SERS, XPS, and DFT Study of Adenine Adsorption on Silver and Gold Surfaces Marco Pagliai,* ,† Stefano Caporali, †,‡ Maurizio Muniz-Miranda, †,§ Giovanni Pratesi, ∥,‡ and Vincenzo Schettino †,§ † Dipartimento di Chimica ″Ugo Schiff″, Universita ̀ degli Studi di Firenze, via della Lastruccia 3, 50019 Sesto Fiorentino (Firenze), Italia ‡ Museo di Scienze Planetarie, via Galcianese 20/h, 59100 Prato, Italia § European Laboratory for Nonlinear Spectroscopy (LENS), via Nello Carrara 1, 50019 Sesto Fiorentino (Firenze), Italia ∥ Dipartimento di Scienze della Terra, Universita ̀ degli Studi di Firenze, via La Pira 4, 50121 Firenze, Italia * S Supporting Information ABSTRACT: The adsorption of adenine on silver and gold surfaces has been investigated combining density functional theory calculations with surface-enhanced Raman scattering and angle-resolved X-ray photoelectron spectroscopy measurements, obtaining useful insight into the orientation and interaction of the nucleobase with the metal surfaces. SECTION: Nanoparticlesand Nanostructures T he adsorption processes of biological molecules on nanostructured surfaces of noble metals like silver and gold are of particular importance for possible applications in diagnostics, regarding medical, biological, and technological sectors. 1,2 For the correct development of systems based on molecules interacting with surfaces employed as nanosensors or drug delivery nanocarriers for biomedical applications, a detailed knowledge of the interactions between adsorbate and metal surface is required. This problem is particularly complex in the case of biomolecules because different interacting sites can exist in the molecular structure. Moreover, these molecules could be present in nature as different tautomers or undergo transformations in response to the environmental variations, like pH changes of the aqueous medium. Adenine, a fundamental constituent of nucleic acids, represents a particularly interesting system to assess the correct application of computational methods and experimental measurements to state the interactions with metal surfaces, as documented by several research groups in recent publica- tions. 3−7 In the present work, the chemical adsorption of adenine (Figure 1) on silver and gold surfaces has been investigated by means of surface-enhanced Raman scattering (SERS) 1,8 that has been shown particularly effective in the study of molecules interacting with metal surfaces, owing to the huge Raman enhancement of the adsorbed ligands. The interpretation of the SERS spectra has been carried out through density functional theory (DFT) approach, which is able to take into account the interactions with the metal substrate by considering complexes constituted by molecules bound to surface active sites. (Experimental and computational details are reported in the Supporting Information). These interactions are basic for the chemical mechanism of the SERS effect 8 and, consequently, for the SERS profile concerning band frequencies and relative intensities. To develop and justify the model systems employed in the DFT calculations and obtain some insight into the interactions Received: November 18, 2011 Accepted: January 4, 2012 Figure 1. Molecular structures of the tautomer N9 (left), the tautomer N7 (center), and the anionic form (right) of adenine. Letter pubs.acs.org/JPCL © XXXX American Chemical Society 242 dx.doi.org/10.1021/jz201526v | J. Phys. Chem.Lett. 2012, 3, 242−245