Charged Silica Research Articles

PH-Dependent Interaction and Resultant Structures of Silica Nanoparticles and Lysozyme Protein

Small-angle neutron scattering (SANS) and UV-visible spectroscopy studies have been carried out to examine pH-dependent interactions and resultant structures of oppositely charged silica nanoparticles and lysozyme protein in aqueous solution. The measurements were carried out at fixed concentration (1 wt %) of three differently sized silica nanoparticles (8, 16, and 26 nm) over a wide concentration range of protein (0-10 wt %) at three different pH values (5, 7, and 9). The adsorption curve as obtained by UV-visible spectroscopy shows exponential behavior of protein adsorption on nanoparticles. The electrostatic interaction enhanced by the decrease in the pH between the nanoparticle and protein (isoelectric point ∼11.4) increases the adsorption coefficient on nanoparticles but decreases the overall amount protein adsorbed whereas the opposite behavior is observed with increasing nanoparticle size. The adsorption of protein leads to the protein-mediated aggregation of nanoparticles. These aggregates are found to be surface fractals at pH 5 and change to mass fractals with increasing pH and/or decreasing nanoparticle size. Two different concentration regimes of interaction of nanoparticles with protein have been observed: (i) unaggregated nanoparticles coexisting with aggregated nanoparticles at low protein concentrations and (ii) free protein coexisting with aggregated nanoparticles at higher protein concentrations. These concentration regimes are found to be strongly dependent on both the pH and nanoparticle size.

Revisit to the self-assembled hybrid acrylate/silica core-shell structured particles in the presence of unmodified silica particles

Polyacrylate/silica nanocomposite particles are synthesized by soap-free emulsion polymerization using negatively charged silica particles, where the anionic initiator, potassium peroxydisulfate (KPS), is used. The influence of the content of hydrophilic silica particles on the composite particle growth and conversion is investigated. Nucleation process of unmodified silica-stabilized emulsion polymerization is studied by characterizing the structures of composite particles forming at different polymerization stages with transmittance electron microscope (TEM) and laser light scattering (DLS). The results show that the presence of silica particles on latex particles reinforces the barrier to radical absorption and retards the polymerization rate and therefore the growth of the latex particles. However, the composite particle size does not decrease much when the content of silica sol is increased to more than 5wt%. A mechanism is proposed to explain the formation of the core-shell structured particles when an electrostatic repulsive force between the silica and polyacrylate particles is present. The polyacrylate precursors forming in the early stage of the soap-free emulsion polymerization are unstable and tend to aggregate and absorb silica particles to lower their interfacial free energy, thus forming the composite particles. In contrast to the monomer droplets dispersed in the sole acrylic soap free emulsion polymerization, the monomer droplets dispersed in the aqueous system containing silica sol can work as polymerization loci due to their relatively small size and the large quantity (or the large specific surface areas), hence enhancing the ability to capture radicals.

Crowding by Anionic Nanoparticles Causes DNA Double-Strand Instability and Compaction

Up to the present, DNA structural transitions caused by cationic polymers as well as in concentrated solutions of neutral polymers are well documented, while a little is known about DNA interaction with like-charge species. Herein, changes in the structure of DNA induced by anionic nanoparticles of different sizes (20-130 nm) were investigated by combining single-molecule DNA fluorescent microscopy, to monitor the conformational dynamics of long-chain DNA, with spectroscopic methods, to gain insight into changes in the secondary structure of DNA. The results showed that several percent of negatively charged silica nanoparticles induced DNA compaction from a coil to a globule, and this change was accompanied by a decrease in the melting temperature of the DNA double helix. DNA was compacted into toroidal condensates with reduced diameters of about 20-30 nm. Smaller 20 nm nanoparticles triggered a DNA coil-globule transition at lower concentrations, but the exclusion volume for each type of nanoparticle at the point of complete DNA collapse, as estimated by taking into account the depth of the ionic atmosphere, was found to be almost the same.

Bridging interactions of proteins with silica nanoparticles: the influence of pH, ionic strength and protein concentration.

Charge-driven bridging of nanoparticles by macromolecules represents a promising route for engineering functional structures, but the strong electrostatic interactions involved when using conventional polyelectrolytes impart irreversible complexation and ill-defined structures. Recently it was found that the electrostatic interaction of silica nanoparticles with small globular proteins leads to aggregate structures that can be controlled by pH. Here we study the combined influence of pH and electrolyte concentration on the bridging aggregation of silica nanoparticles with lysozyme in dilute aqueous dispersions. We find that protein binding to the silica particles is determined by pH irrespective of the ionic strength. The hetero-aggregate structures formed by the silica particles with the protein were studied by small-angle X-ray scattering (SAXS) and the structure factor data were analyzed on the basis of a short-range square-well attractive pair potential (close to the sticky-hard-sphere limit). It is found that the electrolyte concentration has a strong influence on the stickiness near pH 5, where the weakly charged silica particles are bridged by the strongly charged protein. An even stronger influence of the electrolyte is found in the vicinity of the isoelectric point of the protein (pI = 10.7) and is attributed to shielding of the repulsion between the highly charged silica particles and hydrophobic interactions between the bridging protein molecules.

Analytical characteristics of a qPCR-based molecular diagnostic assay - conceptual considerations for laboratory personnel.

Analytical characteristics of a qPCR-based molecular diagnostic assay - conceptual considerations for laboratory personnel.

Experimental investigation of the effects of ionic micelles on colloidal stability

The effects of ionic micelles (tetradecyltrimethylammonium bromide (C14TAB) and sodium dodecylsulfate (SDS)) on the stability of a dispersion of highly charged silica particles was investigated both visually and using UV–vis absorption. As the surfactant concentration increased from 0 to 30cmc, a clear critical flocculation concentration was observed with both surfactants. For SDS, restabilization was observed at 20cmc. These stability results were compared to energy profiles that were calculated using measured force profiles between a single silica sphere and plate obtained with colloid-probe atomic force microscopy. It was found that flocculation would occur once the energy well formed by depletion attraction exceeded 3 kBT, while restabilization occurred once the long range structural energy barrier exceeded 8 kBT. These values are consistent with the energy levels needed for flocculation and restabilization predicted by other researchers.

Deposition of silica nanoparticles onto alumina measured by optical reflectometry and quartz crystal microbalance with dissipation techniques

Understanding of the interactions between particles and the substrate is important for successful sol–gel deposition of thin films. We have studied the deposition of silica nanoparticles on alumina coated surfaces in aqueous electrolytes by optical reflectometry (OR) and a quartz crystal microbalance with dissipation (QCM-D). The deposition of negatively charged silica nanoparticles on positively charged alumina was primarily controlled by the electric diffuse double layer interactions between the substrate and the deposited particles, modulated by the counter-ion release. The build-up of a negative charge on the positively charged substrate resulted in a decrease in the deposition rate with increasing surface coverage. Higher surface coverage of silica nanoparticles was obtained at low pH than at high pH conditions, due to reduced electric diffuse double layer repulsion between the silica nanoparticles. The deposition was enhanced at high pH by increasing the concentration of NaCl due to compression of the electric diffuse double layer. In particular, the repulsion between the silica nanoparticles was efficiently screened at a concentration of NaCl higher than 100mM and thick silica layers could be deposited at pH=6 and 8.

Observation of adsorption versus depletion interaction for charged silica nanoparticles in the presence of non-ionic surfactant

The interaction of anionic silica nanoparticles (Ludox LS30) with non-ionic surfactant decaethylene glycol monododecylether (C12E10) without and in the presence of an electrolyte has been studied by small-angle neutron scattering (SANS) and dynamic light scattering (DLS). The measurements have been carried out for fixed concentrations of nanoparticles (1 wt%), surfactant (1 wt%) and electrolyte (0.1 M NaCl). In SANS, each of these nanoparticle–surfactant systems has been examined for three different contrast conditions where both (nanoparticle and surfactant) as well as individual components (nanoparticle or surfactant) are made visible. It is observed that the nanoparticle–surfactant systems behave very differently without and with the electrolyte. In the absence of salt, the C12E10 micelles form micelle decorated core–shell structures by adsorbing on the nanoparticles. On the other hand, the adsorption of surfactant micelles on nanoparticle is completely suppressed in the presence of salt, leading to the depletion-induced aggregation of nanoparticles. These results have also been corroborated by the DLS data. We thus show that the ionic strength of solution can be used to tune the interaction of ionic silica nanoparticles with non-ionic surfactant.

Nonequilibrium dynamics of a confined colloidal bilayer in a planar shear flow

Using Brownian dynamics (BD) simulations we investigate the impact of shear flow on structural and dynamical properties of a system of charged colloids confined to a narrow slit pore. Our model consists of spherical microions interacting through a Derjaguin-Landau-Verwey-Overbeek (DLVO) and a soft-sphere potential. The DLVO parameters were chosen according to a system of moderately charged silica particles (with valence Z~35) in a solvent of low ionic strength. At the confinement conditions considered, the colloids form two well-pronounced layers. In the present study we investigate shear-induced transitions of the translational order and dynamics in the layers, including a discussion of the translational diffusion. In particular, we show that diffusion in the shear-melted state can be described by an analytical model involving a single shear-driven particle in a harmonic trap. We also explore the emergence of zigzag motion characterized by spatiotemporal oscillations of the particles in the layers in the vorticity direction. Similar behavior has been recently observed in experiments of much thicker colloidal films.

Synthesis of Multifunctional Silica Composites Encapsulating a Mixture Layer of Quantum Dots and Magnetic Nanoparticles

Multifunctional silica colloidal composites with enhanced photoluminescence (PL) and superparamagnetism are reported. Enhanced PL and superparamagnetism were achieved by encapsulating a mixture layer of quantum dots (QDs) and superparamagnetic iron oxide nanoparticles (SPIONs) within a silica sphere, wherein QDs and SPIONs were capped by 3-mercaptopropionic acid (MPA) and 2-carboxy ethyl phosphonic acid (CEPA), respectively. The silica composites encapsulating a mixture layer of QDs and SPIONs, i.e., S(Q,M)S core(layer)shell architectures with various diameters (80, 360, and 900 nm) were successfully prepared by utilizing electrostatic interaction between positively charged amine-functionalized silica (S) and negatively charged mixture of QD–MPA (Q) and SPION–CEPA (M) and then, by forming a silica shell of 10–20 nm. The S(Q,M)S showed more than twice higher PL intensity than MPA-capped QD with the same QD concentration. Increasing the molar ratio of M/Q from 0.02 to 0.05 in the S(Q,M)S increased the saturation magnetization value from 0.15 to 0.62 emu/g. The S(Q,M)S composites with enhanced PL intensity and superparamagnetism are expected to be a plausible probe material for bioimaing and sensing application. Also, the current synthetic strategy for S(Q,M)S composites is expected to be extendible to include other functional nanoparticles.

Switchable Pickering Emulsions Stabilized by Silica Nanoparticles Hydrophobized In Situ with a Switchable Surfactant

Back and forth: The CO2 /N2 trigger of a switchable surfactant (neutral amidine/cationic amidinium) was transferred to mineral nanoparticles through in situ hydrophobization in water. Switchable oil-in-water Pickering emulsions that entail a CO2 /N2 trigger were obtained by using negatively charged silica nanoparticles and a trace amount of the switchable surfactant as the stabilizer.

Polyelectrolyte Adsorption on Solid Surfaces: Theoretical Predictions and Experimental Measurements

This work utilizes a combination of theory and experiments to explore the adsorption of two different cationic polyelectrolytes onto oppositely charged silica surfaces at pH 9. Both polymers, poly(diallyldimethylammonium chloride), PDADMAC, and poly(4-vinyl N-methylpyridinium iodide), PVNP, are highly charged and highly soluble in water. Another important aspect is that a silica surface carries a relatively high surface charge density at this pH level. This means that we have specifically chosen to investigate adsorption under conditions where electrostatics can be expected to dominate the interactions. Of specific focus in this work is the response of the adsorption to the addition of simple salt (i.e., a process where electrostatics is gradually screened out). Theoretical predictions from a recently developed correlation-corrected classical density functional theory for polyelectrolytes are evaluated by direct quantitative comparisons with corresponding experimental data, as obtained by ellipsometry measurements. We find that, at low concentrations of simple salt, the adsorption increases with ionic strength, reaching a maximum at intermediate levels (about 200 mM). The adsorption then drops but retains a finite level even at very high salt concentrations, indicating the presence of nonelectrostatic contributions to the adsorption. In the theoretical treatment, the strength of this relatively modest but otherwise largely unknown nonelectrostatic surface affinity was estimated by matching predicted and experimental slopes of adsorption curves at high ionic strength. Given these estimates for the nonelectrostatic part, experimental adsorption data are essentially captured with quantitative accuracy by the classical density functional theory.

Dynamics in thermo-responsive nanogel crystals undergoing melting

We report here the dynamics in thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) nanogel crystals undergoing melting/freezing and verify the applicability of the dynamical criterion for melting/freezing proposed by Löwen et al. [Phys. Rev. Lett. 70, 1557 (1993)]. According to this criterion the ratio of long time diffusion coefficient (D(L)) to short time diffusion coefficients (D(S)) is ~0.1 for colloidal particles in suspension undergoing melting/freezing. Static and dynamic light scattering techniques have been employed to identify the melting/freezing transition of PNIPAM nanogel colloidal crystals of two different volume fractions φ = 0.49 and 0.79 and to measure D(L) and D(S) across the melting. In dense PNIPAM nanogel crystals undergoing melting, the ratio D(L)/D(S) is found to be less than 0.1 for the first time and this deviation is higher in the suspension with higher φ. We also show that the deviation is genuine by measuring D(L)/D(S) on shear melted charged silica colloidal liquid undergoing freezing. The mean square displacement at shorter times, close to the melting, shows subdiffusive behavior. The subdiffusive behavior, arising due to the overlap of the dangling polymer chains between shells of the neighboring particles, is argued to be the reason for the observed deviation.

Interactions of Hyaluronan Layers with Similarly Charged Surfaces: The Effect of Divalent Cations

We used colloidal probe atomic force microscopy to measure the normal forces between the surface of a silica colloidal particle and a sparse layer of hyaluronan (hyaluronic acid, HA, MW ≈ 10(6) Da) covalently attached to a planar silica surface, both across pure water and following the addition of 1 mM MgCl2. It was found that in the absence of salt the HA layer repelled the colloidal silica surface during both approach and retraction. The addition of the MgCl2, however, changes the net force between the negatively charged HA layer and the opposing negatively charged silica surface from repulsion to adhesion. This interaction reversal is attributed to the bridging effect of the added Mg(2+) ions. Our results provide first direct force data to support earlier simulation and predictions that such divalent cations could bridge between negative charges on opposing surfaces, leading to an overall reversal of force from repulsion to attraction.

Investigation on raspberry-like magnetic-hollow silica nanospheres and its preliminary application for drug delivery

A series of raspberry-like magnetic-hollow silica nanospheres were successfully synthesized via the sol–gel process, which was based on the principle of the electrostatic interaction between negatively charged silica and positively charged polystyrene. The Fe3O4@SiO2 particles as the outer shell were compactly assembled on the surface of PS, and then magnetic-hollow nanospheres were obtained by calcination. Different synthesis conditions including the amount of NH4OH, TEOS, Fe3O4, and the adding time of PS were systematically investigated to discuss the influence of these conditions on the morphology and structure. The prepared magnetic-hollow nanospheres were systematically characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), fourier transform infrared spectrometry, energy-dispersive X-ray analysis, thermogravimetric analysis and nitrogen adsorption–desorption measurement. SEM and TEM images exhibited that the obtained samples with the perfect spherical profile and large cavities structure were well monodisperse and uniform under the optimized condition. Zeta-potential analysis was employed to make clear the formation mechanism of raspberry-like PS@Fe3O4@SiO2 composite nanosphere. Moreover, the drug release of ibuprofen experiment results demonstrated that the magnetic-hollow nanospheres could be used as a drug carrier to slowly release and deliver drugs.

Adsorption of Benzyldimethylhexadecylammonium Chloride at the Hydrophobic Silica–Water Interface Studied by Total Internal Reflection Raman Spectroscopy: Effects of Silica Surface Properties and Metal Salt Addition

The adsorption of the cationic surfactant benzyldimethylhexadecylammonium (BDMHA(+)) chloride was studied at an octadecyltrichlorosilane (OTS)-monolayer-modified silica-water interface by Raman spectroscopy in total internal reflection (TIR) geometry. The present study demonstrates the capabilities of this spectroscopic technique to evaluate thermodynamic and kinetic BDMHA(+)Cl(-) adsorption properties at the hydrophobic silica surface. The surface coverage of BDMHA(+) decreased by 50% at the hydrophobic OTS-silica surface relative to the surface coverage on bare silica; the dominating driving mechanisms for surfactant adsorption were identified as hydrophobic effects and head group charge screening by the electrolyte counterions. Addition of magnesium metal salt (MgCl2) to the aqueous solution (∼ neutral pH) lowered the surface coverage and moderately increased the Langmuir adsorption constants relative to those of the pure surfactant. These trends were previously observed at the hydrophilic, negatively charged silica surface but with a smaller change in the Gibbs free energy of adsorption at the hydrophobic silica surface. The hydrophobic OTS-silica surface properties resulted in shorter times for the surfactant to reach steady-state adsorption conditions compared to the slow adsorption kinetics previously seen with the surfactant at the hydrophilic surface. Adsorption isotherms, based on Raman signal intensities from spectral analysis, were developed according to the Langmuir adsorption model for the pure surfactant at the OTS-silica-water interface; the modified Langmuir model was applied to the surfactant adsorption in the presence of 5, 10, 50, and 100 mM magnesium chloride. Spectral analysis of the Raman scattering intensities and geometric considerations suggests a hemimicelle-type surface aggregate as the most likely surfactant structure at the OTS-silica surface. The different kinetics observed at the hydrophilic versus the hydrophobic silica surface further indicate that the surface charge and potential influence the surfactant diffusion and kinetic rates of adsorption at the silica-water interface.

A dielectric study on colloidal silica nanoparticle Layer-by-Layer assemblies on polycarbonate

This study focuses on the dielectrical characterization of polycarbonate films coated with silica nanoparticle bilayers assembled through the Layer-by-Layer (LbL) technique. This is the first attempt of dielectric characterization performed on LbL-treated plastic substrates. To this aim, LbL coatings consisting of oppositely charged colloidal silica have been built on a polymeric substrate (polycarbonate). Then, dielectric features such as space charge accumulation, electrical voltage breakdown, and resistance to corona discharge (through laser ablation) have demonstrated that the colloidal silica nanoparticle assemblies can influence the nature of the trapped space charges and affect the resistance of polycarbonate to corona discharge, changing the distribution of the laser energy on impact.

Flame Retardancy of Polyester Fabrics Treated by Spray-Assisted Layer-by-Layer Silica Architectures

Silica-based architectures have been assembled through the layer-by-layer (LbL) technique on polyester fabrics by horizontal spray in order to enhance their flame retardancy properties. In particular, five bilayers of oppositely charged silica nanoparticles (namely, alumina-coated silica nanoparticles—positive counterpart—and unmodified silica—negative counterpart) have been employed as a model architecture, aiming to compare the performances achieved with spray-assisted layer-by-layer assembly with conventional dipping. Scanning electron microscopy has shown a better continuous coverage and a more homogeneous distribution of the silica coating, when assembled through horizontal spray compared to dipping. As a consequence, the LbL coatings deposited by spray have exhibited the most efficient protection for PET fabrics, blocking the dripping during the vertical flammability tests, increasing the time to ignition, and reducing the total heat evolved and its rate during combustion, as well as the total smoke...

Optimization of Synthesis Parameters for Mesoporous Shell Formation on Magnetic Nanocores and Their Application as Nanocarriers for Docetaxel Cancer Drug

In this work, Fe3O4@SiO2 nanoparticles were coated with mesoporous silica shell by S−N+I− pathway by using anionic surfactant (S−) and co-structure directing agent (N+). The role of co-structure directing agent (CSDA) is to assist the electrostatic interaction between negatively charged silica layers and the negatively charged surfactant molecules. Prior to the mesoporous shell formation step, magnetic cores were coated with a dense silica layer to prevent iron oxide cores from leaching into the mother system under any acidic circumstances. However, it was found that both dense and mesoporous coating parameters affect the textural properties of the produced mesoporous silica shell (i.e., surface area, pore volume and shell thickness). The synthesized Fe3O4@SiO2@m-SiO2 (MCMSS) nanoparticles have been characterized by low-angle X-ray diffraction, transmission electron microscopy (TEM), and N2 adsorption-desorption analysis, and magnetic properties. The synthesized particles had dense and mesoporous silica shells of 8–37 nm and 26–50 nm, respectively. Furthermore, MCMSS possessed surface area of ca. 259–621 m2·g−1, and pore volume of ca. 0.216–0.443 cc·g−1. MCMSS showed docetaxcel cancer drug storage capacity of 25–33 w/w% and possessed control release from their mesochannels which suggest them as proper nanocarriers for docetaxcel molecules.

Enhancing the barrier properties of colloidosomes using silica nanoparticle aggregates

Recent interest is focused on microcapsules stabilized using colloidal nanoparticles – termed ‘colloidosomes – for encapsulation applications in food, drug and cosmetic industries. However, due to electrostatic repulsion between similarly charged particles, shells composed of single-type nanoparticles tend to be monolayer-thick and relatively permeable. We investigated a self-assembly method for controlling the permeability of colloidal shells using aggregates composed of oppositely charged silica nanoparticles. Using a combination of rapid fluorescence based method and theoretical diffusion models, we found that colloidosomes whose shells contained colloidal silica aggregates displayed lower permeability to peroxyl radicals than ones stabilized by single type of silica nanoparticles. Furthermore, the permeability varied as a function of the ratio of oppositely charged silica nanoparticles in the shell. The ability to control the permeability of colloidosomes, while using a simple self-assembly synthesis method, will enable enhanced control over release kinetics and oxidative stability of encapsulants.