Fabrication of Hollow Inorganic Microspheres by Chemically Induced Self‐Transformation

Two contrasting approaches, involving either polymer‐mediated or fluoride‐mediated self‐transformation of amorphous solid particles, are described as general routes to the fabrication of hollow inorganic microspheres. Firstly, calcium carbonate and strontium tungstate hollow microspheres are fabricated in high yield using sodium poly(4‐styrenesulfonate) as a stabilizing agent for the formation and subsequent transformation of amorphous primary particles. Transformation occurs with retention of the bulk morphology by localized Ostwald ripening, in which preferential dissolution of the particle interior is coupled to the deposition of a porous external shell of loosely packed nanocrystals. Secondly, the fabrication process is extended to relatively stable amorphous microspheres, such as TiO₂ and SnO₂, by increasing the surface reactivity of the solid precursor particles. For this, fluoride ions, in the form of NH₄F and SnF₂, are used to produce well‐defined hollow spheroids of nanocrystalline TiO₂ and SnO₂, respectively. Our results suggest that the chemical self‐transformation of precursor objects under morphologically invariant conditions could be of general applicability in the preparation of a wide range of nanoparticle‐based hollow architectures for technological and biomedical applications.     

One‐Pot Synthesis and Hierarchical Assembly of Hollow Cu2O Microspheres with Nanocrystals‐Composed Porous Multishell and Their Gas‐Sensing Properties

Hierarchical assembly of hollow microstructures is of great scientific and practical value and remains a great challenge. This paper presents a facile and one‐pot synthesis of Cu₂O microspheres with multilayered and porous shells, which were organized by nanocrystals. The time‐dependent experiments revealed a two‐step organization process, in which hollow microspheres of Cu₂(OH)₃NO₃ were formed first due to the Ostwald ripening and then reduced by glutamic acid, the resultant Cu₂O nanocrystals were deposited on the hollow intermediate microspheres and organized into finally multishell structures. The special microstructures actually recorded the evolution process of materials morphologies and microstructures in space and time scales, implying an intermediate‐templating route, which is important for understanding and fabricating complex architectures. The Cu₂O microspheres obtained were used to fabricate a gas sensor, which showed much higher sensitivity than solid Cu₂O microspheres.     

Synthesis of Nickel Sulfide Submicrometer‐Sized Hollow Spheres Using a γ‐Irradiation Route

Nickel sulfide (NiS) hollow spheres have been successfully synthesized by γ‐irradiation, at room temperature, of an aqueous PMMA–CS₂–ethanol solution that contains NiSO₄·6H₂O. Electron microscopy results show that the diameter of the NiS hollow spheres and the thickness of the sphere shells are about 500 nm and 20 nm, respectively. The room‐temperature UV‐vis absorption spectrum of the NiS hollow spheres gives a peak centered at around 233 nm (5.56 eV) with a remarkable blue‐shift relative to that of bulk NiS (2.1 eV). This remarkable blue‐shift may be attributed to the small dimensions of the materials. A possible growth mechanism of NiS hollow spheres by γ‐irradiation method is also presented. The successful preparation of NiS hollow spheres on a large scale under mild conditions could be of interest for both applications and fundamental studies.     

Wavelength and Intensity Multiplexing of Metal Nanoparticles for the Fabrication of Multicolored Micro‐ and Nanospheres

Multicolored micro‐ and nanospheres have been successfully fabricated by a novel method that was developed to broaden the selection of colors available in the application of metal nanoparticles (NPs). The colors of the metal NPs have been systematically tuned over a wide spectral range by wavelength and intensity multiplexing of three‐colored NPs. Multicolor polystyrene micro‐ and nanospheres could be fabricated by combining the systematic color tuning of metal NPs and self‐assembly of metal NPs on the surfaces of the polymer spheres. The agglomeration of metal NPs on the surfaces of polymer spheres could be prevented by optimizing the concentration of the metal NPs and the reaction pH.     

Inside Front Cover: One‐Pot Synthesis and Hierarchical Assembly of Hollow Cu2O Microspheres with Nanocrystals‐Composed Porous Multishell and Their Gas‐Sensing Properties (Adv. Funct. Mater. 15/2007)

On p. 2766, Qinshan Zhu and co‐workers report on multishell hollow Cu₂O microspheres that are synthesized by a facile and one‐pot solvothermal route. A two‐step organization process, in which hollow microspheres of Cu₂(OH)₃NO₃ are formed first followed by reduction to Cu₂O by glutamic acid, leads to the special multishell and hollow microstructures. Interestingly, a Cu₂O gas sensor fabricated with the multishell microspheres shows a much higher sensitivity to ethanol than solid Cu₂O microspheres. Hierarchical assembly of hollow microstructures is of great scientific and practical value and remains a great challenge. This paper presents a facile and one‐pot synthesis of Cu₂O microspheres with multilayered and porous shells, which were organized by nanocrystals. The time‐dependent experiments revealed a two‐step organization process, in which hollow microspheres of Cu₂(OH)₃NO₃ were formed first due to the Ostwald ripening and then reduced by glutamic acid, the resultant Cu₂O nanocrystals were deposited on the hollow intermediate microspheres and organized into finally multishell structures. The special microstructures actually recorded the evolution process of materials morphologies and microstructures in space and time scales, implying an intermediate‐templating route, which is important for understanding and fabricating complex architectures. The Cu₂O microspheres obtained were used to fabricate a gas sensor, which showed much higher sensitivity than solid Cu₂O microspheres.     

Encapsulation and Ostwald Ripening of Au and Au–Cl Complex Nanostructures in Silica Shells

We report a general template strategy for rational fabrication of a new class of nanostructured materials consisting of multicore shell particles. Our approach is demonstrated by encapsulating Au or Pt nanoparticles in silica shells. Other superstructures of these hollow shells, like dimers, trimers, and tetramers can also be formed by nanoparticle‐mediated self‐assembly. We have also used the as‐prepared multicore Au–silica hollow particles to perform the first studies of Ostwald ripening in confined microspace, in which chloride was found to be an efficient mediating ligand. After treatment with aqua regia, Au–Cl complex is formed inside the shell, and is found to be very active under in situ transmission electron microscopy observations while confined in a microcell. This aspect of the work is expected to motivate further in situ studies of confined crystal growth.     

Fabrication of Thermoresponsive Plasmonic Microspheres with Long‐Term Stability from Hydrogel Spheres

Gold nanoparticles have been incorporated with high efficiency into thermoresponsive hydrogel spheres, due to the suitable relationship of the nanoparticles and pore sizes given by the gel network. The hydrogel loaded with nanoparticles remains thermoresponsive, and the loaded gold nanoparticles exhibit little aggregation as detected by plasmon resonance, and are reversibly sensitive to the refractive index and temperature of the surrounding media. The layer‐by‐layer growth of polyelectrolyte multilayers on the composite spheres has also been demonstrated. These thermoresponsive plasmonic microspheres are appealing for application in biological assays.     

Vapor Sorption and Electrical Response of Au‐Nanoparticle– Dendrimer Composites

Films comprising Au nanoparticles and polyphenylene dendrimers (first and second generation) are deposited onto transducer substrates via layer‐by‐layer self‐assembly and characterized by atomic force microscopy and X‐ray photoelectron spectroscopy. Their sorption behavior is studied by measuring the uptake of solvents from the vapor phase with quartz crystal microbalances (QCMs). The resistance of the films is simultaneously monitored. Both sensor types, QCMs and chemiresistors, give qualitatively very similar response isotherms that are consistent with a combination of Henry‐ and Langmuir‐type sorption processes. The sorption‐induced increase in relative differential resistance scales linearly with the amount of analyte accumulated in the films. This result is in general agreement with an activated tunneling process for charge transport, if little swelling and only small changes in the permittivity of the film occur during analyte sorption (a first‐order approximation). The relative sensitivity of the films to different solvents decreases in the order toluene ≈ tetrachloroethylene > 1‐propanol ? water. Films containing the larger second‐generation dendrimers show higher sensitivity than films containing first‐generation dendrimers.     

Synthesis and Assembly of Gold Nanoparticles in Quasi‐Linear Lysine–Keggin‐Ion Colloidal Particles

The formation of fiber‐like colloidal particles of the amino acid lysine complexed with Keggin ions is demonstrated. The lysine–phosphotungstic acid (PTA) colloidal particles act as excellent templates for the synthesis and assembly of gold nanoparticles wherein the lysine‐PTA complex acts as a UV‐switchable reducing agent for gold ions. This novel bio‐organic–inorganic template shows excellent potential as a regulated nanoreactor for application in programmed nanoparticle synthesis and assembly in a single step.     

Assembly of Gold Nanoparticles in a Rod‐Like Fashion Using Proteins as Templates

An area of considerable current interest is the development of a practical approach for assembling inorganic nanoparticles into well‐defined arrays because such a technique would offer immense opportunities leading to applications in microimaging, optoelectronics, therapeutics, etc. This paper illustrates a new, simple one‐step process in which proteins act as templates to assemble gold nanoparticles in a shape‐selective fashion. We show, for the first time, that antibodies to vascular endothelial growth factor 165 isoform, 2C3, and epidermal growth factor receptor can act as templates when present in solution during the synthesis of gold nanoparticles. These proteins direct the assembly of the gold nanoparticles into rod‐like shapes when cooled to –20 °C followed by thawing at room temperature. Immunoglobulin G and bovine serum albumin can also direct the assembly process in a similar fashion; however, small molecules, such as poly(L ‐lysine) and lysine, cannot. The formation of a self‐assembled structure in the form of a continuous rod, or the assembly of discrete nanoparticles in a rod‐like fashion, can be tailored by controlling the ratio of the precursor gold salt, HAuCl₄, to the antibody/protein used as the template. The nanoconjugates are characterized using UV‐vis spectroscopy, transmission electron microscopy, and infrared spectroscopy. The nano‐bioconjugates obtained via this process may find wide application in areas ranging from optoelectronics and biosensors to therapeutics in neoplastic disorders.     

Cover Picture: Vapor Sorption and Electrical Response of Au‐Nanoparticle– Dendrimer Composites (Adv. Funct. Mater. 6/2007)

The cover shows chemiresistors and mass‐sensitive vapor sensors coated with Au‐nanoparticle/dendrimer composites. The Au nanoparticles provide the film with electrical conductivity and the dendrimers control the chemical selectivity, as reported by Nadjedja Krasteva and co‐workers on p. 881. Analyte sorption follows a combined Henry–Langmuir model, and measurements reveal that sorption‐induced increase in film resistance scales linearly with the concentration of analyte sorbed in the film. The background shows a silicon wafer with lithographically defined microelectrode structures for chemiresistor fabrication. Films comprising Au nanoparticles and polyphenylene dendrimers (first and second generation) are deposited onto transducer substrates via layer‐by‐layer self‐assembly and characterized by atomic force microscopy and X‐ray photoelectron spectroscopy. Their sorption behavior is studied by measuring the uptake of solvents from the vapor phase with quartz crystal microbalances (QCMs). The resistance of the films is simultaneously monitored. Both sensor types, QCMs and chemiresistors, give qualitatively very similar response isotherms that are consistent with a combination of Henry‐ and Langmuir‐type sorption processes. The sorption‐induced increase in relative differential resistance scales linearly with the amount of analyte accumulated in the films. This result is in general agreement with an activated tunneling process for charge transport, if little swelling and only small changes in the permittivity of the film occur during analyte sorption (a first‐order approximation). The relative sensitivity of the films to different solvents decreases in the order toluene ≈ tetrachloroethylene > 1‐propanol ? water. Films containing the larger second‐generation dendrimers show higher sensitivity than films containing first‐generation dendrimers.     

ZnO–SnO2 Hollow Spheres and Hierarchical Nanosheets: Hydrothermal Preparation,Formation Mechanism,and Photocatalytic Properties

ZnO–SnO₂ hollow spheres and hierarchical nanosheets are successfully synthesized using an aqueous solution containing ZnO rods, SnCl₄, and NaOH by using a simple hydrothermal method. The effects of hydrothermal temperature and time on the morphology of ZnO–SnO₂ are investigated. The formation process of ZnO–SnO₂ hollow spheres and nanosheets is discussed. The samples are characterized using X‐ray powder diffraction, transmission electron microscopy, scanning electron microscopy, and UV‐vis absorption spectroscopy. Both hollow spheres and hierarchical nanosheets show higher photocatalytic activities in the degradation of methyl orange than that of ZnO rods or SnO₂.     

Preparation and Organization of Nanoscale Polyelectrolyte‐Coated Gold Nanoparticles

The layer‐by‐layer (LbL) desposition of oppositely charged polyelectrolytes from adsorption solutions of different ionic strength onto ～7 nm diameter carboxylic acid‐derivatized gold nanoparticles has been studied. The polyelectrolyte‐modified nanoparticles were characterized by UV‐vis spectrophotometry, microelectrophoresis, analytical ultracentrifugation, and transmission electron microscopy. UV‐vis data showed that the peak plasmon absorption wavelength of the gold nanoparticles red‐shifted after each adsorption step, and microelectrophoresis experiments revealed a reversal in the surface charge of the nanoparticles following deposition of each layer. These data are consistent with the formation of polyelectrolyte layers on the nanoparticles. Analytical ultracentrifugation showed an increase in mean nanoparticle diameter on adsorption of the polyelectrolytes, confirming the formation of gold‐core/polyelectrolyte‐shell nanoparticles. Transmission electron microscopy studies showed no signs of aggregation of the polyelectrolyte‐coated nanoparticles. The adsorption of the polyelectrolyte‐coated gold nanoparticles onto oppositely charged planar supports has also been examined. UV‐vis spectrophotometry and atomic force microscopy showed increased amounts of nanoparticles were adsorbed with increasing ionic strength of the nanoparticle dispersions. This allows control of the nanoparticle surface loading by varying the salt content in the nanoparticle dispersions used for adsorption. The LbL strategy used in this work is expected to be applicable to other nanoparticles (e.g., semiconductors, phosphors), thus providing a facile means for their controlled surface modification through polyelectrolyte nanolayering. Such nanoparticles are envisaged to have applications in the biomedical and bioanalytical fields, and to be useful building blocks for the creation of advanced nanoparticle‐based films.     

Generation of Palladium Nanoparticles within Macrocellular Polymeric Supports: Application to Heterogeneous Catalysis of the Suzuki–Miyaura Coupling Reaction

We present new hybrid organic/inorganic materials dedicated to heterogeneous catalysis. The systems are obtained by the polymerization of a high internal phase reverse emulsion (the so‐called polyHIPE porous materials) and have been further functionalized with various organic groups in order to promote the growth of palladium nanoparticles on its surface. Final supports are then tested for their ability to catalyze the Suzuki–Miyaura coupling reaction, and one material exhibits better activity than the well‐known Pd@C powder system. Furthermore, the catalytic activities of these materials are close to those obtained with their homogeneous catalysis counterpart. These new supports remain active towards a wide range of substrates associated with Suzuki–Miyaura carbon–carbon coupling reactions.     

Preparation of Hollow Silica Nanospheres by Surface‐Initiated Atom Transfer Radical Polymerization on Polymer Latex Templates

Poly(vinylbenzyl chloride), (PVBC) latex particles of about 100 nm in size are prepared by emulsion polymerization. Silyl functional groups are introduced onto the PVBC‐nanoparticle templates via surface‐initiated atom transfer radical polymerization of 3‐(trimethoxysilyl)propyl methacrylate. The silyl groups are then converted into a silica shell, approximately 20 nm thick, via a reaction with tetraethoxysilane in ethanolic ammonia. Hollow silica nanospheres are finally generated by thermal decomposition of the PVBC template cores. Field‐emission scanning electron microscopy and field‐emission transmission electron microscopy are used to characterize the intermediate products and the hollow nanospheres. Fourier‐transform infrared spectroscopy results indicate that the polymer cores are completely decomposed.     

Electrical Properties and Structure of p‐Type Amorphous Oxide Semiconductor xZnO·Rh2O3

p‐Type conduction in amorphous oxide was firstly found in zinc rhodium oxide (ZnO·Rh₂O₃) (Adv. Mater. 2003 , 15, 1409), and it is still the only p‐type amorphous oxide to date. It was reported that an ordered structure at the nanometer scale was contained and its electronic structure is not clear yet. In this paper, optoelectronic and structural properties are reported in detail for xZnO·Rh₂O₃ thin films (x = 0.5–2.0) in relation to the chemical composition x. All the films exhibit positive Seebeck coefficients, confirming p‐type conduction. Local network structure strongly depends on the chemical composition. Transmission electron microscopic observations reveal that lattice‐like structures made of edge‐sharing RhO₆ network exist in 2–3 nm sized grains for rhodium‐rich films (x = 0.5 and 1.0), while the zinc‐rich film (x = 2) is completely amorphous. This result indicates that excess Zn assists to form an amorphous network in the ZnO–Rh₂O₃ system since Zn ions tend to form corner‐sharing networks. The electronic structure of an all‐amorphous oxide p‐ZnO·Rh₂O₃/n‐InGaZnO₄ junction is discussed with reference to electrical characteristics and results of photoelectron emission measurements, suggesting that the p/n junction has large band offsets at the conduction and valence bands, respectively.     

Magnetic‐Core/Porous‐Shell CoFe2O4/SiO2 Composite Nanoparticles as Immobilized Affinity Supports for Clinical Immunoassays

This study demonstrates a novel approach towards the development of advanced protein assay systems based on physically functionalized, magnetic‐core/porous‐shell CoFe₂O₄/SiO₂ composite nanoparticles. The preparation, characterization, and measurement of the relevant properties of the protein assay system is discussed, and the system is used for the detection of cancer antigen 15‐3 (CA 15‐3, used as a model here) in clinical immunoassays. The protein assay system, based on nanometer‐sized magnetic cores and silica shells, shows good adsorption properties for the selective attachment of CA 15‐3 antibodies specific to CA 15‐3. The core/shell nanostructures exhibit good magnetic properties, which enables their integration into a quartz crystal microbalance (QCM) detection cell with the help of a permanent magnet. Under optimal conditions, the resulting immunoassay system presents a good QCM response for the detection of CA 15‐3, and allows the detection of CA 15‐3 at concentrations as low as 1.5 U mL^–1 (U: units). Importantly, the proposed protein assay system can be extended to the detection of other antigens and biological compounds.     

(Pb1–xCdx)S Nanoparticles Embedded in a Conjugated Organic Matrix,as Studied by Photoluminescence and Light‐Induced X‐ray Photoelectron Spectroscopy

Elliptically shaped (Pb_1–xCd_x)S nanoparticles (NPs) of average size 2.3 × 2.9 nm (minor axis × major axis) have been prepared via reaction of a solid [oligo(p‐phenylene‐ethynylene) dicarboxylate]Pb_0.9Cd_0.1 salt matrix, with gaseous H₂S. A significantly long emission lifetime, with multi‐exponential behavior, is detected in time‐resolved photoluminescence measurements, substantially different from the decay patterns of pure PbS and CdS NPs within the same organic matrix. Evidence for the co‐existence of Cd and Pb within the same particle is provided by light‐induced X‐ray photoelectron spectroscopy.     

Single and Multiple‐Step Dip‐Coating of Colloidal Maghemite (γ‐Fe2O3) Nanoparticles onto Si,Si3N4, and SiO2 Substrates

The adsorption behavior of colloidal maghemite (γ‐Fe₂O₃) nanoparticles, passivated by oleic acid and dispersed in octane solution, onto three different substrates (Si, Si₃N₄, and SiO₂) is investigated. The average nanoparticle size is 10 nm, with a size variation (σ) less than 5 %. The adsorption of particles is strongly dependent on both the type of substrate and the particle concentration in solution. By a single‐dipping process, we have obtained a maximum coverage of 0.45 on a Si substrate, but much less on other substrates (0.19 on Si₃N₄ and 0.14 on SiO₂). The particle coverage was drastically increased by the multiple‐adsorption process, where the process of dipping and drying was repeated multiple times. With this process, we can obtain a maximum particle coverage of about 0.76 on a Si substrate and 0.61 on a thermally grown SiO₂ substrate.     

3D MoS2–Graphene Microspheres Consisting of Multiple Nanospheres with Superior Sodium Ion Storage Properties

A novel anode material for sodium‐ion batteries consisting of 3D graphene microspheres divided into several tens of uniform nanospheres coated with few‐layered MoS₂ by a one‐pot spray pyrolysis process is prepared. The first discharge/charge capacities of the composite microspheres are 797 and 573 mA h g^?1 at a current density of 0.2 A g^?1. The 600th discharge capacity of the composite microspheres at a current density of 1.5 A g^?1 is 322 mA h g^?1. The Coulombic efficiency during the 600 cycles is as high as 99.98%. The outstanding Na ion storage properties of the 3D MoS₂–graphene composite microspheres may be attributed to the reduced stacking of the MoS₂ layers and to the 3D structure of the porous graphene microspheres. The reduced stacking of the MoS₂ layers relaxes the strain and lowers the barrier for Na⁺ insertion. The empty nanospheres of the graphene offer voids for volume expansion and pathways for fast electron transfer during repeated cycling.