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At least 757 records · Page 42Linked to original sources
PubMed2006

Sensitive immunoassay of a biomarker tumor necrosis factor-alpha based on poly(guanine)-functionalized silica nanoparticle label.

A novel electrochemical immunosensor for the detection of tumor necrosis factor-alpha (TNF-alpha) based on poly(guanine)-functionalized silica nanoparticles (NPs) label is presented. The detection of mouse TNF-alpha via immunological reaction is based on a dual signal amplification: (1) a large amount of guanine residues introduced on the electrode surface through sandwich immunoreaction and poly(guanine)-functionalized silica NP label; (2) Ru(bpy)3(2+)-induced catalytic oxidation of guanine, which results in great enhancement of anodic current. The synthesized silica NP conjugates were characterized with atomic force microscopy, X-ray photoelectron spectroscopy, and electrochemistry. These experiments confirmed that poly(guanine) and avidin were immobilized on the surface of silica NPs. The performance of the electrochemical immunosensor was evaluated and some experiment parameters (e.g., concentration of Ru(bpy)3(2+), incubation time of TNF-alpha, etc.) were optimized. The detection limit for TNF-alpha is found to be 5.0 x 10(-11) g mL(-1) (2.0 pM), which corresponds to 60 amol of TNF-alpha in 30 microL of sample. This immunosensor based on the poly(guanine)-functionalized silica NP label offers great promise for rapid, simple, cost-effective analysis of biological samples.

Animals↗
PubMed1997

Temperature-responsive liquid chromatography. 2. Effects of hydrophobic groups in N-isopropylacrylamide copolymer-modified silica.

We recently reported the new concept of temperature-responsive liquid chromatography using temperature-responsive poly(N-isopropylacrylamide)-modified surfaces as high-performance liquid chromatography media with aqueous mobile phases. Incorporation of hydrophobic sites is an important factor to improve the efficacy (selectivity and retention) of temperature-responsive chromatography. Toward this goal, we have synthesized semitelechelic copolymers of N-isopropylacrylamide (IPAAm) and butyl methacrylate (BMA) having reactive terminal functional groups using telomerization. The lower critical solution temperatures of the copolymers shift to lower temperatures with increasing hydrophobic BMA content in the poly(IPAAm-co-BMA) relative to that of the IPAAm homopolymer. This temperature-responsive semitelechelic copolymer was grafted to the surface of (aminopropyl)silica through the reaction of activated ester-amine coupling. The polymer-modified silica was used as a column packing material. Separation of a mixture of five steroids having various hydrophobicities was investigated. Retention of steroids on poly(IPAAm-co-BMA)-modified columns is increased with an increase in column temperature. The capacity factors for steroids on the copolymer-modified silica beads was much larger than that on homopolymer PIPAAm-modified columns. The capacity factor for testosterone at 50 degrees C was 33.8 for poly(IPAAm-co-BMA) containing 5 mol% BMA, while that for the PIPAAm homopolymer was 15.0 at the same temperature. The influence of column temperature on steroid retention behavior on copolymer-modified stationary phases was significant compared with the case of homopolymer-modified columns. Furthermore, retention times for steroids increased remarkably with increasing BMA composition. The temperature-responsive elution behavior for the steroids was strongly affected by the hydrophobicity of the grafted polymer chains on silica surfaces. Possible protein separation in temperature-responsive liquid chromatography was explored using insulin chains A and B, and beta-endorphin fragment 1-27. On IBc-3.2-modified silica column, these three peptides were successfully separated at 30 degrees C with 0.5 M NaCl aqueous solution (pH 2.1) as mobile phase. The retention times of these peptides were related to the number of hydrophobic amino acid residues in the peptides. In the proposed chromatography system, elution of target substances is controlled only by a small change in column temperature without any further modification of the aqueous mobile phase.

Acrylamides↗
PubMed2000

Electrogenerated chemiluminescence of tris (2,2'-bipyridyl)ruthenium(II) ion-exchanged in nafion-silica composite films

The voltammetry and electrogenerated chemiluminescence (ECL) of tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)3 2+) ion-exchanged in Nafion and Nafion-silica composite materials have been investigated. The major goal of this work was to investigate and develop new materials and immobilization approaches for the fabrication of ECL-based sensors with improved reactivity and long-term stability. Nation-silica composite materials with varying contents of Nation (53-100 wt% relative to silica) were prepared via the two-step acid/base hydrolysis and condensation of tetramethoxysilane. The Nafion doped sols were spin cast on glassy carbon electrodes, dried, and then ion-exchanged with Ru(bpy)3 2+. The shapes of the cyclic voltammetric curves and the amount of Ru(bpy)3 2+ exchanged into the films strongly depends on the amount of Nafion incorporated into the hybrid sol. Nafion-silica films with a low content of Nafion ion-exchanged less Ru(bpy)3 2+ and exhibited tail-shaped voltammetry at 100 mV/s. The ECL of immobilized Ru(bpy)3 2+ in the presence of either tripropylamine or sodium oxalate in pH 5 acetate buffer was also strongly dependent on the amount of Nafion introduced into the composite with greater ECL observed for the Nafion-silica films relative to pure Nafion.

Journal Article↗
PubMedDate not supplied

Study of the chemical and physical influences upon in vitro peptide-mediated silica formation.

Herein, we report on the ability to create complex 2-D and 3-D silica networks in vitro via polycationic peptide-mediated biosilicification under experimentally altered chemical and physical influences. These structures differ from the sphere-like silica network of particles obtained in vitro under static conditions. Under chemical influences, overall morphologies were observed to shift from a characteristic network of sphere-like silica particles to a sheetlike structure in the presence of -OH groups from additives and to sharp-edged, platelike structures in the presence of larger polycationic peptide matrixes. Under physical influences, using externally applied force fields, overall silica morphologies were observed to transition from sphere-like to fiberlike and dendrite-like structures. These findings could lead to the future development of bio-inspired complex 2-D and 3-D silica micro- and nano-devices.

Amino Acid Sequence↗
PubMed2001

Implications of aqueous silica sorption to iron hydroxide: mobilization of iron colloids and interference with sorption of arsenate and humic substances.

This work highlighted practical implications of aqueous silica sorption to iron hydroxide in natural and engineered systems. Two types of surfaces were prepared by exposing 10 mg/L preformed Fe(OH)3 to aqueous silica (0-200 mg/L as SiO2) for periods of 1.5 h or 50 days. After 1.5 h, the concentration of iron passing through a 0.45 microm pore size filter at pH 6.0-9.5 was always negligible, but if zeta potential < or =-15 mV as much as 35% of the iron passed through filters after 50 days of aging. When arsenate was added to 10 mg/L iron hydroxide particles equilibrated with aqueous silica for 1.5 h, percentage arsenate removals were high. In contrast, if silica was preequilibrated with iron for 50 days, arsenate removals decreased markedly at higher pH and aqueous silica concentrations. Similar trends were observed for humic substances, although their removal was nearly completely prevented at pH 8.5 at SiO2 concentrations above 50 and 10 mg/L at 1.5 h and 50 days exposure, respectively. The mechanism of interference was hindered sorption to the iron hydroxide surface.

Arsenates↗
PubMed2002

Modeling silica sorption to iron hydroxide.

Experiments were conducted to investigate the fundamentals of silica sorption onto preformed ferric hydroxide at pH 5.0-9.5 and silica concentrations of 0-200 mg/L as SiO2. At all pHs studied, sorption densities exceeding monolayer sorption were observed at silica levels typical of natural waters. Under some circumstances, sorption equaled or exceeded a monolayer while the particle zeta potential remained positive, a phenomenon that is completely inconsistent with available surface complexation models. To address this deficiency, an extended surface complexation model was formulated in which soluble dimeric silica (i.e., Si2O2(OH)5-) sorbs directly to iron surface sites. This new model fit sorption density data up to 0.40 mol SiO2/mol Fe and accurately predicted trends in zeta potential and observed H+ release during silica sorption to ferric hydroxide.

Adsorption↗
PubMed2005

Mesoporous silica-supported uranyl: synthesis and photoreactivity.

A mesoporous silica-supported uranyl material (U(aq)O(2)(2+)-silica) was prepared by a co-condensation method. Our approach involves an I(-)M(+)S(-) scheme, where the electrostatic interaction between the anionic inorganic precursor (I(-)), surfactant (S(-)), and cationic mediator (M(+)) provides the basis for the stability of the composite material. The synthesis was carried out under acidic conditions, where the anionic sodium dodecyl sulfate provided the template for the uranyl cation and silicate to condense. Excitation with visible or near-UV light of aqueous suspensions of U(aq)O(2)(2+)-silica generates an excited state that decays with k(0) = 1.5 x 10(4) s(-1). The reaction of the excited state with aliphatic alcohols exhibits kinetic saturation and concentration-dependent kinetic isotope effects. For 2-propanol, the value of k(C)3(H)7(OH)/k(C)()3(D)7(OH) decreases from 2.0 at low alcohol concentrations to 1.0 in the saturation regime at high alcohol concentrations. Taken together, the data describe a kinetic system controlled by chemical reaction at one extreme and diffusion at the other. At low [alcohol], the second-order rate constants for the reaction of silica-U(aq)O(2)(2+) with methanol, 2-propanol, 2-butanol, and 2-pentanol are comparable to the rate constants obtained for these alcohols in homogeneous aqueous solutions containing H(3)PO(4). Under slow steady-state photolysis in O(2)-saturated suspensions, U(aq)O(2)(2+)-silica acts as a photocatalyst for the oxidation of alcohols with O(2).

Journal Article↗
PubMed2003

Preparation of novel silica-cadmium sulfide composite nanoparticles having adjustable void space by size-selective photoetching.

The size-selective photoetching technique was used to control the size of a CdS nanoparticle inside a silica shell. With monochromatic light irradiation, the diffuse reflectance spectra of silica-coated CdS nanoparticles were blue-shifted, and the size of the resulting CdS nanoparticles incorporated in the silica shells was adjustable by varying the wavelength of irradiated light. TEM observation revealed that the original CdS nanoparticle seemed to be in close contact with the amorphous silica shell to leave almost no clearance, while the monochromatic light irradiation caused the decrease in the size of CdS particles, resulting in the formation of a void space between the photoetched CdS core particle and the silica shell. The average void spaces available in the shells were calculated to be ca. 1.4 and 2.4 nm with the irradiation at 514 and 458 nm, respectively. These results indicated that the size-selective photoetching technique enables the regulation of void space formed in the core-shell structure by choosing the wavelength of irradiation light.

Journal Article↗
PubMed2003

Nitric oxide-releasing fumed silica particles: synthesis, characterization, and biomedical application.

The preparation, characterization, and preliminary biomedical application of various nitric oxide (NO)-releasing fumed silica particles (0.2-0.3 microm) are reported. The tiny NO-releasing particles are synthesized by first tethering alkylamines onto the surface of the silica using amine-containing silylation reagents. These amine groups are then converted to corresponding N-diazeniumdiolate groups via reaction with NO(g) at high pressure in the presence of methoxide bases (e.g., NaOMe). N-Diazeniumdiolate groups were found to form more readily with secondary amino nitrogens than primary amino nitrogens tethered to the silica. Different alkali metal cations of the methoxide bases, however, have little effect on the degree of N-diazeniumdiolate formation. The N-diazeniumdiolate moieties attached on the silica surface undergo a primarily proton-driven dissociation to NO under physiological conditions, with an "apparent" reaction order somewhat greater than 1 owing to local increases in pH at the surface of the particles as free amine groups are generated. The rates of N-diazeniumdiolate dissociation are further related to the parent amine structures and the pH of the soaking buffer. The N-diazeniumdiolate groups also undergo slow thermal dissociation to NO, with zero-order dissociation observed at both -15 and 23 degrees C. It is further shown that the resulting NO-releasing fumed silica particles can be embedded into polymer films to create coatings that are thromboresistant, via the release of NO at fluxes that mimic healthy endothelial cells (EC). For example a polyurethane coating containing 20 wt % of NO-releasing particles prepared with pendant hexane diamine structure (i.e., Sil-2N[6]-N(2)O(2)Na) is shown to exhibit improved surface thromboresistivity (compared to controls) when used to coat the inner walls of extracorporeal circuits (ECC) employed in a rabbit model for extracorporeal blood circulation.

Amines↗
PubMed2004

Nano-architectural silica thin films with two-dimensionally connected cagelike pores synthesized from vapor phase.

Novel mesostructured silica thin films were prepared on a Si substrate by a vapor-phase synthesis. Vapor of tetraethoxysilane (TEOS) was infiltrated into a surfactant film consisting of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer. Nanophase transition from a lamellar structure to a two-dimensional cage structure of a silica-surfactant nanocomposite was found under vapor infiltration. The rearrangement into the cage structure implies high mobility of the silica-surfactant composites in solid phase. The silica thin films have two-dimensionally connected cagelike mesopores and are isotropic parallel to the film surface. The structure of pores of the films is advantageous for next-generation low-k films. The mesoporous structure has a large lattice parameter d of approximately 102 A, silica layer thickness of approximately 58 A, pillar diameter in the middle of approximately 60 A, pore size of approximately 72 A, BET surface area of approximately 729 m(2)/g, and pore volume of approximately 1.19 cm(3)/g. The films synthesized by the vapor infiltration show a lower concentration of residual Si-OH groups compared to the films prepared by a conventional sol-gel method. The films show high thermal stability up to 900 degrees C and high hydrothermal stability. This method is a simpler process than conventional sol-gel techniques and attractive for mass production of a variety of organic-inorganic composite materials and inorganic porous films.

Journal Article↗
PubMed2007

Control of mesoporous silica nanostructures and pore-architectures using a thickener and a gelator.

A chiral cationic thickener l-ValPyBr, which was able to enhance the viscosity of water and form loosely physical gel in mixtures of water and alcohols, was synthesized. Sol-gel polymerization of TEOS was carried out in mixtures of water and alcohols under basic conditions using the self-assemblies of l-ValPyBr as template. The left-handed twisted mesoporous silica nanoribbons, which were constructed by nanotubes in monolayer, were obtained, and they tended to self-assemble into bundle structure. Stirring under the preparation process played an important role in the formation of this bundle structure. The obtained silica nanoribbons were uniform in width, thickness, and helical pitch without combining amorphous particles. The helical pitch and pore size of the mesoporous silica nanoribbons sensitively depended on the volume ratio of alcohols to water in the reaction mixtures. With increasing volume ratio of alcohols to water in the reaction mixture, the morphologies of the obtained silica changed from left-handed twisted ribbon to coiled ribbon, then to tubular structure. A compound l-ValPyPF6, structurally related to thickener l-ValPyBr, was able to form physical gel in ethanol, THF, acetonitrile, and the mixtures of ethanol and water. Left-handed multiple helical mesoporous silica nanofibers were prepared by using the self-assemblies of l-ValPyPF6 as template in mixtures of water and alcohols under basic conditions. By controlling both the volume ratio of ethanol to water and the weight ratio of l-ValPyPF6 to TEOS, two- or three-dimensional pore-architecture constructed by porous chiral nanotubes was obtained.

Journal Article↗
PubMed2007

Silica-stabilized gold island films for transmission localized surface plasmon sensing.

Ultrathin gold films prepared by evaporation of sub-percolation layers (typically up to 10 nm nominal thickness) onto transparent substrates form arrays of well-defined metal islands. Such films display a characteristic surface plasmon (SP) absorption band, conveniently measured by transmission spectroscopy. The SP band intensity and position are sensitive to the film morphology (island shape and inter-island separation) and the effective dielectric constant of the surrounding medium. The latter has been exploited for chemical and biological sensing in the transmission localized surface plasmon resonance (T-LSPR) mode. A major concern in the development of T-LSPR sensors based on Au island films is instability, manifested as change in the SP absorbance following immersion in organic solvents and aqueous solutions. The latter may present a problem in the use of Au island-based transducers for biological sensing, usually carried out in aqueous media. Here, we describe a facile method for stabilizing Au island films while maintaining a high sensitivity of the SP absorbance to analyte binding. Stabilization is achieved by coating the Au islands with an ultrathin silica layer, ca. 1.5 nm thick, deposited by a sol-gel procedure on an intermediate mercaptosilane monolayer. The silica coating is prepared using a modified literature procedure, where a change in the reaction conditions from room temperature to 90 degrees C shortened the deposition time from days to hours. The system was characterized by UV-vis spectroscopy, ellipsometry, XPS, HRSEM, AFM, and cyclic voltammetry. The ultrathin silica coating stabilizes the optical properties of the Au island films toward immersion in water, phosphate buffer saline (PBS), and various organic solvents, thus providing proper conditions where the optical response is sensitive only to changes in the effective dielectric constant of the immediate environment. The silica layer is thin enough to afford high T-LSPR sensitivity, while the hydroxyl groups on its surface enable chemical modification for binding of receptor molecules. The use of silica-encapsulated Au island films as a stable and effective platform for T-LSPR sensing is demonstrated.

Journal Article↗
PubMed2005

Experimental and computational studies of trialkylaluminum and alkylaluminum chloride reactions with silica.

Reactions of trimethylaluminum, triethylaluminum, and diethylaluminum chloride and ethylaluminum dichloride with silica gel have been studied experimentally by infrared spectroscopy and elemental analysis. The silica gel was subjected to different pretreatments to alter surface functionalities prior to reaction. In all cases the extent of surface modification reaction follows the trend unmodified > 600 degrees C pretreated > hexamethyldisilazane (HMDZ) pretreated > 600 degrees C/HMDZ pretreated. All of the aluminum compounds studied completely react non-hydrogen-bonded silanols, while also reacting with hydrogen-bonded silanols and siloxanes. Primarily monomeric surface species result from the surface modification reaction. Ethylaluminum chlorides preferentially react with silanols through cleavage of the Al-C bond rather than the Al-Cl bond. Singly bonded Si(s)-O-AlCl(2) surface species are readily synthesized by reaction of ethylaluminum dichloride with HMDZ-pretreated silica gel. Bridged bonded (Si(s)-O)(2)-AlCl surface species are readily synthesized by reaction of diethylaluminum chloride with HMDZ-pretreated silica gel. Computational ab initio studies of the cluster Si(4)O(6)(OH)(4) as a model to study the reaction of monomeric and dimeric methylaluminum dichloride with a silica silanol are also described. Comparison of the potential energy surface (PES) of monomer and dimer indicates that the energetics favor monomer reaction, consistent with experimental results. The energy cost in the dimer reaction is primarily from cleavage of a bridged Al-Cl bond upon adsorption. This does not occur when the monomer adsorbs. A comparison of the PES for the two reaction pathways resulting from cleavage of either an Al-Cl or Al-C bond indicates that while the former reaction is slightly kinetically favored (E(a) = 23.1 kJ/mol for Al-Cl bond cleavage versus E(a) = 31.1 kJ/mol for Al-C bond cleavage), the latter is strongly thermodynamically favored with an overall free energy difference between the two reaction pathways of 135 kJ/mol favorable to Al-C bond cleavage. These reactions are thermodynamically controlled.

Journal Article↗
PubMed2005

Synthesis of FDU-1 silica with narrow pore size distribution and tailorable pore entrance size in the presence of sodium chloride.

Ordered silicas with large (9-15 nm), uniform, cagelike mesopores were synthesized under acidic aqueous conditions from tetraethyl orthosilicate in the presence of sodium chloride using poly(ethylene oxide)-poly(butylene oxide)-poly(ethylene oxide) triblock copolymer B50-6600 (EO39BO47EO39, Dow Chemicals) as a supramolecular template. Except for the use of NaCl in our case, the synthesis mixture composition was the same as that originally reported by Zhao et al. for the synthesis of FDU-1 silica, which was later shown to exhibit a cubic close-packed (Fm3m) structure with stacking faults related to the occurrence of hexagonal close-packed stacking sequences. The copolymer-templated silicas were formed at room temperature and in most cases were subjected to the hydrothermal treatment at 373 or 393 K. The calcined materials were characterized using small-angle X-ray scattering (SAXS) and nitrogen and argon adsorption at 77 K. SAXS patterns were generally similar to those reported for FDU-1 silica, indicating the cubic close-packed (Fm3m) structure, but the presence of stacking faults characteristic of a hexagonal close-packed structure cannot be precluded. The addition of the salt was found to significantly narrow the pore size distributions and to improve the uniformity of entrances to the cagelike mesopores, whereas the pore diameter, specific surface area, and pore volume were similar (in most cases slightly lower) to those for FDU-1 silicas obtained in the absence of NaCl. The materials synthesized in the presence of NaCl also appeared to have better resolved SAXS patterns. The feasibility of tailoring the pore cage diameter (from approximately 9.5 to 14.5 nm) and pore entrance diameter (from below 4 to approximately 8 nm) simply by adjusting the hydrothermal treatment temperature and time was demonstrated, indicating that these simple and convenient ways of structural design of cagelike mesopores are operative in the case of syntheses in the presence of inorganic salts.

Scattering, Radiation↗
PubMed2005

Preferential growth of single-walled carbon nanotubes on silica spheres by chemical vapor deposition.

The preferential growth of single-walled carbon nanotubes (SWNTs) on silica spheres with various diameters was realized for the first time by chemical vapor deposition (CVD) of methane. SWNTs tend to wrap the silica spheres to form a new superstructure of uniform SWNT nanoclaws when the diameters of the silica spheres are larger than 400 nm. The SWNTs obtained on silica spheres have highly graphitic tubular walls as characterized by Raman spectroscopy and HRTEM. This is a new method to obtain tunable uniform elastic deformation of SWNTs, which may act as the model for the study about the effect of delocalized bending on the properties of SWNTs. In addition, the combination of SWNTs with monodispersed silica spheres could conveniently integrate SWNTs into photonic crystals.

Journal Article↗
PubMed2005

The surface of ordered mesoporous benzene-silica hybrid material: an infrared and ab initio molecular modeling study.

Joint IR and computational results allow a detailed characterization of the surface properties of a mesoporous benzene-silica hybrid material with crystal-like wall structure. After outgassing at 450 degrees C, hydroxyl species mainly consist of noninteracting silanols, with both O-H and Si-O stretching modes at lower frequencies than those of SiOH in silica. Interaction with several probe molecules, followed both by experiment and calculus, shows that the aryl group in the coordination sphere of Si imparts a lesser acidity with respect to the isolated silanol in silica. In contrast, adsorption isotherms indicate that the interaction with acetone is stronger with benzene-silica than with silica: this is interpreted in terms of secondary interactions taking place between the slightly acidic CH in acetone and the electronic cloud in benzene-like rings. This suggests that both the inorganic component and the organic one play a role in dictating the surface behavior.

Journal Article↗
PubMed2005

Gas diffusion and microstructural properties of ordered mesoporous silica fibers.

Pore and surface diffusion of carbon dioxide (CO(2)) and ethylene (C(2)H(4)) in the nanopores of ordered mesoporous silica fibers about 200 microm in length was measured by the transient gravimetric method. The experimentally determined pore diffusivity data, coupled with the porosity, pore size, and fiber length, are used to obtain the actual length of the nanopores in silica fibers. These measurements reveal a structure of the ordered nanopores whirling helically around the fiber axis with a spiral diameter of about 15 microm and a pitch value of 1.6 microm. At room temperature the surface diffusion contributes about 10% to the total diffusional flux for these two gases in the nanopores of the ordered mesoporous silica fibers. The surface diffusion coefficients for the ordered mesoporous silica fibers are about 1 order of magnitude larger than the non-ordered mesoporous alumina or silica with similar pore size.

Journal Article↗
PubMed2005

Structure sensitivity of vibrational spectra of mesoporous silica SBA-15 and Pt/SBA-15.

The vibrational properties of mesoporous silica (SBA-15) were investigated by deep ultraviolet (UV) Raman and infrared spectroscopies with and without the presence of platinum nanoparticles in the mesopores that were incorporated by sonication. Raman and IR spectral line assignments were made by comparison to amorphous silicas. This procedure permitted identification of vibrations of longitudinal (LO) and transverse (TO) optical lattice modes, the presence of Si-OH, and vibrational modes associated with the presence of three-, four-, and six-membered siloxane rings. Hydraulic pressing of the mesoporous silica with pressure in the range 3-7 tons cm(-2) destroys the X-ray diffraction pattern and strongly decreases the Raman peak (D2) associated with three-membered rings at the surface. In the presence of platinum nanoparticles in the silica mesopores, a peak attributed to a Pt-O stretching vibration appears at between 530 and 580 cm(-1) in the UV-Raman spectrum, which can be used to monitor the presence of the platinum particles and their interaction with the support. The D2 feature in the UV-Raman spectra also decreases with increasing Pt loading, which is attributed to interactions of the Pt nanoparticles with the silica surface.

Journal Article↗