Mid-infrared plasmonics for ultra-sensitive spectroscopy of biomolecular interactions
Mid-infrared (IR) absorption spectroscopy contrasts with numerous other biosensing methods in that it can directly probe molecular structure via bond specific vibrational modes and monitor structural changes even in the absence of any mass transfer. The technique is therefore a valuable tool for a wide range of applications critical for understanding basic biological function and important aspects of e.g. disease progression and treatment. Despite these attractive features, IR absorption spectroscopy is limited as its acquired signal depends on a molecular bonds intrinsic absorption cross-section and path length (via Beers Law). Sensitivity issues therefore restrict applications to a limited set of strong bands and/or relatively thick samples. Additionally, due to the strong absorption of water in theIR, measurements in fluid are cumbersome, requiring specialized equipment and extremely high analyte concentrations. Plasmonic nano-structures supporting resonances at mid-IR wavelengths offer an attractive means with which to overcome many of these limitations. In particular, plasmonic resonances result in strongly enhanced near-field intensities confined to the surface of metallic particles, which allow one to dramatically increase the absorption signal of molecules. This concept is termed SEIRA (Surface Enhanced Infrared Absorption Spectroscopy). This thesis focuses on leveraging IR-resonant plasmonic nanostructures to enable sensitive SEIRA measurements of molecular monolayers, even in aqueous solutions. In achieving this capability, we first develop methods for utilizing nano-particle interactions in engineered arrays and demonstrate their application to the optimal enhancement of protein absorption bands. We then demonstrate multi-band antennas, capable of simultaneously probing several vibrational bands. Thirdly, we present the first demonstrated use of engineered IR antennas for real-time, in-situ IR spectroscopy measurements on a series of protein and nano-particle binding interactions. By leveraging the far-field scattering properties of plasmonic nano-antennas in addition to the associated near-field enhancement and localization, our method enables a unique chip-based spectroscopy technology that is highly compatible with modern sample preparation and handling techniques. Finally, we present a theoretical treatment of the interaction between our engineered resonances and the natural molecular ones. Our general model correctly predicts detailed absorption spectra and a number of effects dependent the experimental setup and plasmonic antenna design.
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