Anal. Chem. 2006, 78, 5933-5934
Wetting Micro- and Nanofluidic Devices Using Supercritical Water Robert Riehn*,† and Robert H. Austin
Department of Physics, Princeton University, Princeton, New Jersey 08544
We describe a method for wetting micro- and nanofluidic devices with water or any other pure liquid. The process is performed by enclosing the fluidic device in a liquidfilled cell, heating the cell to a temperature above the critical point of the liquid, and subsequent cooling of the cell to room temperature. Because the process liquid is essentially a gas during wetting, arbitrary shapes can be wetted. We demonstrate wetting of micro- and nanostructures in a fused-silica device with only a single inlet. The process is low-cost, fast, safe, and very reliable. A growing variety of bioanalytic devices based on micro-and nanofluidic structures has been developed over the recent years. They have been used for capillary electrophoresis,1 DNA separation in synthetic gels,2 size-deterministic particle sorting,3 and complete self-sufficient analysis systems.4,5 Recently, nanofluidic devices have been used for single-molecule DNA sizing and restricition mapping.6-8 However, with decreasing size and increasing complexity of the fluidic structures, the ability to wet using capillary action has become one of the limiting factors in the design of these devices. In particular, junctions, networks, and regions with decreasing channel dimensions are prone to formation of gas bubbles during wetting.9 Previous approaches toward complete wetting have concentrated on decreasing the free energy of wetting,10 forcing liquid through the device using pressure,11 or removing bubbles from the device by outgassing under reduced pressure.9 * Corresponding author. E-mail:
[email protected]. † Current address: Physics Department, North Carolina State University, Raleigh, North Carolina 27695. (1) Effenhauser, C. S.; Manz, A.; Widmer, H. M. Anal. Chem. 1993, 65, 26372642. (2) Volkmuth, W. D.; Austin, R. H. Nature 1992, 358, 600-602. (3) Huang, L. R.; Cox, E. C.; Austin, R. H.; Sturm, J. C. Science 2004, 304, 987-990. (4) Waters, L. C.; Jacobson, S. C.; Kroutchinina, N.; Khandurina, J.; Foote, R. S.; Ramsey, J. M. Anal. Chem. 1998, 70, 158-162. (5) Unger, M. A.; Chou, H. P.; Thorsen, T.; Scherer, A.; Quake, S. R. Science 2000, 288, 113-116. (6) Turner, S. W.; Perez, A. M.; Lopez, A.; Craighead, H. G. J. Vac. Sci. Technol. B 1998, 16, 3835-3840. (7) Tegenfeldt, J. O.; Prinz, C.; Cao, H.; Chou, S.; Reisner, W. W.; Riehn, R.; Wang, Y. M.; Cox, E. C.; Sturm, J. C.; Silberzan, P.; Austin, R. H. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 10979. (8) Riehn, R.; Lu, M.; Wang, Y. M.; Lim, S. F.; Cox, E. C.; Austin, R. H. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10012-10016. (9) Monahan, J.; Gewirth, A. A.; Nuzzo, R. G. Anal. Chem. 2001, 73, 31933197. (10) Duffy, D. C.; McDonald, J. C.; Schueller, O. J. A.; Whitesides, G. M. Anal. Chem. 1998, 70, 4974-4984. 10.1021/ac0604989 CCC: $33.50 Published on Web 07/04/2006
© 2006 American Chemical Society
Figure 1. Cell for supercritical wetting. (A) Schematic of the cell with device employed in the process. (B) Photograph of cell before assembly.
Here we introduce a drastically different method, in which the device is wetted using supercritical water. The critical point of water is at 374 °C and a pressure of 3212 psi. The liquid form of water is then obtained later by lowering the temperature. The process is thus conceptually related to critical point drying, which is often utilized in the fabrication of microelectromechanical systems.12 EXPERIMENTAL SECTION High-pressure cells compatible with the elevated temperatures during the wetting process were assembled from two Conflat flanges purchased from Varian Inc., separated by a copper seal (Figure 1). The volume of the cells assembled ranged from 5 to 15 mL. The cell was loaded with all parts submerged in degassed and deionized water. After closing the cell, it was placed in an oven, and the temperature was ramped to 400 °C and then slowly lowered down to room temperature. The entire process took ∼4 h, most of which was needed for cooling. We tested our process using mixed nanofluidic and microfluidic devices made of a patterned fused-silica substrate that was (11) Effenhauser, C. S.; Bruin, G. J. M.; Paulus, A.; Ehrat, M. Anal. Chem. 1997, 69, 3451-3457. (12) Mulhern, G. T.; Soane, D. S.; Howe, R. T. Proc. Int. Conf. on Solid-State Sensors and Actuators; 1993; pp 2969.
Analytical Chemistry, Vol. 78, No. 16, August 15, 2006 5933
Figure 2. Annular dark-field micrograph of a microchannel. The region in this image is located ∼1.5 cm left of the inlet single hole of the microchannel. (A) Dry and (B) after wetting. The channels are about 100 µm wide and 500 nm deep.
capped with a fused-silca coverslip, as described in ref 8. The fluidic structure consisted of a microfluidic channel (500 nm deep, 100 µm wide, and 1.5 cm long) that ended in a forked structure. Nanofluidic channels (150 nm in diameter, 100 µm long) that branched off from the microchannel halfway along its length were prepared by focused ion beam milling. The entire fluidic system was only accessible through a single inlet that was drilled through the substrate at one end of the microchannel. At room temperature and atmospheric pressure, this device cannot be wetted fully within a practicable time, or about 1 week. RESULTS AND DISCUSSION Cells were assembled and used according to the above description, and in all cases fully wetted devices were found inside the cells. Figure 2 illustrates complete wetting of the microchannel. Due to the refractive indices of air, water, and fused silica, the fluidic system can be seen as a dark region when dry, and as an outline when wet. The wetting of the nanofluidic channels was visualized by letting the laser dye Rhodamine 610 diffuse into the wet device overnight and then observing the fluorescence under 568-nm irradiation from a Kr+ laser (Figure 3). The main limitation of the process is the temperature that the device needs to withstand. We suspect that for those reasons poly(dimethylsiloxane), SU8, and even poly(tetrafluoroethylene) may be unsuitable materials for a device to be subjected to our method. Furthermore, supercritical water is a bad solvent for salts, and hence, the device can only be wetted with deionized water. Buffer solutions can be brought into wetted devices using diffusion or flow. Another aspect that should be considered is that supercritical
5934 Analytical Chemistry, Vol. 78, No. 16, August 15, 2006
Figure 3. Single-inlet nanochannels. (A) SEM image of nanochannels before sealing and wetting of the device. The region shown in the image is connected to the microfluidic channel via a 100-µmlong nanochannel. (B) Fluorescence micrograph after wetting and diffusion of Rhodamine 610 dye.
water is acidic, and oxidization involving dissolved oxygen proceeds readily. Hence, the device should be clean before it is subjected to the process. Because of the small liquid volume involved, relatively little energy is stored by the supercritical water in the cell. Failure of the seal should thus not lead to catastrophic consequences. Nevertheless, a leak of the cell could still lead to considerable burns, and the oven should not be opened before cooling down to room temperature. We also advise checking the rating of any bolts used for assembling the cell, for use at the appropriate temperature. Applications of our process may be found in the wetting of inorganic nanopores for DNA analysis, wetting of microfluidic structures with decreasing dimensions, wetting of intricate fluidic networks, and possibly wetting of hydrophobic nanotubes. We believe the wetting method may lead to renewed interest in hard microfluidics, which recently had lost in appeal to soft microfluidics because of the problem of gas bubbles after wetting. ACKNOWLEDGMENT We thank Shuang Fang Lim for critical reading of the manuscript. This work was supported by grants from DARPA (MDA972-00-1-0031), NIH (HG01506), NSF Nanobiology Technology Center (BSCECS9876771), the State of New Jersey (NJCST 99-100-082-2042-007). Received for review March 19, 2006. Accepted June 2, 2006. AC0604989
