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dispersion (pH 6.9) of the nanotubes. The concentrations of sulfatelatex beads and ferritin were adjusted to 8 mg mL–1 and 5 mg mL–1, respectively. The mixed solutions of the nanotubes and the nanoparticles were allowed to stand overnight. The solution was then filtered (polycarbonate membrane filter, pore size: 0.2 lm), and the residual nanotubes were washed several times to remove the nanoparticles remaining in bulk water. STEM and TEM Observations: Aqueous dispersions of nanotubes, tapes, and the nanotube-encapsulated nanoparticles were dropped onto a carbon grid, and the excess water was removed by standing at room temperature. Tape and guest-free nanotubes were negatively stained with phosphotungstate solution (2 wt.-%, pH 9), and dried under vacuum. The STEM (Hitachi S-4800) and TEM (Hitachi H7000) were operated at 30 kV and 75 kV, respectively. Received: May 27, 2005 Published online: September 29, 2005
[22] a) D. Letellier, O. Sander, C. Menager, V. Cabuil, M. Lavergne, Mater. Sci. Eng. 1997, 5, 153. b) A. K. Boal, T. J. Headley, R. G. Tissot, B. C. Bunker, Adv. Funct. Mater. 2004, 14, 19. [23] a) R. Djalali, Y. F. Chen, H. Matsui, J. Am. Chem. Soc. 2003, 125, 5873. b) M. Reches, E. Gazit, Science 2003, 300, 625. c) L. Yu, I. A. Banerjee, H. Matsui, J. Mater. Chem. 2004, 14, 739. [24] a) J. H. Jung, K. Yoshida, T. Shimizu, Langmuir 2002, 18, 8724. b) K. J. C. van Bommel, A. Friggeri, S. Shinkai, Angew. Chem. Int. Ed. 2003, 42, 980. c) Q. Ji, R. Iwaura, M. Kogiso, J. H. Jung, K. Yoshida, T. Shimizu, Chem. Mater. 2004, 16, 250. [25] a) W. Shenton, T. Douglas, M. Young, G. Stubbs, S. Mann, Adv. Mater. 1999, 11, 253. b) E. Dujardin, C. Peet, G. Stubbs, J. N. Culver, S. Mann, Nano Lett. 2003, 3, 413. c) M. Knez, A. M. Bittner, F. Boes, C. Wege, H. Jeske, E. Maiss, K. Kern, Nano Lett. 2003, 3, 1079. [26] K. Aoi, K. Itoh, M. Okada, Macromolecules 1997, 30, 8072.
– [1] T. Shimizu, M. Masuda, H. Minamikawa, Chem. Rev. 2005, 105, 1401. [2] a) B. Yang, S. Kamiya, K. Yoshida, T. Shimizu, Chem. Commun. 2004, 500. b) B. Yang, S. Kamiya, Y. Shimizu, N. Koshizaki, T. Shimizu, Chem. Mater. 2004, 16, 2826. [3] a) S. B. Lee, R. Koepsel, D. B. Stolz, H. E. Warriner, A. J. Russell, J. Am. Chem. Soc. 2004, 126, 13 400. b) G. John, M. Mason, P. M. Ajayan, J. S. Dordick, J. Am. Chem. Soc. 2004, 126, 15 012. [4] A. Karlsson, M. Karlsson, R. Karlsson, K. Sott, A. Lundqvist, M. Tokarz, O. Orwar, Anal. Chem. 2003, 75, 2529. [5] a) R. Price, M. Patchan, J. Microencapsulation 1991, 8, 301. b) R. Price, M. Patchan, A. Clare, D. Rittschof, J. Bonaventura, Biofouling 1992, 6, 207. c) R. R. Price, M. Patchan, J. Microencapsulation 1993, 10, 215. d) R. R. Price, M. Patchan, A. Clare, D. Rittschof, J. Bonaventura, Recent Dev. Biofouling Control 1994, 321. [6] a) G. C. L. Wong, J. X. Tang, A. Lin, Y. Li, P. A. Janmey, C. R. Safinya, Science 2000, 288, 2035. b) I. A. Banerjee, L. Yu, M. Shima, T. Yoshino, H. Takeyama, T. Matsunaga, H. Matsui, Adv. Mater. 2005, 17, 1128. [7] a) J.-H. Fuhrhop, D. Fritsch, Acc. Chem. Res. 1986, 19, 130. b) J.-H. Fuhrhop, H. Tank, Chem. Phys. Lipids 1987, 43, 193. [8] J.-H. Fuhrhop, T. Wang, Chem. Rev. 2004, 104, 1201. [9] M. Masuda, T. Shimizu, Chem. Commun. 2001, 2442. [10] M. Masuda, T. Shimizu, Langmuir 2004, 20, 5969. [11] a) D. A. Jaeger, G. Li, W. Subotkowski, K. T. Carron, Langmuir 1997, 13, 5563. b) J. Schneider, C. Messerschmidt, A. Schulz, M. Gnade, B. Schade, P. Luger, P. Bombicz, V. Hubert, J.-H. Fuhrhop, Langmuir 2000, 16, 8575. c) J. Guilbot, T. Benvegne, N. Legros, D. Plusquellec, Langmuir 2001, 17, 613. d) J. Song, Q. Cheng, S. Kopta, R. C. Stevens, J. Am. Chem. Soc. 2001, 123, 3205. [12] J.-H. Fuhrhop, D. Spiroski, C. Boettcher, J. Am. Chem. Soc. 1993, 115, 1600. [13] R. C. Claussen, B. M. Rabatic, S. I. Stupp, J. Am. Chem. Soc. 2003, 125, 12 680. [14] M. Masuda, T. Shimizu, Carbohydr. Res. in press. [15] M. Masuda, T. Shimizu, Carbohydr. Res. 1997, 302, 139. [16] a) J. L. Slater, C. Huang, Prog. Lipid Res. 1988, 27, 325. b) M. Kranenburg, B. Smit, FEBS Lett. 2004, 568, 15. [17] P. M. Proulx-Curry, N. D. Chasteen, Coord. Chem. Rev. 1995, 144, 347. [18] H. Yui, Y. Shimizu, S. Kamiya, I. Yamashita, M. Masuda, K. Ito, T. Shimizu, Chem. Lett. 2005, 34, 232. [19] Y. M. Lvov, R. R. Price, Colloids Surf., B 2002, 23, 251. [20] Y. M. Lvov, R. R. Price, J. V. Selinger, A. Singh, M. S. Spector, J. M. Schnur, Langmuir 2000, 16, 5932. [21] a) P. Ringler, W. Mueller, H. Ringsdorf, A. Brisson, Chem. Eur. J. 1997, 3, 620. b) E. M. Wilson-Kubalek, R. E. Brown, H. Celia, R. A. Milligan, Proc. Natl. Acad. Sci. USA 1998, 95, 8040. c) T. J. Melia, M. E. Sowa, L. Schutze, T. G. Wensel, J. Struct. Biol. 1999, 128, 119.
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Patterning of Conducting Polymers Based on a Random Copolymer Strategy: Toward the Facile Fabrication of Nanosensors Exclusively Based on Polymers** By Bin Dong, Dingyong Zhong, Lifeng Chi,* and Harald Fuchs Conducting polymers have attracted considerable attention owing to the unique combination of their electronic, optical, magnetic, and mechanical properties (lightweight, easily processable, and highly flexible).[1] Anticipated application areas range from microelectronics, electro-optics, and optical electronics to sensors and actuators.[2] In recent years, a significant portion of these studies has been devoted to the fabrication of all-polymer devices.[3] All-polymer devices have been demonstrated to possess properties that may rival those of inorganic devices while still retaining their low cost, flexibility, and disposable nature.[4] Although they are still at the prototype stage, different kinds of devices, such as field-effect transistors (FETs),[5] charge-storage devices,[6] integrated circuits,[7] and optoelectronic devices,[8] have been demonstrated. The emerging field of nanosensors offers the prospect of high sen-
– [*] Prof. L. F. Chi, B. Dong, D. Y. Zhong, Prof. H. Fuchs Physikalisches Institut, Westfälische Wilhelms-Universität Münster, and Center for Nanotechnology (CeNTech) D-48149 Münster (Germany) E-mail:
[email protected] Prof. L. F. Chi, B. Dong Key Lab for Supramolelular Structure and Materials College of Chemistry, Jilin University Changchun 130023 (P.R. China) [**] We thank Dr. Andreas Schäfer for technical assistance. This work was supported by the state of North Rhine-Westphalia (NRW) within a German-Chinese project. Supporting Information is available from Wiley InterScience or from the author.
DOI: 10.1002/adma.200500938
Adv. Mater. 2005, 17, 2736–2741
Adv. Mater. 2005, 17, 2736–2741
www.advmat.de
thickness during deposition this method is not suitable for high-resolution (< 100 nm) fabrication. In the present study we use a copolymer strategy to control the thickness and the adhesive and electrical properties of conducting polymers by incorporating a surface-active monomer into the main chains, thus making them suitable for the fabrication of devices exclusively based on conducting polymers with a sub-100 nm resolution. Due to the generality of the copolymerization process this method is applicable to a wide range of conducting-polymer species. Two different types of nanostructures consisting exclusively of polypyrrole or polyaniline are fabricated and their use as nanoscale sensors is demonstrated. Figure 1a depicts the fabrication process, which, in accordance with the standard lift-off process, includes three steps. In the first step, a pattern is defined on the photoresist using e-beam lithography. Second, a copolymer film thinner than
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sitivity and rapid detection of the analyte.[9] For instance, Craighead and co-workers have reported a polyaniline nanowire chemical sensor more sensitive than the traditional filmbased sensors for the detection of ammonium gas.[10] Huang et al. reported a nanofiber film sensor that responds much faster than the conventional film sensor to HCl gas.[11] Apart from the detection of chemical vapors, improved sensing behavior is also observed when detecting biological molecules, such as glucose[12] and biotin.[13] However, the all-polymer concept has not yet been applied to nanosensors, possibly due to a limitation in resolution and integration. There are basically two approaches to fabricating nanoscale conducting polymeric structures: the synthesis and post-assembly method[14] or the microfabrication method[15–18] (photolithography, soft lithography, etc.). In the former approach, sub-100 nm structures can be readily obtained by either template synthesis[19] or electrospinning.[20] However, manipulation and positioning of the synthesized nanoscale objects with respect to microelectrodes is rather difficult and not precise. In the latter case, conducting polymers can be precisely patterned by a variety of microlithographic methods based on different principles. For example, soft lithography[15] can be used to fabricate conducting-polymer patterns by means of selective deposition, embossing, and so on; photo- or electron (e)-beaminitiated polymerization or degradation[16] can define patterns on conducting polymer films; dip-pen lithography[17] is capable of patterning conducting polymers, based on a principle of ink transfer from the tip to the substrate; electrochemical lithography[18] represents another series of methods of obtaining patterned conducting polymers, which can be obtained by scanning electrochemical microscopy (SECM), electrochemical dip-pen lithography, or scanning tunneling microscopy (STM). Although these microlithographic methods yield conducting-polymer microstructures, they possess significant limitations for patterning structures with sub-100 nm dimensions on insulating surfaces. In the present work we report the fabrication of a nanoscale sensor, consisting exclusively of polymers, by a copolymer strategy, with a resolution of less than 100 nm. Copolymerization of conducting polymers with other components is known to be an effective way to improve the solubility and processability of these polymers.[21] By adjusting the fraction of the two monomers in the resulting copolymer, properties of the conducting polymer, such as conductivity and adhesion, can also be tuned. Adhesion has been demonstrated to be quintessential for microfabrication techniques that involve lift-off processes, e.g., the deposition of metals.[22] Conducting polymers are usually not amenable to the lift-off process because their films do not adhere to the substrate. As a result, they can be easily peeled off from the substrate to give freestanding films. Various methods have been established to improve the adhesion of conducting polymer films, e.g., surface anchoring,[23] addition of additives,[24] etc. By utilizing both silane groups and a surfactant as glue molecules, we reported the fabrication of submicrometer polypyrrole wires through a liftoff process.[25] However, due to a lack of control over the film
a
b
N N m (CH2)3 Si OOO
c N x (CH2)3 Si OOO
n
H N
y
Figure 1. a) Schematic representation of the fabrication process. b,c) Molecular formulae of the copolymers.
the resist layer is deposited by oxidizing pyrrole (or aniline) and N-(3-trimethoxysilylpropyl)pyrrole (abbreviated as py-silane) with iron chloride. Finally, the resist is lifted off in acetone by sonication. Figures 1b,c depict the structures of the copolymers used in this work. In order to realize successful fabrication of conducting-polymer structures by the lift-off process and to facilitate their further application as sensors, conductivity and adhesion, as well as their combination, are the essential parameters that should be taken into account. Here we employed the two-point measurement method and the adhesiontape test[15c] to estimate the electrical and adhesive properties of the copolymer film, respectively. Taking polypyrrole as an example, the introduction of py-silane has a significant influ-
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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ence on the properties of polypyrrole (Table 1). Notably, copolymers that contain only a small amount of py-silane (9 vol.-%) can successfully withstand the adhesion-tape test whereas pure polypyrrole cannot. Moreover, upon increasing Table 1. Influence of copolymer composition on the properties of the resulting polypyrrole film. Binding energies and relative contribution of different elements present in the copolymer deposited at a pyrrole to pysilane ratio of 91:9 as determined by X-ray photoeletron spectroscopy (XPS). Pyrole: py-silane [vol.-%:vol.-%] 100:0 91:9 73:27 55:45 36:64 22:78 0:100
Film conductivity [S cm–1]
Film thickness [nm]
Withstand adhesiontape test [yes/no]
32 ± 5 20 ± 4 5.5 ± 1.5 0.45 ± 0.2 0.013 ± 0.005 2.6610–5 ± 1610–5
