed adsorbent is consistent with improved Brønsted acidity of the silanol in the presence of CuSO(4).

Dielectric properties of polyhedral oligomeric silsesquioxane (POSS)-based nanocomposites at 77k

The goal of this study is to develop dielectric nanocomposites for high energy density applications at liquid nitrogen temperature by utilizing a unique nano-material polyhedral oligomeric silsesquioxanes (POSS). A POSS molecule is consisted of a silica cage core with 8 silicon and 12 oxygen atoms and organic functional groups attached to the corners of the cage. In this study, we utilize POSS for the fabrication of nanocomposites both as a silica nanoparticle filler to enhance the breakdown strength and as a surfactant for effective dispersion of high permittivity ceramic nanoparticles in a polymer matrix. The matrix materials selected for the study are polyvinylidene fluoride (PVDF) and poly(methyl methacrylate) (PMMA). The ceramic nanoparticles are barium strontium titanate (BST 50/50) and strontium titanate. The dielectric properties of the solution-cast nanocomposites films were correlated to the composition and processing conditions. We determined that the addition of POSS did not provide enhanced dielectric performance in PVDF- and PMMA-based materials at either room temperature or 77K. In addition, we found that the dielectric breakdown strength of PMMA is lower at 77K than at room temperature, contradicting literature data.

In-situ immobilization of enzymes in mesoporous silicas

Lipase from Candida antarctica B, horseradish peroxidase and laccase have been entrapped in silica cages rising mesoporous structures. Lipase and laccase yielded the highest structured mesoporous material whereas horseradish peroxidase may have altered the symmetry giving as a result mesocelullar foam (MCF) type of cages. The possible effect in the final structure of the material of the nature, size and surface structure of the proteins as well as the presence of various additives in the enzyme extracts is currently under investigations.

Bottle-around-the-ship: A method to encapsulate enzymes in ordered mesoporous materials

Lipase has been encapsulated in ordered mesoporous materials in a one-step process. The procedure is simple and leads to well structured materials with enzyme loadings higher than those obtained through the post-synthesis approach. In the final bottle-around-the-ship material the enzyme would be entrapped in silica cages whose entrances are smaller than the enzyme itself. Therefore, reagents and products may pass through, while maintaining the active site entrapped and ready for reuse.

Mesoporous Monocrystalline TiO2 and Its Solid-State Electrochemical Properties

Mesoporous monocrystalline rutile TiO2 has been fabricated at low temperature using mesoporous silicas SBA-15 and KIT-6 as hard templates. The key step of the synthetic process was introducing titanium nitrate complex into the template pores and allowing it to dry, dehydrate, decompose, and finally, form TiO2 crystals in the pores. It was found that the reaction temperature and concentration of HNO3 in the used precursor had great effects to the crystallization of TiO2. Removal of the silica templates after the TiO2 crystallization has been investigated. Crystallization of TiO2 in cage-containing mesoporous silicas, FDU-12 and SBA-16 was not successful, further confirming the previous speculation about strong interaction between the crystals and the wall of silica cages. The porous titanium oxide specimens were characterized by using various techniques, including XRD, HRTEM, and nitrogen adsorption/desorption. Proton conductivity and Li-ion insertion property of the samples were also examined. The highest...

Sol-gel reactions of 3-glycidoxypropyltrimethoxysilane in a highly basic aqueous solution

The reactions of 3-glycidoxypropyltrimethoxysilane in a highly basic aqueous solution have been studied by multinuclear magnetic resonance and light scattering techniques. The study has shown that in this peculiar chemical environment the alkoxy groups of 3-glycidoxypropyltrimethoxysilane undergo a fast hydrolysis and condensation which favor the formation of open hybrid silica cages. The silica condensation reaches 90% at a short aging time but does not go to completion even after 9 days. The highly basic conditions also slow down the opening of the epoxies which fully react only after several days of aging. The epoxy opening generates different chemical species and several reaction pathways have been observed; in particular, the formation of polyethylene oxide chains, diols, termination of the organic chain by methyl ether groups and formation of dioxane species. These reactions are slow and proceed gradually with aging; light scattering analysis has shown that clusters of dimensions lower than 20 nm are formed after two days of reactions, but their further growth is hindered by the highly basic conditions which limit full silica condensation and formation of organic chains.

Silica cages with controllable frameworks: synthesis, structure-tailoring, and formation mechanism

A simple method was developed for the synthesis of novel macroporous silica cages with tailored pore architecture and high thermal stability. The silica cages possess anamorphic spherical morphology and narrow particle size distribution (∼1.5 µm), consisting of numerous channels interconnected with one another to form a fully open three-dimensional network. The morphology and structure of macroporous silica cages are strongly affected by synthetic conditions, including the molar compositions of reactants, the evaporation time of solvent, and the aging conditions. The number of silica crystal nuclei in the synthesis system is the major factor that determines the formation of silica cages and the disorder-to-order transformation, which can be controlled by changing the reactant molar concentrations and the solvent evaporation time. Using the appropriate aging time in ethanol provides the possibility of effecting the transformation from disordered twisting silica nanofibers with random branches to silica cages. The transformation from the macroporous cages to the mesostructures and the formation mechanism of the cages are also discussed in detail.

A Mesoporous Pattern Created by Nature in Spicules from Thetya aurantium Sponge

Siliceous or carbonate spicules provide support and defense to marine sponges. The inorganic envelope usually embodies a protein core. Our SAXS study of the siliceous spicules from the demosponge Thetya aurantium proves the very ordered structure assumed by the protein core inside the spicules. Indeed, not only the very sharp diffraction spots already found in previous studies on spicules from different sponges are confirmed, but also the 11 sharp spots in the diffraction pattern recorded after thermal treatment at 250°C can only be interpreted in terms of a natural nanocomposite mesostructure with an hexagonal lattice formed by a three-dimensional periodic arrangement of silica cages in which the protein units act as structure directing agent.

Development status of the differential accelerometer for the MICROSCOPE mission

Tests of the Equivalence Principle are essential to fundamental physics theories, but as performed previously with torsion balances and laser ranging they have been limited by the vibrations inherent to any Earth-based environment. The MICROSCOPE mission will take advantage of the space environment to extend the EP test accuracy to 10 −15, by placing two masses of different materials in a drag compensated orbit. A violation of the Equivalence Principle will appear as a difference in the electrostatic forces necessary to maintain both masses on the same orbit. The satellite will be launched in 2008 and carry as the primary science instrument a differential electrostatic accelerometer. The accelerometer is composed of two coaxial cylindrical proof masses surrounded by silica cages, in a vacuum housing. The two proof masses, one in platinum–rhodium within one in titanium, are maintained at the centre of their cages using electrodes engraved in the silica cage. These electrodes are used to capacitively sense the proof-mass position and to apply electrostatic forces to control the position. The accuracy and stability of the silica cage is therefore essential to the quality of the EP test. This paper presents the current design of the accelerometer, specifically the critical areas for the instrument design, integration, and final performance requirements. Also discussed is the status of the analytical and theoretical models, as well as the experimental investigations, which are developed to overcome these critical areas.

Robust Polyaromatic Octasilsesquioxanes from Polybromophenylsilsesquioxanes, BrxOPS, via Suzuki Coupling

Polybromophenylsilsesquioxane (BrxOPS), where x = 5.3 (p:m = 65:25), because of its octahedral structure and nanometer size, represents a potentially very useful nanoconstruction site. We report here that Br5.3OPS reacts readily with borates of phenyl, biphenyl, naphthyl, 9,9-dimethylfluorene, and thiophene using standard Suzuki conditions to produce the corresponding polyaromatic and heteroaromatics with complete substitution of all bromines. The resulting materials, because of the 3-D octahedral structure, are completely soluble in a variety of common organic solvents. They are also stable to temperatures exceeding 400 °C in air, making them quite robust and easily processable. UV−vis spectra of these materials are red-shifted 20−30 nm from the simple organics, suggesting some conjugation with the silica cage core. Photoluminescence measurements show standard aromatic π−π* behavior with typical quantum efficiencies except for the 9,9-dimethylfluorene compound, which has an unexpectedly high quantum efficiency >95%. The resulting compounds offer potential for further functionalization.

Development of a differential accelerometer to test the equivalence principle in the microscope mission

A violation of the Equivalence Principle (EP), which hypothesizes the equality of inertial mass and gravitational mass, is indicated by current theories in modern physics. The MICROSCOPE mission seeks to extend the accuracy of previous EP tests to 10 - 15 , by avoiding the disturbances inherent to every Earth based test facility. The test will involve the measurement of the electrostatic forces required to maintain two concentric masses on the same orbit. The satellite, to be launched in 2008, will carry two differential accelerometers, one with masses of platinum and titanium, and a second with two platinum masses for baseline measurements. Each accelerometer will contain two coaxial cylindrical proof masses, each encompassed by a silica cage, all in a vacuum housing. The capacitance between electrodes etched into the silica, and the surface of the gold-coated proof masses provides a measurement of the proof mass position, which is then controlled by adjusting the voltages applied to the electrodes. Because an EP violation will appear as a difference between the forces required to keep each mass centred, the quality and stability of the silica cages is essential to achieve the desired test accuracy. This paper presents the overall design of the accelerometer, focusing on areas critical to the instrument core design, integration, and final performance requirements. The models and experimental investigations designed to overcome these issues are also discussed.

Controlled growth of ZnS:Mn nanophosphor in porous silica matrix

The development of nanophosphors of desired sizes and properties for various practical applications and its growth in quantitative amounts inside the pores of an inorganic matrix is presented. By doing so, nanophosphors get surface passivated and are stabilized against environmental attacks. Accordingly, in the present study, the growth parameters for ZnS:Mn nanophosphors were systematically studied inside a SiO2 gel matrix, which can act as a capping agent as well. The samples were prepared using the sol-gel technique, followed by annealing at different temperatures to remove the trapped fluid inside the amorphous silica cage. Two categories of samples with lower (3.11×10−4) and higher (1.5×10−1) ZnS∕SiO2 molar ratios were studied. The x-ray diffraction and scanning electron microscopy observations show that upon annealing, the nanocrystals grow in size and undergo a phase transition from cubic to hexagonal at temperatures between 700 and 900°C. This is one of the very few known reports published on nano hexagonal ZnS formation. The observed phase transition is possibly the combined effect of the high-temperature (∼900°C) and annealing-related compressive stress induced on the nano-ZnS by the silica cage. There has been formation of an intermediate metastable phase of the zinc silicate at annealing temperatures around 700°C. The particle size distribution and emission properties were correlated using the optical absorption and photoluminescence (PL) results. The unannealed cubic nano-ZnS:Mn samples gave a broad PL, peaking at ∼585nm, whereas the samples annealed at 900°C for 5h gave a narrow and sharp PL at ∼590nm. This is attributed to the more efficient T14→A16 transitions of Mn in the resultant hexagonal nano-ZnS matrix.

Synthesis of Nanomagnets Dispersed in Colloidal Silica Cages with Applications in Chemical Separation

This Letter describes a simple, continuous, and generalized method to prepare nanomagnets dispersed in dense submicrospherical silica cages. The method exploits the subtle interplay between the parameters controlling particle formation in aerosol pyrolysis processes. We find that the magnetic properties of the composites can be easily tuned so as to produce suspensions that display reversible sedimentation behavior in an applied magnetic field gradient. Our results indicate that this material can be used for chemical separation in medicine, biology, and environmental protection.

Incorporation of polyhedral oligosilsesquioxane in chemically amplified resists to improve their reactive ion etching resistance

A chemically amplified (CA) methacrylate resist containing polyhedral oligosilsesquioxane (POSS) has been synthesized by AIBN-initiated free radical polymerization. While the polymer of low POSS concentrations showed little improvement in reactive ion etching (RIE) resistance, incorporation of 20.5 wt % of the POSS monomer into methacrylate-based CA resists significantly improved their RIE resistance in the O2 plasma. High-resolution transmission electron microscopy revealed that the RIE resistance improvement was due to the formation of rectangular crystallite-constituting networks of the silica cages uniformly distributed within the polymer matrix.

Surfactant-Induced Modification of Dopants Reactivity in Sol−Gel Matrixes

Organically and bio-organically doped sol−gel materials have attracted much attention due to their ability to reproduce solution molecular activities within the ceramic environment. We take now this methodology one step forward and explore conditions under which the dopant properties can be modified by the matrix. Specifically we report that the co-entrapment of the surfactant cetyltrimethylammonium bromide (CTAB, the modifier) at low concentrations, with an extensive series of pH indicators representing several key molecular families (the primary dopant) within tetramethoxysilane (TMOS)-derived silica sol−gel matrixes, greatly modifies the indicating performance of the primary dopant. Thus, very large pKi shifts of up to 3−4 orders of magnitude obtained upon the co-entrapment cause methyl orange (MO) to become an indicator for higher acidities and phenolphthalein for higher basicities, compared to their solution behavior. In another example, the ΔpKi between the two indicating transitions of alizarin increased from ∼4.5 pH units in solution to ∼10.5 pH units (!) in the glass, transforming it into an indicator for both the high acidic and high basic pH ranges. In yet another example, the two indicating transitions of phenol red were shifted to a more acidic pH range, pushing the tail of the more acidic titration branch into the negative pH values range. These and other effects were found to be more pronounced by the co-entrapment than by the use of CTAB solutions or of sol−gel matrixes without CTAB, pointing to a synergetic effect between the surfactant and the silica cage. The indicators also proved highly sensitive in revealing the properties of the local environment created by the surfactant. Thus, the indicator molecules were shown to migrate and reorient within the hydrophobic and the hydrophilic regions of micellar environment, according to their acquired charge upon pH changes. The concentration-dependent and humidity-dependent surfactant aggregation processes within the silica cage were probed with MO, and the results were compared with the behavior of entrapped MO in sol−gel matrixes of varying hydrophobicities, obtained by the copolymerization of CH3Si(OCH3)3 with TMOS at various ratios, with and without CTAB. Of practical importance have been the observations that the dopant/surfactant co-entrapment greatly improves the leaching profiles: half-lives, ranging from several months to several years, were found for MO and methyl red; and using SAXS, surface area and porosity measurements, it was shown that CTAB can be used to stabilize the microscopic structure of the material upon heat-drying. This provides a potential solution to the problem of continuing structural changes, which take place with sol−gel materials long after the completion of their synthesis.