B<Subscript>12</Subscript>-induced formation of stishovite in sol–gel produced amorphous SiO<Subscript>2</Subscript> matrices

Composites containing vitamin B₁₂ (cyanocobalamin) dispersed in amorphous silica xerogel were studied structurally as a function of annealing temperature. Silica xerogel samples were prepared by the sol–gel method using an ethanol:H₂O:TEOS molar ratio of 4:11.6:1 and loaded with cyanocobalamin. We found that the structure of the cobalamin is unaltered, although decoordination of the benzimidazole nucleobase of B₁₂, whereas the amorphous quartz structure of the matrix is maintained under heat-treatment without low-cristobalite phase transformation, typically of this kind of materials. We found in our samples partial crystallization of the glass matrix in form of stishovite obtained at very lower pressure than those specified by the phase diagram, and temperatures about 400 °C due to the presence of vitamin B₁₂. The presence of stishovite is corroborated by the Rietveld refinement method.     

Electroentrapment of Polyaniline in [3-(2,3-Epoxypropoxy)propyl]trimethoxysilane-Derived Xerogel: A Facile Methodology Towards Molecularly Imprinted Xerogels

A novel polyaniline (PANI)/silica hybrid composite, as a selective coating, was synthesized by the simultaneous sol–gel process and in situ electro-polymerization of aniline and employed for imprinting naproxen. The synthesized composite was chemically bonded inside a copper tube for online capillary microextraction (CME) in combination with high-performance liquid chromatography (HPLC). The copper tube was intended for use as an unbreakable substrate for CME and the HPLC injection loop. Four copper tubes containing different coatings were prepared accordingly to achieve the most appropriate extracting medium for naproxen. Coating 1 was a xerogel of [3-(2,3-epoxypropoxy)propyl]trimethoxysilane (EPPTMOS) synthesized by sol–gel technology and PANI as a conductive polymer (CP) was electrochemically prepared as coating 2. Coating 3 (sol–gel–CP) included the electroentrapped PANI into the xerogel of EPPTMOS, representing the non-imprinted xerogel (NIX) and coating 4, the molecularly imprinted xerogel (MIX), contained entrapped PANI into the xerogel of EPPTMOS, while naproxen template was already imprinted into the composite structure. Extraction efficiency of MIX towards naproxen was about 4–30 times greater than the others. Electroentrapped PANI into the silica xerogel might be responsible for the enhanced extraction efficiency, originating from high surface area of PANI as well as its role in π–π interactions. The mild conditions used in this process enabled the incorporation of organic species such as PANI into the silica particles without degradation. The main advantage of addition of PANI during the sol–gel process was the reduction of gelation time from more than a week to a few hours and, therefore, achieving a facile and controllable method for preparation of selective media. Furosemide, clodinafop-propargyl and haloxyfop-etotyl were also selected to investigate selectivity of the prepared MIX towards naproxen. Ratio of MIX/NIX, a criterion of selectivity, was 3.8, 1.1, 1.2, and 1.4 for naproxen, clodinafop, haloxyfop and furosemide, respectively. The linearity of the HPLC method for naproxen was in the range of 10–1,000 µg L^?1. The limit of detection value was found to be 5 µg L^?1 and RSD % of 2.9 (n = 5) was obtained. Real samples such as blood plasma and urine species were analyzed by the developed method and the relative recovery percentages of 76 and 94 % were obtained for the spiked samples.     

A Polypyrrole-Based Sorptive Microextraction Coating for Preconcentration of Malathion from Aquatic Media

A new micro-solid phase extraction method was developed by combining solid-phase extraction and stir bar sorptive extraction to benefit from the advantages of both techniques. A polypyrrole coating was electrochemically synthesized on the surface of an already used graphite furnace, employed in electro-thermal atomic absorption spectroscopy. The cylindrical geometry of the graphite tube provided a rather huge surface area, suitable for sorptive extraction. The novel sorbent coating was examined as an extracting medium to isolate malathion. Effects of different parameters such as extraction time, salt concentration, sample volume, desorption solvent and time were investigated and optimized. Under the optimized conditions, the limit of detection and limit of quantification of the developed method were 5 and 20?ng?L^?1, respectively. The calibration curve showed linearity in the range of 0.1?C100???g?L^?1 (R ²?=?0.9968). The precision was evaluated at 0.1 and 1???g?L^?1 concentration levels and relative standard deviations (n?=?3) were found to be 10 and 7%, respectively. The developed method was successfully applied to the extraction of malathion from real river water and tap water samples, and relative recoveries at the spiked level of 0.1???g?L^?1 were 94 and 97%, respectively.     

Annealing Behavior of Silica Gel Powders Modified with Silver Crystalline Aggregates

The annealing behaviour of silica powders added with silver, prepared by the sol-gel method, was studied using X-ray diffraction. Partial crystallization of amorphous SiO₂ samples as low as 600°C has been observed. For that, silver needed to be added to the precursor solution in such a way that it formed aggregates. Silica xerogel samples were prepared using a molar ratio ethanol/H₂O/TEOS of 4:11.6:1 and loaded with silver in three different ways: in the form of silver nitrate, silver chloride, or chemically synthesised silver fine particles. The microstructure of the silica xerogel powders was studied as a function of annealing temperature. Attention was paid to the evolution of the glass matrix as well as the silver aggregates in the SiO₂ matrix. Partial crystallization of the glass matrix was achieved at temperatures much lower than those specified by the phase diagram, independently of preparation method of the silver aggregates.     

Structural Study of Silica Xerogel Composites Containing Pd Aggregates

Composites containing Pd aggregates dispersed in amorphous silica are of interest from both a fundamental and applied point of view because of their attractive catalytic properties. The silica powders added with palladium, prepared by the sol-gel method, were studied using X-ray diffraction, and IR- and UV-Vis-spectrophotometry. Silica xerogel samples were prepared using a ethanol/H₂O/TEOS molar ratio of 4:11.6:1 and loaded with sodium tetrachloropalladate. The silica xerogel microstructure of the powders was studied as a function of annealing temperature. Attention was paid to the evolution of the glass matrix as well as the palladium aggregates in the SiO₂ matrix. We found in our samples partial crystallization of the glass matrix in form of quartz and cristobalite phases with palladium oxide and metallic palladium phase at 1000^∘C. The Rietveld refinement method was used in order to determine the percentage of the phase contents.     

Blue-shifted fluorescence spectrum in silica xerogels with incorporation of extract’s leaves

The spectral characteristics of different kind of leaves extracts fluorescence embedded in silica xerogel matrix under structural evolution promoted by heat treatment was studied. We obtain a higher PSII thermostability for extract of leaves, rich in chlorophyll such as spinach, made in darker condition than extract of leaves made in lighter (non-dark) conditions, both embedded in xerogel matrix, which remain bioactive over a very long period of time. In other kind of leaves such geranium, after chlorophyll decomposition the quenching center are formed at temperatures about 800 °C, whereas in buxus sempervirens leaves fluorescing aggregates remain in temperatures as high as 1,000 °C, when their are embedded in silica xerogel matrix. In general blue-shift fluorescence is observed in all cases indicating the PSII denaturizing and formation of fluorescing aggregates in relatively high temperatures.     

Sorption equilibria of CO2 on silica-gels in the presence of water

The sorption equilibria of carbon dioxide on three types of silica gel (SG) with different pore size distributions in the presence of water were studied experimentally using a volumetric method at 275?K with pressures from 0 to 3.7?MPa. Both the pore size distribution of the silica gel and the quantity of pre-sorbed water impact the formation of the CO₂ hydrates. For wet silicon gel A(SG-A) with water loading ratio of 0.75, the highest CO₂ sorption was about 2.5?mmol of CO₂ per gram of dry sorbent at 275?K. Similarly, the highest sorption was about 2.7?mmol for wet SG-B with R _w =0.81. However, CO₂ hydrate did not form on the wet surface of SG-C due to its large pore sizes.     

A crystal chemical study of stishovite

The crystal structure of stishovite, SiO₂, a = 4.1773(1), c = 2.6655(1)Å, space group $\text{P4}{}_2\text{mnm}$, with Z = 2, has been refined with 217 graphite-monochromatized MoKα data (2θ max = 120°) to R = 0.012 (R_w = 0.014). Electron deformation density maps show a modest accumulation of charge density ascribed to partial covalent bonding in both the equatorial and axial bonds together with a delocalization of density in and around the shared octahedron edges with the shorter equatorial bonds showing higher peaks (0.47 eÅ^?3) than the longer axial bonds (0.29 eÅ^?3). Net atomic charges for Si and O determined by a population and κ refinement by varying the occupancy and shape of their valence shells are +1.71 and ?0.86(15) e, respectively. The observed structural distortions and charge distributions conform with the results of ab initio molecular orbital calculations undertaken on molecules designed to mimic the local geometry of the structure of stishovite. The SiO bond length (1.73 Å) calculated for a neutral molecule with 6-coordinated Si is slightly shorter than that observed on the average (1.76 Å) for silicates and molecular crystals with 6-coordinated Si. In addition, the calculated geometrical parameters of two edge-sharing octahedra agree to within ～5% of the observed geometry of stishovite. The calculated charge on Si is indicated to increase with coordination number and bond length in agreement with the larger net charge recorded for stishovite as compared with that recorded for α-quartz, Q(Si) = +1.0 e. Observed deformation maps are compared with theoretical ones calculated in the equatorial plane of the 6-coordinated molecule using molecular orbital theory with a split valence s,p basis supplemented with d-type polarization functions on Si.     

Active media based on SiO2 matrices with incorporated molecules of rhodamine dyes

SiO₂ matrices with incorporated molecules of Rhodamine 6G and Rhodamine 800 laser dyes were synthesized by the sol?Cgel method. The spectral properties of the matrices were studied. It was found that the incorporation of Rhodamine 6G molecules into the xerogel network decreased their association. The influence of the molar ratio of water and formamide on the transparency of the samples was studied. Laser emission of SiO₂ matrices activated with Rhodamine 800 was obtained in the near infrared spectral range. The values of laser spectrum halfwidth of SiO₂ matrices with Rhodamine 6G and Rhodamine 800 are 2?C6?nm. A well-defined structure of the laser emission spectra of the matrices at microsecond pumping duration testifies to their relatively high optical quality.     

Preparation of a portable point-of-care in vitro diagnostic system, for quantification of canine C-reactive protein, based on a magnetic two-site immunoassay

In this study, characterization of the binding kinetics and optimization of a magnetic permeability based point-of-care (POC) immunoassay system for quantification of canine C-reactive protein (cCRP) is described. The reagent is based on a two-site heterogeneous immunoassay system utilizing conjugated superparamagnetic nanoparticles (SPION) and silica particles, both particles carrying covalently linked antibodies directed to the cCRP analyte. Detection is carried out using a magnetic permeability-based small instrument, adjusted in order to apply it in a POC setting near the patients. The kinetic parameters are characterized and applied in the final design of the assay system. In the cCRP system studied, 90 % of the binding between immobilized solid-phase silica antibody and cCRP is complete after only 15 s, and 30 s for the binding between the antibody on the SPION and the bound cCRP on the silica particle. Additionally, the binding rate constants are determined to be 149 and 30 M^?1s^?1, respectively. The analytical sensitivity, clinical sensitivity, and imprecision verifies the clinical usefulness of the system. Also, quantification of cCRP, using the system described, in dog clinical samples from mixed breeds shows a high correlation to a commercially available comparative cCRP ELISA system (y?=?0.98?×?+3.2, R ²?=?0.98, n?=?47). The immunoassay system described can thus provide the veterinarian a valuable tool for rapid diagnosis and monitoring of inflammatory diseases in dogs in a setting near the patients.     

Synthesis,conformational preferences in gas and solution,and molecular gear rotation in 1-(dimethylamino)-1-phenyl-1-silacyclohexane by gas phase electron diffraction (GED), LT NMR and theoretical calculations

1-(Dimethylamino)-1-phenyl-1-silacyclohexane 1, was synthesized, and its molecular structure and conformational properties studied by gas-phase electron diffraction (GED), low temperature ¹³C NMR spectroscopy and quantum-chemical calculations. The predominance of the 1-Ph_ax conformer (1-Ph_eq:1-Ph_ax ratio of 20:80%, ΔG°(317?K)?=??0.87?kcal/mol) in the gas phase is close to the theoretically estimated conformational equilibrium. In solution, low temperature NMR spectroscopy showed analyzable decoalescence of C_ipso and C(1,5) carbon signals in ¹³C NMR spectra at 103?K. Opposite to the gas state in the freon solution employed (CD₂Cl₂/CHFCl₂/CHFCl₂?=?1:1:3), which is still liquid at 100?K, the 1-Ph_eq conformer was found to be the preferred one [(1-Ph_eq: 1-Ph_ax?=?77%: 23%, K?=?77/23?=?2.8; ?ΔG°?=??RT ln K (at 103?K)?=?0.44?±?0.1?kcal/mol]. When comparing 1 with 1-phenyl-1-(X)silacylohexanes (X?=?H, Me, OMe, F, Cl), studied so far, the trend of predominance of the Ph_ax conformer in the gas phase and of the Ph_eq conformer in solution is confirmed.     

Charged silsesquioxane used as a vehicle for gold nanoparticles to perform the synthesis of catalyst xerogels

The water soluble charged silsesquioxane that contains the bridged 1,4-diazoniabicyclo[2.2.2]octane chloride group, was used as stabilizing agent and size controller in the synthesis of gold nanoparticles smaller than 15?nm in aqueous medium. The gold nanoparticle dispersion was converted in solid powder form by evaporation. This powder presented organized structure imposed by the presence of charged organic group, similar to organized structure already observed for pure silsesquioxane. The gold nanoparticles in solid powder form presented high storage stability for several months, at ambient conditions, and can be completely redispersed in water again. After redispersion, the optical properties of gold nanoparticles, observed by ultra-violet and visible spectroscopy, and their morphological characteristics, investigated by transmission electron microscopy, are preserved. The gold nanoparticle aqueous dispersion was used as a vehicle of nanoparticles in the synthesis of sol?Cgel silica based hybrid material. This xerogel was characterized by N₂ adsorption?Cdesorption isotherms, showing 260?m²g^?1, and it was applied in a satisfactory way as catalyst for p-nitrophenol reduction to p-aminephenol.     

Ni-based xero- and aerogels as catalysts for nitroxidation processes

Porous nanocomposites made out of nickel dispersed on silica or alumina matrices were prepared as prospective catalysts for the nitroxidation of hydrocarbons in the form of aerogel or xerogel by adopting either a supercritical or a conventional gel drying procedure. The structural and textural features of the materials were investigated by X-ray diffraction, transmission electron microscopy and N₂ physisorption and combined to the acid/base and reducibility data as deduced by adsorption microcalorimetry and temperature programmed reduction (TPR) profiles. The alumina-based samples are made out of nanocrystalline nickel aluminate and are mesoporous, although the aerogel has larger pore volumes and surface area than the xerogel. On the other hand, in the silica-based samples nickel oxide nanocrystals are dispersed on amorphous silica, the size of the nanocrystals being around 5 nm in the microporous xerogel and 14 nm in the mainly mesoporous aerogel. TPR data point out that the alumina-based samples have similar reducibility, whereas significant differences were observed in the silica-supported composites, the NiO–SiO₂ aerogel exhibiting improved reducibility at low temperature. The NO-catalyst interaction was monitored by temperature programmed NO reaction coupled to mass spectrometry and preliminary tests on the use of the NiO–SiO₂ xerogel and aerogel nanocomposites for the catalytic nitroxidation of 1-methyl-naphthalene to 1-naphthonitrile were obtained in a fixed-bed continuous-flow reactor. The data indicate that the aerogel exhibits larger selectivity than the corresponding xerogel, pointing out the importance of tuning the sol–gel parameters in the design of porous composite materials for catalytic applications.     

Determination of mercury(II) in water samples using dispersive liquid-liquid microextraction and back extraction along with capillary zone electrophoresis

We have developed a method for the determination of mercury in water samples that combines dispersive liquid-liquid microextraction (DLLME) with back-extraction (BE) and detection by capillary zone electrophoresis. DLLME is found to be a simple, cost-effective and rapid method for extraction and preconcentration. The BE procedure is based on the fact that the stability constant of the hydrophilic chelate of Hg(II) with L-cysteine is much larger than that of the respective complex with 1-(2-pyridylazo)-2-naphthol. Factors affecting complex formation and extraction efficiency (such as pH value, concentration of the chelating agent, time of ultrasonication and extraction, and type and quantity of disperser solvent) were optimized. Under the optimal conditions, the enrichment factor is 625, and the limit of detection is 0.62???g?L^?1. The calibration plot is linear in the range between 1 and 1,000???g?L^?1 (R ²?=?0.9991), and the relative standard deviation (RSD, for n?=?6) is 4.1%. Recoveries were determined with tap water and seawater spiked at levels of 10 and 100???g?L^?1, respectively, and ranged from 86.6% to 95.1%, with corresponding RSDs of 3.95?C5.90%.

Tailor-made chiral pyranopyrans based on glucose and galactose and studies on self-assembly of some crystals and low molecular weight organogel (LMOG)

InCl₃ catalyzed reactions of 2-C-acetoxymethylglycal derivatives with different phenolic compounds resulted in the formation of sugar based pyranoarenopyrans in good yields and moderate to excellent diastereoselectivity in favor of 10aR- or 12aR- or 14aR-products. One of the synthesized compounds, viz. (2R,3R,12aR)-2,3,5,12a-tetrahydro-2-methoxymethyl-3-methoxypyrano[2,3-b]naphtho[1,2-e]pyran gelated polar solvents like MeOH, EtOH and non-polar solvents like pentane, hexane, heptane, octane, pet. ether, etc. The SEM picture of the corresponding hexane xerogel exhibited a rare type of microtubular gel assembly, whereas the SEM pictures of MeOH and pet. ether xerogels showed different types of three-dimensional network. Study of the MeOH gel by Fluorescence spectroscopy, ¹H NMR and X-ray powder diffraction analysis indicated that the supramolecular assembly in the MeOH gel can be attributed to π-stacking. The crystal packing of (2R,3R,10aR)-3,10a-dihydro-2-methoxymethyl-3-methoxy-7-methyl-2H,5H-pyrano[2,3-b][1]benzopyran, a benzopyran analogue of the above organogelator was stabilized by C-H?O and C-H?π hydrogen bonds forming one-dimensional columns parallel to the [001] direction, whereas the corresponding benzopyran 3-epimer showed only C-H?O hydrogen bonds among the molecules.     

Effects of sodium salicylate on the microstructure of a novel zwitterionic gemini surfactant and its rheological responses

Solutions of a novel zwitterionic gemini surfactant, 1,2-bis[N-ethyl-N-(sodium 2-hydroxyl-3-sulfopropyl)-dodecyl-ammonium] ethane betaine (DBA_2-12), in presence of sodium salicylate (NaSal) can form wormlike micelles and thereby show a viscoelasticity. At low molar ratio of NaSal/DBA_2-12, R?≤?0.3, the systems behaved like a Newton fluid. However, as the R increased, the systems exhibited shear-thinning behavior. The zero-shear viscosity η ₀ increased dramatically with increasing R and peaked with R?=?0.8 at a value of 18.8 Pa s. This was attributed to the formation of long wormlike micelles (1.6?～?2.9 μm) and network structure confirmed by scanning electron microscopy. Further increase in R beyond 0.8 resulted in a slight decrease in η ₀, which may be caused by the branched micelles formed at high salt concentration. In addition, the tested systems showed a Maxwellian behavior at lower frequencies, but deviated from the Maxwellian mode at higher frequencies.     

Simultaneous Determination of Metformin and Glyburide in Tablets by HPTLC

Simultaneous determination of two antidiabetic drugs, metformin and glyburide, in pharmaceutical tablet formulations were investigated. Normal phase thin layer chromatography plate (silica gel 60 F₂₅₄) was used as stationary phase and water/methanol/ammonium sulfate (2/1/0.5 w/v) as mobile phase to determine two pharmaceutically active ingredients, in three different formulations of Glucovance^®. This system gave a good resolution for metformin (R _f value of 0.43 ± 0.01) and glyburide (R _f value of 0.64 ± 0.02). Determination was by densitometry in the absorbance mode at 237 nm. The linear regression data for the calibration plot showed a good relationship with r = 0.99581 and 0.99982 for metformin and glyburide, respectively. The method was validated for precision and recovery. The limits of detection and quantification were 25.24 and 84.12 ng spot^?1 for metformin and 12.26 and 40.86 ng spot^?1 for glyburide, respectively. Stability study has been carried out for samples and standard solutions.     

Solid-phase extraction of lead(II) ions using multiwalled carbon nanotubes grafted with tris(2-aminoethyl)amine

Multiwalled carbon nanotubes were grafted with tris(2-aminoethyl)amine (MWCNTs-TAA) and employed for solid phase extraction and preconcentration of trace lead ions prior to its determination by inductively coupled plasma optical emission spectrometry. The material was characterized by FT-IR and Raman spectroscopy, thermosgravimetric and elemental analysis. The effects of pH value, shaking time, sample volume, elution conditions and potentially interfering ions were investigated. Under the optimum conditions, the maximum adsorption capacity is 38?mg?g^?1 of Pb(II), the detection limit is 0.32?ng?mL^?1, the enrichment factor is 60, and the relative standard deviation is 3.5% (n?=?6). The method has been applied to the preconcentration of trace amounts of Pb(II) in environmental water samples with satisfactory results.

Gas-phase chemiluminescent determination of inorganic selenium in water samples following flow injection hydride generation and cryotrapping

We describe a method for the determination of inorganic selenium in water samples via gas-phase chemiluminescence (GPCL). Se(IV) was first derivatized with 4-nitro-o-phenylenediamine to form 5-nitropiazselenol. The latter was decomposed by persulfate through photocatalytic oxidation to give Se(VI), which was reduced to Se(IV). Selenium hydride was generated from Se(IV) through reduction with sodium borohydride and then preconcentrated using cryotrapping. The cryotrapped hydride was evaporated and carried to a reaction chamber by a stream of helium, where it produced GPCL as a result of ozonation. The method exhibits a wide linear calibration range (from 0.5?μg?L^?1 to 1.0?mg?L^?1) with a detection limit of 0.12?μg?L^?1 (for n?=?11), and a relative standard deviation of 3.90?% (at n?=?11) at 5.0?μg?L^?1 level of selenium. The method was applied to the determination of inorganic selenium in water samples and gave satisfactory results.

Electroentrapment of Polyaniline in [3-(2,3-Epoxypropoxy)propyl]trimethoxysilane-Derived Xerogel: A Facile Methodology Towards Molecularly Imprinted Xerogels

A novel polyaniline (PANI)/silica hybrid composite, as a selective coating, was synthesized by the simultaneous sol–gel process and in situ electro-polymerization of aniline and employed for imprinting naproxen. The synthesized composite was chemically bonded inside a copper tube for online capillary microextraction (CME) in combination with high-performance liquid chromatography (HPLC). The copper tube was intended for use as an unbreakable substrate for CME and the HPLC injection loop. Four copper tubes containing different coatings were prepared accordingly to achieve the most appropriate extracting medium for naproxen. Coating 1 was a xerogel of [3-(2,3-epoxypropoxy)propyl]trimethoxysilane (EPPTMOS) synthesized by sol–gel technology and PANI as a conductive polymer (CP) was electrochemically prepared as coating 2. Coating 3 (sol–gel–CP) included the electroentrapped PANI into the xerogel of EPPTMOS, representing the non-imprinted xerogel (NIX) and coating 4, the molecularly imprinted xerogel (MIX), contained entrapped PANI into the xerogel of EPPTMOS, while naproxen template was already imprinted into the composite structure. Extraction efficiency of MIX towards naproxen was about 4–30 times greater than the others. Electroentrapped PANI into the silica xerogel might be responsible for the enhanced extraction efficiency, originating from high surface area of PANI as well as its role in π–π interactions. The mild conditions used in this process enabled the incorporation of organic species such as PANI into the silica particles without degradation. The main advantage of addition of PANI during the sol–gel process was the reduction of gelation time from more than a week to a few hours and, therefore, achieving a facile and controllable method for preparation of selective media. Furosemide, clodinafop-propargyl and haloxyfop-etotyl were also selected to investigate selectivity of the prepared MIX towards naproxen. Ratio of MIX/NIX, a criterion of selectivity, was 3.8, 1.1, 1.2, and 1.4 for naproxen, clodinafop, haloxyfop and furosemide, respectively. The linearity of the HPLC method for naproxen was in the range of 10–1,000 µg L⁻¹. The limit of detection value was found to be 5 µg L⁻¹ and RSD % of 2.9 (n = 5) was obtained. Real samples such as blood plasma and urine species were analyzed by the developed method and the relative recovery percentages of 76 and 94 % were obtained for the spiked samples.