Addressable Nanoelectrode Membrane Arrays ... - ACS Publications

Jan 17, 2007 - Institute of Microtechnology, University of Neuchatel, Rue Jaquet-Droz 1, CH-2007 Neuchatel, Switerland. An addressable nanoelectrode ...
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Anal. Chem. 2007, 79, 1474-1484

Addressable Nanoelectrode Membrane Arrays: Fabrication and Steady-State Behavior Cynthia G. Zoski* and Nianjun Yang

Department of Chemistry and Biochemistry, New Mexico State University, MSC 3C, Las Cruces 88003, New Mexico Peixin He

CH Instruments Inc., 3700 Tennison Hill Drive, Austin, Texas 78738 Luca Berdondini and Milena Koudelka-Hep

Institute of Microtechnology, University of Neuchatel, Rue Jaquet-Droz 1, CH-2007 Neuchatel, Switerland

An addressable nanoelectrode membrane array (ANEMA) based on a Au-filled track-etched polycarbonate membrane was fabricated. The Au-filled membrane was secured to a lithographically fabricated addressable ultramicroelectrode (UME) array patterned with 25 regularly spaced (100 µm center to center spacing), 10 µm diameter recessed Pt UMEs to create 25 microregions of 10 µm diameter nanoelectrode ensembles (NEEs) on the membrane. The steady-state voltammetric behavior of 1.0 mM Ru(NH3)6Cl3 and 1.0 mM ferrocene methanol in 0.1 M KCl on each of the micro NEEs resulted in sigmoidalshaped voltammograms which were reproducible across the ANEMA. This reproducibility of the steady-state current was attributed to the overlapping hemispherical diffusion layers at the Au-filled nanopores of each 10 µm diameter NEE of a ANEMA. The track-etched polycarbonate membranes were filled using a gold electroless deposition procedure into the 30 nm diameter pores in the membrane. Electrical connection between the Au-filled template array and the lithographic UME platform array was achieved by potentiostatic electrodeposition of Cu from an acidic copper solution into each of the 25 recessed Pt UMEs on the UME array platform. A multiplexer unit capable of addressing 64 individual micro NEEs on an ANEMA is described. ANEMAs have advantages of high reproducibility, facile fabrication, multitime reuse of lithographically fabricated UME arrays, and purely steady-state behavior. The fabrication and behavior of macrosized nanoelectrode ensembles (NEEs) and arrays (NEAs) remain an active area of interest due to advantages including significantly reduced capacitance and iR effects compared to a single macroelectrode and remarkably enhanced faradaic currents compared to a single nanoelectrode.1-7 The nanoelectrodes in NEEs are randomly spaced in a fixed area, while those in NEAs have an ordered * To whom correspondence should be addressed. E-mail: [email protected].

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spacing.2 Thus NEEs are comparatively easier to fabricate than NEAs. Fabrication methods reported for NEEs include template electroless8-11 and electrochemical deposition,12-14 embedding carbon nanotubes into a SiO2 matrix15-17 and in carbon pastes,18 adsorbing carbon nanotubes onto self-assembled monolayers (SAMs),19 and etching defects into alkane thiol SAMs to expose underlying nano-Au surfaces.20 Fabrication methods for NEAs include dispersing block copolymers onto electrode surfaces21,22 and electrochemical deposition using ideally ordered porous (1) Martin, C. R.; Mitchell, D. T. In Electroanalytical Chemistry, A Series of Advances; Bard, A. J., Rubinstein I., Eds.; Dekker: New York, 1999; Vol. 21, pp 1-74. (2) Ugo, P.; Moretto, L. M.; Vezza, F. Chem. Phys. Chem. 2002, 3, 917-925. (3) Wirtz, M.; Martin, C. R. Adv. Mater. 2003, 15, 455-458. (4) Arrigan, D. W. M. Analyst 2004, 129, 1157-1165. (5) Ugo, P.; Moretto, L. M. In Handbook of Electrochemistry; Zoski, C. G., Ed.; Elsevier: Amsterdam, 2007; Chapter 16, Section 16.2, pp 678-709. (6) Szunerits, S.; Thouin, L. In Handbook of Electrochemistry; Zoski, C. G., Ed.; Elsevier: Amsterdam, 2007; Chapter 10, pp 391-428. (7) Martin, C. R. Acc. Chem. Res. 1995, 28, 61-68. (8) Martin, C. R. Science 1994, 266, 1961-1966. (9) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920-1928. (10) Ugo, P.; Moretto, L. M.; Bellomi, S.; Menon, V. P.; Martin, C. R. Anal. Chem. 1996, 68, 4160-4165. (11) Szunerits, S.; Walt, D. R. Anal. Chem. 2002, 74, 1718. (12) Schonenberger, C.; van der Zande, B. M. I.; Fokkink, L. G. J.; Henny, M.; Schmid, C.; Kruger, M.; Bachtold, A.; Huber, R.; Birk, H.; Staufer, U. J. Phys. Chem. B 1997, 101, 5497-5505. (13) Li, G.-R.; Tong, Y.-X.; Kay, L.-G.; Liu, G.-K. J. Phys. Chem. B 2006, 110, 8965-8970. (14) Tu, Y.; Huang, Z. P.; Wang, D. Z.; Wen, J. G.; Ren, Z. F. Appl. Phys. Lett. 2002, 80, 4018-4020. Li, J.; Ng. H. T.; Cassell, A.; Fan, W.; Chen, H.; Ye, Q.; Koehne, J.; Han, J.; Meyyappan, M. Nano Lett. 2003, 3, 597-602. (15) Li, J.; Stevens, R.; Delzeit, L.; Ng, H. T.; Cassell, A.; Han, J.; Meyyappan, M. Appl. Phys. Lett. 2002, 81, 910-912. (16) Li, J.; Ye, Q.; Cassell, A.; Ng, H. T.; Stevens, R.; Han, J.; Meyyappan, M. Appl. Phys. Lett. 2003, 82, 2491-2493. (17) Li, J.; Ng. H. T.; Cassell, A.; Fan, W.; Chen, H.; Ye, Q.; Koehne, J.; Han, J.; Meyyappan, M. Nano Lett. 2003, 3, 597-602. (18) Gong, K.; Zhang, M.; Yan, Y.; Su, L.; Mao, L.; Xiong, S.; Chen, Y. Anal. Chem. 2004, 76, 6500-6505. (19) Su, L.; Gao, F.; Mao, L. Anal. Chem. 2006, 78, 2651-2657. (20) Baker, W. S.; Crooks, R. M. J. Phys. Chem. B 1998, 102, 10041-10046. (21) Jeoung, E.; Galow, T. H.; Schotter, J.; Bal, M.; Ursache, A.; Tuominen, M. T.; Stafford, C. M.; Russell, T. P.; Rotello, V. M. Langmuir 2001, 17, 63966398. (22) Hirota, K.; Tajima, K.; Hashimoto, K. Langmuir 2005, 21, 11592-11595. 10.1021/ac0619534 CCC: $37.00

© 2007 American Chemical Society Published on Web 01/17/2007

alumina.23 Applications of NEEs and NEAs are reported widely in diverse applications including chemical and biochemical sensors,17,24-31 energy storage,32 and electronics/optoelectronic devices.33-35 Among the earliest reported NEEs are those developed by Martin and co-workers1,8 who filled the randomly spaced nanopores of commercially available track-etched polycarbonate membranes with metals using either electroless or electrochemical deposition. Such membranes are available in several nanopore densities and sizes so that the response of the macromembrane NEE can be controlled, in part, by changing the character of the membrane. Such NEEs have been used in a 2D format where the nanodiscs are coplanar with the membrane surface9,28-31 and in a 3D format where the membrane is either chemically etched36 or plasma etched26,37 so that nanowires protrude approximately 200 nm above the membrane surface or where the nanowire is recessed below the membrane into the nanopore.38-40 In all of these cases, a macrosurface of the NEE array is exposed to a solution containing a redox couple. Theory and models describing the behavior of 2D NEEs have been reported.41-46 The response of macro-2D NEEs critically depends on the time scale (determined by the scan rate in a voltammetric experiment) of the experiment and the nanoelectrode radius and spacing. There are three types of behavior which are possible under voltammetric conditions. At very short times, the diffusion to each nanoelectrode is linear and the diffusion layers do not overlap so that the total current represents the sum of the transient currents from each nanoelectrode. A transient cyclic voltammogram (CV) resulting from this linear diffusion mode is the same as that recorded at a (23) Matsumoto, F.; Harada, M., Koura, N.; Nishio, K.; Masuda, H. Electrochem. Solid-State Lett. 2004, 7, E51-E53. (24) Gasparac, R.; Taft, B. J.; Lapierre-Devlin, M. A.; Lazareck, A. D.; Xu, J. M.; Kelley, S. O. J. Am. Chem. Soc. 2004, 126, 12270-12271. (25) Lin, Y.; Lu, F.; Ren, Z. Nano Lett. 2004, 4, 191-195. (26) Koehne, J.; Chen, H.; Li, J.; Cassell, A. M.; Ye, Q.; Ng, H. T.; Han, J.; Meyyappan, M. Nanotechnology 2003, 14, 1239-1245. (27) Delvaux, M.; Walcarius, A.; Demoustier-Champagne, S. Anal. Chim. Acta 2004, 525, 221-230. (28) Pereira, F. C.; Moretto, L. M.; Leo, M. D.; Zanoni, M. V. B.; Ugo, P. Anal. Chim. Acta 2006, 575, 16-24. (29) Moretto, L. M.; Pepe, N.; Ugo, P. Talanta 2004, 62, 1055-1060. (30) Ugo, P.; Pepe, N.; Moretto, L. M.; Battagliarin, M. J. Electroanal. Chem. 2003, 560, 51-58. (31) Brunetti, B.; Ugo, P.; Moretto, L. M.; Martin, C. R. J. Electroanal. Chem. 2000, 491, 166-174. (32) Fride, V.; Dale, T. J. Power Source 2005, 146, 804-808. (33) Hochbaum, A. I.; Fan, R.; He, R.; Yang, P. Nano Lett. 2005, 5, 457-460. (34) Lexholm, M.; Hessman, D.; Samuelson, L. Nano Lett. 2006, 6, 862-865. (35) Xiao, Z.; Zhang, L.; Meng, G.; Tian, X.; Zeng, H.; Fang, M. J. Phys. Chem. B 2006, 110, 15724-15728. (36) Krishnamoorthy, K.; Zoski, C. G. Anal. Chem. 2005, 77, 5068-5071. (37) Yu, S.; Li, N.; Wharton, J.; Martin, C. R. Nan. Lett. 2003, 3, 815-818. (38) Ito, T.; Aud, A. A.; Dible, G. P. Anal. Chem. 2006, 78, 7048-7053. (39) Evans, U.; Colavita, P. E.; Doescher, M. S.; Schiza, M. Myricks, M. L. Nano Lett. 2002, 2, 641-645. (40) Doescher, M. S.; Evans, U.; Colavita, P. E.; Miney, P. G.; Myricks, M. L. Electrochem. Solid-State Lett. 2003, 6, C112-C115. (41) Gueshi, T.; Tokuda, K.; Matusda, H. J. Electroanal. Chem. 1978, 89, 247260. (42) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J. Electroanal. Chem. 1982, 138, 65-77. (43) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J. Electroanal. Chem. 1982, 161, 247-268. (44) Amatore, C.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39-51. (45) Shoup, D.; Szabo, A. J. Electroanal. Chem. 1984, 160, 19-26. (46) Scharifker, B. R. J. Electroanal. Chem. 1988, 240, 61-76.

planar electrode of comparable active electrode surface area. At intermediate times, hemispherical diffusion occurs at each nanoelectrode, the diffusion layers do not overlap, and the current represents the sum of the steady-state currents from each nanoelectrode. A steady-state voltammogram (SSV) which is similar to that recorded at an ultramicroelectrode (UME) of comparable active area results in this radial diffusion mode. At longer times, the hemispherical diffusion layers of the nanoelectrodes overlap, and the transient current response corresponds to that of the geometric area of the entire NEE surface (active and inactive regions). A transient CV similar to that recorded at a planar electrode of comparable geometric area results in this total overlap diffusion mode. Due to the high density of the nanopores in the track-etched membranes and the macrogeometric area that is accessed by the redox species in solution, the majority of experiments on these macromembrane NEEs are carried out in the total overlap diffusion mode and one works with transient CV data. There has been much interest in working with NEEs in the purely radial diffusion mode where time-independent, steady-state voltammograms are recorded. This type of behavior has been reported on track-etched membranes of very low nanopore density47 but has not been electrochemically exploited. In contrast to the work which has been reported for macroNEEs based on track-etched polycarbonate membranes, we are interested in working on regularly spaced, addressable, microregions (i.e., micro-NEEs) of these membranes. Even though the hemispherical diffusion layers of the nanoelectrodes in each microNEE totally overlap, a SSV is expected because the geometric area of each micro-NEE is small enough to sustain a steady-state flux and current. Moreover, these micro-NEEs are spaced far enough apart (100 µm center-to-center) so that there is no crosstalk between neighboring micro-NEEs. The micro-NEEs and addressing are achieved by securing the metal-filled template to a lithographically fabricated addressable array with 25, 10 µm diameter Pt disc UMEs described previously.48 Addressable NE membrane arrays (ANEMAs) have unique advantages which (a) enable a series of steady-state experiments on one platform to be performed at the nanodiscs in each of the 25 micro-NEEs, (b) feature an easily removed metal-filled template which can be replaced with a new template, and (c) enable a lithographically fabricated array, which serves only as the addressing platform for the metal-filled template, to be reused endlessly. This paper describes the fabrication of ANEMAs, the multiplexer that permits individual interrogation of the micro NEEs of an ANEMA, and their steady-state behavior. EXPERIMENTAL SECTION Chemicals and Materials. Sodium sulfite(Na2SO3), tin(II) chloride (SnCl2), trifluoroacetic acid (TFA), nitric acid (HNO3, Acros Organic, NJ), formaldehyde (37% solution, w/w, HCHO), ammonium hydroxide (29% solution, NH4OH), sulfuric acid (H2SO4), methanol (MeOH), ethanol (EtOH), potassium chloride (KCl), buffer solution pH 10.00, (Fisher Scientific, Fair Lawn, NJ), copper sulfate (CuSO4), acetonitrile (CH3CN), hexaammineruthenium(III) chloride (Ru(NH3)6Cl3, Alfa Aesar, Ward Hill, MA), (47) Hulteen, J. C.; Menon, V. P.; Martin, C. R. J. Chem. Soc., Faraday Trans. 1996, 92, 4029-4032. (48) Zoski, C. G.; Simjee, N.; Guenat, O.; Koudelka-Hep, M. Anal. Chem. 2004, 76, 62-72.

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sodium bicarbonate (NaHCO3, J. T. Baker, Phillipsburg, NJ), and silver nitrate (AgNO3, Johnson Matthey, Ward Hill, MA) were of analytical grade and used as received. Oromerse Solution Part B Gold Replenisher (0.4 troy ounce in 200 mL, Technique Inc., Cranston, RI) was the Au source in the electroless plating of gold inside SPI nanoporous polycarbonate membranes (West Chester, PA) of pore density 6 × 108 cm-2, pore diameter 30 nm, geometric diameter 13 mm, and thickness 6.0 µm. Aqueous solutions were prepared with Milli-Q water (18.2 MΩ cm, Millipore Co., Bedford, MA). Q-tips (Johnson & Johnson), one-sided adhesive copper tape (3M), and clear view premium tape (Staples) were also used. Instrumentation. Electrochemical experiments were performed using a CHI 660A electrochemical work station and CHI 1030 multipotentiostat (CH Instrument., Austin, TX) interfaced with a 64-channel multiplexer. All experiments involving UMEs, UME arrays, and ANEMAs were performed in a Faraday cage. An AR pH meter (Fisher Scientific) was used in adjusting the pH of electroless gold plating solutions. Thermal treatment of goldfilled polycarbonate membranes was carried out in an Isotemp oven Model 615G (Fisher Scientific). An Olympus BX 51 optical microscope (Olympus American Inc., Melville, NY) equipped with an Olympus U-DCIR (Nomarski differential interference contrast imaging) unit was used to inspect the smoothness of electrodeposited copper and to observe the Au-filled membranes. Digital simulations were performed using DigiElch simulation software49 which is capable of simulating linear (1D) and radial (2D) semiinfinite diffusion on a disk electrode. A LEO 1530 scanning electron microscope was used in obtaining images of the polycarbonate membranes before and after gold electroless deposition. Multiplexer Unit. A 1-of-8 multiplexer (e.g., a MAX308) was used to allow eight working electrodes to be turned on in sequence, one at a time. For a 64-electrode array, for example, eight 1-of-8 multiplexers allow eight groups of eight UMEs to be turned on in sequence with the ability to control each electrode individually in the designated group. A typical multiplexer has a resistance of less than 100 Ω. Since the current for a 10 µm diameter microelectrode is in the nA range, the voltage drop across the multiplexer is less than 0.1 mV. This configuration allows the potential of each electrode to be controlled and scanned. The control logic is also relatively simple. The multiplexer has a built-in decoder, so that only three I/O lines are needed to control the 64 channels. No other circuitry or drivers are needed. A key question concerns electrode potential control. In electrochemical measurements, the cell is usually connected all of the time to allow the electrode surface to reach equilibrium. This reduces the charging current and residual current and allows steady-state current measurements. With working electrode multiplexing, a sufficiently long waiting period is needed to allow surface equilibrium to be established. For a 10 µm diameter microelectrode, it takes about 0.2 s for the current to reach a steady state under diffusion-controlled conditions. For 64-channel multiplexing, 32 s are required to complete the measurement for all channels. The measurement time can be reduced if an 8-channel potentiostat (CHI 1030) is used. Each of eight working electrodes are connected to a 1-of-8 multiplexer. This allows 8 electrodes to be connected simultaneously. When one channel measurement is finished, it is switched to another electrode. After (49) website: http://www.digielch.de/

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one round of 8 measurements, this channel should reach equilibrium. With each electrode connected for 0.2 s to reach equilibrium, only 4 s are needed for all 64 electrode measurements. For smaller electrodes, less time is needed. Software was also developed to run the CHI 1030 multipotentiostat in the multiplexer mode. Electrodes. The reference and counter electrodes were Ag/ AgCl (saturated, 4 M) and a Pt wire, respectively. A lithographically fabricated addressable Pt UME array described previously48 was used as the platform for the ANEMAs. The addressable Pt UME array platform contained 25 Pt UME discs of 10 µm diameter which were recessed 200 nm below an insulator coating and with a 100 µm center-to-center spacing. Gold Nanoelectrode Ensembles. The electroless deposition of gold into nanoporous polycarbonate membranes and the subsequent fabrication of gold nanoelectrode ensembles were carried out as described previously,36 with some modifications. Briefly, after soaking in MeOH for 20-30 min, three polycarbonate membranes were sensitized in a 50 mL SnCl2-TFA solution for 45 min. The SnCl2-TFA solution was prepared by dissolving 0.2464 g of SnCl2 (0.026 M) and 0.3 mL of TFA in 50 mL of water/ MeOH (volume ratio of 1:1). The sensitized membranes were then washed consecutively in three separate beakers filled with 200 mL of MeOH for 10 min per beaker. The washed membranes were then put into 75 mL of a clear AgNO3-NH4OH solution for 10 min. The clear AgNO3-NH4OH solution was made by dissolving 0.3698 g of AgNO3 (0.029 M) into 75 mL of water and then titrating slowly with NH4OH drop by drop. The titration proceeds through the formation of a brown color (signaling the formation of AgOH) and is stopped when the brown color disappears (signaling the formation of Ag(NH3)2NO3). After 10 min, the membranes were taken out of the clear Ag(NH3)2NO3 solution, and washed consecutively in two beakers filled with 200 mL of MeOH for 10 min per beaker. Each membrane was then hung vertically (using a clip) in a precooled 20 mL gold plating solution for 24 h at a temperature of 4-5 °C in a refrigerator. The gold plating solution was prepared by mixing 1.6007 g of Na2SO3 (0.127 M), 0.2100 g of NaHCO3 (0.025 M), and 5 mL of HCHO (0.625 M) with 90 mL of water in a 100 mL volumetric flask, it was adjusted to pH 10.00 with 1.8 M sulfuric acid, and the solution was made up to 100 mL with water. A volume of 0.5 mL of Oromerse Solution Part B Gold Replenisher solution was then added to 20 mL of the freshly made plating solution in a sample vial, and the pH value was readjusted with 1.8 M sulfuric acid to pH ) 10.00. This gold plating solution was precooled in a refrigerator at 5 °C, while the polycarbonate membranes were being treated. After the electroless deposition, the gold-coated membranes were rinsed with 150 mL of Milli-Q water in three separate beakers for 10 min per beaker and then in 100 mL of 25% HNO3 for 12 h uncovered at room temperature. The Au-coated and -filled membranes were then rinsed in a single beaker of 150 mL of water for 15 min. The washed membranes were then allowed to air-dry before heating in an oven at 150-155 °C for 15 min. The SPI membranes have a dull and shiny side which can be clearly distinguished even when covered with Au. The gold layer on the dull face and/or shiny face of the membranes was removed gently and slowly (with the Q-tip moving in a single direction only) using a Q-tip wetted with EtOH. The Q-tip was

Figure 2. Photograph of a representative ANEMA showing the macro-NE membrane, the UME array platform, and the connecting ribbon cables to the multiplexer unit.

Figure 1. Schematic diagram of the fabrication of an ANEMA.

first shaken in air to remove excess EtOH from the Q-tip swab. The Au-filled membranes were then used to make gold nanoelectrode ensembles according to the procedure developed by Martin and co-workers9 in order to characterize the membranes and to make ANEMAs using the method described below. In fabricating ANEMAs, the gold layer was initially removed from the dull side of the Au-filled membrane only and left on the shiny side of the Au-filled membrane. RESULTS AND DISCUSSION Fabrication of Addressable NE Membrane Arrays. Figure 1 shows the steps involved in fabricating ANEMAs. These steps include (i) the use of a lithographically fabricated addressable Pt ultramicroelectrode array platform, (ii) copper deposition to fill the 200 nm recess in each UME of the Pt UME array platform, (iii) positioning a Au-filled membrane on top of the copper deposited UME array platform, (iv) attachment of the membrane to the UME array platform with clear tape, and (v) removal of the surface gold layer from the solution side of the membrane. The details of each step follow below. The platform for an ANEMA is a lithographically fabricated addressable Pt UME array as previously described.48 This UME array platform consists of 25 Pt UME discs (10 µm diameter, recessed 200 nm below the insulating surface) with a regular 100 µm center-to-center spacing. The UME array platform was checked optically using an Olympus microscope and electrochemically in a solution of 1.0 mM Ru(NH3)6Cl3/0.1 M KCl to confirm the size of the Pt UMEs and the electrical connections of each Pt UME within the array.48 The main body of the array (Figure 2) containing addressing tracks which were not protected by insulation was wrapped with Teflon tape to avoid corrosion of the tracks and prevent leakage currents between neighboring UMEs on the array. After copious rinsing of this UME array platform with Millipore water, copper was electrodeposited from

a solution of 2.4 mM CuSO4/0.1 M H2SO4 into the recessed microdiscs under potentiostatic control for 50 min with each UME disc set at -0.2 V vs Ag/AgCl. A 50 min deposition ensured that each recess was overfilled with copper which then protruded above the array surface. This copper overgrowth was wiped away slowly and gently using a membrane wetted with methanol. The surface of the UME array platform was then checked under a microscope equipped with an Olympus U-DICR (Nomarski differential interference contrast imaging) unit to check the smoothness of the remaining copper and to ensure that there was no extraneous copper between the Cu-deposited disks after washing copiously with water. The surface of the UME array platform was then wetted with one small drop of CH3CN (too much CH3CN will cause the membrane to shrivel up), and one fresh piece of the Au-filled membrane (one-fourth of a 13 mm diameter membrane) was laid onto the platform with the Au-covered side facing upward so that the Cu-deposited discs were covered. The CH3CN wetting holds the Au-filled membrane in place until it can be secured with clear tape. One piece of clear tape with a hole of area 0.031 cm2 (2 mm diameter) was then positioned over the Au-filled membrane and used to secure the Au-membrane to the UME platform array. In this way electrical contact was established between each underlying Cu-disk and a corresponding microregion of the Au-membrane lying on top. The exposed region of the NE membrane established by the clear tape was viewed under a microscope to check the integrity of the seal. At this point, there is a uniform gold layer on the membrane enclosed in a circular region defined by the clear tape. We refer to this assembly, shown in Figure 2, as an addressable NE membrane array (ANEMA). The ANEMA was electrochemically evaluated in a solution of 1.0 mM Ru(NH3)6Cl3/0.1 M KCl, which was purged with argon. In the first evaluation, a uniform Au surface area of 0.031 cm2 is exposed to the solution. When the potential is swept linearly from 0 to -0.35 V vs Ag/AgCl at a sweep rate of 15 mV/s, a cyclic voltammogram as shown schematically in Figure 1 appears if there is electrical contact between the gold nanowires in the membrane and the electrodeposited copper in the UME array platform. When this occurred, the Au layer on the exposed membrane was removed slowly and carefully with a Q-tip wetted with ethanol to expose the separate gold nanodiscs. Microregions of these Au nanodiscs were connected electrically underneath by each of the 25 Cu deposited UMEs on the UME array platform. After rinsing the surface-attached membrane of the ANEMA with water and re-immersing in 1.0 mM Ru(NH3)6Cl3/0.1 M KCl solution, the Analytical Chemistry, Vol. 79, No. 4, February 15, 2007

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potential of each of the 25 micro-nanoelectrode ensembles (NEEs) was scanned from 0 to -0.40 to 0 V vs Ag/AgCl at a scan rate of 5 mV/s. Steady-state I-E voltammograms similar to that shown schematically in Figure 1 were recorded. The experimental details of each of these steps in the ANEMA fabrication follow below. Copper Electrodeposition Into Recessed Pt UMEs. Copper was chosen as the conducting material to fill the recessed discs on the UME array platform because (1) the deposition can be carried out quantitatively by controlling the deposition time under potentiostatic conditions, (2) copper metal is highly conductive, and (3) copper has a low oxidation potential in acidic media which facilitates copper stripping so that it can be replated easily into the recesses of the Pt UME array platform. Electrodeposition of copper from an acidic solution also has the advantages of a fast plating rate which prevents the hydrolysis of copper ions into copper oxides and also reduces the size of copper particles, which leads to a smoother surface; these aspects have been found to contribute to the excellent conductivity of copper.50 Optimum deposition conditions including copper deposition potential and time, and concentrations of sulfuric acid and copper sulfate were chosen after examining the growth rate of copper into the recessed Pt UMEs in the presence and absence of hydrogen evolution and observing the morphology of electrodeposited copper using an Olympus microscope with a U-DICR unit. We observed that copper deposition potentials more positive than -0.2 V vs Ag/ AgCl resulted in a very slow growth rate of copper into the recessed Pt UMEs, whereas deposition potentials more negative than -0.2 V led to hydrogen evolution, faster deposition rates, enlarged copper particles, and failure in achieving a good deposition in the recessed discs. The concentrations of 0.1 M sulfuric acid and 2.4 mM copper sulfate were found to be optimum for copper deposition. Figure 3 shows the current-time curves for copper deposition at a constant potential of -0.2 V vs Ag/AgCl from 2.4 mM CuSO4/ 0.1 M H2SO4 solutions into each of the 25 Pt UME recessed discs for 50 min. For convenience of plotting, the results for deposition into the first 24 Pt UME recessed discs only are shown; the deposition result for the 25th Pt UME is similar. The currents for copper deposition into the different recessed Pt UMEs were found to be similar. However, the current-time curves included two time regimes. During the first 10 min, the current for copper deposition increased rapidly with time as the recess filled and then slowed once the recess was filled and the copper deposition increased the surface area of the copper overflowing the recess. These growth stages were confirmed by viewing the copper deposition under an Olympus microscope/U-DICR unit. Initially, copper growth started at the edge of the recessed disc due to the large current density on the edge of Pt UMEs, followed by overlapped growth as copper grew toward the center of each Pt UME. This growth process follows the results reported by others using scanning electron microscopy51 and compared with mathematically simulated results.52 Deposition times greater than 30 min resulted in overfilling the 200 nm recess so that the electrodeposited copper (50) Bertocci, U.; Turner, D. S. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Dekker: New York, 1974; Vol. II, p 384. (51) Hasegawa, M.; Negishi, Y.; Nakanishi, T.; Osaka, T. J. Electrochem. Soc. 2005, 152, C221-C228. (52) West, A. C.; Mayer, S.; Reid, J. Electrochem. Solid-State Lett. 2001, 4, C50C53.

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Figure 3. Current-time curves for copper electrodeposition at -0.2 V vs Ag/AgCl for 50 min from a 2.4 mM CuSO4/0.2 M H2SO4 solution into recessed Pt UMEs on the addressable UME array platform. All plots have the same potential ranges and current scales.

spilled over onto the surface of the UME array platform. We found that the UME array platform with overfilled copper never resulted in electrical connection between the gold nanowires in the NE membranes with the underlying Cu-deposited UMEs. This failure may have been caused by nonuniform heights of Cu deposition above the array surface. Since deposition times longer than 30 min ensured total filling of the recessed UME discs, a deposition time of 50 min was used and the excess copper was carefully wiped off the UME array surface. A Q-tip wetted with water or solvents such as MeOH was found to clear the copper deposited over the array surface but also removed the copper from the recessed channels. Instead, a polycarbonate membrane (onefourth of a 13 mm diameter membrane) wetted with MeOH was found to efficiently remove the overfilled copper on the array surface by moving the membrane slowly and gently across the UME array surface. Figure 4 shows a single Pt UME on the addressable UME array platform (a) before Cu deposition, (b) after a 50 min Cu deposition, and (c) after removing overdeposited Cu. Under a microscope, a smooth Cu surface was observed and electrical connection was consistently achieved between the Au nanowires of a NE membrane with the underlying electrodeposited copper in each of the 25 UMEs on the array platform.

Figure 4. Cu deposition on a single 200 nm recessed 10 µm diameter Pt UME on an addressable UME array platform. A Pt UME on the addressable UME array platform: (a) before Cu deposition, (b) after a 50 min Cu deposition, and (c) after removal of overdeposited Cu.

Figure 6. Cyclic voltammograms of macro-Au NEEs of geometric area of 0.031 cm2 for cases I, V (s), III, VII (--), and II, IV, VI, and VIII, (- ‚ -). The CV of a polycrystalline Au macrodisk of comparable geometric area (‚‚‚) is shown for comparison. The solution was 1.0 mM Ru(NH3)6Cl3/0.1 M KCl. Scan rate ) 15 mV/s; quiet time ) 30 s. Figure 5. Illustration of NE membrane configurations used for the fabrication of macro-Au NEEs for membrane characterization.

Characterization of NE Membranes. Prior to attaching the Au-filled membranes to the addressable UME array platform, the NE membranes were characterized electrochemically. We were interested in exploring whether there was any difference in electrical contact through the shiny or dull face of the NE membrane and if electrical contact could be made through individual nanowires rather than through a uniform gold surface layer as reported in the literature.9 Figure 5 shows membrane configurations that were considered. In I-IV of Figure 5, the shiny face of the membrane faced the solution and was either covered by a uniform layer of electrolessly plated Au (I, III) or was stripped of this Au layer so that only 30 nm diameter Au discs (II, IV) were exposed to the solution. Electrical contact was made through the dull side of the membrane, which was either a uniform layer of electrolessly plated Au (I, II) or was stripped of this Au layer so that electrical contact was made through the exposed 30 nm diameter Au discs (III, IV). The dull side of the membrane was attached to a piece of single sided-adhesive conductive copper tape. A similar strategy was followed in V-VIII with the dull side of the membrane facing the solution and electrical connection being made through the shiny side of the membrane. A geometric electrode area of 0.031 cm2 was exposed to a solution by punching a hole in a piece of clear tape and wrapping this around each fabricated membrane/ conductive tape assembly as described in the literature.9 Cyclic voltammograms at these Au macromembrane NEEs were recorded in an aqueous solution of 1.0 mM Ru(NH3)6Cl3/ 0.1 M KCl. Figure 6 shows representative CVs of cases I and V

(solid line) where solution and electrical contact was made through a uniform Au layer, III and VII (dashed line) where electrical contact was made through Au nanowires and the uniform Au surface layer was exposed to solution, and II, IV, VI, and VIII (dashed-dotted line) where electrical contact was made either through a uniform Au layer or through Au nanowires and Au nanodiscs were exposed to the solution. For these macro-NEEs, Figure 6 also demonstrates that there is no difference in the CVs when electrical contact is made from either the dull or shiny side of the membrane. The reproducibility of the CVs in all of these cases is very good in terms of CV shape and distinguishing features such as peak currents and potential. However, the CVs of macromembrane NEEs where the Au nanodiscs were exposed to the solution (dashed-dotted line) were consistently shifted approximately 20 mV more negative from those recorded with a uniform Au surface layer exposed to the solution (solid and dashed lines). Membranes prepared at different times and cut from different sections of one membrane did not alter the results described here. The CV in Figure 6 for a polycrystalline macroAu electrode of similar geometric area (solid dots) shows that this shift is unique to the macromembrane NEEs, as is the slight change in shape of the CV from that recorded at macro-Au electrodes. This can be attributed to a change in diffusional behavior and an apparent slow reaction rate of Ru(NH3)6Cl3 on a macro-NEE which are characteristic of CVs recorded on partially blocked surfaces.44 The important conclusion from these results in terms of fabricating ANEMAs is that electrical connection can be made exclusively through the Au nanowires of the membrane without the presence of a uniform Au surface coating, which is typically reported in the literature. Analytical Chemistry, Vol. 79, No. 4, February 15, 2007

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Figure 7. Comparison of experimental cyclic voltammograms (s) for the reduction of 1.0 mM Ru(NH3)6Cl3/0.1 M KCl at a (a) macroAu NEE I and (b) macro-Au NEE IV at a scan rate of 15 mV/s with digitally simulated cyclic voltammograms (--). The rate constant for the simulations in a and b were 0.1 and 0.01 cm/s, respectively; E° for (a) was -0.18 V and for (b) was -0.2 V. Other parameters include the following: scan rate, 15 mV/s; electrode area, 0.031 cm2; electron transferred, 1; charge-transfer coefficient, 0.5, diffusion coefficient, 7.1 × 10-6 cm2/s; geometric area, 0.031 cm2.

Comparison of these experimental CVs (solid line) with those generated from digital simulations (dashed line) is shown in Figure 7. Figure 7a shows the experimental CV (solid line) recorded for case I, where both electrical and solution contacts are made through a uniform Au layer, and a digitally simulated CV (dashed line) based on a linear diffusive (1D) semiinfinite model using the values of rate constant k° ) 0.1 cm/s, E° ) -0.18 V, scan rate v ) 15 mV/s, electrode area A ) 0.03 cm2, electrons transferred n ) 1, charge-transfer coefficient R ) 0.5, and diffusion coefficient D ) 7.1 × 10-6 cm2/s (measured from the limiting current, iT,∞ ) 4nFDC*a, of a 1.0 mM Ru(NH3)6Cl3/ 0.1 M KCl solution at a 10 µm diameter gold inlaid disk electrode where the radius of the glass insulation compared to that of the Au disc was >10 and where C* ()1.0 mM) is the concentration of Ru(NH3)6Cl3 and a ()10 µm) is the diameter of a gold microelectrode). We found that k° values ranging from 0.1 to 10 cm/s resulted in negligible changes in the simulated CVs shown in Figure 7a. Figure 7b shows the experimental CV (solid line) for case IV, where both electrical and solution contacts are made through 30 nm diameter Au discs and the digitally simulated CV (dashed line) based on a linear diffusive (1D) semiinfinite model using the values of rate constant k° ) 0.01 cm/s, E° ) -0.20 V and all other parameters as in a. The different k° and E° values used in the simulations for reduction at the 30 nm diameter Au discs in the macro-NEE can be explained as follows. Templatesynthesized macro-Au NEEs belong to the class of partially blocked electrodes where the shape of the cyclic voltammogram compared to that at a bare electrode of similar geometric area is governed in a complicated way by the parameters θ (the fraction of surface that is blocked), v (sweep rate), k° (standard potential), and R0 (half the distance between the centers of two adjacent active sites of radius a). The dominating factors that come into play when recording CVs at these macro-NEEs are the current density and the magnitude of the total diffusion layer thickness ()(DRT/ 1480

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Fv)1/2 for cyclic voltammetry) compared to the radius of and distance between active sites. First, for a given total current, the current density at the active sites of a blocked electrode will be larger than that at a bare electrode. Because the effects of heterogeneous kinetics at an electrode surface are determined by the overpotential required for a given current, and overpotential depends upon the current density, the effects of heterogeneous electrontransfer kinetics may be larger at a partially covered electrode. Second, when individual sites behave as nanoelectrodes (NEs), and they are spaced at a distance such that their diffusion layers exceed the dimension of the active site but do not overlap, the CV will represent that of a collection of NEs and produce a steadystate voltammogram. However, when the diffusion layers surrounding the NEs completely overlap, the voltammogram will represent that of the geometric area of the macro-NEE and a transient CV will be recorded. For a given macro-NEE geometry, the diffusion layer thickness is determined by the sweep rate in a cyclic voltammetric experiment and, depending on the NE radius and spacing, a transition from a transient to a steady-state CV is possible. Due to this transition and the influence of kinetics, one might expect a shift in the voltammogram along the potential axis, relative to that recorded at a bare electrode, which is not entirely related to kinetics. Dimensionless constants for disk-type active sites which are useful in predicting the behavior of macro-NEEs are44

Kλ1/2 )

(1 - θ)1/2 DRT 0.6Ro Fv

( )

1/2

Λ(1 - θ) ) k°(1 - θ)(RT/DFv)1/2

(1) (2)

For Kλ1/2 . 1, peak-shaped transient CVs result with an apparent decrease in the heterogeneous rate constant compared to that for an unblocked electrode

kapp° ) k°(1 - θ)

(3)

If k° is sufficiently large and/or v is sufficiently small so that Λ(1 - θ) . 1, nernstian CVs are obtained. If k° is sufficiently small and/or v is sufficiently large so that Λ(1 - θ) , 1, the CVs become kinetically irreversible. Apparently the blocking of the electrode surface does not affect the peak current heights but will affect the peak-to-peak separation on transient CVs.44 For Kλ1/2 , 1, sigmoidal-shaped steady-state voltammograms are observed. The fraction of surface area that is blocked is also related to the active and geometric areas (Aact., Ageo), the fractional electrode area f, the radius of the nanopores ro, and the pore density p as follows:5

f ) Aact/Ageo ) πro2p

(4)

θ)1-f

(5)

kapp° ) fkο

(6)

so that

With a pore density p ) 6 × 108 cm-2 (provided by SPI) and ro ) 15 nm, f was calculated to be 4.2 × 10-3. From eq 6 and with kapp°

Figure 8. Formation and reduction of Au oxide layers on a macroAu NEE (s) and a polycrystalline Au disk electrode (--) of similar geometric areas (0.031 cm2) in 0.1 M sulfuric acid at a scan rate of 100 mV/s.

) 0.01 cm s-1 from the simulation of the CV shown in Figure 7b for a macro-Au NEE, a value of kο ) 2 cm s-1 is calculated as the electron-transfer rate constant of Ru(NH3)6Cl3 on a macro-Au NEE. This value is reasonable for a reversible electrochemical reaction and in agreement with that found from the range of kο values used in fitting CVs from the macro-Au surface-covered template and the Au polycrystalline electrode of similar geometric area. In a similar way, the E° used in simulating the CV in Figure 7b for the macro-NEE can be thought of as an apparent standard potential Eapp°. The comparisons of the experimental and simulated CVs of the macro-NEEs with those from a bare Au electrode of similar geometric area are in general agreement with a previously reported model44 and confirm that the Au-filled membranes were behaving electrochemically as predicted. Scanning electron microscopy (SEM) images (not shown) of case IV membranes show the presence of a distribution of Aufilled pores with a diameter of approximately 30 nm, in agreement with our previous observations.48 Figure 8 further demonstrates the presence of Au in the nanopores through voltammograms which show the formation of Au oxide and its reduction on the macromembrane NEEs in 0.1 M sulfuric acid over the potential range of -0.2 to +1.2 V vs Ag/AgCl at a scan rate of 100 mV/s (solid line). Comparison with that obtained at a polycrystalline Au electrode of similar geometric area (i.e., 2 mm diameter, dashed line) is also shown. In both cases, a sharp anodic wave at 1.1 V indicates the formation of Au oxides which are reduced between 0 and 0.4 V and at 0.52 V, respectively. The charging current response of these macromembrane NEEs was investigated by recording CVs in 0.1 M KCl solution over the potential range of -0.2 to +0.2 V at scan rates ranging from 10 to 200 mV/s, an example of which is shown as Figure 9. The charging currents at a macro-Au NEE and a polycrystalline Au disk electrode of corresponding geometric area were found to increase proportionally with scan rate, as expected for charging currents recorded under cyclic voltammetric conditions.53 The significantly smaller charging current at the macro-Au NEE compared to that at the macro-Au electrode is due to the smaller Aact. of the macro-Au NEE. This active area, based on eq 4, calculates to 1.33 × 10-4 cm2, while the geometric area Ageo, defined by the open circular area of the masking tape, is 0.0314 cm2. In contrast, Aact. and Ageo of the macro-polycrystalline Au disk are 0.0314 cm2. (53) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; Chapter 6, pp 233-234.

Figure 9. Comparison of the charging currents at a macro-Au NEE (s) and a polycrystalline gold disk electrode (--) of similar geometric areas of 0.031 cm2 in 0.1 M KCl at a scan rate of 40 mV/s.

Collectively, these methods of characterizing macro-Au NEEs indicate that their faradaic behavior is similar to a macropolycrystalline Au electrode of similar geometric area for an electrochemically reversible system, whereas the charging currents of macro-Au NEEs are comparatively much smaller, in agreement with reported results.9 Our success rate in fabricating Au-filled membranes with similar characteristics was 90%. Response of an Addressable NE Membrane Array. A Aufilled membrane with the surface Au layer removed from the dull side and remaining on the shiny side of the membrane was secured to the UME array platform as described in the previous section. When the membrane was immersed in a 1.0 mM Ru(NH3)6Cl3/0.1 M KCl solution, we initially looked for a transient CV response at each of the 25 connections on the ANEMA when the potential was scanned linearly at a scan rate of 15 mV/s. Figure 10 shows these transient CVs, all of which are peak-shaped with similar peak potentials and peak currents due to the fact that each copper connection contacts the same macro-Au surface area through the nanowires of the membrane. For convenience of plotting, the results for the first 24 connections only are shown; the result for the 25th connection is similar. These results indicated good electrical contact between the copper-filled UMEs of the UME array platform with corresponding microregions of the Au nanowires of the NE membrane. Once achieved, the uniform Au layer on the surface of the NE membrane was carefully removed as described earlier. Figure 11 shows the steady-state voltammograms (SSVs) recorded at the micro-NEEs on an ANEMA after removing the surface Au layer and exposing microregions of Au nanodiscs to a solution of 1.0 mM Ru(NH3)6Cl3/0.1 M KCl and a potential scan rate of 5 mV s-1. All of the SSVs have the retraceable sigmoidal shape characteristic of a steady-state voltammogram. For convenience of plotting, the results for the first 24 connections only are shown; the result for the 25th connection is similar. A half-wave height potential of -0.2 V vs Ag/AgCl and limiting currents of 1.3 ( 0.2 nA (determined by doubling the current at the halfwave height potential) were measured for the SSVs recorded on the ANEMA. The sigmoidal-shaped voltammograms shown in Figure 11 recorded at the ANEMA indicate that radial diffusion of Ru(NH3)6Cl3 toward each micro-Au NEE is the primary mode of mass transport. To the best of our knowledge, this is the first report on the steady-state behavior of NEEs achieved in this way. Each micro-Au NEE is in contact with a 10 µm diameter Cu conductor below the membrane. Thus Ageo of each micro-NEE corresponds to πa2 or 7.9 × 10-7 cm2, where a is the radius of the underlying Cu conductor. A steady-state limiting current Analytical Chemistry, Vol. 79, No. 4, February 15, 2007

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Figure 10. Cyclic voltammograms recorded on an ANEMA with a uniform Au film of area 0.03 cm2 exposed to a 1.0 mM Ru(NH3)6Cl3/ 0.1 M KCl solution. Scan rate ) 15 mV/s; quiet time, 30 s. All plots have the same potential and current scales.

equation for well-separated nanodiscs so that there is no overlap of radial diffusion layers has been reported as9

iss ) 4nFDC*roAgp

(7)

where ro is the radius (15 nm) of each nanodisc on the membrane, p is the membrane density (6 × 108 cm-2) of pores, and all other variables have their usual significance. The current given by eq 7 is essentially a summation of the steady-state currents generated by each nanoelectrode in a given geometric area. Using the parameters in this work (n ) 1, D ) 7.1 × 10-6 cm2/s), the steadystate current from eq 7 calculates to a value of 1.9 nA, which is in reasonable agreement with the experimentally determined value of 1.3 ( 0.2 nA. However, from the product of the membrane density (p ) 6 × 108 cm-2) and the geometric area, the calculated number of nanodiscs in this geometric area is 471. These nanodiscs are randomly distributed in the polycarbonate membrane. An expression for the inter-nanoelectrode distances in such a random array has been reported to be

d ) 0.5p 1482

-1/2

Analytical Chemistry, Vol. 79, No. 4, February 15, 2007

(8)

Figure 11. Steady-state voltammograms recorded on an ANEMA in a solution of 1.0 mM Ru(NH3)6Cl3/0.1 M KCl. Scan rate, 5 mV/s; quiet time, 60 s. All plots have the same potential ranges and current scales.

from the nearest-neighbor distribution46 and calculates to an average distance between nearest nanodiscs of 0.2 µm. Considering an ordered array with either a square or hexagonal lattice, the inter-nanodisc spacing is approximately twice that for the random array.46 The distance that a diffusing species travels in time t is given as approximately ∆ ) 2(Dt)1/2, 54 and calculates to 0.05 cm for the conditions used in recording the steady-state voltammograms shown in Figure 11. Thus, the hemispherical diffusion layers surrounding the individual nanodiscs overlap and each of the 25 micro-NEEs can be considered to be in the total overlap diffusion regime. In such a situation, the current responds to the geometric electrode area of the micro-NEE rather than the active electrode area. Thus, the magnitude of the steady-state current for each micro-NEE reported here is expected to be closer to that predicted for a UME

iss ) 4nFDCa

(9)

which calculates to 1.4 nA for a 10 µm diameter UME and is in better agreement with the experimentally determined value of 1.3 ( 0.2 nA for each of the 25 micro-NEEs on the ANEMA. (54) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; Chapter 4, p. 147; Chapter 5, p 164.

Figure 12. Experimental (s) and digitally simulated (--) SSVs recorded at a (a) 10 µm diameter Au disk UME and (b) 10 µm diameter NEE on an Au ANEMA in a solution of 1.0 mM Ru(NH3)6Cl3/0.1 M KCl and at a scan rate of 5 mV/s. The value of the rate constant in simulation a was 0.3 cm/s and in simulation b, 0.05 cm/s. Other simulation parameters include the following: E°, -0.2 V; scan rate, 5 mV/s; electrode radius, 5 µm; electrons transferred, 1; chargetransfer coefficient, 0.5; diffusion coefficient, 7.1 × 10-6 cm2/s.

The steady-state behavior of Ru(NH3)6Cl3 on each micro-Au NEE on the ANEMA was digitally simulated using the DigiElch software package17 and specifying radial (2D) semiinfinite diffusion on a disk UME. Figure 12 compares experimental (solid lines) and simulated (dashed lines) SSVs for a 10 µm diameter Au disk UME (a) and a micro-NEE on the ANEMA (b). The lower limit of the rate constant k° on the Au disk UME was found to be 0.3 cm/s, and this simulated voltammogram was found to be indistinguishable from that with k° as large as 10 cm/s. In comparison, an apparent rate constant kapp° of 0.05 cm/s was found for a micro-Au NEE of the ANEMA. This kapp°, from eq 6, corresponds to k° )12 cm/s, in reasonable agreement with that found for the Au disk UME and verifying that the redox reaction is reversible on the Au NEE operating in a steady-state mode. Changing the membrane on the ANEMA resulted in similar limiting currents, half-wave potentials, and simulation fittings. Stripping and re-deposition of the Cu on the UME array platform also had no effect on the features of the steady-state waves. In contrast to the macro-NEE arrays, in fabricating these ANEMAs, we found that consistent electrical contact between the underlying Cu and the Au membrane array was achieved when the contact was made through the dull side of the membrane. We also had the best results in making electrical contact to these membranes when the surface Au layer was left on the shiny side of the membrane and removed from the dull side of the membrane before attachment to the UME platform array. To avoid leakage currents between neighboring micro-NEEs on the ANEMA, it was necessary to pay close attention to the soldering of the ribbon cable to the pin connectors that were used in connecting the multiplexer unit to the UME array platform. It was also necessary to wrap the main body of the UME array platform in Teflon tape, a good insulator, to prevent leakage currents on the UME array platform itself. These leakage currents shifted the steady-state currents of neighboring micro-NEEs on the ANEMA by equal magnitudes positive and negative of zero current, without chang-

Figure 13. Steady-state voltammograms of 1.0 mM FcMeOH/0.1 M KCl at a (a) single 10 µm diameter NEE on an ANEMA, (b) single 10 µm diameter recessed UME on a bare UME array platform, and (c) 10 µm diameter Au disk electrode at scan rates of 5 (black solid lines), 20 (blue dashed lines), 40 (red dotted lines), and 80 mV/s (brown solid lines).

ing the shape of the recorded SSV. Using the fabrication method described for the ANEMA and paying close attention to the cable connections from the multiplexer unit, we had a consistent 80% success rate in fabricating ANEMAs with the responses shown in Figures 10 and 11. Figure 11 shows the steady-state response for the one-electron reversible reduction of Ru(NH3)6Cl3 at a sweep rate of 5 mV/s. We also looked at the one-electron reversible oxidation of ferrocene methanol over the potential range of 0-0.4 V vs Ag/ AgCl at several sweep rates (Figure 13). Figure 13a shows the SSVs at one micro-NEE on the ANEMA, which is representative of the 25 micro-NEEs available on the array at increasing sweep rates of 5, 20, 40, and 80 mV/s. A charging current contribution to the faradaic current, which is characterized by jumps in the Analytical Chemistry, Vol. 79, No. 4, February 15, 2007

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total current at the initial and reversal potentials, was found to be significant at sweep rates as low as 20 mV/s and to increase linearly with sweep rate as expected for charging currents.53,55 Similar behavior in the current was found for the recessed Pt discs on the bare UME array platform, as shown in Figure 13b and is representative of those found across the entire array. This indicates that there is a large capacitance associated with the UME array platform itself, which can be attributed to the large area Pt addressing pads leading from the recessed UME discs and covered by a 200 nm layer of silicon nitride (Si3N4)48 having a dielectric constant of 6-8.56 In comparison, Figure 13c shows the steady-state currents for a 10 µm diameter Pt disc UME recorded with the same potential sweep rates. There are no obvious charging current contributions at any of the sweep rates, and the observable change in the faradaic steady-state current as sweep rate increases is due to the change in diffusion from pure radial diffusion to a mixed radial-linear diffusion.57,58 The NE membrane and the securing tape can be removed from the UME array platform by wetting sufficiently with MeOH and pulling carefully on the clear tape until it releases from the platform surface. Another NE membrane can then be attached as described above. The copper deposited in the recessed Pt UME discs on the UME array platform can be easily stripped potentiostatically (at 0.3 V for 30 min) in an acidic solution (0.6 M H2SO4), and fresh copper can be re-deposited into the recessed Pt UME discs. However, to date, we have found the copper deposition to be very stable, withstanding the removal and replacement of many NE membranes. Additionally, the NE membrane itself, when secured to the UME array platform, is also very stable and can be used for several days. (55) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; Chapter 1, pp 16-18. (56) Siliconfareast.com (http://www.siliconfareast.com/sio2si3n4.html); TimeDomain CVD Inc. (http://timedomaincvd.com/CVD_Fundamentals/ films/ SiN_properties.html). (57) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; Chapter 6, p 232. (58) Zoski, C. G.; Bond, A. M.; Colyer, C. L.; Myland, J. C.; Oldham, K. B. J. Electroanal. Chem. 1989, 263, 1-21.

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CONCLUSIONS Microregions of a macro-nanoelectrode (NE) membrane were individually addressed by securing a gold-filled nanoporous tracketched polycarbonate membrane to a lithographically fabricated addressable Pt UME array platform. Electrical contact between the macro-NE membrane and the UME array platform was established through the potentiostatic electrodeposition of copper from an acidic solution into each of the 25 recessed 10 µm diameter Pt UMEs on the addressable Pt UME array platform. Sigmoidal-shaped steady-state voltammograms (SSVs) for the oneelectron reversible reduction of Ru(NH3)63+ and the one-electron reversible oxidation of ferrocene methanol were recorded on the addressable nanoelectrode membrane arrays (ANEMAs). The steady-state limiting currents indicate that each individually addressed micro-nanoelectrode ensemble (NEE) operates in a total overlap radial diffusion mode so that the limiting steady-state current is given by that for a UME of comparable geometric area. A multiplexing unit coupled to a multichannel potentiostat was designed and used in performing the Cu deposition and in recording the SSVs at the 25 individually addressable micro-NEEs on the macro-NE membrane. These addressable nanoelectrode membrane arrays (ANEMAs) permit the multiuse and reproducible use of lithographically fabricated UME arrays, which are not easily fabricated, by interchanging metal-filled macro-NE membranes, which are comparatively easier to fabricate. Present studies which are focused on catalytic, biological, and fundamental applications of these novel ANEMAs will be reported in forthcoming publications. ACKNOWLEDGMENT The financial support of the National Science Foundation (Grants CHE-0210315, EPS-0447691/NMSU Costshare, and ADVANCE-0123690 to C.G.Z.) and the Internet accessible digital simulation software package from Digi-Elch are greatfully acknowledged. Received for review December 8, 2006. AC0619534

October

17,

2006.

Accepted