Funding was provided by the Defense Threat Reduction Agency, Physical Science and Technology Division, Protection and Hazard Mitigation technical area. This research was supported in part by an appointment to the Postgraduate Research Participation Program at the Air Force Research Laboratory administered by the Oak Ridge Institute for Science and Education (ORISE) through an interagency agreement between the U.S. Department of Energy and the Air Force Research Laboratory, Materials and Manufacturing Directorate, Airbase Technologies Division (AFRL/RXQ).

1.&#160;Preparation and Calculations
<ol>
	<li>Prepare 1 mM HCl solution using concentrated hydrochloric acid, 37%, and water. 
	Note: Caution should always be used when handling concentrated acids. Concentrated acid should always be added to water, never add water to concentrated acid.</li>
	<li>Determine the desired concentration of TMOS, siloxane precursor, for the microwave reaction. A TMOS concentration of 25 mM will be used for procedural demonstrations.</li>
</ol>
2.&#160;Hydrolysis of TMOS and Preparation of Reaction Solution
<ol>
	<li>For a 25 mM reaction solution of TMOS in acetone, obtain a 50 ml&#160;plastic conical tube, TMOS, and the prepared 1 mM HCl solution.</li>
	<li>Using a micropipetter, add 850 &#181;l&#160;of the 1 mM HCl solution to the plastic conical tube.</li>
	<li>Using a micropipetter, add 150 &#181;l&#160;of TMOS to the plastic conical tube containing the 850 &#181;l&#160;of 1 mM HCl.</li>
	<li>Observe that the mixture of TMOS and acid are originally immiscible, resulting in two clear colorless layers.</li>
	<li>Vortex the solution. A quick burst will cause solution to turn slightly cloudy and pale white. Vortexing the solution again will yield a clear colorless solution. Total vortex time is ~10 sec. At this point, the TMOS is hydrolyzed and in the form of silicic acid.</li>
	<li>Add acetone to the plastic conical tube containing the silicic acid solution and dilute to a total volume of 40 ml. Ensure proper mixing by vortexing the solution after diluting the solution with acetone.</li>
</ol>
Note: Prior to microwave heating, the silicic acid / acetone solution will be stable for several hours. Silica condensation was not observed in similar prepared solutions which were stable for several weeks without any evidence of condensation. Although for best results, reaction solutions should be used upon day of preparation to ensure reproducible results.
3.&#160;Create Microwave Method on Microwave Reactor
<ol>
	<li>Create a microwave method within the provided software for SiO2 NP formation. Typical reaction parameters include: power = 300 W, temperature = 125 &#176;C and time = 60 sec. Using an external laptop the temperature, pressure, and power profiles can be stored, plotted, and compared with future experiments as in Figure 1.</li>
</ol>
4.&#160;Microwave-assisted Synthesis of SiO2&#160;Nanoparticles
<ol>
	<li>Obtain a 10 ml&#160;microwave reaction vial, stir bar, and snap cap.</li>
	<li>Add the stir bar to the vial and using a micropipetter add 5 ml&#160;of the silicic acid/acetone reaction solution to the microwave vial. A reaction volume of 5 ml&#160;should be used to provide adequate headspace to accommodate the generation of pressure from superheated acetone.</li>
	<li>Place the snap cap on 10 ml&#160;vial. Ensure cap seats properly on the vial to reduce loss of solvent when superheated.</li>
	<li>Place vial in microwave reactor.</li>
	<li>Press the start button on the laptop to start the microwave heating procedure. Observe proper placement of pressure head unit. Observe that the proper microwave power settings are correct once the reaction begins: correct power setting, increase in reaction temperature, and generation of reaction pressure. When the reaction time has expired, compressed air will quickly cool the reaction.</li>
	<li>Remove the reaction vial from microwave reactor after cooling.</li>
</ol>
Note: Typically for a 5 ml&#160;reaction, a measured conversion of ~40-50% has been reported19 in the preparation of SiO2 NPs by these microwave-assisted methods.
5.&#160;DLS Size Analysis of Reaction Solutions
<ol>
	<li>Obtain a polystyrene disposable cuvette.</li>
	<li>Add 900 &#181;l&#160;of distilled water to the cuvette.</li>
	<li>Add 100 &#181;l&#160;of reaction solution containing SiO2 NP to the cuvette containing the water and mix.</li>
	<li>Setup procedures for particle measurements by following DLS analyzer software and analyze the solution for particle size.</li>
</ol>
6.&#160;SiO2 NP Cleanup Procedure
<ol>
	<li>Add 1 ml&#160;of SiO2 NP reaction solution to an Eppendorf tube.</li>
	<li>Centrifuge at 15.7k x g for 30-60 min&#160;depending on particle diameter.</li>
	<li>Carefully decant excess solution from SiO2 NP pellet.</li>
	<li>Add 1 ml&#160;fresh acetone and resuspend SiO2 NP by placing in an ultrasonic bath for 5-10 min.</li>
	<li>Repeat (centrifuging, decanting, resuspending) two more times for adequate removal of residual silicic acid.</li>
</ol>
7.&#160;SEM Sample Preparation
<ol>
	<li>Clean the highly polished single crystal silicon wafers in an ultrasonic bath for approximately 30 min prior to use.</li>
	<li>Place the silicon wafers in piranha solution (3:1 conc. H2SO4:30% H2O2) at a temperature of 80 &#176;C for 1 hr after removing from the ultrasonic bath. Wash the silicon wafers with distilled water and dry. 
	Note: Caution should be used when handling concentrated acids, especially piranha solution.</li>
	<li>Drop-cast clean or not clean solutions of SiO2 NPs onto the clean prepared silicon wafer.</li>
	<li>Place prepared samples on silicon wafer in the cylindrical tube of the sputter system.</li>
	<li>Vacuum the cylindrical tube of the sputter system to 40 mTorr.</li>
	<li>Purge the cylindrical tube of the sputter system with Argon gas until a pressure of approximately 200 mTorr is reached.</li>
	<li>Decrease the pressure to 80 mTorr and apply 15 mA of voltage and keep constant. Sputter platinum on prepared samples for 60 sec.</li>
</ol>

<ol>
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We have nothing to disclose.

The microwave-assisted methods described in this manuscript are advantageous over conventional heating methods because SiO2 NPs can be synthesized accurately, precisely, and quickly. The following criteria should be followed to eliminate any potential issues associated with SiO2 NPs formation by these microwave-associated techniques: 1) use of a catalyst such as 1 mM HCl, 2) hydrolysis of the TMOS should be completed before addition of acetone, 3) use of an aprotic solvent such as acetone, 4) use of...

Silica nanoparticles (SiO2 NPs) were first synthesized by St&#246;ber1 and through modifications2-7 have become the preferred method for SiO2 NPs synthesis. Typically, St&#246;ber reactions are catalyzed by alkaline conditions where silica sols are formed. Acid-catalyzed reactions are used less frequently than alkaline-catalyzed reactions due to the greater degree of difficulty of hydrolysis of the siloxane precursor. Unlike alkaline-catalyzed reactions, acid-catalyzed reactions preferentially form silica gels.8
Microwave-assisted chemical reactions are an emerging technique within the scientific community and in literature due to the associated benefits to the techniques9-18. Specifically, microwave-assisted techniques have been shown to be advantageous in the synthesis of nanomaterials where the promotion of spontaneous nucleation events is desired. Microwave conditions are advantageous because microwave reactors deliver controlled power quickly to the reaction10. Until recently19, the synthesis of SiO2 NPs using microwave reactors have been used with limited success mainly as a result of issues with reproducibility20-22.
The details and procedural methods often reported in the literature on the synthesis of nanomaterials often tend to be obscure and sometimes seen as an &#34;art-form.&#34; Combining microwave-assisted synthetic techniques and nanomaterial synthesis can compound the subject even further. The purpose of this manuscript is to guide researchers in the synthesis of nanomaterials by microwave-assisted techniques, eliminate obscurity associated with these techniques, and point out common mistakes associated with these techniques.
In a microwave chemical reaction, any molecular species containing a permanent dipole is capable of interacting and perturbing the electromagnetic (EM) field. These species are not limited solely to the reagents and solvents used within the reaction, but can be any substance placed in the EM field i.e. glass vials, salts, ionic liquids.
The ability for a specific substance to effectively convert EM energy into heat is defined as the loss factor of a material or tan &#948;. Solvents are commonly classified by their loss factor where values for tan &#948; &#62; 0.5 are considered high, 0.1 &#62; tan &#948; &#62; 0.5 are considered medium, and tan &#948; &#60; 0.1 are considered low. These loss factors values relate the ability for a particular solvent to couple or absorb microwave energy and convert that energy into heat. Thermal energy is generated through molecular friction of species attempting to align with the oscillating EM field. If solvents with high tan &#948; values are used within a reaction, the solvent will dominate the microwave absorption events, masking the precursor or reagents, leading to bulk heating as a result of the solvent strongly coupling with the EM field.
Typically, polar solvents are used in SiO2 NP synthesis to ensure reagent solubility and for donation of labile protons23. Common solvents used in SiO2 NP syntheses are alcohol solvents such as ethanol, methanol or 2-propanol. These solvents all have high tan &#948; values (0.941, 0.799, and 0.659 for ethanol, 2-propanol and methanol, respectively) making them poor solvent choices for microwave-assisted chemical reactions of SiO2 NPs as they efficiently couple with the EM field. It is our belief that microwave-assisted reactions are most effective when low tan &#948; solvents are used in combination with polar molecular precursors in synthetic reactions. These circumstances allow for the molecular precursors to couple with the EM field, providing molecular heating, while the solvent interacts minimally. For microwave-assisted reactions in this manuscript, acetone is used as an alternative to the commonly used alcohol solvents associated with SiO2 NPs synthesis. Acetone is considered a low loss factor solvent (tan &#948; = 0.054), which limits the solvent interactions within the EM field allowing selective microwave absorption with the reactants meditating silica condensation reactions.
In this manuscript, we outline the procedures associated with the microwave-assisted synthesis of SiO2 NPs which are accurate, precise and quick. SiO2 NP growth is achieved by effective coupling of the precursor with the EM field while the solvent has a minimal role in heating. Hydrolysis of the siloxane precursor is achieved by using hydrochloric acid, which leads to slower rates of hydrolysis and limits further condensation reactions. Alkaline-catalyzed reactions have much faster reaction rates and can complicate growth processes when used with microwave-assisted techniques. The resultant SiO2 NPs synthesized by these techniques range in diameters from 30 nm up to diameters greater than 250 nm. SiO2 NP size is controlled by varying the precursor concentration and exposure time to the microwave radiation.

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	</colgroup>
	<tbody>
		<tr height="25">
			<td height="25" style="height:25px;width:247px;">Name of the reagent</td>
			<td style="width:207px;">Company</td>
			<td style="width:153px;">Catalogue number</td>
			<td style="width:161px;">Comments (optional)</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:247px;">Tetramethylorthosilicate</td>
			<td style="width:207px;">Sigma Aldrich</td>
			<td style="width:153px;">218472</td>
			<td style="width:161px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:247px;">Hydrochloric acid, 37%</td>
			<td style="width:207px;">Sigma Aldrich</td>
			<td style="width:153px;">435570</td>
			<td style="width:161px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:247px;">Acetone</td>
			<td style="width:207px;">Fisher</td>
			<td style="width:153px;">A949SK</td>
			<td style="width:161px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:247px;">Sulfuric acid</td>
			<td style="width:207px;">EMD Millipore</td>
			<td style="width:153px;">SX1244</td>
			<td style="width:161px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:247px;">Hydrogen peroxide, 30%</td>
			<td style="width:207px;">EMD Millipore</td>
			<td style="width:153px;">HX0635</td>
			<td style="width:161px;"></td>
		</tr>
		<tr height="25">
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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.

Temperature, pressure, and microwave power traces for a representative SiO2 NP microwave-assisted reaction are presented in Figure 1. The microwave reaction plot is divided into three sections - ramping, reaction, and cooling. A reaction temperature of 125 &#176;C and reaction time of 60 sec are used in this representative SiO2 NP reaction. During the ramping portion, the power is maximized at 300 W (or near max power) so that the reaction temperature can be reached quickly without ...

Watch this Scientific Journal Video about Preparation of Silica Nanoparticles Through Microwave-assisted Acid-catalysis at JoVE.com

Preparation of Silica Nanoparticles Through Microwave-assisted Acid-catalysis

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 ...

preparation-silica-nanoparticles-through-microwave-assisted-acid

Research

JoVE Journal

Chemistry

18.5K vistas.  Oak Ridge Institute for Science and Education. 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.