Electrical percolation during gelation of lithium doped siloxane-poly(oxyethylene) hybrids

Gelation mechanisms of lithium-doped Siloxane-Poly(oxyethylene) (PEO) hybrids containing polymer of two different molecular weight (500 and 1900 g/mol) were investigated through the evolution of the electrical properties during the solgel transition. The results of electrical measurements, performed by in-situ complex impedance spectroscopy, were correlated with the coordination and the dynamical properties of the lithium ions during the process as shown by Li-7 NMR measurements. For both hybrids sols, a decrease of the conductivity is observed at the initial gelation stage, due to the existence of an inverted percolation process consisting of the progressive separation of solvent molecules containing conducting species in isolated islands during the solid network formation. An increase of conductivity occurs at more advanced stages of gelation and aging, attributed to the increasing connectivity between PEO chains promoted by the formation of crosslinks of siloxane particles at their extremities, favoring hopping motions of lithium ions along the chains.

Titania-based organic-inorganic hybrid planar waveguides

Eu3+ -doped titania-silica planar waveguides were prepared from tetraethylorthotitanate (TEOT) and modified silane 3-amino-propyltriethoxysilane (APTS). Films were deposited on borosilicate glass substrates by a dip-coating technique. The refractive index, the thickness and the total attenuation coefficient of the waveguides were measured at 632.8 and 1550 nm by prism coupling technique. Starting from pure titania films, the addition of modified silane leads to a decrease in the refractive index and an increase in thickness. Squared electric field simulation has shown that the light confinement in the waveguide increases with the silane content of the so]. Emission spectra present a broad emission band due to the modified silane and EU emission transitions arising mainly from the D-5(0) level to the F-7(J) (J = 0-4) manifolds. The dependence of transition intensities and excited state lifetimes on the initial composition and also on the heat treatment performed was interpreted in terms of structural changes occurring during the preparation process. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Small-angle X-ray scattering study of the smart thermo-optical behavior of zirconyl aqueous colloids

The smart thermo-optical systems studied here are based on the unusual thermoreversible sol-gel transition of zirconyl chloride aqueous solution modified by sulfuric acid in the molar ratio Zr/SO4:3/1. The transparency to the visible light changes during heating due to light scattering. This feature is related to the aggregates growth that occurs during gelation. These reversible changes can be controlled by the amount of chloride ions in solution. The thermoreversible sol-gel transition temperature increases from 323 to 343 K by decreasing the molar ratio Cl/Zr from 7.0 to 1.3. In this work the effect of the concentration of chloride ions on the structural characteristics of the system has been analyzed by in situ SAXS measurements during the sol-gel transition carried out at 323 and 333 K. The experimental SAXS curves of sols exhibit three regions at small, medium and high scattering vectors characteristics of Guinier, fractal and Porod regimes, respectively. The radius of primary particles, obtained from the crossover between the fractal and Porod regimes, remains almost invariable with the chloride concentration, and the value (4 Angstrom) is consistent with the size of the molecular precursor. During the sol-gel transition the aggregates grow with a fractal structure and the fractal dimensionality decreases from 2.4 to 1.8. This last value is characteristic of a cluster-cluster aggregation controlled by a diffusion process. Furthermore, the time exponent of aggregate growth presents values of 0.33 and 1, typical of diffusional and hydrodynamic motions. A crossover between these two regimes is observed.

Influence of membrane-solution interface on the selectivity of SnO2 ultrafiltration membranes

SnO2 supported membranes, presenting 3.0 nm average pore size, have been produced by sol casting on alumina tubular substrate using aqueous colloidal suspensions prepared by sol-gel route. The selectivity and flux throughout SnO2 membrane were analyzed by permeation experiments, using a laboratory tangential filtration pilot equipped with a monotubular membrane. To evaluate the effect of the surface charge at the membrane-solution interface, aqueous salt solutions (NaCl, Na2SO4, CaCl, and CaSO4) of different ionic strength have been filtered and the results correlated with the values of zeta potential measured at several pH. The results show that the retention coefficient is dependent on the electrolyte present in aqueous solution decreasing as: (dication, monoanion) > (monocation, monoanion) approximate to (monocation, dianion) > (dication, dianion). The surface charge and the cation adsorption capacity play a determinant role in these selectivity sequences. (C) 2001 Elsevier B.V. B.V. All rights reserved.

Degradation of metallophtalocyanine dye by combined processes of electrochemistry and photoelectrochemistry

Photoelectrocatalytic degradation of metallophtalocyanine reactive dye (turquoise blue 15) was performed using a Ti/TiO2 thin film photoanode prepared by sol-get method. Hundred percent of color removal and almost complete mineralization (95% at pH 2 and 85% at pH 8) where achieved after 6 h of photolectrocatalytic oxidation of 2.5 x 10(-5) mol L-1 AT15 dye in Na2SO4 mol L-1 under E = +1.2 V versus SCE. The method limitation occurs at dye concentration higher than 4 x 10-5 mol L-1, where the degradation rate becomes markedly slower. An important improvement in color removal and TOC reduction for 1 x 10(-3) mol L-1 metallophtalocyanine dye was achieved using a combined process. After 4 h of potential controlled electrolysis at -1.2 V on a cathode of platinum followed by 6 h of photoelectrocatalytic oxidation leads to 100% of color removal and 83% of TOC decay and eletrodeposition of 69% of the released copper originally presented as copperphtalocyanine complex, by electrodeposition on the cathode without any other treatment. (C) 2005 Elsevier Ltd. All rights reserved.

Elaboration of boehmite nano-powders by spray-pyrolysis

Boehmite (gamma-AlOOH) synthesis have been investigated using a spray pyrolysis (SP) device starting from a stable sol of Al-tri-sec-butoxide peptized by nitric acid. Free spherical particles from 100 to 500 nm have been elaborated. Particles sub-structure is made of nano-crystalline boehmite with very small average crystallite size (one crystal cell along the b axis). The nano-crystalline boehmite synthesized by SP at low temperature (200 degrees C) is spontaneously dispersible in water without any surface treatment. Boehmite powder may be transformed to transition gamma-alumina by heat post-treatment. Powders of sub-micrometric and spherical gamma-alumina particles may also be synthesized by SP at 700 degrees C. (C) 2008 Elsevier B.V. All rights reserved.

Development and Evaluation of a Nanoporous Iron (Hydr)oxide Electrode for Phosphate Sensing

Nanoporous iron (hydr) oxide electrodes are evaluated as phosphate sensors using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The intensity of the reduction peak current (I-cp) of the ferrihydrite working electrode is tied to phosphate concentration at low pH; however, a hematite electrode combined with the use of EIS provided reliable sensing data at multiple pH values. Nanoporous hematite working electrodes produced an impedance phase component (theta) that shifts with increasing phosphate, and, at chosen frequencies, theta values were fitted for the range 1 nM to 0.1 mM phosphate at pH 4 and pH 7 in 5 mM NaClO4.

Gelation and drying of weakly bonded silica-PPO nanocomposites

Silica-poly(oxypropylene) (PPO) nanocomposites containing PPO with weak physical bonds between the organic (PPO) and inorganic (silica) phases were obtained by the sol-gel procedure. Three precursor sols containing silica and PPO with molecular weights of 1000, 2000 and 4000g/mol were prepared. The structure changes during the whole sol-gel process, i.e. sol formation, sol-gel transition and gel aging and drying were investigated in situ by small angle X-ray scattering (SAXS). The experimental SAXS curves corresponding to sols and wet gels containing PPO of molecular weight 1000g/mol indicate that the aggregates formed during the studied process are fractal objects. Close to the sol-gel transition and during gel aging the fractal dimension is D=2.5. A clearly different structure evolution occurs in samples prepared with PPO with molecular weights 2000 and 4000 g/mol. Our SAXS results indicate the presence of two coexisting and well-defined structure levels, one of them corresponding to small silica clusters and the other to large silica aggregates. These two levels remain along the whole transformation. The SAXS curves of all dry samples are similar to those of the corresponding wet gels suggesting that no significant changes at nanoscopic scale occur during the drying process.

Tin oxide nanoparticle formation using a surface modifying agent

This work presents results concerning the preparation of redispersible tin oxide nanoparticles achieved by using Tiron molecule ((OH)(2)C(6)H(2) (SO(3)Na)(2)) as surface modifying agent. The adsorption isotherm measurements show that an amount of 10 wt.% of Tiron is need to recover the SnO(2) nanoparticles surface with a monolayer. These nanoparticles can be easily redispersed in tetramethyl ammonium hydroxide at pH greater than or equal to11 until a powder concentration of 12 vol.% of tin. Under these conditions, hydrodynamic particle size is about 7 nm and increases until 52 nm at pH 6 due to the aggregation phenomenon. The time evolution of the viscoelastic properties indicates that the suspensions at pH 12.5, containing 12 vol.% tin oxide and 10 wt.% of surface modifier are kinetically stable. After thermal treatment at different temperature the powder characterisation evidences that the presence of Tiron monolayer at the nanoparticles surface increases the thermal stability of the porous texture and prevent the micropore size growth. This set of results contributes to satisfy the demand for more controlled synthesis of nanoparticles with high thermal stability as required for fabrication of ultrafiltration ceramic membranes. (C) 2004 Elsevier Ltd. All rights reserved.

Stimuli-responsive controlled growth of mono- and bidimensional particles from basic zirconium sulfate hydrosols

A thermostimulated sol-gel transition in a system prepared by mixing a ZrOCl(2) acidified solution to a hot H(2)SO(4) aqueous solution was studied by dynamic theological measurements and quasi-elastic light scattering. The effect of temperature and of molar ratio R(S) = [Zr]/[SO(4)] on the gelation kinetics was analyzed using the mass fractal aggregate growth model. This study shows that the linear growth of aggregates occurs at the early period of transformation, while bidimensional growth occurs at the advanced stage. The bidimensional growth can be shifted toward monodimensional growth by decreasing the aggregation rate by controlling the temperature and/or molar ratio R(S). EXAFS and Raman results gave evidence that the linear chain growth is supported by covalent sulfate bonding between primary building blocks. At the advanced stage of aggregation, the assembly of linear chains through hydrogen bonding gave rise to the growth of bidimensional particles.

Self-supported bacterial cellulose/boehmite organic-inorganic hybrid films

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Polysiloxane-poly(propylene oxide) hybrid discs as solid phase in anti-HCV detection using a recombinant core protein

In this work, siloxane-poly(propylene oxide) discs (PPO disc) prepared using the sol-gel process were used as solid phase in enzyme-linked immunosorbent assays (ELISA) for the detection of anti-hepatitis C virus (HCV) antibodies. The HCV RNA from serum (genotype 1b) was submitted to the RT-PCR technique and subsequent amplification of the HCV core 408 pb. This fragment was cloned into expression vector pET42a and expressed in Escherichia coli as recombinant protein with glutathione S-transferase (GST). Cell cultures were grown and induced having a final concentration of 0.4 x 10(-3) mol L-1 of IPTG. After induction, the cells were harvested and the soluble fraction was analyzed using polyacrilamide gel 15% showing a band with an approximate molecular weight of 44 kDa, the expected size for this GST-fused recombinant protein. The recombinant protein was purified and continued by immunological detection using HCV-positive serum and showed no cross-reactivity with positive samples for other infectious diseases. An ELISA was established using 1.25 ng of recombinant protein per PPO disc, a dilution of 1: 10,000 and 1:40 for a peroxidase conjugate and serum, respectively, and solutions of hydrogen peroxide and 3,3',5,5'-tetra-methylbenzidine in a ratio of 1: 1. The proposed methodology was compared with the ELISA conventional polystyrene-plate procedure and the performance of the PPO discs as a matrix for immunodetection gave an easy synthesis, good performance and reproducibility for commercial application. (c) 2007 Elsevier B.V. All rights reserved.

Drug-matrix interaction of sodium diclofenac incorporated into ureasilpoly(ethylene oxide) hybrid materials

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Effect of TiO2 surface modification in Rhodamine B photodegradation

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Controlling the growth of zirconia needles precursor from a liquid crystal template

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)