Evidence for rigid binding of rhodamine 6G to silica surfaces in aqueous solution based on fluorescence anisotropy decay analysis

Strong ionic binding of the cationic probe rhodamine 6G (R6G) to the anionic surface of silica particles in water provides a convenient labeling procedure to study both particle growth kinetics and surface modification by time-resolved fluorescence anisotropy (TRFA). The decays for R6G dispersed in diluted Ludox silica sols usually fit to a sum of picosecond and nanosecond decay components, along with a significant residual anisotropy component. The origin of the nanosecond decay component (phi2) is not fully understood, and has been ascribed to wobbling of the probe on the silica surface, the presence of a subpopulation of small nanoparticles in the Ludox sol, or rapid exchange between free and bound R6G. To elucidate the physical meaning of phi2, measurements were performed in various silica-based colloidal systems using different concentrations of silica. We found that the fraction of phi2 was generally higher in Ludox than in aqueous sodium silicate and decreased with increasing silica concentration; phi2 vanished upon gelation of sodium silicate at pH 7 leading to a total loss of R6G depolarization (r(t) = const). These results rule out the presence of local R6G wobbling when bound ionically to colloidal silica and support the rigid sphere model to describe the TRFA decays for R6G-Ludox. This conclusion is entirely supported by steady-state anisotropy data and structural considerations for the R6G molecule and the silica surface.     

Influences of colloidal stability and electrokinetic property on electrodialysis performance in the presence of silica sol

Negatively charged silica sol is known to lead to fouling of anion exchange membranes during electrodialysis (ED) as a result of its deposition on the membrane surface. It is known that the fouling potential is related to the physical and electrochemical properties of the silica particles as well as those of the anion exchange membranes. In this study, the properties of the silica sol were characterized in terms of its particle size, turbidity, and zeta potential in order to predict their effects on the electrodialysis performance. In the stability of colloidal particles, the critical coagulation concentrations of silica sol were determined as functions of ionic strength, cation species, and solution pH. In the electrodialysis of NaCl solution containing silica sol with various concentrations of CaCl(2), the colloidal behavior related to deposition and transport was examined during and after electrodialysis. The electrodialysis experiments clearly showed that the deposition and transport of silica sol during electrodialysis were related to the colloidal stability of dispersion.     

Experimental study of ultrafiltration membrane fouling by sodium alginate and flux recovery by backwashing

Membrane fouling is the major limitation for a broader application of membrane technology. One of the main causes of membrane fouling in advanced wastewater reclamation and in membrane bioreactors (MBR) are the extracellular polymeric substances (EPS). Among the main constituents in EPS, polysaccharides are the most ubiquitous. This study aims at a better understanding of the fouling mechanisms of EPS and the efficiency of backwashing technique, which is applied in practice to restore membrane flux. For that purpose, the evolution of fouling by sodium alginate, a microbial polysaccharide, is studied in ultrafiltration. Fouling experiments are carried out in a single fiber apparatus, aiming at identifying the significance of distinct fouling mechanisms and their degree of reversibility by backwashing. An important parameter considered in the study is the concentration of calcium ions, which promote sodium alginate aggregation and influence the rate of flux decline, the reversibility of fouling and rejection. A rapid irreversible fouling takes place due to internal pore constriction, at the beginning of filtration, followed by cake development on the membrane surface. With increased calcium addition, cake development becomes the dominant mechanism throughout the filtration step. Furthermore, fouling reversibility is increased with the increase of calcium concentration. A unique behavior of sodium alginate solution in the absence of calcium is also noted, i.e. the formation of a labile layer on the membrane surface, which is affected by the small cross-flow that exists inside hollow fibers, even in the nominally dead-end mode of operation.     

Preparation and characteristics of spinnable sol and continuous mullite fibers with nano silica addition

The spinning precursor sols for the continuous mullite-based fibers were prepared by adding nano-silica to substitute part of silica sol. The effect of SiO₂ nanoparticles on the particle evolution models, polymerization degree and solid content of the sol,and the spinning length and sintering behavior of the fibers was investigated. The results were shown that the addition of nano silica enhanced the polymerization degree and extended the spinnable range of the sol. The appropriate polymerization degree (B value) for this sol system was 1.885–2.145. The grain diameter decreased from 39.6 to 25.9 nm with increasing the nano-silica content to 20 %, and then, it increased to 41.2 nm with increasing the nano-silica content to 100 %. The appropriate content of nano-silica powders would reduce the grain diameter. However, it had no influence on the linear growth model, homogeneity and solid content of the precursor sol.     

Quantification of Polymer Fouling on Heat Transfer Surfaces During Synthesis

During the synthesis and processing of polymers, a significant amount of polymer may be deposited on the heat transfer surfaces. This deposit is unwanted and is usually named fouling. This study deals with the fouling behavior of a vinyl acetate/vinyl ester copolymer during emulsion polymerization and compares this to fouling of an already reacted vinyl acetate/ethylene copolymer dispersion. Besides the state of polymerization, also the influence of the wall temperature is investigated. The experiments are performed on cooled and heated surfaces. The deposition process is quantified by the mass‐based fouling resistance, the height of the fouling layer, and the surface coverage using a digital microscope. It is found that the state of polymerization and the temperature gradient between wall and bulk exert a strong influence on the fouling behavior, especially on the structure of the fouling layer.     

Adsorption kinetics of laponite and ludox silica nanoparticles onto a deposited poly(diallyldimethylammonium chloride) layer measured by a quartz crystal microbalance and optical reflectometry

A quartz crystal microbalance with dissipation (QCM-D) and an optical reflectometer (OR) have been used to investigate the adsorption behavior of Laponite and Ludox silica nanoparticles at the solid-liquid interface. The adsorption of both Laponite and Ludox silica onto poly(diallyldimethylammonium chloride) (PDADMAC)-coated surfaces over the first few seconds were studied by OR. Both types of nanoparticles adsorbed rapidly and obtained a stable adsorbed amount after only a few minutes. The rate of adsorption for both nanoparticle types was concentration dependent. The maximum adsorption rate of Ludox nanoparticles was found to be approximately five times faster than that for Laponite nanoparticles. The QCM data for the Laponite remained stable after the initial adsorption period at each concentration tested. The observed plateau values for the frequency shifts increased with increasing Laponite particle concentration. The QCM data for the Ludox nanoparticles had a more complex long-time behavior. In particular, the dissipation data at 3 ppm and 10 ppm Ludox increased slowly with time, never obtaining a stable value within the duration of the experiment. We postulate here that this is caused by slow structural rearrangements of the particles and the PDADMAC within the surface adsorbed layer. Furthermore, the QCM dissipation values were significantly smaller for Laponite when compared with those for Ludox for all nanoparticle concentrations, suggesting that the Laponite adsorbed layer is more compact and more rigidly bound than the Ludox adsorbed layer.     

Hybrid silica coatings on polycarbonate: enhanced properties

Hybrid silica coatings based on 3- glycidoxypropyltriethoxysilane (GPTES), tetraethylorthosilicate (TEOS) and colloidal silica were deposited on polycarbonate (PC) by the sol–gel method, in order to obtain a material with enhanced properties with respect to raw PC (mainly scratch resistance, hydrophobicity and density), and consequently reach increased durability. The necessity of performing a N₂-plasma treatment on PC (before coating deposition) was highlighted in order to obtain a good adherence between the coating and the substrate: XPS measurements showed that after treatment, nitrogenous radicals had formed on the PC surface and were able to link covalently with the sol during its deposition. Adherence was also higher when young sols (<8-day-old) were used. Different alkoxysilanes/colloidal silica ratios were tested to optimize the coating resistance: crack resistance of the coatings was found to be greater when the ratio was high. Scratch resistance of raw PC was enhanced as soon as PC was coated, irrespective of the alkoxysilanes/colloidal silica ratio or the sol ageing time. The density of the coatings was assessed by environmental ellipsometric porosimetry and found to be very high. Water contact angle measurements showed that the hydrophobicity of the coatings was inferior to raw PC. The addition in the sol of a small wt% of octyltriethoxysilane (OTES), 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FTES) and silicone surface additive (BYK-306) allowed a significant increase in hydrophobicity of the samples.     

The gel swelling properties of alginate fibers and their applications in wound management

Calcium alginate fibers have a novel gel‐forming capability in that, upon the ion exchange between sodium ions in the contact solution and calcium ions in the fiber, the fiber slowly transforms into a fibrous gel. This paper reviews the principles of the gel‐forming process for alginate fibers and analyzed the gelling behavior of various types of alginate fibers. The absorption characteristics of alginate wound dressings were analyzed and it was found that alginate wound dressings absorb a large quantity of liquid into the fiber structure, in addition to those held between the fibers in the textile structure. This gives rise to the unique gel blocking properties of alginate wound dressings. In addition, alginate wound dressings also have novel hemostatic and antimicrobial properties as well as the ability to promote wound healing. They are now widely used in the management of highly exuding wounds such as leg ulcers, pressure sores, and surgical wounds. Copyright © 2007 John Wiley & Sons, Ltd.     

Surface ATRP of hydrophilic monomers from ultrafine aqueous silica sols using anionic polyelectrolytic macroinitiators

A convenient two-step route was developed to prepare new anionic ATRP macroinitiators from near-monodisperse poly(2-hydroxyethyl methacrylate) precursors by partial esterification with 2-bromoisobutyryl bromide, followed by esterification of the remaining hydroxyl groups using excess 2-sulfobenzoic acid cyclic anhydride. These new macroinitiators can be electrostatically adsorbed onto ultrafine cationic Ludox CL silica sols; subsequent surface polymerization of various hydrophilic monomers in aqueous solution at room temperature afforded a range of polymer-grafted ultrafine silica sols. The resulting sterically stabilized particles were characterized by dynamic light scattering, transmission electron microscopy, aqueous electrophoresis, FTIR spectroscopy, and elemental microanalyses.     

Effect of High-Temperature Treatment on Optical Properties of Silica Films Doped with Al2O3, P2O5 and Rare-Earth Elements

The paper reports effects of high-temperature treatment of silica layers doped with Al₂O₃, P₂O₅, Yb and Er ions, which are coated inside silica substrate tubes and transformed through viscous flow into the core of a preform, on properties of these cores. The gel layers are applied inside the silica substrate tubes from sols based on tetraethoxysilane and corresponding chlorides in isopropanol by controlled lowering the column of the sol. The layers are thermally treated at 500°C and subject to high-temperature treatment from 1000 to 1300°C. The tube with the layers is collapsed into a preform from which fibers are drawn at about 1950°C. The results of concentration measurements show that during the high-temperature treatment the content of silica in the layer increases at least five times in comparison with that of the sol composition. This effect depends on the sol composition and the number of the coated gel layers. Measurements show that in the prepared fibers there is some remaining unsaturated absorption of erbium ions at 1535 nm, which is related to quenching of the erbium ions.     

Influence of natural organic matter on the deposition kinetics of extracellular polymeric substances (EPS) on silica

The influence of humic acid and alginate, two major components of natural organic matter (NOM), on deposition kinetics of extracellular polymeric substances (EPS) on silica was examined in both NaCl and CaCl(2) solutions over a wide range of environmentally relevant ionic strengths utilizing a quartz crystal microbalance with dissipation. Deposition kinetics of both soluble EPS and bound EPS extracted from four bacterial strains with different characteristics was investigated. EPS deposition on humic acid-coated silica surfaces was found to be much lower than that on bare silica surfaces under all examined conditions. In contrast, pre-coating the silica surfaces with alginate enhanced EPS deposition in both NaCl and CaCl(2) solutions. More repulsive electrostatic interaction between EPS and surface contributed to the reduced EPS deposition on humic acid-coated silica surface. The trapping effect induced by the rough alginate layer resulted in the greater EPS deposition on alginate-coated surfaces in NaCl solutions, whereas surface heterogeneities on alginate layer facilitated favorable interactions with EPS in CaCl(2) solutions. The presence of dissolved background humic acid and alginate in solutions both significantly retarded EPS deposition on silica surfaces due to the greater steric and electrostatics repulsion.     

Effect of sepiolite fiber on the structure and properties of the sepiolite/silica aerogel composite

Sepiolite fiber-reinforced silica aerogel composites for thermal insulators were prepared by dispersing sepiolite fiber in silica sol, aging, solvent exchanging, and drying in supercritical fluid. The surface treated sepiolite fiber and sepiolite/silica aerogel composite were characterized by scanning electron microscope; transmission electron microscope and Fourier transform infrared spectroscopy. The influence of surface treated sepiolite fiber on the mechanical and thermal properties of the aerogel composite was studied. The results indicate the hydroxyl groups on silica sol particles surface able to condense with the hydroxyls of sepiolite fibers with forming Si–O–Si between sepiolite fibers and aerogel matrix in the sol–gel process, which achieves excellent interfacial interaction in the sepiolite/silica aerogel composite, so the mechanical properties of the aerogel composite have been improved effectively without sacrificing much thermal insulating performance.     

Controlled electrochemically-assisted deposition of sol-gel biocomposite on electrospun platinum nanofibers

The modification of platinum nanofibers by silica using the electrochemically-assisted deposition is reported here. Pt nanofibers are obtained by electrospinning and deposited on a glass substrate. The electrochemically-assisted deposition of the sol-gel material then gives the unique possibility to finely tune the silica film thickness around these nanofibers. It also allows the successful encapsulation of a biomolecule (glucose oxidase was chosen here as a model) while retaining its biological activity, as pointed out via the electrochemical monitoring of H(2)O(2) produced upon addition of glucose in the medium. This silica-glucose oxidase composite offers the possibility of comparing systematically the influence of the deposition time on the bioelectrode response and to compare it with the particular features of the deposits. It was found that the film first grew uniformly around the nanofibers and then started to deposit between them, covering the whole sample (fibers and glass substrate), and tended to fully embed the nanofibers for prolonged deposition. The thickness of the silica film is critical for the electroactivity of the biocomposite, the best response being obtained for a silica layer thickness in the range of the fiber diameter (～50 nm).     

Surface silylation and pore structure development of silica aerogel composites from colloid and TEOS-based precursor

Flexible aerogel-fiber composites were prepared by silylation and ambient drying of colloidal silica and tetraethylorthosilicate (TEOS)-based sol. After immersing glass fiber matrices into silica sol with colloid-based, colloid/TEOS-based, and TEOS-based silica sol, it was surface-modified in a trimethylchlorosilane/n-hexane solution and heat-treated at 230 °C in ambient atmosphere. Surface silylation of silica aerogel synthesized from colloid and TEOS-based silica sols showed different behaviors. For colloid silica gel, it was comprised of small sized mesopores because colloid-based silica gel has dense networks through great degrees of hydrolysis and condensation. On the contrary, TEOS-based aerogel was consisted of relatively large-sized pores because of comparatively lesser degree of hydrolysis and condensation. Through this study, we can know that the pore structures of silica aerogel could be controlled by choosing colloid or TEOS-based precursor and surface silylation reaction.     

Preparation of core-shell and hollow fibers using layer by layer (LbL) self-assembly of polyelectrolytes on electrospun submicrometer-scale silica fibers

Herein, fabrication of hollow fibers made of polyelectrolyte multilayers is reported. Silica submicrometer-scale fibers were fabricated by electrospinning and layer by layer deposition of polyelectrolytes were performed to coat silica fibers with polyelectrolyte multilayers, which were prepared by consecutive deposition of poly(ethyleneimine) and poly(styrene sulfonate sodium salt)/sodium dodecyl sulfate onto the surface of the silica fibers. In order to obtain hollow fibers, the core removal was carried out by introducing the core-shell fibers to a hydrofluoric acid solution. The hollow fibers were stable in hydrofluoric acid solution and displayed pH-dependent structural changes. SEM microscopy indicated the formation of the glass fibers and the fibers coated with polyelectrolyte multilayers (Silica—polyelectrolyte multilayers (PEM) fibers). The diameter of the core-shell fibers was increased after layer-by-layer coating. ATR-FTIR was performed for characterization of the glass fibers before and after layer-by-layer coating as well as after selective core removal. IR spectrum of the Silica-PEM fibers indicates C-H stretching modes of saturated hydrocarbons, confirming multilayers formation. Core removal was also confirmed by IR spectroscopy as Si-O-Si band disappears for the IR spectrum of the fibers after core-removal.     

Direct and indirect monitoring of peptide-silica interactions using time-resolved fluorescence anisotropy

The present work extends the application of time-resolved fluorescence anisotropy (TRFA) of a cationic probe rhodamine 6G (R6G) in aqueous Ludox to in situ monitoring of peptide adsorption onto the silica particles. Steady-state anisotropy and TRFA of R6G in Ludox sols were measured to characterize the extent of the ionic binding of the probe to silica particles in the presence of varying levels of tripeptides of varying charge, including Lys-Trp-Lys (KWK), N-acetylated Lys-Trp-Lys (Ac-KWK), Glu-Trp-Glu (EWE), and N-acetylated Glu-Trp-Glu (Ac-EWE). The results were compared to those obtained by direct observation of peptide adsorption using the steady-state anisotropy of the intrinsic tryptophan residue. Ionic binding of the peptides to Ludox particles produced an increase in the steady-state Trp anisotropy that was dependent on the number of cationic groups present, but the limiting anisotropy values were relatively low, indicating significant rotational freedom of the indole residue in the adsorbed peptides. On the other hand, R6G showed significant decreases in anisotropy in the presence of cationic peptides, consistent with the cationic peptides blocking the adsorption of the dye to the silica surface. Thus, R6G is able to indirectly report on the binding of peptides to Ludox particles. It was noteworthy that, while there were similar trends in the data obtained from steady-state anisotropy and TRFA studies of R6G, the use of steady-state anisotropy to assess binding of peptides overestimated the degree of peptide adsorption relative to the value obtained by TRFA. The study shows that the competitive binding method can be used to assess the binding of various biologically relevant compounds onto silica surfaces and demonstrates the potential of TRFA for probing peptide-silica and protein-silica interactions.     

Sol–gel preparation of double-layer tri-wavelength antireflective coating

Double-layer tri-wavelength antireflective (AR) coating effective simultaneously at 351, 527 and 1,053?nm has been designed and prepared by the sol?Cgel process. The refractive index and film thickness of bottom layer and up layer are 1.27 and 113?nm, and 1.17 and 245?nm, respectively. The bottom layer with refractive index of 1.27 was prepared from a mixture of acid-catalyzed and base-catalyzed silica sols, and the up layer with refractive index of 1.17 was prepared from polypropylene oxide modified silica sol. It was found that the addition of polypropylene oxide into the sol significantly decreased the refractive index and increased the hydrophobicity of the AR coating. The obtained tri-wavelength AR coating gives very high transmittance of 99.7, 99.1 and 98.0?% at 351, 527 and 1,053?nm, simultaneously.     

Effects of aging time on V2O5 sol-gel coatings

Vanadium pentoxide (V₂O₅) sols have been used in conductive coatings and related applications [1, 2, 3]; however, the changes which may occur in the properties of these coatings with sols of different ages have not been carefully shown. Properties which may be dependent on the age of the sol (sol viscosity, film morphology and conductance) were measured in this work. The coating sols were prepared by ion-exchange of sodium metavanadate solutions.It was found that the coating thickness, sol viscosity, and surface morphology of the coatings were directly related to the age of the sol used. The dried coating thickness increased from 3 microns to 20 microns over a 30 day period. The sol viscosity increased from 1 centipoise to near 2 centipoise. The surface morphology of the coatings changed from that of a featureless surface at day 1 to a continual coverage of micron-sized fibers as the coating sols aged. The conductivity of the coatings, though, was unaffected by the age of the sol. The coating properties were correlated to the amount of polymerization of dissolved precursors in solution.     

The effect of electrolyte conductivity on electrophoretic deposition

In this work, the influence of electrolyte conductivity on the electrophoretic deposition of alumina particles from ethanol suspensions was studied. Deposition experiments in a Hull cell showed that high-conductivity ethanol-based suspensions yield uniform deposits, while low-conductivity suspensions result in nonuniform deposits. The difference in the deposition, behavior is due to the resistance increase over the deposit during polarization. Impedance measurements during electrophoretic deposition showed that during EPD the relative deposit resistance increases much faster for the high- than for the low-conductivity suspension. The impedance measurements also showed that the resistance increase dropped almost to the suspension resistance after the electric field was turned off. This means that the resistance over the deposit is caused by the interaction of ions with the deposit and by the depletion of ions at the deposition electrode. Negatively charged ions are depleted in the deposit by migration toward the positively charged counterelectrode, while positively charged ions undergo electrochemical reactions at the deposition electrode. This change in ion concentrations near the deposition electrode changes the acid/base properties of the particles in the deposit, as proven by adsorbed pH indicators on the particles. The change in acid/base behavior is quasi-irreversible and results in a memory effect of the deposit resistance when the voltage is reapplied.     

CuCl2硅溶胶和水溶液中铜的电化学行为研究

采用循环伏安法和计时安培法研究了CuCl2硅溶胶和水溶液中铜在玻碳电极上的电沉积和电结晶行为.结果表明在两种CuCl2电解质中,铜的电沉积分两个步骤完成,Cu2+还原为Cu+在硅溶胶中较水溶液中容易;采用吸附-成核模型解析电流-时间暂态曲线,并确定铜的电结晶机理为扩散控制下的连续成核三维生长(3DP),Cu2+在水溶液中的扩散系数较硅溶胶中的大,但相同电位下在硅溶胶中的饱和成核数密度高于水溶液中.