Surface-Enhanced Raman Spectroscopy
The Raman signal from certain molecules adsorbed on certain metal surfaces, such ad silver or gold, can be enhanced five-to-six orders of magnitude compared to the Raman signal from the same molecules in bulk volume. One theory that has been put forth to explain this phenomenon is that the interaction of the laser beam with irregularities on the metal surface excites conduction electrons leading to a surface plasmon resonance and a strong enhancement of electric field. Choosing an appropriate surface substrate for SERS is very important. The most popular and universal substrates used for SERS are electrochemically-etched silver electrodes, as well as silver and gold colloids with an average particle size below 20 nm.
A problem with SERS spectra is that they can be difficult to interpret since the normal Raman spectrum of a molecule is not necessarily the same as that of the same molecule adsorbed on a SERS substrate. Because of such complications, Surface-Enhanced Resonance Raman spectroscopy (SERRS) has been developed. It utilizes both the Surface-Enhancement effect and the Raman Resonance effect, so the resulting enhancement in Raman signal intensity can be as high as 1014. But the main advantage of SERRS is its spectra resemble regular Resonance Raman spectra, which makes it much easier to interpret.
Array of golden chiral nanoparticles - Image courtesy of Saulius Juodkazis, Swinburne University of Technology, Melbourne, Victoria, Australia.
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