Anal. Chem. 1999, 71, 4294-4299
Electrochemical Detection in Polymer Microchannels J. S. Rossier, M. A. Roberts, R. Ferrigno, and H. H. Girault*
Laboratoire d'EÄ lectrochimie, EÄ cole Polytechnique Fe´ de´ rale de Lausanne, 1015 CH-Lausanne, Switzerland
A method, using UV laser photoablation, is presented for the fabrication and the integration of an electrochemical detector in a microchannel device, where carbon microband electrodes are placed either in the bottom or in the side walls of the rectangular microchannel. The different electrochemical cell geometries are tested with a model compound (ferrocenecarboxylic acid) in 40- and 100-µm-wide capillaries fabricated in planar polymer substrates. The experimental results are compared to numerical simulations for stagnant stream conditions. Depending on the scan rate and on the microchannel depth, the system behaves as a microband electrode until a linear diffusion field develops within the channel. The limit of detection for a one electron redox species within the 120-pL detection volume is ∼1 fmol with both cyclic voltammetry and chronoamperometric detection. The miniaturization of analytical tools has played a significant role in the development of fast analysis systems using ever decreasing volumes.1-4 The advantages of speed and separation efficiency in such systems have been well documented;5 however, problems have also arisen concerning the handling and detection of very small amounts of sample, inherent to a miniaturized device. Electroosmotic pumping has been used to efficiently handle small sample volumes, thereby addressing the handling problem.6 Miniaturized analytical systems have also been coupled with various detectors including laser-induced fluorescence (LIF) for ultrasensitive detection7,8 or even mass spectrometry,9,10 demonstrating the ability to detect small sample amounts. These detection techniques require significant off-chip instrumentation which increases expense and often necessitates time-consuming alignment procedures. (1) Manz, A.; Verpoorte, E.; Effenhauser, C. S.; Burggraf, N.; Reymond, D. E.; Harrison, D. J.; Widmer, H. M. J. High Resolut. Chromatogr. 1993, 16, 433436. (2) Harrison, D. J.; Fluri, K.; Seiler, K.; Fan, Z.; Effenhauser, C. S.; Manz, A. Science 1993, 261, 895-897. (3) Woolley, A. T.; Mathies, R. A. Anal. Chem. 1995, 67, 3676-3680. (4) Woolley, A. T.; Sensabaugh, G. F.; Mathies, R. A. Anal. Chem. 1997, 69, 2181-2186. (5) Jacobson, S.; Hergenroder, R.; Koutny, L. B.; Ramsey, J. M. Anal. Chem. 1994, 66, 1114-1118. (6) Manz, A.; Effenhauser, C. S.; Burggraf, N.; Harrison, D. J.; Seiler, K.; Fluri, K. J. Micromech. Microeng. Microfluid. 1994, 1-19 (special issue). (7) Chiem, N.; Harrison, D. J. Anal. Chem. 1997, 69, 373-378. (8) Haab, B. B.; Mathies, R. A. Anal. Chem. 1995, 67, 3253-3260. (9) Ramsey, R. S.; Ramsey, J. M. Anal. Chem. 1997, 69, 1174-1178. (10) Xue, Q.; Foret, F.; Dunayevskiy, Y. M.; Zavracky, P. M.; McGruer, N. E.; Karger, B. L. Anal. Chem. 1997, 69, 426-430.
4294 Analytical Chemistry, Vol. 71, No. 19, October 1, 1999
Electroanalytical tools have taken advantage of microfabrication techniques for many years in order to produce microelectrodes. The ability to minimize iR drop distortion of experimental data and their low capacitance is part of the properties that are suited for a fast analysis.11 The possibility of micromachining electrodes by standard microfabrication techniques,12 photoablation,13 or using glass or epoxy resin encapsulation14-17 is now well established. Furthermore, the integration of microelectrodes in picoliter vials has been demonstrated18,19 for single-cell analysis.20 These volumes are also typical volumes of plugs that have to be detected in fast-analysis microchip separations.21 In chromatography or capillary electrophoresis, several strategies have been developed for a number of years in order to optimize cell design for the detection of molecules such as neurotransmitters,22-25 carbohydrates,26-34 amino acids,35 or even proteins.36 For all these electrochemical measurements, two main strategies have been proposed, either precisely positioning external microelectrodes (11) Montenegro, M. I., Queiros, M. A., Daschbach, J. L., Eds. Microelectrodes: Theory and Applications; NATO ASI Series E 197; Kluwer Academic Publishers: Boston, 1991. (12) Romankiw, L. T. Electrochim. Acta 1997, 42, 2985-3005. (13) Seddon, B. J.; Shao, Y.; Fost, J.; Girault, H. H. Electrochim. Acta 1993, 39, 783-791. (14) Howell, J. O.; Wightmanh, R. M. Anal. Chem. 1984, 56, 524-529. (15) Ewing, A. G.; Dayton, M. A.; Wightman, R. M. Anal. Chem. 1981, 53, 18421847. (16) Ponchon, J.-L.; Cespuglio, R.; Gonon, F.; Jouvet, M.; Pujol, J.-F. Anal. Chem. 1979, 51, 1483-1486. (17) Dayton, M. A.; Stutts, B., J. C.; Wightman, R. M. Anal. Chem. 1980, 52, 946-950. (18) Bratten, C. D. T.; Cobbold, P. H.; Cooper, J. M. Anal. Chem. 1997, 69, 253-258. (19) Clark, R. A.; Hietpas, P. B.; Ewing, A. G. Anal. Chem. 1997, 69, 259-263. (20) Schlue, W.-R.; Kilb, W.; Gu ¨ nzel, D. Electrochim. Acta 1997, 42, 3197-3205. (21) Haswell, S. J. Analyst 1997, 122, R1-R10. (22) Blank, C. L. J. Chromatogr. 1976, 117. (23) Chen, M. C.; Huang, H. J. Anal. Chem. 1995, 67, 4010-4014. (24) Kristensen, H. K.; Lau, Y. Y.; Ewing, A. G. J. Neurosci. Methods 1994, 51, 183-188. (25) Lunte, S. M.; Oshea, T. J. Electrophoresis 1994, 15, 79-86. (26) Cassidy, R. M.; Lu, W. Z.; Tse, V. P. Anal. Chem. 1994, 66, 2578-2583. (27) Fermier, A. M.; Colon, L. A. J. High-Resolut. Chromatogr. 1996, 19, 613616. (28) Lu, W. Z.; Cassidy, R. M. Anal. Chem. 1993, 65, 2878-2881. (29) O’Shea, T. J.; Lunte, S. M.; Lacourse, W. R. Anal. Chem. 1993, 65, 948951. (30) Santos, L. M.; Baldwin, R. P. Anal. Chem. 1987, 59, 1766-1770. (31) Weber, P. L.; Lunte, S. M. Electrophoresis 1996, 17, 302-309. (32) Ye, J. N.; Baldwin, R. P. J. Chromatogr., A 1994, 687, 141-148. (33) Zhong, M.; Lunte, S. M. Anal. Chem. 1996, 68, 2488-2493. (34) Zhou, W. H.; Baldwin, R. P. Electrophoresis 1996, 17, 319-324. (35) Zhou, S. Y.; Zuo, H.; Stobaugh, J. F.; Lunte, C. E.; Lunte, S. M. Anal. Chem. 1995, 67, 594-599. (36) Ye, J. N.; Baldwin, R. P. Anal. Chem. 1994, 66, 2669-2674. 10.1021/ac981382i CCC: $18.00
© 1999 American Chemical Society Published on Web 08/27/1999
in the small volume or integrating the electrodes in the nanoliter device. The first approach has the advantage of flexible implementation in microvolumes but needs very precise and reproducible placement of the microelectrodes.19,37,38 The second strategy provides a better measurement stability and does not require cumbersome placement procedures before the analysis. Some promising examples of the integration of electrodes on a chip for the detection of electrophoretically separated compounds have already been shown.39-41 It has previously been demonstrated that polymer photoablation is suitable for generating microfluidic structure and that the electroosmotic flow can be controlled under a variety of electrophoretic conditions.42 Furthermore, the same technique has also been used for the micropatterning of proteins.43 The present work introduces a novel fabrication technique and design of microfabricated electrodes within a polymer microchannel element. The method for the integration of carbon ink electrodes is shown here to be an advantageous design for detection in small-volume devices. Even if the geometry of the microchannel is suited for the detection of plugs driven under electrophoretic conditions,44 as a prerequisite, the electrochemical characterization of the stagnant system is investigated. Recent developments have shown that, in such ultrasmall volumes (
