Nanometer-sized Silica Particles Research Articles

Encapsulation of biomolecules for bioanalytical purposes: Preparation of diclofenac antibody-doped nanometer-sized silica particles by reverse micelle and sol–gel processing

In recent years, the sol–gel technique has attracted increasing interest as a unique approach to immobilize biomolecules for bioanalytical applications as well as biochemical and biophysical studies. For this purpose, crushed biomolecule-doped sol–gel glass monoliths have been widely used. In the present work, for the first time, the encapsulation of anti-diclofenac antibodies in silica nanoparticles was carried out by a combination of reverse micelle and sol–gel technique. Cyclohexane was used for the preparation of the microemulsion as organic solvent, while surfactant Igepal CO-520 was found to be the optimal stabilizer. The antibody source was a purified IgG fraction originating from a polyclonal rabbit antiserum. Tetramethyl orthosilicate (TMOS) was used as precursor. Rather uniform, monodispersed and spherical silica particles of about 70 nm diameter size were fabricated, as was demonstrated by transmission electron microscopy (TEM) and scanning electron microscopy/energy dispersive X-ray fluorescence analysis (SEM/EDX). The biological activity of the encapsulated antibodies was evaluated by incubation of the nanoparticles with a diclofenac standard solution and analysis of the filtrate and followed washing solutions by a highly sensitive enzyme-linked immunosorbent assay (ELISA), using non-doped particles as blanks. While only about 6% of the added diclofenac was nonspecifically retained by the blank, the corresponding amount of about 66% was much higher with the antibody-doped particles. An obvious advantage of this approach is the general applicability of the developed technique for a mild immobilization of different antibody species.

Visible luminescence from octadecylsilane monolayers on silica surfaces: Time-resolved photoluminescence characterization

We have found that the adsorption of octadecyltrichlorosilane (OTS) monolayers on nanometer-sized silica particles yields a broad blue photoluminescence (PL) with a time scale of nanoseconds. The observed PL intensity increases after curing at temperatures from ∼100 to ∼300°C, suggesting that condensations between adjacent OTS molecules on the silica surface are related to the PL. The PL decay curve of the cured samples remains unchanged from 77 to 450 K, whereas the time-integrated PL intensity shows a monotonous decrease with increasing temperature. From these experimental results, a model of radiative and nonradiative process associated with the PL is presented.

Electron spin resonance probing of fundamental point defects in nanometer-sized silica particles

Electron spin resonance probing of fundamental point defects in nanometer-sized silica particles

Electron spin resonance probing of E′-type defects in fumed silica nanoparticles

Results are presented of an electron spin resonance (ESR) characterization of E′ type defects observed in fumed ∼ 7-nm sized silica nanoparticles. This is accomplished through K- and Q-band ESR following 10-eV irradiation used to photo-dissociate H from passivated defects. The defect analysis provides access to the structure of the nanoscale on atomic level, herewith adding an extra dimension for analysis. To assess specific physico-chemical structural aspects of the particles, the E′ centers have been monitored as a function of thermal treatment in vacuum in the range 850–1115 °C, revealing the presence of two systems of E′ centers. The first one exhibits ESR parameters very similar to those of the familiar E′ γ center encountered in bulk fused silica, while the second bath exhibits an altered zero crossing g value and line shape, attributed to variations in local structure. While the first E′ system most likely resides in the core regions of the nanoparticles, the second system may mainly be confined to the outer SiO 2 layers exposing a structurally different, possibly more strained nature than macroscopic glassy SiO 2. Probing of fundamental point defects in nanometer-sized silica particles thus reveals structural variations and non-uniformity, indicative of a structure different from bulk material.

Photoluminescence decay dynamics of transparent silica glass prepared from nanometer-sized silica particles

The time-resolved photoluminescence (PL) decays are measured for transparent amorphous silica prepared from solid-phase sintering of nanometer-sized silica particles, which has recently been shown to exhibit a unique white PL emission under ultraviolet excitation [T. Uchino and T. Yamada, Appl. Phys. Lett. 85, 1164 (2004)]. Unlike usual PL processes observed in normal silica glass, it is shown that the present PL results from trapping-controlled migration of photoexcited carriers and their radiative recombination.

Polymerization of w/o Microemulsions for the Preparation of Transparent SiO2/PMMA Nanocomposites

Reverse w/o microemulsions composed of methyl methacrylate (MMA) forming the oil phase, nonionic surfactants, and water are used for the synthesis of transparent SiO2/PMMA nanocomposites. An inorganic precursor, tetraethoxysilane (Si(OEt)(4), TEOS), is hydrolyzed in the reverse micelles containing aqueous ammonia. During the hydrolysis of TEOS, polymerization of the continuous MMA phase is initiated using AIBN (azobisisobutyronitrile), and after thermal polymerization at 333 K for 12 h, solid blocks of PMMA are obtained in which nanometer-sized silica particles are trapped in the solid polymer matrix. According to small-angle X-ray and dynamic light scattering experiments, the water droplets in MMA microemulsions are 12 nm (R(W) = 13) in diameter, whereas after polymerization of the microemulsion, the SiO2 particles in the transparent SiO2/PMMA composites are 26 nm in diameter. Transmission electron micrographs demonstrate a low degree of agglomeration in the composites. In comparison with materials generated from micelle-free solutions, the particle size distribution is narrow. The reverse micelle-mediated approach produces composites of high transparency comparable with that of pure PMMA.

Investigation of Copper Removal Mechanisms during CMP

In this paper we continue our (CMP) of copper mechanism study of the chemical mechanical polishing using a novel experimental approach. We investigated the effects of chemical reagents in the polishing slurry on the removal behavior of copper film during its CMP. More specifically, the friction and wear of the copper surface against a polyurethane pad were measured in the presence of slurries containing various chemical constituents and surface-treated abrasive particles. These slurries are typically formulated for Cu CMP in the industry. The surface qualities and wear mechanisms were then investigated using an atomic force microscope. The experimental results indicate that the copper removal mechanism in the presence of an abrasive and DI water alone was dominated by mechanical abrasion, while chemical dissolution of copper is much more pronounced when the slurry was added a complexing agent such as glycine, regardless the involvement of the strong oxidizer hydrogen peroxide. In the presence of a slurry consisting of hydrogen peroxide, glycine, and nanometer-sized silica particles, a series of electrochemical reactions promotes the rapid formation of copper oxide and the redeposition of precipitation. Based on these tribological investigations we attempt to describe the nature of the material removal, which is one of the key unanswered questions in the CMP community. © 2004 The Electrochemical Society. All rights reserved.

White light emission from transparent SiO2 glass prepared from nanometer-sized silica particles

We report that nanometer-sized silicon-dioxide particles are sintered to optical transparency at temperatures even below 1000°C, forming nonporous bulk silica glass. The resultant silica glass exhibits visible emission, which appears white to the naked eye, in the wavelength range from ∼400to∼700nm at room temperature under ultraviolet excitation. The observed emission is quite stable after prolonged exposure to the atmosphere and shows no appreciable light-induced degradation. The present photoluminescence characteristics are found to be basically different from those reported previously for silica nanoparticles and silica-based porous materials.

Coalescence suppression in model immiscible polymer blends by nano-sized colloidal particles

The effect of nanometer sized silica particles (R≈16 nm) on the flow-induced morphology of immiscible polymer blends is studied. Polydimethylsiloxane (PDMS) and polyisobutylene (PIB) are chosen as model components. A stable droplet/matrix microstructure is obtained for blends of 30% PIB in 70% PDMS or vice versa. Rheological measurements are used to show that the silica particles alter the sensitivity of the of dispersed phase/matrix microstructure to shear flow. Coalescence is suppressed or at least slowed down on a practical time scale, especially when PDMS is the matrix phase. The effect of mixing conditions, pre-shear rate and particle concentration on the blend morphology are studied. Cryo-SEM is used to observe the accumulation of the particles at the interface. Blends stabilized by solid particles could provide an interesting alternative to blends compatibilized by block-copolymers.

Microscopic structure of nanometer-sized silica particles

We have studied the structure of nanometer-sized silica particles called fumed silica, which is a synthetic amorphous silicon dioxide produced by burning silicon tetrachloride in an oxygen-hydrogen flame, using infrared and Raman spectroscopies and a high-energy x-ray diffraction method. It has been demonstrated that the structure of fumed silica is not identical to that of the normal bulk silica glass in terms especially of the distribution of the size of silica rings. Three- and four-membered rings are more frequent in fumed silica than in the bulk silica glass. It has also been shown that the network structure of fumed silica is more flexible than that of the bulk one, probably explaining the reason why fumed silica can accommodate a large number of three- and four-membered rings in the structure.

Localized changes in the structural stability of myoglobin upon adsorption onto silica particles, as studied with hydrogen/deuterium exchange mass spectrometry

A new method is presented for monitoring the conformational stability of various parts of a protein that is physically adsorbed onto nanometer-sized silica particles. The method employs hydrogen/deuterium (H/D) exchange of amide hydrogens, a process that is extremely sensitive to structural features of proteins. The resulting mass increase is analyzed with Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. Higher structural specificity is obtained by enzymatically cleaving the adsorbed proteins prior to mass spectrometric analysis. The mass increases of four peptic fragments of myoglobin are followed as a function of the H/D exchange time. The four peptic fragments cover 90% of the myoglobin structure. Two of the peptic fragments, located in the middle of the myoglobin sequence and close to the heme group, do not show any adsorption-induced changes in their structural stability, whereas the more stable C- and N-terminal fragments are destabilized. Interestingly, for the N-terminal fragment, comprising residues 1–29, two distinct and equally large conformational populations are observed. One of these populations has a stability similar to that in solution (−23 kJ/mol), whereas the other population is highly destabilized upon adsorption (−11 kJ/mol).

Nanometer fluorescent hybrid silica particle as ultrasensitive and photostable biological labels.

Nanometer-sized fluorescent hybrid silica (NFHS) particles were prepared for use as sensitive and photostable fluorescent probes in biological staining and diagnostics. The first step of the synthesis involves the covalent modification of 3-aminopropyltrimethoxysilane with an organic fluorophore, such as fluorescein isothiocyanate, under N2 atmosphere for getting a fluorescent silica precursor. Then the NFHS particles, with a diameter of well below 40 nm, were prepared by controlled hydrolysis of the fluorescent silica precursor with tetramethoxysilane (TMOS) using the reverse micelle technique. The fluorophores are dispersed homogeneously in the silica network of the NFHS particles and well protected from the environmental oxygen. Furthermore, since the fluorophores are covalently bound to the silica network, there is no migration, aggregation and leakage of the fluorophores. In comparison with common single organic fluorophores, these particle probes are brighter, more stable against photobleaching and do not suffer from intermittent on/off light emission (blinking). We have used these newly developed NFHS particles as a fluorescent marker to label antibodies, using silica immobilization method, for the immunoassay of human alpha-fetoprotein (AFP). The detection limit of this method was down to 0.05 ng mL(-1) under our current experimental conditions. We think this material would attract much attention and be applied widely in biotechnology.

Anelastic compression of nanometer-sized silica particles under high pressure: A high-energy x-ray diffraction measurement

We compressed nanometer-sized silica particles, fumed silica, using a cubic-anvil-type apparatus up to pressures of 8 GPa at room temperature. The high-energy (61.7 keV) x-ray diffraction measurements of the recovered samples were carried out to determine the x-ray structure factor and in the real space correlation functions. These results indicate that anelastic compression occurs in silica nanoparticles in the pressure range from 4 to 8 GPa, which is well below the pressure regime of the anelastic compression (\ensuremath{\sim}10 to \ensuremath{\sim}20 GPa) observed for bulk amorphous silica.

Influence of Particle Wettability on the Type and Stability of Surfactant-Free Emulsions

A systematic investigation into the influence of the wettability of spherical, nanometer-sized silica particles on the type and stability of water−toluene emulsions is described. The particles rang...

Catastrophic Phase Inversion of Water-in-Oil Emulsions Stabilized by Hydrophobic Silica

A short review of the experimental findings concerning the stabilization of emulsions by solid particles is given. We then describe the preparation and properties of water-in-oil (w/o) emulsions stabilized by nanometer-sized hydrophobic silica particles alone. Emulsions of median diameter equal to 0.6 μm are completely stable to coalescence as a result of an adsorbed layer of particles at the oil−water interface. Their stability to sedimentation increases with particle concentration due to network formation of the particles in the continuous oil phase. The w/o emulsions catastrophically invert, without hysteresis, to oil-in-water (o/w) at volume fractions of water around 0.7, i.e., as soon as the drops begin to deform. The drops in o/w emulsions are larger (100 μm) and cream rapidly but remain stable to coalescence. We demonstrate that for emulsions stabilized by hydrophilic silica particles, phase inversion from o/w to w/o occurs at the same dispersed phase volume fraction as above. It is therefore suggested that the system hydrophile−lipophile balance is determined by the particle wettability. Comparisons with the behavior of surfactant-stabilized emulsions are given throughout.

Atomic and electronic structures of nanometer sized silica particles

The Austin Model 1 semi-empirical method combined with geometry optimization procedure was used to study atomic and electronic structures of nanoscale silica particles containing up to 192 atoms (64 Si and 128 O). Twofold Si–O rings were found to dominate in nanometer sized particles. The fraction of larger rings increases with increasing particle sizes. Bond length and bond angle distributions were calculated. The shortest Si–O, O–O and Si–Si interatomic distances are approximately 1.7, 3.0 and 3.3 Å, respectively. Average O–Si–O and Si–O–Si bond angles are approximately 110 ∘ and 130 ∘. Average electronic energy band gap is 6.9 eV.

Effects of Polymers on Particle Adsorption on Macroscopic Surfaces Studied by Optical Reflectometry

The adsorption of nanometer-sized silica particles on a macroscopic silica surface pretreated with different polymers was studied using optical reflectometry in a stagnation point flow cell. Efficient sticking of particles occurs only when no electrostatic repulsion between particles and surface exists. Cationic polyelectrolytes, e.g., partially methylated poly(vinylimidazole), are very efficient in inducing adsorption of negatively charged SiO2particles. Competition between the negatively charged SiO2particles and an anionic polymer present in the suspension reduces the amount of particles adsorbed on the surface. Adsorbed nanometer-sized silica particles do not become embedded in the adsorbed polymer layer but rather stay on the periphery of this layer on the time scale of the experiment (15 min). Therefore, repeated sequential addition of adsorbing polymer and particles will lead to controllable high amounts of particles on the surface.

Molecular State and Mechanical Properties of Epoxy Hybrid Composite

A hybrid nanocomposite was prepared from the combination of an epoxy resin with a dispersion of nanometer-sized silica particles prepared by hydrolysis of silicon tetra alkoxides. A silane coupling agent was used to improve the filler-matrix interface by crosslinking into the epoxy networks. Silica dispersion caused the increase of nanometer size voids evaluated by the positron annihilation method and the decrease of Vickers hardness. The use of coupling agent also brought about the decrease of nanometer size voids and the increase in hardness.

Properties of low-density bulk silica gel: lyosil

Bulk silica gels prepared by hydrolysis, gelation and curing of tetramethoxy silane were further characterized. The pore size distribution is narrow in the range of 13.1 nm and the BET surface area is 622 m 2/g. Visible light transmittance for bulk lyosil of 0.5 cm thick plate is 80% with UV cut off at 300 nm of a lyosil having bulk density of 0.40 g/cm 3. Thermal conductivity of a dried monolithic lyosil is 0.065 W/m K and remained unchanged after firing in air at 600°C. Electron micrographs show that lyosils have uniform distribution of several nanometer sized silica particles linked together.