Hybrid Formulation of Cu Nanoparticles and Labdane Diterpene from Alpinia Nigra: a Vibrational Spectroscopic Study

The vibrational characteristics of a labdane diterpene, (E)-labda-8(17),12-diene-15,16-dial, have been studied using FT-IR, FT-Raman and surface-enhanced Raman spectroscopy (SERS). A good yield of the compound has been sourced and isolated from the seeds of Alpinia nigra. Experimentally obtained vibrational modes of the compound obtained by IR and Raman were verified by theoretical calculations at density functional theory (DFT) level using Gaussian 09 software package. Copper nanoparticles have been used to enhance the inherently weak Raman signals of the compound. In the SERS spectra, intense enhancement (up to 10 3 fold) was observed in the bands at lower wavenumbers, corresponding to the in-plane deformations, which indicated perpendicular orientation of the compound on the metal surface. However, some bands were weakened by the proximity of the metal surface. This interaction was assessed by IR and visualized by electron microscopy, revealing that Cu nanoparticles lie in the vicinity of the compound, which is responsible for the SERS phenomenon.