Credit: © 2008 APS

In theory, infrared spectroscopy can detect the vibrations of a very small number of molecules, but this requires some way of enhancing the vibrational signals. Now Annemarie Pucci at the University of Heidelberg in Germany and co-workers1 have used a gold nanowire as a 'nanoantenna' to amplify vibrational signals from less than one attogram (10-18 grams) of molecules.

Researchers have previously managed to enhance molecular vibrations by three orders of magnitude by using metal particles that increase the local electromagnetic field. Pucci and colleagues achieve enhancements of over five orders of magnitude with their gold nanoantenna comprising just a few crystalline gold grains. The nanoantenna is coated with the long-chain molecules octadecanthiol, and exposed to infrared light, which excites resonances in the gold.

The nanoantenna greatly enhances the signal strength from vibrations of the octadecanthiol molecules. The amplification is far greater than on a flat gold surface, because the field is significantly enhanced at the ends of the nanoantenna where there are about 150,000 molecules — less than one attogram. However, for this low detection limit to be maintained for other molecules, the length of the nanoantenna must be carefully tailored so that its infrared resonance matches the frequencies of the molecular vibrations.