Name: Preparation of Janus Particles and Alternating Current Electrokinetic Measurements with a Rapidly Fabricated Indium Tin Oxide Electrode Array (Video) | JoVE Journal
Uploaded: 2017-06-23T14:00:00.000+00:00
Duration: 9 min 55 s
Description: Learn to fabricate ITO electrodes using laser engraving and prepare metal-coated Janus particles for AC electrokinetic measurements. This method enables electrorotation and dielectrophoresis experiments in microfluidics.

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5 minutes

This work was supported by the Ministry of Science and Technology, Taiwan, R.O.C., under Grant NSC 103-2112-M-002-008-MY3.

1. Fabrication of the Microchip
<ol>
	<li>Preparation of the ITO electrode
	<ol>
		<li>Use commercial illustration software to draw a cross pattern. Set the distance between the diagonal electrodes to 160 &#181;m 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.</li>
		<li>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 the ITO glass several times.</li>
		<li>Put the ITO glass onto the pulsed-fiber laser marking machine. Focus the laser on the surface of the ITO glass by adjusting the distance between the ITO glass and the laser to 279.5 mm. 
		NOTE: The laser used here has the following specifications: 1,064 nm, 20 W, 90 to 120 ns pulse width, and 20 to 80-kHz pulse repetition frequency, with a pulsed-light intensity at about 5 x 105 W/cm2).</li>
		<li>Directly input the illustration file (DXF file) onto the computer of the laser marking machine. Click the &#34;Mark Parameter&#34; button and input the following parameters: speed, &#34;800 mm/s;&#34; power, &#34;60%;&#34; and frequency, &#34;40 kHz.&#34; Tick the &#34;Frame,&#34; &#34;Fill,&#34; and &#34;Fill First&#34; terms.
		<ol>
			<li>Click the &#34;Preview&#34; button position the pattern at the ITO glass center. Click the &#34;Mark Sample&#34; button to pattern the ITO glass (Figure 1A).</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Setting up a four-phase generator and connecting the experimental microchip
	<ol>
		<li>Build the circuits of the invertors, as shown in Figure 2A.</li>
		<li>Connect the 4 wires on the four-phase electrode by direct contact with tape, as shown in Figure 2C. Divide the &#34;Channel 1&#34; of the function generator into two branches using a double BNC connector.
		<ol>
			<li>Connect one branch to the wire (#1) attached to the ITO electrode and the other to the input of the invertor. Connect the output of the invertor to the wire (#3), as shown in Figure 2B.</li>
		</ol>
		</li>
		<li>Connect &#34;Channel 2&#34; with the same procedure as in step 1.2.2, but connect to the wires (#2 and #4) as shown in Figure 2B.</li>
		<li>For the EROT experiments, set the phase shift between the two channels to 90&#176;, directly on the function generator. Apply a sine wave at 0.5-4 Vpp voltage amplitude and a frequency ranging from 100 Hz to 5 MHz during the experiments, as shown in Figure 2D.</li>
		<li>For the DEP experiments, connect one branch of Channel 1 to the wire (#1) attached to the ITO electrode and the other to the input of the invertor. Connect the output of the invertor to the wire (#2). Connect Channel 2 using the same procedure, but connect to the wires (#3 and #4).</li>
		<li>Set the phase shift between the 2 channels at 0&#176;, directly on the function generator. Apply a sine wave at 0.5-4 Vpp voltage amplitude and a frequency ranging from 100 Hz to 5 MHz during the experiments, as shown in Figure 2D.</li>
	</ol>
	</li>
</ol>
2. Preparation of the Samples
<ol>
	<li>Preparation of Janus particles
	<ol>
		<li>Centrifuge the 2 &#181;m silica particle aqueous suspension (10% w/w) at 2,200 x g for 1 min.</li>
		<li>Pipette 2 &#181;L of sedimentary silica particles into a 1.5 mL microcentrifuge tube and add 500 &#181;L of ethanol (99.5% v/v). 
		NOTE: The supernatant does not need to discarded; just save it in the refrigerator at 4 &#176;C. It does not need to be resuspended before pipetting.</li>
		<li>Sonicate the ethanol-silica particle suspension using an ultrasonicator (43 kHz, 50 W) for 1 min and then centrifuge it at 2,200 x g for 3 min.</li>
		<li>Substitute the supernatant with 500 &#181;L of ethanol and repeat step 2.1.3 three times.</li>
		<li>Substitute the supernatant with 8 &#181;L of ethanol and sonicate the ethanol-silica particle suspension using an ultrasonicator (43 kHz, 50 W) for 3 min. 
		NOTE: About 10 &#181;L of ethanol-silica particle suspension should remain in the tube at this step.</li>
		<li>Pipette 2 &#181;L of ethanol-silica particle suspension and drop it on a normal glass slide (width: 25 mm, length: 75 mm, and thickness: 1.2 mm) to form a droplet. 
		Note: This quantity of particle suspension is enough to prepare the monolayers for 5-6 slides (2 &#181;L for each slide).</li>
		<li>Slowly drag the ethanol-silica particle droplet slightly with a cover glass to form a monolayer of silica particles (Figure 3A).</li>
		<li>Put the slide with the monolayer of silica particles into a sputtering device to be coated with Au.
		<ol>
			<li>Remove the air from the sputter chamber at 100 mTorr and inject argon for 10 min (replace the air with argon). Stop injecting argon and then remove the argon from the chamber at 70 mTorr.</li>
			<li>Set the current to 15 mA for 200 s. (Figure 3B); Janus particles are already prepared at this step.</li>
		</ol>
		</li>
		<li>Drop 20 &#181;L of DI water on the sputtered slide and scrape the Janus particles from the monolayer using a normal 200 &#181;L pipette tip. 
		NOTE: The Janus particles scraped from the monolayer suspend in the DI water droplet at this step.</li>
		<li>Pipette the Janus particle suspension droplet and drop it into another 1.5-mL centrifuge tube.</li>
		<li>Use the Janus particle suspension to prepare the sample by diluting it with DI water to a suitable concentration for experiments. 
		NOTE: The concentration of the particle suspension in the experiments described here is about 2,000 counts/&#181;L.</li>
	</ol>
	</li>
	<li>Preparation of fully coated metallic particles14
	<ol>
		<li>Mix the PDMS polymer base and the curing agent at a 10:1 weight ratio.</li>
		<li>Tape around the glass slide to form the lateral walls of the container. Pour the mixture of PDMS onto a taped slide to achieve a PDMS layer of 2-3 mm, as shown in Figure 4A.</li>
		<li>Put the taped slide (container) with the PDMS mixture in an airtight chamber and run the vacuum pump for 30 min to remove the bubbles in the PDMS mixture.</li>
		<li>Put the taped slide (container) with the PDMS mixture (step 2.2.3) into the oven. Cure the PDMS mixture at 70 &#176;C for 2 h to form a PDMS stamp.</li>
		<li>After the PDMS stamp is cured, remove the slide and tape to get the PDMS stamp, the surface of which attached originally to the glass slide, forming a flat surface, as shown in Figure 4B.</li>
		<li>Follow the steps from 2.1.1-2.1.8 to prepare a monolayer of Janus particles on a slide.</li>
		<li>Use the flat surface of the PDMS stamp to stamp the monolayer with uniform pressure.</li>
		<li>Put the PDMS stamp with the monolayer of Janus particles, which is inverted from the glass slide made in step 2.1.8, into the sputtering device to be coated with Au.
		<ol>
			<li>Remove air from the sputter chamber at 100 mTorr and inject argon for 10 min (replace the air with argon). Stop injecting argon and then remove the argon from the chamber at 70 mTorr.</li>
			<li>Set the current to 15 mA for 200 s (Figure 4C); the fully coated metallic particles are already prepared at this step.</li>
		</ol>
		</li>
		<li>Follow the steps from 2.1.9-2.1.11 to prepare the sample for experiments.</li>
	</ol>
	</li>
</ol>
3. Experiments for AC Electrokinetic Measurement
<ol>
	<li>Wrap 5 pieces of paraffin film to prepare a spacer. Combine the ITO electrode array with the film spacer of 500-&#181;m thickness using a heat gun and place the electrode on the microscope stage.
	<ol>
		<li>Drop 8 &#181;L of particle suspension, which was prepared in steps 2.1 and 2.2, onto the center of the cross electrode array. Place a cover glass on the spacer.</li>
	</ol>
	</li>
	<li>For the EROT experiments, on the function generator, set the phase shift between the 2 channels to 90&#176;. Apply a sine wave at 0.5-4 Vpp voltage amplitude and a frequency ranging from 100 Hz to 5 MHz during the experiments (based on the connection in steps 1.2.2-1.2.3).
	<ol>
		<li>Choose the wave form by clicking the &#34;waveform&#34; button on the function generator. Input the voltage and frequency value using the numbered buttons on the function generator and then turn on the AC signal by clicking &#34;output&#34; button.</li>
	</ol>
	</li>
	<li>For the DEP experiments, set the phase shift between the 2 channels to 0&#176;. Apply a sine wave at 0.5-4 Vpp voltage amplitude and a frequency of 100 Hz to 5 MHz during the experiments (based on the connections in step 1.2.5) by setting up the function generator as in step 3.2.1.</li>
	<li>Turn on the AC signal by clicking the &#34;output&#34; button and capture the images of particle motion and rotation under an inverted optical microscope with a 40X, NA 0.6 objective using a camera.</li>
	<li>Input the images of particle motion and rotation to the software and analyze the trajectory of particles by particle tracking to obtain the particle and angular velocities. 
	Note: The &#34;Image J&#34; software and its &#34;MultiTracker&#34; plugin were used here for image binarization and particle tracking.</li>
</ol>

<ol><li>Walther, A., Müller, A. H. <a target="_blank" href="http://pubs.rsc.org/en/Content/ArticleLanding/2008/SM/b718131k#!divAbstract">Janus particles.</a> Soft Matter. 4 (4), 663-668 (2008).</li>
<li>Chen, Y. -L., Jiang, H. -R. <a target="_blank" href="http://aip.scitation.org/doi/abs/10.1063/1.4967740?journalCode=apl">Electrorotation of a metallic coated Janus particle under AC electric fields.</a> Appl Phys Lett. 109 (19), 191605(2016).</li>
<li>Zhang, L., Zhu, Y. <a target="_blank" href="https://www.ncbi.nlm.nih.gov/pubmed/22924894">Directed assembly of janus particles under high frequency ac-electric fields: Effects of medium conductivity and colloidal surface chemistry.</a> Langmuir. 28 (37), 13201-13207 (2012).</li>
<li>Gangwal, S., Cayre, O. J., Velev, O. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Dielectrophoretic+assembly+of+metallodielectric+Janus+particles+in+AC+electric+fields%5BTitle%5D)+AND+%22Langmuir%22%5BJournal%5D)">Dielectrophoretic assembly of metallodielectric Janus particles in AC electric fields.</a> Langmuir. 24 (23), 13312-13320 (2008).</li>
<li>Zhang, L., Zhu, Y. <a target="_blank" href="http://aip.scitation.org/doi/abs/10.1063/1.3378687?journalCode=apl">Dielectrophoresis of Janus particles under high frequency ac-electric fields.</a> Appl Phys Lett. 96 (14), 141902(2010).</li>
<li>Chen, J., Zhang, H., Zheng, X., Cui, H. <a target="_blank" href="http://aip.scitation.org/doi/full/10.1063/1.4868373">Janus particle microshuttle: 1D directional self-propulsion modulated by AC electrical field.</a> AIP Adv. 4 (3), 031325(2014).</li>
<li>Gangwal, S., Cayre, O. J., Bazant, M. Z., Velev, O. D. <a target="_blank" href="http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.100.058302">Induced-charge electrophoresis of metallodielectric particles.</a> Phys Rev Lett. 100 (5), 058302(2008).</li>
<li>Peng, C., Lazo, I., Shiyanovskii, S. V., Lavrentovich, O. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Induced-charge+electro-osmosis+around+metal+and+Janus+spheres+in+water%3A+Patterns+of+flow+and+breaking+symmetries%5BTitle%5D)+AND+%22Phys+Rev+E%22%5BJournal%5D)">Induced-charge electro-osmosis around metal and Janus spheres in water: Patterns of flow and breaking symmetries.</a> Phys Rev E. 90 (5), 051002(2014).</li>
<li>Ramos, A., García-Sánchez, P., Morgan, H. <a target="_blank" href="https://www.infona.pl/resource/bwmeta1.element.elsevier-629cc57b-551e-3eca-bc03-4c5aae304080">AC electrokinetics of conducting microparticles: A review.</a> Curr Opin Colloid Interface Sci. 24, 79-90 (2016).</li>
<li>Hong, L., Jiang, S., Granick, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Simple+method+to+produce+Janus+colloidal+particles+in+large+quantity%5BTitle%5D)+AND+%22Langmuir%22%5BJournal%5D)">Simple method to produce Janus colloidal particles in large quantity.</a> Langmuir. 22 (23), 9495-9499 (2006).</li>
<li>Bhaskar, S., Hitt, J., Chang, S. W. L., Lahann, J. <a target="_blank" href="http://onlinelibrary.wiley.com/doi/10.1002/anie.200806241/full#references">Multicompartmental microcylinders.</a> Angewandte Chemie International Edition. 48 (25), 4589-4593 (2009).</li>
<li>Nie, Z., Li, W., Seo, M., Xu, S., Kumacheva, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Janus+and+ternary+particles+generated+by+microfluidic+synthesis%3A+design%2C+synthesis%2C+and+self-assembly%5BTitle%5D)+AND+%22J+Am+Chem+Soc%22%5BJournal%5D)">Janus and ternary particles generated by microfluidic synthesis: design, synthesis, and self-assembly.</a> J Am Chem Soc. 128 (29), 9408-9412 (2006).</li>
<li>Jiang, H. -R., Yoshinaga, N., Sano, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Active+motion+of+a+Janus+particle+by+self-thermophoresis+in+a+defocused+laser+beam%5BTitle%5D)+AND+%22Phys+Rev+Lett%22%5BJournal%5D)">Active motion of a Janus particle by self-thermophoresis in a defocused laser beam.</a> Phys Rev Lett. 105 (26), 268302(2010).</li>
<li>Pawar, A. B., Kretzschmar, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Multifunctional+patchy+particles+by+glancing+angle+deposition%5BTitle%5D)+AND+%22Langmuir%22%5BJournal%5D)">Multifunctional patchy particles by glancing angle deposition.</a> Langmuir. 25 (16), 9057-9063 (2009).</li>
<li>García-Sánchez, P., Ren, Y., Arcenegui, J. J., Morgan, H., Ramos, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Alternating+current+electrokinetic+properties+of+gold-coated+microspheres%5BTitle%5D)+AND+%22Langmuir%22%5BJournal%5D)">Alternating current electrokinetic properties of gold-coated microspheres.</a> Langmuir. 28 (39), 13861-13870 (2012).</li>
<li>Ren, Y. K., Morganti, D., Jiang, H. Y., Ramos, A., Morgan, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Electrorotation+of+metallic+microspheres%5BTitle%5D)+AND+%22Langmuir%22%5BJournal%5D)">Electrorotation of metallic microspheres.</a> Langmuir. 27 (6), 2128-2131 (2011).</li>
<li>Jones, T. B., Jones, T. B. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aJones%2C+TB+%22Electromechanics+of+particles%22">Electromechanics of particles</a>. , Cambridge University Press. Cambridge, UK. (2005).</li>
<li>Morganti, D. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aMorganti%2C+D+%22AC+electrokinetic+analysis+of+chemically+modified+microparticles%22">AC electrokinetic analysis of chemically modified microparticles</a>. , University of Southampton. (2012).</li>
<li>Morgan, H., Hughes, M. P., Green, N. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Separation+of+submicron+bioparticles+by+dielectrophoresis%5BTitle%5D)+AND+%22Biophys+J%22%5BJournal%5D)">Separation of submicron bioparticles by dielectrophoresis.</a> Biophys J. 77 (1), 516-525 (1999).</li>
<li>Ren, Y., et al. <a target="_blank" href="http://pubs.rsc.org/en/Content/ArticleLanding/2015/LC/c5lc00058k#!divAbstract">Induced-charge electroosmotic trapping of particles.</a> Lab Chip. 15 (10), 2181-2191 (2015).</li>
</ol>

The authors have nothing to disclose.

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...

Janus particles, named after the Roman god with a double face, are asymmetric particles whose two sides have physically or chemically different surface properties1,2. Due to this asymmetric feature, Janus particles exhibit special responses under electric fields, such as DEP3,4,5,6, 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 Au2,13. 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 used9,15,16,17,18,19. For example, EROT is the steady-state rotational response of a particle under an externally imposed rotating electric field2,9,15,16. 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 movement3,4,5,9,15. 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, &#969;c = 2&#963; / aCDL, of DEP and EROT, where &#963; is the liquid conductivity, a is the particle radius, and CDL is the capacitance of the electrical double layer15,16. 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 development15,18,19,20.
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 &#181;m, which can be adjusted to suit the experiments. The four-phase electrode array can be used to characterize and concentrate particles in different microfluidic devices15,16,18. 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&#176; (for EROT) or 180&#176; (for DEP)15. The AC signal is applied at a 0.5 to 4 Vpp 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 &#181;m radially away from the array center, and EROT is recorded at the circular region, 65 &#181;m 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.

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

preparation of janus particles and alternating current electrokinetic measurements with a rapidly fabricated indium tin oxide electrode array

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

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&#176; (for EROT) or 180&#176; (for DEP). Representative results of AC electrokinetic measurements with a four-phase ITO electrode array are presented.

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.
<img alt="Laser measurement setup and ITO electrode diagram, showcasing alignment and substrate details." class="xfigimg" src="/files/ftp_upload/55950/55950fig1.jpg" d...

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.

This article provides a simple method to prepare partially or fully coated metallic particles and to perform the rapid fabrication of electrode arrays, ...

The overall goal of this procedure is to rapidly fabricate indium tin oxide electrodes and prepare metal-coated particles for electrokinetic experiments. This method can facilitate experiment in electrokinetics field, such as electrorotation, dielectrophoresis, and microfluidics. The main advantage of this technique is that it provides a rapid and convenient method to fabricate ITO electrode and prepare metal coat particles.To begin fabricating the electrode, place a clean piece of 25 millimeter by 50 millimeter ITO-coated glass on the stage of a pulsed fiber laser engraving machine. Position the laser 279.5 millimeters above the ITO glass. Import the electrode pattern into the instrument control software.Select Frame, Fill, and Fill First. Set the engraving speed to 800 millimeters per second, the laser power to 60%and the frequency to 40 kilohertz. Preview the engraving pattern, and manually adjust the glass position to center the pattern on the glass.Once the pattern is centered, perform the engraving to divide the conductive surface into quadrants. Use polyimide tape to affix an insulated stranded wire to each conductive quadrant of the engraved ITO glass. Then, split each channel output of a two-function generator with double BNC connectors.Equip one output of each channel with an inverting operational amplifier. To prepare an electrode array for electrorotation measurements, first connect the upper left electrode directly to the double BNC connector of the first channel. Connect the corresponding inverter output to the non-adjacent electrode.Then, connect the lower left electrode to the second channel. Connect the corresponding inverter output to the last wire to produce an electrode array with a 90 degree phase shift between adjacent electrodes. To prepare an electrode array for dielectrophoresis, instead connect the inverted first channel to the lower left electrode.Connect the lower right electrode to the second channel. Connect the last electrode to the corresponding inverter output to produce an array with a 180 degree phase shift between adjacent electrodes. To begin preparing the Janus particles, centrifuge a 10%by weight aqueous suspension of two micrometer silica particles at 2, 200 times g for one minute.Transfer two microliters of the resulting sedimentary silica particles to a 1.5 milliliter microcentrifuge tube. Add 500 microliters of 99.5%ethanol to the silica particles. Sonicate the suspension for one minute, and then centrifuge the suspension at 2, 200 times g for three minutes.Decant the supernatant, and add another 500 microliters of ethanol to the sample. Repeat the sonication and centrifugation three more times, refreshing the ethanol each time. Then, decant the supernatant and add eight microliters of ethanol to the sample.Sonicate the sample for three minutes. Pipette two microliters of the resulting suspension onto a glass slide. Use a cover slide to slowly spread the droplet over the slide to form a monolayer of silica particles.Once the solvent has evaporated, place the slide in a sputter coater with a gold target. Sputter coat the slide at 15 milliamps for 200 seconds to prepare the Janus particles. Apply 20 microliters of deionized water to the slide.Carefully dislodge the Janus particles with a 200 microliter pipette tip so that the particles are suspended in the water droplet. Then, transfer the Janus particle suspension to a 1.5 milliliter microcentrifuge tube. Dilute the suspension with deionized water as needed to achieve a suitable Janus particle concentration for the experiment.To begin preparing fully-coated metallic particles, mix together a polydimethylsiloxane polymer base and its curing agent in a 10 to one weight ratio. Apply tape to the edges of a glass slide to form a container. Pour the PDMS mixture into the tape-walled container to a depth of two to three millimeters.Degas the PDMS under vacuum for 30 minutes, and cure the PDMS at 70 degrees Celsius for two hours. Then, separate the cured PDMS from the tape and the slide to obtain a smooth PDMS stamp. Prepare a monolayer of Janus particles on a glass slide.Then, lay the PDMS stamp smooth side down on the slide and apply even pressure to transfer the Janus particles to the stamp. Carefully separate the slide from the PDMS stamp to expose the silica faces of the Janus particles. Sputter coat the particles on the PDMS stamp with another thin layer of gold to obtain the fully-coated particles.Use a droplet of deionized water and a pipette tip to dislodge the particles from the stamp surface to be suspended in the droplet. Transfer the suspension to a microcentrifuge tube, and dilute the suspension as needed. To begin the experiment, cover the ITO four-phase electrode array with five sheets of paraffin laboratory film.Use a heat gun to fix the paraffin layers to the electrode array, forming a 500 micrometer spacer. Then, place the electrode array on the stage of an inverted microscope with a 40X 0.6 NA objective lens. Place eight microliters of a particle suspension at the center of the array, and cover it with a cover glass.Set the function generator phase shift to 90 degrees for an electrorotation experiment or zero degrees for a dielectrophoresis experiment. Select a sine wave as the waveform, and set the peak-to-peak voltage and frequency. Start the signal generator, and use the inverted microscope camera to capture images of particle motion and rotation.Use image processing software to determine the particle velocities for further analysis. Alternating current electrokinetic property measurements were performed on silica particles with varying degrees of gold coating. Janus particles were found to rotate in the same direction as the electric field at low frequencies and counter-field at higher frequencies.The maximum angular speed was observed at a characteristic frequency. Electrorotation measurements of bare silica particles showed co-field rotation at all measured frequencies. The maximum angular speed was observed at the lowest measured frequency.In contrast, gold-coated silica particles showed counter-field rotation at all measured frequencies. The maximum angular speed for the gold-coated particles was observed at a lower characteristic frequency than in the Janus particles. These results, overall, suggested that the Janus particle behavior could not be attributed to a simple, two-hemisphere superposition model.Dielectrophoresis measurements of the fully gold-coated particles showed an N-dielectrophoresis response at low frequencies and a P-dielectrophoresis response at high frequencies. The crossover frequency closely matched the characteristic frequency at which the maximum angular speed had been observed. After its development, these techniques paved the way for the researchers in the field of soft matter to explore electrode canopy for laminar in microfluidic synthesis.After watching this video, you should have a good understanding of how to fabricate ITO electrode with a laser marking machine and prepare metal coat particle for electrokinetic experiments.

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Preparation of Janus Particles and Alternating Current Electrokinetic Measurements with a Rapidly Fabricated Indium Tin Oxide Electrode Array (Video) | JoVE Journal

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

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