Batch Fabrication of Nanopillars for Autonomous Nanofluidic SERS Arrays
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Batch Fabrication of Nanopillars for Autonomous Nanofluidic SERS Arrays Michael S. Pio, Sunghoon Kwon, Yang-Kyu Choi, and Luke P. Lee Berkeley Sensor and Actuator Center, Department of Bioengineering University of California at Berkeley, Berkeley, CA 94720, U. S. A. ABSTRACT We have investigated a nanopillar-based surface enhanced Raman scattering (SERS) for future multiplexed nanofluidic SERS (nanoSERS) arrays. Without using e-beam or focus ion beam method, we have accomplished this simple and batch processed nanoSERS arrays on a chip, which is economical and mass producible. The polysilicon nanopillar structures are fabricated on top of a silicon wafer using optical lithography, reactive ion etching and passivation steps. High aspect ratio pillar-like nanostructures and spacing are controlled by the reactive ion etching gas and passivation steps. The heights of nanopillars ranged from 0.1 µm to 0.3 µm and their diameters ranged from 20 nm to 100 nm. A thin gold (10-20 nm) layer is evaporated on top of the nanopillar surfaces for further surface enhancements. The Raman shifts were measured for 10-3 M of 4, 6-diamidino-2-phenylindole dihydrochloride dye in solution using a 500 mW near infrared laser (785 nm). A direct correlation between the density of nanopillars and the intensity of Raman enhancement is observed. The SERS spectra through thin PyrexTM glass and polydimethylsiloxane (PDMS) are characterized to find the optimized nanofluidic materials for an autonomous multiplexing biomolecular detection schemes. INTRODUCTION With the recent explosion of genomic and proteomic information, there is an increasingly important need for fast, efficient, inexpensive and multiplexing biomolecular detection schemes [1]. Current biological detection methods are still encumbered by conventional labeling such as fluorescence, radioisotope, and enzymatic methods while Raman spectroscopy can identify biomolecules without any labeling by examining signatures from vibration spectra [2-4]. Surface-Enhanced Raman Scattering (SERS) is an increasingly appealing detection method for various biosensor applications because of its dramatic enhancement of Raman signals, which eventually increases the detection limit of the sensing system. In one example, researchers have combined SERS and Capillary Electrophoresis (CE) to avoid fluorescence labeling and also to obtain structure specific information [4]. Several research groups have reported single molecule detection and spectroscopy using SERS [5-6]. Up to 1014 enhancements have been reported using silver colloidal solution [4]. Other researchers have used annealed gold films to attain Raman enhancements [7]. Most of these methods have limited mass production value as well as low controllability of feature sizes for integrated nanoSERS arrays. Any future analytical device should be able to feature nano-scale surface structures with precisely controllable features inside microfluidic environments. This huge enhancement factor is critical in light of many biopolymers such as DNA having weak R
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