SIZE-SPECIFIC SEPARATION OF BIO-MOLECULES USING POROUS ALUMINA MEMBRANE Yosep Choi, Misun Cha, Prashant Purwar and Junghoon Lee Seoul National University, Korea ABSTRACT This paper reports an on-chip nano-porous membrane that can separate bio-molecules based on their sizes. The nano- porous membrane is one of key elements for the separation of bio-molecules. In conventional approaches, however, there are many limitations such as difficulty in integrating with a lab-on-a-chip (LOC) device and the lack of precision in separations. In this work, porous alumina membrane was directly fabricated on a microfluidic platform to be integrated with LOC. We demonstrate an effective, size-specific separation of DNA and protein with such device. KEYWORDS: Bio-molecule Separation, Porous Alumina Membrane, Lab-on-a-chip INTRODUCTION An efficient bio-molecular separation through the nano-porous membrane requires the material with controllable pore size, length and surface chemistry. The pore size needs to be in the range of a few tens of nanometers. The size of pores should have a uniform distribution in order to achieve high selectivity of separation. Finally, high porosity is required for obtaining enough analyte flux. Commercially available nano-porus membranes generally exhibit large size distribution and relatively large thickness values [1]. Furthermore it is cumbersome to integrate such membrane with an on-chip device to construct a re- liable one. Recently bio-molecule separation driven by diffusion or pressure using such approach have been demonstrated [2]. In order to take full advantage of on-chip microfluidic system it is necessary to develop fabrication process suitable to inte- grating the nano-porous membrane. Here we describe an on-chip microfluidic device with built-in porous alumina membrane of precisely controlled nano- pores. The nano-porous membrane was directly fabricated on a microfluidic platform resulting in an integrated architecture with a thin porous alumina membrane. The size of nano-pores has narrow size distribution. Pore widening and atomic layer deposition were used to precisely control the size. Bio-molecule separation through the nano-pores driven by electric potential was performed and characterized. We demonstrate that while single strand DNAs can easily pass through the porous alumina membrane, aptamer-protein complexes are effectively prevented from such separation due to their size. EXPERIMENTAL Porous alumina was adopted as a membrane material that enables bulk and low cost fabrication without high cost lithographic techniques such as nano-imprint lithography and e-beam lithography. Highly dense and uniform pores of porous alumina spontaneously formed from aluminum layer by an anodizing process. Porous alumina membrane was directly fabricated on silicon substrate using micro fabrication process (Fig. 1(a)). Nitride membrane was suspended by KOH anisotropic wet-etching, then titanium and aluminum were deposited by a metal evaporator with the thickness of 50 nm and 1 μm, respectively. The first anodizing process was carried out in 0.3 M oxalic acid (H 2 C 2 O 4 ) at 40 V. Pores were formed in a relatively regular shape as the anodizing time elapses due to the self-alignment of stressed pores [3]. 6 wt% phosphoric acid (H 3 PO 4 ) and 8 wt% chromic acid (H 2 CrO 4 ) were mixed at 60 °C and used to selectively remove the first alumina layer. Periodic vertical pores were formed during second anodizing process with same condition as the first step. The uniformity of the pore size is critical to the accurate separation of molecules based on their sizes. Pores were slightly widened to immerse the porous alumina membrane into 10 wt% phosphoric acid at 30 °C for 15 min to enhance uniformity of pores. Etching rate of widening process was about 0.8 nm/min at room temperature. For a controllable size of pores, Ruthenium(Ru) was deposited by an atomic layer deposition (ALD) process. Finally, several steps of top/back side RIE etching were performed to open back side of the membrane. Fig. 1(b) shows the configuration of a single chip device that includes two PDMS chambers between porous alumina membrane-silicon substrate. Each chambers consist of two reservoirs, venting holes and microfluidic channels. The upper chamber is directly connected to the bottom chamber so that a sample injected is spontaneously flows into the bottom chamber via the hole in the silicon substrate by capillary effect. The PDMS structures were effectively bonded to porous alumina membrane and silicon substrate through O 2 plasma treatment. 978-0-9798064-4-5/μTAS 2011/$20©11CBMS-0001 404 15th International Conference on Miniaturized Systems for Chemistry and Life Sciences October 2-6, 2011, Seattle, Washington, USA