Single-molecule-level detection of interfacial molecular structures and ultrafast dynamics†

Elucidating the ultrafast dynamics of interfacial molecules at the single-molecule level is pivotal for advancing our understanding of fundamental chemical and biological processes. Here, for the first time, we realized detection of ultrafast vibrational dynamics by a novel technique that integrates femtosecond sum frequency generation vibrational spectroscopy (SFG-VS) with nanoparticle-on-mirror (NPoM) nanocavities (NPoM-SFG-VS). Using a symmetric stretching vibrational mode of para-nitrothiophenol (ν_NO2) as a probe, we have successfully identified signals from self-assembled monolayers (SAMs) comprising ∼60 molecules, demonstrating the single-molecule-level sensitivity of the NPoM-SFG-VS. The dephasing time and vibrational relaxation time of ν_NO2 at the single-molecule level were determined to be 0.33 ± 0.01 ps and 2.2 ± 0.2 ps, respectively. By controlling the solution concentration used to prepare SAMs (C), a correlation between peak frequency of ν_NO2 and C is established. It was found that single-molecule-level detection was achieved at C ≤ 10⁻¹⁰ M. With this protocol, microregion distribution of interfacial molecule number can be mapped using NPoM-SFG imaging. This work provides insights into the structures and vibrational dynamics of individual interfacial molecules, aiding in precise engineering of surface properties and reactivity.