1. Introduction

We developed a reliable but simple integration method of nanostructure using either polymeric materials or functionalized microbeads in a poly-dimethylsiloxane (PDMS)-based microfluidic channel, for nanofluidic applications. In order to realize PDMS nanofluidic application, various methods, including the junction gap breakdown [1] and cracking of oxidized PDMS [2] have been reported. However, because those techniques create nanostructures at the interface between PDMS and cover substrates, nanofluidic junctions are sequestered at the corner of the microchannel and the coupling between the microchannel and the nanojunction is poor. In this work, the nanoporous junction was created by infiltrating polymer solution between the gaps created simply by mechanical cutting, without cleanroom processes. The PDMS can seal itself with the heterogeneous nanoporous material between PDMS/PDMS gap, due to its flexibility without any (covalent) chemical bonding between PDMS and porous materials. This allows one to integrate nanoporous-junction into PDMS microchip in a leak-free manner with excellent repeatability. In addition, vertical nanojunctions are better coupled with the microchannel, allows efficient operations.

2. Experimental Method

Schematics of fabrication processes were shown in Figure 1. Desired PDMS microchannels can be obtained from the standard PDMS chip fabrication processes (Figure 1(a)). Since the nanofluidic applications usually consisted of the microchannels connected by nano-structures [3], we mechanically cut across the microchannels using conventional razor blades for guiding porous material infiltration after punching sample loading holes (Figure 1(b)). Once the gap was created, PDMS tends to restore its inherent geometric structure due to its flexibility. By bending the chip, the gap was opened and a drop of 1.5uL Nafion was put on the edge of the gap (Figure 1(c)). Then the solution can immediately fill both the gap and a portion of microchannels by capillary forces. After 10 minutes of curing at 95C, solvents would evaporate. The elastic nature of PDMS seals the nanoporous junction tightly between the PDMS gap. Any remaining material on the top of the PDMS surface and inside microchannel can be removed at once by taping (Figure 1(d)). Finally, glass plate can be bonded on top of the device using plasma treatment (Figure 1(e)). The microscope image of fabricated nanoporous-junctions and cooperating microchannels were shown in Figure 2(a). Three microchannels were connected each other by self-sealed nanoporous junction which lay across the microchannels.

Figure 1. Schematics of fabrication processes.

3. Results

The DC ion current through the nanoporous-junction can be an excellent indicator for testing reliability and repeatability. The current was proportional to the applied voltage and showed an excellent linearity from randomly selected microchips as shown in Figure 2(b). As a practical example, we demonstrated nanofluidic preconcentration [4] of BODIPY dye and proteins (b-phycoerythrin) using the device (Figure 3(a), 3(b)). Because the polymeric junction spans across the entire microchannel height, the preconcentration was achieved with high pressure field or even in large channels, with the dimension of 1000um width X 100um depth (Figure 3(c)) which has never been demonstrated before. This method could be a simple but generic method for integrating various polymeric and other nanomaterials within the PDMS microfluidic channels.

Figure 2. (a) Microscope image of fabricated PDMS microchannels and self sealed nanoporous-junction and
(b) I-V plot for confirming fabrication repeatability.

Figure 3: Preconcentration factors of (a) BODIPY dye molecules and (b) b-PE proteins.
Ion preconcentration in microchannels have the dimension of 1000um (width) X 100um (depth).

References

1. H. Lee, S. Chung, S. J. Kim, and J. Han, Analytical Chemistry, 79, 6868 (2007).
2. D. Huh, K. L. Mills, X. Zhu, M. A. Burns, M. D. Thouless, and S. Takayama, Nature Materials, 6, 424 (2007).
3. S. J. Kim, Y. C. Wang, J. H. Lee, H. Jang, and J. Han, Physical Review Letters, 99, 045501 (2007).
4. Y. C. Wang, A. Stevens, and J. Han, Analytical Chemistry, 77, 4293 (2005).