Positively Thermo‐Sensitive Monodisperse Core–Shell Microspheres

In this paper, we report on a novel family of monodisperse thermo‐sensitive core–shell hydrogel microspheres that is featured with high monodispersity and positively thermo‐responsive volume phase transition characteristics with tunable swelling kinetics, i.e., the particle swelling is induced by an increase rather than a decrease in temperature. The microspheres were fabricated in a three‐step process. In the first step, monodisperse poly(acrylamide‐co‐styrene) seeds were prepared by emulsifier‐free emulsion polymerization. In the second step, poly(acrylamide) or poly[acrylamide‐co‐(butyl methacrylate)] shells were fabricated on the microsphere seeds by free radical polymerization. In the third step, the core–shell microspheres with poly‐ (acrylamide)/poly(acrylic acid) based interpenetrating polymer network (IPN) shells were finished by a method of sequential IPN synthesis. The proposed monodisperse core–shell microspheres provide a new mode of the phase transition behavior for thermo‐sensitive “smart” or “intelligent” monodisperse micro‐actuators that is highly attractive for targeting drug delivery systems, chemical separations, sensors, and so on.     

A Micellar Route to Ordered Arrays of Magnetic Nanoparticles: From Size‐Selected Pure Cobalt Dots to Cobalt–Cobalt Oxide Core–Shell Systems

Starting with Co‐salt‐loaded inverse micelles, which form if the diblock copolymer polystyrene‐block‐poly(2‐vinylpyridine) is dissolved in a selective solvent like toluene and CoCl₂ is added to the solution, monomicellar arrays of such micelles exhibiting a significant hexagonal order can be prepared on top of various substrates with tailored intermicellar distances and structure heights. In order to remove the polymer matrix and to finally obtain arrays of pure Co nanoparticles, the micelles are first exposed to an oxygen plasma, followed by a treatment in a hydrogen plasma. Applying in‐situ X‐ray photoelectron spectroscopy, it is demonstrated that: 1) The oxygen plasma completely removes the polymer, though conserving the original order of the micellar array. Furthermore, the resulting nanoparticles are entirely oxidized with a chemical shift of the Co 2p_3/2 line pointing to the formation of Co₃O₄. 2) By the subsequent hydrogen plasma treatment the nanoparticles are fully reduced to metallic Co. 3) By exposing the pure Co nanoparticles for 100 s to various oxygen partial pressures pequation/tex2gif-inf-5.gif, a stepwise oxidation is observed with a still metallic Co core surrounded by an oxide shell. The data allow the extraction of the thickness of the oxide shell as a function of the total exposure to oxygen (pequation/tex2gif-inf-7.gif × time), thus giving the opportunity to control the ferromagnetic–antiferromagnetic composition of an exchange‐biased magnetic system.     

Synthesis of Core–Shell Inorganic Nanotubes

New materials and techniques pertaining to the synthesis of inorganic nanotubes have been ever increasing since the initiation of the field in 1992. Recently, WS₂ nanotubes, which are produced now in large amounts, were filled with molten lead iodide salt by a capillary wetting process, resulting in PbI₂@WS₂ core–shell nanotubes. This work features progress in the synthesis of new core–shell nanotubes, including BiI₃@WS₂ nanotubes produced in a similar same manner. In addition, two new techniques for obtaining core–shell nanotubes are presented. The first is via electron‐beam irradiation, i.e., in situ synthesis within a transmission electron microscope. This synthesis results in SbI₃ nanotubes, observed either in a hollow core of WS₂ ones (SbI₃@WS₂ nanotubes), or atop of them (WS₂@SbI₃ nanotubes). The second technique involves a gaseous phase reaction, where the layered product employs WS₂ nanotubes as nucleation sites. In this case, the MoS₂ layers most often cover the WS₂ nanotube, resulting in WS₂@MoS₂ core–shell nanotubes. Notably, superstructures of the form MoS₂@WS₂@MoS₂ are occasionally obtained. Using a semi‐empirical model, it is shown that the PbI₂ nanotubes become stable within the core of MoS₂ nanotubes only above a critical core diameter of the host (>12 nm); below this diameter the PbI₂ crystallizes as nanowires. These model calculations are in agreement with the current experimental observations, providing further support to the growth mechanism of such core–shell nanotubes.     

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.     

Switching of Electrically Responsive,Light‐Sensitive Colloidal Suspensions of Anisotropic Pigment Particles

An unusual electro‐optical behavior of colloidal suspensions of dichroic, elongated (rod‐shaped) pigment particles is reported. These suspensions exhibit nematic liquid crystal order at low volume fraction of the suspended particles (<15 wt%) and show a strong electric and optical response to an external electric field. Additionally, the characteristics of the optical response can be reversibly manipulated by illuminating the sample with light in its absorption band. The suspensions show a number of interesting phenomena like homeotropic‐planar orientational transitions and light‐induced pattern formation.     

Colloidal Suspensions of Nanometer‐Sized Mesoporous Silica

Nanometer‐sized surfactant‐templated materials are prepared in the form of stable suspensions of colloidal mesoporous silica (CMS) consisting of discrete, nonaggregated particles with dimensions smaller than 200 nm. A high‐yield synthesis procedure is reported based on a cationic surfactant and low water content that additionally enables the adjustment of the size range of the individual particles between 50 and 100 nm. Particularly, the use of the base triethanolamine (TEA) and the specific reaction conditions result in long‐lived suspensions. Dynamic light scattering reveals narrow particle size distributions in these suspensions. Smooth spherical particles with pores growing from the center to the periphery are observed by using transmission electron microscopy, suggesting a seed‐growth mechanism. The template molecules could be extracted from the nanoscale mesoporous particles via sonication in acidic media. The resulting nanoparticles give rise to type IV adsorption isotherms revealing typical mesopores and additional textural porosity. High surface areas of over 1000 m² g^–1 and large pore volumes of up to 1 mL g^–1 are obtained for these extracted samples.     

Core–Shell Colloids and Hollow Polyelectrolyte Capsules Based on Diazoresins

Hollow polyelectrolyte microcapsules containing diazoresins (DZR) were fabricated by the layer‐by‐layer self‐assembly of a polycation, DZR, in alternation with poly(styrenesulfonate) (PSS) onto polystyrene (PS) particles, followed by dissolution of the PS core by tetrahydrofuran (THF). The multilayer film buildup on the colloids was observed by UV‐visible spectroscopy, single particle light scattering (SPLS), and transmission electron microscopy (TEM). The data confirmed regular and stepwise layer formation of DZR and PSS on the colloid particles, with a thickness of about 10 nm for each DZR/PSS bilayer when exposed to aqueous solution, and approximately 5 nm in the “dry state”. The photosensitive nature of the DZR layers was exploited to construct highly stable, covalently attached (polymerized) films by exposure of the ionic self‐assembled DZR/PSS multilayer films to UV‐irradiation. TEM and atomic force microscopy (AFM) confirmed the formation of hollow DZR/PSS multilayer capsules. Osmotic pressure experiments followed by confocal laser scanning microscopy revealed a high mechanical stability of the hollow DZR/PSS capsules. The mechanically robust polymerized multilayer films on the colloids and as free‐standing three‐dimensional hollow capsules are more stable in various chemical environments (i.e., resistant to etching by solvents) than their ionically linked counterparts.     

Synthesis of Magnetic,Up‐Conversion Luminescent,and Mesoporous Core–Shell‐Structured Nanocomposites as Drug Carriers

The synthesis (by a facile two‐step sol–gel process), characterization, and application in controlled drug release is reported for monodisperse core–shell‐structured Fe₃O₄@nSiO₂@mSiO₂@NaYF₄: Yb³⁺, Er³⁺/Tm³⁺ nanocomposites with mesoporous, up‐conversion luminescent, and magnetic properties. The nanocomposites show typical ordered mesoporous characteristics and a monodisperse spherical morphology with narrow size distribution (around 80 nm). In addition, they exhibit high magnetization (38.0 emu g^?1, thus it is possible for drug targeting under a foreign magnetic field) and unique up‐conversion emission (green for Yb³⁺/Er³⁺ and blue for Yb³⁺/Tm³⁺) under 980 nm laser excitation even after loading with drug molecules. Drug release tests suggest that the multifunctional nanocomposites have a controlled drug release property. Interestingly, the up‐conversion emission intensity of the multifunctional carrier increases with the released amount of model drug, thus allowing the release process to be monitored and tracked by the change of photoluminescence intensity. This composite can act as a multifunctional drug carrier system, which can realize the targeting and monitoring of drugs simultaneously.     

A Novel Conductive Core–Shell Particle Based on Liquid Metal for Fabricating Real‐Time Self‐Repairing Flexible Circuits

As a critical part of flexible electronics, flexible circuits inevitably work in a dynamic state, which causes electrical deterioration of brittle conductive materials (i.e., Cu, Ag, ITO). Recently, gallium‐based liquid metal particles (LMPs) with electrical stability and self‐repairing have been studied to replace brittle materials owing to their low modulus and excellent conductivity. However, LMP‐coated Ga₂O₃ needs to activate by external sintering, which makes it more complicated to fabricate and gives it a larger short‐circuit risk. Core–shell structural particles (Ag@LMPs) that exhibit excellent initial conductivity(8.0 Ω sq^?1) without extra sintering are successfully prepared by coating nanosilver on the surface of LMPs through in situ chemical reduction. The critical stress at which rigid Ag shells rupture can be controlled by adjusting the Ag shell thickness so that LM cores with low moduli can release, achieving real‐time self‐repairing (within 200 ms) under external destruction. Furthermore, a flexible circuit utilizing Ag@LMPs is fabricated by screen printing, and exhibits outstanding stability and durability (R/R₀ < 1.65 after 10 000 bending cycles in a radius of 0.5 mm) because of the functional core–shell structure. The self‐repairable Ag@LMPs prepared in this study are a candidate filler for flexible circuit design through multiple processing methods.     

Synthesis of Au–CdS Core–Shell Hetero‐Nanorods with Efficient Exciton–Plasmon Interactions

The synthesis of large lattice mismatch metal‐semiconductor core–shell hetero‐nanostructures remains challenging, and thus the corresponding optical properties are seldom discussed. Here, we report the gold‐nanorod‐seeded growth of Au–CdS core–shell hetero‐nanorods by employing Ag₂S as an interim layer that favors CdS shell formation through a cation‐exchange process, and the subsequent CdS growth, which can form complete core–shell structures with controllable shell thickness. Exciton–plasmon interactions observed in the Au–CdS nanorods induce shell thickness‐tailored and red‐shifted longitudinal surface plasmon resonance and quenched CdS luminescence under ultraviolet light excitation. Furthermore, the Au–CdS nanorods demonstrate an enhanced and plasmon‐governed two‐photon luminescence under near‐infrared pulsed laser excitation. The approach has potential for the preparation of other metal‐semiconductor hetero‐nanomaterials with complete core–shell structures, and these Au–CdS nanorods may open up intriguing new possibilities at the interface of optics and electronics.     

Stabilization of Isolated Hydrous Amorphous and Tetragonal Zirconia Nanoparticles Through the Formation of a Passivating Alumina Shell

In this work, we explore the high‐temperature phase stability of isolated, alumina‐coated zirconia nanocrystals with a goal of understanding how interfacial energy affects phase stability. Isolated tetragonal and hydrous amorphous zirconia colloids were synthesized and coated with alumina through the hydrolysis of aluminum isopropoxide. Alumina‐coated samples exhibited phase behavior that was markedly different from that of the uncoated analogs. Uncoated tetragonal particles transformed to the monoclinic phase at 1100 °C while alumina‐coated tetragonal particles did not transform until 1400 °C. Uncoated hydrous amorphous particles crystallized to the tetragonal phase after heating at 600 °C and transformed to the monoclinic phase after heating at 800 °C. Alumina‐coated hydrous amorphous particles crystallized only after heating at 1050 °C, and transformed to the monoclinic phase after heating at 1400 °C. Differences in phase behavior are postulated to depend on the zirconia–alumina interface, which must be disrupted before zirconia particles can fuse and facilitate the tetragonal‐to‐monoclinic phase transition. By coating the nanocrystals with a thin alumina shell and studying the resultant phase stability, we explore the effect of reproducibly modified interfacial chemistry on phase behavior in nanoscale ceramic composites.     

Electro‐optical Materials: Switching of Electrically Responsive,Light‐Sensitive Colloidal Suspensions of Anisotropic Pigment Particles (Adv. Funct. Mater. 3/2011)

An unusual electro‐optical behavior of colloidal suspensions of dichroic, elongated (rod‐shaped) pigment particles is reported. These suspensions exhibit nematic liquid crystal order at low volume fraction of the suspended particles (<15 wt%) and show a strong electric and optical response to an external electric field. Additionally, the characteristics of the optical response can be reversibly manipulated by illuminating the sample with light in its absorption band. The suspensions show a number of interesting phenomena like homeotropic‐planar orientational transitions and light‐induced pattern formation.     

Biopebbles: DNA‐Functionalized Core–Shell Silica Nanospheres for Cellular Uptake and Cell Guidance Studies

The development of a versatile class of silica nanoparticles for cell studies is reported. The particles contain a fluorescent dye‐encoded core and a single‐stranded DNA oligonucleotide‐displaying shell. They are accessible in arbitrary size and color through robust protocols for Stöber‐based colloidal synthesis and sturdy chemical surface functionalization. Silica particles in the size range of 100 nm to 1.5 µm diameter containing fluorescein, Cy3 oder Cy5 dye‐encoded cores are synthesized and functionalized with DNA oligonucleotides. These silica biopebbles are conveniently traceable by microscopy and have a high affinity to live cells, which makes them ideal for cell uptake studies, as demonstrated for MCF7 and A431 cancer cells. The biopebbles can be utilized as building blocks for the self‐assembled formation of arbitrary surface patterns on glass substrates. With these architectures, the privileged internalization of the biopebbles can be exploited for improved adhesion and guidance of cells because the particles are no longer ingested by adhered cells due to their physical connection with the solid support. It is believed that the biopebble approach will be useful for a variety of applications, fundamental studies in cell biology and tissue engineering.     

Ultralong CdTe Nanowires: Catalyst‐Free Synthesis and High‐Yield Transformation into Core–Shell Heterostructures

A novel catalyst‐free synthetic strategy for producing high‐quality CdTe nanowires in solution is proposed. A special reaction condition is intentionally constructed in the reaction system to induce the formation of nanowires through oriented in situ assembly of tiny particles. To establish such special synthetic conditions in the CdTe system, not only are its typical features and possible solutions deeply analyzed, but also related factors, such as the ligand environment, injection and growth temperature, and Cd‐to‐Te precursor ratio, are systemically investigated. High‐quality ultralong (up to 10 μm) and ultrathin (less than 10 nm) CdTe nanowires are produced in solution under optimal reaction conditions. Morphological, spectral, and compositional analyses are performed to examine the products formed at different reaction stages in order to clarify the formation mechanism of the CdTe nanowires. Furthermore, the transformation of the CdTe nanowires into CdTe/CdSe core–shell heterostructures is intensively explored, and the CdSe epitaxial growth process is specially tracked by morphological and spectral characterization techniques. Finally, CdTe nanowires coated with a continuous and dense CdSe shell are successfully fabricated by using a proper coating protocol.     

Water‐Stable,Magnetic Silica–Cobalt/Cobalt Oxide–Silica Multishell Submicrometer Spheres

A method to produce monodisperse magnetic composite spheres with diameters from less than 100 nm to more than 1 μm in water solution is reported. The spheres consist of a dielectric silica core and a cobalt/cobalt oxide shell which can be protected from further oxidation with an outer shell of silica or, alternatively, they can be covered with the polymer polyvinylpyrrolidone as a stabilizer. The formation of a uniform magnetic shell proceeds with the adsorption of metallic cobalt seeds, produced by the reduction of cobalt chloride with sodium borohydride, on a self‐assembled layer of polyelectrolytes on the silica core. In the second step, an outer silica shell can be formed by the hydrolysis and condensation of (3‐aminopropyl)trimethoxysilane and tetraethoxysilane. The double‐shell composite spheres show excellent sphericity, monodispersity, and a magnetic hysteresis loop at room temperature.     

Large‐Scale Highly Ordered Chitosan‐Core Au‐Shell Nanopatterns with Plasmonic Tunability: A Top‐Down Approach to Fabricate Core–Shell Nanostructures

A new strategy for fabricating highly ordered chitosan–Au core–shell nano­patterns with tunable surface plasmon resonance (SPR) properties is developed. This strategy combines fabrication of a chitosan nanopattern by using a soft‐nanoimprint technique with selective deposition of Au nanoparticles onto the patterned chitosan surface. The SPR response can be tuned by controlling the features of the resulting Au shell/polymer hybrid pattern, which makes these materials potentially useful in ultrasensitive optical sensors for molecular detection.     

Formation Mechanism of Epitaxial Palladium–Platinum Core–Shell Nanocatalysts in a One‐Step Supercritical Synthesis

The scarcity of platinum group metals provides a strong incentive to optimize the catalytic activity and stability, e.g., through nanoalloys or core–shell nanoparticles. Here, time‐resolved X‐ray total scattering and transmission electron microscopy characterization are used to study the formation of palladium–platinum core–shell nanoparticles under solvothermal conditions. It is shown that Pd rapidly forms small (5–10 nm), disordered primary particles, which agglomerate and crystallize when reaching 20–25 nm. The primary Pd particles provide nucleation sites for Pt, and, with extended reaction time, the Pd cores become fully covered with Pt shells. The observed core–shell material is surprising when considering the Pt–Pd phase diagram and relative surface energies, but it can be rationalized through the kinetics of precursor conversion. To bridge the gap between scientific studies and industrial demand for large‐scale production, the synthesis process is successfully transferred to a continuous flow supercritical reactor providing a simple scalable and green process for production of bimetallic nanocatalysts.     

A Novel Route to Thermosensitive Polymeric Core–Shell Aggregates and Hollow Spheres in Aqueous Media

Poly(ε‐caprolactone)/poly(N‐isopropylacrylamide) (PCL/PNIPAM) core–shell particles are obtained by localizing the polymerization of NIPAM and crosslinker methylene bisacrylamide around the surface of PCL nanoparticles. The resultant particles are converted to hollow PNIPAM spheres by simply degrading the PCL core with an enzyme. The hollow spheres are thermosensitive and display a reversible swelling and de‐swelling at ～ 32 °C.     

Luminescent Core/Shell Nanoparticles with a Rhabdophane LnPO4‐xH2O Structure: Stabilization of Ce3+‐Doped Compositions

The core/shell strategy has been successfully developed for rhabdophane lanthanide phosphate aqueous colloids. The growth of a LaPO₄‐xH₂O shell around Ce,Tb‐doped core nanoparticles increases their stability against oxidation. A bright green luminescence is thus preserved in sol–gel films whose fabrication requires silica coating and thermal treatment of the core/shell nanoparticles.