Core Size Distribution Research Articles

Synthesis of small palladium nanoparticles stabilized by bisphosphine BINAP bearing an alkyl chain and their palladium nanoparticle-catalyzed carbon–carbon coupling reactions under room-temperature

A new bisphosphine ligand, C8-BINAP, and C8-BINAP-stabilized palladium nanoparticles have been prepared; C8-BINAP was found to be an effective protecting ligand for preparing and stabilizing palladium nanoparticles with very small core size and narrow size distribution and the C8-BINAP-Pd nanoparticles behave as an efficient catalyst for carbon-carbon coupling reactions at room temperature.

Preparation of magnetic nanoparticles with large specific loss power for heating applications

Glass crystallisation method is used as a relatively new technique for the preparation of nanocrystalline magnetic iron oxide. Properties of glassy flakes, the precursor state, prepared powders as well as a water-based ferrofluid were analysed by XRD, TEM and magnetic methods. Different contributions to specific loss power (SLP) are measured by AC-susceptometry, hysteresis loops and a calorimetric method. Values of SLP in the order of 100 W/g maghemite may be achieved with the option of further increase by improving the core size distribution.

Laser Light Scattering Study on the Structure of a Poly(vinylidene fluoride) Aggregate in the Dilute Concentration State

By means of laser light scattering, poly(vinylidene fluoride) (PVDF) chains were found to form complex aggregates of very narrow size distribution even at dilute concentrations of good solvents such as propylene carbonate (PC) and γ-butyrolactone (BL). Almost the same radii of gyration for aggregates in both solvents were obtained as RG ∼ 220 ± 10 nm. Their static sizes appeared to be concentration-independent as well as temperature-independent at a wide temperature range of 25−160 °C in each solvent. As for solvent effects, the static scattering intensities had different scattering patterns especially at high-q regimes and also different hydrodynamic radii were observed as RH = 380 ± 10 nm in propylene carbonate and RH = 320 ± 10 nm in γ-butyrolactone, respectively. By the introduction of both core−shell structure and core size distribution, the intensity patterns of scattered light and large ratios of RH/RG could be explained. The attractive inter-/intramolecular force between four consecutive sequences...

Enhancement of AC-losses of magnetic nanoparticles for heating applications

Aqueous ferrofluids of maghemite nanoparticles coated with carboxydextran were investigated with respect to their specific loss power (SLP) in dependence on frequency and field amplitude of magnetic AC-fields. In order to elucidate the effect of the size distribution on SLP fluid fractions with different mean particle core size were prepared by a magnetic separation procedure from the original ferrofluid. Structural characterisation by means of TEM and XRD as well as reconstruction of core size distributions from magnetisation curves reveals that the narrow size distributions of the fractions cover a range of mean core sizes from about 8 up to 20nm. Spectra of the complex susceptibility were measured for a frequency range of 20Hz to 1 MHz. From the imaginary part of the susceptibility the specific loss power is calculated in dependence on frequency. The results are compared with calorimetrical measurements performed in dependence on field amplitude up to 11kA/m at 410kHz. A very high specific loss power in the order of 400W per gram maghemite was found at 410kHz and 11kA/m for the fluid fraction having the largest mean core diameter. A deviation from linear response behaviour is found for this sample showing a power law field dependence of the specific loss power SLP∼H2.5. In addition to liquid suspensions measurements were performed with particles immobilised in mannitol or gel in order to elucidate the role of Brownian relaxation. The experimentally found dependence of SLP on the mean particle core diameter may be understood in the frame of the Debye dispersion model. Results are discussed with respect to applications of ferrofluids in RF-magnetic hyperthermia.

One-Pot Preparation of Subnanometer-Sized Gold Clusters via Reduction and Stabilization by meso-2,3-Dimercaptosuccinic Acid

We report herein a simple method to prepare subnanometer-sized gold clusters by the reactions between hydrogen tetrachloroaurate (HAuCl4) and meso-2,3-dimercaptosuccinic acid (DMSA; HO2CCH(SH)CH(SH)CO2H) in water. It is demonstrated that DMSA molecules efficiently reduce the Au(III) species and stabilize the resultant gold clusters. The Au:DMSA clusters, which tend to aggregate into assemblages in water, can be well-isolated from each other by ion-pair formation between the carboxyl groups of DMSA and tetraoctylammonium (TOA) cations. This surface modification enables us to examine the core size distributions of the individual clusters by mass spectrometry and TEM. It is revealed that the gold clusters comprised of 10-13 atoms (ca. 0.8 nm) are the main products of the reactions.

Relative contributions to the plasmon line shape of metal nanoshells

Nanoshells are mesoscopic particles consisting of a dielectric core coated with a metal shell, in particular gold or silver, of uniform nanometer scale thickness. This topology supports plasmon excitations with frequencies that are sensitively dependent on the relative radii of the nanoparticle's core and shell. The plasmon linewidth for this geometry is typically quite broad, nominally 100 nm or more in wavelength at plasmon resonance wavelengths in the near infrared. Several distinct physical mechanisms control the plasmon lineshape: phase retardation effects, including multipolar plasmon contributions; inhomogeneous broadening due to core and shell size distributions; and electron scattering at the shell interfaces. These mechanisms are examined in terms of their relative contributions to the plasmon line shape for nanoshells fabricated with diameters of 100--250 nm.

Solution Self-Assembly of Poly(ferrocenylphenylphosphine)-block-polydimethylsiloxane: Formation of Spherical Micelles with an Organometallic Poly(ferrocenylphenylphosphine) Core

The novel organometallic-inorganic diblock copolymer, poly(ferrocenylphenylphosphine)-block-polydimethylsiloxane (PFP-b-PDMS), with narrow molecular weight distribution has been synthesized by living anionic polymerization through sequential monomer addition. These block copolymers self-assemble into “star-like” spherical micelles in hexane with a dense organometallic PFP core surrounded by a swollen corona of the PDMS chains. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) were used to characterize these micellar aggregates. It was found that the block copolymer micelles have a relatively narrow core size distribution, but an overall broader distribution of hydrodynamic size in hexane. Significantly, the preparation method of the micelle solution was also found to have an influence on the size and size distribution of the resulting micellar structures.

Synthesis and Self-Assembly of the Organic−Organometallic Diblock Copolymer Poly(isoprene-b-ferrocenylphenylphosphine): Shell Cross-Linking and Coordination Chemistry of Nanospheres with a Polyferrocene Core

Narrow polydispersity samples of the organic-organometallic diblock copolymer, poly(isoprene-b-ferrocenylphenylphosphine) (PI-b-PFP), have been synthesized by sequential living anionic polymerizations in tetrahydrofuran. These block copolymers form “starlike” spherical micelles in hexane with a dense PFP core surrounded by a swollen corona of PI chains as characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements. Although the TEM results indicated a very narrow micelle core size distribution, DLS studies indicated a monomodal, but relatively broad distribution of the overall hydrodynamic size of the micelles in solution. The hydrodynamic size of the micelles and the broad size distribution in solution suggest that DLS measurements might be detecting aggregates of individual micelles. The PI corona chains were found to undergo a cross-linking reaction under UV irradiation in the presence of the radical initiator AIBN. The micelle solution concentration and UV irradiation time affect this cross-linking reaction. Furthermore, the phosphorus sites in the block copolymers could be coordinated to a transition metal, such as Au. The resulting Au-containing block copolymers still form spherical micelles, but with a different size and size distribution.

Structure of Alkanoic Acid Stabilized Magnetic Fluids. A Small-Angle Neutron and Light Scattering Analysis

Small-angle neutron scattering and dynamic and static light scattering measurements were used to probe the structures of aqueous and organic-solvent-based magnetic fluids comprising dispersed magnetite nanoparticles (∼10 nm in diameter) stabilized against flocculation by adsorbed alkanoic acid layers. A core−shell model fitted to a set of neutron scattering spectra obtained from contrast variation experiments allowed the determination of the iron oxide core size and size distribution, the thicknesses of the surfactant shells, and the spatial arrangement of the individual particles. The magnetic colloidal particles appear to form compact fractal clusters with a fractal dimension of 2.52 and a correlation length of ∼350 Å in aqueous magnetic fluids, consistent with the structures of clusters observed directly using cryo-TEM (transmission electron microscopy), whereas chainlike clusters with a fractal dimension of 1.22 and a correlation length of ∼400 Å were found for organic-solvent-based magnetic fluids. The differences in cluster structure were attributed to the relative strengths of the particle−particle interaction energies. Weak interactions in the organic-solvent-based systems dictate the formation of small structures for which the apparent fractal dimensions are naturally small, whereas significantly stronger interparticle interactions in aqueous magnetic fluids result in larger, more compact clusters with higher fractal dimensions. The growth of the aqueous clusters beyond a certain size was inhibited by an increasingly high energy barrier (balance between repulsive electrostatic and attractive van der Waals interactions) with increasing cluster size. The aqueous clusters were stable against further growth when diluted with a surfactant solution but grew in time when diluted with pure water. In the latter case, the loss of part of the stabilizing secondary surfactant layer to the aqueous phase to satisfy thermodynamic partitioning constraints led to a destabilization in its dispersion. Light scattering studies indicated a change in the fractal dimension from 2.52 to about 1.20 as the clusters grew.

Small, Water-Soluble, Ligand-Stabilized Gold Nanoparticles Synthesized by Interfacial Ligand Exchange Reactions

A general synthetic approach leading to well-defined, water-soluble gold nanoparticles is described that involves a simple, interfacial ligand exchange reaction between a 1.4 nm phosphine-passivated precursor and an anionic or cationic thiol-containing ligand. We demonstrate the utility of this route by synthesizing water-soluble gold nanoparticles that are stabilized by either an anionic ligand (2-mercaptoethanesulfonate), a cationic ligand (2-(dimethylamino)ethanethiol hydrochloride), or a mixture of both ionic and phosphine ligands. Although the course of the ligand exchange process depends on the nature of the incoming ligand, each of these nanoparticle products retain the small core size and narrow size distribution of the starting particle (1.4 ± 0.4 nm). The stabilities of these nanoparticles to elevated temperature, extremes of pH, and added salt are reported and found to depend on the nature of the exposed headgroups on the ligand shell. Salt-induced aggregation is not observed in any of the case...

Comparison of the core size distribution in iron dextran complexes using Mössbauer spectroscopy and X-ray diffraction

Mössbauer spectra of a series of iron dextran complexes in the intermediate temperature range where both sextet and doublet coexist may be used to obtain a qualitative description of the distribution of core sizes in these samples. Eight samples from five suppliers have been examined at 100 and 77 K. These differ markedly in the relative doublet contribution to the total spectral area and also in the hyperfine fields characterizing the sextets. The results indicate three distinct types of distribution. One sample from each type has also been examined at 4 K, where the doublet component has vanished and the hyperfine field distribution has become narrow and symmetric. These data are compared with estimates of average core diameters from X-ray line broadening.

Phase Separation Behavior and Crew-Cut Micelle Formation of Polystyrene-b-poly(acrylic acid) Copolymers in Solutions

Static light scattering was used to study the phase separation behavior of homopolystyrene and polystyrene-b-poly(acrylic acid) (PS-b-PAA) in DMF as a function of added water content. It was found that the critical water content (cwc), at which phase separation starts, depends on both the polymer concentration and the molecular weight. The higher the polymer concentration and the molecular weight, the lower the cwc. For PS homopolymer, phase separation involves the precipitation of the polymer chains. In the copolymer solution, phase separation results in the formation of regular crew-cut micelles consisting of a PS core and a PAA corona; thus, it is preferably referred to as microphase separation. The change of the micelle fraction as a function of water addition can be estimated from the relationship between the cwc and the initial copolymer concentration. The influence of added electrolytes, i.e., NaOH, HCl, NaCl, CaCl2, or Ca(Ac)2, on the self-assembly process of the copolymers in DMF was also explored. Since the micelle cores are highly swollen by DMF in the early stages of their formation, the structures are labile. As the added water content increases, the cores become gradually less swollen and the mobility of the polymer chains in the cores decreases. A study of polymer chain exchange among the micelles was performed by mixing two solutions of micelles of different sizes at different water contents and studying the micelle core size distribution by transmission electron microscopy. It is shown that the copolymer chain exchange within a 1 day period is significant at a water content of 6 wt %, but becomes negligible when the water content is increased to ∼11 wt %.

High-resolution electron microscopy and electron energy-loss spectroscopy of giant palladium clusters

Combined structural and chemical characterization of cationic polynuclear palladium coordination compounds Pd561L60(OAc)180, where L=1,10-phenantroline or 2,2′-bipyridine has been carried out by high-resolution electron microscopy (HREM) and analytical electron microscopy methods including electron energy-loss spectroscopy (EELS), zero-loss electron spectroscopic imaging, and energy-dispersive X-ray spectroscopy (EDX). The cell structure of the cluster matter with almost completely uniform metal core size distributions centered around 2.3 ±0.5 nm was observed. Zero-loss energy filtering allowed to improve the image contrast and resolution. HREM images showed that most of the palladium clusters had a cubo-octahedral shape. Some of them had a distorted icosahedron structure exhibiting multiple twinning. The selected-area electron diffraction patterns confirmed the face centered cubic structure with lattice parameter close to that of metallic palladium. The energy-loss spectra of the populations of clusters contained several bands, which could be assigned to the delayed Pd M4, 5-edge at 362 eV, the Pd M3-edge at 533 eV and the Pd M2-edge at 561 eV, the NK-edge at about 400 eV, the O K-edge at 532 eV overlapping with the Pd M3-edge and the carbon C K-edge at 284 eV. Background subtraction was applied to reveal the exact positions and fine structure of low intensity elemental peaks. EELS evaluations have been confirmed by EDX. The recorded series of the Pd M-edges and the N K-edge in the spectra of the giant palladium clusters obviously were related to Pd-Pd- and Pd-ligand bonding.

Reconstitution of manganese oxide cores in horse spleen and recombinant ferritins

The formation of Mn(III) oxyhydroxide (MnOOH) cores within the nanoscale cavity of the iron storage protein ferritin has been investigated by electron microscopy and visible absorption spectroscopy. At pH 8.9, discrete amorphous MnOOH cores were formed within horse spleen apoferritin at a range of metal:protein ratios, as well as in ferritin molecules seeded with a small ferrihydrite nucleus. Analysis of the resultant core size distributions showed that the reconstitution of horse spleen apoferritin with Mn(II) was similar to that observed previously for Fe(II) reconstitution in recombinant human L-chain ferritin, suggesting that horse spleen apoferritin does not exhibit Mn(II) oxidase activity at pH 8.9. Reconstitution with MnOOH shows essentially “all-or-nothing” behavior in which many protein molecules remain unmineralized whilst others are loaded to maximum capacity. Kinetic studies showed no significant differences between horse spleen ferritin, recombinant H- and L-chain homopolymers, and H-chain variants containing site-directed modifications at the ferroxidase and putative Fe nucleation centers. Our results indicate that the reconstitution of ferritin with MnOOH cores proceeds by a nonspecific pathway. We propose that the outer surface of the protein inhibits the development of MnOOH nuclei in bulk solution whereas the inner surface is inactive, enabling nucleation and growth to proceed unperturbed within the cavity. One possibility is that differences in the general polyelectrolyte properties of these two surfaces, rather than site-specific charges, account for the “Janus” behavior of the molecule. A similar mechanism might also increase the specificity of iron oxide mineralization in ferritins that lack ferroxidase centers.

The investigation of the pore structure of silica gels with long-chain alkyl grafts by thermal analysis

The total pore volumes and the core size distributions were calculated for the silica gel Si-100 and chemically modified silica gels RP-8 and RP-18 on the basis of the thermal desorption data of various liquids. The results obtained are compared with those from the nitrogen adsorption/desorption method. Good consistency between the parameters obtained by using different methods is observed in the case of unmodified silica gel. The presence of long alkyl chains on the silica surface strongly influences the pore diameter calculated from thermogravimetric data.

Comment on "Kinetics of depletion of a population of hydrating cement particles"

Reading the article by Rubend D. Cohen and Menashi D. Cohen entitled: "Kinetics of depletion of a population of hydrating cement particles" [1], I found the following points on which I would like to comment. Through derivations incorporating the concept o f core-size distribution as a function of time the authors arrive at the usual integral connecting single-particle kinetics and PSD with the total degree of hydration

Pore Structure Analysis of Porous Particles by Modelless and Micropore Method

AbstractThe modelless (ML) and micropore (MP) methods for the pore structure analysis of porous particles have been studied. Concerning the ML method, it has been shown that the Kiselev equation can only be used to describe condensation and evaporation in capillaries, but cannot be used to describe the increase and decrease of the thickness of the adsorption layer. Strictly speaking, therefore, the ML method is not modelless even for the pore core size distribution. A comparison between the ML method and methods using an equivalent pore model is given. The results of using the ML method in conjunction with a certain pore model are little less accurate than those obtained by adopting the equivalent pore model straightaway. The parameters required for the calculation of both the pore core size and the pore size distribution as well as the conversion between these two calculations are given.Concerning the MP method, it has been shown that the three types of v‐t curves, the theoretical foundation of the MP method, are not in one to one correspondance to the three kinds of adsorption mechanism. From the viewpoint of accuracy, the MP method has no advantage over the methods which are based on the condensation mechanism, both have their own merits.

The evolution of an elliptic vortex ring

The evolution of a vortex ring in an ideal fluid under self-induction from a flat and elliptic configuration is followed numerically using the cut-off approximation (Crow 1970) for the velocity at the vortex. Calculations are presented for four different axes ratios of the initial ellipse. A particular choice is made for the core size and vorticity distribution in the core of the vortex ring. When the initial axes ratio is close to 1, the vortex ring oscillates periodically. The periodicity is lost as more eccentric cases are considered. For initial axes ratio 0·2, the calculations suggest a break-up of the ring through the core at one portion of the ring touching that at another, initially distant, portion of the ring.Results from quantitative experiments, conducted at moderate Reynolds number with the vortex rings produced by puffing air through elliptic orifices, are compared with the calculations. The agreement is fairly good and it is found that a vortex ring produced from an orifice of axes ratio 0·2 breaks up into two smaller rings. The relevance of the results to the vortex trail of an aircraft is discussed.

Single‐mode propagation in multi‐layer elliptical fiber waveguides

Single mode propagation in three‐layer step index elliptical fiber waveguides is investigated theoretically. Previous analysis of the elliptical dielectric tube waveguide is extended to include cladded, W‐type and tube‐type fibers. Numerical results on cutoff characteristics of the first higher‐order mode and phase coefficient, group velocity and power distribution characteristics of the dominant modes are presented over a wide range of parameters. A two‐layer step index elliptical fiber waveguide is presented as a special case. High eccentricity single mode fibers have small modal beat lengths but have small core size and greater power distribution in the claddings. W‐type fiber in the middle range of eccentricities is suggested for single mode single polarization applications.

Quantitative differences between sympathetic nerve terminals

The fluorescence histochemical method for catecholamines shows differences in both innervation density and fluorescence intensity between sympathetic nerve terminals in different organs. Quantitative electron microscopy shows differences in vesicle density in varicosities of nerve terminals in heart, spleen and vas deferens of the rat. The electron-dense core of the small noradrenergic vesicles has a different mean volume and a different core-size distribution in different nerve terminals. The significance of these findings is discussed.