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How to shine a light on a single-molecule motor

The ability to observe the motion of molecular machines is likely to accelerate practical applications. Joel F. Hooper reports.

Direct imaging of nanoscale machinery would open many doors for research.
Laguna Design / Getty

Back in 1959 physicist Richard Feynman, speaking of the exciting opportunities for innovation in microscopic engineering, noted there was “plenty of room at the bottom”. He imagined a future where chemists would manipulate individual molecules to create nano-scale machines combat sickness and disease.

For that optimistic vision to become reality, however, will require the right optical equipment, with nano-scientists facing the problem of creating molecular machines so tiny they can’t see them properly.

Traditional microscopy techniques cannot resolve objects smaller than about 200 nm, due to the diffraction limits of light. Electron microscopes can go much smaller than this, but single-molecule imaging below the nanometer scale is still at the very cutting edge of this field and limited to a few labs.

However, an international team led by Johan Hofkens, of the University of Leuven, Belgium, and Nobel Prize winner Ben Feringa report in the Journal of the American Chemical Society that they have successfully used an unusual technique called “defocused wide-field imaging” to observe a single molecular rotor in real time, using UV/visible light.

Left: Structure of the surface-bound molecular motor. Right: Orientations of the motor were revealed via wide-field defocused fluorescence microscopy.
Krajnik et al.

A UV laser drives the motor, while a second laser excites a light-emitting molecule at the end of its “propeller”. The orientation of this beacon can be determined by looking at the pattern of photons beyond the focal plane, allowing the movement of the molecule to be observed. Using this technique, the team have observed a molecular motor rotating in a single direction around a carbon-carbon double bond, driven by UV light.

The ability to easily observe the motion of molecular machines could bring researchers out of the dark, and accelerate this field of research and its practical applications, including nano-machines that can fight disease. Perhaps in the not-to-distant future, Feynman’s vision of “swallowing the doctor” will become a reality.

Joel Hooper is a senior research fellow at Monash University, in Melbourne, Australia.
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