Silicatein, an enzymatic biocatalyst purified from the glassy skeletal elements of a marine sponge, and previously shown capable of catalyzing and structurally directing the hydrolysis and polycondensation of silicon alkoxides to yield silica and silsesquioxanes at low temperature and pressure and neutral pH, is shown to be capable of catalyzing and templating the hydrolysis and subsequent polycondensation of a water-stable alkoxide-like conjugate of titanium to form titanium dioxide. The structure and behavior of the TiO 2 formed through this biocatalytic route, including thermally induced crystal grain growth and phase transformation from anatase to rutile, differ from those of TiO 2 formed from the same precursor via alkali catalysis or thermal pyrolysis. This enzymatic route affords a path to templated synthesis that avoids the high temperatures and extremes of pH typically required for synthesis of metallo-oxanes from the corresponding alkoxide-like precursors, and thus provides access to a new and potentially useful parameter space of structures and properties. The proteins may also be nanoscopically structure-directing, as evidenced by the formation of nanocrystallites of anatase, a polymorph usually formed at much higher temperatures. The summation of weak interactions between the protein and mineral may induce this stabilization and thus may afford a new level of nanostructural control, with associated enhancement of selected performance properties.

Two uncharged microcrystalline substances, pyrene (PYR) and fluorescein diacetate (FDA), were rendered water dispersible by treatment with various amphiphilic substances and subsequently encapsulated by exposure to an alternating sequence of cationic and anionic polyelectrolytes. The amphiphilic compounds employed to charge the microcrystals were ionic surfactants, phospholipids, and polyelectrolytes with an amphiphilic nature. Polyelectrolyte layers were self-assembled onto the pre-charged microcrystalline templates by means of electrostatic layer-by-layer deposition, thus forming a multilayered polymeric shell around the crystalline cores. The semipermeable nature of the polymer multilayer shell was thereafter exploited to remove the templated core by exposure to a mild organic solvent. The release behavior of solubilized PYR and FDA from the crystalline core was examined by monitoring their fluorescence after dissolution with ethanol. Complete removal of the core yielded hollow polymer capsules of micrometer dimensions. The capsule porosity was found to be influenced by the amphiphile used to pre-charge the microcrystal surface. The strategy presented is expected to be a general approach for the encapsulation of hydrophobic, low molecular weight compounds such as drugs, as well as providing a novel and facile pathway to the fabrication of polymer multilayered microcapsules with controlled release properties for drug delivery.

The molecular mechanisms underlying the biological synthesis of nanostructured mineral/organic composites have long been recognized to offer exciting prospects for materials science.[1±8] In addition to their benign conditions for synthesis (including neutral pH, low temperature, low pressure, and the absence of caustic chemicals), these mechanisms often reveal a precision of nanostructural control not yet achievable in anthropogenic syntheses. Investigations into such mechanisms have shown that protein filaments occluded within the silica skeletal elements of a marine sponge consist of structure-directing enzymes capable of catalyzing, in vitro, the hydrolysis and polycondensation of molecular precursors of silica, silsesquioxanes, [5,9±11] and titania. [12] We show here that these protein filaments are not only capable of the hydrolysis and polycondensation of a gallium oxide molecular precursor to yield (depending on the reaction conditions) either gallium oxo-hydroxide (GaOOH) or spinel gallium oxide (c-Ga 2 O 3 , a gassensing semiconductor) at room temperature, but also to direct their resulting structures. This control is seen in the defined orientation of nanocrystals with respect to the surface of the protein, suggesting that structural determinants on the surface of the protein catalyze the formation of the c-Ga 2 O 3 polymorph at low temperatures and may direct its crystallographic orientation. These results demonstrate the feasibility of a low-temperature catalytic route to the synthesis and COMMUNICATIONS 314

Silicatein is an enzyme isolated from the biosilica produced by the marine demosponge, Tethya aurantia. Once isolated from the sponge, silicatein can be used in vitro to catalyze the hydrolysis and direct polycondensation of a wide variety of alkoxide, ionic, and organometallic precursors to the corresponding chalcogens at standard temperature and pressure and neutral pH. On the basis of these results, an array of small molecules that mimic the unique physiochemical environment found in the enzyme active site was investigated for catalytic activity in the formation of silica from silicon alkoxides at neutral pH. The most successful of these biomimetic catalysts (cysteamine) was used to encapsulate firefly luciferase, green and blue fluorescent proteins (GFP, BFP), and Escherichia coli cells expressing GFP in silica matrixes. The benign conditions required for the catalysis of synthesis of these silica composites does not impair the activities of the encapsulated enzyme, fluorescent proteins, or live cells as shown by fluorescence measurements. In conjunction with microcontact printing, this biomimetically catalyzed encapsulation method has been used to produce patterned functional arrays of silica nanoparticulate composite materials.

Any catalyst should be efficient and stable to be implemented in practice. This requirement is particularly valid for manganese hydrogenation catalysts. While representing a more sustainable alternative to conventional noble metal-based systems, manganese hydrogenation catalysts are prone to degrade under catalytic conditions once operation temperatures are high. Herein, we report a highly efficient Mn(I)-CNP pre-catalyst which gives rise to the excellent productivity (TOF° up to 41 000 h−1) and stability (TON up to 200 000) in hydrogenation catalysis. This system enables near-quantitative hydrogenation of ketones, imines, aldehydes and formate esters at the catalyst loadings as low as 5–200 p.p.m. Our analysis points to the crucial role of the catalyst activation step for the catalytic performance and stability of the system. While conventional activation employing alkoxide bases can ultimately provide catalytically competent species under hydrogen atmosphere, activation of Mn(I) pre-catalyst with hydride donor promoters, e.g. KHBEt3, dramatically improves catalytic performance of the system and eliminates induction times associated with slow catalyst activation.

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We report on the preparation and utilization of a novel class of particulate labels based on nanoencapsulated organic microcrystals with the potential to create highly amplified biochemical assays. Labels were constructed by encapsulating microcrystalline fluorescein diacetate (FDA; average size of 500 nm) within ultrathin polyelectrolyte layers of poly(allylamine hydrochloride) and poly(sodium 4-styrenesulfonate) via the layer-by-layer technique. Subsequently, the polyelectrolyte coating was used as an "interface" for the attachment of anti-mouse antibodies through adsorption. A high molar ratio of fluorescent molecules present in the microcrystal core to biomolecules on the particle surface was achieved. The applicability of the microcrystal-based label system was demonstrated in a model sandwich immunoassay for mouse immunoglobulin G detection. Following the immunoreaction, the FDA core was dissolved by exposure to organic solvent, leading to the release of the FDA molecules into the surrounding medium. Amplification rates of 70-2000-fold (expressed as an increase in assay sensitivity) of the microcrystal label-based assay compared with the corresponding immunoassay performed with direct fluorescently labeled antibodies are reported. Our approach provides a general and facile means to prepare a novel class of biochemical assay labeling systems. The technology has the potential to compete with enzyme-based labels as it does not require long incubation times, thus speeding up bioaffinity tests.

Homogeneously catalyzed reactions often make use of additives and promotors that affect reactivity patterns and improve catalytic performance. While the role of reaction promotors is often discussed in view of their chemical reactivity, we demonstrate that they can be involved in catalysis indirectly. In particular, we demonstrate that promotors can adjust the thermodynamics of key transformations in homogeneous hydrogenation catalysis and enable reactions that would be unfavorable otherwise. We identified this phenomenon in a set of wellestablished and new Mn pincer catalysts that suffer from persistent product inhibition in ester hydrogenation. Although alkoxide base additives do not directly participate in inhibitory transformations, they can affect the equilibrium constants of these processes. Experimentally, we confirm that by varying the base promotor concentration one can control catalyst speciation and inflict substantial changes to the standard free energies of the key steps in the catalytic cycle. Despite the fact that the latter are universally assumed to be constant, we demonstrate that reaction thermodynamics and catalyst state are subject to external control. These results suggest that reaction promotors can be viewed as an integral component of the reaction medium, on its own capable of improving the catalytic performance and reshaping the seemingly rigid thermodynamic landscape of the catalytic transformation.