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