Prof. Ryota Iino Profile

Prof. Ryota Iino

at Institute for Molecular Science

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Proceedings Article | 3 October 2023 Presentation + Paper

Sub-tip-radius near-field interactions in nano-FTIR vibrational spectroscopy on single protein particles

Jun Nishida, Akihiro Otomo, Ryota Iino, Takashi Kumagai

Proceedings Volume 12654, 1265403 (2023) https://doi.org/10.1117/12.2676631

KEYWORDS: Vibration, Proteins, Spectroscopy, Infrared spectroscopy, Near field, Atomic force microscopy, Infrared radiation, Molecular spectroscopy, Demodulation, Analytic models

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While tip-enhanced Raman spectroscopy (TERS) has enabled vibrational spectroscopy with single molecule sensitivity and even atomic-scale spatial resolution, a counterpart for infrared spectroscopy has remained elusive regardless of advantages in terms of its versatility and compatibility with unique sets of the ultrafast and nonlinear spectroscopy. Here, we apply nano-FTIR spectroscopy, based on infrared scattering scanning near-field optical microscopy (IR s-SNOM), to detect amide-I vibration modes from a single protein consisting of ~500 amide groups. A protein with an effective radius of a few nm was deposited on an atomically flat gold substrate in the air. By lock-in demodulating the tip-scattered field with harmonics (n = 2 ~ 7) of the tip-tapping frequency, we identify a strong enhancement of the vibrational amplitude for higher harmonics associated with the increasingly localized tip-sample near-field interactions confined below < 10 nm. We then regard the protein as an infrared-resonant dielectric sphere and calculate complex-valued scattering spectra from a tip-protein-substrate hybrid system based on a few analytical models under quasi-static approximation. Such a calculation semi-quantitatively reproduces key features in the observed vibrational response from single protein particles, including the magnitude of the vibrational response in the optical phase and its dependence on the demodulation harmonics. This work bridges the currently established application of nano-FTIR to a relatively large protein complex or molecular monolayer to studies of objects significantly smaller than the tip radius at a few-nm level, paving a path toward single-molecule and atomic-scale infrared vibrational spectroscopy.

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