The styrylpyridine dye for the silane sol-gel transition studies by time-dependent fluorescence

A trans-4-(p-N,N-dimethylaminostyryl)-N-vinylbenzylpyridinium chloride (vbDMASP) fluorescence probe was optimized in ground and excited state as a function of change in the microenvironment polarity, using the Amsol HyperChem program package. In the calculations, protic and aprotic solvents were used. On this basis a change in the molecule geometry after excitation, depending on the surrounding solvent, was determined. Absorption and steady-state fluorescence spectra of vbDMASP in the solvent of different polarity and in the model water-glycerol solutions were also recorded. On the basis of Stokes' shift change with the Onsager polarity scale a change in the dipole moment of the probe during transition from ground to excited state, in protic and aprotic solvents was determined. Since during the sol-gel transition of tetraethylorthosilane in the acidic environment both polarity and viscosity of the microenvironment change the vbDMASP probe was applied and fluorescence time-resolved measurements were done. On this basis the correlations between the results of time-resolved measurements for the multichromophoric probe applied in the gelation process and molecular optimization data are discussed.     

Cobalt–silicon mixed oxide nanocomposites by modified sol–gel method

Cobalt–silicon mixed oxide materials (Co/Si=0.111, 0.250 and 0.428) were synthesised starting from Co(NO₃)₂·6H₂O and Si(OC₂H₅)₄ using a modified sol–gel method. Structural, textural and surface chemical properties were investigated by thermogravimetric/differential thermal analyses (TG/DTA), XRD, UV–vis, FT-IR spectroscopy and N₂ adsorption at −196 °C. The nature of cobalt species and their interactions with the siloxane matrix were strongly depending on both the cobalt loading and the heat treatment. All dried gels were amorphous and contained Co²⁺ ions forming both tetrahedral and octahedral complexes with the siloxane matrix. After treatment at 400 °C, the sample with lowest Co content appeared amorphous and contained only Co²⁺ tetrahedral complexes, while at higher cobalt loading Co₃O₄ was present as the only crystalline phase, besides Co²⁺ ions strongly interacting with siloxane matrix. At 850 °C, in all samples crystalline Co₂SiO₄ was formed and was the only crystallising phase for the nanocomposite with the lowest cobalt content. All materials retained high surface areas also after treatments at 600 °C and exhibited surface Lewis acidity, due to cationic sites. The presence of cobalt affected the textural properties of the siloxane matrix decreasing microporosity and increasing mesoporosity.     

Synthesis of silica–silver wires by a sol–gel technique

SiO₂–Ag wires were synthesized by a sol–gel technique. A two step approach was followed, focusing mainly on the effect of acid concentration on the first stage and processing temperature on the second. This acid-catalyzed reaction on the first stage yielded SiO₂–AgCl wires with diameters as low as 800 nm average, and lengths ranging up to 100 μm, as determined by LV-SEM and TEM. A thermal treatment at different temperatures on the second step, under H₂ atmosphere, yields silica–silver unidirectional structures. The chemical composition of these structures was determined by EDS, indicating the presence of Si, O and Ag. The transformation of the wires as a function of temperature under reducing atmosphere was followed by electron microscopy analysis. At 400 °C and above the silica starts to cover the reduced silver while maintaining the unidirectional conformation, suggesting a tendency to form silver wires covered by a silica layer.     

A synthetic hydrogel with thermoreversible gelation,III: an NMR study of the sol–gel transition

The sol–gel transition mechanism of a thermoreversible hydrogel composed of a copolymer comprising poly(N-isopropylacrylamide) and poly(ethylene glycol) (PNIPAAm–PEG) was studied by NMR. The ¹H– and ¹³C–NMR spectra measured on a PNIPAAm–PEG solution in 99.9% D₂O showed a remarkable line width broadening of the PNIPAAm block of more than that of the PEG block, during thermally induced hydrogel formation. This result suggested that the mobility of the PNIPAAm block is more restricted than that of the PEG block during gelation. A crosslinked polymer network formation was ascertained by a sudden reduction in the spin-lattice relaxation time (T₁) of the residual HDO proton during gelation. The temperature dependency of the T₁ values for the PNIPAAm and PEG blocks revealed that the microscopic condition of the PNIPAAm block in water was drastically changed during gelation, while that of the PEG block was unchanged. The experimental results from NMR supported the following gelation mechanism; that an aggregation of PNIPAAm blocks in the separate copolymers caused by hydrophobic interaction forms crosslinking points to give an infinite three-dimensional network structure. The hydrated PEG chains in the copolymers provide the network with a swelling property in water, and prevent the aggregation from causing a macroscopic phase separation.     

Renewable silica sol–gel derived carbon composite based glucose biosensor

An amperometric mediated glucose biosensor has been developed based on a sol–gel derived carbon composite material. Glucose oxidase and the mediator vinylferrocene have been immobilised within the porous, rigid and organically modified silicate network in the composite material. The organic group in the silicate network controls the hydrophobicity of the electrode surface and thus limits the wettability of the electrode surface. Various important fabrication factors controlling the biosensor performance have been investigated systematically. The glucose biosensor can be renewed easily in a reproducible manner by a simple polishing step and it has a long operational lifetime. Applicability of the biosensor has been demonstrated in real samples and the results obtained by this biosensor corroborate well with a classical UV spectrophotometric technique.     

TGA–FTIR study of a lead zirconate titanate gel made from a triol-based sol–gel system

A Pb(Zr,Ti)O₃ precursor gel made from a sol prepared using 1,1,1,-tris(hydroxymethyl)ethane, lead acetate and zirconium and titanium propoxides, stabilised with acetylacetone, was analysed using TGA–FTIR analysis. Decomposition under nitrogen (N₂) gave rise to evolved gas absorbance peaks at 215 °C, 279 °C, 300 °C and 386 °C, but organic vapours continued to be evolved, along with CO₂ and CO until 950 °C. The final TGA step in N₂ is thought to relate to decomposition of an intermediate carbonate phase and the final elimination of residues of triol or acetylacetonate species which form part of the polymeric gel structure. By contrast, heating in air promoted oxidative pyrolysis of the final organic groups at ≤450 °C. In air, an intermediate carbonate phase was decomposed by heating at 550 °C, allowing Pb(Zr,Ti)O₃ to be produced some 400 °C below the equivalent N₂ decomposition temperature.     

High temperature oxidation of chromium with nanometric ceria sol–gel coating

Isothermal oxidation behavior of chromium with and without nanometric sol-gel CeO2 coating is studied at 1000℃ in air. Scanning electron microscopy （SEM） and transmission electron microscopy （TEM） are used to examine the surface morphology and microstructure of their oxide films. It is found that ceria coating greatly improves the anti-oxidation property of chromium. Laser Raman spectrometer and X-ray diffraction spectrometer （XRD） are also used to study the stress level in oxide films formed on ceria-coated and ceria-free Cr. The difference in oxidation behavior is mainly attributed to the fact that ceria greatly reduces the growth speed and grain size of Cr2O3 film, and this fine grain-sized Cr2O3 film probably has better high temperature plasticity, i.e. oxide film can relieve parts of compressive stress by means of creeping. XRD and Raman testing results both show the stress declination due to nano-CeO2 application, and their deviation is analyzed conceming to the rare earth effect.     

Optochemical sensing by immobilizing fluorophore-encapsulating liposomes in sol–gel thin films

The chemical stability of optochemical sensors depends largely on the physiochemical properties of the supportive matrix of the sensor and on the method used to immobilize sensing reagents to the supportive matrix of the sensor. Leaking of physically immobilized sensing reagents from the matrix support decreases the stability of the sensor and its overall usefulness. Covalent immobilization eliminates leakage of the sensing reagent from the support but may lead to alteration of spectral properties and loss of analyte response. This paper presents a new method for physical immobilization of polar fluorescence dyes in a sensing support. The method is based on the immobilization of fluorescent dye encapsulating liposomes in a sol–gel film of micrometer thickness. The encapsulation of the dye molecules in the liposomes effectively increases the molecular dimensions of the sensing reagent, thus preventing its leakage from the matrix support. This paper describes the analytical properties of a pH sensor fabricated by immobilizing carboxyfluorescein-encapsulating liposomes in a sol–gel thin film. The sensor shows excellent stability with respect to dye leaking which in turn leads to high reproducibility and sensitivity of about 0.01 pH units. The linear dynamic range of the sensor is between pH 6 and 7.5 and its response time is at the sub-seconds time scale.     

Tryptophan photooxidation promoted by new hybrid materials prepared by condensation of naphthalene imides with silicate by the sol–gel process

Three novel hybrid organic/inorganic materials were synthesized from 4-substituted (NO₂, Br, H) 1,8-naphthalene imide-N-propyltriethoxysilane by the sol–gel process. These materials were obtained as a xerogel and partially characterized. The ability to photosensitize the oxidation and degradation of tryptophan indole ring by these materials was studied through photophysical and photochemical techniques. Although the derivatives containing Br and NO₂ as substituent do not cause efficient tryptophan photodamage, the hybrid material obtained from 1,8-naphthalic anhydride is very efficient to promote tryptophan photooxidation. By using laser flash photolysis it was possible to verify the presence of naphthalene imide transient radical species. The presence of oxygen causes an increase of the yield of radical formation. These results suggest that the mechanism of photodegradation of tryptophan occurs by type I, i.e. the transient radical (TrpH⁺) formed by the direct reaction of the triplet state of the naphthalene imide moiety with tryptophan. Thus a inorganic–organic hybrid material that can be used to promote the oxidation of biomolecules was obtained.     

Photopolymerized sol–gel monoliths for separations of glycosylated proteins and peptides in microfluidic chips

Photopolymerized silica sol–gel monoliths, functionalized with boronic acid ligands, have been developed for protein and peptide separations in polydimethylsiloxane microfluidic devices. Pore size characterization of the monoliths was carried out with SEM, image analysis, and differential scanning calorimetry to evaluate both the micron‐sized macropores and the nanometer‐sized mesopores. Monoliths were functionalized with boronic acid using three different immobilization techniques. Batch experiments were conducted to determine the capacity of the monoliths and selectivity toward cis‐diol‐containing compounds. Conalbumin was used as a model glycoprotein, and a tryptic digest of the glycoprotein horseradish peroxidase was used as a peptide mixture to demonstrate proof‐of‐concept extraction of glycoproteins and glycopeptides by the monoliths formulated in polydimethylsiloxane microfluidic chips. For proteins, fluorescence detection was used, whereas the peptide separations employed off‐line analysis using MALDI‐MS.     

A facile synthesis of highly water-soluble, core–shell organo-silica nanoparticles with controllable size via sol–gel process

A series of highly water-soluble organo-silica nanoparticles, ranging from 2 to 10 nm in diameter, were synthesized by the cohydrolysis and copolycondensation reactions. ω-methoxy(polyethyleneoxy)propyltrimethoxysilane (PEG6-9) and hydroxymethyltriethoxysilane (HMTEOS) mixtures were catalyzed by sodium hydroxide in the presence of surfactant benzethonium chloride (BTC) with various ratios of PEG6-9/HMTEOS at room temperature. The synthesized organo-silica nanoparticles possess a core–shell structure with a core of organo-silica resulting from HMTEOS and a monolayer shell of PEG6-9. The chemo-physical characteristics of the particles were studied by gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, ²⁹Si nuclear magnetic resonance (NMR), dynamic light scattering (DLS), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The molecular weight and particle size of the particles increased with increasing HMTEOS molar ratios. The richest HMTEOS composition for the water-soluble particles was found to be HMTEOS:PEG6-9 = 80:20, where the particles had a 6 nm diameter core and a 0.8 nm thick shell. We propose that these water-soluble organo-silica nanoparticles will be suitable for biomedical applications.     

Ionic transport phenomenon across sol–gel derived organic–inorganic composite mono-valent cation selective membranes

Organic–inorganic composite mono-valent cation selective membranes (MCSMs) were prepared by sol–gel under acidic conditions, in which sulfonic acid groups were introduced at the inorganic segment. Studies on physicochemical and electrochemical properties revealed their excellent mechanical, thermal, and oxidative stabilities, high conductivity, ion-exchange capacity, permselectivity for mono-valent cations, ionic diffusion and water transport number. These properties suggested the suitability of MCSMs, especially Si-65%, for electro-separation of Na⁺ from Ca²⁺, Mg²⁺, and Fe³⁺. The effect of electrolyte solution on the characteristics of the current–voltage (i–v) curve in MCSM was studied based on the concentration polarization. Electro-transport of different ions in terms of plateau length and concentration profiles for different ions in the solution phase, diffusion boundary layer and membrane phase were presented. Information obtained from i–v curve analysis were validated by electrodialysis (ED) experiments for individual or mixed electrolyte solutions. Electro-transport efficiency and separation factor of different ions for MCSM and Nafion117 (N117) membranes were compared, which suggested suitability of MCSMs for separating cations.     

An optical pH sensor with extended detection range based on fluoresceinamine covalently bound to sol–gel support

An optical pH sensor was developed based on the fluorophor, fluoresceinamine isomer II (FA), covalently immobilized in a sol–gel matrix. This sol–gel matrix was created by the copolymerization of two precursors, methyltriethoxysilane (MTES) and 3-glycidoxypropyltrimethoxysilane (GPTMS), in an ethanolic solution. Fluoresceinamine was covalently bound to the glycidoxypropyl chain group of GPTMS, thereby preventing it from leaching. Moreover, the immobilization of the fluoresceinamine also extended its linear detection range. The sensor showed good repeatability, a short response time of less than 8 s, high long-term stability and no temperature effect in the biologically relevant range. In the pH range of 4–10, the sensor was very sensitive and its linear range was found to be between pH 6 and 9 (R² = 0.995).     

Effect of the sol–gel route on the textural characteristics of silica imprinted with Rhodamine B

A series of silica xerogels that support Rhodamine B as a template were synthesized using distinct sol–gel routes, namely, acid‐catalyzed routes, a base‐catalyzed route, acid‐catalyzed with base‐catalyzed (two steps) hydrolytic routes, and a FeCl₃‐catalyzed nonhydrolytic route. The extraction methods (thermal, Soxhlet, water washing, and ultrasound) were also evaluated. The resulting xerogels were characterized through porosimetry using nitrogen adsorption/desorption. The samples were further analyzed through small‐angle X‐ray scattering, Fourier transform infrared spectroscopy, and SEM. The preparation route affected the materials’ textural properties. Extraction was optimized using acid and two‐step routes. The acid route from Rhodamine B to Rhodamine 6G generated the highest selectivity factor (2.5). The nonhydrolytic route produced the best imprinting factor. Competitive adsorption was also used, from which the approximate imprinting factor was 2. The cavity shape generated during the production of the imprinted silica dictates the adsorption behavior, not the magnitude of the surface area.     

Biodegradable and stimuli sensitive amphiphilic graft copolymers and their sol–gel phase transition behavior

pH and temperature‐sensitive biodegradable poly(β‐aminoester)‐graft‐poly(ε‐caprolactone)‐block‐methoxy poly(ethylene glycol) (PBAE‐g‐PCL‐b‐mPEG) amphiphilic graft copolymers with different molecular weights were synthesized. The structure of these copolymers was adjusted by varying the feed ratios of ε‐caprolactone to methoxy poly(ethylene glycol)s (mPEG), amine and diacrylate monomer amounts and the molecular weight of mPEG. Aqueous solutions of these copolymers formed micelles at lower concentrations; however, the concentrated solutions showed a reversible sol–gel transition property depending on both pH and temperature changes under representative physiological conditions (pH 7.4, 37°C). The effects of the molecular weight of pH‐sensitive poly(β‐aminoester) block and mPEG group, the hydrophobic to hydrophilic block ratio (PCL/mPEG) and the concentration of the copolymer on the sol–gel transition were investigated. Proton nuclear magnetic resonance (¹H NMR) and gel permeation chromatography measurements were used to characterize the structure of the synthesized copolymers. The self‐assemble behavior and critical micelle concentration of the amphiphilic copolymers were estimated in phosphate buffer solution using fluorescence spectroscopy. The gelling behavior was measured by using tube inversion method. At pH 7.4, all copolymer solutions prepared 20 wt% concentration indicated sol–gel transition with increasing temperature. In vitro degradation experiments displayed that the synthesized graft copolymers mostly degraded hydrolytically within 20 days under physiological conditions. In order to investigate the potential application of synthesized hydrogels in drug delivery, Methylene Blue was used and approximately 70% of the loaded amount was released in 120 hr. The findings indicate that obtained graft copolymers can be used as injectable biodegradable carriers for pharmaceutical drugs. Copyright © 2015 John Wiley & Sons, Ltd.     

Polyvinylimidazole/sol–gel composite as a novel solid‐phase microextraction coating for the determination of halogenated benzenes from aqueous solutions

A polyvinylimidazole/sol–gel composite is proposed as a novel solid‐phase microextraction fiber to extract five halobenzenes from the headspace of aqueous solutions in combination with gas chromatography with mass spectrometry. The prepared fiber was characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. The obtained results showed that porous polyvinylimidazole/sol–gel composite was chemically deposited on fused silica fiber. The effect of important extraction parameters including extraction temperature, extraction time, and salt content were investigated. The optimum conditions were as follows: extraction temperature 25°C, extraction time 20 min, and salt concentration 30 w/v%. Detection limits and relative standard deviations of the developed method for halogenated benzenes were below 0.1 pg/mL and 15%, respectively. Repeatability of the proposed method, explained by relative standard deviation, varied between 5.48 and 9.15% (n = 5). The limits of detection (S/N = 3) ranged between 0.01 and 0.10 ng/L using gas chromatography with mass spectrometry with selected ion monitoring mode. For real sample analysis, three types of water samples with different matrices (ground, surface, and tap water) were studied. The optimized procedure was applied to extraction and method validation of halogenated benzenes in spiked water samples.     

The alkoxide sol–gel process in the calcium phosphate system and its applications†

A single calcium glycolate was synthesized. The alkoxide was stable under ambient atmosphere. The calcium glycolate, phosphoric acid and P(OH) _x (OEt)_{3− x} were used as the precursors. Acetic acid was used as a reagent to modify the calcium glycolate and to change the acidity of the mixtures of the precursors. Mixtures of the calcium glycolate and phosphoric acid in a Ca/P ratio of 1.67 showed unusual sol–gel behavior. A transparent gel could be formed depending on the content of acetic acid and the extent of stirring. The behavior is attributed to a high viscosity and a large molecular size of the ethylene glycol solvent, leading to a strong dependence of the reactions in the mixtures on the diffusion process, greatly affected by stirring. When the mixtures of the calcium glycolate and PO(OH) _x (OEt) _{3− x} contained acetic acid at an acetic acid/Ca ratio of 3, stable alkoxide solutions with Ca/P ratios of 1.0, 1.5 and 1.67 could be formed. Different calcium phosphate compounds and hydroxyapatite coatings on alumina substrates could easily be formed from the alkoxide solutions. The chemical homogeneity provided by the alkoxide route leads to easy formation of the required products. Copyright © 1999 John Wiley & Sons, Ltd.     

Preparation of stable second‐order nonlinear optical films by sol–gel technique

A stable nonlinear optical (NLO) film containing “T” type alkoxysilane dye was prepared by sol–gel technology. This crosslinked “T” type alkoxysilane dye was synthesized and fully characterized by FTIR, UV–Vis spectra, and ¹H‐NMR. Followed by hydrolysis and copolymerization processes of the alkoxysilane with γ‐glycidoxypropyl trimethoxysilane (KH560) and tetraethoxysilane (TEOS), high quality inorganic–organic hybrid second‐order NLO films were obtained by spin coating. The “T” type structure of the alkoxysilane was found to be effective for improving the temporal stability of the optical nonlinearity due to the reduction in the relaxation of the chromophore in the film materials. Copyright © 2009 John Wiley & Sons, Ltd.     

Polymer–filler interactions in sol–gel derived polymer/silica hybrid nanocomposites

The effect of polymer–filler interaction on solvent swelling and dynamic mechanical properties of the sol–gel derived acrylic rubber (ACM)/silica, epoxidized natural rubber (ENR)/silica, and poly (vinyl alcohol) (PVA)/silica hybrid nanocomposites has been described for the first time. Tetraethoxysilane (TEOS) at three different concentrations (10, 30, and 50 wt %) was used as the precursor for in situ silica generation. Equilibrium swelling of the hybrid nanocomposites in respective solvents at ambient condition showed highest volume fraction of the polymer in the swollen gel in PVA/silica system and least in ACM/silica, with ENR/silica recording an intermediate value. The Kraus constant (C) also followed a similar trend. In dynamic mechanical analysis, the storage modulus dropped at higher strain (>1%), which indicated disengagement of polymer segments from the filler surfaces. This drop was maximum in ACM/silica, intermediate in ENR/silica, and minimum in PVA/silica, both at 50 and 70 °C. The drop in modulus with theoretical volume fraction of silica (ϕ) was interpreted with the help of a Power law model ΔE′ = a₁ϕ, where a₁ was a constant and b₁ was primarily a filler attachment parameter. Strain dependence of loss modulus was observed in ACM/silica hybrid nanocomposites, while ENR/silica and PVA/silica nanocomposites showed almost strain‐independent behavior. The storage modulus showed sharp increase with increasing frequency in ACM/silica system, while that was lower in both ENR/silica (at higher frequency) and PVA/silica systems (in the entire frequency spectrum). The increase in modulus with ϕ also followed similar model ΔE′ = a₂ϕ proposed in the strain sweep mode. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2399–2412, 2005     

The sol–gel synthesis of perovskites by an EDTA/citrate complexing method involves nanoscale solid state reactions

Nowadays, sol–gel procedures are well established in the synthesis of complex oxides as they allow to obtain phase pure products and to control precisely their stoichiometry. This quality makes them a tool of choice for the preparation of perovskite-type oxides. To optimize the functional properties of these materials, it is essential to set accurately their possible complex stoichiometries. However, details of the formation of the perovskite crystal remain obscure. Different stages of an ethylene-diamine-tetraacetic acid (EDTA)/citrate-gel based synthesis process for mixed conducting (Ba_0.5Sr_0.5)(Fe_0.8Zn_0.2)O_3−δ of cubic perovskite structure are elucidated. The combination of analytical transmission electron microscopy with X-ray diffraction reveals that the perovskite-type oxide is formed already at moderate temperatures at around 700 °C via nanoscale solid state reactions between finely-dispersed crystalline intermediates identified as a spinel and a carbonate. The reaction scheme, however, is intricate and includes stuffed tridymite structures as transient phases. The ultrafine intermixing of extremely small reactants makes EDTA/citrate-gel based procedures superior to classical solid state routes with respect to applications that demand phase purity and stoichiometry control.