Showing papers by "Younan Xia published in 2008"

Gold nanocages: synthesis, properties, and applications.

Abstract: Noble-metal nanocages comprise a novel class of nanostructures possessing hollow interiors and porous walls. They are prepared using a remarkably simple galvanic replacement reaction between solutions containing metal precursor salts and Ag nanostructures prepared through polyol reduction. The electrochemical potential difference between the two species drives the reaction, with the reduced metal depositing on the surface of the Ag nanostructure. In our most studied example, involving HAuCl4 as the metal precursor, the resultant Au is deposited epitaxially on the surface of the Ag nanocubes, adopting their underlying cubic form. Concurrent with this deposition, the interior Ag is oxidized and removed, together with alloying and dealloying, to produce hollow and, eventually, porous structures that we commonly refer to as Au nanocages. This approach is versatile, with a wide range of morphologies (e.g., nanorings, prism-shaped nanoboxes, nanotubes, and multiple-walled nanoshells or nanotubes) available upon...

Rapid synthesis of silver nanowires through a CuCl- or CuCl2-mediated polyol process

Abstract: The presence of various ions has been shown to have a strong impact on the shape and size of silver nanostructures produced via the polyol reduction of AgNO3. Here we report a simple and rapid (reaction time ∼1 h) route to Ag nanowires, in which ethylene glycol serves as the solvent and a precursor to the reducing agent. The reaction could be performed in disposable glass vials, with all the reagents being delivered using pipettes. In addition to the use of poly(vinyl pyrrolidone) as a stabilizer, copper (I) or copper (II) chloride had to be added to the reaction to reduce the amount of free Ag+ during the formation of initial seeds and scavenge adsorbed oxygen from the surface of the seeds once formed. In doing so, Ag nanowires were grown preferentially.

Functionalization of Electrospun TiO2 Nanofibers with Pt Nanoparticles and Nanowires for Catalytic Applications

Abstract: This paper reports a simple procedure for derivatizing the surface of anatase TiO2 nanofibers with Pt nanoparticles and then Pt nanowires. The nanofibers were prepared in the form of a nonwoven mat by electrospinning with a solution containing both poly(vinyl pyrrolidone) and titanium tetraisopropoxide, followed by calcination in air at 510 degrees C. The fiber mat was then immersed in a polyol reduction bath to coat the surface of anatase fibers with Pt nanoparticles of 2-5 nm in size with controllable density of coverage. Furthermore, the coated fibers could serve as a three-dimensional scaffold upon which Pt nanowires of roughly 7 nm in diameter could be grown at a high density and with a length up to 125 nm. The fiber membranes functionalized with Pt nanoparticles and nanowires are interesting for a number of catalytic applications. It was found to show excellent catalytic activity for the hydrogenation of azo bonds in methyl red, which could be operated in a continuous mode by passing the dye solution through the membrane at a flow rate of 0.5 mL/s.

Controlling the Thickness of the Surface Oxide Layer on Cu Nanoparticles for the Fabrication of Conductive Structures by Ink‐Jet Printing

Abstract: With the aim of preparing a high performance conductive ink, we sought to control the surface chemistry of Cu nanoparticles so as to minimize surface oxidation. Specifically, the surface oxide layer on Cu nanoparticles synthesized in ambient atmosphere was minimized by adjusting the molecular weight of poly(N-vinylpyrrolidone) capping molecules, as confirmed by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy analyses. In addition, we demonstrate that by minimizing the thickness of the surface oxide layer, Cu granular films with good conductivity could be obtained by sintering nanoparticle assembles. Finally, we fabricated highly conductive Cu patterns on a plastic substrate by ink-jet printing.

Ultrathin gold nanowires can be obtained by reducing polymeric strands of oleylamine-AuCl complexes formed via aurophilic interaction.

Abstract: This Communication describes a facile route to the preparation of ultrathin gold nanowires using linear chains formed from [(oleylamine)AuCl] complex via aurophilic interaction. The linear chains, with AuI···AuI bonds as the backbone and surrounded by oleylamines, can group together to form bundles of polymeric strands. When the AuI was reduced to Au0 by reacting with Ag nanoparticles in hexane, the polymeric strands functioned as both the source of Au and the template to mediate the nucleation and growth of Au nanowires. Using this method, we were able to produce Au nanowires with an average diameter of ∼1.8 nm and an aspect ratio of >1000 in high yields (∼70%).

Dark-field microscopy studies of single metal nanoparticles: understanding the factors that influence the linewidth of the localized surface plasmon resonance

Abstract: This article provides a review of our recent Rayleigh scattering measurements on single metal nanoparticles. Two different systems will be discussed in detail: gold nanorods with lengths between 30 and 80 nm, and widths between 8 and 30 nm; and hollow gold-silver nanocubes (termed nanoboxes or nanocages depending on their exact morphology) with edge lengths between 100 and 160 nm, and wall thicknesses of the order of 10 nm. The goal of this work is to understand how the linewidth of the localized surface plasmon resonance depends on the size, shape, and environment of the nanoparticles. Specifically, the relative contributions from bulk dephasing, electron-surface scattering, and radiation damping (energy loss via coupling to the radiation field) have been determined by examining particles with different dimensions. This separation is possible because the magnitude of the radiation damping effect is proportional to the particle volume, whereas, the electron-surface scattering contribution is inversely proportional to the dimensions. For the nanorods, radiation damping is the dominant effect for thick rods (widths greater than 20 nm), while electron-surface scattering is dominant for thin rods (widths less than 10 nm). Rods with widths in between these limits have narrow resonances-approaching the value determined by the bulk contribution. For nanoboxes and nanocages, both radiation damping and electron-surface scattering are significant at all sizes. This is because these materials have thin walls, but large edge lengths and, therefore, relatively large volumes. The effect of the environment on the localized surface plasmon resonance has also been studied for nanoboxes. Increasing the dielectric constant of the surroundings causes a red-shift and an increase in the linewidth of the plasmon band. The increase in linewidth is attributed to enhanced radiation damping.

Putting Electrospun Nanofibers to Work for Biomedical Research

Abstract: Electrospinning has been exploited for almost one century to process polymers and related materials into nanofibers with controllable compositions, diameters, porosities, and porous structures for a variety of applications. Owing to its high porosity and large surface area, a non-woven mat of electrospun nanofibers can serve as an ideal scaffold to mimic the extracellular matrix for cell attachment and nutrient transportation. The nanofiber itself can also be functionalized through encapsulation or attachment of bioactive species such as extracellular matrix proteins, enzymes, and growth factors. In addition, the nanofibers can be further assembled into a variety of arrays or architectures by manipulating their alignment, stacking, or folding. All these attributes make electrospinning a powerful tool for generating nanostructured materials for a range of biomedical applications that include controlled release, drug delivery, and tissue engineering.

Integration of photonic and silver nanowire plasmonic waveguides

Abstract: Future optical data transmission modules will require the integration of more than 10,000 x 10,000 input and output channels to increase data transmission rates and capacity. This level of integration, which greatly exceeds that of a conventional diffraction-limited photonic integrated circuit, will require the use of waveguides with a mode confinement below the diffraction limit, and also the integration of these waveguides with diffraction-limited components. We propose to integrate multiple silver nanowire plasmonic waveguides with polymer optical waveguides for the nanoscale confinement and guiding of light on a chip. In our device, the nanowires lay perpendicular to the polymer waveguide with one end inside the polymer. We theoretically predict and experimentally demonstrate coupling of light into multiple nanowires from the same waveguide, and also demonstrate control over the degree of coupling by changing the light polarization.

On the polyol synthesis of silver nanostructures: glycolaldehyde as a reducing agent.

Abstract: The polyol synthesis is a popular method of preparing metal nanostructures, yet the mechanism by which metal ions are reduced is poorly understood. Using a spectrophotometric method, we show, for the first time, that heating ethylene glycol (EG) in air results in its oxidation to glycolaldehyde (GA), a reductant capable of reducing most noble metal ions. The dependence of reducing power on temperature for EG can be explained by this temperature-dependent oxidation, and the factors influencing GA production can have a profound impact on the nucleation and growth kinetics. These new findings provide critical insight into how the polyol synthesis can be used to generate metal nanostructures with well-controlled shapes. For example, with the primary reductant identified, it becomes possible to evaluate and understand its explicit role in generating nanostructures of a specific shape to the exclusion of others.

Methods of etching articles via microcontact printing

Abstract: Improved methods of forming a patterned self-assembled monolayer on a surface and derivative articles are provided. According to one method, an elastomeric stamp is deformed during and/or prior to using the stamp to print a self-assembled molecular monolayer on a surface. According to another method, during monolayer printing the surface is contacted with a liquid that is immiscible with the molecular monolayer-forming species to effect controlled reactive spreading of the monolayer on the surface. Methods of printing self-assembled molecular monolayers on nonplanar surfaces and derivative articles are provided, as are methods of etching surfaces patterned with self-assembled monolayers, including methods of etching silicon. Optical elements including flexible diffraction gratings, mirrors, and lenses are provided, as are methods for forming optical devices and other articles using lithographic molding. A method for controlling the shape of a liquid on the surface of an article is provided, involving applying the liquid to a self-assembled monolayer on the surface, and controlling the electrical potential of the surface.

A quantitative study on the photothermal effect of immuno gold nanocages targeted to breast cancer cells.

Abstract: Gold nanocages with an average edge length of 65 +/- 7 nm and a strong absorption peak at 800 nm were conjugated with monoclonal antibodies (anti-HER2) to target breast cancer cells (SK-BR-3) through the epidermal growth factor receptor (in this case, HER2), which is overexpressed on the surfaces of the cells. Both the number of immuno Au nanocages immobilized per cell and the photothermal therapeutic effect were quantified using flow cytometry. The targeted cells were irradiated with a pulsed near-infrared laser, and by varying the power density, the duration of laser exposure, and the time of response after irradiation, we were able to optimize the treatment conditions to achieve effective destruction of the cancer cells. We found that cells targeted with the immuno Au nanocages responded immediately to laser irradiation and that the cellular damage was irreversible at power densities greater than 1.6 W/cm(2). The percentage of dead cells increased with increasing exposure time up to 5 min and then became steady. By quantifying the photothermal effect of immuno Au nanocages, critical information with regards to both the optimal dosage of nanocages and parameters of the laser irradiation has been garnered and will be applied to future in vivo studies.

A Comparative Study of Galvanic Replacement Reactions Involving Ag Nanocubes and AuCl2− or AuCl4−

Abstract: The galvanic replacement reaction provides a simple and effective method for preparing hollow nanostructures of noble metals including Au, Pd, and Pt when Ag nanostructures are used as sacrificial templates.[1] These hollow nanostructures are enclosed by continuous or porous walls with a tunable/controllable thickness. For Au-based hollow nanostructures such as nanoboxes and nanocages, reduction of the wall thickness would lead to a red-shift for the localized surface plasmon resonance (LSPR) peak.[2] Significantly, when the LSPR peak is tuned into the near-infrared region from 800 to 900 nm (the so-called transparent window of soft tissues), these nanostructures hold great promise for a variety of biomedical applications that may include drug delivery,[3] contrast-enhanced optical imaging,[4] and photothermal therapy.[5]

Optical near-field mapping of plasmonic nanoprisms.

Abstract: The optical local-field enhancement on nanometer length scales provides the basis for plasmonic metal nanostructures to serve as molecular sensors and as nanophotonic devices. However, particle morphology and the associated surface plasmon resonance alone do not uniquely reflect the important details of the local field distribution. Here, we use interferometric homodyne tip-scattering near-field microscopy for plasmonic near-field imaging of crystalline triangular silver nanoprisms. Strong spatial field variation on lengths scales as short as 20 nm are observed sensitively depending on structural details and environment. The poles of the dipole and quadrupole plasmon modes, as identified by phase-sensitive probing and calculations performed in the discrete dipole approximation (DDA), reflect the particle symmetry. Together with the observation that the largest enhancement is not necessarily found to be associated with the tips of the nanoprisms, our results provide critical information for the selection of particle geometries as building blocks for plasmonic device applications.

Coating electrospun poly(ε-caprolactone) fibers with gelatin and calcium phosphate and their use as biomimetic scaffolds for bone tissue engineering

Abstract: Electrospinning was employed to fabricate fibrous scaffolds of poly(epsilon-caprolactone) in the form of nonwoven mats. The surfaces of the fibers were then coated with gelatin through layer-by-layer self-assembly, followed by functionalization with a uniform coating of bonelike calcium phosphate by mineralization in the 10 times concentrated simulated body fluid for 2 h. Transmission electron microscopy, water contact angle, and scanning electron microscopy measurements confirmed the presence of gelatin and calcium phosphate coating layers, and X-ray diffraction results suggested that the deposited mineral phase was a mixture of dicalcium phosphate dehydrate (a precursor to apatite) and apatite. It was also demonstrated that the incorporation of gelatin promoted nucleation and growth of calcium phosphate. The porous scaffolds could mimic the structure, composition, and biological function of bone extracellular matrix. It was found that the preosteoblastic MC3T3-E1 cells attached, spread, and proliferated well with a flat morphology on the mineralized scaffolds. The proliferation rate of the cells on the mineralized scaffolds was significantly higher (by 1.9-fold) than that on the pristine fibrous scaffolds after culture for 7 days. These results indicated that the hybrid system containing poly(epsilon-caprolactone), gelatin, and calcium phosphate could serve as a new class of biomimetic scaffolds for bone tissue engineering.

Facile synthesis of highly faceted multioctahedral Pt nanocrystals through controlled overgrowth.

Abstract: Highly faceted Pt nanocrystals with a large number of interconnected arms in a quasi-octahedral shape were synthesized simply by reducing H2PtCl6 precursor with poly(vinyl pyrrolidone) in aqueous solutions containing a trace amount of FeCl3. The iron species (Fe3+ or Fe2+) play a key role in inducing the formation of the multioctahedral structure by decreasing the concentration of Pt atoms and keeping a low concentration for the Pt seeds during the reaction. This condition favors the overgrowth of Pt seeds along their corners and thus the formation of multiarmed nanocrystals. Electron microscopy studies revealed that the multioctahedral Pt nanocrystals exhibit a large number of edge, corner, and surface step atoms. The size of the multioctahedral Pt nanocrystals can be controlled by varying the concentration of FeCl3 added to the reaction and/or the reaction temperature. These multioctahedral Pt nanocrystals were tested as electrocatalysts for the oxygen reduction reaction in a proton exchange membrane fu...

Nanomaterials at work in biomedical research.

Abstract: With some nanomaterial-based medicines having entered the marketplace, and more on the verge of doing so, nanomedicine is expected to become an exciting playground for chemists and material scientists.

Facile synthesis of bimetallic nanoplates consisting of Pd cores and Pt shells through seeded epitaxial growth.

Abstract: Pd−Pt core−shell nanoplates with hexagonal and triangular shapes were synthesized through the heterogeneous, epitaxial growth of Pt on Pd nanoplates. The Pd nanoplates were synthesized by reducing Na2PdCl4 precursor with PVP as a reducing agent, which then served as seeds for the nucleation of Pt atoms formed by reducing H2PtCl6 with citric acid. Characterization of the as-prepared Pd−Pt nanoplates by scanning transmission electron microscopy and high-resolution transmission electron microscopy reveals that a thin, uniform Pt shell was formed around the Pd nanoplate, demonstrating the layer-by-layer epitaxial growth of Pt on Pd surface in this approach. The close lattice match between Pd and Pt (lattice mismatch of only 0.77%) and the slow reduction rate associated with the mild reducing power of citric acid play key roles in achieving the epitaxial growth of Pt shells on Pd nanoplates.

Quantitative Analysis of Dipole and Quadrupole Excitation in the Surface Plasmon Resonance of Metal Nanoparticles

Abstract: Metal nanoparticles have received increasing attention for their peculiar capability to control local surface plasmon resonance (SPR) when interacting with incident light waves. In this article, we calculate the optical extinction spectra of silver nanocubes with the edge length ranging from 15 to 200 nm by using the discrete dipole approximation method. An increasing number of SPR peaks appear in the optical spectra, and their positions change when the nanocube size increases. We have developed a method to quantitatively separate the contributions of the individual dipole component and quadrupole component of the optical extinction cross sections. This allows us to specify unambiguously the physical origin of each SPR peak in the spectra. We have also extensively analyzed the distribution patterns of electric fields and electric charges within and around the silver nanoparticle. These patterns clearly show the dipole and quadrupole excitation features at the SPR peaks. The near-field analyses are consistent with the far-field extinction spectra analyses. This suggests that the combination of far-field spectra and near-field pattern analysis can greatly help to uncover the intrinsic physics behind light interaction with metal nanoparticles and excitation dynamics of local surface plasmonic waves.

Self-Assembly, Molecular Packing, and Electron Transport in n-Type Polymer Semiconductor Nanobelts

Abstract: We have found that poly(benzobisimidazobenzophenanthroline) (BBL) nanobelts can be prepared by a simple high-yield, solution-phase process, which enables dispersions of the nanobelts in a large number of solvents including environmentally benign solvents such as methanol and water. Characterization of the nanobelts by transmission electron and atomic force microscopies, electron diffraction, and X-ray diffraction showed that the BBL polymer chains are oriented parallel to the long axis of each nanobelt. This unique packing motif is unlike the reported packing of polymer chains in other nanostructures, such as poly(3-hexylthiophene) nanowires, where the polymer backbone packs face-to-face along the nanowire direction. This unusual molecular packing in BBL nanobelts is explained by the rather strong intermolecular interactions, which are a result of the rigid and planar polymer chains. We investigated electron transport in single nanobelts and nanobelt networks via field-effect transistors and observed mobi...

High Mobility Single-Crystal Field-Effect Transistors from Bisindoloquinoline Semiconductors

Abstract: A novel heptacyclic bisindoloquinoline-based organic semiconductor has been synthesized, characterized, and used to fabricate single-crystal field-effect transistors. A synthetic route was developed for the synthesis of heptacyclic bis(indolo{1,2-a})quinoline via an intramolecular cyclization of anthrazoline derivatives. Single-crystal X-ray structure revealed that the seven fused rings of bis(indolo{1,2-a})quinoline are relatively coplanar and lead to a slipped face-to-face π-stacking with the shortest intermolecular spacing of 3.3 A. Single-crystal field-effect transistors based on the bis(indolo{1,2-a})quinoline had carrier mobility as high as 1.0 cm2/V·s with on/off ratios greater than 104.

Facile Synthesis of Gold Nanoparticles with Narrow Size Distribution by Using AuCl or AuBr as the Precursor

Abstract: Gold(I) halides, including AuCl and AuBr, were employed for the first time as precursors in the synthesis of Au nanoparticles. The synthesis was accomplished by dissolving Au(I) halides in chloroform in the presence of alkylamines, followed by decomposition at 60 degrees C. The relative low stability of the Au(I) halides and there derivatives eliminated the need for a reducing agent, which is usually required for Au(III)-based precursors to generate Au nanoparticles. Controlled growth of Au nanoparticles with a narrow size distribution was achieved when AuCl and oleylamine were used for the synthesis. FTIR and mass spectra revealed that a complex, [AuCl(oleylamine)], was formed through coordination between oleylamine and AuCl. Thermolysis of the complex in chloroform led to the formation of dioleylamine and Au nanoparticles. When oleylamine was replaced with octadecylamine, much larger nanoparticles were obtained due to the lower stability of [AuCl(octadecylamine)] complex relative to [AuCl(oleylamine)]. Au nanoparticles can also be prepared from AuBr through thermolysis of the [AuBr(oleylamine)] complex. Due to the oxidative etching effect caused by Br(-), the nanoparticles obtained from AuBr exhibited an aspect ratio of 1.28, in contrast to 1.0 for the particles made from AuCl. Compared to the existing methods for preparing Au nanoparticles through the reduction of Au(III) compounds, this new approach based on Au(I) halides offers great flexibility in terms of size control.

Excitation enhancement of CdSe quantum dots by single metal nanoparticles

Abstract: We study plasmon-enhanced fluorescence from CdSe∕CdS∕CdZnS∕ZnS core/shell quantum dots near a variety of Ag and Au nanoparticles. The photoluminescence excitation (PLE) spectrum of quantum dots closely follows the localized surface plasmon resonance (LSPR) scattering spectrum of the nanoparticles. We measure excitation enhancement factors of ∼3 to 10 for different shapes of single metal nanoparticles.

Polyol synthesis of Cu2O nanoparticles: use of chloride to promote the formation of a cubic morphology

Abstract: We have employed the polyol method to synthesize copper(I) oxide (Cu2O) nanostructures with well-defined shapes and in large quantities. Transmission electron microscopy and X-ray powder diffraction studies show that polycrystalline colloidal spheres could be prepared in high yields by simply reducing copper nitrate with ethylene glycol heated to 140 °C in the presence of poly(vinyl pyrrolidone). When a small amount of sodium chloride was introduced, single-crystal nanocubes were obtained. In this case, chloride seems to play a pivotal role in controlling the formation of seeds and the growth rates of various crystallographic planes to shape the Cu2O nanostructures into nanocubes. Based on the structural analysis of samples obtained at different growth times, we also proposed a plausible mechanism to account for the formation of these two distinctive morphologies under different conditions.

Mechanistic study of the synthesis of Au nanotadpoles, nanokites, and microplates by reducing aqueous HAuCl4 with poly(vinyl pyrrolidone).

Abstract: This article describes a simple approach to anisotropic Au nanostructures with various shapes by reducing HAuCl4 with poly(vinyl pyrrolidone) (PVP) in aqueous solutions without the use of any additional capping agent or reductant. In this approach, the commercially available PVP servers as a mild reducing agent thanks to its hydroxyl (−OH) end groups, enabling kinetic control over both nucleation and growth. As the volume of HAuCl4 solution added to the reaction was increased, the morphology of Au nanostructures evolved from nanotadpoles to nanokites and then triangular and hexagonal microplates. The slow reduction rate associated with the mild reducing power of PVP plays a critical role in forming nanoplates during nucleation as well as their growth into highly anisotropic nanostructures. Electron microscopy studies reveal that the nanotadpoles and nanokites are formed through the linear fusion of small Au particles (<10 nm) to the initially formed nanoplates, whereas the microplates result from the cont...

Synthesis and Optical Properties of Cubic Gold Nanoframes.

Abstract: This paper describes a facile method of preparing cubic Au nanoframes with open structures via the galvanic replacement reaction between Ag nanocubes and AuCl2−. A mechanistic study of the reaction revealed that the formation of Au nanoframes relies on the diffusion of both Au and Ag atoms. The effect of the edge length and ridge thickness of the nanoframes on the localized surface plasmon resonance peak was explored by a combination of discrete dipole approximation calculations and single nanoparticle spectroscopy. With their hollow and open structures, the Au nanoframes represent a novel class of substrates for applications including surface plasmonics and surface-enhanced Raman scattering.

A facile, water-based synthesis of highly branched nanostructures of silver.

Abstract: We report a facile and environmentally friendly method of preparing highly branched silver nanostructures. By reducing AgNO3 with l-ascorbic acid in an aqueous solution, silver particles having a coral-like morphology were formed in a few minutes. A mechanistic study of the growth process revealed that the silver branches grew from a bulbous seed formed through aggregation, and that by changing the concentrations of the reagents, the degree of particle branching could be altered. With their potentially high surface areas, these branched structures could find use as catalysts or as substrates for surface-enhanced Raman scattering applications.

Electrocatalytic Properties of Pt Nanowires Supported on Pt and W Gauzes

Abstract: This paper describes the preparation of Pt- or W-supported Pt nanowires by directly growing them on the surface of Pt or W gauze. The growth direction of the nanowires was determined to be along the axis. Electrochemical measurements were performed to investigate their catalytic performance toward methanol oxidation. It was found from cyclic voltammetry that the Pt nanowires supported on Pt gauze had the largest electrochemically active surface area with the greatest activity toward methanol oxidation reaction. They also exhibited a slightly slower current decay over time, indicating a higher tolerance to CO-like intermediates. Furthermore, electrochemical impedance spectroscopy measurements showed that the catalytic performance of the supported Pt nanowires prepared with a H2PtCl6 precursor concentration of 40 mM is significantly better for methanol oxidation than the samples prepared at a concentration of 80 mM. This was due partially to the incomplete removal of poly(vinyl pyrrolidone) (PVP) from...