Preventing the Galvanic Replacement Reaction toward Unconventional Bimetallic Core-Shell Nanostructures.

A bimetallic core-shell nanostructure is a versatile platform for achieving intriguing optical and catalytic properties. For a long time, this core-shell nanostructure has been limited to ones with noble metal cores. Otherwise, a galvanic replacement reaction easily occurs, leading to hollow nanostructures or completely disintegrated ones. In the past few years, great efforts have been devoted to preventing the galvanic replacement reaction, thus creating an unconventional class of core-shell nanostructures, each containing a less-stable-metal core and a noble metal shell. These new nanostructures have been demonstrated to show unique optical and catalytic properties. In this work, we first briefly summarize the strategies for synthesizing this type of unconventional core-shell nanostructures, such as the delicately designed thermodynamic control and kinetic control methods. Then, we discuss the effects of the core-shell nanostructure on the stabilization of the core nanocrystals and the emerging optical and catalytic properties. The use of the nanostructure for creating hollow/porous nanostructures is also discussed. At the end of this review, we discuss the remaining challenges associated with this unique core-shell nanostructure and provide our perspectives on the future development of the field.

Coherent hexagonal platinum skin on nickel nanocrystals for enhanced hydrogen evolution activity.

Metastable noble metal nanocrystals may exhibit distinctive catalytic properties to address the sluggish kinetics of many important processes, including the hydrogen evolution reaction under alkaline conditions for water-electrolysis hydrogen production. However, the exploration of metastable noble metal nanocrystals is still in its infancy and suffers from a lack of sufficient synthesis and electronic engineering strategies to fully stimulate their potential in catalysis. In this paper, we report a synthesis of metastable hexagonal Pt nanostructures by coherent growth on 3d transition metal nanocrystals such as Ni without involving galvanic replacement reaction, which expands the frontier of the phase-replication synthesis. Unlike noble metal substrates, the 3d transition metal substrate owns more crystal phases and lower cost and endows the hexagonal Pt skin with substantial compressive strains and programmable charge density, making the electronic properties particularly preferred for the alkaline hydrogen evolution reaction. The energy barriers are greatly reduced, pushing the activity to 133 mA cmgeo-2 and 17.4 mA µgPt-1 at -70 mV with 1.5 µg of Pt in 1 M KOH. Our strategy paves the way for metastable noble metal catalysts with tailored electronic properties for highly efficient and cost-effective energy conversion.

Seed-Mediated Synthesis of Thin Gold Nanoplates with Tunable Edge Lengths and Optical Properties.

Thin Au nanoplates show intriguing localized surface plasmon resonance (LSPR) properties with potential applications in various fields. The conventional synthesis of Au nanoplates usually involves the formation of spherical nanoparticles or produces nanoplates with large thicknesses. Herein, we demonstrate a synthesis of uniform thin Au nanoplates by using Au-Ag alloy nanoframes obtained by the galvanic replacement of Ag nanoplates with HAuCl4 as the seeds and a sulfite (SO32-) as a ligand. The SO32- ligand not only complexes with the Au salt for the controlled reduction kinetics but also strongly adsorbs on Au {111} facets for effectively constraining the crystal growth on both basal sides of the Au nanoplates for controlled shape and reduced thicknesses. This seed-mediated synthesis affords Au nanoplates with a thickness of only 7.5 nm, although the thickness increases with the edge length. The edge length can be customizable in a range of 48-167 nm, leading to tunable LSPR bands in the range of 600-1000 nm. These thin Au nanoplates are applicable not only to surface-enhanced Raman spectroscopy with enhanced sensitivity and reliability but also to a broader range of LSPR-based applications.

Lattice Mismatch-Induced Formation of Copper Nanoplates with Embedded Ultrasmall Platinum or Palladium Cores for Tunable Optical Properties.

Although noble metal nanocrystals have been studied extensively in the past decades, the shape-controlled synthesis of non-noble metal nanocrystals has remained challenging with limited success, which is a grand obstacle to their wide applications. Herein, a novel lattice mismatch-involved shape-control mechanism of Cu nanocrystals in a seed-mediated synthesis is reported, which can produce Cu nanoplates in high yield with tailored sizes (28-130 nm), holding great potential in optical and catalytic applications. The lattice mismatch between Cu and the seed is found effective in inducing crystallographic defects for symmetry breaking toward anisotropic nanocrystals. While a too-large lattice mismatch (11.7% for Au seeds) leads to multiple twin defects to form quasi-spherical Cu nanocrystals, an appropriately large lattice mismatch (7.7% for Pt and 6.9% for Pd seeds) successfully induces planar defects for the formation of Cu nanoplates. The size of the Cu nanoplates is customizable by controlling the concentration of the seeds, leading to tunable optical properties. A prototype of a colorimetric indicator with Cu nanoplates, potentially applicable to the safety control of foods and drugs is demonstrated. This mechanism paves a new way for the shape-controlled synthesis of Cu and other metal nanocrystals for a broad range of applications.

Promoting nickel oxidation state transitions in single-layer NiFeB hydroxide nanosheets for efficient oxygen evolution.

Promoting the formation of high-oxidation-state transition metal species in a hydroxide catalyst may improve its catalytic activity in the oxygen evolution reaction, which remains difficult to achieve with current synthetic strategies. Herein, we present a synthesis of single-layer NiFeB hydroxide nanosheets and demonstrate the efficacy of electron-deficient boron in promoting the formation of high-oxidation-state Ni for improved oxygen evolution activity. Raman spectroscopy, X-ray absorption spectroscopy, and electrochemical analyses show that incorporation of B into a NiFe hydroxide causes a cathodic shift of the Ni2+(OH)2 → Ni3+δOOH transition potential. Density functional theory calculations suggest an elevated oxidation state for Ni and decreased energy barriers for the reaction with the NiFeB hydroxide catalyst. Consequently, a current density of 100 mA cm-2 was achieved in 1 M KOH at an overpotential of 252 mV, placing it among the best Ni-based catalysts for this reaction. This work opens new opportunities in electronic engineering of metal hydroxides (or oxides) for efficient oxygen evolution in water-splitting applications.

Counting charges per metal nanoparticle.

Charges on a metal nanoparticle are measured with precision by electron holography.

Highly Strained Au-Ag-Pd Alloy Nanowires for Boosted Electrooxidation of Biomass-Derived Alcohols.

Although strain engineering is effective in boosting the activities of noble metal catalysts, it remains desirable to construct fully strained catalysts to push the activity to even higher levels. Herein, we report a novel route to strong lattice strains of a Pd-based catalyst by radial growth of a Pd-rich phase on Au-Ag alloy nanowires that are no thicker than 1.5 nm. It creates not only tensile strains in the Pd-rich sheath due to the core-sheath lattice mismatch but also distortion and twinning of the lattice, producing nonhomogeneous local strains as hotspots for the catalysis. Toward the electrochemical oxidation of biomass-derived alcohols including ethanol, ethylene glycol, and glycerol, the highly strained nanowires outperformed their less strained counterparts and reached up to 13.6, 18.2, and 11.1 A mgPd-1, respectively. This strain engineering strategy may open new avenues to highly efficient catalysts for direct alcohol fuel cells and many other applications.

Encapsulated Metal Nanoparticles for Catalysis.

Metal nanoparticles have drawn great attention in heterogeneous catalysis. One challenge is that they are easily deactivated by migration-coalescence during the catalysis process because of their high surface energy. With the rapid development of nanoscience, encapsulating metal nanoparticles in nanoshells or nanopores becomes one of the most promising strategies to overcome the stability issue of the metal nanoparticles. Besides, the activity and selectivity could be simultaneously enhanced by taking advantage of the synergy between the metal nanoparticles and the encapsulating materials as well as the molecular sieving property of the encapsulating materials. In this review, we provide a comprehensive summary of the recent progress in the synthesis and catalytic properties of the encapsulated metal nanoparticles. This review begins with an introduction to the synthetic strategies for encapsulating metal nanoparticles with different architectures developed to date, including their encapsulation in nanoshells of inorganic oxides and carbon, porous materials (zeolites, metal-organic frameworks, and covalent organic frameworks), and organic capsules (dendrimers and organic cages). The advantages of the encapsulated metal nanoparticles are then discussed, such as enhanced stability and recyclability, improved selectivity, strong metal-support interactions, and the capability of enabling tandem catalysis, followed by the introduction of some representative applications of the encapsulated metal nanoparticles in thermo-, photo-, and electrocatalysis. At the end of this review, we discuss the remaining challenges associated with the encapsulated metal nanoparticles and provide our perspectives on the future development of the field.

Customizable Ligand Exchange for Tailored Surface Property of Noble Metal Nanocrystals.

It is highly desirable, while still challenging, to obtain noble metal nanocrystals with custom capping ligands, because their colloidal synthesis relies on specific capping ligands for the shape control while conventional ligand exchange processes suffer from "the strong replaces the weak" limitation, which greatly hinders their applications. Herein, we report a general and effective ligand exchange approach that can replace the native capping ligands of noble metal nanocrystals with virtually any type of ligands, producing flexibly tailored surface properties. The key is to use diethylamine with conveniently switchable binding affinity to the metal surface as an intermediate ligand. As a strong ligand, it in its original form can effectively remove the native ligands; while protonated, it loses its binding affinity and facilitates the adsorption of new ligands, especially weak ones, onto the metal surface. By this means, the irreversible order in the conventional ligand exchange processes could be overcome. The efficacy of the strategy is demonstrated by mutual exchange of the capping ligands among cetyltrimethylammonium, citrate, polyvinylpyrrolidone, and oleylamine. This novel strategy significantly expands our ability to manipulate the surface property of noble metal nanocrystals and extends their applicability to a wide range of fields, particularly biomedical applications.

Non­covalent proteasome inhibitor PI­1840 induces apoptosis and autophagy in osteosarcoma cells.

Osteosarcoma (OS) is the predominant form of primary bone malignancy in children and adolescents. Although the combination of chemotherapy and modified surgical therapy leads to marked improvements in the survival rate, the therapeutic outcomes remain unsatisfactory. Therefore, the identification of novel drugs with higher efficacy and fewer side­effects is urgently required. Proteasome inhibitors have been approved by the Food and Drug Administration (FDA) for the treatment of certain cancers, although none of them are directed against OS. Non­covalent proteasome inhibitors, such as PI­1840, are superior to covalent ones in numerous respects in view of their chemical structure; however, to date, no studies have been published on the effects of non­covalent proteasome inhibitors on OS cells. In the present study, the antineoplastic effects of PI­1840 were systematically evaluated in the OS cell lines, MG­63 and U2­OS. Cell viability and morphological changes were assessed by Cell Counting Kit­8 (CCK­8) and live/dead assays. The cell cycle was analyzed using flow cytometry (FCM) and western blot analysis (assessing the levels of the proteins p21, p27, and the tyrosine kinase, WEE1). The extent of cell apoptosis and autophagy were assessed by FCM, western blot analysis [of the apoptosis­associated proteins, microtubule­associated protein 1 light chain 3 α (LC3) and Beclin1], and mRFP­GFP­LC3 adenovirus transfection assay. Transwell and wound healing assays, and western blot analysis of the matrix metalloproteinases (MMPs)2 and 9 were performed to preliminarily evaluate the migration and invasion capability of the cells. In the present study, our results revealed that PI­1840 inhibited the proliferation of OS cells and induced apoptosis, partly due to attenuation of the nuclear factor­κB (NF­κB) pathway. In addition, PI­1840­induced autophagy was detected, and inhibiting the autophagy of the OS cells led to an increase in the survival rate of the U2­OS cells rather than of the MG­63 cells. Furthermore, PI­1840 attenuated the migration and invasion capabilities of the OS cells. In conclusion, the present study revealed PI­1840 to be a promising drug for the treatment of OS.

Dynamically Switchable Multicolor Electrochromic Films.

The dynamic optical switch of plasmonic nanostructures is highly desirable due to its promising applications in many smart optical devices. To address the challenges in the reversibility and transmittance contrast of the plasmonic electrochromic devices, here, a strategy is reported to fabricate color switchable electrochromic films through electro-responsive dissolution and deposition of Ag on predefined hollow shells of Au/Ag alloy. Using the hollow Au/Ag alloy nanostructures as stable seeds for site-specific deposition of Ag, elimination of the random self-nucleation events is enabled and optimal reversibility in color switching is allowed. The hollow structure further enables excellent transmittance contrast between the bleached and colored states. With its additional advantages such as the convenience for preparation, high sensitivity, and field-tunable optical property, it is believed that this new electrochromic film represents a unique platform for designing novel smart optical devices.

Stabilization of noble metal nanostructures for catalysis and sensing.

Noble metal nanocrystals have been widely used as active components in catalysis and chemical/bio-sensing. The sizes, structures, and shapes of noble metal nanocrystals are crucial to their electronic, optical, and catalytic properties. However, metal nanocrystals tend to lose their structural and morphological properties when they are subjected to thermal and chemical treatment. Therefore, stabilization of noble metal nanostructures remains a challenge. In this feature article, we present our recent efforts on the stabilization of noble metal nanocrystals, i.e., using inorganic and non-metal solids as supports and physical barriers, protecting the nanocrystal surface by a metal coating, and forming alloys with other metals. At the end of this review, we provide our perspectives on the future development of effective methods for nanocrystal stabilization.

Ultra-Small Platinum Nanoparticles Encapsulated in Sub-50 nm Hollow Titania Nanospheres for Low-Temperature Water-Gas Shift Reaction.

Ultra-small platinum nanoparticles loaded over titania is a promising catalyst for the low-temperature water-gas shift (WGS) reaction and shows the potential to work in a mobile hydrogen fuel cell system. Their precise size engineering (<3 nm) and reliable stabilization remain challenging. To address these issues, we report a reverse-micelle synthesis approach, which affords uniform ultra-small platinum nanoparticles (tunable in ∼1.0-2.6 nm) encapsulated in hollow titania nanospheres with a shell thickness of only ∼3-5 nm and an overall diameter of only ∼32 nm. The Pt@TiO2 yolk/shell nanostructured catalysts display extraordinary stability and monotonically increasing activity with the decreasing size of the Pt nanoparticles in the WGS. The size-dependent variation in the electronic property of the Pt nanoparticles and the reducible oxide encapsulation that prevents the Pt nanoparticles from sintering are ascribed as the main reasons for the excellent catalytic performance.

Solid-to-Hollow Conversion of Silver Nanocrystals by Surface-Protected Etching.

Although hollow silver nanocrystals possess unique plasmonic properties, there is a lack of robust strategies to synthesize such nanocrystals with high efficiency and controllability. To solve this problem, a new surface-protected etching strategy to convert solid Ag nanocrystals, which are widely available from conventional syntheses, into their hollow counterparts, producing a family of hollow Ag nanocrystals is reported. Hollow Ag nanospheres and nanotubes were prepared conveniently in this way. The key was the surface modification of Ag nanocrystals by a minor amount of Pt prior to a controllable etching process, which accounts for enhanced stability of the Ag surface and subsequent etching of Ag from the inner part of the nanocrystals while retaining the overall crystal morphology. These hollow Ag nanocrystals showed distinctive optical properties, as demonstrated by the enhanced optical transmittance of flexible electrodes fabricated with Ag nanotubes, compared to nanowires. These hollow Ag nanocrystals hold promise in different plasmonic and electronic applications.

Aqueous Synthesis of Ultrathin Platinum/Non-Noble Metal Alloy Nanowires for Enhanced Hydrogen Evolution Activity.

Although aqueous synthesis of nanocrystals is advantageous in terms of the cost, convenience, environmental friendliness, and surface cleanness of the product, nanocrystals of Pt and non-noble metal alloys are difficult to obtain with controlled morphology and composition from this synthesis owing to a huge gap between the reduction potentials of respective metal salts. This huge gap could now be remedied by introducing a sulfite into the aqueous synthesis, which is believed to resemble an electroless plating mechanism, giving rise to a colloid of Pt-M (M=Ni, Co, Fe) alloy nanowires with an ultrasmall thickness (ca. 2.6 nm) in a high yield. The sulfite also leads to the formation of surface M-S bonds and thus atomic-level Pt/M-S(OH) interfaces for greatly boosted hydrogen evolution kinetics under alkaline conditions. An activity of 75.3 mA cm-2 has been achieved with 3 µg of Pt in 1 m KOH at an overpotential of 70 mV, which is superior to previously reported catalysts.

Gold and gold-silver alloy nanoparticles enhance the myogenic differentiation of myoblasts through p38 MAPK signaling pathway and promote in vivo skeletal muscle regeneration.

Under the severe trauma condition, the skeletal muscles regeneration process is inhibited by forming fibrous scar tissues. Understanding the interaction between bioactive nanomaterials and myoblasts perhaps has important effect on the enhanced skeletal muscle tissue regeneration. Herein, we investigate the effect of monodispersed gold and gold-silver nanoparticles (AuNPs and Au-AgNPs) on the proliferation, myogenic differentiation and associated molecular mechanism of myoblasts (C2C12), as well as the in vivo skeletal muscle tissue regeneration. Our results showed that AuNPs and Au-AgNPs could support myoblast attachment and proliferation with negligible cytotoxicity. Under various incubation conditions (normal and differentiation medium), AuNPs and Au-AuNPs significantly enhanced the myogenic differentiation of myoblasts by upregulating the expressions of myosin heavy chain (MHC) protein and myogenic genes (MyoD, MyoG and Tnnt-1). The further analysis demonstrated that AuNPs and Au-AgNPs could activate the p38α mitogen-activated protein kinase pathway (p38α MAPK) signaling pathway and enhance the myogenic differentiation. Additionally, the AuNPs and Au-AgNPs significantly promote the in vivo skeletal muscle regeneration in a tibialis anterior muscle defect model of rat. This study may provide a nanomaterials-based strategy to improve the skeletal muscle repair and regeneration.

Synthesis of ultrathin platinum nanoplates for enhanced oxygen reduction activity.

Ultrathin Pt nanostructures exposing controlled crystal facets are highly desirable for their superior activity and cost-effectiveness in the electrocatalytic oxygen reduction reaction (ORR), and they are conventionally synthesized by epitaxial growth of Pt on a limited range of templates, such as Pd nanocrystals, resulting in a high cost and less structural diversity of the ultrathin Pt nanostructures. To solve this problem, we demonstrate that ultrathin Pt nanostructures can be synthesized by templating conveniently available Ag nanocrystals without involving galvanic replacement, which enables a much-reduced cost and controllable new morphologies, such as ultrathin Pt nanoplates that expose the {111} facets. The resulting ultrathin Pt nanoplates are ∼1-2 nm in thickness, which show an ∼22-fold increase in specific activity (5.3 mA cm-2), an ∼9.5-fold increase in mass activity (1.62 A mg-1) and significantly enhanced catalytic stability in the ORR, compared with the commercial Pt/C catalyst. We believe this strategy opens a door to a highly extendable family of ultrathin noble metal nanostructures, thus promising excellent activity and stability in a broad range of catalytic applications.

Fully alloyed Ag/Au nanorods with tunable surface plasmon resonance and high chemical stability.

Limited success has been achieved in preparing nanorods of silver with uniform sizes and tunable localized surface plasmon resonances. Also, the practical applications of silver nanostructures have been hindered by their poor chemical stability in a corrosive environment. Here we address these issues by converting Au@Ag core/shell nanorods into fully alloyed ones through controlled high-temperature annealing in confined spaces. Compared with their core/shell counterparts, the obtained alloy nanorods demonstrated significantly enhanced stability toward oxidative etching. We also systematically investigated their novel plasmonic properties, and revealed that the band positions of both longitudinal and transverse modes can be readily tuned by either manipulating the Ag/Au ratio or starting with gold cores of different aspect ratios. Moreover, we have achieved widely adjusted peak intensity ratios between the transverse and longitudinal bands from 0.14 to 1.22, which is impossible for nonalloyed nanorods. The alloy nanorods developed in this work are believed to find great uses in fundamental spectroscopic studies as well as many attractive plasmonic applications.

Gold nanoshurikens with uniform sharp tips for chemical sensing by the localized surface plasmon resonance.

Creation of uniform sharp tips in noble metal nanostructures is highly desirable for chemical sensing applications that rely on their localized surface plasmon resonance (LSPR), while it remains a great challenge as typically it is not energetically favorable. Herein, we report a robust synthesis route to a novel family of unique shuriken-shaped Au nanostructures with four in-plane sharp tips in high yield and uniformity. The success of the synthesis relies on the anisotropic crystal growth of quasi-planar Au seeds by taking advantage of the capping effect of a ligand on the specific facets, as well as the predominant deposition of Au over its surface diffusion that accounts for the formation of the sharp tips. The resulting Au nanoshurikens show remarkable LSPR in the near-infrared range of the spectrum, which proves to be sensitive to a minor change in the sharp tips, thus enabling superior chemical sensing activity, as demonstrated by detection of mercury of ultralow concentrations. This novel nanostructure promises not only great potential in monitoring mercury in aquatic ecosystems, but also wide applicability to many other sensing scenarios, such as analyzing various chemicals and biologically active species, with excellent sensitivity.