Scattering-type scanning near-field optical microscopy (s-SNOM) is based on AFM and tip-enhanced IR-scattering. It has emerged to the forefront of key technologies to study the chemical composition of synthetic and biological materials, not on the macroscopic scale like conventional IR-spectroscopy, but far beyond the diffraction limit of light. Detection of the elastically tip-scattered infrared light simultaneous to AFM imaging yields nanoscale resolved near-field images and broadband near-field spectra (nano-FTIR) with &lt;20nanometer spatial resolution. Thereby we achieve high impact in the field of biochemistry. The upcoming technology is able to demonstrate acquisition of material-specific nano-FTIR absorption spectra of biomaterials at unmatched spatial resolution and sensitivity. It is demonstrated that nano-FTIR can probe protein secondary structure on the nanometer scale and with sensitivity to individual protein complexes. Ferritin, a globular protein complex of 12nm in diameter can clearly be identified by nano-spectroscopy due to the dominant α-helical protein structure. This indicates that nano-FTIR is sensitive to about 4000 amino acids, corresponding to about 5000 C=O and N-H bonds, respectively. Studying the protein structure in purple membranes show that s-SNOM can also reveal the well-defined orientation of the transmembrane α-helices by coupling selectively the amide I molecular vibrations normal to the membrane surface. Also, an investigation of a mixture of tobacco mosaic virus (TMV) and insulin aggregates on silicon enabled unambiguous identification of the respective structures. Even though the morphology of TMV and insulin resembles, the difference in ratio of absorption of amide I and amide II allowed to visualize the nanoscale distribution of TMV and insulin in the heterogeneous sample. The discussed studies consitute a solid foundation for infrared nanobiospectroscopy, which is a prerequisite for highly impactful future nano-FTIR applications in biochemical and biomedical research.

nano-FTIR: Nanometer-scale near-field spectroscopy for probing protein secondary structure of individual protein complexes

255th National Meeting (2018)

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