Mesoporous Silica Applications

Sol-Gel processing of silica nanoparticles and their applications

Recently, silica nanoparticles (SNPs) have drawn widespread attention due to their applications in many emerging areas because of their tailorable morphology. During the last decade, remarkable efforts have been made on the investigations for novel processing methodologies to prepare SNPs, resulting in better control of the size, shape, porosity and significant improvements in the physio-chemical properties. A number of techniques available for preparing SNPs namely, flame spray pyrolysis, chemical vapour deposition, micro-emulsion, ball milling, sol-gel etc. have resulted, a number of publications. Among these, preparation by sol-gel has been the focus of research as the synthesis is straightforward, scalable and controllable. Therefore, this review focuses on the recent progress in the field of synthesis of SNPs exhibiting ordered mesoporous structure, their distribution pattern, morphological attributes and applications. The mesoporous silica nanoparticles (MSNPs) with good dispersion, varying morphology, narrow size distribution and homogeneous porous structure have been successfully prepared using organic and inorganic templates. The soft template assisted synthesis using surfactants for obtaining desirable shapes, pores, morphology and mechanisms proposed has been reviewed. Apart from single template, double and mixed surfactants, electrolytes, polymers etc. as templates have also been intensively discussed. The influence of reaction conditions such as temperature, pH, concentration of reagents, drying techniques, solvents, precursor, aging time etc. have also been deliberated. These MSNPs are suitable for a variety of applications viz., in the drug delivery systems, high performance liquid chromatography (HPLC), biosensors, cosmetics as well as construction materials. The applications of these SNPs have also been briefly summarized.

Functionalized self-assembled monolayers on mesoporous silica nanoparticles with high surface coverage

Mesoporous silica nanoparticles (MSNs) containing vinyl-, propyl-, isobutyl- and phenyl functionalized monolayers were reported. These functionalized MSNs were prepared via molecular self-assembly of organosilanes on the mesoporous supports. The relative surface coverage of the organic monolayers can reach up to 100% (about 5.06 silanes/nm.

Pseudomorphic synthesis of monodisperse magnetic mesoporous silica microspheres for selective enrichment of endogenous peptides

In this work, we describe a novel synthetic strategy of magnetic mesoporous silica spheres (Fe3O4@mSiO2) for the selective enrichment of endogenous peptides. Fe3O4 particles were coated with silica shell by a sol-gel method, followed by pseudomorphic synthesis to transform nonporous silica shell into ordered mesoporous silica shell. The core/shell structure and mesostructure were individually fabricated in two steps, which can be expedient to independently optimize the properties of monodispersion, magnetization and mesostructure. Actually, it was confirmed that the produced Fe3O4@mSiO2 particles possess good monodispersion, high magnetization, superparamagnetism, uniform accessible mesopores, and large surface area and pore volume. With these good properties, Fe3O4@mSiO2 spheres were applied to the rapid enrichment of peptides. Based on the size-exclusion mechanism and hydrophobic interaction with siloxane bridge group mainly on the surface of inside pores, Fe3O4@mSiO2 can selectively capture peptides and exclude high-MW proteins and salts. Furthermore, peptides in human plasma were successfully enriched by Fe3O4@mSiO2.

Do H-bond features of silica surfaces affect the H2O and NH3 adsorption? Insights from periodic B3LYP calculations

The adsorption of a single H(2)O and NH(3) molecule on different fully hydroxylated α-quartz, cristobalite, and tridymite surfaces has been studied at the B3LYP level of theory, within a periodic approach using basis sets of polarized triple-ζ quality and accounting for basis set superposition error (BSSE). Fully hydroxylated crystalline silica exhibits SiOH as terminal groups whose distribution and H-bond features depend on both the considered silica polymorph and the crystallographic plane, which gives rise to isolated, H-bond interacting SiOH pairs or infinitely connected H-bond chains. A key point of the present study is to understand how the H-bond features of a dry crystalline silica surface influence its adsorption properties. Results reveal that the silica-adsorbate (H(2)O and NH(3)) interaction energy anticorrelates with the density of SiOH groups at the surface. This counterintuitive observation arises from the fact that pre-existing H-bonds of the dry surface need to be broken to establish new H-bonds between the surface and the adsorbate, which manifests in a sizable energy cost due to surface deformation. A simple method is also proposed to estimate the strength of the pre-existing H-bonds at the dry surfaces, which is shown to anticorrelate with the adsorbate interaction energy, in agreement with the above trends.

Rapid synthesis of SBA-15 rods with variable lengths, widths, and tunable large pores

Dispersed SBA-15 rods have been synthesized with varying lengths, widths, and pore sizes in a low-temperature synthesis in the presence of heptane and NH(4)F. The pore size of the material can systematically be varied between 11 and 17 nm using different hydrothermal treatment times and/or temperatures. The particle length (400-600 nm) and width (100-400 nm) were tuned by varying the HCl concentration. All the synthesized materials possess a large surface area of 400-600 m(2)/g and a pore volume of 1.05-1.30 cm(3). A mechanism for the effect of the HCl concentration on the particle morphology is suggested. Furthermore, it is shown that the reaction time can be decreased to 1 h, with well-retained pore size and morphology. This work has resulted in SBA-15 rods with the largest pore size reported for this morphology.

Strong silica monoliths with large mesopores prepared using agarose gel templates

Mesoporous silica pellets with controllable shape and pore size were prepared using agarose gel templates. Robust (compressive strength of 3.3-25.1 MPa), crack-free silica monoliths have been produced with large mesopores (14-23 nm), high surface areas (410-540 m(2) g(-1)), and large pore volumes (1.1-1.2 cm(3) g(-1)). The synthesis was achieved by infusing preformed agarose gels with tetraethyl orthosilicate and the nonpolar condensation catalyst tetrabutyl ammonium fluoride. The infiltrated gels were transferred to water to initiate hydrolysis and condensation of the silica precursor. Fluoride catalyzed the gelation of silica in a matter of minutes; hence, the oxide maintained the shape of the agarose pellet. The mesopore size could be modified by altering the weight percent of agarose gel used. The method employed here is simple and reproducible. As these materials have such large mesopore dimensions, they could be used as hard templates or could be specifically functionalized for use in environmental remediation, as environmentally responsive materials, biocatalysts, or catalysts.

Ordered mesoporous polymer-silica hybrid nanoparticles as vehicles for the intracellular controlled release of macromolecules

A two-dimensional hexagonal ordered mesoporous polymer-silica hybrid nanoparticle (PSN) material was synthesized by polymerization of acrylate monomers on the surface of SBA-15 mesoporous silica nanoparticles. The structure of the PSN material was analyzed using a series of different techniques, including transmission electron microscopy, powder X-ray diffraction, and N(2) sorption analysis. These structurally ordered mesoporous polymer-silica hybrid nanoparticles were used for the controlled release of membrane-impermeable macromolecules inside eukaryotic cells. The cellular uptake efficiency and biocompatibility of PSN with human cervical cancer cells (HeLa) were investigated. Our results show that the inhibitory concentration (IC(50)) of PSN is very high (>100 μg/mL per million cells), while the median effective concentration for the uptake (EC(50)) of PSN is low (EC(50) = 4.4 μg/mL), indicating that PSNs are fairly biocompatible and easily up-taken in vitro. A membrane-impermeable macromolecule, 40 kDa FITC-Dextran, was loaded into the mesopores of PSNs at low pH. We demonstrated that the PSN material could indeed serve as a transmembrane carrier for the controlled release of FITC-Dextran at the pH level inside live HeLa cells. We believe that further developments of this PSN material will lead to a new generation of nanodevices for intracellular controlled delivery applications.

Synthesis of Polymer-Mesoporous Silica Nanocomposites

Polymer nanocomposites show unique properties combining the advantages of the inorganic nanofillers and the organic polymers. The mesoporous silica nanofillers have received much attention due to their ordered structure, high surface area and ease for functionalization of the nanopores. To accommodate macromolecules, the nanopores lead to unusually intimate interactions between the polymer and the inorganic phase, and some unusual properties can be observed, when compared with nonporous fillers. Whereas many review articles have been devoted to polymer/nonporous nanofiller nanocomposites, few review articles focus on polymer/mesoporous silica nanocomposites. This review summarizes the recent development in the methods for synthesizing polymer/mesoporous silica nanocomposites based on the papers published from 1998 to 2009, and some unique properties of these composites are also described.

Encapsulation of protein in silica matrices: structural evolution on the molecular and nanoscales

The immobilization of biological species such as proteins and enzymes in sol-gel hosts is currently an area of intense research activity. However, the majority of these studies have been directed toward investigating the biological activity or physicochemical properties of the encapsulated species, with much less attention having been directed toward the effect of proteins on the structural evolution of the sol-gel matrix. This study investigates the structural evolution of sol-gel matrices in the presence of a model protein, bovine serum albumin (BSA). The sol-gel matrices were produced via the NaF-catalyzed hydrolysis of a mixture of tetramethyoxysilane (TMOS) and methyltrimethoxysilane (MTMS), yielding nanohybrid matrices with controlled pore sizes, pore volumes, and surface chemistry. The structural evolution of the matrix was investigated using a complementary suite of techniques, including solid-state (29)Si NMR, FTIR, SANS contrast variation, and N(2) sorption. A novel approach was developed to model the SANS data, to extract key structural parameters. The results indicated that the structural evolution of the matrices was modulated by a series of complex interactions between the enzyme and the evolving sol-gel nanohybrid: On the molecular scale, increasing BSA content led to an associated increase in both the abundance of linear Si-O-Si species (FTIR) and the Qn network connectivity ((29)Si NMR). However, only minor changes in the connectivity of the evolving Tn network were evident with varying BSA content. The selective role of the protein in these systems, where the approach of the methylated monomer to the vicinity of the protein's surface is presumably impeded by the hydrophobicity of the monomer, will be discussed. On the nanoscale, N(2) sorption data were consistent with an initial increase in the mesopore volume and surface area at low BSA loadings, followed by a subsequent monotonic decrease with increasing BSA content. In contrast, no such trends were evident in the in situ SANS data obtained from these samples, suggesting that modulation of the evolving network structure of the silica matrix by BSA during condensation prevents collapse of the nanoscale gel structure during freeze-drying. This latter comparison reflects the important role of in situ techniques such as small angle scattering (which can be used to study both open and closed porosity and probe nanostructure on length scales from approximately 1 nm to >100 nm) in investigating such complex, multicomponent systems, and techniques for modeling such data in sol-gel systems will be discussed.

2H-solid-state-NMR study of hydrogen adsorbed on catalytically active ruthenium coated mesoporous silica materials

(2)H solid-state NMR measurements were performed on three samples of ruthenium nanoparticles synthesized inside two different kinds of mesoporous silica, namely SBA-3 silica materials and SBA-15 functionalized with -COOH groups and loaded with deuterium gas. The line-shape analyses of the spectra reveal the different deuteron species. In all samples a strong -OD signal is found, which shows the catalytic activity of the metal, which activates the D-D bond and deuterates the -SiOH groups through the gas phase, corroborating their usability as catalysts for hydrogenation reactions. At room temperature the mobility of the -Si-OD groups depends on the sample preparation. In addition to the -Si-OD deuterons, the presence of different types of deuterons bound to the metal is revealed. The singly coordinated -Ru-D species exhibit several different quadrupolar couplings, which indicate the presence of several non-equivalent binding sites with differing binding strength. In addition to the dissociated hydrogen species there is also a dihydrogen species -Ru-D(2), which is attributed to defect sites on the surface. It exhibits a fast rotational dynamics at all temperatures. Finally there are also indications of three-fold coordinated surface deuterons and octahedrally coordinated deuterons inside the metal.

A solid-state NMR investigation of the structure of mesoporous silica nanoparticle supported rhodium catalysts

A detailed study of the chemical structure of mesoporous silica catalysts containing rhodium ligands and nanoparticles (RhP-MSN) was carried out by multi-dimensional solid-state NMR techniques. The degree of functionalization of the rhodium-phosphinosilyl complex to the surface of the RhP-MSN channels was determined by (29)Si NMR experiments. The structural assignments of the rhodium-phosphinosilyl complex were unambiguously determined by employing the novel, indirectly detected heteronuclear correlation ((13)C-(1)H and (31)P-(1)H idHETCOR) techniques, which indicated that oxidation of the attached phosphinosilyl groups and detachment of Rh was enhanced upon syngas conversion.

Vault Isomerization and its Application to the Synthesis of a Dideoxydysoxysulfone Precursor

Novel silica-induced controlled disproportionation of α-mercaptosulfones produces α-mercapto α′-sulfonyl sulfides. Its discovery and exploitation, as the heart of a new process – vault isomerization – permits the elaboration of a more efficient synthesis of a previously described synthetic precursor.