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/cm 3 ), N 2 -adsorption surface area (870 m 2 / g), and thermogravimetric and scanning electron micrographic characteristics of the dyemodified aerogels remain identical to those of the unmodified silica aerogels. The bulk concentration of the dye in the aerogels was e6.6 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 O 2 -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 O 2 or Ar in the DAPaerogel monoliths was estimated at g0.01-0.02 cm 2 /s, which is only 10× less than the unimpeded diffusion coefficient of Ar in air.

Synthesis of Aerogel-Metal Cluster Composites by Gamma Radiolysis

“…In SEM micrographs of the Ag-aerogel composites, the bead structure of silica is evident, and similar to that reported for native silica aerogelsJ31 The density values of the metal doped aerogels are larger, but still comparable with those of pure silica aerogels (0.003-0.010 g/cm 3 ). [71 Surface areas and mean pore sizes are in the 700-900 m 2 /g and 7-10 nm range respectively, close to the values reported for native silica aerogels (700-1000 m 2 /g and, 8-14 nm, respectively).…”

“…In situ formation and entrapment of metal clusters in aerogels is desirable for application in catalysis [I,2], sensors [3], waste disposal [4,5], and electromagnetic shielding. Metal clusters have been formed in aerogels by adding to a hydrogel a precursor, generally an organometallic compound that does not leach out during the wash cycles required prior to the final supercritical drying procedure.…”

“…[10] Experimental Details Silica aerogel composites were prepared by a modification of previously published procedures in which the contents of vial A (4.514 mL of tetramethoxysilane; 3.839 mL of methanol) and of vial B (4.514 mL of methanol; 1.514 mL of water, and 20 p L of concentrated NH 4 OH) are mixed thoroughly to form a sol that gels at room temperature in 10-15 min. [3] The gels were left to age at room temperature then were removed from their molds and soaked in water. The water-washed gels were washed again, with an aqueous solution of the desired metal ion.…”

“…The pore-filling acetone was removed by supercritical drying with CO 2 . [3] The samples were irradiated with y radiation from the fission products of the campus' nuclear reactor (a pool reactor with a maximum power of 200 kW that employs 235Ur fuel rods.) Typical total doses were up to about 3.5 kGy per run, as measured by Thermoluminescent Dosimeters (TLD) placed in vials adjacent to those containing the samples.…”

Encapsulating Cytochrome <em>c</em> in Silica Aerogel Nanoarchitectures without Metal Nanoparticles while Retaining Gas-phase Bioactivity

“…All of the pores in solid aerogels result in much available surface area making aerogels extremely useful for any applications where surface reactions are important. When chemical or biochemical functionality is assembled within the aerogel nanoarchitecture, it has been shown that the physical porosity and enhanced surface area of the aerogels help to improve sensors, as well as electrodes for battery, fuel cell, and supercapacitor applications 11,[15][16][17][18][19][20][21][22][23] . In order to dry aerogels in a way that leaves the porous solid matrix unchanged, it is typical to remove the solvent that remains in the pores after sol-gel synthesis through supercritical solvent extraction.…”