Preparation of titania hollow particles with independently controlled void size and shell thickness by catalytic templating core–shell polymer particles

Organic/inorganic hybrids were prepared by catalytic hydrolysis and subsequent polycondensation of tetra-n-butyl titanate (TnBT) in shell layers grafted on core particles. The core particles were synthesized by emulsifier-free emulsion polymerization of styrene, N-n-butyl-N-2-methacryloyloxyethyl-N,N-dimethylammonium bromide (C₄DMAEMA), and 2-chloropropionyloxyethyl methacrylate using 2,2′-azobis(2-amidinopropane) dihydrochloride as an initiator. The core diameters were controlled in the range of 70–550 nm by adjusting a C₄DMAEMA feed concentration. The core–shell particles were prepared by surface-initiated activator generated electron transfer–atom transfer radical polymerization of 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA). The sizes of core–shell particles were found to increase monotonically with an increase in a DMAEMA concentration. The hybrid particles were fabricated by adding TnBT into a water/ethanol dispersion of core–shell particles. The amounts of titania deposited increased in proportion to the grafted amounts of poly[2-(N,N-dimethylamino)ethyl methacrylate] on the core particles. The X-ray diffraction measurement revealed that the hollow titania particles obtained by heat treatment of hybrids have an anatase crystallographic phase.     

Biomimetic synthesis of raspberry-like hybrid polymer–silica core–shell nanoparticles by templating colloidal particles with hairy polyamine shell

The nanoparticles composed of polystyrene core and poly[2-(diethylamino)ethyl methacrylate] (PDEA) hairy shell were used as colloidal templates for in situ silica mineralization, allowing the well-controlled synthesis of hybrid silica core–shell nanoparticles with raspberry-like morphology and hollow silica nanoparticles by subsequent calcination. Silica deposition was performed by simply stirring a mixture of the polymeric core–shell particles in isopropanol, tetramethyl orthosilicate (TMOS) and water at 25 °C for 2.5 h. No experimental evidence was found for nontemplated silica formation, which indicated that silica deposition occurred exclusively in the PDEA shell and formed PDEA–silica hybrid shell. The resulting hybrid silica core–shell particles were characterized by transmission electron microscopy (TEM), thermogravimetry, aqueous electrophoresis, and X-ray photoelectron spectroscopy. TEM studies indicated that the hybrid particles have well-defined core–shell structure with raspberry morphology after silica deposition. We found that the surface nanostructure of hybrid nanoparticles and the composition distribution of PDEA–silica hybrid shell could be well controlled by adjusting the silicification conditions. These new hybrid core–shell nanoparticles and hollow silica nanoparticles would have potential applications for high-performance coatings, encapsulation and delivery of active organic molecules.     

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.     

Preparation of core–shell composite polymer particles by a novel heterocoagulation based on hydrophobic interaction

Submicron-sized cationic polystyrene shell particles with active ester groups were effectively self-assembled on hydrophobic surfaces of cross-linked polystyrene (PST) particles, uncharged core particles with ca. 8.5-µm diameter in aqueous systems. The hydrophobic interactions between the shell particles and core particles play a key role in heterocoagulation. The resulting heterocoagulates were highly physically stable in water, and the morphology was controlled by several factors including the solid content of latex, self-assembling time, and electrolyte concentration. Composite polymer particles with a core–shell structure were successfully obtained from the heterocoagulates by heat treatment for 3 h at a temperature above the glass transition temperature (T_g) of the cationic polymer shell particles.     

Core–shell particles with conductive polymer cores

Heterocoagulation of large cationic polypyrrole particles with small anionic polyacrylate beads followed by heat processing of the heterocoagulate units proved to be an efficient method in the preparation of core-shell particles with conductive cores and dielectric shells. Specific conditions required for producing monodispersed composite particles with uniform shells were examined in the stages of the synthesis of polypyrrole particles with controlled size, in "one-step" heterocoagulation of oppositely charged polypyrrole microbeads with small polyacrylic particles, and during shell formation by spreading of the acrylic polymer on the surface of polypyrrole cores.     

Synthesis of bifunctional core–shell particles with a porous zeolite core and a responsive polymeric shell

The aim of our work is the synthesis and characterization of colloidal core–shell particles with a zeolite core and an environmentally responsive shell. We have synthesized colloidal ZSM-5 zeolite and modified the surface with 3-(trimethoxysilyl)propyl methacrylate in order to introduce double bonds at the surface. The cross-linked polymeric shell was prepared by precipitation polymerization using the functionalized zeolite particles as seeds. We employed thermoresponsive poly(N-isopropylacrylamide) and pH-responsive poly(vinylpyridine) as the polymeric shell, respectively. The temperature- and pH-depending swelling and deswelling of the core–shell particles were characterized with dynamic light scattering techniques. Transmission electron microscopy pictures show the morphology of the synthesized particles. It is proposed that these types of bifunctional core–shell particles could be of use for controlled uptake and release applications and separation of molecules.     

X-ray imaging of newly-developed gadolinium compound/silica core–shell particles

A preparation method for gadolinium compound (Gd) nanoparticles coated with silica (Gd/SiO₂) is proposed. Gd nanoparticles were prepared with a homogeneous precipitation method at 80 °C using 1.0 × 10⁻³ M Gd(NO₃)₃ and 0.5 M urea in the presence of 1.0 g/L stabilizer. Among stabilizers examined. Sodium n-dodecyl sulfate (SDS) was suitable as the stabilizer for preparing small Gd nanoparticles, and consequently Gd nanoparticles with a size of 46.2 ± 12.4 nm were prepared using the SDS. Silica-coating of the Gd nanoparticles was performed by a St?ber method at room temperature using 0.013 M TEOS and 2.0 × 10⁻³ M NaOH in water/1-propanol solution in the presence of 1.0 × 10⁻³ M Gd nanoparticles, which resulted in production of Gd/SiO₂ particles with an average size of 64.2 ± 14.4 nm. The Gd/SiO₂ particles were surface-modified with 3-aminopropyltrimethoxysilane and succinic anhydride. It was confirmed by measurement of electrophretic light scattering that amino group or carboxyl group was introduced onto the Gd/SiO₂ particles. The gadolinium concentration of 1.0 × 10⁻³ M in the as-prepared colloid solution was increased up to a gadolinium concentration of 0.4 M by centrifugation. The core–shell structure of Gd/SiO₂ particles was undamaged, and the colloid solution was still colloidally stable, even after the concentrating process. The concentrated Gd/SiO₂ colloid solution showed an X-ray image with contrast as high as a commercial Gd complex contrast agent. Internal organs in a mouse could be imaged injecting the concentrated colloid solution into it.     

Silica–silver core–shell particles for antibacterial textile application

The silica–silver core–shell particles were synthesized by simple one pot chemical method and were employed on the cotton fabric as an antibacterial agent. Extremely small (1–2 nm) silver nanoparticles were attached on silica core particles of average 270 nm size. The optimum density of the nano silver particles was found which was sufficient to show good antibacterial activity as well as the suppression in their surface plasmon resonance responsible for the colour of the core–shell particle for antibacterial textile application. The change in the density and size of the particles in the shell were monitored and confirmed by direct evidence of their transmission electron micrographs and by studying surface plasmon resonance characteristics. The colony counting method of antibacterial activity testing showed excellent results and even the least silver containing core–shell particles showed 100% activity against bacterial concentration of 10⁴ colony counting units (cfu). The bonding between core–shell particles and cotton fabric was examined by X-ray photoelectron spectroscopy. The antibacterial activity test confirmed the firm attachment of core–shell particles to the cotton fabric as a result 10 times washed sample was as good antibacterial as that of unwashed sample. The bacterial growth was inhibited on and beneath the coated fabric, at the same time no zone of inhibition which occurs due to the migration of silver ions into the medium was observed indicating immobilization of silver nanoparticles on silica and core–shell particles on fabric by strong bonding.     

Stability of Ag@SiO_2 core–shell particles in conditions of photocatalytic overall water-splitting

Core–shell nanoparticles containing plasmonic metals(Ag or Au) have been frequently reported to enhance performance of photo-electrochemical(PEC) devices. However, the stability of these particles in water-splitting conditions is usually not addressed. In this study we demonstrate that Ag@SiO_2 core–shell particles are instable in the acidic conditions in which WO_3-based PEC cells typically operate, Ag in the core being prone to oxidation, even if the SiO_2 shell has a thickness in the order of 10 nm. This is evident from in situ voltammetry studies of several anode composites. Similar to the results of the PEC experiments, the Ag@SiO_2 core–shell particles are instable in slurry-based, Pt/ZnO induced photocatalytic water-splitting. This was evidenced by in situ photodeposition of Ag nanoparticles on the Pt-loaded ZnO catalyst, observed in TEM micrographs obtained after reaction. We explain the instability of Ag@SiO_2 by OH-radical induced oxidation of Ag, yielding dissolved Ag+. Our results imply that a decrease in shell permeability for OH-radicals is necessary to obtain stable, Ag-based plasmonic entities in photo-electrochemical and photocatalytic water splitting.     

Characterization and preparation of core–shell type nanoparticle for encapsulation of anticancer drug

The aim of this study is to prepare delivery vehicles of paclitaxel using low molecular weight water-soluble chitosan (LMWSC) and evaluate them as an anticancer drug delivery system. LMWSC was modified with methoxy polyethylene glycol (LMWSC-MPEG, ChitoPEG), and then it was conjugated with cholesterol (LMWSC-MPEG-Chol). Core–shell type LMWSC-MPEG-Chol nanoparticles (LMWSC-NPs) were prepared by the dialysis method, and the core–shell structure was confirmed by ¹H NMR analysis. To this polymer, paclitaxel was encapsulated and core–shell type nanoparticles were prepared. The release tests indicated that release of paclitaxel from the core–shell type nanoparticles and its transport across the dialysis membrane was slower than dialysis of free paclitaxel. In a cytotoxicity study using CT26 cell, the paclitaxel-encapsulated core–shell type nanoparticles (LMWSC-NPs) showed a toxicity against tumor cells similar to paclitaxel itself. The results of a tumor inhibition test with CT26 implanted upon mouse tumor models in vivo indicated that the application of a dose of 10 mg/kg of LMWSC-NPT showed a superior survival rate, and a slower tumor growth than when paclitaxel alone was administered, although the tumor growth and survival rate were not significantly changed at a dose of 2 mg/kg. The LMWSC-NPT dose above 10 mg/kg showed a superior antitumor activity.     

Thermoresponsive submicron-sized core–shell hydrogel particles with encapsulated olive oil

Thermoresponsive submicron-sized core–shell hydrogel particles with incorporated olive oil were synthesised and studied. The microspheres having poly(N-isopropylacrylamide-co-methyl methacrylate) core and poly(N-isopropylacrylamide) shell were synthesised by emulsifier-free seed polymerisation method. The morphology, particle size and distribution characteristics of the core microspheres were studied with different amount of initiator, monomer–solvent ratio and polymerisation time using scanning electron microscopy and dynamic light scattering particle size analysis. The prepared core and core–shell microspheres were regularly spherical with average size of around 190.0 and 320.0 nm respectively and nearly monodispersed size distribution. Transmission electron microscopy study revealed the core–shell structure of the microspheres. The thermoresponsive transition temperature (T _t) of the core–shell microspheres was determined as 33 °C by optical absorbance measurement, dynamic light scattering particle size analysis and differential scanning calorimetry. The release rate of olive oil from core–shell microspheres was accelerated by squeezing out the entrapped olive oil as the temperature was increased above T _t. Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy study indicated the formation of copolymer.     

Electrical surface charge and potential of hematite/yttrium oxide core–shell colloidal particles

Interest in the synthesis of composite colloidal particles consisting of a core and shell with different compositions stems from the fact that such particles can be useful in processes where the properties of both core (e.g., size and shape homogeneity, ease of preparation in large amounts, magnetic characteristics, etc.) and shell (interfacial properties, porosity, chemical stability, etc.) might be of interest. However, the applicability must be based on a proper characterization of those properties. In this work, colloidal spheres of hematite (α-Fe₂O₃) were used as nuclei of mixed particles where the shell is yttrium oxide. The electrical properties of the aqueous interface are compared to those of the pure oxides by means of potentiometric titration of their surface charge and potential against pH, as a function of indifferent electrolyte concentration. It is found that the mixed particles efficiently mimic yttrium oxide, since the behavior of their surface electrical characteristics closely resembles that of the latter compound. Differences are found, however, that can be ascribed to an incomplete or porous coverage, but such divergences are of little significance when an overall comparison is carried out. Received: 30 January 2001 Accepted: 11 July 2001     

Morphology and film formation of poly(butyl methacrylate)–polypyrrole core–shell latex particles

 Core–shell latex particles made of a poly(butyl methacrylate) (PBMA) core and a thin polypyrrole (PPy) shell were synthesized by two-stage polymerization. In the first stage, PBMA latex particles were synthesized in a semicontinuous process by free-radical polymerization. PBMA latex particles were labeled either with an energy donor or with an energy acceptor, in two different syntheses. These particles were used in a second stage as seeds for the synthesis of the core–shell particles. The PPy shell was polymerized around the PBMA core latex in an oxidative chemical in situ polymerization. Proofs for the success of the core–shell synthesis were obtained using nonradiative energy transfer (NRET) and atomic force microscopy (AFM). NRET gives access to the rate of polymer chain migration between adjacent particles in a film annealed at a temperature above the glass-transition temperature T _g of the particles. Slower chain migration of the PBMA polymer chains was obtained with the PBMA–PPy core–shell particles compared to rate of the PBMA polymer chain migration found with the pure, uncoated PBMA particles. This result is due to the coating of PBMA by PPy, which hinders the migration of the PBMA polymer chains between adjacent particles in the film. This observation has been confirmed by AFM measurements showing that the flattening of the latex film surface is much slower for the core–shell particles than for the pure PBMA particles. This result can again be explained by the presence of a rigid PPy shell around the PBMA core. Thus, these two complementary methods have given evidence that real core–shell particles were synthesized and that the shell seriously hinders film formation of the particles in spite of the fact that it is very thin (thickness close to 1 nm) compared to the size (750 and 780 nm in diameter) of the PBMA core. Transparency measurements confirm the results obtained by NRET and AFM. When the films are placed at a temperature higher than the T _g of PBMA, the increase in transparency is faster for films made with the uncoated PBMA particles than for films made with the coated PBMA particles. This result indicates again that the presence of the rigid PPy layer around the PBMA core reduces considerably the speed at which the structure of the film is modified when heated above the T _g of PBMA. Received: 02 September 1999 Accepted: 21 December 1999     

Sol–gel synthesis of porous inorganic materials using “core–shell” latex particles as templates

Here we report on the sol–gel synthesis of porous inorganic materials based on manganese, molybdenum, and tungsten compounds using the “core–shell” siloxane-acrylate latex as a template. The chemical composition and structural characteristics of the materials obtained have been investigated. It was shown that temperature conditions and gaseous media composition during the template destruction controlled the composition and structure of porous materials. To obtain porous inorganic materials for catalytic applications, the “core–shell” latex template was preliminarily functionalized by gold and palladium nanoparticles obtained by thermal reduction of noble metal ions-precursors in a polycarboxylic “shell”. Upon the template removal, noble metals nanoparticles of a size of dozens of nanometers were homogeneously distributed in the material porous structure. The evaluation of the catalytic activity of macroporous manganese, tungsten, and molybdenum oxides under the conditions of liquid phase catalytic oxidation of organic dyes has been performed. The prospects of employing macroporous oxide systems with immobilized nanoparticles of noble metals in the processes of hydrothermal oxidation of radionuclide organic complexes in radioactive waste decontamination have been demonstrated.     

The volume transition in thermosensitive core–shell latex particles containing charged groups

An investigation of the volume transition in thermosensitive core–shell particles by dynamic light scattering (DLS) is presented. The core of the particles consists of polystyrene (diameter 118 nm), whereas the thermosensitive shell is composed of a network of poly (N-isopropylacrylamide) containing 2 mol% acrylic acid counits. The hydrodynamic radius of these particles as determined by DLS decreases in a continuous manner when raising the temperature. It is shown that the volume transition in the core–shell microgels remains continuous for a wide range of ionic strengths and pH values. This behavior is opposite to that of macrogels of the same chemical composition, which undergo a discontinuous volume transition. The present investigation therefore demonstrates that affixing the network to solid colloidal particles profoundly alters the volume transition of thermosensitive networks. The reason is that shrinking can take place only along the radial direction of the particles. The solid core thus exerts a strong spatial constraint onto the network, which leads to the observed behavior. Received: 29 March 1999 Accepted in revised form: 16 July 1999     

Controlled preparation of core–shell polystyrene/polypyrrole nanocomposite particles by a swelling–diffusion–interfacial polymerization method

Polystyrene/polypyrrole (PS/PPy) core–shell nanocomposite particles with uniform and tailored morphology have been successfully synthesized using the “naked” PS particulate substrate with the aid of a proposed strategy, the so-called swelling–diffusion–interfacial polymerization method. After initially forming pyrrole-swollen PS particles, diffusion of the monomer toward the aqueous phase was controlled through the addition of hydrochloric acid, eventually leading to its polymerization on the substrate particle surface. This process allows the nanocomposite particles to possess uniform and intact PPy overlayer and affords much more effective control over the structure and morphology of the resultant nanocomposites by simply changing the PS/pyrrole weight ratio or the addition amount of the doping acid. In particular, the nanocomposite particles with a thin, uniform, and intact PPy overlayer and their corresponding PPy hollow particles were obtained at a low addition amount of pyrrole. The resultant nanocomposite particles have been extensively characterized using scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and thermogravimetry.     

Poly(styrene-b-ethylene oxide) copolymers as stabilizers for the synthesis of silica–polystyrene core–shell particles

Following previous works [1, 2], silica–polystyrene core–shell particles have been synthesized by dispersion polymerization of styrene in an ethanol/water mixture in the presence of a poly(styrene-b-ethylene oxide) block copolymer as stabilizer. Besides the formation of composite core–shell particles, a large number of free latex particles that do not contain silica were also formed. This number decreases as the size of the silica beads decreases from 300 to 29 nm in diameter, and becomes very low compared to the number of composite particles for the smallest silica beads used. In every case, the composite particles could be easily separated from the free latex particles by centrifugation, providing a material made of regular core–shell composite particles. On the basis of the mechanisms involved in dispersion polymerization, hypotheses were formulated to account for the formation of the silica–polystyrene composite particles. Received: 6 May 1999 Accepted in revised form: 29 June 1999     

Modified reverse microemulsion synthesis for iron oxide/silica core–shell colloidal particles

Iron oxide/silica core–shell colloidal particles were prepared by basic reverse microemulsion (RM) method and two modified RM methods. By basic RM method, maximum particle size obtained was mere 40 nm. For building photonic crystals working in the visible range, the colloidal particles must be larger than 100 nm. Thus two modified RM methods were used. By alcohol modified RM method, short chain alcohols were used as co-surfactant. The particle size rose to near 100 nm, but the core–shell structure was comparatively poor. By emulsifier pair modified RM method, the particle size leapt to over 200 nm and a narrow growth window was found favorable to enhance the stability and rigidity of the surfactants layers. The core–shell mechanism was also discussed and a new four-step mechanism was proposed.     

Amphiphilic polymeric particles with core–shell nanostructures: emulsion-based syntheses and potential applications

The design and synthesis of amphiphilic nano- to micro-sized polymeric particles with core–shell nanostructures have attracted more and more attention because of their wide applicability in modern material science and their technological importance in the areas of colloid and interface science. Many synthetic strategies have been developed for the preparation of amphiphilic core–shell particles that consist of hydrophobic polymer cores and hydrophilic polymeric shells. In this review, we focus on emulsion-based approaches and properties of particles produced. These methods are: (1) grafting to functionalized particle that produces a corona-like particle, (2) grafting from reactive seed particle that produces a brush-like particle, (3) copolymerization of reactive macro-monomer with hydrophobic monomer that produces a corona-like particle, (4) emulsion polymerization in the presence of block or comb-like copolymer containing controlled free-radical moiety that produces a multi-layered particle, and (5) redox-initiated graft polymerization of vinyl monomer from a water-soluble polymer containing amino groups that produces a hairy-like particle. Potential applications of some of these particles in drug and gene deliveries, enzyme immobilization, colloidal nanocatalyst, chemical sensing, smart coating, and thermal laser imaging will be discussed.