Sol-gel-derived biomaterials of silica and carrageenans

A new precursor, tetrakis(2-hydroxyethyl) orthosilicate (THEOS), introduced by Hoffmann et al. (J. Phys. Chem. B 106 (2002) 1528-1533), was used to synthesize monolithic hybrid biomaterials on the basis of silica and three main types of carrageenans, kappa-, iota-, and lambda-carrageenans. The advantage of THEOS over the currently applied TEOS and TMOS is in its complete solubility in water. This negated the need to add organic solvents, thus excluding a denaturating effect on biopolymers. In their turn, carrageenans introduced into the precursor solution made use of common catalysts unneeded to trigger the sol-gel transition. It was found that they promoted the mineralization, acting as a template for the inorganic component. The kinetics of sol-gel processes, mechanical properties, phase behavior, and structure of novel hybrid biomaterials were studied by dynamic rheology, differential scanning calorimetry, and scanning electron microscopy. The material properties were regulated by both the precursor and carrageenan. The increase of silicate concentration led to a rise in the stiffness and brittleness of the material, whereas the polysaccharide addition made it softer and more elastic. It was shown that the formation and properties of mixed gels were determined by the nature of carrageenan. kappa-Carrageenans brought about shrinkage of hybrid materials that led to water separation, while iota- and lambda-carrageenans did not induce the syneresis. This is in line with the difference in polysaccharide properties when they are in aqueous solutions without silicate. Furthermore, kappa- and iota-carrageenans experienced a thermoreversible phase transition in the hybrid materials owing to the helix-coil transition. This resulted in a step like change in the mechanical properties of mixed systems in the corresponding temperature range. lambda-Carrageenan is a nongelling polysaccharide, which is why the rheological parameters of its hybrid gel were unchanged with the temperature. It was found that the polysaccharides modified the structure of silica-based materials. They transformed a three-dimensional network of connected silica particles into that consisting of crossed fibers.     

Hybrid silica-PVA nanofibers via sol-gel electrospinning

We report on the synthesis of poly(vinyl alcohol) (PVA)-silica hybrid nanofibers via sol-gel electrospinning. Silica is synthesized through acid catalysis of a silica precursor (tetraethyl orthosilicate (TEOS) in ethanol-water), and fibers are obtained by electrospinning a mixture of the silica precursor solution and aqueous PVA. A systematic investigation on how the amount of TEOS, the silica-PVA ratio, the aging time of the silica precursor mixture, and the solution rheology influence the fiber morphology is undertaken and reveals a composition window in which defect-free hybrid nanofibers with diameters as small as 150 nm are obtained. When soaked overnight in water, the hybrid fibers remain intact, essentially maintaining their morphology, even though PVA is soluble in water. We believe that mixing of the silica precursor and PVA in solution initiates the participation of the silica precursor in cross-linking of PVA so that its -OH group becomes unavailable for hydrogen bonding with water. FTIR analysis of the hybrids confirms the disappearance of the -OH peak typically shown by PVA, while formation of a bond between PVA and silica is indicated by the Si-O-C peak in the spectra of all the hybrids. The ability to form cross-linked nanofibers of PVA using thermally stable and relatively inert silica could broaden the scope of use of these materials in various technologies.     

Influence of polymeric additives on biomimetic silica deposition on patterned microstructures

Microstructured polymer films prepared by photochemical grafting of different polymers were used as restricted reaction areas in silica deposition experiments. Linear and branched poly(alkyleneimines) and poly(allylamine hydrochloride) in pure aqueous or phosphate-containing solutions were used as additives to silica precursor solutions. The silica deposits obtained by spin-coating these solutions onto microstructured polymer films were investigated by scanning electron microscopy and atomic force microscopy. Experiments with poly(alkylene imines) in the silica precursor solution show the deposition of smooth and granular silica structures that closely mimic the natural patterns. The structure formation can be explained by physicochemical processes. Hypotheses that have been made for the natural silification processes can be evaluated on this basis.     

On the interaction of encapsulated pH indicator species within a silica matrix produced by three sol-gel routes

Solid acid-base sensors were prepared by encapsulating two pH indicators (brilliant yellow or acridine) within a silica matrix by the sol-gel method using three different routes: (1) non-hydrolytic, (2) acid catalyzed and (3) base catalyzed. The interactions of the silica-indicator with the resulting materials were then investigated by cyclic and differential pulse voltammetry. Complementary, ultraviolet-visible, photoacoustic spectroscopy was employed for the characterization of the interactions by monitoring the band shifts (bathochromic or hypsochromic, depending on the sol-gel route) between the neat pH indicators and those encapsulated within the silica network. Furthermore, X-ray photoelectron spectroscopy showed that the N 1s binding energy in brilliant yellow was shifted for the material resulting from the acid route. The electrochemical behavior and the pH indicator interactions with the silica network were dependent on the nature of the employed sol-gel route. For the sensors prepared with acridine, the interactions with the silica network took place through the nitrogen group from the pyridinic ring. For the brilliant yellow indicator, different behaviors were observed depending on the route, suggesting different processes during preparation or analysis. For the basic catalyzed and non-hydrolytic routes, it was not possible to assign a specific interaction. Nevertheless, it seemed that interactions might have taken place through the hydroxyl and/or sulphonic groups. Furthermore, for the brilliant yellow sensor prepared through the acid route, it was possible to show that the interaction probably or partially occurred through the azo groups.     

Synthesis of hierarchical hollow silica microspheres containing surface nanoparticles employing the quasi-hard template of poly(4-vinylpyridine) microspheres

A facile method of preparing hierarchical hollow silica microspheres containing surface silica nanoparticles (HHSMs) through the sol-gel process of tetraethylorthosilicate employing a quasi-hard template of non-cross-linking poly(4-vinylpyridine) microspheres is proposed. The quasi-hard template contains the inherent catalyst of the basic pyridine group, and a few of the polymer chains can escape from the template matrix into the aqueous phase, which initiates the sol-gel process spontaneously both on the surface of the template used to prepare the hollow silica shell and in the aqueous phase to produce the surface silica nanoparticles. By tuning the weight ratio of the silica precursor to the quasi-hard template, HHSMs with a size of about 180 nm and a shell thickness ranging from 14 to 32 nm and surface silica nanoparticles ranging from 17 to 36 nm are produced initially through the deposition of surface silica nanoparticles onto the silica shell, followed by template removal either by calcination or solvent extraction. The synthesized HHSMs are characterized, and a possible mechanism for the synthesis of HHSMs is proposed.     

Status and dreams of photonics polymer for IT

We have proposed a low-loss, high-bandwidth and large-core graded-index plastic optical fiber (GI POF) in data-corn, area. The GI POF enables us to eliminate the “modal noise” problem which is observed in medium-core silica fibers. Therefore, stable high-speed data transmission can be realized by the GI POF rather than medium-core silica fibers. Furthermore, advent of perfluorinated (PF) polymer based GI POF network can support higher transmission than silica fibers network because of the small material dispersion of PF polymer compared with silica. In addition, we proposed a “highly scattering optical transmission (HSOT) polymer” and applied it to a light guide plate of a liquid crystal display backlight. The HSOT polymer backlight that was designed using the HSOT designing simulator demonstrated twice the brightness of the conventional taransparent backlight with sufficient color uniformity. Furthermore, we proposed the two types of zero-birefringence polymers synthesized by the random copolymerization method and the anisotropic molecule dopant method. Both of the polymers exhibited no orientational birefringence for any orientation of polymer chains.     

Synthesis of Ordered Biosilica Materials

Biogenic silica with amazing diversity of nanostructure shells,fibers and granules in diatoms and sponges is mediated by proteins and polysaccharides and forms at ambient pressure and temperatures.Chemical synthetic methods,in contrast,have to rely on extreme pH and /or surfactants to induce the condensation of silica precurors into specific patterns.One kind of benign synthesis method through plant cell wall template-directed ordered biosilica materials under ambient conditions in intriguing in this context.Organized silica materials in intercellular spaces of epidermal cells of tall fescue leaves were synthesized through molecular recognition between Si-OH and polysaccharide-OH or glycoprotein-OH of main components of plant cell walls and cellular processing as well when Si(OEt)4 was supplied rather than monosilicic acid.The biosynthesis of structural silica in tall fescue plant was correlated with the Si species applied,reflecting the slower condensation from tetraethoxysilane (TEOS) and thus providing greater opportunities for structural control by the underlying matrix of cell walls.The composition was estimated by energy dispersive X-ray(EDX) spectra on a scanning electron microscope.All organized structures showed carbon,oxygen and silicon peaks,indication that their formations differ from natural siliceous process.     

Preparation of dendritic and network PANI/silica composite fibers

Dendritic and network PANI fibers with controlled diameters from nanosize to sub-micrometer-size were prepared at room temperature. Conducting polyaniline (PANI)/silica composite fibers were synthesized via a sol-gel progress thereafter. Structural characterization was performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). FT-IR and UV-vis were used to verify the incorporation of the silica.     

The role of initiation in the synthesis of silica/poly(methyl methacrylate) nanocomposite latex particles through emulsion polymerization

Silica/poly(methyl methacrylate) nanocomposite latex particles have been synthesized by emulsion polymerization of methyl methacrylate using a nonionic surfactant: nonylphenol poly(oxyethylene) and three different initiators, namely: 2,2′-azobis(2-amidinopropane) dihydrochloride (AIBA), potassium persulfate (KPS) and azobis(isobutyronitrile) (AIBN), being cationic, anionic and nonionic, respectively. A silica sol with an average diameter of 68 nm was used as the seed. The polymerization reaction was conducted under alkaline conditions in order to evaluate the role of the surface charge of the hydrophilic silica on the coating reaction. AIBA was found to be adsorbed on the silica surface owing to electrostatic interactions of the amidine function of the cationic initiator with the silanolate groups of the oxide surface, while the anionic and the nonionic initiators did not adsorb on silica under the same conditions. Nonetheless, whatever the nature of the initiator, polymerization took place on the silica particles as evidenced by transmission electron microscopy. The extent of interaction between the inorganic surface and the polymer particles was quantified by means of ultracentrifugation and a material balance. As much as 65% by weight of the total polymer formed was found to be present at the silica surface using AIBA, while only 40% for KPS and 25% for AIBN was found to cover the silica particles under alkaline conditions. We demonstrate that by using a cationic initiator and by controlling the pH of the suspension it is possible to significantly decrease the amount of free polymer. Coating of the silica particles took place through a kind of in situ heterocoagulation mechanism. Received: 8 December 2000 Accepted: 22 February 2001     

Sol–gel silica hybrid biomaterials for application in biodegradation of toxic compounds

The purpose of the present work is the sol–gel synthesis, structure characterization and potential application of hybrid biomaterials based on silica precursor (MTES) and natural polymers such as gelatin or pectin. The structure formation in the biomaterials was investigated by XRD, FTIR, BET and AFM. The results showed that all studied hybrid biomaterials have an amorphous structure. The FT-IR spectra of the obtained materials with MTES showed chemical bonds at 2,975, 1,255, 880 and 690 cm⁻¹ due to the presence of Si–O–R (CH₃ and C₂H₅) and Si–C bonds. In the samples synthesized with TEOS the inorganic and organic components interact by hydrogen bonding, Van der Waals or electrostatic forces. Surface area of investigated samples decreases with increasing of the natural polymers content. The structure evolution was studied by AFM and roughness analysis. Depending on the chemical composition a different design and size of particles and their aggregates on the surface structure were established. The hybrid biomaterials were used for immobilization of bacterial cells and applied in the biodegradation of the toxic compound 4-chlorobutyronitrile, possible constituent of waste water effluents in a laboratory glass bioreactor. Optimization of the process at different temperatures was carried out.     

Effect of silicon precursor on silicon incorporation in SAPO-11 and their catalytic performance for hydroisomerization of n-octane on Pt-based catalysts

SAPO-11 molecular sieves were synthesized using silica sol, hydrophilic fumed silica, and tetraethyl orthosilicate(TEOS) as silicon precursors. Their physicochemical properties were characterized using XRD,SEM, nitrogen adsorption-desorption, Py-IR, NH_3-TPD, EDS, and ~(27)Al,~(31)P,~(29)Si MAS NMR techniques. The catalytic performance was assessed in the hydroisomerization of n-octane. The results showed that the silicon precursors influenced the physicochemical properties and catalytic performance of SAPO-11. SAPO-11 synthesized using hydrophilic fumed silica as silicon precursor showed higher silicon distribution and had more medium acid sites. SAPO-11 synthesized using TEOS as silicon precursor had more silicon content, but more silicon islands formed in its framework. The depolymerization of silicon precursors might affect the silicon content and distribution in SAPO-11. In the hydroisomerization of n-octane, the catalytic activity strongly depended on the number of medium acid sites instead of the number of total acid sites.SAPO-11 synthesized using hydrophilic fumed silica as silicon precursor exhibited higher catalytic activity than the other samples because it has more medium acid sites.     

Low‐Temperature Sol‐Gel Synthesis of Nanostructured Polymer/Titania Hybrid Films based on Custom‐Made Poly(3‐Alkoxy Thiophene)

A low‐temperature route to directly obtain polymer/titania hybrid films is presented. For this, a custom‐made poly(3‐alkoxy thiophene) was synthesized and used in a sol‐gel process together with an ethylene‐glycol‐modified titanate (EGMT) as a suitable titania precursor. The poly(3‐alkoxy thiophene) was designed to act as the structure‐directing agent for titanium dioxide through selective incorporation of the titania precursor. The nanostructured titania network, embedded in the polymer matrix, is examined with atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements. By means of the scattering technique grazing incidence wide‐angle X‐ray scattering (GIWAXS), a high degree of crystallinity of the polymer as well as successful transformation of the precursor into the rutile phase of titania is verified. UV/Vis measurements reveal an absorption behavior around 500 nm which is similar to poly(3‐hexyl thiophene), a commonly used polymer for photoelectronic applications, and in addition, the typical UV absorption behavior of rutile titania is observed.     

Bioassisted synthesis of catalytic gold/silica nanotubes using layer-by-layer assembled polypeptide templates

We report the bioassisted synthesis of gold nanoparticle/silica (Au NP/silica) tubes using layer-by-layer (LBL) assembled poly(L-lysine)/poly(L-tyrosine) (PLL/PLT) multilayer films deposited on the polycarbonate (PC) membrane pores as both mediating agents and templates. The novelty of this approach is the in situ synthesis of Au NP/silica tubes using PLL/PLT multilayer films for sequential growth of Au NPs and silicas. The experimental data revealed that the buildup of the LBL multilayer films was mainly driven by the formation of hydrogen bond and the polypeptide macromolecular assemblies adopted mainly β-sheet conformation. The as-prepared Au NP/silica tubes possessed promising catalytic activity toward the reduction of p-nitrophenol. The synthesis conditions such as the concentration of gold precursor and polypeptide molecular weight were found to influence the gold weight ratio and particle size in the tubes and the catalytic properties of the Au NP/silica tubes. This approach provides a facile, robust, and green method to obtain nonaggregated metal nanoparticles immobilized in porous oxide network at ambient conditions. Using the synergy between biomimetic or bioassisted synthesis of nanostructured materials and LbL assembly technique, a variety of structures such as films, tubes, and capsules comprising of multiple compositions can be obtained.     

Heat of adsorption of cysteine enzymes on mesoporous silica with high specific surface area

The adsorption of a cysteine enzyme, on mesoporous silica with high specific surface area synthesized by the sol-gel method, was studied in a heat flow calorimeter, to determine the energy involved in the adsorption process of the protein. The adsorption was carried out at a constant temperature of 30°C to avoid the denaturation of the enzyme. The observed results indicate that the obtained biomaterials (silica-enzyme) have possibilities for their application in several biotechnology processes. The heat of papain adsorption and the solid-enzyme (SiO₂-Papain) interactions at different pH are presented.     

UV accelerated aging and aging resistance of dihydroxy poly(p‐phenylene benzobisoxazole) fibers

By introducing binary hydroxyl groups into poly(p‐phenylene benzoxazole) (PBO) macromolecular chains, we synthesized dihydroxy poly(p‐phenylene benzobisoxazole) (DHPBO) polymers and then prepared DHPBO fibers by dry‐jet wet‐spinning. Comparative studies were performed between intrinsic PBO fibers and DHPBO fibers. The effects of hydroxyl polar groups on improving the UV aging resistance of PBO fibers were investigated. With the introduction of hydroxyl groups, substantial changes in the chemical structures and surface morphologies of DHPBO fibers were observed. As proved by tensile testing and intrinsic viscosity measurement, the UV resistance of DHPBO fibers is obviously improved compared to that of intrinsic PBO fibers. XRD results indicate that the UV aging of these fibers occurs mainly on the surfaces of fibers. Based on these results, the mechanism of UV aging of PBO fibers was discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Manipulating formation and drug-release behavior of new sol-gel silica matrix by hydroxypropyl guar gum

To develop biocompatible sol-gel silica matrix for the encapsulation of biomolecules or drugs, a novel water-soluble silica precursor, tetrakis(2-hydroxyethyl)orthosilicates (THEOS), was used in combination with a water-soluble polysaccharide derivative, hydroxypropyl guar gum (HPGG). We found that the introduction of HPGG could trigger and accelerate the sol-gel transition of THEOS in water and induce rapid formation of homogeneous gel matrix without the addition of any organic solvents or catalysts. Moreover, added HPGG macromolecules had a great influence on the network structure and particle dimension in the silica gel matrix, as confirmed by scanning electron microscope (SEM) observation. From the time sweep rheological measurements, it was found that a higher HPGG amount could lead to shorter gelation time for the sol-gel transition. From the strain and frequency sweep rheological experiments, it was found that the resultant silica matrix containing a higher amount of HPGG exhibited a narrower linear viscoelastic region, a higher dynamic muduli, and greater complex viscosity. In particular, the gel strength of the silica matrix could be modulated by the amount of HPGG. By investigating the controlled release of vitamin B12 from the sol-gel silica matrixes, a strong dependence of the release profile on the amount of introduced HPGG was observed. In this case, a higher HPGG amount resulted in lower release rate.     

The formation mechanism of cobalt silicide on silica from Co(SiCl3)(CO)4 by in situ Fourier transform infrared spectroscopy

Silica supported CoSi particles were synthesized by metal organic chemical vapor deposition of the Co(SiCl(3))(CO)(4) precursor carried in hydrogen at atmospheric pressure and moderate temperature in a fluidized bed reactor. In contrast, CoCl(2) supported on silica was formed by using argon as the carrier gas. The samples were characterized by X-ray diffraction, transmission electron microscopy, ultraviolet-visible spectroscopy, and thermogravimetric/differential thermogravimetric analysis. The precursor Co(SiCl(3))(CO)(4) reacted with the hydroxyl groups of amorphous silica via loss of HCl and introduced cobalt species onto the surface. The decomposition mechanism of the supported precursor on silica was investigated by in situ Fourier transform infrared spectroscopy from room temperature to 300 °C in a hydrogen or argon atmosphere. The results showed that CO and HCl elimination occurred in a hydrogen atmosphere, while only CO elimination occurred in Ar. All of the results showed that it was possible to prepare supported CoSi at lower temperatures via changing the carrier gas.     

Preparation and surface properties of low-density gels synthesized using prepolymerized silica precursors

Properties of silica xerogels and aerogels synthesized using a number of prepolymerized silica precursors were probed by 29Si magic-angle spinning (MAS) NMR spectroscopy, the small-angle X-ray scattering (SAXS) method, the nitrogen adsorption method, and transmission electron microscopy (TEM) to show that xerogels with attractive textural properties can easily be prepared using this type of precursors and the conventional one-step, base procedure. Pore sizes and overall pore volumes in these materials can be notably larger than those in the corresponding materials synthesized using tetraethoxysilane. This positive effect stems from the stronger structure of the polymeric network due to a higher degree of silica condensation on one side and a larger thickness of polymeric chains on the other. The thorough investigations of the fine silica structure demonstrate, however, that the relationship between the microstructure of the silica precursor and the micro- and macrostructures of dry gels is complex and the use of more condensed precursors favors, but does not necessarily ensure, more porous dry materials, under the same reaction conditions. Ethyl silicate 40 may be recommended as a low-cost precursor suitable for applications in this situation.     

Preparation and characterization of gold nanoparticles and nanowires loaded into rod-shaped silica by a one-step procedure

Rod-shaped mesoporous silica nanoparticles (RMSN) with built-in gold nanoparticles or thin gold nanowires in the pore channels were in situ synthesized via a one-step procedure. The insertion of a hydrophobic gold precursor into the mesopores of RMSN was reached through a micellar solubilization mechanism and gold nanoparticles were achieved through a thermal reduction. The resulting RMSN and Au-RMSN samples were characterized by using X-ray diffraction, transmission and scanning microscopies (TEM and SEM), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption and solid-state Nuclear Magnetic Resonance (NMR). The interaction of Au precursor (a carbene complex) with the thiol group at the silica surface was identified and found to play a crucial role in the dispersion of the uniform metal nanoparticles at the internal surface of RMSN. Moreover, TEM micrographs revealed the absence of large gold particles outside the mesopore network. The shape of Au nanoparticles and their loading amount in the mesoporous silica could be easily tuned by altering the concentration of gold precursor.     

Highly Porous Silica Monoliths from Ethyl (L)-Lactate Modified Silanes

Summary. A new type of silica precursor was synthesized by (trans)alkoxylation of alkoxy- and chlorosilanes with ethyl (L)-lactate. This novel ethyl lactate modified silane was hydrolyzed and condensed in the presence of a non-ionic surfactant – poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymer (P123) – to give monolithic silica gels. The wet gels were dried using two different drying techniques resulting in crack-free monoliths: a) supercritical drying with CO₂ to yield a porous inorganic material and b) surface silylation with trimethylchlorosilane to yield an inorganic–organic nanocomposite material. The obtained porous gels were characterized by different techniques including thermal analysis, nitrogen sorption, and electron microscopy (TEM, SEM).