[Thymine on silver island films: surface-enhanced Raman scattering and surface-enhanced infrared absorption studies]

Thymine, being one of the four nucleobases in the nucleic acid of DNA, has received an increasing interest due to its potential importance in genetics and possible applications in medical science. In the present study, adsorption of thymine on laser-ablated silver island films was comparatively analyzed by means of surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA). Two new bands at 1305 and 1573 cm(-1) appeared in the SERS spectrum of thymine on silver island films and represented that the molecule configuration of thymine hydrolyzed ion compound changed its structure from a keto form to an enol one. The greater enhancements of in-plane ring breath vibration bands and of 1649 cm(-1) band associated with C(4)==O stretching vibration clearly suggested that the thymine molecules were adsorbed on the silver surface via O(8). The interaction of N(3) with silver surface could be inferred by the emergence of a stronger peak at 773 cm(-1), which was assigned to the ring breath caused by the stretching vibration of the C(6)--N(3)--C(2). The blue shift of ring breath vibration from 749 to 773 cm(-1) indicated that thymine ring should have some weak interaction with silver surface, i.e., there was a certain angle between the thymine ring and the silver surface. To sum up, the SERS spectrum indicated that thymine adsorbed on the silver surface as enolate through the O(8) and N(3) atoms. The molecular plane assumed a tilted orientation with respect to the silver surface. Laser-ablated silver island films also exhibited a good surface enhancement to infrared absorption for thymine. Enhancement factor was estimated to be about 200 under our experimental condition. There was no shift of C(2)==O stretching vibration in SEIRA spectrum, revealing that O(7) was not involved in the adsorption process. Consistent with our SERS spectrum, the greater enhancement of the thymine ring deformation vibration (1730 cm(-1)) in SEIRA agreed a tilted orientation of thymine on silver surface.