(103c) Gucha-Gucha Extraction Microchip

Chemical processes utilizing microfluidic systems have been investigated intensively. We have investigated fast solvent extraction process utilizing parallel two-phase flows, which have high specific interface area. By utilizing the two-phase flow, we have demonstrated cobalt ion analysis [1], phase transfer catalysis reaction [2], liquid membrane transport [3], interfacial polymerization for construction of polymer membrane [4], counter-current extraction [5] and so on.

In order to extend applicability of this technique, we have developed a novel extraction microchip having droplet formation, coagulation, and phase-separation parts. In this microchip, organic phase forms droplets in aqueous phase and, therefore, higher specific interface area is expected. Since the two-phase forms disordered state at the first part, which is called as "gucha-gucha" in Japanese, we named the microchip as "gucha-gucha extraction microchip".

Experimental:

Microchannels were fabricated on glass substrates by ordinary lithographic wet-etching technique. In order to make two different depths, two-step etching method was applied [6]. The microchannels were fabricated on a bottom substrate and liquid inlets were fabricated on a top substrate. There two substrates were thermally bonded. Organic and aqueous solutions were driven by syringe pumps and fed into the microchannel via polymer capillary and some connection parts.

Results and discussion:

The gucha-gucha extraction microchip consists of three parts; (1) droplet formation, (2) coagulation of the droplets, and (3) two-phase separation. For the droplet formation part, we designed Y-shape configuration. Two inlet microchannels having a width of 250 µm and depth of 100 µm were connected by shallow Y-shaped microchannel having a width of 50 µm and depth of 10 µm. The output of the shallow microchannel was connected to a microchannel having a width of 250 µm and depth of 100 µm. In order to form oil-in-water (O/W) droplets, the shallow part and output microchannel were washed by sodium hydroxide solution to obtain clean hydrophilic surface. At the output of the shallow channel, organic phase was divided to form droplets. When hexane and water was introduced with 1 µl/min flow rate, droplets having average diameter of 43 µm was formed.

In the second part, the droplets were coagulated to form plug or parallel two-phase flow. From the droplet formation part to the coagulation part, the droplets were flowed in the hydrophilic microchannel. In order to coagulate the organic droplets, the microchannel surface was modified with hydrophobic octadecylsilane group. It the hydrophobic microchannel, the droplet adsorbed to the microchannel wall and the following droplets attached to the adsorbed organic phase. By repeating these processes, the droplet flow was converted to plug or parallel flow.

In the third part, the organic and aqueous phases were separated by utilizing hydrophilic-hydrophobic patterning microchannel. In our previous paper, gas-liquid separation by utilizing the hydrophilic-hydrophobic patterning, where a microchannel having deep and shallow parts was used and only the shallow part was modified with hydrophobic group utilizing capillarity [6]. By utilizing the patterned structure, gas bubbles were purged to the shallow part of the microchannel completely. In the present study, we tried to use the similar structure, but the structure did not work very well. In that structure, viscosity of the organic phase in the shallow part affected the phase separation, which could be neglected in gas bubble purge. Therefore, we connected a deep microchannel to the shallow part. In brief, hydrophilic deep channel (sample flow) was connected to hydrophilic shallow channel (separation part), which connected to hydrophobic deep channel (separated organic flow). This structure worked very well. The organic and aqueous phases in a microchannel having a width of 250 µm and depth of 100 µm could be separated with very high flow rate up to 70 µl/min.

After investigating these three unit operations, these operations were integrated into a single microchip and we have confirmed the flow control as we designed. In order to demonstrate effectiveness of our gucha-gucha extraction microchip, cobalt complex was extracted from aqueous phase to organic phase.

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[2] "Fast and high conversion phase-transfer synthesis exploiting the liquid-liquid interface formed in a microchannel chip", H. Hisamoto, T. Saito, M. Tokeshi, A. Hibara and T. Kitamori, Chemical Communications (24), 2662-2663 (2001).

[3] "Three-layer flow membrane system on a microchip for investigation of molecular transport", M. Surmeian, M. N. Slyadnev, H. Hisamoto, A. Hibara, K. Uchiyama and T. Kitamori, Analytical Chemistry 74(9), 2014-2020 (2002).

[4] "Chemicofunctional membrane for integrated chemical processes on a microchip", H. Hisamoto, Y. Shimizu, K. Uchiyama, M. Tokeshi, Y. Kikutani, A. Hibara and T. Kitamori, Analytical Chemistry 75(2), 350-354 (2003).

[5] "Countercurrent laminar microflow for highly efficient solvent extraction", A. Aota, M. Nonaka, A. Hibara, and T. Kitamori, Angewandte Chemie International Edition, 46(6), 878-880 (2007).

[6] "Surface modification method of microchannels for gas-liquid two-phase flow in microchips", A. Hibara, S. Iwayama, S. Matsuoka, M. Ueno, Y. Kikutani, M. Tokeshi and T. Kitamori, Analytical Chemistry 77(3), 943-947 (2005).