Silica Gel Glass Research Articles

The Influences of Structural Evolution of Silica Gel Glass on the Photophysical Properties of Benzoic Acid

Benzoic acid was dispersed into silica gel glass at molecular level by sol-gel process. The luminescence properties of benzoic acid in gel glass heat-treated at various temperatures were studied. The changes in gel glass structure and the surrounding environment of benzoic acid resulted in different electronic transitions, and thus the different luminescence spectra. The fluorescence bands centered at 320 nm and 355 nm, respectively induced by the S1ππ* → S0 transition of benzoic acid and S1nπ* → S0 transition of benzoate anions, respectively, were observed when the doped gel glass was heat-treated at the temperatures below 200°C and at the temperature range 300 ∼ 400°C, respectively. On heating at 500°C, benzoate anions were steadily immobilized in the cages of the Si–O network with the SiO4 tetrahedra formation and the RTP of benzoate anions is observed. It is proposed that the luminescence spectra of benzoic acid can be used as a structural evolution probe of gel glass.

Photoluminescence properties of alkaline metallic ions doped sol–gel silica glasses

K + and Na +-doped silica glasses have been prepared through sol–gel processes employing Si(OC 2H 5) 4, K 2S and Na 2S as precursor, with HCl as catalyst. The UV–visible absorptive spectra, XRD patterns and Fourier transform infrared spectra of the samples have all been investigated. The photoluminescence spectra of all the samples show that the fluorescent intensity of K- and Na-doped samples is about 2–3 times of that of un-doped sol–gel silica glass. K + and Na + ions can act as a good sensitizer and activator in the sol–gel silica glasses.

From Synthetic Methods to γ-Peptides – From Chemistry to Biology

The research activities of our group are demonstrated by examples in the following fields: (i) TADDOL auxiliary system (combinatorial synthesis, a chiral hydroperoxide, immobilization on controlled-pore glass silica gel); (ii) a geminally diphenyl-substituted 4-isopropyl-1,3-oxazolidinone as a superior Evans-type auxiliary (for enantioselective enolate alkylation, aldol addition, Michael addition, and Diels-Alder reactions); (iii) enantioselective reactivity umpolung (with a lithiated methylthiomethyl derivative of an oxazolidinone), and (iv) chemical and biological investigations of γ-peptides (folding to helices and turns, stability against peptidases). The impact of biology on the projects of a synthetic organic group is discussed.

The luminescence of PbS nanoparticles embedded in sol–gel silica glass

PbS nanoparticles embedded in a novel silica glass have been achieved in the present study by sol–gel processing. Their fluorescence properties have been evaluated and compared with those of un-doped glass. A novel luminescent phenomenon has been observed from the composite of PbS nanoparticles and the sol–gel silica glass. The photoluminescence (PL) spectrum of the composite consists of two emission peaks (440 and 605 nm). The Pb 2+ ions in the sol–gel silica glass show sharp emission band. This novel luminescence from the samples is assigned to the composite structure of PbS nanoparticles and porous silica glasses.

Sol–gel silica glasses with nanoparticles of copper selenide: synthesis, optics and structure

The silica glasses with copper selenide nanoparticles were fabricated by the sol–gel technique. The samples were characterized with transmission electron microscopy, Rutherford backscattering spectroscopy and optical absorption. The composition of nanoparticles is close to Cu2Se stoichiometry and can be varied under tuning of the preparation steps. These glasses exhibit the new features in visible and IR ranges.

Defects and Photoluminescence of Ni2+ and Mn2+-Doped Sol–Gel SiO2 Glass

SiO2 glass doped with Ni2+ and Mn2+ has been prepared by the sol–gel process. Very strong visible light (its fluorescence efficiency is about 4 times of that of undoped sol–gel SiO2 glass and about 160 times of that of ZnS nanocrystallites) from the SiO2 glass doped with Ni2+ and Mn2+ has been observed. The emission wavelength of Ni2+-doped, Mn2+-doped, and undoped glass samples is about 450 nm. However, the co-doping of Ni2+ and Mn2+ shifts the emission wavelength to 420 nm. The sol–gel SiO2 glass is a porous phosphor material. Very strong photoluminescence of the sol–gel SiO2 glass mainly comes from structural defects. Because the doped samples contain more Si dangling bonds, nonbridging oxygen, and oxygen vacancy in its structure and surface defects inside the nanometer-scale hole in the sol–gel SiO2 glass than undoped glass sample, the fluorescence efficiency of doped samples has been remarkably increased. Because the doped ions affect the band gap structure of the host materials, the emission wavelength of the co-doped samples is different from that of Ni2+- and Mn2+-doped samples. Because the emission wavelength of Ni2+ and Mn2+ luminescent centers in the sol–gel silica glass is almost the same as that of undoped sample, the fluorescence intensities of Ni2+- and Mn2+-doped samples are higher than those of the co-doped samples.

Spectroscopic studies of chromium-doped silica sol–gel glasses

The results of EPR and optical measurements of Cr-doped silica glasses obtained by means of sol–gel technology are presented. The effect of Cr concentration on optical behavior was studied. Thermoluminescence was observed after heating previously cooled and UV irradiated samples. The EPR measurements were performed for UV irradiated samples and a strong temperature dependence of the signal was noticed. The nature of optically active Cr centers was discussed. It is concluded that the EPR signal is associated with the Cr 5+ ions at tetrahedral sites whereas the emission is attributed to the ligand–metal charge transfer transitions of Cr 6+ ions coupled to Cr 5+. No evidence was found for the presence of Cr 3+ ions in silica gel glass.

Enantioselective aminolysis of an alpha-chloroester catalyzed by Candida cylindracea lipase encapsulated in sol-gel silica glass.

[reaction: see text] Lipase from Candida cylindracea (CcL) encapsulated in porous silica glass by a sol-gel method catalyzes enantioselective aminolysis of ethyl 2-chloropropionate. A silica matrix enhances the enzyme activity, i.e., improves the yield. The scope and limitations of the aminolysis reaction were investigated, and dynamic kinetic resolution of the ester was achieved. Encapsulated lipase remains active when used repeatedly. Encapsulation much improves the chiral discrimination by lipase and makes this enzyme even more useful in organic chemistry.

Sol–gel glass is not necessarily an inert matrix for enzyme encapsulation. Catalysis of sulfoxidation by silica

In sulfoxidation of thioanisole (PhSMe) by hydrogen peroxide, horseradish peroxidase (HRP) encapsulated in sol–gel silica glass exhibits the same yield as dissolved HRP does but lower excess of the ( S)-enantiomer (68% versus 91% at 22°C and 43% versus 86% at 40°C). Unexpectedly, the yield of methyl phenyl sulfoxide, PhS(O)Me, is the same in the presence of undoped and HRP-doped glasses. All of these findings result from the ability of the silica matrix itself to catalyze the sulfoxidation reaction. The drop in the enantiomeric excess is due to two simultaneous processes, namely enantioselective HRP catalysis and non-enantioselective silica catalysis. This side effect of the silica matrix makes possible an efficient method for the sulfoxidation of aryl methyl sulfides by hydrogen peroxide in water. In the presence of silica gel, the corresponding sulfoxides are obtained in the 76–90% yield. Evidently, sol–gel glass may not be an inert matrix for the encapsulated enzyme. The possible catalysis by silica should be taken into consideration in the design of biosensors, supported catalysts, and other composite materials.

Probing the chemical environment at the porous cage of ormosils through the fluorescence of oxazine 1

Ormosils (organically modified silicates) were prepared by the sol–gel process through the incorporation of non-ionic surfactants in the starting solution. Oxazine 1 was used as the fluorescent probe for the achievement of this study. Thus, recording of the absorption and emission fluorescence spectra, as well as measurement of the fluorescence lifetimes of oxazine 1, enabled us to determine the allocation of organic molecules within the porosity of silica gel-glasses. Furthermore, the reorientational correlation times of the fluorescent probe have been obtained through fluorescence anisotropy. According to the microviscosity data calculated from the fluorescent anisotropy measurements, movement of oxazine 1 is more restricted in the porous cage of ormosils than it is in silica gel-glasses.

Behavior of organic compounds confined in monoliths of sol–gel silica glass. Effects of guest–host hydrogen bonding on uptake, release, and isomerization of the guest compounds

Various proteins, catalysts, and other compounds can be encapsulated or diffused into porous sol–gel glasses, but little is known about their interactions with the glass matrix. We report unexpectedly large effects that hydrogen bonding between organic compounds and sol–gel silica has on equilibria and reactions involving these guest compounds. Silica monolith immersed in a solution takes up the organic solute. Styrene, which is incapable of hydrogen bonding, becomes evenly distributed between external solution and the glass. Aniline and N,N-diethyl-p-methoxybenzamide, which are capable of hydrogen bonding, become extracted into the glass when the solvent (neat CCl4) does not interfere with their hydrogen bonding with silica. They become evenly distributed between the solution and the glass when a component of the solvent (DMF added to CCl4) or chemical modification (trimethylsilylation) of the silica surface suppresses hydrogen bonding of the guests with the surface. Ultraviolet spectra show that silica–guest interactions are present when the guest uptake is excessive and absent when this uptake is balanced. Ultraviolet spectra of aniline show that the hydrogen atoms are donated by silica to the guest. Not only the extent, but also the rate, of uptake is enhanced when the guest makes hydrogen bonds to the silica matrix; suppression of these bonds lowers the uptake rate. Silica monolith extracts trans-3,3′-diacetylazobenzene from a CCl4 solution 1250-fold. Upon addition of DMF, hydrogen bonds are broken, and the monolith completely releases the solute into the external solution. Five derivatives of azobenzene (3,3′-dimethyl-, 3-acetyl-, 3,3′-diacetyl-, 3,5-diacetyl-, and 3,3′,5,5′-tetraacetylazobenzene), which differ in the propensity for accepting hydrogen atoms, served as photochromic probes and showed the effect of hydrogen bonding on reactivity. Both the photoinduced (trans-to-cis) and the subsequent thermal (cis-to-trans) isomerizations of the five derivatives obey the first-order law in glass as well as in free solution. When the solvent (neat CCl4) allows hydrogen bonding, the proportion of the isomers in the photostationary state differs between the glass and solution, and the rate constant for the thermal reaction is two to four times (in different derivatives) smaller in the glass than in solution. Evidently, hydrogen bonding retards the rearrangement of the probe molecules in the silica matrix. When hydrogen bonding is abolished (by addition of DMF to CCl4), the compositions of the photostationary state in the glass and in solution become equal, and so do the rate constants. Effects of hydrogen bonding on enzymes encapsulated in sol–gel glass and on the distribution of analytes between the glass monolith and the sample solution should be taken into consideration when designing accurate biosensors.

Unexpected Interactions between Sol−Gel Silica Glass and Guest Molecules. Extraction of Aromatic Hydrocarbons into Polar Silica from Hydrophobic Solvents

Properties of a solute may differ greatly between a free solution and that solution confined in pores of a sol−gel glass. We studied the entry of various aromatic organic compounds from solution into the monolith of sol−gel silica immersed in this solution. Partitioning of the solute is quantified by the uptake coefficient, the ratio of its concentrations in the glass and in the surrounding solution at equilibrium. The dependence of this coefficient on the solvent gives insight into possible interactions between the solute and the silica matrix. We report the uptake of 31 compounds altogether: 18 halogen derivatives of benzene; 5 condensed (fused) aromatics; and stilbene and three substituted derivatives of it, each in both cis and trans configurations. When the solvent is hexane, the uptake coefficients are as follows: 1.0−1.9 for the halobenzenes; 3.0−4.6 for the hydrocarbons; and 3.3−4.9 for the stilbenes. When the solvent is carbon tetrachloride or dichloromethane, the uptake coefficients become 0.82−1.39 for the hydrocarbons and 0.90−1.25 for the stilbenes. The excessive uptake of organic compounds from hexane is unexpected, for it amounts to extraction of nonpolar or slightly polar solutes from a nonpolar solvent into a polar interior of silica glass. The solute−silica interactions responsible for this extraction are not of the van der Waals type. Our findings are consistent with hydrogen bonding between the aromatic π system in the solutes and the hydroxyl groups on the silica surface. Hexane cannot interact with this surface but dichloromethane and carbon tetrachloride can: they shield the hydroxyl groups from the organic solvents and thus suppress the hydrogen bonding. This explanation is supported by the emission spectra of the aromatic compound pyrene when it is dissolved in acetonitrile, dichloromethane, cyclohexyl chloride, and hexane and when it is taken up by monoliths of sol−gel silica from these four solutions. The relative intensities of the emission bands designated III and I change greatly when pyrene is taken up from hexane but remain unchanged when it is taken up from the other three solvents. Evidently, hexane does not, whereas the other three solvents do, line the silica surface and shield it from approach by pyrene molecules. Even though solute molecules are much smaller than the pores in the sol−gel glass, diffusion of these molecules into the monolith may result in an uneven partitioning at equilibrium. This fact must be taken into consideration in the design of biosensors, immobilized catalysts, and other composite materials because their function depends on the entry of analytes, substrates, and other chemicals into the glass matrix.

Fiber optic oxygen sensor based on phosphorescence quenching of erythrosin B trapped in silica-gel glasses

Experimental results from a phosphorescence-based fiber-optical oxygen sensor are presented. The optical oxygen sensor module has overall dimensions of 6 × 6 × 12 mm 3. Erythrosin B immobilized in sol–gel silica, which showed strong phosphorescence quenching by oxygen, was used in the sensor probe. Oxygen sensing is effective from 0.014 to 600 mbar. On account of the long phosphorescence lifetime (∼0.28 ms) and high phosphorescence yield (∼2%) of erythrosin B in sol–gel silica at room temperature, the sensitivity of the sensor improves by a factor of 10 as compared to transition-metal complex-based optical fiber oxygen sensors. The phosphorescence quenching effect is highly selective to oxygen. The sensor is inert to commonly found gases such as nitrogen and argon. Time-decay of phosphorescence is also studied.

Effects of Encapsulation in Sol−Gel Silica Glass on Esterase Activity, Conformational Stability, and Unfolding of Bovine Carbonic Anhydrase II

Bovine carbonic anhydrase II retains its overall conformation when encapsulated in silica monoliths by the sol−gel method. Upon gradual heating the enzyme in solution precipitates at 64 °C, whereas the encapsulated enzyme does not; it unfolds, with the nominal melting temperature of 51 ± 3 °C. Even at 74.0 °C, the encapsulated enzyme is only ∼77% unfolded, but it does not refold upon cooling. Upon treatment with guanidinium chloride, the degree of enzyme unfolding is 100% in solution but only ∼83% within the silica matrix. Again, the enzyme does not refold upon removal of the denaturing agent from the glass. The glass matrix constrains the motions of the encapsulated protein molecules and prevents both their full unfolding and refolding. The former may be taken for “stabilization”, the later for “destabilization” of the native conformation. Evidently, the effect of the glass on the encapsulated protein cannot be described in these general terms. The encapsulated enzyme obeys the Michaelis−Menten kinetic l...

Temperature-dependent luminescence and energy transfer in europium and rare earth codoped nanostructured xerogel and sol–gel silica glasses

Luminescence and decay times of Eu 3+ ions codoped with other rare-earth ions (Nd 3+, Sm 3+, Dy 3+, Er 3+ and Yb 3+) were measured at temperatures between 10 and 300 K in order to investigate the excitation energy transfer processes in the nanostructured xerogel and annealed sol–gel silica glasses. Decay-time profiles were analyzed as the sum of two exponents by applying two models for the electric dipole–dipole type of resonant energy transfer. The critical distances ( R o) of energy transfer in Dexter model are confined to be in the range of 0.88±0.07 nm (at 300 K), 0.92±0.08 nm (at 10 K) for annealed glasses and 0.93±0.11 nm at 300 K for xerogels. The energy transfer processes are considered to reflect optical properties of confined nanostructured networks in the sol–gel SiO 2 glasses.

Size and shape distribution of magnetic nanoparticles in disordered systems: computer simulations of superparamagnetic resonance spectra

A new model is proposed in order to account for the superparamagnetic resonance spectra arising from the ferromagnetic particles dispersed in a diamagnetic matrix (e.g., ferrofluids, partially devitrified glasses). The model explicitly takes into account non-sphericity of magnetic nanoparticles in a joint distribution density of sizes and demagnetising factors. The distribution in particle shapes is shown to take an important part in determining the superparamagnetic resonance spectra. By computer simulating the resonance spectra of annealed iron-containing borate and sol–gel silica glasses, characteristics of size and shape distributions of magnetic nanoparticles are evaluated. As the anneal temperature is increased from 456°C to 480°C, the most probable diameter of the magnetic particles formed in the borate glass is increased from 3.4 to 4.7 nm, while their shapes become better ordered. The mean size of particles formed in the sol–gel silica glass is small, ca. 3.0 nm, and their shapes are less ordered in comparison with the borate glass.

Optical properties of Eu(III) chelates trapped in silica gel glasses

Preparation and optical properties of silica gel glasses doped with Eu(III) chelates (the complexes of β-diketones) are described. In particular the emission and excitation spectra of the precursor salts and glasses are reported. The assignment of electronic levels was proposed and the intensity analysis of the f–f transitions was performed. The decay times of 5D 0 levels of Eu(III) complexes in gels are analyzed and compared with those reported for precursor salts.

Spectroscopy and optical properties of HITC-doped silica sol–gel glasses

The photophysical behavior of the organic dye HITC (1,1′,3,3,3′,3′-hexamethylindotrycarbocyanine iodide) incorporated into silica gel glasses prepared from different starting compositions was studied in detail by fluorescence spectroscopy and lifetime measurements. The structural and morphologic characterization of the hosting matrix by FT-IR, 29Si-NMR, UV–VIS–NIR spectroscopies, SEM and TEM techniques and measurements of the BET specific surface area, revealed the important influence on the optical properties of the water content in the starting solution of the different sol–gel compositions. An increase in the transmission at the UV cut-off as well at the NIR wavelengths was achieved in samples prepared from the specific composition of the gel produced from low water to monomer molar ratio ( r w/m=1 *), and avoiding during the reaction water adsorption from the air. This specific method of preparation is adequate to achieve the chemical stabilization of the HITC dye in gel glasses. The poor mechanical properties of these gel glasses can be improved when drying is performed under controlled pressure and humidity conditions.

Identification of Oblique and Coplanar Inclined Fluorescent J-Dimers in Rhodamine 110 Doped Sol-Gel-Glasses

Rhodamine 110 (R110) fluorescent J-dimers have been first observed in doped silica gel-glasses made by sol-gel process. The R110 doped gel-glasses photophysical properties such as excitation/fluorescence spectra and lifetimes were studied in a wide range of dye concentrations. It has been determined that the formation of R110 dimers is a consequence of the dye adsorption on the porous surface of the gel-glasses. The adsorption geometry, which is established by the angle formed between the longitudinal axis of the molecule and the gel surface, determined the formation of either R110 fluorescent J-dimers or nonfluorescent H-dimers. Since the adsorption angle was related to the dye concentration, the transition from J-dimers to H-dimers was found for a specific R110 concentration. Formation of J-dimers with a preferential oblique or coplanar inclined configuration was induced by the change of the polar character of the adsorption surface.

The microstructural dependence of intramolecular photoautomerism of salicylic acid doped in silica gel glass prepared by a sol–gel process

Many fluorophores will show photoautomerism if irradiated by light, which results in some changes of the fluorescence emission spectra of these fluorophores. The intramolecular photoautomerism of salicylic acid in silica gel glass prepared by a sol–gel process and an ethanol solution were studied comparatively. Doped in silica gel glass, salicylic acid, as well as water and ethanol, co-exists in the pores of the gel glass, and only one broad band due to the zwitterion is observed and shifts to longer wavelength with respect to that in ethanol solution as a result of solvent effects. After the doped gel glass being heat treated at 150°C, some salicylic acid molecules are trapped in the tiny pores with little polarity, and the broad band around 370 nm due to the neutral molecule appears. The number of salicylic acid molecules trapped in the low polar pores of the gel glass increases after the sample is heat treated at elevated temperature, which increases in the fluorescence intensity of 370 nm emission band.