Subnanometer-Scale Visualization of Interaction Forces at Self-Assembled Molecules/Water Interfaces with 3D-AFM

The formation of self-assembled structures with organic building blocks on solid materials through a bottom-up process has been studied by many researchers. Three-dimensional (3D) self-assembled structures are promising for functional devices such as separation and storage. Molecular recognition utilizing nanospaces in self-assembled structures is an important research topic for their applications. However, understanding the recognition mechanism, for example, how the nanospace in a host structure interacts with the guest molecule, is still challenging. One of the main obstacles is a lack of analytical methods for investigating interaction forces acting in 3D nanospaces. 3D scanning atomic force microscopy (3D-AFM) has been developed as a "structural measurement" technique that allows us to visualize the spatial distributions of hydration and surface fluctuating structures in the nanospaces at the solid/liquid interfaces. Tip scanning of conventional AFMs is only in the XY direction, whereas the tip is scanned in the Z direction in addition to the XY direction in the 3D-AFM imaging. The Z + XY tip scanning makes it possible to obtain 3D force distribution images. Combining the 3D-AFM with a frequency-shift detection method, the interaction forces acting on the tip are detectable at the pico-Newton (pN) level. Based on the principle and performance, we believe that the 3D-AFM technique can be applied as an analytical tool for not only "structural measurement " but also "interaction force measurement.”<br/>In this study, we demonstrated the direct visualization of the interaction forces in 3D nanospaces at self-assembled molecules/water interfaces with the 3D-AFM technique. Two types of tetrakis4-ethynylphenylmethane (TEPM) derivatives with oligo(ethyleneglycol) (EG_n) chain (EG_2-TEPM, EG₄-TEPM) were used as the building blocks. The TEPM derivatives form self-assembled monolayers on graphite. In the monolayers, there are hydrophobic pockets (diameter: approx. 1.7 nm) surrounded by the aromatic structure of the TEPM derivatives. The interaction forces in hydrophobic pockets were measured by the 3D-AFM in water. As a result, characteristic localized attractive interaction was visualized only at the EG₂-TEPM monolayer/water interface, and analysis of the 3D-AFM images suggests that the attractive interactions originate from hydrophobic pockets. The number of hydrogen bond networks of water in the hydrophobic pockets of EG₂-TEPM monolayer is small compared to bulk water, resulting in the low density of water molecules. The attractive interactions are likely caused by the low density of water molecules in the hydrophobic pockets. Our results demonstrate that the 3D-AFM technique has the capability of visualizing the characteristic interaction forces at the single molecule level, suggesting future contributions to the elucidation of the molecular recognition mechanisms.