Nature 469, 385–388 (2011)

Surface-enhanced Raman spectroscopy (SERS) allows researchers to detect small numbers of molecules because the light scattered by the molecules can be enhanced by factors of up to a million. SERS relies on light being concentrated into hotspots by nanoscale features on the surface, but these hotspots — which are electromagnetic rather than material structures — are too small to be imaged with existing optical techniques. Now Xiang Zhang and co-workers at the University of California in Berkeley and the Lawrence Berkeley Lab have overcome this obstacle by using molecules to measure the electromagnetic field of individual hotspots.

The Berkeley team submerged their samples — thin layers of aluminium films and silver nanoparticle clusters — in a solution of dye molecules and recorded the fluorescence from the molecules with a charge-coupled device. The molecules were moving so fast that their fluorescence was a background blur. However, when one of the molecules was adsorbed at the hotspot, it stopped moving and its fluorescence was enhanced by the electromagnetic field there, so it showed up as a bright spot on the image. By using a dye solution that was so weak that only one molecule was adsorbed at a given time, Zhang and co-workers were able to image single hotspots as small as 15 nm with an accuracy down to 1.2 nm.