A study of the effects of organic modification and processing technique on the luminescence quenching behavior of sol-gel oxygen sensors based on a Ru(II) complex

The luminescence decay behavior of ruthenium(II)-tris-4,7-diphenyl-1,10-phenanthroline dichloride dissolved in different inorganic and organically modified silicate (ORMOSIL) sol-gel matrixes was investigated. Bulk xerogels and spin-coated thin films were synthesized from methyltrimethoxysilane (MTMS), tetraethyl orthosilicate (TEOS), and various molar ratios of the two precursors. Systematic changes in composition were conducted to examine the structural properties of sol-gel silicates for possible oxygen sensor supports. The luminescence behavior of the ruthenium complex quenched by oxygen was analyzed as a function of the sol-gel composition and processing technique. The Stern-Volmer ratio, indicative of the degree of luminescence quenching by oxygen, was found to increase with increasing additions of MTMS in the bulk xerogels. This improvement in oxygen sensitivity was attributed to the increased diffusivity in the less polar sol-gel matrix. Conversely, spin-coated thin films showed a decreasing Stern-Volmer ratio with increasing MTMS addition and, thus, a lower oxygen sensitivity than the inorganic TEOS samples. This contradictory quenching behavior was attributed to structural changes in the sol-gel matrix caused by the spin-coating process. In addition to luminescence decay measurements, XPS and ellipsometric studies were performed to evaluate the structural effects of MTMS on the properties of TEOS-derived xerogels and spin-coated thin films.