Showing papers by "Younan Xia published in 2007"

Synthesis of silver nanostructures with controlled shapes and properties.

Abstract: Mastery over the shape of a nanostructure enables control over its properties and usefulness for a given application. By controlling the crystallinity of the seeds from which nanostructures grow and the rate of atomic addition to seeds, we selectively produced pentagonal nanowires, cuboctahedra, nanocubes, nanobars, bipyramids, and nanobeams of silver with a solution-phase polyol synthesis. The example of nanobars illustrates how the shape of a silver nanostructure affects the color of light that it scatters. We further show how silver nanowires and nanobeams can serve as conduits for both electrons and photons.

Immuno Gold Nanocages with Tailored Optical Properties for Targeted Photothermal Destruction of Cancer Cells

Abstract: Gold nanocages with a relatively small size (e.g., ∼45 nm in edge length) have been developed, and the structure of these nanocages was tailored to achieve strong absorption in the near-infrared (NIR) region for photothermal cancer treatment. Numerical calculations show that the nanocage has a large absorption cross section of 3.48 × 10-14 m2, facilitating conversion of NIR irradiation into heat. The gold nanocages were conjugated with monoclonal antibodies (anti-HER2) to target epidermal growth factor receptors (EGFR) that are overexpressed on the surface of breast cancer cells (SK-BR-3). Our preliminary photothermal results show that the nanocages strongly absorb light in the NIR region with an intensity threshold of 1.5 W/cm2 to induce thermal destruction to the cancer cells. In the intensity range of 1.5−4.7 W/cm2, the circular area of damaged cells increased linearly with the irradiation power density. These results suggest that this new class of bioconjugated gold nanostructures, immuno gold nanocag...

Superparamagnetic Colloids: Controlled Synthesis and Niche Applications**

Abstract: The aim of this article is to provide a comprehensive review of current research activities that center on superparamagnetic colloids. We begin with an overview of synthetic strategies that have been developed for generating both nanoscale and mesoscale superparamagnetic colloids, with a focus on those systems that can be prepared as monodisperse samples and in relatively large quantities. We then discuss a variety of techniques that have been exploited for modifying surface properties, as well as for controlling the assembly and patterning of these magnetically active colloids. Towards the end, we highlight a range of innovative applications enabled by the unique combination of superparamagnetism and colloidal suspension. We conclude this review article with personal remarks and perspectives on the directions toward which future research in this area might be directed.

Facile synthesis of Ag nanocubes and Au nanocages

Abstract: This protocol describes a method for the synthesis of Ag nanocubes and their subsequent conversion into Au nanocages via the galvanic replacement reaction. The Ag nanocubes are prepared by a rapid (reaction time < 15 min), sulfide-mediated polyol method in which Ag(I) is reduced to Ag(0) by ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP) and a trace amount of Na2S. When the concentration of Ag atoms reaches supersaturation, they agglomerate to form seeds that then grow into Ag nanostructures. The presence of both PVP and Na2S facilitate the formation of nanocubes. With this method, Ag nanocubes can be prepared and isolated for use within approximately 3 h. The Ag nanocubes can then serve as sacrificial templates for the preparation of Au nanocages, with a method for their preparation also described herein. The procedure for Au nanocage preparation and isolation requires approximately 5 h.

Shape‐Controlled Synthesis of Metal Nanostructures: The Case of Palladium

Abstract: This article features shape-controlled synthesis of Pd nanostructures. Similar to the Ag system, both the crystallinity of seeds and the growth rates of different crystallographic facets play a vital role in determining the final shape of a resultant nanostructure. We specifically discuss how the reduction rate can be controlled to maneuver the crystallinity (i.e., single-crystal, single twinned, and multiple twinned) of seeds in the nucleation stage. The distribution of multiple twinned and single-crystal seeds can be further manipulated by employing oxidative etching. As the seed grows into a nanocrystal, the growth rates of different facets (e.g., {111} versus {100}) can be altered with capping agents to control the final shape. The ability to generate Pd nanostructures with a variety of geometrical shapes provides a great opportunity to systematically evaluate their electrical, plasmonic, and catalytic properties, as well as to fully explore their applications.

Synthesis and optical properties of silver nanobars and nanorice.

Abstract: Silver nanobars with rectangular side facets and an average aspect ratio of 2.7 have been synthesized by modifying the concentration of bromide added to a polyol synthesis. Subsequent rounding of nanobars transformed them into nanorice. Due to their anisotropy, nanobars and nanorice exhibit two plasmon resonance peaks, scattering light both in the visible and in the near-infrared regions. With a combination of discrete-dipole approximation calculations and single-nanoparticle spectroscopy, we explored the effect of nanostructure aspect ratio and corner sharpness on the frequency of plasmon resonance. Near-field calculations and surface-enhanced Raman scattering measurements on single particles were performed to show how local field enhancement changes with both the wavelength and polarization of incident light.

Synthesis and mechanistic study of palladium nanobars and nanorods.

Abstract: This paper describes a simple and versatile method for growing highly anisotropic nanostructures of Pd, single-crystal nanobars bounded by {100} facets and single-crystal nanorods with their side surfaces enclosed by {100} and {110} facets. According to thermodynamic arguments, Pd atoms should nucleate and grow in a solution phase to form cuboctahedrons of spherical shape with their surfaces bounded by a mix of {111} and {100} facets. Anisotropic nanostructures can only form under kinetically controlled conditions, while the cubic symmetry is broken. In the present system, we found that one-dimensional growth could be induced and maintained through an interplay of the following processes: (i) speedy reduction of the precursor to ensure prompt addition of atoms to the seed; (ii) chemisorption of bromide on the seed to promote the formation of {100} and {110} facets; and (iii) localized oxidative etching on one specific face of the seed to initiate preferential growth on this face. Experimentally, the anisotropic growth can be achieved by varying the type and concentration of reducing agent, as well as by adjusting the reaction temperature. This methodology developed for Pd has also been extended to both Au and Pt. As expected for a kinetically controlled product, the anisotropic nanostructure evolved into the thermodynamically favored shape during an aging process.

Gold Nanocages for Biomedical Applications

Abstract: Nanostructured materials provide a promising platform for early cancer detection and treatment. Here we highlight recent advances in the synthesis and use of Au nanocages for such biomedical applications. Gold nanocages represent a novel class of nanostructures, which can be prepared via a remarkably simple route based on the galvanic replacement reaction between Ag nanocubes and HAuCl4. The Au nanocages have a tunable surface plasmon resonance peak that extends into the near-infrared, where the optical attenuation caused by blood and soft tissue is essentially negligible. They are also biocompatible and present a well-established surface for easy functionalization. We have tailored the scattering and absorption cross-sections of Au nanocages for use in optical coherence tomography and photothermal treatment, respectively. Our preliminary studies show greatly improved spectroscopic image contrast for tissue phantoms containing Au nanocages. Our most recent results also demonstrate the photothermal destruction of breast cancer cells in vitro by using immuno-targeted Au nanocages as an effective photo-thermal transducer. These experiments suggest that Au nanocages may be a new class of nanometer-sized agents for cancer diagnosis and therapy.

Perylenediimide nanowires and their use in fabricating field-effect transistors and complementary inverters.

Abstract: Perylenetetracarboxyldiimide (PTCDI) nanowires self-assembled from commercially available materials are demonstrated as the n-channel semiconductor in organic field-effect transistors (OFETs) and as a building block in high-performance complementary inverters. Devices based on a network of PTCDI nanowires have electron mobilities and current on/off ratios on the order of 10-2 cm2/Vs and 104, respectively. Complementary inverters based on n-channel PTCDI nanowire transistors and p-channel hexathiapentacene (HTP) nanowire OFETs achieved gains as high as 8. These results demonstrate the first example of the use of one-dimensional organic semiconductors in complementary inverters.

Photoacoustic tomography of a rat cerebral cortex in vivo with au nanocages as an optical contrast agent.

Abstract: Poly(ethylene glycol)-coated Au nanocages have been evaluated as a potential near-infrared (NIR) contrast agent for photoacoustic tomography (PAT). Previously, Au nanoshells were found to be an effective NIR contrast agent for PAT; however, Au nanocages with their more compact sizes ( 100 nm for Au nanoshells) and larger optical absorption cross sections should be better suited for in vivo applications. We sequentially injected Au nanocages into the circulatory system of a rat in three administrations and in vivo PAT was conducted immediately prior to the first injection and continued until 5 h after the final injection. A gradual enhancement of the optical absorption in the cerebral cortex, by up to 81%, was observed over the course of the experiment.

Fabrication of Cubic Nanocages and Nanoframes by Dealloying Au/Ag Alloy Nanoboxes with an Aqueous Etchant Based on Fe(NO3)3 or NH4OH

Abstract: This paper describes a two-step procedure for generating cubic nanocages and nanoframes. In the first step, Au/Ag alloy nanoboxes were synthesized through the galvanic replacement reaction between Ag nanocubes and an aqueous HAuCl4 solution. The second step involved the selective removal (or dealloying) of Ag from the alloy nanoboxes with an aqueous etchant based on Fe(NO3)3 or NH4OH. The use of a wet etchant other than HAuCl4 for the dealloying process allows one to better control the wall thickness and porosity of resultant nanocages because there is no concurrent deposition of Au. By increasing the amount of Fe(NO3)3 or NH4OH added to the dealloying process, nanoboxes derived from 50-nm Ag nanocubes could be converted into nanocages and then cubic nanoframes with surface plasmon resonance (SPR) peaks continuously shifted from the visible region to 1200 nm. It is also possible to obtain nanocages with relatively narrow SPR peaks (with a full width at half-maximum as small as 180 nm) by controlling the amount of HAuCl4 used for the galvanic replacement reaction and thus the optimization of the percentage of Au in the alloy nanoboxes.

One-Dimensional Nanostructures of Metals: Large-Scale Synthesis and Some Potential Applications

Abstract: We review recent developments in our group regarding the solution-phase synthesis of one-dimensional nanostructures of metals. The synthetic approaches include solution-liquid-solid growth for nanowires of low-melting-point metals such as Pb; seed-directed growth for Ag nanowires, nanobeams, and nanobelts; kinetically controlled growth for Pt nanorods, nanowires, and multipods; and galvanic replacement for nanotubes of Au, Pt, and Pd. Both characterization and mechanistic studies are presented for each nanostructure. Finally, we highlight the electrical and plasmonic properties of these metal nanostructures and discuss their potential applications in nanoscale devices.

Mechanistic studies on the galvanic replacement reaction between multiply twinned particles of Ag and HAuCl4 in an organic medium.

Abstract: This article presents a mechanistic study on the galvanic replacement reaction between 11- and 14-nm multiply twinned particles (MTPs) of Ag and HAuCl4 in chloroform. We monitored both morphological and spectral changes as the molar ratio of HAuCl4 to Ag was increased. The details of reaction were different from previous observations on single-crystal Ag nanocubes and cuboctahedrons. Because Au and Ag form alloys rapidly within small MTPs rich in vacancy and grain boundary defects, a complete Au shell did not form on the surface of each individual Ag template. Instead, the replacement reaction resulted in the formation of alloy nanorings and nanocages from Ag MTPs of decahedral or icosahedral shape. For the nanorings and nanocages derived from 11-nm Ag MTPs, the surface plasmon resonance (SPR) peak can be continuously shifted from 400 to 616 nm. When the size of Ag MTPs was increased to 14 nm, the SPR peak can be further shifted to 740 nm, a wavelength sought by biomedical applications. We have also investigated the effects of capping ligands and AgCl precipitate on the replacement reaction. While hollow structures were routinely generated from oleylamine-capped Ag MTPs, we obtained very few hollow structures by using a stronger capping ligand such as oleic acid or tri-n-octylphosphine oxide (TOPO). Addition of extra oleylamine was found to be critical to the formation of well-controlled, uniform hollow structures free of AgCl contamination thanks to the formation of a soluble complex between AgCl and oleylamine.

The SERS Activity of a Supported Ag Nanocube Strongly Depends on Its Orientation Relative to Laser Polarization

Abstract: Silver nanocubes with sharp or truncated corners were synthesized, deposited on silicon substrates, and functionalized with Raman-active thiols for surface-enhanced Raman scattering (SERS) studies. The use of substrates with registration marks allowed us to correlate the SERS spectra from individual nanocubes to their physical parameters revealed by high-resolution SEM imaging. We observed dramatic variations in SERS intensity when the nanocubes were oriented at different angles relative to the polarization of excitation laser. This angular dependence was less significant when the nanocubes were truncated and became nearly spherical in profile. Numerical calculations were employed to confirm our observations, and to attribute the source of variation to the difference in near-field distribution between different laser polarizations.

Fabrication of Field-Effect Transistors from Hexathiapentacene Single-Crystal Nanowires

Abstract: This paper describes a simple, solution-phase route to the synthesis of bulk quantities of hexathiapentacene (HTP) single-crystal nanowires. These nanowires have also been successfully incorporated as the semiconducting material in field-effect transistors (FETs). For devices based on single nanowires, the carrier mobilities and current on/off ratios could be as high as 0.27 cm2/Vs and >103, respectively. For transistors fabricated from a network of nanowires, the mobilities and current on/off ratios could reach 0.057 cm2/Vs and >104, respectively. We have further demonstrated the use of nanowire networks in fabricating transistors on mechanically flexible substrates. Preliminary results show that these devices could withstand mechanical strain and still remain functional. The results from this study demonstrate the potential of utilizing solution-dispersible, nanostructured organic materials for use in low-cost, flexible electronic applications.

Synthesis of palladium icosahedra with twinned structure by blocking oxidative etching with citric acid or citrate ions

Abstract: For this reason, anicosahedron with 12 corners and 30 edges is anticipated tobe a superior substrate for SERS detection. These opportu-nities inspired us to target the synthesis of Pd icosahedra.Pd is a face-centered cubic (fcc) noble metal, and Pdnanoparticles may take three different shapes when fastnucleation and growth are involved: truncated cubes oroctahedra, decahedra, and icosahedra.

Nanocrystals with unconventional shapes--a class of promising catalysts.

Abstract: Research into noble-metal nanocrystals is stimulated by the fascinating sizeand shape-dependent properties of these nanomaterials. Because of their unique and tunable properties, they hold promise for various applications in optics, electronics, information storage, biological labeling, imaging, and surface-enhanced Raman scattering (SERS). Catalysis has also long relied on noble-metal nanocrystals for a wide variety of organic and inorganic reactions. Nanocrystals of noble metals are attractive for use as catalysts because of their high surface-to-volume ratios and high surface energies, which in turn cause their surface atoms to be highly active. So far, they have been used to catalyze many types of reactions including oxidation, cross-coupling, electron-transfer, and hydrogenation. In particular, as an active component in catalytic converters, Pt nanocrystals already drastically reduce pollution from automobiles. The use of fuel cells in the future to reduce the dependence on gasoline and the output of greenhouse gases makes it especially significant to manufacture Pt nanocrystals with superb performance for electrocatalysis. Catalysis requires the use of a noble metal in a finely divided state, where both the size and shape of the nanocrystals are critical parameters that must be controlled to maximize their activity. Shape control could enable the properties of a nanocrystal to be tuned with a greater versatility than can be achieved otherwise. For example, both the reactivity and selectivity of a nanocatalyst can be tailored by controlling the shape, as shape determines the number of atoms located at the edges or corners. Recent work by ElSayed and Narayanan correlating the catalytic activity of Pt nanocrystals with the number of surface atoms indicates that a large number of edge and corner atoms holds the key to improving their catalytic performance. Their study was limited to shapes bound only by {111} and {100} facets. In general, high-index planes have a greater density of unsaturated atomic steps, ledges, and kinks which can serve as active sites for breaking chemical bonds. Fundamental studies on the single-crystal surfaces of bulk Pt have shown that high-index planes exhibit much higher catalytic activity than common, stable, low-index planes, such as {111} and {100}. If one can create shapes with high-index surface facets, the catalytic activity can be further enhanced. Hence, it is clear that maximization of high-index surfaces and abundant corner and edge sites should be the criteria for selection of an excellent nanocatalyst. However, the common shapes of face-centered cubic (fcc) metals are enclosed by {111} and {100} facets and contain a low percentage of corner and edge sites (see Figure 1). These

Gold Nanostructures: Engineering Their Plasmonic Properties for Biomedical Applications

Abstract: The surface plasmon resonance peaks of gold nanostructures can be tuned from the visible to the near infrared region by controlling the shape and structure (solid vs. hollow). In this tutorial review we highlight this concept by comparing four typical examples: nanospheres, nanorods, nanoshells, and nanocages. A combination of this optical tunability with the inertness of gold makes gold nanostructures well suited for various biomedical applications.

A Water‐Based Synthesis of Octahedral, Decahedral, and Icosahedral Pd Nanocrystals

Abstract: Noble-metal nanocrystals have attracted increasing attention owing to their potential use in catalysis, electronics, and biology. The physicochemical properties of these nanocrystals are highly sensitive to their shape and size. For example, the number, location, and intensity of surface plasmon resonance (SPR) bands of Au and Ag nanocrystals display a strong correlation with the shape of the particle. Furthermore, the reactivity and selectivity of metal nanocatalysts depend strongly on the crystallographic planes exposed on the surface of the particles and can therefore be tuned by controlling the morphology of these particles. An exquisite shape control of noble metal nanocrystals is therefore highly desirable for tailoring their properties and is also required for high performance in many applications. Palladium nanocrystals are widely used as primary catalysts for the low-temperature reduction of automobile pollutants, hydrogenation reactions, and organic reactions such as Suzuki, Heck, and Stille coupling. Pd is also wellknown for its remarkable capacity in hydrogen absorption. Most of these applications are related to the adsorption of hydrogen onto the surface of Pd nanocrystals. Recent studies have revealed that the hydrogen-absorption capacity and surface-enhanced Raman scattering (SERS) activity of Pd nanocrystals are dependent on their shape. A wide variety of Pd nanocrystals, including cuboctahedra, cubes, rods, and bars, have been prepared to date, mostly by the polyol method, in which ethylene glycol (EG) serves as both a reductant and a solvent. However, the major products of a polyol synthesis are often restricted to shapes such as truncated cubes or cuboctahedra, owing to the fast reduction and growth rate associated with the polyol process. An alternative water-based system could provide a more convenient and environmentally benign route to the synthesis of noble-metal nanocrystals, because it does not involve toxic organic solvents or reagents. Recently, our group reported the syntheses of Pd thin plates and icosahedra in aqueous solution using poly(vinyl pyrrolidone) (PVP) and citric acid as the reducing agent, respectively. It is worth pointing out that an octahedron and a decahedron are two particle shapes that face-centered cubic (fcc) noble metals can potentially take, although high-yield syntheses of these two types of nanostructures are yet to be realized for Pd. The formation of a particular shape in the synthesis of metal nanocrystals is often explained in terms of the presence of surfactants or capping agents, which can change the order of free energies of different facets through their interactions with the metal surface in a solution-phase synthesis. This alteration may significantly affect the relative growth rates of different facets and thus lead to different morphologies for the final products. To achieve shape control of a nanocrystal, however, not only the thermodynamics or physical restrictions imposed by the surface stabilizing agent must be considered, but also nucleation and kinetics. Herein, we report on a water-based system for the facile synthesis of Pd nanocrystals with various shapes by reducing a Pd precursor with citric acid. Citric acid or citrate ion can also serve as a capping agent in this system thanks to their strong binding to the {111} facets of Pd. More specifically, we demonstrate that citric acid favors the formation of a structure such as an octahedron, icosahedron, or decahedron whose surface is covered by {111} planes. We also demonstrate that the shape of the Pd nanocrystals can be controlled by varying the concentrations of the Pd precursor and citric acid. We have been able to selectively produce Pd octahedra, icosahedra, and decahedra in high yields using this simple approach. The synthesis was conducted in an aqueous solution containing Na2PdCl4, PVP, and citric acid at 90 8C for 26 h. Na2PdCl4 and PVP are used as a Pd precursor and a stabilizer, respectively. Citric acid serves as a mild reductant and a capping agent in a manner similar to the mechanism of a conventional citrate-based synthesis of gold or other noblemetal nanoparticles. Several hours into the reaction, the color of the solution changed from light yellow to deep brown, thereby indicating the formation of Pd nanocrystals. We explored a range of concentrations for both Na2PdCl4 (5.8– 17.4 mm) and citric acid (0.13–0.39m) to determine the optimal conditions for the preparation of Pd nanocrystals with different shapes. Figure 1 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Pd octahedra synthesized in the presence of 7.3 mm Na2PdCl4 and 0.13m citric acid with a molar ratio of the repeating unit of PVP to the Pd precursor of 5:1. The sample contains approximately 90% octahedra with edge lengths of approximately 20 nm and around 10% other shapes, including triangular plates and decahedra (see also Figure S1 in the Supporting Information). Although an octahedral shape has [*] Dr. B. Lim, Dr. Y. Xiong, Prof. Y. Xia Department of Chemistry, University of Washington Seattle, Washington 98195 (USA) E-mail: xia@chem.washington.edu

Synthesis of silver nanoplates at high yields by slowing down the polyol reduction of silver nitrate with polyacrylamide

Abstract: This communication describes a simple route to Ag nanoplates by slowing the polyol reduction of silver nitrate with polyacrylamide (PAM). In addition to its role as a stabilizer for Ag colloids, the amino groups of PAM can form complexes with Ag+ ions in the solution to substantially reducing the reduction rate, leading to the formation of thin nanoplates.

Growing Pt nanowires as a densely packed array on metal gauze.

Abstract: This paper describes the synthesis of single-crystal nanowires of Pt directly on the surface of metal gauze made of Pt or W and demonstrates a more attractive approach for the growth of Pt nanowires on functional solid supports. There is evidence that the growth mechanism of the nanowires is correlated to the concentration of the Pt precursor and the surface roughness of the substrate. Electrochemical measurements indicate that the active surface area of the Pt nanowire-coated gauze is about 2−3 orders of magnitude greater than the pristine gauze. The results described herein demonstrate a new type of conductive support that can be used as active components for fuel cell applications and as an ideal 3D model catalyst.

Trimeric Clusters of Silver in Aqueous AgNO3 Solutions and Their Role as Nuclei in Forming Triangular Nanoplates of Silver

Abstract: The extensive use of silver nanostructures as optical labels, substrates for surface-enhanced Raman scattering (SERS), near-field optical probes, and contrast agents for biomedical imaging has led to a steadily growing interest in the chemical synthesis of such species. More importantly, the optical properties of silver nanostructures can be tailored with great versatility by controlling their shapes during synthesis. A remarkable example is that of triangular nanoplates of silver, a class of nanostructures with two-dimensional anisotropy. The nanoplates exhibit fascinating optical properties, such as intense quadrupole resonance peaks that are absent in small nanospheres and have found use in chemical and biological sensing. Since the first publication on this subject by Mirkin and co-workers in 2001, a number of different synthetic routes have been demonstrated, including those based on photoor thermally induced transformation and on direct chemical reduction. All of these methods rely on the slow generation of neutral silver atoms to enable kinetic control. Although kinetic control has also been used in the synthesis of platelike nanostructures from other noble metals, it remains largely unresolved how this process works. 8] It has been proposed that light of proper wavelengths or that certain capping ligands, such as citrate, are responsible for the formation of silver nanoplates. 6e] However, our most recent work demonstrated that the platelike morphology could also be obtained in the absence of both light and citrate. Herein, we elucidate the mechanism of nanoplate formation by focusing on the silver clusters that dominate nucleation. Despite the technological importance of nanocrystals and the extensive efforts that have been devoted to studying them, attempts to synthetically and systematically control their shapes and properties have met with limited success. One barrier to success is the fact that very little is known about the details of nucleation involved in the formation of nanocrystals. In the case of a metal, it is still unclear how a precursor salt is reduced into neutral atoms that then aggregate and evolve into nanoscale crystals. The g-radiation-based synthesis developed by Henglein has shed some light on the nucleation process by controlling the generation of zero-valent atoms and thus their agglomeration into small clusters. Both UV/Vis spectroscopic and scanning tunneling microscopic studies of these clusters suggested that Ag4 2+ and Ag8 4+ were the most abundant species involved in the nucleation stage. Growth of these clusters into nanocrystals likely occurred through a combination of aggregation and atomic addition. Herein we demonstrate, for the first time, that there exists a smaller cluster, Ag3 + or Ag3, in the nucleation stage of a solution-phase synthesis that employs AgNO3 as a precursor to silver. These trimeric clusters can serve as nuclei for the addition of newly formed silver atoms and eventually lead to the formation of triangular nanoplates. Mass spectrometry provides a tool for simple identification and characterization of silver clusters possibly contained in aqueous AgNO3 solution. Since a mass spectrometer can separate and detect ions of different masses, it allows the different isotopes of a given element to be easily distinguished. It is also feasible to quickly identify clusters of different sizes by analyzing the isotope patterns. Natural silver comprises a nearly 1:1 mixture of two isotopes with atomic masses of 106.9 and 108.9 amu. A trimeric cluster of silver contains three silver atoms that may come in four different combinations: three Ag atoms, two Ag atoms plus one Ag atom, one Ag atom plus two Ag atoms, or three Ag atoms. As a result, Ag ions are expected to appear in the mass spectrum as a doublet (with two peaks located at m/z 106.9 and 108.9), while Ag3 + clusters give rise to a quadruplet (with four peaks located at m/z 320.7, 322.7, 324.7, and 326.7 with a ratio of 1:3:3:1). Figure 1a shows a positive-mode mass spectrum taken from an aqueous solution of AgNO3 immediately after its preparation. In the m/z range from 80 to 1100, there are four sets of peaks with distinct isotope patterns. According to the isotope patterns and their corresponding m/z ratios, the peaks can be assigned to Ag, [Ag2NO3] , Ag3 , and [Ag3(NO3)2] . The insets show the doublet and quadruplet patterns characteristic of Ag and Ag3 . The positive charge on Ag3 + might be intrinsic to the trimeric cluster, or it might be caused by oxidation during the electrospray ionization process. Therefore, the trimeric clusters of silver in aqueous AgNO3 can be either positively charged (Ag3 ) or neutral (Ag3). The negative-mode mass spectrum (Figure S1 in the Supporting Information) indicates that there are also a number of negatively charged complexes in the aqueous AgNO3 solution, with notable examples including [Ag(NO3)2] , [Ag2(NO3)3] , and [Ag3(NO3)4] . As established in previous work, [*] Dr. Y. Xiong, I. Washio, Dr. J. Chen, Dr. M. Sadilek, Prof. Y. Xia Department of Chemistry University of Washington Seattle, WA 98195 (USA) Fax: (+1)206-685-8665 E-mail: xia@chem.washington.edu

Cation exchange: a simple and versatile route to inorganic colloidal spheres with the same size but different compositions and properties.

Abstract: This paper describes a cation exchange approach to the synthesis of metal chalcogenide core−shell particles with the same size but a number of different compositions. This method begins with the preparation of colloidal spheres of amorphous Se (a-Se), followed by their reaction with Ag atoms to form Se@Ag2Se spheres. These core−shell spheres are then converted into Se@MSe (M = Zn, Cd, and Pb) via cation exchange with Zn2+, Cd2+, and Pb2+. All the colloidal spheres prepared using this method are monodispersed in size and characterized by a spherical shape and a smooth surface. Starting from the same batch of Se@Ag2Se, the resultant Se@MSe samples were essentially the same in size. Furthermore, these core−shell colloidal spheres can be easily made superparamagnetic by incorporating Fe3O4 nanoparticles into the a-Se cores. This synthetic approach provides a simple and versatile route to magnetoactive core−shell spheres with the same size but a range of different compositions. This study also implies that it ...

Facile synthesis of tadpole-like nanostructures consisting of Au heads and Pd tails.

Abstract: This communication describes an investigation of the galvanic replacement reaction between single-crystal Pd nanorods and AuCl4- ions. We found that the Au atoms resulting from the galvanic replacement reaction did not coat the entire surface of a Pd nanorod to generate a core-sheath or hollow nanostructure. In the earlier stages of the reaction, Au deposition was localized to both ends of the Pd nanorod. Then, a transition from two-end to one-end growth was observed, producing a new type of hybrid nanostructure in the shape of a tadpole consisting of a Au head and a Pd tail. Beyond this point, the Au served as nucleation sites for further Au deposition until the hybrid nanostructure was dismantled into round Au nanoparticles and smaller/shorter Pd fragments.

Microscale fish bowls: a new class of latex particles with hollow interiors and engineered porous structures in their surfaces.

Abstract: Microscale fish bowls, hollow particles with engineered holes in their surfaces, were prepared using two different methods. In the first method, commercial latex beads suspended in water were swollen with a good solvent of the polymer, followed by freezing with liquid nitrogen and evaporation of the solvent below 0 °C. While one big hole was generated when the amount of solvent used for the swelling was relatively low, small holes could be produced in the outer surface of each bowl by increasing the degree of swelling. The porosity and pore structure show a similar dependence on the degree of swelling for both amorphous and semicrystalline polymers even though they are supposed to exhibit different phase behaviors during the freezing and solvent evaporation processes. In the second method, a polymer emulsion in water was prepared and then frozen with liquid nitrogen, followed by solvent evaporation below 0 °C. The porosity and pore structure could be controlled by adjusting the concentration of the polyme...

Correlated Rayleigh Scattering Spectroscopy and Scanning Electron Microscopy Studies of Au−Ag Bimetallic Nanoboxes and Nanocages

Abstract: The optical properties of hollow nanoparticles (Au-Ag nanoboxes and nanocages) were investigated by recording Rayleigh scattering spectra of single particles, whose morphology and composition had been analyzed by scanning electron microscopy (SEM). This was achieved by depositing the particles on optically transparent substrates with registration marks, which are compatible with SEM imaging. Fitting the experimental spectra to a Lorentzian function yields the frequencies and homogeneous line widths of the plasmon resonance for the particles. The resonances are extremely broad, with dephasing times of 2-5 fs. Analysis of the line width data using the dimensions determined by SEM shows that the broadening is due to a combination of electron-surface scattering and radiation damping. The sensitivity of the plasmon resonance to the dielectric constant of the environment was also investigated by adding a drop of water to the substrate. The nanoboxes show similar dielectric sensitivities compared to other metal nanoparticle systems. A significant increase in the line width was also observed for the nanoboxes in water compared with air. This was attributed to increased radiation damping in the environment with a higher dielectric constant. Both the red shift and the increase in line width are reversible.

Time-resolved spectroscopy of silver nanocubes: Observation and assignment of coherently excited vibrational modes

Abstract: The response of single crystal, cubic silver particles to ultrafast laser-induced heating has been examined experimentally and theoretically. The transient absorption traces display clear modulations due to coherently excited vibrational modes. Nanocube samples with edge lengths smaller than 50 nm show a single modulation, whereas samples larger than 50 nm show two vibrational modes. The results are compared to finite element calculations, where the cubes are modeled as having cubic crystal symmetry with the principal axes parallel to the sides of the particle. The action of the laser pulse is treated in two ways, first, as creating a uniform initial strain. In this case the predominant mode excited is the breathing mode. The period of this mode is in reasonable agreement with the vibrational periods measured for the smaller cubes and with the higher frequency modulation observed for the larger cubes. A nonuniform initial strain is also considered, which could arise from nonuniform heating for particles larger than the optical skin depth of the metal. In this case the predominant mode excited is a nontotally symmetric mode. The calculated periods from this analysis are in reasonable agreement with the lower frequency modulations observed for the larger samples. The results from this study show that, to within the accuracy of these measurements, the elastic constants of cubic silver nanoparticles are the same as bulk silver.

Galvanic replacement reaction: A simple and powerful route to hollow and porous metal nanostructures

Abstract: Galvanic replacement reaction provides a remarkably simple and versatile route to metal nanostructures with controllable hollow interiors and porous walls. The key step of this process involves the replacement reaction between a suspension of nanoscale metal templates and a salt precursor containing a relatively less active metal. This method has been successfully applied to prepare gold-based hollow nanostructures with a wide range of different morphologies, including cubic nanoboxes, cubic nanocages, triangular nanorings, prism-shaped nanoboxes, single-walled nanotubes, and multiple-walled nanoshells or nanotubes. In addition to gold, hollow platinum and palladium nanostructures have also been prepared by using appropriate salt precursors for the replacement reaction. These hollow and porous metal nanostructures show intriguing optical and mechanical properties, with their surface plasmon resonance peaks tunable from the visible to the near-infrared region. These materials are expected to find a...