Karin Möller

Colloidal Suspensions of Functionalized Mesoporous Silica Nanoparticles

The synthesis and characterization of colloidal mesoporous silica (CMS) functionalized with vinyl-, benzyl-, phenyl-, cyano-, mercapto-, aminopropyl- or dihydroimidazole moieties is reported. Uniform mesoporous particles ranging in size from 40 to 150 nm are generated in a co-condensation process of tetraethylorthosilicate (TEOS) and organotriethoxysilanes (RTES) in alkaline aqueous media containing triethanolamine (TEA) in combination with cetyltrimethylammonium chloride (CTACl) serving as a structure-directing agent. The materials are obtained as colloidal suspensions featuring long-term stability after template removal by ion exchange with an ethanolic solution of ammonium nitrate or HCl. The spherical particles exhibit a wormlike pore system with defined pore sizes and high surface areas. Samples are analyzed by a number of techniques including TEM, SEM, DLS, TGA, Raman, and cross-polarized (29)Si-MAS NMR spectroscopy, as well as nitrogen sorption measurements. We demonstrate that co-condensation and grafting methods result in similar changes in the nitrogen adsorption behavior, indicating a successful internal lining of the pores with functional groups through both procedures.

One-step synthesis of hierarchical zeolite beta via network formation of uniform nanocrystals.

A hierarchical mesoporous network of zeolite beta with very high micropore as well as mesopore volume was synthesized without the need of a porogen at near 100% yield in the form of easily retrievable micrometer-sized particles. This was achieved by a dense-gel synthesis utilizing steam-assisted conversion (SAC) to induce a burst of nucleation. During the first phase of the synthesis, individual, evenly sized zeolite beta nanoparticles are formed that subsequently condense into a porous network displaying uniform mesopores. The final product consists of hierarchical self-sustaining macroscopic zeolite aggregates assembled from 20 nm crystalline domains of zeolite beta. The small size of the zeolite crystals in the resulting materials gives rise to mesopores with dominant pore sizes of about 13 nm. Large surface areas between 630 and 750 m(2)/g and total pore volumes up to 0.9 mL/g were obtained without sacrificing the microporosity (usually larger than 0.20 mL/g). Crystallization conditions were optimized for different Si/Al ratios between 10 and 33. A complete conversion into hierarchical zeolite beta was achieved in only a few hours at 170-180 °C if the amount of water present during the steam-assisted conversion was adequately adjusted. This dense gel steam conversion process proves to be a highly efficient strategy for fabricating hierarchical zeolite beta networks in a single step.

Three-dimensionally confined diluted magnetic semiconductor clusters: Zn1−xMnxS

We report the first example of a dilute magnetic semiconductor (DMS) confined in all three dimensions (DMS quantum dot). Zn0.93Mn0.07S clusters of not, vert, similar 25 A diameter are successfully synthesized inside a glass matrix and fully characterized by chemical analysis, x-ray diffraction, extended x-ray absorption fine structure (EXAFS), and photoluminescence spectroscopy. Effect of size quantization on the exciton energy has been observed. Preliminary magnetic susceptibility data are presented and discussed.

Colloidal suspensions of mercapto-functionalized nanosized mesoporous silica

Different synthesis routes yielding colloidal suspensions of mercapto-functionalized mesoporous silica particles of nanosize dimensions between 50 and 200 nm are presented. Mercaptopropylsilane was introduced into the silica networks through co-condensation. The mercapto group induces significant changes in the morphology, transforming highly ordered hexagonal particles either into filaments or small agglomerates with nanoscale domains. The addition of triethanolamine base instead of NaOH leads to the formation of colloidal suspensions of spherical particles having a diameter of ca. 100 nm. The grafting of the thiol group into the colloidal mesoporous silica was characterized with sorption measurements, thermal analysis, 29Si-MAS-NMR, and Raman spectroscopy. Oxidative treatment of the thiol groups anchored to the internal surface of the nanoscale mesoporous particles results in the formation of grafted sulfonic acid residues. The latter reaction proceeds to complete conversion as demonstrated with FT Raman spectroscopy.

Pores Within Pores—How to Craft Ordered Hierarchical Zeolites

Thin walls of crystalline zeolites can be assembled into hexagonal nanopore networks, which expands the range of their catalytic reactions to larger molecules. Zeolites are aluminosilicate crystals that have internal networks of angstrom-size pores, similar to the dimensions of small molecules. They are among the most widely used materials in heterogeneous catalysis (1, 2) because of their defined structure and composition. Although zeolites are very potent solid-acid catalysts, their catalytic applications have been limited to processing smaller molecules; their internal pores are not readily accessed by molecules exceeding 1 nm in size. Major efforts have been directed to overcoming this limitation. On page 328 of this issue, Na et al. (3) present a new strategy for creating thin zeolite walls, containing small pores, that grow into structures forming larger pores that can catalyze reactions with larger molecules.

Highly Efficient siRNA Delivery from Core-Shell Mesoporous Silica Nanoparticles with Multifunctional Polymer Caps

A new general route for siRNA delivery is presented combining porous core-shell silica nanocarriers with a modularly designed multifunctional block copolymer. Specifically, the internal storage and release of siRNA from mesoporous silica nanoparticles (MSN) with orthogonal core-shell surface chemistry was investigated as a function of pore-size, pore morphology, surface properties and pH. Very high siRNA loading capacities of up to 380 μg per mg MSN were obtained with charge-matched amino-functionalized mesoporous cores, and release profiles show up to 80% siRNA elution after 24 h. We demonstrate that adsorption and desorption of siRNA is mainly driven by electrostatic interactions, which allow for high loading capacities even in medium-sized mesopores with pore diameters down to 4 nm in a stellate pore morphology. The negatively charged MSN shell enabled the association with a block copolymer containing positively charged artificial amino acids and oleic acid blocks, which acts simultaneously as capping and endosomal release agent. The potential of this multifunctional delivery platform is demonstrated by highly effective cell transfection and siRNA delivery into KB-cells. A luciferase reporter gene knock-down of up to 80-90% was possible using extremely low cell exposures with only 2.5 μg MSN containing 0.5 μg siRNA per 100 μL well.

Nanofusion: Mesoporous Zeolites Made Easy

A new path to zeolites: Nanofusion is a template-free, one-step process that gives mesoporous zeolite beta with tunable mesopore diameters in very short reaction times. The hierarchical materials are built from zeolite beta nanocrystals in concentrated gels. The nanocrystals can either be retrieved as individual particles in a colloidal suspension or are directly fused into mesoporous zeolitic materials (see scheme).

Zeolite encapsulated vanadium oxo species for the catalytic reduction of NO by NH3

Vanadium oxo species have been introduced into the supercages of Y-zeolite by adsorption and decomposition of VOC13 to produce catalysts for the study of the effects of oxo vanadium site isolation on catalytic reduction of NO with NH3. 51V solid-state MAS-NMR of calcined samples with up to 4 V atoms per supercage show that Y-zeolite stabilizes vanadium with octahedral, V2O5-like square pyramidal, and most commonly, tetrahedral coordinations. Two distinct isolated tetrahedral environments were identified with NMR resonances around −530 and −830 ppm. There was no NMR evidence for oxygen bridge-bonded vanadia structures in the tetrahedral species in these catalysts. DRIFTS shows that ammonia adsorbs primarily on Bronsted acid sites associated with both the zeolite and the vanadium oxo species. DRIFTS during NO reaction with preadsorbed NH3 shows the production of a hydroxyl species at 3690 cm−1, which was assigned to V+4-OH, in agreement with the literature. Turnover frequencies, based on NO conversion and total vanadium loading, range from 1 × 10−5 to 3 × 10−4 s−1 with selectivity to N2 of 90–100%. Steady-state reaction experiments indicate that isolated vanadia species are viable sites for the catalytic reduction of NO.

REACTIVITY OF A TRIMETHYLSTANNYL MOLYBDENUM COMPLEX IN MESOPOROUS MCM-41

A highly thermostable tin–molybdenum complex is encapsulated into the hexagonal mesoporous channel host MCM-41 and thermally transformed into supported metal clusters.

EXAFS ANALYSIS OF SIZE-CONSTRAINED SEMICONDUCTING MATERIALS

Semiconducting materials such as CdSe, CdS, PbS and GaP are included in crystalline zeolite Y and mordenite and structurally flexible ethylene-methacrylic acid copolymer solid matrices. EXAFS analysis reveals formation of species with dimensions of molecular size up to ca. 13 A in the crystalline hosts, while the polymer matrices allow agglomeration of larger semiconducting particles. Zeolite anchored structures are distinctively different to small particles with bulk crystal structure as usually found in colloidal systems.