Name: aqueous droplets used as enzymatic microreactors and their electromagnetic actuation
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The authors would like to acknowledge the DFG for the support.

1. Hydrophobization of Magnetic Nanoparticles
<ol>
	<li>For the synthesis of the hydrophobic magnetic particles, add 0.85 g FeCl3 hexahydrate (3.14 mmol) and 0.30 g FeCl2 tetrahydrate (1.51 mmol) to 200 mL water/ethanol solution (4:1 v/v).</li>
	<li>To this mixture, add 0.20 mL 1H,1H,2H,2H-Perfluorooctyltriethoxysilane (PFOTES) (5.23 mmol) with vigorous stirring by a magnetic stirrer (500 rpm). Carry out the synthesis in an inert gas atmosphere (N2) by using a round-bottom flask with a ...

<ol>
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For the successful use of microfluidic technologies, it is important to move the reaction volume corresponding to the requirements of the biotechnological synthesis and analyses. The actuation platform presented here makes it possible to move microfluidic droplets by magnetic force. The movement can be performed freely in two dimensions on a planar surface of a reaction platform by enclosing the liquid drop with a magnetic superhydrophobic shell. Thus an alternative system to predefined microfluidic channels with complex...

Technical applications with micro reactions are predominantly carried out in predefined microchannel chips. These systems are widely established and comprehensively described in the literature (inter alia 1,2,3). In 2011, the turnover of microfluidic technologies worldwide totaled 6.2 billion euro 4. In contrast, the use of freely movable micro reactor compartments was previously only examined and published to a limited extent. The most common method for moving aqueous micro droplets is electrowetting 5. Other methods for the motion of drops on surfaces are based on electric fields 6, magnetic force 7 or acoustic actuation 8. Due to their unfavorable surface to volume ratio, these droplet-based microreactor systems are exposed to strong evaporation effects. Thus, the drop motion is usually established as a liquid two-phase system, where the upper phase has a high boiling point protecting the aqueous phase from evaporation. Nevertheless, this approach involves a high risk of contaminating the reaction droplet by uncontrolled diffusion. This is a significant obstacle for the technical establishment of the mentioned systems.
Recent work is concerned with non-adherent liquid-solid phase transitions. A highly effective approach is the use of superhydrophobic surfaces, allowing the formation of spherical aqueous droplets. An extension of this reaction concept is the use of micro reaction compartments with a superhydrophobic surface or shell, which may for example consist of polytetrafluoroethylene (PTFE) particles 9. Their contact angles on surfaces are usually in the range of 160&#176; (depending on the surface roughness). The spherical compartments thus provide minimal resistance to movement on a surface and simultaneously provide protection against water evaporation.
Aqueous drops coated with micro sized PTFE particles may maintain their spherical shape up to a diameter of around 2 mm. At higher volumes, the hydrophobic shell is usually not completely closed anymore 10. The influence of other shell materials and the expansion of the field of application of the liquid marble to nonpolar solvents was implemented by Gao and McCarthy by using ionic liquids 12. For the formation of hydrophobic particle-based shells, so far particle diameters in sizes of 10 nm- 30 &#181;m have been described 11,14,16. New studies showed that hydrophobic nanoparticles as shell material are of even better use than that of microparticles 13. First stability studies confirmed an increase in stability when the particle size is reduced from ca. 600 nm to ca. 100 nm. This likely results from the denser particle distribution around the aqueous sphere 15.
The protection of aqueous reaction compartments by a hydrophobic shell and their designation as liquid marbles was first described in 2001 by Aussillous et al. and Mahadevan et al. 17,18. Since then, few applications of these defined reaction compartments have been described. For example, a gas sensor based on liquid marbles 19 and a detection method for water contamination based on an optically qualitative basis have been developed 20. The authors distinguish the advantages of high reaction rates and the low consumption of chemicals of their micro reaction systems. Recent publications deal with the production of pH-sensitive liquid marbles 16 or the representation of &#39;Janus particles&#39; with two different coatings of different functionality. For example, Bormashenko et al. could synthesize a microreactor with shells made of Teflon and semiconducting Carbon Black 21. Furthermore it was demonstrated that microreactors can efficiently and convenient synthesize polyperoxides by absorbing external oxygen as comonomer through the permeable gas-liquid interface 24. In another approach the shell of silica-particle-based liquid marbles provide the reactive substrate surfaces to regulate the classical silver mirror reaction 26. Current problems for research and development in the field of hydrophilic-core-hydrophobic-shell droplets are the particle size adjustment, the reproducible production of monodisperse droplets, the wettability of surfaces and the effect of a second hydrophilic shell on the micro reaction compartments 22, as well as a better control of the droplet trajectories, e.g. for the development of continuous microPCR-systems 4.
A magnetic actuation of these microreactors offers the advantage of relatively high movement ranges and a good selectivity of the force when working in biochemical systems. When using hydrophobic magnetite particles, they fulfil both the function of the magnetic force transmission to the movement of the microreactors, as well as the function of a hydrophobic shell. The magnetic movement of droplets with magnetic particles inside a droplet was postulated for the first time in 2006 by Lehmann et al. 23 and Shikida et al. 25, who used manually moved permanent magnets as actuators for the mobilization of a single droplet. Another approach to move a small amount of liquid was realized by Zhao et al., who used the hydrophobic Fe3O4 particles as magnetic shell. The shell of the magnetic liquid marble was opened on the upper side of the drop by a vertical reverse magnetic field 27. Based on this concept, Xue et al. were able to develop particles which form a microreactor with a surface tension of 20.1 dyne cm&#8722;1 28. Lin et al. fabricated novel cellulose-based micro/nano hierarchical spheres with both superparamagnetism and superhydrophobicity which provide god stability for magnetic liquid droplet transportation and manipulation 31.This was so far only released as a proof-of-principle study and not used for any application. The magnetic and electrical control of the liquid marbles is currently pursued in first approaches. Zhao et al. in 2010 15 and Zhang et al. 2012 29 were able to develop a droplet manipulation by the manual (hand-operated) movement of a permanent magnet beneath core-shell droplets. Bormashenko et al. 11 achieved the acceleration of a ferromagnetic liquid marble to a speed of 25 cm s-1 by approaching a neodymium magnet. The above mentioned principle studies were carried out exclusively by the manual movement of a small permanent magnet. As a next development step, Zhao et al. were recently able to estimate the required magnetic flux density for the movement of magnetic liquid marble by varying the distance of a permanent magnet 30. For a reaction control comparable to that of common lab-on-a-chip systems, it seems inevitable to provide the means of automated control of the discrete liquid volumes. To satisfy this need, we developed a new control system based on variable field gradients to fixate, move and open the magnetic microreactors.

<table border="0" cellpadding="0" cellspacing="0" width="903">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:363px;">3D-printer</td>
			<td style="width:163px;">FelixPrinters</td>
			<td style="width:211px;">Pro1</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">10-acetyl-3,7-dihydroxyphenoxazine (Amplex Red)</td>
			<td>Life Technologies</td>
			<td>A12222</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ammonium hydroxide</td>
			<td style="width:163px;">TU-KL</td>
			<td>1072</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">CAD software</td>
			<td style="width:163px;">Siemens</td>
			<td style="width:211px;">Soled edge</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Contact angle measuring device</td>
			<td style="width:163px;">Dataphysics</td>
			<td style="width:211px;">OCA 20</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:363px;">Cylinder magnet</td>
			<td style="width:163px;">Webcraft GmbH</td>
			<td style="width:211px;">S-04-13-N</td>
			<td>https://www.supermagnete.de/stabmagnete-neodym-rund/stabmagnet-durchmesser-4mm-hoehe-12.5mm-neodym-n42-vernickelt_S-04-13-N</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Dipotassium phosphate</td>
			<td>Bernd Kraft</td>
			<td>7758-11</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Drying oven</td>
			<td>Binder</td>
			<td>FD 115</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Ethanol</td>
			<td>Sigma-Aldrich</td>
			<td>68-17-5</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">FeCl2 tetrahydrate</td>
			<td style="width:163px;">TU-KL</td>
			<td>1625</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">FeCl3 hexahydrate</td>
			<td style="width:163px;">TU-KL</td>
			<td>1622</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Fluorescence probe</td>
			<td style="width:163px;">PerkinElmer</td>
			<td style="width:211px;">LS 55</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Horseradish peroxidase</td>
			<td>Carl Roth</td>
			<td>9003-99-0</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Hydrogen peroxide</td>
			<td>Th.Geyer GmbH &#38; Co</td>
			<td>7722-84-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Monopotassium phosphate</td>
			<td>Bernd Kraft</td>
			<td>7778-77-0</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Peltier element</td>
			<td style="width:163px;">Conrad&#160;</td>
			<td style="width:211px;">193569</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Perfluoroctyltriethoxysilane</td>
			<td>Sigma-Aldrich</td>
			<td>51851-37-7</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Scanning Electron Microscope</td>
			<td style="width:163px;">FEI</td>
			<td style="width:211px;">Helios NanoLab 650 DualBeam</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:363px;">Separation bar magnet</td>
			<td style="width:163px;">Webcraft GmbH</td>
			<td style="width:211px;">Q-40-20-10-N,&#160;</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Winding machine</td>
			<td style="width:163px;">IWT GmbH</td>
			<td style="width:211px;">FW122</td>
			<td></td>
		</tr>
	</tbody>
</table>

aqueous droplets used as enzymatic microreactors and their electromagnetic actuation

For the successful implementation of microfluidic reaction systems, such as PCR and electrophoresis, the movement of small liquid volumes is essential. In conventional lab-on-a-chip-platforms, solvents and samples are passed through defined microfluidic channels with complex flow control installations. The droplet actuation platform presented here is a promising alternative. With it, it is possible to move a liquid drop (microreactor) on a planar surface of a reaction platform (lab-in-a-drop). The actuation of microreactors on the hydrophobic surface of the platform is based on the use of magnetic forces acting on the outer shell of the liquid drops which is made of a thin layer of superhydrophobic magnetite particles. The hydrophobic surface of the platform is needed to avoid any contact between the liquid core and the surface to allow a smooth movement of the microreactor. On the platform, one or more microreactors with volumes of 10 &#181;L can be positioned and moved simultaneously. The platform itself consists of a 3 x 3 matrix of electrical double coils which accommodate either neodymium or iron cores. The magnetic field gradients are automatically controlled. By variation of the magnetic field gradients, the microreactors' magnetic hydrophobic shell can be manipulated automatically to move the microreactor or open the shell reversibly. Reactions of substrates and corresponding enzymes can be initiated by merging the microreactors or bringing them into contact with surface immobilized catalysts.

The shell particles have a diameter of around 640 nm. The magnetizable nanoparticles enclosed in this fluorosilane shell particles have diameters in a range between 22 nm and 37 nm. A 5 &#181;L microreactor with water as a liquid core had a contact angle of around 160&#176;.
The force needed to move a 10 &#181;L microreactor as described above is 1.34 &#177; 0.08 &#181;N. Figure 1 shows the electrom...

Watch this Scientific Journal Video about Aqueous Droplets Used as Enzymatic Microreactors and Their Electromagnetic Actuation at JoVE.com

Aqueous Droplets Used as Enzymatic Microreactors and Their Electromagnetic Actuation

Lab-in-a-drop reaction systems allow the versatile implementation of complex reactions in a microfluidic scale. An automated actuation platform consisting of a 3 x 3 matrix of electromagnetic coils was developed and successfully used to merge two 10 &#181;L microreactors and thereby initiate an enzymatic reaction in the resulting liquid marbles.

For the successful implementation of microfluidic reaction systems, such as PCR and electrophoresis, the movement of small liquid volumes is essential. In ...

The overall purpose of this device is to provide a new microfluidic platform technology that realizes automated cascade reactions in small aqueous droplets. This method could help answer key questions to the field of synthetic biology by reproducing reaction sequences of microorganisms performing complex reaction sequences with enzymes and screening new catalysts. The main advantage of our lab-in-a-drop approach is that it is very adaptable.As opposed to channel-based microfluidic chips, the only prerequisite is a hydrophilic-reaction environment. Though this method can provide insight into enzymatic reactions, it can also be applied to other systems, such as inorganic catalysts or stringing of ligands. I first had the idea for this method when I tried to design a microfluidic device with integrated product separation but without a complex system of valves and separators.By moving the reaction solution as a droplet, reaction control becomes very simple. First, design in 3D print the coil bodies. Then, use a winding machine to wrap the bodies with a 08-millimeter copper wire 45 hundred times each.Place a Peltier element on an electric board. Screw the first layer of coils onto the Peltier element to form a square. Then, secure the second layer of coils on top with the 3D-printed plastic frame or other non-conductive material.Connect the electric board to the magnet control with a ribbon cable. Insert neodymium magnets into the coils. Place a one-millimeter thick quartz glass or plastic plate on top of the coil matrix.Secure the cover in place with screws to finish assembling the actuation platform. To begin the nanoparticle synthesis under an inert atmosphere, first suspend 85 grams Iron(III)chloride hexahydrate and 3 grams Iron(II)chloride tetrahydrate in 200 milliliters of a four-to-one water-to-ethanol solution. Then, add 0.2 milliliters PFOTES and stir the mixture at 500 RPM with a magnetic stir bar.Add 1.5 molar ammonium hydroxide to the mixture to achieve a pH of 8.0. And then continue stirring the solution for 24 hours to obtain the hydrophobic magnetic nanoparticles. Affix a bar magnet with an adhesive force of 25 kilograms to the bottom of the reaction flask.Pour off the solution and wash the particles three times with a four-to-one water-to-ethanol solution. Remove the magnet and dry the particles at 60 degree Celsius for 24 hours. Characterize the particles with scanning electron microscopy.Prepare a solution of 0.1 micrograms per milliliter horseradish peroxidase in 0.1 molar pH 6.5 potassium phosphate buffer with 10 millimolar hydrogen peroxide. Then, prepare a 20-micromolar solution of the fluorescent probe 10-Acetyl-3, 7-dihydroxyphenoxazine in potassium phosphate buffer. Then, gently grind the dry nanoparticles using a glass mortar and pestle.Transfer the particles to a polystyrene weighing pan and add 10 microliters of the peroxidase solution. Gently swirl the pan for about 10 seconds to achieve nanoparticle self assembly around the peroxidase solution. Transfer this microreactor to the actuation platform.Repeat this process with 10 microliters of the fluorescent probe solution and place the second microreactor on the platform. Store the remaining particles at room temperature. Use the Peltier element to keep the reaction solutions in the microreactors at 25 degree Celsius.Mount a fluorescence microscope about 10 millimeters above the microreactors and connect the microscope to a computer. Using the magnet control activate the magnets and turn to position the microreactor of fluorescent probe solution under the microscope. Turn on the excitation light.Raise the coil magnet to open the microreactor and begin recording the fluorescence microscopy image. After two-to-five seconds, lower the magnet to close the microreactor. Then, use the magnet control to move the peroxidase microreactor adjacent to the fluorescence microreactor.Open the fluorescent probe microreactor. The peroxidase microreactor is pulled into the probe microreactor and opened, initiating the reaction. Monitor the reaction by fluorescence microscopy opening and closing the merged microreactor as needed.Microreactors were created by coding droplets of solution with hydrophobic iron nanoparticles. These microreactors can be moved, opened, closed, and merged by manipulation of neodymium magnets under an actuation platform. The actuation platform can also be constructed with iron cores in the coil bodies.This provides enough magnetic force to move a microreactor by more than 10 millimeters but is insufficent to open the microreactor. Thus, a neodymium magnet must be used to perform reactions in small solution volumes. A representative enzymatic reaction between a peroxidase and a fluorescent label was performed and monitored by fluorescent microscopy.The Michaelis-mentioned kinetics observed during the reaction agreed well with literature values, indicating the the microreactor setup does not influence the reaction progress. After watching this video, you should have a good understanding of how to synthesize hydrophobic microparticles and to generate aqueous microdroplets with them. The platform technology can be used for automated reaction control by droplet merging.Additionally, single microreactors filled with substrate can moved on top of small surface zones with mobilized enzymes. Using multiple immobilzation zones with different enzymes enables a sequential reaction cascade with transportable and immediate products. This prototype development is just the beginning.Currently, we are implementing an automated dispenser system above the reaction platform to reposit immobilized enzymes at any position. The coil matrix will be increased to 10 times 10 positions. This technique can pave the way for computer-controlled planning and execution of biochemical reaction sequences.

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