The optical, chemical and photochemical properties of organic molecules trapped in sol-gel materials for fiber optic chemical sensors

Effects of the sol-gel matrix on the optical properties of nile blue A, thionin and ruthenium(II)-tris(4,7-diphenyl-1,10-phenanthroline) were investigated. Three xerogel compositions doped with nile blue A and thionin were initially examined by absorption and fluorescence spectroscopy. The silicate and aluminosilicate xerogels exhibited similar spectral properties whereas the organically modified xerogels, synthesized from both tetraethyl orthosilicate (TEOS) and methyltrimethoxysilane (MTMS), shifted the absorbance and emission peaks towards the blue. The blue shift in the spectra was attributed to less polar xerogels that resulted from network modifying Si-CH{dollar}sb3{dollar} groups. Preliminary testing of the nile blue A and thionin containing xerogels for potential fiber optic sensor applications yielded negative results. The nile blue A xerogels, examined for pH sensing, were characterized by poor chemical stability in alkaline solutions. On the other hand, the photochemical properties of thionin prevented its use for hydrogen sulfide gas sensing.; The second part of experimentation evaluated the effect of MTMS on the properties of TEOS-derived xerogels. In thionin doped xerogels, increasing the concentration of MTMS was found to enhance the formation of thionin dimers. Secondly, FTIR spectroscopy revealed that MTMS additions reduced the polarity of the silica cage. Furthermore, atomic force microscopy (AFM) imaging of the xerogel surface exhibited surface roughening with the addition of MTMS.; The final stage of research instituted an organically modified silicate composition, doped with a ruthenium complex, that was coated onto a porous fiber substrate for fiber optic oxygen sensor measurements. Due to the reduced signal-to-noise ratio in the solid state instrumentation (a blue LED and a silicon detector), the ultimate sensitivity of this device was below 1 ppm. However, the sensor suffered from a relatively long response time of approximately 10 seconds and a poor Stern-Volmer ratio. Finally, preliminary results demonstrated a collimator device and a planar waveguide for oxygen sensor applications. The non-linear Stem-Volmer plots of these sensors were supported by a double exponential lifetime decay of the ruthenium complex that was trapped in the organically modified silicate composition.