Self-Assembled Tetrapod Molecules on Graphite Form Nanopockets with Attractive Force Distribution Studied by 3D Scanning AFM

The precise design of molecular architectures through molecular self-assembly on solid substrates is a promising approach for developing next-generation materials and their functions. The molecular architecture having nanopockets that can capture target molecules with high selectivity and affinity has immense potential for use in sensing devices and separation materials with molecular recognition functions. Toward the design and construction of molecular architecture with nanopockets, we have been exploring molecular building blocks having three-dimensional (3D) extended structures. Furthermore, we have developed a molecular-scale analytical technique based on atomic-resolution atomic force microscopy in liquid.<br/>For the applications of molecular nanopockets, not only nanoscale surface structures such as molecular orientation, but also the recognition mechanism of how guest molecules and host nanospaces interact is an important research topic. However, the lack of a measurement method for interaction forces in 3D nanospace has made it difficult to improve the understanding of the recognition mechanism. We have developed a 3D scanning atomic force microscope (3D-AFM) that can visualize hydration structures and surface fluctuation structures at the solid-liquid interface with sub-nanometer resolution. Conventional AFMs scan the tip only in the XY direction, but 3D-AFM scans the tip in the Z direction in addition to the XY direction, making it possible to obtain 3D force distribution information. Combined with the frequency shift detection method, 3D-AFM can also detect the interaction force acting on the tip at the picoNewton (pN) level. Based on this principle and performance, we believe that 3D-AFM technology can be applied as an analysis tool for "interaction force measurement" as well as "structural measurement."<br/>We have performed direct visualization with 3D-AFM of the characteristic interaction force in 3D nanospaces at the self-assembled molecules/water interface. Two types of tetrakis(4-ethynylphenyl)methane (TEPM) derivatives with oligo(ethyleneglycol) (EG_n) chain (EG₂-TEPM, EG₄-TEPM) were used as the building blocks and these TEPM derivatives form self-assembled monolayers on graphite. In the monolayers, there are hydrophobic nanopockets (diameter: approx. 1.7 nm) surrounded by the aromatic structure of the TEPM derivatives. We performed 3D-AFM measurement at the self-assembled monolayers/water interface. 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 interaction forces originate from the nanopockets and that the nanopockets of EG₂-TEPM are slightly smaller than those of EG₄-TEPM. The number of hydrogen bonds forming the water molecule network and the water molecule density are reduced in restricted spaces, such as inside the EG₂-TEPM nanopockets, compared to the bulk water. The visualized attractive interaction forces seem to originate from the water molecule behavior in the nanopockets. 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 and the design of novel nanodevices.