Biomineralized Nanopore Membranes on Silicon for Nanoparticle Translocation

and Introduction: Nanopores have many biological applications. They can be used as single molecule detectors, deoxyribonucleic acid (DNA) sequencing, and potentially be functionalized to simulate lipid bilayers or nuclear pore complexes. However, these nanopores are expensive and time consuming to make using conventional microfabrication techniques. An alternative to these top-down processed nanopores is diatoms. Diatoms are a major group of algae that synthesize a threetier network of silica pores for their cell wall, which can be seen in Figure 1. They can grow up to 300 μm in diameter, yet the smallest pores are approximately 40 nm wide. In order to have access to these nanopores, the diatoms need to be positioned and immobilized over 20 μm pores etched through silicon wafers. Currently, the diatoms are manually placed over the pores and a UV curable epoxy, Norland optical adhesive-60 (NOA-60), is manually dispensed around the diatom. In order to create a more efficient process with a higher yield, standard contact lithography was explored to immobilize the diatoms over the micropore. The criteria used to determine the effectiveness of the process was to check for absence of leakages, breaking of the diatom or clogging of the nanopores. Two photoresists were tested, SU-8 and NOA-60. SU-8 is a negative UV-curable photoresist that is well established in contact lithography. NOA-60 is polyurethane-based resin that adheres to glass. Therefore, it requires an anti-adhesion layer to enable release from the glass mask and the sample. Polydimethylsiloxane (PDMS) was used as the anti-adhesion layer as suggested in [1]. How ever, our experiments proved that the PDMS layer was ineffective in enabling separation of photomask and substrate after UV exposure. SU-8 proved to be successful in meeting the criteria in combination with a sulfuric-peroxide mixture (SPM) treatment. Fabrication Process: Oxidized silicon wafers with through-wafer pores were used as substrates. The pore diameter on the back side of the wafer was 100 μm and the pore diameter on the front side was 20 μm. Details on the fabrication process of the silicon micropores can be found in [2]. Diatoms were deposited on the wafer from a 1:1 water:ethanol solution and positioned over the silicon the micropore using a micromanipulator. Subsequently, positively charged polyL-lysine was used to form a temporary bond between the negatively charged diatom and the oxidized silicon wafer. After poly-L-lysine was deposited on the chip, SU-8 3005 was spun on at 3,000 rpm for 30 seconds. Once the post-exposure bake was completed, the mask consisting of a 100 μm dot was aligned over the 20 μm through-wafer pore. Exposure was completed on an EVG-620 with a dose of 350 mJ. After the post-exposure bake, the chip was developed in SU-8 developer for two minutes and rinsed with isopropyl alcohol. A schematic of the process can be seen in Figure 2. The SPM treatment was completed using a 3:1 ratio of sulfuric acid to Figure 2: Schematic diagram of fabrication process. PROCESS & CHARACTERIZATION 207 2012 NNIN REU RESEARCH ACCOMPLISHMENTS Acknowledgements: I would like to thank my PI, Dr. Michael Goryll, and my mentor, Xiaofeng Wang, for their guidance and time, and the staff at the Center for Solid State Electronics Research at Arizona State University. I would also like to thank the National Nanotechnology Infrastructure Network Research Experience for Undergraduates Program and the National Science Foundation for funding this great experience.