Enhancement of infrared absorption of the low-temperature uracil thin films by nanostructured silver surface

Enhancement of infrared absorption (SEIRA) of biological molecules by a nanostructured metal surface is one of the main directions of increasing the sensitivity of modern optical biosensors. The FTIR absorption spectra of thin films of uracil (a RNA base) deposited on low-temperature substrate (T = 6 K) with nanoscale silver structures were investigated in the spectral range 2700-600 cm-1. It was shown that the intensity of the absorption bands of stretching vibrations νCO (range 1800-1600 cm-1) of uracil thin films (Ur) increases by a factor 3-4. For multilayer films the influence of nanostructures on the vibrational spectra is weakening and for the layers of the film that are spaced from the surface more than 15 nm the gain is absent practically. The energies and the vibrational spectra of complexes uracil monomers and dimers with 20 tetrahedral atomic nanoclusters of silver were calculated by the quantum-mechanical method DFT/B3LYP. The most stable complexes have the coordination bond between the top of the tetrahedron of silver and oxygen of the carbonyl group C4O. It was found that the formation of such complexes significantly (3-5 times) amplifies the intensity of the stretching vibration νC4O of uracil, while the intensity of the deformation vibrations βNH, βCH, and the rings are not experiencing significant growth. PACS: 78.66.-w Optical properties of specific thin films; 78.68.+m Optical properties of surfaces; 33.20.Ea Infrared spectra.