Financial support from DFG (projects SM 82/13-1) is gratefully acknowledged.

1. Preparation of Amidated Pectin Stock Solution
<ol>
	<li>Mix 20 g amidated pectin with 980 g water (2.0 wt%). The degree of amidation is 25 wt%.</li>
	<li>Homogenize the solution with a high speed stirrer (10,000 rpm) for 2 min to obtain a homogenous viscous solution.</li>
	<li>Measure pH using pH strips or pH meter. If the pH is lower than 6.5, titrate with 0.5 M NaOH to neutralize the solution (to pH 7.0).</li>
	<li>Add calcium carbonate in a ratio of 0.1825 g per gram of dry amidated pectin (q=1). &#34;q&#34; denotes the degree of crosslinking.</li>
	<li>For 1 kg 2.0 wt% amidated pectin solution, add 3.65 g calcium carbonate (q=1)7 per 20.0 g dry pectin.</li>
	<li>For greater crosslinking, add 0.3650 g calcium carbonate per gram of dry amidated pectin (q=2).</li>
</ol>
2. Production of Hydrogels
<ol>
	<li>Homogenize the amidated pectin/calcium carbonate mixture using the high speed homogenizer (10,000 rpm) to obtain a white homogenous dispersion.</li>
	<li>Transfer the suspension into open polypropylene molds or glass Petri dishes.</li>
	<li>Place the molds in the high pressure autoclave. Seal the autoclave.</li>
	<li>Pressurize the autoclave with gaseous CO2 up to 5 MPa at RT. Refer to Gurikov et al.7 for further information. Maintain the pressure for 24 hr.</li>
	<li>Slowly depressurize the autoclave at 0.2 MPa/min.</li>
	<li>Open the autoclave and remove the molds. Remove the hydrogels from the molds by turning them over. If necessary, use a spatula.</li>
</ol>
3. Solvent Exchange Procedure
<ol>
	<li>Prepare 10 g of 10:90 (w/w) ethanol/water mixture per gram of hydrogel.</li>
	<li>Immerse the hydrogels in the 10:90 (w/w) ethanol/water mixture for 12 hr.</li>
	<li>Continue this process with increasing ethanol concentrations, i.e., from 10:90 (w/w) ethanol/water mixture to 30:70 (w/w) ethanol/water mixture. After 12 hr, transfer to 50:50 (w/w) ethanol/water mixture, then to 70:30 (12 hr), then 90:10 (12 hr) and then to 100% ethanol solution (12 hr).</li>
	<li>Soak the gel further in pure ethanol so that the final concentration inside the gel is more than 98% (w/w). Measure the concentration using the density meter. The alcogel is now ready for supercritical CO2 drying.</li>
</ol>
4. Production of Aerogels by Supercritical CO2 Drying
<ol>
	<li>Place the samples in the same high pressure autoclave used for hydrogel preparation (see step 2.3).</li>
	<li>Fill the autoclave with additional ethanol (2-10% of the autoclave volume) to prevent premature solvent evaporation from the gels. Complete immersion of gel in the solvent is not required.</li>
	<li>Seal the autoclave. Turn on the autoclave heating. Set the autoclave working temperature to 323 K. Pressurize the autoclave with carbon dioxide to 12 MPa using a compressor or pump.</li>
	<li>Periodically replace the CO2 inside the autoclave10,11 with fresh CO2 keeping the pressure constant. 6-7 residence volumes are required over a period of 6 hr. Refer to Gurikov et al.7 for further information.</li>
	<li>Slowly depressurize the autoclave at 0.2 MPa/min.</li>
	<li>Open the autoclave and collect the aerogel. Store the aerogel in an exicator or a sealed container.</li>
</ol>

<ol>
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I., Skj&#229;k-Br&#230;k, G., Smidsr&#248;d, O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Alginate+based+new+materials.">Alginate based new materials.</a> Int. J. Biol. Macromol. 21 (1), 47-55 (1997).</li><li>Garc&#237;a-Gonz&#225;lez, C. A., Alnaief, M., Smirnova, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Polysaccharide-based+aerogels-Promising+biodegradable+carriers+for+drug+delivery+systems.">Polysaccharide-based aerogels-Promising biodegradable carriers for drug delivery systems.</a> Carbohydr. Polym. 86 (4), 1425-1438 (2011).</li><li>Alnaief, M., Alzaitoun, M. A., Garc&#237;a-Gonz&#225;lez, C. A., Smirnova, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+of+biodegradable+nanoporous+microspherical+aerogel+based+on+alginate.">Preparation of biodegradable nanoporous microspherical aerogel based on alginate.</a> Carbohydr. Polym. 84 (3), 1011-1018 (2011).</li><li>Gurikov, P., Raman, S. P., Weinrich, D., Fricke, M., Smirnova, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+novel+approach+to+alginate+aerogels:+carbon+dioxide+induced+gelation.">A novel approach to alginate aerogels: carbon dioxide induced gelation.</a> RSC Adv. 5, 7812-7818 (2015).</li><li>Meyssami, B., Balaban, M. O., Teixeira, A. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Prediction+of+pH+in+model+systems+pressurized+with+carbon+dioxide.">Prediction of pH in model systems pressurized with carbon dioxide.</a> Biotechnol. Prog. 8 (2), 149-154 (1992).</li><li>Subrahmanyam, R., Gurikov, P., Dieringer, P., Sun, M., Smirnova, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+Road+to+Biopolymer+Aerogels-Dealing+with+the+Solvent.">On the Road to Biopolymer Aerogels-Dealing with the Solvent.</a> Gels. 1 (2), 291-313 (2015).</li><li>Garc&#237;a-Gonz&#225;lez, C. A., Camino-Rey, M. C., Alnaief, M., Zetzl, C., Smirnova, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Supercritical+drying+of+aerogels+using+CO2:+Effect+of+extraction+time+on+the+end+material+textural+properties.">Supercritical drying of aerogels using CO2: Effect of extraction time on the end material textural properties.</a> J. Supercrit. Fluids. 66, 297-306 (2012).</li><li>&#214;zbak&#305;r, Y., Erkey, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+and+theoretical+investigation+of+supercritical+drying+of+silica+alcogels.">Experimental and theoretical investigation of supercritical drying of silica alcogels.</a> J. Supercrit. Fluids. 98, 153-166 (2015).</li><li>Rudaz, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Aeropectin:+Fully+Biomass-Based+Mechanically+Strong+and+Thermal+Superinsulating+Aerogel.">Aeropectin: Fully Biomass-Based Mechanically Strong and Thermal Superinsulating Aerogel.</a> Biomacromolecules. 15 (6), 2188-2195 (2014).</li><li>Quraishi, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Novel+non-cytotoxic+alginate-lignin+hybrid+aerogels+as+scaffolds+for+tissue+engineering.">Novel non-cytotoxic alginate-lignin hybrid aerogels as scaffolds for tissue engineering.</a> J. Supercrit. Fluids. 105, 1-8 (2015).</li><li>Martins, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+of+macroporous+alginate-based+aerogels+for+biomedical+applications.">Preparation of macroporous alginate-based aerogels for biomedical applications.</a> J. Supercrit. Fluids. 106, 152-159 (2015).</li><li>Davidovich-Pinhas, M., Bianco-Peled, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+quantitative+analysis+of+alginate+swelling.">A quantitative analysis of alginate swelling.</a> Carbohydr. Polym. 79 (4), 1020-1027 (2010).</li><li>Mallepally, R. R., Bernard, I., Marin, M. A., Ward, K. R., McHugh, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Superabsorbent+alginate+aerogels.">Superabsorbent alginate aerogels.</a> J. Supercrit. Fluids. 79, 202-208 (2013).</li><li>Porta, G. D., Del Gaudio, P., De Cicco, F., Aquino, R. P., Reverchon, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Supercritical+Drying+of+Alginate+Beads+for+the+Development+of+Aerogel+Biomaterials:+Optimization+of+Process+Parameters+and+Exchange.">Supercritical Drying of Alginate Beads for the Development of Aerogel Biomaterials: Optimization of Process Parameters and Exchange.</a> Ind. Eng. Chem. Res. 52 (34), 12003-12009 (2013).</li><li>Brown, Z. K., Fryer, P. J., Norton, I. T., Bridson, R. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Drying+of+agar+gels+using+supercritical+carbon+dioxide.">Drying of agar gels using supercritical carbon dioxide.</a> J. Supercrit. Fluids. 54 (1), 89-95 (2010).</li><li>Betz, M., Garc&#237;a-Gonz&#225;lez, C. A., Subrahmanyam, R. P., Smirnova, I., Kulozik, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+of+novel+whey+protein-based+aerogels+as+drug+carriers+for+life+science+applications.">Preparation of novel whey protein-based aerogels as drug carriers for life science applications.</a> J. Supercrit. Fluids. 72, 111-119 (2012).</li><li>Antonyuk, S., Heinrich, S., Gurikov, P., Raman, S., Smirnova, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Influence+of+coating+and+wetting+on+the+mechanical+behaviour+of+highly+porous+cylindrical+aerogel+particles.">Influence of coating and wetting on the mechanical behaviour of highly porous cylindrical aerogel particles.</a> Powder Technol. 285, 34-43 (2015).</li></ol>

The authors declare that they have no competing financial interests.

By using the CO2 induced gelation technique, one can eliminate the need for chemical substitutes (for example acetic acid or glucono delta-lactone (GDL)) required for inducing the crosslinking of the biopolymer. The surface areas of the amidated pectin aerogels are in the higher ranges of literature values5, however the pore volumes are much higher than those presented in literature5. Higher pore volumes were also observed for alginate aerogels prepared by CO2 induced gelation7. However, it remains to be verified whether the reason for this high pore volumes (4-150 nm pore size range) is due to the gelation technique or an inherent property of the biopolymers previously not addressed in literature. Pectin aerogels have been reported in literature to possess superinsulating properties12 and alginate aerogels prepared by this technique also possess thermal conductivities in the superinsulating range3,7. Therefore, the amidated pectin aerogels produced by this technique may also be envisaged to possess superinsulating properties.
The rate of depressurization in Protocol Section 2 is an important step in the hydrogel preparation. Fast depressurization can lead to increased macroporosity of the gels. This phenomena can be applied for tissue engineering applications where macroporosity of the material with interconnectivity is an important feature for the growth and proliferation of cells13,14. In addition, the crosslinking degree in Protocol Section 1 plays an important role in the syneresis and swelling property of the amidated pectin hydrogels. This is similar to alginate hydrogels whose swelling behavior is influenced by the crosslinker concentration as well15. Thereby aerogels made by amidated pectin can also be tuned to possess superabsorbent property similar to those reported for alginate aerogels16.
By using the CO2 induced gelation considering amidated pectin (or alginate) as the primary system, further diversity can be incorporated into the aerogels by introducing different cross linking agents and biopolymer combinations. Several metal carbonates (e.g., zinc, nickel, cobalt, copper, strontium, barium) could be used for cross linking3, where cations can be released by pH lowering in aqueous media with pressurized CO2 (3-5 MPa). However, insoluble salts of some of these cations may not form stable dispersions for lower biopolymer concentrations and can settle down at the bottom leading to inhomogeneous gels. This is a general issue with the internal setting gelation method including CO2 induced gelation3 and thereby, the technique&#39;s usability for an application should be evaluated on a case to case basis.
Various blends prepared using water soluble biopolymers such as starch, carrageenan, methyl and carboxy methyl cellulose, gellan gum, lignin, gelatin and others; water soluble synthetic polymers such as polyethylene glycol (PEG), polyvinyl alcohol (PVA), Pluronic P-123 and others; and water soluble inorganic precursors such as sodium silicate can be also mixed with amidated pectin to produce hybrid aerogels similar to alginate2 with tunable properties.
As supercritical CO2 drying (scCO2 drying) is a quintessential step in aerogel production, any combination of pre-processing steps such as solvent exchange and drying using CO217,18 or gelation, solvent exchange and drying using CO27 could provide a clear processing advantage. The advantage is envisioned as integrated one pot process: where biopolymer dispersions can be converted into biopolymer aerogels using CO2 as the main processing medium in a single autoclave. For certain pharmaceutical applications, one can also envision performing a four step: gelation, solvent exchange, supercritical drying and active component loading5,19 process in a single autoclave using CO2 as the processing medium. Post treatment such as protective coating of drug loaded aerogels in certain cases is necessary for targeted drug release20.
To conclude, the present work demonstrates the use of pressurized CO2 for gelation of amidated pectin based systems. In addition, the use of pressurized CO2 as a common medium for precursor to product conversion for target applications in a single autoclave is envisaged.

Aerogels are a class of porous materials that can be prepared using a variety of precursors ranging from inorganic (such as silica, titania, zirconia and others), synthetic (such as resorcinol formaldehyde, polyurethane and others) or biopolymers (polysaccharides, proteins and others)2. What sets them apart from the conventional porous materials is their ability to simultaneously possess all the three characteristics; namely high surface area, ultra-low density and mesoporous pore size distribution (i.e., pore sizes from 2-50 nm). With aforementioned characteristics, aerogels are extensively applied in the fields of insulation, biomedicine, catalysis, adsorption and absorption applications, pharmaceuticals and neutraceuticals2. Taking into account the above possibilities, the production of biopolymer gel systems and their subsequent transformation to aerogels opens up a multitude of opportunities towards high value added bio based materials. Such an endeavor is taken up in this study using amidated pectin as an example.
Aerogels are typically produced by the sol-gel technique. Gels are systems consisting of liquid entrapped in a matrix and can be prepared by covalent, ionic, pH induced, thermal or cryo cross linking3. For this specific system, we utilize ionic crosslinking; i.e., a bivalent cation (e.g., calcium) to crosslink biopolymeric chains together. To perform controllable ionic crosslinking of biopolymers such as amidated pectin or alginate, one can utilize the diffusion method or the internal setting method4. In the diffusion method, gelation occurs at first in the outer layer followed by diffusion propagation, as the cations diffuse from outer solution into an amidated pectin or alginate droplet or layer4. In the internal setting method, the insoluble form of the crosslinker is homogenously dispersed in the biopolymer solution and cations are released by initiating a pH change4,5,6. However, both techniques face an issue regarding the homogeneity of the final gel when produced in slab or monolithic form. This work demonstrates the use of high pressure CO2 (5 MPa) for the production of amidated pectin hydrogels building further on previous works on alginate gels3,7. In brief, it is an internal setting gelation technique that utilizes pressurized CO2 to reduce pH instead of weak acids to produce homogeneous gels. With an increase in the pressure, the solubility of carbon dioxide in water increases accompanied by a lowering of pH to 3.08. This causes calcium carbonate to solubilize, releasing the calcium ions. The calcium ions crosslink with the amidated pectin biopolymer to yield hydrogels. Stable homogeneous gels down to very low biopolymer concentrations (0.05 wt%) could be produced using this technique7.
As gelation takes place in an aqueous medium, solvent exchange to an organic solvent is required due to a miscibility gap in the CO2/water system. Typically low molecular weight alcohols (methanol/ethanol/isopropanol) and ketones (acetone) can be used for the solvent exchange process. However, direct soaking in a bath with pure ethanol or other organic solvents leads to significant irreversible shrinkage. To avoid this drawback, stepwise solvent exchange is performed5,9. When the solvent concentration inside the gel reaches &#62;98%, the organic solvent is dried with supercritical CO2 (12 MPa) leaving behind an aerogel.

<table><tbody><tr><td>&lt;strong&gt;Equipment&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Ultraturrax homogenizer</td><td>IKA, Germany</td><td>T 25 Digital</td><td></td></tr><tr><td>Polypropylene molds</td><td>TH. Geyer, Germany</td><td>9,033,201</td><td></td></tr><tr><td>High pressure autoclave&amp;nbsp;</td><td>Ernst Haage, Germany</td><td>custom made</td><td>Setup constuction done in-house</td></tr><tr><td>Compressor</td><td>Andreas Hofer</td><td>MKZ 185-40</td><td>Setup constuction done in-house</td></tr><tr><td>Nitrogen adsorption</td><td>Quantachrome</td><td>Nova 4000e</td><td></td></tr><tr><td>Density meter</td><td>Anton Paar</td><td>DMA 4000</td><td></td></tr><tr><td>&lt;strong&gt;Chemicals&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Amidated pectin</td><td>Herbstreith and Fox, Germany</td><td>CU 025</td><td>CAS # 56645-02-4; provided by company for research purposes</td></tr><tr><td>Sodium Hydroxide</td><td>Sigma Aldrich, Germany</td><td>S8045</td><td>CAS # 1310-73-2; required only in case the pectin solution needs to be neutralized to pH 6.5-7.5</td></tr><tr><td>Calcium carbonate</td><td>Magnesia GmbH, Germany</td><td>4421 Calcium carbonat, leicht, pr&amp;auml;zipitiert, EP, E170</td><td>CAS # 471-34-1</td></tr><tr><td>Ethanol, 99.8%</td><td>Sigma Aldrich, Germany</td><td>32205</td><td>CAS # 64-17-5</td></tr><tr><td>Carbon dioxide, 99.9%</td><td>AGA Gas GmbH, Germany</td><td></td><td>CAS # 124-38-9; in-house tank available (3 ton)</td></tr><tr><td>Deionised Water</td><td></td><td></td><td>CAS # 7732-18-5;&amp;nbsp; available in-house (6.4-7.0 pH)</td></tr></tbody></table>

preparation of biopolymer aerogels using green solvents

Although the first reports on aerogels made by Kistler1 in the 1930s dealt with aerogels from both inorganic oxides (silica and others) and biopolymers (gelatin, agar, cellulose), only recently have biomasses been recognized as an abundant source of chemically diverse macromolecules for functional aerogel materials. Biopolymer aerogels (pectin, alginate, chitosan, cellulose, etc.) exhibit both specific inheritable functions of starting biopolymers and distinctive features of aerogels (80-99% porosity and specific surface up to 800 m2/g). This synergy of properties makes biopolymer aerogels promising candidates for a wide gamut of applications such as thermal insulation, tissue engineering and regenerative medicine, drug delivery systems, functional foods, catalysts, adsorbents and sensors. This work demonstrates the use of pressurized carbon dioxide (5 MPa) for the ionic cross linking of amidated pectin into hydrogels. Initially a biopolymer/salt dispersion is prepared in water. Under pressurized CO2 conditions, the pH of the biopolymer solution is lowered to 3 which releases the crosslinking cations from the salt to bind with the biopolymer yielding hydrogels. Solvent exchange to ethanol and further supercritical CO2 drying (10 - 12 MPa) yield aerogels. Obtained aerogels are ultra-porous with low density (as low as 0.02 g/cm3), high specific surface area (350 - 500 m2/g) and pore volume (3 - 7 cm3/g for pore sizes less than 150 nm).

The typical hydrogels obtained after gelation step with higher crosslinking degree (q=2) (as instructed in Protocol section 2) are shown in Figure 1. The samples on the left (sample A and B) are the 2 wt% and 1 wt% pectin gels obtained by CO2 induced gelation. By decreasing the biopolymer concentration (0.5 wt% or lower), the gels become transparent (sample C). Further reduction in the biopolymer concentration (0.25 wt%) also yields stable hydrogels (sample D) but these gels are very fragile and can break when handling. The bubbles observed inside the hydrogels are created during depressurization when the dissolved CO2 leaves the gel water system due to decrease in CO2 solubility.
The amidated pectin aerogel characteristics are presented in the Table 1. The obtained aerogels are ultra-porous with low density (as low as 0.013 g/cm3) measured as the ratio between the mass of the aerogel and its volume. The surface area is measured by the nitrogen adsorption. For pectin aerogels, it yielded a specific surface area between 350 - 500 m2/g. The pore volume for pore sizes in the 4-150 nm range is measured by the Kelvin model of pore filling using nitrogen (BJH method). The pore volume for the amidated pectin aerogels was between 3 - 7 cm3/g for pore sizes between 4 and 150 nm.
<img alt="Figure 1" src="/files/ftp_upload/54116/54116fig1.jpg" /> 
Figure 1. Amidated pectin hydrogels with higher crosslinking degree (q=2). Top left: 2 wt% (Sample A); top right: 1 wt% (Sample B); bottom left: 0.5 wt% (Sample C); bottom right: 0.25 wt% (Sample D). Gels become transparent with decreasing biopolymer concentration. The bubbles are produced during CO2 depressurization. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54116/54116fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Pectin concentration [wt%]</td>
			<td>Cross-linking degree q</td>
			<td>Bulk density [g/cm3]</td>
			<td>Specific surface area [m2/g]</td>
			<td>Specific pore volume [cm3/g]</td>
			<td>Average pore size (diameter) [nm]</td>
		</tr>
		<tr>
			<td>2.00</td>
			<td>1</td>
			<td>0.081</td>
			<td>502</td>
			<td>4.1</td>
			<td>14</td>
		</tr>
		<tr>
			<td>1.00</td>
			<td>1</td>
			<td>0.044</td>
			<td>491</td>
			<td>7.1</td>
			<td>27</td>
		</tr>
		<tr>
			<td>0.50</td>
			<td>1</td>
			<td>0.035</td>
			<td>357</td>
			<td>3.8</td>
			<td>27</td>
		</tr>
		<tr>
			<td>0.25</td>
			<td>1</td>
			<td>0.013</td>
			<td>335</td>
			<td>4.9</td>
			<td>41</td>
		</tr>
		<tr>
			<td>2.00</td>
			<td>2</td>
			<td>0.069</td>
			<td>447</td>
			<td>3.1</td>
			<td>13</td>
		</tr>
		<tr>
			<td>1.00</td>
			<td>2</td>
			<td>0.048</td>
			<td>441</td>
			<td>3.6</td>
			<td>26</td>
		</tr>
		<tr>
			<td>0.50</td>
			<td>2</td>
			<td>0.030</td>
			<td>429</td>
			<td>5.8</td>
			<td>25</td>
		</tr>
		<tr>
			<td>0.25</td>
			<td>2</td>
			<td>0.017</td>
			<td>347</td>
			<td>5.0</td>
			<td>24</td>
		</tr>
	</tbody>
</table>
Table 1. Characteristics of the amidated pectin aerogels.

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Preparation of Biopolymer Aerogels Using Green Solvents

A new way for the production of biopolymer-based aerogels by carbon dioxide (CO2) induced gelation is shown. The technique utilizes pressurized carbon dioxide (5 MPa) for the production of biopolymer hydrogels and supercritical CO2 (12 MPa) to convert gels into aerogels. The only solvents needed besides CO2 are water and ethanol.

Although the first reports on aerogels made by Kistler1 in the 1930s dealt with aerogels from both inorganic oxides (silica and others) and biopolymers ...

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JoVE Journal

Chemistry

17.6K Views.  Hamburg University of Technology. A new way for the production of biopolymer-based aerogels by carbon dioxide (CO2) induced gelation is shown. The technique utilizes pressurized carbon dioxide (5 MPa) for the production of biopolymer hydrogels and supercritical CO2 (12 MPa) to convert gels into aerogels. The only solvents needed besides CO2 are water and ethanol.

Video: Preparation of Biopolymer Aerogels Using Green Solvents

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.

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 ...

Synthesis and Functionalization of 3D Nano-graphene Materials: Graphene Aerogels and Graphene Macro Assemblies

Efforts to assemble graphene into three-dimensional monolithic structures have been hampered by the high cost and poor processability of graphene. Additionally, most reported graphene assemblies are held together through physical interactions (e.g., van der Waals forces) rather than chemical bonds, which limit their mechanical strength and conductivity. This video method details recently developed strategies to fabricate mass-producible, graphene-based bulk materials derived from either polymer foams or single layer graphene oxide. These materials consist primarily of individual graphene sheets connected through covalently bound carbon linkers. They maintain the favorable properties of graphene such as high surface area and high electrical and thermal conductivity, combined with tunable pore morphology and exceptional mechanical strength and elasticity. This flexible synthetic method can be extended to the fabrication of polymer/carbon nanotube (CNT) and polymer/graphene oxide (GO) composite materials. Furthermore, additional post-synthetic functionalization with anthraquinone is described, which enables a dramatic increase in charge storage performance in supercapacitor applications.

This video method describes the synthesis of high surface area, monolithic 3D graphene-based materials derived from polymer precursors as well as single layer graphene oxide.

Efforts to assemble graphene into three-dimensional monolithic structures have been hampered by the high cost and poor processability of graphene. ...

Engineering

Fabrication and Testing of Catalytic Aerogels Prepared Via Rapid Supercritical Extraction

Protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms are presented. Three preparation methods are described: (a) the incorporation of metal salts into silica or alumina wet gels using an impregnation method; (b) the incorporation of metal salts into alumina wet gels using a co-precursor method; and (c) the addition of metal nanoparticles directly into a silica aerogel precursor mixture. The methods utilize a hydraulic hot press, which allows for rapid (&#60;6 h) supercritical extraction and results in aerogels of low density (0.10 g/mL) and high surface area (200-800 m2/g). While the work presented here focuses on the use of copper salts and copper nanoparticles, the approach can be implemented using other metal salts and nanoparticles. A protocol for testing the three-way catalytic ability of these aerogels for automotive pollution mitigation is also presented. This technique uses custom-built equipment, the Union Catalytic Testbed (UCAT), in which a simulated exhaust mixture is passed over an aerogel sample at a controlled temperature and flow rate. The system is capable of measuring the ability of the catalytic aerogels, under both oxidizing and reducing conditions, to convert CO, NO and unburned hydrocarbons (HCs) to less harmful species (CO2, H2O and N2). Example catalytic results are presented for the aerogels described.

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 ...

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 ...

Synthesis Method for Cellulose Nanofiber Biotemplated Palladium Composite Aerogels

Here, a method to synthesize cellulose nanofiber biotemplated palladium composite aerogels is presented. Noble metal aerogel synthesis methods often result in fragile aerogels with poor shape control. The use of carboxymethylated cellulose nanofibers (CNFs) to form a covalently bonded hydrogel allows for the reduction of metal ions such as palladium on the CNFs with control over both nanostructure and macroscopic aerogel monolith shape after supercritical drying. Crosslinking the carboxymethylated cellulose nanofibers is achieved using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) in the presence of ethylenediamine. The CNF hydrogels maintain their shape throughout synthesis steps including covalent crosslinking, equilibration with precursor ions, metal reduction with high concentration reducing agent, rinsing in water, ethanol solvent exchange, and CO2 supercritical drying. Varying the precursor palladium ion concentration allows for control over the metal content in the final aerogel composite through a direct ion chemical reduction rather than relying on the relatively slow coalescence of pre-formed nanoparticles used in other sol-gel techniques. With diffusion as the basis to introduce and remove chemical species into and out of the hydrogel, this method is suitable for smaller bulk geometries and thin films. Characterization of the cellulose nanofiber-palladium composite aerogels with scanning electron microscopy, X-ray diffractometry, thermal gravimetric analysis, nitrogen gas adsorption, electrochemical impedance spectroscopy, and cyclic voltammetry indicates a high surface area, metallized palladium porous structure.

A synthesis method for cellulose nanofiber biotemplated palladium composite aerogels is presented.The resulting composite aerogel materials offer potential for catalysis, sensing, and hydrogen gas storage applications.

Here, a method to synthesize cellulose nanofiber biotemplated palladium composite aerogels is presented. Noble metal aerogel synthesis methods often ...

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

Applications such as sensors, batteries, and fuel cells have been improved through the use of highly porous aerogels when functional compounds are encapsulated within the aerogels. However, few reports on encapsulating proteins within sol&#8211;gels that are processed to form aerogels exist. A procedure for encapsulating cytochrome c (cyt. c) in silica (SiO2) sol-gels that are supercritically processed to form bioaerogels with gas-phase activity for nitric oxide (NO) is presented. Cyt. c is added to a mixed silica sol under controlled protein concentration and buffer strength conditions. The sol mixture is then gelled and the liquid filling the gel pores is replaced through a series of solvent exchanges with liquid carbon dioxide. The carbon dioxide is brought to its critical point and vented off to form dry aerogels with cyt. c encapsulated inside. These bioaerogels are characterized with UV-visible spectroscopy and circular dichroism spectroscopy and can be used to detect the presence of gas-phase nitric oxide. The success of this procedure depends on regulating the cyt. c concentration and the buffer concentration and does not require other components such as metal nanoparticles. It may be possible to encapsulate other proteins using a similar approach making this procedure important for potential future bioanalytical device development.

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

This procedure describes how to encapsulate cytochrome c (cyt. c) in silica (SiO2) sol-gels, process these gels to form bioaerogels, and use these bioaerogels to rapidly recognize nitric oxide (NO) through a gas-phase reaction. This type of protocol may aid in the future development of biosensors or other bioanalytical devices.

Applications such as sensors, batteries, and fuel cells have been improved through the use of highly porous aerogels when functional compounds are ...

Bioengineering

Forming Giant-sized Polymersomes Using Gel-assisted Rehydration

Polymer vesicles, or polymersomes, are being widely explored as synthetic analogs of lipid vesicles based on their stability, robustness, barrier properties, chemical versatility and tunable physical characteristics. Typical methods used to prepare giant-sized (&gt; 4 &#181;m) vesicles, however, are both time and labor intensive, yielding low numbers of intact polymersomes. Here, we present for the first time the use of gel-assisted rehydration for the rapid and high-yielding formation of giant (&gt;4 &#181;m) polymer vesicles (polymersomes). Using this method, polymersomes can be formed from a wide array of rehydration solutions including several different physiologically-compatible buffers and full cell culture media, making them readily useful for biomimicry studies. This technique is also capable of reliably producing polymersomes from different polymer compositions with far better yields and much less difficulty than traditional methods. Polymersome size is readily tunable by altering temperature during rehydration or adding membrane fluidizers to the polymer membrane, generating giant-sized polymersomes (&gt;100 &#181;m).

We present a protocol to rapidly form giant polymer vesicles (pGVs). Briefly, polymer solutions are dehydrated on dried agarose films adhered to coverslips. Rehydration of the polymer films results in rapid formation of pGVs. This method greatly advances the preparation of synthetic giant vesicles for direct applications in biomimetic studies.

Polymer vesicles, or polymersomes, are being widely explored as synthetic analogs of lipid vesicles based on their stability, robustness, barrier ...

Aesthetically Enhanced Silica Aerogel Via Incorporation of Laser Etching and Dyes

A procedure for aesthetically enhancing silica aerogel monoliths by laser etching and incorporation of dyes is described in this manuscript. Using a rapid supercritical extraction method, large silica aerogel monolith (10 cm x 11 cm x 1.5 cm) can be fabricated in about 10 h. Dyes incorporated into the precursor mixture result in yellow-, pink- and orange-tinged aerogels. Text, patterns, and images can be etched onto the surface (or surfaces) of the aerogel monolith without damaging the bulk structure. The laser engraver can be used to cut shapes from the aerogel and form colorful mosaics.

This protocol describes a method for etching text, patterns, and images onto the surface of silica aerogel monoliths in native and dyed form and assembling the aerogels into mosaic designs.

A procedure for aesthetically enhancing silica aerogel monoliths by laser etching and incorporation of dyes is described in this manuscript. Using a rapid ...

Designed for Molecular Recycling: A Lignin-Derived Semi-aromatic Biobased Polymer

The development of chemically recyclable biopolymers offers opportunities within the pursuit of a circular economy. Chemically recyclable biopolymers make a positive effort to solve the issue of polymer materials in the disposal phase after the use phase. In this paper, the production of biobased semi-aromatic polyesters, which can be extracted entirely from biomass such as lignin, is described and visualized. The polymer poly-S described in this paper has thermal properties similar to certain commonly used plastics, such as PET. We developed a Green Knoevenagel reaction, which can efficiently produce monomers from aromatic aldehydes and malonic acid. This reaction has been proven to be scalable and has a remarkably low calculated E-factor. These polyesters with ligno-phytochemicals as a starting point show an efficient molecular recycling with minimal losses. The polyester poly(dihydrosinapinic acid) (poly-S) is presented as an example of these semi-aromatic polyesters, and the polymerization, depolymerization, and re-polymerization are described.

An example of a closed-loop approach towards a circular materials economy is described here. A whole sustainable cycle is presented where biobased semi-aromatic polyesters are designed by polymerization, depolymerization, and then re-polymerized with only slight changes in their yields or final properties.

The development of chemically recyclable biopolymers offers opportunities within the pursuit of a circular economy. Chemically recyclable biopolymers make ...

Synthesis of Hydrogels with Antifouling Properties As Membranes for Water Purification

Hydrogels have been widely utilized to enhance the surface hydrophilicity of membranes for water purification, increasing the antifouling properties and thus achieving stable water permeability through membranes over time. Here, we report a facile method to prepare hydrogels based on zwitterions for membrane applications. Freestanding films can be prepared from sulfobetaine methacrylate (SBMA) with a crosslinker of poly(ethylene glycol) diacrylate (PEGDA) via photopolymerization. The hydrogels can also be prepared by impregnation into hydrophobic porous supports to enhance the mechanical strength. These films can be characterized by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) to determine the degree of conversion of the (meth)acrylate groups, using goniometers for hydrophilicity and differential scanning calorimetry (DSC) for polymer chain dynamics. We also report protocols to determine the water permeability in dead-end filtration systems and the effect of foulants (bovine serum albumin, BSA) on membrane performance.

This paper reports practical methods to prepare hydrogels in freestanding films and impregnated membranes and to characterize their physical properties, including water transport properties.

Hydrogels have been widely utilized to enhance the surface hydrophilicity of membranes for water purification, increasing the antifouling properties and ...

Preparation of Biopolymer Aerogels Using Green Solvents

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