Surface-enhanced Raman spectroscopy (SERS) typically utilizes surface-immobilized metal nanoparticles to enhance the detection sensitivity (lower concentration limit of molecular detection) of Raman spectroscopy. A molecule's vibrational properties are probed by Raman spectroscopy, which provides complex yet unambiguous molecular identification.
Immobilizing silver (or certain other metal nanoparticles) on a planar surface enables an enhanced electric field to be generated near the surface. The end result is vastly improved molecular detection sensitivity, sometimes even down to the single-molecule level.
The combination of unambiguous molecular identification with low detection limits is not found in other spectroscopy techniques. This is a major reason why surface-enhanced Raman spectroscopy is widely utilized, when feasible, as a powerful analytical detection technique.
Henry Du, Svetlana Sukhishvili (Stevens Institute of Technology, New Jersey), and coworkers have discovered an easy way to greatly improve the detection sensitivity of surface-enhanced Raman spectroscopy. Inhibiting oxidation of the silver nanoparticles facilitates charge transfer between the molecule to be detected and the nanoparticles, which can increase the detection sensitivity by a factor of 100,000.
The surfaces and molecules.
Silver nanoparticles, approximately 45 nanometer diameter, were densely coated onto a planar glas surface (approximately 135 nanoparticles per square micrometer). These nanoparticle-coated surfaces were purged with either argon gas or oxygen gas for up to five minutes.
As their model molecules, the scientists chose to study nitrophenols. These molecules are important components of explosives, drugs, and other useful molecules.
Enhanced detection limit.
Utilizing an oxygen-depleted surface, the scientists observed out-of-plane vibrations of some of the chemical bonds (carbon-hydrogen and carbon-oxygen) in para-nitrophenol. These vibration bands are indicative of strong charge transfer between the para-nitrophenol molecule and the silver nanoparticles, and a flat orientation of the molecule on the nanoparticles.
The high-sensitivity vibration bands were observed in the para-nitrophenol concentration range of 1 part per trillion to 1 part per billion, but became overwhelmed by other bands at higher concentrations. The latter are indicative of vertical molecular orientation, such as on a crowded surface.
The high-sensitivity vibration bands were not observed on the oxygenated surface, and the detection limit was only approximately 100 parts per billion. Thus, para-nitrophenol can be detected on an unoxygenated surface by surface-enhanced Raman spectroscopy at a concentration as low as 1 part per trillion, 100,000 times lower than on oxygenated surfaces.
Similar results were observed with dinitrophenol and trinitrophenol. The effect therefore seems to be general, at least for nitrophenol molecules.
Implications.
Many environmental and biological applications of surface-enhanced Raman spectroscopy utilize silver surfaces that are oxygenated by air or water. Removing oxygen gas from a sample is easy to do, and yields a tremendous increase in detection sensitivity, for nitrophenols and likely for other broadly similar molecules.
for more information:
Erol, M., Han, Y., Stanley, S. K., Stafford, C. M., Du, H., & Sukhishvili, S. (2009). SERS Not To Be Taken for Granted in the Presence of Oxygen Journal of the American Chemical Society, 131 (22), 7480-7481 DOI: 10.1021/ja807458x