Tip-enhanced near-field Raman analysis of tip-pressurized adenine molecule

We report on near-field Raman spectra of a single nanocrystal of DNA-base adenine molecules using a silver-layer-coated apertureless probe tip of an atomic force microscope. The tip-enhanced near-field Raman spectrum shows eight Raman bands that are assigned to the normal modes of adenine molecules based on the density-functional theory (DFT) calculations. The vibrational frequencies of several bands are observed to have unambiguously shifted to the values of the corresponding bands, observed using the conventional surface-enhanced Raman-scattering spectrum of adenine molecules adsorbed on colloidal silver particles. The DFT vibrational calculations of adenine complexes involving a silver atom suggest that the Raman band shifts occur due to the deformation of adenine molecules by the silver atoms coated in the probe tip.