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Free-Standing, Erect Ultrahigh-Aspect-Ratio Polymer Nanopillar and Nanotube Ensembles Guofang Chen, Steven A. Soper, and Robin L. McCarley* Department of Chemistry and Center for Biomodular Multi-Scale Systems, Louisiana State UniVersity, Baton Rouge, Louisiana 70803-1804 ReceiVed May 22, 2007. In Final Form: August 14, 2007 Free-standing polymer (poly(methyl methacrylate) or cyclic olefin copolymer) nanopillar and nanotube ensembles with previously unreported, ultrahigh aspect ratios (300 to >1600) were fabricated via anodic aluminum oxide (AAO) template-based methods that utilize a dilute, aqueous H3PO4 template etchant followed by freeze drying removal of the aqueous medium. Good replication of the AAO template by either solutions of the polymeric materials or molten polymer was achieved by using ultrasonic degassing and vacuum conditions. Classical surface wetting and viscoelastic fluid rheology theories were applied to explain the formation of polymer nanopillars and nanotubes in the aluminum oxide templates. The utilization of dilute H3PO4 for etching the AAO template and freeze-drying removal of the environmental liquid allows for the preparation of free-standing, erect, and ordered polymeric nanopillars or nanotubes that show much promise for use in biological microelectromechanical systems that target biological analyses.
Introduction Nanoscale structures patterned in periodic arrays or ensembles, such as nanoparticles, nanorods, nanopillars, nanowires, and nanotubes, have attracted extensive attention due to their novel size-dependent properties.1-9 Of this group, nanopillar and nanotube arrays are among the most studied because of their potential applications in photonic crystals, data storage, sensors, microfluidic devices, bioreactors, biomolecular separations, and biomimetic surfaces.10-14 The formation of nanometer-scale structures can be achieved by sol-gel routes, self-assembly, lithography, and replication methods, to name a few.10,15-21 * To whom correspondence should be addressed. Telephone: +1-225578-3239. Fax: +1-225-578-3458. E-mail:
[email protected]. (1) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078-1080. (2) Mieszawska, A. J.; Jalilian, R.; Sumanasekera, G. U.; Zamborini, F. P. J. Am. Chem. Soc. 2005, 127, 10822-10823. (3) Berdichevsky, Y.; Lo, Y.-H. AdV. Mater. 2006, 18, 122-125. (4) Lee, P.-S.; Lee, O.-J.; Hwang, S.-K.; Jung, S.-H.; Jee, S. E.; Lee, K.-H. Chem. Mater. 2005, 17, 6181-6185. (5) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N.; Nevanen, T. K.; Soederlund, H.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11864-11865. (6) Nicewarner-Pena, S. R.; Freeman, R. G.; Reiss, B. D.; He, L.; Pena, D. J.; Walton, I. D.; Cromer, R.; Keating, C. D.; Natan, M. J. Science 2001, 294, 137-141. (7) Dersch, R.; Steinhart, M.; Boudriot, U.; Greiner, A.; Wendorff, J. H. Polym. AdV. Technol. 2005, 16, 276-282. (8) Baker, L.; Jin, P.; Martin, C. Crit. ReV. Solid State Mater. Sci. 2005, 30, 183-205. (9) Xiang, H.; Shin, K.; Kim, T.; Moon, S. I.; McCarthy, T. J.; Russell, T. P. Macromolecules 2004, 37, 5660-5664. (10) Lee, W.; Jin, M.-K.; Yoo, W.-C.; Lee, J.-K. Langmuir 2004, 20, 76657669. (11) Kaji, N.; Tezuka, Y.; Takamura, Y.; Ueda, M.; Nishimoto, T.; Nakanishi, H.; Horiike, Y.; Baba, Y. Anal. Chem. 2004, 76, 15-22. (12) Heule, M.; Rezwan, K.; Cavalli, L.; Gauckler, L. J. AdV. Mater. 2003, 15, 1191-1194. (13) Cheek, B. J.; Steel, A. B.; Torres, M. P.; Yu, Y.-Y.; Yang, H. Anal. Chem. 2001, 73, 5777-5783. (14) Lellouche, J.-P.; Govindaraji, S.; Joseph, A.; Jang, J.; Lee, K. J. Chem. Commun. 2005, 4357-4359. (15) Kuo, C.-W.; Shiu, J.-Y.; Chen, P. Chem. Mater. 2003, 15, 2917-2920. (16) Lee, S. B.; Koepsel, R.; Stolz, D. B.; Warriner, H. E.; Russell, A. J. J. Am. Chem. Soc. 2004, 126, 13400-13405. (17) Gibson, J. M. Phys. Today 1997, 50, 56-61. (18) Kramer, N.; Birk, H.; Jorritsma, J.; Schonenberger, C. Appl. Phys. Lett. 1995, 66, 1325-1327. (19) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453-457. (20) Steinhart, M.; Wendorff, J. H.; Greiner, A.; Wehrspohn, R. B.; Nielsch, K.; Schilling, J.; Choi, J.; Go¨sele, U. Science 2002, 296, 1997.
Template-assisted replication approaches are cost-effective and allow for high-throughput fabrication of a variety of materials. In addition, template-assisted techniques possess the advantage of being able to readily create large-area, ordered nanostructures and, in some cases, vertically aligned structures having high aspect ratios.10,19,20,22 There are three critical issues for the successful fabrication of polymeric nanostructuresssuch as pillars and tubesswith high aspect ratios using techniques based on the filling of a nanochanneled substrate (nanotemplate) with a precursor material: the first is control over the type of architecture to be produced (nanopillar or nanotube), the second is facilitation of template filling by the precursor material (pillars/tubes that conform to the template’s contour), and the third is retention of physical integrity of the polymeric nanostructures after the removal of the template (noncollapsed/erect pillars or tubes). To date, precise control over the fabrication of polymeric nanotube or nanopillar ensembles still remains a challenge. In this work, using electrochemically produced anodic aluminum oxide (AAO) templates, we describe the unprecedented fabrication of free-standing, polymeric ultrahigh-aspect-ratio nanostructures (PUHARNs), namely, nanopillars and nanotubes composed of poly(methyl methacrylate) or cyclic olefin copolymer, with aspect ratios as high as 1670 (60 nm diameter and 100 µm height). Issues key to the successful production of the PUHARNs described here include control over filling of the AAO template with the polymer or polymer precursor and optimization of the conditions for freeing the PUHARNs from the template. In the latter case, which is crucial to the successful fabrication of polymeric nanopillars and nanotubes, we demonstrate that upright (noncollapsed or nonaggregated) nanostructures with ultrahigh aspect ratios can be obtained by manipulating solution conditions for template removal and solution pH, so as to minimize surface tension forces acting on the nanostructures. Control over filling of the AAO template results in the formation of nanopillars or nanotubes that completely follow the contour of the template, with the resulting structure (21) Steinhart, M.; Wehrspohn, R. B.; Go¨sele, U.; Wendorff, J. H. Angew. Chem., Int. Ed. 2004, 43, 1334-1344. (22) Johnson, S. A.; Khushalani, D.; Coombs, N.; Mallouk, T. E.; Ozin, G. A. J. Mater. Chem. 1998, 8, 13-14.
10.1021/la701502m CCC: $37.00 © 2007 American Chemical Society Published on Web 10/12/2007
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type being a function of the nature of the liquid introduced into the AAO template and its method of introduction. Nanopillars of poly(methyl methacrylate), PMMA, were fabricated using a polymerization pattern-transfer technique that has as key elements the capillary force-induced delivery of neat monomer into the nanochannels of the AAO template and subsequent rapid photopolymerization of the monomer. Nanotube ensembles of PMMA and cyclic olefin copolymer (COC) were prepared using pressure-driven AAO template wetting by polymer melts,20 which is similar in many ways to nanoimprint lithography.23 These nanostructures and their production are important to a variety of fields, particularly to that focusing on microanalysis devices made by embossing/molding routes that are used in the detection of disease state or for drug screening.24 In that vein, we envision these ultrahigh surface area nanostructures being used as scaffolds for enzymes and molecular recognition agents, so as to minimize the footprint of the enzymatic reactor or capture element region in a microfluidic device.25 Experimental Section Preparation of Polymer Nanotubes. A piece of 0.25-mm thick polymer sheet, PMMA (Goodfellow, Devon, PA), or Topas 6013D61 (COC, TOPAS Advanced Polymers GmbH, Florence, KY) was placed in contact with anodic aluminum oxide surfaces prepared as described below, and the assembly was then clamped together between two glass microscope slides using binding clips (ACCO, Linconshire, IL). The AAO template was prepared via a two-step anodization process,26 followed by removal of the Al support with saturated aqueous mercuric chloride for 5-24 h; subsequent dissolution of the oxide barrier layer with 6 wt % H3PO4 at 40 °C for 30 min yielded AAO templates possessing nanochannels with both ends open. The polymer was melted by heating at 230 °C for 10 min under vacuum (170 Pa) in a standard vacuum oven. After that, the polymer/AAO composite was cooled to room temperature and removed from the glass slide support by soaking the assembly in an ultrasonic water bath. The AAO template pattern was dissolved in 0.6 M H3PO4 solution at ambient temperature. The erect polymer nanotubes were obtained by washing with Nanopure water (Barnstead, 18 MΩ cm) followed by freeze-drying removal of the water using a Labconco FreezeZone freeze dry system. Fabrication of Polymeric Nanopillars. The AAO templates were prepared via a two-step anodization process26 and consisted of an Al substrate with an AAO layer with nanochannels open on one end. Methyl methacrylate monomer (Fisher Scientific, Pittsburgh, PA) containing 1% w/v benzoin methyl ether (Fisher Scientific, Pittsburgh, PA) and 1% w/v PMMA (Mw 93 300 and Mn 46 400; Scientific Polymer Products Inc., Ontario, NY; catalog #037Sb) was introduced into the nanochannels of the AAO templates by ultrasonication (Branson Ultrasonic Cleaner model 2510) of the template and methyl methacrylate/benzoin methyl ether/PMMA solution for 30 min. The sample was then immediately placed on a glass support either in the laboratory ambient atmosphere or under N2, and polymerization was achieved by a 30-min exposure to 254 nm light (15 mW/cm2, DUV Exposure System, ABM, Inc., San Jose, CA). Subsequently, the AAO template containing the PMMA structures was bonded to a 0.25-mm thick PMMA sheet with epoxy resin (ITW, Devcon, IL). The replication template consisting of Al and AAO was then removed from the assembly by use of saturated aqueous mercuric chloride followed by 0.6 M phosphoric acid. Finally, the PMMA replicas were carefully washed with 18 MΩ cm water, and the water was removed by freeze drying at -80 °C as described above. (23) Park, S.; Padeste, C.; Schift, H.; Gobrecht, J.; Scharf, T. AdV. Mater. 2005, 17, 1398-1401. (24) Soper, S. A.; Ford, S. M.; Qi, S.; McCarley, R. L.; Kelly, K.; Murphy, M. C. Anal. Chem. 2000, 72, 642A-651A. (25) Chen, G.; McCarley, R. L.; Soper, S. A.; Situma, C.; Bolivar, J. G. Chem. Mater. 2007, 19, 3855-3857. (26) Masuda, H.; Yada, K.; Osaka, A. Jpn. J. Appl. Phys., Part 2 1998, 37, L1340-L1342.
Chen et al. Characterization. The morphology of the PUHARNs was investigated with a Hitachi S-3600N extra-large chamber variablepressure scanning electron microscope (SEM) at an accelerating voltage of 15 kV. Samples were overcoated with Pd/Au before examination with the SEM.
Results and Discussion To fully exploit template-based synthesis of polymer nanostructures, it would be highly desirable to have the capability to predictively produce different classes of PUHARNs (nanopillar or nanotube ensembles) that are ordered and erect, using a common fabrication approach. To this end, we examined the results from introducing solutions (monomer or polymer melt) into the nanochannels of AAO templates under various conditions, which allowed for the fabrication of a variety of architectures in several polymeric materials. Our initial studies demonstrated that the observed lengths of PUHARNs are smaller than the length of the nanochannel of the AAO replication templates due to trapped gases inside the nanochannel, similar to what has been found for polystyrene melts.10 To remove trapped gases in the channels during filling of the nanotemplate for fabrication of nanopillars and nanotubes, we employed ultrasonic and high-vacuum degassing, respectively. These treatments result in a lack of back pressure when the liquid wets and flows into the channels. After the AAO was removed using aqueous NaOH and then air-dried (as described in reports on the fabrication of CdS nanowires,27 block copolymer nanotubes,9 and metallic nanotubes),28 we found that the ensembles of the polymeric highaspect-ratio nanopillars/nanotubes routinely collapsed into an entangled mesh or clusters due to the surface tension forces acting on the nanopillars/nanotubes during the evaporation of the rinse liquid. SEM images of such samples demonstrate that PMMA nanopillars collapsed (Figure 1A) and nanotubes congregated into 1-2 µm bundles clustered into disordered domains (Figure 1B). According to the lever principle,29 the force F acting on nanostructures in contact with an evaporating rinse liquid of given surface tension γ is proportional to the pillar/tube aspect ratio A, the inverse of interpillar spacing d, and the cosine of the contact angle θ of the rinse liquid at the nanopillar/ nanotube surface as follows: F ≈ 2Aγ cos θ/d. To suppress this effect, the surface tension must be reduced or the contact angle of the rinse liquid with respect to the nanostructures must be brought close to 90° (cos θ ) 0). We have discovered that the latter can be accomplished by simple freeze-drying removal at -80 °C of the liquid in contact with the PUHARNs instead of supercritical drying.27 However, we found that in order to obtain ordered and erect ultrahigh-aspect-ratio nanopillars, NaOH is not a good choice for the removal of AAO. In Figure 2, are shown SEM micrographs of PMMA nanopillars following complete removal of the template with NaOH or H3PO4. When NaOH (10 wt %) is used, the resulting nanopillars are severely deformed, leading to twisted bundles composed of multiple PMMA nanopillars (Figure 2A). Free-standing nanopillars can be produced with an aspect ratio of up to 1667 (100 µm height and 60 nm diameter) when H3PO4 is used to remove the template (Figure 2B). This can be attributed to the relatively hydrophobic surface of the PMMA nanopillars at low pH due to the presence of nonionized carboxylic acid (27) Liang, Y.; Zhen, C.; Zou, D.; Xu, D. J. Am. Chem. Soc. 2004, 126, 16338-16339. (28) Lee, W.; Scholz, R.; Nielsch, K.; Go¨sele, U. Angew. Chem., Int. Ed. 2005, 44, 6050-6054. (29) Tanaka, T.; Morigami, M.; Atoda, N. Jpn. J. Appl. Phys., Part 1 1993, 32, 6059-6064.
Polymer Nanopillar and Nanotube Ensembles
Figure 1. SEM images of PMMA structures prepared by natural drying: (A) nanopillars and (B) nanotubes. The template possessed channels that were 175 nm in diameter and 60 µm in depth (aspect ratio ) 343), with a 460 nm interchannel center-to-center spacing.
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Figure 2. SEM images of PMMA nanopillar ensembles after removal of the AAO template with (A) NaOH (2.5 mol L-1) or (B) H3PO4 (0.6 mol L-1). The template had channels that were 60 nm in diameter and 100 µm in depth (aspect ratio ) 1667), with a 100 nm interchannel spacing.
groups.30 During the process of polymerization of methyl methacrylate under 254 nm light, some photo-oxidation of the resulting PMMA occurs, leading to surface carboxylic groups. In addition, we have found that commercial polymer sheets have varying degrees of surface carboxylic acids present as the result of photoinduced oxidation during their exposure to ambient conditions.30 Due to the differences in the wettabilities of carboxylic acids and carboxylate ions,31 the contact angle of water on the surface is a function of solution pH. Thus, when NaOH is used to remove the AAO template, the carboxylic acids on the surfaces of PMMA nanopillars are completely ionized to carboxylate ions, and the magnitude of the force that exists between the water and the PMMA nanopillar surfaces (adhesive force) becomes larger due to the contact angle of water on the surfaces of the PMMA nanopillars being very low. On the other hand, effectively all of the surface carboxylic acid groups are protonated when H3PO4 is employed to remove the AAO replication template, and the contact angle of water on such PMMA nanopillar surfaces is much larger; thus, the force produced by surface tension between nanopillars is much weaker. Therefore, PMMA nanopillar arrays stand erect when dilute H3PO4 (instead of NaOH) is used for the removal of the AAO template. Through the use of dilute H3PO4 to remove the AAO template and freeze-drying of the environmental liquid, we have successfully suppressed the surface tension force and the interactions
between individual nanopillars/nanotubes for a variety of polymeric materials, such as PMMA and COC. Large-area, erect, and free-standing polymeric nanopillar/nanotube arrays can be routinely formed, allowing for the PUHARNs to maintain their hexagonally packed nature upon removal of the AAO template (see Figure 3). The lengths of the nanopillars/nanotubes are quite uniform and equal to the thickness of the original AAO template (within experimental error). For example, the dimensional parameters of the nanopillars in Figure 3 are as follows: 175 nm diameter, 285 nm edge-to-edge spacing, and ∼1 µm (see Figure 3A) and 60 µm (see Figure 3B) heights with the aspect ratios of 6 and 343, respectively. In Figure 3C are shown free-standing PMMA nanotubes with a wall thickness of 50 nm, a height of 60 µm, and an outer diameter of 232 nm that resulted from introducing polymer melts to the AAO templates. The latter dimension is larger than the channel diameter of the AAO template of 175 nm and can be attributed to the “die swell” property of viscoelastic polymers.32 Virtually identical results were obtained for Topas 6013D61, a cyclic olefin copolymer (COC), Figure 3D. The dimensions of the COC nanotubes were effectively the same as those of the PMMA nanotubes: 50 nm wall thickness, 60 µm height, and 232 nm outer diameter. The outcomes from these studies led us to pose the question: why are nanotubes formed when using either a polymer melt20 or a polymer solution (polymer dissolved in solvent),33 whereas nanopillars result when using a monomer solution (low-molecularweight fluids)34,35 followed by polymerization? To answer this
(30) Wei, S.; Vaidya, B.; Patel, A. B.; Soper, S. A.; McCarley, R. L. J. Phys. Chem. B 2005, 109, 16988-16996. (31) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370-1378.
(32) Chiu, W. Y.; Shyu, G. D. J. Appl. Polym. Sci. 1988, 35, 847-862. (33) Song, G.; She, X.; Fu, Z.; Li, J. J. Mater. Res. 2004, 19, 3324-3328. (34) Masuda, H.; Fukuda, K. Science 1995, 268, 1466-1468.
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Figure 3. SEM images of free-standing, erect polymer nanopillar and nanotube ensembles: 175-nm diameter PMMA nanopillars with aspect ratios of (A) 6 and (B) 343- and 232-nm diameter, 50-nm wall thickness nanotubes with an aspect ratio of 259 made from (C) PMMA and (D) COC.
question, information regarding interactions of the liquids with the template and the properties of the liquids must be delineated. Below, we discuss the formation mechanisms of polymer (35) Jiang, K.; Wang, Y.; Dong, J.; Gui, L.; Tang, Y. Langmuir 2001, 17, 3635-3638.
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nanotubes and pillars based on theories of surface wetting and the rheology of viscoelastic fluids. Due to the high surface energy of AAO (1340 mN m-1 for γ-Al2O336), organic materials with a low surface energy (
