SERS Substrates Self-Organized Hexagonal-Nanopore SERS Array** Dukhyun Choi, Yeonho Choi, Soongweon Hong, Taewook Kang, and Luke P. Lee* A real-time label-free detection method is in critical demand for biological, chemical, and medical applications, and environ- mental monitoring. Surface-enhanced Raman spectroscopy (SERS) is one of the best label-free detection methods. [1–3] Accordingly, a variety of fabrication methods of SERS substrates have been demonstrated. [4–6] Among the many such fabrication methods, self-assembly of nanoparticles is domi- nant. However, due to the uncontrollable bottom-up approach, the self-assembly method forms random hot spots for electro- magnetic (EM) field enhancement. [6] Since the disordered random hot spots cause a nonuniform response of SERS signals, it is required to find a solution for forming precisely pre- determined, reproducible, and organized nanoarchitectures for SERS substrates. For these purposes, focused-ion-beam [7,8] and electron-beam lithography [9] were applied for precision nanopatterning with the advantage of high resolution without the need for a physical mask, since the pattern can be changed at any time by using computer-aided design (CAD) software. However, the disadvantages of these two methods are the long exposure time due to pixel-by-pixel scanning steps, high cost, and substantial maintenance. To overcome these limitations, phase-shift lithography, [10] nanosphere lithography, [11] and nonlithographic self-organized nanoscale templating [12] have been introduced as effective fabrication methods for plasmonic devices or SERS substrates. Among the nonlithographic fabrication methods for SERS substrates, the self-organized nanoporous template of anodized aluminum oxide (AAO) has attracted great attention due to self-organized nanostructures with long-range ordering and facile controllability of various configuration shapes. [13–15] Furthermore, AAO templates are rigid, well defined, self- supporting, and self-stable. [12–17] The precise control of the anodization process can provide a highly tunable nanopore geometry of AAO, which leads to a promising nanotemplate for plasmonic nanostructures. Recently, a few groups have demonstrated AAO-based SERS substrates. [18–20] Some examples of the SERS nanostructures are silver nanowires with less than 10 nm coupling gap distance, [18] hybrid nanowire clusters of silver and silica, [19] and mesostructured plasmonic nanowires through mesoporous silica. [20] These structures were all prepared by electrochemically depositing metals inside the pores of AAO, thus scarifying their pores to generate plasmonic nanowires. However, it is desirable to extend the AAO template-based SERS substrate to develop innovative inte- grated plasmonic optical antennas on top of nanopores, which may provide new solutions for next-generation DNA sequen- cing. Integrated nanopore structures with plasmonic optical antennas might also provide an exciting nanofluidic platform for the elucidation of the mechanism of biomolecular transport phenomena at the nanoscale. [21,22] Herein, we report the fabrication of self-aligned nanoplas- monic SERS structures on a nanopore array and the characterization of the SERS responses by controlling two key parameters: gold thickness and AAO thickness. The enhancement factor (EF) of the self-aligned SERS substrate on the nanopore array reaches 10 7 . Furthermore, the structures are highly reproducible and uniform over a large area as a result of high-throughput and simple fabrication steps. As shown in Figure 1, the self-aligned SERS substrate on a nanopore array is accomplished by depositing a thin layer of Au on a self-organized hexagonal AAO nanopore array (the details can be found in the Experimental Section and the Supporting Information, Figures S1 and S2). To avoid blocking the nanopores during the evaporation process of Au, due to coalescence of the grains under the condition that T s /T m > 0.24 (where T s is the substrate temperature during evaporation and T m is the melting point of the target metal), [23] we maintained the condition T s /T m < 0.24 for Au (T m ¼ 1337 K). The final structure consisted of a plasmonic nanopore Au layer on the AAO layer. To study the geometrical effects of the plasmonic nanostructure, we deposited a different thickness of Au (t Au ) and controlled the time of anodization for different thicknesses [  ] Prof. L. P. Lee, Prof. D. Choi, S. G. Hong Biomolecular Nanotechnology Center Berkeley Sensor and Actuator Center Department of Bioengineering University of California Berkeley, CA 94720–1762 (USA) E-mail: lplee@berkeley.edu Prof. Y. Choi Department of Biomedical Engineering College of Health Science Korea University Seoul 136-703 (Republic of Korea) Prof. T. W. Kang Department of Chemical and Biomolecular Engineering Sogang University Seoul 121-742 (Republic of Korea) [  ] The authors thank Liz Wu for the technical support. This work was supported by DARPA SERS Fundamental and by a grant (code #: 2009K00470) from the ‘‘Center for Nanostructured Materials Tech- nology’’ under the ‘‘21st Century Frontier R&D Programs’’ of the Ministry of Education, Science, and Technology, Korea. SERS¼ surface-enhanced Raman spectroscopy. : Supporting Information is available on the WWW under http:// www.small-journal.com or from the author. DOI: 10.1002/smll.200901937 small 2010, 6, No. 16, 1741–1744 ß 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1741