Figure 3. Nano-FTIR sensitivity to protein orientation demonstrated with PM.

(a) Topography of PMs on a silicon substrate. Scale bar, 400 nm. (b) Illustration of near-field probing of PM. (c) Infrared near-field phase images ϕ₃ at two different frequencies, 1,660 cm⁻¹ (left) and 1,720 cm⁻¹ (right; 19 min total image acquisition time). (d) Nano-FTIR (red) and GI-FTIR spectra of an ensemble of horizontally absorbed PMs on a gold substrate (blue). The red dot in the topography image marks the position where the nano-FTIR spectra (average of 60 interferograms; 23 min total acquisition time; 8 cm⁻¹ resolution; × 4 zero filling) was recorded. The position marked by the yellow dot and R indicates where the reference spectrum was recorded.