Science 330, 353–356 (2010)

Detecting single molecules is a critical tool for molecular biology and materials science. It is usually accomplished by coaxing the molecule to emit light. However, many important molecules (including DNA) are not efficient emitters. An alternative approach, called photothermal imaging, involves heating the molecule with a pump beam and observing the effects of this heat on a probe beam. Michel Orrit and colleagues at Leiden University have now shown that this technique can detect a single molecule at room temperature.

The researchers chose to image a molecule specifically designed to quench emission and this molecule was immersed in glycerol, a poor conductor of heat whose refractive index changes strongly as its temperature changes. A pump beam heated the molecule, causing a refractive index change in the surrounding glycerol, creating in turn an effective scatterer of an intense probe beam to which the molecules themselves were transparent. The technique's high signal-to-noise ratio and single-molecule discrimination was made possible by the use of glycerol, as well as by a synchronous detection scheme that modulated the pump beam at a high frequency to eliminate background noise.