Water features on silica gel surfaces investigated by delayed luminescence (original) (raw)
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Delayed luminescence induced by complex domains in water and in aqueous solutions
Many recent studies on water have conjectured a complex structure composed of hydrogen bonded low- and high-density domains. In this work the structure of pure water and aqueous solutions of silica gel (TEOS) has been investigated by using delayed luminescence, which previously have showed significant increase in aqueous salt solutions where low-density domain formation is expected. Photon emission shows an Arrhenius trend with an activation energy in water-TEOS solutions larger than in pure water and salt-water solutions. Moreover, delayed photon emission decay shows an intrinsic lifetime of about 5 microseconds both in solutions and in pure water that, along with secondary lifetimes induced by the presence of TEOS, could be related to the formation of different domains.
Current Applied Physics, 2003
Development of optical biosensors is an active area of research in the field of medical technology. Sol-gel matrices made from alkoxide silicates, tetraethyl orthosilicate (TEOS) appear to be suitable glassy host matrix for the sensing system. However, the major problem in the TEOS based sol-gel matrices is stability. So it is important to study dopant-matrix interaction as a function of time. In the present study, we report fluorescence emission and excited state lifetime measurements on fluorescent probes entrapped in TEOS sol-gel for monitoring the physico-chemical processes for characterization and monitoring of local environment (pores) of dopant molecule (fluorescent probes) for construction of sensing layer for optical transducer. Different types of fluorescent probes viz., Hoechst 33258 (H258) and pyranine (PY) were used. Sol-gels containing these probes were prepared at pH-6 : 0 and the physical and spectroscopic parameters were monitored as a function of storage time (days). The emission intensity from entrapped H258 has shown relatively higher extent of decrease during aging. The excited state fluorescence lifetime measurements on these probes depicted single exponential decay component at 5.4 ns (PY) and 3.6 ns (H258) in fresh sol-gels. After a few days of storage the sol-gel containing H258 revealed an additional short decay component whereas no such alteration could be observed with the probe molecule PY. Further confirmation of multicomponents decay was obtained by distribution analysis of lifetime of H258 where an increase in width of mean lifetime was observed with storage whereas no such change was indicated from PY. Thus it appears that H258 is a better probe molecule for characterizing and monitoring local environment of pores in sol-gel.
Radiative and Nonradiative Excited State Processes for Studying the Sol to Gel Evolution
Langmuir, 2002
The sol to gel evolution of systems based on the hydrolysis of titanium n-butoxide, Ti(OBu n)4, in 1-butanol was investigated by monitoring the changes of the radiative and nonradiative electronic excited state processes of embedded dyes (cresyl violet and 4-(dicyanomethylene)-2-methyl-6(p-dimethylaminostyryl)-4H-pyran). Fluorescence anisotropy experiments (FA) allow determination of changes in the microviscosity of the medium surrounding the fluorophore through the sol-gel evolution. The increase of the anisotropy parameter, 〈r〉, is explained in terms of solvent confinement in cavities enclosed within cross-linked polymeric chains. The acoustic signal recorded in laser-induced optoacoustics experiments (LIOAS) is attenuated as the system loses fluidity, with a minimum at t g, thus providing an alternative method for determining the gelation point. In addition, within the theoretical approach of percolation theory, the exponent of the viscosity power law is obtained from the attenuation of the sound wave. Although both FA and LIOAS provide information on the degree of cross-linking between polymeric chains, there is a clear difference between the behavior of the macroscopic shear viscosity determined by LIOAS and the local friction or microviscosity obtained from FA.
In situ fluorescence probing of the chemical changes during sol-gel thin film formation
Journal of The American Ceramic Society, 1995
Pyranine (8-hydroxy-l,3,6-trisulfonated pyrene) was used as an in situ fluorescence probe to monitor the chemical evolution during sol-gel thin film deposition of silica by the dip-coating process. The sensitivity of pyranine luminescence to protonation/deprotonation effects was used to quantify changes in the water/alcohol ratio in real time as the substrate was withdrawn from the sol reservoir. The spatially resolved spectral results showed that preferential evaporation of alcohol occurred, and that the solvent composition in the vicinity of the drying line reached values in excess of 80 vol% water. Correlation of the luminescence results with the interference pattern of the depositing film allowed the solvent composition to be mapped as a function of film thickness.
Biosensors and Bioelectronics, 2005
Characterization of the internal environment of a sol-gel matrix is an important area of investigation in optical biosensors. In the present study, different sol-gel compositions were prepared by varying the water (H 2 O) to tetraethyl-orthosilicate (TEOS) ratio (R) from 1 to 16 and the changes in the internal environment of the sol-gel both in bulk and thin films as a function of aging (storage) were investigated using fluorescence spectroscopy. We focussed on the fluorescence characteristics , viz. emission and excited state lifetime of Hoechst 33258 (H258), a bisbenzimidazole derivative, which was used as fluorescence probe entrapped in the TEOS derived sol-gel bulk and thin films. These sols were prepared at a low pH (∼2.0) and the thin films were coated by dip coating technique at withdrawal speeds of 1 cm/min and 0.1 cm/min. Usually, uniform thin films were obtained at a high speed (1 cm/min) and partially cracked film at a low speed (0.1 cm/min) as observed by fluorescence microscope. These observations did not change during aging. On the contrary, three months long observations on steady-state fluorescence emission measurements on H258 depicted a blue shift from 535 nm to 508 nm at R = 1 in the sol-gel bulk, whereas at higher ratios this was not prominent. At all ratios, dual emission bands were observed in thin films. This may be due to faster sol-gel to xerogel transition during aging depending on the ratio (R). Analysis of the excited state decay profiles of H258 revealed a double exponential fitting having a short (τ 1) and a long (τ 2) component in both fresh and during aging, in the sol-gel bulk and thin films, indicating heterogeneity in the internal environment. The value of τ 1 increased from 0.4 ns to 1.2 ns whereas τ 2 attained a value from 3.0 ns to 3.6 ns at R = 1 upon aging in the sol-gel bulk. The corresponding values of τ 1 and τ 2 in thin films were 0.3 ns and 3.5 ns, respectively. The values of these decay components in thin films did not alter much due to storage, but their relative contributions showed more systematic changes in the thin films. The observed changes could be correlated to rigidification in the bulk depending on the ratio (R). This process was very slow at R ≥ 4. The heterogeneity in the internal environment of bulk and thin films upon aging appeared to be different as revealed from analysis of excited-state lifetime. Thus, the bisbenzimidazole derivative H258 appears to be very useful probe for characterizing the internal environment of both the sol-gel bulk and thin films.
A structural study of the sol-gel process by optical fluorescence and decay time spectroscopy
Journal of Non-Crystalline Solids, 1991
The opucal spectroscopy of the europmm ion has been used as a probe of the structure of sol-gel derived sdlca glasses The fluorescence and optical decay times of the Eu 3 + ion were monitored during the different stages of the sol-gel process and changes m these opucal properues were interpreted in terms of the constantly changing environment of the Eu 3+ ion.
Journal of Non-Crystalline Solids, 1986
We report the use of photophysical probes for studying the sol/gel transition of silicon tetra-alkoxide undergoing hydrolysis and condensation-polymerization, for studying the properties of the final silica gel glass, and for studying the formation of reversed-phase organic silica gel glasses during hydrolysis of phenyl and ethyl trimethoxysilanes. Pyrene fluorescence and excimerization process were used to probe, stage by stage, the dynamics of the structural changes that occur during the sol to wet gel to dry gel transitions and to probe the effects of parameter changes such as pH, water/silane ratio and alkoxy groups. Unlike adsorption on silica surfaces, pyrene trapped in the silica glass shows monomeric emission only. Structural oscillations are observed upon addition of surface active agent. To the best of our knowledge, this is the first far-from-equilibrium dissipative structure, in which a polymeric system oscillates. We also report the uses of the final gel glass as a novel room-temperature trap of organic molecules. The possibility to trap an organic molecule in inorganic glass opens many new opportunities and applications, two of which are reported: (a) Room temperature phosphorescence is obtained for a number of molecules, e.g., pyrene. (b) The first organic-dye (spiropyrane)/inorganic glass (SiO 2) photochromic material is described.
Delayed luminescence: a novel technique to obtain new insights into water structure
Journal of Biological Physics, 2012
Fully understanding the structure of water is a crucial point in biophysics because this liquid is essential in the operation of the engines of life. Many of its amazing anomalies seem to be tailored to support biological processes and, during about a century, several models have been developed to describe the water structuring. In particular, a theory assumes that water is a mixture of domains constituted by two distinct and interconverting structural species, the low-density water (LDW) and the high-density water (HDW). According to this theory, by using some particular solutes or changing the water temperature, it should be possible to modify the equilibrium between the two species, changing in this way the water behavior in specific biological processes, as in governing the shape and stability of the structures of proteins. In this work, we assess the possibility of obtaining information on the structures induced in water by specific salts or by temperature by measuring the delayed luminescence (DL) of some salt solutions and of water in the super-cooled regime. Previous works have demonstrated that the delayed luminescence of a system is correlated with its dynamic ordered structures. The results show significant DL signals only when the formation of LDW domains is expected. The measurement reveals a similar activation energy for the domains both in aqueous salt solutions and super-cooled
Probing Si and Ti Based Sol-Gel Matrices by Fluorescence Techniques
Journal of Fluorescence, 2002
The photophysical behavior of several probes incorporated in sol-gel-derived matrices (both monoliths and thin films) has been studied using steady-state and time-resolved fluorescence, along with fluorescence anisotropy to study the matrix structure and to elucidate probe-matrix interactions. The probes studied include laser and solvatochromic dyes along with porphyrins and phthalocyanines. It was found that spectral shifts, time-resolved decays, and quantum yields depend on the type of matrix and its preparation conditions combined with the drying time and the nature of retained solvent, which can be added to act as an anticracking agent. The differences between the results in the TiO 2 matrix, where electron transfer is most probably present, and SiO 2 are shown.