Synthesis and sensing properties of D5h pentagonal silver star nanoparticles.

In this work, we use silver decahedral nanoparticle (AgDeNP) seeds to synthesize pentagonal silver stars (AgStDeNPs) and study the sensing properties of these nanoparticles. The regrowth process of AgStDeNPs is kinetically-controlled, so the purity of the seed NPs is critical to avoid secondary deposition in the highly non-equilibrium reduction. To control the regrowth process, surface blocking with sodium polyacrylate (PANa) was implemented. PANa moderates rough silver nanostructures typically obtained by reduction with ascorbic acid. To modulate polymer binding to the surface and thus to tune surface blocking, pH served as a key synthetic parameter. Under optimal regrowth conditions, new sliver was deposited on the highest energy sites of the decahedra - the vertices of the rims - to yield pentagonal stars. The universality of this regrowth process was established with several different seed particles. The sharpness and size of the stellated tips are tunable by the amount of added silver. Gold deposition onto AgStDeNPs enables the preparation of diverse structures with enhanced stability. Ease of transformation, e.g. rounding, of star branches opens a promising venue for enhanced SPR sensing. Also, AgStDeNPs enable femtomolar detection of 5,5-dithiobis(2-nitrobenzoic acid) in SERS.

A design strategy for the hierarchical fabrication of colloidal hybrid mesostructures.

Advances in nanotechnology depend upon expanding the ability to create new and complex materials with well-defined multidimensional mesoscale structures. The creation of hybrid hierarchical structures by combining colloidal organic and inorganic building blocks remains a challenge due to the difficulty in preparing organic structural units of precise size and shape. Here we describe a design strategy to generate controlled hierarchical organic-inorganic hybrid architectures by multistep bottom-up self-assembly. Starting with a suspension of large inorganic nanoparticles, we anchor uniform block copolymer crystallites onto the nanoparticle surface. These colloidally stable multi-component particles can initiate the living growth of uniform cylindrical micelles from their surface, leading to three-dimensional architectures. Structures of greater complexity can be obtained by extending the micelles via addition of a second core-crystalline block copolymer. This controlled growth of polymer micelles from the surface of inorganic particles opens the door to the construction of previously inaccessible colloidal organic-inorganic hybrid structures.

Light-triggered carbon monoxide delivery with Al-MCM-41-based nanoparticles bearing a designed manganese carbonyl complex.

A photoactive manganese carbonyl complex namely, fac-[Mn(pqa)(CO)3]ClO4 (abbreviated as {Mn-CO}, pqa = (2-pyridylmethyl)(2-quinolylmethyl)amine) has been incorporated in the pores of 60 nm mesoporous Al-MCM-41 nanoparticles. Strong electrostatic interactions with the negatively charged walls of the aluminosilicate host entrap the cationic carbonyl complex in the resulting material {Mn-CO}@Al-MCM-41 which has been characterized by various physical techniques and chemical analysis. The sample morphology and microstructure of the material have been established by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results of powder X-ray diffraction (PXRD) data and the SEM-EDX elemental maps of Si, O, Al and Mn confirm that the carbonyl complex is within the pores of the nanoparticles. The 2% manganese content of {Mn-CO}@Al-MCM-41, determined by acid digestion followed by inductively coupled plasma-optical emission spectroscopy (ICP-OES), indicates efficient loading of the carbonyl complex in the nanoparticles. In biological buffer solutions, less than 2% leaching of {Mn-CO} from the nanoparticles was noted within a period of 24 h. When exposed to a broadband visible light source (λ > 350 nm), {Mn-CO}@Al-MCM-41 rapidly releases CO, as confirmed by the reduced myoglobin assay. The utility of light-induced CO delivery by {Mn-CO}@Al-MCM-41 has been established by its capacity to relax rat aorta muscle rings in tissue bath experiments.

Gold plating of silver nanoparticles for superior stability and preserved plasmonic and sensing properties.

Gold-plated silver nanoparticles (Au@AgNPs, shell@core) with high stability in various environments detrimental to AgNPs have been produced with preserved plasmonic and sensing properties.

Iron nanoparticles catalyzing the asymmetric transfer hydrogenation of ketones.

Investigation into the mechanism of transfer hydrogenation using trans-[Fe(NCMe)CO(PPh(2)C(6)H(4)CH═NCHR-)(2)][BF(4)](2), where R = H (1) or R = Ph (2) (from R,R-dpen), has led to strong evidence that the active species in catalysis are iron(0) nanoparticles (Fe NPs) functionalized with achiral (with 1) and chiral (with 2) PNNP-type tetradentate ligands. Support for this proposition is given in terms of in operando techniques such as a kinetic investigation of the induction period during catalysis as well as poisoning experiments using substoichiometric amounts of various poisoning agents. Further support for the presence of Fe(0) NPs includes STEM microscopy imaging with EDX analysis, XPS analysis, and SQUID magnetometry analysis of catalytic solutions. Further evidence of Fe NPs acting as the active catalyst is given in terms of a polymer-supported substrate experiment whereby the NPs are too large to permeate the pores of a functionalized polymer. Final support is given in terms of a combined poisoning/STEM/EDX experiment whereby the poisoning agent is shown to be bound to the Fe NPs. This paper provides evidence of a rare example of asymmetric catalysis with nonprecious metal, zerovalent nanoparticles.

Nanocrystal self-assembly with rod-rod block copolymers.

Two distinct morphologies of hexylselenophene-hexylthiophene rod-rod block copolymer films can be prepared depending on the molecular weight of the sample (see picture: left M(n) =12.9, right M(n) =3.9 kg mol(-1)). These polymers can be used to organize spherical CdSe nanocrystals (yellow) into either dispersed or aligned hierarchical structures. Scale bars: 200 nm.

Ultrathin gold nanoframes through surfactant-free templating of faceted pentagonal silver nanoparticles.

Ultrathin gold nanoframes (up to 1.6 nm) were prepared via templating upon well-defined faceted silver morphologies. Starting with silver decahedra, small quantities of gold (1-10 mol% relative to the amount of silver) were selectively deposited on the nanoparticle edges under optimized reducing conditions. Silver dissolution in hydrogen peroxide yielded well-defined gold frames that retained their structural integrity in the ultrathin nanowire regime below 2 nm. The frame formation protocol was also successfully applied to other silver nanoparticle shapes featuring pentagonal twinning and (111) facets (e.g., pentagonal faceted rods and icosahedra). The demonstrated approach can be applied in the controlled preparation of ultrathin metal nanowires complementary to lithography and in the production of ultrafine noble-metal nanostructures for catalytic applications.

Probing dynamic generation of hot-spots in self-assembled chains of gold nanorods by surface-enhanced Raman scattering.

Further progress in the applications of self-assembled nanostructures critically depends on developing a fundamental understanding of the relation between the properties of nanoparticle ensembles and their time-dependent structural characteristics. Following dynamic generation of hot-spots in the self-assembled chains of gold nanorods, we established a direct correlation between ensemble-averaged surface-enhanced Raman scattering and extinction properties of the chains. Experimental results were supported with comprehensive finite-difference time-domain simulations. The established relationship between the structure of nanorod ensembles and their optical properties provides the basis for creating dynamic, solution-based, plasmonic platforms that can be utilized in applications ranging from sensing to nanoelectronics.

Graphene oxide-periodic mesoporous silica sandwich nanocomposites with vertically oriented channels.

This paper describes the synthesis and characterization of single-layer graphene oxide-periodic mesoporous silica sandwich nanocomposites. Through a comprehensive exploration of the synthesis conditions, it has proven possible to create the first example of a graphene oxide-periodic mesoporous silica nanocomposite in which hexagonal symmetry PMS film grows on both sides of the graphene oxide sheets with the mesoporous channels vertically aligned with respect to the graphene oxide surface. The formation of this novel architecture is found to be very sensitive to pH, the ratio of surfactant template to graphene oxide, the amount of silica precursor, and the temperature of the synthesis. On the basis of the collected data of a multi-technique analysis, it is proposed that the mode of formation of the nanocomposite involves the co-assembly of silicate-surfactant admicelles on opposite sides of graphene oxide platelets acting thereby as a template for growth of vertical mesopores off the platelet surface. These composites showed semiconductive behavior with electrical conductivity sensitively responding to analyte vapor exposure. The discovery of graphene oxide-periodic mesoporous silica sandwich nanocomposites will provide new opportunities for research that exploits the synergism of the graphene oxide and periodic mesoporous silica parts.

Catalyst-free synthesis of transparent, mesoporous diamond monoliths from periodic mesoporous carbon CMK-8.

We report on the synthesis of optically transparent, mesoporous, monolithic diamond from periodic mesoporous carbon CMK-8 at a pressure of 21 GPa. The phase transformation is already complete at a mild synthesis temperature of 1,300 degrees C without the need of a catalyst. Surprisingly, the diamond is obtained as a mesoporous material despite the extreme pressure. X-ray diffraction, SEM, transmission electron microscopy, selected area electron diffraction, high-resolution transmission electron microscopy, and Z-contrast experiments suggest that the mesoporous diamond is composed of interconnected diamond nanocrystals having diameters around 5-10 nm. The Brunauer Emmett Teller surface area was determined to be 33 m2 g(-1) according Kr sorption data. The mesostructure is diminished yet still detectable when the diamond is produced from CMK-8 at 1,600 degrees C and 21 GPa. The temperature dependence of the porosity indicates that the mesoporous diamond exists metastable and withstands transformation into a dense form at a significant rate due to its high kinetic inertness at the mild synthesis temperature. The findings point toward ultrahard porous materials with potential as mechanically highly stable membranes.

Controlling phase separation and optical properties in conjugated polymers through selenophene-thiophene copolymerization.

Selenophene-thiophene block copolymers were synthesized and studied. The properties of these novel block copolymers are distinct from those of statistical copolymers prepared from the same monomers with a similar composition. Specifically, the block copolymers exhibit broad and red-shifted absorbance features and phase-separated domains in the solid state. Scanning transmission electron microscopy and topographic elemental mapping confirmed that the domains are either rich in selenophene or thiophene, indicating that the blocks of distinct heterocycles preferentially associate with one another in the solid state. This preference is surprising in view of the chemical similarities between repeat units. The overall results demonstrate a phase separation that is controlled by elemental differences. As a result of this phase separation, these novel conjugated block copolymers should find utility in a variety of studies and optoelectronics uses.

Lamellar envelopes of semiconductor nanocrystals.

We report the solution-phase formation of ordered lamellar nanocrystal (NC) arrays. These semiconductor lamellae exhibit structural integrity and temporal stability, without recourse to chemical cross-linking. They are many micrometers in diameter yet only two or three nanocrystal layers in thickness. We show that these structures can integrate a cargo of water-soluble ions, molecules, metal nanoparticles, or biomolecules. Encapsulation of gold nanoparticles is found to cause photoluminescence enhancement of the host CdSe nanocrystals. These easily prepared nanocrystal "envelopes" promise properties modulated through the integration of a vast array of water-soluble species. Our approach marks a step toward ordered, compartmentalized, NC-based complexes with controlled architectures.

Oligomerized Tie2 localizes to clathrin-coated pits in response to angiopoietin-1.

The tyrosine kinase receptor Tie2 is expressed on endothelial cells, and together with its ligand angiopoietin-1 (Ang1), is important for angiogenesis and vascular stability. Upon activation by Ang1, Tie2 is rapidly internalized and degraded, a mechanism most likely necessary to attenuate receptor activity. Using immunogold electron microscopy, we show that on the surface of endothelial cells, Tie2 is arranged in variably sized clusters containing dimers and higher order oligomers. Clusters of Tie2 were expressed on the apical and basolateral plasma membranes, and on the tips of microvilli. Upon activation by Ang1, Tie2 co-localized with the clathrin heavy chain at the apical and basolateral plasma membranes and within endothelial cells indicating that Tie2 internalizes through clathrin-coated pits. Inhibiting cellular endocytosis by depleting cellular potassium or by acidifying the cytosol blocked the internalization of Tie2 in response to Ang1. Our results suggest that one pathway mediating the internalization of Tie2 in response to Ang1 is through clathrin-coated pits.

Chiral thiol-stabilized silver nanoclusters with well-resolved optical transitions synthesized by a facile etching procedure in aqueous solutions.

A novel approach of cyclic reduction in oxidative conditions has been developed to prepare a single dominant species of chiral thiol-stabilized silver nanoclusters (AgNCs). Such AgNCs, which are stable in solution for up to a few days, have been obtained for the first time. The generality of the established procedure is proven by using several enantiomeric water-soluble thiols, including glutathione, as protective ligands. The prepared AgNCs featured prominent optical properties including a single pattern of UV-vis absorption with well-resolved peaks. The chirality of the clusters has been investigated by circular dichroism (CD) spectroscopy. CD spectra displayed strong characteristic signatures in the visible range. Tentative identification of the cluster composition is discussed.

Block copolymers under periodic, strong three-dimensional confinement.

In this communication we study the influence of strong 3D confinement on the self-assembly of diblock copolymers containing a polyferrocenylsilane metallopolymer segment. Both silica colloidal crystals and silica inverse colloidal crystals, having nanometer-scale interconnected pore networks, are used as molds to direct the self-assembly. Unusual morphologies, such as concentric shells and branched lamellae, result from the interaction of the polymer with the high surface area topologically periodic templates.

Redox-induced synthesis and encapsulation of metal nanoparticles in shell-cross-linked organometallic nanotubes.

A new approach to encapsulate silver nanoparticles inside block copolymer nanotubes is reported and involves an in situ redox reaction between a polyferrocenylsilane (PFS) inner wall and silver ions. Partial preoxidation of the PFS domains was found to be a key step for the efficient formation of one-dimensional arrays of silver nanoparticles confined within the nanotubes.

Towards flexible inorganic "mesomaterials": one-pot low temperature synthesis of mesostructured nanocrystalline titania.

We hereby report a simple route for the low temperature synthesis of mesoporous nanocrystalline titania involving brief hydrothermal treatment of butanolic precursors and non-ionic tri-block-copolymer surfactant at 100 degrees C, followed by evaporation induced self assembly to make a crack-free flexible film. At no time in the film-forming process is a temperature of more than 120 degrees C reached, thereby permitting the use of substrates that are not stable to higher temperatures.

Novel route to periodic mesoporous aminosilicas, PMAs: ammonolysis of periodic mesoporous organosilicas.

A new route to periodic mesoporous aminosilicas (PMAs) that contain amine functional groups in the framework of a mesoporous network is reported. The materials are prepared via thermal ammonolysis of periodic mesoporous organosilicas (PMOs) under a flow of ammonia gas. PMOs integrate similar or even higher quantities of nitrogen-containing groups upon ammonolysis than similarly treated ordered mesoporous silicas (MCM-41). The quantity of amine groups introduced into the materials was found to depend strongly on the ammonolysis temperature. The largest loading of amine groups was obtained when a well-ordered cubic methylene PMO material without prior vacuum-drying was thermolyzed in ammonia. The ordered mesoporosity of PMOs was preserved during the ammonolysis with only a slight decrease in the mesopore size and the degree of mesostructural ordering. The extent of substitution of framework oxygen by amine and nitride groups was established by solid-state (29)Si CP-MAS, (29)Si MAS, (15)N MAS, and (13)C CP-MAS NMR spectroscopies, elemental analysis, and X-ray photoelectron spectroscopy. In some cases, methylene and methyl functional groups were also present in the PMAs along with amine functional groups, as inferred from elemental analysis and gas adsorption, particularly in cases where PMOs were subjected to ammonolysis at 400 and 550 degrees C for several hours. This resulted in new multifunctional mesoporous organoaminosilica nanomaterials with properties that could be tuned by systematically varying the relative amounts of hydrophilic amine and hydrophobic hydrocarbon pendent and framework groups. The stability upon storage was found to be much higher for PMAs obtained from PMOs than for those obtained from MCM-41 silicas under the same conditions.