Surface Enhanced Vibrational Spectroscopy of Proteins with Plasmonic Nanoantenna Arrays
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Surface Enhanced Vibrational Spectroscopy of Proteins with Plasmonic Nanoantenna Arrays Ronen Adato1,7, Ahmet A. Yanik1,7, Jason J. Amsden2, David Kaplan2, Fiorenzo Omenetto2,3, Mi K. Hong4,7, Shyamsunder Erramilli4,5,7 and Hatice Altug1,6,7,* 1 Department of Electrical and Computer Engineering, Boston University,8 St. Mary’s St., Boston, MA 02215, U.S.A 2 Department of Biomedical Engineering, Tufts University, 4 Colby St., Medford, MA, 02155 3 Department of Physics, Tufts University, 4 Colby St., Medford, MA, 02155 4 Department of Physics, Boston University, 590 Commonwealth Ave., Boston, MA, 02215 5 Department of Biomedical Engineering, Boston University, 44 Cummington St., Boston, MA 6 Materials Science Division, Boston University, 8 St. Mary’s St., Boston, MA, 02215 7 Photonics Center, Boston University, 8 St. Mary’s St., Boston, MA, 02215 *Corresponding author, [email protected] ABSTRACT Infrared absorption spectroscopy is a powerful tool for structural and functional studies of biomolecules. The technique enables direct access to the vibrational fingerprints of molecular bonds in the mid-infrared spectral region (3-20µm). Although intrinsic absorption cross-sections are nearly ten orders of magnitude greater than corresponding Raman cross-sections, they are still small in comparison with those of fluorescent molecules. Sensitivity improvements are therefore required for the method to be applicable to single molecule / molecular layer studies. In this work, we demonstrate the use of lithographically patterned arrays of nanoantennas to enhance the absorption signature of the protein amide-I and II backbone vibrations. Strong absorption signals from monolayer thickness films are obtained. By arranging ensembles of tailored antennas in specific lattices, higher quality factor resonances and increased near-field intensities are possible. These features are leveraged to obtain 104-105 fold signal enhancements and the direct measurement of vibrational spectra of proteins at zepto-mole sensitivity levels. INTRODUCTION Plasmonic nanoparticles offer the unique ability to focus light beyond the diffraction limit owing to their antenna like functionality [1]. Exploiting these strong, localized fields that accompany the plasmonic resonances of metallic nanoparticles has led to the demonstration of signal enhancements of several orders of magnitude in the Raman scattering process, an effect known as surface enhanced Raman scattering (SERS) [2]. This enhancement process can also be extended to the infrared. In analogy to SERS, the application to infrared absorption spectroscopy (SEIRA), has also been demonstrated [3-5]. The bulk of previous SEIRA experiments, however, have obtained enhancements via chemically prepared or roughened metal surfaces. The stochastic nature of these substrates limits enhancement due to the uncertain spatial profile of the enhancement and degree of spectral overlap of the plasmonic resonances with the vibrational modes of interest. The result is weaker absorption signals and a lack of repro
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