Showing papers by "Younan Xia published in 2011"

Controlling the synthesis and assembly of silver nanostructures for plasmonic applications

Abstract: Coinage metals, such as Au, Ag, and Cu, have been important materials throughout history.1 While in ancient cultures they were admired primarily for their ability to reflect light, their applications have become far more sophisticated with our increased understanding and control of the atomic world. Today, these metals are widely used in electronics, catalysis, and as structural materials, but when they are fashioned into structures with nanometer-sized dimensions, they also become enablers for a completely different set of applications that involve light. These new applications go far beyond merely reflecting light, and have renewed our interest in maneuvering the interactions between metals and light in a field known as plasmonics.2–6 In plasmonics, the metal nanostructures can serve as antennas to convert light into localized electric fields (E-fields) or as waveguides to route light to desired locations with nanometer precision. These applications are made possible through a strong interaction between incident light and free electrons in the nanostructures. With a tight control over the nanostructures in terms of size and shape, light can be effectively manipulated and controlled with unprecedented accuracy.3,7 While many new technologies stand to be realized from plasmonics, with notable examples including superlenses,8 invisible cloaks,9 and quantum computing,10,11 conventional technologies like microprocessors and photovoltaic devices could also be made significantly faster and more efficient with the integration of plasmonic nanostructures.12–15 Of the metals, Ag has probably played the most important role in the development of plasmonics, and its unique properties make it well-suited for most of the next-generation plasmonic technologies.16–18 1.1. What is Plasmonics? Plasmonics is related to the localization, guiding, and manipulation of electromagnetic waves beyond the diffraction limit and down to the nanometer length scale.4,6 The key component of plasmonics is a metal, because it supports surface plasmon polariton modes (indicated as surface plasmons or SPs throughout this review), which are electromagnetic waves coupled to the collective oscillations of free electrons in the metal. While there are a rich variety of plasmonic metal nanostructures, they can be differentiated based on the plasmonic modes they support: localized surface plasmons (LSPs) or propagating surface plasmons (PSPs).5,19 In LSPs, the time-varying electric field associated with the light (Eo) exerts a force on the gas of negatively charged electrons in the conduction band of the metal and drives them to oscillate collectively. At a certain excitation frequency (w), this oscillation will be in resonance with the incident light, resulting in a strong oscillation of the surface electrons, commonly known as a localized surface plasmon resonance (LSPR) mode.20 This phenomenon is illustrated in Figure 1A. Structures that support LSPRs experience a uniform Eo when excited by light as their dimensions are much smaller than the wavelength of the light. Figure 1 Schematic illustration of the two types of plasmonic nanostructures discussed in this article as excited by the electric field (Eo) of incident light with wavevector (k). In (A) the nanostructure is smaller than the wavelength of light and the free electrons ... In contrast, PSPs are supported by structures that have at least one dimension that approaches the excitation wavelength, as shown in Figure 1B.4 In this case, the Eo is not uniform across the structure and other effects must be considered. In such a structure, like a nanowire for example, SPs propagate back and forth between the ends of the structure. This can be described as a Fabry-Perot resonator with resonance condition l=nλsp, where l is the length of the nanowire, n is an integer, and λsp is the wavelength of the PSP mode.21,22 Reflection from the ends of the structure must also be considered, which can change the phase and resonant length. Propagation lengths can be in the tens of micrometers (for nanowires) and the PSP waves can be manipulated by controlling the geometrical parameters of the structure.23

Gold Nanocages: From Synthesis to Theranostic Applications

Abstract: Gold nanostructures have garnered considerable attention in recent years for their potential to facilitate both the diagnosis and treatment of cancer through their advantageous chemical and physical properties. The key feature of Au nanostructures for enabling this diverse array of biomedical applications is their attractive optical properties, specifically the scattering and absorption of light at resonant wavelengths due to the excitation of plasmon oscillations. This phenomenon is commonly known as localized surface plasmon resonance (LSPR) and is the source of the ruby red color of conventional Au colloids. The resonant wavelength depends on the size, shape, and geometry of the nanostructures, providing a set of knobs to manipulate the optical properties as needed. For in vivo applications, especially when optical excitation or transduction is involved, the LSPR peaks of the Au nanostructures have to be tuned to the transparent window of soft tissues in the near-infrared (NIR) region (from 700 to 900 ...

Gold nanostructures: a class of multifunctional materials for biomedical applications

Abstract: Gold nanostructures have proven to be a versatile platform for a broad range of biomedical applications, with potential use in numerous areas including: diagnostics and sensing, in vitro and in vivo imaging, and therapeutic techniques. These applications are possible because of the highly favorable properties of gold nanostructures, many of which can be tailored for specific applications. In the first part of this tutorial review, we will discuss the most critical properties of gold nanostructures for biomedical applications: surface chemistry, localized surface plasmon resonance (LSPR), and morphology. In the second part of the review, we will discuss how these properties can be harnessed for a selection of biomedical applications, aiming to give the reader an overview of general strategies as well as highlight some recent advances in this field.

The effect of sedimentation and diffusion on cellular uptake of gold nanoparticles

Abstract: In vitro experiments typically measure the uptake of nanoparticles by exposing cells at the bottom of a culture plate to a suspension of nanoparticles, and it is generally assumed that this suspension is well-dispersed. However, nanoparticles can sediment, which means that the concentration of nanoparticles on the cell surface may be higher than the initial bulk concentration, and this could lead to increased uptake by cells. Here, we use upright and inverted cell culture configurations to show that cellular uptake of gold nanoparticles depends on the sedimentation and diffusion velocities of the nanoparticles and is independent of size, shape, density, surface coating and initial concentration of the nanoparticles. Generally, more nanoparticles are taken up in the upright configuration than in the inverted one, and nanoparticles with faster sedimentation rates showed greater differences in uptake between the two configurations. Our results suggest that sedimentation needs to be considered when performing in vitro studies for large and/or heavy nanoparticles.

Metal nanocrystals with highly branched morphologies.

Abstract: Metal nanocrystals with highly branched morphologies are an exciting new class of nanomaterials owing to their unique structures, physicochemical properties, and great potential as catalysts, sensing materials, and building blocks for nanoscale devices. Various strategies have recently been developed for the solution-phase synthesis of metal nanocrystals with branched morphologies, such as multipods and nanodendrites. In this Minireview, the procedures and mechanisms underlying the formation of branched metal nanocrystals are presented in parallel with recent advances in synthetic approaches based on kinetically controlled overgrowth, aggregation-based growth, heterogeneous seeded growth, selective etching, and template-directed methods, as well as their properties for catalytic or electrocatalytic applications.

Platinum concave nanocubes with high-index facets and their enhanced activity for oxygen reduction reaction

Abstract: Platinum nanoparticles are widely used as the primary catalysts in a myriad of industrial processes such as CO/NOx oxidation in catalytic converters, nitric acid production, petroleum cracking, as well as hydrogen (or alcohol) oxidation and oxygen reduction reactions in fuel-cell technology. For most catalytic reactions, it has been shown that high-index planes, which are associated with large numbers of atomic steps, edges, and kinks hold the key to the enhancement of catalytic performance in terms of activity and/or selectivity. A number of protocols have been demonstrated for generating Pt nanoparticles enclosed by high-index facets, including those based on electrochemical reduction and heat treatment. For example, Sun and co-workers have reported the synthesis of tetrahexahedral (THH) Pt nanocrystals with high-index facets, such as {730}, {210}, and {520}, by applying a square-wave potential to polycrystalline Pt microspheres supported on a glassy carbon electrode. Although these Pt nanocrystals have been shown to have high catalytic activity, their sizes are still relatively too large and the method of preparation is rather limited in terms of production volume. It still remains a challenge to produce Pt nanocrystals with high-index facets by using a simple, scalable route based on wet chemical reduction. Over the past several years, kinetic control has been demonstrated as a simple and versatile approach to the shapecontrolled synthesis of noble-metal nanocrystals in the solution phase. In general, kinetic control can be achieved by: 1) substantially slowing down the formation rate of atoms, 2) using a weak reducing agent, 3) introducing an oxidation process, and 4) taking advantage of Ostwald ripening. When the concentration of metal atoms in the solution is low, the atoms tend to add to the edges and corners of a seed rather than the entire surface, thus leading to the formation of nanocrystals with thermodynamically unfavorable morphologies, including rods, plates, multipods, and dendritic structures. In recent years, nanocrystals with concave rather than flat faces have attracted attention because of their high-index facets. To this end, Zheng and co-workers have demonstrated the synthesis of concave Pd polyhedral nanocrystals with high electrocatalytic activity for formic acid oxidation. Mirkin and co-workers have also reported the synthesis of concave cubic Au nanocrystals, and demonstrated higher chemical activity compared to octahedra enclosed by lowindex {111} facets. Herein we report the first synthesis of Pt concave nanocubes enclosed by high-index facets including {510}, {720}, and {830} by slowly adding an aqueous NaBH4 solution and a mixture containing K2PtCl4, KBr, and Na2H2P2O7 into deionized water by using two syringe pumps. In this synthesis, the formation of a Pt pyrophosphato complex (that is formed by mixing K2PtCl4 and Na2H2P2O7) and the slow addition of this precursor by a syringe pump are believed to play a key role in the formation of Pt concave nanocubes. In this case, the seeds selectively overgrow from corners and edges, and the Br ion serves as a capping agent to block the growth of the h100i axis. The Pt concave nanocubes exhibited substantially enhanced specific activity (per unit surface area) relative to those of Pt nanocubes, cuboctahedra, and commercial Pt/C catalysts that are bounded by low-index facets such as {100} and {111} toward the oxygen reduction reaction (ORR), which is the ratedetermining step in a proton-exchangemembrane (PCM) fuel cell. In a typical synthesis, an aqueous NaBH4 solution and a mixture containing K2PtCl4, KBr, and Na2H2P2O7 were prepared separately and then injected simultaneously at an injection rate of 67 mLmin 1 by using two syringe pumps into deionized water maintained at 95 8C. The color of the solution immediately turned from light pink to black upon the addition of the reactant solutions, thus indicating rapid reduction of PtCl4 2 into elemental Pt by NaBH4. Figure 1a shows a typical transmission electron microscopy (TEM) image of the product that contains Pt nanocubes with a concave structure. [*] Dr. T. Yu, D. Y. Kim, Prof. H. Zhang, Prof. Y. Xia Department of Biomedical Engineering Washington University Saint Louis, MO 63130 (USA) E-mail: xia@biomed.wustl.edu D. Y. Kim Department of Chemical and Biomolecular Engineering (BK21 graduate program) Korea Advanced Institute of Science and Technology (KAIST) 335 Gwahangro, Yuseong-gu, Daejeon 305-701 (Korea)

Shape-Controlled Synthesis of Copper Nanocrystals in an Aqueous Solution with Glucose as a Reducing Agent and Hexadecylamine as a Capping Agent†

Abstract: NSF [DMR 0804088, 1104614, ECS-0335765]; Washington University in St. Louis; China Scholarship Council (CSC)

Synthesis of Pd Nanocrystals Enclosed by {100} Facets and with Sizes < 10 nm for Application in CO Oxidation

Abstract: The catalytic activity of noble-metal nanocrystals is mainly determined by their sizes and the facets exposed on the surface. For single crystals, it has been demonstrated that the Pd(100) surface is catalytically more active than both Pd(110) and Pd(111) surfaces for the CO oxidation reaction. Here we report the synthesis of Pd nanocrystals enclosed by {100} facets with controllable sizes in the range of 6–18 nm by manipulating the rate of reduction of the precursor. UV-vis spectroscopy studies indicate that the rate of reduction of Na2PdCl4 can be controlled by adjusting the concentrations of Br− and Cl− ions added to the reaction mixture. Pd nanocrystals with different sizes were immobilized on ZnO nanowires and evaluated as catalysts for CO oxidation. We found that the activity of this catalytic system for CO oxidation showed a strong dependence on the nanocrystal size. When the size of the Pd nanocrystals was reduced from 18 nm to 6 nm, the maximum conversion rate was significantly enhanced by a factor of ∼10 and the corresponding maximum conversion temperature was lowered by ∼80 °C.

Synthesis of Pd−Pt Bimetallic Nanocrystals with a Concave Structure through a Bromide-Induced Galvanic Replacement Reaction

Abstract: This article describes a systematic study of the galvanic replacement reaction between PtCl62− ions and Pd nanocrystals with different shapes, including cubes, cuboctahedrons, and octahedrons. It was found that Br− ions played an important role in initiating, facilitating, and directing the replacement reaction. The presence of Br− ions led to the selective initiation of galvanic replacement from the {100} facets of Pd nanocrystals, likely due to the preferential adsorption of Br− ions on this crystallographic plane. The site-selective galvanic replacement resulted in the formation of Pd−Pt bimetallic nanocrystals with a concave structure owing to simultaneous dissolution of Pd atoms from the {100} facets and deposition of the resultant Pt atoms on the {111} facets. The Pd−Pt concave nanocubes with different weight percentages of Pt at 3.4, 10.4, 19.9, and 34.4 were also evaluated as electrocatalysts for the oxygen reduction reaction (ORR). Significantly, the sample with a 3.4 wt.% of Pt exhibited the lar...

Palladium Concave Nanocubes with High-Index Facets and Their Enhanced Catalytic Properties

Abstract: NSF[DMR-0804088, ECS-0335765]; Washington University in St. Louis; China Scholarship Council (CSC)

A New Theranostic System Based on Gold Nanocages and Phase-Change Materials with Unique Features for Photoacoustic Imaging and Controlled Release

Abstract: This communication reports a new theranostic system with a combination of capabilities to both enhance the contrast of photoacoustic (PA) imaging and control the release of a chemical or biological effector by high-intensity focused ultrasound (HIFU). The fabrication of this system simply involves filling the hollow interiors of gold nanocages with a phase-change material (PCM) such as 1-tetradecanol that has a melting point of 38−39 °C. The PCM can be premixed and thus loaded with a dye, as well as other chemical or biological effectors. When exposed to direct heating or HIFU, the PCM will melt and escape from the interiors of nanocages through small pores on the surface, concurrently releasing the encapsulated molecules into the surrounding medium. We can control the release profile by varying the power of HIFU, the duration of exposure to HIFU, or both.

Structure sensitivity of alkynol hydrogenation on shape- and size-controlled palladium nanocrystals: which sites are most active and selective?

Abstract: The activity and selectivity of structure-sensitive reactions are strongly correlated with the shape and size of the nanocrystals present in a catalyst. This correlation can be exploited for rational catalyst design, especially if each type of surface atom displays a different behavior, to attain the highest activity and selectivity. In this work, uniform Pd nanocrystals with cubic (in two different sizes), octahedral, and cuboctahedral shapes were synthesized through a solution-phase method with poly(vinyl pyrrolidone) (PVP) serving as a stabilizer and then tested in the hydrogenation of 2-methyl-3-butyn-2-ol (MBY). The observed activity and selectivity suggested that two types of active sites were involved in the catalysis--those on the planes and at edges--which differ in their coordination numbers. Specifically, semihydrogenation of MBY to 2-methyl-3-buten-2-ol (MBE) occurred preferentially at the plane sites regardless of their crystallographic orientation, Pd(111) and/or Pd(100), whereas overhydrogenation occurred mainly at the edge sites. The experimental data can be fit with a kinetic modeling based on a two-site Langmuir-Hinshelwood mechanism. By considering surface statistics for nanocrystals with different shapes and sizes, the optimal catalyst in terms of productivity of the target product MBE was predicted to be cubes of roughly 3-5 nm in edge length. This study is an attempt to close the material and pressure gaps between model single-crystal surfaces tested under ultra-high-vacuum conditions and real catalytic systems, providing a powerful tool for rational catalyst design.

Ceramic nanofibers fabricated by electrospinning and their applications in catalysis, environmental science, and energy technology

Abstract: This paper provides a brief review of current research activities that focus on the fabrication of ceramic nanofibers by electrospinning, as well as their applications in various areas. We begin with a brief introduction to electrospinning in the context of ceramic nanofibers, and the methods for preparing aligned and/or hollow nanofibers. We then discuss approaches to the fabrication of nanofibers with a hierarchical structure. We continue with a highlight of some recent applications enabled by electrospun ceramic nanofibers, with a focus on three areas: catalysis, environmental science, and energy technology, which are expected to become the most important and exciting subjects of research in this century. In the end, we conclude this review with some perspectives on the future directions and implications for this new class of functional nanomaterials. Copyright © 2010 John Wiley & Sons, Ltd.

Label-free oxygen-metabolic photoacoustic microscopy in vivo

Abstract: Almost all diseases, especially cancer and diabetes, manifest abnormal oxygen metabolism. Accurately measuring the metabolic rate of oxygen (MRO2) can be helpful for fundamental pathophysiological studies, and even early diagnosis and treatment of disease. Current techniques either lack high resolution or rely on exogenous contrast. Here, we propose label-free metabolic photoacoustic microscopy (mPAM) with small vessel resolution to noninvasively quantify MRO2 in vivo in absolute units. mPAM is the unique modality for simultaneously imaging all five anatomical, chemical, and fluid-dynamic parameters required for such quantification: tissue volume, vessel cross-section, concentration of hemoglobin, oxygen saturation of hemoglobin, and blood flow speed. Hyperther- mia, cryotherapy, melanoma, and glioblastoma were longitudinally imaged in vivo. Counterintuitively, increased MRO2 does not necessarily cause hypoxia or increase oxygen extraction. In fact, early-stage cancer was found to be hyperoxic despite hypermetabolism. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE). (DOI: 10.1117/1.3594786)

Generation of Hot Spots with Silver Nanocubes for Single-Molecule Detection by Surface-Enhanced Raman Scattering

Abstract: This paper presents a simple strategy for the formation of surface-enhanced Raman scattering (SERS) hot spots, or regions with extraordinary large electric-field enhancements, by depositing a silver nanocube on a metal substrate. Our experimental and theoretical results show that hot spots form at the corners of a nanocube in contact with the substrate and the hot spots derived from a single silver nanocube are capable of detecting SERS from a single molecule. By varying the electrical property of the substrate, and the distance between the nanoparticle and the substrate, we show that the substrate can dramatically affect the SERS from a supported nanoparticle. In addition, by comparing the SERS for nanocubes and nanospheres of similar sizes, we show that this effect is also sensitive to the shape of the supported nanoparticle, and enhancement factors of 9.7×106 and 2.1×108 were obtained for a nanosphere and a nanocube on a metal substrate, respectively. This new approach requires minimum fabrication efforts and offers great simplicity for the formation of robust and fully accessible hot spots, providing an effective SERS platform for single-molecule detection.

Facile Synthesis of Pd-Pt Alloy Nanocages and Their Enhanced Performance for Preferential Oxidation of CO in Excess Hydrogen

Abstract: This article describes a new method for the facile synthesis of Pd–Pt alloy nanocages with hollow interiors and porous walls by using Pd nanocubes as sacrificial templates. Differing from our previous work (Zhang, H.; Jin, M. S.; Wang, J. G.; Li, W. Y.; Camargo, P. H. C.; Kim, M. J.; Yang, D. R.; Xie, Z. X.; Xia, Y. Synthesis of Pd-Pt Bimetallic Nanocrystals with a Concave Structure through a Bromide-Induced Galvanic Replacement Reaction. J. Am. Chem. Soc.2011, 133, 6078–6079), we complemented the galvanic replacement (between Pd nanocubes and PtCl42–) with a coreduction process (for PdCl42– from the galvanic reaction and PtCl42– from the feeding) to generate Pd–Pt alloy nanocages in one step. We found that the rate of galvanic replacement (as determined by the concentrations of Br– and PtCl42– and temperature) and the rates of coreduction (as determined by the type of reductant and temperature) played important roles in controlling the morphology of resultant Pd–Pt alloy nanocages. The Pd–Pt nanocages ex...

Structural Dependence of Oxygen Reduction Reaction on Palladium Nanocrystals

Abstract: We have synthesized sub-10 nm Pd cubic and octahedral nanocrystals and then evaluated their activities towards oxygen reduction reaction (ORR). The ORR activity of Pd nanocubes was one order of magnitude higher than that of Pd octahedra, and comparable to that of the state-of-the-art Pt catalysts.

Successive Deposition of Silver on Silver Nanoplates: Lateral versus Vertical Growth

Abstract: NSF [DMR-0804088, ECS-0335765]; Washington University in St. Louis; Ministry of Education, Science and Technology [R32-20031]; China Scholarship Council

UV–Ozone Cleaning of Supported Poly(vinylpyrrolidone)-Stabilized Palladium Nanocubes: Effect of Stabilizer Removal on Morphology and Catalytic Behavior

Abstract: Poly(vinylpyrrolidone) (PVP)-stabilized Pd nanocubes were synthesized, deposited on a carbon-based support, and subsequently treated with UV-ozone (UVO) in order to eliminate the traces of PVP still present on the surface. Cubes, being a thermodynamically unfavorable shape, are very prone to restructuring to minimize the interfacial free energy and thus allow the assessment of their morphological stability during UVO cleaning. The process of PVP removal was monitored by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and in situ attenuated total reflection infrared spectroscopy (ATR-IR). High-resolution scanning electron microscopy (SEM) imaging was used to evaluate the morphology of the nanocubes. The effect of PVP removal was also studied in the hydrogenation of acetylene, showing a 4-fold increase of activity. This method can be applied to nanoparticles of other common shapes, which expose different crystal planes, in order to study the structure sensitivity of chemical reactions.

Silver nanocrystals with concave surfaces and their optical and surface-enhanced Raman scattering properties.

Abstract: Shaped and dimpled: Silver nanocrystals enclosed by concave surfaces and thus high-index facets have been prepared by simply controlling the growth habit of Ag cubic seeds. Four types of concave nanocrystals, including octahedron, cube, octapod, and trisoctahedron, were obtained (see picture).

Controlling the Morphology of Rhodium Nanocrystals by Manipulating the Growth Kinetics with a Syringe Pump

Abstract: Noble-metal nanocrystals with well-defined and controllable morphologies are of great importance to applications in catalysis, plasmonics, and surface-enhanced spectroscopy. Many synthetic approaches have been demonstrated for controlling the growth habit and thus morphology of metal nanocrystals, but most of them are based on a thermodynamic approach, including the use of a capping agent. While thermodynamic control has shown its power in generating nanocrystals with a myriad of different morphologies, it is ultimately limited by the obligation to minimize the surface energy of a system. As a result, it is impractical to use thermodynamic control to generate nanocrystals having high-energy facets and/or a negative curvature. Using rhodium as an example, here we demonstrate a general method based on kinetic control with a syringe pump that can be potentially extended to other noble metals and even other solid materials. For the first time, we were able to produce concave nanocubes with a large fraction of...

Ordered Zigzag Stripes of Polymer Gel/Metal Nanoparticle Composites for Highly Stretchable Conductive Electrodes

Abstract: Stretchable electrodes have gained ever-increasing interest for a wide range of applications including smart clothing, [ 1 ] dielectric elastomer actuators (DEAs), [ 2 ] stretchable and rollable displays, [ 3 ] and fl exible electronics. [ 4 ] The required strain varies depending on the application. For example, a stretchable display needs about 10 to 30% strain, but more than 100% strain is desired for DEAs. Realization of stretchable electrodes has been explored with a number of different approaches. One of them is to generate the stretchable structures of metals. [ 5 ] Inplane S-shaped [ 6 ] or z-shaped [ 7 ] metal strips, or out-of-plane wavy geometry [ 8 ] can have a large net elongation. Although a large strain ( ≈ 60%) without electrical failure has been reported, fabrication of a fi ne-structured electrode is yet to be achieved. Another approach is to use a composite material made of an elastomer and conducting metal fi llers or carbon nanotubes. [ 9 ]

Smart Multifunctional Hollow Microspheres for the Quick Release of Drugs in Intracellular Lysosomal Compartments

Abstract: Prepared to self-destruct: when poly(D, L-lactic-co-glycolic acid) (PLGA) hollow microspheres containing NaHCO(3) entered the endocytic organelles of a live cell, the NaHCO(3) in the aqueous core reacted with protons that infiltrated from the compartment to generate CO(2) gas. The evolution of CO(2) bubbles led to the formation of small holes in the PLGA shell and thus rapid release of the encapsulated drug doxorubicin.

Gold nanocages covered with thermally-responsive polymers for controlled release by high-intensity focused ultrasound

Abstract: This paper describes the use of Au nanocages covered with smart, thermally-responsive polymers for controlled release with high-intensity focused ultrasound (HIFU). HIFU is a highly precise medical procedure that uses focused ultrasound to heat and destroy pathogenic tissue rapidly and locally in a non-invasive or minimally invasive manner. The released dosage could be remotely controlled by manipulating the power of HIFU and/or the duration of exposure. We demonstrated localized release within the focal volume of HIFU by using gelatin phantom samples containing dye-loaded Au nanocages. By placing chicken breast tissues on top of the phantoms, we further demonstrated the feasibility of this system for controlled release at depths up to 30 mm. Because it can penetrate more deeply into soft tissues than near-infrared light, HIFU is a potentially more effective external stimulus for rapid, on-demand drug release.

Nanocrystals Composed of Alternating Shells of Pd and Pt Can Be Obtained by Sequentially Adding Different Precursors

Abstract: This paper describes a layer-by-layer epitaxial approach to the synthesis of multishelled nanocrystals composed of alternating shells of Pd and Pt by starting with seeds made of Pd or Pt nanocrystals. The synthesis was conducted by sequentially adding PtCl42– and PdCl42– salt precursors into a system containing either Pd or Pt seeds (in the shape of cuboctahedrons, octahedrons, plates, or cubes) together with a weak reducing agent such as citric acid (CA). The slow reduction kinetics associated with CA played an important role in the epitaxial growth of one metal on the other, resulting in the formation of Pd–Pt multishelled nanocrystals. Owing to the capping effect of CA for {111} facets of Pd and Pt, the multishelled nanocrystals tended to be enclosed by {111} facets in the form of octahedrons or thin plates, depending on the shapes of the Pd or Pt seeds: octahedrons for cuboctahedral, cubic, or octahedral seeds, and plates for platelike seeds.

In vivo integrated photoacoustic and confocal microscopy of hemoglobin oxygen saturation and oxygen partial pressure

Abstract: We developed dual-modality microscope integrating photoacoustic microscopy (PAM) and fluorescence confocal microscopy (FCM) to noninvasively image hemoglobin oxygen saturation (sO2) and oxygen partial pressure (pO2) in vivo in single blood vessels with high spatial resolution. While PAM measures sO2 by imaging hemoglobin optical absorption at two wavelengths, FCM quantifies pO2 using phosphorescence quenching. The variations of sO2 and pO2 values in multiple orders of vessel branches under hyperoxic (100% oxygen) and normoxic (21% oxygen) conditions correlate well with the oxygen–hemoglobin dissociation curve. In addition, the total concentration of hemoglobin is imaged by PAM at an isosbestic wavelength.

Selective sulfuration at the corner sites of a silver nanocrystal and its use in stabilization of the shape.

Abstract: This paper describes a new approach to site-selective sulfuration at the corner sites of Ag nanocrystals including triangular nanoplates and nanocubes. The reaction simply involved mixing an aqueous suspension of the Ag nanocrystals with an aqueous solution of polysulfide at room temperature. As a precursor to elemental S, polysulfide is highly soluble in water and can directly react with elemental Ag upon contact to generate Ag2S in the absence of oxygen. The reaction was easily initiated at the corner sites and then pushed toward the center. By controlling the reaction time and/or the amount of polysulfide added, the reaction could be confined to the corner sites only, generating Ag–Ag2S hybrid nanocrystals with greatly improved stability against aging at 80 and 100 °C in air than their counterparts made of pure Ag.

Gold nanocages as contrast agents for photoacoustic imaging

Abstract: Gold nanoparticles with tunable absorption and scattering properties have been developed as contrast agents for various optical imaging techniques. As a hybrid modality that combines the merits of both optical and ultrasonic imaging, photoacoustic (PA) imaging also benefits from the use of these nanoparticles to greatly enhance the contrast for visualization of structures and biomarkers in biological tissues. Gold nanocages characterized by hollow interiors, ultrathin and porous walls are of particular interest for in vivo PA imaging because of their compact sizes, bio-inertness and well-defined surface chemistry, as well as their strong and highly wavelength-tunable optical absorption in the near-infrared (NIR) optical window of soft tissues. This review discusses the application of gold nanocages as a new class of contrast agents for PA imaging in the context of cancer diagnosis.