Showing papers on "Nanoparticle published in 2001"

Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles

Abstract: It is shown that a “nanofluid” consisting of copper nanometer-sized particles dispersed in ethylene glycol has a much higher effective thermal conductivity than either pure ethylene glycol or ethylene glycol containing the same volume fraction of dispersed oxide nanoparticles. The effective thermal conductivity of ethylene glycol is shown to be increased by up to 40% for a nanofluid consisting of ethylene glycol containing approximately 0.3 vol % Cu nanoparticles of mean diameter <10 nm. The results are anomalous based on previous theoretical calculations that had predicted a strong effect of particle shape on effective nanofluid thermal conductivity, but no effect of either particle size or particle thermal conductivity.

Photoinduced conversion of silver nanospheres to nanoprisms.

Abstract: A photoinduced method for converting large quantities of silver nanospheres into triangular nanoprisms is reported. The photo-process has been characterized by time-dependent ultraviolet-visible spectroscopy and transmission electron microscopy, allowing for the observation of several key intermediates in and characteristics of the conversion process. This light-driven process results in a colloid with distinctive optical properties that directly relate to the nanoprism shape of the particles. Theoretical calculations coupled with experimental observations allow for the assignment of the nanoprism plasmon bands and for the first identification of two distinct quadrupole plasmon resonances for a nanoparticle. Unlike the spherical particles they are derived from that Rayleigh light-scatter in the blue, these nanoprisms exhibit scattering in the red, which could be useful in developing multicolor diagnostic labels on the basis not only of nanoparticle composition and size but also of shape.

Dendrimer-Encapsulated Metal Nanoparticles: Synthesis, Characterization, and Applications to Catalysis

Abstract: This Account reports the synthesis and characterization of dendrimer-encapsulated metal nanoparticles and their applications to catalysis. These materials are prepared by sequestering metal ions within dendrimers followed by chemical reduction to yield the corresponding zerovalent metal nanoparticle. The size of such particles depends on the number of metal ions initially loaded into the dendrimer. Intradendrimer hydrogenation and carbon−carbon coupling reactions in water, organic solvents, biphasic fluorous/organic solvents, and supercritical CO2 are also described.

Synthesis of highly crystalline and monodisperse maghemite nanocrystallites without a size-selection process.

Abstract: The synthesis of highly crystalline and monodisperse γ-Fe2O3 nanocrystallites is reported. High-temperature (300 °C) aging of iron−oleic acid metal complex, which was prepared by the thermal decomposition of iron pentacarbonyl in the presence of oleic acid at 100 °C, was found to generate monodisperse iron nanoparticles. The resulting iron nanoparticles were transformed to monodisperse γ-Fe2O3 nanocrystallites by controlled oxidation by using trimethylamine oxide as a mild oxidant. Particle size can be varied from 4 to 16 nm by controlling the experimental parameters. Transmission electron microscopic images of the particles showed 2-dimensional and 3-dimensional assembly of particles, demonstrating the uniformity of these nanoparticles. Electron diffraction, X-ray diffraction, and high-resolution transmission electron microscopic (TEM) images of the nanoparticles showed the highly crystalline nature of the γ-Fe2O3 structures. Monodisperse γ-Fe2O3 nanocrystallites with a particle size of 13 nm also can be...

Nanocrystalline semiconductors: Synthesis, properties, and perspectives

Abstract: The synthesis and study of so-called “nanoparticles”, particles with diameters in the range of 1−20 nm, has become a major interdisciplinary area of research over the past 10 years. Semiconductor nanoparticles promise to play a major role in several new technologies. The intense interest in this area derives from their unique chemical and electronic properties, which gives rise to their potential use in the fields of nonlinear optics, luminescence, electronics, catalysis, solar energy conversion, and optoelectronics, as well as other areas. The small dimensions of these particles result in different physical properties from those observed in the corresponding macrocrystalline, “bulk”, material. As particle sizes become smaller, the ratio of surface atoms to those in the interior increase, leading to the surface properties playing an important role in the properties of the material. Semiconductor nanoparticles also exhibit a change in their electronic properties relative to that of the bulk material; as th...

Fungus-mediated synthesis of silver nanoparticles and their immobilization in the mycelial matrix: a novel biological approach to nanoparticle synthesis

Abstract: A novel biological method for the synthesis of silver nanoparticles using the fungus Verticillium is reported. Exposure of the fungal biomass to aqueous Ag+ ions resulted in the intracellular reduction of the metal ions and formation of silver nanoparticles of dimensions 25 ± 12 nm. Electron microscopy analysis of thin sections of the fungal cells indicated that the silver particles were formed below the cell wall surface, possibly due to reduction of the metal ions by enzymes present in the cell wall membrane. The metal ions were not toxic to the fungal cells and the cells continued to multiply after biosynthesis of the silver nanoparticles.

Crystallographically oriented mesoporous WO3 films: synthesis, characterization, and applications.

Abstract: Mesoporous semiconducting films consisting of preferentially orientated monoclinic-phase nanocrystals of tungsten trioxide have been prepared using a novel version of the sol−gel method. Transformations undergone by a colloidal solution of tungstic acid, stabilized by an organic additive such as poly(ethylene glycol) (PEG) 300, as a function of the annealing temperature have been followed by means of a confocal Raman microscope. The shape and size of WO3 nanoparticles, the porosity, and the properties of the films depend critically on preparation parameters, such as the tungstic acid/PEG ratio, the PEG chain length, and the annealing conditions. Well-crystallized WO3 films combine excellent photoresponse to the blue region of the solar spectrum, up to 500 nm, with good transparency at wavelengths larger than 550 nm. Particular applications of these nanocrystalline WO3 films include photoelectrochemical and electrochromic devices.

Growth of Single-Walled Carbon Nanotubes from Discrete Catalytic Nanoparticles of Various Sizes

Abstract: Discrete catalytic nanoparticles with diameters in the range of 1−2 nm and 3−5 nm respectively are obtained by placing controllable numbers of metal atoms into the cores of apoferritin, and used for growth of single-walled carbon nanotube (SWNTs) on substrates by chemical vapor deposition (CVD). Atomic force microscopy (AFM), transmission electron microscopy (TEM), and micro-Raman spectroscopy are used to characterize isolated nanotubes grown from the discrete nanoparticles. The characterizations, carried out at single-tube and single-particle level, obtain clear evidence that the diameters of nanotubes are determined by the diameters of catalytic nanoparticles. With nanoparticles placed on ultrathin alumina membranes, isolated SWNTs are grown and directly examined by transmission electron microscopy. For the first time, both ends of an as-grown single-walled nanotube are imaged by TEM, leading to a microscopic picture of nanotube growth mechanism. It is shown that controlling the structures of catalytic ...

Metal Nanoparticles: Synthesis, Characterization, and Applications

Abstract: Introduction to particle synthesis, optical, and electronic properties solution-phase nanocluster synthesis and mechanisms of formation magic number metal clusters electrochemical synthesis of high aspect ratio gold particles template synthesis of metal nanostructures synthesis of metal nanoclusters using dendrimer templates nanosphere lithography electrochemistry of monolayer protected gold clusters modelling of nanoparticle optical properties hyper-Raleigh scattering of nanoparticles nanoparticle single electron devices DNA detection using gold nanoparticles conductance-based nanoparticle chemical sensors surface plasmon resonance detection of biomolecules synthesis of conductive polymer-gold particle composites optical properties of complex metal nanostructures si-coated nanoparticles nanoparticle arrays mixed particle arrays nanoparticles in electronic devices.

Nanoscopic Metal Particles − Synthetic Methods and Potential Applications

Abstract: Mono- and bimetallic colloidal particles have gained increasing attention in science and application throughout the last several years. In this contribution, we present a synopsis of the wet chemical syntheses of these materials and survey potential applications in catalysis and materials science. Methods for the characterization of these particles and their surfaces are not reviewed here.

Photoactivated fluorescence from individual silver nanoclusters.

Abstract: Fluorescence microscopy of nanoscale silver oxide (Ag2O) reveals strong photoactivated emission for excitation wavelengths shorter than 520 nanometers. Although blinking and characteristic emission patterns demonstrate single-nanoparticle observation, large-scale dynamic color changes were also observed, even from the same nanoparticle. Identical behavior was observed in oxidized thin silver films that enable Ag2O particles to grow at high density from silver islands. Data were readily written to these films with blue excitation; stored data could be nondestructively read with the strong red fluorescence resulting from green (wavelengths longer than 520 nanometers) excitation. The individual luminescent species are thought to be silver nanoclusters that are photochemically generated from the oxide.

Evidence for Seed-Mediated Nucleation in the Chemical Reduction of Gold Salts to Gold Nanoparticles

Abstract: Central to the concept of seed-mediated growth of nanoparticles is that small nanoparticle seeds serve as nucleation centers to grow nanoparticles to a desired size. We have examined this common assumption in a model system, the wet chemical synthesis of gold nanoparticles via reduction of a gold salt, by transmission electron microscopy and electronic absorption spectroscopy. We find that changing the seed concentration does affect the size of the product nanoparticles, but the method of reagent addition drastically affects the outcome even more. For fast addition of reducing agent, the presence of seeds appears to promote the formation of more seeds instead of growth. The observed nucleations are drastically enhanced (99%) compared to particle growth. For slow addition of reducing agent, the seeds do grow, but the product nanoparticle's degree of homogeneity in shape is compromised. For higher concentrations of seeds, nanoparticle growth is better controlled for slow addition of reducing agent compared ...

Formation of Gold Nanoparticles by Laser Ablation in Aqueous Solution of Surfactant

Abstract: Gold nanoparticles were produced by laser ablation of a gold metal plate in an aqueous solution of sodium dodecyl sulfate. The absorption spectrum of the gold nanoparticles was essentially same as that of gold nanoparticles chemically prepared in a solution. The size distribution of the nanoparticles thus produced was measured by an electron microscope and was found to shift to a smaller size with an increase in surfactant concentration. This behavior is explained in terms of the dynamic formation model. Dependence of the nanoparticle abundance on surfactant concentration in the solution shows that stable gold nanoparticles tend to be formed as the surfactant concentration exceeds 10-5 M. The gold nanoparticles having diameters larger than 5 nm were pulverized into those having diameters of 1−5 nm by a 532-nm laser.

Magnetic Nanocomposite Particles and Hollow Spheres Constructed by a Sequential Layering Approach

Abstract: The fabrication of magnetic composite core−shell particles and hollow spheres with tailored dimensions and compositions has been accomplished by a multistep (layer-by-layer) strategy. Composite particles were prepared by coating submicrometer-sized anionic polystyrene (PS) latices with magnetite (Fe3O4) nanoparticle layers alternately adsorbed with polyelectrolyte from aqueous solution. The thickness of the deposited multilayers could be finely tuned with nanoscale precision, either by selection of the number of adsorption cycles performed or by the number of polyelectrolyte layers deposited between each nanoparticle layer (i.e., interlayer). As demonstrated by transmission electron microscopy, a marked improvement in the growth, uniformity, and regularity of the composite multilayers was achieved when the number of polyelectrolyte interlayers was increased from one [(poly(diallyldimethylammonium chloride) (PDADMAC)] to three [(PDADMAC/poly(styrenesulfonate) (PSS)/PDADMAC)]. Hollow, intact magnetic sphere...

Multilayered Titania, Silica, and Laponite Nanoparticle Coatings on Polystyrene Colloidal Templates and Resulting Inorganic Hollow Spheres

Abstract: The applicability of the layer-by-layer (LbL) technique for the formation of a range of polymer-core inorganic-shell particles and inorganic hollow spheres is demonstrated Titanium dioxide, silica, and Laponite nanoparticles were used as the inorganic building blocks for multilayer formation on polystyrene (PS) sphere templates Composite organic−inorganic particles were formed by the controlled assembly of the preformed nanoparticles in alternation with oppositely charged polyelectrolytes onto PS microspheres The influence of nanoparticle type, shape (spherical to sheetlike), and size (3−100 nm), and the diameter of the PS sphere templates (210−640 nm) on the formation of multilayer shells was examined by transmission and scanning electron microscopy In addition, the LbL technique for coating polymer spheres has been shown to be adaptable with small variations in the coating steps used to optimize the nanoparticle coatings of the different materials For example, the number of polyelectrolyte multilay

Monodisperse 3 d Transition-Metal (Co,Ni,Fe) Nanoparticles and Their Assembly intoNanoparticle Superlattices

Abstract: Magnetic colloids, or ferrofluids, have been studied to probe the fundamental size-dependent properties of magnetic particles and have been harnessed in a variety of applications. The magnetorheological properties of magnetic colloids have been exploited in high-performance bearings and seals. The deposition of magnetic dispersions on platters and tapes marked the earliest embodiments of magnet information storage. Magnetic particles enhance contrast in magnetic resonance imaging and promise future diagnostic and drug delivery applications. The need to explore the scaling limits of magnetic storage technology has motivated the preparation of size-tunable monodisperse magnetic nanoparticles with controlled internal structures. The study of these nanoparticles is critical to efforts to separate the role of defects from intrinsic, finite size effects.

Alkyl Phosphonate/Phosphate Coating on Magnetite Nanoparticles: A Comparison with Fatty Acids

Abstract: Coated magnetite nanoparticles with a 6−8 nm average diameter were prepared. The surfactants used to stabilize the nanoparticles and disperse them in organic solvents were oleic acid (OA), lauric acid , dodecyl phosphonate, hexadecyl phosphonate, and dihexadecyl phosphate. Transmission electron microscopy analyses of the aggregation of the coated particles suggest that carboxylate surfactants provide the particles with better isolation and dispersibility as compared with phosphonate surfactants. However, Fourier transform infrared spectra of the phosphonate and phosphate coated particles suggest that these surfactants cover the surface of the nanoparticles in islands of high packing density. The thermogravimetric and differential scanning calorimetry measurements suggest that there is a quasi-bilayer of these surfactants covering the surface of the nanoparticles, with varying amounts of surfactant in the outer layer and with the second layer weakly bound to the primary layer through hydrophobic interactio...

Core–Shell Assembled Nanoparticles as Catalysts

Abstract: The deliberate tailoring of nanoparticles supported on oxides, dispersed in dendrimers and encapsulated in monolayer shells could lead to novel catalytic applications. The study of core–shell assembled gold or alloy nanoparticles for electrocatalytic oxidation of carbon monoxide and methanol stems, in part, from recent insights in core–shell reactivities and, in part, from surprising findings of catalytic activities of oxide-supported gold nanoparticles. Monolayer-encapsulated metallic nanoparticles serve as intriguing model building blocks towards catalysts. Whether such core–shell nanoparticles can in general be developed into aggregation- and poison-resistant catalysts of high catalytic activities rests with our capabilities in catalytic activation and structural manipulation.

Synthesis and Magnetic Properties of Nickel Nanorods

Abstract: Nickel nanoparticles of tunable shape have been synthesized in THF, in the presence of hexadecylamine (HDA) or trioctylphosphineoxide (TOPO), in mild conditions and characterized by HREM and SQUID measurements. The formation of nanorods is promoted by a high amine content in the reaction medium. In contrast to what is observed for TOPO-protected nickel particles, the saturation magnetization of HDA-capped nanoparticles is comparable to that of bulk nickel, which demonstrates that the coordination of an amine ligand does not alter the magnetic properties of nickel.

Electrophoretic displays using nanoparticles

Abstract: An electrophoretic display comprises a fluid and a plurality of nanoparticles having diameters substantially less the wavelengths of visible light such that, when the nanoparticles are in a dispersed state and uniformly dispersed throughout the fluid, the fluid presents a first optical characteristic, but when the nanoparticles are in an aggregated state in which they are gathered into aggregates substantially larger than the individual nanoparticles, the fluid presents a second optical characteristic different from the first optical characteristic. The electrophoretic display further comprises at least one electrode arranged to apply an electric field to the nanoparticle-containing fluid and thereby move the nanoparticles between their dispersed and aggregated states. Various compound particles comprising multiple nanoparticles, alone or in combination with larger objects, and processes for the preparation of such compound particles, are also described.

Size Effects on the Optical Properties of Organic Nanoparticles

Abstract: Nanoparticles of 1-phenyl-3-((dimethylamino)styryl)-5-((dimethylamino)phenyl)-2-pyrazoline (PDDP) ranging from tens to hundreds of nanometers were prepared by using the reprecipitation method. Their excitonic transitions responsible for absorption and emission, as compared with those of dilute solution, have been investigated as a function of nanoparticle size. We found that PDDP nanoparticles possess a special size dependence in their optical properties. We identified an extended charge-transfer (CT) state stemming from PDDP molecules closely stacking in nanoparticles and observed its shift to the high-energy side with decreasing nanoparticle size due to exciton confinement. At the same time, the molecular π−π* absorption of the nanoparticles was also blue-shifted, accompanied by an almost unchanging n−π* absorption as a result of the reduced overlap of the pyrazoline ring π orbital and decreased intermolecular interactions. Moreover, S1 and CT states were in equilibrium in the nanoparticles, and the pro...

Semiconductor−Metal Nanocomposites. Photoinduced Fusion and Photocatalysis of Gold-Capped TiO2 (TiO2/Gold) Nanoparticles

Abstract: Semiconductor/metal composite nanoparticles have been synthesized by chemically reducing HAuCl4 on the surface of preformed TiO2 nanoparticles. These gold-capped TiO2 nanoparticles (particle diameter 10−40 nm) were stable in acidic (pH 2−4) aqueous solutions. At low [TiO2]:[Au] ratio (<1:1), these composite nanoparticles are bigger in size (20−40 nm) and readily undergo aggregation. The TiO2/Au nanoparticles with TiO2:Au ratio ≥1:10 were relatively stable to 532 nm laser irradiation. On the other hand 532 nm laser pulse excitation of aggregated nanoparticles ([TiO2]:[Au] ≤ 1:1) led to morphological changes as they fused to produce large-size particles. Laser-induced melting/fusion was confirmed from the growth in the particle size as well as the disappearance of the aggregation absorption band. The process of melting and fusion occurred with an apparent rate constant of 1.25 × 109 s-1. The role of the gold layer in promoting the photocatalytic charge transfer has been probed using thiocyanate oxidation at...

Biochemically functionalized silica nanoparticles

Abstract: In this report, we demonstrate the biochemical modification of silica based nanoparticles. Both pure and dye-doped silica nanoparticles were prepared, and their surfaces were modified with enzymes and biocompatible chemical reagents that allow them to function as biosensors and biomarkers. The nanoparticles produced in this work are uniform in size with a 1.6% relative standard deviation. They have a pure silica surface and can thus be modified easily with many biomolecules for added biochemical functionality. Specifically, we have modified the nanoparticle surfaces with enzyme molecules (glutamate dehydrogenase (GDH) and lactate dehydrogenase (LDH)) and a biocompatible reagent for cell membrane staining. Experimental results show that the silica nanoparticles are a good biocompatible solid support for enzyme immobilization. The immobilized enzyme molecules on the nanoparticle surface have shown excellent enzymatic activity in their respective enzymatic reactions. The nanoparticle surface biochemical functionalization demonstrates the feasibility of using nanoparticles for biosensing and biomarking applications.

Synthesis of Monodisperse Nanoparticles of Barium Titanate: Toward a Generalized Strategy of Oxide Nanoparticle Synthesis

Abstract: The physical properties of materials as they progress from the bulk to the nanoscale regime (1 00 nm) continue to be of immense interest and increasing importance for future technological applications. Nanocrystals display properties generally found to be scientifically different from the bulk material or the atomic or molecular species from which they can be derived. Examples of this phenomenon are manifold. 1-3 The study of semiconductor nanocrystal quantum dots is a well-established field, yielding rich, useful ,and application-oriented research. 4 A coherent field of study is currently emerging for the systematic examination of nanocrystal oxides with the aim of producing nanoparticles with narrow size distributions and size tunability in the nanoscale regime. Among the important characteristics of nanoparticle systems are facile manipulation and reversible assembly which allow for the possibility of incorporation of nanoparticles into electric, electronic, or optical devices. Such “bottom up” or “self-assembly” approaches are the benchmark of nanotechnology. Ferroelectric materials have been under investigation due to the prospect that the stable polarization states could be used to encode the 1 and 0 of the Boolean algebra that form the basis of memory and logic circuitry (FRAM). The family of complex ferroelectric oxides such as BaTiO 3, Pb(Zr,Ti)O3, and (Ba,Sr)TiO3 has far reaching applications in the electronics industry for transducers, actuators, and high-k dielectrics. 5 Ferroelectricity is the phenomenon designated to crystals in possession of a spontaneous polarization and hysteresis effects with respect to the dielectric displacement in the presence of an applied electric field. The precise nature of ferroelectricity at the nanoscale, such as critical size dependent suppression in particles and thin films, 6

Hybrid organic/inorganic reverse osmosis (RO) membrane for bactericidal anti-fouling. 1. Preparation and characterization of TiO2 nanoparticle self-assembled aromatic polyamide thin-film-composite (TFC) membrane.

Abstract: Hybrid organic/inorganic reverse osmosis (RO) membranes composed of aromatic polyamide thin films underneath titanium dioxide (TiO2) nanosized particles have been fabricated by a self-assembly process, aiming at breakthrough of biofouling problems. First, positively charged particles of the colloidal TiO2 were synthesized by a sol−gel process, and the diameter of the resulting particles in acidic aqueous solution was estimated to be ≈2 nm by analyzing the UV−visible absorption characteristics with a quantum mechanical model developed by Brus. Transmission electron microscopy (TEM) further confirmed the formation of the quantum-sized TiO2 particles (∼10 nm or less). The TiO2 particles appeared to exist in the crystallographic form of anatase as observed with the X-ray diffraction (XRD) pattern in comparison with those of commercial 100% rutile and commercial 70:30% anatase-to-rutile mixture. The hybrid thin-film-composite (TFC) aromatic polyamide membranes were prepared by self-assembly of the TiO2 nanopar...

The Effect of Stabilizers on the Catalytic Activity and Stability of Pd Colloidal Nanoparticles in the Suzuki Reactions in Aqueous Solution

Abstract: Transition metal nanoparticles used in catalysis in solution are stabilized by capping the surfaces that are supposed to be used for catalysis. Determining how these two properties, that is, the catalytic activity and stability of nanoparticles, change as different capping materials are used is the aim of this work. Pd nanoparticles prepared by the reduction of metal salts in the presence of three different stabilizershydroxyl-terminated poly(amido-amine) (PAMAM) dendrimers (Gn-OH, where Gn represents the nth generation), block copolymer polystyrene-b-poly(sodium acrylate) and poly(N-vinyl-2-pyrrolidone) (PVP)are used as catalysts in the Suzuki reactions in an aqueous medium to investigate the effects of these stabilizers on the metallic nanoparticle catalytic activity and stability. The stability of the Pd nanoparticles is measured by the tendency of the nanoparticles to give Pd black powder after the catalytic reaction. The Suzuki reaction is a good “acid test” for examining the stability of these nanop...

Preparation of Monodispersed Fe−Mo Nanoparticles as the Catalyst for CVD Synthesis of Carbon Nanotubes

Abstract: Uniform iron−molybdenum nanoparticles were prepared by thermal decomposition of metal carbonyl complexes using a mixture of long-chain carboxylic acid and long-chain amine as protective agents. The sizes of the nanoparticles can be systematically varied from 3 to 14 nm by changing the experimental conditions. High-resolution TEM images and EDX data show that the prepared nanoparticles are highly crystalline iron nanoparticles containing ≈4% molybdenum. The effects of the concentration, reaction time, the ratio of metal carbonyl complexes versus protective agents, and the ratio of acid/amine of the protective agents on the sizes of the produced nanoparticles were systematically studied. The prepared nanoparticles were used as catalysts for single-walled carbon nanotube growth and the results indicate that there is an upper limit for the size of the catalyst particles to nucleate single-walled carbon nanotubes.