Hierarchically structured ceria-silica : synthesis and thermal properties

A one-pot method for the synthesis of hierarchically structured ceria-silica composite materials is reported along with the results of their characterization by a variety of physical techniques. Crystallization of ceria in an aqueous mixture of anionic, cationic, and neutral surfactants, namely Pluronic P123 (EO 20 PO 70 EO 20 ), CTAB (hexadecyltrimethylammonium bromide), and SDS (sodium dodecyl sulfate) leads to the formation of a suspension of capped ceria nanoparticles. Subsequent addition of tetraethoxysilane followed by aging at 40 − 80 ° C leads to the condensation of silica. After thermal removal of the organic species, the formation of high surface area composites directed by the interaction of the capped nanoparticles and the remaining surfactants is possible. The thermal stability and redox activity of the composite materials have been studied by in situ powder X-ray di ff raction, TGA/DSC, transmission electron microscopy, Ce L III -edge XANES, and temperature-programmed reduction under H 2 /N 2 . Encapsulation of the ceria nanoparticles in the templated silica matrix leads to high thermal stability with the nanocrystalline nature of the ceria retained upon heating to 900 ° C in air with no annealing evident by in situ thermodi ff ractometry. Temperature-programmed reduction shows large hydrogen uptake at around 600 ° C, corresponding to complete reduction of all Ce(IV) to Ce(III) in the case of a cerium-rich sample (Ce:Si = 5:12). This reduction leads to amorphization of the ceria followed by the collapse of the hierarchical structure with formation of Ce 2 Si 2 O 7 crystallites embedded in amorphous silica. For a sample of lower cerium content, crystalline Ce 6 (Si 4 O 13 )(SiO 4 ) 2 is formed under reductive condition

Additive-induced morphological tuning of self-assembled silica-barium carbonate crystal aggregates

Crystallisation of barium carbonate from alkaline silica solutions results in the formation of extraordinary micron-scale architectures exhibiting non-crystallographic curved shapes, such as helical filaments and worm-like braids. These so-called "silic

Additive-induced morphological tuning of self-assembled silica-barium carbonate crystal aggregates

Crystn. of barium carbonate from alk. silica solns. results in the formation of extraordinary micron-scale architectures exhibiting non-crystallog. curved shapes, such as helical filaments and worm-like braids. These so-called "silica biomorphs" consist of a textured assembly of uniform elongated witherite nanocrystallites, which is occasionally sheathed by a skin of amorphous silica. Although great efforts have been devoted to clarifying the phys. origin of these fascinating materials, to date little is known about the processes underlying the obsd. self-organization. Herein, we describe the effect of two selected additives, a cationic surfactant and a cationic polymer, on the morphol. of the forming crystal aggregates, and relate changes to expts. conducted in the absence of additives. Minor amts. of both substances are shown to exert a significant influence on the growth process, leading to the formation of predominantly flower-like spherulitic aggregates. The obsd. effects are discussed in terms of feasible morphogenesis pathways. Based on the assumption of a template membrane steering biomorph formation, it is proposed that the two additives are capable of performing specific bridging functions promoting the aggregation of colloidal silica which constitutes the membrane. Morphol. changes are tentatively ascribed to varying colloid coordination effecting distinct membrane curvatures

DNA compaction induced by a cationic polymer or surfactant impact gene expression and DNA degradation

There is an increasing interest in achieving gene regulation in biotechnological and biomedical applications by using synthetic DNA-binding agents. Most studies have so far focused on synthetic sequence-specific DNA-binding agents. Such approaches are relatively complicated and cost intensive and their level of sophistication is not always required, in particular for biotechnological application. Our study is inspired by in vivo data that suggest that DNA compaction might contribute to gene regulation. This study exploits the potential of using synthetic DNA compacting agents that are not sequence-specific to achieve gene regulation for in vitro systems. The semi-synthetic in vitro system we use include common cationic DNA-compacting agents, poly(amido amine) (PAMAM) dendrimers and the surfactant hexadecyltrimethylammonium bromide (CTAB), which we apply to linearized plasmid DNA encoding for the luciferase reporter gene. We show that complexing the DNA with either of the cationic agents leads to gene expression inhibition in a manner that depends on the extent of compaction. This is demonstrated by using a coupled in vitro transcription-translation system. We show that compaction can also protect DNA against degradation in a dose-dependent manner. Furthermore, our study shows that these effects are reversible and DNA can be released from the complexes. Release of DNA leads to restoration of gene expression and makes the DNA susceptible to degradation by Dnase. A highly charged polyelectrolyte, heparin, is needed to release DNA from dendrimers, while DNA complexed with CTAB dissociates with the non-ionic surfactant C12E5. Our results demonstrate the relation between DNA compaction by non-specific DNA-binding agents and gene expression and gene regulation can be achieved in vitro systems in a reliable dose-dependent and reversible manner

Co-precipitation of silica and alkaline-earth carbonates using TEOS as silica source

We explore the use of tetraethoxysilane (TEOS) as a silica source for the formation of carbonate-silica composite materials known as ‘biomorphs’. The basic hydrolysis of TEOS furnishes silica in a controllable fashion, allowing a significantly higher reproducibility of the obtained silica–barium and silica–strontium carbonate co-precipitates compared to commercial water glass silica used so far. We further discuss the influence of ethanol used as a co-solvent on the morphologies of biomorphs, which are examined by optical microscopy, field emission scanning electron microscopy (FESEM) and energy dispersive X-ray analysis (EDX)

Stevensite in the modern thrombolites of Lake Clifton, Western Australia: A missing link in microbialite mineralization?

Microbialites form the earliest macroscopic evidence of life, and have always been important in particular aquatic ecosystems. They demonstrate the remarkable ability of microorganisms to provide the foundation for structures that can rival coral reefs in size. Microbialites are generally assumed to form by microbial trapping and binding of detrital grains, by carbonate organomineralization of microbial biofi lms, or by inorganic mineralization around microbial templates. Here we present a signifi cant discovery that modern thrombolitic microbialites in Lake Clifton, Western Australia, gain their initial structural rigidity from biofi lm mineralization by the trioctahedral smectite mineral stevensite. This nucleates in and around microbial fi lament walls when biological processes suppress carbon and Ca activities, leaving Mg to bind with silica and form a microporous framework that replaces and infi lls the fi lament web. After microbial materials are entombed, local carbon and Ca activities rise suffi ciently for aragonite microcrystals to grow within the stevensite matrix and perhaps replace it entirely, with eradication of biogenic textural features. This may explain why many ancient microbialite carbonates lack clear evidence for biogenicity. Stevensite may provide the missing link between microbial organomineralization and subsequent abiotic calcifi cation

Condensed Supramolecular Helices: The Twisted Sisters of DNA

Condensation of DNA helices into hexagonally packed bundles and toroids represents an intriguing example of functional organization of biological macromolecules at the nanoscale. The condensation models are based on the unique polyelectrolyte features of DNA, however here we could reproduce a DNA-like condensation with supramolecular helices of small chiral molecules, thereby demonstrating that it is a more general phenomenon. We show that the bile salt sodium deoxycholate can form supramolecular helices upon interaction with oppositely charged polyelectrolytes of homopolymer or block copolymers. At higher order, a controlled hexagonal packing of the helices into DNA-like bundles and toroids could be accomplished. The results disclose unknown similarities between covalent and supramolecular non-covalent helical polyelectrolytes, which inspire visionary ideas of constructing supramolecular versions of biological macromolecules. As drug nanocarriers the polymer–bile salt superstructures would get advantage of a complex chirality at molecular and supramolecular levels, whose effect on the nanocarrier assisted drug efficiency is a still unexplored fascinating issue