Enhanced resonator sensitivity with nanostructured porous silica coatings

We present a strategy to increase the sensitivity of resonators to the presence of specific molecules in the gas phase, measured by the change in resonant frequency as the partial pressure of the molecule changes. We used quartz crystals as the resonators and coated them with three different thin films (<1 microm thick) of porous silica: silica xerogel, silica templated by an ordered hexagonal phase of surfactant micelles, and silica templated by an isotropic L3 phase surfactant micellar system. We compared the sensitivity of coated resonators to the presence of water vapor. The crystals coated with hexagonal phase-templated silica displayed a sensitivity enhancement up to 100-fold compared to an uncoated quartz crystal in the low-pressure regime where adsorption played a dominant role. L3 phase-templated silica displayed the highest sensitivity (up to a 4000-fold increase) in the high partial pressure regimes where capillary condensation was the main accumulation mechanism. Three parameters differentiate the contributions of these coatings to the sensitivity of the underlying resonator: (i) specific surface area per unit mass of the coating, (ii) accessibility of the surfaces to a target molecule, and (iii) distribution in the characteristic radii of curvature of internal surfaces, as measured by capillary condensation.     

Synthesis of silica gels with a controlled porous structure

Silica gels with a developed porous structure were synthesized by a sol-gel method with the use of templates, drying in supercritical CO₂, and spray drying. A correlation between the synthesis conditions and the textural characteristics of the resulting material was established.     

Photochemistry of ketones adsorbed on porous silica

The Phetochemical reactions of several families of ketones adsorbed on various forms of silica have been investigated. The influence of surface coverage, silica average pore diameter, temperature, applied magnetic fields, and additives on the product distributions have been determined. The results are consistent with a model in which the silica surface consists of two major regions, one consisting of stronger binding sites (active silanol groups) and the other consisting of weaker binding sites (free silanol groups). The relative proportions of these two types of sites are related to the pore size of the silica, smaller pores correlating with a larger fraction of the stronger binding sites and larger pores correlating with a smaller fraction of the stronger binding sites. At low coverage, the initial location of a ketone in one of the two binding sites is an important factor in determining the product distributions. Some preliminary results are reported on the time resolved spectroscopy of ketones adsorbed on silica investigated by diffuse reflectance absorption spectrescopy.     

Modification of porous silica particles with poly(acrylic acid)

The surface of porous silica particles was modified with poly(acrylic acid) by reacting the carboxyl groups on poly(acrylic acid) with the amino groups of pregrafted aminopropyltriethoxysilane (APS). The chemical modifications by APS and polymer were characterized by infrared spectroscopy and the amount of APS and poly(acrylic acid) grafted to the surface were determined by thermal gravimetric analyses. The wettability of the modified silica particles, based on the rate of water penetration, was pH‐dependent with PAA; at pH 1.5 the wettability increased but at pH 5.5 it decreased dramatically. The pore size and size distribution of the silica particles decreased with APS and polymer grafting. Copyright © 2000 John Wiley & Sons, Ltd.     

Small-angle neutron scattering of colloidal silica dispersions in a non-polar solvent

Small-angle neutron scattering studies were performed on dilute dispersions of colloidal silica spheres in mixtures of h₁₂- and d₁₂-cyclohexane. The particles consisted of a SiO₂-core and a layer of stearyl alcohol molecules terminally attached with a chemical bond (Si-O) to the particle surface. The contrast variation method was applied to reveal the internal structure of the particles. The matchpoints determined with this method were in accordance with those calculated from the mass density of the particles and the atomic composition, as determined with elemental analysis.For a detailed interpretation of the scattering curves, we assumed that the particles were spherosymmetrical and consisted of two concentric layers. With the relation we derived between the radius of gyration and the reciprocal contrast for such a model, it was possible to determine all the parameters characterizing the particle in terms of this model. The model calculations performed using these parameters fitted very well to the experimental intensities for high contrasts. For lower contrasts, the fit was somewhat less good. This is probably due to random fluctuations in the scattering length density within the particle core.The different radii as found by neutron scattering, agreed very well with those determined using other techniques, such as light scattering (static and dynamic) and electron microscopy.     

Luminescent porous silica fibers as drug carriers

Luminescent and porous silica fibers have been successfully prepared by using the electrospinning process. The obtained multifunctional silica fibers, which possess a porous structure and display blue luminescence, can serve as a drug delivery host carrier, using ibuprofen (IBU) as a model drug, allowing the investigation of storage/release properties. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), N(2) adsorption/desorption, photoluminescence (PL) spectra, and kinetic decay were used to characterize the structural, morphological, and optical properties of the as-obtained samples. The results reveal that the multifunctional silica materials exhibit an irregular porous structure, and display a fiberlike morphology with dimensions of several hundred nanometers in width and several millimeters in length. The obtained silica fibers exhibit an intense broad bluish emission, which might be attributed to impurities and/or defects in the silica fibers. The IBU-loaded silica fiber system shows blue luminescence under UV irradiation and controlled release behavior for IBU. In addition, the emission intensities of silica fibers in the drug carrier system vary with the released amount of IBU, thus allowing the drug release to be easily tracked and monitored by the change of the luminescence intensity.     

Concentration effects on irreversible colloid cluster aggregation and gelation of silica dispersions

Effects of particle concentration on the irreversible aggregation of colloidal silica are studied using in situ destabilization via the ionic strength increase derived from the enzymatic hydrolysis of urea by urease. Aggregation is monitored by time-resolved optical density and dynamic light scattering measurements. It terminates at a gel boundary, signaled by a prominent increase of the optical density and incipient non-ergodicity. Raman scattering is used to demonstrate that the enzymatic reaction continues, well beyond gelation for the compositions studied here, until the urea is consumed. Calibration of the ionic conductivity permits for constructing stability diagrams in terms of particle and salt concentration. As with reversible gelation, the process exhibits a collective character in that lower ionic strengths are required for gelation of concentrated dispersions and vice versa. However, light scattering demonstrates that the gel boundary is preceded here by a line marking the transition from reversible to irreversible cluster formation, with the two transition boundaries tracking each other. Comparisons are made with dispersions destabilized by direct addition of salt solutions, which gel under very different conditions.     

DNMR study of hydrophilic and hydrophobic silica dispersions in EBBA liquid crystals

Dispersions of hydrophilic (A300) and hydrophobic (R812) silica aerosils in a Schiff-base-type liquid crystal (LC), p-ethoxy(benzylidene)-p-n-butylaniline (2O.4), EBBA, were characterized by deuterium nuclear magnetic resonance (DNMR). The formation and stability of random (RAN) versus anisotropic (AAN) aerosil networks under zero- versus in-field cooling was studied as a function of aerosil density and compared to previous studies of n-alkylcyanobiphenyl (nCB) dispersions. Whereas the LC directors of the hydrophobic R812 dispersions are almost completely annealed after in-field cooling, the hydrophilic A300 silica in EBBA gives rise to a mixture of RAN and AAN. The more complete R812 AAN partially breaks under in-field sample rotation, but the partial AAN formed by the A300 silica is stable. Weakening the aerosil network to compensate for weaker LC surface anchoring results in a complete network, but a strong LC/silica surface interaction must be combined with hydrophilic aerosils to produce AANs which are both complete and stable.     

Liquid chromatography of dextrans on porous silica beds

Kinetics, equilibrium isotherms and chromatography retention times for sorption of dextrans T-10, T-20, T-40, T-70, T-110, T-161, T-250 and T-500 on porous silica were measured at 25 degrees C. The Henry constant and retention factors for the dextrans were obtained. The values of the partition coefficient for the distribution of the dextrans between the bulk solution and the pore space were calculated within the framework of a pore volume filling model with consideration given to the ratio between the sizes of the macromolecular coils and the pore inlet. The measurements showed that this parameter depends on the structure of the sorbent and the molecular mass distribution of the dextran. The interaction of aqueous dextran solution with porous silica is characterized by the sieve effect. Large macromolecular coils of dextran T-161 cannot penetrate into the pore space of the silica sorbent with pore diameter 14 nm. The difference in Henry law constants calculated from adsorption and chromatographic data for dextrans T-70 and T-110 can be explained by the slow diffusion of dextran macromolecules into silica pores under chromatographic conditions.     

Adsorption of charged diblock copolymers on porous silica

Block copolymers of styrene and 2-vinylpyridine of different molecular weights were synthesized and chemically modified to poly(vinyl pyridine)/poly(styrene sulfonate) or polystyrene/poly(2-vinyl pyridinium) salts. Adsorption on “Spherosil” silica with a high specific surface area was performed from aqueous solutions of polyelectrolyte copolymers or from noncharged copolymer in an organic medium and subsequent sulfonation or quaternization in the adsorbed state. The adsorption mechanism was studied under various solvent conditions to give silica maximal ion-exchange properties. Modification of silica resulted in a highly stable coated material which combines the mechanical properties of the porous beads and the ion-exchange properties of the “supported” and “pellicular” ion-exchangers and should have potential use in chromatography.     

Dispersity of platinium on porous glasses and silica

Hydrogen adsorption has been investigated in order to determine the dispersity of platinum supported on different silicon-containing materials, such as porous glasses, silica and acid treated silica. The acid treatment of silica results in more dispersed platinum as compared with silica and porous glasses. The influence of pore size distribution as a further important factor on Pt-crystallite distribution and dispersity is discussed.

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Production of porous silica microparticles by membrane emulsification

A method for the production of near-monodispersed spherical silica particles with controllable porosity based on the formation of uniform emulsion droplets using membrane emulsification is described. A hydrophobic metal membrane with a 15 μm pore size and 200 μm pore spacing was used to produce near-monodispersed droplets, with a mean size that could be controlled between 65 and 240 μm containing acidified sodium silicate solution (with 4 and 6 wt % SiO(2)) in kerosene. After drying and shrinking, the final silica particles had a mean size in the range between 30 and 70 μm. The coefficient of variation for both the droplets and the particles did not exceed 35%. The most uniform particles had a mean diameter of 40 μm and coefficient of variation of 17%. By altering the pH of the sodium silicate solution and aging the gel particles in water or acetone, the internal structure of the silica particles was successfully modified, and both micro- and mesoporous near-monodispersed spherical particles were produced with an average internal pore size between 1 and 6 nm and an average surface area between 360 and 750 m(2) g(-1). A material balance and particle size analysis provided identical values for the internal voidage of the particles, when compared to the voidage as determined by BET analysis.     

Aqueous dip-coating route to dense and porous silica thin films using silica nanocolloids with an aid of polyvinylpyrrolidone

An aqueous dip-coating route without using volatile alcohols for preparing dense and porous silica thin films is presented. The films were prepared by dip-coating on Si(100) and silica glass substrates using aqueous suspension of 8–11 nm colloidal silica where no alcohols were added. The addition of polyvinylpyrrolidone (PVP) in the suspension greatly improved the wettability, allowing homogeneous films to be formed on the substrates while the substrates showed poor wettability without PVP. When the as-deposited films were fired at 500–1,000 °C, optically transparent porous films with smooth surface were obtained. The porous films had homogeneous distribution and packing of silica nanoparticles, refractive indices of 1.24–1.33, and pencil hardness lower than 6B. The porous films underwent significant densification by viscous sintering when fired at temperatures over 1,000 °C. The viscous sintering was accompanied by the progress of condensation reaction and by the structural relaxation of siloxane network, which was revealed in the infrared absorption spectra. The resulting dense films had optical transparency, smooth surface, pencil hardness over 9H and refractive indices close to that of silica glass.     

Preparation and evaluation of solid dispersions of nitrendipine prepared with fine silica particles using the melt-mixing method

Solid dispersions (SD) of nitrendipine (NTD), a poorly water-soluble drug, were prepared using the melt-mixing method with hydrophilic silica particles (Aerosil and Sylysia) with different particle size and specific surface areas as carriers. Powder X-ray diffraction and differential scanning calorimetry evaluation showed that NTD in the SDs treated with the melt-mixing method was dispersed in the amorphous state. FT-IR spectroscopy obtained with the SDs indicated the presence of hydrogen bonding between the secondary amine groups of NTD and silanol groups of silica particles. The dissolution property of NTD in the SDs was remarkably improved regardless of the grade of silica. At the end of the dissolution test (60 min) the concentrations of NTD for the SDs with Aerosil 200 and Sylysia 350 were 8.88 and 10.09 microg/ml, corresponding to 28 and 31 times that of the original NTD crystals, respectively. The specific surface area and the adsorbed water amount of the SDs were also significantly improved. The rapid dissolution rate from the SDs was attributed to the amorphization of drug, improved specific surface area and wettability than the original drug crystals. In the stability test, powder X-ray diffraction pattern indicated that amorphous NTD in the SD with Aerosil 200 was stable for at least 1 month under the humid conditions (40 degrees C/75% RH).     

The effect of nanoparticle growth on rheological properties of silica and silicate dispersions

CdS and ZnS nanoparticles were prepared in the solid–liquid interfacial adsorption layer as a nanophase reactor. The substrates were hydrophilic and hydrophobic aerosils and hydrophilic layer silicates dispersed in ethanol–cyclohexane mixtures. The growth of particles at various surface concentration of precursor ions was monitored by absorption spectroscopy, band-gap-energy measurements and particle diameter measurements. Also, the rheological properties of nanoparticle–support composites in organic and aqueous dispersions were measured. The energy of separation between the nanoparticles depended on the particle diameter. The intercalation of nanoparticles in the layered silicates yielded a nanostructured two-phase system. The presence of semiconductive subcolloids was proven by transmission electron microscopy measurements, which offer an excellent possibility for the determination of the particle size distribution. Received: 20 July 1999/Accepted in revised form: 22 September 1999     

Nonstoichiometric interpolyelectrolyte complexes as colloidal dispersions based on NaPAMPS and their interaction with colloidal silica particles

Preparation and characterization of some nonstoichiometric interpolyelectrolyte complexes (NIPECs) as stable colloidal dispersions by the interaction between poly(sodium 2-acrylamido-2-methylpropanesulfonate) (NaPAMPS) and three strong polycations bearing quaternary ammonium salt centres in the backbone, poly(diallyldimethylammonium chloride) (PDADMAC) and two polycations containing N,N-dimethyl-2-hydroxypropyleneammonium chloride units (PCA₅ and PCA₅D₁), have been followed in this study as a function of the polycation structure and polyelectrolyte concentration. Complex characteristics were followed by polyelectrolyte titration, turbidity and quasi-ellastic light scattering. Almost monodisperse NIPECs nanoparticles with a good storage stability were prepared when total concentration of polyelectrolyte was varied in the range 0.85-6.35 mmol/L, at a ratio between charges (n⁻/n⁺) of 0.7. NIPECs as a new kind of flocculants were used to flocculate a stable monodisperse silica suspension. The main advantage of NIPECs as flocculants is the broad flocculation window, which is a very important aspect for industrial applications.     

Monodispersed submicron porous silica particles functionalized with CD derivatives for chiral CEC

Rapid and efficient enantioseparation of halogen aryl alcohols and β‐blockers propranolol and pindolol in packed bed CEC (p‐CEC) using as‐prepared submicron porous silica chiral stationary phases (CSPs) has been achieved. Monodispersed 0.66 and 0.81 μm chiral submicron porous silica spheres were prepared using tetramethoxysilane and hexadecyltrimethylammonium bromide, followed by a hydrothermal treatment method with ammonia–ethanol to expand the pore of silica spheres without changing their spherical morphology. A proper specific surface of ca. 230 m²/g and pore sizes average of 6–8 nm were obtained by this method. The submicron porous silica spheres were modified with mono‐6‐phenylcarbamoylated β‐CD via thiol‐en radical addition. They were packed into 9 cm 50 μm id capillary columns with photopolymerized monolithic frits. These submicron CSPs showed greater column efficiency (about 476 000 plates/m for 4‐iodophenyl‐1‐ethanol) and higher resolution than the corresponding 3 μm CSP.     

Vitalising porous inorganic silica networks with organic functions--PMOs and related hybrid materials

Mesoporous organic-inorganic hybrid materials are an interesting class of materials, which combine the advantages of two worlds; the inorganic part builds a robust substrate while organic functions make them alive in a way that they can be potentially used in a number of applications. In this tutorial review, we provide an overview of how mesoporous materials are synthesised via the 'soft template' path and how the incorporation of organic functions can be achieved. Furthermore, a colourful survey full of examples is presented, providing a small overview of field of applications that can potentially benefit from the use of periodic mesoporous organosilicas (PMOs) and related materials.