1. Anderson M. L., Morris C. A., Stroud R. M., Merzbacher C. I., Rolison D. R., Langmuir 15, 674 (1999).
2. Base- and acid-catalyzed gels were derived from published procedures (3) and are described in detail elsewhere (1). Particle suspensions (Au or Pt sol) were combined with an equal vol % of silica sol. The mixture was stirred for 1 min then poured into vials (subsequently sealed with Parafilm) where gelation occurs. For solid-silica composites the volume percent of the solid ranged from 1 to 90 vol %. When adding solid at <50 vol % the solid was added rapidly to the sol while stirring. After mixing for 15 s the dispersion was poured into small vials just before gelation. When adding solid at >50 vol % the sol was added to a vial containing the solid which was then sealed and shaken vigorously until gelation occurred in order to ensure homogenous dispersion of the solid in the gel. Stirring or shaking is preferred over ultrasonication because certain solids (carbons or polymers) will phase-separate during sonication. The gelation time is dependent on the type and amount of catalyst used and on the solid incorporated: typically for a 15-ml total volume of silica sol 0.7 ml of 30% NH 4 OH (base-catalyzed gels; gelation time of 1.5 min) or 0.07 g of HCl (acid-catalyzed gels; gelation time of 3 min) was used. To date the only solid we have found that affects the gelation time relative to neat silica sol is titania aerogel for which the gelation time decreases proportionately to the amount of solid added. The composite gels are aged for no longer than 10 to 20 min before washing steps are initiated.
3. Russo R., Hunt A. J., J. Non-Cryst. Solids 86, 219 (1986);
4. Ellerby L. M., et al., Science 225, 1113 (1992).
5. C. J. Brinker and G. W. Scherer Sol-Gel Science (Academic Press New York 1990).