One-step synthesis of highly monodisperse hybrid silica spheres in aqueous solution

An effective and reproducible method of preparing highly monodisperse organic-inorganic hybrid silica spheres was studied. One process, one precursor (organosilane) and one solvent (water) were used in our experiments. The size of hybrid silica spheres could be adjusted from 360 to 770 nm with relative standard deviation below 2% by controlling the concentration of the organosilane precursor and the ammonia catalyst. The increasing of the precursor concentration increases the particle size while the catalyst concentration has a reverse effect on the particle size. The concept of homogeneous nucleation and growth processes are introduced to explain the formation mechanism and the effect of reaction conditions. The scanning electron microscopy (SEM) images illustrate the copiousness in quantity and the uniformity in size/shape of the particles that could be routinely accomplished in this synthesis. Fourier transform infrared (FT-IR) and (29)Si nuclear magnetic resonance (NMR) spectra confirm the structure of vinyl hybrid silica spheres, where the vinyl group (-CH=CH(2)) exists and connects to the silicon atom. This method has also been extended to design and prepare other organic-inorganic hybrid materials especially in monodisperse surface-modified silica spheres.     

Silica coating of nanoparticles by the sonogel process

A modified aqueous sol-gel route was developed using ultrasonic power for the silica coating of indium tin oxide (ITO) nanoparticles. In this approach, organosilane with an amino functional group was first used to cover the surface of as-received nanoparticles. Subsequent silica coating was initiated and sustained under power ultrasound irradiation in an aqueous mixture of surface-treated particles and epoxy silane. This process resulted in a thin but homogeneous coverage of silica on the particle surface. Particles coated with a layer of silica show better dispersability in aqueous and organic media compared with the untreated powder. Samples were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and the zeta potential.     

Colloidal nanocomposite hydrogel particles

Quaternary ammonium salt, (3-acrylamidopropyl)-trimethylammonium chloride was used to synthesize nanohydrogel and composite particles such as inorganic–organic hybrid composites and hydrogel nanoparticles with magnetic properties utilizing a water-in-oil microemulsion system. The positively charged cationic monomer was chosen to promote silica hydrolysis and condensation to prepare silica-hydrogel nanocomposite particles with interesting morphologies. It was shown that highly monodisperse, completely charged nanohydrogel can be used to encapsulate ferrite particles. Furthermore, it was also confirmed that cationic nanohydrogel particles with variant morphology can be prepared by employing suitable silica precursor. Morphology, structure, properties, and size of nanocomposite materials were explored utilizing transmission electron microscopy, atomic force microscopy, and vibrating sample magnetometer.     

Preparation and characterization of a novel nanocomposite particles via in situ emulsion polymerization of vinyl functionalized silica nanoparticles and vinyl acetate

A new class of poly(vinyl acetate) (PVAc)/silica nanocomposite particles was successfully prepared in aqueous solution through a facile synthetic process. First, vinyl functionalized silica nanoparticles (VFSs) were synthesized using one-step method in aqueous emulsion, and then the vinyl groups located on the surface of VFSs were used to induced in situ polymerization of vinyl acetate. Scanning electron microscopy (SEM) images showed that VFSs and PVAc/silica nanocomposite particles all revealed highly monodispersed and uniform spheres. Especially, PVAc/silica nanocomposite particles obtained from transmission electron microscopy images presented an obvious core–shell structure, and the thickness of PVAc shell grafting on the surface of VFSs core was about 17 nm. In addition, the influence of the hydrolyzed and condensed time of vinyl triethoxysilane on the size and size distribution of VFSs was also investigated. The results of dynamic light scattering and SEM analysis indicated that the size and size distribution of VFSs decreased gradually with the extension of the reaction time from 6 to 48 h. Moreover, the structures and thermal properties of the samples were characterized via FT-IR and heat-flow DSC–TG.     

Facile synthesis of nanocrystal encoded fluorescent silica microspheres

Monodisperse CdTe composite microspheres with a spherical shape were prepared using organosilane chemicals in aqueous solution. CdTe nanocrystals (NCs) were loaded into the matrix of silica microspheres during the formation of composite microspheres. Detailed characterization of the CdTe composite microspheres by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and spectrofluorimeter was performed to elucidate the morphology and fluorescence of the composite microspheres. In contrast to CdTe NCs in aqueous solution, CdTe NCs in the composite microspheres revealed high stability and fluorescence due to the confined effects of silica matrix. In addition, multicolored CdTe QDs were encoded into the microspheres at precise ratios.     

Hydrophobicity modification of woven cotton fabric by hydrophobic fumed silica coating

Water repellency of woven cotton fabric was achieved by coating with the aqueous dispersion containing organosilane agent (HDTMS) and fumed silica. The coating agents were applied using padding method and then followed by batching the coated fabric at the ambient temperature for 24 h to allow the condensation reaction between HDTMS silanol group and fumed silica silanol group, rendering silica particles hydrophobic. An ultrasonicator was employed to prepare the homogenous coating dispersion. The water repellency evaluated by water contact angle determination which showed the contact angle over 110° was obtained with low amount of applied HDTMS of 1 wt%. The effect of fumed silica addition on an increase in fiber surface roughness geometry showed the influential result in improving the water contact angle. From durability to washing test, the hydrophobic coatings evidenced from SEM and ATR/FTIR remained adhering to fiber surface, indicating the durability. After washing, the coating on the fabric with fumed silica addition appeared to be scatter particles which made a contribution to the higher contact angle value when compared to sheet-like layer coating in case of HDTMS coating alone.     

Size-controllable synthesis of monodispersed colloidal silica nanoparticles via hydrolysis of elemental silicon

A new method is presented for preparing monodisperse and uniform-size silica nanoparticles using a two-stage hydrolysis of silicon powder in aqueous medium. The influence of synthesis conditions including solution composition and temperature on the formation of silica nanoparticles were systematically investigated. The structure and morphology of the silica particles were characterized via transmission electron microscopy (TEM) and dynamic light scattering (DLS). Various-sized particles in the range 10–100 nm were synthesized. The size of the nanoparticles can be precisely controlled by using a facile regrowth procedure in the same reaction media.     

Surface ATRP of hydrophilic monomers from ultrafine aqueous silica sols using anionic polyelectrolytic macroinitiators

A convenient two-step route was developed to prepare new anionic ATRP macroinitiators from near-monodisperse poly(2-hydroxyethyl methacrylate) precursors by partial esterification with 2-bromoisobutyryl bromide, followed by esterification of the remaining hydroxyl groups using excess 2-sulfobenzoic acid cyclic anhydride. These new macroinitiators can be electrostatically adsorbed onto ultrafine cationic Ludox CL silica sols; subsequent surface polymerization of various hydrophilic monomers in aqueous solution at room temperature afforded a range of polymer-grafted ultrafine silica sols. The resulting sterically stabilized particles were characterized by dynamic light scattering, transmission electron microscopy, aqueous electrophoresis, FTIR spectroscopy, and elemental microanalyses.     

Synthesis of poly-L-glutamic acid grafted silica nanoparticles and their assembly into macroporous structures

Polypeptide-coated silica nanoparticles represent an interesting class of organic-inorganic hybrids since the ordered secondary structure of the polypeptide grafts imparts functional properties to these nanoparticles. The synthesis of a poly-l-glutamic acid (PLGA) silica nanoparticle hybrid by employing N-carboxyanhydride (NCA) polymerization to synthesize the polypeptide chains and Cu catalyzed azide alkyne cycloaddition reaction to graft these chains onto the silica surface is reported. This methodology enables the synthesis of well-defined polypeptide chains that are attached onto the silica surface at high surface densities. The PLGA-silica conjugate particles are well dispersed in water, and have been thoroughly characterized using multinuclear ((13)C, (29)Si) solid state NMR, thermogravimetric analysis, Fourier transform infrared, dynamic light scattering, and transmission electron microscopy. The pH-dependent reversible aggregation of the PLGA-silica particles, driven by the change in PLGA structure, has also been studied. Preliminary results on the use of aqueous dispersions of silica-PLGA for the preparation of three-dimensional macroporous structures with oriented pores by ice templating methodology are also demonstrated. These macroporous materials, comprising a biocompatible polymer shell covalently attached to rigid inorganic cores, adopts an interesting lamellar structure with fishbone-type architecture.     

Changes of methylated silica surfaces on ageing

Silica rendered hydrophobic by organosilanes is a widely used model material in colloid chemistry, biological research, catalysis, etc. However, it is often overlooked that the surface properties of silica, and silica made hydrophobic be reacting with silane, change with time when the substrate is immersed in aqueous solution. Therefore the experimental conditions when such model systems are employed have to be carefully assessed. This paper summarizes the findings of the force measurement tests between air bubbles and silica particles hydrophobized with organosilanes such as trimethylchlorosilane and 1,1,1,3,3,3-hexamethyl-disilazane. The results showed that the attractive forces as well as the adhesion between the air bubbles and silica particles decrease with the time of aging in aqueous solution. The silica surfaces rendered hydrophobic with organosilanes become hydrophilic with time due to hydration. The hydrophobicity could be restored by heating the samples at 190?^○C. The atomic force microscopy imaging on silica plates revealed that in addition to hydration, decomposition of the organosilane layer also takes place.     

Photodegradation of rhodamine B in aqueous solution via SiO2@TiO2 nano-spheres

Titania coated silica spheres (SiO₂@TiO₂) prepared by heterocoagulation of silica and titania nano-particles were investigated as catalyst in the photodegradation of rhodamine B (RB) in aqueous solution. The silica spheres were prepared by the well-known Stöber method and titania sol by a hydrolysis–condensation reaction in acidic media. The uncoated and coated particles were characterized by zeta potential measurements, acoustic attenuation spectroscopy and scanning electron microscopy. The degradation of the dye was induced by illuminating the coated spheres in aqueous solution with artificial solar light. The spectral distribution of the applied light corresponds to the sunlight spectrum on the earth's surface. Rhodamine B was used as model dye and decomposed completely to colourless end products after illumination. The decrease in concentration of rhodamine B was monitored by UV–vis spectroscopy and the total organic carbon (TOC) was determined in order to verify the degradation mechanism described elsewhere.     

Template assisted synthesis of silica-coated molecular crystal nanorods: From hydrophobic to hydrophilic nanorods

We report a method for the preparation of silica-coated molecular crystal nanorods. A sol-gel method was used to make silica nanotubes inside anodized alumina templates. The nanotubes were then loaded with 9-anthracene carboxylic acid (9-AC) and solvent annealed to produce silica-coated organic nanorods. The core-shell structure was confirmed using electron microscopy, and the highly crystalline organic core was characterized using powder X-ray diffraction and transmission electron microscopy. The silica-coated 9-AC rods had much improved dispersal properties in aqueous solution, and were also able to undergo reversible bending under UV illumination, as observed previously for uncoated 9-AC rods. This work demonstrates that it is possible to make surface-coated molecular crystal nanorods that retain their useful functionalities.     

Preparation and properties of poly(styrene-co-maleic anhydride)/silica hybrid materials by the in situ sol–gel process

Poly(styrene-co-maleic anhydride)/silica hybrid material has been successfully prepared from styrene–maleic anhydride copolymer and tetraethoxysilane (TEOS) in the presence of a coupling agent (3-aminopropyl)triethoxysilane (APTES) by an in situ sol–gel process. It was observed that the gel time of sol–gel solution was dramatically influenced by the amount of APTES. The hybrid material exhibits optical transparency almost as good as both silica gel and the copolymer. The covalent bonds between organic and inorganic phases were introduced by the aminolysis reaction of the amino group with maleic anhydride units of copolymer to form a copolymer bearing trimethoxysilyl groups, which undergo hydrolytic polycondensation with TEOS. The differential scanning calorimetry (DSC) showed that the glass transition temperature of the hybrid materials increases with increasing of SiO₂ composition. Photographs of scanning electron microscopy (SEM) and atomic force microscopy (AFM) inferred that the size of the inorganic particles in the hybrid materials was less than 20 nm. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1607–1613, 1998     

Thermoresponsive transport through ordered mesoporous silica/PNIPAAm copolymer membranes and microspheres

Thermosensitive inorganic-organic hybrid polymers and gels can be used for controlled molecular transport in a variety of applications that require robust, mechanically stable materials. Silica and poly(N-isopropylacrylamide) (PNIPAAm) precursors were copolymerized in the presence of surfactant supramolecular assemblies to form hybrid gels with ordered nanostructure. This method was less complicated and results in enhanced reversible transport properties compared to previous approaches noted herein. In this study, the thermoresponsive polymer, PNIPAAm, was incorporated into polymerizing silica networks using the coupling agent 3-methacryloxypropyltrimethoxysilane. The hydration transition of PNIPAAm associated with its lower critical solution temperature (LCST) in aqueous solution was retained in the hydrated silica matrices and was used to control the permeability of membranes and molecular release behavior of particles. This report presents new methods for formation of hybrid silica/PNIPAAm membranes and particles, characterization of these materials, and documentation of reversible molecular transport properties of these new hybrid materials.     

A facile method of synthesis of asymmetric hollow silica spheres

This paper presents a "one-step" method to synthesize asymmetric hollow silica spheres. In this method, when positively charged polystyrene particles were blended with mercaptopropyltriethoxysilane and stirred at 50 °C in alkaline ethanol/water medium for a period of time, Janus or lobed asymmetric hollow silica spheres could be directly obtained, just changing the ratio of ethanol to water in the reaction medium. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to characterize the morphology and topography of the asymmetric hollow silica spheres. The formation mechanism was described in detail.     

Sol–gel nanocoating on commercial TiO<Subscript>2</Subscript> nanopowder using ultrasound

The surface of commercial titania particles was coated by a layer of silica by a two-step process which involved a power ultrasound initiated sol–gel reaction. In the first step of this solution process, aminosilane, i.e. organosilane with amino functional group, was used to modify the surface of pristine nanoparticles. Subsequent silica nanocoating was initiated and sustained under power ultrasound agitation in a mixture of surface modified particles and epoxysilane. As a result, a homogenous coverage of silica on the nanoparticles’ surface, with thickness controllable from one to several nanometers, was obtained. Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and particle surface zeta potential measurements were employed to follow steps in the process and to confirm the reaction mechanism.     

Encapsulation of aluminum flakes with hybrid silica/poly(acrylic acid) nanolayers by combination of sol–gel and in situ polymerization methods: a corrosion behavior study

A combination of sol–gel method and in situ polymerization was used to form a hybrid silica/poly(acrylic acid) nanolayer for the corrosion protection of aluminum pigments. To this end, the pigment particles were first coated with a silica layer by sol–gel method. Tetraethylorthosilicate was used as a precursor and during a condensation reaction, an inorganic silica layer was formed. Then, 3-methacryloxypropyltrimethoxysilane was attached on the surface and in situ polymerization of acrylic acid (AA), as a hydrophile monomer, was performed. The obtained Al/Si/PAA flakes were characterized by different methods such as Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, scanning electron microscopy, and transmission electron microscopy (TEM). The attached PAA chains on the surface were deattached by HF aqueous solution and analyzed by gel permeation chromatography. Also, the surface energy of samples was measured using Owens and Wendt equation by means of contact angle data. As results, the characterizing tests approved the successful encapsulation of Al pigments and TEM image showed a 10–15 nm silica layer and a 20–25 nm PAA layer. Although the Al/Si pigments showed a quantity of evolved hydrogen, the hybrid coated pigments had excellent anticorrosive properties in acidic and alkaline solutions. Also, the surface free energy of Al/Si/PAA showed an increase compared to that of Al.     

Superparamagnetic and fluorescent thermo-responsive core-shell-corona hybrid nanogels with a protective silica shell

We present the preparation and the characterization of the solution behavior and functional properties of superparamagnetic and/or fluorescent, thermo-responsive inorganic/organic hybrid particles with an intermediate protective silica shell and a smart polymer corona. These well-defined multifunctional nanogels were prepared via two consecutive encapsulation processes of superparamagnetic Fe(2)O(3) nanoparticles (NPs) and/or fluorescent CdSe(ZnS) semiconductor nanocrystals with a silica layer and a crosslinked poly(N-isopropylacrylamide) (PNIPAAm) polymer shell. First, the different NPs were entrapped into a silica shell using a microemulsion process. Therein, the precise adjustment of the conditions allows to entrap either several particles or single ones and to tailor the thickness of the silica shell in the range of 20-60 nm. In a second step, a polymer coating, i.e. thermosensitive PNIPAAm, was attached onto the surface of the multifunctional core-shell particles via free radical precipitation polymerization, furnishing multifunctional core-shell-corona hybrid nanogels. Analyses of the functional properties, i.e. optical brightness and magnetic moments, along with transmission electron microscopy reveal near monodisperse hybrid nanoparticles that retain the intrinsic properties of the original nanocrystals. Additionally, we demonstrate the drastically increased chemical stability due to the barrier properties of the intermediate silica layer that protects and shields the inner functional nanocrystals and the responsive character of the smart PNIPAAm shell.     

Poly(dimethylsiloxane) Star Polymers Having Nanosized Silica Cores

Summary: Poly(dimethylsiloxane) (PDMS) star polymers having a nanosized silica particle as a core were prepared by reacting silica nanoparticles with monoglycidylether‐terminated poly(dimethylsiloxane). This star polymer was a hybrid material having an extremely high content of silica. The PDMS arms formed an organic domain to separate the silica particles and to prevent particle aggregation. The star polymers exhibited good thermal stability and high activation energy of their degradation reaction, in comparison to the linear PDMS polymer and the PDMS/silica blending materials. This star polymer can be used as a flame retardant for polymeric materials and this preparation technique can be applied to prepare other star polymers.

An SEM image of poly(dimethylsiloxane) star polymers having nanosized silica particles as a core.     

Conjugated polyelectrolyte-grafted silica microspheres

A direct method for preparation of conjugated polymer-grafted silica particles is reported. Silica particles (0.3 and 5 mum diameter) are treated with a 3-(trimethoxysilyl)propylamine derivative that is functionalized with an aryl iodide unit. A solution step-growth polymerization reaction is performed in solution that contains a dispersion of the aryl iodide-functionalized particles. The reaction is a Pd(0)-catalyzed (Sonogashira) A-B-type polymerization of an oligo(ethylene glycol)-fuctionalized diiodobenzene and a bis(propyloxy)sulfonate-substituted diethynylbenzene. The overall process affords silica particles that feature a surface graft layer of an anionic poly(phenylene ethynylene)-type conjugated polyelectrolyte. The particle surface modification process was monitored by infrared (FTIR) spectroscopy, and the polymer-grafted silica particles were characterized by thermogravimetric analysis, scanning and transmission electron microscopy, confocal fluorescence microscopy, and absorption and fluorescence spectroscopy. The conjugated polyelectrolyte-grafted silica particles are highly fluorescent, and a Stern-Volmer quenching study of the particles' fluorescence with electron-transfer- and energy-transfer-type quenchers shows that the quenching response depends on the type of quenching mechanism.