Fabrication of 3D Gold Nanoelectrode Ensembles by Chemical Etching

Anal. Chem. 2005, 77, 5068-5071

Fabrication of 3D Gold Nanoelectrode Ensembles by Chemical Etching Kothandam Krishnamoorthy and Cynthia G. Zoski*,†

Department of Chemistry, Georgia State University, P.O. Box 4098, Atlanta, Georgia 30302-4098

A simple chemical etching procedure based on the solubility of polycarbonate membranes in solvent mixtures is reported for fabricating 3D gold nanoelectrode ensembles. A solvent ratio of 50:50 dichloromethane/ ethanol was found to be optimum for selective controlled etching of the surface layers of the polycarbonate membranes to expose up to 200-nm lengths of gold nanowires. The absence of double layer charging current in cyclic voltammograms of the resulting 3D nanoelectrode ensemble verified that the seal between the gold nanowires and the polycarbonate membrane was not compromised as a result of the chemical etching. Recently, there has been intense interest in the fabrication of three-dimensional (3D) nanostructured arrays for broad applications ranging from the development of microbatteries1 and DNA sensors2 to platforms for single-particle deposition.3 Methods of fabrication depend on the geometry of the active 3D site and have included microfabrication,4 breath figures on conducting polymers,5 electrochemical synthesis of conducting polymers in the presence of surfactant,6conducting polymer hydrogels,7 chemical vapor deposition,8 and plasma etching of metal-filled membranes9 and diamond films.10 The most general method for fabricating 3D nanostructured ensembles of wires or tubes is template synthesis in which metals are grown within the cylindrical and monodisperse pores of a nanoporous membrane by electroless deposition. For example, metal cylindrical nanotubes,11 conical nanotubes,12 and nanowires13 have been synthesized using porous templates such * To whom correspondence should be addressed. E-mail: [email protected]. † Present address: Department of Chemistry and Biochemistry, New Mexico State University, P.O. Box 30001, MSC 3C, Las Cruces, NM 88003-8001. (1) Long, J. W.; Dunn, B.; Rolison, D. R.; White, H. S. Chem. Rev. 2004, 104, 4463. (2) 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. (3) Sapp, S. A.; Mitchell, D. T.; Martin, C. R. Chem. Mater. 1999, 11, 1183. (4) Isik, S.; Berdondini, L.; Oni, J.; Blochl, A.; Koudelka-Hep, M.; Schuhmann, W. Biosens. Bioelectron. 2005, 20, 1566. (5) Erdogan, B.; Song, L.; Wilson, J. N.; Park, J. O.; Srinivasarao, M.; Bunz, U. H. F. J. Am. Chem. Soc. 2004, 126, 3678. (6) Qu, L.; Shi, G.; Chen, F.; Zhang, J. Macromolecules 2003, 36, 1063. (7) Ghosh, S.; Inganas, O. Adv. Mater. 1999, 11, 1214. (8) Correa-Duarte, M. A.; Wagner, N.; Rojas-Chapana, J.; Morsczeck, C.; Thie, M.; Giersig, M. Nano Lett. 2004, 4, 2233. (9) Yu, S.; Li, N.; Wharton, J.; Martin, C. R. Nano Lett. 2003, 3, 815. (10) Honda, K.; Rao, T. N.; Tryk, D. A.; Fujishima, A.; Watanabe, M.; Yasui, K.; Masuda, H. J. Electrochem. Soc. 2000, 147, 659. (11) Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268, 700. (12) Harrel, C. C.; Kohli, P.; Siwy, Z.; Martin, C. R. J. Am. Chem. Soc. 2004, 126, 15646. (13) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920.

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as polycarbonate,14 polyester,15 and alumina membranes.16 Gold has most often been used in electroless deposition, whereas metals such as platinum,17 gold,18,19 and metal-semiconductor Au-Te,20 Au-Sn-Au21 have been deposited using electrodeposition procedures. The diameter and length of the nanostructure are adjusted by varying the pore diameter and thickness of the membrane, respectively.22,23 The metal-filled membrane is then etched by exposing the membranes to O2 plasma such that the polymer at the membrane surface is selectively removed, and the ends of the nanowires or nanotubes are exposed revealing a 3D nanostructure.9 The length of the exposed nanowire or nanotube is controlled by varying the plasma etch time. The resulting nanoelectrode ensembles (NEEs) with protruding metal nanowires have been referred to as nano wheat fields.9 Martin and co-workers initiated the fabrication of 3D NEEs using gold electroless deposition in polycarbonate membranes followed by O2 plasma etching.9 A drawback to their method is that voids are created around the base of the gold nanowires, which become larger as etching time is increased. Kelley and co-workers refined Martin’s fabrication strategy, reporting significantly lower detection limits of 3D over 2D NEEs in developing a DNA-specific biosensor.2 It is clear that development of fabrication methods of 3D NEEs that do not rely on costly instrumentation would significantly facilitate further investigation and rapid production of 3D nanostructured devices. We report here a simple method of fabricating 3D NEEs using Au-filled polycarbonate membranes. Instead of an etching procedure involving O2 plasma, we rely on a chemical procedure that takes advantage of the solubility of the polycarbonate membranes in different solvents. Chemical procedures have been used for total dissolution of membranes in order to release nanowires or nanotubes grown within the membrane. Anopore membranes have been dissolved either totally or partially by exposure to NaOH24,25 or a mixture of phosphoric acid and (14) Moretto, L. M.; Pepe, N.; Ugo, P. Talanta 2004, 62, 1055. (15) Hulteen, J. C.; Menon, V. P.; Martin, C. R. Faraday Trans. 1996, 92, 4029. (16) Kohli, P.; Wharton, J. E.; Braide, O.; Martin, C. R. J. NanoSci. Nanotechnol. 2004, 4, 605. (17) Penner, R. M.; Martin, C. R. Anal. Chem. 1987, 59, 2625. (18) Wang, J.; Tian, M.; Mallouk, T. E.; Chan, M. H. W. J. Phys. Chem. B 2004, 108, 841. (19) Kautek, W.; Reetz, S.; Pentzien, S. Electrochim. Acta 1995, 40, 1461. (20) Ku, J. R.; Vidu, R.; Talroze, R.; Stroeve, P. J. Am. Chem. Soc. 2004, 126, 15022. (21) Wang, J. G.; Tian, M. L.; Mallouk, T. E.; Chan, M. H. W. Nano Lett. 2004, 4, 1313. (22) Sukeerthi, S.; Contractor, A. Q. Anal. Chem. 1999, 71, 2231. (23) Delvaux, M.; Walcarius, A.; Demoustier-Champagne, S. Biosens. Bioelectron. 2005, 20, 1587. (24) Liang, Y.; Zhen, C.; Zou, D.; Xu, D. J. Am. Chem. Soc. 2004, 126, 16338. 10.1021/ac050604r CCC: $30.25

© 2005 American Chemical Society Published on Web 06/17/2005

chromic acid in order to fabricate 3D carbon nanotube arrays.26-29 Polycarbonate membranes were dissolved completely either in chloroform or in dicholoromethane.30,31 Polycarbonate membranes have several advantages over alumina membranes including flexibility, easy and controllable electroless gold deposition,13,14 and the possibility of making many devices by cutting small pieces out of a full membrane.32 EXPERIMENTAL SECTION Instrumentation. SEM images of gold-filled etched and unetched polycarbonate membranes were obtained using a LEO 1530 scanning electron microscope. Cyclic voltammograms were recorded using a CHI 660A electrochemical work station with Ag/ AgCl reference and Pt counter electrodes. Reagents. Trifluoroacetic acid (Across Organics), nitric acid (Across Organics), tin(II) chloride (Across Organics),sodium sulfite (Across Organics), sulfuric acid (Fisher), methanol (Fisher), formaldehyde (Fisher), and sodium bicarbonate (J.T. Baker) were of analytical grade and used as received. SPI polycarbonate membranes with a density of 6 × 108 pores/cm2 were used. Oromerse Part B gold solution (0.4 troy ounce/200 mL) was purchased from Technic Inc. and used as received. Electroless Deposition of Gold Nanowires. Gold nanowires were grown inside polycarbonate membranes by modifying the procedure reported by Martin and co-workers.12,33-37 A gold plating solution was prepared by dissolving Na2SO3 (3.2014 g, 0.127 M), NaHCO3 (0.42005 g, 0.025 M), and 10 mL of HCHO (0.625 M) in 180 mL of water. The pH of this solution was adjusted to 10 by adding 1.8 M H2SO4 dropwise. The volume was adjusted to 200 mL by adding water. Twenty milliliters of this solution was pipetted and mixed with 0.5 mL of Oromerse Part B gold solution. The pH of the solution was again adjusted to 10 by the dropwise addition of 1.8 M H2SO4 (∼0.1 mL). The sample vial was kept at 5 °C in a refrigerator while the polycarbonate membrane was prepared for electroless deposition of gold. A polycarbonate membrane (diameter 13 mm) with 30-nm pores was immersed in a solution of SnCl2 and trifluoroacetic acid for 45 min. This solution was prepared by dissolving SnCl2 (0.2464 g, 0.026 M) and 0.3 mL of CF3COOH (TFA) in a 50-mL standard measuring flask using a mixture of methanol and water (50:50). In the meantime, an ammonical AgNO3 solution was prepared by titrating 50 mL of 0.029 M AgNO3 with concentrated ammonium hydroxide solution. The solution turned brown when one or two drops of ammonia were added and became colorless upon further addition of ammonia (∼0.5 mL). The membrane was removed from the (25) Klein, J. D.; Herrick, R. D.; Palmer, D.; Sailor, M. J.; Brumlik, C. J.; Martin, C. R. Chem. Mater. 1993, 5, 902. (26) Kang, D. W.; Suh, J. S. J. Appl. Phys. 2004, 96, 5234. (27) Yoon, S. M.; Chae, J.; Suh, J. S. Appl. Phys. Lett. 2004, 84, 825. (28) Suh, J. S.; Lee, J. S. Appl. Phys. Lett. 1999, 75, 2047. (29) Lee, J. S.; Suh, J. S. J. Appl. Phys. 2002, 92, 7519. (30) Menon, V. P.; Lei, J.; Martin, C. R. Chem. Mater. 1996, 8, 2382. (31) Duvail, J. L.; Retho, P.; Fernandez, V.; Louarn, G.; Molinie, P.; Chauvet, O. J. Phys. Chem. B 2004, 108, 18552. (32) Krishnamoorthy, K.; Gokhale, R. S.; Contractor, A. Q.; Kumar, A. Chem. Commun. 2004, 820. (33) Brumlik, C. J.; Menon, V. P.; Martin, C. R. J. Mater. Res. 1994, 9, 1174. (34) Delvaux, M.; Walcarius, A.; Demoustier-Champagne, S. Biosens. Bioelectron. 2005, 20, 1587. (35) Hou, Z.; Abbot, N. L.; Stroeve, P. Langmuir 2000, 16, 2401. (36) Hou, Z.; Abbot, N. L.; Stroeve, P. Langmuir 1998, 14, 3287. (37) Jirage, K. B.; Hulteen, J. C.; Martin, C. R. 1999, 71, 4913.

SnCl2/TFA solution, washed with methanol for 10 min, and then immersed in the ammoniacal AgNO3 solution for 10 min. The membrane was again washed with methanol thoroughly to remove the excess AgNO3. The membrane was hung into the gold plating solution vertically using a clip. This resulted in even formation of gold on both sides of the membrane. The gold-filled membrane was taken out after 24 h, and the gold on both faces of the membrane was gently removed using Q-Tips wetted with ethanol. Using this procedure to remove the surface layer of gold from the membrane, we observed that the gold wires are not pulled out from the pores. This procedure differs from that reported in previous work where Scotch tape was used to remove the gold,13 which resulted in the removal of gold wires from the pores.3,9 It was also crucial that the pH of the plating solution be maintained at 10 so that the gold preferentially deposited into the pores rather than on the surface of the membrane. This pH led to the formation of gold layers, which were easily removed with an ethanol soaked Q-Tip; other pH values were found to produce gold layers that were significantly difficult to remove in this way. The membranes were then immersed in 25% HNO3 for 12 h to remove the surfacebound chemicals from the gold plating solution. Finally, the membrane was heated at 150 °C, the glass transition temperature of polycarbonate, for 10 min. This resulted in sealing of the polycarbonate membrane around the gold nanowires as has been reported previously.13 Etching Procedure. Etching of the polycarbonate membrane (PCM) was carried out by using a mixture of dichloromethane (DCM) and ethanol (EtOH). The PCM was found to be highly soluble in chloro solvents such as tetrachloromethane, chloroform, and dichloromethane and insoluble in methanol, EtOH, and ethyl acetate. We found that by mixing a solvent that dissolves a PCM with a solvent in which the PCM is insoluble allows the PCM to be etched, ultimately producing Au nano wheat fields. We chose DCM and EtOH because of their good miscibility. The PCM is soluble in DCM and insoluble in EtOH. A solvent mixture of 50: 50 DCM and EtOH was prepared. The etching was carried out by gently wiping the surface of the PC membrane using a Q-Tip wetted in the solvent mixture. The Q-Tip was shaken in air to remove the excess solvent mixture. If the Q-Tip is saturated, the solvent mixture spreads on the membrane so that the etching cannot be controlled. Controlled etching also depends on the state of the membrane after consecutive wipes. For example, in the case of 25:75 DCM and ethanol, the membrane shrinks after 50 wipes. In contrast, the membrane started curling after ∼12 wipes in 50:50 DCM/EtOH. Therefore, 10 and 50 wipes were chosen for 50:50 and 25:75 DCM/EtOH, respectively. To perform the etching, a square was cut from the PCM and secured at one end on a glass slide with a copper tape. Copper tape was preferred over Scotch tape which dissolves, detaches from the glass slide during wiping, but stays attached to the PCM. Nano wheat fields were created by wiping the surface 10 times with Q-Tips wetted with the solvent mixture. The SEM of this membrane shows that the Au nanowires protrude from the pores due to removal of polycarbonate layers. However, voids are created on the surface of PC membrane as a result of solvent evaporation from the surface. To avoid this void formation, a dry Q-Tip was used and the solvent was wiped immediately from the surface. Analytical Chemistry, Vol. 77, No. 15, August 1, 2005

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Nanoelectrode Ensembles. Nanoelectrode ensembles (NEEs) were prepared using two types of membranes. In one, gold was removed from both faces of the Au filled PCM. The etching was done on one side of the membrane. The unetched side was adhered to a strip of adhesive conducting copper tape. With a second Au-filled PCM, the gold from only one face of the PCM was removed and the etching was carried out on the same side. This leaves a gold layer on one side of the membrane, and this side was adhered to a strip of adhesive conducting copper tape. With both PCMs, the etched side is the upper side, which was exposed to analyte. To define the area of the NEE exposed to solution, a method developed in Martin’s laboratory was used.13 A 0.031-cm2 hole was punched in a piece of cello tape (Office Depot) and then pasted onto the membrane so that the exposed copper tape was also covered completely on both sides to prevent exposure to the analyte solution. RESULTS AND DISCUSSION In principle, the polycarbonate surface should be removable by a solvent in which the membrane dissolves. It is well known that polycarbonate dissolves in solvents such as chloroform and dichloromethane.30,31 Therefore, we first attempted to remove surface layers from a gold-filled polycarbonate membrane using dichloromethane. Immediately upon exposure to the solvent, the polycarbonate membrane dissolved completely. Therefore, it is not possible to etch the polycarbonate using dichloromethane alone. Polycarbonate membranes are also insoluble in solvents such as ethanol, methanol, acetonitrile, and ethyl acetate.38 Thus, by mixing a dissolving with a nondissolving solvent, we found that it was possible to selectively etch the surface layers of Aufilled polycarbonate membranes. For example, a mixture of DCM and EtOH in a solvent ratio of 50:50 resulted in controlled etching of the surface layers of Au-filled polycarbonate membranes. Preparation of gold nanowires in polycarbonate membranes and details of the chemical etching method are described in the Experimental Section. Briefly, the surface of the polycarbonate membrane was wiped with a Q-Tip dipped in the solvent mixture. After this solvent wipe, the solvent was allowed to dry and followed by another solvent wipe; the same procedure was repeated 10 times. The SEM of this membrane revealed Au nanowires protruding from the polycarbonate membrane. However, the slow evaporation of solvent after each solvent wipe left undesired voids on the polycarbonate surface (Figure 1). Such voids could be eliminated by wiping the polycarbonate membrane with a dry Q-Tip immediately after exposure to the solvent mixture. Thus, the surface of the Au-filled polycarbonate membrane was solvent wiped with a Q-Tip dipped in 50:50 DCM/EtOH mixture, followed immediately by a dry wipe of the surface with a dry Q-Tip. The SEM picture (Figure 2) of this membrane shows the absence of voids on the surface, indicating that this method efficiently produces 3D NEEs with protruding Au wires of ∼200 nm in length. The etching rate was reproducibly controlled by restricting the quantity of solvent mixture applied to the membrane surface, as described in the Experimental Section. An experimental density of 320 ( 20 filled pores (unetched membrane) or nanowires in (38) The solubility of polycarbonate membrane was tested in ethanol, methanol, acetonitrile, and ethyl acetate by taking a small piece of membrane and immersing it in the solvents for 24 h.

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Figure 1. SEM image of etched Au-filled polycarbonate using a 50:50 DCM/EtOH mixture.

Figure 2. SEM image of 3D NEEs created using a 50:50 DCM/ EtOH mixture applied to a Au-filled polycarbonate membrane (30nm-diameter pores). The solvent was removed immediately by wiping with a Q-Tip.

54-µm2 areas of either unetched or etched membranes is in excellent agreement with a reported pore density of 6 × 108 cm-2. To investigate the role of solvent ratios in producing 3D NEEs, ratios of 25:75, 50:50, and 75:25 DCM/EtOH were selected. For 25:75 DCM/EtOH, 50 solvent/dry wipes were required to produce protruding Au nanowires from the surface of the polycarbonate membrane (Figure 3). In comparison, nanowires protruded after 10 solvent/dry wipes with a 50:50 DCM/EtOH mixture (Figure 2). Only five solvent/dry wipes were necessary to expose the gold nanowires with a 75:25 DCM/EtOH mixture; however, there were a significant number of voids on the polycarbonate membrane surface as shown in the SEM picture (Figure 4). Additionally, the polycarbonate dissolved so rapidly in the solvent mixture that it was difficult to efficiently remove from the surface by dry wipe, and small lumps of polycarbonate remained on the surface, which are apparent in Figure 4. We also observed that the polycarbonate membranes began to shrink immediately after the first solvent wipe, indicating that this 75:25 DCM/EtOH solvent ratio is not suitable for etching. Of the three solvent mixtures, a 50:50 DCM/ EtOH mixture was found to be optimum for fabrication of 3D NEEs.

Figure 5. Cyclic voltammograms of a 3D NEE (solid line) and 2D NEE (b) in 10 mM ferrocenemethanol and 0.5 M KCl. Scan rate 10 mV/s. Cyclic voltammograms of 3D NEE (2) and 2D NEE (-) in 0.5 M KCl at a scan rate of 10 mV/s. Figure 3. SEM image of etched Au-filled PCM using 25:75 DCM/ EtOH mixture. The solvent was removed immediately using a dry Q-Tip.

Figure 4. SEM image of 3D NEEs created using a 75:25 DCM/ EtOH mixture applied to a Au-filled polycarbonate membrane (30nm-diameter pores). The solvent was removed immediately by wiping with a Q-Tip.

To record cyclic voltammograms (CVs) of these 3D NEEs, the unetched side of the membrane was adhered to a piece of adhesive conducting copper tape and the entire assembly insulated with cellophane tape. A 0.031-cm2-diameter hole (ensemble geometric area) was punched in the cellophane tape to allow exposure of the etched side to an aqueous solution of 10 mM ferrocenemethanol/0.5 M KCl. Cyclic voltammograms were recorded by cycling the potential of the working electrode (NEE) between 0 and 0.5 V versus Ag/AgCl. The absence of a double layer charging current in the CV of an etched membrane (Figure 5, solid line) is indicative of a good seal between the conducting Au nanowires and the polycarbonate membrane. Double layer charging was found to be absent at sweep rates ranging from 10 mV/s up to the largest sweep rate of 5000 V/s.

The larger peak currents of the etched membrane relative to a representative unetched membrane (Figure 5, dots) demonstrate that the area of each Au-filled pore in the membrane has increased in the case of the 3D NEE. A peak-shaped CV results due to the close spacing of the ∼2 × 107 Au-filled pores in the exposed geometric area so that the overlap of individual diffusion layers results in the creation of an apparent planar diffusion layer that extends over the entire NEE. Thus, the NEE behaves like a large electrode with a surface area equal to the total surface area of the ensemble, including the active and nonactive surface areas.13,39 CONCLUSIONS A simple chemical etching procedure has been developed for fabricating 3D NEEs. The procedure is based on the solubility of polycarbonate membranes in solvent mixtures. A solvent ratio of 50:50 DCM/EtOH was found to be optimum for selectively etching the surface layers of polycarbonate membranes, thus producing exposed 3D Au nanowires ∼200 nm in length. These 3D NEEs are free from voids on the polycarbonate surface and around the base of the nanowires, which were previously observed after O2 plasma etching. This simple chemical etching procedure is an interesting and viable alternative to the presently used O2 plasma etching and will enable the fabrication and design of 3D NEEs for a multitude of applications to be more readily accessible to those investigators without access to sophisticated microfabrication facilities. ACKNOWLEDGMENT Funding by the National Science Foundation (CHE-0210315) is gratefully acknowledged. Received for review April 10, 2005. Accepted May 17, 2005. AC050604R (39) Amatore, C. In Physical Electrochemistry; Rubenstein, I., Ed. Marcel Dekker: New York, 1995; pp 131-209.

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