META 2021, META'12

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Nanoantenna enhanced infrared spectroscopy of molecules
Jörg Bochterle, Frank Neubrech, Annemarie Pucci

Last modified: 2011-12-09

Abstract


Under certain conditions light interacts with the conduction electrons in metals. Such conditions are fulfilled for small nanoparticles with dimensions in the order of the wavelength of light or smaller. The resonant excitation is called localized surface plasmon resonance (LSPR) and features a large enhancement of the local electrical fields in the vicinity of the metal nanoparticles, which act in this case as nanoantennas. The strong fields are of great interest for sensing applications. Placing molecules in these hotspots, the plasmon can couple to vibrational bands of the molecule, which are in the infrared spectral range. When both resonance frequencies match, the coupling is maximal, giving rise to the strongest enhanced signal of the molecule. This principle has been proved for example to detect a self assembled monolayer of Octadecanthiol molecules which were chemisorbed to the surface of a gold nanoantenna.[1] It can also be applied to detect more complex molecules, like biologically relevant proteins or antibodies. Therefore the surface of the nanoantenna is functionalized with the corresponding antibody so that only its antigen will bind. Any change in the enhanced spectral signature can then be assigned to the analyte.

To optimize the sensing of biomolecules using nanoantennas we covered nanoantennas with GIPC1 protein. To do so, we immersed the nanoantennas supported on CaF2 substrate aqueous solution of the protein for 2h. After rinsing the sample with pure water and drying with nitrogen gas, we performed surface enhanced infrared spectroscopy. The lengths of the antennas are chosen to tune the plasmonic resonance to the amide I and amide II bands of the protein. We see their enhanced signal in the form of Fano-type antiabsorption peaks in the infrared spectrum.

In contrast to these rather big molecules, we performed similar measurements with adsorbed carbon monoxide (CO) molecules. To achieve binding of the CO to the gold surface of the nanoantennas we had to work in ultra high vacuum (UHV) conditions and cool the samples with liquid nitrogen. The differences arising from the size of the adsorbed molecule will be presented in this contribution.

Keywords


plasmonic sensing; biomedical application