Synthesis and characterization of TiO<Subscript>2</Subscript>-incorporated silica foams

Titania-incorporated silica (TiO₂–SiO₂) porous materials have great applications in diverse areas. In this work, TiO₂–SiO₂ porous materials with tunable Si/Ti molar ratio (R) have been successfully prepared through a one-pot method under a near-neutral condition. With decreasing Si/Ti R, a phase transition from a macroporous foam-like structure to mesostructure is observed. The resultant TiO₂–SiO₂ porous materials possess large surface areas and high pore volumes. In addition, the titania species are homogenously dispersed in silica matrix when Si/Ti R ≥ 10. Our contribution provides a convenient method to synthesize TiO₂/SiO₂ porous materials with very large pore size, high pore volume, and relatively high titania content well dispersed in the silica wall framework.     

Angiogenic potential of boron-containing bioactive glasses: in vitro study

Boron-containing bioactive glasses (BGs) are being extensively researched for the treatment and regeneration of bone defects because of their osteostimulatory and neovascularization potential. In this study, we report the effects of the ionic dissolution products (IDPs) of different boron-doped, borosilicate, and borate BG scaffolds on mouse bone marrow stromal cells in vitro, using an angiogenesis assay. Five different BG scaffolds of the system SiO₂–Na₂O–K₂O–MgO–CaO–P₂O₅–B₂O₃ (with varying amounts of SiO₂ and B₂O₃) were fabricated by the foam replication technique. Bone marrow stromal cells were cultivated in contact with the IDPs of the boron-containing BG scaffolds at different concentrations for 48 h. The expression and secretion of vascular endothelial growth factor (VEGF) from the cultured cells was measured quantitatively using the VEGF ELISA Kit. Cell viability and cell morphology were determined using WST-8 assay and H&E staining, respectively. The cellular response was found to be dependent on boron content and the B release profile from the glasses corresponded to the positive or negative biological activity of the BGs.     

Determination of CaCO<Subscript>3</Subscript> and SiO<Subscript>2</Subscript> content in the binders of historic lime mortars

The binders of historic mortars composed of small grain sized silica (SiO₂) and carbonated lime (CaCO₃) are considered as the main part that give hydraulic character and high strength to the mortar. In this study, FTIR, SEM–EDS, LIBS and XRD spectroscopy were used to find out the weight ratios of CaCO₃ to SiO₂ in the binders of historic lime mortars. For this purpose, a series of pure calcium carbonate and silica mixture were prepared in ten combinations in varying ratios from 0.5 to 5. Calibration curve was prepared for each analysis by plotting the peak area or intensity ratios of CaCO₃ to SiO₂ versus the weight ratios of CaCO₃ to SiO₂. A good linear correlation coefficient was obtained for each analysis respectively. The analyses were then tested on the binder of the Roman mortar samples. The results indicated that FTIR, SEM–EDS and LIBS spectroscopy are convenient tools to determine the weight ratios of CaCO₃ to SiO₂ in the binders of mortars. But XRD spectroscopy is not convenient for quantitative analysis of binders due to the presence of varied amounts of amorphous or poor crystalline silica in their compositions.     

Low-temperature sintering of SiC reticulated porous ceramics with MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> additives as sintering aids

SiC reticulated porous ceramics (SiC RPCs) was fabricated with polymer replicas method by using MgO–Al₂O₃–SiO₂ additives as sintering aids at 1,000∼1,450 °C. The MgO–Al₂O₃–SiO₂ additives were from alumina, kaolin and Talc powders. By employing various experimental techniques, zeta potential, viscosity and rheological measurements, the dispersion of mixed powders (SiC, Al₂O₃, talc and kaolin) in aqueous media using silica sol as a binder was studied. The pH value of the optimum dispersion was found to be around pH 10 for the mixtures. The optimum condition of the slurry suitable for impregnating the polymeric sponge was obtained. At the same time, the influence of the sintering temperature and holding time on the properties of SiC RPCs was investigated. According to the properties of SiC RPCs, the optimal sintering temperature was chosen at 1,300 °C, which was lower than that with Al₂O₃–SiO₂ additives as sintering aids.     

A wrinkle to sub-100 nm yolk/shell Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>@SiO<Subscript>2</Subscript> nanoparticles

Yolk/shell nanoparticles (NPs), which integrate functional cores (likes Fe₃O₄) and an inert SiO₂ shell, are very important for applications in fields such as biomedicine and catalysis. An acidic medium is an excellent etchant to achieve hollow SiO₂ but harmful to most functional cores. Reported here is a method for preparing sub-100 nm yolk/shell Fe₃O₄@SiO₂ NPs by a mild acidic etching strategy. Our results demonstrate that establishment of a dissolution–diffusion equilibrium of silica is essential for achieving yolk/shell Fe₃O₄@SiO₂ NPs. A uniform increase in the silica compactness from the inside to the outside and an appropriate pH value of the etchant are the main factors controlling the thickness and cavity of the SiO₂ shell. Under our “standard etching code”, the acid-sensitive Fe₃O₄ core can be perfectly preserved and the SiO₂ shell can be selectively etched away. The mechanism of regulation of SiO₂ etching and acidic etching was investigated.

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Microstructural and <Emphasis Type="Italic">in vitro</Emphasis> characterization of SiO<Subscript>2</Subscript>-Na<Subscript>2</Subscript>O-CaO-MgO glass-ceramic bioactive scaffolds for bone substitutes

In the present research work, the preparation and characterization of bioactive glass-ceramic scaffolds for bone substitutes are described. The scaffolds were prepared by starch consolidation of bioactive glass powders belonging to the SiO₂-Na₂O-CaO-MgO system using three different organic starches (corn, potatoes and rice) as reported in a previous screening process [1]. The scaffolds, characterized by scanning electron microscopy, showed a porous structure with highly interconnected pores. The pores sizes assessed by mercury intrusion porosimetry put in evidence the presence of pores of 50–100 μm. The structure of the scaffolds was investigated by X-ray diffraction and revealed the glass-ceramic nature of the obtained material. The mechanical properties of the scaffolds were evaluated by means of compressive tests on cubic samples and the obtained results demonstrated their good mechanical strength. The in vitro bioactivity of the scaffolds was tested by soaking them in a simulated body fluid (SBF) and by subsequently characterizing the soaked surfaces by SEM, EDS and X-ray diffraction. Good in vitro bioactivity was found for the starting glass and for the obtained scaffolds. Moreover, the scaffold bioresorption, tested by measuring the samples weight loss in SBF at different periods of time, showed a partial resorption of the scaffolds. Cell culture testing of the three different scaffolds indicated no differences in cell number and in alkaline phosphatase activity; the morphology of the osteoblasts showed good spreading, comparable to bulk material which was used as the control.     

Preparation of lamellar-stacked TS-1 and its catalytic performance for the ammoximation of butanone with H<Subscript>2</Subscript>O<Subscript>2</Subscript>

A SiO₂ particle was prepared with different alkali sources, and then lamellar-stacked TS-1 catalysts were hydrothermally synthesized using the SiO₂ particle as a silica source. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectra, nitrogen adsorption–desorption and UV–vis absorption spectra were used to characterize the TS-1 catalysts. The effect of the alkali source during the preparation of the SiO₂ particle on the textural properties and catalytic performance of the TS-1 catalyst was thoroughly investigated. The TS-1 catalyst that was prepared with a SiO₂ particle using tetrapropylammonium hydroxide (TPAOH) as an alkali source (TS-1-TPAOH) possessed more meso- and macro-pores and a higher framework Ti content than the catalyst that was prepared with a SiO₂ particle using NH₃·H₂O as an alkali source (TS-1-NH₃·H₂O). As a result, the TS-1-TPAOH catalyst had a better catalytic performance for butanone ammoximation with H₂O₂ than conventional TS-1 and TS-1-NH₃·H₂O catalysts. Furthermore, the influences of reaction conditions, including reaction temperature, reaction time, the amount of catalyst and the molar ratio between H₂O₂ and butyl ketone oxime on the catalytic performance of the TS-1-TPAOH catalyst were evaluated. The unique structure of the lamellar-stacked TS-1 catalyst can effectively avoid the diffusing of large reactant molecules into zeolite channels and has potential applications in other oxidation reactions.     

Nacre-inspired lightweight and high-strength AZ91D/Mg<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>5</Subscript>w composites prepared by ice templating and pressureless infiltration

We developed a facile and low-cost approach to prepare lightweight and high-strength magnesium–matrix composites with a nacre-inspired laminated structure. First, lamellar Mg₂B₂O₅ whisker (Mg₂B₂O₅w) scaffolds with initial solid loadings of 10, 15 and 20 vol% were prepared by ice templating. The wettability between a molten AZ91D alloy and the Mg₂B₂O₅w scaffold was greatly improved by the incorporation of nano-SiO₂ sol in the aqueous slurry, making the preparation of nacre-mimetic AZ91D/Mg₂B₂O₅w composite by way of pressureless infiltration feasible. The SiO₂ content in the Mg₂B₂O₅w scaffold has a significant effect on the processing and the microstructure and properties of the composites. The optimum SiO₂ content was about 6–8 wt% of the total ceramic loading. A lower SiO₂ content resulted in incomplete infiltration, while a higher content led to the formation of a large quantity of Mg₂Si in the composite. The flexural strength of the composites seemed independent of the initial ceramic loading (10–20 vol%), whereas the compressive strength and elastic modulus increased considerably and the crack-growth fracture toughness decreased with increasing ceramic content. The mechanism for such variations was addressed.     

SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> fibers from titanium-modified polycarbosilane

We developed a process for preparing SiO₂/TiO₂ fibers by means of precursor transformation method. After mixing PCS and titanium alkoxide, continuous SiO₂/TiO₂ fibers were fabricated by the thermal decomposition of titanium-modified PCS (PTC) precursor. The tensile strength and diameter of SiO₂/TiO₂ fibers are 2.0 GPa, 13 μm, respectively. Based on X-ray diffraction (XRD), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM) measurements, the microstructure of the SiO₂/TiO₂ fibers is described as anatase–TiO₂ nanocrystallites with the mean size of ~10 nm embedded in an amorphous silica continuous phase.     

High-temperature oxidation of MoSi<Subscript>2</Subscript>

Oxidation behavior of MoSi₂ was investigated in air over the temperature range of 1400–1700 °C. Spallation of the SiO₂ scale did not occur at any temperature, and Mo₅Si₃ formation did not happen below 1700 °C. A change in the rate-controlling mechanism was detected within the temperature range of this study. Activation energy for oxidation of MoSi₂ at high temperatures was determined to be 204 kJ/mol. This value is less than the value of activation energy for oxidation of MoSi₂ controlled by diffusion of O₂ through amorphous SiO₂ layer reported at lower temperatures. The decrease in activation energy is attributed to the increased degree of crystallization of amorphous silica to β-cristobalite at high temperatures resulting in enhanced O₂ diffusion through SiO₄⁻⁴ tetrahedral structure.     

Surface-modified 3D scaffolds for tissue engineering

The aim of this work was to use sol–gel processing to develop bioactive materials to serve as scaffolds for tissue engineering that will allow the incorporation and release of proteins to stimulate cell function and tissue growth. We obtained organofunctionalized silica with large content of amine and mercaptan groups (up to 25%). The developed method can allow the incorporation and delivery of proteins at a controlled rate. We also produced bioactive foams with binary SiO₂–CaO and ternary SiO₂–CaO–P₂O₅ compositions. In order to enhance peptide–material surface properties, the bioactive foams were modified with amine and mercaptan groups. These materials exhibit a highly interconnected macroporous network and high surface area. These textural features together with the incorporation of organic functionally groups may enable them to be used as scaffolds for the engineering of soft tissue.     

A novel bioactive porous bredigite (Ca<Subscript>7</Subscript>MgSi<Subscript>4</Subscript>O<Subscript>16</Subscript>) scaffold with biomimetic apatite layer for bone tissue engineering

The aim of this study was to develop a novel bioactive, degradable and cytocompatible bredigite (Ca₇MgSi₄O₁₆) scaffold with biomimetic apatite layer for bone tissue engineering. Porous bredigite scaffolds were prepared using polymer sponge method. The bredigite scaffolds with biomimetic apatite layer (BTAP) were obtained by soaking bredigite scaffolds in simulated body fluid (SBF) for 10 days. The porosity and in vitro degradability of BTAP scaffolds were investigated. In addition, osteoblast-like cell morphology, proliferation and differentiation on BTAP scaffolds were evaluated and compared with β-tricalcium phosphate (β-TCP) scaffolds. The results showed that BTAP scaffolds possessed 90% of porosity. The degradation of BTAP scaffolds was comparable to that of β-TCP scaffolds. Cells on BTAP scaffolds spread well and presented a higher proliferation rate and differentiation level as compared with those on β-TCP scaffolds. Our results indicated that BTAP scaffolds were degradable and possessed the function to enhance cell proliferation and differentiation, and might be used as bone tissue engineering materials.     

Study on dielectric and tunable properties of Cr-doped Ba<Subscript>0</Subscript><Subscript>.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript> thin films by rf sputtering

Ba_0.6Sr_0.4TiO₃ dielectric thin films doped by Cr(0, 1, 2.5, 5, 10 mol%) (BSTC) were prepared by radio frequency magnetron sputtering on Pt/Ti/SiO₂/Si substrates. The structure and morphology of the BSTC thin films were studied by atomic force microscopy and X-ray diffraction. The effect of Cr doping on the dielectric properties of BST thin films were analyzed. The results show that the dielectric loss of Cr doping BST thin films is lower than that undoped, and the tunability increased with Cr doping. The thin film doped with 5 mol% Cr has the best dielectric properties. The tunability, loss and figure of merit (FOM) at 1 MHz were 38.9%, 0.0183, and 21.3, respectively.     

Rietveld analysis of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> thin films obtained by RF-sputtering

Calcium copper titanate, CaCu₃Ti₄O₁₂, CCTO, thin films with polycrystalline nature have been deposited by RF sputtering on Pt/Ti/SiO₂/Si (100) substrates at a room temperature followed by annealing at 600 °C for 2 h in a conventional furnace. The crystalline structure and the surface morphology of the films were markedly affected by the growth conditions. Rietveld analysis reveal a CCTO film with 100 % pure perovskite belonging to a space group Im3 and pseudo-cubic structure. The XPS spectroscopy reveal that the in a reducing N₂ atmosphere a lower Cu/Ca and Ti/Ca ratio were detected, while the O₂ treatment led to an excess of Cu, due to Cu segregation of the surface forming copper oxide crystals. The film present frequency -independent dielectric properties in the temperature range evaluated, which is similar to those properties obtained in single-crystal or epitaxial thin films. The room temperature dielectric constant of the 600-nm-thick CCTO films annealed at 600 °C at 1 kHz was found to be 70. The leakage current of the MFS capacitor structure was governed by the Schottky barrier conduction mechanism and the leakage current density was lower than 10^?7 A/cm² at a 1.0 V. The current–voltage measurements on MFS capacitors established good switching characteristics.     

In vitro evaluation of gentamicin release from a bioactive tricalcium silicate bone cement

Tricalcium silicate (Ca₃SiO₅) cement, a novel self-setting biomaterial, has been shown to exhibit good hydraulic properties and excellent bioactivity. In this study, gentamicin sulfate (GS) was integrated into cement pastes and in vitro release of GS from the Ca₃SiO₅ cement was performed in deionized water, phosphate buffer saline (PBS) and HCl solutions with different pH at 37 °C, respectively. The results showed that the initial fast release of GS was restricted to a low level and prolonged release of drugs was achieved in water and PBS. The prolonged GS release is attributed to the interaction of GS with the calcium silicate hydrate network and the formation of unique nano-to-micro porous structure after hydration. Furthermore, GS release from milled powders of the hydrated cement suggested that the constrained GS could be released at low pH environment or during the degradation of the cement. When the samples were soaked in PBS, a nano-structured apatite layer was formed on the surface of the cement, which resulted in a relatively lower GS release rate as compared to that in water. The results suggest that Ca₃SiO₅ cement might be used as bioactive bone implant materials with drug loading and prolonged release properties.     

Preparation and characterization of Cu/SiO<Subscript>2 </Subscript>catalyst by co-gelation process

Cu/SiO₂ catalyst with bimodal pore structure was prepared by co-gelation reactions of tetramethoxysilane (TMOS) and copper nitrate in the presence of poly (ethylene oxide) (PEO) with an average molecular weight of 10,000 and the catalyst of acetic acid. In this process, the interconnected macroporous morphology was formed when transitional structures of spinodal decomposition were frozen by the sol–gel transition of silica. The addition of copper into the silica–PEO system had a negligible effect on the morphology formation. In gel formation, it was found that the crystallite sizes of the CuO estimated from the peak width in the Cu/SiO₂ with the presence of PEO were not small as expected. It was considered that there was no obvious interaction between the Cu cation and PEO, most of the copper ions in wet silica gel were present in the outer solution. They easily aggregated as copper salts in the drying process of wet gel and decomposed into CuO particles in heating. While in the Cu/SiO₂ with the absence of PEO, the Cu was selectively entrapped as small particles in the gel skeleton due to the interaction between Cu aqua complex and silica gel network.     

In vitro release of cisplatin from sol–gel processed organically modified silica xerogels

SiO₂, SiO₂/PEG and SiO₂/PDMS xerogels were examined as polymeric carriers for the controlled release of cisplatin—an antineoplasmic medicine. Drug/carrier systems were prepared by the sol–gel method. The effect of organic substitution of the silica xerogel matrix and drying conditions on the release of cisplatin was evaluated. Based on the presented results of the study it may be stated that sol–gel method is useful for entrapping a cisplatin in the pores of organically modified silica gels and for releasing cisplatin mainly in the way of diffusion from the pores of the lattice under the in vitro conditions. The use of organic impurities in silica gel increased the release of cisplatin from xerogel (from 62% to 97% within 7 days), and thermal treatment of all xerogels with cisplatin at the temperature of 80 °C resulted in the acceleration of the drug release (2 days) and increase of the released drug (89–98%).     

Microstructural evolution during direct laser sintering in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system

The microstructural evolution during direct laser sintering of LSD (Layerwise Slurry Deposition)—samples in the Al₂O₃–SiO₂ system has been investigated. Slurries with a water content of 34 wt.% and a SiO₂/Al₂O₃—ratio of about 3:1 have been used to manufacture layers which—after consecutive drying—have been sintered and laminated by laser treatment. Densified samples can be obtained with laser irradiances from 190 to 270 kW/cm² and scan velocities between 35 and 65 mm/s. Elemental mappings of the layers’ cross sections suggest an inhomogeneous phase distribution in the laser sintered LSD samples with a slight alumina concentration gradient. A lower degree of particle melting in the bottom region of the layers is plausible due to attenuation of the laser beam intensity. SEM and HRTEM micrographs show that after a few seconds of laser treatment relictic starting phase, crystalline alumina plus amorphous silica, occur together with needle like mullite, the latter formed within an amorphous aluminosilicate phase. The resulting phase assemblage reflects the non-equilibrium conditions which can be expected for short time laser treatments. Mullite nucleation within the bulk of the liquid phase rather than in the vicinity of the parent alumina phase suggests that dissolution of alumina is the rate controlling step. Subsequent thermal post treatment in air in a conventional sintering furnace causes an increase of density to about 96% and leads to additional phase reactions. Amorphous silica transforms into cristobalite and the amount of alumina is reduced by additional mullite formation. By both coalescence of individual crystals and grain growth the morphology of the newly formed mullite changes during post heat treatment.     

Ultra-long SiO<Subscript>2</Subscript> and SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> tubes embedded with Pt nanoparticles using magnus green salt as templating structures

For the first time, magnus green salt (MGS, [Pt(NH₃)₄][PtCl₄]) fibers precipitated by solvent modification have been employed as a structure-directing modifier to synthesize single silica and silica/titania microtubes via a sol–gel process. In the case of titania tubes, tetraethylorthosilicate must be used as a capping agent to hinder the aggregation of primary MGS fibers and to serve as a protective layer against thermal stress during the metal salt fiber reduction. This implies that SiO₂/TiO₂ tubes result. The synthesized tubular materials were imaged by scanning and transmission electron microscopy, while their composition was determined by energy dispersive X-ray analysis and thermogravimetric analysis. Crystallinity and thermal stability of the tube walls were studied using X-ray diffraction analysis. The obtained oxide tubes possess high aspect ratios (80–200) because they are up to 60 μm in length, but only 300–700 nm in thickness. The key aspects of the synthesis approach are that the templating MGS fibers control the internal diameter of the oxide tubes, while the synthesis conditions control their wall thickness. The suggested method is a simple approach which produces, at low temperatures, very long oxide tubes with a very high amount of Pt (48–51 wt%) directly incorporated inside the tubes. To the best of our knowledge, filling of SiO₂ or SiO₂/TiO₂ nanotubes with such a dense population of Pt metal nanoparticles has not been demonstrated so far; our own experiments with [Pt(NH₃)₄](HCO₃)₂ as templating salt formed only tubes containing about 40 wt% Pt and were only about 20 μm long. The now formed more Pt-rich tubes are expected to have vivid applications in (photo)catalysis and in fabricating novel devices, such as nano- or sub-microcables.