CPM Seminar

High-resolution Investigations of Molecular Systems in Various Environments by FM-AFM

Kei Kobayashi, Hirofumi Yamada

Department of Electronic Science & Engineering
Kyoto University

In this talk our recent research activities on FM-AFM investigations of organic molecules are presented. Since the measured interaction forces by AFM can include a wide variety of local interactions such as a simple van der Waals force, various electric forces and short-range chemical forces, various material properties on a nanometer scale can be also detected simultaneously with AFM imaging of surface structures. In particular, Kelvin probe force microscopy (KFM) and dissipation energy mapping method are powerful tools for the investigations of the surface properties. In KFM, surface charge density, contact potential difference and various electrical properties can be directly studied by the measured electrostatic forces. On the other hand, the dissipation of the vibration energy which is measured by the amplitude reduction of the cantilever vibration reflects the local mechanical properties such as molecular-scale deformations.

In contrast to atomic-scale imaging capabilities of FM-AFM in UHV environments, high-resolution imaging in liquids is severely hindered by the extreme reduction of the Q-factor due to the hydrodynamic interaction between the cantilever and the liquid. Recently the use of the small amplitude mode and the large noise reduction in the cantilever deflection sensor brought great progress in FM-AFM imaging in liquids. We found that the noise was effectively suppressed by decreasing the laser light coherence, which was experimentally performed by modulating the laser power with a high frequency signal.

We have so far succeeded in imaging several biomolecules in liquids on a molecular scale using the improved FM-AFM. Purple membranes consisting of hexagonally packed bacteriorhodopsin (bR) protein molecules were imaged in phosphate buffer solution. The individual bR protein trimers making a regular array were clearly resolved. The difference in structure between the cytoplasmic and the extracellular sides of the membrane was differentiated by the high-resolution images obtained. In addition, by force mapping method in FM-AFM we recently succeeded in visualizing the hydration structures, which play essential roles in the stabilization of higher-order protein structures as well as in various bio-function processes. The frequency shift signal was recorded while the AFM tip was scanned with in the vertical and horizontal directions on a mica substrate in water. The results reveal that there are four water layer structures with slightly different spacings on the surface and that the closest layer to the surface has an atomic-scale horizontal structure reflecting the surface crystal structure of muscovite mica.