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39796-B10
Spectroscopic Investigation of Aerogel Structure and Suitability for Sensor Platforms

Mary K. Carroll, Union College

We are investigating probe-doped silica aerogels prepared via the Union College rapid supercritical extraction (RSCE) method. In this method, aerogels are prepared from liquid precursors within a mold in a hot press, in a process that takes approximately eight hours. Luminescent probes included in the precursor mixture become entrapped within the sol-gel matrix, and are accessible to gas-phase analytes of interest.

During the third year of the grant period, we have (1) entrapped a variety of luminescent probes in RSCE aerogels; (2) continued spectral characterization of sol-gel materials doped with the oxygen-sensitive luminescent probes platinum(II) octaethylporphyrin (PtOEP) and platinum(II) meso-tetra(pentafluororphenyl)porphine (PtTFPP): luminescence excitation and emission spectra, time-based emission scans as a function of quencher concentration, and luminescence lifetime analysis; (3) compared probe-doped silica aerogels made via our RSCE method with those prepared from the same TMOS-based precursor mixture through (a) conventional supercritical CO₂ extraction in a critical point dryer and (b) drying under ambient conditions to form xerogels; and (4) undertaken physical characterization, including FTIR spectra, of these materials.

There are two main advantages to using aerogels prepared via the RSCE method as platforms for gas sensors. First, there is no opportunity for the probe to leach out of the sol-gel matrix during this method, whereas leaching of a substantial fraction of the probe moieties occurs in the solvent-exchange steps required for the SC CO₂ method, which limits significantly the analytical response achievable with those sensors. Second, the processing time for the RSCE method is much shorter; the conventional method involves formation of a wet sol-gel, solvent exchange to liquid CO₂, and supercritical extraction of the CO₂, a process that takes several days in total. The major disadvantage of the RSCE method is that one must employ high temperatures (up to 290?C), so this approach is limited to thermally stable probes.

Silica aerogel materials prepared via the two methods have comparable bulk and skeletal densities, FTIR spectra, and optical transmittance in the visible region. Aerogels prepared via either method have higher surface areas than xerogels; however, the SC CO₂ aerogels have significantly higher surface areas (925 m²/g) than the RSCE aerogels (560 m²/g). This difference in surface area may be the result of differing times for the sol gels to “age” prior to the extraction step (1-2 days for SC CO₂ vs. 1-2 hours for RSCE aerogels). We are investigating this further.

Aerogels doped with PtTFPP show considerable promise as oxygen sensors. They exhibit more uniform response to changes in oxygen concentration than do the comparable PtOEP-doped aerogels, and have Stern-Volmer quenching constants comparable to the better PtOEP-based sensors we have prepared. Moreover, the PtTFPP-doped materials have better photostability.

The funding of this research (Jan. 2004 to present) has had a significant positive impact on the PIs research program and career. She included results obtained through ACS-PRF funding in successful NSF RUI (CHE-0514527) and NSF MRI (CMMI-0722842) grant proposals, and she was promoted to full professor while the grant was active.

During 2006-2007, five undergraduate students were directly involved in the project: three seniors, one junior and one sophomore. Of the seniors, one (Reeve) is now a chemistry graduate student at Carnegie Mellon, another (Ferrarone) is in an NIH postbaccalaureate research position. There were five presentations at national and international meetings related to this project [1-5], as well as several presentations by students in seminar and symposium venues at Union College, and two invited lectures by the PI at Hamilton College and Connecticut College.

[1] Mary K. Carroll, Amanda J. Barrow**, John R. Ferrarone*, Sadie G. Gorman*, Aaron F. Phillips* and Ann M. Anderson “Use of luminescent probes for comparison of supercritical extraction techniques in silica aerogel preparation.” Paper INOR 92 at the 233rd ACS National Meeting, Chicago, IL, March 2007.

[2] Adam E. Reeve**, Mary K. Carroll, David C. Korim* and Ann M. Anderson “Characterization of silica-based aerogels: Surface area, pore size distribution and infrared spectra.” Paper CHED 1373 at the 233rd ACS National Meeting, Chicago, IL, March 2007.

[3] Jessica B. Reichbind**, Shawn W. Bartok*, Melissa K. Passarelli*, Mary K. Carroll and Ann M. Anderson “Suitability of silica aerogels as platforms for sensors based on phosphorescent probes.” Paper CHED 370 at the 233rd ACS National Meeting, Chicago, IL, March 2007.

[4] John R. Ferrarone**, Amanda J. Barrow*, Mary K. Carroll and Ann M. Anderson “Spectroscopic Analysis of Oxygen-Sensing Aerogels.” Oral presentation at the 20th National Conference on Undergraduate Research, San Rafael, CA, April 2007.

[5] A. J. Barrow*, J. R. Ferrarone*, A. F. Phillips*, S. Baig*, M. K. Carroll (presenting) and A. M. Anderson “Comparison of Silica Aerogels Prepared via Two Supercritical Extraction Methods” Poster paper 82 at the XIVth International Sol-Gel Conference, Montpellier, France, September 2007.

** indicates presenting undergraduate author; * indicates current or former undergraduate student co-author

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