Nucleation of silica Stöber particles in the presence of methacryloxypropyltrimethoxysilane

The effect of 3-methacryloxypropyltrimethoxysilane (MPTMOS) on the nucleation of silica particles synthesized in a water?ethanol?ammonia?tetraethoxysilane (TEOS) mixture by the Stöber?Fink?Bohn method has been studied. It has been shown, using atomic force microscopy, that, as the content of MPTMOS in a TEOS + MPTMOS precursor mixture is increased from 0 to 12.5 mol %, the final silica particle size decreases from 470 to 10 nm, because the number of nucleation centers increases by several orders of magnitude. In contrast to TEOS, hydrolysis of MPTMOS yields a smaller amount of deprotonated orthosilicic acid monomers, the condensation of which is hindered by electrostatic repulsion. The polycondensation of electrically neutral products of MPTMOS hydrolysis gives rise to a larger number of nucleation centers in the reaction mixture.     

Smart control of monodisperse Stöber silica particles: effect of reactant addition rate on growth process

Control over the synthesis of monodisperse silica particles up to mesoscopic scale is generally made difficult due to intrinsic limitation to submicrometric dimensions and secondary nucleation in seeded experiments. To investigate this issue and overcome these difficulties, we have implemented single step processing by quantifying the effects of the progressive addition of a diluted tetraethyl orthosilicate solution in ethanol on the size and monodispersity of silica particles. Contrary to particles grown in seeded polymerization, monodisperse particles with size up to 2 microm were synthesized. Moreover, the particles exhibit a final diameter (d(f)), which varies with V(-1/3) over more than 2 orders of magnitude in rate of addition (V). On the basis of a kinetic study in the presence of addition showing that particle growth is limited by the diffusion of monomer species, we developed a diffusion-limited growth model to theoretically explain the observed d(f)(V) behavior and quantitatively retrieve the measured amplitude and exponent. Using a single parameter procedure, we can therefore predict and generate in the room temperature range, monodisperse particles of a targeted size by simply adjusting the rate of addition.     

Preparation of spherical silica particles by Stöber process with high concentration of tetra-ethyl-orthosilicate

In this paper, Stöber process with high concentration of tetra-ethyl-orthosilicate (TEOS) up to 1.24 M is used to prepare monodisperse and uniform-size silica particles. The reactions are carried out at [TEOS] = 0.22–1.24 M, low concentrations of ammonia ([NH₃] = 0.81[TEOS]), and [H₂O] = 6.25[TEOS] in isopropanol. The solids content in the resulting suspension achieves a maximum value of 7.45% at 1.24 M TEOS. Various-sized particles in the range of 30–1000 nm are synthesized. The influences of TEOS, NH₃, and H₂O on the size and size distribution of the particles are discussed. A modified monomer addition model combined with aggregation model is proposed to analyze the formation mechanism of silica particles.     

ESI-TEM imaging of surfactants and ions sorbed in Stöber silica nanoparticles

The sorption of surfactants and NaCl in silica nanosized particles creates unexpected spatial distributions of solutes that were evidenced by electron spectroscopy imaging in the transmission electron microscope (ESI/TEM). The spectral images show that simple ions (Na(+), Cl(-), Br(-)) are actually absorbed within the particles irrespective of their charges, while surfactant chains are adsorbed at the particle surfaces. The expected effect of the surfactants on particle aggregation is also observed in the micrographs. In the case of salt, close-packed silica particle arrays are formed at low ionic strength, but only coarse aggregates form at higher salt concentrations. The particles absorb both Na(+) and Cl(-) ions in similar amounts, from 0.5 mol L(-)(1) NaCl, but Na(+) ions are depleted from the particles' immediate outer vicinity, where Cl(-) ions are in turn accumulated. These results confirm that St?ber silica nanoparticles are highly porous and reveal their potential usefulness as carriers of small molecules and ions, due to the small particle size, exceptional colloidal stability, and this newly found sorption behavior.     

Revisiting the Stöber method: inhomogeneity in silica shells

We demonstrate that the silica shell on nanoparticles formed by a typical St?ber method is inhomogeneous in nature. The outer layer of the shell is chemically more robust than the inner layer, which can be selectively etched by hot water. Methods are developed to "harden" the soft silica shells. These new understandings are exploited to develop versatile and template-free approaches for fabricating sophisticated yolk-shell nanostructures.     

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.

     

Effect of pH on the Final Connectivity Distribution of the Silicon Atoms in the Stöber Particles

Using ²⁹Si MAS-NMR we investigate the effect of pH on the final connectivity distribution of the silicon atoms in the Stöber particles. Our data suggest that the fraction of the silicon atoms that are ully-connected decreases as the ammonia concentration is increased. This suggests a more negative first shell substitution effect in the precipitated phase (liquid droplet) if condensation reactions are irreversible. A simple model is developed to describe the condensation kinetics in the precipitated phase, and the results support the negative first shell substitution effect. These findings challenge the notion of a positive first shell substitution effect in alkaline conditions.     

Stöber synthesis of monodispersed luminescent silica nanoparticles for bioanalytical assays

We have developed a simple method to prepare bright and photostable luminescent silica nanoparticles of different sizes and narrow size distribution in high yield. The method is based on the use of St?ber synthesis in the presence of a fluorophore to form bright silica nanoparticles. Unlike micro-emulsion-based methods often used to prepare luminescent silica particles, the St?ber method is a one-pot synthesis that is carried out at room temperature under alkaline conditions in ethanol:water mixtures and avoids the use of potentially toxic organic solvents and surfactants. Our luminescent particles contained the transition metal complex tris(1,10-phenanthroline) ruthenium(II) chloride, [Ru(phen)3]Cl2. They showed higher photostability and a longer fluorescence lifetime compared to free Ru(phen)3 solutions. Leakage of dye molecules from the silica particles was negligible, which was attributed to strong electrostatic attractions between the positively charged ruthenium complex and the negatively charged silica. To demonstrate the utility of the highly luminescent silica nanoparticles in bioassays, we further modified their surface with streptavidin and demonstrated their binding to biotinylated glass slides. The study showed that digital counting of the luminescent nanoparticles could be used as an attractive alternative to detection techniques involving analogue luminescence detection in bioanalytical assays.     

Fabrication of teardrop-shaped silica particles in polyelectrolyte diluted solution through in situ sol–gel process

A new strategy to fabricate teardrop-shaped silica particles is presented. Monodispersed teardrop-like silica particles were obtained through basic catalyzed sol–gel process of tetraethoxysilane by employed sodium polyacrylate as soft template. Increasing the salt concentration of solution, the morphology of silica particles can transform from teardrop-like to hollow structures, and finally formed solid particles. The morphologies of silica particles are characterized by TEM and SEM. Our finding can be scaled up for large-scale synthesis of unusual structures of inorganic or composite materials in a predictable manner. This study is expected to provide further understanding of the role of polyelectrolyte in the synthesis of inorganic materials towards design of unusual architectures and functional materials.     

Synthesis of mesoporous Stöber silica nanoparticles: the effect of secondary and tertiary alkanolamines

The effect of secondary (diethanolamine) and tertiary (triethanolamine) alkanolamines as catalysts on the formation of mesoporous Stöber silica nanoparticles by sol–gel method was studied. The particles were characterized by thermogravimetry and differential thermal analysis, Fourier transform infrared spectroscopy, N₂ physisorption measurements, and field emission scanning electron microscopy. By using ammonia and different alkanolamines as catalysts, the Brunauer–Emmet–Teller (BET) surface area and pore volume increased in the order of ammonia < diethanolamine < triethanolamine. A maximum BET surface area of 140.1 m² g^?1 and pore volume of 0.66 cm³ g^?1 were obtained from triethanolamine catalyzed silica particles. The average particle size of silica prepared by ammonia and different alkanolamines as catalysts decreased in the order of ammonia > diethanolamine > triethanolamine. The role of different alkanolamines on the textural properties and particle size of silica is explained in terms of their relative steric hindrance and basicity.     

Synthesis of monodispersed silica nanoparticles with high concentration by the Stöber process

Silica nanoparticles with high concentration were prepared by the sol–gel process based on the Stöber method using tetraethoxysilane as a starting material. It was found that silica sol with about 4 wt% in concentration and with a diameter of about 10 nm was obtained by controlling the reaction conditions in the Stöber process. By removing the solvent under a reduced pressure, the particle concentration was increased up to 15 wt% without aggregation.     

Effect of exposure to growth media on size and surface charge of silica based Stöber nanoparticles: a DLS and ζ-potential study

Journal of Sol-Gel Science and Technology - In this paper the fate (as size and surface charge changes) of Stöber silica based nanoparticles in contact with a growth medium is studied through...     

Stöber silica particles as basis for redox modifications: particle shape, size, polydispersity, and porosity

The synthesis of Stöber silica particles as basis for redox modifications is optimized for desired properties, in particular diameter in a wide sub-micrometer range, spherical shape, monodispersity, the absence of porosity, and aggregation free isolability for characterization and later covalent modification. The materials are characterized by SEM, DLS, nitrogen sorption isotherms, helium as well as Gay-Lussac (water) pycnometry, and DRIFT spectroscopy. Particles with diameters between approximately 50 and 800 nm are obtained by varying the concentrations of the reagents and reactants, the type of solvent as well as the temperature. The use of high water concentrations and post-synthetic calcination at 600 °C results in silica particles that can be considered as nonporous with respect to the size of the active molecules to be immobilized. The effect of reaction temperature on size distribution is identified. Low polydispersity is achieved by performing the reaction in a temperature range in which a change in temperature has only a weak or no effect on the final particle diameter. Upon optimization of the sol–gel process, the shape of the particles is still spherical. The agreement between experimental and geometric data is within the expected precision of the characterization techniques.     

über das Stäubungsvermögen der Stoffe

Zusammenfassung Es wird der Begriff: Stäubungsvermögen der verschiedenen Stoffe aufgestellt und etwas näher beleuchtet, sowie eine Apparatur zur Festlegung dieser Stoffeigenschaft vorgeschlagen. Mit dem danach gebauten Me\gerät werden 40 Stoffe auf ihr Stäubungsvermögen untersucht, wobei diese Stoffe nach verschiedenen Gesichtspunkten ausgew ahlt und daher in verschiedene Versuchsreihen eingeordnet wurden. Es wird vorgeschlagen, jedem einzelnen Stoff eine Kennziffer für sein Stäubungsvermögen zu erteilen, und es wird gezeigt, wie dies Vermögen mit der Feinheitskennlinie des betreffenden Stoffes in Zusammenhang steht. Als besonders bemerkenswert wird hervorgehoben, da\ bei einem gewöhnlichen, ungefähr zu Mehlfeinheit vermahlenen Stoff das Stäubungsvermögen mit wachsender Feinheit abnimmt, und da\ eine starke Zunahme des Stäubungsvermögens eintritt, wenn bei einem solchen Stoff die feinsten Anteile entfernt werden. Es wird endlich versucht, die gefundenen Ergebnisse von einem physikalischen Gesichtspunkt aus zu erörtern.     

Redox-active silica nanoparticles. Part 6. Synthesis and spectroscopic and electrochemical characterization of viologen-modified Stöber silica particles with diameters of approximately 125 nm

The synthesis of Stöber silica particles and their redox behavior after viologen modification are reported. The particles were synthesized using high reaction temperatures and a postsynthetic calcination step to yield nonporous silica spheres with d of approximately 125 nm and low polydispersity. As the immobilization strategy, the condensation of surface hydroxyl groups with an alkoxysilane-substituted viologen modifier was chosen. The successful attachment was confirmed by ¹³C-CP/MAS-NMR and diffuse reflectance Fourier transform infrared as well as UV/Vis and energy dispersive X-ray spectroscopy. The surface concentration was estimated to be between 48 and 100  $\upmu$ mol/g. The modified material is redox-active. Differential pulse voltammetry and cyclic voltammetry experiments in nonaqueous solvents at Pt and mercaptopropionic acid-modified gold electrodes reveal two separated and pronounced signals of the first and second reduction of the viologen unit. The observed potentials are in good agreement with those of the free modifier. Furthermore, the treatment of the viologen-modified material with Na₂S₂O₄ leads to the formation of a nanoparticulate silica material with stable free radicals located at the particle surface. In addition, such radicals are generated by the in situ electrolysis at Pt net electrodes in acetonitrile or dimethylformamide. The radical formation was confirmed by EPR and UV/Vis spectroscopy. The reduction is reversed by air oxidation.     

Size regulation and prediction of the SiO2 nanoparticles prepared via Stöber process

In this study, monodispersed SiO₂ nanoparticles with controllable size ranging from 20 to 100 nm have been successfully prepared by using a Stöber sol-gel process. The particle size, distribution, and morphology were examined by dynamic light scattering (DLS) and transmission electron microscopy (TEM). In order to accurately regulate the particle size, the effects of various reaction parameters on the particle size and morphology of SiO₂ nanoparticles were systematically investigated. It was found that, to some extent, the SiO₂ particle size increased linearly with the increase of ammonia or water content, as well as the decrease of reaction temperature, while nearly kept constant with the increase of tetraethyl orthosilicate (TEOS). Based upon the results, an empirical formula has been developed to model/predict the particle size of the SiO₂ nanoparticles over a wide range of various reaction conditions.     

Über die Acetylierung von löslicher Stärke

Ohne Zusammenfassung