Anal. Chem. 2003, 75, 2224-2230
Nanocapillary Array Interconnects for Gated Analyte Injections and Electrophoretic Separations in Multilayer Microfluidic Architectures Donald M. Cannon, Jr.,‡ Tzu-Chi Kuo,†,‡ Paul W. Bohn,*,†,‡ and Jonathan V. Sweedler*,†,‡
Department of Chemistry and the Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801
An electrokinetic injection technique is described which uses a nuclear track-etched nanocapillary array to inject sample plugs from one layer of a microfluidic device into another vertically separated layer for electrophoretic separations. Gated injection protocols for analyte separations, reported here, establish nanocapillary array interconnects as a route to multilevel microfluidic analytical designs. The hybrid nanofluidic/microfluidic gated injection protocol allows sample preparation and separation to be implemented in separate horizontal planes, thereby achieving multilayer integration. Repeated injections and separations of FITC-labeled arginine and tryptophan, using 200-nm pore-diameter capillary array injectors in place of traditional cross injectors are used to demonstrate gated injection with a bias configuration that uses relay switching of a single high-voltage source. Injection times as rapid as 0.3 s along with separation reproducibilities as low as 1% for FITC-labeled arginine exemplify the capability for fast, serial separations and analyses. Impedance analysis of the micro-/nanofluidic network is used to gain further insight into the mechanism by which this actively controlled nanofluidic-interconnect injection method works. Gated sample introduction via a nanocapillary array interconnect allows the injection and separation protocols to be optimized independently, thus realizing the versatility needed for real-world implementation of rapid, serial microchip analyses. Microfluidic analyses are becoming an integral technology in the field of analytical chemistry and are expanding into diverse areas of science, as noted in numerous reviews.1-5 Controlling fluid flow is fundamental to the design of microfabricated analytical devices. Flexibility in fluid transport protocols enhances the * Corresponding authors. E-mails:
[email protected],
[email protected]. † Department of Chemistry. ‡ Beckman Institute for Advanced Science and Technology. (1) Bousse, L.; Cohen, C.; Nikiforov, T.; Chow, A.; Kopf-Sill, A. R.; Dubrow, R.; Parce, J. W. Annu. Rev. Biophys. Biomol. Struct. 2000, 29, 155-181. (2) Gawron, A. J.; Martin, R. S.; Lunte, S. M. Eur. J. Pharm. Sci. 2001, 14, 1-12. (3) Verpoorte, E. Electrophoresis 2002, 23, 677-712. (4) Reyes, D. R.; Iossifidis, D.; Auroux, P.-A.; Manz, A. Anal. Chem. 2002, 74, 2623-2636. (5) Auroux, P.-A.; Iossifidis, D.; Reyes, D. R.; Manz, A. Anal. Chem. 2002, 74, 2637-2652.
2224 Analytical Chemistry, Vol. 75, No. 10, May 15, 2003
versatility of the device as well as makes it easier to optimize specific analyses. For separations, electroosmotic flow has traditionally been chosen over pressure-induced flow, because less band broadening is observed, and it is easier to implement. Sequential chemical manipulations and multidimensional analysis schemes require that multiple fluidic tasks be integrated onto a single two-dimensional platform, thus placing extreme demands on design. Consequently, three-dimensional (multilayer) microfluidic devices have been pursued to address the complexity required for increasingly sophisticated analytical schemes.6-15 Fast microchip-based electrophoretic separations rely on fluid transport protocols that inherently determine the injected sample size and reproducibility. Sample injections on single-layer planar microfluidic devices have been achieved by various tailored fluid flow designs. Numerous papers have described T-type designs, including the single-T,16 cross-T,17 double-T,18,19 and multi-T configurations,20,21 for electrokinetic injection of sample volumes in the picoliter range. The main premise of the T design is to use the intersection of the sample channel and the separation channel (6) Kugelmass, S. M.; Lin, C.; DeWitt, S. H. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3877, 88-94. (7) Jo, B.-H.; Beebe, D. J. In Proc. SPIE-Int. Soc. Opt. Eng., 1999; Vol. 3877, pp 222-229. (8) Anderson, J. R.; Chiu, D. T.; Jackman, R. J.; McDonald, J. C.; Wu, H.; Whitesides, S. H.; Whitesides, G. M.; Cherniavskaya, O. Anal. Chem. 2000, 72, 3158-3164. (9) Chiu, D. T.; Li Jeon, N.; Huang, S.; Kane, R. S.; Wargo, C. J.; Choi, I. S.; Ingber, D. E.; Whitesides, G. M. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 2408-2413. (10) Dharmatilleke, S.; Henderson, H. T.; Bhansali, S.; Ahn, C. H. Proc. SPIE 2000, 4177, 90-97. (11) Gray, B. L.; Jaeggi, D.; Mourlas, N. J.; van Drieenhuizen, B. P.; Williams, K. R.; Maluf, N. I.; Kovacs, G. T. A. Sens. Actuators, A 2000, 77, 57-65. (12) Kuo, T.-C.; Cannon, D. M., Jr.; Feng, W.; Shannon, M. A.; Sweedler, J. V.; Bohn, P. W. In Micro Total Analysis Systems 2001; Ramsey, J. M., van den Berg, A., Eds.; Kluwer Academic Publishers, 2001; pp 60-62. (13) Cannon, D. M., Jr.; Kuo, T.-C.; Feng, W.; Shannon, M. A.; Sweedler, J. V.; Bohn, P. W. In Micro Total Analysis Systems 2001; Ramsey, J. M., van den Berg, A., Eds.; Kluwer Academic Publishers, 2001; pp 199-200. (14) Kuo, T.-C.; Cannon, D. M., Jr.; Shannon, M. A.; Bohn, P. W.; Sweedler, J. V. Sens. Actuators, A 2003, 102, 223-233. (15) Kuo, T.-C.; Cannon, D. M., Jr.; Chen, Y.; Tulock, J. J.; Shannon, M. A.; Sweedler, J. V.; Bohn, P. W. Anal. Chem. 2003, in press. (16) Harrison, D. J.; Manz, A.; Fan, Z.; Luedi, H.; Widmer, H. M. Anal. Chem. 1992, 64, 1926-1932. (17) Jacobson, S. C.; Hergenroder, R.; Moore, A. W., Jr.; Ramsey, J. M. Anal. Chem. 1994, 66, 4127-4132. (18) Effenhauser, C. S. Anal. Methods Instrum. 1993, 1, 172-176. (19) Koutny, L. B.; Schmalzing, D.; Taylor, T. A.; Fuchs, M. Anal. Chem. 1996, 68, 18-22. 10.1021/ac020629f CCC: $25.00
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to create a physically defined sample plug. Accordingly, the double-T intersection allows larger injection volumes relative to the single T, and the multi-T design provides for additional choices of injection volume on a single device. In the simple T-type injection designs, electrophoretic transport biasing can cause differential loading of analytes and require injection times of up to a few seconds,22 ultimately limiting temporal resolution and throughput. To circumvent these and other limitations of the simple T design, pinched T-type injections have become routine.17 Pinched injections utilize a more complicated voltage configuration to provide backflow into the injection channel; however, recent studies have shown that utilizing sample channels narrower than the separation channel can eliminate leakage between sample and separation channels and offer enhanced separation performance.23 Injections for rapid, serial separations in microchips have also been achieved with various gated-injection protocols to provide reproducible injection plugs that are readily varied by field strength and duration,24-27 analogous to capillary zone electrophoresis injections.28 Initial gated-injection studies have used a T-type configuration, but with a different sample loading procedure.24-26 In these protocols, the sample stream, originating at one end of the separation channel, is directed through the 90° turn at the intersection toward the sample waste reservoir while electroosmotic buffer flow is maintained in the other two arms. Injection occurs during the fast (
