Until now it has been virtually impossible to record meaningful and reproducible data from single non-bonded molecules. A group of scientists from China and Sweden have successfully used surfaced enhanced Raman spectroscopy to probe single molecular behavior [Luo, et al., Chem. Comm. (2009), doi: 10.1039/b819402e].
Single molecule surface enhanced Raman spectroscopy (SM-SERS) is already over a decade old, and is now experiencing a renaissance due in part to some of the unique physical and chemical properties of nanosystems and some developments in plasmonics.
SM-SERS is not without its problems however, and these range from spectral fluctuations (signal blinking), non-reproducibility, through to a lack of understanding of the origin of the sites suitable for SM-SERS. So because of these technicalities SM-SERS until now has been quite limited in its application.
Luo et al., set out to combat some of these spectral fluctuations which overwhelm the reproducibility and quality of most experiments. They comment on a joint experimental and theoretical study on temperature-dependent SM-SERS involving a non-polar organic molecule of perylene on colloidal Ag nanoparticles.
At room temperature, the SERS spectra of perylene displayed strong spectral fluctuations; these were effectively eliminated by lowering the temperature of the substrate resulting in a stable SERS spectrum over a long period of time.
This study presents the first successful SM-SERS for a non-polar molecule, and more importantly the first blinking-free SM-SERS. It shows that the thermal effect is not derived from laser heating or plasmon excitation as originally thought, but mainly from local conditions. This can be easily controlled through adjusting, where appropriate, the experimental setup procedure.
Laser induced thermal effects were analyzed by reducing the laser power and conducting the experiments at a lower temperature, variations in concentration were measured in adiabatic conditions.
Based on the results, the researchers have proposed a model to describe the dynamic process of a single perylene molecule on Ag nanoparticles under low temperature conditions.
In combination with first principle calculations, the stable structure of molecule was determined to be one trapped in the gap of two nanoparticles.
This work has not only broadened the scope and practical application of the SM-SERS technique, but it also provides a valuable protocol for engineering single-molecular Raman behavior of small organic molecules on metal nanoparticles.