Abstract
Surface enhanced infrared absorption spectroscopic studies (SEIRAS) as a technique to study biological molecules in extremely low concentrations is greatly evolving. In order to use the technique for identification of the structure and interactions of such biological molecules, it is necessary to identify the effects of the plasmonic electric-field enhancement on the spectral signature. In this study the spectral properties of 1,2-Dipalmitoyl-sn-glycero-3 phosphothioethanol (DPPTE) phospholipid immobilized on gold nanoantennas, specifically designed to enhance the vibrational fingerprints of lipid molecules were studied. An AFM study demonstrates an organization of the DPPTE phospholipid in bilayers on the nanoantenna structure. The spectral data were compared to SEIRAS active gold surfaces based on nanoparticles, plain gold and plain substrate (Si) for different temperatures. The shape of the infrared signals, the peak positions and their relative intensities were found to be sensitive to the type of surface and the presence of an enhancement. The strongest shifts in position and intensity were seen for the nanoantennas, and a smaller effect was seen for the DPPTE immobilized on gold nanoparticles. This information is crucial for interpretation of data obtained for biological molecules measured on such structures, for future application in nanodevices for biologically or medically relevant samples.
Highlights
1662 cm−1 and 1258 cm−1, tailored to excite the plasmonic resonance around 2814 cm 1, 1662 cm−1 and 1258 cm−1, which correspond to the enhancement of the electric field (EF) in the vibrational bands which correspond to the enhancement of the electric field (EF) in the vibrational bands regions associated with the stretching modes of the CH2, C=O, and PO2−−, that correspond regions associated with the stretching modes of the CH, C=O, and PO2, that correspond to the most important functional groups of the lipids. 2 to the most important functional groups of the lipids
We have used the enhancement of plasmonic infrared nanoantennas and the chemical specificity of infrared spectroscopy to investigate the organization of Dipalmitoyl-sn-glycero-3 phosphothioethanol (DPPTE) tethering lipid model membrane
The combination of SEIRAS with temperature dependence analysis allows to monitor in situ the conformation and dynamics of the functional groups of the thiophospholipid on the nanostructured surface and determine its phase transition temperature
Summary
Plasmonics have become one of the most vibrant areas in research with technological innovations impacting fields from telecommunications to medicine. Gold nanoantennas control infrared and visible light at the nanoscale through excitation of collective electron oscillations known as plasmons (or localized surface plasmons), concentrating into small subdiffraction-limited “hot spots” in the feed-gap of the antenna. Huge local field intensities of several orders of magnitude higher than the incident one have been achieved within these structures, allowing the study of light-matter interaction at the nanoscale and to detect picomolar amounts of biological samples, including large protein complexes. An important achievement is that the optical resonances can be tuned with the geometrical shape and dielectric constant of the nanostructures and with the illumination properties (polarization, wavelength) and can be used for sensitive infrared detection
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