Molecular state and distribution of fullerenes entrapped in sol-gel samples

A novel synthetic method that can encapsulate fullerene molecules (pure C60, pure C70, or their mixture) over a wide range of concentrations ranging from micromolar to millimolar in hybrid glass by a sol-gel method without any time-consuming, complicated, and unwanted extra steps (e.g., addition of a surfactant or derivatization of the fullerenes) has been successfully developed. The molecular state and distribution of encapsulated fullerene molecules in these sol-gel samples were unequivocally characterized using newly developed multispectral imaging techniques. The high sensitivity (single-pixel resolution) and ability of these instruments to record multispectral images at different spatial resolutions (approximately 10 microm with the macroscopic instrument and approximately 0.8 microm with the microscopic instrument) make them uniquely suited for this task. Specifically, the imaging instruments can be used to simultaneously measure multispectral images of sol-gel-encapsulated C60 and C70 molecules at many different positions within a sol-gel sample in an area either as large as 3 mm x 4 mm (with the macroscopic imaging instrument) or as small as 0.8 microm x 0.8 microm (with the microscopic instrument). The absorption spectrum of the fullerene molecule at each position can then be calculated either by averaging the intensity of a 15 x 15 square of pixels (which corresponds to an area of 3 mm x 4 mm) or from the intensity of a single pixel (i.e., an area of about 0.8 microm x 0.8 microm), respectively. The molecular state and distribution of fullerene molecules within sol-gel samples can then be determined from the calculated spectra. It was found that spectra of encapsulated C60 and C70 measured at five different positions within a sol-gel sample were similar not only to one another but also to spectra measured at six different times during the sol-gel reaction process (from t = 0 to 10 days). Furthermore, these spectra are similar to the corresponding spectra of monomeric C60 or C70 molecules in solution. Similarly, spectra of sol-gel samples containing a mixture of C60 and C70 were found to be the same at five different positions, as well as similar to spectra calculated from an average of the spectra of C60 and C70 either encapsulated in a sol-gel or in solution. It is evident from these results that C60 and C70 molecules do not undergo aggregation upon encapsulation into a sol-gel but rather remain in their monomeric state. Furthermore, entrapped C60 and C70 molecules in their monomeric state were distributed homogeneously throughout the entire sol-gel samples. Such a conclusion can be readily, quickly, and easily obtained, not with traditional spectroscopic techniques based on the use of a single-channel detector (absorption, fluorescence, infrared, Raman) but rather with the newly developed multispectral imaging technique. More importantly, the novel synthetic method reported here makes it possible, for the first time, to homogenously entrap monomeric fullerene molecules (C60, C70, or their mixture) in a sol-gel at various concentrations ranging from as low as 2.2 mM C60 (or 190 microM C70) to as high as 4.2 mM C60 (or 360 microM C70).     

The mechanism of pyrene fluorescence quenching by selective and nonselective quenchers during sol–gel transition

The process of pyrene fluorescence quenching by potassium iodide, acrylamide, and gaseous oxygen in silane sol during sol–gel transition was investigated. Pyrene fluorescence emission spectra were recorded vs concentrations of the added quencher on consecutive days of the gelling process. When using 0, 20, and 100% oxygen in the gas mixture as a quencher, time-resolved measurements were also made. On this basis, Stern–Volmer constants were determined on consecutive days of the gelling process, availability of fluorophore molecules to the quencher, and also rate constants of pyrene quenching by acrylamide, iodide ions, and oxygen were specified. Moreover, diffusion coefficients were determined for acrylamide and potassium iodide in the silane gel formed. The mechanism of fluorophore quenching in sol of growing viscosity and in gel was discussed in view of using this carrier in the construction of sensors and optical biosensors.     

Phase Transitions in Pure and Hybrid Hydrogels: A Fluorescence Study

Summary: Recent observations on phase transitions around the critical point showed that the critical exponents differed drastically from percolation results and classical results for pure hydrogel systems, depending on monomer concentration. In addition to pure hydrogels, the sol-gel phase transition during radical crosslinking copolymerization of acrylamide (AAm) and N-isopropylacrylamide (NIPA) hybrid was studied by using the steady state fluorescence (SSF) technique. N,N′-methylenebis(acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and initiator, respectively. Pyranine (trisodium 8-hydroxypyrene-1,3,6-trisulfonate acid, HPTS) was added as a fluoroprobe for monitoring the polymerization. It was observed that pyranine binds to AAm and NIPA chains on the initiation of the polymerization, thus the fluorescence spectra of the bonded pyranines shift to shorter wavelengths. Fluorescence spectra of the bonded pyranines allowed to monitor the sol-gel phase transition without disturbing the system mechanically and to test the universality of the sol-gel transition as a function of polymer concentration ratios. The observations around the gel point of PAAm-PNIPA hybrid show that the gel fraction exponent β obeyed the percolation result.     

Relationships Between Fluorescence Properties of Benzoquinolines and Physicochemical Changes in the Sol–Gel–Xerogel Transitions of Silicon Alkoxide Systems

Fluorescence and excitation spectra of 3,4-, 5,6- and 7,8-benzoquinolines (BQs) dispersed in the individual sol–gel–xerogel transitions systems of silicon alkoxide have been observed as a function of the reaction time. The fluorescence spectra of excited-state species (neutral, ion-pair and protonated species) of each BQ have been obtained clearly. In the starting sol–gel systems, the fluorescence mainly originated from the neutral species of each BQ. As hydrolysis of the silicon alkoxide proceeded, an interaction between the resulting silanol group and the neutral species led to a formation of the ion-pair species in their ground state. Upon excitation, a part of the ion pair relaxed to the protonated species during the fluorescence lifetime, and the resulting excited species emitted a corresponding fluorescence. As the sol–gel reaction proceeded further, the geometrical relaxation was gradually prevented with an increase in rigidity around the BQ molecules, so that a fluorescence tended to be observed from an unrelaxed conformer of the ion pair. During the gel to xerogel transition, a part of the BQ molecules exists in relatively large spaces of pores, where water plays an important role, and slowly came to exhibit the fluorescence from the protonated species because of concentrated water in the spaces. Contributions of the three excited-state species to the total fluorescence spectra have been estimated by spectral curve fitting. Consequently, the sol–gel reaction has been resolved into four physicochemical reaction stages, which govern the fluorescence behaviors of BQs. Differences in the fluorescence behaviors among the BQ isomers reflect the acid-dissociation constants in the excited states.     

Contributions to the Gelling Process in the Bi-Sr-Ca-Cu-O System, Via Acetate Route

Metal acetates represent suitable precursors in the sol-gel method. Generally, acetates are used in multicomponent systems with alkoxides or other carboxylic salts. However, Bi−Sr−Ca−Cu−O superconductors have been obtained by the sol-gel method starting only from acetate mixture. This work was intended to study the gelling process of acetates involved in the formation of bismuth-based superconducting phases, both individually and in the bi-and multicomponent systems. Probably, copper acetate is mainly responsible for the gel formation in these systems. The gelling process was monitored by measuring the pH, density and viscosity changes. The gels obtained were characterized by UV-VIS and IR spectra, DTA/TGA and powder X-ray diffraction.     

Iodide fluorescence quenching of sol-gel immobilized BSA

Several potential applications of sol-gel immobilized proteins may require drying the gel before use. The attendant shrinkage may sterically restrict the penetration of solutes to the entrapped protein. In this study, collisional quenching of intrinsic bovine serum albumin (BSA) fluorescence by dissolved iodide was used to quantitatively compare the accessibility of protein molecules entrapped in hydrated and dried sol-gel monoliths. The results show that iodide penetration to immobilized BSA is sterically restricted by approximately 25% in hydrated gels and 50% in dried gels, relative to the quenching behavior of dissolved BSA. The further decrease upon drying is consistent with the overall degree of monolith shrinkage.     

醋酸和丙三醇对制备SrTiO3的影响

醋酸和丙三醇对制备ＳｒＴｉＯ_３的影响颜秀茹，霍明亮，王建萍，王菲，曾淑兰（天津大学化学系天津３０００７２）关键词　冰醋酸，溶胶－凝胶过程，丙三醇，ＳｒＴｉＯ_３ＳｒＴｉＯ３是电子材料中使用最广的材料之一，有关它的制备已有不少报道。一般可以通过ＳｒＣＯ...     

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.     

ESR Study of a Nitroxide Radical in Sol-Gel Glasses

A spin probe TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy) was dissolved in a tetraethyl orthosilicate sol-gel reaction system and measured by electron spin resonance spectroscopy at 295 K. The nitrogen hyperfine coupling constant was from 1.64–1.66 mT in the sol-gel solutions. The values were sensitive to the ethanol-to-water ratio of the solutions. The hyperfine coupling constant in the xerogels was 1.70 mT, which was almost the same as that in water, indicating that the probe molecules were trapped in silica pores with water adsorbed on the silica surfaces. The motion of TEMPOL in the xerogels was considerably slower than in the sol-gel solutions. The local viscosity estimated was from 70–90 cP. The ESR spectra of TEMPOL were altered during the sol-gel process, indicating that adsorbed water on the silicas surfaces has an important role for trapping organic molecules in sol-gel glasses.     

溶胶凝胶法制备CdS/SiO2量子点玻璃的研究

通过溶胶凝胶转变过程制备了硫化镉／二氧化硅量子点复合玻璃材料（硫化镉在复合材料中的最大掺杂质量比可达到30％）；并针对两种催化剂对复合体系的不同影响对这一类体系的物理化学性质进行了较为系统的研究．复合体系的溶胶凝胶转变点可通过对复合溶胶粘度随时间变化的观测而确定．TEM法与低频Raman散射法的测定表明实验制备的样品中CdS颗粒基本为nm级的球形粒子。其CdS颗粒粒径随CdS含量增加而增大。随CdS／SiO2量子点玻璃材料中CdS颗粒粒径的减小，其吸收光谱中的吸收边界与常规尺寸颗粒的吸收光谱边界相比有明显的蓝移，体现出显著的量子效应．适宜的热处理过程对复合材料中CdS颗粒粒径的减小和量子效应的增强起着有利的作用．     

Polyelectrolytes conformational probes: Auramine O and ethidium bromide interactions with hypercoiling maleic acid–olefin copolymers

Two cationic dyes—auramine O (AuO) and ethidium bromide (EBr)—have been tested for their ability to respond to conformational states of maleic acid–olefin copolymers in aqueous solution. The fraction of bound dye and the intensity of fluorescence of AuO are extremely sensitive to the presence of polyelectrolyte compact coils and to changes in ionic strength and counterion species. A related behavior is shown by the shift in the absorption visible spectrum of EBr, whereas its fluorescence seems to depend somewhat on the “structure” of the compact coils. These properties of the two dyes, which find a convincing correlation with viscosity and equilibrium thermodynamic data for the copolymers considered, are thus proved to be useful in probing the degree of compactness of polyelectrolyte coiled conformations in dilute aqueous solutions.     

A Spectroscopic Study of the Effects of Various Solvents and Sol-Gel Hosts on the Chemical and Photochemical Properties of Thionin and Nile Blue A

Tetraethyl orthosilicate (TEOS)-based gels were doped with two optically active organic indicators, thionin and nile blue A. Before trapping in a sol-gel host, thionin and nile blue A were both evaluated for solvent and protonation effects on their spectral properties. Only extreme pH values provided by HCl, NaOH, and NH₄OH produced new absorption and/or fluorescence bands. Introduction of nile blue A into alkaline environments (0.1N NaOH, NH₄OH) results in the appearance of a broad absorption band centered near 520 nm whereas highly acidic environments (1N HCl) show a reduction of the 635 nm absorption peak accompanied by an absorption band located near 460 nm. A marked decrease is observed in the optical density of thionin in 1N HCl solution which results in a reduction in the fluorescence intensity. The absorption and fluorescence spectra also reveal a decrease in a pH 11 solution of NH₄OH as compared to neutral conditions. Both dyes formed dimers when the sol-gel host, initially synthesized with TEOS, was organically modified with methyltrimethoxysilane (MTMS). However, thionin dimers were present in all silica-based sol-gel compositions, as evidenced by the absorption and fluorescence spectra. Substitution of MTMS for some of the TEOS in the gel matrix resulted in blue shifts in the absorption and fluorescence spectra of nile blue A. The absorption peak shifted 50 nm to 596 nm whereas the fluorescence shifted around 40 nm to 635 nm. These blue shifts resulted from the reduced polarity of the silica-based xerogel. Thionin also exhibited shifts in its absorption and fluorescence spectra with organic modification by MTMS. The absorption shifted approximately 3 nm to 595 nm while the fluorescence maximum decreased 7 nm to 630 nm. The blue shifts in the spectra of thionin with additions of MTMS were attributed to surface sites that altered the molecular structure of the adsorbed thionin molecules.     

Effect of pressure on the Prodan fluorescence in bilayer membranes of phospholipids with varying acyl chain lengths

The fluorescence spectra of 6-propionyl-2-(dimethylamino)naphthalene (Prodan) were observed as a function of pressure for the bilayer membrane systems of dilauroylphosphatidylcholine (DLPC), dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), and distearoylphosphatidylcholine (DSPC). The wavelength of the emission maximum, lambdamax, was found to be 480, 430, and 500 nm for the liquid crystalline (Lalpha), ripple gel (P'beta), and pressure-induced interdigitated gel (LbetaI) phase, respectively. Since the lambdamax reflects the solvent property around the probe molecules, we could speculate on the location of the Prodan molecules in the bilayer membranes; in the Lalpha phase of the lipid bilayer, the Prodan molecules distribute around the phosphate of the lipids (i.e. the polar region). The Lalpha/P'beta phase transition caused the Prodan molecules to move into the less polar region near the glycerol backbone. The fluorescence intensity of the Prodan in the P'beta phase was dependent on the chain length of the lipids and on pressure; the shorter the chain length of the lipid, the stronger the fluorescence intensity of the Prodan. Moreover, for the DLPC bilayer membrane system, the fluorescence intensity at 430 nm increased with increasing pressure, indicating that the partition of Prodan into the DLPC bilayer membrane is promoted by applying pressure. In the case of the DPPC and DSPC bilayers, as the pressure increased further, the pressure-induced interdigitation caused the Prodan molecules to squeeze out of the glycerol backbone region and to move the hydrophilic region near the bilayer surface. The ratio of fluorescence intensity at 480 nm to that at 430 nm, F480/F430, showed a sharp change at the phase-transition pressure. In the case of the DPPC and DSPC bilayers, the values of F480/F430 showed an abrupt increase above a certain pressure higher than the Lalpha/P'beta transition pressure, which corresponds to the interdigitation from the P'beta to the LbetaI phase. The plot of F480/F430 versus pressure is available for recognition of the bilayer phase transitions, especially the bilayer interdigitation.     

Characteristic changes in the fluorescence spectra of 1-naphthol at the gelation point during the sol-gel-xerogel transitions

The fluorescence spectra of 1-naphthol were observed during the sol-gel-xerogel transitions of two different systems as a function of time; one is in the silicon and titanium (Si:Ti = 4500:1) binary systems involving no catalysts and the silicon and lithium (Si:Li = 99:1) binary systems involving HC1 as the catalyst. During the first stage of the sol-gel reaction of the 1-naphthol system, the fluorescence spectra mainly originated from the broad ’L₂ state. The fluorescence spectrum originating from the anionic species at around 470 nm increased as the reaction proceeded. It was found that the fluorescence spectra originating from the anionic species of 1-naphthol drastically decreased in both systems just after gelation. These findings indicate that it becomes difficult for the dissociation of the excited state of 1-naphthol to give a dissociative proton to the surrounding matrix. The fluorescence-excitation spectra for the Si/Ti system indicated that the main route for the excited state of 1-naphthol to form a dissociative proton is through the excited state of the contact ion pair, while the main route in the Si/Li system is via the direct excitation of the neutral 1-naphthol and its dissociation. The observed changes in the fluorescence spectra of 1-naphthol in these sol-gel systems provide a sensitive means to monitor changes during the sol-gel transition process.     

掺杂Ce的TiO_2纳米粒子的光致光及其光催化活性

采用sol-gel法制备了纯的和掺杂不同量Ce的TiO_2纳米粒子，并利用XRD， TEM，BET，XPS和PL光谱对样品进行表征，主要考察焙烧温度和含量对掺杂Ce的 TiO_2纳米粒子性质以及光催化降解苯酚活性的影响，并探讨了Ce的掺杂对TiO_2相 变的作用机制以及PL光谱与光催化活性的关系，结果表明，掺杂的Ce~(4+)没有进 入到TiO_2晶格中，而是以小团簇的CeO_2化学态均匀地弥散在TiO_2纳米粒子中， 这可能导致了Ce的掺杂对TiO_2的相变有很大的抑制作用；Ce的掺杂没有引起新的 光致发光现象，而适量Ce的掺杂能够降低TiO_2纳米粒子PL光谱的强度，这是因为 掺杂的Ce~(4+)易于捕获光生电子而生成Ce~(3+);600℃处理的掺杂Ce的TiO_2纳米 粒子表现出较高的光催化活性，这说明600℃是比较合适的焙烧温度，而掺杂不同 量的Ce的TiO_2样品的光催化活性顺序是：3 mol%＞4 mol%＞2 mol%＞5 mol%＞1 mol%＞0 mol%，这与它们的PL光谱强度的顺序是相反的，即PL光谱强度越低，其 光催化活性越高，这说明PL光谱与其光催化活性间有着必然的联系，这是因为掺杂 剂Ce~(4+)能够捕获光生电子，在光致发光过程中使PL光谱强度下降，而在光催化 反应过程中使有机污染物加快氧化。     

Mapping viscosity in cells using molecular rotors

This article describes an emerging method for quantitative measurement and spatial imaging of microviscosity within individual domains of live cells. The method is based on fluorescence detection from small synthetic molecules termed 'molecular rotors', which are characterised by a strong response of fluorescence lifetimes or spectra to the viscosity of their immediate environment. Alongside this new method, two complementary techniques are discussed, which provide further insights into diffusion controlled processes on a microscopic scale in a biological environment. These are time resolved fluorescence anisotropy and imaging of short-lived excited state of molecular oxygen, termed 'singlet oxygen'. It is possible to utilise all three approaches for the quantitative determination of viscosity in individual organelles of live cells. Finally, it is discussed how the major advantage of molecular rotor imaging, fast signal acquisition, can be used to monitor changing viscosity during dynamic biological processes within cells, such as photoinduced cell death.     

Thermosensitive Gels for the Topical Administration of Diltiazem

In the present work, thermosensitive systems were prepared, characterized, and proposed for diltiazem administration in the topical treatment of anal fissures. Methylcellulose and PluronicF127 were used as gelling polymers. Some low-toxicity molecules, such as sodium glycocholate, citric acid, and lactic acid, were added to gel formulations as counterions to enhance diltiazem lipophilicity. The systems were characterized by sol-gel transition temperature, viscosity, and rheological studies. The resulting data allowed us to determine which systems presented sol-gel transition. A change from Newtonian to plastic behavior at sol-gel transition temperature was observed. An increase in diltiazem pig skin permeability and two-fold skin accumulation was observed in the presence of citric acid.     

Monomer-Dimer Equilibrium of Rhodamine B Encapsulated in Si–Al and Si–Ti Binary-Oxide Thin Films

Si—Al and Si—Ti binary-oxide thin films including Rhodamine B (RB) have been prepared. They were dip-coated as a function of time after mixing of each sol-gel reaction system. The absorption and fluorescence spectra of the individual films have been observed. These spectra were analyzed in order to clarify the behavior of RB along with the change in the environment around the RB molecules, caused by the progress of the sol-gel reaction, in the fluid sol and the prepared thin films. Some amount of the RB dimers (H- and J-types) were formed in the Si—Al and Si—Ti binary-oxide films (Si : M = 75 : 25) prepared at the initial stage of the sol-gel reaction and aged under relative humidity of 60%. In the case of Si—Al binary-oxide films, the amount of the J-dimer decreased along with the reaction time at which the films were prepared, indicating that growing polymer networks of metal alkoxides around the RB molecules prevent the formation of the J-dimer. On the other hand, larger amounts of the H- and J-dimers were formed in the Si—Ti binary-oxide films prepared at longer reaction time of the solution. RB interacts more strongly with —TiOH compared with —AlOH. In the case of the Si—Ti binary-oxide films, with the progress of the sol-gel reaction, RB molecules in the prepared films easily cohere around the —TiOH and form the dimers because of increase in the amount of the —TiOH and contraction in the volume of the spaces where RB molecules exist.     

Drying Study of Siloxane-PPG Nanocomposites

This is a study of the structural transformations occurring in hybrid siloxane-polypropyleneglycol (PPG) nanocomposites, with different PPG molecular weight, along the drying process. The starting materials are wet gels obtained by the sol-gel procedure using as precursor the 3-(trietoxysilyl)propylisocyanate (IsoTrEOS) and polypropylenglycol bis(2-amino-propyl-ether) (NH₂-PPG-NH₂). The shrinkage and mass loss measurements were performed using a temperature-controlled chamber at 50°C. The nanostructural evolution of samples during drying was studied in situ by small angle x-ray scattering (SAXS). The experimental results demonstrate that the drying process is highly dependent on the molecular weight of polymer. After the initial drying stage, the progressive emptying of pores leads to the formation of a irregular drying front in gels prepared from PPG of high molecular weight, like 4000 g/mol. As a consequence, an increase of the SAXS intensity due to the increase of electronic density contrast between siloxane clusters and polymeric matrix is observed. For hybrids containing PPG of low molecular weight, the pore emptying process is fast, leading to a regular drying front, without isolated nanopockets of solvents. SAXS intensity curves exhibit a maximum, which was associated to the existence of spatial correlation of the silica clusters embedded in the polymeric matrix. The spatial correlation is preserved during drying. These results also reveal that the structural transformation during drying is governed by capillary forces and depends on the entanglement of polymer chains.     

Self-assembly of chiral trans-cyclobutane-containing β-dipeptides into ordered aggregates

Two chiral synthetic β-dipeptides have been constructed, one with two trans-cyclobutane residues and the other with one trans and one cis fragment, 1 and 2, respectively, and investigated to get insight into the non-covalent interactions responsible for their self-assembly to form ordered aggregates, as well into parameters such as their morphology and size. Experimental evidence of the formation of these assemblies was provided by spectroscopy, microscopy and X-ray diffraction experiments that suggest the formation of nanoscale helical aggregates. This process involves a conformational change in the molecules of each dipeptide with respect to the preferred conformation of the isolated molecules in solution. A high-resolution NMR spectroscopy study allowed the determination of the dynamics of the gelation process in [D(8)]toluene and the sol-gel transition temperature, which was around 270 K in this solvent at a concentration of 15 mM. NMR spectroscopy experiments also provided some information about conformational changes involved in the sol-gel transition and also suggested a different gel packing for each dipeptide. These observations have been nicely explained by computational studies. The self-assembly of the molecules has been modelled and suggested a head-to-head molecular arrangement for 1 and a head-to-tail arrangement for 2 to give helical structures corresponding to hydrogen-bonded single chains. These chains interact with one another in an antiparallel way to afford bundles, the significant geometry parameters of which fit well to the main peaks observed in wide-angle X-ray diffraction spectra of the aggregates in the solid state.