99th CEMS Colloquium

Title

Visualizing Unexplored Nanoworld by Innovative In-liquid AFM Technologies

Abstract

Atomic force microscopy (AFM) has a unique capability of imaging atomic-scale structures in a liquid environment regardless of the conductivity of the sample. Thus, it has been widely used for various studies in life science, interfacial science and electrochemistry. However, there have remained many unseen structures and dynamics in liquid environments. Examples include protein dynamics inside living cells, charge distributions inside electric double layer, the spatial distribution of molecular chains inside swollen polymers at a polymer/water interface. These unseen structures mostly consist of atoms, ions, or molecules not firmly fixed onto a substrate. To visualize such unexplored nanoworld by AFM, we have been developing various innovative nanoprobe technologies. For example, we have developed in-liquid frequency modulation AFM (FM-AFM) and enabled atomic-resolution imaging even in liquid environments. In addition, we have improved the speed of FM-AFM from 1 frame/min to 1 frame/s and succeeded in visualizing calcite crystal growth and dissolution processes with atomic-scale resolution¹. These observations clarified that calcite forms a Ca(OH)₂ monolayer as an intermediate state of growth and dissolution processes. Meanwhile, we have also extended the operation principle of FM-AFM from 2D to 3D and made it possible to visualize subnanometer-scale 3D hydration structures². This work has stimulated many subsequent works on the development of similar AFM techniques and their applications to the studies of various hydration structures on minerals and biomolecules. Furthermore, we also started to apply this technique to visualize the 3D distribution of ionic liquid on a gold surface and its dependence on the applied bias voltage. These observations suggest that 3D-AFM should be capable of imaging inside of various 3D objects as long as they have a self-organizing capability. To explore this possibility, we so far imaged the 3D structure of isolated ethylene glycol chains, lubricants adsorbed on a hard disk, artificially created a 3D model structure made of carbon nanotubes, and interfaces between polymers and water. More recently, we demonstrated 3D imaging of intra-cellular nanostructures by 3D-AFM³. By looking at these unseen structures, we are investigating nanoscale mechanisms of various in-liquid phenomena.

1. K. Miyata et al.: Nano Lett. 17 (2017) 4083.
2. T. Fukuma et al.: Phys. Rev. Lett. 104 (2010) 016101.
3. M. Penedo et al.: Sci. Adv. 7 (2021) eabj4990.