Sol–gel encapsulation of acid phosphatase in the presence of the ionic liquid [BMIM][BF4]

Abstract  

The effect of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) on acid phosphatase (APase) from wheat germ in solution and in the sol–gel-encapsulated form was investigated to explore new methods of enzyme preparation with improved catalytic performance. APase was encapsulated in hydrogel beads made from tetramethyl orthosilicate. Compared with free APase, 20–28% enzymatic activity was retained in over nine catalytic cycles. Sol–gel encapsulation improved the thermal stability of APase. Heat shock exposure at 60 °C for 1 h resulted in activity decreasing by a factor of three, only, for sol–gel-encapsulated APase, in contrast with a factor of nine decrease for free APase. Addition of 10% v/v [BMIM][BF₄] did result in a slight decrease of enzymatic activity for free and sol–gel-encapsulated APase, but resulted in a remarkable increase in alkaline pH tolerance of sol–gel-encapsulated APase.     

Morphology and photoluminescence study of titania nanoparticles

Titania nanoparticles are prepared by sol–gel chemistry with a poly(ethylene oxide) methyl ether methacrylate-block-poly(dimethylsiloxane)-block-poly(ethylene oxide) methyl ether methacrylate triblock copolymer acting as the templating agent. The sol–gel components—hydrochloric acid, titanium tetraisopropoxide, and triblock copolymer—are varied to investigate their effect on the resulting titania morphology. An increased titania precursor or polymer content yields smaller primary titania structures. Microbeam grazing incidence small-angle X-ray scattering measurements, which are analyzed with a unified fit model, reveal information about the titania structure sizes. These small structures could not be observed via the used microscopy techniques. The interplay among the sol–gel components via our triblock copolymer results in different sized titania nanoparticles with higher packing densities. Smaller sized titania particles, (∼13–20 nm in diameter) in the range of exciton diffusion length, are formed by 2% by weight polymer and show good crystallinity with less surface defects and high oxygen vacancies.     

Effect of silica particles on stability of highly concentrated water-in-oil emulsions with non-ionic surfactant

Water-in-oil, high internal phase emulsion made of super-cooled aqueous solution containing a mixture of inorganic salts and stabilized with non-ionic surfactant (sorbitan monooleate) alone was investigated. It was not possible to produce a highly concentrated emulsion (with aqueous phase fraction = 94 wt %), stabilized with surface-treated silica, solely: we were able to form an emulsion with a maximal aqueous phase mass fraction of 85 wt % (emulsion inverts/breaks above this concentration). The inversion point is dependent on the silica particle concentration, presence of salt in the aqueous phase, and does not depend on the pH of the dispersed phase. All emulsions stabilized by the nanoparticles solely were unstable to shear. So, the rheological properties and stability of the emulsions containing super-cooled dispersed phase, with regards to crystallization, were determined for an emulsion stabilized by non-ionic surfactant only. The results were compared to the properties obtained for emulsions stabilized by surface treated (relatively hydrophobic) silica nanoparticles as a co-surfactant to sorbitan monooleate. The influence of the particle concentration, type of silica surface treatment, particle/surfactant ratio on emulsification and emulsion rheological properties was studied. The presence of the particles as a co-stabilizer increases the stability of all emulsions. Also, it was found that the particle/surfactant ratio is important since the most stable emulsions are those where particles dominate over the surfactant, when the surfactant’s role is to create bridging flocculation of the particles. The combination of the two types of hydrophobic silica particles as co-surfactants is: one that resides at the water/oil interface and provides a steric boundary and another that remains in the oil phase creating a 3D-network throughout the oil phase, which is even more beneficiary in terms of the emulsion stability.     

Biodegradability of sol–gel silica microparticles for drug delivery

The biodegradability of porous sol–gel silica microparticles in physiological buffers has been investigated using a USP4 flow-through dissolution tester. In the open configuration, which most closely models in-vivo conditions, the particles dissolved rapidly at pH 7.4, with a rate dependent on the surface area and media flow rate. In the closed configuration, the fastest dissolving 4 mg silica sample was almost completely dissolved in 100 mL of buffer after 36 h. The initial dissolution rates appeared relatively linear but dropped off as dissolved SiO₂ concentrations approached 20–25 ppm. Addition of serum proteins acted to slow dissolution by 20–30%, suggesting a slower degradation in vivo. Silica microparticles administered for controlled release drug delivery would therefore be expected to be eliminated relatively rapidly from the body, depending on the sample size and local fluid flow conditions.     

Preparation of calcium plumbate via different route

Calcium plumbate Ca₂PbO₄ was prepared by sol–gel methods (Pechini complex route with two varieties and water soluble polymer method) and by solid state reaction. The sol–gel prepared samples contained calcium plumbate as the only one phase as early as after 2 h annealing at 800 °C. Phase composition was detected by XRD measurement and by Raman spectroscopy. The next annealing at 800 °C for 24 h induced weak Pb losses displayed by present CaO (according to phase equilibrium) and confirmed by XRF measurement. The Pb losses were smaller for the sol–gel prepared samples probably due to earlier formation of Ca₂PbO₄. Microstructure and grain size were also established. Sol–gel prepared samples had regularly distributed grains with a small distribution interval with median value in order of 1 μm. Differences in microstructure of solid state and sol–gel samples are presented on SEM micrographs.     

Sol–gel and impregnated prepared silver TiO<Subscript>2</Subscript> semiconductors as photocatalysts for the UV decomposition of 2,4-<Emphasis Type="SmallCaps">d</Emphasis>: a comparative study of the preparation method

UV-photocatalytic oxidation of 2,4-dichlorophenoxyacetic acid (2,4-d) was studied on Silver–TiO₂ semiconductors prepared by co-gelling silver acetylacetonate and titanium butoxide (TiO₂–Ag) or by incipient impregnation with silver acetylacetonate (0.5 wt% Ag) of bare TiO₂ sol–gel and TiO₂–P25 supports. The comparative study shows that only a slight modification on the specific surface area was obtained on the TiO₂–Ag sample (60 m²/g) as compared to the TiO₂ sol–gel bare support (65 m²/g). XRD patterns of the samples show anatase as the crystalline phase present in the sol–gel TiO₂ preparations. By means of HRTEM and HAADF-STEM electron microscopy techniques, silver nanoparticles (<12.0 nm) were identified which are very small to be detected by XRD. The photocatalytic oxidation of the herbicide 2,4-d used as a model of contaminant show for the kinetic parameter t_1/2 values of 45 min for the TiO₂–Ag sample, while for the impregnated Ag/TiO₂ and Ag/P25 nanomaterials t_1/2 was 124 and 66 min, respectively. The higher photoactivity of the TiO₂–Ag photocatalyst is attributed to a combined effect of the silver nanoparticles in interaction with the titania semiconductor.     

Photochromic hybrid sol–gel films containing naphthopyrans

Abstract  

Several photochromic benzo- and naphthopyrans were embedded in hybrid organic–inorganic ureasilicate based films obtained by a sol–gel process. The resulting films, containing 0.1% (w/w) of the photochromic compound, were transparent and flexible and for naphthopyrans they were colourless or presented just a slight colouration. Under UV continuous irradiation the films developed a significant absorption in the visible region that fades in the dark with variable kinetics depending on the naphthopyran structure. 2,2-Diphenyl-3H-naphtho[2,1-b]pyran films showed a very fast colouration under UV light giving rise to coloured open forms, containing mainly the TC isomer (98%) along with a very small amount of the more stable TT isomer (2%), that practically returned to the initial absorbance in 1 min.     

A simple template-free sol–gel synthesis of spherical nanosilica from agricultural biomass

Mesoporous silica nanoparticles with a spherical morphology have been synthesized from rice husk (agricultural biomass) by a simple, template-free synthetic approach, which was carried out via sol–gel technique at ambient condition. Transmission electron micrographs revealed the formation of spherical silica nanoparticles with an average diameter of 50.9 nm. From the nitrogen adsorption–desorption analysis, the rice husk silica shows a high specific BET surface area of 245 m² g⁻¹. The silica nanoparticles have a narrow pore size distribution of 5.6–9.6 nm.     

Structural and low temperature Raman scattering studies in SrTi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> nanoparticles synthesized by sol–gel method

We investigate effects of Co dopant concentration on the structure and low temperature Raman scattering properties in SrTi_1−x Co _x O₃ (x = 0.00, 0.10, 0.20, 0.30) nanoparticles prepared by sol–gel method. The dopant induced changes are studied by XRD, and Raman scattering measurements. The results show an average particle size of about 20 nm depending on the Co content and the lattice parameters decrease as increasing the Co content. In the Raman spectra, a broad structure in the region 100–500 cm⁻¹ is almost absent and the peaks in the region 600–800 cm⁻¹ show different weights with respect to SrTiO₃, relating to structural changes. The anomalous change in the area ratio of Raman peaks as function of temperature suggests a phase transition in our samples in the range of 110–130 K. These results indicate that the Co ion has replaced the site of Ti in unit cell. This novel route also demonstrates the advantage of synthesizing the compound with low annealing temperature.     

Pickering emulsions stabilized by surface-modified Fe3O4 nanoparticles

Unmodified Fe(3)O(4) nanoparticles do not stabilize Pickering emulsions of a polar oil like butyl butyrate. In order to obtain stable emulsions, the Fe(3)O(4) nanoparticles were modified by either carboxylic acid (RCOOH) or silane coupling agents (RSi(OC(2)H(5))(3)) to increase their hydrophobicity. The influence of such surface modification on the stability of the resultant Pickering emulsions was investigated in detail for both a non-polar oil (dodecane) and butyl butyrate in mixtures with water. The stability of dodecane-in-water emulsions in the presence of carboxylic acid-coated particles decreases as the length of the alkyl group (R) and the coating extent increase. However, such particles are incapable of stabilizing butyl butyrate-water emulsions even when the carboxylic acid length is decreased to two. However, the silane-coated Fe(3)O(4) nanoparticles can stabilize butyl butyrate-in-water emulsions, and they also increase the stability of dodecane-in-water emulsions. Thermal gravimetric analysis indicates that the molar quantity of silane reagent is much higher than that of carboxylic acid on nanoparticle surfaces after modification, raising their hydrophobicity and enabling enhanced stability of the resultant polar oil-water emulsions.     

The calculation of the optimum pH range for synthesizing BST nanopowders by sol–gel auto-combustion process

The optimum pH range for synthesizing barium strontium titanate (Ba_0.5Sr_0.5TiO₃, BST) nanopowders by sol–gel auto-combustion method was calculated based on the principles of matter balance and charge balance. The effectiveness of the calculation was proved by the successful synthesis of highly dispersive, spherically shaped and pure BST nanoparticles with setting the pH in the range pre-decided by theoretical evaluation. Ours might have provided an effective path for pre-deciding the solution pH in synthesizing various oxide nanopowders by sol–gel auto-combustion method.     

Microencapsulation of an acrylate monomer in silica particles by sol–gel process

The sol–gel synthesis strategies combined with the templated growth of organic–inorganic hybrid networks provide access to an immense new area of innovative multi-functional advanced materials. One possible way to prepare such new advanced materials is to encapsulate liquid active agents (such as monomers, dyes, catalysts and hardeners) in microcapsules. Silica microcapsules of tetraethylortosilicate (TEOS) and 3-(trimethoxysilyl)propyl methacrylate (MPTS) were prepared in a precursor-monomer/NH₄OH water microemulsion system. Trimethylolpropane triacrylate (TMPTA)—a trifunctional monomer useful in manufacturing of coatings, inks and adhesives—and a corresponding photoinitiator (DAROCUR 1173) were entrapped inside the obtained microcapsules. MPTS was used to increase compatibility between TMPTA and the sol–gel precursors. As stability agent we added a “home made” product resulted from functionalization of poly (ethylene glycol) methyl ether (MPEG) with (3-isocyanatopropyl) triethoxysilane (NCOTEOS). Were obtained microcapsules containing incorporated monomer and having average particle size in range of 0.5–50 μm. Thermal analysis, morphology study and the increase of the silica microcapsules average diameter, measured by DLS technique confirm the monomer encapsulation.     

Improved biocompatibility of protein encapsulation in sol–gel materials

By using the fluorescent dye 6-propionyl-2-(N,N-dimethylamino) naphthalene (PRODAN) to monitor methanol generated during tetramethyl orthosilicate polymerization we have optimised the encapsulation of protein in silica sol–gel monoliths with respect to completion of hydrolysis and distillation in order to remove methanol such that protein can be added without denaturation. A minimum of 24 h at +4 °C was found to be required before hydrolysis is complete and 3–5 min of vacuum distillation at 50 °C and 300 mbar needed to remove methanol before the gel is formed. The biocompatibility of a tetramethyl orthosilicate sol–gel monolith was demonstrated by preserving the trimer protein allophycocyanin (APC) in its native form for up to 500 h. This obviates the previously essential requirement of covalently binding the trimer together in order to prevent dissociation into monomers and has enabled observation of native APC trimer in a sol–gel pore for the first time down to the single molecule level using combined fluorescence spectroscopy and confocal microscopy. The higher stability afforded by the protocol we describe could impact on the application of sol–gel materials to single-molecule studies of wider bearing such as protein folding and aggregation.     

Simultaneous formation of ferrite nanocrystals and deposition of thin films via a microwave-assisted nonaqueous sol–gel process

Combination of the surfactant-free nonaqueous sol–gel approach with the microwave technique makes it possible to synthesize Fe₃O₄, CoFe₂O₄, MnFe₂O₄, and NiFe₂O₄ nanoparticles of about 5–6 nm and with high crystallinity and good morphological uniformity. The synthesis involves the reaction of metal acetates or acetylacetonates as precursors with benzyl alcohol at 170 °C under microwave irradiation of 12 min. Immersion of glass substrates in the reaction solution results in the deposition of homogeneous metal ferrite films whose thickness can be adjusted through the precursor concentration. If preformed nickel nanoparticles are used as a type of curved substrate, the ferrite nanoparticles coat the seeds and form core–shell structures. These results extend the microwave-assisted nonaqueous sol–gel approach beyond the simple synthesis of nanoparticles to the preparation of thin films on flat or curved substrates.     

Selective oxidation of aromatic hydrocarbons by potassium and phosphorous-modified iron oxide–silica nanocomposite

Abstract  

Supported iron catalysts are active for hydrocarbon oxidation with H₂O₂, but the hydrogen peroxide dismutation is a shortcoming that may constrain their applications. Herein, we attempted to address this problem using potassium and phosphate-doped iron oxide–silica nanocomposite (KPFeSi) synthesized via sol–gel methods. The promoted silica–iron oxide nanocomposite has been characterized by elemental analyses, FTIR, X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) surface-size determination. The synthesized KPFeSi was an active catalyst in the low-temperature liquid phase oxidation of various alkyl aromatics with hydrogen peroxide in conversions of 31–78%. Furthermore, the direct oxidation of benzene into phenol using hydrogen peroxide has been achieved in the absence of any acid with this KPFeSi compound.     

From Inulin to Fructose Syrups Using Sol–Gel Immobilized Inulinase

The present work aims to provide the basic characterization of sol–gel immobilized inulinase, a biocatalyst configuration yet unexploited, using as model system the hydrolysis of inulin to fructose. Porous xerogel particles with dimensions in slight excess of 10 μm were obtained, yielding an immobilization efficiency of roughly 80%. The temperature– and pH–activity profiles displayed a broader bell-shaped pattern as a result of immobilization. In the latter case, a shift of the optimal pH of 0.5 pH units was observed towards a less acidic environment. The kinetic parameters estimated from the typical Michaelis–Menten kinetics suggest that immobilization in sol–gel did not tamper with the native enzyme conformation, but on the other hand, entrapment brought along mass transfer limitations. The sol–gel biocatalyst displayed a promising operational stability, since it was used in more than 20 consecutive 24-hour batch runs without noticeable decay in product yield. The performance of sol–gel biocatalyst particles doped with magnetite roughly matched the performance of simple sol–gel particles in a single batch run. However, the operational stability of the former proved poorer, since activity decay was evident after four consecutive 24-hour batch runs.     

Simple sol–gel route to synthesis of mesoporous TiO<Subscript>2</Subscript>

Mesoporous TiO₂ with a high specific surface area was prepared from titanium sulfate solution in a simple sol–gel route, where formamide was used as pH adjusting agent. TiO₂ had a high resistance to phase transformation, and maintained monophasic anatase after calcinating at 600 °C. The highest specific surface area achieved on the prepared samples is 231.90 m² g⁻¹ after calcinating at 450 °C.     

Sol–gel transitions of sodium montmorillonite dispersions by cationic end-capped poly(ethylene oxides) (surface modification of bentonites, IV)

Poly(ethylene oxides), PEO, end-capped with hexadecyl dimethylammonium, HDDMA⁺, and trimethylammonium hexyldimethylammonium, THA²⁺, changed distinctly the sol–gel transition of sodium montmorillonite dispersions. In the presence of HDDMA⁺-PEO 1,500 and 4,000, domains of sol with increased salt tolerance (c _k=550–1,000 mmol NaCl/l) were found at high polymer and low montmorillonite contents. The corresponding PEO of higher molar mass (20,000 and 35,000) led to extended fields of flocs. THA²⁺-PEO 1,500 formed attractive gels at polymer concentrations >2–5 g/l and montmorillonite contents >0.5%. These gels showed very high yield values. THA²⁺-PEO of higher molar mass acted as stabilizing agents. The salt tolerance was highest (300–750 mmol/l) in the presence of THA²⁺-PEO 20,000. The observed sol–gel diagrams reveal the interplay between polymer end-group fixation on the clay mineral particles, polymer conformation, and colloidal stabilization and destabilization mechanisms.

Characterisation of nickel–zinc ferrite/silica nanocomposites with low ferrite concentration obtained by an improved modified sol–gel method

The paper presents a study regarding the structure, morphology and magnetic behaviour of x% (Ni_0.65Zn_0.35Fe₂O₄)/(100 − x)% SiO₂ ferrimagnetic nanocomposites for low Ni–Zn ferrite concentration (x = 5, 10, 15, 20 and 30 mass percent) obtained by an improved modified sol–gel method. The obtained gels and nanocomposites have been characterized by fast Fourier transform-infrared (FT-IR) spectrometry, X-ray diffraction (XRD), transmission electron microscopy (TEM) and magnetic measurements (MM). The addition of a supplementary quantity of diol in the synthesis, corresponding to a molar ratio EG : TEOS = 1:1, and the control of the thermal treatment applied to the precursor xerogels tetraethylortosilicate (TEOS)–metal nitrates (MN)–ethylene glycol (EG) leads to fine (~2–9 nm), almost spherical Ni–Zn ferrite nanoparticles homogenously dispersed inside the amorphous SiO₂ matrix. TEM images reveal the fine nature and the narrow size distribution of the ferrite nanoparticles. Nanoparticles diameter increases with the ferrite concentration and with the annealing temperature. For all concentrations of ferrite in SiO₂ and all annealing temperature, we have obtained Ni_0.65Zn_0.35Fe₂O₄ ferrite as single phase (proven by XRD) in the amorphous silica matrix, only after a pre-treatment of synthesized gels, at 573 K, for 3 h. The magnetic behaviour of ferrite nanoparticles in quasi-static magnetic fields is very particular, depending on the annealing temperature and the ferrite content in silica matrix. We have obtained superparamagnetic behaviour for the nanocomposites, for a concentration of 30% ferrite in SiO₂ at high annealing temperature, of 1,273 K.     

Fabrication and performance of nanoporous TiO2/SnO2 electrodes with a half hollow sphere structure for dye sensitized solar cells

The incorporation of nano-crystalline semiconductors with novel kinds of ordered microstructure is a very important area of research in the field of dye sensitized solar cells. A sol–gel method involving hydrolysis of titanium isopropoxide was used to form TiO₂ nanoparticles on the surface of SiO₂ spheres. In this process, 1, 5, or 10 wt% of SnCl₂.2H₂O was added to the sol–gel solution. To prepare TiO₂/SnO₂ nanoparticles with a half hollow sphere structure, SiO₂ was removed with NaOH solution. The crystal phase, crystal shape, and surface properties of the metal oxide nanocrystals were studied by x-ray diffraction and scanning electron microscopy. The photovoltaic performance of the TiO₂/SnO₂ nanoparticles with half hollow sphere structures was measured. The dye sensitized solar cell using nanoporous TiO₂ as electrode materials exhibits an overall conversion efficiency of 7.36% with a light intensity of 100 mW/cm². The short circuit photocurrent (I_sc), open circuit photovoltage (V_oc), and conversion efficiency (η) of these solar cells were improved over conventional materials.