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PubMed2005

Shell click-crosslinked (SCC) nanoparticles: a new methodology for synthesis and orthogonal functionalization.

A new methodology for the preparation of well-defined core-shell nanoparticles was developed, based upon the employment of a multifunctional crosslinker to coincidently stabilize supramolecular polymer assemblies and imbed into the shell unique chemical functionalities. Amphiphilic diblock copolymers of poly(acrylic acid)(80)-b-poly(styrene)(90) that had been assembled into micelles and partially functionalized throughout the corona with alkynyl groups were utilized as Click-readied nanoscaffolds for the formation of shell Click-crosslinked nanoparticles (SCCs). Divergently grown dendrimers of the zero, first, second, and third generations having increasing numbers of azide terminating groups ((N(3))(2)-[G-0], (N(3))(4)-[G-1], (N(3))(8)-[G-2], and (N(3))(16)-[G-3], respectively) were investigated as crosslinkers via Click reactions with the alkynyl groups to form covalent linkages throughout the block copolymer micelle corona, thus forming a crosslinked shell. The crosslinking reactions were characterized by (1)H NMR and IR spectroscopies, differential scanning calorimetry (DSC), and dynamic light scattering (DLS) measurements. Only the first generation dendrimer ((N(3))(4)-[G-1]) possessed a sufficient balance of polyvalency and water solubility to achieve crosslinking and establish a robust nanostructure. The resulting SCC was further characterized with atomic force microscopy (AFM), transmission electron microscopy (TEM), and analytical ultracentrifugation (AU). The dendritic crosslinker is important as it also allows for the incorporation of excess functionality that can undergo complementary reactions. Within the shell of this SCC the remaining azide termini of the dendrimer crosslinker were then consumed in a secondary Click reaction with an alkynyl-functionalized fluorescein to yield a fluorescently labeled SCC that was characterized with DLS, AFM, TEM, AU, UV-vis, and fluorescent measurements as a function of pH.

Acrylic Resins↗

PubMed2006

Size-selective shell cross-linked interior functionalized siloxane nanocages.

A new structure, consisting of a shell cross-linked, 2 nm size siloxane nanocage containing propylamine groups tethered to the interior face of the shell was synthesized, starting with micelles of the surfactant molecule, (triethoxysilyl)propylcetylcarbamate. After hydrolysis of the ethoxysilyl groups and condensation and capping of the silanols to form a cross-linked, one-atom-layer-thick siloxane shell, the carbamate was converted to amine, releasing the cetyl group from the structure and resulting in the desired spherical nanocage. The intermediates in the synthesis process and the final structure were characterized by 1H and 29Si NMR, DLS, TEM, and mass spectroscopy. The amine groups tethered to the interior surface of the shell react readily with ninhydrin but do not interact with the larger ZnTPP, indicating molecular size selectivity by the cross-linked shell. The structure also exhibits confinement effect in the amine-catalyzed decarboxylation of acetoacetic acid, exhibiting higher activity and higher selectivity for acetal than (aminopropyl)triethoxysilane.

Journal Article↗

PubMed2005

Using MD snapshots in ab initio and DFT calculations: OH vibrations in the first hydration shell around Li+(aq).

The average OH stretching vibrational frequency for the water molecules in the first hydration shell around a Li(+) ion in a dilute aqueous solution was calculated by a hybrid molecular dynamics + quantum-mechanical ("MD + QM") approach. Using geometry configurations from a series of snapshots from an MD simulation, the anharmonic, uncoupled OH stretching frequencies were calculated for 100 first-shell OH oscillators at the B3LYP and HF/6-31G(d,p) levels of theory, explicitly including the first shell and the relevant second shell water molecules into charge-embedded supermolecular QM calculations. Infrared intensity-weighting of the density-of-states (DOS) distributions by means of the squared dipole moment derivatives (which vary by a factor of 20 over the OH stretching frequency band at the B3LYP level), changes the downshift from approximately -205 to -275 cm(-1) at the B3LYP level. Explicit inclusion of relevant third-shell water molecules in the supermolecular cluster leads to a further downshift by approximately -30 cm(-1). Our final estimated average downshift is approximately -305 cm(-1). The experimental value lies somewhere in the range between -290 and -420 cm(-1). Also, the absolute nu(OH) frequency is well reproduced in our calculations. "In-liquid" instantaneous correlation curves between nu(OH) and various typical H-bond strength parameters such as R(O...O), R(H...O), the intramolecular OH bond length, and the IR intensity are presented. Some of these correlations are robust and persist also for the rather distorted instantaneous geometries in the liquid; others are less so.

Journal Article↗

PubMed2005

Aligned Zn-Zn2SiO4 core-shell nanocables with homogeneously intense ultraviolet emission at 300 nm.

Aligned coaxial nanocables were grown on Si substrates by a vapor-deposition technique. The lengths of the nanocables increased as the distance between the substrate and the source decreased. The nanocables were characterized as homogeneously crystallized shells of about 25 nm thick, diameters of about 100 nm, and round top ends. It was found that the shell emits an intense middle-ultraviolet about 300 nm at room temperature. This emission was attributed to the thin double-layer structure in the Zn-Zn2SiO4 core-shell nanocable where the Zn2SiO4 shell has the potential to serve as more ideal luminophors. The results demonstrated that the nanocable density could be changed by altering nucleation density at the steps on the substrate surface. The unique growth manner described herein provides a new technique for the homogeneous crystallization of Zn-Zn2SiO4 core-shell nanocables.

Journal Article↗

PubMed2005

Magnetic properties of monodispersed Ni/NiO core-shell nanoparticles.

We have recently developed a method to fabricate monodispersed Ni/NiO core-shell nanoparticles by pulsed laser ablation. In this report, the size-dependent magnetic properties of monodispersed Ni/NiO core-shell nanoparticles were investigated. These nanoparticles were formed in two steps. The first was to fabricate a series of monodispersed Ni nanoparticles of 5 to 20 nm in diameter using a combination of laser ablation and size classification by a low-pressure differential mobility analyzer (DMA). The second step was to oxidize the surfaces of the Ni particles in situ to form core-shell structures. A superconducting quantum interference device (SQUID) magnetometer was used to measure the magnetic properties of nanostructured films prepared by depositing the nanoparticles at room temperature. Ferromagnetism was observed in the magnetic hysteresis loop of the nanostructured films composed of core-shell nanoparticles with core diameters smaller than the superparamagnetic limit, which suggests the spin of Ni core was weakly exchange coupled with antiferromagnetic NiO shell. In contrast, smaller nanoparticles with core diameters of 3.0 nm exhibited superparamagnetism. The drastic change in the hysteresis loops between field-deposited and zero-field-deposited samples was attributable to the strong anisotropy that developed during the magnetic-field-assisted nanostructuring process.

Letter↗

PubMed2006

Fabrication of nanorattles with passive shell.

This investigation describes the formation of a metal nanorattle with a pure metal shell by varying experimental parameters. The galvanic replacement reaction between silver and chloroauric acid was adopted to prepare hollow metal nanoparticles. This approach is extended to produce nanorattles of Au cores and Au shells by starting with Au(core)Ag(shell) nanoparticles as templates. The effect of temperature on the nanostructure of the final product is also considered. The composition of the shell in nanorattles can be controlled by varying the reaction temperature (to form pure gold or gold-silver alloy, for example). X-ray absorption fine structure spectroscopy is conducted to elucidate the fine structure of these nanoparticles. Partial alloying between the Au core and the Ag shell is observed by extended X-ray absorption fine structure (EXAFS).

Journal Article↗

PubMed2004

Preparation of submicrometer-sized monodispersed thermoresponsive core-shell hydrogel microspheres.

We have successfully prepared monodispersed thermoresponsive core-shell hydrogel microspheres with a mean diameter of 200-400 nm with poly(N-isopropylacrylamide-co-styrene) [P(NIPAM-co-St)] cores and poly(N-isopropylacrylamide) (PNIPAM) shells. The submicrometer-sized monodispersed P(NIPAM-co-St) core seeds were prepared by using a surfactant-free emulsion polymerization method, and the PNIPAM shell layers were fabricated onto the core seeds by using a seed polymerization method. The particle size, morphology and monodispersity, and thermoresponsive characteristics of the prepared microspheres were experimentally studied. In the preparation of P(NIPAM-co-St) seeds, with increasing the initiator dosage, the mean diameters and the dispersal coefficients were almost at the same levels at first; however, when the initiator dosage increased further to a critical amount, the mean diameters decreased drastically and the monodispersity became worse significantly. With increasing the stirring rate, the particle diameter decreased, and when the stirring rate was larger than 600 rpm, the monodispersity became worse obviously. With increasing the phase ratio, the mean diameter became larger simply, and the monodispersity became worse first and then became better again. With increasing the reaction time, the particle sizes nearly did not change, while the monodispersity gradually became better slightly. For the core-shell microspheres, with increasing the NIPAM dosage in the preparation of the PNIPAM shell layers, the mean diameters became larger simply, the monodispersity became better, and the thermoresponsive swelling ratio of the hydrodynamic diameters increased.

Acrylamides↗

PubMed2005

Self-assembled shells composed of colloidal particles: fabrication and characterization.

We construct shells with tunable morphology and mechanical response with colloidal particles that self-assemble at the interface of emulsion droplets. Particles self-assemble to minimize the total interfacial energy, spontaneously forming a particle layer that encapsulates the droplets. We stabilize these layers to form solid shells at the droplet interface by aggregating the particles, connecting the particles with adsorbed polymer, or fusing the particles. These techniques reproducibly yield shells with controllable properties such as elastic moduli and breaking forces. To enable diffusive exchange through the particle shells, we transfer them into solvents that are miscible with the encapsulant. We characterize the mechanical properties of the shells by measuring the response to deformation by calibrated microcantilevers.

Colloids↗

PubMed2004

Synthesis of dumbbell-shaped Au-Ag core-shell nanorods by seed-mediated growth under alkaline conditions.

We report a simple synthesis of Au-Ag core-shell nanorods (NRs) under alkaline conditions (pH 8.0-10.0) from silver and ascorbate ions using gold nanorods (GNRs) as the seeds. The silver ions that are reduced by the ascorbate ions become deposited on the surfaces of the GNRs to form differently dumbbell-shaped Au-Ag core-shell NRs and nanoparticles, depending on the pH and the concentration of silver ions. The longitudinal plasmon absorbance bands of the Au-Ag core-shell NRs are stronger and appear at shorter wavelengths than those for the original GNRs. We confirmed the formation of Au-Ag core-shell NRs by both energy-dispersive X-ray spectrometry and inductively coupled plasma mass spectrometry measurements, which indicate that the 109Ag/197Au ratios are 0.046, 0.085, and 0.097 at pH 8.0, 9.0, and 10.0, respectively. The transmission electron microscopy measurements show that the Au-Ag core-shell NRs are monodispersed (>90%).

Journal Article↗

PubMed2005

Preparation of sub-micrometer silica shells using poly(1-methylpyrrol-2-ylsquaraine).

Poly(1-methylpyrrol-2-ylsquaraine) precipitates from reaction solution as uniformly spherical particles with a diameter of 1.3 microm. Upon heating, the particles reduce in diameter until extinction at approximately 630 degrees C. Treatment of the particles with 9:1 tetraethoxysilane:ethanol solution, and subsequent hydrolysis in dilute acid, results in a polymer core-silica shell structure. Removal of the core, upon heating to 660 degrees C, results in an amorphous silica shell with a diameter half that of the initial template sphere. It has been found that the silica shells produced by this method are able to encapsulate organic dyes upon soaking of the shells in chloroform solutions of the dyes, and further washings with fresh chloroform did not remove the dyes. The production of crystalline titanium dioxide shells was also achieved through the use of the polysquaraine particles as a spherical template.

Journal Article↗

PubMed2006

Highly luminescent, stable, and water-soluble CdSe/CdS core-shell dendron nanocrystals with carboxylate anchoring groups.

A dendron ligand with two carboxylate anchoring groups at its focal point and eight hydroxyl groups as its terminal groups was found to efficiently convert as-synthesized CdSe/CdS core-shell nanocrystals in toluene to water-soluble dendron-ligand stabilized nanocrystals (dendron nanocrystals). The resulting dendron nanocrystals retained 60% of the photoluminescence value of the original CdSe/CdS core-shell nanocrystals in toluene and were significantly brighter than the similar dendron nanocrystals with thiolate (deprotonated thiol group) as the anchoring group which retained just 10% of the photoluminescence value of the original CdSe/CdS core-shell nanocrystals in toluene. The carboxylate-based dendron nanocrystals survived UV irradiation in air for at least 13 days, about 9 times better than the thiolate-based dendron nanocrystals (35 h) and similar to that of the thiolate-based dendron-box stabilized CdSe/CdS core-shell nanocrystals (box nanocrystals). Upon UV irradiation, the dendron nanocrystals became even 2 times brighter than the original CdSe/CdS core-shell nanocrystals in toluene, and the UV-brightened PL can retain the brightness for at least several months. These stable and bright dendron nanocrystals were soluble in various aqueous media, including all common biological buffer solutions tested, for at least 1.5 years. In addition to their superior performance, the synthetic chemistry of carboxylate dendron ligands and the corresponding dendron nanocrystals is relatively simple and with high yield.

Cadmium Compounds↗

PubMed2006

Thermosensitive pluronic micelles stabilized by shell cross-linking with gold nanoparticles.

Gold nanoparticles were employed to prepare shell cross-linked Pluronic micelles that exhibit a reversibly thermosensitive swelling/shrinking behavior. Two terminal hydroxyl groups of Pluronic F127 were thiol-functionalized to form self-assembling Pluronic micelles in aqueous solution with exposed -SH groups in an outer shell layer. The thiol groups present in the outer shell were cross-linked by gold nanoparticles synthesized through NaBH4 reduction of gold precursor anions. The resultant shell cross-linked gold-Pluronic micelles exhibited a temperature-dependent volume transition: their hydrodynamic diameter was changed from 157.1 +/- 15.6 nm at 15 degrees C to 53.4 +/- 5.5 nm at 37 degrees C as determined by dynamic light scattering. The critical micelle temperature measured by a pyrene solubilization technique suggested that the reversible swelling/shrinking behavior of the micelles was caused by hydrophobic interactions of cross-linked or grafted Pluronic copolymer chains in the micelle structure with increasing temperature. Transmission electron microscopy directly revealed that the shell cross-linked micelles were indeed produced by gold nanoparticles covalently clustered on the surface. These novel self-assembled organic/inorganic hybrid micelles would hold great potential for diagnostic and therapeutic applications.

Borohydrides↗

PubMed2006

Macrophage uptake of core-shell nanoparticles surface modified with poly(ethylene glycol).

The in vitro uptake of core-shell nanoparticles encapsulated in a bio-macromolecular nanoshell assembled from multilayered polyelectrolytes was studied. Sulfate modified fluorescent polystyrene nanobeads (diameter 200 nm) were used as a solid core upon which charged multilayers of poly-l-lysine, chitosan, and heparin sulfate are electrostatically deposited utilizing a layer-by-layer (LbL) self-assembly process. The nanoshell composed of the multilayered polyelectrolytes was modified with poly(ethylene glycol) (PEG) of varying molecular weights (either MW 2000, 5000, or 20 000 Da) to form a hydrophilic and long-circulating nanoparticle. The assembly of the nanoshell was confirmed by zeta potential, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The reversal in charge upon the deposition of alternating polyelectrolytes was observed by zeta potential measurements. The nanometer thickness of the nanoshell was confirmed by TEM. The presence of the (C-C-O)(n)() backbone in PEG at the surface of the nanoshell was confirmed by the increase in (C-O,N) peak area concentrations compared to (C-C) peak area, and these results were gathered from XPS. In vitro studies between suspension macrophages and core-shell nanoparticles were performed to determine how the hydrophilicity and the charge on the nanoshell can promote or reduce uptake. Results showed that after 24 h uptake was decreased 3-fold when PEGs of 2000 and 20 000 Da were chemisorbed to the nanoshell, as opposed to a nanoshell with either a positive or highly negative charge. Confocal microscopy aided in verifying that core-shell nanoparticles were internalized within the cell cytoplasm and were not attached to the cell surface. Protein adhesion studies with bovine serum albumin were performed to determine the relationship between surface charge and opsonization of core-shell nanoparticles. It was found that a hydrophilic surface with a low negative charge reduced protein adsorption and uptake. The in vitro uptake of macrophages and protein adsorption onto core-shell nanoparticles formed using layer-by-layer assembly has not been previously studied.

Adsorption↗

PubMed2006

Localization/Quasi-delocalization transitions and quasi-mobility-edges in shell-doped nanowires.

We propose a novel concept, namely, shell-doping of nanowires, for control of carrier mobility in nanowires. Different from traditional doping, where dopant atoms are distributed uniformly inside nanowires, shell-doping spatially confines dopant atoms within a few atomic layers in the shell region of a nanowire. Our numerical results based on the Anderson model of electronic disorder show that electrons in a shell-doped nanowire exhibit a peculiar behavior very different from that of uniformly doped nanowires. Beyond some critical doping, electron dynamics in a shell-doped nanowire undergoes a localization/quasi-delocalization transition, namely, the electron diffusion length decreases in the regime of weak disorder but increases in the regime of strong disorder. This transition is a result of the existence of quasi-mobility-edges in the energy spectrum of the system, which can be exploited experimentally through control of electron concentration, carrier density, and degree of disorder.

Diffusion↗

PubMed2006

Phase segregation in AlInP shells on GaAs nanowires.

We have studied morphology and phase segregation of AlInP shells on GaAs nanowires. Photoluminescence measurements on single core-shell nanowires indicated variations in the shell composition, and phase segregation was confirmed by cross-sectional scanning transmission electron microscopy on 30 nm thin slices of the wires. It was discovered that Al-rich domains form in the <112> directions where two {110} shell facets meet during growth. We propose that the mechanism behind this phase segregation is a variation in the chemical potential along the circumference of the nanowire together with a difference in diffusion lengths for the different growth species. From the morphology of the core and the shell, we conclude that the side facet growth is temperature dependent forming {112}facets at low growth temperature and {110} facets at high growth temperature.

Journal Article↗

PubMed2003

In vitro glutamate-stimulated release of dopamine from nucleus accumbens core and shell of spontaneously hypertensive rats.

Spontaneously hypertensive rats (SHR) are used as a model for attention-deficit hyperactivity disorder (ADHD) since SHR display the major symptoms of ADHD (hyperactivity, impulsivity, inablity to sustain attention during behavioral tasks). We previously showed that electrical and/or K+-stimulated release of dopamine (DA) from nerve terminals in the prefrontal cortex, nucleus accumbens, and caudate-putamen of SHR was significantly lower than that of Wistar-Kyoto (WKY) control rats. The aim of the present investigation was to determine whether glutamate-stimulated release of DA from nucleus accumbens core and shell of SHR was significantly different from that of WKY. Using an in vitro superfusion technique, we showed that glutamate-stimulated release of [3H]DA from striatal slices is mediated by glutamate activation of AMPA receptors and that glutamate-stimulated release of [3H]DA from nucleus accumbens core and shell of 4-6-week-old SHR and WKY is not significantly different. Glutamate-stimulated release of [3H]DA from SHR shell is significantly lower than SHR core and there is also a tendency for glutamate-stimulated [3H]DA release from SHR shell to be lower than release from WKY shell.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed2004

Effect of sodium lauryl sulfate in dissolution media on dissolution of hard gelatin capsule shells.

PURPOSE: Sodium lauryl sulfate (SLS) is a commonly used surfactant in dissolution media for poorly water soluble drugs. However, it has occasionally been observed that SLS negatively impacts the dissolution of drug products formulated in gelatin capsules. This study investigated the effect of SLS on the dissolution of hard gelatin capsule shells. METHODS: The USP paddle method was used with online UV monitoring at 214 nm (peptide bond). Empty size #0 capsule shells were held to the bottom of the dissolution vessel by magnetic three-prong sinkers. RESULTS: SLS significantly slowed down the dissolution of gelatin shells at pH < 5. Visually, the gelatin shells transformed into some less-soluble precipitate under these conditions. This precipitate was found to contain a higher sulfur content than the gelatin control sample by elemental analysis, indicating that SLS is part of the precipitate. Additionally, the slowdown of capsule shell dissolution was shown to be dependent on the SLS concentration and the ionic strength of the media. CONCLUSIONS: SLS interacts with gelatin to form a less-soluble precipitate at pH < 5. The use of SLS in dissolution media at acidic pH should be carefully evaluated for gelatin capsule products.

Capsules↗

PubMed2005

Hyperphagia induced by GABAA receptor-mediated inhibition of the nucleus accumbens shell: dependence on intact neural output from the central amygdaloid region.

To investigate the role of corticolimbic input in modulating feeding-related nucleus accumbens (Acb) circuitry, researchers temporarily deactivated sites within the basolateral amygdaloid complex (BLA) or central amygdaloid region (CeA) via GABA(A) agonist (muscimol) infusions and measured feeding responses following muscimol infusions into the Acb shell. Hyperphagia elicited by intra-Acb shell muscimol was not altered by coinfusions of intra-BLA muscimol. In contrast, muscimol infusions into the CeA dose-dependently reduced feeding elicited either by intra-Acb shell GABA(A) receptor stimulation or by food deprivation and produced a syndrome of forepaw treading. Intra-CeA tetrodotoxin infusions also blocked intra-Acb shell muscimol-induced hyperphagia. Hence, feeding elicited by intra-Acb shell GABA(A) receptor stimulation requires intact neural output from the CeA but not the BLA.

Amygdala↗