M. Ramírez-del-Solar

Sonogels and derived materials

Nowadays the sol–gel process is used by an increasing number of researchers for the preparation of various products, including bulk materials, films, membranes or fibers. The application of ultrasound (sonocatalysis) to the precursors gives rise to materials with new properties, known as sonogels. The absence of additional solvent and, mainly, the effects of ultrasonic cavitation create a unique environment for sol–gel reactions leading to particular features in the resulting gels: high density, fine texture, homogeneous structure etc. These properties determine the evolution of sonogels on further processing and the final material structure. In this sense, the full exploitation of sonocatalysis requires a thorough understanding of the processes involved and their sensitivity to reaction parameters. For this purpose, we have used diverse techniques to investigate the microstructural evolution during different steps in the sonogel process. The results of these studies are reported here. Finally, we present an overview of some applications for which the mechanical, textural and optical characteristics of sonogels are quite useful. Copyright

Effect of the method of preparation on the texture of TiO2SiO2 gels

Abstract The chemical reactivity of Ti(OBun)4 was modified through the addition of acetic acid to obtain sonogels of the TiO2SiO2 system by ultrasonic treatment of the alkoxide-water mixture. The characteristics of the resulting gels were compared with those of similar standard gels prepared in alcoholic solution. The influence of composition and method of preparation on the gels was studied. The results of density and BET measurements, thermal analysis and IR spectroscopy indicate that ultrasound drastically affects the texture.

Kinetic study of gelation of solventless alkoxide-water mixtures☆

Abstract A very high-speed rotatory blender was used to enhance the homogenization of TEOS + H 2 O mixtures. The effects of several reaction parameters on the gelation rate of these solutions and those submitted to ultrasonic treatment were studied. Comparison of the results points to differing accelerating mechanisms in each case.

Size and surface effects in the magnetic properties of maghemite and magnetite coated nanoparticles

An increasing number of promising applications for future technology is arising from size constraints in nanoparticles (NPs) and from the chemical manipulation of their surfaces. In this work, we analyse the finite-size and surface effects on polyacrylic acid-coated Fe3O4 NPs and oleic acid-coated γ-Fe2O3 NPs by studying their magnetization curves at different temperatures. The measured thermal dependence of the saturation magnetization is no longer explained by the typical T3/2 Bloch law, yielding higher values than those expected for its exponent. When incorporated in polymeric matrixes to form magnetic transparent nanocomposites, the oleic acid-coated γ-Fe2O3 NPs also deviate from Bloch’s law, but following the opposite trend observed in free coated NPs.

Trapping Copper Phthalocyanine in a Silica Sono-Xerogel

A copper phthalocyanine (CuPc) has been encapsulated in silica. The trapping effects were studied comparing the UV-Vis absorption spectra of some CuPc solutions and composites prepared under different conditions. The trapped organic molecules’ stability was monitored during the gelation and drying processes. Leachability test have been carried out with the aim of checking the trapping efficiency. Hydrolysis water of pH=2 and a molar ratioRw=6 mol acid H2O/mol TMOS leads to a higher CuPc stacking angle homogeneity. The increase of Pc induces a narrower mesopore distribution and helps the stabilization of the composite.

Preparation and Characterization of Fluorescent CdS Quantum Dots used for the Direct Detection of GST Fusion Proteins

Advances in the life sciences are now closely linked to the availability of new experimental tools that allow for the manipulation of biomolecules and the study of biological processes at the molecular level. In this context, we have optimized a synthesis process to obtain glutathione-capped fluorescent CdS nanoparticles to specifically detect Glutathione S-Transferase (GST) -tagged proteins. Using our method, based on five different heating steps, brightly fluorescent and biocompatible CdS quantum dots of different sizes can be obtained. QD optical behaviour has been evaluated studying both absorbance and fluorescence. For all the samples, the excitonic absorption onset clearly shows a blue shift at 512nm in comparison with that of bulk CdS, due to the quantum confinement effect. At increased average sizes of the nanocrystal, the emission fluorescent band shows a red shift, from 440nm to 540nm. Among different QD solutions, we demonstrate an expansion of the emission range up to �˝ 100 nm, thus improving their features as biomarkers. Moreover we show that optimized glutathione- capped quantum dots can directly bind GST blotted onto polyvinylidene difluoride (PVDF) membranes, and thus are suitable for the direct detection of GST fusion proteins.

INFLUENCE OF PROCESSING INDUCED TEXTURAL EFFECT ON AGGREGATION LEVEL IN CUPC–SIO2 SONOGEL COMPOSITES

Abstract Aggregation effects in metallophthalocyanine/SiO 2 composites play an important role in the optical properties of these materials. With the aim to decrease phthalocyanine self-association induced in aqueous solution into the network pores, alternative processing methods for trapping the organic in the matrices have been carried out. Monolithic xerogels have been obtained by three chemically different processing routes, preserving their homogeneity and transparency, even under extremely acid conditions. Micro, meso and non-porous silica xerogels have been obtained, depending on processing method. However, the organic doping highly modifies the textural features of the final composite, increasing the pore volume of composites even in more than 50%. Phthalocyanine aggregation degree has been tested by UV–VIS absorption spectroscopy and correlated to the textural parameters and pore size distribution of the composites. The effect of the textural features on molecular self-association level is evaluated.

Confinement of CdS Nanocrystals in a Sonogel Matrix

CdS/silica xerogel composites have been prepared using a sonocatalytic method. The confinement effects of CdS semiconductor nanocrystallites have been analyzed through UV-VIS absorption and Raman scattering. The blue shift of the absorption band and the shape of the Low Frequency Inelastic Raman Scattering (LOFIRS) spectra make it possible to evaluate the size of nanocrystallites, which are contrasted with previous results obtained through other techniques. Moreover, in the 200–700 cm−1 region of Raman shift, resonant effects are discussed, through the longitudinal optical mode lines.

Ultrastructural evolution during gelation of TiO2-SiO2 sols

Abstract Small angle X-ray scattering was used to examine in situ formation of mixed TiO2-SiO2 gels. In order to elaborate the homogeneous solution, either ultrasonic radiation or alcoholic dilution of the precursors was carried out. The evolution of the typical sizes calculated at low and high q-regions were correlated. This lead to an approximate model for the aggregation process. The fit of the experimental data to a simple growth law was attempted allowing a kinetic rate constant to be estimated. This permits the evaluation of the differences induced in titanium doped silica sono- and classic gels.

CdS semiconductor nanoparticles in silica sonogel matrices: Code: F12

SiO2 gels obtained by sonocatalytic method combined with DCCA were used as host-matrices for extremely fine dispersions of CdS semiconductor particles. Small crystallites were produced “in situ” by H2S gas diffusion method. The particles were characterized by TEM and HRTEM, EXAFS, UV-Vis and Raman spectroscopies. The size of crystallites ranged from 5 to 10 nm. The optical transmission spectra showed the characteristic blue shift as a function of the particles size, as predicted by the theory. The optical and mechanical qualities of the samples were substantially improved by an infiltration method using a sono-sol which sealed the superficial pores thus ensuring greater longevity and the possibility of obtaining transparent gels by polishing.