Well‐Defined Metal–Organic Framework Hollow Nanocages

Metal–organic frameworks (MOFs) have demonstrated great potentials in a variety of important applications. To enhance the inherent properties and endow materials with multifunctionality, the rational design and synthesis of MOFs with nanoscale porosity and hollow feature is highly desired and remains a great challenge. In this work, the formation of a series of well‐defined MOF (MOF‐5, Fe^II‐MOF‐5, Fe^III‐MOF‐5) hollow nanocages by a facile solvothermal method, without any additional supporting template is reported. A surface‐energy‐driven mechanism may be responsible for the formation of hollow nanocages. The addition of pre‐synthesized poly(vinylpyrrolidone)‐ (PVP) capped noble‐metal nanoparticles into the synthetic system of MOF hollow nanocages yields the yolk–shell noble metal@MOF nanostructures. The present strategy to fabricate hollow and yolk–shell nanostructures is expected to open up exciting opportunities for developing a novel class of inorganic–organic hybrid functional nanomaterials.     

Gold and silver nanoparticles: a class of chromophores with colors tunable in the range from 400 to 750 nm

This paper presents several solution-phase methods for the large-scale synthesis of metal nanoparticles with controllable compositions (e.g., spherical nanoparticles of gold/silver alloys), morphologies (e.g., nanospheres, triangular nanoplates, circular nanodisks, and nanocubes of silver), and structures (e.g., solid vs. hollow colloids). Spectral measurements indicated that the positions of surface plasmon resonance (SPR) bands for these nanoparticles could be tuned by varying all these parameters. The number of SPR peaks was found to increase as the symmetry of the nanoparticles decreased. In addition to their use as chromophores with strong extinction coefficients, these nanoparticles could serve as a platform to probe binding events of chemical and biochemical species on their surfaces. Gold nanoshells with hollow interiors were, in particular, shown to exhibit a much higher sensitivity to environmental changes than gold solid colloids with roughly the same size.     

Photothermal ablation therapy for cancer based on metal nanostructures

Besides conventional surgery, radiation therapy, and chemotherapy, which all tend to have side-effects and damage normal tissues, new medical strategies, such as photothermal sensitization and photo-thermal ablation therapy (PTA) with near-IR laser light, have been explored for treating cancer. Much of the current excitement surrounding nanoscience is directly connected to the promise of new nanotechnology for cancer diagnosis and therapy. The basic principle behind PTA is that heat generated from light can be used to destroy cancer cells. Strong optical absorption and high efficiency of photothermal conversion at the cancer sites are critical to the success of PTA. Because of their unique optical properties, e.g., strong surface plasmon resonance (SPR) absorption, noble metal nanomaterials, such as gold and silver, have been found to significantly enhance photothermal conversion for PTA applications. Substantial effort has been made to develop metal nanostructures with optimal structural and photothermal properties. Ideal metal nanostructures should have strong and tunable SPR, be easy to deliver, have low toxicity, and be convenient for bioconjugation for actively targeting specific cancer cells. This review would highlight some gold nanostructures with various shapes and properties, including nanoparticles (NPs), nanorods (NRs), nanoshells, nanocages, and hollow nanospheres, which have been studied for PTA applications. Among these structures, hollow gold nanospheres (HGNs) exhibit arguably the best combined properties because of their small size (30―50 nm), spherical shape, and strong, narrow, and tunable SPR absorption.     

Ag@C core/shell structured nanoparticles: controlled synthesis, characterization, and assembly

Silver nanoparticles with tunable sizes were encapsulated in a carbonaceous shell through a green wet chemical route-the catalyzed dehydration of glucose under hydrothermal condition. In this one-pot synthesis, glucose was used as the reducing agent to react with Ag+ or Ag(NH3)2+, and it also served as the source of carbonaceous shells. The effects of hydrothermal temperature, time, and the concentrations of reagents on formation of the final nanostructures were systematically studied. The presence of competitive molecules poly(vinyl pyrrolidone) was found to be able to relieve the carbonization process, to incorporate themselves into carbonaceous shell, and to make the carbonaceous shell colorless. All these approaches provided diverse means to tailor the Ag@C nanostructures. By evaporation of the solvents gradually in a moist atmosphere, the monodispersed nanoparticles could self-assemble into arrays. Transmission electron microscopy, scanning electron microscopy, and UV-vis extinction spectra and surface-enhanced Raman spectra were used to characterize the core/shell nanostructures. These Ag@C core/shell nanoparticles have hydrophilic, organic-group-loaded surfaces and characteristic optical properties, which indicated their promising applications in optical nanodevices and biochemistry.     

Effect of concentrations of silver(I) and gold(III) initial compounds on the properties of AgAu bimetal nanocages

Results of studying kinetics of formation of hollow bimetal nanoparticles (nanocages) based on silver and gold that are obtained by the three-stage synthesis are presented. The factors defining sizes, structure, composition, and optical properties of nanocages were revealed by means of high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy.     

Double‐Shelled Nanocages with Cobalt Hydroxide Inner Shell and Layered Double Hydroxides Outer Shell as High‐Efficiency Polysulfide Mediator for Lithium–Sulfur Batteries

Lithium–sulfur (Li‐S) batteries have been considered as a promising candidate for next‐generation electrochemical energy‐storage technologies because of their overwhelming advantages in energy density. Suppression of the polysulfide dissolution while maintaining a high sulfur utilization is the main challenge for Li–S batteries. Here, we have designed and synthesized double‐shelled nanocages with two shells of cobalt hydroxide and layered double hydroxides (CH@LDH) as a conceptually new sulfur host for Li–S batteries. Specifically, the hollow CH@LDH polyhedra with complex shell structures not only maximize the advantages of hollow nanostructures for encapsulating a high content of sulfur (75 wt %), but also provide sufficient self‐functionalized surfaces for chemically bonding with polysulfides to suppress their outward dissolution. When evaluated as cathode material for Li–S batteries, the CH@LDH/S composite shows a significantly improved electrochemical performance.     

Yolk/shell nanoparticles: new platforms for nanoreactors, drug delivery and lithium-ion batteries

Yolk/shell or 'rattle-typed' nanomaterials with nanoparticle cores inside hollow shells are interesting among the complex hollow nanostructures. Yolk/shell nanoparticles (YSNs) are promising functional nanomaterials for a variety of applications such as catalysis, delivery, lithium-ion batteries and biosensors due to their tailorability and functionality in both the cores and hollow shells. This feature article provides an overview of advances in this exciting area of YSNs, covering systematic synthesis approaches and key promising applications based on the literature and our own recent work. We present some strategies for the synthesis of YSNs with controllable sizes, compositions, geometries, structures and functionalities. Applications of these new materials in a wide range of potential areas are discussed including nanoreactors, biomedicine and lithium-ion batteries. Promising future directions of this active research field are also highlighted.     

Silica-metal core-shell nanostructures

Silica-metal nanostructures consisting of silica cores and metal nanoshells attract a lot of attention because of their unique properties and potential applications ranging from catalysis and biosensing to optical devices and medicine. The important feature of these nanostructures is the possibility of controlling their properties by the variation of their geometry, shell morphology and shell material. This review is devoted to silica-noble metal core-shell nanostructures; specifically, it outlines the main methods used for the preparation and surface modification of silica particles and presents the major strategies for the formation of metal nanoshells on the modified silica particles. A special emphasis is given to the St?ber method, which is relatively simple, effective and well verified for the synthesis of large and highly uniform silica particles (with diameters from 100 nm to a few microns). Next, the surface chemistry of these particles is discussed with a special focus on the attachment of specific organic groups such as aminopropyl or mercaptopropyl groups, which interact strongly with metal species. Finally, the synthesis, characterization and application of various silica-metal core-shell nanostructures are reviewed, especially in relation to the siliceous cores with gold or silver nanoshells. Nowadays, gold is most often used metal for the formation of nanoshells due to its beneficial properties for many applications. However, other metals such as silver, platinum, palladium, nickel and copper were also used for fabrication of core-shell nanostructures. Silica-metal nanostructures can be prepared using various methods, for instance, (i) growth of metal nanoshells on the siliceous cores with deposited metal nanoparticles, (ii) reduction of metal species accompanied by precipitation of metal nanoparticles on the modified silica cores, and (iii) formation of metal nanoshells under ultrasonic conditions. A special emphasis is given to the seed-mediated growth, where metal nanoshells are formed on the modified silica cores with deposited metal nanoparticles. This strategy assures a good control of the nanoshell thickness as well as its surface properties.     

Unusual Formation of CoSe@carbon Nanoboxes,which have an Inhomogeneous Shell,for Efficient Lithium Storage

Hybrid hollow nanostructures with tailored shell architectures are attractive for electrochemical energy storage applications. Starting with metal–organic frameworks (MOFs), we demonstrate a facile formation of hybrid nanoboxes with complex shell architecture where a CoSe‐enriched inner shell is intimately confined within a carbon‐enriched outer shell (denoted as CoSe@carbon nanoboxes). The synthesis is realized through manipulation of the template‐engaged reaction between Co‐based zeolitic imidazolate framework (ZIF‐67) nanocubes and Se powder at elevated temperatures. By virtue of the structural and compositional features, these unique CoSe@carbon nanoboxes manifest excellent lithium‐storage performance in terms of high specific capacity, exceptional rate capability, excellent cycling stability, and high initial Coulombic efficiency.     

Unusual Formation of CoSe@carbon Nanoboxes,which have an Inhomogeneous Shell,for Efficient Lithium Storage

Hybrid hollow nanostructures with tailored shell architectures are attractive for electrochemical energy storage applications. Starting with metal–organic frameworks (MOFs), we demonstrate a facile formation of hybrid nanoboxes with complex shell architecture where a CoSe‐enriched inner shell is intimately confined within a carbon‐enriched outer shell (denoted as CoSe@carbon nanoboxes). The synthesis is realized through manipulation of the template‐engaged reaction between Co‐based zeolitic imidazolate framework (ZIF‐67) nanocubes and Se powder at elevated temperatures. By virtue of the structural and compositional features, these unique CoSe@carbon nanoboxes manifest excellent lithium‐storage performance in terms of high specific capacity, exceptional rate capability, excellent cycling stability, and high initial Coulombic efficiency.     

Nanosized Heterostructures of Au@Prussian Blue Analogues: Towards Multifunctionality at the Nanoscale

Access to multifunctionality at the nanoscale requires the development of hybrid nanostructures that combine materials of different natures. In this line of thought, current research on coordination polymers is not only focusing on their synthesis at the nanoscale, but also on combining these polymers with other materials. According to a novel and rational approach, single‐layer Au@Prussian blue analogue (PBA) and double‐layer Au@PBA@PBA′ core–shell nanoparticles (NPs) may be obtained through the growth of a cyano‐bridged coordination network on the gold surface. The nanosized heterostructures combine the plasmonic optical properties of the gold core and the magnetic properties of the PBA shell. Whereas the single‐layer nanoparticles are paramagnetic, the double‐layer nanostructures display ferromagnetism; therefore, the overall structural motif may be considered as multifunctional. The developed synthetic concept also includes an easy access to hollow PBA NPs.     

Electronic and Vibrational Dynamics of Hollow Au Nanocages Embedded in Cu2O Shells

We have synthesized hollow Au nanocages embedded within thick porous shells of cuprous oxide (Cu₂O). The shell causes a significant redshift of the localized surface plasmon resonance of Au into the near‐IR. Electron–phonon coupling in the Au nanocage is 3–6 times faster in the core–shell structure due to the higher thermal conductivity of Cu₂O compared to water. Coherent phonon oscillations within the Au lattice are characterized by a breathing mode of the entire structure for both bare and core–shell nanocages, an assignment made through the use of structural mechanics simulations. The experimental frequencies are obtained through simulations by selectively applying a force to the shell of the core–shell structure. We interpret this as rapid thermal expansion of the gold leading to a mechanical force that acts on the shell.     

Synthesis and Characterization of Pd@MxCu1−x (M=Au,Pd, and Pt) Nanocages with Porous Walls and a Yolk–Shell Structure through Galvanic Replacement Reactions

This paper describes the synthesis of Pd@M_xCu_1?x (M=Au, Pd, and Pt) nanocages with a yolk–shell structure through galvanic replacement reactions that involve Pd@Cu core–shell nanocubes as sacrificial templates and ethylene glycol as the solvent. Compared with the most commonly used templates based on Ag, Cu offers a much lower reduction potential (0.34 versus 0.80 V), making the galvanic reaction more easily to conduct, even at room temperature. Our structural and compositional characterizations indicated that the products were hollow inside, and each one of them contained porous M–Cu alloy walls and a Pd cube in the interior. For the Pd@Au_xCu_1?x yolk–shell nanocages, they displayed broad extinction peaks extending from the visible to the near‐IR region. Our mechanistic study revealed that the dissolution of the Cu shell preferred to start from the slightly truncated corners and then progressed toward the interior, because the Cu {100} side faces were protected by a surface capping layer of hexadecylamine. This galvanic approach can also be extended to generating other hollow metal nanostructures by using different combinations of Cu nanostructures and salt precursors.     

Preparation and properties of silica–gold core–shell particles

Silica-metal core–shell particles, as for instance those having siliceous core and nanostructured gold shell, attracted a lot of attention because of their unique properties resulting from combination of mechanical and thermal stability of silica and magnetic, electric, optical and catalytic properties of metal nanocrystals such as gold, silver, platinum and palladium. Often, the shell of the core–shell particles consists of a large number of metal nanoparticles deposited on the surface of relatively large silica particles, which is the case considered in this work. Namely, silica particles having size of about 600 nm were subjected to surface modification with 3-aminopropyltrimethoxysilane. This modification altered the surface properties of silica particles, which was demonstrated by low pressure nitrogen adsorption at ?196 °C. Next, gold nanoparticles were deposited on the surface of aminopropyl-modified silica particles using two strategies: (i) direct deposition of gold nanoparticles having size of about 10 nm, and (ii) formation of gold nanoparticles by adsorption of tetrachloroauric acid on aminopropyl groups followed by its reduction with formaldehyde.The overall morphology of silica–gold particles and the distribution of gold nanoparticles on the surface of modified silica colloids were characterized by scanning electron microscopy. It was shown that direct deposition of colloidal gold on the surface of large silica particles gives more regular distribution of gold nanopartciles than that obtained by reduction of tetrachloroauric acid. In the latter case the gold layer consists of larger nanoparticles (size of about 50 nm) and is less regular. Note that both deposition strategies afforded silica–gold particles having siliceous cores covered with shells consisting of gold nanoparticles of tunable concentration.     

Recent advances in polymer protected gold nanoparticles: synthesis, properties and applications

The use of various functionalized polymers as stabilizers to design metal core-organic shell hybrid nanoparticle architectures has attracted increasing interest for different applications. The feature article reviews recent reports published from 2004 to the beginning of 2007 on the synthesis of polymer protected gold nanoparticles (AuNPs), and also comments their properties and applications.     

From Precision Synthesis of Block Copolymers to Properties and Applications of Nanoparticles

Inorganic nanoparticles have become a research focus in numerous fields because of their unique properties that distinguish them from their bulk counterparts. Controlling the size and shape of nanoparticles is an essential aspect of nanoparticle synthesis. Preparing inorganic nanoparticles by using block copolymer templates is one of the most reliable routes for tuning the size and shape of nanoparticles with a high degree of precision. In this Review, we discuss recent progress in the design of block copolymer templates for crafting spherical inorganic nanoparticles including compact, hollow, and core–shell varieties. The templates are divided into two categories: micelles self‐assembled from linear block copolymers and unimolecular star‐shaped block copolymers. The precise control over the size and morphology of nanoparticles is highlighted as well as the useful properties and applications of such inorganic nanoparticles.     

Synthesis of Monodisperse Hollow Carbon Nanocapsules by Using Protective Silica Shells

Monodisperse hollow carbon nanocapsules (<200 nm) with mesoporous shells were synthesized by coating their outer shells with silica to prevent aggregation during their high‐temperature annealing. Monodispersed silica nanoparticles were used as starting materials and octadecyltrimethoxysilane (C18TMS) was used as a carbon source to create core–shell nanostructures. These core–shell nanoparticles were coated with silica on their outer shell to form a second shell layer. This outer silica shell prevented aggregation during calcination. The samples were characterized by TEM, SEM, dynamic light scattering (DLS), UV/Vis spectroscopy, and by using the Brunauer–Emmett–Teller (BET) method. The as‐synthesized hollow carbon nanoparticles exhibited a high surface area (1123 m² g^?1) and formed stable dispersions in water after the pegylation process. The drug‐loading and drug‐release properties of these hollow carbon nanocapsules were also investigated.     

Ordered two-dimensional ensembles of nanoparticles with Au-core/Ag-shell structure. Optical and sensor properties

Optical spectroscopy and atomic force microscopy are used to study the formation of silver shells on gold nanoparticles in their highly ordered two-dimensional ensembles produced by the method of block copolymer micellar lithography. The shells are synthesized using formaldehyde or hydroquinone as a reducing agent. The spectral characteristics of the nanoparticle ensemble are shown to be essentially sensitive to the silver shell thickness. As a result, working units of solid-state optical sensors for detecting trace amounts of metal ions and different reductants may be created based on highly ordered 2D ensembles of gold nanoparticles.     

Hollow Zn/Co Zeolitic Imidazolate Framework (ZIF) and Yolk–Shell Metal@Zn/Co ZIF Nanostructures

Metal–organic frameworks (MOFs) feature a great possibility for a broad spectrum of applications. Hollow MOF structures with tunable porosity and multifunctionality at the nanoscale with beneficial properties are desired as hosts for catalytically active species. Herein, we demonstrate the formation of well‐defined hollow Zn/Co‐based zeolitic imidazolate frameworks (ZIFs) by use of epitaxial growth of Zn‐MOF (ZIF‐8) on preformed Co‐MOF (ZIF‐67) nanocrystals that involve in situ self‐sacrifice/excavation of the Co‐MOF. Moreover, any type of metal nanoparticles can be accommodated in Zn/Co‐ZIF shells to generate yolk–shell metal@ZIF structures. Transmission electron microscopy and tomography studies revealed the inclusion of these nanoparticles within hollow Zn/Co‐ZIF with dominance of the Zn‐MOF as shell. Our findings lead to a generalization of such hollow systems that are working effectively to other types of ZIFs.     

Fabrication of nanorattles with passive shell

This investigation describes the formation of a metal nanorattle with a pure metal shell by varying experimental parameters. The galvanic replacement reaction between silver and chloroauric acid was adopted to prepare hollow metal nanoparticles. This approach is extended to produce nanorattles of Au cores and Au shells by starting with Au(core)Ag(shell) nanoparticles as templates. The effect of temperature on the nanostructure of the final product is also considered. The composition of the shell in nanorattles can be controlled by varying the reaction temperature (to form pure gold or gold-silver alloy, for example). X-ray absorption fine structure spectroscopy is conducted to elucidate the fine structure of these nanoparticles. Partial alloying between the Au core and the Ag shell is observed by extended X-ray absorption fine structure (EXAFS).