Aqueous Droplets Used as Enzymatic Microreactors and Their Electromagnetic Actuation

Summary

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 µL microreactors and thereby initiate an enzymatic reaction in the resulting liquid marbles.

Abstract

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 µL 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.

Introduction

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 mo....

Protocol

1. Hydrophobization of Magnetic Nanoparticles

1. 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).
2. 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 cork in order to prevent the secondary oxidation of the magnetite particles.
3. Adjust the solution drop wise with an ammonium hydroxide soluti....

Results

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 µL microreactor with water as a liquid core had a contact angle of around 160°.

The force needed to move a 10 µL microreactor as described above is 1.34 ± 0.08 µN. Figure 1 shows the electromagnetic force of a coil w.......

Discussion

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.......

Materials

List of materials used in this article

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