Highly transparent sol-gel derived ureasilicate monoliths exhibiting long-term optical stability

Optically clear and elastic organic-inorganic hybrid materials were synthesized by hydrolysis and condensation of ureasilicate precursors. Ammonia or citric acid was used as a catalyst for hydrolysis and condensation reactions. The influence of the catalyst nature and the precursor content on the optical, mechanical and thermal properties of samples was investigated. It was shown that it is possible to significantly change the elastic properties of the ureasilicates keeping their good optical transparency in the visible range. The preparation conditions assuring long-term optical and mechanical stability of the samples were determined.     

Drying and sintering of sol-gel derived large SiO2 monoliths

This review article summarizes the development of drying and sintering techniques for the production of sol-gel derived, large silica glass components. Gels may be synthesized using particulate or metal alkoxide precursors, or both in combination. Rapid fracture-free drying has been achieved easily with particulate gels because of their large pore size (100–6000 Å). Alkoxide gels, which generally have small pores (<200 Å), were initially difficult to dry without cracking. However, recent studies have shown that large alkoxide gel monoliths can also be dried in reasonably short times (<10 days). During subsequent heat treatment, alkoxide gels tend to have high shrinkage rates, which may cause trapping of hydroxyl ions or organic groups remaining on the gel surface. Although the removal of these species is easier for particulate gels, their large pore size necessitates heating above 1400°C to achieve full consolidation. Sintering at such temperatures was observed to deteriorate glass quality, through crystallization, warping, and/or sagging. Extensive optimization of the entire process has shown that on a laboratory scale, high-optical-quality glass can be produced from both alkoxide and particulate gels. It remains to be seen whether sol-gel process will be feasible for the manufacture of high-quality glass products on a commercial scale.     

Transition from transparent aerogels to hierarchically porous monoliths in polymethylsilsesquioxane sol-gel system

A transition from hierarchical pore structures (macro- and meso-pores) to uniform mesopores in monolithic polymethylsilsesquioxane (PMSQ, CH(3)SiO(1.5)) gels has been investigated using a sol-gel system containing surfactant Pluronic F127. The precursor methyltrimethoxysilane (MTMS) undergoes an acid/base two-step reaction, in which hydrolysis and polycondensation proceed in acidic and basic aqueous media, respectively, as a one-pot reaction. Porous morphology is controlled by changing the concentration of F127. Sufficient concentrations of F127 inhibit the occurrence of micrometer-scale phase separation (spinodal decomposition) of hydrophobic PMSQ condensates and lead to well-defined mesoporous transparent aerogels with high specific pore volume as a result of the colloidal network formation in a large amount of solvent. Phase separation regulates well-defined macropores in the micrometer range on decreasing concentrations of F127. In the PMSQ-rich gelling domain formed by phase separation, the PMSQ colloidal network formation forms mesopores, leading to monolithic PMSQ gels with hierarchical macro- and meso-pore structures. Mesopores in these gels do not collapse on evaporative drying owing to the flexible networks and repulsive interactions of methyl groups in PMSQ.     

Silica gel-based monoliths prepared by the sol-gel method: facts and figures

There is a great deal of interest in continuous beds as stationary phases for both HPLC and CEC. There are various ways to prepare monoliths, by polymerization of organic species or by polymerization of silicon alkoxides. The former method has recently been reviewed, while silica based monoliths are now commercially available. The purpose of this paper is to deal with the problems associated with silica based monoliths. The most important problem is obviously the cracking and the shrinkage of the bed during drying. The second problem is monolith cladding. Much literature has been published but no definitive solution is available and thus a wide research area remains open. Monoliths are a compromise between loadability, permeability and mass transfer kinetics. Due to the better mass transfer properties of a monolithic skeleton over distinct particles, high flow rates and high speed separations are possible.     

Separation behavior of electron-beam curing derived, acrylate-based monoliths

Electron beam (EB) curing-derived monolith materials were prepared from ethyl methacrylate (EMA), trimethylolpropane triacrylate (TMPTA), 2-propanol, 1-dodecanol, and toluene within the confines of 3 mmx100 mm id glass columns, applying a total dose of 22 kGy for curing. Monolithic columns were checked for their separation behavior for selected dansylated (DNS)-amino acids as well as for cyclophilin 18. Their separation performance was compared to that of a C18-modified silica-based rigid rod (Chromoliths). In the separation of dansylated amino acids, retention times were reduced on EB-derived columns, where the peak resolution was significantly better than on a Chromolith. This finding was attributed to a larger fraction of small pores (<2.15 nm) in the EB curing-derived monoliths. Finally, EB curing-derived monoliths have been used to separate cyclophilin 18 from crude cell lysis mixtures.     

Dynamical processes in sol-gel derived rare-earth doped glasses

We have investigated the dynamical processes of rare-earth photoluminescence in silica glasses produced by the sol-gel method. Generally, static spectroscopic properties are used as a tool to probe the local structure near dopants as well as for the study of glass formation. Rare earth ions are preferred structural probes, especially the Eu³⁺ ion for its sensitivity to local structural modifications and its spectroscopic simplicity. By an analysis of the decaying emission that follows a pulsed laser excitation, we have studied several dynamical processes. Importance of a distribution of different environments for the dopant is clearly seen even for the wet gels. The evolution of the lifetimes allow to study the densification process.     

Optical determination of Cr(VI) using regenerable, functionalized sol-gel monoliths

Transparent, pyridine-functionalized sol-gel monoliths have been formed and their use in Cr(VI) sensing applications is demonstrated. The monoliths were immersed in acidic Cr(VI)-containing solutions, and the Cr(VI) uptake was monitored using UV-vis and atomic absorption spectroscopies. At concentrations at the ppm level, the monoliths exhibit a yellow color change characteristic of Cr(VI) uptake, and this can be measured by monitoring the absorption change at about 350 nm using UV-vis spectroscopy. Concentrations at the ppb level are below the limit of detection using this wavelength of 350 nm for measurement. However, by adding a diphenylcarbazide solution to monoliths that have been previously immersed in ppb-level Cr(VI) solutions, a distinct color change takes place within the gels that can be measured at about 540 nm using UV-vis spectroscopy. Concentrations as low as 10 ppb Cr(VI) can be measured using this method. The monoliths can then be regenerated for subsequent sensing cycles by thorough washing with 6.0 M HCl. The factors affecting monolith uptake of Cr(VI) have been explored. In addition, the gels have been characterized using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) measurements.     

Enzymatic activity of oxalate oxidase and kinetic measurements by optical methods in transparent sol-gel monoliths

Novel synthetic techniques are used for the encapsulation of the enzymes oxalate oxidase and peroxidase in stable, optically transparent porous silica glass matrices. The large enzymes are fully immobilized in the porous glass but small molecules such as oxalate ions pass readily through the pores in the glass. The enzymes catalyze the reactions leading to the formation of a colored dye product. Upon exposure of the doped glass to oxalate solutions, a colored glass is formed. The absorption spectrum of the colored product and the changes of absorbance with time are measured within the glass matrix. The sensitivity and the time-dependence of the response are discussed.     

In situ sol-gel preparation of porous alumina monoliths for chromatographic separations of adenosine phosphates

A method enabling the in situ preparation of porous alumina monoliths within 100 μm i.d. fused silica capillaries has been developed. These monoliths were prepared using the sol-gel process from a mixture consisting of an inorganic aluminum salt, a porogen, an epoxide, and a solvent. We investigated the effects of varying the preparation conditions on the physical characteristics of the monoliths with respect to their potential application in chromatographic separations. The best columns were obtained from a mixture of aluminum chloride hexahydrate, N,N-dimethylformamide, water, ethanol and propylene oxide. Adenosine phosphates were then separated in the optimized column with retention increasing according to number of phosphate functionalities.     

Effect of preparatory conditions on the performance of photopolymerized sol-gel monoliths for capillary electrochromatography

We prepared different photopolymerized sol-gel (PSG) columns by varying the amount of monomer (methacryloxypropyltrimethoxysilane), porogen (toluene) and catalyst (hydrochloric acid) in the reaction solution containing a photoinitiator (Irgacure 1800). The effects of these variations on the chromatographic behavior of the PSG columns were studied. All of the columns studied exhibited reversed-phase character. The concentration of hydrochloric acid was important for the rigidity of the columns, although it did not affect the separation property. The ratio of monomer solution to porogen was a critical factor in controlling the through-pore size and the surface area of PSG, which were found to significantly affect the separation properties, such as permeability, theoretical plate number, retention time, and separation efficiency, of a mixture of test analytes-thiourea, benzene, and naphthalene. There was no change in the retention order for the test analytes. Short separation times were achieved on PSG columns made from a 10% monomer stock solution and 90% porogen with 1 M hydrochloric acid. Mixtures of polycyclic aromatic hydrocarbons and alkylbenzenes were separated with theoretical plate numbers greater than 100 000 plates/m.     

Fabrication of anisotropic porous silica monoliths by means of magnetically controlled phase separation in sol-gel processes

Sol-gel accompanied by phase separation is an established method for the preparation of porous silica monoliths with well-defined macroporosity, which find numerous applications. In this work, we demonstrate how the addition of (superpara)magnetic nanocolloids as templates to a system undergoing a sol-gel transition with phase separation leads to the creation of monoliths with a strongly anisotropic structure. It is known that magnetic nanocolloids respond to the application of an external magnetic field by self-assembling into columnar structures. The application of a magnetic field during the chemically driven spinodal decomposition induced by the sol-gel transition allows one to break the symmetry of the system and promote the growth of elongated needle-like silica domains incorporating the magnetic nanocolloids, aligned in the direction of the field. It is found that this microstructure imparts a strong mechanical anisotropy to the materials, with a ratio between the Young's modulus values measured in a direction parallel and perpendicular to the one of the field as high as 150, and an overall smaller average macropores size as compared to isotropic monoliths. The microstructure and properties of the porous monoliths can be controlled by changing both the system composition and the strength of the applied magnetic field. Our monoliths represent the first example of materials prepared by magnetically controlling a phase transition occurring via spinodal decomposition.     

Pyrolysis study of sol-gel derived zirconia by TG-GC-MS

Summary A homogeneous ZrO₂ gel was obtained by hydrolysis-condensation of zirconium(IV) n-propoxide previously reacted with acetic acid. Dried zirconia powders were characterized by Fourier transformed infra-red (FTIR) and X-ray diffraction (XRD) analyses. Thermogravimetric (TG) and differential thermal analysis (DTA) coupled with mass spectrometric (MS) and gas chromatographic (GC) measurements were carried out in order to identify and quantify the organic products released during the ZrO₂ gel pyrolysis. The TG-MS semi-quantitative analysis of the main released species allowed to describe the chemical rearrangement occurring in the solid during heating and to determine the chemical composition of the initial gel.     

Preparation and microstructure of sol-gel derived silver-doped silica

Silver-doped silica was prepared by hydrolysis and condensation of tetraethyl orthosilicate (TEOS, Si(OC₂H₅)₄) in the presence of a silver nitrate (AgNO₃) solution by two different synthesis methods. In the first synthesis route, sol-gel mixtures were prepared using an acid catalyst. In the second synthesis route, silver-doped silica gels were formed by two-step acid/base catalysis. For the same concentration of silver dopant [AgNO₃]/[TEOS] = 0.015 acid-catalyzed sol-gel formed a microporous silica with an average pore size of <25 Å whereas the two-step catalyzed silica had an average pore size of 250 Å and exhibited a mesoporous structure when fully dried. The differences in the pore size affected the silver particle formation mechanism and post-calcination silver particle size. After calcination at 800 °C for 2 h the acid-catalyzed silica contained metallic silver particles size with an average particle size of 24 ± 2 nm whereas two-step catalyzed silica with the same concentration of [AgNO₃]/[TEOS] = 0.015 contained silver nanoparticles with an average size of approximately 32 ± 2 nm. Mechanisms for silver particle formation and for silica matrix crystallization with respect to the processing route and calcination temperature are discussed.     

Platinum acetylacetonate effect on sol-gel derived titania catalysts

The band gap (Eg) of sol-gel derived titania has been determined using UV-Vis spectroscopy. When platinum acetylacetonate was added during titanium tetrabutoxide gelation, a significant diminution of the band gap was observed. Low Eg values were stabilized on heat-treated platinum/titania samples.     

Dual-analyte spectroscopic sensing in sol-gel derived polyelectrolyte-silica composite thin films

Ferrozine (3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p′-disulfonic acid, monosodium salt hydrate), an iron indicator, and HTPS (8-hydroxyl-1,3,6-pyrenetrisulfonic acid, trisodium salt), a pH indicator, were immobilized in sol–gel derived PDMDAAC-SiO₂ (where PDMDAAC stands for poly(dimethyldiallylammonium chloride), composite thin films via ion-exchange. The two indicators were immobilized in two adjacent sections of the same PDMDAAC-SiO₂ film which was supported on a glass optical substrate. The spectroscopic response of the film to both Fe²⁺ and H⁺ in solutions was investigated by attenuated total reflection (ATR) spectrometry at two well-separated wavelengths, 562 nm for Fe²⁺ and 460 nm for H⁺. The Ferrozine/HPTS immobilized PDMDAAC-SiO₂ films had the following characteristics: linear range, 2.5×10⁻⁶–5.0×10⁻⁵ M for Fe²⁺, pH 4.1–6.8 for H⁺; sensitivity, 2.2×10⁴ ΔA/M for Fe²⁺, 0.583 ΔA/pH for H⁺.     

Encapsulation of the ferritin protein in sol-gel derived silica glasses

A significant recent development in sol-gel science has been the encapsulation of biomolecules such as proteins and enzymes in optically transparent silica glasses. This paper reports on the encapsulation of an iron (Fe) storage protein, ferritin, to develop a magnetic silica glass. Native ferritin, which has a nanometer-sized microcrystalline Fe oxide core, was encapsulated in optically transparent silica glasses using the sol-gel process. Fe could be released from ferritin but could not be reconstituted into apoferritin when the protein was trapped in the pores of the glass. Transmission electron microscopy of ferritin-doped aged silica gels indicated that crystallinity of the Fe oxide core was retained upon sol-gel encapsulation. Magnetic measurements on ferritin-doped silica gels indicated the material to be paramagnetic, but not superparamagnetic.     

Controlling the primary particle evolution process towards silica monoliths with tunable hierarchical structure

In order to establish the hierarchical structure in multiple levels on mesoporous silica, this article reports a new strategy to prepare the monolith with the pore configuration in nanometer scale, micro-morphology in micrometer level and macroscopic shape in millimeter or larger grade. These hierarchical monoliths are synthesized in a weak acidic condition by using triblock copolymer P123, hydroxyl carboxylic acid and tetramethyl orthosilicate (TMOS), and the textural properties of the mesostructure can be facilely adjusted by simply controlling the synthesis condition without any additive. During the synthesis, the primary particles can be selectively synthesized as monodispersed sphere, noodle, prism, straight rods with different size or irregular bars, and their connection plus arrangement in 3D directions can be also regulated. Therefore, various textural properties of mesopore are able to be altered including pore size (5.5-10.6 nm), total pore volume (0.48-1.2 cm(3) g(-1)), micropore surface area (47-334 m(2) g(-1)), and pore shape (from 2D or 3D straight channel to plugged channel). Moreover, these monoliths exhibit a considerable mechanical strength; they are also applied in eliminating particulate matters and tobacco special nitrosamines (TSNA) in tobacco smoke, exhibiting various morphology-assisted functions.     

Structural evolution during pyrolysis of sol-gel derived hybrid materials

Siloxane-titania materials have been prepared starting from diethoxydimethylsilane and titanium isopropoxide. The transparent starting gels can be described as nanocomposites formed with siloxane chains and TiO₂-based particles. The pyrolysis process under argon atmosphere up to 1600°C has been followed by various techniques such as infrared and solid state NMR spectroscopies, X-ray diffraction and Ti K-edge X-ray Absorption. Si-C bond cleavage begins at low temperature leading to redistribution reactions with the formation of new Si-O bonds. Crystallization of TiO₂ at low temperature is prevented and titanium carbide starts to form around 800°C. Samples pyrolyzed at 1600°C show the presence of SiO₂ and TiC crystalline phases.