Sub-Tip-Radius Near-Field Interactions in Nano-FTIR Vibrational Spectroscopy on Single Proteins

• DOI: 10.1021/acs.nanolett.3c03479
• Publication Date: 2024-01-09T14:03:35Z
• Authors: Jun Nishida; Akihiro Otomo; Takanori Koitaya; Akitoshi Shiotari; Taketoshi Minato; Ryota Iino; Takashi Kumagai
• Keywords: Radius; Spectroscopy, Fourier Transform Infrared; Nanotechnology; Microscopy, Atomic Force; Vibration

Tip-enhanced vibrational spectroscopy has advanced to routinely attain nanoscale spatial resolution, with tip-enhanced Raman even achieving atomic-scale and submolecular sensitivity. infrared techniques, such as nano-FTIR AFM-IR spectroscopy, have also enabled the chemical analysis of molecular monolayers, inorganic nanoparticles, protein complexes. However, fundamental limits nanospectroscopy in terms resolution sensitivity remained elusive, calling for a quantitative understanding near-field interactions nanocavities. Here, we demonstrate application probe amide-I vibration single consisting ∼500 amino acid residues. Detection higher tip tapping demodulation harmonics up seventh order leads pronounced enhancement peak amplitude resonance, originating from sub-tip-radius geometrical effects beyond dipole approximations. This characterization single-nanometer opens path toward employing at subnanoscale single-molecule levels.