This work was supported by a grant from the RGC-NSFC Joint Fund sponsored by the Research Grants Council of Hong Kong (Project No. N_HKUST615/14). We would like to acknowledge Nanosystem Fabrication Facility (NFF) of HKUST for the device / system fabrication.

1. Fabrication of a Cr/Au adhesion layer using liftoff
<ol>
	<li>Spincoat HPR504 positive photoresist of 1.2 &#181;m thickness onto a quartz wafer using a spread speed of 1,000 rpm for 5 s and a spin speed of 4,000 rpm for 30 s.</li>
	<li>Softbake the photoresist on the quartz wafer at 110 &#176;C for 5 min on a hot plate.</li>
	<li>Using a mask aligner, expose the wafer such that locations for Cr/Au deposition are exposed with ultraviolet (UV) light. The exposure power density and time is 16 mW/cm2 and 7.5 ...

<ol>
	<li>Bakker, E., Telting-Diaz, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+sensors.">Electrochemical sensors.</a> Analytical Chemistry. 74 (12), 2781-2800 (2002).</li><li>Jobst, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thin-Film+Microbiosensors+for+Glucose-Lactate+Monitoring.">Thin-Film Microbiosensors for Glucose-Lactate Monitoring.</a> Analytical Chemistry. 68 (18), 3173-3179 (1996).</li><li>Matsumoto, T., Ohashi, A., Ito, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Development+of+a+micro-planar+Ag/AgCl+quasi-reference+electrode+with+long-term+stability+for+an+amperometric+glucose+sensor.">Development of a micro-planar Ag/AgCl quasi-reference electrode with long-term stability for an amperometric glucose sensor.</a> Analytica Chimica Acta. 462 (2), 253-259 (2002).</li><li>Suzuki, H., Hirakawa, T., Sasaki, S., Karube, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+integrated+three-electrode+system+with+a+micromachined+liquid-junction+Ag/AgCl+liquid-junction+Ag/AgCl+reference+electrode.">An integrated three-electrode system with a micromachined liquid-junction Ag/AgCl liquid-junction Ag/AgCl reference electrode.</a> Analytica Chimica Acta. 387 (1), 103-112 (1999).</li><li>Ives, D. J. G., Janz, G. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Reference Electrodes - theory and practice. , (1961).</li><li>Huynh, T. M., Nguyen, T. S., Doan, T. C., Dang, C. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+of+thin+film+Ag/AgCl+reference+electrode+by+electron+beam+evaporation+method+for+potential+measurements.">Fabrication of thin film Ag/AgCl reference electrode by electron beam evaporation method for potential measurements.</a> Advances in Natural Sciences: Nanoscience and Nanotechnology. 10 (1), 015006 (2019).</li><li>Katan, T., Szpak, S., Bennion, D. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Silver/silver+chloride+electrode:+Reaction+paths+on+discharge.">Silver/silver chloride electrode: Reaction paths on discharge.</a> Journal of The Electrochemical Society. 120 (7), 883-888 (1973).</li><li>Katan, T., Szpak, S., Bennion, D. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Silver/silver+chloride+electrodes:+Surface+morphology+on+charging+and+discharging.">Silver/silver chloride electrodes: Surface morphology on charging and discharging.</a> Journal of The Electrochemical Society. 121 (6), 757-764 (1974).</li><li>Polk, B. J., Stelzenmuller, A., Mijares, G., MacCrehan, W., Gaitan, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ag/AgCl+microelectrodes+with+improved+stability+for+microfluidics.">Ag/AgCl microelectrodes with improved stability for microfluidics.</a> Sensors and Actuators B: Chemical. 114 (1), 239-247 (2006).</li><li>Mechaour, S. S., Derardja, A., Oulmi, K., Deen, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+the+wire+diameter+on+the+stability+of+micro-scale+Ag/AgCl+reference+electrode.">Effect of the wire diameter on the stability of micro-scale Ag/AgCl reference electrode.</a> Journal of The Electrochemical Society. 164 (14), E560-E564 (2017).</li><li>Brewer, P. J., Leese, R. J., Brown, R. J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+improved+approach+for+fabricating+Ag/AgCl+reference+electrodes.">An improved approach for fabricating Ag/AgCl reference electrodes.</a> Electrochimica Acta. 71, 252-257 (2012).</li><li>Safari, S., Selvaganapathy, P. R., Derardja, A., Deen, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+growth+of+high-aspect+ratio+nanostructured+silver+chloride+on+silver+and+its+application+to+miniaturized+reference+electrodes.">Electrochemical growth of high-aspect ratio nanostructured silver chloride on silver and its application to miniaturized reference electrodes.</a> Nanotechnology. 22 (31), 315601 (2001).</li><li>Tjon, K. C. E., Yuan, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impedance+characterization+of+silver/silver+chloride+micro-electrodes+for+bio-sensing+applications.">Impedance characterization of silver/silver chloride micro-electrodes for bio-sensing applications.</a> Electrochimica Acta. 320, 134638 (2019).</li><li>Pargar, F., Kolev, H., Koleva, D. A., van Breugel, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microstructure,+surface+chemistry+and+electrochemical+response+of+Ag+|+AgCl+sensors+in+alkaline+media.">Microstructure, surface chemistry and electrochemical response of Ag | AgCl sensors in alkaline media.</a> Journal of Materials Science. 53 (10), 7527-7550 (2018).</li><li>Hassel, A. W., Fushimi, K., Seo, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+agar-based+silver+|+silver+chloride+reference+electrode+for+use+in+micro-electrochemistry.">An agar-based silver | silver chloride reference electrode for use in micro-electrochemistry.</a> Electrochemistry communications. 1 (5), 180-183 (1999).</li></ol>

The authors have nothing to disclose.

The physical properties of an Ag/AgCl electrode is controlled by the morphology and the structure of the AgCl deposited on the electrode. In this paper, we presented a protocol to precisely control the coverage of a single layer of AgCl on the surface of the silver electrode. An integral part of the protocol is a modified form of the Faraday&#39;s Law of Electrolysis, which is used to control the degree of AgCl on the thin film silver electrodes. It can be written as:
<img alt="Equation" src="...

The Ag/AgCl electrode is one of the most used electrodes in the field of electrochemistry. It is most commonly used as the reference electrode in electrochemical systems due to its ease of fabrication, non-toxic property and stable electrode potential1,2,3,4,5,6.
Researchers have attempted to understand the mechanism of Ag/AgCl electrodes. The layer of chloride salt on the electrode has been found to be a fundamental material in the characteristic redox reaction of the Ag/AgCl electrode in a chloride containing electrolyte. For the oxidation path, the silver at the imperfection sites on the surface of the electrode combines with the chloride ions in the solution to form soluble AgCl complexes, in which they diffuse to the edges of the AgCl deposited on the surface of the electrode for precipitation in the form of AgCl. The reduction path involves the formation of soluble AgCl complexes using the AgCl on the electrode. The complexes diffuse to the silver surface and reduces back to elemental silver7,8.
The morphology of the AgCl layer is a pivotal influence in the physical property of Ag/AgCl electrodes. Various works showed that the large surface area is key to form reference Ag/AgCl electrodes with highly reproducible and stable electrode potentials9,10,11,12. Therefore, researchers have investigated methods to create Ag/AgCl electrodes with a large surface area. Brewer et al. discovered that using constant voltage instead of constant current to fabricate Ag/AgCl electrodes would result in a highly porous AgCl structure, increasing the surface area of the AgCl layer11. Safari et al. took advantage of the mass transport limitation effect during AgCl formation on the surface of silver electrodes to form AgCl nanosheets on top of them, increasing the surface area of the AgCl layer significantly12.
There is a rising trend to design AgCl electrode for sensing applications. A low contact impedance is crucial for sensing electrodes. Thus, it is important to understand how the surface coating of AgCl would affect its impedance property. Our previous research showed that the degree of AgCl coverage on the silver electrode has a pivotal influence on the impedance characteristic of the electrode/electrolyte interface13. However, to correctly estimate the contact impedance of thin film Ag/AgCl electrodes, the AgCl layer formed must be smooth and have well-controlled coverage. Therefore, a method to form smooth AgCl layers with designated degrees of AgCl coverage is needed. Works have been done to address this need partially. Brewer et al. and Pargar et al. discussed that a smooth AgCl can be achieved using a gentle constant current, fabricating the AgCl layer on top of the silver electrode11,14. Katan et al. formed a single layer of AgCl on their silver samples and observed the size of individual AgCl particles8. Their research found that the thickness of a single layer of AgCl is around 350 nm. The aim of this work is to develop a protocol to form fine and well-controlled films of AgCl with predicted impedance properties on top of silver electrodes.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>AST Peva-600EI E-Beam Evaporation System</td>
			<td>Advanced System Technology</td>
			<td></td>
			<td>For Cr/Au Deposition</td>
		</tr>
		<tr>
			<td>AZ 5214 E Photoresist</td>
			<td>MicroChemicals</td>
			<td></td>
			<td>Photoresist for pad opening</td>
		</tr>
		<tr>
			<td>AZ P4620 Photoresist</td>
			<td>AZ Electronic Materials</td>
			<td></td>
			<td>Photoresist for Ag liftoff</td>
		</tr>
		<tr>
			<td>Branson/IPC 3000 Plasma Asher</td>
			<td>Branson/IPC</td>
			<td></td>
			<td>Ashing</td>
		</tr>
		<tr>
			<td>Branson 5510R-MT Ultrasonic Cleaner</td>
			<td>Branson Ultrasonics</td>
			<td></td>
			<td>Liftoff</td>
		</tr>
		<tr>
			<td>CHI660D</td>
			<td>CH Instruments, Inc</td>
			<td></td>
			<td>Electrochemical Analyser</td>
		</tr>
		<tr>
			<td>Denton Explorer 14 RF/DC Sputter</td>
			<td>Denton Vacuum</td>
			<td></td>
			<td>For Ag Sputtering</td>
		</tr>
		<tr>
			<td>FHD-5</td>
			<td>Fujifilm</td>
			<td>800768</td>
			<td>Photoresist Development</td>
		</tr>
		<tr>
			<td>HPR 504 Photoresist</td>
			<td>OCG Microelectronic Materials NV</td>
			<td></td>
			<td>Photoresist for Cr/Au liftoff</td>
		</tr>
		<tr>
			<td>Hydrochloric acid fuming 37%</td>
			<td>VMR</td>
			<td>20252.420</td>
			<td>Making diluted HCl for cathodic cleaning</td>
		</tr>
		<tr>
			<td>J.A. Woollam M-2000VI Spectroscopic Elipsometer</td>
			<td>J.A. Woollam</td>
			<td></td>
			<td>Measurement of silicon dioxide passivation layer thickness on dummy</td>
		</tr>
		<tr>
			<td>Multiplex CVD</td>
			<td>Surface Technology Systems</td>
			<td></td>
			<td>Silicon dioxide passivation</td>
		</tr>
		<tr>
			<td>Oxford RIE Etcher</td>
			<td>Oxford Instruments</td>
			<td></td>
			<td>For Pad opening</td>
		</tr>
		<tr>
			<td>Potassium Chloride</td>
			<td>Sigma-Aldrich</td>
			<td>7447-40-7</td>
			<td>Making KCl solutions</td>
		</tr>
		<tr>
			<td>SOLITEC 5110-C/PD Manual Single-Head Coater</td>
			<td>Solitec Wafer Processing, Inc.</td>
			<td></td>
			<td>For spincoating of photoresist</td>
		</tr>
		<tr>
			<td>SUSS MA6</td>
			<td>SUSS MicroTec</td>
			<td></td>
			<td>Mask Aligner</td>
		</tr>
		<tr>
			<td>Sylgard 184 Silicone Elastomer Kit</td>
			<td>Dow Corning</td>
			<td></td>
			<td>Adhesive for container on chip</td>
		</tr>
	</tbody>
</table>

fabrication of thin film silver/silver chloride electrodes with finely controlled single layer silver chloride

This paper aims to present a protocol to form smooth and well-controlled films of silver/silver chloride (Ag/AgCl) with designated coverage on top of thin film silver electrodes. Thin film silver electrodes sized 80 &#181;m x 80 &#181;m and 160 &#181;m x 160 &#181;m were sputtered on quartz wafers with a chromium/gold (Cr/Au) layer for adhesion. After passivation, polishing and cathodic cleaning processes, the electrodes underwent galvanostatic oxidation with consideration of Faraday&#39;s Law of Electrolysis to form smooth layers of AgCl with a designated degree of coverage on top of the silver electrode. This protocol is validated by inspection of scanning electron microscope (SEM) images of the surface of the fabricated Ag/AgCl thin film electrodes, which highlights the functionality and performance of the protocol. Sub-optimally fabricated electrodes are fabricated as well for comparison. This protocol can be widely used to fabricate Ag/AgCl electrodes with specific impedance requirements (e.g., probing electrodes for impedance sensing applications like impedance flow cytometry and interdigitated electrode arrays).

Figure 1 shows an 80 &#181;m x 80 &#181;m Ag/AgCl electrode with a designed AgCl coverage of 50% fabricated following this protocol. By observation, the area of the AgCl patch is around 68 &#181;m x 52 &#181;m, which corresponds to around 55% of AgCl coverage. This shows that the protocol can finely control the amount of AgCl coverage on the thin film Ag electrodes. The AgCl layer fabricated is also very smooth, as evident by the clumping of adjacent AgCl par...

Watch this Scientific Journal Video about Fabrication of Thin Film Silver/Silver Chloride Electrodes with Finely Controlled Single Layer Silver Chloride at JoVE.com

Fabrication of Thin Film Silver/Silver Chloride Electrodes with Finely Controlled Single Layer Silver Chloride

This paper aims to present a method to form smooth and well-controlled films of silver chloride (AgCl) with designated coverage on top of thin film silver electrodes.

This paper aims to present a protocol to form smooth and well-controlled films of silver/silver chloride (Ag/AgCl) with designated coverage on top of thin ...

This protocol enables the deposition of a smooth single layer of silver chloride with designated coverage on thin film silver electrodes. This is the first time a technique that can precisely control the coverage of single layer silver chloride on thin films of electrode is introduced. To begin, flush the chip using isopropanol followed by DI water.Pour 0.01 molar hydrochloric acid solution into the acrylic container. Using laboratory clean wipes, wipe dry the macro silver/silver chloride reference electrodes pipette exterior and electrode. Connect the chip and the macro electrodes to the analyzer such that a thin film silver electrode on the chip is defined as the working electrode, the macro silver/silver chloride reference electrode is defined as the reference electrode, and the bare macro silver/silver chloride electrode is defined as the counter-electrode.Place the macro electrodes into the container. Use Blu-Tack as the lid of the container to anchor the macro electrodes. Place the setup into a Faraday cage.In the CHI660D software, click on the setup tab in the top left corner of the window. Then click technique, amperometric IT curve and OK to perform cathodic cleaning of the electrodes. In the popup menu, modify the parameters for cathodic cleaning.Set the initial voltage as minus 1.5, the sample interval as 0.1 seconds, the run time as 900 seconds, the quiet time as zero second, and the scales during run as one. For an 80 micrometers by 80 micrometers electrode, set the sensitivity as one e minus 006. Press OK.Start the process by pressing the start icon under the menu bar.Let the experiment run and finish. Open the Faraday cage, remove the macro reference and counter-electrodes and wipe them dry. Pour the used electrolyte in a waste container and flush the acrylic container using DI water.Pour 0.1 molar potassium chloride solution into the acrylic container. Connect the chip and the macro electrodes to the analyzer such that the cleaned thin film silver electrode on the chip is defined as the working electrode, the macro silver/silver chloride reference electrode is defined as the reference electrode, and the bare macro silver/silver chloride electrode is defined as the counter-electrode. Place the macro electrodes into the container.Use Blu-Tack as the lid of the container to anchor the macro electrodes. Place the setup into a Faraday cage. In the CHI660D software, click on the setup tab at the top left corner of the window, click technique, chronopotentiometry, and OK to perform galvanostatic fabrication of single layer silver chloride on the silver electrodes.In the popup menu, modify the parameters. Set the cathodic current as zero amps. Set the anodic current such that the current density applied to the thin film electrode is 0.5 milliamps per square centimeter.Keep the high end low voltage limit and hold time as default. Set the cathodic time as 10 seconds. Set the anodic time correspondingly to achieve the degree of silver chloride coverage needed.Set the initial polarity as anodic, the data storage interval as 0.1 seconds, the number of segments as one, and the current switching priority as time. Uncheck the auxiliary signal recording when sample interval is greater than or equal to 0.0005 seconds. Press OK.Start the process by pressing the start icon under the menu bar.Let the experiment run and finish. Open the Faraday cage and remove the macro reference and counter-electrodes and wipe dry their surfaces. Submerge the macro electrodes in 3.5 molar potassium chloride solution for storage.Then discard the used electrolyte in a waste container and flush the acrylic container using DI water. Cover the opening of the acrylic container using paraffin film until ready to use. This image shows an 80 micrometer by 80 micrometer silver/silver chloride electrode with a designed silver chloride coverage of 50%There was a clumping of adjacent silver chloride particles, but stacked silver chloride particles were not observed.A distinctive silver/silver chloride intersection could be seen. More successful examples of thin film silver/silver chloride electrodes fabricated are shown here. 80 micrometer by 80 micrometer electrodes with a designated silver chloride coverage of 70%and 30%together with 160 micrometer by 160 micrometer electrodes with a designated silver chloride coverage of 75%and 90%Shown here is a polished electrode surface and an unpolished electrode surface.For the unpolished electrode, finger-like structures were observed on the surface whereas the polished electrode surface is smooth with minor scratch marks caused by the polishing process. Here is an unpolished 80 micrometer by 80 micrometer silver/silver chloride electrode with a designed silver chloride coverage of 50%The silver chloride formed appeared to be recessed inwards instead of protruding outwards. When forming the silver chloride layer on electrodes of different surface areas, it's important to remember to retune the applied current to maintain the same current density.This protocol enables researchers to the assigned thin film silver/silver chloride electrodes for impedance sensing as a previous research demonstrated that the silver/silver chloride electrodes impedance depend on silver chloride coverage.

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13.8K Views.  The Hong Kong University of Science and Technology. This protocol enables the deposition of a smooth single layer of silver chloride with designated coverage on thin film silver electrodes. This is the first time a technique that can precisely control the coverage of single layer silver chloride on thin films of electrode is introduced. To begin, flush the chip using isopropanol followed by DI water.Pour 0.01 molar hydrochloric acid solution into the acrylic container. Using laboratory clean wipes, wipe dry the macro silver/silver chlor...