Credit: © 2010 AAAS

Condensed-phase single-molecule spectroscopy, developed over the past two decades, allows molecular properties and dynamics to be observed without the averaging effects associated with ensemble measurements. The fluorescence of molecules is the property most often exploited in this technique, because the low background signal allows the targeted molecule to stand out from its surroundings. Unfortunately, fluorescence-based techniques have inherent problems such as quenching reactions and an obvious reliance on efficient fluorophores.

Other single-molecule techniques are being developed that rely more directly on absorption, and now Michel Orrit and colleagues from Leiden University have imaged non-fluorescing molecules by exploiting their photothermal properties1. The technique relies on the heat released by a molecule after absorbing light, which alters the refractive index of the surrounding medium (glycerol), causing detectable changes in the intensity of a probe beam.

The sensitivity of the technique was first tested using 10- and 5-nm gold spheres, and was then used to image single-strand DNA molecules tagged with two azo dye molecules and fixed to a glass substrate. The dye was chosen because of its excellent heat-dissipation properties, which are inversely related to its fluorescence efficiency. The molecules appeared as spots with a signal-to-noise ratio of approximately 10, and did not fade for at least an hour.