Luminescent probes of hydrophobic silica aerogels

Aerogels have many applications such as platforms for sensing chemical species as well as insulation between windows. When Silica aerogels are exposed to water their nanostructure deteriorates. However, we can fabricate hydrophobic aerogels using Union’s patented rapid supercritical extraction (RSCE) technique, in a hydraulic hot press. Varying degrees of hydrophobic aerogels have been fabricated using different mixtures of tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) via a base-catalyzed reaction. Sessile drop tests reveal that water beads on these hydrophobic aerogels with a contact angle of above 150 degrees. But does this surface hydrophobicity render the porous nanostructure impermeable to water? We describe here a series of spectroscopic experiments in order to ascertain the extent to which water vapor penetrates hydrophobic aerogels of varying degrees of hydrophobicity. We have entrapped and detected Rhodamine 6G, Fluorescein, and Rhodamine B in hydrophilic and hydrophobic aerogels to be used as humidity-sensitive probes. We have shown that the fluorescence emission signal of Fluorescein increases in response to increasing relative humidity. We have shown that hydrophobic aerogels are not impervious to water vapor. A humidity apparatus has been set up that will allow us to quantify the relative humidity of the aerogel environment while collecting fluorescence spectra. We describe here our spectroscopic evaluation of humidity-sensitive probes immobilized within hydrophobic and hydrophilic silica aerogels that are exposed to varying amounts of water vapor.

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