Showing papers by "Younan Xia published in 2013"
TL;DR: A progress report on the use of galvanic replacement for generating complex hollow nanostructures with tunable and well-controlled properties and its capability to fabricate nanomaterials with complex structures and/or compositions by coupling with other processes such as co-reduction and the Kirkendall effect.
Abstract: This article provides a progress report on the use of galvanic replacement for generating complex hollow nanostructures with tunable and well-controlled properties. We begin with a brief account of the mechanistic understanding of galvanic replacement, specifically focused on its ability to engineer the properties of metal nanostructures in terms of size, composition, structure, shape, and morphology. We then discuss a number of important concepts involved in galvanic replacement, including the facet selectivity involved in the dissolution and deposition of metals, the impacts of alloying and dealloying on the structure and morphology of the final products, and methods for promoting or preventing a galvanic replacement reaction. We also illustrate how the capability of galvanic replacement can be enhanced to fabricate nanomaterials with complex structures and/or compositions by coupling with other processes such as co-reduction and the Kirkendall effect. Finally, we highlight the use of such novel metal nanostructures fabricated via galvanic replacement for applications ranging from catalysis to plasmonics and biomedical research, and conclude with remarks on prospective future directions.
803 citations
TL;DR: A deep understanding of the shape-dependent catalytic properties, together with an ability to experimentally maneuver the shape of metal nanocrystals, will eventually lead to rational design of advanced catalysts with substantially enhanced performance.
Abstract: Palladium is a marvelous catalyst for a rich variety of reactions in industrial processes and commercial devices. Most Pd-catalyzed reactions exhibit structure sensitivity, meaning that the activity or selectivity depends on the arrangement of atoms on the surface. Previously, such reactions could only be studied in ultrahigh vacuum using Pd single crystals cut with a specific crystallographic plane. However, these model catalysts are far different from real catalytic systems owing to the absence of atoms at corners and edges and the extremely small specific surface areas for the model systems. Indeed, enhancing the performance of a Pd-based catalyst, in part to reduce the amount needed of this precious and rare metal for a given reaction, requires the use of Pd with the highest possible specific surface area. Recent advances in nanocrystal synthesis are offering a great opportunity to investigate and quantify the structural sensitivity of catalysts based on Pd and other metals. For a structure-sensitive reaction, the catalytic properties of Pd nanocrystals are strongly dependent on both the size and shape. The shape plays a more significant role in controlling activity and selectivity, because the shape controls not only the facets but also the proportions of surface atoms at corners, edges, and planes, which affect the outcomes of possible reactions. We expect catalysts based on Pd nanocrystals with optimized shapes to meet the increasing demands of industrial applications at reduced loadings and costs. In this Account, we discuss recent advances in the synthesis of Pd nanocrystals with controlled shapes and their resulting performance as catalysts for a large number of reactions. First, we review various synthetic strategies based on oxidative etching, surface capping, and kinetic control that have been used to direct the shapes of nanocrystals. When crystal growth is under thermodynamic control, the capping agent plays a pivotal role in determining the shape of a product by altering the order of surface energies for different facets through selective adsorption; the resulting product has the lowest possible total surface energy. In contrast, the product of a kinetically controlled synthesis often deviates from the thermodynamically favored structure, with notable examples including nanocrystals enclosed by high-index facets or concave surfaces. We then discuss the key parameters that control the nucleation and growth of Pd nanocrystals to decipher potential growth mechanisms and build a connection between the experimental conditions and the pathways to different shapes. Finally, we present a number of examples to highlight the use of these Pd nanocrystals as catalysts or electrocatalysts for various applications with structure-sensitive properties. We believe that a deep understanding of the shape-dependent catalytic properties, together with an ability to experimentally maneuver the shape of metal nanocrystals, will eventually lead to rational design of advanced catalysts with substantially enhanced performance.
546 citations
TL;DR: Au nanohexapods are promising candidates for cancer theranostics in terms of both photothermal destruction and contrast-enhanced diagnosis as well as in vitro and in vivo capabilities.
Abstract: Gold nanohexapods represent a novel class of optically tunable nanostructures consisting of an octahedral core and six arms grown on its vertices. By controlling the length of the arms, their localized surface plasmon resonance peaks could be tuned from the visible to the near-infrared region for deep penetration of light into soft tissues. Herein we compare the in vitro and in vivo capabilities of Au nanohexapods as photothermal transducers for theranostic applications by benchmarking against those of Au nanorods and nanocages. While all these Au nanostructures could absorb and convert near-infrared light into heat, Au nanohexapods exhibited the highest cellular uptake and the lowest cytotoxicity in vitro for both the as-prepared and PEGylated nanostructures. In vivo pharmacokinetic studies showed that the PEGylated Au nanohexapods had significant blood circulation and tumor accumulation in a mouse breast cancer model. Following photothermal treatment, substantial heat was produced in situ and the tumor metabolism was greatly reduced for all these Au nanostructures, as determined with ^(18)F-flourodeoxyglucose positron emission tomography/computed tomography (^(18)F-FDG PET/CT). Combined together, we can conclude that Au nanohexapods are promising candidates for cancer theranostics in terms of both photothermal destruction and contrast-enhanced diagnosis.
533 citations
TL;DR: The synthesis of uniform 9 nm Pt-Ni octahedra with the use of oleylamine and oleic acid as surfactants and W(CO)6 as a source of CO that can promote the formation of {111} facets in the presence of Ni is reported.
Abstract: Nanoscale Pt–Ni bimetallic octahedra with controlled sizes have been actively explored in recent years owning to their outstanding activity for the oxygen reduction reaction (ORR). Here we report the synthesis of uniform 9 nm Pt–Ni octahedra with the use of oleylamine and oleic acid as surfactants and W(CO)6 as a source of CO that can promote the formation of {111} facets in the presence of Ni. Through the introduction of benzyl ether as a solvent, the coverage of both surfactants on the surface of resultant Pt–Ni octahedra was significantly reduced while the octahedral shape was still attained. By further removing the surfactants through acetic acid treatment, we observed a specific activity 51-fold higher than that of the state-of-the-art Pt/C catalyst for the ORR at 0.93 V, together with a record high mass activity of 3.3 A mgPt–1 at 0.9 V (the highest mass activity reported in the literature was 1.45 A mgPt–1). Our analysis suggests that this great enhancement of ORR activity could be attributed to th...
518 citations
TL;DR: It is demonstrated that ROS-responsive PATK functionalized with a cancer-targeting peptide is a promising gene carrier for safe, efficient, and cancer- targeted gene delivery.
Abstract: The high intracellular oxidative stress in a cancer cell is a biologically relevant stimulus for efficient intracellular delivery of therapeutic genes. In this study, reactive oxygen species (ROS)-responsive poly(amino thioketal) (PATK) was synthesized to achieve efficient and safe intracellular gene delivery in prostate cancer cells. The DNA/PATK polyplexes were efficiently disassembled upon exposure to high levels of ROS in prostate cancer cells, leading to enhanced intracellular release of DNA in the cells. As a result, DNA/PATK polyplexes showed significantly higher gene transfection efficiency than their non-degradable counterparts did. In addition, conjugation of GRP78 protein-targeting peptide to the PATK not only increased its cellular uptake in prostate cancer cells but also enhanced gene transfection efficiency. This study demonstrates that ROS-responsive PATK functionalized with a cancer-targeting peptide is a promising gene carrier for safe, efficient, and cancer-targeted gene delivery.
335 citations
TL;DR: It is demonstrated that the growth pathway of a seed is indeed determined by the ratio between the rates for atom deposition and surface diffusion, suggesting that surface diffusion needs to be taken into account when controlling the shape or morphology of metal nanocrystals.
Abstract: Controlling the shape or morphology of metal nanocrystals is central to the realization of their many applications in catalysis, plasmonics, and electronics. In one of the approaches, the metal nanocrystals are grown from seeds of certain crystallinity through the addition of atomic species. In this case, manipulating the rates at which the atomic species are added onto different crystallographic planes of a seed has been actively explored to control the growth pattern of a seed and thereby the shape or morphology taken by the final product. Upon deposition, however, the adsorbed atoms (adatoms) may not stay at the same sites where the depositions occur. Instead, they can migrate to other sites on the seed owing to the involvement of surface diffusion, and this could lead to unexpected deviations from a desired growth pathway. Herein, we demonstrated that the growth pathway of a seed is indeed determined by the ratio between the rates for atom deposition and surface diffusion. Our result suggests that surface diffusion needs to be taken into account when controlling the shape or morphology of metal nanocrystals.
326 citations
TL;DR: An optofluidic system was constructed from a diode laser as the energy source, an aqueous suspension of plasmonic nanostructures as the photothermal transducer, and a glass capillary for measuring the volumetric expansion of the suspension.
Abstract: An optofluidic system was constructed from a diode laser as the energy source, an aqueous suspension of plasmonic nanostructures as the photothermal transducer, and a glass capillary for measuring the volumetric expansion of the suspension. The suspension could be driven to move up the capillary by more than 30 mm and be used to control the operation of an electrical switch.
266 citations
TL;DR: The results suggest that the high sensitivity of synthesis outcomes to the trace amounts of impurities in a polyol, a major issue for reproducibility and scale up synthesis, did not exist in the present system.
Abstract: This article describes a robust method for the facile synthesis of small Ag nanocubes with edge lengths controlled in the range of 18–32 nm. The success of this new method relies on the substitution of ethylene glycol (EG)—the solvent most commonly used in a polyol synthesis—with diethylene glycol (DEG). Owing to the increase in hydrocarbon chain length, DEG possesses a higher viscosity and a lower reducing power relative to EG. As a result, we were able to achieve a nucleation burst in the early stage to generate a large number of seeds and a relatively slow growth rate thereafter; both factors were critical to the formation of Ag nanocubes with small sizes and in high purity (>95%). The edge length of the Ag nanocubes could be easily tailored in the range of 18–32 nm by quenching the reaction at different time points. For the first time, we were able to produce uniform sub-20 nm Ag nanocubes in a hydrophilic medium and on a scale of ∼20 mg per batch. It is also worth pointing out that the present protoc...
264 citations
TL;DR: A thermoresponsive bubble-generating liposomal system is proposed for triggering localized extracellular drug delivery and may also provide an ability to monitor a temperature-controlled drug delivery process.
Abstract: The therapeutic effectiveness of chemotherapy is optimal only when tumor cells are subjected to a maximum drug exposure. To increase the intratumoral drug concentration and thus the efficacy of chemotherapy, a thermoresponsive bubble-generating liposomal system is proposed for triggering localized extracellular drug delivery. The key component of this liposomal formulation is the encapsulated ammonium bicarbonate (ABC), which is used to create the transmembrane gradient needed for a highly efficient encapsulation of doxorubicin (DOX). At an elevated temperature (42 °C), decomposition of ABC generates CO(2) bubbles, creating permeable defects in the lipid bilayer that rapidly release DOX and instantly increase the drug concentration locally. Because the generated CO(2) bubbles are hyperechogenic, they also enhance ultrasound imaging. Consequently, this new liposomal system encapsulated with ABC may also provide an ability to monitor a temperature-controlled drug delivery process.
235 citations
TL;DR: In this article, the electrochemical surface area (ECA) calculation was studied using the charges associated with stripping of CO and underpotentially deposited H and Cu on highly dispersed Pt- and Pd-based nanoparticles.
181 citations
TL;DR: In this paper, a systematic study was conducted on small Pd nanocrystals (5-6 nm) to understand the effects of catalyst structure and electrolyte on the oxygen reduction reaction (ORR) and formic acid oxidation (FAO).
Abstract: A systematic study was conducted on small Pd nanocrystals (5–6 nm) to understand the effects of catalyst structure and electrolyte on the oxygen reduction reaction (ORR) and formic acid oxidation (FAO) The ORR activities of Pd catalysts strongly depended on their structure and the electrolyte used It was found that Pd cubes were 10 times more active than Pd octahedra for ORR in an aqueous HClO4 solution due to higher onset potential of OHad formation on the cubic surface In the case of a H2SO4 solution, the ORR activity of Pd cubes was 17 times higher than that of Pd octahedra due to the stronger adsorption of (bi)sulfate on the surface of octahedral nanocrystals in addition to OHad In alkaline solutions, however, no structure dependence was observed for ORR due to the outer-sphere electron-transfer mechanism in the potential region for Pd oxide formation For FAO, no advantage was observed on shape-controlled Pd nanocrystals in comparison to conventional Pd catalysts The FAO current densities, both
TL;DR: For the first time, this work was able to precisely control the edge lengths of Ag octahedra below 100 nm, and the lower limit of size could even be pushed down to 20 nm.
Abstract: Silver octahedra with edge lengths controlled in the range of 20–72 nm were synthesized via seed-mediated growth. The key to the success of this synthesis is the use of single-crystal Ag seeds with uniform and precisely controlled sizes to direct the growth and the use of citrate as a selective capping agent for the {111} facets. Our mechanistic studies demonstrated that Ag seeds with both cubic and quasi-spherical shapes could evolve into octahedra. For the first time, we were able to precisely control the edge lengths of Ag octahedra below 100 nm, and the lower limit of size could even be pushed down to 20 nm. Using the as-obtained Ag octahedra as sacrificial templates, Au nanocages with an octahedral shape and precisely tunable optical properties were synthesized through a galvanic replacement reaction. Such hollow nanostructures are promising candidates for a broad range of applications related to optics, catalysis, and biomedicine.
TL;DR: The ability to control the shape of metal nanocrystals is central to advances in many areas of modern science and technology, including catalysis, plasmonics, electronics, and biomedicine as mentioned in this paper.
Abstract: The ability to control the shape of metal nanocrystals is central to advances in many areas of modern science and technology, including catalysis, plasmonics, electronics, and biomedicine. This article provides a brief overview of our recent efforts toward the development of solution-phase methods for shape-controlled synthesis of metal nanocrystals. While the synthetic methods only involve simple redox reactions, we have been working diligently to understand the complex nucleation and growth mechanisms leading to the formation of metal nanocrystals with desired shapes and related properties. We hope this review will inspire new ideas and concepts in the general area of nanomaterial synthesis, expand our ability to engineer the properties of metals for various applications, and contribute to the realization of sustainable use for some of the scarcest materials.
TL;DR: An approach based on a combination of inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy for quantitative analysis of the role played by Br(-) ions in the synthesis of Pd nanocrystals is reported, potentially extendable to other systems involving chemisorbed capping agents.
Abstract: We report an approach based on a combination of inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy for quantitative analysis of the role played by Br– ions in the synthesis of Pd nanocrystals. The Br– ions were found to adsorb onto Pd{100} facets selectively with a coverage density of ca. 0.8 ion per surface Pd atom. The chemisorbed Br– ions could be removed via desorption at an elevated temperature under reductive conditions. They could also be gradually released from the surface when Pd cubic seeds grew into cuboctahedrons and then octahedrons. On the basis of the coverage density information, we were able to estimate the minimum concentration of Br– ions needed for the formation of Pd nanocubes with a specific size. If the concentration of Br– ions was below this minimum value, not all of the {100} facets could be stabilized by the capping agent, leading to the formation of nanocubes with truncated corners. The quantitative analysis developed in this study is potentially ...
TL;DR: The novel layered scaffold developed has the potential for improving tendon healing due to its ability to deliver both cells and growth factors simultaneously in a surgically convenient manner.
TL;DR: This research highlights the need to understand more fully the chiral stationary phase transition between Na6(CO3)(SO4) and Na3(SO4), which is important for the efficient and efficient design of materials for electronics.
Abstract: Sergio Navalón 1 and H. García 1,2,3,* 1 Deparment of Chemistry, Universidad Politécnica de Valencia, C/Camino de Vera, s/n, 46022 Valencia, Spain; sernaol@doctor.upv.es 2 Instituto Universitario de Tecnología Química CSIC-UPV, Universitat Politécnica de Valencia, Av. De los Naranjos s/n, 46022 Valencia, Spain 3 Center of Excellence for Advanced Materials Research, King Abdulaziz University, 21589 Jeddah, Saudi Arabia * Correspondence: hgarcia@qim.upv.es; Tel.: +34-96-387-7807
TL;DR: A facile route based on polyol reduction is reported for the synthesis of Pd RBPs with purity >90% and sizes controlled in the range 5-15 nm and the success of the synthesis relies on the use of iodide ions to manipulate the strength of an oxidative etchant and selectively cap the Pd{100} facets.
Abstract: Controlling the shape and thus facets of metal nanocrystals is an effective way to enhance their performance in catalytic reactions. While Pd nanocrystals with a myriad of shapes have been successfully prepared with good uniformity and in high yield, Pd right bipyramids (RBPs) that have a singly twinned structure have been elusive. We report a facile route based on polyol reduction for the synthesis of Pd RBPs with purity >90% and sizes controlled in the range 5-15 nm. The success of our synthesis relies on the use of iodide ions to manipulate the strength of an oxidative etchant and selectively cap the Pd{100} facets. The as-prepared RBPs could serve as seeds to generate a set of Pd nanocrystals with novel shapes and structures. The RBPs also exhibited enhanced catalytic activity toward formic acid oxidation, with a current density 2.5 and 7.1 times higher than those of the single-crystal Pd nanocubes (which were also mainly covered by {100} facets) and commercial Pd black, respectively.
TL;DR: It is demonstrated that vapor-induced phase separation played a pivotal role in generating the yarns with a unique structure and the low vapor pressure of N,N-dimethylformamide (DMF) was critical to the evolution of pores in the interiors.
Abstract: This article presents a simple and reliable method for generating polystyrene (PS) yarns composed of bundles of nanofibrils by using a proper combination of solvent and relative humidity. We elucidated the mechanism responsible for the formation of this new morphology by systematically investigating the molecular interactions among the polymer, solvent(s), and water vapor. We demonstrated that vapor-induced phase separation played a pivotal role in generating the yarns with a unique structure. Furthermore, we discovered that the low vapor pressure of N,N-dimethylformamide (DMF) was critical to the evolution of pores in the interiors. On the contrary, the relatively high vapor pressure of tetrahydrofuran (THF) hindered the formation of interior pores but excelled in creating a rough surface. In all cases, our results clearly indicate that the formation of either internal porosity or surface roughness required the presence of water vapor, a nonsolvent of the polymer, at a proper level of relative humidity. The exact morphology or pore structure was dependent on the speed of evaporation of the solvent(s) (DMF, THF, and their mixtures) as well as the interdiffusion and penetration of the nonsolvent (water) and solvent(s). Our findings can serve as guidelines for the preparation of fibers with desired porosity both internally and externally through electrospinning.
TL;DR: Under biological conditions the radioactive Au nanocages were shown to emit light with wavelengths in the visible and near-infrared regions, enabling luminescence imaging of the whole mice in vivo, as well as the organs ex vivo.
Abstract: Cerenkov luminescence imaging based on light emission from the decay of radionuclides has recently drawn great interest in molecular imaging. In this paper, we report for the first time the Cerenkov luminescence phenomenon of 198Au isotope, as well as a facile route to the preparation of radioluminescent Au nanocages without additional radiolabeling or dye conjugation. The specific radioactivity of the Au nanocages could be easily and precisely controlled by varying the concentration of H198AuCl4 precursor used for the galvanic replacement reaction. The direct incorporation of 198Au atoms into the structure of Au nanocages enabled the ability of accurate analysis and real-time imaging in vivo. Furthermore, under biological conditions the radioactive Au nanocages were shown to emit light with wavelengths in the visible and near-infrared regions, enabling luminescence imaging of the whole mice in vivo, as well as the organs ex vivo. When combined with their favorable scattering and absorption properties in ...
TL;DR: In this paper, the authors studied the properties of faceted nanocrystals with high-index facets and showed that the size of a facet determines the surface-to-bulk atomic ratio and the proportions of different types (e.g., vertex, edge, and face) of atoms while the shape governs the types of facets on its surface and thus the arrangement of face atoms.
Abstract: Experimental studies with single-crystal surfaces and computational simulations have long established that the activity and selectivity of a heterogeneous catalyst for a structure-sensitive reaction can be maximized by controlling the arrangement of atoms on the surface. The essence of these studies, however, could not be materialized until very recently when it became possible to generate faceted nanocrystals with controlled shapes and sizes, including those with high-index facets. In principle, the size of a faceted nanocrystal determines the surface-to-bulk atomic ratio and the proportions of different types (e.g. vertex, edge, and face) of atoms while the shape governs the types of facets on its surface and thus the arrangement of face atoms. Additionally, the adsorption of atomic/molecular species during the synthesis of nanocrystals and operation of a catalytic reaction can alter their shape and thus their durability as a catalyst. All of these issues need to be fully understood and addressed in the design, synthesis, and utilization of faceted noble-metal nanocrystals for catalytic applications.
TL;DR: A model is proposed to describe vascularization in a 3D porous scaffold, which can potentially serve as a guideline for future design of porous scaffolds.
Abstract: The formation of a stable vascular network in a scaffold is one of the most challenging tasks in tissue engineering and regenerative medicine. Despite the common use of porous scaffolds in these applications, little is known about the effect of pore size on the neovascularization in these scaffolds. Herein is fabricated poly(D, L-lactide-co-glycolide) inverse opal scaffolds with uniform pore sizes of 79, 147, 224, and 312 μm in diameter and which are then used to systematically study neovascularization in vivo. Histology analyses reveal that scaffolds with small pores ( 200 μm) favor the formation of vascular networks with large blood vessels at low densities and deep penetration depth. Based on the different patterns of vessel ingrowth as regulated by the pore size, a model is proposed to describe vascularization in a 3D porous scaffold, which can potentially serve as a guideline for future design of porous scaffolds.
TL;DR: This work demonstrates the feasibility of using a new triphasic system with uncovered catalyst to maximize the thermal stability and catalytic activity and offers a general approach to the synthesis of high-performance catalytic systems with tunable compositions.
Abstract: A triphasic catalytic system (Pt/TiO2–SiO2) with an “islands in the sea” configuration was fabricated by controlling the selectivity of SiO2 deposition onto the surface of TiO2 versus the surface of Pt nanoparticles. The Pt surface was exposed, while the nanoparticles were supported on TiO2 and isolated from each other by SiO2 to achieve both significantly improved sinter resistance up to 700 °C and outstanding activity after high-temperature calcination. This work not only demonstrates the feasibility of using a new triphasic system with uncovered catalyst to maximize the thermal stability and catalytic activity but also offers a general approach to the synthesis of high-performance catalytic systems with tunable compositions.
TL;DR: This work advances the understanding of oxidative etching in nanocrystal synthesis but also offers a powerful means for controlling the shape and size of metal nanocrystals simply by adjusting the rates of etching and regrowth.
Abstract: Palladium octahedra with controlled edge lengths were obtained from Pd cubes of a single size. The success of this synthesis relies on a transformation involving oxidative etching and regrowth. Because the {100} side faces of the Pd nanocubes were capped by Br– ions, Pd atoms were removed from the corners during oxidative etching, and the resultant Pd2+ ions could be reduced and deposited back onto the nanocubes, but preferentially on the {100} facets. We could control the ratio of the etching and regrowth rates (Retching and Rregrowth) simply by varying the amount of HCl added to the reaction solution. With a large amount of HCl, etching dominated the process (Retching ≫ Rregrowth), resulting in the formation of Pd octahedra with an edge length equal to 70% of that of the cubes. In contrast, with a small amount of HCl, all of the newly formed Pd2+ ions could be quickly reduced and deposited back onto the Pd cubes. In this case, Retching ≈ Rregrowth, and the resultant Pd octahedra had roughly the same vol...
TL;DR: This work not only advances the understanding of the growth mechanism of tetrahedrons but also offers a new approach to controlling the shape of metal nanocrystals.
Abstract: Palladium octahedrons and tetrahedrons enclosed by eight and four {111} facets have been synthesized from cuboctahedral Pd seeds by using Na2PdCl4 and Pd(acac)2, respectively, as the precursors. Our mechanistic studies indicate that the cuboctahedral seeds were directed to grow into octahedrons, truncated tetrahedrons, and then tetrahedrons when Pd(acac)2 was used as a precursor. In contrast, the same batch of seeds only evolved into octahedrons with increasing sizes when the precursor was switched to Na2PdCl4. The difference in growth pattern could be attributed to the different reduction rates of these two precursors. The fast reduction of Pd(acac)2 led to a quick drop in concentration for the precursor in the very early stage of a synthesis, forcing the growth into a kinetically controlled mode. In comparison, the slow reduction of Na2PdCl4 could maintain this precursor at a relatively high concentration to ensure thermodynamically controlled growth. This work not only advances our understanding of the...
TL;DR: PAM of cytochromes in cytoplasm is expected to be a high-throughput, label-free technique for studying live cell functions, which cannot be accomplished by conventional histology.
Abstract: Photoacoustic microscopy (PAM) has achieved submicron lateral resolution in showing subcellular structures; however, relatively few endogenous subcellular contrasts have so far been imaged. Given that the hemeprotein, mostly cytochromes in general cells, is optically absorbing around the Soret peak (∼420 nm), we implemented label-free PAM of cytochromes in cytoplasm for the first time. By measuring the photoacoustic spectra of the oxidized and reduced states of fibroblast lysate and fitting the difference spectrum with three types of cytochromes, we found that the three cytochromes account for more than half the optical absorption in the cell lysate at 420 nm wavelength. Fixed fibroblasts on slides were imaged by PAM at 422 and 250 nm wavelengths to reveal cytoplasms and nuclei, respectively, as confirmed by standard staining histology. PAM was also applied to label-free histology of mouse ear sections by showing cytoplasms and nuclei of various cells. PAM of cytochromes in cytoplasm is expected to be a high-throughput, label-free technique for studying live cell functions, which cannot be accomplished by conventional histology.
TL;DR: Keep your wine chilled!
Abstract: Keep your wine chilled! Microscale polystyrene (PS) bottles are loaded with dye molecules and then corked with a phase-change material (PCM). When the temperature is raised beyond its melting point, the PCM quickly melts and triggers an instant release of the encapsulated dye. The release profiles can be manipulated by using a binary mixture of PCMs with different melting points.
TL;DR: A new class of contrast agents based on gold nanocages with hollow interiors and porous walls to label human mesenchymal stem cells for both in vitro and in vivo tracking using two-photon microscopy and photoacoustic microscopy is introduced.
Abstract: Stem cell tracking is a highly important subject. Current techniques based on nanoparticle-labeling, such as magnetic resonance imaging, fluorescence microscopy, and micro-computed tomography, are plagued by limitations including relatively low sensitivity or penetration depth, involvement of ionizing irradiation, and potential cytotoxicity of the nanoparticles. Here we introduce a new class of contrast agents based on gold nanocages (AuNCs) with hollow interiors and porous walls to label human mesenchymal stem cells (hMSCs) for both in vitro and in vivo tracking using two-photon microscopy and photoacoustic microscopy. As demonstrated by the viability assay, the AuNCs showed negligible cytotoxicity under a reasonable dose, and did not alter the differentiation potential of the hMSCs into desired lineages. We were able to image the cells labeled with AuNCs in vitro for at least 28 days in culture, as well as to track the cells that homed to the tumor region in nude mice in vivo.
TL;DR: The ability to transform a bulk synthesis into a droplet-based system holds great potential for the development of a new route to the high-volume production of nanocrystals.
Abstract: Droplet-based microreactors are used for the continuous production of Pd nanocrystals. Specifically, commercially available polytetrafluoroethylene (PTFE) tube and silica capillaries are utilized to fabricate a fluidic device capable of generating water-in-oil droplets. In addition to the feasibility of using such droplets as microreactors for conducting a synthesis, the ability to control the composition and concentration of reagents by adjusting the flow rates is demonstrated; reagents are mixed by periodically pinching the PTFE tube, and nanocrystals are produced with uniform size distribution in a continuous fashion. The capability to tailor the size and shape of the resultant nanocrystals is further demonstrated by introducing the reducing agent and capping agent at different flow rates to control the nucleation and growth processes. The ability to transform a bulk synthesis into a droplet-based system holds great potential for the development of a new route to the high-volume production of nanocrystals.
TL;DR: This study clearly demonstrates the importance of facet capping, surface diffusion, and reaction kinetics in controlling the morphologies of bimetallic nanocrystals during a seed-mediated process and provides a new direction for the rational design and synthesis of nanocry crystals with spatially controlled distributions of elements for a variety of applications.
Abstract: This article describes a systematic study of the spatially confined growth of Rh atoms on Pd nanocrystal seeds, with a focus on the blocking effect of a surface capping agent and the surface diffusion of adatoms. We initially used Pd cuboctahedrons as the seeds to illustrate the concept and to demonstrate the capabilities of our approach. Because the Pd{100} facets were selectively capped by a layer of chemisorbed Br(–) or I(–) ions, we were able to confine the nucleation and deposition of Rh atoms solely on the {111} facets of a Pd seed. When the synthesis was conducted at a relatively low temperature, the deposition of Rh atoms followed an island growth mode because of the high Rh–Rh interatomic binding energy. We also facilitated the surface diffusion of deposited Rh atoms by increasing the reaction temperature and decreasing the injection rate for the Rh precursor. Under these conditions, the deposition of Rh on the Pd{111} facets was switched to a layered growth mode. We further successfully extended this approach to a variety of other types of Pd polyhedral seeds that contained Pd{111} and Pd{100} facets in different proportions on the surface. As expected, a series of Pd–Rh bimetallic nanocrystals with distinctive elemental distributions were obtained. We could remove the Pd cores through selective chemical etching to generate Rh hollow nanoframes with different types and degrees of porosity. This study clearly demonstrates the importance of facet capping, surface diffusion, and reaction kinetics in controlling the morphologies of bimetallic nanocrystals during a seed-mediated process. It also provides a new direction for the rational design and synthesis of nanocrystals with spatially controlled distributions of elements for a variety of applications.
TL;DR: Multiscale photoacoustic microscopy (PAM) is used to chronically monitor neovascularization in an inverse opal scaffold implanted in a mouse model up to 6 weeks by taking advantage of the optical absorption contrast intrinsic to hemoglobin molecules in red blood cells.
Abstract: It is a grand challenge to visualize and assess in vivo neovascularization in a three-dimensional (3D) scaffold noninvasively, together with high spatial resolution and deep penetration depth. Here we used multiscale photoacoustic microscopy (PAM), including acoustic-resolution PAM (AR-PAM) and optical-resolution PAM (OR-PAM), to chronically monitor neovascularization in an inverse opal scaffold implanted in a mouse model up to 6 weeks by taking advantage of the optical absorption contrast intrinsic to hemoglobin molecules in red blood cells. By combining with optical coherence tomography (OCT) based on optical scattering contrast, we also demonstrated the capability to simultaneously image and analyze the vasculature and the scaffold in the same mouse. The hybrid system containing OR-PAM and OCT offered a fine lateral resolution of ∼5 μm and a penetration depth of ∼1 mm into the scaffold/tissue construct. AR-PAM further extended the penetration depth up to ∼3 mm at a lateral resolution of ∼45 μm. By quantifying the 3D PAM data, we further examined the effect of pore size (200 vs. 80 μm) of a scaffold on neovascularization. The data collected from PAM were consistent with those obtained from traditional invasive, labor-intensive histologic analyses.