Self-assembly of Colloids for Plastic Capillary Electrochromatography Chip

In this work we have developed and characterized a novel fabrication process for producing dense, porous array of silica particles packed inside a plastic microfluidic channel. The novel approach relies on in-situ crystallization of colloidal silica particles within specific regions of a microchannel defined by plasma processing. These selfassembled silica colloids through a plastic microchannel are suitable for the application of plastic capillary electrochromatography chips. kXYWORDS: Capillary elect~oc~~o~zatog~up~y (CEC), capillary force, colloid, surface modljkation INTRODUCTION Plastic micromachining is a rapidly expanding area within the BioMEMS application field for developing inexpensive and disposable uTAS devices. Plastic chromatographic separation chips including plastic CE chips have been studied by many groups [l]. Implementing a packed column on a plastic chromatography chip is still considered a challenging -task. Conventional packing methods such as slurry packing may give chip-to-chip variation of packing density even if the plastic chips are prepared through microfabrication techniques. Figure 1 shows a schematic sketch of the plastic chip developed in this work. Here we have employed a self-assembly technique to fabricate a dense porous nanostructure through the microchannel. The main advantage of our approach is that neither branch charmel nor external frits are required for creating the packing structure. Furthermore, the morphology of the packed column can be easily examined to ensure Figure 1. Schematic sketch of plastic based electrochromatography chip with self-assembled column. 7th lnternat~onal Conference on Miniaturized Chemical and Blochemlcal Analysts Systems October 5-9, 2003, Squaw Valley, Callfornla USA O-974361 I-0.O/~TAS2003/$15.0002003TRF 417 high quality. Such dense porous nanostructures are very useful as preconcentrators and microreactors as well as chromatographic separation columns in chemical and biological application [2, 31. FABRICATION Colloidal suspensions spontaneously form a three dimensional array after crystallization on a hydrophilic substrate along the meniscus (4 (b) CC) in microfluidic channels. In this study, we have applied this self-assembly process to Figure 2. Contact angle modification microchannels formed on cyclic olefin by plasma treatment: (a) native COC; copolymer (COC) substrate. The surface (b) hydrophobic; and (c) hydrophilic. properties of plastic substrates can be easily tailored to suit biocompatibility requirements by using batch-processing techniques such as plasma treatment [4]. As shown in Figure 2, contact angle of COC can be controlled using plasma treatment. Plastic chips having a cross channel structure were fabricated using injection molding [5]. The dimensions of channel are 60 urn in width and 60-70 urn in depth. To define the wetting section of plastic substrate, plastic chips were sequentially modified with plasma as shown in Figure 3(a) and (b). Since the hydrophobic front of treated microchannel works as a passive stop 3 ^‘.‘. :.:: + ‘ ‘_‘. ̂,‘, , -c3 ‘.‘.‘. T -E t-2 r -a e-0 +:[:::: Plasma Mask (b) Figure 3. Chip fabrication: (a) hydrophobic plasma treatment; (b) selective hydrophilic treatment; and (c) colloidal self-assembly. valve against capillary flow, we can expect self-assembly of colloids to occur only in the hydrophilic channel. A beaker containing 1.0 wt% solution of uniform silica spheres in water was sonicated in ultrasonic bath for 10 minutes, placed on a hot plate to keep solution temperature at 55 “C and stirred to prevent gravitational sedimentation. Then the modified chip was immersed vertically into the beaker until the bottom of hydrophilic channel reached the solution surface as shown in Figure 3(c). Capillary suction force sucked up the suspension solution to fill the hydrophilic section of the lane. The chips were removed from the solution after 1 hour. 7th lnternat~onal Conference on Miniaturized Chemical and Blochemlcal Analysts Systems October 5-9, 2003, Squaw Valley, Callfornla USA