Detection of biologically active diterpenoic acids by Raman Spectroscopy

Three poorly detectable, biologically active diterpenoic acids, kaurenoic, abietic, and gibberellic acid, were studied by using different modes of Raman spectroscopy. Because of their structural similarities, in the absence of strongly polarizable groups, conventional Raman spectroscopy is not suitable for their unambiguous identification, especially not in solution.

We attempted to increase the sensitivity by applying UV-resonance Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS) techniques. The UV-Raman spectra of the three compounds in ethanol/water 50 : 50 showed only very few enhanced Raman lines. SERS spectra with 514-nm excitation with Ag colloids were also relatively weak.

The best SERS spectrawere obtained with 785-nm excitation on a novel nanostructured substrate, 'black silicon' coated with a 400-nm gold layer. The spectra showed clear differences, and these 'fingerprints' would be suitable for the unambiguous identification of these diterpenoic acids.