Durable Modification of Silica Aerogel Monoliths with Fluorescent 2,7-Diazapyrenium Moieties. Sensing Oxygen near the Speed of Open-Air Diffusion

Abstract

Silica sol−gels were covalently modified with N-(3-trimethoxysilylpropyl)-2,7-diazapyrenium bromide (DAP). Luminescent aerogels are created in which none of the fluorophore leaches from the gels during either the washing or supercritical drying necessary to prepare DAP-modified aerogels. The bulk density (0.17 g/cm3), N2-adsorption surface area (870 m2/g), and thermogravimetric and scanning electron micrographic characteristics of the dye-modified aerogels remain identical to those of the unmodified silica aerogels. The bulk concentration of the dye in the aerogels was ≤6.6 mM; at these loadings the aerogels demonstrate bulk photoluminescence. As based on the mesoporous surface area, the surface coverage of the dye is 7−8% of a monolayer. The absorption, emission, and O2-quenching characteristics of the diazapyrenium dye in the aerogels parallel those obtained in alcoholic (rather than aqueous) solution, which further indicates that the dopant molecules are isolated from each other and that they see an environment with a ∼OH polarity. Time-resolved emission studies indicate that all DAP moieties reside in a single type of microenvironment. Emission quenching of ∼1-cm-diameter monoliths of DAP−silica aerogel is complete in <15 s, which compares very favorably with the best response times for pyrene guests in micrometer-thick xerogel films. The apparent diffusion coefficient of O2 or Ar in the DAP−aerogel monoliths was estimated at ≥0.01−0.02 cm2/s, which is only 10× less than the unimpeded diffusion coefficient of Ar in air.