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PubMed2006

MnS hierarchical hollow spheres with novel shell structure.

High yields of MnS microspheres with novel hierarchical structure were prepared through a simple solution method. Field emission scanning electron microscopy and transmission electron microscopy analyses reveal that the microsphere has a core-shell structure: the interior hollow sphere is covered by a shell consisting of nanorod arrays. Interestingly, the nanorod is a wurtzite (WZ)/zinc blende (ZB) phase admixture with a large amount of stacking faults/twins. The alternation of WZ and ZB along the growth direction of the nanorod enables it to exhibit the features of a quantum well. Furthermore, the WZ/ZB admixture structure could also be regarded as a type II homomaterial heterostructure. All these features imply that the novel core-shell structure has great potential for applications, among them the quantum well photoelectrical and heterostructure photoconduction fields.

Journal Article↗

PubMed2006

Preparation of carbon-supported core-shell Au-Pt nanoparticles for methanol oxidation reaction: The promotional effect of the Au core.

Core-shell Au-Pt nanoparticles with intimate contact of Pt and Au were prepared by a displacement reaction without formation of monometallic Au nanoparticles. The Au-Pt nanoparticles were dispersed on carbon (Au@Pt/C) and were used to catalyze methanol electrooxidation in acidic solutions at room temperature. The core-shell nanostructure was confirmed by transmission electron microscopy and X-ray photoelectron spectroscopy, and specific catalytic activities were evaluated by CO anodic stripping voltammetry in 0.5 M H(2)SO(4) and by cyclic voltammetry in 1 M CH(3)OH + 0.5 M H(2)SO(4). The Au@Pt/C catalyst demonstrated enhanced specific activity in methanol electrooxidation and showed multiple CO stripping peaks which were all negatively shifted with respect to a similarly prepared Ag@Pt/C catalyst. The activity enhancement is attributed to the presence of Au underneath a very thin Pt shell where electron exchange between Au and Pt had promoted the formation of active oxygen species on Pt, which facilitated the removal of inhibiting CO-like reaction intermediates.

Journal Article↗

PubMed2006

Air-stable PbSe/PbS and PbSe/PbSexS1-x core-shell nanocrystal quantum dots and their applications.

The optical properties and functionality of air-stable PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell nanocrystal quantum dots (NQDs) are presented. These NQDs showed chemical robustness over months and years and band-gap tunability in the near infrared spectral regime, with a reliance on the NQD size and composition. Furthermore, these NQDs exhibit high emission quantum efficiencies of up to 65% and an exciton emission band that is narrower than that of the corresponding PbSe NQDs. In addition, the emission bands showed a peculiar energy shift with respect to the relevant absorption band, changing from a Stokes shift to an anti-Stokes shift, with an increase of the NQD diameter. The described core-shell structures and the corresponding PbSe core NQDs were used as passive Q-switches in eye-safe lasers of Er:glass, where they act as saturable absorbers. The absorber saturation investigations revealed a relatively large ground-state cross-section of absorption (sigma gs = 10(-16) - 10(-15) cm2) and a behavior of a "fast" absorber with an effective lifetime of tau eff approximately 4.0 ps is proposed. This lifetime is associated with the formation of multiple excitons at the measured pumping power. The product of sigma gs and tau eff enables sufficient Q-switching performance and tunability in the near infrared spectral regime. The amplified spontaneous emission properties of PbSe NQDs were examined under continuous illumination by a diode laser at room temperature, suitable for standard device conditions. The results revealed a relatively large gain parameter (g = 2.63 - 6.67 cm-1). The conductivity properties of PbSe NQD self-assembled solids, annealed at 200 degrees C, showed an Ohmic behavior at the measured voltages (up to 30 V), which is governed by a variable-range-hopping charge transport mechanism.

Air↗

PubMed1997

Dissolution testing of soft shell capsules-acetaminophen and nifedipine.

The problem of pelliculation of soft gelatin capsules on aging has been studied. Pelliculation frequently differs in soft shell capsules from hard shell capsules because of the larger mass of gelatin in the softshell dosage form. Two batches of nifedipine soft shell capsules are described. One batch failed in vivo testing and the other batch did not, after storage at 25 degrees C, yet both batches failed dissolution testing by the USP apparatus II method. The USP dissolution apparatus III-method, however, distinguished correctly between the batches. The study reported here was carried out with acetaminophen and was set up to establish whether the USP apparatus III testing method would be more suitable, and if so, what the most desirable dissolution medium should be. To create controlled pelliculation, samples were made with 0, 20, and 80 ppm of formaldehyde. The resulting capsules were subjected to ambient and accelerated storage and dissolution testing was carried out. It was found that the USP apparatus III method best reflected distinction between acceptable and substandard capsules.

Acetaminophen↗

PubMed2004

Steric stabilization of core-shell nanoparticles in liquid carbon dioxide at the vapor pressure.

Nondilute nanoparticle dispersions were stabilized in liquid CO2 at 25 degrees C at pressures as low as the vapor pressure for greater than 30 min. By modifying hydrophilic silica with a trifunctional silylating agent, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxy silane, a cross-linked polymer shell was formed around the silica core. The presence of the shell led to weaker Hamaker interactions between approaching fluoro-silica composite particles and enabled dispersibility at weaker solvent conditions (low pressures) than for metals with larger Hamaker constants. Steric stabilization of the nanoparticles was provided by low-molecular-weight perfluorodecane side chains at the surface of the fluoro-silica composite shell. Compared to polymeric chains, the perfluorodecane side chains are more easily solvated and thus stabilize nanoparticle dispersions in CO2 at much lower pressures, even down to the vapor pressure.

Carbon Dioxide↗

PubMed2004

Patterned dual pH-responsive core-shell hydrogels with controllable swelling kinetics and volumes.

Dual pH-responsive core-shell hydrogels containing both a vinyl pyridine component and a 2-dimethylaminoethyl methacrylate component were prepared using an in situ photopolymerization process. Complementary photomasks were utilized to prepare hydrogels with core/shell volume ratios of 2:1, 1:1, and 1:2. Depending on the location of each polymer component, dramatically different swelling profiles were achieved. Selective swelling of the shell followed by the core components allowed the hydrogel to expand with the usual kinetics; however, by switching the location of each polymer component and swelling the core first, swelling rates decreased by over 1 order of magnitude and were dependent on the shell component's volume. The ability to pattern core/shell volumes also provided the ability to fabricate hydrogels that possess a constant maximum diameter but different cutoff points between its first and its second transition volumes. These materials may be of interest for controlled release applications.

Ethylamines↗

PubMed2005

Preparation of hollow polymer particles with a single hole in the shell by SaPSeP.

The influence of sodium dodecyl sulfate (SDS) on the shell formation of hollow polymer particles prepared by the SaPSeP method, which was proposed by the authors for the preparation of micrometer-sized hollow polymer particles, was investigated. A single hole was observed in the shell of the hollow particles prepared by seeded polymerization of micrometer-sized, monodisperse divinylbenzene/p-xylene droplets in aqueous medium in the presence of SDS at concentrations above 43.3 mM, which were prepared by the dynamic swelling method. The fraction of the hollow particles having a single hole in the shell and the area of the hole increased with the SDS concentration.

Journal Article↗

PubMed2005

Gold nanoparticle localization at the core surface by using thermosensitive core-shell particles as a template.

We report novel thermosensitive hybrid core-shell particles via in situ gold nanoparticle formation using thermosensitive core-shell particles as a template. This method for the in situ synthesis of gold nanoparticles with microgel interiors offers the advantage of eliminating or significantly reducing particle aggregation. In addition, by using thermosensitive microgel structures in which the shell has thermosensitive and gel properties in water--whereas the core itself is a water-insoluble polymer--we were able to synthesize the gold nanoparticles only at the surface of the core, which had reactive sites to bind metal ions. After the gold nanoparticles were synthesized, electroless gold plating was carried out to control the thickness of the gold nanoshells. The dispersions of the obtained hybrid particles were characterized by dynamic light scattering and UV-vis absorption spectroscopy, and the dried particles were also observed by electron microscopy. Adaptation of the technique shown here will create a number of applications as optical, electronic, and biomedical functional materials.

Gold↗

PubMed2005

Investigation of the core-shell interface in gold@silica nanoparticles: a silica imprinting approach.

The nature of the self-assembled core-shell interface in gold@silica nanoparticles synthesized via a 3-aminopropyltrimethoxysilane (APTMS) route is investigated using materials synthesis as a sensitive tool for elucidating interfacial composition and organization. Our approach involves condensation of the gold@silica nanoparticles within a silica framework for synthesis of a composite gold-silica material containing approximately 30 wt % gold. This material contains one of the highest gold loadings reported, but maintains gold core isolation as ascertained via a single surface plasmon resonance absorption band frequency corresponding to that of gold nanoparticles in dilute aqueous solution. The immobilized gold cores are subsequently etched using cyanide anion for the synthesis of templated porosity, which corresponds to the space that was occupied by the gold. Characterization of immobilized amines is performed using probe molecule binding experiments, which demonstrate a lack of accessible amines after gold removal. Solid-state 13C CPMAS NMR spectroscopy on these materials demonstrates that the amount of amine immobilization must be less than 10% of the expected yield, assuming that all of the APTMS becomes bound to the gold nanoparticle template. These results require a core-shell interface in the gold@silica nanoparticles that is predominantly occupied by inorganic silicate species, such as Si-O-Si and Si-OH, rather than primary amines. Such a result is likely a consequence of the weak interaction between primary amines and gold in aqueous solution. Our method for investigating the core-shell interface of gold@silica nanoparticles is generalizable for other interfacial structures and enables the synthesis of bulk imprinted silica using colloidal templates.

Journal Article↗

PubMed2006

Synthesis of hollow silica spheres with mesostructured shell using cationic-anionic-neutral block copolymer ternary surfactants.

Hollow silica spheres with mesostructured shells (HSSMS) were prepared with a vesicle template of cetyltrimethylammonium bromide-sodium dodecyl sulfate-Pluronic P123 (C(16)TMAB-SDS-EO(20)PO(70)EO(20)) at a SDS/C(16)TMAB ratio of 0.6-0.8 following a fast silicification in dilute silicate solution at pH approximately 5.0. The mesostructure of the shell is disordered, and the mesopore size is about 5.5-7.5 nm. Moreover, the direction and length of the nanochannels of the shell change with the SDS/C(16)TMAB ratios. A bi-template model, in which the C(16)TMA(+)-DS(-) form the stable bilayer vesicle structure and the P123 copolymers anchored on C(16)TMA(+)-DS(-) vesicle act as the template for the mesoporous silica, was proposed to explain the formation of the HSSMS. This bi-template model can be applied extensively to prepare the HSSMS with different diameters and pore sizes by using other C(n)TMAX-SDS-EO(n)PO(m))EO(n) ternary-surfactant mixtures.

Journal Article↗

PubMed2006

Au nanoparticles encapsulated in Ru carbonyl carboxylate shells.

A two-step surface functionalization approach has been used to encase Au nanoparticles in monolayer organometallic Ru-complex shells by the reaction of an intermediate surface-bound mercaptopropanoic acid capping species with Ru dodecacarbonyl (Ru3(CO)12) clusters. Vibrational (infrared and Raman) spectroscopy shows that insertion of carboxylate groups into the Ru clusters results in their fragmentation and the formation of a shell of Ru dicarbonyl carboxylate oligomers that remain attached to the Au nanoparticles through the original Au-alkanethiolate bonds. The structural integrity of the metallic nanoparticulate Au cores has been verified by X-ray photoelectron spectroscopy, X-ray diffraction, and transmission electron microscopy. The organometallic Ru-complex shell may be decomposed thermally to eliminate the mercaptopropanoate and carbonyl groups and leave a mixed phase of Au and RuO2.

Carboxylic Acids↗

PubMed2006

Facile preparation method of SiO2/PS/TiO2 multilayer core-shell hybrid microspheres.

Organic-inorganic hybrid particles have many potential applications, but almost all of this research was focused on the hybrid particles containing one kind of inorganic nanoparticles. This paper presented a facile preparation method for SiO2/PS/TiO2 multilayer core-shell hybrid microspheres. In this approach, positively charged SiO2/PS core-shell hybrid particles were first synthesized by miniemulsion polymerization using cationic initiator and emulsifier. These positively charged SiO2/PS hybrid particles were mixed with tetra-n-butyl titanate for sol-gel reaction to directly form SiO2/PS/TiO2 multilayer core-shell hybrid microspheres. Some influencing parameters such as surfactant concentration, tetra-n-butyl titanate amount, and glacial acetic acid amount were investigated. TEM, TGA, and EDX analyses indicated that titania layers were successfully coated onto the surfaces of hybrid microspheres.

Journal Article↗

PubMed2005

Fabrication of hybrid nanocapsules by calcium phosphate mineralization of shell cross-linked polymer micelles and nanocages.

Self-assembled shell cross-linked poly(acrylic acid-b-isoprene) (PAA78-b-PI97) micelles or cross-linked PAA nanocages in aqueous solution were used as templates for the preparation of novel polymer-inorganic nanocapsules. The hybrid nanostructures were typically 50-70 nm in diameter and consisted of spherical polymer nanoparticles or nanocages enclosed within a continuous 10-20 nm thick surface layer of amorphous calcium phosphate. Nucleation of calcium phosphate specifically in association with the polymer nanoparticles was facilitated by low supersaturation levels and by sequestration of Ca2+ ions within the carboxylate-rich PAA domains prior to addition of HPO4(2-). Modifications in ionic concentrations were used to control the calcium phosphate surface layer thickness and prepare mineralized cross-linked PAA-b-PI micelles with variable shell permeability. The permeability of beta-carotene into the hydrophobic PI core of mineralized shell cross-linked PAA-b-PI micelles was reduced by approximately 50 or 100% respectively for hybrid nanostructures enclosed within 10 or 20 nm thick calcium phosphate layers. Our results suggest that calcium phosphate-polymer cross-linked nanocapsules could have potential applications as pH-responsive biocompatible hybrid nanostructures for use in applications such as drug delivery, bioimaging, and therapeutics.

Biocompatible Materials↗

PubMed2005

Growth and optical properties of strained GaAs-GaxIn 1-x P core-shell nanowires.

We have synthesized GaAs-Ga(x)In(1-x)P (0.34 < x < 0.69) core-shell nanowires by metal-organic vapor phase epitaxy. The nanowire core was grown Au-catalyzed at a low temperature (450 degrees C) where only little growth takes place on the side facets. The shell was added by growth at a higher temperature (600 degrees C), where the kinetic hindrance of the side facet growth is overcome. Photoluminescence measurements on individual nanowires at 5 K showed that the emission efficiency increased by 2 to 3 orders of magnitude compared to uncapped samples. Strain effects on the band gap of lattice mismatched core-shell nanowires were studied and confirmed by calculations based on deformation potential theory.

Journal Article↗

PubMed2005

Assembly and interaction of Au/C core-shell nanostructures: in situ observation in the transmission electron microscope.

Using transmission electron microscopy, we identify the temperature-dependent interaction pathway of carbon-supported Au nanoparticles. At low temperature (room temp. to 400 degrees C), Au nanoparticles predominantly interact by coalescence initiated by an atomic Au bridge. At high temperature (400-800 degrees C), the particles assemble into Au/C core-shell nanostructures. C-shells around individual nanoparticles passivate their surface and prevent coalescence. Ultimately coalescence does occur via rupture of the passivating shells, which invariably follows the assembly of C sheets enveloping and compressing multiple closely spaced Au/C nanoparticles.

Journal Article↗

PubMed2005

Self-organized growth of Si/Silica/Er2Si2O7 core-shell nanowire heterostructures and their luminescence.

Self-organized Si-Er heterostructure nanowires showed promising 1.54 microm Er(3+) optical activity. Si nanowires of about 120-nm diameter were grown vertically on Si substrates by the vapor-liquid-solid mechanism in an Si-Er-Cl-H(2) system using an Au catalyst. Meanwhile, a single-crystalline Er(2)Si(2)O(7) shell sandwiched between nanometer-thin amorphous silica shells was self-organized on the surface of Si nanowires. The nanometer-thin heterostructure shells make it possible to observe a carrier-mediated 1.53 microm Er(3+) photoluminescence spectrum consisting of a series of very sharp peaks. The Er(3+) spectrum and intensity showed absolutely no change as the temperature was increased from 25 to 300 K. The luminescence lifetime at room temperature was found to be 70 micros. The self-organized Si nanowires show great potential as the material basis for developing an Si-based Er light source.

Crystallization↗

PubMed2006

Parallel core-shell metal-dielectric-semiconductor germanium nanowires for high-current surround-gate field-effect transistors.

Core-shell germanium nanowires (GeNW) are formed with a single-crystalline Ge core and concentric shells of nitride and silicon passivation layer by chemical vapor deposition (CVD), an Al2O3 gate dielectric layer by atomic layer deposition (ALD), and an Al metal surround-gate (SG) shell by isotropic magnetron sputter deposition. Surround-gate nanowire field-effect transistors (FETs) are then constructed using a novel self-aligned fabrication approach. Individual SG GeNW FETs show improved switching over GeNW FETs with planar gate stacks owing to improved electrostatics. FET devices comprised of multiple quasi-aligned SG GeNWs in parallel are also constructed. Collectively, tens of SG GeNWs afford on-currents exceeding 0.1 mA at low source-drain bias voltages. The self-aligned surround-gate scheme can be generalized to various semiconductor nanowire materials.

Journal Article↗

PubMed2004

Core-shell-type resins for solid-phase peptide synthesis: comparison with gel-type resins in solid-phase photolytic cleavage reaction.

[reaction: see text] Novel core-shell-type resins with a rigid core and amino-functionalized flexible shell were prepared with 2,4,6-trichloro-1,3,5-triazine (CNC) and Jeffamine ED-600 starting from 1% cross-linked aminomethyl (AM) polystyrene resins. All of the amino groups were located outside the resin beads, and the loading capacity was 0.2-0.4 mmol/g. The amount of CNC treated was a determining factor in the properties of the final resins. The core-shell-type resins showed superior performances in terms of the initial loading of amino acid and the photocleavage reaction compared to the gel-type resins.

Catalysis↗