New Research


2009/03/01

Mapping of Chemical Interactions Sensed by Single Molecular Probe

−For General Understanding of Chemical Reactions and Functions at Surfaces and Interfaces−

Professor Ken-ichi Fukui, Associate Professor Akihito Imanishi

 Functional groups of molecules have essential importance on how the molecule interacts with other materials or molecules. For example, mesoscopic-scale biomolecules such as DNA or proteins form their predetermined structures for expected functions by hydrogen bonding between the constituent molecular chains.
 We have been developing a novel technique to analyze the chemical properties as well as local structures of material surfaces in nanometer scale by direct measurements of the force sensed by a single molecule with a designed functional group on its head. The force acting between a pair of functional groups is very small, but recent development of atomic force microscopy (AFM), such as noncontact AFM (NC-AFM), has given an opportunity for reliable measurements in several pN magnitude. As is schematically shown in the figure, we have synthesized novel tripod molecules with a photo-actuating elbow, which enables to place or store the functional group (carboxylic group (-COOH) for the bottom figure) depending on the wavelength of light irradiation. By measuring the TiO2 photo-catalyst surface by the carboxylic-group-terminated molecular tip, we have succeeded in identifying surface hydroxyl groups on TiO2 in sub-nanometer scale. Hydrogen bonding is preferentially formed and sensed as larger attractive force when the straight molecule comes on the surface hydroxyl group, but not for the bent molecule. By comparing the two images obtained in the same area for the straight and bent molecules, respectively, hydroxyl species can be chemically identified.
 Various types of functional groups can be chemically coupled on the same tripod. For example, a molecular tip with a redox-active group has been synthesized for studying the electron transfer at liquid-solid interface. Our molecular tip technique can be adopted for analyses of the reactive center of catalysts, chemical species identification during surface reactions, mapping of functional groups and charges on surfaces or mesoscopic scale molecules.
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