Sol-gel Silica Glass Research Articles

Effect of silver ions on the energy transfer from host defects to Tb ions in sol–gel silica glass

Plasmonic metal structures have been suggested to enhance the luminescence from rare-earth (RE) ions, but this enhancement is not yet well understood or applied to phosphor materials. Although some reports using Ag nanoparticles (NPs) in glass have attributed enhancement of RE emission to the strong electric fields associated with Ag NPs, it has also been proposed that the enhancement is instead due to energy transfer from Ag ions to RE ions. Our work using sol–gel silica shows a third possibility, namely that enhancement of the RE (e.g. Tb) emission is due to energy transfer from defects of the host material to the Tb ions, where the addition of Ag influences the silica host defects. The oxidation state of Ag as a function of annealing temperature was investigated by x-ray diffraction, transmission electron microscopy, UV–vis measurements and x-ray photoelectron spectroscopy, while optical properties were investigated using a Cary Eclipse fluorescence spectrophotometer or by exciting samples with a 325nm He–Cd laser. The results showed that Ag ions have a significant effect on the silica host defects, which resulted in enhancement of the green Tb emission at 544nm for non-resonant excitation using a wavelength of 325nm.

Characterizing Solvent Dynamics in Nanoscopic Silica Sol–Gel Glass Pores by 2D-IR Spectroscopy of an Intrinsic Vibrational Probe

Porous sol–gel matrices were synthesized with IR-active Si–H vibrational chromophores integrated into the silica network. The Si–H vibrational mode was found to be highly accessible to solvents within the nanoscopic pores. Vibrational solvatochromism of the silane vibration was controlled largely by interactions between infiltrating solvents and the oxygen and hydroxide sites in the silica. Exchanging solvents in the silica matrices produced reversible solvatochromic shifts in some cases but led to irreversible shifts when strongly interacting solvents were tested, suggesting that a layer of solvent was not exchangeable. 2D-IR spectroscopy was used to monitor spectral diffusion and extract the homogeneous line widths of the Si–H mode for a range of infiltrating solvents as well as solvent-free aerogel samples. It was demonstrated that the silane vibration is sensitive to the nature of the infiltrating solvent, making these vibrationally active sol–gel films a general platform for solvent dynamics in nanoscopic confined volumes.

Effects of densification atmosphere on optical properties of ionic copper-activated sol–gel silica glass: towards an efficient radiation dosimeter

Ionic copper-doped pure silica glass, with high photoluminescence (PL) quantum yield (QY), has been prepared using the sol–gel technique and densification of the porous xerogel under inert atmosphere. The optical properties of the obtained glass, and especially the green light emission due to Cu+ ions under UV excitation, have been studied and compared to those of the glass produced in air atmosphere conditions. We have shown that the sintering process under helium increases the quantum efficiency of this emission by a factor 40. Moreover, the resulting copper-doped silica rod has been used in the fabrication of a high numerical aperture microstructured optical fiber, which shows similar luminescent properties to the sol–gel preform. The obtained fiber exhibits promising characteristics to be employed in an all-fibred UV sensor. In particular, the high emission efficiency and the specific air-clad geometry of the fiber made it possible to perform measurements without removing the polymer coating.

Highly Porous Silica Glasses and Aerogels Made Easy: The Hypersaline Route

Through the addition of inorganic salts as porogens during an alcohol-based sol-gel process high surface area silica glasses and aerogels with varying pore sizes are synthesized. In the presence of ZnCl2 and CaCl2 respectively, tetraethyl orthosilicate (TEOS) dissolved in ethanol hydrolyzes also in the absence of any acid or base catalysts, eventually forming high surface area monolithic silica networks. The porosity can be controlled by the gel ageing step as well as the salt nature. Two different gel drying conditions were investigated, i.e., room temperature (RT) air drying at ambient pressure and supercritical (sc) CO2 drying. While a reference solution which only contains ethanol and TEOS does not gel at all, the addition of ZnCl2 leads to mesoporous aerogels with high surface areas of 1400 m2 g−1 after sc CO2 drying. Interestingly, the utilization of CaCl2 however, produces aerogel-like materials of high porosity for both drying methods under preservation of the nano-and macroscopic features. Thereby, it is shown that inorganic salts can influence the structure formation during the sol-gel process where the type of salt controls the framework connectivity/stability and tunes the porosity towards smaller pore sizes than usually obtained. At the same time, the herein presented hypersaline aerogel-synthesis is very simple, fast and in the case of CaCl2 drying can even be performed in air

Long-lived emission from Eu3+:PbF2 nanocrystals distributed into sol–gel silica glass

This paper reports an optical investigation of Eu3+:PbF2 nanocrystals distributed into silica glasses fabricated by sol–gel methods. The sample microstructure was investigated using scanning transmission electron microscopy. The β-cubic PbF2 crystalline phase was identified using X-ray diffraction analysis. The observed emission bands correspond to 5D0 → 7FJ (J = 0–4) transitions of Eu3+. The spectroscopic parameters for Eu3+ ions were determined based on excitation and emission measurements as well as luminescence decay analysis. Emission originating from 5D0 state of Eu3+ ions in sample containing PbF2 nanocrystals is long-lived in comparison to precursor sol–gel silica glasses.

Silica sol–gel glasses incorporating dual-luminescent Yb quinolinolato complex: processing, emission and photosensitising properties of the ‘antenna’ ligand

The [Yb(5,7ClQ)(2)(H5,7ClQ)(2)Cl] (1) complex, that exhibits dual-luminescence in the visible (ligand-centered) and in the NIR (Yb-centered), has been incorporated into a silica sol-gel glass obtaining 1-doped glassy material which is optically transparent and homogeneous and with good mechanical properties. The doped sol-gel glass can be considered a "solid state solution" and photophysical studies demonstrate that the emissive properties of the dopant complex are preserved in the silica matrix. Observed NIR decay times fall in the μs range and are likely limited by "second-sphere" matrix interactions. The ligand-to-metal energy transfer mechanism occurs on ultrafast timescale and involves ligand triplet states. The sensitization efficiency of the antenna quinolinolato ligand toward Yb(3+) is estimated to be as high as ~80%. The Yb natural radiative lifetime observed for 1 in MeCN-EtOH solution (τ(rad) = 438 μs) is the shortest reported so far for ytterbium complexes.

Size-dependent luminescence of Sm3+ doped SnO2 nano-particles dispersed in sol-gel silica glass

Sol-gel glasses with composition (100−x)SiO2–xSnO2 doped with 0.4 mol% of Sm3+, with x ranging from 1 to 10, have been successfully synthesized. Transparent doped nano-glass-ceramics were prepared by thermal treatment of the precursor glasses at 900°C during 4 hours, leading to nanocomposites comprising SnO2 nanocrystals embedded into an amorphous SiO2 phase. A structural analysis in terms of X-ray Diffraction and High Resolution Transmission Electron Microscopy confirms the precipitation of SnO2 nanocrystals within the glassy matrix. The mean radius of the obtained SnO2 nanocrystals, ranging from 2.1 to 4.7 nm calculated by the Scherrer and Brus equations, similar to the Bohr’s exciton radius, constitutes a wide band-gap semiconductor quantum-dot system. Energy transfer from SnO2 nanocrystal host to Sm3+ ions is confirmed by luminescence spectra and analyzed as a function of SnO2 concentration, showing an evolution that could be ascribed to selective excitation of nanocrystal sets with predetermined size. Besides, a study of the luminescence as a function of temperature helps to clarify the involved energy transfer mechanisms.

Silica-based nanoporous sol-gel glasses: from bioencapsulation to protein folding studies

Organic-inorganic nanoporous silica sol-gel glasses constitute the ideal support for protein bio-encapsulation and to study the different factors influencing the protein folding process in a crowded environment. Due to the facile silica surface modifications with desired Si-substituted organic groups from organosilanes precursors, organically modified 'wet-aged' silica-based glasses obtained via the sol-gel process, can be used as host materials to mimic the crowded environment of the proteins and cells that can be found in the cytoplasm for instance. Numerous studies to date showed that silica-based nanoporous glasses can stabilise bioactive proteins. However, it is important to know about the different factors affecting the protein stability and therefore its properly-folded state. In this review, we report the recent results on the influence of different parameters (such as surface hydration, hydrophobicity, solute effects, thermal stability, porosity, macromolecular crowding) on the protein conformation based on the design and the characterisation of nanoporous silica-based materials containing different functional groups (e.g., hydrophobic alkyl, phosphate and fluorinated groups). The enhancement of the protein folding owing to the physical properties and microstructure of the host matrix induced by the nature of the functional groups and the siloxane network play a major role on the protein biological activity and therefore to the development efficient bionanodevices such as biocatalysts, sensors, drug delivery systems or implanted devices. It is also the opportunity to understand the different factors influencing the protein misfolding that are the cause of devastating diseases such as Alzheimer or Huntington.

Luminescence and X-ray Diffraction Studies of Rare Earth Doped Silica Sol-Gel Glass

We present recent work exploring the effects of aluminum co-doping on the luminescent properties of rare earth ions in sol-gel glasses. Using x-ray diffraction techniques we have identified the formation of aluminum rich nano-crystals in silica glass annealed to high temperatures. We discuss implications of these results in understanding the impact aluminum co-doping has on rare earth luminescence in sol-gel glass.

Effect of reducing sintering atmosphere on Ce-doped sol–gel silica glasses

Abstract The effect of sintering atmosphere on the optical and structural properties of Cerium doped sol–gel silica was investigated. Two sets of xerogels with Ce concentration from 0 to 10 mol% were prepared and densified in different conditions. The effect of a post-densification rapid thermal treatment was also considered. Optical absorption measurements evidenced that in oxidizing conditions Cerium is preferably incorporated inside the silica matrix as Ce4+ while the trivalent state is favoured by reducing conditions. Transmission electron microscopy images show rare-earth clusters formation whose nature was investigated by means of Raman, EDS and microdiffraction measurements. In order to evaluate the efficiency of the glasses as scintillating materials radio-luminescence measurements were carried out. A numerical fit of RL spectra showed the presence of two components peaking at 2.7 and 3.1 eV. After RTT, the relative intensity of the 3.1 eV component increases with Ce concentration.

The Effect of Salts and Co-Solvents on the Cytochrome c Folding Pathway within a Sol-gel Glass

The folding reaction of ferric cytochrome c (cyt c) was examined in the presence of several salts and co-solvents in both solution and within porous silica sol-gel glasses to characterize the conformational changes that cyt c can undergo if constrained to remain in a compact state. The sol-gel pores restrict protein motions to a volume slightly larger than the native state. Unfolding was induced by changing pH and addition of guanidine. The populations of four species characterized by their heme ligation were determined using UV/VIS absorption spectroscopy: the native HM state (His18/Met80), the partially folded HW (His18/water) and HH (His18/His33) intermediates, and the 5C (water) unfolded state. In solution, the native HM state unfolds primarily into the HH species, while in the gel the HW species is formed preferentially. This indicates that the steric constraints within the gel pores hinder some backbone motions. We and others have previously shown that the water solvating the protein within the gel pores is more ordered than bulk water, leading to a decreased hydrophobic effect. Here we present the effects of several salts and co-solvents on the folding pathway of cyt c. We find that addition of some, but not all, salts and co-solvents can alter both the folding kinetics and which conformations are accessed by the protein. The results are discussed in the context of molten globule folding models and the Hofmeister ranking of chaotropic agents.

PS22 ゾルゲルシリカガラスの構造と弾性率(ポスターセッション)

PS22 ゾルゲルシリカガラスの構造と弾性率(ポスターセッション)

Structural and optical characterization of oxygenated CdTe/Sm3+ in sol–gel silica glasses

Abstract Oxygenated cadmium telluride co-doped with rare earth element samarium is prepared by sol–gel method. Sample is air annealed at 200, 500 and 800 °C. The absorption and fluorescence spectra are analysed using Judd–Ofelt analysis to obtain J–O parameters. X-ray spectroscopy analysis shows the presence of thermally grown CdTeO 3 phase developed at the expense of CdTe phase on heat treatment for samples annealed at 800 °C. The FT-IR spectrum of the sample is taken at different annealing temperatures and the assignments of the absorption bands are compiled which shows the gradual evolution of the rigid glassy network formed. Enhanced fluorescence from air annealed Co-doped samples are also investigated.

Gd-incorporation and luminescence properties in sol–gel silica glasses

A thorough study was performed on Gd-doped sol–gel silica glasses, in the concentration range 0–8 mol% Gd. The analyses were carried out as a function of Gd content, before and after a post-densification thermal treatment. Different results concerning optical, vibrational, magnetic, and structural features were gathered in correlated experiments. The presence of Gd-rich nanoclusters was revealed. The size of nanoclusters increases by increasing the dopant concentration and by performing a rapid thermal treatment (RTT) at 1800 °C in air, which causes also a remarkable intensity increase of the 6P 7/2 → 8S 7/2 radioluminescence transition of Gd 3+. Nanoclusters are amorphous, possibly close to a Gd 2SiO 5 stoichiometry as suggested by fast Fourier transform infrared spectroscopy (FTIR). FTIR studies revealed also the presence of Gd-dimers interacting with OH − groups. Moreover, the presence of isolated Gd 3+ ions was detected by electron paramagnetic resonance investigations. Based on the results obtained with different techniques, the Gd-incorporation in silica glass host and the resulting optical properties are discussed.

Folding Myoglobin within a Sol-Gel Glass: Protein Folding Constrained to a Small Volume

The unfolding and refolding reaction of myoglobin was examined in solution and within a porous silica sol-gel glass. The sol-gel pores constrain the protein to a volume that is the same size and shape as the folded native state accompanied by a few layers of water solvation. Denaturants such as low pH buffers can be diffused through the gel pores to the protein to initiate unfolding and refolding. Acid-induced unfolding was hindered by the steric constraints imposed by the gel pores such that more denaturing conditions were required within the gel than in solution to create the unfolded state. No new folding intermediates were observed. Refolding of myoglobin was not complete in millimolar pH 7 buffer alone. Addition of 25% glycerol to the pH 7 buffer resulted in nearly complete refolding, and the use of 1M phosphate buffer resulted in complete refolding. The role of this cosolvent and salt in disrupting the ordered water surrounding the protein within the gel is discussed in light of the Hofmeister series and entropic trapping via a diminished hydrophobic effect within the gel. These results are consistent with the premises of folding models in which secondary and tertiary structures are considered to form within a compact conformation of the protein backbone.

Water Diffusion in Nanoporous Glass: An NMR Study at Different Hydration Levels

By pulsed field gradient nuclear magnetic resonance measurements, we investigated the translational diffusion of water confined in the 200 A diameter pores of a sol-gel silica glass. The experiments, performed as a function of the hydration level, showed an enhancement of the self-diffusion coefficient when the water content corresponds to one or fewer monolayers. An explanation for this occurrence has been given in terms of a two-phase process involving a fast molecular exchange between the liquid and the vapor phase. Moreover, in partially filled pores, the surface water diffusion coefficient was measured, and was 4 times lower than the diffusion of liquid confined water in saturated spaces.

Synthesis and Characterization of Highly Transparent Sol-Gel Glass for Photovoltaic Applications

ABSTRACTRoom temperature sol-gel synthesis and optical characterization of highly transparent silica glass for photovoltaic (PV) applications is presented in this paper. Tetraethyl orthosilicate (TEOS), Ethanol, Hydrochloric acid (HCl) and deionized water were used as precursors in the volumetric ratio of 4:4.3:0.1:3.2 ml. Silica glass of thickness in the range of 0.5-1 cm were obtained with an average transmittance of 93% and absorption coefficient (α) of 0.08 cm−1 in a wide wavelength window of 350-1100 nm. Application of the developed sol-gel silica glass on solar ray concentration, anti-reflective coating (ARC) and effect of surface passivation on silicon wafers were examined. Carrier lifetime of the sol-gel silica passivated silicon substrate was 16 s and the calculated surface recombination velocity of the was 2200 cm/sec. Very low value of α, high transparency in a wide spectral window and effective surface passivation on silicon suggest that sol-gel processed silica glass can be a potential cost effective candidate for different PV applications.

Detection of thrombin-specific oligonucleotides using quartz crystal microbalances coated with silica sol–gel glass thin films

A silica sol–gel glass containing thrombin was spin-coated on a quartz crystal and used to measure the binding of thrombin with thrombin-specific aptamers. The porous structures of the silica thin films were optimized to facilitate binding of the aptamers with thrombin by varying the solution pH and the concentration of polyethylene glycol. The hydrophilic silica sol–gel glass prevented aptamers from adsorbing nonspecifically and the absence of covalent bonding allowed the entrapped thrombin molecules to retain their reactivity. Further, the mass sensitivity of the quartz crystal microbalance was enhanced by encapsulating more thrombin molecules inside the porous sol–gel glasses.

Microwave studies on the dielectric properties of Sm3+ and Sm3+/CdTe doped sol–gel silica glasses

Abstract Complex permittivity and conductivity studies of Samarium and Samarium/semiconductor cadmium telluride sol–gel silica glass samples were done. We use cavity perturbation technique at S band frequencies using TE 10p Mode. Structural evolution of the matrix on annealing is discussed based on FTIR analysis/XRD power diffraction. In cavity perturbation technique dielectric parameters like complex permittivity and conductivity are determined by measuring changes in resonant frequency due to small perturbation inside the cavity produced by the introduction of the samples. The addition of the semiconductor along with the samarium was found to lower the permittivity, loss factor and conductivity. Variations of permittivity values with annealing temperature find applications in IC Technology, optic fibre communication, etc. The Sm 3+ /CdTe doped glasses can also be used in the fabrication of new and improved materials for microwave electronic circuits and in electromagnetic shielding devices.

Energy transfer from the host to Er3+ dopants in semiconductor SnO2 nanocrystals segregated in sol–gel silica glasses

Undoped and Er3+-doped glass–ceramics of composition (100−x)SiO2–xSnO2, with x = 5 or 10 and with 0.4 or 0.8 mol% of Er3+ ions, were synthesised by thermal treatment of precursor sol–gel glasses. Structural studies were developed by X-Ray Diffraction. Wide band gap SnO2 semiconductor quantum-dots embedded in the insulator SiO2 glass are obtained. The mean radius of the SnO2 nanocrystals, ranging from 2 to 3.2 nm, is comparable to the exciton Bohr radius. The luminescence properties have been analysed as a function of sample composition and thermal treatment. The results show that Er3+ ions are partially partitioned into the nanocrystalline phase. An efficient UV excitation of the Er3+ ions by energy transfer from the SnO2 nanocrystal host is observed. The Er3+ ions located in the SnO2 nanocrystals are selectively excited by this energy transfer mechanism. On the other hand, emission from the Er3+ ions remaining in the silica glassy phase is obtained by direct excitation of these ions.