Showing papers by "Younan Xia published in 2018"

The physical chemistry and materials science behind sinter-resistant catalysts

Abstract: Catalyst sintering, a main cause of the loss of catalytic activity and/or selectivity at high reaction temperatures, is a major concern and grand challenge in the general area of heterogeneous catalysis. Although all heterogeneous catalysts are inevitably subjected to sintering during their operation, the immediate and drastic consequences can be mitigated by carefully engineering the catalytic particles and their interactions with the supports. In this tutorial review, we highlight recent progress in understanding the physical chemistry and materials science involved in sintering, including the discussion of advanced techniques, such as in situ microscopy and spectroscopy, for investigating the sintering process and its rate. We also discuss strategies for the design and rational fabrication of sinter-resistant catalysts. Finally, we showcase recent success in improving the thermal stability and thus sinter resistance of supported catalytic systems.

Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants.

Abstract: There is a growing interest in controlling the synthesis of colloidal metal nanocrystals and thus tailoring their properties toward various applications. In this context, choosing an appropriate combination of reagents (e.g., salt precursor, reductant, capping agent, and stabilizer) plays a pivotal role in enabling the synthesis of metal nanocrystals with diversified sizes, shapes, and structures. Here we present a comprehensive review that highlights one of the key reagents for the synthesis of metal nanocrystals via chemical reduction: the reductants. We start with a brief introduction to the compounds commonly employed as reductants in the colloidal synthesis of metal nanocrystals by showing their oxidation half-reactions and the corresponding oxidation potentials. Then we offer specific examples pertaining to the controlled synthesis of metal nanocrystals, followed by some fundamental aspects covering the general mechanisms of metal ion reduction based on the Marcus Theory. Afterwards, we present a case-by-case discussion on a wide variety of reductants, including their major properties, reduction mechanisms, and additional effects on the final products. We illustrate these aspects by selecting key examples from the literature and paying close attention to the underlying mechanism in each case. At the end, we conclude by summarizing the highlights of the review and providing some perspectives on future directions.

Shape‐Controlled Synthesis of Colloidal Metal Nanocrystals by Replicating the Surface Atomic Structure on the Seed

Abstract: Controlling the surface structure of metal nanocrystals while maximizing the utilization efficiency of the atoms is a subject of great importance. An emerging strategy that has captured the attention of many research groups involves the conformal deposition of one metal as an ultrathin shell (typically 1-6 atomic layers) onto the surface of a seed made of another metal and covered by a set of well-defined facets. This approach forces the deposited metal to faithfully replicate the surface atomic structure of the seed while at the same time serving to minimize the usage of the deposited metal. Here, the recent progress in this area is discussed and analyzed by focusing on the synthetic and mechanistic requisites necessary for achieving surface atomic replication of precious metals. Other related methods are discussed, including the one-pot synthesis, electrochemical deposition, and skin-layer formation through thermal annealing. To close, some of the synergies that arise when the thickness of the deposited shell is decreased controllably down to a few atomic layers are highlighted, along with how the control of thickness can be used to uncover the optimal physicochemical properties necessary for boosting the performance toward a range of catalytic reactions.

Hollow Metal Nanocrystals with Ultrathin, Porous Walls and Well-Controlled Surface Structures.

Abstract: Recent developments of a novel class of catalytic materials built on hollow nanocrystals having ultrathin, porous walls, and well-controlled surface structures are discussed, with a focus on platinum and the oxygen reduction reaction (ORR). An introduction is given to the critical role of platinum in the proton exchange membrane fuel cells, and the pressing need to develop a strategy for achieving cost-effective and sustainable use of this precious metal. How to maximize the mass activity of ORR catalysts based on platinum by rationally engineering the surface structure while increasing the utilization efficiency of atoms is then discussed. After reporting on the synthetic methods involving galvanic replacement and seed-mediated growth followed by etching, respectively, a number of examples to demonstrate the enhancement in activity and durability for this new class of catalytic materials are showcased. The feasibility to have the methodology extended from platinum to other precious metals such as gold and ruthenium is highlighted. In conclusion, some of the remaining issues and emerging solutions are examined.

Design and Fabrication of a Hierarchically Structured Scaffold for Tendon‐to‐Bone Repair

Abstract: A hierarchically structured scaffold is designed and fabricated for facilitating tendon-to-bone repair. The scaffold is composed of three regions with distinct functions: (i) an array of channels to guide the in-growth of cells and aligned deposition of collagen fibers, as well as integration of the scaffold with the tendon side, (ii) a region with a gradient in mineral composition to facilitate stress transfer between tendon and bone, and (iii) a mineralized inverse opal region to promote the integration of the scaffold with the underlying bone. Cell culture experiments confirm that adipose-derived stromal cells are able to infiltrate and proliferate through the entire thickness of the scaffold without compromised cell viability. The seeded stem cells exhibit directed differentiation into tenocytes and osteoblasts along the mineral gradient as a response to the gradient in Young's modulus. This novel scaffold holds great promise to promote the formation of a functional tendon-to-bone attachment by offering a structurally and compositionally appropriate microenvironment for healing.

Integration of Phase-Change Materials with Electrospun Fibers for Promoting Neurite Outgrowth under Controlled Release.

Abstract: We report a temperature-regulated system for the controlled release of nerve growth factor (NGF) to promote neurite outgrowth. The system is based upon microparticles fabricated using coaxial electrospray, with the outer solution containing a phase-change material (PCM) and the inner solution encompassing payload(s). When the temperature is kept below the melting point of the PCM, there is no release due to the extremely slow diffusion through a solid matrix. Upon increasing the temperature to slightly pass the melting point, the encapsulated payload(s) can be readily released from the melted PCM. By leveraging the reversibility of the phase transition, the payload(s) can be released in a pulsatile mode through on/off heating cycles. The controlled release system is evaluated for potential use in neural tissue engineering by sandwiching the microparticles, co-loaded with NGF and a near-infrared dye, between two layers of electrospun fibers to form a tri-layer construct. Upon photothermal heating with a near-infrared laser, the NGF is released with well-preserved bioactivity to promote neurite outgrowth. By choosing different combinations of PCM, biological effector, and scaffolding material, this controlled release system can be applied to a wide variety of biomedical applications.

Synthesis of Ru Icosahedral Nanocages with a Face-Centered-Cubic Structure and Evaluation of Their Catalytic Properties

Abstract: Owing to the presence of {111} facets, twin boundaries, and strain fields on the surface, noble-metal nanocrystals with an icosahedral shape have been reported with stellar performance toward an array of catalytic reactions. Here, we report the successful synthesis of Ru icosahedral nanocages with a face-centered cubic (fcc) structure by conformally coating Pd icosahedral seeds with ultrathin Ru shells, followed by selective removal of the Pd cores via chemical etching. We discovered that the presence of bromide ions was critical to the layer-by-layer deposition of Ru atoms. According to in situ XRD, the fcc structure in the Ru nanocages could be retained up to 300 °C before it was transformed into the conventional hexagonal close-packed (hcp) structure. Additionally, the icosahedral shape of the Ru nanocages could be largely preserved up to 300 °C. The Ru icosahedral nanocages with twin boundaries on the surface exhibited greatly enhanced activities toward both the reduction of 4-nitrophenol and decompos...

Enabling Complete Ligand Exchange on the Surface of Gold Nanocrystals through the Deposition and Then Etching of Silver

Abstract: We report an indirect method for the effective replacement of ligands on the surface of Au nanocrystals with different morphologies. The method involves the deposition of an ultrathin layer of Ag to remove a strong capping agent such as cetyltrimethylammonium chloride (CTAC), followed by selective etching of the Ag layer in the presence of citrate ions as a stabilizer. Using multiple characterization techniques, we confirm that the surface of the Au nanocrystals is covered by citrate ions after the indirect ligand exchange process, and there is essentially no aggregation during the entire process. We also demonstrate that this method is effective in suppressing the toxicity of Au nanospheres by completely replacing the initially used CTAC with citrate.

A Droplet-Reactor System Capable of Automation for the Continuous and Scalable Production of Noble-Metal Nanocrystals

Abstract: Noble-metal nanocrystals with well-controlled shapes or morphologies are of great interest for a variety of applications. To utilize these nanomaterials in consumer products, one has to produce the colloidal nanocrystals in large quantities while maintaining good control over their physical parameters and properties. Droplet reactors have shown great potential for the continuous and scalable production of colloidal nanocrystals with controlled shapes. However, the efficiencies of most previously reported systems are still limited because of the complex post-treatment procedures. For example, the mixture of silicone oil and an aqueous suspension of solid products has to be separated by leveraging their miscibility and difference in density, while the solid products often need to be purified and concentrated by centrifugation. Herein, we report the design and construction of a droplet-reactor system that include new features such as a homemade unit for the automatic separation of silicone oil from the aqueo...

General Method for Generating Circular Gradients of Active Proteins on Nanofiber Scaffolds Sought for Wound Closure and Related Applications.

Abstract: Scaffolds functionalized with circular gradients of active proteins are attractive for tissue regeneration because of their enhanced capability to accelerate cell migration and/or promote neurite extension in a radial fashion. Here, we report a general method for generating circular gradients of active proteins on scaffolds composed of radially aligned nanofibers. In a typical process, the scaffold, with its central portion raised using a copper wire to take a cone shape, was placed in a container (upright or up-side-down), followed by dropwise addition of bovine serum albumin (BSA) solution into the container. As such, a circular gradient of BSA was generated along each nanofiber. The bare regions uncovered by BSA were then filled with an active protein of interest. In demonstrating their potential applications, we used different model systems to examine the effects of two types of protein gradients. While the gradient of laminin and epidermal growth factor accelerated the migration of fibroblasts and ke...

Perspective: Aligned arrays of electrospun nanofibers for directing cell migration.

Abstract: Cell migration plays an important role in a wide variety of biological processes, including embryogenesis, wound healing, inflammation, cancer metastasis, and tissue repair. Electrospun nanofibers have been extensively explored as scaffolds to manipulate cell migration owing to their unique characteristics in mimicking the hierarchical architecture of extracellular matrix. In particular, aligned arrays of electrospun nanofibers are capable of guiding and promoting the directional migration of cells. The physical parameters and properties of the aligned nanofibers, including their size, modulus, and surface chemistry, can all affect the migratory behaviors of cells, while the controlled release of growth factors and drugs from the nanofibers can also be utilized to influence cell migration. By manipulating cell migration, electrospun nanofibers have been applied to promote tissue repair and help eradicate tumors in vivo. In this perspective, we highlight recent developments in collecting electrospun nanofibers as aligned arrays and then illustrate how the aligned nanofibers can be utilized to manipulate cell migration.

Direct in Situ Observation and Analysis of the Formation of Palladium Nanocrystals with High-Index Facets

Abstract: Synthesizing concave-structured nanoparticles (NP) with high-index surfaces offers a viable method to significantly enhance the catalytic activity of NPs. Current approaches for fabricating concave NPs, however, are limited. Exploring novel synthesis methods requires a thorough understanding of the competing mechanisms that contribute to the evolution of surface structures during NP growth. Here, by tracking the evolution of Pd nanocubes into concave NPs at atomic scale using in situ liquid cell transmission electron microscopy, our study reveals that concave-structured Pd NPs can be formed by the cointroduction of surface capping agents and halogen ions. These two chemicals jointly create a new surface energy landscape of Pd NPs, leading to the morphological transformation. In particular, Pd atoms dissociate from the {100} surfaces with the aid of Cl– ions and preferentially redeposit to the corners and edges of the nanocubes when the capping agent polyvinylpyrrolidone is introduced, resulting in the for...

The effect of adipose-derived stem cell sheets and CTGF on early flexor tendon healing in a canine model.

Abstract: Intrasynovial tendon injuries are among the most challenging in orthopedics. Despite significant improvements in operative and rehabilitation methods, functional outcomes continue to be limited by adhesions, gap formation, and rupture. Adhesions result from excessive inflammation, whereas tendon gapping and rupture result from inflammation-induced matrix degradation and insufficient regeneration. Therefore, this study used a combined treatment approach to modulate inflammation with adipose-derived mesenchymal stromal cells (ASCs) while stimulating tendon regeneration with connective tissue growth factor (CTGF). ASCs were applied to the repair surface via cell sheets and CTGF was delivered to the repair center via porous sutures. The effect of the combined treatment was assessed fourteen days after repair in a canine flexor tendon injury model. CTGF, either alone or with ASCs, reduced inflammatory (IL1B and IL6) and matrix degrading (MMP3 and MMP13) gene expression, while increasing anti-inflammatory gene (IL4) expression and collagen synthesis compared to control repairs. The combined treatment was more effective than CTGF treatment alone, reducing the inflammatory IFNG and scar-associated COL3A1 gene expression and increasing CD146+ tendon stem/progenitor cells at the tendon surface and interior along the core suture tracks. Therefore, the combined approach is promising in promoting early flexor tendon healing and worthy of further investigation.

Inverse Opal Scaffolds with Gradations in Mineral Content for Spatial Control of Osteogenesis.

Abstract: The design and fabrication of inverse opal scaffolds with gradations in mineral content to achieve spatial control of osteogenesis are described. The gradient in mineral content is established via the diffusion-limited transport of hydroxyapatite nanoparticles in a closely packed lattice of gelatin microbeads. The mineral-graded scaffold has an array of uniform pores and interconnected windows to facilitate efficient transport of nutrients and metabolic wastes, ensuring high cell viability. The graded distribution of mineral content can provide biochemical and mechanical cues for spatially regulating the osteogenic differentiation of adipose-derived stromal cells. This new class of scaffolds holds promise for engineering the interfaces between mineralized and unmineralized tissues.

A Rationally Designed Route to the One-Pot Synthesis of Right Bipyramidal Nanocrystals of Copper

Abstract: Lined with planar defects, nanoscale right bipyramids (RBPs) are of great interest for a variety of applications owing to the presence of a single type of {100} facets and two oppositely positioned right-angle corners as well as a higher surface-to-volume ratio than that of a cube. Here we report a rationally designed route to the facile synthesis of Cu RBPs with edge lengths controlled in the range 30–70 nm. The one-pot protocol involves the sequential reduction of Pd(II) and Cu(II) precursors, mixed at a molar ratio of 1:200, by glucose in the presence of hexadecylamine, a capping agent with strong binding to the Cu(100) surface. Because of the difference in standard reduction potential between the Pd(II)/Pd and Cu(II)/Cu pairs, the Pd(II) precursor is reduced first for the generation of Pd seeds lined with parallel planar defects, followed by the slow deposition of Cu atoms for the formation of RBPs. By varying the growth time and/or the amount of Pd(II) relative to Cu(II), Cu RBPs with tunable edge le...

Site-selective growth of Ag nanocubes for sharpening their corners and edges, followed by elongation into nanobars through symmetry reduction

Abstract: It remains a challenge to synthesize Ag nanocubes with sharp corners and edges while retaining a compact size below 20 nm. Here we demonstrate the use of site-selective growth to sharpen the corners and edges of truncated Ag nanocubes with sizes down to 18 nm, followed by their elongation into nanobars with aspect ratios up to 2. The key to the success of this synthesis is the site-selective deposition at corners and edges, as enabled by cetyltrimethylammonium chloride (CTAC). While CTA+ is an effective colloidal stabilizer, Cl− can react with Ag+ to generate AgCl precipitates, slowing down the reduction kinetics. In addition, Cl− can serve as a facet-selective capping agent towards the {100} side faces and thereby confine the growth mainly to corners and edges. Interestingly, once all the corners and edges have been sharpened, the growth is switched to an asymmetric mode to favor deposition on one of the six side faces only, leading to the formation of Ag nanobars with controllable aspect ratios. The symmetry reduction takes place as a result of the limited supply of Ag atoms, the strong capping of Cl− ions towards the {100} facets, and the possible involvement of localized oxidative etching caused by Cl−/O2. We also demonstrate that the Ag nanocubes with sharp corners and edges can serve as a better sacrificial template than their truncated counterparts in generating Au hollow nanostructures with ultrathin walls.

Continuous processing of phase-change materials into uniform nanoparticles for near-infrared-triggered drug release

Abstract: We report a method based on interfacial, anti-solvent-induced precipitation in a fluidic device for the continuous and scalable processing of phase-change materials (PCMs) into uniform nanoparticles with controlled diameters in the range of 10–100 nm. A eutectic mixture of lauric acid and stearic acid, with a well-defined melting point at 39 °C, serves as an example to demonstrate the concept. In the fluidic device, a coaxial flow is created by introducing a PCM solution in ethanol and a lipid solution in water (the anti-solvent) as the focused and focusing phases, respectively. The formation of lipid-capped PCM nanoparticles is governed by diffusion-controlled mixing of ethanol and water. During the production, both doxorubicin (DOX, an anticancer drug) and indocyanine green (ICG, a near-infrared dye) can be readily loaded into the PCM nanoparticles to give a smart drug release system. Upon irradiation with near-infrared light, the photothermal heating caused by ICG can melt the PCM and thereby trigger the release of DOX. This work not only provides a new technique for the continuous processing of PCMs and other soft materials into uniform nanoparticles with controlled sizes but also demonstrates a biocompatible system for controlled release and related applications.

A facile, robust and scalable method for the synthesis of Pd nanoplates with hydroxylamine as a reducing agent and mechanistic insights from kinetic analysis

Abstract: Nanocrystals lined with stacking faults have received much attention in recent years due to their typical anisotropic, plate-like geometry and their perplexing formation mechanism. In this report, we introduce a simple and reliable method for the scalable production of Pd nanoplates containing stacking faults. The success of our protocol was reliant on the use of hydroxylamine as a reducing agent, which allowed the nucleation and growth of well-defined Pd nanoplates with an average edge length of 24.5 ± 6.6 nm and a thickness of 4.7 ± 0.4 nm. We conducted a kinetic analysis to validate the importance of an appropriate initial reduction rate in determining the formation of seeds lined with stacking faults. To demonstrate the robustness of this synthesis, we conducted a set of control experiments under different experimental conditions such as acidity, temperature, and chemical environment and demonstrated that Pd nanoplates could be obtained as final products in all scenarios. We further extended the batch-based synthesis to a continuous flow reactor, moving one step closer towards high-volume production. Taken together, this method offers both simplicity and reproducibility for the synthesis of Pd nanoplates, which will enable future mechanistic studies and applications.

Synthesis of Palladium Nanoscale Octahedra through a One-Pot, Dual-Reductant Route and Kinetic Analysis.

Abstract: Shape-controlled synthesis of colloidal metal nanocrystals has traditionally relied on the use of an approach that involves the reduction of a metal precursor by a single reductant. Once the concentration of atoms surpasses supersaturation, they will undergo homogeneous nucleation to generate nuclei and then seeds, followed by further growth into nanocrystals. In general, it is a grand challenge to optimize such an approach because the kinetic requirement for nucleation tends to be drastically different from what is needed to guide the growth process. In this work, we overcome this difficulty by switching to a dual-reductant approach, in which both strong and weak reductants are added into the same reaction solution. By controlling their amounts to program the reduction kinetics, the strong reductant only regulates the homogeneous nucleation process to generate the desired seeds, and once consumed, the weak reductant takes over to control the growth pattern and thereby the shape of the resulting nanocrystals.

Understanding the Stability of Pt-Based Nanocages under Thermal Stress Using In Situ Electron Microscopy

Abstract: We report a systematic study of the thermal stability of Pt-based cubic, octahedral, and icosahedral nanocages using high-resolution electron microscopy coupled with in situ heating. Our results indicate that all these nanocages could be used with no observable changes up to 150 °C, with their facets still preserved at even higher temperatures. We observed the same behavior in all the nanocages under thermal stress: hole enlargement and atom migration toward the edges to create nanoframes. This transformation could be rationalized by the thermodynamic driving force to minimize surface free energy. The octahedral nanocages were found to be more stable by 50 oC than the icosahedral nanocages, suggesting that the thermal stability of such catalysts can be potentially optimized to extend their usage toward different applications at elevated temperatures.

Nanofiber-Based Multi-Tubular Conduits with a Honeycomb Structure for Potential Application in Peripheral Nerve Repair.

Abstract: Peripheral nerve injury is a large-scale problem and it is a great challenge to repair the long lesion in a thick nerve. The design of a multi-tubular conduit with a honeycomb structure by mimicking the anatomy of a peripheral nerve for the potential repair of large defects in thick nerves has been reported. A bilayer mat of electrospun nanofibers is rolled up to form a single tube, with the inner and outer layers comprised aligned and random nanofibers, respectively. Seven such tubes are then assembled into a hexagonal array and encased within the lumen of a larger tube to form the multi-tubular conduit. By introducing an adhesive to the regions between the tubes, the conduit is robust enough for handling during surgery. The seeded bone marrow stem cells (BMSCs) are able to proliferate in all the tubes with even circumferential and longitudinal distributions. Under chemical induction, the BMSCs are transdifferentiated into Schwann-like cells in all the tubes. While the cellular version holds great promise for peripheral nerve repair, the multi-tubular conduit can also be used to investigate the fundamental aspects involved in the development of peripheral nervous system and migration of cells.

Melanocortin 1 Receptor Targeted Imaging of Melanoma With Gold Nanocages and Positron Emission Tomography.

Abstract: Purpose:Melanoma is a lethal skin cancer with unmet clinical needs for targeted imaging and therapy. Nanoscale materials conjugated with targeting components have shown great potential to improve t...

Rhodium Decahedral Nanocrystals: Facile Synthesis, Mechanistic Insights, and Experimental Controls

Abstract: Decahedral nanocrystals have received great attention owing to their unique symmetry and strain-energy distribution. In contrast to other noble metals, it has been difficult to synthesize decahedral Rh nanocrystals. We report a robust, one-pot method based on polyol reduction for the facile synthesis of Rh decahedral nanocrystals in high purity, with sub-20 nm sizes. The success of the synthesis relied on our ability to manipulate reduction kinetics by systematically tuning experimental parameters. We found that the yield of Rh decahedral nanocrystals could be maximized by optimizing: i) the concentration of Rh(acac)3 (metal precursor); ii) the molecular weight and amount of poly(vinyl pyrrolidone) (colloidal stabilizer/capping agent); and iii) the chain length of the polyol (solvent/reducing agent), with tetraethylene glycol being the best. We believe the mechanisms elucidated herein can be extended to other syntheses to produce metal nanocrystals with multiply twinned structures.

Enhancing the tactile and near-infrared sensing capabilities of electrospun PVDF nanofibers with the use of gold nanocages

Abstract: Owing to its piezoelectric and pyroelectric properties, poly(vinylidene fluoride) (PVDF) has been extensively explored for applications related to tactile sensing, energy harvesting, and thermal imaging. However, PVDF cannot be directly used to detect light because of its weak absorption in the visible and near-infrared (NIR) regions, preventing effective conversion from light to heat and then electrical signal. In this work, we address this issue by incorporating Au nanocages (AuNCs) into PVDF nanofibers during electrospinning. The strong and tunable optical absorption associated with AuNCs makes them an effective transducer for converting light to heat and then electrical signal. The presence of AuNCs and the strong electric field inherent to electrospinning both promote the formation of the ferroelectric β phase for maximal piezoelectric and pyroelectric conversions. With the incorporation of AuNCs, the electrospun PVDF nanofibers show enhanced capabilities for tactile and NIR sensing. While the voltage output under the tactile force is increased by 12.6-fold relative to the case of pristine PVDF nanofibers, a voltage output of 7.2 V is achieved when the hybrid device is subjected to the on/off cycles of NIR irradiation by an 808 nm diode laser at a power density of 0.2 W cm−2.

Quantitative analysis of the reduction kinetics of a Pt(II) precursor in the context of Pt nanocrystal synthesis

Abstract: In this letter, we report a quantitative analysis of how a Pt(II) precursor is reduced to atoms at different temperatures for the formation of Pt nanocrystals with different morphologies and sizes. Our results suggest that in the early stage of a synthesis, the Pt(II) precursor is reduced to atoms exclusively in the solution phase, followed by homogeneous nucleation to generate nuclei and then seeds. At a relatively low reaction temperature such as 22°C, the growth of the seeds is dominated by autocatalytic surface reduction that involves the adsorption and then reduction of the Pt(II) precursor on the surface of the just-formed seeds. This particular growth pathway results in relatively large assemblies of Pt nanocrystals. When the reaction temperature is increased to 100°C, the dominant reduction pathway will be switched from surface to solution phase, producing much smaller assemblies of Pt nanocrystals. Our results also demonstrate that a similar trend applies to the seed-mediated growth of Pt nanocrystals in the presence of Pd nanocubes.