Preparation of Janus Particles and Alternating Current Electrokinetic Measurements with a Rapidly Fabricated Indium Tin Oxide Electrode Array

Summary

In this article, a simple method to prepare partially or fully coated metallic particles and to perform AC electrokinetic property measurements with a rapidly fabricated indium tin oxide (ITO) electrode array is demonstrated.

Abstract

This article provides a simple method to prepare partially or fully coated metallic particles and to perform the rapid fabrication of electrode arrays, which can facilitate electrical experiments in microfluidic devices. Janus particles are asymmetric particles that contain two different surface properties on their two sides. To prepare Janus particles, a monolayer of silica particles is prepared by a drying process. Gold (Au) is deposited on one side of each particle using a sputtering device. The fully coated metallic particles are completed after the second coating process. To analyze the electrical surface properties of Janus particles, alternating current (AC) electrokinetic measurements, such as dielectrophoresis (DEP) and electrorotation (EROT)- which require specifically designed electrode arrays in the experimental device- are performed. However, traditional methods to fabricate electrode arrays, such as the photolithographic technique, require a series of complicated procedures. Here, we introduce a flexible method to fabricate a designed electrode array. An indium tin oxide (ITO) glass is patterned by a fiber laser marking machine (1,064 nm, 20 W, 90 to 120 ns pulse-width, and 20 to 80 kHz pulse repetition frequency) to create a four-phase electrode array. To generate the four-phase electric field, the electrodes are connected to a 2-channel function generator and to two invertors. The phase shift between the adjacent electrodes is set at either 90° (for EROT) or 180° (for DEP). Representative results of AC electrokinetic measurements with a four-phase ITO electrode array are presented.

Introduction

Janus particles, named after the Roman god with a double face, are asymmetric particles whose two sides have physically or chemically different surface properties^1,2. Due to this asymmetric feature, Janus particles exhibit special responses under electric fields, such as DEP^3,4,5,6, EROT², and induced-charge electrophoresis (ICEP)^7,8,9. Recently, several methods to prepare Janus particles have been reported, including the Pickering emulsion method¹⁰, the electrohydrodynamic co-jetting method¹¹, and the microfluidic photopolymerization method¹². However, these methods require a series of complicated apparatus and procedures. This article introduces a simple method to prepare Janus particles and fully coated metallic particles. A monolayer of micro-scaled silica particles is prepared in a drying process and is put into a sputtering device to be coated with Au. One hemisphere of the particle is shaded, and only the other hemisphere is coated with Au^2,13. The monolayer of the Janus particle is stamped with a polydimethylsiloxane (PDMS) stamp and then treated with a second coating process to prepare fully coated metallic particles¹⁴.

To characterize the electrical properties of a Janus particle, different AC electrokinetic responses, such as DEP, EROT, and electro-orientation, are widely used^{9,15,16,17,18,19}. For example, EROT is the steady-state rotational response of a particle under an externally imposed rotating electric field^2,9,15,16. By measuring the EROT, the interaction between the induced dipole of the particles and the electric fields can be obtained. DEP, which arises from the interaction between the induced dipoles and a non-uniform electric field, is capable of leading to particle movement^3,4,5,9,15. Different kinds of particles can be attracted to (positive DEP) or repelled from (negative DEP) the electrode edges, which serves as a general method for manipulating and characterizing particles in the microfluidic device. The translational (DEP) and rotational (EROT) characteristics of the particle under the electric field are dominated by the real and imaginary part of the Clausius-Mossotti (CM) factor, respectively. The CM factor depends on the electrical properties of the particles and the surrounding liquid, which are revealed from the characteristic frequency, ω_c = 2σ / aC_DL, of DEP and EROT, where σ is the liquid conductivity, a is the particle radius, and C_DL is the capacitance of the electrical double layer^15,16. To measure the EROT and DEP of particles, specially designed electrode array patterns are needed. Traditionally, a photolithographic technique is used to create electrode arrays and requires a series of complicated procedures, including photoresist spin-coating, mask alignment, exposure, and development^15,18,19,20.

In this article, the rapid fabrication of electrode arrays is demonstrated by direct optical patterning. A transparent thin-film ITO layer, which is coated on the glass substrate, is partially removed by a fiber laser marking machine (1,064 nm, 20 W, 90 to 120 ns pulse width, and 20 to 80 kHz pulse repetition frequency) to form a four-phase electrode array. The distance between the diagonal electrodes is 150-800 µm, which can be adjusted to suit the experiments. The four-phase electrode array can be used to characterize and concentrate particles in different microfluidic devices^15,16,18. To generate the four-phase electric field, the electrode array is connected to a 2-channel function generator and to two invertors. The phase shift between the adjacent electrodes is set at either 90° (for EROT) or 180° (for DEP)¹⁵. The AC signal is applied at a 0.5 to 4 V_pp voltage amplitude, and the frequency ranges from 100 Hz to 5 MHz during the operation process. Janus particles, metallic particles, and silica particles are used as samples to measure their AC electrokinetic properties. Suspensions of the particles are placed on the center region of the electrode array and are observed under an inverted optical microscope with a 40X, NA 0.6 objective. Particle motion and rotation are recorded with a digital camera. The DEP movement is recorded at the annular region, between 40 and 65 µm radially away from the array center, and EROT is recorded at the circular region, 65 µm radially away from the array center. Particle velocity and angular velocity are measured by the particle-tracking method. The particle centroids are distinguished by gray scale or geometry of particles using software. The particle velocity and angular velocity are obtained by measuring the movements of the particle centroids.

This article provides a simple method to rapidly fabricate arbitrarily patterned electrode arrays. It introduces the preparation of fully or partially coated metallic particles, which can be used in different fields, with uses ranging from biology to industry applications.

Protocol

1. Fabrication of the Microchip Preparation of the ITO electrode Use commercial illustration software to draw a cross pattern. Set the distance between the diagonal electrodes to 160 µm and make the arms of the cross pattern 30 mm wide and 55 mm long, as shown in Figure 1. Save the illustration file as a DXF file. Use a glass cutter to trim the ITO glass to a size of 25 mm x 50 mm (width x length). Use 75% ethanol and DI water to rinse…

Representative Results

The four-phase electrode array is created by a fiber laser marking machine. The ITO conductive layer coated on the glass is removed by a focus laser to form a cross pattern with a gap of 160 µm, as shown in Figure 1B. Figure 1: Preparation of the ITO El…

Discussion

Fabricating ITO electrode arrays using the fiber laser marking machine provides a rapid method to prepare electrodes with arbitrary patterns. However, there are still some disadvantages to this method, such as fewer charge carriers and the lower fabrication accuracy of ITO electrodes compared to metal electrodes created by traditional methods. These disadvantages could limit some experiments. For example, fewer charges carriers could affect the distribution of the electric field when there is a large distance between ele…

Materials

Silica Microsphere-2.34 µm	Bangs Laboratories	SS04N
Ethyl Alcohol (99.5%)	KATAYAMA CHEMICAL	E-0105
SYLGARD 184 A&B Silicone Elastomer(PDMS)	DOW CORNING		PDMS
ITO glass	Luminescence Technology	LT-G001
Fiber laser marking machine	Taiwan 3Axle Technology	TAFB-R-20W
2-channel function generator	Gwinsek	AFG-2225
CMOS camera	Point Grey	GS3-U3-32S4M-C
Sputter	JEOL	JFC-1100E
Operational Amplifiers	Texas Instruments	LM6361N	OP invertor
Ultrasonic Cleaner	Gui Lin Yiyuan Ultrasonic Machinery Co.	DG-1
Microcentrifuge	Scientific Specialties, Inc.		1.5ml
Mini Centrifuge	LMS	MC-MCF-2360
Microscope cover glass	Marienfeld-Superior		18*18mm
Inverted optical microscope	Olympus	OX-71
Parafilm	bemis		spacer