Anal. Chem. 2006, 78, 1904-1912
Fabrication and Characterization of a Nanometer-Sized Optical Fiber Electrode Based on Selective Chemical Etching for Scanning Electrochemical/Optical Microscopy Kenichi Maruyama,† Hiroyuki Ohkawa,† Sho Ogawa,† Akio Ueda,† Osamu Niwa,‡ and Koji Suzuki*,†,§
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan, National Institute of Advanced Industrial Science and Technology, Central 6, Tsukuba, Ibaraki 305-8566 Japan, and JST-CREST, 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan
We have already reported a method for fabricating ultramicroelectrodes (Suzuki, K. JP Patent, 2004-45394, 2004). This method is based on the selective chemical etching of optical fibers. In this work, we undertake a detailed investigation involving a combination of etched optical fibers with various types of tapered tip (protrudingshape, double- (or pencil-) shape and triple-tapered electrode) and insulation with electrophoretic paint. Our goal is to establish a method for fabricating nanometersized optical fiber electrodes with high reproducibility. As a result, we realized pencil-shaped and triple-tapered electrodes that had radii in the nanometer range with high reproducibility. These nanometer-sized electrodes showed well-defined sigmoidal curves and stable diffusion-limited responses with cyclic voltammetry. The pencil-shaped optical fiber, which has a conical tip with a cone angle of 20°, was effective for controlling the electrode radius. The pencil-shaped electrodes had higher reproducibility and smaller electrode radii (rapp < 1.0 nm) than those of other etched optical fiber electrodes. By using a pencil-shaped electrode with a 105-nm radius as a probe, we obtained simultaneous electrochemical and optical images of an implantable interdigitated array electrode. We achieved nanometer-scale resolution with a combination of scanning electrochemical microscopy SECM and optical microscopy. The resolution of the electrochemical and optical images indicated sizes of 300 and 930 nm, respectively. The neurites of living PC12 cells were also successfully imaged on a 1.6-µm scale by using the negative feedback mode of an SECM. The introduction of ultramicroelectrodes (UMEs) has contributed to significant advances in electrochemistry.1-3 The use of * To whom correspondence should be addressed. E-mail: suzuki@ applc.keio.ac.jp. † Keio University. ‡ National Institute of Advanced Industrial Science and Technology. § JST-CREST. (1) Suzuki, K. JP Patent 2004-45394, 2004. (2) Morris, R. B.; Franta, D. J.; White, H. S. J. Phys. Chem. 1987, 91, 3559. (3) Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. S. Science 1990, 250, 1118.
1904 Analytical Chemistry, Vol. 78, No. 6, March 15, 2006
UMEs has enhanced mass transport and reduced the IR drop and double-layer charging effects. These UMEs have been employed to study rapid electron-transfer kinetics4,5 where the enhanced mass transport rates enable the measurement of fast heterogeneous electron-transfer kinetics, the detection of single molecules,6 and the provision of high-resolution images for scanning probe microscopy studies.7-9 The reduction in electrode dimensions from submicrometer to nanometer order has enabled single cells to be measured in microenvironments.10-12 These nanometer-sized electrodes have been used for living tissue since they cause minimal physical damage.13 Quantitative measurements with nanometer-sized electrodes also provide an insight into the electrochemical behavior of the nanostructured materials used in energy conversion and chemical sensing. Many researchers have reported the preparation of band-, disk-, or hemisphericalshaped electrodes with submicrometer dimensions by insulating the metal surface (Pt, Au) and carbon with such coatings as glass,14 nail varnish,15,16 wax,17 epoxy,18 and copolymers.19 (4) Shao, Y.; Mirkin, M. V.; Fish, G.; Kokotov, S.; Palanker, D.; Lewis, A. Anal. Chem. 1997, 69, 1627. (5) (a) Howell, J. O.; Wightman, R. M. Anal. Chem. 1984, 56, 524. (b) Wipf, D. O.; Kristensen, E. W.; Deakin, M. R.; Wightman, R. M. Anal. Chem. 1988, 60, 306. (c) Bond, A. M.; Henderson, T. L. E.; Mann, D. R.; Mann, T. F.; Thormann, W.; Zoski, C. G. Anal. Chem. 1988, 60, 1878. (d) Bowyer, W. J.; Engelman, E. E.; Evans, D. H. J. Electroanal. Chem. 1989, 262, 67. (e) Chen, S.; Kucernak, A. J. Phys. Chem. 2002, 106, 9396. (6) (a) Fan, F.-R. F.; Bard, A. J. Science 1995, 267, 871. (b) Fan, F.-R. F.; Kwak, J.; Bard, A. J. J. Am. Chem. Soc. 1996, 118, 9669. (7) Heben, M. J.; Dovek, M. M.; Lewis, N. S.; Penner, R. M.; Quate, C. F. J. Microsc. 1988, 152, 651. (8) (a) Bach, C. E.; Nichols, R. J.; Beckmann, W.; Meyer, H.; Schulte, A.; Besenhard, J. O. J. Electrochem. Soc. 1993, 140, 1281. (b) Mao, B. W.; Ye, J. H.; Zhuo, X. D.; Mu, J. Q.; Fen, Z. D.; Tian, Z. W. Ultramicroscopy 1992, 42, 464. (9) (a) Lee, C.; Miller, C. J.; Bard, A. J. Anal. Chem. 1991, 63, 78. (b) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 2000, 72, 276. (10) Giros, B.; Jaber, M.; Jones, S. R.; Wightman, R. M. Nature 1996, 379, 606. (11) Kuras, A.; Gutmaniene, N. J. Neurosci. Methods 2000, 96, 143. (12) Ghien, J. B.; Wallingford, R. A.; Ewing, A. G. J. Neurochem. 1990, 54, 633. (13) Wong, D. K. Y.; Xu, L. Y. F. Anal. Chem. 1995, 67, 4086. (14) Penner, R. M.; Heben, M. J.; Lewis, N. S. Anal. Chem. 1989, 61, 1630. (15) Green, M. P.; Hanson, K. J.; Scherson, D. A.; Xing, X.; Richter, M.; Ross, P. N.; Carr, R.; Lindau, I. J. Chem. Phys. 1989, 93, 2181. (16) Vitus, C. M.; Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 7559. (17) (a) Wiechers, J.; Twomey, T.; Kolb, D. M.; Behm, R. J. J. Electroanal. Chem. 1988, 248, 451. (b) Nagahara, L. A.; Thundat, T.; Lindsay, S. M. Rev. Sci. Instrum. 1989, 60, 3128. 10.1021/ac0502549 CCC: $33.50
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Slevin et al. reported a relatively straightforward, inexpensive, and reproducible procedure for fabricating nanometer-sized electrodes. The electrodes were prepared by electrodepositing insulation layers onto a metal surface.20 Lee and Bard also fabricated gold-coated ring electrodes by insulating optical fibers with electrophoretic paint.21 Their ring electrodes were based on pulling optical fiber for scanning electrochemical/optical microscopy (SECM/OM).22 Pulled optical fiber probes were fabricated using a carbon dioxide laser and a pipet puller.23 With this procedure, complicated pulling parameters such as incident laser power, laser spot size, pulling power, and the velocity criterion24 must be controlled to obtain tapered fiber probes. The fiber-pulling method could only be used for pencil-shaped probes, and it was difficult to produce
