Electrochemical oxidation of aniline in a silica sol–gel matrix

Electrochemical oxidation of aniline encapsulated in a silica solid electrolyte prepared by a sol–gel process yielded products that were dependent on the pore size. An acid-catalyzed process that used tetramethyl orthosilicate as the precursor and aniline as a dopant yielded the silica. When the aging time was limited to one day so that a mesoporous solid was obtained, the potentiodynamic oxidation of aniline at a carbon fiber electrode resulted in the formation of polyaniline. With aging times of 3–5 days, microporous silica was obtained. In this electrolyte, the formation of dimers and other oligomers was observed by cyclic voltammetry. Evidence for these products was the presence of a quasi-reversible redox couple at 0.2 V vs Ag/AgCl that was previously related to oligomeric aniline by Raman spectroscopy. The results supported the hypothesis that the pore structure of sol–gel electrolytes can influence the pathways of electrode reactions therein.     

Fluorescent organic nanocrystal confined in sol–gel matrix for bio-imaging

The sol–gel chemistry combined to a spray-drying process allowed us to control the formation of original hybrid core–shell nanoparticles constituted by molecular nanocrystals of rubrene embedded in biocompatible silicate spheres. With a good management of all the physical (gas flows, temperatures) and chemical (dye, solvent and alkoxide natures, concentrations, and hydrolysis and condensation conditions) parameters, we optimized a one-step and self-assembly process allowing to obtain nanoparticles exhibiting promising optical properties such as highly fluorescent labels (two-photon excitation) for medical imaging. Moreover, the presence of Si–OH functions on the silicate shell surface make easy to functionalize these fluorescent nanoparticles by grafting biomolecules for targeting properties. The confined nucleation and growth of rubrene nanocrystals in sol–gel silicate spheres during their drying in the air laminar flows, prevents any phase segregation or particle coalescence and stabilizes mechanically and chemically the organic cores. The first particle sizes obtained in these first experiments are ranging between 80 and 600 nm, but lower diameters will be easily prepared by increasing the solvent amount. Transmission electron microscopy was used to characterize the rubrene organic cores. The electron diffraction patterns performed at 100 K, under low-dose illumination to avoid amorphization of the samples during electron irradiation, have shown the good crystallinity of the NP rubrene cores that seem to be constituted by single rubrene nanocrystals. Finally, optical confocal microscopy, used in reflection and fluorescence modes, showed that all the core–shell particles are strongly fluorescent. This high fluorescence intensity arises from the high molecule numbers of rubrene nanocrystals, which enhance the absorption and emission cross sections.     

Synthesis of superhydrophobic fluoro-containing silica sol coatings for cotton textile by one-step sol–gel process

Journal of Sol-Gel Science and Technology - Superhydrophobic coatings were successfully fabricated on cotton textiles through a simple one-step sol–gel process. A fluorinated copolymer...     

Solid-phase microextraction Ni–Ti fibers coated with functionalised silica particles immobilized in a sol–gel matrix

One of the possible approaches for the development of novel solid-phase microextraction (SPME) fibers is the physical deposition of porous materials onto a support using high-temperature epoxy glue. However, a major drawback arises from decomposition of epoxy glue at temperatures below 300 °C and instability in some organic solvents. This limitation motivated us to explore the possibility of replacing the epoxy glue with a sol–gel film, thermally more stable and resistant to organic solvents. We found that functionalised silica particles could be successfully attached to a robust Ni–Ti wire by using a UV-curable sol–gel film. The particles were found to be more important than the sol–gel layer during the microextraction process, as shown by competitive extraction trials and by the different extraction profiles observed with differently functionalised particles. If a quality control microscopic-check aiming at the rejection of fibers exhibiting unacceptably low particle load was conducted, acceptable (6–14%) reproducibility of preparation of C₁₈-silica fibers was observed, and a strong indication of the durability of the fibers was also obtained. A cyclohexyldiol-silica fiber was used, as a simple example of applicability, for the successful determination of benzaldehyde, acetophenone and dimethylphenol at trace level in spiked tap water. Recoveries: 95–109%; limits of detection: 2–7 μg/L; no competition effects within the studied range (≤125 μg/L).     

Surfactants for CNTs dispersion in zirconia-based ceramic matrix by sol–gel method

Zirconium oxide is a ceramic material widely studied due to its mechanical and electrical properties that can be improved with the use of carbon nanotubes (CNTs) as reinforcement. The synthesis of CNT/zirconia composites by sol–gel method is still very scarce, due to the hydrophobic nature of the CNTs, being their dispersion in aqueous medium an intrinsic difficulty to the synthesis. In this work, we present a sol–gel synthesis for MWCNTs/zirconia composites, where two kinds of surfactants, sodium and ammonium stearates dissolved in water (1 g/100 mL), were used as dispersant agents for multiwall carbon nanotubes (MWCNTs). They are cheap and easy to prepare, and were very effective in dispersing the MWCNTs. Different quantities of MWCNTs (up to 5 wt%) were added in the solution of stearate/water and this solution with the highly dispersed MWCNTs was added to the zirconia sol–gel, producing composites of MWCNTs/zirconia with different concentrations of MWCNTs. All the powders were heat treated at 300 and 500 °C and the powder characterization was performed by transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and infrared spectroscopy (FTIR). The composite MWCNTs/zirconia remained amorphous at 300 °C and presented a tetragonal phase at 500 °C with an average grain size of about 20 ± 3 nm, determined by the Scherrer equation from the XRD patterns. For these crystalline samples, TEM images suggest a more effective interaction between MWCNTs with ZrO₂ matrix, where it can be observed that the carbon nanotubes are fully coated by the matrix.     

Quinine doped hybrid sol–gel coatings for wave guiding and optical applications

Pure and quinine doped silica coatings have been prepared over sodalime glasses. The coatings were consolidated at low temperature (range 60–180 °C) preserving optical activity of quinine molecule. We designed a device to test the guiding properties of the coatings. We confirmed with this device that light injected in pure silica coatings is guided over distances of meters while quinine presence induces isotropic photoluminescence. With the combined use of both type of coatings, it is possible to design light guiding devices and illuminate regions in glass elements without electronic circuits.     

Polyoxometalates entrapped in sol–gel matrices for reducing electron exchange column applications

Electron exchange columns were developed by utilizing the redox properties of polyoxometalates (POMs) entrapped in silica matrices via the sol–gel route. The properties of the columns strongly depend on the composition of the precursors used to prepare the matrices. The columns exhibit good reversibility and are the first ‘reducing’ electron exchange columns ever prepared. They are also the first columns where both the matrix and the entrapped redox agent are inorganic compounds. This increases their stability. However, the redox properties of the entrapped POMs in the matrices are affected by the composition of the matrices.     

Hybrid sol–gel silica adsorbent materials synthesized by molecular imprinting for tannin removal

Journal of Sol-Gel Science and Technology - This study reports the development of a functional adsorbent synthesized by the molecular imprinting method in a sol–gel matrix. The adsorption...     

Fast sol–gel technology: from fabrication to applications

A novel method to prepare crack-free sol–gel materials without shrinkage is reviewed. The method allows fabrication of a viscous sol–gel resin in a few minutes followed by either thermal-curing or UV-curing requiring several hours or several minutes, respectively. The method is distinguished by the short time required to achieve a solid monolith. The fast sol–gel method uses a combination of organically modified alkoxides with traditional alkoxides as precursors, to produce a final product which is an organic-inorganic hybrid with properties that vary from silicone rubbers to silica glass. Optical and physical properties, such as refractive index and thermal expansion, can be engineered by controlling the ratio between the precursors. This class of materials is a promising candidate for preparation of optical elements such as waveguides and submicron structured replicas and can also be used as an optical bonding material. This paper reviews the fast sol–gel technology, as well as methods to characterize the process and its final products. Various applications of fast sol–gel materials are presented.     

Polycaprolactone membranes reinforced by toughened sol–gel produced silica networks

The aim of this work is to develop polycaprolactone based porous materials with improved mechanical performance to be used in bone repair. The hybrid membranes consist in a polymeric porous material in which the pore walls are coated by a silica thin layer. Silica coating increases membrane stiffness with respect to pure polymer but in addition filling the pores of the polymer with a silica phase improves bioactivity due to the delivery of silica ions in the neighborhood of the material in vivo. Nevertheless silica network, even that produced by sol–gel, might be too stiff and brittle what is not desirable for its performance as a coating. In this work we produced a toughened silica coating adding chitosan and 3-glycidoxypropyltrimethoxysilane (GPTMS) to the precursor solution looking for having polymer chains linked by covalent bonding to the silica network. Hybrid polymer–silica coating was produced by in situ sol–gel reaction using Tetraethyl orthosilicate (TEOS), GPTMS and chitosan. Chemical reaction between amine groups of chitosan chains and epoxy groups of GPTMS allowed covalent bonding of polymer chains to the silica network. Physical properties of the hybrid membranes were characterized and cell attachment of MC3T3-E1 pre-osteoblastic cells on the surface of these supports was assessed.     

Deposition of silica thin films formed by sol–gel method

Deposition of silica thin films on silicon wafer was investigated by in situ mass measurements with a microbalance configured for dip coating. Mass change was recorded with respect to deposition time when the substrate was fully immersed in the silica sol. Mass gain during deposition was higher than predicted from monolayer coverage of silica nano particles. This implied that deposition was facilitated by gelling of the nanoparticles on the substrate. The rate of deposition was enhanced by increasing the particle concentration in the sol and by decreasing the particle size from 12 to 5 nm. Increasing the salt concentration of the silica sol at constant pH enhanced the deposition of the silica particles. Reducing the pH of the sol from 10 to 6 decreased the deposition rate due to aggregation of the primary silica particles.     

Role of pore size on the electrochemical oxidation of 5-hydroxytryptophan in a silica sol–gel matrix

Silica sol–gels that are prepared by room-temperature processing are suited as matrices for voltammetry in the absence of a contacting liquid phase because the electrolyte encapsulated within the pore volume supports charge transport. We hypothesized that when the electrode process involves a coupled chemical reaction, the pore size can influence the product distribution. This hypothesis was tested with 5-hydroxytryptophan (5-HTPP) as the electroactive species because the cation radical formed by its oxidation can react to form a simple dione or larger dimer, trimer, and oligomer products. Microporous and mesoporous silica sol–gels, which have respective pore diameters in the Å and nm ranges, were the solid-state electrolytes. Glassy carbon (GC) electrodes were used initially; however, experiments in aqueous solution showed that passivation precluded applications in a solid electrolyte. Boron-doped diamond (BDD) electrodes yielded the same voltammetric signature as that of GC but passivated more slowly. In accord with the hypothesis, the oxidation of 5-HTPP at BDD in microporous silica yielded voltammetry suggesting that dione formation was the primary oxidation pathway. With mesoporous silica, the analogous experiment suggested dimer formation. The voltammetric interpretation was consistent with the results obtained by chemical oxidation of 5-HTPP in these matrices followed by product identification by mass spectrometry.     

Surface sol–gel synthesis of silica films on polyimide substrate

Silica films were grown on polyimide substrate using surface sol–gel reaction, and the film growth process was characterized by ellipsometry, atomic force microscopy, and X-ray photoelectron spectroscopy. On the activated polyimide surface, silica film was grown by sequential immersion in SiCl₄ solution and H₂O. The thickness of silica films is linear with the depositing cycle, about 5.0 nm per cycle. The silica films present an island-like growth type and are not a strict equilibrium SiO₂ structure. Moreover, the result of the tensile test suggests that the silica films have a good adhesion to the polyimide substrate.     

Microwave sintering of nickel ferrite nanoparticles processed via sol–gel method

Magnetic nickel ferrite (NiFe₂O₄) was prepared by sol–gel process and calcined in the 2.45 GHz singlemode microwave furnace to synthesize nickel nanopowder. The sol–gel method was used for the processing of the NiFe₂O₄ powder because of its potential for making fine, pure and homogeneous powders. Sol–gel is a chemical method that has the possibility of synthesizing a reproducible material. Microwave energy is used for the calcining of this powder and the sintering of the NiFe₂O₄ samples. Its use for calcination has the advantage of reducing the total processing time and the soak temperature. In addition to the above combination of sol–gel and microwave processing yields to nanoscale particles and a more uniform distribution of their sizes. X-ray diffraction, energy dispersive X-ray spectroscopy, transmission electron microscopy and vibrating sample magnetometer were carried out to investigate structural, elemental, morphological and magnetic aspects of NiFe₂O₄. The results showed that the mean size and the saturation magnetization of the NiFe₂O₄ nanoparticles are about 30 nm and 55.27 emu/g, respectively. This method could be used as an alternative to other chemical methods in order to obtain NiFe₂O₄ nanoparticles.     

The role of the sol–gel route on the interaction between rhodamine B and a silica matrix

A series of silica xerogels having rhodamine B (RhB) as a template and Ti centers were synthesized by distinct sol–gel routes, namely, acid-catalyzed, base-catalyzed, acid-catalyzed with base-catalyzed (two steps) hydrolytic routes and a FeCl₃-catalyzed non-hydrolytic route. The interaction of RhB with the prepared silica matrix was investigated by Fourier transform infrared spectroscopy, attenuated total reflectance, diffuse reflectance spectroscopy in the ultraviolet–visible region, Raman spectroscopy, mass spectrometry, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and confocal microscopy. Raman spectroscopy suggested the presence of Ti–O and Si–O–Ti moieties within the silica matrix. Infrared band shifts provided insight into potential interaction sites. Taking into account the results from ART, XPS, PL and confocal microscopy, encapsulation of RhB preferentially occurs inside the silica network for acid 1, basic and two-steps routes, and the presence of Ti occurs on the surface of the silica occurs for acid 2, basic and two-steps routes. Also, we have shown that although the structural characteristics of the encapsulated and extracted systems are affected by the route, the molecular structure is conserved during and after the encapsulation process.     

Modified procedure for the sol–gel processing of indium–tin oxide (ITO) films

Dispersions of crystalline nanoscaled ITO and solutions of amorphous precursors were used for the preparation of transparent conductive films on glass both by large area dip-coating (30 × 30 cm²) and pad-printing. The advantages of both types of coating materials regarding single layer thickness and conductivity were combined by a coating-infiltration procedure. The resulting films showed a specific resistivity down to 1.77 10E−03 Ω cm after treatment in reducing atmosphere and an increased resistance to subsequent storage in air.