Liquid-Phase Peak Force Infrared Microscopy for Chemical Nanoimaging and Spectroscopy.

Peak force infrared (PFIR) microscopy is an emerging atomic force microscopy that bypasses Abbe's diffraction limit in achieving chemical nanoimaging and spectroscopy. The PFIR microscopy mechanically detects the infrared photothermal responses in the dynamic tip-sample contact of peak force tapping mode and has been applied for a variety of samples, ranging from soft matters, photovoltaic heterojunctions, to polaritonic materials under the air conditions. In this article, we develop and demonstrate the PFIR microscopy in the liquid phase for soft matters and biological samples. With the capability of controlling fluid compositions on demand, the liquid-phase peak force infrared (LiPFIR) microscopy enables in situ tracking of the polymer surface reorganization in fluids and detecting the product of click chemical reaction in the aqueous phase. Both broadband spectroscopy and infrared imaging with ∼10 nm spatial resolution are benchmarked in the fluid phase, together with complementary mechanical information. We also demonstrate the LiPFIR microscopy on revealing the chemical composition of a budding site of yeast cell wall particles in water as an application on biological structures. The label-free, nondestructive chemical nanoimaging and spectroscopic capabilities of the LiPFIR microscopy will facilitate the investigations of soft matters and their transformations at the solid/liquid interface.