Nanoimprint lithography Fabrication of Polymeric Nanorods Using Bilayer Nanoimprint Lithography** Fatih Buyukserin, Mukti Aryal, Jinming Gao, and Wenchuang Hu* Polymeric nanoparticles are becoming increasingly important in a variety of biological applications, such as biomolecular sensing, diagnostic imaging, and therapeutic drug delivery. [1,2] For these applications, the mass production of multifunctional nanocomposite materials with precise control of particle size, shape, and composition is a significant challenge. [3,4] For instance, the use of conventional bottom-up strategies (e.g., emulsion polymerization) to fabricate polymeric nanostruc- tures with nonspherical geometry and a uniform size distribution is difficult because these methods are typically driven by the minimization of interfacial free energy that yields spherical particles with a size variation. [3] Moreover, the formation of nanocomposite materials is difficult due to the challenge in assembling multiple components from large volume fractions of solvent. On the other hand, in the field of microelectronics, polymers as resist can be precisely patterned to have arbitrary shapes using state-of-the-art photo-, e-beam, and X-ray lithographic technologies. [5] They are limited either by high cost, poor accessibility, slow speed, or radiation damage to functional polymers. In the past decade, many low- cost nanopatterning techniques have been invented to pattern polymer structures, such as nanoimprint lithography (NIL) [6,7] and soft lithography, [8] among many others. [9–15] These methods are capable of making nanostructures of desired shape and size. However, it is not straightforward to produce large quantities of biofunctional nanoparticles using them. Most of these nanopatterning approaches, either imprinting or soft lithography, result in a residual layer that connects the periodic structures on a surface. Furthermore, they are limited by the lack of large-scale nanopatterned molds for mass production and a reliable method to transfer particles from surface to solution. Recently, several top-down engineering methods for producing size-controlled, nonspherical polymeric particles have been reported. These techniques involve the use of photolithography, [16,17] microfluidics, [18,19] soft lithography on non-wetting surfaces (PRINT), [20] step-and-flash imprint lithography (S-FIL), [21] and stretching of spherical parti- cles. [4,22,23] Although these methods demonstrated different degrees of success, they also showed certain limitations in particle-size control, cost, and throughput. For example, the throughputs of the photolithography and microfluidic approaches are relatively low at this stage and the particle sizes are above 5 mm. [16–19] The stretching method requires prefabrication of uniform spherical particles to maintain size uniformity, [4] and the PRINT and S-FIL methods require liquid precursors and costly nanopatterned molds. [20,21,24] In this Communication, we report a bilayer nanoimprint lithography (B-NIL) method with large-scale, low-cost Si molds of high-density nanopores transferred from anodic alumina for the fabrication of free-standing polymeric nanorods with tunable lengths from 100 nm to 1 mm. A sacrificial polymer layer is introduced to the imprinting procedure to form free-standing nanoparticles with the functional polymer. Large-scale Si molds with high densities (10 10 cm 2 ) and ordered arrays of nanopores are fabricated by plasma etching using anodic alumina membranes as a mask, which enables the fabrication of large quantities of nanorods (>10 10 per imprint cycle) using nanoimprints on bilayer polymers. The same mold can be reused to prepare particles with different lengths by controlling the initial polymer thickness. Fluorescent nanoparticles are also fabricated by incorporating a dye molecule into the polymer matrix. In a typical procedure, a nanoporous silicon mold is first prepared by inductively coupled plasma (ICP) etching with Cl 2 :Ar gases using an anodized alumina membrane (AAM) [25] as an etch mask (Figure 1a). The Si mold is then modified with a fluorocarbon-based silane to form a self-assembled mono- layer (F-SAM) for ease of demolding. As a proof of principle, a bilayer polymer substrate is prepared in which Si or quartz is spin coated first with a sacrificial poly(methyl methacrylate) (PMMA) layer and then a UV-curable SU-8 polymer layer. SU-8 is used in this study as a model polymer system because it communications [  ] Prof. W. Hu, Dr. F. Buyukserin, + M. Aryal Department of Electrical Engineering University of Texas at Dallas Richardson, TX 75080 (USA) E-mail: walter.hu@utdallas.edu Prof. J. Gao Department of Pharmacology Simmons Comprehensive Cancer Center University of Texas Southwestern Medical Center Dallas, TX 75390 (USA) [+] Current address: Gazi University Nanomedicine Research Center 06830, Ankara (Turkey) [  ] This research is supported by the Montcrief Foundation. We thank C. Kessinger for assistance with confocal laser scanning micros- copy and E. Kildebeck for helpful discussions. : Supporting Information for this article is available online under http://www.small-journal.com or from the author. DOI: 10.1002/smll.200801822 1632 ß 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2009, 5, No. 14, 1632–1636