Preparation of hollow silica microspheres with controlled shell thickness in supercritical fluids

This article presents a novel route to prepare hollow silica microspheres with well-defined wall thickness by using cross-linked polystyrene (PS) microspheres as templates with the assistance of supercritical carbon dioxide (SC-CO₂). In this approach, the cross-linked PS templates can be firstly prepared via emulsifier-free polymerization method by using ethylene glycol dimethacrylate or divinylbenzene as cross-linkers. Then, the silica shell from the sol–gel process of tetraethyl orthosilicate (TEOS) which was penetrated into the PS template with the assistance of SC-CO₂ was obtained. Finally, the hollow silica spheres were generated after calcinations at 600 °C for 4 h. The shell thickness of the hollow silica spheres could be finely tuned not only by adjusting the TEOS/PS ratio, which is the most frequently used method, but also by changing the pressure and aging time of the SC-CO₂ treatment. Fourier transform infrared spectroscopy, transmission electron microscopy, and scanning electron microscope were used to characterize these hollow silica spheres.     

Preparation of hollow composite spheres with raspberry‐like structure based on hydrogen‐bonding interaction

The hollow composite spheres with a raspberry‐like structure were prepared by a self‐assemble heterocoagulation based on the inter‐particle hydrogen‐bonding interaction between the amide groups of hollow poly (N,N′‐methylenebisacrylamide‐co‐N‐isopropyl acrylamide) (P(MBA‐co‐NIPAAm)) microspheres and the carboxylic acid groups of poly(ethyleneglycol dimethacrylate‐co‐methacrylic acid) (P(EGDMA‐co‐MAA)) nanoparticles, in which P(EGDMA‐co‐MAA) nanoparticle acted as the corona and the hollow P(MBA‐co‐NIPAAm) microsphere behaved as the core. The control coverage of the corona particles on the surface of hollow core microspheres of P(MBA‐co‐NIPAAm)/P(EGDMA‐co‐MAA) hollow composite sphere was studied in detail through adjustment of the mass ratio between the core and corona particles. The effect of the pH on the stability of the raspberry‐like hollow spheres was investigated. The polymer particles and the resultant heterocoagulated raspberry‐like hollow spheres were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Copyright © 2008 John Wiley & Sons, Ltd.     

N,P dual-doped hollow carbon spheres for high-performance supercapacitors

Journal of Solid State Electrochemistry - A series of nitrogen (N) and phosphorus (P) dual-doped hollow carbon spheres (NPHCSs) with different contents of N and P were synthesized by Stöber...     

A scanning electron microscopy study on hollow silica microspheres: defects and influences of the synthesis composition

Defects on hollow silica spheres synthesized in a tetraethylorthosilicate-octylamine-HCl-H₂O system were recorded by scanning microscope. Based on the results, influences of synthesis composition on the formation of these defects are discussed. It is evidenced that products prepared with different octylamine-to-tetraethylorthosilicate ratios may have surface depressions, cracks and non-hollow microspheres. However, by changing water and acid additions, these defects could be reduced or eliminated. Generally, samples synthesized with a large octylamine addition commonly exhibit surface depressions. A small octylamine or a large water addition benefits the formation of solid silica microspheres among the product. Acid, although is not indispensable for the formation of hollow spheres, helps to eliminate or reduce depressions on the hollow shells. It is explained that the added acid gives rise to a relative localized fast hydrolysis versus condensation, facilitating an easy mobility of hydrolyzed silica species, and consequently the shell surface is smoothened.     

Preparation of Porous Hollow SiO2 Spheres by a Modified St?ber Process Using MF Microspheres as Templates

Using the surface charged and acid dissolvable melamine formaldehyde (MF) microspheres as sacrificial hard templates, silica coated MF core?Cshell composite microspheres, denoted as MF@SiO₂, were synthesized via a surfactant-assisted sol?Cgel process by using tetraethyl orthosilicate (TEOS) as silica source. Hollow SiO₂ spheres with mesoporous shells were then obtained after selective removal of the MF cores and the pore directing surfactant by hydrochloric acid etching or calcinations in air. Interesting shrinkage phenomena were observed in both the hollow products derived from hydrochloric acid etching and calcinations. The influence of the ratio of MF sphere to TEOS and the removal method of the MF core on the size of the hollow spheres, the shell thickness and the shell surface roughness have been studied. The composition, the thermal stability, the morphology, the surface area and pore size distribution, the wall thickness and adsorption properties of the hollow spheres derived from hydrochloric acid etching and calcinations were also investigated and compared based on the FTIR, SEM, TEM, TGA, Nitrogen adsorption?Cdesorption and spectrophotometer techniques or measurements.     

Synthesis of double mesoporous core-shell silica spheres with tunable core porosity and their drug release and cancer cell apoptosis properties

In this work, we demonstrate a simple two-pot approach to double mesoporous core–shell silica spheres (DMCSSs) with uniform size of 245–790 nm, shell thickness of 41–80 nm and surface area and total pore volume of 141–618 m² g^?1 and 0.14–0.585 cc g^?1, respectively. First, solid silica spherical particles were synthesized by the Stöber method and used as a core. Second, a mesoporous shell could be formed around the silica cores by using an anionic surfactant and a co-structure directing agent. It was found that mesopores can be anchored within dense silica cores during mesoporous silica shell formation, synchronously the base group with surfactant assistant can etch the dense silica cores to re-organize new mesostructure, so that double mesoporous core–shell silica sphere (DMCSS) structure can be obtained by a single surfactant-templating step. The spherical size and porosity of the silica cores of DMCSS together with shell thickness can be tuned by controlling Stöber parameters, including the concentrations of ammonia, solvent and tetraethoxysilane and the reaction time. DMCSS were loaded with ketoprofen and thymoquinone, which are an anti-inflammatory and a potential novel anti-cancer drug, respectively. Both drugs showed controlled release behavior from the pores of DMCSS. Drug uptakes within DMCSS were ～27 and 81 wt.% for ketoprofen and thymoquinone, respectively. Furthermore, DMCSS loaded with thymoquinone was more effective in inducing cancer cell apoptosis than uncontained thymoquinone, because of the slow release of the drug from the mesoporous structure.     

Structural characterization of nanosized silica spheres

Silica nanospheres with wide applications on chemistry have been synthesized using the Stöber method and have been modified with several organic functional groups by post-synthesis reaction in anhydrous environments. The surface analysis of the silica nanosized spheres leads to low surface area values and large skeletal densities, pointing out to dense nanospheres. High-resolution transmission electron microscopy (HRTEM) and small-angle X-ray scattering (SAXS) prove the microstructure level of the surface porosity that is clearly affected by the organic functionalization.     

Preparation of thermoresponsive core–shell polymeric microspheres and hollow PNIPAM microgels

A reliable and efficient route for preparing thermoresponsive hollow microgels based on cross-linked poly(N-isopropyl acrylamide) (PNIPAM) was developed. Firstly, monodisperse thermoresponsive core–shell microspheres composed of a P(styrene (St)-co-NIPAM) core and a cross-linked PNIPAM shell were prepared by seeded emulsion polymerization using P(St-co-NIPAM) particles as seeds. The size of the P(St-co-NIPAM) core can be conveniently tuned by different dosages of sodium dodecyl sulfate. The thickness of the cross-linked PNIPAM shell can be controlled by varying the dosage of NIPAM in the preparation of PNIAPM shell. Then, hollow PNIPAM microgels were obtained by simply dissolving the P(St-co-NIPAM) core with tetrahydrofuran. The core–shell microspheres and the hollow microgels were characterized by transmission electron microscopy, dynamic light scattering, atomic force microscopy, and Fourier-transform infrared spectroscopy.     

Hydrazide-functionalized hollow porous poly(2-hydroxyethyl methacrylate-co-ethylene dimethacrylate) spheres for immobilization of enzyme

Hollow porous poly(2-hydroxyethyl methacrylate-co-ethylene dimethacrylate)(HEMA-co-EDMA) spheres were prepared by emulsifier-free emulsion polymerization, swelling, seed emulsion polymerization and extraction. Then the spheres activated with 2,4,6-trichloro-1,3,5-triazine were functioned with adipohydrazide (AH). After periodate oxidation of its carbohydrate moieties, horseradish peroxidase was immobilized on the hydrazide-functionalized hollow porous poly(HEMA-co-EDMA) spheres. The amount of immobilized enzyme was up to 43.4 μg of enzyme/g of support. Moreover, the immobilized horseradish peroxidase exhibited high activity and good stability.     

Fabrication of anisotropic silica hollow microspheres using polymeric protrusion particles as templates

Anisotropic polystyrene/poly(styrene-co-divinylbenzene) (PS/P(S-DVB)) protrusion particles with various morphologies such as eyeball-like, snowman-like, and raspberry-like were synthesized using a modified seeded polymerization method by dynamically controlling and stabilizing the phase separation. The effects of swelling agent, crosslinker, and monomer concentrations on the particle morphologies were studied. Using the PS/P(S-DVB) protrusion particles as templates, anisotropic silica (SiO₂) hollow microspheres were fabricated facilely. The obtained anisotropic silica hollow spheres had a potential application in rapid waste removal and detoxification extraction with a very simple procedure.     

一种制备单分散SiO2空心微球的新方法

在乙醇/氨水介质中, 分别以分散聚合和无皂乳液聚合方法制得的不同粒径聚苯乙烯(PS)微球为模板, 以正硅酸乙酯(TEOS)为前驱体, 通过控制介质中氨水的初始体积, 一步法制得了不同粒径的单分散SiO2空心微球. 整个过程无需添加其它溶剂溶解或高温煅烧的方法来除去模板微球. 对SiO2空心微球进行测试表征, 提出了SiO2空心微球的可能形成机制.     

Facile,single-pot preparation of nanoporous SiO2 particles (carrier) with AgNPs at core and crust for controlled disinfectant release

This study demonstrates a novel, facile and one-pot approach to synthesize silica nanoparticles with silver at core and crust (SiNP-AgCC). A modified Stöber method was used to make SiNP-AgCC. A significant reduction in the size of SiO₂ nanoparticles was seen, with 2–5 nm AgNPs being uniformly distributed on the surface and 10–20 nm AgNPs in the center. A typical mesoporous SiO₂ particle (SiNP) produced using the Stöber method was transformed to nanoporous SiO₂ by this modified Stöber method. Nanoporous SiO₂ particles with silver in the center are advantageous for slow and consistent Ag⁺ release, which was confirmed by Ag⁺ ion release test. Antibacterial activities of the samples were tested to evaluate the disinfection performance of the samples on gram-negative bacteria (Escherichia coli) using disk diffusion and the LB-agar method. SiNP-AgCC showed prolonged silver release for more than 20 days and improved antibacterial properties even after five days of incubation.     

Facile synthesis of monodisperse microspheres and gigantic hollow shells of mesoporous silica in mixed water-ethanol solvents

Mesoporous silica materials with a variety of morphologies, such as monodisperse microspheres, gigantic hollow structures comprising a thin shell with a hole, and gigantic hollow structures consisting of an outer thin shell and an inner layer composed of many small spheres, have been readily synthesized in mixed water-ethanol solvents at room temperature using cetyltrimethylammonium bromide (CTAB) as the template. The obtained mesoporous silica generally shows a disordered mesostructure with typical average pore sizes ranging from 3.1 to 3.8 nm. The effects of the water-to-ethanol volume ratio (r), the volume content of tetraethyl orthosilicate TEOS (x), and the CTAB concentration in the solution on the final morphology of the mesoporous silica products have been investigated. The growth process of gigantic hollow shells of mesoporous silica through templating emulsion droplets of TEOS in mixed water-ethanol solution has been monitored directly with optical microscopy. Generally, the morphology of mesoporous silica can be regulated from microspheres through gigantic hollow structures composed of small spheres to gigantic hollow structures with a thin shell by increasing the water-to-ethanol volume ratio, increasing the TEOS volume content, or decreasing the CTAB concentration. A plausible mechanism for the morphological regulation of mesoporous silica by adjusting various experimental parameters has been put forward by considering the existing state of the unhydrolyzed and partially hydrolyzed TEOS in the synthesis system.     

A facile two-step etching method to fabricate porous hollow silica particles

We report here the fabrication of hollow silica particles with mesopores larger than 10 nm on their wall via a facile two-step etching method. Different from the conventional template method, the new method uses the silica particles as starting materials, which were synthesized using the well-known Stöber method. In the hollow silica preparation, first, we gently etch the silica particles with a NaOH solution without using template molecules to make them porous. Then, we coat the porous silica particles with poly-dimethyldiallylammonium chloride (PDDA) and treat the PDDA-coated porous silica with an ammonia solution to form the hollow silica nanospheres. In this study, we found that the NaOH dosage and ammonia concentration have significant impact on the morphology of the final products. Adsorption was also studied and results show that the hollow nanospheres can effectively uptake protein-based biomolecules (hemoglobin).     

Stöber silica’s microporosity

Substantial knowledge gap still exists in understanding Stöber silica’s confusing microporosity. In this work, we utilized simultaneous thermal analysis coupled with Fourier transform infrared spectroscopy to characterize Stöber silica samples prepared with various post-treatments including water or ethanol washing and drying at different temperatures. The results suggest that ammonia-catalyzed ethoxylation between ethanol and silanol groups can take place during drying, and the resulting ethoxyl groups along with Si-containing oligomers may contribute to serious micropore blocking. On the other hand, water washing is effective to hydrolyze and remove these pore-blocking materials and thus enable cleared micropores. Several interesting findings including the very sharp DSC peaks, high evolving temperature of ethanol, and pyrolysis of organic matters are linked to Stöber silica’s micropores. Our work has undoubtedly improved the mechanistic understanding of Stöber silica’s microporosity and will thus facilitate the practical optimization and application of this material.

     

Synthesis of stable hollow silica microspheres with mesoporous shell in nonionic W/O emulsion

Stable hollow silica microspheres were synthesized by a solgel method in nonionic W/O emulsion; the mesoporous shell wall of the spheres could have potential applications as controlled release capsules for drugs, dyes, cosmetics and inks, artificial cells, catalysts, and fillers.     

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.     

Non-Porous versus Mesoporous Siliceous Materials for CO2 Capture

In this study, the adsorption properties of a Stöber silica-based material towards CO₂ were evaluated for the first time. The use of Stöber silica as support is interesting for real technological applications mainly due to economic factors. Furthermore, a direct comparison between the non porous Stöber sample with an ordered porous material (based on MCM-41 silica) allowed to evaluate the effect of the porosity on the CO₂ adsorption properties. In order to make silica materials reactive towards CO₂, they were functionalized by introducing amino groups via grafting of 3-[2-(2-aminoethyl)aminoethyl]aminopropyltrimethoxysilane. After a qualitative study of the CO₂ adsorption, the quantitative determination of CO₂ adsorption capacity at 35 °C revealed that the mesoporous material is more efficient compared to the Stöber-based one (adsorption capacity values of 0.49 and 0.58 mol/kg for Stöber-based and mesoporous samples). However, since the difference in the adsorption capacity is only about 15 % and the Stöber-based sample is considerably cheaper, the non-porous sample should be considered as a favourable adsorbent material for CO₂ capture applications.     

Synthesis of tri-layer hybrid microspheres with magnetic core and functional polymer shell

Tri-layer magnetite/silica/poly(divinylbenzene) (Fe₃O₄/SiO₂/PDVB) core-shell hybrid microspheres were prepared by distillation precipitation polymerization of divinylbenzene (DVB) in the presence of magnetite/3-(methacryloxyl)propyl trimethoxysilane (MPS) modified silica core-shell particles as seeds. The polymerization of DVB was performed in neat acetonitrile with 2,2′-azobisisobutyronitrile (AIBN) as initiator to coat magnetite/MPS-modified silica particles through the capture of DVB oligomers with the aid of vinyl groups on the surface of inorganic seeds in absence of any stabilizer or surfactant. Other magnetite/silica/polymer tri-layer hybrid particles, such as magnetite/silica/poly(ethyleneglycol dimethacrylate) (Fe₃O₄/SiO₂/PEGDMA) and magnetite/silica/poly(ethyleneglycol dimethacrylate-co-methacrylic acid) (Fe₃O₄/SiO₂/P(EGDMA-co-MAA)) with various polarity and functionality, were also prepared by this procedure. Magnetite/silica/poly(N,N′-methylenebisacrylamide-co-methacrylic acid) (Fe₃O₄/SiO₂/P(MBAAm-co-MAA)) were synthesized with unmodified magnetite/silica particles as seeds. The resultant tri-layer hybrid particles were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectra (FT-IR), dynamic light scattering, and vibrating sample magnetometer (VSM).     

Acid–base sensor based on sol–gel encapsulation of bromothymol blue in silica: application for milk spoilage detection

Journal of Sol-Gel Science and Technology - In this work, silica capsules containing bromothymol blue (BTB) were synthesized by a modified Stöber sol–gel method. These silica capsules...