Nanoimprinted SERS Sensors for Chemical and Biological Detection
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Nanoimprinted SERS Sensors for Chemical and Biological Detection Guinevere Strack1,2, Michaela Fitzgerald1,2, Junwei Su3, Margery G. H. Pelletier4, Peter Gaines4, Hongwei Sun3, Pradeep Kurup2*, Ravi Mosurkal1* 1 Bio-Science & Technology Team, Materials Science and Engineering Branch, US Army Natick Soldier RDEC, Natick, MA 01760, U.S.A. Departments of 2Civil & Environmental Engineering, 3Mechanical Engineering, and 4Biological Sciences, University of Massachusetts Lowell, Lowell, MA 01854, U.S.A. *Corresponding authors: [email protected]; [email protected] ABSTRACT Herein, we demonstrate a facile, rapid, and scalable method to fabricate polymer-based gratings for surface-enhanced Raman spectroscopy (SERS) sensors. To accomplish this, epoxy nanostripe arrays on silicon substrates were prepared using thermal annealing and UV-crosslinking. After preparation of the nanostripe arrays, the surface was briefly treated with oxygen plasma, which decreased the surface energy and enabled the growth of AgNPs on the polymer surface using a simple, low-cost, aqueous-based synthesis procedure. The SERS substrates exhibited a detection limit of ~1 pM using rhodamine 6G (R6G). In addition, preliminary work with E. coli DH5 showed that the nanoimprinted substrates can be used to obtain Raman spectra of washed bacteria cells. INTRODUCTION Surface-enhanced Raman scattering (SERS) provides sensitive and non-destructive characterization of various molecules, ranging from simple compounds composed of a few atoms to whole bacteria cells.[1-11] Using this method, single molecule-level detection has been reported with enhancement factors (EFs) of several orders of magnitude.[7] The EF is dependent on the formation of “hot spots” provided by noble metallic nanostructures; however, hotspots are typically not evenly distributed throughout the surface of the sensor, and therefore, the EF is inconsistent. Although, roughing the surface of the sensor using a repeating pattern can improve the EF, it is difficult to obtain a consistent Raman signal. Furthermore, even if a consistent signal is obtained, techniques used to fabricate SERS-based sensors are typically not amendable to scale-up and are therefore, not viable options for technological development. Herein, we demonstrate a facile, rapid, and scalable method to fabricate polymer-based gratings for SERS analysis. To accomplish this, cross-linked epoxy “nanostripe” arrays were prepared using thermal nanoimprint lithography and then decorated with silver nanoparticles (AgNPs). Compared to a planar surface, nanometric gratings, which are high surface area structured substrates, offer a higher density of hotspots and a greater number of binding sites for molecules. Moreover, such periodic arrays (or gratings) have unique optical resonance features that have been shown to further enhance the SERS signal.[12, 13] After preparation of the nanostripe arrays, the surface was briefly treated with oxygen plasma, which decreased the surface energy and enabled the growth of AgNPs on the polyme
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