SiO2 Matrices Research Articles

The Release and Bioactivity of Lysozyme from Selected Sol-Gel Derived SiO<sub>2</sub> Matrices

Local drug release has many benefits – a steadier distribution, improved compliance, but most importantly it allows the convenient use of protein based molecules as therapeutic agents. Many different types of materials have been studied as drug carriers, including sol-gel derived SiO2 matrices. In this study lysozyme was used as a model protein and its release from prepared SiO2 monoliths and its biological activity thereafter was studied spectroscopically. Sucrose was used in some preparations to assess its ability to function as a protective agent during storing. Lysozyme release and bioactivity was similar in both preparations containing it when tested fresh. In monoliths stored for ten weeks, however, differences were observed in the biological activity of released lysozyme. In the preparations containing sucrose, lysozyme had retained its activity, while it was virtually nil in the preparations containing only lysozyme. This shows that sol-gel derived SiO2 matrices can be used as carriers for small proteins and that sucrose can function as a protective agent in them.

Composition of Ge+ and Si+ implanted SiO2/Si layers: Role of oxides in nanocluster formation

X-ray photoelectron spectroscopy (XPS) has been used to examine the atomic content of implanted SiO2/Si layers. In particular, an XPS analysis permits to identify elemental Ge and Si, as well as GeO2 precipitations in SiO2 matrices. The XPS results reveal valuable information not only about the formation mechanism of Ge and Si nanoclusters but also on the annealing kinetics of SiO2 whose properties are known to be significantly altered during the process of ion implantation and subsequent annealing. The composition of ion beam-modified SiO2 layers strongly depends on the annealing temperature. With respect to germanium implanted samples a possibility of Ge nanocrystals formation appears at high (above 1000°C) annealing temperatures. It has been shown that an intermediate step in the Ge oxide formation is necessary for the creation of Ge nanoclusters. Additionally, the presence of a subsurface zone GeOx (about 100nm thick) predicted in kinetic three-dimensional lattice simulations has been confirmed. In the case of Si+ implanted samples substoichiometric silicon oxide lines in the XPS spectra of a SiO2 layer for all samples have been observed. No evidence of a line connected to the Si–Si bonding has been observed even at the highest annealing temperatures, at which only stoichiometric SiO2 has been detected.

Direct photo-oxidation of benzene to phenol over Ti/Si binary oxide prepared by sol-gel method

Titanium-silicon (Ti/Si) binary oxides having a varying Ti content were prepared using the sol-gel method and used as photocatalysts. The photo-oxidation of benzene to phenol was carried out using Ti/Si binary oxide catalysts in the presence of benzene, water and gaseous oxygen. The amounts of benzene used hardly affected the phenol yield. On the other hand, the addition of sulfuric acid into reaction solution led to the improvement of phenol formation. The H2O2 formation rate seemed to be related to the phenol yeilds. The photocatalytic reactivity of Ti/Si binary oxide having different TiO2 contents was investigated and it was found to be dramatically enhanced in the range of lower TiO2 contents. XANES, ESR and XRD spectroscopic investigations of these Ti/Si binary oxide catalysts indicated that Ti species were highly dispersed in SiO2 matrices and existed in a tetrahedral coordination. The photo-irradiation of catalysts having tetrahedral Ti species seemed to effectively lead to the formation of H2O2 as an intermediate.

Solar selective absorber functionality of carbon nanoparticles embedded in SiO2, ZnO and NiO matrices

AbstractIt is possible to design an optical material, through control of the substrate and coatings, to absorb a large fraction of the solar spectrum and to emit very little radiation energy in the near and far infra‐red wavelength range. This requires a coating structure that transmits in the ultraviolet and visible, but reflects in the near and far infrared, with a sharp drop in transmission near 2 (m. In this paper, we report on selective solar absorber coatings of carbon nano‐particles embedded in SiO2, ZnO and NiO matrices, fabricated by a sol‐gel technique. The coatings were tested on aluminium substrates. UV‐VIS and FTIR spectroscopies were used to determine the spectral response of the composite coatings, from which the solar absorptance and thermal emittance were determined. We show that the NiO matrix samples have the best solar selective behaviour, and suggest a theoretical evaluation framework for future comparison of such materials. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Highly Dispersed Mixed Zirconia and Hafnia Nanoparticles in a Silica Matrix: First Example of a ZrO2–HfO2–SiO2 Ternary Oxide System

AbstractZrO2 and HfO2 nanoparticles are homogeneously dispersed in SiO2 matrices (supported film and bulk powders) by copolymerization of two oxozirconium and oxohafnium clusters (M4O2(OMc)12, M = Zr, Hf; OMc = OC(O)–C(CH3)CH2) with (methacryloxypropyl)trimethoxysilane (MAPTMS, (CH2C(CH3)C(O)O)–(CH2)3Si(OCH3)3). After calcination (at a temperature ≥800 °C), a silica matrix with homogeneously distributed MO2 nanocrystallites is obtained. This route yields a spatially homogeneous dispersion of the metal precursors inside the silica matrix, which is maintained during calcination. The composition of the films and the powders is studied before and after calcination by using Fourier transform infrared (FTIR) analysis, X‐ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS). The local environment of the metal atoms in one of the calcined samples is investigated by using X‐ray Absorption Fine Structure (XAFS) spectroscopy. Through X‐ray diffraction (XRD) the crystallization of Hf and Zr oxides is seen at temperatures higher than those expected for the pure oxides, and transmission electron microscopy (TEM) shows the presence of well‐distributed and isolated crystalline oxide nanoparticles (5–10 nm).

Excited-state Intramolecular Proton Transfer of 1,5- and 1,8-Dihydroxyanthraquinones Chemically Adsorpted onto SiO2, SiO2-Al2O3, and Al2O3Matrices

In order to investigate the excited-state intramolecular proton transfer (ESIPT) process of dihydroxyanthraquinones (DHAQ; 1,5-DHAQ and 1,8-DHAQ) in organic-inorganic hybrid matrices, transparent SiO2, SiO2Al2O3, and Al2O3 matrices chemically bonded with DHAQ were prepared using a sol-gel technique. The absorption maxima of 1,5- and 1,8-DHAQ in SiO2 matrices are observed at around 420 nm, whereas those of DHAQ in both SiO2-Al2O3 and Al2O3 matrices are markedly shifted to longer wavelength compared with those in SiO2 matrix. This indicates that DAHQ forms a chemical bond with an Al atom of Al2O3. The DHAQ in SiO2 matrix shows a markedly Stokes-shifted emission which is originated from the ESIPT in DHAQ. Based on the emission lifetimes of DHAQ, the ESIPT of DHAQ was found to be strongly affected by the chemical interaction with Al atom in the Al2O3-related matrices.

Nonlinear Optical Absorption and Switching Properties of Gold Nanoparticle Doped SiO<SUB>2</SUB>–TiO<SUB>2</SUB> Sol–Gel Films

The nonlinear optical absorption and switching properties of sol-gel derived of Au nanoparticle doped SiO2-TiO2 sol-gel films having different Au-surface plasmon absorption positions are reported in this paper. The Au nanoparticles are embedded in SiO2 and SiO2-TiO2 mixed glassy film matrices with different refractive index values. To study the nonlinear absorption properties, lasers with three different wavelengths are used. The optical switching behavior is studied by using the pump-probe technique with 532 nm as the excitation wavelength. Ground state conduction band, surface plasmon band, and the free carrier band are taken as three level model to explain theoretically the obtained RSA and SA behaviors.

Concentration and size dependence of the dynamics of the Mn 3d5 luminescence in wire‐like arrangements of (Zn,Mn)S nanoparticles

Abstract(Zn,Mn)S nanoparticles with Mn concentrations ranging from 1% to 30% and in a wire‐like arrangement were formed inside mesoporous SiO2 matrices of various pore diameters. The nanoparticles were characterised using photoluminescence and excitation spectroscopy. It is found, that the Mn2+ ions are incorporated on cation lattice sites replacing Zn. The decay times of the internal Mn2+ (3d5) luminescence are studied in detail by time resolved spectroscopy over more than 5 orders of magnitude in intensity. A concentration and a remarkable size dependence of the time behaviour has been observed indicating a geometry dependence of the energy transfer processes within the Mn system. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

ZnS/Si composite nano-structured thin films and their photoluminescence

Thin films of ZnS/Si composite were prepared using pulsed laser deposition by alternately ablating ZnS and Si materials on a rotary target holder onto an ultra-clean glass substrate in vacuum. The film structure consisted of a mixture of Si and ZnS nanocrystals with some segregation of S and Zn. Most of the particles in the film were found to be less than 15 nm in diameter even after annealing for 6 hours at 500°C in air. With excitation wavelengths of 280 nm and 380 nm, the composite film yielded largely independent photoluminescent (PL) emission peaks originating from the Si and ZnS nanoparticles. There is a slight difference in the Si and ZnS PL results compared with those obtained from thin films of Si nanocrystals and ZnS nanoparticles embedded in SiO2 matrices. This difference is attributed to the different segregation and oxidation behaviors of the materials in different neighborhood environments. Our results open up the possibility of modifying the PL of ZnS/Si composite films for potential applications in multiple-function optoelectronic devices.

Adjustably Bioresorbable Sol-Gel Derived SiO2 Matrices for Release of Large Biologically Active Molecules

Amorphous, sol-gel derived SiO2 are known to biocompatible and bioresorbable materials. Bioresorbable materials have potential applications as implants or injectable matrices in the controlled delivery of biologically active agents in the living tissue. Bioresorbable matrices provide desirable properties, e.g., extra removal operations that have to be done with biostable matrices are avoided and the release of large therapeutic molecules can be controlled by matrix degradation rather than by diffusion. New important groups of drugs, such as biotechnically-produced peptides and proteins, are potential to be encapsulated in bioresorbable SiO2, because they are typically larger in size and their direct oral administration without protecting matrix is difficult due to digestion. The methods to achieve a wide range of SiO2 bioresorption rates (from days to months) are introduced in this study. This is done by a “conventional” alkoxy-based sol-gel method at protein-friendly conditions by adjusting the precursor ratios, aging of the sol and by using different preparation methods (casting, spray-drying and freeze-drying). The prepared morphologies include implantable monolithic sticks and injectable microspheres. The importance of chemical structure is shown in comparison with the specific surface area and pore volume.

Chemical characterization of bioresorbable sol–gel derived SiO2 matrices prepared at protein-compatible pH

Sol–gel derived SiO2 have been shown to be biocompatible and bioresorbable and they have potential use in living tissue, e.g., in bone regeneration and controlled drug delivery. Bioresorbable SiO2 is a potential alternative for the controlled delivery of large biologically active agents, such as proteins and peptides. The aim was to prepare SiO2 matrices with varying bioresorption rate at protein-compatible conditions and to characterize the chemical features of the matrices. SiO2 was prepared in two morphologies, monoliths by casting and microparticles by spray drying. A model protein was encapsulated into the SiO2 matrices. Materials were carefully characterized with FTIR- and Raman spectroscopy, TGA-FTIR, solid state 29Si MAS NMR, SEM and matrix dissolution was measured in simulated physiological conditions. It is shown that both fast and slowly dissolving SiO2 matrices could be prepared at protein-compatible conditions. Fast-dissolving SiO2 microparticles contained a high proportion of Q3 and a low portion of Q4 indicating poor cross-linking of SiO2 species and an increased amount of hydrolysable terminal groups. Spectroscopic techniques and TGA-FTIR show that organic residues and moisture are left in the matrices. The amount of organic residues is larger in the fast-dissolving SiO2 matrices, but it does not significantly affect the bioresorption rate.

Laser and Properties of Nanostructured Materials

We review optical and electronic properties of Si-based nanostructured materials, such as O-dimensional Si nanocrystallites with sizes from 1 nm to 10 nm and 1-dimensional Si nanowires, that are synthesized using laser ablation techniques. Characteristics of clusters consisting of several atoms to hundreds of atoms are also described in order to understand the physical and chemical properties of Si-based nanostructured materials. Si nanocrystallites with sizes below 5 nm widen band gaps arising from the quantum confinement effect. P and Er impurities exhibit novel properties in SiO2 matrices including Si nanocrystallites. Si nanowire can be grown via vapor-liquid-solid mechanism. Quantized conductance and Coulomb blockade are described for the nanowires having interesting quantum properties.

Preparation of CdS-containing silica–titania composites by the sol–gel process

Silica–titania composite materials containing 20mol% TiO2 were prepared by sol–gel process using tetraethoxysilane (TEOS) and titanium isopropoxide (Ti(OPr)4) as precursors. A two-stage hydrolysis procedure was performed in acid conditions for preparation of the TiO2–SiO2 oxides. Nanometer sized crystallites of CdS were incorporated into TiO2–SiO2 matrices by complex formation with thiourea, SC(NH2)2. The influence of the preparation route and calcination temperature on the structural characteristics and physico-chemical properties of TiO2–SiO2 and CdS/TiO2–SiO2 materials was studied by means of nitrogen physisorption, X-ray diffraction, infrared spectroscopy and atomic force microscopy. The results indicate that anatase phase is stable at temperatures up to 600°C for TiO2–SiO2 matrices and the addition of CdS particles affects the system crystallization.

Brillouin scattering and x-ray photoelectron studies of germanium nanoclusters synthesized in SiO2 by ion implantation

Brillouin scattering and x-ray photoelectron spectroscopy (XPS) have been utilized to characterize Ge+-implanted thermal SiO2 layers on a Si substrate with subsequent annealing at 500°C and 1100°C. Sputtering depth profiling in conjunction with XPS studies have been applied to identify the chemical state of elemental Ge and GeO2 precipitations in the SiO2 matrices. The presence of a subsurface GeOx zone as predicted in kinetic 3-dimensional lattice simulations has been confirmed. It is concluded that the intermediate step of Ge oxide formation seems necessary for the creation of Ge nanoclusters. The Ge atomic concentrations obtained from XPS were used to compute the bulk and shear moduli, and consequently the surface acoustic wave (SAW) velocities, for the Ge∕GeO2∕SiO2 systems. These calculations confirm the character of SAW velocity softening as determined from the Brillouin scattering investigations.

Excitation of Er atoms by energy transfer from Si nanocrystallites embedded in SiO2 matrices fabricated by laser ablation

We have investigated excitation of Er3+ ions via energy transfer from Si nanocrystallites embedded in SiO2 films. The Er-doped films were fabricated using a laser ablation technique. We found that a photoluminescence (PL) excitation spectra of Er3+ ions coincides with that of Si nanocrystallites. Thus, it is evident that Er3+ ions are excited via the luminescent singlet state in Si nanocrystallites. Furthermore, we obtained the results that support the energy transfer mechanism. PL intensity of Er3+ ions increases with Er concentration while that of Si nanocrystallites decrease inversely. PL intensity of Er3+ ions increases with temperature from cryogenic to room temperature under photo-excitation at power density higher than 110 mW/cm2. The increase is characteristic of the luminescent state in Si nanocrystallites but not any state in Er3+ ions.

Preparation, Characterization, and Reactivities of Highly Functional Titanium Oxide-Based Photocatalysts Able to Operate under UV–Visible Light Irradiation: Approaches in Realizing High Efficiency in the Use of Visible Light

Abstract A review of the research advances made in the design and development of highly reactive and functional titanium oxide photocatalysts, which can utilize not only UV but also visible or solar light, and a clarification of the active sites as well as the detection of the reaction intermediates at the molecular level have been presented here. The potential for the effective utilization and conversion of solar energy into useful and safe chemical energy by the modification of the electronic properties of such TiO2 photocatalysts is great when one considers their myriad applications as well as their non-polluting qualities. Such a modification process by methods such as ion-implantation can be applied not only for semiconducting bulk TiO2 photocatalysts but also for TiO2 thin film photocatalysts and titanium oxide photocatalysts highly dispersed within zeolite frameworks. Moreover, the photocatalytic reactivity of semiconducting TiO2 nano-powders was found to be dramatically enhanced by the loading of small amounts of Pt. This worked to enhance the reduction reaction, resulting in the charge separation of the electrons and holes generated by light irradiation. In addition, highly dispersed titanium oxide species prepared within zeolite frameworks as well as SiO2 or Al2O3 matrices showed much higher and unique photocatalytic performances as compared to semiconducting bulk TiO2 photocatalysts. Significantly, a new alternative method to directly prepare such visible light-responsive TiO2 thin film photocatalysts on various substrates has been successfully developed by applying a RF magnetron sputtering deposition method.

Analysis of silicates using wavelength‐dispersive x‐ray fluorescence spectrometry for major elements: effects of loss elimination and catch‐weights

AbstractAnalysis of silicates using wavelength‐dispersive x‐ray fluorescence spectrometry was evaluated for the determination of the 10 most common major elements, Fe, Mn, Ti, P, Ca, K, Si, Al, Mg and Na and also Zr, Sr, S and Cl. The calibration was based on de Jongh alpha coefficients for loss‐eliminated matrix correction, a flux containing Li2B4O7 and LiBO2 and two‐level synthetic standards prepared in SiO2 matrices (except for S and Cl). Sulphur and Cl standards were prepared in Al2O3 matrices because of poor reproducibility of duplicates prepared in an SiO2 matrix. Backgrounds for major elements (high peak‐to‐background ratio) were not measured. Trace elements (low peak‐to‐background ratio) were not determined and their matrix effects were not considered. The method was validated for the 10 most common major elements using 23 Certified Reference Materials covering a wide range of concentrations. The absolute error was <±0.2% for most determinations and within experimental error. Sulphur and Cl in high silicate concentrations were not accurately quantifiable owing to variable retention during the vigorous fusion of the SiO2 matrix. For samples with high loss‐on‐fusion (LOF), catch‐weights caused a significant and systematic error for the major element with highest concentration. The presence of undetermined trace elements showed no significant effect except slightly overestimated LOF. For an accurate analysis, it is therefore necessary that nominal weights be used when LOF is high and all major elements are determined. The method is suitable for the ‘as received’ analysis of a wide variety of silicates for major elements. Copyright © 2004 John Wiley & Sons, Ltd.

Preparation of Titanium−Silicon Binary Oxide Thin Film Photocatalysts by an Ionized Cluster Beam Deposition Method. Their Photocatalytic Activity and Photoinduced Super-Hydrophilicity

Titanium−silicon binary oxide thin films of differing TiO2 content were prepared by using an ionized cluster beam (ICB) deposition method using multi-ion sources under a dry preparation process. From the results of UV−vis absorption measurements, binary oxide thin films with low TiO2 content showed much higher transmittance as compared with the pure TiO2 thin film, indicating that the Ti−oxide species of these Ti−Si binary oxide thin films exist in a highly dispersed state within the SiO2 matrices. Ti K-edge XAFS measurements revealed that such binary oxide thin films with low TiO2 content (less than 20%) showed only a sharp preedge peak attributed to the highly dispersed Ti−oxide species which are different from the peaks of anatase TiO2 thin films. Moreover, they were found to exhibit much higher and more efficient photocatalytic reactivity for the decomposition of NO into N2 and O2 under UV light irradiation than the pure TiO2 thin films.

Structural studies of bismuth nanocrystals embedded in SiO2 or GeO2 matrices

The layer of bismuth nanoclusters embedded in glass matrices and the surface layer of bismuth grains have been obtained by thermal treatment in hydrogen atmosphere of Bi0.33Ge0.67O1.84 and Bi0.57Si0.43O1.72 glass. The thickness and structure of such layers strongly depend on temperature and time of reduction. The structural studies of bismuth nanocrystals embedded in SiO2 or GeO2 matrices were performed with optical microscopy and atomic force microscopy. By the use of a slow-positron beam we monitored the structural changes undergoing in near-to-surface layers after the first steps of isothermal annealing. A simple two-layer model of reduced glasses explains the evolution of the surface layer and electrical properties of the material during the reduction process.

Self-organized growth and optical emission of silicon-based nanoscale β-SiC quantum dots

Si-based β-SiC quantum dots (QDs) were fabricated for exploring efficient blue emission from β-SiC nanostructures. Microstructural observations and x-ray photoemission spectroscopy reveal that the β-SiC QDs with sizes of 5–7 nm are embedded in the SiO2 and graphite matrices, displaying a locally tetragonal symmetry. Photoluminescence spectral examinations show two narrow blue-emitting bands at 417 and 436 nm, which are determined by both quantum confinement and surface structure of the β-SiC QDs. Electron spin resonance investigation demonstrates that the photoexcited carriers partially come from the β-SiC QD core with a widened band gap, whereas the radiative recombination occurs in Si excess defect centers at the β-SiC QD surface. A theoretical calculation about electronic states caused by the vacancy defects in the gap of balls formed with excess Si atoms at the surfaces of the β-SiC QDs supports our assignment to the two blue-emitting origin.