SERS, XPS, and DFT Study of Adenine Adsorption on Silver and

Letter pubs.acs.org/JPCL

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″, Università 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, Università 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: Nanoparticles and 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 publications.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 © 2012 American Chemical Society

Figure 1. Molecular structures of the tautomer N9 (left), the tautomer N7 (center), and the anionic form (right) of adenine.

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 effect8 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 Published: January 4, 2012 242

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from DFT calculations18 performed by B3LYP/LANL2DZ, but quite similar results can be obtained by adopting different functionals and basis sets (as shown in Figures S2 and S3 of the Supporting Information) and point to an interaction of both neutral and anionic molecules involving the nitrogen atoms in position 3 and 9. The normal Raman spectra of adenine in aqueous solutions appear quite similar (Figure 2) by increasing the pH value from pH 6, where adenine is present as a neutral molecule, to pH 13, where the anionic form is predominant. In addition, also the calculated normal modes of the Ag+ complexes are quite similar, as shown in Figure 3 for the strongest SERS band

between adenine and metal surfaces, X-ray photoelectron spectroscopy (XPS) measurements have been performed on metal plates with and without adsorbed nucleobase. XPS measurements show that both silver and gold surfaces present a sizable amount of metal atoms with oxidation number +1. (The deconvolution of the characteristic XPS peaks of the silver and gold plates is reported in Figure S1 of the Supporting Information.) This result allows modeling of the metal active sites as involving Ag+ or Au+ ions. These models were found able to reproduce accurately the SERS spectra of different ligands.3,9−17 Figure 2 shows the Raman spectra of adenine in aqueous solutions at pH 6 and 13 values, along with the SERS spectra of

Figure 3. Cartesian displacements of the calculated ring breathing modes of adenine as tautomer N7 (left) and anion (right) linked to Ag+.

around 735 cm−1. (A comparison between calculated and experimental frequencies for the most prominent SERS bands is reported in Table S1 of the Supporting Information. The computed normal modes for the tautomer N7 linked to Ag+ model are shown in Figure S4 of the Supporting Information.) The proposed adsorption on silver is further justified by calculating the electrostatic potential for adenine in both neutral and anionic forms. By considering the adsorption on metal as a reaction between one ligand molecule as a nucleophile and one metal active site as a Lewis acid, the tautomer N7 can interact with the metal substrate through the nitrogen atoms in position 3 and 9, that constitute the most negative part of adenine, as shown in Figure 4. A similar interaction is expected for adenine in the anionic form. (See Figure 4.) Figure 2. Upper panel: Raman spectra of adenine in aqueous solutions at pH 6 (blue line) and 13 (red line), respectively. Middle panel: SERS spectra of adenine adsorbed on silver plates after incubation in aqueous solutions at pH 6 (blue line) and 13 (red line), respectively. Lower panel: DFT-simulated SERS spectra for the tautomer N7 (blue line), the tautomer N9 (green line), and the anionic form (red line) of adenine, respectively.

adenine adsorbed on silver plates. The key feature of adenine SERS spectra is the intense band at ∼735 cm−1, assigned to the ring breathing mode. The surfaces have been prepared following the procedure described by Muniz-Miranda et al.3 by incubation in 10−3 M adenine aqueous solutions at pH 6 and 13, respectively. The experimental features of the SERS spectra are faithfully reproduced by DFT calculations only if adenine is assumed as tautomer N7 (where a hydrogen atom is bonded to nitrogen in position 7) and anion, for the two different pH values, respectively. The calculated SERS spectrum of tautomer N9 (where a hydrogen atom is bonded to nitrogen in position 9), instead, does not reproduce the observed intensities, mainly for the strongest band at ∼735 cm−1. These conclusions derive

Figure 4. Negative part of the electrostatic potential of the tautomer N7 (left) and the anionic form (right) of adenine.

The present considerations on the electrostatic potential could represent a useful guideline for identifying the molecular sites involved in the interaction of a ligand with the metal surface. To verify the possibility of tautomer N7 to act as a bidentate ligand, we have also performed DFT calculations by modeling one active-site of the metal surface as Ag3+ cluster. This species has been observed experimentally,19 and it has been successfully applied in the simulation of the SERS spectra of oxazole and thiazole.10,12 This model of interaction provides 243

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quite similar results with respect to those reported in Figure 2 for tautomer N7 of adenine linked to one Ag+ ion, regarding both band positions and intensities. The optimized structure of the model is reported in Figure S5 of the Supporting Information, along with the proposed structure of the Ag(I)/ adenine coordination compound, whose Raman spectrum closely resembles the SERS spectrum of adenine on silver. This latter is shown in Figure S6 of the Supporting Information, in comparison with the simulated Raman spectra of the Ag3+/adenine system and of the Ag(I)/adenine coordination compound. To consolidate these conclusions further, we have performed an attempt to obtain some information on the orientation of adsorbed adenine by means of angle-resolved XPS measurements. Figure 5 reports the XPS spectra related to the nitrogen

Figure 6. Upper panel: SERS spectrum of adenine adsorbed on gold plate. Middle panel: DFT-simulated SERS spectra for tautomer N7 (red line) and tautomer N9 (blue line) of adenine interacting with Au(0), respectively. Lower panel: DFT-simulated SERS spectra for tautomer N7 (red line) and tautomer N9 (blue line) of adenine interacting with Au(I).

Au(I) is quite lower than that of Ag(I) on the silver plate. Hence, the DFT calculations have been carried out here considering the gold active site constituted by one Au+ ion or by one Au(0) atom. As shown in Figure 6, the adenine/Au(0) complexes provide for both tautomers N7 and N9 a better agreement with the observed SERS spectrum than those with Au(I) ion. The optimized structures of the proposed adenine/ gold complexes are reported in Figure S7 of the Supporting Information. In conclusion, SERS spectroscopy combined with DFT approaches is able to describe adequately the chemisorption of adenine on silver and gold surfaces on the basis of the agreement between experimental and computational results. Moreover, the electrostatic potential of the molecule, as calculated by DFT, provides a useful indication on the molecular active sites involved in the interaction with metal. The models adopted in the calculations are justified by XPS measurements, both to describe the active sites of the metal substrates correctly and to obtain information on the adsorption geometry of the ligand with respect to the surfaces. This study has shown how XPS measurements could provide a valuable support for the complete comprehension of the adsorption process on metal surfaces. Albeit it could be useful to perform other measurements with different sources, in particular, with synchrotron X-ray radiation, the adopted procedure is quite encouraging.

Figure 5. XPS measurements on silver plates after incubation in adenine aqueous solutions at pH 6 (upper panel) and 13 (lower panel). The left and right panels refer to 0 and 60° takeoff angles, respectively. The overall fitted spectra are reported as a red line; the XPS peaks of the amino group and of the purine skeleton are reported with blue and green lines, respectively.

atoms of adenine and adenine anion, showing two different peaks attributable to the amino group (∼400 eV) and to the nitrogen atoms of the purine skeleton (∼398 eV).20 Changing the orientation of the silver plate with respect to the electron analyzer21 allows varying the electron takeoff angle and, consequently, the probing depth. Moving the takeoff angle from 0 (normal emission) to 60°, the relative ratio of the nitrogen XPS peaks increases after deconvolution from ∼1:4 to ∼1:3. This result could be explained by assuming that the NH2 group is located in the opposite side with respect to the metal surface for both adenine and adenine anion, in agreement with the DFT results. We have also performed DFT calculations for adenine adsorbed on gold plates. Our results could be explained on the basis of a model in which adenine is still assumed as tautomer N7, as shown in Figure 6. Our calculations correctly predict the ring breathing mode observed at ∼735 cm−1 as the most intense band of the SERS spectrum. The calculations performed on the interaction model proposed by Kundu et al.,4 instead, with the nitrogen atom in position 3 for the tautomer N9 of adenine linked to one Au+ ion, does not correctly reproduce the SERS profile. However, our XPS measurements on the gold plate show that the percentage of

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ASSOCIATED CONTENT

S Supporting Information *

Computational and experimental details. This material is available free of charge via the Internet at http://pubs.acs. org/. 244

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Study Aided by Density Functional Theory. Vib. Spectrosc. 2010, 52, 85−92. (17) Aroca, R. F.; Clavijo, R. E.; Hallas, M. D.; Schlegel, H. B. Surface-Enhanced Raman Spectra of Phthalimide. Interpretation of the SERS Spectra of the Surface Complex Formed on Silver Islands and Colloids. J. Phys. Chem. A 2000, 104, 9500−9505. (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (19) Xiong, Y.; Washio, I.; Chen, J.; Sadilek, M.; Xia, Y. Trimeric Clusters of Silver in Aqueous AgNO3 Solutions and Their Role as Nuclei in Forming Triangular Nanoplates of Silver. Angew. Chem., Int. Ed. 2007, 46, 4917−4921. (20) Furukawa, M.; Yamada, T.; Katano, S.; Kawai, M.; Ogasawara, H.; Nilsson, A. Geometrical Characterization of Adenine and Guanine on Cu(110) by NEXAFS, XPS, and DFT Calculation. Surf. Sci. 2007, 601, 5433−5440. (21) Chiappe, C.; Malvaldi, M.; Melai, B.; Fantini, S.; Bardi, U.; Caporali, S. An Unusual Common Ion Effect Promotes Dissolution of Metal Salts in Room-Temperature Ionic Liquids: A Strategy to Obtain Ionic Liquids Having Organic-Inorganic Mixed Cations. Green Chem. 2010, 12, 77−80.

AUTHOR INFORMATION

Corresponding Author

*E-mail: marco.pagliai@unifi.it.

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ACKNOWLEDGMENTS This work was supported by the Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR). We thank Regione Toscana for financial support of the project LTSP through the fund POR FSE 2007/2013 (Obiettivo 2, Asse IV).

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