Showing papers by "Younan Xia published in 2009"
TL;DR: A comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals, including a brief introduction to nucleation and growth within the context of metal Nanocrystal synthesis, followed by a discussion of the possible shapes that aMetal nanocrystal might take under different conditions.
Abstract: Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.
4,927 citations
TL;DR: Pd-Pt bimetallic nanodendrites consisting of a dense array of Pt branches on a Pd core by reducing K2PtCl4 with L-ascorbic acid in the presence of uniform Pd nanocrystal seeds in an aqueous solution showed relatively large surface areas and particularly active facets toward the oxygen reduction reaction (ORR), the rate-determining step in a proton-exchange membrane fuel cell.
Abstract: Controlling the morphology of Pt nanostructures can provide a great opportunity to improve their catalytic properties and increase their activity on a mass basis. We synthesized Pd-Pt bimetallic nanodendrites consisting of a dense array of Pt branches on a Pd core by reducing K2PtCl4 with L-ascorbic acid in the presence of uniform Pd nanocrystal seeds in an aqueous solution. The Pt branches supported on faceted Pd nanocrystals exhibited relatively large surface areas and particularly active facets toward the oxygen reduction reaction (ORR), the rate-determining step in a proton-exchange membrane fuel cell. The Pd-Pt nanodendrites were two and a half times more active on the basis of equivalent Pt mass for the ORR than the state-of-the-art Pt/C catalyst and five times more active than the first-generation supportless Pt-black catalyst.
2,695 citations
TL;DR: This work develops a platform based on the photothermal effect of gold nanocages that works well with various effectors without involving sophiscated syntheses, and is well-suited for in vivo studies due to the high transparency of soft tissue in NIR.
Abstract: Photosensitive caged compounds have enhanced our ability to address the complexity of biological systems by generating effectors with remarkable spatial/temporal resolutions. The caging effect is typically removed by photolysis with ultraviolet light to liberate the bioactive species. Although this technique has been successfully applied to many biological problems, it suffers from a number of intrinsic drawbacks. For example, it requires dedicated efforts to design and synthesize a precursor compound for each effector. The ultraviolet light may cause damage to biological samples and is suitable only for in vitro studies because of its quick attenuation in tissue. Here we address these issues by developing a platform based on the photothermal effect of gold nanocages. Gold nanocages represent a class of nanostructures with hollow interiors and porous walls. They can have strong absorption (for the photothermal effect) in the near-infrared while maintaining a compact size. When the surface of a gold nanocage is covered with a smart polymer, the pre-loaded effector can be released in a controllable fashion using a near-infrared laser. This system works well with various effectors without involving sophisticated syntheses, and is well suited for in vivo studies owing to the high transparency of soft tissue in the near-infrared region.
1,305 citations
TL;DR: A number of chemical routes have been developed to produce Pt nanocrystals with well-defined and controllable shapes to improve their performance in terms of catalytic activity and selectivity as mentioned in this paper.
791 citations
TL;DR: An etching solution based on I2 and KI that can selectively dissolve the Au nanospheres on the cell surface within a short period of time is introduced that is capable of etching away a relatively large amount of Au Nanospheres at a low molar concentration.
Abstract: This Letter presents a new method for differentiating the Au nanospheres attached to the cell surface from those being internalized into the cells. We introduced an etching solution based on I2 and KI that can selectively dissolve the Au nanospheres on the cell surface within a short period of time. The advantage of this etchant is its low toxicity to the cells because it is capable of etching away a relatively large amount of Au nanospheres at a low molar concentration. By combining with quantitative elemental analysis, we found that the deposition of Au nanospheres on the surface of cancer cells was highly dependent on the sign of surface charges on the Au nanospheres. In addition, by fitting the uptake data with a kinetic model, we were able to derive the overall and internalization rate constants for Au nanospheres and both of them were found to be governed by the surface charges on Au nanospheres.
746 citations
TL;DR: This work illustrates how oxidative etching and kinetic control can be employed to manipulate the shapes and optical responses of plasmonic nanoparticles made of either Ag or Pd.
Abstract: Under the irradiation of light, the free electrons in a plasmonic nanoparticle are driven by the alternating electric field to collectively oscillate at a resonant frequency in a phenomenon known as surface plasmon resonance. Both calculations and measurements have shown that the frequency and amplitude of the resonance are sensitive to particle shape, which determines how the free electrons are polarized and distributed on the surface. As a result, controlling the shape of a plasmonic nanoparticle represents the most powerful means of tailoring and fine-tuning its optical resonance properties. In a solution-phase synthesis, the shape displayed by a nanoparticle is determined by the crystalline structure of the initial seed produced and the interaction of different seed facets with capping agents. Using polyol synthesis as a typical example, we illustrate how oxidative etching and kinetic control can be employed to manipulate the shapes and optical responses of plasmonic nanoparticles made of either Ag or Pd. We conclude by highlighting a few fundamental studies and applications enabled by plasmonic nanoparticles having well-defined and controllable shapes.
635 citations
TL;DR: A simple, one-pot method that generates dimers of silver nanospheres in one step without any additional assembly steps that provides a well-defined system for studying the hot spot phenomenon in surface-enhanced Raman scattering (SERS).
Abstract: This paper describes a simple, one-pot method that generates dimers of silver nanospheres in one step without any additional assembly steps. The dimers are consisted of single-crystal silver nanospheres ∼30 nm in diameter and separated by a gap of 1.8 nm wide. The key to the success of this method lies in the control of colloidal stability and oxidative etching by optimizing the amount of chloride added to a polyol synthesis. The dimers provide a well-defined system for studying the hot spot phenomenon (hot spot: the gap region of a pair of strongly coupled silver or gold nanoparticles), an extremely important but poorly understood subject in surface-enhanced Raman scattering (SERS). Because of the relatively small size of the silver nanospheres, only those molecules trapped in the hot spot region are expected to contribute to the detected SERS signals. By correlating SERS measurements with SEM imaging, we found that the SERS enhancement factor within the hot spot region of such a dimer was on the order o...
591 citations
TL;DR: In this article, the shape control of Pd nanocrystals with well-controlled shapes in aqueous solutions is discussed. But the shape of the final shape taken by a nanocrystal is determined by the twin structures of seeds and the growth rates of different crystallographic facets.
Abstract: This article provides an overview of recent developments regarding synthesis of Pd nanocrystals with well-controlled shapes in aqueous solutions. In a solution-phase synthesis, the final shape taken by a nanocrystal is determined by the twin structures of seeds and the growth rates of different crystallographic facets. Here, the maneuvering of these factors in an aqueous system to achieve shape control for Pd nanocrystals is discussed. L-ascorbic acid, citric acid, and poly(vinyl pyrrolidone) are tested for manipulating the reduction kinetics, with citric acid and B-ions used as capping agents to selectively promote the formation of {111} and {100} facets, respectively. The distribution of single-crystal versus multiple-twinned seeds can be further manipulated by employing or blocking oxidative etching. The shapes obtained for the Pd nanocrystals include truncated octahedron, icosahedron,octahedron, decahedron, hexagonal and triangular plates, rectangular bar, and cube. The ability to control the shape of Pd nanocrystals provides a great opportunity to systematically investigate their catalytic, electrical, and plasmonic properties.
573 citations
448 citations
TL;DR: This study examines the use of electrospun biodegradable polymers as scaffolds not only for enhancing the differentiation of mouse ES cells into neural lineages but also for promoting and guiding the neurite outgrowth.
439 citations
TL;DR: A facile synthesis of anatase TiO(2) nanocrystals with exposed, chemically active {001} facets with a strong dependence on the pH value of the solution used for hydrothermal treatment is reported.
Abstract: This paper reports a facile synthesis of anatase TiO2 nanocrystals with exposed, chemically active {001} facets. The nanocrystals were prepared by digesting electrospun nanofibers consisting of amorphous TiO2 and poly(vinyl pyrrolidone) with an aqueous acetic acid solution (pH = 1.6), followed by hydrothermal treatment at 150 °C for 20 h. The as-obtained nanocrystals exhibited a truncated tetragonal bipyramidal shape with 9.6% of the surface being enclosed by {001} facets. The use of electrospinning is critical to the success of this synthesis as it allows for the generation of very small particles of amorphous TiO2 to facilitate hydrothermal crystallization, an Ostwald ripening process. The morphology of the nanocrystals had a strong dependence on the pH value of the solution used for hydrothermal treatment. Low pH values tended to eliminate the {001} facets by forming sharp corners while high pH values favored the formation of a rodlike morphology through an oriented attachment mechanism. When acetic ac...
TL;DR: This work demonstrated the use of Au nanocages as a new class of lymph node tracers for noninvasive photoacoustic (PA) imaging of a sentinel lymph node (SLN) with a number of attractive features: noninvasiveness, strong optical absorption in the near-infrared region (for deep penetration), and the accumulation of Aunanocages with a higher concentration than the initial solution for the injection.
Abstract: This work demonstrated the use of Au nanocages as a new class of lymph node tracers for noninvasive photoacoustic (PA) imaging of a sentinel lymph node (SLN) Current SLN mapping methods based on blue dye and/or nanometer-sized radioactive colloid injection are intraoperative due to the need for visual detection of the blue dye and low spatial resolution of Geiger counters in detecting radioactive colloids Compared to the current methods, PA mapping based on Au nanocages shows a number of attractive features: noninvasiveness, strong optical absorption in the near-infrared region (for deep penetration), and the accumulation of Au nanocages with a higher concentration than the initial solution for the injection In an animal model, these features allowed us to identify SLNs containing Au nanocages as deep as 33 mm below the skin surface with good contrast Most importantly, compared to methylene blue Au nanocages can be easily bioconjugated with antibodies for targeting specific receptors, potentially eliminating the need for invasive axillary staging procedures in addition to providing noninvasive SLN mapping
TL;DR: The localized surface plasmon resonance of a silver nanoparticle is responsible for its ability to strongly absorb and scatter light at specific wavelengths as discussed by the authors, and the absorption and scattering spectra of a particle can be predicted using Mie theory (for a spherical particle) or the discrete dipole approximation method (for particles in arbitrary shapes).
Abstract: The localized surface plasmon resonance of a silver nanoparticle is responsible for its ability to strongly absorb and scatter light at specific wavelengths. The absorption and scattering spectra (i.e., plots of cross sections as a function of wavelength) of a particle can be predicted using Mie theory (for a spherical particle) or the discrete dipole approximation method (for particles in arbitrary shapes). In this review, we briefly discuss the calculated spectra for silver nanoparticles with different shapes and the synthetic methods available to produce these nanoparticles. As validated in recent studies, there is good agreement between the theoretically calculated and the experimentally measured spectra. We conclude with a discussion of new plasmonic and sensing applications enabled by the shape-controlled nanoparticles.
TL;DR: In this paper, the authors discuss the Prinzipien von Kristallkeimbildung and -wachstum in Nanokristall formen and diskutieren wir einzelne Arten von Nanokistallformen and betrachten verschiedene experimentelle Parameter, bei denen die Formkontrolle von Metallnanokristalen ein entscheidender Faktor ist.
Abstract: Nanokristalle sind von grundlegender Bedeutung in der modernen Wissenschaft und Technologie. Die Eigenschaften eines Nanokristalls lassen sich uber seine Form beeinflussen und auf eine bestimmte Anwendung hin zuschneiden. Ziel dieses Aufsatzes ist es, einen umfassenden Uberblick uber den aktuellen Stand der Forschung auf dem Gebiet der formkontrollierten Synthese von Metallnanokristallen zu geben. Nach einer kurzen Einleitung uber die Prinzipien von Kristallkeimbildung und -wachstum diskutieren wir einzelne Arten von Nanokristallformen und betrachten verschiedene experimentelle Parameter, uber die die Keimbildung und das Wachstum von Metallnanokristallen in der Losungssynthese gezielt manipuliert werden kann. Wir erlautern diese Methoden anhand von Beispielen, wo zumindest ein Grundverstandnis fur die beobachtete Formkontrolle vorhanden ist. Schlieslich stellen wir Anwendungen vor, bei denen die Formkontrolle von Metallnanokristallen ein entscheidender Faktor ist, und beenden den Aufsatz mit einem personlichen Ausblick auf kunftig zu erwartende Entwicklungen auf diesem Gebiet.
TL;DR: A simple and versatile method for generating a continuously graded, bonelike calcium phosphate coating on a nonwoven mat of electrospun nanofibers, which can potentially be employed for repairing the tendon-to-bone insertion site via a tissue engineering approach.
Abstract: We have demonstrated a simple and versatile method for generating a continuously graded, bonelike calcium phosphate coating on a nonwoven mat of electrospun nanofibers. A linear gradient in calcium phosphate content could be achieved across the surface of the nanofiber mat. The gradient had functional consequences with regard to stiffness and biological activity. Specifically, the gradient in mineral content resulted in a gradient in the stiffness of the scaffold and further influenced the activity of mouse preosteoblast MC3T3 cells. This new class of nanofiber-based scaffolds can potentially be employed for repairing the tendon-to-bone insertion site via a tissue engineering approach.
TL;DR: It is found that explanted dorsal root ganglia (DRG) adhere well to the conductive core–sheath nanofibers and generate neurites across the surface when there is a nerve growth factor in the medium.
Abstract: We have prepared conductive core-sheath nanofibers via a combination of electrospinning and aqueous polymerization. Specifically, nanofibers electrospun from poly(e-caprolactone) (PCL) and poly((L)-lactide) (PLA) were employed as templates to generate uniform sheaths of polypyrrole (PPy) via in situ polymerization. These conductive core-sheath nanofibers offer a unique system for studying the synergistic effect of different cues on neurite outgrowth in vitro. We found that explanted dorsal root ganglia (DRG) adhered well to the conductive core-sheath nanofibers and generated neurites across the surface when there was a nerve growth factor in the medium. Furthermore, the neurites could be oriented along one direction and enhanced by 82% in terms of maximum length when uniaxially aligned conductive core-sheath nanofibers are compared with their random counterparts. Electrical stimulation, when applied through the mats of conductive core-sheath nanofibers, was found to further increase the maximum length of neurite for random and aligned samples by 83% and 47%, respectively, relative to the controls without electrical stimulation. Combined together, these results suggest the potential use of the conductive core-sheath nanofibers as scaffolds in applications such as neural tissue engineering.
TL;DR: It is found that the neurites extended radially outward from the DRG main body without specific directionality when cultured on a nonwoven mat of randomly oriented nanofibers, and this biaxial pattern clearly demonstrates that neurite outgrowth can be influenced by nan ofibers in different layers of a scaffold, rather than the topmost layer only.
Abstract: Electrospun nanofibers can be readily assembled into various types of scaffolds for applications in neural tissue engineering. The objective of this study is to examine and understand the unique patterns of neurite outgrowth from primary dorsal root ganglia (DRG) cultured on scaffolds of electrospun nanofibers having different orders, structures, and surface properties. We found that the neurites extended radially outward from the DRG main body without specific directionality when cultured on a nonwoven mat of randomly oriented nanofibers. In contrast, the neurites preferentially extended along the long axis of fiber when cultured on a parallel array of aligned nanofibers. When seeded at the border between regions of aligned and random nanofibers, the same DRG simultaneously expressed aligned and random neurite fields in response to the underlying nanofibers. When cultured on a double-layered scaffold where the nanofibers in each layer were aligned along a different direction, the neurites were found to b...
TL;DR: In this article, a simple galvanic replacement reaction between solutions containing metal precursor salts and Ag nanostructures prepared through polyol reduction is described, with the reduced metal depositing epitaxially on the surface of the Ag nanocubes, adopting their underlying cubic form.
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...
TL;DR: In this paper, the authors synthesized Pd-Pt bimetallic nanodendrites consisting of a dense array of Pt branches on a Pd core by reducing K2PtCl4 with Lascorbic acid in the presence of uniform Pd nanocrystal seeds in an aqueous solution.
Abstract: Controlling the morphology of Pt nanostructures can provide a great opportunity to improve their catalytic properties and increase their activity on a mass basis. We synthesized Pd-Pt bimetallic nanodendrites consisting of a dense array of Pt branches on a Pd core by reducing K2PtCl4 with L-ascorbic acid in the presence of uniform Pd nanocrystal seeds in an aqueous solution. The Pt branches supported on faceted Pd nanocrystals exhibited relatively large surface areas and particularly active facets toward the oxygen reduction reaction (ORR), the rate-determining step in a proton-exchange membrane fuel cell. The Pd-Pt nanodendrites were two and a half times more active on the basis of equivalent Pt mass for the ORR than the state-of-the-art Pt/C catalyst and five times more active than the first-generation supportless Pt-black catalyst.
TL;DR: A technique for fabricating chitosan inverse opal scaffolds characterized by a biodegradable material, uniform pore size, well-controlled interconnectivity, and nanofibrous texture on the wall surface is described.
Abstract: Tissue engineering is a promising approach to the development of biological substitutes that regenerate, replace, maintain, or improve the function of damaged tissues. Among various topics related to tissue engineering, the structures and properties of the scaffolds have been studied extensively in the context of material science and biomedical engineering. There are a number of generic requirements for the scaffold: 1) the material used for fabricating the scaffold must be biocompatible and biodegradable, together with positive responses from the seeded cells; 2) the scaffold should contain a network of pores, and favorably in the form of 3D interconnected architecture; and 3) the scaffold should have proper mechanical properties to suit the specific applications, including the generation of cartilage, bone, artificial blood vessel, among others.[1] In order to generate a well-defined scaffold, numerous methods have been proposed, including emulsion freeze drying,[2] high pressure processing,[3] particulate leaching,[4] gas foaming, [5] phase separation,[6] and electrospinning.[7] However, most of these methods are rather limited in terms of capability and feasibility. For example, the electrospinning method can hardly be extended to fabricate truly 3D scaffolds. Many of the other methods typically lead to the formation of irregular pore sizes, shapes, and structures, as well as poor connectivity. The pore size and structure of a scaffold are known to play a vital role in cell culture because they are responsible for not only the adhesion, migration, and distribution of cells, but also for the exchange of nutrients and metabolite wastes. Despite extensive efforts to control the pore sizes and structures, the issues related to uniformity and interconnectivity are yet to be solved as pointed out by many researchers.[8] In addressing these issues, the inverse opal structure can be considered as an ideal system, which has the most uniform pore size and regular 3D interconnectivity. Several groups have already shown the potential of an inverse opal as scaffold for 3D tissue engineering.[9] However, the used materials, silicate and polyacrylamide, are not biodegradable and can thus limit their potential use in clinical applications. Here we describe a technique for fabricating chitosan inverse opal scaffolds characterized by a biodegradable material, uniform pore size, well-controlled interconnectivity, and nanofibrous texture on the wall surface. We used uniform poly(caprolactone) (PCL) microspheres, prepared using a simple fluidic device, as the template.[10] Chitosan was chosen as a scaffold material because of its unique nanofibrous structure that typically develops during freeze-drying, as well as its nontoxic, anti-microbial, biocompatible, and biodegradable properties. Moreover, chitosan does not need a cross-linking procedure because it is only soluble in an acidic solution. We have also evaluated the potential use of the chitosan inverse opal as 3D scaffolds in the culture of preosteoblastic cells.
TL;DR: A new strategy based on plasma etching can be used to isolate and exclusively probe the SERS-active molecules adsorbed in the hot-spot region between two silver nanocubes.
Abstract: This paper reports a new strategy based on plasma etching that allows us to exclusively probe the SERS-active molecules adsorbed in the hot-spot region formed between two Ag nanocubes. Experimentally, we verified that the enhancement factor of the hot spot (EFhot-spot) was strongly dependent on its orientation relative to the laser polarization. For the hot spot formed between two Ag nanocubes of 100 nm in edge length, the EFhot-spot was found to vary from 1.0×108 to 4.1×106 and 4.4×105 as the long axis of the dimer was changed from 0 (parallel) to 45 and 90 (perpendicular) degrees relative to the direction of laser polarization. These results suggest a maximum enhancement of Raman signals by ~170 folds for the hot spot relative to the EF obtained for a single Ag nanocube of similar size. While the hot spot made a major contribution to the observed SERS signals when the dimer's long axis was parallel to the laser polarization, the hot spot did not contribute additionally to the detected signals when the dimer was in other orientations relative to the laser polarization.
TL;DR: A simple approach to the synthesis of asymmetric, hybrid colloidal particles by precipitation polymerization, where the key is to introduce Au or Ag colloids 2 min after (rather than before) starting the polymerization.
Abstract: This paper describes a simple approach to the synthesis of asymmetric, hybrid colloidal particles by precipitation polymerization. The key is to introduce Au or Ag colloids 2 min after (rather than before) starting the polymerization. The hybrid particles were uniform in size, and each one of them only contained one Au (or Ag) nanoparticle in its surface. Due to the simplicity of this procedure, it should be possible to use it for large-scale production. This method can be extended to metal nanoparticles other than Au and Ag and with a range of sizes, as long as they have appropriate stabilizers on the surface.
TL;DR: Through analysis of SERS bands, it is concluded that sharp features on the Ag nanocubes could greatly increase the contribution of the chemical enhancement to the SERS intensity.
Abstract: We have investigated the surface-enhanced Raman scattering (SERS) of chemically prepared single-crystal nanocubes and nanospheres of Ag with three different molecules to quantitatively understand the effect of sharp features on the SERS enhancement factor. Both experimental measurements and theoretical calculations confirmed a higher SERS activity for the nanocubes as a result of sharp features on their surfaces. We also found major discrepancies between the measured SERS intensities and those predicted from the electromagnetic mechanism. Through analysis of SERS bands, we concluded that sharp features on the Ag nanocubes could greatly increase the contribution of the chemical enhancement to the SERS intensity.
TL;DR: The ratios of absorption to extinction obtained from experimental and theoretical approaches agreed well, demonstrating the potential use of this method in determining the optical absorption and scattering properties of gold nanostructures and other types of nanomaterials.
Abstract: This paper presents a method for measuring the optical absorption cross sections (σa )o f Au-Ag nanocages and Au nanorods. The method is based on photoacoustic (PA) imaging, where the detected signal is directly proportional to the absorption coefficient (µa) of the nanostructure. For each type of nanostructure, we first obtained µa from the PA signal by benchmarking against a linear calibration curve (PA signal versus µa) derived from a set of methylene blue solutions with different concentrations. We then calculated σa by dividing the µa by the corresponding concentration of the Au nanostructure. Additionally, we obtained the extinction cross section (σe, sum of absorption and scattering) from the extinction spectrum recorded using a conventional UV-vis-NIR spectrometer. From the measurements of σa and σe, we were able to easily derive both the absorption and scattering cross sections for each type of gold nanostructure. The ratios of absorption to extinction obtained from experimental and theoretical approaches agreed well, demonstrating the potential use of this method in determining the optical absorption and scattering properties of gold nanostructures and other types of nanomaterials.
TL;DR: Template-assisted self-assembly (TASA) is a process in which colloidal aggregates with well-controlled sizes, shapes, and structures are fabricated by dewetting aqueous dispersions of building blocks across surfaces patterned with two-dimensional arrays of templates as mentioned in this paper.
Abstract: Template-assisted self-assembly (TASA) is a process in which colloidal aggregates with well-controlled sizes, shapes, and structures are fabricated by dewetting aqueous dispersions of building blocks across surfaces patterned with two-dimensional arrays of templates. Since our first demonstration in 2001, the capability and feasibility of this approach has been illustrated with the organization of polymer latex spheres or silica beads into homo-aggregates, including circular rings; polygonal and polyhedral clusters; and linear, zigzag, and spiral chains. It has been demonstrated to produce hetero-aggregates in the configuration of HF and H2O molecules that contained colloidal spheres of different sizes, compositions, densities, optical properties, or a combination of these features. More recently, TASA has also been successfully extended to colloidal building blocks with sizes well below 100 nm, expanding this technique to the nanoscale where it is expected to impact on a broad range of applications such as surface-enhanced Raman scattering (SERS) detection.
TL;DR: From optical fluorescence microscopy and scanning electron microscopy images, it is confirmed that the emulsion-templated microbeads (W-W/ O-W) have larger and better interconnected pores than the W-O-W microb Meads.
Abstract: Poly(d,l-lactide-co-glycolide) (PLGA) microbeads with a hollow interior and porous wall are prepared using a simple fluidic device fabricated with PVC tubes, glass capillaries, and a needle. Using the fluidic device with three flow channels, uniform water-in-oil-in-water (W-O-W) emulsions with a single inner water droplet can be achieved with controllable dimensions by varying the flow rate of each phase. The resultant W-O-W emulsions evolve into PLGA microbeads with a hollow interior and porous wall after the organic solvent in the middle oil phase evaporates. Two approaches are employed for developing a porous structure in the wall: emulsion templating and fast solvent evaporation. For emulsion templating, a homogenized, water-in-oil (W/O) emulsion is introduced as the middle phase instead of the pure oil phase. Low-molecular-weight fluorescein isothiocyanate (FITC) and high-molecular-weight fluorescein isothiocyanate-dextran conjugate (FITC-DEX) is added to the inner water phase to elucidate both the pore size and their interconnectivity in the wall of the microbeads. From optical fluorescence microscopy and scanning electron microscopy images, it is confirmed that the emulsion-templated microbeads (W-W/O-W) have larger and better interconnected pores than the W-O-W microbeads. These microstructured microbeads can potentially be employed for cell encapsulation and tissue engineering, as well as protection of active agents.
TL;DR: Recent advances in seeded growth are described as the ultimate approach to producing metal nanocrystals with precisely controlled sizes, shapes, and composition, the necessary first step toward their use and assembly for large-scale applications.
Abstract: The potential of using nanocrystals in applications within the fields of catalysis, electronics, medicine, and others has fueled research into the preparation and assembly of these materials. For most applications, it is necessary to have nanocrystal samples in which the size, shape, composition, and structure are tightly controlled within a narrow distribution. This need has motivated researchers to explore different synthesis protocols, including a method featured in this issue of ACS Nano by Kitaev and co-workers, where decahedral silver nanoparticles were used as seed particles for the growth of faceted silver rods. In this Perspective, we describe recent advances in seeded growth as the ultimate approach to producing metal nanocrystals with precisely controlled sizes, shapes, and compositions—the necessary first step toward their use and assembly for large-scale applications.
TL;DR: A significant dependency on excitation wavelength is found for the Au-Ag nanocages, independent of the underlying LSPR, which can be extended to other bimetallic SERS substrates.
Abstract: The surface-enhanced Raman scattering (SERS) properties of bimetallic Au–Ag nanocages has been thoroughly investigated by changing the nanocage composition, localized surface plasmon resonance (LSPR) peak position and excitation wavelength. We found a significant dependency on excitation wavelength for the Au–Ag nanocages, independent of the underlying LSPR, which can be extended to other bimetallic SERS substrates. While it is well-understood that plasmon damping can occur for Au nanoparticles when their LSPR peaks are close to interband transition frequencies and thereby attenuate SERS intensities, this study probes an additional aspect and shows that SERS intensities are reduced when the excitation light source is near interband transition frequencies regardless of the LSPR location.
TL;DR: A versatile technique for producing monodisperse microspheres from both hydrophobic and hydrophilic polymers using a simple fluidic device fabricated with a poly(vinyl chloride) (PVC) tube, a syringe needle, and a glass capillary tube is described.
Abstract: A versatile technique for producing monodisperse microspheres from both hydrophobic and hydrophilic polymers using a simple fluidic device fabricated with a poly(vinyl chloride) (PVC) tube, a syringe needle, and a glass capillary tube is described. The technique is successfully applied to a variety of different materials, including poly(e-caprolactone) (PCL) as an example of a hydrophobic polymer, ethyl-2cyanoacrylate (ECA) as an example of organic monomer, and gelatin as an example of a hydrophilic, natural polymer. From the calculated capillary number (Ca) andWeber number (We), the system is confirmed to work in the dripping regime. Precise control over particle size can be achieved by varying the polymer concentration and/or the flow rate for the continuous phase. An increase in flow rate for the continuous phase or a decrease in polymer concentration results in the reduction of particle size. The production of raspberry-like microspheres with a mixture of PCL and ECA is also demonstrated. In addition, we have developed a tapping method based on solvent evaporation on a concave glass for crystallizing these microspheres into close-packed lattices. Microspheres with uniform diameters are of great importance in many applications, including, among others, cosmetics, printing, coating, drug delivery, tissue engineering, and photonics. The best established method for producing such spheres relies on the formation of stable oil-in-water (O/ W) or water-in-oil (W/O) emulsions. Two approaches are commonly employed to make these emulsions: emulsification under amechanical or shear force and emulsification involving uniform breakup of a stream of the discontinuous phase as