Infrared (IR) absorption spectroscopy directly probes the vibrational modes associate with the molecular bonds in a sample by measuring absorption in the mid-infrared spectral region, ~ 3 - 20 microns. IR spectroscopic measurements are thus intrinsically endowed with a level of chemical specificity and information content far exceeding that of most other optical measurement techniques. Despite their potential, IR absorption measurements suffer not only from limited sensitivity, but are severely hindered by the strong, broad absorption of water that overlaps the bands of most organic compounds of interest. While recent surface enhanced infrared absorption (SEIRA) spectroscopy measurements have shown that IR resonant nanoantennas can be leveraged to dramatically increase sensitivity, [2-3] these have all been performed in dry environments and without time-resolution.
In this work we show that utilizing plasmonic nanoantennas not only to for intensity enhancement, but also to efficiently localize and redirect incident radiation enables a highly sensitive, versatile means to perform SEIRA measurements while discarding interfering water absorption. This opens the door for highly sensitive measurements of proteins and other biological significant molecules and nano particles in their native environment.
These capabilities are ideally suited to time-resolved studies of biomolecules and other chemical species at the monolayer level. We demonstrate the performance of our approach by monitoring protein-binding interactions via their specific amide band absorption. Highlighting the exquisite chemical sensitivity of infrared spectroscopy, we also perform measurements on chemically distinct particles. Here we are able to chart the movement of various molecular groups down to the displacement of minute volumes of water.
Our plasmonic technology thus overcomes the limitations of infrared spectroscopy in performing sensitive measurements on trace samples in aqueous environments. Finally, unlike traditional, bulky infrared sampling accessories based on traditional effects such as total internal reflection, our plasmonic approach is chip-based and represents a dramatic improvement in the compatibility of infrared absorption spectroscopy with modern sample preparation and handling technologies, as well as complementary optical measurement techniques.
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 Neubrech, F, et al. Resonant plasmonic and vibrational coupling in a tailored nanoantenna for infrared detection. Phys Rev Lett, 101, 157403 (2008)
 Adato, R, et al. Ultra-sensitive vibrational spectroscopy of protein monolayers with plasmonic nanoantenna arrays. Proc Natl Acad Sci USA, 106, 19227-32 (2009)
 Adato, R and Altug, H. In-situ ultra-sensitive infrared absorption spectorsocpy of biomolecule interactions in real time with plasmonic nanoantennas. Nat Commun. 4, 2154, (2013).