Three-Dimensional Electrochemical Functionality of an Interdigitated

Article pubs.acs.org/JPCC

Three-Dimensional Electrochemical Functionality of an Interdigitated Array Electrode by Scanning Electrochemical Microscopy Fraser P. Filice, Michelle S. M. Li, Jeffrey D. Henderson, and Zhifeng Ding* Department of Chemistry, The University of Western Ontario, 1151 Richmond Street, London, ON N6A 5B7, Canada

Downloaded by UNIV OF SUSSEX on September 8, 2015 | http://pubs.acs.org Publication Date (Web): September 3, 2015 | doi: 10.1021/acs.jpcc.5b05568

S Supporting Information *

ABSTRACT: As interdigitated array electrodes (IDAEs) become increasingly common in analytical systems for rapid characterization of samples, physical insight into locationspecific electrochemical performance and functionality of these IDAEs is vital. Such characterization can be performed through the use of scanning electrochemical microscopy (SECM), which is a powerful noninvasive physical methodology for determining electrochemical and topographic characteristics of complex samples. Depth scan SECM imaging was performed for the generation of 2D current maps of an IDAE relative to an ultramicroelectrode (UME) position in the x−z plane. Hundreds of probe approach curves (PACs) and horizontal sweeps were obtained from one depth scan image by simply extracting vertical and horizontal cross-sectional lines. These experimental PACs and sweeps were further characterized through comparison with simulation generated curves through modeling of the experimental system. An UME approach to and horizontal sweep across asymmetric systems such as an IDAE were explored in this paper. Full 3D models of finite element analysis were developed for the above SECM system, providing a deeper understanding of the above PACs and horizontal sweeps by means of SECM feedback in correlation with local concentration profiles. Especially, the transition region of an IDAE, where conductive and insulating substrates meet, was extensively investigated in this work.

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INTRODUCTION

SECM is commonly used to image electrochemical and topographical characteristics of a substrate, providing precise localized analysis of complex substrates.21,38−42 SECM operates by moving a biased ultramicroelectrode (UME) (25 μm diameter electrode or smaller) over a substrate with extreme precision.21,39,40 The UME tip and substrate are submerged in an electrolyte solution containing a redox agent. Current is monitored with reference to UME position. The precise positioning of a small-scale UME allows for localized characterization of electrochemical traits of a nonuniform substrate. IDAEs can be integrated into more complex electrochemical systems for more specialized analysis. Combinations such as generation at the SECM probe electrode with collection at the IDAE are anticipated to provide high collection efficiency.7,17,21 Traditionally, SECM probe approach curves (PACs) involve a biased UME approaching in a linear fashion, toward a single point on a sample vertically.43−45 To characterize a single sample at different locations, multiple approaches would be required, which is time-consuming. In addition, by completing multiple approaches, there is an increased risk of crashing the electrode into the sample or substrate, due to the possible

Microstructure arrays, such as interdigitated array electrodes (IDAEs), have been employed for a variety of applications including redox cycling,1,2 dielectrophoretic microfluidic separation for cell or particle sorting,3−7 biological sensing applications,8−14 and biomolecule delivery.15 With the miniaturization of high-powered electronics and the need for rapid automated categorization of samples, IDAEs have been increasingly used for lab on a chip sensors and microfluidics systems.16−18 Various traits of IDAEs are characterized using different methods. However, to electrochemically characterize an IDAE, few techniques are as powerful as scanning electrochemical microscopy (SECM).19,20 SECM is a noninvasive analytical method in the scanning probe microscopy (SPM) family. Developed greatly by Bard et al. since 1989,21 SECM has been employed for chemical kinetics studies, chemical imaging, potential distributions, and microfabrications.22−26 Recent developments toward various biological applications have been pursued,27−33 such as electron transfer kinetics and molecular transports.34−36 SECM has previously been used in concert with micropatterned IDAEs to create powerful biosensing analytical tools.9,11,37 IDAEs can be a powerful electroanalytical tool, allowing for self-contained collection and generation in alternating bands or more complex electrochemical analysis.7,8 © XXXX American Chemical Society

Received: June 11, 2015 Revised: August 28, 2015

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DOI: 10.1021/acs.jpcc.5b05568 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article


The Journal of Physical Chemistry C

tubes (o.d.: 2.00 mm, i.d.: 1.16 mm, length: 10.00 cm, Sutter Instruments, Novato, CA) were pulled to get a pair of capillary tips that were sealed with a micropipette puller (PP-83, Narishige, Japan). Five micrometer UMEs were created by inserting a 5 μm Pt wire (Goodfellow Metals, Cambridge, U.K.) into the heat-pulled and sealed capillary. The capillary assembly was placed under internal vacuum via its open end and heated to form a tight glass sheathing around the Pt wire. The sealed glass tube was then manually polished using a custom-made polishing wheel with alumina polishing pads (3.0, 0.3, and 0.05 μm, Buehler, ON) to expose the Pt wire at the tip of the glass electrode. Electrodes were characterized by the ratio of insulating glass sheath radius to conductive Pt wire radius (RG), eq 1.

height variations of the sample surface, which can be damaging to the sample or the electrode itself. In our group, however, a different method of SECM approaches called depth scan imaging has been previously developed.46,47 Depth scan mode passes the UME in the x-axis direction to perform a horizontal line scan at a constant height. The electrode is then lowered in the z-axis by a set distance, and another line scan is carried out. This action is repeated until the desired depth is reached. The repeated horizontal sampling at various approaching heights to the sample produces a two-dimensional image of the current feedback in the x−z plane in real-time. The depth scan imaging strategy allows the distance to the sample to be more easily gauged during the scan, removing the limitation of conventional SECM approach methods. In one depth image, up to hundreds of PACs can be obtained, limited only by the user defined image width and resolution. By simply selecting vertical cross sections from the image, experimental PAC data can be extracted, producing a plot of current vs the distance to the substrate. Extracted experimental PACs can be compared to simulated PACs to obtain quantitative analysis of sample traits, such as the kinetic and physical properties.36,40,41,48 Conventionally, simulations of this system under study are designed and computed using finite element analysis.49 Simulation of a complex system such as a UME approach to an IDAE requires the use of 3D model geometry to be computed and advanced software, such as COMSOL Multiphysics. This is very computationally demanding, requiring long compute times and large amounts of memory. Until recently, simulations of this complexity were not possible to perform using consumer grade hardware. We optimized the simulation to allow for such simulations to be computed in reasonable time on a custombuilt workstation PC. The 3D nature of the model allows analysis of any location in solution above the array under study, with the effects of adjacent structures accounted for. Herein for the first time, we report the 3D modeling for the IDAE-SECM system useful in revealing IDAE functionality information in conjunction with SECM depth scans. The simulation can generate multiple PACs to any location over the sample for interpretation of experimental results. We will first demonstrate the 3D model simulations to give insight into the design of an IDAE. The alternating conductive and insulating bands of the IDAE illustrated blended electrochemical behaviors due to the diffusion of redox species from adjacent bands and the resolution limits of a chosen UME size.

RG =

radius of glass sheath at the tip radius of Pt wire

(1)

The electrode RG was polished to approximately 2.19 Electrode tips were examined during the polishing process using an optical microscopy and tested for functionality using cyclic voltammetry (CV). Instrumentation. SECM experiments were all conducted on an adapted Alpha-SNOM (WITec, Ulm, Germany) with a homemade UME holder in place of a primary lens above the sample.47 Optical images of the samples were taken from the inverted objective lens (50× lens, N.A. 0.55, W.D. 10.1 mm, Nikon, Japan) and camera positioned below the sample. Positioning of the sample and electrode were performed by the Alpha-SNOM piezoelectric xy-stage and z-axis with extreme precision of 1 nm. All electrochemical experimentation was performed using a CH Instruments Electrochemical Analyzer (CHI800B, CH Instruments, Austin, TX) with a CHI200 Picoamp Booster to reduce noise. A Ag/AgCl wire suspended from the UME holder into solution was used as a combined reference and counter electrode. The potentiostat output signal was transported to a data acquisition channel of the AlphaSNOM microscope. Atomic force microscopy (AFM) was also carried out using the Alpha-SNOM. SECM Experiments. SECM of the interdigitated array was performed by securing a Petri dish containing the IDAE on the scanning stage and under the SECM probe of the microscope. The electrochemical analyzer was set to produce a constant voltage corresponding to the oxidation plateau of the electrochemical mediator (0.300 V vs Ag/AgCl for FcMeOH, determined by CV). The electrode is then lowered into solution using the microscope z axis until it is in close proximity to the sample. The optical objective lens is calibrated to be centered on the position of the electrode tip. This allows for visual tracking of both the electrode tip and the IDAE for analysis by SECM. The scan width and depth can be set in the WITec software, as well as the integration time and resolution of the scan image. Each scan has 256 × 256 pixels, with a scan scale of 90 μm in width and 20 μm in depth and an integration time of 0.05 s. To optimize the electrode distance, depth scans are performed above the sample. The electrode height is gradually lowered until feedback is seen. Care is taken to ensure contact with the sample does not occur. Once the electrode height has been optimized, the microscope is zeroed at this position. This makes it easy to return to this height if the electrode has to be raised at any point during the experiment. The Simulation Workstation Computer. COMSOL Multiphysics (version 4.4) software was used for all simulations.

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EXPERIMENTAL SECTION Materials. Ferrocenemethanol (FcMeOH) (97%) and potassium chloride (KCl) (99%) were purchased from SigmaAldrich (Mississauga, ON). A stock solution of 0.9 mM FcMeOH with 0.1 M KCl as a supporting electrolyte was prepared in deionized water (18 MΩ Milli-Q water, Millipore, Etobicoke, ON). The Au-coated IDAE was purchased from ABTECH Scientific, Inc. (Richmond, VA). Each IDAE electrode has 25 alternating digitized pairs of 20 μm Au digit width and 20 μm glass interdigit space. For the SECM depth scan study, the IDAE was adhered onto a Petri dish (P35G-020-C, MatTek Corperation, Ashland, MA) using double-sided tape (nonconductive surface to the Petri dish). The SECM approaches to the IDAE were conducted in 3 mL of the 0.9 mM FcMeOH electrolyte solution and carried out at 22 °C. Electrode Fabrication. Electrodes were fabricated as reported elsewhere.19,47,50,51 Briefly, borosilicate glass capillary B


Article



Figure 1. (A) Optical image of a gold interdigitated array electrode (IDAE), (B) labeled IDAE 3D COMSOL geometry along with the SECM probe, (C) meshed 3D IDAE-SECM probe with finer mesh at regions below the SECM probe scanning area and three edges of the two gold bands, and (D) labeled meshed SECM probe with finer mesh at Pt disk.

simulations performed, and the sides of the gold bands were also treated as gold. The gold substrate possesses conductive properties allowing for the regeneration of an electrochemical mediator (FcMeOH) in solution. The glass bands alternately have insulating characteristics and limit diffusion toward the electrode tip. A simulation approach is performed to only two adjacent bands in this model, one insulating and one conducting. However, an additional conducting and insulating band is simulated on either side of these two to allow for the compensation of proximity effects (Figure 1B). Two 20 μm × 20 μm sections of the bands (one gold and one glass) were created directly below the region the electrode would operate in for the purposes of finer meshing at these more dynamic locations. The bulk boundaries of the simulation model were defined distant from the electrode tip, to ensure negligible effect on the area under study. The electrode modeled in this simulation is a Pt disk with a diameter of 5 μm with an RG of 2. This corresponds to the physical electrode used in experimental analysis of the array. The electrode z and x positions are parametrized in COMSOL, which allows for parametric movement of the electrode position, automating the approach to, and sweep across, the substrate. A bulk solution concentration of FcMeOH was set at 0.9 mM to match the stock solution used in the physical experiment. A diffusion coefficient for FcMeOH was set at 7.6 × 10−10 m2/s.39,44,53 At 0.300 V, the one-electron oxidation of FcMeOH to [FcMeOH]+ is a diffusion-controlled process which follows Fick’s second law (eq 2 in solution):

The custom-built workstation computer for the COMSOL simulations uses an Intel Core I7 4930K (6-core, 12-thread) and 32 GB Kingston HyperX Fury Black memory on the LGA2011 platform. A Mushkin Chronos Sandforce 480 GB SSD was also added to increase swap speeds for large simulations and to reduce load and save times for commonly used models. All parts used in the construction of the workstation are consumer available, “gaming grade” hardware. Parts were selected, acquired, and assembled in-house with optimization for the COMSOL simulations prioritized. Ubuntu Linux 14.04.1 LTS was installed as a reliable platform for running simulations.

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SIMULATIONS AND THEORY Model of Interdigitated Array Electrode. A model of an IDAE was constructed in 3D based on its physical device (Figure 1A), for the purpose of characterizing the system that cannot be simulated using 2D or 2D Axial models. The array is composed of 20 μm gold bands and 20 μm glass bands in an alternating pattern. The deposited gold is 100 nm thick on the glass backing.19 Notice that presented domain size is with 16*a (a is the electrode radius), significantly smaller than the suggested 1000*a by Cornut and Lefrou,52 because of the demand on computer system resources in 3D simulations. Simulations for extended geometries have been performed. It was found that between our bulk solution side length (80 μm) and a side length of 5000 μm (1000*a), a bulk solution current deviation of less than 3% was observed. The thickness of this gold was accounted for relative to the glass backing in the C


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⎛ ∂ 2C ∂C B ∂ 2C B ∂ 2C B ⎞ ⎟ + = DB⎜ 2B + 2 ∂t ∂y ∂z 2 ⎠ ⎝ ∂x

Horizontal sweeps are obtained by plotting the normalized tip current eq 5 versus its x position when it sweeps across the IDAE bands. Experimental PACs are similarly normalized to the bulk solution current value taken from the steady state region of a PAC. Experimental PACs are then overlaid on top of the simulated theoretical PACs. Theoretical curves are simulated with an absolute distance from the substrate designed into the model geometry. Experimental PACs are matched to these curves to determine the tip to substrate distance of the closest point in the approach. It is important to note that the defined simulation model does not account for forced convection occurring at the electrode tip by the motion of the electrode.56 Each individual electrode position is simulated as an independent simulation. It is assumed that the influences of forced convection are negligible at the electrode scan rate used experimentally (

Three-Dimensional Electrochemical Functionality of an Interdigitated

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