The authors thank undergraduate students Lutao Xie, for physical characterization of the aerogel materials, and Aude Bechu, for testing the draft procedure. We are grateful to the Union College Engineering Laboratory for machining the stainless steel mold. The Union College Aerogel Laboratory has been funded by grants from the National Science Foundation (NSF MRI CTS-0216153, NSF RUI CHE-0514527, NSF MRI CMMI-0722842, NSF RUI CHE-0847901, NSF RUI DMR-1206631, and NSF MRI CBET-1228851). This material is based upon work supported by the NSF under Grant No. CHE-0847901.

Safety Considerations: Safety glasses or goggles should be worn at all times during the preparative work with solutions and the hydraulic hot press. Laboratory gloves should be worn when preparing the chemical reagent solution and when pouring the solution into the mold in the hot press. TMOS, methanol and concentrated ammonia, and solutions containing these reagents, must be handled within a fume hood. The supercritical extraction process releases hot methanol, so it is necessary both to vent the hydraulic hot press, and to ensure that there are no ignition sources within the vent path of the hot press. In addition, we recommend installation of a safety shield around the hot press. In the event of a gasket failure, the shield will help contain the resulting gasket pieces and thereby protect anyone working near the hot press.
1. Prepare Reagents and Other Supplies
<ol>
	<li>Gather the reagents needed for the recipe: tetramethylorthosilicate, methanol, deionized water, and ammonia.</li>
	<li>Make 100.0 ml of a 1.5&#160;M ammonia solution. To do so, dilute 10.1 ml of 14.8 M concentrated ammonia to 100 ml with deionized water.</li>
	<li>Acquire a square stainless steel mold, 12.7 cm x 12.7 cm x 1.9 cm high, with 9 circular wells of 2.2 cm diameter (see Figure 3). Wipe the mold with a clean, damp rag to remove any surface oil or dust. Spray inside of each circular well with high-temperature mold release spray to ease in removal of aerogels from the mold after processing.</li>
	<li>Prepare three sets of sealing gaskets from 1/16 in&#160;(1.6 mm) thick graphite sheet and 0.0005 in&#160;(0.012 mm) thick stainless steel foil. Cut three pieces of each material sufficient to cover the mold completely (&#62;12.7 cm x &#62;12.7cm).</li>
</ol>
2. Prepare Instruments
<ol>
	<li>Program the hot press sealing and extraction programs. First set up a sealing program that will be used to seal the bottom of the open mold. See Table 1 for the necessary program values. Next set up the extraction program with the correct parameters for the silica aerogels using the mold described above. See Table 2 for these parameters.</li>
	<li>Prepare glassware. To avoid contamination, four glass beakers will be needed, one 250&#160;ml beaker labeled &#39;precursor solution,&#39; one 100&#160;ml beaker labeled &#39;methanol,&#39; one 20&#160;ml beaker labeled &#39;DI water,&#39; and one 10&#160;ml beaker labeled &#39;1.5&#160;M ammonia.&#39; Make sure all beakers are clean and dry.</li>
	<li>Prepare pipettes. Digital pipettes should be used for ease. A 10&#160;ml digital pipette and a 1,000&#160;&#956;l&#160;pipette will be used. Make sure multiple pipette tips are available.</li>
	<li>Prepare sonicator by adding water to the fill line.</li>
</ol>
3. Seal Mold Bottom
<ol>
	<li>Place mold and gasket material in hot press. First center a graphite sheet on the lower platen, add a sheet of stainless steel foil and place the mold on top of the stainless steel foil. Add another set of gasket material (stainless steel then graphite) on top of the mold. (Note: used gasket material can be used on the top in this step, but new gasket material must be used on the bottom.)</li>
	<li>Start the hot press sealing program, using the parameters shown in Table 1. This program seals the bottom of the mold to prevent the liquid precursor chemicals from leaking when the mold is filled with precursor solution.</li>
</ol>
4. Make Precursor Solution
The recipe for TMOS-based silica aerogels is shown in Table 3. All solution preparation work is performed in a fume hood.
<ol>
	<li>First pipette aliquots of TMOS totaling 17.0 ml from the reagent bottle into the 250&#160;ml&#160;glass beaker labeled &#39;precursor solution&#39;.</li>
	<li>Pour some methanol into the 100&#160;ml glass beaker and then pipette aliquots of methanol totaling 55.0 ml into the 250&#160;ml glass beaker labeled &#39;precursor solution.&#39;</li>
	<li>Pour some deionized water into the 20&#160;ml beaker labeled &#39;DI water&#39; and from that beaker pipette 7.2 ml of water into the 250&#160;ml beaker.</li>
	<li>Lastly, pour some 1.5&#160;M NH3 into the 10&#160;ml beaker and from that beaker pipette 270 &#181;l&#160;of the solution into the 250&#160;ml beaker.</li>
	<li>Seal the beaker with plastic paraffin film.</li>
	<li>Mix reagents to ensure that hydrolysis occurs by sonicating the precursor solution for at least 5 min. Prior to sonication, two liquid layers are sometimes visible in the precursor mixture. Following 5 min of sonication, the solution should appear to be monophasic. If it does not, sonicate the mixture for an additional 5 min.</li>
</ol>
5. Pour Precursor Solution into the Mold in the Hot Press
<ol>
	<li>At the end of the mold sealing program the hot press platens will open. Remove the top-side gasket material and set aside. Leave the mold as is in the hot press so that the bottom side of the mold remains sealed.</li>
	<li>Fill each well of the mold completely with the precursor solution. (Note: there will be about 10 ml of aerogel precursor solution left over after filling the mold. This can be discarded or processed under ambient conditions to make xerogels.)</li>
	<li>Put fresh gasket material on the top of the mold: the stainless steel foil first and then the graphite on top.</li>
	<li>Run the hot press extraction program (shown in Table 2). This program seals the mold, heats the contents to a supercritical state, performs the supercritical extraction and then cools the mold.</li>
</ol>
6. Remove the Aerogels from the Mold
<ol>
	<li>When the extraction process is complete, remove the mold and gasket material from the hot press.</li>
	<li>Remove the top gasket material from the mold. Set this aside.</li>
	<li>Gently loosen the mold from the bottom gasket material.</li>
	<li>Carefully remove each aerogel from the mold, one at a time, by firmly pushing them through from one side with a gloved finger.</li>
	<li>When the aerogels are removed from the mold, the process is complete.</li>
</ol>

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K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Alumina+aerogels+prepared+via+rapid+supercritical+extraction.">Alumina aerogels prepared via rapid supercritical extraction.</a> J. Sol-Gel Sci. Technol. 53 (2), 216-226 (2010).</li><li>Dunn, N. J. H., Carroll, M. K., Anderson, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Characterization+of+alumina+and+nickel-alumina+aerogels+prepared+via+rapid+supercritical+extraction.">Characterization of alumina and nickel-alumina aerogels prepared via rapid supercritical extraction.</a> Polymer Preprints. 52 (1), 250-251 (2011).</li><li>Plata, D. L., Briones, Y. 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The authors declare that they have no competing financial interests.

The RSCE method produces consistent batches of monolithic silica aerogels using an automated and simple process. The method as presented here requires an eight-hour processing step. It is possible to speed up the heating and cooling steps to make monolithic aerogels in as little as 3 hr22; however, when an 8 hr procedure is employed, more consistent batches of aerogel monoliths result. Small variations in the process parameters do not affect the physical properties of the resulting aerogels, indicating that the process is robust22.
The precursor recipe employed here results in monolithic silica aerogels; however, the rapid supercritical extraction process can be employed to make a variety of other types of aerogels20 for a wide range of potential uses including hydrophobic silica aerogels23 (for applications ranging from chemical spill cleanup to enhanced daylighting), titania and titania-silica aerogels24 (for photocatalysis), and alumina and alumina-based aerogels25,26 (for catalysis applications). It is possible to place a liquid precursor mixture, as in this work, or a previously prepared wet gel26 in the wells of the mold for processing. The main limitation is that the chemicals involved in the formation of the sol-gel matrix not react with either the metal mold or gasket material at the temperatures employed in the extraction process. In addition, it is necessary to ensure that the supercritical point of the solvent or solvent mixture within the pores of the aerogel can be exceeded during the hot press process.
The aerogels can be doped with other chemicals (for example to make sensors as in Plata et al.27) by using a solution of the dopant molecule in methanol or water in place of pure solvent in the precursor mixture; however, the added chemicals must be thermally stable up to the maximum temperature employed in the hot press program in order to survive RSCE processing.
When using the Union RSCE process it is important to supply the appropriate amount of restraining force. Different sized and shaped molds can be used but the hot press restraining force must be adjusted accordingly28. If the force is too low, then the solvent will vent sub-critically and the wet gel will shrink within the mold. If the force is too high, then excessive pressure will build up in the mold and the aerogel will be destroyed upon extraction. The maximum aerogel size is limited by the maximum restraining force of the hot press. With a 24&#160;ton hot press, we have prepared monoliths as large as 7.6 cm&#160;x 7.6 cm x 1.3 cm. Roth et al.28&#160;provide more information on appropriate processing conditions.
The maximum temperature used in this protocol is 288 &#176;C, which is well above the critical temperature of methanol (240 &#176;C) but below the supercritical temperature of water (374 &#176;C). The wet gel likely contains some water so we have increased the maximum temperature in order to exceed the supercritical point of the solvent mixture. It is possible to heat to a lower maximum temperature (~250 &#176;C) if needed; however, if this is done a longer dwell time (~60 min) in Protocol 2 of the hot press extraction program (see Table 2) is recommended to ensure that mold and wet gel reach a sufficiently high temperature.
If the processing steps are not consistently producing translucent monoliths, then use of an instrumented mold, equipped with pressure and temperature sensors (as in Anderson et al.22 or Roth et al.28) is recommended to confirm in-mold conditions. If aerogel monoliths are observed to be cloudier than usual, consider preparing a fresh batch of catalyst solution. Over time, the 1.5&#160;M ammonia solution can become less concentrated due to reaction of ammonia with atmospheric CO2. The lower-concentration solution of the ammonia catalyst results in longer gelation times, but this is not visually apparent when gelation occurs within the mold in the hot press.
We use a mold with wells that go completely through the block of metal. Such a mold allows for easy removal of intact monolithic aerogels after processing; it is also easier to machine than a mold in which each well has a solid, flat bottom. A disadvantage of this mold design is that if the mold is not properly sealed in Protocol 3 of the Procedure, the liquid precursors will leak from the bottom of the mold onto the lower hot press platen. When it is not important in the desired application to obtain intact monolithic aerogels, a mold with closed-bottom wells can be employed. In this case, monoliths will still form in the mold, but because of the lack of shrinkage of the matrix, one will have to break the aerogels in order to remove them from the mold.
In summary, the Union College rapid supercritical extraction method for aerogel fabrication has several advantages. It is fast: the protocol described here results in high-quality silica aerogel monoliths in eight hours. It is environmentally friendlier and potentially more cost-effective than other aerogel fabrication methods that require solvent exchanges: the RSCE method is not labor-intensive, requiring less than 20 min of preparation time per batch of aerogels, and generates little solvent waste. Finally, this RSCE method has promise for automation and scale-up: hydraulic hot presses come in many sizes, from bench-top models to production line equipment.

Silica aerogel materials have low density, high surface area, and low thermal and electrical conductivity combined with a nanoporous structure with excellent optical properties. The combination of these properties in one material makes aerogels attractive in a large number of applications1. In a recent review article, Gurav et al. describe in detail the current and potential applications of silica aerogel materials, both in scientific research and in development of industrial products2. For example, silica aerogels have been used as absorbents, as sensors, in low-dielectric materials, as storage media for fuels, and for a wide array of thermal insulating applications2.
Aerogels are typically fabricated using a two-step process. The first step involves mixing appropriate chemical precursors, which then undergo condensation and hydrolysis reactions to form a wet gel. To prepare silica gels, the hydrolysis reactions occur between water and a silica-containing precursor, in this case tetramethylorthosilicate (TMOS, Si(OCH3)4), in the presence of acid or base catalyst. 
Si(OCH3)4 + H2O <img alt="arrow" fo:content-width=".2in" src="/files/ftp_upload/51421/arrow.jpg" width="20" /> Si(OCH3)4-n(OH)n + n CH3OH
TMOS is insoluble in water. In order to facilitate hydrolysis, it is necessary to include another solvent, in this case methanol (MeOH, CH3OH), and to stir or sonicate the mixture. Base-catalyzed polycondensation reactions then occur between the hydrolyzed silica species:
R3SiOH + HOSiR3 <img alt="arrow" fo:content-width=".2in" src="/files/ftp_upload/51421/arrow.jpg" width="20" /> R3Si-O-SiR3 + H2O
R3SiOH + CH3OSiR3 <img alt="arrow" fo:content-width=".2in" src="/files/ftp_upload/51421/arrow.jpg" width="20" /> R3Si-O-SiR3 + CH3OH
The polycondensation reactions result in the formation of a wet gel, comprised of a porous SiO2 solid matrix, in which the pores are filled with the solvent byproducts of the reaction, in this case methanol and water. The second step involves drying the wet gel to form an aerogel: removing the solvent from the pores without altering the solid matrix. The drying process is critically important to the formation of the aerogel. If not carried out correctly the fragile nanostructure collapses and a xerogel is formed as illustrated schematically in Figure 1.
There are three basic methods for drying sol-gel materials to produce aerogels: supercritical extraction, freeze drying and ambient pressure drying. The supercritical extraction methods avoid crossing the liquid-vapor phase line so that surface tension effects do not cause the nanostructure of the gel to collapse. Supercritical extraction methods can be performed at high temperature (250-300 &#176;C) and pressure with direct extraction of the alcohol solvent byproduct of the condensation and hydrolysis reactions3-7. Alternatively, one can perform a set of exchanges and replace the alcohol solvent with liquid carbon dioxide, which has a low supercritical temperature (~31 &#176;C). The extraction can then be performed at relatively low temperature8,9, albeit at high pressure. Freeze drying methods10,11 first freeze the wet gel at low temperature and then allow the solvent to sublimate directly to a vapor form, again avoiding crossing the liquid-vapor phase line. The ambient pressure method uses surfactants to reduce the surface tension effects or polymers to strengthen the nanostructure, followed by drying of the wet gel at ambient pressure12-16.
The Union College Rapid Supercritical Extraction (RSCE) process is a one-step (precursor to aerogel) method17-19. The method employs high-temperature supercritical extraction, which allows fabrication of monolithic aerogels in hours, rather than the days to weeks required by other methods. The method utilizes a confined metal mold and a programmable hydraulic hot press. Chemical precursors are mixed and poured directly into the mold, which is placed between the platens of the hydraulic hot press. The hot press is programmed to close and apply a restraining force to seal the mold. The hot press then heats the mold at a specified rate to a temperature, Thigh, above the critical temperature of the solvent (see Figure 2 for a plot of the process). During the heatup period the chemicals react to form a gel and the gel strengthens and ages. As the mold is heated the pressure also rises, eventually reaching a supercritical pressure. Upon reaching Thigh, the hot press dwells at a fixed state while the system equilibrates. Next the hot press force is decreased and the supercritical fluid escapes, leaving behind a hot aerogel. The press then cools the mold and its contents to room temperature. At the end of the process (which can take 3-8 hr) the press opens and monolithic aerogels are removed from the mold.
This RSCE method offers significant advantages over other aerogel fabrication methods. It is fast (&#60;8 hr total) and not very labor intensive, typically requiring only 15-20 min preparation time followed by 3-8 hr processing time. It does not require solvent exchanges, which means that relatively little solvent waste is generated during the process.
In the section that follows, we describe a protocol for preparing a set of cylindrical silica aerogel monoliths via the Union RSCE method from a precursor mixture comprised of TMOS, methanol, and water with aqueous ammonia employed as the catalyst for the hydrolysis and polycondensation reactions (with a TMOS:MeOH:H2O:NH3 molar ratio of 1.0:12:3.6:3.5 x 10-3). We note that the Union RSCE method can be used to prepare aerogels of various different sizes and shapes, depending on the metal mold and hydraulic hot press employed. This RSCE method has also been used to prepare other types of aerogels (titania, alumina, etc.) from different precursor recipes20.

<table border="0" cellpadding="0" cellspacing="0" width="1344">
	<colgroup>
		<col />
		<col span="2" />
		<col />
	</colgroup>
	<tbody>
		<tr height="61">
			<td height="61" style="height:61px;width:504px;">Tetramethylorthosilicate&#160; (TMOS)</td>
			<td style="width:252px;">Sigma Aldrich&#160;&#160; www.sigmaaldrich.com</td>
			<td style="width:252px;">218472-500G</td>
			<td style="width:336px;">98% purity, CAS 681-84-5&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;</td>
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:504px;">Methanol&#160; (MeOH)</td>
			<td style="width:252px;">Fisher Scientific&#160; www.fishersci.com</td>
			<td style="width:252px;">A412-20</td>
			<td style="width:336px;">Certified ACS Reagent Grade, &#8805;99.8%</td>
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:504px;">Ammonium Hydroxide (aqueous ammonia)</td>
			<td style="width:252px;">Fisher Scientific&#160; www.fishersci.com</td>
			<td style="width:252px;">A669S212</td>
			<td style="width:336px;">Certified ACS Plus, about 14.8N, 28.0-20.0 w/w%</td>
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:504px;">Deionized Water</td>
			<td style="width:252px;">On tap in house</td>
			<td style="width:252px;"></td>
			<td style="width:336px;"></td>
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:504px;">Flexible Graphite Sheet</td>
			<td style="width:252px;">Phelps Industrial Products</td>
			<td style="width:252px;">7500.062.3</td>
			<td style="width:336px;">1/16&#34; thick</td>
		</tr>
		<tr height="61">
			<td height="61" style="height:61px;width:504px;">Stainless Steel Foil</td>
			<td style="width:252px;">Various</td>
			<td style="width:252px;"></td>
			<td style="width:336px;">.0005&#34; thick, 304 Stainless Steel</td>
		</tr>
		<tr height="110">
			<td height="110" style="height:111px;width:504px;">High Temperature Mold Release Spray</td>
			<td style="width:252px;">Various&#160; (for example, CRC Industrial Dry PTFE Lube)</td>
			<td style="width:252px;"></td>
			<td style="width:336px;">Should be able to withstand high temperatures.</td>
		</tr>
	</tbody>
</table>

preparing silica aerogel monoliths via a rapid supercritical extraction method

A procedure for the fabrication of monolithic silica aerogels in eight hours or less via a rapid supercritical extraction process is described. The procedure requires 15-20 min of preparation time, during which a liquid precursor mixture is prepared and poured into wells of a metal mold that is placed between the platens of a hydraulic hot press, followed by several hours of processing within the hot press. The precursor solution consists of a 1.0:12.0:3.6:3.5 x 10-3 molar ratio of tetramethylorthosilicate (TMOS):methanol:water:ammonia. In each well of the mold, a porous silica sol-gel matrix forms. As the temperature of the mold and its contents is increased, the pressure within the mold rises. After the temperature/pressure conditions surpass the supercritical point for the solvent within the pores of the matrix (in this case, a methanol/water mixture), the supercritical fluid is released, and monolithic aerogel remains within the wells of the mold. With the mold used in this procedure, cylindrical monoliths of 2.2 cm diameter and 1.9 cm height are produced. Aerogels formed by this rapid method have comparable properties (low bulk and skeletal density, high surface area, mesoporous morphology) to those prepared by other methods that involve either additional reaction steps or solvent extractions (lengthier processes that generate more chemical waste).The rapid supercritical extraction method can also be applied to the fabrication of aerogels based on other precursor recipes.

Following the procedure described here results in consistent batches of monolithic silica aerogels. Figure 4 shows images of typical silica aerogels made via this process. Each aerogel takes on the shape and size of the well in the processing mold with no shrinkage. The images show that the silica aerogels are translucent.
The physical properties of these aerogels are summarized in Table 4. They are comparable to those of silica aerogels produced from similar precursor recipes using low temperature supercritical extraction21. Figure 5 shows a typical pore distribution acquired by BJH analysis of the desorption isotherm acquired with a Micromeritics ASAP 2010.The aerogels are mesoporous with a peak in pore diameter near 20 nm.
<table border="0" cellpadding="0" cellspacing="0">
	<tbody align="center">
		<tr>
			<td>Step #</td>
			<td>Temp</td>
			<td>Temp Rate</td>
			<td>Force</td>
			<td>Force Rate</td>
			<td>Dwell time (min)</td>
			<td>Step Duration (min)</td>
		</tr>
		<tr>
			<td>1</td>
			<td>off</td>
			<td>--</td>
			<td>20,000 lb 
			(89 kN)</td>
			<td>600 k lb/min* 
			(2,669 N/min)</td>
			<td>10</td>
			<td>10</td>
		</tr>
		<tr>
			<td>2</td>
			<td colspan="6">End Step</td>
		</tr>
	</tbody>
</table>
Table 1. Hot Press Mold Sealing Program Settings. 
* this rate represents the maximum press rate


<table border="0" cellpadding="0" cellspacing="0" fo:keep-together.within-page="always">
	<tbody>
		<tr>
			<td>Step #</td>
			<td style="width:100px;">Temperature</td>
			<td>Temp Rate</td>
			<td>Press Force</td>
			<td>Force Rate</td>
			<td>Dwell time (min)</td>
			<td>Step Duration (min)</td>
		</tr>
		<tr>
			<td>1 - Seal Mold</td>
			<td>90 &#176;F 
			(32 &#176;C)</td>
			<td>200 &#176;F/min 
			(111 &#176;C/min)*</td>
			<td>40,000 lb 
			(178 kN)</td>
			<td>600,000 lb/min 
			(2,669 kN/min)*</td>
			<td>2</td>
			<td>2</td>
		</tr>
		<tr>
			<td>2 - Heat and Equilibrate</td>
			<td>550 &#176;F 
			(288 &#176;C)</td>
			<td>2 &#176;F/min 
			(1.1 &#176;C/min)</td>
			<td>40,000 lb 
			(178 kN)</td>
			<td>--</td>
			<td>30</td>
			<td>260</td>
		</tr>
		<tr>
			<td>3 - Extract and Equilibrate</td>
			<td>550 &#176;F 
			(288 &#176;C)</td>
			<td>--</td>
			<td>1,000 lb 
			(4.4 kN)</td>
			<td>1,000 lb/min 
			(4.4 kN/min)</td>
			<td>30</td>
			<td>69</td>
		</tr>
		<tr>
			<td>4 - Cool Down</td>
			<td>100 &#176;F 
			(38 &#176;C)</td>
			<td>3 &#176;F/min 
			(1.7 &#176;C/min)</td>
			<td>1,000 lb 
			(4.4 kN)</td>
			<td>--</td>
			<td>1</td>
			<td>151</td>
		</tr>
		<tr>
			<td>5 - Finish</td>
			<td>End Step</td>
			<td></td>
			<td></td>
			<td></td>
			<td>Total Time:</td>
			<td>482 min 
			(8 hr)</td>
		</tr>
	</tbody>
</table>


Table 2. Hot Press Extraction Program Settings. 
* these rates represent maximum press rates


<table border="0" cellpadding="0" cellspacing="0" width="500">
	<tbody>
		<tr>
			<td>Chemical</td>
			<td>Amount (ml)</td>
		</tr>
		<tr>
			<td>TMOS</td>
			<td>17</td>
		</tr>
		<tr>
			<td>MeOH</td>
			<td>55</td>
		</tr>
		<tr>
			<td>H2O</td>
			<td>7.2</td>
		</tr>
		<tr>
			<td>1.5 M NH3</td>
			<td>0.27</td>
		</tr>
	</tbody>
</table>


Table 3. Recipe for 80&#160;ml Silica Precursor Solution.


<table border="0" cellpadding="0" cellspacing="0" width="500">
	<tbody>
		<tr>
			<td>Property</td>
			<td>Typical Value</td>
		</tr>
		<tr>
			<td>Bulk Density</td>
			<td>0.1 g/cm3</td>
		</tr>
		<tr>
			<td>Skeletal Density</td>
			<td>1.9 g/cm3</td>
		</tr>
		<tr>
			<td>BET Surface Area</td>
			<td>560 m2/g</td>
		</tr>
		<tr>
			<td>Cumulative Pore Volume</td>
			<td>3.9 cm3/g</td>
		</tr>
		<tr>
			<td>Average BJH Desorption Pore Diameter</td>
			<td>21 nm</td>
		</tr>
		<tr>
			<td>Average BJH Adsorption Pore Diameter</td>
			<td>27 nm</td>
		</tr>
	</tbody>
</table>


Table 4. Properties of Silica Aerogels Prepared via RSCE Process.
<img alt="Figure 1" fo:content-width="5in" fo:src="/files/ftp_upload/51421/51421fig1highres.jpg" src="/files/ftp_upload/51421/51421fig1.jpg" /> 
Figure 1. Schematic of sol-gel drying process.
<img alt="Figure 2" fo:content-width="5in" fo:src="/files/ftp_upload/51421/51421fig2highres.jpg" src="/files/ftp_upload/51421/51421fig2.jpg" /> 
Figure 2. Hot press processing parameters used during RSCE process. (Note: English units are employed in this figure because the hot press is programmed in those units.)
<img alt="Figure 3" fo:content-width="5in" fo:src="/files/ftp_upload/51421/51421fig3highres.jpg" src="/files/ftp_upload/51421/51421fig3.jpg" /> 
Figure 3. Schematic of mold used in RSCE process. Aerogels are formed in each of the nine wells, which pass through the entire mold height (all dimensions are in cm).
<img alt="Figure 4" fo:content-width="5in" fo:src="/files/ftp_upload/51421/51421fig4highres.jpg" src="/files/ftp_upload/51421/51421fig4.jpg" /> 
Figure 4. Images of silica aerogels prepared via this RSCE process.
<img alt="Figure 5" fo:content-width="5in" fo:src="/files/ftp_upload/51421/51421fig5highres.jpg" src="/files/ftp_upload/51421/51421fig5.jpg" /> 
Figure 5. Typical BJH pore distribution (desorption) results for silica aerogel prepared via RSCE.

Watch this Scientific Journal Video about Preparing Silica Aerogel Monoliths via a Rapid Supercritical Extraction Method at JoVE.com

Preparing Silica Aerogel Monoliths via a Rapid Supercritical Extraction Method

This article describes a rapid supercritical extraction method for fabricating silica aerogels. By utilizing a confined mold and hydraulic hot press, monolithic aerogels can be made in eight hours or less.

A procedure for the fabrication of monolithic silica aerogels in eight hours or less via a rapid supercritical extraction process is described. The ...

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20.1K Views.  Union College. This article describes a rapid supercritical extraction method for fabricating silica aerogels. By utilizing a confined mold and hydraulic hot press, monolithic aerogels can be made in eight hours or less.

Video: Preparing Silica Aerogel Monoliths via a Rapid Supercritical Extraction Method

Preparation of Silica Nanoparticles Through Microwave-assisted Acid-catalysis

Microwave-assisted synthetic techniques were used to quickly and reproducibly produce silica nanoparticle sols using an acid catalyst with nanoparticle diameters ranging from 30-250 nm by varying the reaction conditions. Through the selection of a microwave compatible solvent, silicic acid precursor, catalyst, and microwave irradiation time, these microwave-assisted methods were capable of overcoming the previously reported shortcomings associated with synthesis of silica nanoparticles using microwave reactors. The siloxane precursor was hydrolyzed using the acid catalyst, HCl. Acetone, a low-tan &#948; solvent, mediates the condensation reactions and has minimal interaction with the electromagnetic field. Condensation reactions begin when the silicic acid precursor couples with the microwave radiation, leading to silica nanoparticle sol formation. The silica nanoparticles were characterized by dynamic light scattering data and scanning electron microscopy, which show the materials&#39; morphology and size to be dependent on the reaction conditions. Microwave-assisted reactions produce silica nanoparticles with roughened textured surfaces that are atypical for silica sols produced by St&#246;ber&#39;s methods, which have smooth surfaces.

Silica nanoparticles were prepared using acid-catalysis of a siloxane precursor and microwave-assisted synthetic techniques resulting in the controlled growth of nanomaterials ranging from 30-250 nm in diameter. The growth dynamics can be controlled by varying the initial silicic acid concentration, time of the reaction, and temperature of reaction.

Microwave-assisted synthetic techniques were used to quickly and reproducibly produce silica nanoparticle sols using an acid catalyst with nanoparticle ...

Preparing Adherent Cells for X-ray Fluorescence Imaging by Chemical Fixation

X-ray fluorescence imaging allows us to non-destructively measure the spatial distribution and concentration of multiple elements simultaneously over large or small sample areas. It has been applied in many areas of science, including materials science, geoscience, studying works of cultural heritage, and in chemical biology. In the case of chemical biology, for example, visualizing the metal distributions within cells allows us to study both naturally-occurring metal ions in the cells, as well as exogenously-introduced metals such as drugs and nanoparticles. Due to the fully hydrated nature of nearly all biological samples, cryo-fixation followed by imaging under cryogenic temperature represents the ideal imaging modality currently available. However, under the circumstances that such a combination is not easily accessible or practical, aldehyde based chemical fixation remains useful and sometimes inevitable. This article describes in as much detail as possible in the preparation of adherent mammalian cells by chemical fixation for X-ray fluorescent imaging.

Here, we present a protocol on how to determine the quantity and distribution of metals in a sample using synchrotron X-ray fluorescence. We focus on adherent cells, and describe the chemical fixation method to prepare this sample. We then describe how to mount and image the sample using synchrotron X-rays.

X-ray fluorescence imaging allows us to non-destructively measure the spatial distribution and concentration of multiple elements simultaneously over ...

Supercritical Nitrogen Processing for the Purification of Reactive Porous Materials

Supercritical fluid extraction and drying methods are well established in numerous applications for the synthesis and processing of porous materials. Herein, nitrogen is presented as a novel supercritical drying fluid for specialized applications such as in the processing of reactive porous materials, where carbon dioxide and other fluids are not appropriate due to their higher chemical reactivity. Nitrogen exhibits similar physical properties in the near-critical region of its phase diagram as compared to carbon dioxide: a widely tunable density up to ~1 g ml-1, modest critical pressure (3.4 MPa), and small molecular diameter of ~3.6 &#197;. The key to achieving a high solvation power of nitrogen is to apply a processing temperature in the range of 80-150 K, where the density of nitrogen is an order of magnitude higher than at similar pressures near ambient temperature. The detailed solvation properties of nitrogen, and especially its selectivity, across a wide range of common target species of extraction still require further investigation. Herein we describe a protocol for the supercritical nitrogen processing of porous magnesium borohydride.

Nitrogen is an effective supercritical fluid for extraction or drying processes due to its small molecular size, high density in the near-liquid supercritical regime, and chemical inertness. We present a supercritical nitrogen drying protocol for the purification treatment of reactive, porous materials.

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A Method to Manipulate Surface Tension of a Liquid Metal via Surface Oxidation and Reduction

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where interfacial tension is a dominant force. A variety of methods exist for controlling the interfacial tension of aqueous and organic liquids on this scale; however, these techniques have limited utility for liquid metals due to their large interfacial tension.
Liquid metals can form soft, stretchable, and shape-reconfigurable components in electronic and electromagnetic devices. Although it is possible to manipulate these fluids via mechanical methods (e.g., pumping), electrical methods are easier to miniaturize, control, and implement. However, most electrical techniques have their own constraints: electrowetting-on-dielectric requires large (kV) potentials for modest actuation, electrocapillarity can affect relatively small changes in the interfacial tension, and continuous electrowetting is limited to plugs of the liquid metal in capillaries.
Here, we present a method for actuating gallium and gallium-based liquid metal alloys via an electrochemical surface reaction. Controlling the electrochemical potential on the surface of the liquid metal in electrolyte rapidly and reversibly changes the interfacial tension by over two orders of magnitude ( &#820;500 mN/m to near zero). Furthermore, this method requires only a very modest potential (&#60; 1 V) applied relative to a counter electrode. The resulting change in tension is due primarily to the electrochemical deposition of a surface oxide layer, which acts as a surfactant; removal of the oxide increases the interfacial tension, and vice versa. This technique can be applied in a wide variety of electrolytes and is independent of the substrate on which it rests.

We present a method to control the interfacial energy of a liquid metal in an electrolyte via electrochemical deposition (or removal) of a surface oxide layer. This simple method can control the capillary behavior of gallium-based liquid metals by tuning the interfacial energy rapidly, significantly, and reversibly using modest voltages.

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where ...

A Straightforward Method for Glucosinolate Extraction and Analysis with High-pressure Liquid Chromatography (HPLC)

Glucosinolates are a well-studied and highly diverse class of natural plant compounds. They play important roles in plant resistance, rapeseed oil quality, food flavoring, and human health. The biological activity of glucosinolates is released upon tissue damage, when they are mixed with the enzyme myrosinase. This results in the formation of pungent and toxic breakdown products, such as isothiocyanates and nitriles. Currently, more than 130 structurally different glucosinolates have been identified. The chemical structure of the glucosinolate is an important determinant of the product that is formed, which in turn determines its biological activity. The latter may range from detrimental (e.g., progoitrin) to beneficial (e.g., glucoraphanin). Each glucosinolate-containing plant species has its own specific glucosinolate profile. For this reason, it is important to correctly identify and reliably quantify the different glucosinolates present in brassicaceous leaf, seed, and root crops or, for ecological studies, in their wild relatives. Here, we present a well-validated, targeted, and robust method to analyze glucosinolate profiles in a wide range of plant species and plant organs. Intact glucosinolates are extracted from ground plant materials with a methanol-water mixture at high temperatures to disable myrosinase activity. Thereafter, the resulting extract is brought onto an ion-exchange column for purification. After sulfatase treatment, the desulfoglucosinolates are eluted with water and the eluate is freeze-dried. The residue is taken up in an exact volume of water, which is analyzed by high-pressure liquid chromatography (HPLC) with a photodiode array (PDA) or ultraviolet (UV) detector. Detection and quantification are achieved by conducting comparisons of the retention times and UV spectra of commercial reference standards. The concentrations are calculated based on a sinigrin reference curve and well-established response factors. The advantages and disadvantages of this straightforward method, when compared to faster and more technologically advanced methods, are discussed here.

A Straightforward Method for Glucosinolate Extraction and Analysis with High-pressure Liquid Chromatography HPLC

Here, we describe in great detail an established and robust protocol for the extraction of glucosinolates from ground plant materials. After an on-column sulfatase treatment of the extracts, the desulfoglucosinolates are eluted and analyzed by high-pressure liquid chromatography.

Glucosinolates are a well-studied and highly diverse class of natural plant compounds. They play important roles in plant resistance, rapeseed oil ...

A Convenient Method for Extraction and Analysis with High-Pressure Liquid Chromatography of Catecholamine Neurotransmitters and Their Metabolites

The extraction and analysis of catecholamine neurotransmitters in biological fluids is of great importance in assessing nervous system function and related diseases, but their precise measurement is still a challenge. Many protocols have been described for neurotransmitter measurement by a variety of instruments, including high-pressure liquid chromatography (HPLC). However, there are shortcomings, such as complicated operation or hard-to-detect multiple targets, which cannot be avoided, and presently, the dominant analysis technique is still HPLC coupled with sensitive electrochemical or fluorimetric detection, due to its high sensitivity and good selectivity. Here, a detailed protocol is described for the pretreatment and detection of catecholamines with high pressure liquid chromatography with electrochemical detection (HPLC-ECD) in real urine samples of infants, using electrospun composite nanofibers composed of polymeric crown ether with polystyrene as adsorbent, also known as the packed-fiber solid phase extraction (PFSPE) method. We show how urine samples can be easily precleaned by a nanofiber-packed solid phase column, and how the analytes in the sample can be rapidly enriched, desorbed, and detected on an ECD system. PFSPE greatly simplifies the pretreatment procedures for biological samples, allowing for decreased time, expense, and reduction of the loss of targets. 
Overall, this work illustrates a simple and convenient protocol for solid-phase extraction coupled to an HPLC-ECD system for simultaneous determination of three monoamine neurotransmitters (norepinephrine (NE), epinephrine (E), dopamine (DA)) and two of their metabolites (3-methoxy-4-hydroxyphenylglycol (MHPG) and 3,4-dihydroxy-phenylacetic acid (DOPAC)) in infants' urine. The established protocol was applied to assess the differences of urinary catecholamines and their metabolites between high-risk infants with perinatal brain damage and healthy controls. Comparative analysis revealed a significant difference in urinary MHPG between the two groups, indicating that the catecholamine metabolites may be an important candidate marker for early diagnosis of cases at risk for brain damage in infants.

We present a convenient solid-phase extraction coupled to high-pressure liquid chromatography (HPLC) with electrochemical detection (ECD) for simultaneous determination of three monoamine neurotransmitters and two of their metabolites in infants' urine. We also identify the metabolite MHPG as a potential biomarker for the early diagnosis of brain damage for infants.

The extraction and analysis of catecholamine neurotransmitters in biological fluids is of great importance in assessing nervous system function and ...

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Here we present protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms. Methods for preparing materials using copper salts and copper-containing nanoparticles are featured. Catalytic testing protocols demonstrate the effectiveness of these aerogels for three-way catalysis applications.

Protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms are presented. Three ...

Separation of Aldehydes and Reactive Ketones from Mixtures Using a Bisulfite Extraction Protocol

The purification of organic compounds is an essential component of routine synthetic operations. The ability to remove contaminants into an aqueous layer by generating a charged structure provides an opportunity to use extraction as a simple purification technique. By combining the use of a miscible organic solvent with saturated sodium bisulfite, aldehydes and reactive ketones can be successfully transformed into charged bisulfite adducts that can then be separated from other organic components of a mixture by the introduction of an immiscible organic layer. Here, we describe a simple protocol for the removal of aldehydes, including sterically-hindered neopentyl aldehydes and some ketones, from chemical mixtures. Ketones can be separated if they are sterically unhindered cyclic or methyl ketones. For aliphatic aldehydes and ketones, dimethylformamide is used as the miscible solvent to improve removal rates. The bisulfite addition reaction can be reversed by basification of the aqueous layer, allowing for the re-isolation of the reactive carbonyl component of a mixture.

Here, we present a protocol to remove aldehydes and reactive ketones from mixtures by a liquid-liquid extraction protocol directly with saturated sodium bisulfite in a miscible solvent. This combined protocol is rapid and facile to perform. The aldehyde or ketone can be re-isolated by the basification of the aqueous layer.

The purification of organic compounds is an essential component of routine synthetic operations. The ability to remove contaminants into an aqueous layer ...

Preparation of Functional Silica Using a Bioinspired Method

The goal of the protocols described herein is to synthesize bioinspired silica materials, perform enzyme encapsulation therein, and partially or totally purify the same by acid elution. By combining sodium silicate with a polyfunctional bioinspired additive, silica is rapidly formed at ambient conditions upon neutralization. 
The effect of neutralization rate and biomolecule addition point on silica yield are investigated, and biomolecule immobilization efficiency is reported for varying addition point. In contrast to other porous silica synthesis methods, it is shown that the mild conditions required for bioinspired silica synthesis are fully compatible with the encapsulation of delicate biomolecules. Additionally, mild conditions are used across all synthesis and modification steps, making bioinspired silica a promising target for the scale-up and commercialization as both a bare material and active support medium.
The synthesis is shown to be highly sensitive to conditions, i.e., the neutralization rate and final synthesis pH, however tight control over these parameters is demonstrated through the use of auto titration methods, leading to high reproducibility in reaction progression pathway and yield. 
Therefore, bioinspired silica is an excellent active material support choice, showing versatility towards many current applications, not limited to those demonstrated here, and potency in future applications.

Here, we present a protocol to synthesize bioinspired silica materials and immobilize enzymes therein. Silica is synthesized by combining sodium silicate and an amine &#39;additive&#39;, which neutralize at a controlled rate. Material properties and function can be altered either by in situ enzyme immobilization or post-synthetic acid elution of encapsulated additives.

The goal of the protocols described herein is to synthesize bioinspired silica materials, perform enzyme encapsulation therein, and partially or totally ...

A Rapid Synthesis Method for Au, Pd, and Pt Aerogels Via Direct Solution-Based Reduction

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various precursor noble metal ions with reducing agents in a 1:1 (v/v) ratio results in the formation of metal gels within seconds to minutes compared to much longer synthesis times for other techniques such as sol-gel. Conducting the reduction step in a microcentrifuge tube or small volume conical tube facilitates a proposed nucleation, growth, densification, fusion, equilibration model for gel formation, with final gel geometry smaller than the initial reaction volume. This method takes advantage of the vigorous hydrogen gas evolution as a by-product of the reduction step, and as a consequence of reagent concentrations. The solvent accessible specific surface area is determined with both electrochemical impedance spectroscopy and cyclic voltammetry. After rinsing and freeze drying, the resulting aerogel structure is examined with scanning electron microscopy, X-ray diffractometry, and nitrogen gas adsorption. The synthesis method and characterization techniques result in a close correspondence of aerogel ligament sizes. This synthesis method for noble metal aerogels demonstrates that high specific surface area monoliths may be achieved with a rapid and direct reduction approach.

A rapid, direct solution-based reduction synthesis method to obtain Au, Pd, and Pt aerogels is presented.

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various ...