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Showing papers by "Younan Xia published in 2005"


Journal ArticleDOI
TL;DR: Investigating the growth mechanisms for silver nanocubes, nanowires, and nanospheres produced through a polymer-mediated polyol process found that the crystallinity of a seed and the extent of the PVP coverage on the seed were both instrumental in controlling the morphology of final product.
Abstract: The concept of shape-controlled synthesis is discussed by investigating the growth mechanisms for silver nanocubes, nanowires, and nanospheres produced through a polymer-mediated polyol process. Experimental parameters, such as the concentration of AgNO(3) (the precursor to silver), the molar ratio between poly(vinylpyrrolidone) (PVP, the capping agent) and AgNO(3), and the strength of chemical interaction between PVP and various crystallographic planes of silver, were found to determine the crystallinity of seeds (e.g., single crystal versus decahedral multiply twinned particles). In turn, the crystallinity of a seed and the extent of the PVP coverage on the seed were both instrumental in controlling the morphology of final product. The ability to generate silver nanostructures with well-defined morphologies provides a great opportunity to experimentally and systematically study the relationship between their properties and geometric shapes.

1,474 citations


Journal ArticleDOI
TL;DR: In this work, dark-field microscopy is used to observe a new plasmon resonance effect for a single silver nanocube in which the plAsmon line shape has two distinct peaks when the particles are located on a glass substrate.
Abstract: In this work, we use dark-field microscopy to observe a new plasmon resonance effect for a single silver nanocube in which the plasmon line shape has two distinct peaks when the particles are located on a glass substrate. The dependence of the resonance on nanocube size and shape is characterized, and it is found that the bluer peak has a higher figure of merit for chemical sensing applications than that for other particle shapes that have been studied previously. Comparison of the measured results with finite difference time domain (FDTD) electrodynamics calculations enables us to confirm the accuracy of our spectral assignments.

1,370 citations


Journal ArticleDOI
TL;DR: By controlling the molar ratio between Ag and HAuCl4, the gold nanocages could be tuned to display surface plasmon resonance peaks around 800 nm, a wavelength commonly used in optical coherence tomography (OCT) imaging.
Abstract: Gold nanocages of <40 nm in dimension have been synthesized using the galvanic replacement reaction between Ag nanocubes and HAuCl4 in an aqueous solution. By controlling the molar ratio between Ag and HAuCl4, the gold nanocages could be tuned to display surface plasmon resonance peaks around 800 nm, a wavelength commonly used in optical coherence tomography (OCT) imaging. OCT measurements on phantom samples indicate that these gold nanocages have a moderate scattering cross-section of ∼8.10 × 10-16 m2 but a very large absorption cross-section of ∼7.26 × 10-15 m2, suggesting their potential use as a new class of contrast agents for optical imaging. When bioconjugated with antibodies, the gold nanocages have also been demonstrated for specific targeting of breast cancer cells.

956 citations



Journal ArticleDOI
TL;DR: Both triangular and hexagonal nanoplates of Pd exhibited surface plasmon resonance (SPR) peaks in the visible region, and their positions matched with the results of discrete dipole approximation (DDA) calculation.
Abstract: The rapid reduction of Na2PdCl4 by ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP) has recently been demonstrated as a convenient method of generating Pd cubooctahedra and twinned nanoparticles. Here we describe a new procedure where Pd triangular or hexagonal nanoplates could be selectively synthesized by manipulating the reduction kinetics of the polyol process. More specifically, the reduction rate was substantially reduced through the introduction of Fe(III) species and the O2/Cl- pair, two wet etchants for Pd(0). The etching power of the O2/Cl- pair could be further enhanced by adding an acid to lower the pH of the reaction solution. Unlike the previously reported synthesis of Ag and Au nanoplates, light was found to have no indispensable role in the formation of Pd nanoplates. Both triangular and hexagonal nanoplates of Pd exhibited surface plasmon resonance (SPR) peaks in the visible region, and their positions matched with the results of discrete dipole approximation (DDA) calcula...

616 citations


Journal ArticleDOI
TL;DR: The galvanic replacement reaction between a Ag template and HAuCl4 in an aqueous solution transforms 30-200 mn Ag nanocubes into Au nanoboxes and nanocages.
Abstract: The galvanic replacement reaction between a Ag template and HAuCl4 in an aqueous solution transforms 30-200 mn Ag nanocubes into Au nanoboxes and nanocages (nanoboxes with porous walls). By controlling the molar ratio of Ag to HAuCl4, the extinction peak of resultant structures can be continuously tuned from the blue (400 nm) to the near-infrared (1200 nm) region of the electromagnetic spectrum. These hollow An nanostructures are characterized by extraordinarily large cross sections for both absorption and scattering. Optical coherence tomography measurements indicate that the 36 nm nanocage has a scattering cross-section of similar to 0.8 X 10(-15) m(2) and an absorption cross-section of similar to 7.3 X 10(-15) m(2). The absorption cross-section is more than five orders of magnitude larger than those of conventional organic dyes. Exposure of Au nanocages to a camera flash resulted in the melting and conversion of Au nanocages into spherical particles due to photothermal heating. Discrete-di poleapproximation calculations suggest that the magnitudes of both scattering and absorption cross-sections of An nanocages can be tailored by controlling their dimensions, as well as the thickness and porosity of their walls. This novel class of hollow nanostructures is expected to find use as both a contrast agent for optical imaging in early stage tumor detection and as a therapeutic agent for photothermal cancer treatment.

541 citations


Journal ArticleDOI
TL;DR: After galvanic replacement, Pd-Ag and Pt-Ag nanostructures remain SERS active, suggesting their use as a SERS probe for studying the dependence of interfacial chemistry on composition.
Abstract: Silver nanocubes dispersed in water were transformed into Pd−Ag or Pt−Ag nanoboxes by adding either Na2PdCl4 or Na2PtCl4 By controlling the amount of noble metal salt added, and therefore the molar ratio of Na2PdCl4 or Na2PtCl4 to Ag, we could tune the surface plasmon resonance peak of the nanostructures across the entire visible spectrum, from 440 to 730 nm Replacement of Ag with Pd resulted in the formation of a nanobox composed of a Pd−Ag alloy single crystal, but the nanobox formed after replacement of Ag with Pt was instead composed of distinct Pt nanoparticles DDA calculations suggest that both nanoboxes absorb light strongly, with Qabs/Qsca ≈ 5 After galvanic replacement, Pd−Ag and Pt−Ag nanostructures remain SERS active, suggesting their use as a SERS probe for studying the dependence of interfacial chemistry on composition

508 citations



Journal ArticleDOI
TL;DR: In this paper, the use of electrospinning in the fabrication of polymeric, ceramic, and composite nanofibers with core-sheath, hollow, or porous structures, as well as the efforts made to improve their morphological homogeneity, functionality, and device performance are highlighted.
Abstract: Recently there has been a great deal of progress in the production of one-dimensional nanostructures with core-sheath structures from a rich variety of materials. Here we highlight the use of electrospinning in the fabrication of polymeric, ceramic, and composite nanofibers with core-sheath, hollow, or porous structures, as well as the efforts made to improve their morphological homogeneity, functionality, and device performance.

422 citations


Journal ArticleDOI
TL;DR: Uniform cubooctahedral nanoparticles of Pd were synthesized using a modified polyol process, and like the silver system, the Pd nanoparticles underwent dissolution due to oxidative etching by Cl-/O2.
Abstract: Uniform cubooctahedral nanoparticles of Pd were synthesized using a modified polyol process. Like the silver system, the Pd nanoparticles underwent dissolution due to oxidative etching by Cl-/O2. In the early stage of etching, the twinned particles in a sample were selectively removed to leave behind uniform, single-crystal cubooctahedra. If the etching was allowed to continue, the single-crystal cubooctahedra were also attacked and partially dissolved. Understanding the role of oxidative etching is critical to the achievement of both uniform shape and size.

414 citations


Journal ArticleDOI
TL;DR: It was established that the fibers tended to be oriented along a direction such that the net torque of electrostatic forces applied to the two ends of a discrete segment of the fiber were minimized.
Abstract: Electrospinning is a simple, versatile, and useful technique for fabricating nanofibers from a rich variety of functional materials. The nanofibers are usually collected as nonwoven mats, in which the fibers are randomly oriented. We have recently demonstrated that the nanofibers can be uniaxially aligned by introducing an insulating gap into the conductive collector. To elucidate the mechanism of alignment, we have systematically studied the effect of the area and geometric shape of the insulating gap on the deposition of fibers. By modeling the electrostatic forces acting on the fiber, it was established that the fibers tended to be oriented along a direction such that the net torque of electrostatic forces applied to the two ends of a discrete segment of the fiber were minimized. By varying the design of electrode pattern, it was possible to control both alignment and assembly of the electrospun nanofibers.

Journal ArticleDOI
05 Aug 2005-Langmuir
TL;DR: Adjusting the concentration of Fe(II) or Fe(III) in the polyol reduction of silver nitrate allows for the production of either nanocubes or nanowires, and the addition of chloride prevented rapid aggregation of seeds and facilitated their growth into discrete nanostructures.
Abstract: Adjusting the concentration of Fe(II) or Fe(III) in the polyol reduction of silver nitrate allows for the production of either nanocubes or nanowires. Reduction by ethylene glycol maintained the iron ions in the reduced Fe(II) form, which in turn reacted with and removed adsorbed atomic oxygen from the surface of silver nanoparticles. Relatively high concentrations (2.2 microM) of iron ions facilitated the growth of multiply twinned seeds to form nanowires by removing oxygen from the surface of twinned seeds and preventing their dissolution by oxidative etching. Lower concentrations (<0.44 microM) of iron ions only partially removed oxygen from the surface, resulting in selective etching of twinned seeds and accelerated formation of single-crystal nanocubes. In each case, the addition of chloride prevented rapid aggregation of seeds and facilitated their growth into discrete nanostructures.

Journal ArticleDOI
TL;DR: Pd nanocubes between 8 and 50 nm in size were synthesized at the same concentration of Na2PdCl4 precursor by controlling the number of seeds formed in the nucleation stage by increasing the concentration of FeCl3, an oxidative etchant for Pd, and exhibited surface plasmon resonance peaks in the visible region.
Abstract: Pd nanocubes between 8 and 50 nm in size were synthesized at the same concentration of Na2PdCl4 precursor by controlling the number of seeds formed in the nucleation stage. Increasing the concentration of FeCl3, an oxidative etchant for Pd, reduced the number of seeds and led to formation of larger Pd nanocubes. The larger nanocubes exhibited surface plasmon resonance peaks in the visible region, the locations of which matched with the results of the discrete dipole approximation calculation. While the nanocubes of 25 and 50 nm in size oxidized in air to form Pd@PdO core-shell structures, the 8-nm nanocubes were stable in air for over 90 days.

Journal ArticleDOI
TL;DR: While many solution-phase methods have been developed for synthesizing nanoparticles with well controlled shapes, it is still a challenge to generate hollow nanostructures without involving exotic templates.
Abstract: Corrosion can be broadly defined as "the damage to metalcaused by reactions with its environment," and may take place in manydifferent forms such as pitting, crevice etching, intergranular exchange,galvanic replacement, and dealloying. Although corrosion is generallyundesirable (automobile corrosion alone costs $16 billion annually!), itcan be exploited as a versatile route to porous structures such as metalmembranes. While many solution-phase methods have been developed forsynthesizing nanoparticles with well controlled shapes, it is still agreat challenge to generate hollow nanostructures without involvingexotic templates.

Journal ArticleDOI
TL;DR: A brief account of solution-phase methods that generate silver and gold nanostructures with well-controlled shapes is given in this article, which is organized into five sections: the first section discusses the nucleation and formation of seeds from which nanostructure grow.
Abstract: This article provides a brief account of solution-phase methods that generate silver and gold nanostructures with well-controlled shapes. It is organized into five sections: The first section discusses the nucleation and formation of seeds from which nanostructures grow. The next two sections explain how seeds with fairly isotropic shapes can grow anisotropically into distinct morphologies. Polyol synthesis is selected as an example to illustrate this concept. Specifically, we discuss the growth of silver nanocubes (with and without truncated corners), nanowires, and triangular nanoplates. In the fourth section, we show that silver nanostructures can be transformed into hollow gold nanostructures through a galvanic replacement reaction. Examples include nanoboxes, nanocages, nanotubes (both single- and multi-walled), and nanorattles. The fifth section briefly outlines a potential medical application for gold nanocages.We conclude with some perspectives on areas for future work.

Journal ArticleDOI
TL;DR: Monodispersed gold nanocages of an approximately 35 nm edge length exhibit strong optical resonance, with the peak wavelength tunable in the near-infrared range, revealing an absorption cross section approximately 5 orders of magnitude larger than conventional dyes.
Abstract: We describe gold nanocages as a new class of potential contrast agent for spectroscopic optical coherence tomography (OCT). Monodispersed gold nanocages of an approximately 35 nm edge length exhibit strong optical resonance, with the peak wavelength tunable in the near-infrared range. We characterized the optical properties of the nanocage by using OCT experiments along with numerical calculations, revealing an absorption cross section approximately 5 orders of magnitude larger than conventional dyes. Experiments with tissue phantoms demonstrated that the nanocages provide enhanced contrast for spectroscopic as well as conventional intensity-based OCT imaging.

Journal ArticleDOI
TL;DR: In this article, two series of binary blends of poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylenevinylene] (MEH−PPV) with regioregular poly(3-hexylthiophene) (PHT) and with poly(9,9-dioctylfluorene)(PFO) were prepared, and their morphology and optical and electrical properties were characterized.
Abstract: Electrospun nanofibers of two series of binary blends of poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylenevinylene] (MEH−PPV) with regioregular poly(3-hexylthiophene) (PHT) and with poly(9,9-dioctylfluorene) (PFO) were prepared, and their morphology and optical and electrical properties were characterized. Morphological and photophysical studies showed that the phase-separated domains in MEH−PPV/PHT nanofibers (30−50 nm) are much smaller as compared to blend thin films (100−150 nm), and efficient energy transfer was observed in these blend nanofibers. The MEH−PPV/PFO blend nanofibers had cocontinuous or core−shell structures, and significant energy transfer was absent in these blend nanofibers as compared to bulk thin films. Field-effect transistors based on MEH−PPV/PHT blend nanofibers showed exponential dependence of hole mobility on blend composition. The hole mobility decreased from 1 × 10-4 cm2/(V s) in 20 wt % MEH−PPV blend nanofibers to 5 × 10-6 cm2/(V s) at 70 wt %. If corrected for the reduced chann...

Journal ArticleDOI
TL;DR: In this article, the authors provide an overview of some recent developments related to the synthesis and functionalization of monodisperse colloidal spheres, a class of colloidal materials that has found widespread use in applications such as the fabrication of photonic crystals, optical sensing, and drug delivery.
Abstract: This article provides an overview of some recent developments related to the synthesis and functionalization of monodisperse colloidal spheres, a class of colloidal materials that has found widespread use in applications such as the fabrication of photonic crystals, optical sensing, and drug delivery. Traditionally, the choice of materials has been limited to polystyrene and silica. We and other groups have recently expanded the scope of materials by developing a number of methods for producing monodisperse colloidal spheres from various semiconductors and metals. This article is confined to our own work; it covers three different synthetic strategies: the bottom–up approach, the top–down approach, and template-directed synthesis. The colloidal spheres may have a solid, hollow, or core–shell structure, and the chemical compositions can include Se, Bi, Pb, In, Sn, Cd, Pt, Ag2Se, CdSe, PbS, or TiO2. As an example to illustrate the attractive features of these colloidal spheres, we demonstrate the fabrication of Ag2Se-based photonic crystals whose stop bands can be thermally switched between two spectral positions.

Journal ArticleDOI
TL;DR: In this paper, a study on superhydrophobic silica surfaces based on roughness created by assembling different nanostructured materials was reported, where four different surface structures were created on a silica base material, which was subsequently covered with a self-assembled monolayer of tridecafluoro-1,1,2,2- tetrahydrooctyldimethylchlorosilane (TFCS).
Abstract: A study on superhydrophobic silica surfaces based on roughness created by assembling different nanostructured materials was reported. Four different surface structures were created on a silica base material, which was subsequently covered with a self-assembled monolayer of tridecafluoro-1,1,2,2- tetrahydrooctyldimethylchlorosilane (TFCS). The surfaces consisting of a nanofiber mesh demonstrated the highest static contact angle, when covered with TFCS monolayers. However, the surfaces consisting of nanorod arrays were found to be possessing the best dynamic hydrophobicity due to their continuous surface structure.

Journal ArticleDOI
TL;DR: A number of synthetic strategies for generating nanotubes from inorganic materials that do not have a layered structure are developed.

Journal ArticleDOI
TL;DR: In this article, the enzyme-polymer composites were prepared by electrospinning a solution of surfactant-stabilized enzyme and polymer in toluene and showed great improvement in the enzyme activity over bulk films as a result of increased mass transfer for substrate molecules to and from the enzyme reactive sites.
Abstract: Nanofibers consisting of enzyme–polymer composites have been prepared by directly electrospinning a solution of surfactant-stabilized enzyme and polymer in toluene. Additional treatment with glutaraldehyde could greatly stabilize the enzyme activity of the fibers, which could be maintained in a buffer under shaking conditions for more than two weeks. The nanofibers also showed great improvement in the enzyme activity over bulk films as a result of increased mass-transfer for substrate molecules to and from the enzyme reactive sites. These stable and catalytically active nanofiber-based mats were highly durable and could be easily recovered from solution, making them ideal candidates for large scale applications.

Journal ArticleDOI
TL;DR: In this paper, the Stober process was used to synthesize silica colloids embedded with superparamagnetic iron oxide nanoparticles, which were then added to an alcoholic medium to produce emulsion drops consisting of aggregates of IR nanoparticles and toluene.



Journal ArticleDOI
TL;DR: The galvanic replacement reaction between a Ag template and HAuCl4 in an aqueous solution transforms 30-200 mn Ag nanocubes into Au nanoboxes and nanocages as discussed by the authors.
Abstract: The galvanic replacement reaction between a Ag template and HAuCl4 in an aqueous solution transforms 30-200 mn Ag nanocubes into Au nanoboxes and nanocages (nanoboxes with porous walls). By controlling the molar ratio of Ag to HAuCl4, the extinction peak of resultant structures can be continuously tuned from the blue (400 nm) to the near-infrared (1200 nm) region of the electromagnetic spectrum. These hollow An nanostructures are characterized by extraordinarily large cross sections for both absorption and scattering. Optical coherence tomography measurements indicate that the 36 nm nanocage has a scattering cross-section of similar to 0.8 X 10(-15) m(2) and an absorption cross-section of similar to 7.3 X 10(-15) m(2). The absorption cross-section is more than five orders of magnitude larger than those of conventional organic dyes. Exposure of Au nanocages to a camera flash resulted in the melting and conversion of Au nanocages into spherical particles due to photothermal heating. Discrete-di poleapproximation calculations suggest that the magnitudes of both scattering and absorption cross-sections of An nanocages can be tailored by controlling their dimensions, as well as the thickness and porosity of their walls. This novel class of hollow nanostructures is expected to find use as both a contrast agent for optical imaging in early stage tumor detection and as a therapeutic agent for photothermal cancer treatment.

Journal ArticleDOI
TL;DR: Because of the uniformity in size and high refractive index associated with the Se@CdSe core-shell colloids, they could serve as a new class of building blocks to fabricate photonic crystals with wide and strong stop bands.
Abstract: Monodispersed spherical core−shell colloids of Se@Ag2Se have been exploited as a chemical template to synthesize Se@CdSe core−shell particles using a cation-exchange reaction. A small amount of tributylphosphine could facilitate the replacement of Ag+ by Cd2+ in methanol at 50 °C to complete the conversion within 150 min. The orthorhombic structure of β-Ag2Se changed to a well-defined wurtzite lattice for CdSe. The CdSe shells could be converted back to β-Ag2Se by reacting with AgNO3 in methanol at room temperature. Because of the uniformity in size and high refractive index associated with the Se@CdSe core−shell colloids, they could serve as a new class of building blocks to fabricate photonic crystals with wide and strong stop bands.

Patent
29 Mar 2005
TL;DR: In this article, a method for electrospinning nanofibers having a core-sheath, tubular, or composite structure is described, where a high voltage is applied between the spinneret and a spaced conductive collector.
Abstract: A method for electrospinning nanofibers having a core-sheath, tubular, or composite structure is disclosed. The process uses a spinneret having first and second capillaries that channel first and second fluids in the spinneret, the second capillary surrounding the first. A high voltage is applied between the spinneret and a spaced conductive collector. In one embodiment, the first fluid is a mineral oil and the second fluid is a polymeric solution that may include a polymer, a catalyst, a solvent, and a sol-gel precursor. The as-spun nanofiber includes an oil core and a composite sheath. The oil may be removed to produce a composite tubular fiber or the polymer and oil may be removed by calcination to produce a ceramic tubular fiber. In other embodiments, miscible fluids are used to produce porous nanofibers, selected additives functionalize the surfaces of the nanofibers and/or conjugated polymers are used.

Journal ArticleDOI
TL;DR: In this paper, a Cation exchange reaction between Ag{sup +} and Cd{sup 2+} was employed to transform singlecrystal Ag{sub 2}Se nanowires into single-crystal CdSe nanowsires as a case study of transformation from one chalcogenide to another.


Journal ArticleDOI
15 Sep 2005-Talanta
TL;DR: There is a linear correlation between the intensity ratio of SiOEP at 580nm to PtO EP at 650nm and the concentration of oxygen, which could be used to reliably monitor the partial pressure of oxygen in a system.