Stober Synthesis Research Articles

Bioinspired synthesis of bioactive glass nanocomposites for hyaluronic acid delivery to bone and skin

In this study, we present nanocomposites of bioactive glass (BG) and hyaluronic acid (HA) (nano-BGHA) for effective delivery of HA to skin and bone. The synthesis of the nanocomposites has been carried out through the bio-inspired method, which is a modification of the traditional Stober's synthesis as it avoids using ethanol, ammonia, synthetic surfactants, or high-temperature calcination. This environmentally friendly, bio-inspired route allowed the synthesis of mesoporous nanocomposites with an average hydrodynamic radius of ∼190 nm and an average net surface charge of ∼−21 mV. Most nanocomposites are amorphous and bioactive in nature with over 70 % cellular viability for skin and bone cell lines even at high concentrations, along with high cellular uptake (90–100 %). Furthermore, the nanocomposites could penetrate skin cells in a transwell set-up and artificial human skin membrane (StratM®), thus depicting an attractive strategy for the delivery of HA to the skin. The purpose of the study is to develop nanocomposites of HA and BG that can have potential applications in non-invasive treatments that require the delivery of high molecular weight HA such as in the case of osteoarthritis, sports injury treatments, eye drops, wound healing, and some anticancer treatments, if further investigated. The presence of BG further enhances the range to bone-related applications. Additionally, the nanocomposites can have potential cosmeceutical applications where HA is abundantly used, for instance in moisturizers, dermal fillers, shampoos, anti-wrinkle creams, etc.

Synthesis of Stober silica nanoparticles in solvent environments with different Hansen solubility parameters

• Changing the solvent in the Stober synthesis is a good tool to control particle size. • The addition of non-polar liquids results formation of smaller particles. • Dispersion and polar thermodynamic parameters influence on nanoparticle size. • A decrease of hydrogen interaction index leads to size increase of nanoparticles. The most commonly used technique for obtaining SiO 2 nanoparticles of controlled size is the Stober synthesis. In this paper, a series of Stober syntheses in media with different thermodynamic parameters are described and the obtained nanoparticles were characterized by electron microscopy. As a result, it was shown that the solvent variation in the Stober synthesis is a working tool for controlling the particle size of silica. The addition of nonpolar liquids to the reaction mixture led to the formation of smaller particles. Also, it was found that an increase in the dispersion and polar parameters of the solvent leads to an increase in the size of the resulting SiO 2 nanoparticles. The same observations were obtained with a decrease in the index of hydrogen interactions between solvent molecules.

Restricted and Unrestricted Migration Mechanisms of Silica Nanoparticles in Agarose Gels and Their Utilization for the Separation of Binary Mixtures

The migration of SiO2 nanoparticles (NP) was investigated during agarose gel electrophoresis with various gel mesh sizes. As a model system, spherical SiO2 NP of different sizes were synthesized by Stober synthesis, and surface-modified by covalently binding different types of ligands to tailor the zeta potential as required. In the following step, agarose gels were cast with TBE (Tris/Borate/EDTA) buffer solution, thereby controlling the mesh sizes by varying agarose concentration, with higher concentrations leading to smaller mesh sizes. Additionally, the applied voltage could be varied for different electrophoretic experiments. Generally, small mesh sizes and high applied voltages lead to faster particle migration. The migration of SiO2 NP occurs under two regimes: restricted migration, where small mesh size interferes with large particles so that the gel is acting similar to a sieve; and unrestricted migration, where mesh size does not influence the particles’ migration behavior and they move accordin...

Study on optical nonlinearity of Au@SiO2 composite nanoparticles towards photonic applications

This work describes investigations on the optical nonlinearity of Au@SiO2 composite nanoparticles synthesized by citrate reduction followed by Stober's synthesis technique. Their optical, structural and morphological properties were examined by collection of joint techniques, including UV–vis spectroscopy, powder XRD and transmission electron microscopic analysis, respectively. The optical nonlinearity of the sample is analyzed by z-scan technique using pulsed Nd-YAG laser at the excitation wavelength of 1064 nm. The Au@SiO2 composite nanoparticles exhibit two times higher nonlinear index of refraction than the Au nanoparticles.

ChemInform Abstract: Recent Advances in Carbon Nanospheres: Synthetic Routes and Applications

AbstractReview: [66 refs.

The cause of absorption and luminescence band shift of disperse Red-13 in silica spheres

Optical absorption and luminescence band shifts have been observed for a disperse red-13 (DR-13) attached on silica spheres. A covalent bond is formed between an isocyanatopropyl triethoxysilane (ICPTES) and a DR-13 prior to DR-13 being incorporated inside the spheres. The Stober synthesis process was performed with the synthesized ICPTES-DR-13 (ICPDR), tetraethoxy orthosilane (TEOS), 2-propanol and NH4OH solution. The final silica spheres with ICPDR (ICPDRSS) were analyzed with Fourier transform infrared, UV-visible and fluorescence spectrometers. Characteristic absorption peaks at 1700 and 1704 cm−1 representing −C=O stretching vibrations for ICPDR and ICPDRSS indicate the formation of urethane linkages. Although the absorption band at 502 nm did not shift with increasing concentration of DR-13 in methanol, the absorption band for the ICPDRSS shifted 28 nm toward shorter wavelength due to the antiparallel geometry of the azochromophores. A broad luminescence was observed for ICPDRSS and shifted toward the blue with increasing excitation wavelength, which was due to a transformation of the DR-13 in silica spheres from a trans-form to a cis-form.

Au/SiO2/QD core/shell/shell nanostructures with plasmonic-enhanced photoluminescence

A sol–gel method has been developed to fabricate Au/SiO2/quantum dot (QD) core–shell–shell nanostructures with plasmonic-enhanced photoluminescence (PL). Au nanoparticle (NP) was homogeneously coated with a SiO2 shell with adjusted thickness through a Stober synthesis. When the toluene solution of hydrophobic CdSe/ZnS QDs was mixed with partially hydrolyzed 3-aminopropyltrimethoxysilane (APS) sol, the ligands on the QDs were replaced by a thin functional SiO2 layer because the amino group in partially hydrolyzed APS has strong binding interaction with the QDs. Partially hydrolyzed APS plays an important role as a thin functional layer for the transfers of QDs to water phase and the subsequent connection to aqueous SiO2-coated Au NPs. Although Au NPs were demonstrated as efficient PL quenchers when the SiO2 shell on the Au NPs is thin (less than 5 nm), we found that precise control of the spacing between the Au NP core and the QD shell resulted in QDs with an enhancement of 30 % of PL efficiency. The Au/SiO2/QD core/shell/shell nanostructures also reveal strong surface plasmon scattering, which makes the Au/SiO/QD core–shell–shell nanostructures an excellent dual-modality imaging probe. This technology can serve as a general route for encapsulating a variety of discrete nanomaterials because monodispersed nanostructures often have a similar surface chemistry.

Aptamer-Functionalized Silica Nanoparticles for Targeted Cancer Therapy

Target-specific drug delivery system that can transport an effective dosage of anti-cancer drugs to the targeted tumor cells can significantly reduce drug toxicity to the normal cells and increase the therapeutic effect of the drug. In our work, we have evaluated the cytotoxic potential of paclitaxel-loaded silica nanoparticles (Si-PTX NPs) prepared by template-directed stober synthesis technique. The particles were characterized using transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. We have modified the surface of the drug-loaded particles chemically and conjugated a tumor-specific aptamer (Apt-Si-PTX NPs) to facilitate targeted drug delivery to the cancer cells. In vitro studies carried out demonstrated that the aptamer-conjugated paclitaxel-loaded silica nanoparticles could target the cancer cells with high specificity and destroy them effectively, while sparing the normal cells. This work concludes that the aptamer-tagged paclitaxel-loaded silica nanoparticles are excellent targeting moieties for targeted drug delivery to tumor cells.

Modified Stöber synthesis of highly luminescent dye-doped silica nanoparticles

Dye-doped silica nanoparticles (NPs) have been demonstrated to be sensitive labeling markers for biosensing and bioimaging. Their flexible conjugation, excellent photostability and ultrasensitivity make them a powerful tool in biological analysis. Although there have been many reports on the basic research and application of these NPs, they are far from reaching their full potential. Silica NPs can be obtained through two principal approaches: the Stober synthesis and the microemulsion method. The Stober synthesis has the advantage of being easily scaled up for commercial applications and the possibility to effortlessly transfer the NPs into aqueous solutions (typical of bioanalysis). However, further investigation on the impact of the synthesis parameters on the particles size and on the doping process are needed in order to obtain highly luminescent particles. In this study a modified Stober synthesis is proposed and a systematic study of the different reagents is reported, which provides a better picture on the influence of ethanol, ammonia, water, silica precursors, and dye concentration on the final morpho-optical properties. As a result, samples of luminescent silica NPs from 10 to 300 nm have been synthesized and optimized to be highly promising labels for biological applications.

A Generic Single‐Step Synthesis of Monodisperse Core/Shell Colloids Based on Spontaneous Pickering Emulsification

Monodisperse core–shell colloids are widely applied in various fields such as biomedicine, catalysis, and optics, and are also important for fundamental studies in colloid and interface science. The synthesis of particles with a well-defined core–shell morphology still comprises several separate steps such as core preparation, its subsequent functionalization with a coupling agent and the deposition of one or several shells. This multi-step approach towards composite colloids has some serious drawbacks. First, both coupling chemistry and synthetic pathways need to be specifically designed for the materials involved. Second, multi-step reactions often require intermediate purifications or separations that not only reduce the net yield but also hamper the up-scaling of composite-colloid synthesis for applications such as those referred to above. In this communication we report a novel single-step synthesis of monodisperse latex-based core–shell colloids that completely relies on self-assembly of the components involved (Fig. 1). Recently we discovered that nano-particles may induce spontaneous emulsification of oil in water to thermodynamically stable, monodisperse Pickering emulsions. This novel emulsification phenomenon, first observed for magnetite (Fe3O4) nano-particles, turns out to be quite generic since also cobalt ferrite (CoFe2O4) and commercial silica particles (Ludox) lead to stable Pickering emulsions. It was also shown that using the chemically reactive oil (methacryloxypropyltrimethoxysilane, TPM), Pickering emulsions could be polymerized by addition of a radical initiator (potassiumpersulphate, KPS) resulting in stable colloidal dispersions (Fig. 2a and b). Here we investigate the use of such particle-stabilized emulsions (hereafter TPM-emulsions) in a seeded growth polymethylmethacrylate (PMMA) polymerization, to obtain in one single and simple step, PMMA colloids with various inorganic cores. Initially we focused on TPM-emulsions merely as a source for monodisperse inorganic cores to seed a surfactant-free PMMA polymerization. However, exploring what would happen when TPM-emulsions are prepared directly in the presence of methylmethacrylate (MM) we noticed that, surprisingly, MM does not interfere at all with the TPM emulsification. In fact, during the latter emulsification, MM remains present as a separate oil phase (Fig. 2c insert). Addition of a water-soluble radical initiator, however, not only induces polymerization of the TPM-emulsion droplets to solid spheres, but simultaneously also triggers the polymerization of PMMA shells onto these spheres. Thus the floating MM phase (Fig. 2) acts as a monomer source for this PMMA shell formation and, indeed, the shell thickness continues to grow until the whole MM phase is consumed. It turns out that, in addition, polymerized TPM-emulsions are also excellent seeds dispersions for growing polystyrene shells via surfactant free emulsion polymerization, as well as silica shells via the classical Stober synthesis, showing that the spontaneous Pickering emulsification allows for the preparation of polymeric as well as inorganic composite model colloids. C O M M U N IC A TI O N

Silica nanoparticles encapsulating near-infrared emissive cyanine dyes

We show that efficient near-infrared (NIR) cyanine fluorophores ( 1 and 2) can be encapsulated into silica nanoparticles providing a highly versatile and unique platform for in vivo diagnostics. Utilizing this platform, multiple fluorophores can be loaded within a single particle allowing the light absorption and emission properties of the nanoparticle to be controlled independent of particle size. Furthermore, such dyed nanoparticles may have extinction coefficients as high as about 100 × 10 6 L mol −1 cm −1 in the NIR (on a per mole of particles basis), with quantum yields from about 8–10%. A simple synthetic method for varying particle size and dye-loading level is presented, and a modified Stober synthesis reduces deleterious exposure of the dye to the highly alkaline conditions used. The cyanine dyes are encapsulated in silica in a non-aggregated state and the fluorescence brightness is largely maintained to nominal dye concentrations approaching 50 μM. The ability to control light absorption and emission properties independent of particle size, and convenient access to particle sizes in the range of 20–100 nm (a size regime difficult to access with other nanoparticle approaches such as quantum dots), are important features for anatomical targeting in in vivo diagnostics and targeted therapeutic applications.

Manipulation of the Magnetic Properties of Magnetite−Silica Nanocomposite Materials by Controlled Stober Synthesis

The paper describes the synthesis and characterization of the magnetic properties of magnetite/silica nanocomposites using a modified Stober method. Magnetite nanoparticles averaging 8-10 nm in diameter and stabilized with oleic acid in toluene were used as the magnetic component of the nanocomposites. SQUID magnetic measurements and ferromagnetic resonance spectroscopy measurements were performed at each stage of the synthesis to understand the properties of the formed composites. Changes of blocking temperature in ZFC/FC SQUID curves correlated with corresponding changes of the resonance field in the ferromagnetic spectra of the sample at each stage of formation. The paper concludes that it is possible to manipulate the magnetic properties of silica/magnetite composite materials by controlling their surface properties and silica coating thickness.

Bright and Stable Core−Shell Fluorescent Silica Nanoparticles

A class of highly fluorescent and photostable core-shell nanoparticles from a modified Stober synthesis in the size range of 20-30 nm is described. These nanoparticles are monodisperse in solution, 20 times brighter, and more photostable than their constituent fluorophore, and are amenable to specific labeling of biological macromolecules for bioimaging experiments. The photophysical characteristics of the encapsulated fluorophores differ from their solution properties. This raises the possibility of tuning nanoparticle structure toward enhanced radiative properties, making them an attractive material platform for a diverse range of applications.

Chemical reaction kinetics leading to the first Stober silica nanoparticles – NMR and SAXS investigation

29Si-NMR and 13C-NMR were used in methanol and ethanol to monitor the intermediates or hydrolyzed monomers that lead to the formation of the first primary particles as detected by small angle X-ray scattering. This identification was facilitated by using initial NH 3 and H 2O levels at the lower end of those experienced in Stober synthesis to slow the reaction kinetics. We found that [NH 3] and [H 2O] control the balance between hydrolysis of tetraethylorthosilicate (TEOS) and the condensation of its hydrolyzed monomers. Transesterification between methanol and TEOS did occur; however, it was negligible compared to the production of hydrolyzed intermediates. The first nanostructures appear at a hydrolyzed monomer concentration around 0.1 M, indicating that formation of the primary structures is thermodynamically controlled by a supersaturation of the intermediate species. Differences in particle size between methanol and ethanol are attributed to thermodynamic interactions between the solvent and the hydrolyzed intermediates.

Covalent Surface Functionalization and Self-Organization of Silica Nanoparticles.

More than 10 000 carboxylic acid groups are covalently coated on monodisperse silica nanoparticles (see schematic diagram) prepared by a Stober synthesis. Colloidal super crystals form spontaneously on dissociation in aqueous dispersion with low ionic strength.

Paramagnetic Silica Particles: Synthesis and Grafting of a Silane Coupling Agent Containing Nickel Ions onto Colloidal Silica Particles

A method is presented for the preparation of stable model dispersions of paramagnetic silica particles. A specially designed silane coupling agent containing Ni 2+ ions is grafted onto silica particles which are either commercially obtained or prepared by the Stober synthesis. A wet process is described which avoids aggregation of ungrafted and grafted silica particles. The resulting magnetic silica particles have been characterized by dynamic light scattering, transmission electron microscopy, elemental analysis, and infrared spectroscopy. Preliminary magnetization measurements and magnetic relaxation (NMRD) experiments illustrate some of the possibilities for studying model silica colloids with a covalently bonded paramagnetic surface layer.

Spherical dye-doped silica particles

Organic dyes such as water soluble porphyrins were incorporated into monodispersed SiO2 particles using the sol-gel technique (the Stober synthesis) and their monomeric and dimeric properties were investigated by aiming at the microcavity effect. Doping efficiencies of dyes into SiO2 particles depended on dye's hydrophilic properties. The doping efficiency of a porphyrin, TTMAPP, decreased with increasing the nominal content in the starting solution and the doping content showed the maximum value of 1.6×10−5 mol/g SiO2. With increasing the TTMAPP content, spectra of doped particles were changed from dimeric to monomeric nature; the absorption peak due to dimers disappeared and halfwidth of absorption at Soret band decreased. This unusual change was caused by the decrease of particle sizes by dye doping. The TTMAPP-doped sample having monomeric spectrum showed the fluorescence life time of 8.5 nsec and photochemical hole burning at about 5 K.