Loops of Light Optically Detect Single Molecules

A new nanoscale light-manipulation method that optically detects single molecules could be used in a variety of photochemistry applications and could help advance technologies for visualizing single molecules and multiple-molecule interactions.

Progress in optically detecting single molecules has been hindered by their weak optical response. Currently, researchers use metal nanostructures to focus light into tiny zones called “hot spots,” which excite electrons on the surface, causing them to oscillate coherently. When shone on a molecule, and with the help of these oscillating electrons, the focused light can increase a molecule’s optical signal to 100 billion times its normal strength, a level detectable by optical microscopes.

The current method, however, has two limitations: The first is that hot spots can become too hot; the second is that they are very small. This means that the heat from hot spots can melt the nanostructure, destroying its ability to channel light effectively. In addition, hot spots produce only a very small cross section in which interaction with molecules can take place. For a single molecule to become detectable, it needs to find the hot spot.

Shining circularly polarized light on ring-shaped nanostructures increases the opportunity for interaction with molecules. (Image: Katholieke Universiteit Leuven)

To overcome these drawbacks, Dr. Ventsislav Valev and colleagues at Katholieke Universiteit Leuven sought to nanoengineer larger spots. The international team began by shining circularly polarized light on nanostructures and found that this could increase the useful area. When they shone light on square-ring shaped gold nanostructures, the scientists observed that the entire surface of the nanostructures was successfully activated.

“Essentially, light is constituted of electric and magnetic fields moving through space,” Valev said. “While with linearly polarized light the fields move in a linear, forward direction, with circularly polarized light, they rotate in a spiral-like motion.”

The circularly polarized light imparts a sense of rotation on the electron density in ring-shaped gold nanostructures, thus trapping the light in the rings and forming “loops of light.” The loops of light cause excited electrons to oscillate coherently on the full surface of the square-ringed nanostructures, rather than in a few concentrated hot spots. This increases the opportunity for interaction with molecules.

“The trick is to try to activate the whole surface of the nanostructure so that whenever a molecule attaches, we will be able to see it,” Valev said. “That is precisely what we did.”

The study appeared online in Advanced Materials.

For more information, visit: www.kuleuven.be/english