Durable modification of silica aerogel monoliths with fluorescent 2,7-diazapyrenium moieties. Sensing oxygen near the speed of open-air diffusion

Silica sol-gels were covalently modified with N-(3-trimethoxysilylpropyl)-2 ,7-diazapyrenium bromide (DAP). Luminescent aerogels are created in which n one of the fluorophore leaches from the gels during either the washing or s upercritical drying necessary to prepare DAP-modified aerogels, The bulk de nsity (0.17 g/cm(3)), N-2-adsorption surface area (870 m(2)/g), and thermog ravimetric and scanning electron micrographic characteristics of the dye-mo dified aerogels remain identical to those of the unmodified silica aerogels , The bulk concentration of the dye in the aerogels was 16.6 mM; at these l oadings the aerogels demonstrate bulk photoluminescence. As based on the me soporous surface area, the surface coverage of the dye is 7-8% of a monolay er. The absorption, emission, and O-2-quenching characteristics of the diaz apyrenium dye in the aerogels parallel those obtained in alcoholic (rather than aqueous) solution, which further indicates that the dopant molecules a re Isolated from each other and that they see an environment with a similar to OH polarity, Time-resolved emission studies indicate that all DAP moiet ies reside in a single type of microenvironment. Emission quenching of simi lar to 1-cm-diameter monoliths of DAP-silica aerogel is complete in <15 s, which compares very favorably with the best response times for pyrene guest s in micrometer-thick xerogel films. The apparent diffusion coefficient of O-2 or Ar in the DAP-aerogel monoliths was estimated at greater than or equ al to 0.01.-0.02 cm(2)/s, which is only 10x less than the unimpeded diffusi on coefficient of Ar in air.