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1998
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November
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In the Laboratory
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Fluorescent Probes and the Structural Evolution of Hybrid and Nonhybrid Sol-Gel Systems
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L. M. Ilharco, J. M. Gaspar Martinho, and C. I. Martins
Complexo I, Instituto Superior Tecnico, Centro de Quimica-Fisica Molecular
Av. Rovisco Pais,
Lisboa Codex 1096,
PORTUGAL
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November 1998
Vol. 75 No. 11
p. 1466
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| Abstract |
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A pedagogical approach to the sol-gel process is suggested, taking into account the interest of such an interdisciplinary subject for an advanced undergraduate course in chemistry. A comparison is made between pure inorganic and hybrid sol-gel systems, obtained by incorporation of polymers and exhibiting interesting properties. In these systems, the structural evolution during the sol-gel-dry gel (xerogel) process is a conditioning factor of the final properties. As case studies, we have selected sol-gel systems consisting of tetraethyl orthosilicate (TEOS, a widely used precursor), water, and ethanol at pH 1.2 and 2.5, and the same systems modified by addition of polytetrahydrofuran (PTHF). The sol-gel-xerogel structural evolution was followed using a carefully chosen fluorescent probe, Py-(CH2)12-Py, incorporated in a very low concentration (of the order of 10-6 M). The ratio between fluorescence intensities of excimer and monomer of pyrene, IE/IM, was followed with time. The increase of this ratio with time for pH 1.2, in contrast to pH 2.5, has been interpreted in terms of the different sizes of the primary silica particles, which have a hydrophobic character, and in which the fluorescent probe is encapsulated. A slower increase of the IE/IM ratio for hybrid systems was explained by the presence of PTHF: the polymer acts as a spacer between silica particles, and, owing to its higher hydrophobic character when compared to the solvent, retains a significant part of the probe molecules. Based on the fluorescence results, structural models are proposed for the two types of systems.
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| More Information |
 Citation |
Ilharco, Laura M.; Martinho, J. M. Gaspar; Martins, C. I. J. Chem. Educ. 1998 75 1466.
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Keywords |
fluorescence spectrometry, laboratory instruction
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History |
Created:
Last Updated: |
June 18, 1999
June 24, 2005
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| Home > JCE Print > Journal of Chemical Education > Issues >
1998
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November
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1466
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