Surface-enhanced Raman spectroscopy for trace identification of controlled substances: Morphine, codeine, and hydrocodone.

We obtain the normal Raman and surface-enhanced Raman spectrum of three controlled substances: morphine, codeine, and hydrocodone. The spectra are assigned with the aid of density functional theory. Because of rather intense fluorescence, normal Raman spectra suffer from poor signal-to-noise, even when differential subtraction techniques are employed. On the other hand, surface enhancement by Ag nanoparticles both enhances the Raman signal and suppresses the fluorescence, enabling far more sensitive detection and identification. We also present a set of discriminant bands, useful for distinguishing the three compounds, despite the similarities in their structures.