Photoswitchable Molecular Tip Designed for AFM Chemical Identification

Daniel S. Burgess

Chemists at Tokyo Institute of Technology and at the University of California, Santa Barbara, have designed and synthesized a photoswitchable molecule that they suggest could be used in conjunction with atomic force microscopy (AFM) to identify chemical species with submolecular resolution.

“The current state of the art in molecular identification is like trying to recognize someone by feeling their face while wearing boxing gloves,” said Yoko Yamakoshi, an associate researcher at the university’s Center for Polymers and Organic Solids. The new approach, she said, not only offers scientists thinner gloves, but also enables them to change them without stopping the experiment.

The photoswitchable molecule features a rigid adamantane “core” section and a tripod formed by three linear acetylene “legs” that terminate in thiol “feet” for stable adsorption on the atomic force microscope (AFM) probe surface. When exposed to 360-nm radiation (hν₁), the azobenzene “elbow” section converts to the cis form, bending the tip toward the base of the molecule. Upon exposure to 450-nm light (hν₂), it returns to the trans form. The photoswitching behavior makes it possible to change the function of the tip and observe the same area on the sample. Courtesy of Ken-ichi Fukui.

The 4-nm-long molecule features a rigid adamantane section that connects the tip and a tripod formed by three linear legs that terminate in thiols for stable adsorption on the AFM probe surface. The tip incorporates an azobenzene section that transforms when exposed to 360-nm radiation, bending the tip toward the base of the molecule. Upon exposure to 450-nm light, it returns to its original shape.

That photoswitching behavior makes it possible to change the function of the tip and then observe the same area on the sample. Ken-ichi Fukui, an associate professor at the institute’s department of chemistry, said that it is impractical if not nearly impossible to do this using standard scanning probe techniques such as AFM; for example, by switching tips in the instrument and returning to the feature of interest.

The molecular tip was used to image an Au(111) surface. Using the change in length of the molecule (Δh) as it switched between its two isomers, the researchers resolved a virtual step of approximately 1 nm in height.

“It takes too much time to scan the macroscopic area in high resolution,” he explained, “and usually the observation area is limited to less than the scale of a micrometer.”

In proof-of-principle experiments using an ultrahigh-vacuum AFM from JEOL Ltd. of Tokyo, the researchers imaged an Au(111) surface with the molecular tip. Using the change in length of the molecule as it switched between its two isomers, they resolved a virtual step of approximately 1 nm in height. The excitation source in the study was a 100-W xenon lamp, and the desired wavelengths were selected using bandpass filters from Asahi Spectra Co. Ltd. of Tokyo.

Yamakoshi said that the next step is to synthesize functional groups to be used at the end of the molecular tip for the chemical identification of a sample by monitoring its noncovalent interaction with the probe. In the biochemical realm, she suggested, the switchable functionalized probes could be used to investigate substrate/enzyme, ligand/receptor, DNA/DNA and DNA/protein interactions.

Fukui added that the approach could enable the user to identify reaction intermediates that would offer insights into catalytic reactions. He also envisions it being adopted for such applications as the manipulation of individual molecules.

Journal of Physical Chemistry B, Feb. 9, 2006, pp. 1968-1970.