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Fabrication and Characterization of Nanoelectrode Arrays Formed via Block Copolymer Self-Assembly Eunhee Jeoung,† Trent H. Galow,† Joerg Schotter,‡ Mustafa Bal,‡ Andrei Ursache,‡ Mark T. Tuominen,‡ Christopher M. Stafford,§ Thomas P. Russell,§ and Vincent M. Rotello*,† Department of Chemistry, Department of Physics, and Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003 Received April 9, 2001. In Final Form: August 6, 2001 A novel approach to highly ordered and modular nanoelectrode arrays (NEAs) has been developed using block copolymer self-assembly. Variable scan rate cyclic voltammetry studies were performed to characterize the NEA. At low scan rates, the NEA behaves similar to a macroelectrode, while at high scan rates the nanoelectrodes act independently. This is an important feature for real-time in vivo sensing and other electroanalytical applications.
Introduction Nanoelectrode ensembles (NEEs) are versatile electroanalytical tools that have applications ranging from in vivo sensors to “smart” electronic noses.1 Benefits of NEEs over macro-sized electrodes include enhanced sensitivities, fast response times, and convection-independent responses.2 Typically, NEEs are prepared in two ways. In the “template synthesis” approach developed by Martin et al., gold or platinum metal is electrodeposited into the pores of nanoporous polycarbonate membranes.3 In another approach, Crooks et al. have used microphase separation to create molecule-sized defects on a gold surface, using a hydrophilic and a hydrophobic organothiol (self-assembled monolayer based NEE).4 While both of these approaches are quite effective, neither allows for simultaneous control over both pore size and pore dispersity.5 Additionally, the disorder inherent in these systems results in the random distribution of pores over the electrode surface. * To whom correspondence should be addressed. E-mail: rotello@ chem.umass.edu. † Department of Chemistry. ‡ Department of Physics. § Department of Polymer Science and Engineering. (1) (a) Martin, C. R. Science 1994, 266, 1961-1966. (b) Martin, C. R. Acc. Chem. Res. 1995, 28, 61-68. (c) Kobayashi, Y.; Martin, C. R. Anal. Chem. 1999, 71, 3665-3672. (d) Parthasarathy, R. V. Nature 1994, 369, 298-301. (e) Martin, C. R.; Mitchell, D. T. Anal. Chem. 1998, 70, 322A-327A. (2) (a) Wightman, R. M. Anal. Chem. 1981, 53, 1125A-1134A. (b) Oldham, K. B. J. Electroanal. Chem. 1992, 323, 53-76. (c) Fleischmann, M.; Pons, S.; Rolison, D. R.; Schmidt, P. P. Ultramicroelectrodes; Datatech Systems, Inc.: Morganton, NC, 1987; Chapter 3, pp 65-106. (3) (a) Penner, R. M.; Martin, C. R. Anal. Chem. 1987, 59, 26252630. (b) Cheng, I. F.; Whiteley, L. D.; Martin, C. R. Anal. Chem. 1989, 61, 762-766. (c) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920-1928. (d) Hulteen, J. C.; Menon, V. P.; Martin, C. R. J. Chem. Soc., Faraday Trans. 1996, 92, 4029-4032. (e) Martin, C. R. Chem. Mater. 1996, 8, 1739-1746. (4) (a) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 13291340. (b) Chailapakul, O.; Crooks, R. M. Langmuir 1993, 9, 884-888. (c) Che, G. L.; Cabrera, C. R. J. Electroanal. Chem. 1996, 417, 155-161. (d) Hayes, W. A.; Kim, H.; Yue, X. H.; Perry, S. S.; Shannon, C. Langmuir 1997, 13, 2511-2518. (e) Chailapakul, O.; Sun, L.; Xu, C. J.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459-12467. (f) Sun, L.; Crooks, R. M. Langmuir 1993, 9, 1951-1954. (g) Baker, W. S.; Crooks, R. M. J. Phys. Chem. B 1998, 102, 10041-10046. (h) Yang, D. H.; Zi, M. X.; Chen, B. S.; Gao, Z. Q. J. Electroanal. Chem. 1999, 470, 114-119. (i) Yang, Z. P.; Engquist, I.; Kauffmann, J. M.; Liedberg, B. Langmuir 1996, 12, 1704-1707. (5) Seshadri, K.; Wilson, A. M.; Guiseppi-Elie, A.; Allara, D. L. Langmuir 1999, 15, 742-749.
In addition to their favorable electrochemical properties, nanoelectrode systems have potential applications in the creation of organized nanodevices such as data storage systems,6 sensor arrays, and electrochemically controlled drug delivery systems.7 For these goals to be realized, however, the electrodes must be produced in ordered arrays (nanoelectrode arrays, NEAs), featuring regular pore sizes and spacing.8 We report here the application of highly ordered self-assembling block copolymer films for the creation of these NEA systems. In recent studies, a straightforward, versatile, and robust method for the fabrication of highly ordered, densely packed nanoporous arrays was introduced9 (Figure 1). Extensive characterization of these nanoporous arrays by small-angle X-ray scattering (SAXS) indicates that the cross-linked polystyrene matrix contained hexagonally close-packed nanopores (1.88 × 1011 electrodes cm-2, an approximately 200-fold increase on current limit10). The X-ray studies, in conjunction with transmission electron microscopy (TEM), also provided a lattice constant of 24 nm (pore-to-pore distance) and a high aspect ratio (14 nm diameter × 1000 nm deep pores). In addition to the ease of preparation, this technique is rapid, scalable, reproducible, and highly modular. This modularity is derived from the ability to control pore size and spacing through variation in block length, coupled with the ability to control polymer thickness during the spin-coating process. (6) Thurn-Albrecht, T.; Schotter, J.; Kastle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2001, 290, 2126-2129. (7) (a) Montelius, L.; Heidari, B.; Graczyk, M.; Maximov, I.; Sarwe, E. L.; Ling, T. G. I. Microelectron. Eng. 2000, 53, 521-524. (b) Oyama, M.; Masuda, T.; Mitani, M.; Okazaki, S. Electrochemistry 1999, 67, 1211-1213. (c) Porath, D.; Bezryadin, A.; de Vries, S.; Dekker, C. Nature 2000, 403, 635-638. (d) Wang, J.; Naser, N.; Renschler, C. L. Anal. Lett. 1993, 26, 1333-1346. (e) Zhang, X. J.; Wang, J.; Ogorevc, B.; Spichiger, U. E. Electroanalysis 1999, 11, 945-949. (f) Santini, J. T.; Cima, M. J.; Langer, R. Nature 1999, 397, 335-338. (g) Dickinson, T. A.; White, J.; Kauer, J. S.; Walt, D. R. Trends Biotechnol. 1998, 16, 250-258. (h) Steemers, F. J.; Walt, D. R. Mikrochim. Acta 1999, 131, 99-105. (8) While the nanoelectrodes cannot be individually addressed, it is possible to pattern the polymer film and the underlying Au film. This should enable very small regions of the nanoelectrode array to be addressed, offering the potential for the films to act as multisensing devices. (9) Thurn-Albrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell, T. Adv. Mater. 2000, 12, 787-791. (10) Ugo, P.; Moretto, L. M.; Bellomi, S.; Menon, V. P.; Martin, C. R. Anal. Chem. 1996, 68, 4160-4165.
10.1021/la010531g CCC: $20.00 © 2001 American Chemical Society Published on Web 09/14/2001
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Figure 1. (a) Cross-sectional TEM image of an oriented diblock copolymer film on a dark Au film. (b) AFM images obtained from the top surface of a porous template on a gold substrate after removal of the PMMA material comprising the cylinders shows the height image and (c) the phase image.
Experimental Section Nanoporous arrays were fabricated by spin-coating a polystyrene (PS)/poly(methyl methacrylate) (PMMA) diblock copolymer solution (10% w/v in toluene) (Mw ) 42 000 g/mol, Mw/Mn ) 1.04, PMMA 30 vol %) onto a gold-plated silicon film (Au/Si film). The dried films are then annealed in the presence of a strong electric field to orient the PMMA cylinders perpendicular to the surface. Finally, exposure to UV radiation (254 nm) simultaneously cross-linked the PS and degraded the PMMA that was removed by rinsing with glacial acetic acid. Tetrabutylammonium perchlorate (TBAP) was obtained from SACHEM and purified via recrystallization from ethyl acetate. Dried acetonitrile (99.99%) was purchased from VWR and used as received. Cyclic voltammetry was performed on a BAS Potentiostat CV-50W. For the nanoelectrode system, the electrode area was demarcated by a 7 mm diameter circle in a Teflon mask. Ninety percent iR compensation was employed for all experiments. Atomic force microscopy (AFM) images were obtained using a Digital Instruments Dimension 3100 atomic force microscope, while TEM images were obtained using a JEOL 100CX electron microscope.
Results and Discussion To confirm the applicability of our polymer films as NEAs, variable scan rate cyclic voltammetry (CV) studies were carried out using ferrocene as a redox-active molecular probe. Previous investigations have shown that NEEs exhibit a scan-rate dependence with three distinct voltammetric response regimes.3b In the low scan rate regime, radial diffusion boundary layers totally overlap (linear diffusion, Figure 2b) inducing the NEE to behave as a planar macroelectrode (Figure 2a). At higher scan rates, the current response is dominated by radial diffusion at the mouth of each pore. Since these zones of radial diffusion are independent of each other, the nanoelectrodes act as individual entities and provide an important trait for real-time monitoring for in vivo biosensors and other electroanalytical applications.6 At very high scan rates, the “linear active” state is reached in which the current response is governed by linear diffusion within the individual array pores. Again, the nanoelectrodes are acting independently. Cyclic voltammetry studies employing our electrode systems revealed a very similar scan-rate dependence to that previously seen in NEE systems.3,4 At low scan rates
Figure 2. Schematic illustration showing the diffusion pattern to electrodes: (a) linear diffusion to planar electrode, (b) overlapping radial diffusion to porous electrode, (c) radial diffusion to porous electrode, and (d) linear-active diffusion to porous electrode.
(Figure 3a), the voltammograms are peak-shaped. This behavior is similar to planar macroelectrodes, signifying that current at these slow scan rates is primarily controlled by linear diffusion arising from overlapping radial diffusion layers. At intermediate scan rates (250-3000 mV/ s), the voltammograms become less peak-shaped and more sigmoidal. This is consistent with mixed nonlinear and linear diffusion based diffusion, with nonlinear diffusion becoming predominant as scan speeds are increased. At 5000 mV/s, the sigmoidal-shaped voltammogram is characteristic of essentially nonlinear diffusion, indicating that the nanoelectrodes are only drawing current from local domains and are behaving almost independently. For comparison, a planar Au/Si film containing no PS matrix was examined. Normal linear diffusion based voltammograms were obtained at all scan rates (Figure 3b). Semiquantitative analysis of the voltammetric data was achieved by plotting log(ipa) versus log(v), where ipa is the anodic peak current in the ferrocene voltammogram, for both the macrosized electrode (4) and nanoelectrode array (O) (Figure 4). As expected for the macroelectrode system, a slope of 0.5 is obtained at all scan rates, associated with linear diffusion to the electrode. However, this is not the case for the nanoelectrode array system. At low scan rates, the slope equals 0.5, consistent with linear diffusion. As sweep rates are increased, the slope gradually decreases because of nonlinear diffusion becoming more dominant as the elements become isolated. Since these elements are immediately surrounded by polymer, geometric constraints preclude pure radial diffusion where the slope reaches zero. Therefore, we anticipate that at very high scan rates the slope would return to 0.5 as the linear active case is reached, affording a sigmoidal curve.11 At this point, (11) Higher scan rate studies were not performed due to phenomena arising from uncompensated resistance effects.
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Figure 5. Capacitance current vs scan rate for cyclic voltammograms containing 0.1 M TBAP in acetonitrile. Ar atmosphere, 298 K.
system and blank Au film were determined using eq 1, where i(A) is capacitive current contribution to anodic current, v (V s-1) is sweep rate, A (cm-2) is electrode area, and Cd (F cm-2) is the double-layer capacitance.
ic ) vACd
Figure 3. Cyclic voltammograms of 6.34 mM ferrocene obtained using (a) a nanoporous array electrode and (b) a naked Au surface as a working electrode at three different scan rates. The data were obtained in acetonitrile solution containing 0.1 M TBAP. Ar atmosphere, 298 K.
(1)
Cyclic voltammetry was performed for solutions containing only electrolyte (0.1 M TBAP) in MeCN. Sweeping from -100 to 600 mV (vs saturated calomel electrode), the charging current at 100 mV (well removed from the switching potentials) was measured. A plot of current versus voltage was obtained (Figure 5) and afforded double-layer capacitances of 20 and 36 µF cm-2 (based on active electrode areas) for the Au film and NEA system, respectively. The similar capacitance current obtained with the NEA and the blank Au surface clearly shows that the blockage of pores is limited and that the etching process is very efficient.12 Conclusions In summary, we have used polymer self-assembly to create a highly ordered nanoelectrode array. These NEAs provide tools for both the understanding of solution properties in nanoscale matrixes as well as the creation of nanoscopic devices. We are currently investigating the further optimization of the electrochemical properties of these arrays, as well as applying these systems to the creation of miniaturized multisensing units for application in in vivo and flow processes.
Figure 4. Log(anodic peak current) vs log(scan rate) for cyclic voltammograms of 6.34 mM ferrocene in acetonitrile solution containing 0.1 M TBAP. Ar atmosphere, 298 K. The curve fitted for the NEA system is a guide to the eye only.
the nanoelectrodes would be completely independent and should once again provide peaked voltammograms.3b Since capacitance current is proportional to electrode area, the capacitance current obtained for our NEA system provides a direct measure of etching efficiency during NEA preparation. The double-layer capacitances of the NEA
Acknowledgment. This research was supported by the National Science Foundation (NANO CTS-9871782, MRSEC DMR-9809365, CHE-9905492 to V.R.) and the Department of Energy (DE-FG02-96ER45612). V.R. acknowledges support from the Alfred P. Sloan Foundation and the Camille and Henry Dreyfus Foundation. V.R. and M.T. are Cottrell Scholars of the Research Corporation. We thank Professor Maroney for providing access to BAS Potentiostat CV-50W. LA010531G (12) The higher capacitance of the NEA could arise from three different effects: (a) the polystyrene matrix is not perfectly sealed to the Au surface; (b) the pores are not perfectly cylindrical at the interface; (c) boundary effects on capacitance.
