Anal. Chem. 2009, 81, 2022–2026
Centrifugal Sedimentation for Selectively Packing Channels with Silica Microbeads in Three-Dimensional Micro/Nanofluidic Devices Maojun Gong,† Paul W. Bohn,‡ and Jonathan V. Sweedler*,† Department of Chemistry, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, and Department of Chemical and Biomolecular Engineering and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Incorporation of nanofluidic elements into microfluidic channels is one approach for adding filtration and partition functionality to planar microfluidic devices, as well as providing enhanced biomolecular separations. Here we introduce a strategy to pack microfluidic channels with silica nanoparticles and microbeads, thereby indirectly producing functional nanostructures; the method allows selected channels to be packed, here demonstrated so that a separation channel is packed while keeping an injection channel unpacked. A nanocapillary array membrane is integrated between two patterned microfluidic channels that cross each other in vertically separated layers. The membrane serves both as a frit for bead packing and as a fluid communication conduit between microfluidic channels. Centrifugal force-assisted sedimentation is then used to selectively pack the microfluidic channels using an aqueous silica bead suspension loaded into the appropriate inlet reservoirs. This packing approach may be used to simultaneously pack multiple channels with silica microbeads having different sizes and surface properties. The chip design and packing method introduced here are suitable for packing silica particles in sizes ranging from nanometers to micrometers and allow rapid (∼10 min) packing with high quality. The liquid/analyte transport characteristics of these packed micro/nanofluidic devices have potential utility in a wide range of applications, including electroosmotic pumping, liquid chromatographic separations, and electrochromatography. Incorporation of nanofluidic components into microfluidic networks has attracted attention in biomolecular separations,1,2 sample enrichment,3 and even single molecule analysis,4,5 primarily because the unique properties of nanochannels are comparable * To whom correspondence should be addressed. Professor Jonathan V. Sweedler, E-mail:
[email protected]. Fax: +1-217-244-8068. † University of Illinois at Urbana-Champaign. ‡ University of Notre Dame. (1) Fu, J.; Schoch, R. B.; Stevens, A. L.; Tannenbaum, S. R.; Han, J. Nat. Nanotech. 2007, 2, 121–128. (2) Zeng, Y.; Harrison, D. J. Anal. Chem. 2007, 79, 2289–2295. (3) Wang, Y. C.; Stevens, A. L.; Han, J. Anal. Chem. 2005, 77, 4293–4299. (4) Jo, K.; Dhingra, D. M.; Odijk, T.; de Pablo, J. J.; Graham, M. D.; Runnheim, R.; Forrest, D.; Schwartz, D. C. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 2673–2678. (5) Zhang, H.; Wirth, M. J. Anal. Chem. 2005, 77, 1237–1242.
2022
Analytical Chemistry, Vol. 81, No. 5, March 1, 2009
to the characteristic scaling lengths of large molecules such as proteins and DNA. The nanochannels are usually either patterned purposefully using high-resolution lithography or obtained as random cylindrical pore arrays through nuclear track-etching, as in nanocapillary array membranes (NCAMs). An interesting alternative, microbeads packed into a microchannel indirectly produce randomly structured nanofiltration architectures in microfluidic chips,2 thereby achieving the same functionality but with a simple fabrication protocol. Microbeadpacked columns have been widely used in high performance liquid chromatography (HPLC) to separate, quantify, and identify compounds.6 Both particle sizes and packing quality are important factors affecting the ultimate performance of packed columns.6 Smaller particles improve HPLC separation efficiency; however, a larger pressure drop requires more powerful pumps to deliver mobile phase through the column. A promising alternative is capillary electrochromatography (CEC), which is based on electroosmosis and electrophoresis, to transport liquid and analytes. Although there are several methods for packing capillary columns with microbeads for CEC, pressure packing,7,8 electrokinetic packing, centripetal forces,9 and gravity packing,10 it remains a challenge to pack small particles (
