Born of the marriage of two cutting edge techniques, a new method can image bundles of DNA strands by sensing vibrations within the molecules. Reported in the 4 June PRL, the method can detect objects only dozens of atoms wide. It might someday enable researchers to map out the constituents of individual biomolecules.
In recent years, researchers have developed techniques for obtaining images sharper than those produced by ordinary light microscopes, which cannot detect features smaller than roughly half the wavelength of the light. In one, they have exploited the fact that when light shines on a nanometer-sized metal point, or “tip,” hovering above a surface, electrons sloshing in the metal pump up the electric field in the light. That effectively casts a spotlight on the material below, illuminating a patch much less than half a wavelength wide. Such “tip enhancement” allows researchers to see how the light interacts with just the tiny volume of material near the tip, so by moving the tip across the sample they can map out exceptionally sharp images.
Meanwhile, others have striven to detect molecular vibrations with greater sensitivity. Light can make molecules vibrate, which in turn changes the frequency of the light–a phenomenon known as Raman scattering. That process can be made more efficient by combining a pair of lasers of two different frequencies. The key is to tune the frequencies so that the difference exactly equals a frequency at which the molecules naturally vibrate. When that happens, a combination of three photons–two from one laser and one from the other–can jostle a molecule to produce a fourth photon. That “anti-Stokes” photon reveals the presence of the vibrating molecule. Such “coherent anti-Stokes Raman scattering” (CARS) can detect and identify tiny amounts of material.
When combined, CARS and tip enhancement can image individual molecules by feeling their vibrations, report Yasushi Inouye and Satoshi Kawata of Osaka University in Japan and their colleagues. They used a pair of near-infrared lasers and a silver-coated silicon tip to elicit CARS from strands of DNA attached to a glass surface. They moved the sample back and forth beneath the illuminated tip and recorded the locations where their detector picked up anti-Stokes photons associated with a particular DNA vibration. The researchers then plotted the data to form an image. They achieved resolution of roughly 15 nanometers, says Inouye, which was good enough to see individual bundles of DNA.
The team hopes to improve the resolution and to detect vibrations at other frequencies. That might ultimately enable them to feel out the individual components of biomolecules, such as the base-pairs in DNA, Inouye says. An alternative technique called scanning tunneling spectroscopy (STS) can already reveal such detail by sending a tiny electric current into different parts of a molecule and measuring the resistance. But STS works only on materials that conduct electricity and provides information about the electrons, not the vibrations.
In principle, the new technique has the potential to reach molecular resolution, says Vahid Sandoghdar of the Swiss Federal Institute of Technology, Zurich. But even with the current resolution, he says, the method would be a valuable technique for imaging biological molecules and materials–if it can be made reliable and reproducible. “Rather than trying to push the resolution to the molecular level,” Sandoghdar says, researchers “should focus on making this a practical tool.”