Spectroscope twist sees single molecules
Since chipmakers extensively use surface effects in building up complex structures on silicon, a recent innovation in observing the behavior of atoms at a material's surface could prove to be an important breakthrough.
Putting a new spin on a standard technique, researchers at the University of Illinois at Urbana-Champaign replaced single-molecule fluorescence spectroscopy with an absorption technique that has the same resolution as atomic-force microscopy, today's highest resolution for observing surface features.
"Single-molecule absorption spectroscopy is an extremely sensitive technique in analytical chemistry for measuring electrical properties of molecules and studying energy transfer on surfaces," said Martin Gruebele, an Illinois professor of chemistry, physics and biophysics. "While most molecules don't fluoresce—limiting the usefulness of single-molecule fluorescence spectroscopy—all molecules absorb, making single-molecule absorption spectroscopy a much more general approach."
Single-molecule fluorescence is highly selective, detecting only molecules that fluoresce at a specific frequency. Researchers consider absorption to be far more sensitive and versatile.
The problem has been thermal noise in the sample and detection system, which masks the signal representing the presence of a molecule. Since the noise is generated by the illuminating laser light, it has been difficult to control.
The new approach, which makes photon absorption clearly observable, tackles the thermal noise problem by illuminating the sample through the back of a transparent silicon substrate or by modulating the light.
- Chappell Brown