Li, J., Pang, J., Yan, Z., Jahng, J., Li, J., Morrison, W., Liang, J., Zhang, Q., Xia, X.,
Tip-enhanced infrared (IR) spectra and imaging have been widely used in cutting-edge studies for the in-depth understanding of the composition, structure, and function of interfaces at the nanoscale. However, molecular monolayer sensitivity has only been demonstrated at solid/gas interfaces. In an aqueous environment, reduced sensitivity limits the practical applications of tip-enhanced IR nanospectroscopy. Here, we present an approach to hypersensitive nanoscale IR spectra and imaging in an aqueous environment with a combination of photoinduced force (PiF) microscopy and resonant antennas. The highly confined electric field in between the tip and antenna amplified the PiF, while the excitation via plasmon internal reflection mode minimized the environmental absorption. A polydimethylsiloxane (PDMS) layer (∼1–2 nm thickness) functionalized on the atomic force microscopy tip and a self-assembled monolayer of bovine hemoglobin on the antenna was identified successfully in water with varying antenna sizes. Sampling volume of ∼604 chemical bonds from PDMS was demonstrated with sub-10 nm spatial resolution, confirmed by electric field distribution mapping on the antennas. For the first time, this platform presents the application of PiF microscopy in aqueous environments, providing a brand-new configuration to achieve highly enhanced nanoscale IR signals, promising for future research of interfaces and nanosystems in aqueous environments.