Local Science – International Impacts – Single Molecules

Eric Le Ru, Matthias Meyer, and Pablo Etchegoin from the MacDiarmid Institute at Victoria University have developed a method of detecting and tracking single molecules using Surface-Enhanced Raman Spectroscopy (SERS).

SERS is being developed now as a possible technique to monitor extremely small quantities of molecules. Possible applications could be: revealing the presence of performance enhancing drugs, tracing illegal substances in forensic science and the treatment/detection of diseases by tracking single molecules as they travel through the body.

This breakthrough made an international impact when it was reported in the prestigious Journal of Physical Chemistry with a news story in Analytical Chemistry. The article is copied here with permission from the journal publishers.

Single Molecule SERS

Although several researcgers have reported single-molecule (SM) sensitivity in surface-enhanced Raman scattering (SERS), all of the studies have relied on indirect evidence.   To provide more direct and convincing evidence of SM-SERS, Eric Le Ru, Matthias Meyer, and Pablo Etchegoin and colleagues at Victoria University of Wellington (New Zealand) carried out SERS measurement on a two-analyte mixture.  The results provide the first unambiguous proof of SM-SERS,from the MacDiarmid Institute at Victoria University have developed a method of detecting and tracking single molecules using Surface-Enhanced Raman Spectroscopy (SERS).

In the new approach, the researchers prepared a colloidal solution with equal concentrations of two dyes – BTZ and RH6G.  They used a relatively high concentration (100 nM) of dyes to ensure that at least one molecule was present at most active sites.  They reasoned that the ERS signal should always be a mixture of the two dyes because of the large number of molecules.  Thus, if a SERS signal was observed for just one dye, it would provide evidence for a small number of molecules.

The researchers collected a series of 1000 SERS spectra from the two-dye mixture as well as from control samples containing each of the dyes alone.  The control samples showed that the two dyes have clearly distinguishable SERS spectra.  The mixture spectra were identical to the sum of the individual dye spectra; this suggests that the two dyes did not interact with each other.

In the mixture spectra, the researchers observed fluctuations in intensity and shape, which they attributed to changing colloid configurations in the scattering volume. They also noticed large fluctuations in the relative proporation of signal from each of the two dyes.  In some scans, the signals were from only one dye.  This observation, which indicates that the SERS signal is dominated by a small number of molecules, provides the first direct evidence for SM-SERS.

(Analytical Chemistry, April 2006, 2083)