SERS of guanine and its alkyl derivatives on gold sols

SER spectra of guanine, 7-methylguanine and 9-ethylguanine on a gold colloid are reported. An assignment of these spectra is carried out by comparison with the corresponding Raman spectra in aqueous solution. The interpretation of the SER spectra reveals the influence that the alkyl substituents exert on the electrodynamic behaviour of the guanine derivatives when adsorbed on the gold surface. Thus, guanine and 7-methylguanine adsorb mainly in their ketonic form, adopting a perpendicular arrangement on the gold surface, whereas 9-ethylguanine lies nearly flat on this support. In addition, the comparison with results obtained previously using silver and copper as supports demonstrates the different affinity that these bases show towards the three metal supports commonly employed in SERS.