Assessing the Electrochemical Behavior of Microcontact-Printed Silver

Jan 10, 2018 - Students will assist in the development of a process for creating silver nanogrids on glass for use as an ultramicroelectrode. 2. Stude...
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Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Assessing the Electrochemical Behavior of Microcontact-Printed Silver Nanogrids Wesley C. Sanders,*,† Peter Iles,‡ Ron Valcarce,§ Kyle Salisbury,† Glen Johnson,† Aubry Lines,† John Meyers,† Cristofer Page,† Myles Vanweerd,† and Davies Young† †

Engineering Department, Salt Lake Community College, Salt Lake City, Utah 84123, United States Division of Natural Sciences, Salt Lake Community College, Salt Lake City, Utah 84123, United States § Chemistry Department, Salt Lake Community College, Salt Lake City, Utah 84123, United States ‡

S Supporting Information *

ABSTRACT: This paper describes a laboratory exercise used to address the ongoing need for nanotechnology-related, hands-on laboratory experiences for undergraduate students. Determination of the electrochemical behavior of student-fabricated silver nanogrids is reported. Students successfully used cyclic voltammetry to analyze silver nanogrids printed using microcontact printing and subsequent metallization. The silver nanogrids exhibit electrochemical behavior similar to that of electrodes manufactured in industry. Additionally, optical microscopy, atomic force microscopy, and scanning electron microscopy were used to assess nanogrid quality and dimensions.

KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Analytical Chemistry, Materials Science, Laboratory Instruction, Hands-On Learning/Manipulatives, Nanotechnology, Surface Science, Polymer Chemistry

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INTRODUCTION

It is reported that 6 million “nanotech workers” will be needed by the year 2020.1 These workers will create and improve products that will be worth approximately $3 trillion/year by that time.1 For this reason, nanotechnology plays a pivotal role in undergraduate education.1 For graduating students to be successful in this emerging workforce, integration of nanotechnology into science and engineering curricula is a necessity.1 The fields of nanoscience and nanotechnology have been incorporated in the syllabi of many upper-division undergraduate and master level courses all over the world.2 However, these nanotechnology-related courses have a need for practical exercises that include the fabrication, characterization, and assessment of nanomaterials.2 The authors developed a laboratory exercise to meet this need. The laboratory exercise reported in this paper involves the electrochemical assessment of student-fabricated ultramicroelectrodes (UMEs) assembled using a microcontact-printed, polymeric template metallized using redox chemistry. This laboratory investigation can be easily adopted in undergraduate analytical chemistry, materials science, or nanotechnology courses to provide students with an opportunity to witness the intersection of nanomaterials synthesis, nanomaterial characterization, and electrochemical analysis. © XXXX American Chemical Society and Division of Chemical Education, Inc.

LEARNING OBJECTIVES

The laboratory exercise presented in this paper provides students with experiences related to nanofabrication, characterization, and electrochemical measurements. These experiences allow students to witness the intersection of nanotechnology and the natural sciences. There are six associated learning objectives: 1. Students will assist in the development of a process for creating silver nanogrids on glass for use as an ultramicroelectrode. 2. Students will use microcontact printing and redox chemistry to create silver nanogrids. 3. Students will demonstrate the ability to successfully obtain cyclic voltammograms of a silver nanogrid printed on glass. 4. Students will compare the cyclic voltammograms of silver macroelectrodes to the voltammograms of silver nanogrids. 5. Students will identify the type of mass transport occurring at a nanoelectrode surface based on the shape obtained in a cyclic voltammogram. Received: June 10, 2017 Revised: December 6, 2017

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DOI: 10.1021/acs.jchemed.7b00403 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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Laboratory Experiment

6. Students will evaluate the effectiveness of using cyclic voltammetry to assess the electrochemical behavior of silver nanogrids printed on glass.

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PREVIOUS UME FABRICATION UMEs manufactured by electrochemical suppliers are metal electrodes with at least one dimension smaller than 25 μm housed in a glass capillary.3 The small electrode area results in the production of currents on the order of picoamperes to nanoamperes.4 UMEs can be used in a wide variety of applications including high-resolution electrochemical imaging and sensing in biological cells.5 UMEs have also been used for electrochemical micromachining, scanning electrochemical microscopy, and single-molecule detection.6,7 An environmental UME application includes heavy metal detection in natural water systems where very little or no analyte is present.8,9 Fabrication of UMEs in undergraduate laboratories is a difficult and time-consuming process that requires experimental skill and patience.3 These tedious methods can involve the use of dangerous chemicals, multiple fabrication steps, and the creation of waste.10 Researchers have the option of purchasing commercially available UMEs, but costs can reach in excess of 400 USD.3 Such exorbitant costs can stifle introduction of nanoscale, voltammetric experiments into laboratory classes at the undergraduate level.4 To combat these issues, lithography, template methods, and self-assembly have been explored as options for microelectrode fabrication.3−5,11 Disc-shaped electrodes have been produced in undergraduate laboratories by sealing conductive wires in an insulating sheath; this is the most frequently used fabrication technique.5 Reports state that UMEs have been fabricated in this manner using carbon fiber, gold, platinum, and silver.3 Sur et al. describe a laboratory experiment for students involving UME fabrication where students fabricate disc-shaped gold and platinum microelectrodes by sealing 10−50 μm diameter platinum and gold wires in soda lime glass.4 UMEs have also been fabricated using self-assembly.11 This involves modifying gold electrodes with alkanethiol monolayers and selectively removing areas to provide bare gold areas, which served as microelectrodes.11 Conversely, microcontact printing has been used to selectively form alkanethiol self-assembled monolayers on designated areas on gold surfaces.11 This paper describes UME fabrication involving metallization of a molecular template printed on a glass substrate. This experiment utilizes two processes reported previously in the literature, microcontact printing of polymer grids (1) and metallization of the polymer grid (2).12 The previous paper described the creation of a conductive micro/nanoscale grids using a molecular template, with no subsequent electroanalytical or chemical analysis.12 Although these processes have been reported previously, this experiment is novel because, to date, there have been no reports in this Journal or elsewhere describing assessment of the electrochemical behavior of a metallized, molecular template printed using soft lithography. The following list highlights the significant differences between the experiment described in this paper and previously reported UME fabrication:4 • UME fabrication does not involve an existing platinum, gold, silver, or carbon nanowire. • Metallic structures are grown on a microcontact-printed template. No other reports describe the use of a template for UME fabrication.

• Metallic micro/nanostructures are not housed in a glass capillary. • No postprocessing steps (i.e., heat treatment) are required prior to UME use.

EXECUTION

UME Fabrication with Microcontact-Printed PVP Template

The UME fabrication reported in this paper starts with microcontact printing. Microcontact printing involves “inking” the protrusions on the surface of a patterned polydimethylsiloxane (PDMS) stamp for the direct transfer of patterns.13 Polyvinylpyrrolidone (PVP) is the ink used in this experiment; it serves as a molecular template that directs the formation of the silver nanogrid. PVP facilitates the metallization of the grid because silver ions are able to coordinate along the polymer chain via the carbonyl group.14 Metallization occurs when the silver ions are reduced to form nanoscale silver particles in the presence of a reducing agent.14 Cyclic Voltammetry Analysis of UMEs

Cyclic voltammetry was used to assess the electrochemical behavior of silver nanogrids printed on glass. Cyclic voltammetry involves the application of electrical potential to a working electrode and measurement of the corresponding current.15 The applied potential is measured against a reference electrode which is usually a silver/silver chloride electrode. Negative potentials are initially applied to the working electrode; this is referred to as the forward scan. Once the negative potentials are sufficiently strong, a reduction reaction occurs, and a cathodic current is observed in a cyclic voltammogram indicated by IC in Figure 1. A reverse scan

Figure 1. Cyclic voltammogram of a silver wire (178 μm diameter) in 5 mM Ru(NH3)6Cl3/0.05 M KNO3. Scan rate 50 mV/s.

involves application of potentials to the working electrode in the positive direction, encouraging oxidation. This process produces an anodic current that is observed in the cyclic voltammogram indicated by IA in Figure 1.15 Voltammograms are shaped differently for ultramicroelectrodes.16 Not only is the magnitude of the current much smaller due to the decreased electrode area, but the current goes to a steady-state value resulting in the formation of voltammograms that are Sshaped,16 as shown in Figure 2.17 A steady-state current is produced because the UME behaves as a “dot” with the surrounding diffusion layer being hemispherical in shape (Figure 2 inset) and extending out into the solution.16 The amount of electroactive species diffusing to the electrode surface is defined by the volume enclosed by this expanding B

DOI: 10.1021/acs.jchemed.7b00403 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Figure 4. Microcontact printing of the PVP grid over the transparent tape mask.

aqueous solutions used for metallization, a Denton Desktop V sputter coater operating at 40 mA for 30 s was used to deposit a thin copper layer to serve as a barrier as reported previously.12 After sputtering, the transparent tape mask was removed, and the PVP grids were immersed in a 100 °C solution (heated using a water bath) containing 1 mL of 5 mM silver nitrate and 1 mL of 0.6 M sodium citrate for 5 min (Figure 5a), resulting in

Figure 2. Cyclic voltammogram of 5 mM rubrene in toluene collected using a 10 μm Pt-UME. Scan rate 10 mV/s. Reprinted with permission from ref 17. Copyright 2016 American Chemical Society.

hemisphere, rather than a plane projecting into the solution as for a planar electrode (Figure 1 inset).16

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EXPERIMENT This faculty-led research project was conducted by seven engineering and materials science students during the Fall 2016 and Spring 2017 semesters. Initially, faculty prepared PDMS stamps using blank, unused, commercially available CDs and DVDs. Specific procedures for PDMS stamp preparation can be found in the Supporting Information. The “ink” used for microcontact printing was prepared by dissolving 0.15 g of 55 000 molecular weight PVP (Sigma-Aldrich) in 30 mL of 100% isopropyl alcohol. Next, 15 mL of the original PVP solution was diluted with 15 mL of 100% IPA. Faculty also prepared substrates by cutting glass microscope slides with a diamond scribe into several square pieces approximately 3 cm × 3 cm in size. Cleaning procedures for the glass substrates and PDMS stamps included sonicating in 100% isopropyl alcohol for 10 min and drying with a stream of nitrogen. After the cleaning procedure, residual moisture was driven from the glass substrates and PDMS stamps by placing them on a hot plate and moderately heating for 10 min prior to the experiment as reported previously.12 Before printing the PVP grid, glass substrates were covered with two pieces of smooth, transparent tape, leaving a 2 mm gap for printing the grid on bare glass (Figure 3). This allowed the creation of a narrow grid (Figure

Figure 5. Heating the polymer nanogrid in silver nitrate/sodium citrate solution (a). Silver grid strip and corresponding optical microscope image collected using a 40× objective lens (b).

the formation of a narrow strip on the glass substrate containing an array of silver structures arranged in a “gridlike” pattern (Figure 5b). Afterward, the glass plates were immersed in a beaker of deionized water several times to rinse and were dried with a stream of nitrogen. It was necessary to coat the silver grid with a passivating layer to prevent stripping of silver from the nanogrid array during voltammetric scans. The protecting layer, however, needed to allow the redox probe, hexamine ruthenium chloride (Ru(NH3)6Cl3), to diffuse to the surface of the silver grid. Previous reports have described the use of commercially available nail polish to insulate UMEs.19−21 A similar technique was used in this experiment. Breathable nail polish (Orly) was used as the protecting layer. Breathable nail polish was used because traditional nail polish consists of tight molecular bonds.22 However, breathable nail polish contains molecules with a staggered structure, like a bricklaying pattern, creating a film that is permeable to water and gaseous molecules.22 One drop of breathable nail polish was spin coated on the silver nanorid. Details regarding the application of nail polish can be found in the Supporting Information. Silver nanogrids created using CDand DVD-molded PDMS stamps were characterized using atomic force microscopy and scanning electron microscopy prior to the addition of the breathable nail polish. After nail polish was added, cyclic voltammetry analyses were conducted.

Figure 3. Addition of the transparent tape mask to the glass substrate.

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4) rather that a grid covering the entire glass substrate. The authors believe that a smaller nanogrid array decreased the likelihood of planar mass transport, which is believed to occur if the grid covers the entire surface of the glass. Procedures associated with microcontact printing of the PVP grid are available in the Supporting Information. The PVP polymer used to create the molecular template is watersoluble.18 To prevent removal of the molecular template in the

HAZARDS Isopropyl alcohol is flammable; do not use alcohol near an open flame or heat source. Silver nitrate is very hazardous to skin, eyes, and throat. Sodium citrate and potassium chloride are moderately hazardous to skin, eyes, and throat. Hexammine ruthenium(III) chloride is a skin, eye, and throat irritant. Nail polish contains volatile organic compounds (VOCs); use in a C

DOI: 10.1021/acs.jchemed.7b00403 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Cyclic Voltammetry Data

well-ventilated area. Avoid inhalation and skin contact with all chemicals used in the laboratory experiment. Wear gloves and protective eyewear when handling these chemicals.

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An Epsilon EC potentiostat (BASI) with a Ag/AgCl reference electrode was used to perform cyclic voltammetry. Electrochemical measurements were conducted in an aqueous solution of 1 mM Ru(NH3)6Cl3, containing 0.05 M KNO3. The silver wire used as a control in this experiment exhibited voltammetric behavior characteristic of planar mass transport (Figure 8a). A cathodic peak at −0.05 V and an anodic peak at

RESULTS

Microscopy Data

Under faculty guidance, students collected SEM images of a silver wire and nanogrid electrodes. SEM imaging was performed using a Hitachi TM 3000 Benchtop SEM. A silver wire was used as a control electrode, and SEM data indicates a diameter of 178 μm (Figure 6a).

Figure 6. SEM image (60×) of silver wire (178 μm diameter) used as a control (a) and 60× image of silver nanogrid and corresponding 10,000× image (b).

SEM data suggests the silver nanogrid is approximately 2 mm wide and consists of an array of silver structures arranged in a grid-like pattern (Figure 6b). Under faculty guidance, students utilized AFM to obtain images of the silver nanogrids (Figure 7a,c). Images were

Figure 8. Cyclic voltammograms for silver microwire (178 μm diameter) (a), CD nanogrid (b), and DVD nanogrid (c). Scan rate: 50 mV/s.

0.10 V are observed in the voltammogram for the silver wire. The characteristic sigmoidal shape specific for commercially available UMEs is observed in the voltammograms for the CD nanogrid (Figure 8b) and for the DVD nanogrid (Figure 8c). CV data for the CD and DVD nanogrids suggests hemispherical diffusion as the primary means of mass transport to the microcontact-printed electrodes. However, the steady-state current obtained from the DVD nanogrid is higher than the steady-state current obtained from the CD nanogrid. When identical cross-sectional 4 μm distances in the AFM data of the CD nanogrid (Figure 7b) and the DVD nanogrid (Figure 7d) are compared, more silver structures are present in the DVD nanogrids. It is believed that the increased number of silver structures in the DVD nanogrid array results in a higher steadystate current. Voltammograms for additional CD and DVD nanogrid samples exhibiting this behavior are available in the Supporting Information. Silver nanogrid UMEs were coated with permeable nail polish to prevent anodic stripping of the silver from the PVP grid during voltammetric scans. Addition of breatheable nail polish to the silver nanogrids allowed multiples scans on individual electrodes without the removal of silver metal. Although the reported voltammograms are S-shaped, they are not as defined as UME voltammograms presented in other reports. This is likely due to slightly inhibited diffusion of the redox mediator through the breatheable nail polish. Despite this fact, reproducible, steady-state voltammograms were obtained.

Figure 7. AFM image of CD nanogrid (a) and corresponding crosssectional profile (b). AFM image of DVD nanogrid (c) and corresponding cross-sectional profile (d).

obtained using a Nanosurf Easyscan 2 AFM (Nanoscience, Inc.) operating in contact mode. Figure 7a shows an AFM image of a CD silver nanogrid. The corresponding crosssectional analysis (Figure 7b), obtained using WSXM image processing software,23 suggests that the heights of the silver structures in the grid are slightly taller than 200 nm, and approximately 1 μm wide. Figure 7c shows an AFM image of DVD silver nanogrids. The corresponding cross-sectional analysis (Figure 7d), also obtained using WSXM image processing software,23 suggests that the heights of the silver structures in the grids are between 60 and 100 nm in height, and approximately 500 nm wide.

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ASSESSMENT A postlab handout is included in the Supporting Information section. This handout prompts students to review pertinent literature, tests student knowledge on associated electroD

DOI: 10.1021/acs.jchemed.7b00403 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Fabricated Iridium-Based Ultramicroelectrode Arrays. Electroanalysis 1998, 10, 89−93. (10) Feeney, R.; Kounaves, S. P. Microfabricated Ultramicroelectrode Arrays: Developments, Advancements, and Applications in Environmental Analysis. Electroanalysis 2000, 12, 677−684. (11) He, H. X.; Li, Q. G.; Zhou, Z. Y.; Zhang, H.; Li, S. F. Y.; Liu, Z. F. Fabrication of Microelectrode Arrays Using Microcontact Printing. Langmuir 2000, 16, 9683−9686. (12) Sanders, W. C.; Valcarce, R.; Iles, P.; Smith, J. S.; Glass, G.; Gomez, J.; Johnson, G.; Johnston, D.; Morham, M.; Befus, E.; Oz, A.; Tomaraei, M. Printing Silver Nanogrids on Glass. J. Chem. Educ. 2017, 94, 758−763. (13) Johannes, M. S.; Cole, D. G.; Clark, R. L. Atomic Force Microscope Based Nanofabrication of Master Pattern Molds for Use in Soft Lithography. Appl. Phys. Lett. 2007, 91, 123111-1−123111-3. (14) Zhu, J. J.; Kan, C. X.; Wan, J. G.; Han, M.; Wang, G. H. HighYield Synthesis of Uniform Ag Nanowires with High Aspect Ratios by Introducing the Long-Chain PVP in an Improved Polyol Process. J. Nanomater. 2011, 2011, 1−7. (15) Kissinger, P. T.; Heineman, W. R. J. Chem. Educ. 1983, 60, 702− 706. (16) Cyclic Voltammetry with a Microelectrode. http://www.asdlib. org/onlineArticles/elabware/kuwanaEC_lab/PDF-20-Experiment2. pdf (accessed Oct 2017). (17) Deng, H.; Dick, J. E.; Kummer, S.; Kragl, U.; Strauss, S. H.; Bard, A. J. Probing Ion Transfer across Liquid-Liquid Interfaces by Monitoring Collisions of Single Femtoliter Oil Droplets on Ultramicroelectrodes. Anal. Chem. 2016, 88, 7754−7761. (18) Khan, M. S.; Gul, K.; Rehman, N. U. Interaction of Polyvinylpyrrolidone with Metal Chloride Aqueous Solutions. Chin. J. Polym. Sci. 2004, 22, 581−584. (19) Foord, J. S.; Hu, J.; Holt, K. B. Diamond Ultramicroelectrodes. Phys. Status Solidi A 2007, 204, 2940−2944. (20) Liljeroth, P.; Johans, C.; Slevin, C. J.; Quinn, B. M.; Kontturi, K. Disk-Generation/Ring Collection Scanning Electrochemical Microscopy: Theory and Application. Anal. Chem. 2002, 74, 1972−1978. (21) Duay, J.; Goran, J. M.; Stevenson, K. J. Facile Fabrication of Carbon Ultramicro- to Nanoelectrode Arrays with Tunable Voltammetric Response. Anal. Chem. 2014, 86, 11528−11532. (22) Hubbard, L. What’s the Deal with Breathable Nail Polish?. https://www.allure.com/story/do-you-need-breathable-nail-polish (accessed Oct 2017). (23) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705-1−013705-8.

chemical phenomena, and includes a data page for microscopy and cyclic voltammetry uploads.

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SUMMARY This experiment allows students to witness the intersection of pure and applied science. Students participating in this experiment acquire experience with nanofabrication and characterization. Additionally, students are provided with an opportunity to use an electroanalytical technique to characterize the behavior of student-fabricated nanostructures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00403. Instructor notes (PDF, DOCX) Student handout (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wesley C. Sanders: 0000-0003-4472-3086 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Utah Engineering Initiative for generous support. REFERENCES

(1) Morris, J.; Moeck, P.; Weasel, L.; Straton, J. Nanotechnology Courses for General Education. In Proceedings of the 122nd ASEE Annual Conference and Exposition, Seattle, WA, June 14−17, 2015. https://peer.asee.org/nanotechnology-courses-for-general-education (accessed Oct. 2017). (2) Cea, P.; Martín, S.; Gonzalez-Orive, A.; Osorio, H. M.; Quintín, P.; Herrer, L. Nanofabrication and Electrochemical Characterization of Self-Assembled Monolayers Sandwiched between Metal Nanoparticles and Electrode Surfaces. J. Chem. Educ. 2016, 93, 1441−1445. (3) Danis, L.; Polcari, D.; Kwan, A.; Gateman, S. M.; Mauzeroll, J. Fabrication of Carbon, Gold, Platinum, Silver, and Mercury Ultramicroelectrodes with Controlled Geometry. Anal. Chem. 2015, 87, 2565−2569. (4) Sur, U. K.; Dhason, A.; Lakshminarayanan, V. A Simple and LowCost Ultramicroelectrodes Fabrication and Characterization Method for Undergraduate Students. J. Chem. Educ. 2012, 89, 168−172. (5) Demaille, C.; Brust, M.; Tsionsky, M.; Bard, A. J. Fabrication and Characterization of Self-Assembled Spherical Gold Ultramicroelectrodes. Anal. Chem. 1997, 69, 2323−2328. (6) Yoon, Y. H.; Shin, T.; Shin, E. Y.; Kang, H.; Yoo, J. S.; Park, S. M. A Nanometer Potential Probe for the Measurement of Electrochemical Potential of Solution. Electrochim. Electrochim. Acta 2007, 52, 4614− 4621. (7) Shao, Y.; Mirkin, M. V.; et al. Nanometer-Sized Electrochemical Sensors. Anal. Chem. 1997, 69, 1627−1634. (8) Orozco, J.; Suarez, G.; Fernandez-Sanchez, C.; McNeil, C.; Jimenez-Jorquera, C. Characterization of Ultramicroelectrode Arrays Combining Electrochemical Techniques and Optical Microscopy Imaging. Electrochim. Electrochim. Acta 2007, 53, 729−736. (9) Feeney, R.; Herdan, J.; Nolan, M. A.; Tan, S. H.; Tarasov, V. V.; Kounaves, S. P. Analytical Characterization of Microlithographically E

DOI: 10.1021/acs.jchemed.7b00403 J. Chem. Educ. XXXX, XXX, XXX−XXX