A new Raman technique has allowed Swiss chemists to localize and determine with precision the chemical nature of individual molecules on a surface. Renato Zenobi of ETH Zurich and colleagues say their approach will be of special interest in nanotechnology applications as identification is now available on a scale of just 10 nanometres.

Single-molecule detection has until now relied on fluorescence techniques, but these do not allow direct identification of the substances in question. In contrast, Raman spectroscopy, can provide a true fingerprint of the molecules in a sample. Zenobi and his colleagues have now taken the method down to the level of individual molecules.

Put simply, the ETH team's approach involves amplifying considerably the weak signal emitted by a single molecule bathed in laser light. To do so, they combined two phenomena - the intensity boost experienced when a sample is supported on a silver or gold substrate and the scanning of a sample with a silver or gold tip moved above it during the Raman measurement. A silver tip of similar in size to that used in a scanning tunnelling microscope and is moved to scan an area of the gold substrate, approximately 10 by 10 nanometres square. By applying this approach, Zenobi's team was able to amplify the attendant Raman signal by a factor of 10⁷.

The team demonstrated proof of principle using two different substances, and suggest that the technique would be amenable to all compounds. They have proved that it is single molecules that are being detected on the basis of dilution studies in which signals of the same intensity as before are recorded, but at fewer locations on the substrate. The team also observed the signals to fluctuate over a period of a few seconds, this they say is additional proof that individual molecules are being addressed; clusters of molecules would not show such evidence of molecular motion. the spontaneous loss of signal from points on the substrate is indicative of laser decomposition of an individual molecule at that point.

The researchers envisage numerous applications for their new method. In principle, it is now possible to determine with high precision on thin samples of a material which substances are present and where. Such measurements could supply useful information in biology, environmental analysis, and also in the preparation of new materials.

"There are two very important applications for our work," Zenobi told SpectroscopyNOW, "molecular electronics and catalysis."

"In molecular electronics, for instance, individual molecules are being used as electronic elements, switching such elements would be accompanied by a change in conformation that would be immediately reflected in a change in the Raman spectrum," he adds, "Molecular diagnostic and analytical methods for this emerging field are largely missing, our methodology could make an important contribution." Similarly, in the field of catalysis, studies of the precise course taken by reactants as they interact with a catalytic surface to form a product might made accessible as never before. "Identification of small numbers of molecules in tight spaces, i.e. the nanoscale metallic "active sites" on catalysts, could be achieved with our methodology," Zenobi told us.

Related links:

• J Phys Chem C, 2007, 111, 1733-1738
• Zenobi Group Page

Article by David Bradley