Is It Possible to Observe Surface-Enhanced Raman Scattering from Buried Molecules?

There are several literature reports on enhanced Raman spectra of molecules behind metal films more than 20 nm thick. We have reproduced these results for the molecule 4,4'-bipyridine (bipy). The most important issue of this article is, whether these molecules are really buried or have sneaked out to the top of the metal films. The dedicated experiments on samples in which bipy is supposedly at rough interfaces did not yield a clear result. Therefore, bipy was buried below a smooth Au film, and roughness is provided by gold nanoparticles on top of the smooth gold film. The access of bipy to the hot spots between the top Au layer and the Au nanoparticles was blocked by benzenedithiol. In this case, we could not confirm SERS from buried bipy.