Adenine Adsorption at Single Crystal and Thin-Film Gold Electrodes

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Adenine Adsorption at Single Crystal and Thin-Film Gold Electrodes: An In Situ Infrared Spectroscopy Study Antonio Rodes,‡ Manuela Rueda,*,† Francisco Prieto,† Ce´sar Prado,† Juan Miguel Feliu,‡ and Antonio Aldaz‡ Department of Physical Chemistry, UniVersity of SeVille, C/ Profesor Garcıá Gonza´lez no. 2, 41012 SeVilla, Spain, and Department of Physical Chemistry and UniVersity Institute of Electrochemistry, UniVersity of Alicante, Apart 99, E-03080 Alicante, Spain ReceiVed: July 1, 2009; ReVised Manuscript ReceiVed: September 12, 2009

Adenine adsorption at gold electrodes is studied in neutral sodium fluoride solutions by cyclic voltammetry and in situ infrared spectroscopy. External reflection measurements were performed with Au(111) and Au(100) single crystal electrodes in D2O solutions, whereas surface enhanced infrared reflection absorption spectroscopy experiments under attenuated total reflection conditions were carried out with sputtered gold thin-film electrodes, both in D2O and H2O solutions. The results clearly show the specific adsorption of the molecule providing significant surface enhancement of the strong ring stretching band and the scissoring mode of the amino group. On the basis of the results, an adsorption model is proposed in which the molecular plane and the C-N(H2) bond are tilted on the electrode surface. Coordination is proposed to take place via the N(amino) atom upon sp3 hybridization and the N7 atom of the ring. The model applies irrespective of the surface crystallographic orientation of the electrode or the applied potential. The influence of the reconstructed or the unreconstructed state of the surface is inferred from the analyses of the spectra recorded while scanning the potential in opposite directions. Introduction The interest for DNA bases organization on well-defined surface electrodes is increasing nowadays for technical reasons such as the preparation of biocompatible materials and biosensors and also for their fundamental biological relevance. Remarkable advances in the level of understanding of the adsorption of biologic compounds on metal electrodes has been driven by the development of in situ techniques as vibrational spectroscopies and probe microscopies and by the improvement in the preparation of reproducible metal surfaces with welldefined crystallographic orientations. In a previous paper1 the adsorption of adenine on Au(111) and Au(100) single crystal electrodes was studied by electrochemical techniques as a first step in understanding the behavior at a microscopic level. The adsorption was found to depend not only on the crystallographic orientation of the electrode but also on the reconstructed or unreconstructed state of the surface. Adsorption on reconstructed areas is accompanied by the lifting of the reconstruction. The results of the thermodynamic analysis on Au(111) electrodes indicate a weak chemical interaction with the metal probably involving a covalent bond in which the adsorbate will act as the donor agent. The adsorption was stronger than on mercury electrodes2-6 and originates a higher negative shift of the potential of zero charge. For the adsorption on mercury, an orientation of adenine molecule lying flat on the surface was assumed2 however, on the gold electrode the thermodynamic data are more in favor of a “low perpendicular” or a “tilted” orientation of the molecule at the surface. The aim of this paper is to extend previous studies of adenine adsorption at single crystal gold electrodes1 to well-defined gold thin-film * To whom correspondence should be addressed. E-mail: [email protected]. ‡ University of Alicante. † University of Seville.

surfaces using in situ infrared techniques in order to search for structural microscopic information. Infrared spectroscopy is a powerful tool for the in situ characterization of electrode/solution interfaces.7-11 When applied to the study of adlayers at metal electrodes, the potentialdependent in situ infrared spectra can provide information on the nature and bonding geometry of the adsorbates, and on their interactions with neighbor adsorbed species and solvent molecules.7-11 In this paper, the in situ infrared external reflection experiments carried out in adenine-containing solutions with Au(111) and Au(100) single crystal electrodes with the basal crystallographic orientations will be reported first. Then the results of the experiments performed under internal reflection conditions with gold thin-film electrodes will be described. In a previous paper,12 we showed that quasi-Au(111) thin film electrodes can be obtained by argon sputtering, giving rise to nanostructured gold films that are similar to those obtained by thermal,13,14 or electron beam,15 evaporation. Internal reflection experiments under attenuated total reflection (ATR) conditions with the thin metal film deposited on a high refractive index material (the so-called Kretschmann configuration) overcomes the overlapping between solution and surface bands related to the thin layer configuration required for the external reflection experiments.9,10,15 The nanostructure of the thin film causes an important increase of the infrared absorption by species adsorbed at the film/ solution interface (the so-called surface enhanced infrared reflection absorption (SEIRA) effect),9,15-18 which increases the sensitivity of the in situ infrared spectra, thus facilitating the detection of weak adsorbate bands. Finally, the diminution of the strong interference coming from the infrared absorption by water,9,10,15 makes possible the study of water-metal and wateradsorbate interactions. Combined with the low time constant of the spectroelectrochemical cell used in the attenuated total

10.1021/jp906672u CCC: $40.75  2009 American Chemical Society Published on Web 10/07/2009

Adenine Adsorption at Single Crystal and Thin-Film Gold Electrodes

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reflection experiments, the SEIRA effect allows the performance of time-resolved step-scan experiments in the submillisecond range. In this way, ATR-SEIRA spectroscopy has been applied in the past to the study of the adsorption of p-nitrobenzoic acid,19 uracil,20-22 self-assembled mercapto monolayers formation and deprotonation,21 trimesic acid,23 viologen self-assembled monolayers,24 methylaminopyridine derivatives,25 among others. Experimental Section Test solutions were 0.5 M NaF (Merck Suprapur) and Purelab Ultra (Elga-Vivendi) water. Adenine (Merck pro analisi or Fluka puriss, recrystallized in a 90:10 water + methanol mixture) was added to the sodium fluoride solution to reach the desired concentration. Solutions were deaerated by bubbling argon (L’Air Liquid N50). The gold single crystals used as working electrodes in the external reflection (spectro)electrochemical experiment were grown by melting a high purity Au wire (99.9998%, AlfaAesar), which was subsequently oriented, cut, and polished following the method developed by Clavilier.26,27 The diameters of the samples for the electrochemical and in situ infrared measurements were around ca. 2.0 and 4.5 mm, respectively. Prior to each experiment, the working electrode was heated in a gas-oxygen flame, cooled down in air, and protected with ultra pure water.28-30 Gold thin-film electrodes were deposited by argon sputtering on one of the sides of a silicon prism bevelled at 60° (Kristallhandel Kelpin, Germany).12 Deposition was carried out in the vacuum chamber of a MED020 coating system (BALTEC AG) equipped with a turbomolecular pump. The deposition rate and thin film thickness were controlled with a quartz crystal microbalance. In all the experiments reported in this paper, the film thickness was ca. 20 nm and the deposition rate 0.006 ( 0.001 nm/s. The gold film was electrochemically annealed by cycling at 50 mV/s between 0.10 and 1.20 V for several hours in a 10 mM CH3COONa + 0.1 M HClO4 solution. Spectroelectrochemical experiments were carried out in glass cells at room temperature. The cells for the internal31 and the external10,32 reflection experiments were provided, respectively, with Si and CaF2 prismatic windows bevelled at 60°. A thin gold foil allows the electrical contact with the gold film electrodes. A saturated mercurous sulfate electrode and a gold foil were used as reference and counter electrodes, respectively. All potentials are referred to saturated calomel electrode (SCE). Spectroelectrochemical experiments were carried out with a Nicolet Magna 850 spectrometer equipped with a narrow-band MCT-A detector. Spectral resolution was 8 cm-1. The in situ spectra were collected with either p- or s-polarized light and are presented as the ratio -log(R2/R1), where R2 and R1 are the reflectance values corresponding to the single-beam spectra recorded at the sample and reference potentials, respectively. In the external reflection experiments the so-called SNIFTIR (subtractively normalized interfacial Fourier transform infrared) technique was employed. Sets of 50 interferograms were collected alternately at the sample and reference potential and then coadded. One of these sets was enough to reach a good signal-to-noise ratio in the ATR-SEIRAS experiments. In another set of experiments, the single beam spectra were obtained during a slow potential scan between two potential limits at a scan rate of 2 mV s-1. Either 104 or 52 interferograms were averaged to calculate each single beam spectrum, which corresponds to a potential interval of either 20 or 10 mV. Potential scan experiments are to be preferred when the reconstruction at the more negative reference potential wants to be avoided.

Figure 1. Cyclic voltammograms for (a) Au(111), (b) Au(100), and (c) thin-film gold electrodes in a 10 mM adenine + 0.5 M NaF solution. Sweep rate: 50 mV s-1. Immersion potentials: 0.20 V (a,b) or 0.10 V (c).

Transmission infrared experiments were performed with a transmission cell for liquids (Spectratech) provided with circular CaF2 windows separated by a 15 µm Teflon spacer. The transmission spectra for the 10 mM adenine + 0.5 M NaF solutions were collected with nonpolarized light and presented as -log(I2/I1), where I1 holds for the intensity of the single beam spectrum obtained with the 0.5 M NaF solution prepared either in water or in deuterium oxide. The transmission spectrum for solid adenine was obtained in KBr pellet. Results Voltammetric Results. Figure 1 shows voltammetric curves obtained with Au(111) and Au(100) single crystal electrodes in adenine containing 0.5 M NaF solution. They were obtained with flame-annealed electrodes immersed in the adenine solution at a positive potential (0.2 V vs SCE) and kept at this potential for a few minutes so that the surface has the time to lose the reconstruction. The curves correspond to the first scan, and at the slow sweep rate of the experiments the stationary state is reached. These curves show features in the negative- and positive-going scan that correspond, respectively, to desorption and adsorption of adenine at the gold surface. In this way, the Au(100) electrode shows a desorption peak at very negative potentials at which most probably the surface rereconstruction can take place while in the reverse scan two well developed adsorption peaks can be observed, which were ascribed to the adsorption on unreconstructed and reconstructed surfaces.1 In the case of the Au(111) electrode, the two adsorption peaks are more overlapped and both the adsorption and desorption take place at less negative potentials than on Au(100) electrodes. As previously discussed in ref 1, the relative intensity and peak potential of the features appearing in the voltammogram depend on the experimental conditions, namely, the immersion potential, the lower limit during the potential scan and the direction of the scan (positive- or negative-going sweep). The experiments suggest that although adenine adsorption seems to induce the lifting of the reconstructed surfaces of flame-annealed electrodes, reconstructed domains reappears latter upon polarization at the lower potential limit.

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Figure 2. Transmission infrared spectra for (a) solid adenine; (b,c) 10 mM adenine + 0.5 M NaF solutions prepared in (b) water or (c) deuterium oxide. One hundred interferograms were collected to obtain each spectrum, which in the case of adenine solutions is referred to the single beam spectrum collected for the adenine-free test electrolyte solution.

In Figure 1, a stationary voltammetric curve obtained under similar conditions with the gold thin-film electrode is also shown so that it can be compared with the ones obtained with Au(100) and Au(111) electrodes. The results clearly corroborate the existence of a quasi-Au(111) surface in the case of thin film electrodes because the voltammogram in this case (Figure 1c) is very similar to the one obtained with the Au(111) electrode (Figure 1b). Spectroscopic Results. For the sake of comparison with the in situ spectra reported below for adenine adsorbed at the Au(111), Au(100), and gold thin-film electrodes, the transmission spectra for solid adenine and for 10 mM adenine + 0.5 M solutions prepared either in water or in deuterium oxide were obtained. These spectra (in the region 1800 to 1200 cm-1) are shown in Figure 2. It is well known that adenine dissolved in deuterium oxide exchanges all nitrogen bound hydrogen atoms, resulting in adenine-d3.33 The literature about the interpretation of adenine IR spectra is abundant.33-40 Main absorption bands in the spectral region between 1800 and 1200 cm-1 are associated to scissoring of the amino group and to skeletal stretching within the rings. The assignments made by Giese et al.41 for the deuterated (adenine-d3) and nondeuterated (adenined0) forms are given in Table 1. The main differences between the spectra of adenine-d3 and adenine-d0 concern the bands between 1700 and 1600 cm-1. The intense band at ca. 1670 cm-1 of nondeuterated solid adenine is mainly related to the scissoring mode of the amino group, which is displaced toward significantly lower wavenumbers in the deuterated molecule (bellow 1200 cm-1). Giese et al.41 assign also contributions of the C6-N10 and C5-C6 stretching modes to this band. The intense band at 1604 cm-1 for solid adenine is due almost exclusively to skeletal stretching within the purine ring.41 In the case of the dissolved adenine-d0, these two bands are

Rodes et al. overlapped by the intense band for the bending mode of the H2O molecules, so that the three bands appear as a wide unresolved band in the transmission spectra in water. The spectra in D2O solutions show only one band in this region at 1623 cm-1 due to the purine ring stretching of the adenine-d3 molecule. According to Geise et al. this band does not correspond exactly to the same mode that the band at 1604 cm-1 in the solid adenine-d0 but also includes contributions of the C6-N10 and C5-C6 stretching modes as the band at 1670 cm-1 in the solid nondeuterated adenine. The blue shift of this band in the NaF-containing deuterium oxide solution is the same as observed in the spectra of the deuterated sodium salt and has been ascribed to the loss of intermolecular interactions.33,35 Figure 3 shows in situ potential-difference external reflection infrared spectra obtained with Au(111) and Au(100) electrodes in contact with a 10 mM adenine + 0.5 M NaF solution prepared in deuterium oxide. The choice of the solvent in the external reflection experiments is determined by the appearance of the main absorption bands for adenine in the spectral region between 1700 and 1600 cm-1, which is strongly perturbed by the interference from an uncompensated absorption associated to the O-H bending mode of liquid water at ca. 1640 cm-1. This band is shifted to ca. 1200 cm-1 for the O-D bending. The spectra reported in Figure 3 for the Au(111) and Au(100) electrodes were collected, respectively, at 0.20 and 0.10 V and are referred to the single beam collected either at -0.65 V (Au(111)) or -0.80 V (Au(100)) in the same solution. These latter potentials are in the region where adenine is not expected to be adsorbed at the corresponding gold electrode surface. The bands observed in the spectra obtained for Au(111) and Au(100) are qualitatively similar. All the spectra show a negative-going band at 1616 cm-1. From the comparison of this band with those appearing in the spectra reported in Figure 2, this band can be assigned to adenine molecules in solution at the reference potential that are consumed at the sample potential. This assignment is confirmed by the persistence of this band in the spectra collected with s-polarized light (see spectrum b in Figure 3 for the Au(111) electrode). The spectra obtained with Au(111) and Au(100) and p-polarized light show a main positive-going band at ca. 1640 cm-1 corresponding to species formed at the sample potential. The absence of this band in the spectra collected with s-polarized light indicates that it can be ascribed to adsorbed species. The in situ spectra for Au(111) and Au(100) electrodes show also a series of small positivegoing features in the spectral region between 1600 and 1200 cm-1 that could also be related to absorption bands from adsorbed adenine. The existence of these bands will be confirmed later in the ATR-SEIRA spectra collected with the gold thin-film electrode, for which the signal-to-noise ratio is much higher than for the external reflection spectra. The dependence with the electrode potential of the external reflection spectra obtained with the Au(111) and Au(100) electrodes in the adenine containing solution has been studied. Figure 4 shows a set of the potential-dependent spectra collected with the Au(111) electrode during a slow potential scan between 0.40 and -0.55 V. These infrared spectra show an increase of the adsorbate band with increasing electrode potential together with a slight blue-shift of the band frequency. At the same time, the intensity of the adenine consumption band also increases. The external reflection spectra reported above have shown characteristic features for adenine adsorbed at gold single crystal electrodes. However, these spectra suffer from a relatively low signal-to-noise ratio and from the overlapping between adsorbate and solution bands. These drawbacks are minimized in the ATR-



TABLE 1: Vibration Modes of Solid Adenine-d0 and of Adenine-d3 in D2O Solutions and Theoretical Assignments According to Giese et al41

SEIRA spectra, since a significant enhancement of the band intensities takes place, which in addition is surface specific.9,15-17 Spectrum a in Figure 5 was obtained at 0 V with the gold thinfilm electrode in contact with the 10 mM adenine + 0.5 M NaF solution prepared in deuterium oxide. In addition to an intense positive band at 1643 cm-1, well-defined bands also appear at 1517, 1449, 1378, and 1317 cm-1, which cannot be clearly observed in the external reflection experiments. In contrast with the external reflection spectra, no bands related to the consumption of adenine in solution can be observed at 1616 cm-1 in the ATR-SEIRA spectra collected in deuterium oxide solution. The negative-going band appearing at 2559 and 1201 cm-1 can be related, respectively, to the O-D stretching and bending modes of interfacial deuterium oxide molecules at the reference potential being displaced/reoriented upon adenine adsorption. It has to be noted that some interference from uncompensated Si-O stretching band coming from the silicon substrate may appear at around 1200 cm-1 in the ATR-SEIRA spectra. Spectrum b in Figure 5 was obtained under the same conditions as spectrum a except for the solvent, which was water. Now, the main adsorbate band appears at 1677 cm-1. Other positivegoing features in the region between 1600 and 1200 cm-1 are observed approximately at the same frequencies than in the deuterium oxide solutions. The spectrum collected in water shows negative-going bands corresponding to interfacial water molecules adsorbed at the reference potential. The frequency values for the δ(OH) and ν(OH) bands, appearing, respectively, at ca. 1626 and 3439 cm-1, are characteristic of weakly hydrogen-bonded water molecules predominating at the electrified metal/solution interface at potentials below the potential of zero charge.13-15,42 Positive-going features overlapping with the O-H stretching band at 3354 and 3208 cm-1 could be related to the symmetric and asymmetric N-H stretching modes of adsorbed adenine, which usually provide weak bands in this

spectral region. C-H stretching modes can also contribute but usually they are very weak signals. Figure 6 shows a set of spectra collected at different potentials for the gold thin-film electrodes in a 10 mM adenine + 0.5 M NaF solutions prepared in deuterium oxide. In this experiment, the spectra were collected during a slow potential scan from 0.00 down to -0.80 V and correspond to a potential interval of 10 mV around the marked value. All the spectra have been referred to the single-beam spectrum collected at -0.80 V. As in Figure 5, the positive-going bands in the spectra collected at potentials above -0.70 V are the signature of adsorbed species coming from adenine. The intensity of all these features, together with those described above for interfacial solvent molecules, decrease when decreasing the electrode potential down to -0.80 V. The absence of positive-going solvent bands at potentials at which adenine is adsorbed indicates the existence of weak interactions between adsorbed adenine and interfacial solvent molecules.15,43-48 This behavior is in contrast to that observed for uracil,42,44,47,49 which incorporates coadsorbed water molecules upon adsorption on gold electrodes, as witnessed by the appearance of positive-going water bands around 3400 cm-1 (O-H stretching) in the corresponding ATR-SEIRA spectra. But probably, coadsorption of water is also related to coadsorption of anions, as observed with SO4) and ClO4- and Cl-, Br-, and I-,15,43,44,49 which in the case of the anion used in this work is hardly plausible. Discussion Band Assignments and Antecedents about Adenine Adsorption. Because of the biological relevance of adenine and of its derivatives, the assignment of the vibration spectra signals is now well documented. Initially, assignments were done by comparisons with spectra of pure adenine and of its derivatives

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Figure 3. Potential-difference spectra collected either with p- (a,c) or s-polarized (b) light with Au(111) (a,b) and Au(100) (c) electrodes in a 1 mM adenine + 0.5 M NaF solution prepared in deuterium oxide. Sample potential: 0.20 V (Au(111)) or 0.10 V (Au(100)). Reference potential: -0.65 V (Au(111)) or -1.00 V (Au(100)); 2000 interferograms collected at each potential.

(deuterated, methylated, halogenated, etc.) under different experimental conditions.35-37,40,50,51 Later, the increasing computation facilities allowed the theoretical calculation of spectra and so the atom positions involved in the vibration modes producing the spectral bands.38,39,41,52-56 The most recent studies41,55 make use of the density functional theory (DFT), which takes into account electron correlation. In Table 1 are reported the assignment of Giese et al.41 for the most representative bands in the region 1800 to 1200 cm-1 of nondeuterated adenine (d0-adenine) and of deuterated d3-adenine, together with the experimental IR bands for solid d0-adenine and for d3adenine in NaF D2O solutions. Adenine is a planar molecule with a C5 group symmetry so most of the signals correspond to in-plane vibration modes. The vast majority of vibrations involve

strongly coupled motions. The assignments made by Giese et al. are similar to the ones reported by Nowak et al.,54 which use an ab initio method. The main difference is that for the 4-d0 mode Nowak did not include any contribution of the scissoring NH2 mode. In fact, Giese et al. assumed NH2 contributions in all the modes 1-d0 to 5-d0, although they are only predominant in the bands at 1672 and 1590 cm-1 of the solid adenine-d0 spectrum; hence they are so strongly affected by deuteration. However, the vibrations at 1604 cm-1 in the solid adenine-d0 is due almost exclusively to skeletal stretching within the purine ring. In the case of adenine-d3, the mode at a similar frequency was assumed to include also contribution of the C6-N10 and C5-C6 stretching. As shown in Table 1, this band for adenine-d3 in D2O solution is blue shifted as compared


Figure 4. Potential-difference spectra collected with a Au(111) electrode during a slow potential scan at 2 mV s-1 from 0.40 to -0.65 V in a 1 mM adenine + 0.5 M NaF solution prepared in deuterium oxide. Reference potential: -0.65 V. 104 interferograms were collected with p-polarized light for each spectrum which corresponds to a potential interval of 20 mV.

Figure 5. Potential-difference ATR-SEIRA spectra collected at 0.00 V with an electrochemically annealed gold thin-film electrode in a 1 mM adenine + 0.5 M NaF solution prepared in (a) deuterium oxide and (b) water. Reference potential: -0.80 V; 100 interferograms collected at each potential with p-polarized light.

to the band in the solid adenine-d0 spectrum and in the solid adenine-d3,41 due to the weakness of H-bonds at the basic pH of the NaF solution. At wavenumbers lower than 1450 cm-1, bending of the external NH and CH angles becomes important. These motions are coupled to stretching and deformation motions of the rings.


Figure 6. Potential-difference ATR-SEIRA spectra collected with an electrochemically annealed gold thin-film electrode during a slow potential scan from 0 to -0.80 V in a 1 mM adenine + 0.5 M NaF solution prepared in deuterium oxide. Reference potential: -0.80 V; 52 interferograms collected at each potential.

The complexity of the adenine molecule and the high number of bonds involved in the vibration modes due to the high conjugation of the molecule, make the formulation of a structural model including the identification of the coordination sites of the molecule a difficult task. In order to pursue this objective it can also be convenient to consider the spectroscopic results of similar systems involving adenine in an adsorbed state. The discussion is aimed to decide about the orientation of the molecule with the molecular plane parallel or perpendicular to the surface and about the coordination sites, either the π-electrons ring, the imidazol nitrogen atoms (N7, N9), the pyrimidin nitrogen atoms (N3, N1),or the NH2 group. Most of the antecedents about vibrational spectra of adsorbed adenine are related to SERS.41,57-64 FT-IR data are scarcer.65-67 In this respect, Liedberg et al.65 and Whitman et al.66 have studied mono- and multilayers of DNA bases incubated on gold films, and Raval et al.67 have considered adenine adsorption on Cu(110) substrates in ultra high vacuum. In the SERS experiments the enhanced bands for adsorbed adenine are the ones at 739, 1338, and 1454 cm-1. These bands were assigned to ring skeletal vibration modes of adsorbed adenine on silver electrodes.64 Particularly, the strong Raman signal at 739 cm-1 was considered to be due to symmetrical ring breathing vibration, so that a vertical or slightly tilted orientation of the molecule on the surface was assumed based on the selection rules. However, Moskovits et al.62 pointed out the absence of any C-H signal around 3000 cm-1, which should significantly contribute to the Raman polarizability in case of a perpendicular orientation of the molecule. Therefore they proposed a different explanation for the 739 cm-1 (based on previous interpretation by Speca et al.68) consisting in the coupling of NH2 bending and ring stretching modes. The NH2 group indeed will contribute also to other surface active ring modes at higher frequencies, so that a perpendicular orientation is not required to explain the Raman spectra. Watanabe et al.63 studied the adsorption of different nucleotide bases on silver electrodes and they suggested the interaction of adenine molecule with the surface by the N7 position. Taniguchi et al.57

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also proposed the interaction of the surface to the adenine imidazol ring by the N7 site in adsorbed nicotin-adenindicucleotide (NAD) on silver electrodes. In addition, and because the blue-shift of the breathing band typical of compounds in which the pyrimidine ring is directly linked to the metal is not observed in the SERS experiments, they conclude that the pyrimidine ring is linked to the metal by the NH2 group. However, on gold electrodes they observed different behavior depending on the electrode potential. At positive potentials, the adenine fraction of the NAD molecule interacts with the surface by the N1 position and the NH2 group, thus favoring also the approximation of the nicotinamide fraction to the electrode. At more negative potentials, they proposed the interaction of the adenine part with the metal exclusively by the NH2 group. Implication of the NH2 group in the coordination to the metal was also concluded by Otto et al.58 on the base of SERS experiments with adenine derivatives with partially or totally methylated NH2 group and purines that do not contain the NH2 group, among others. They do not exclude the implication of the N7 atom in the coordination. However, the interaction by N9 can be ruled out because the spectra of compounds as adenosine- monophosphate (AMP), which has the N9 atom bonded to the sugar chain, provide the same features that the adenine spectrum. Coordination by NH2 and N7 of the adenine part was also proposed by Xiao et al.59-61 for NAD adsorption on gold electrodes, based on their near IR- SERS experiments and by Giese et al.41 for adenine adsorption on silver colloids. FT-IR spectra have the advantage of less ambiguous selection rules and in the case of adenine adsorption the fact that the intrinsically most intense bands are the ring stretching bands and the scissoring NH2 mode (appearing at 1604 and 1673 cm-1, respectively, in the solid). However, different conclusions about the molecular orientation and the coordination sites of adsorbed adenine are reached in the literature. Liedberg et al.65 reflection-absorption FT-IR spectra of monolayers of adenine on gold in air show that the two signals are practically coincident at 1647 cm-1. Other surface active signals, although much weaker, appear at 1450, 1395, and 1300 cm-1. The authors proposed, according to ab initio calculations,58,69 that the sp3 hybridization of the amine nitrogen allows the binding of the amine nitrogen atom to the metal by the pair of nonbonding electrons. In this way, both the ring and the NH2 group have some inclination on the surface and their signals are surface active. Recently, Omanovic et al.70 proposed the adsorption of the adenine part of the NAD molecule on gold electrodes at positive potentials by the NH2, based on their PM-IRRAS experiments. At negative potential however the reorientation of the molecule with a flat orientation of the adenine part on the surface takes place. On the other hand, Raval et al.67 proposed a perpendicular orientation of adenine molecule with coordination to the metal by the N9 and N3 atoms to explain their experiments of adenine monolayers on Cu(110) crystal in ultrahigh vacuum. The main spectroscopic evidence in which the authors based this conclusion is the fact that they were unable to detect the asymmetric NH2 vibration mode (at 3557 cm-1 in Ar matrix), but only the symmetric NH2 vibration mode (at 3425 cm-1), so that the NH2 group should be completely perpendicular to the surface with the two N atoms being equidistant with respect to the surface. On the other hand, the authors took into consideration the antecedents about the coordination sites of adenine in the complex to metal ions, in which the N9 position seems to be the most favorable chelating site. Effectively, there are X-ray, UV-vis, and IR evidence supporting that in Ag+ and Cu+ complexes71-73 and in some

Rodes et al. monodental Cu2+74,75 or Co2+76 complexes adenine uses the N9 position. The N3 and N9 positions seem to be preferential in the case of bidental complexes of Cu+74,77,78,80-83 and Cd2+. However, for divalent cations such as Co2+, Pd2+, Pt2+, and Ti2+ the N7 and N10 coordination sites have been proposed.71,77,78,84 Coordination by the amine group is less probable, but there is evidence supporting this coordination in the case of some ruthenium and mercury complexes.85 The IR spectra of some adenine metal complexes show strong wavenumber shifts of the signal corresponding to the pyrimidine and imidazol ring vibration modes with respect to the ligant signals;68 this was interpreted as an indication of the implication of the N atoms of the respective rings in coordination. The bands characteristic of the NH2 group show only slight deviations in the case of the Fe2+ and Zn2+ complexes; however in the Co2+ and Ni2+ complexes all the bending bands associated to this group are red shifted while the C-NH2 stretching band is blue-shifted. This is indicative of the NH2 implication in the coordination in the case of the two latter complexes. Adsorption Model. In Table 2 are summarized the active bands obtained in the external reflection and in the ATRSEIRAS experiments. The existence of active bands related to stretching modes of the imidazol ring, particularly the strong band at 1640 cm-1 observed for adenine-d3, clearly exclude a flat orientation of adenine on the surface and the interaction by the π-electrons with the metal. On the other hand, the surface activity of the scissoring mode of the NH2 group is inferred from the ATR experiments in water solutions. Effectively, although the band at 1670 cm-1 assigned to this mode together with the band at 1640 cm-1 assigned to stretching mode within the ring are in part masked by the negative band due to the bending mode of H2O molecule, clearly the positive band observed at 1677 cm-1 is wider and shifted to higher wavenumbers than in the case of adenine-d3 adsorption. Hence, the NH2 group cannot be coplanar with the electrode surface. Only upright or tilted orientations of the molecule on the electrode surface are compatible with the experiments. However, a largely upright ring orientation of the molecule in which the two amino hydrogens are equidistant from the surface, as proposed by Raval et al.67 for the adsorption on Cu(110) surfaces (the coordination points suggested to be the N9 and N3 positions), can be ruled out because it will imply that all the in-plane ring C-C and C-N stretching modes should be surface actives, which does not correspond to the results shown in Table 2. Moreover, both the symmetric and the asymmetric N-H vibrations seem to be active in our case. In addition, coordination of adenine to the surface via N9 is unlikely since adsorption on gold of the adenosine molecule, in which the N9 position is sterically blocked by the sugar moiety, gives an IR spectra essentially identical to adenine molecule.65 The coordination through the amino group seems more feasible, assuming that the N atom in the amino group adopts a pyramidal sp3, rather than a planar sp2, hybridization, as suggested for the adsorption on other substrates.41,58-61,65,66,68,69 This deformation of the molecule implies that the C6-N10 bond is tilted with respect to the surface plane, so that the strong ring band around 1606-1640 cm-1 with a high participation of this bond41 is surface enhanced. Moreover, the blue shift of this band as compared with the value in the solid or in D2O and H2O solutions indicates a strong interaction with the metal. More difficult is to decide about the shift of the scissoring mode of the NH2 group because the band only can appear in H2O solutions but it is masked by the bending band of the solvent. Interestingly, the IR spectra of a thiol-derivatized adenine



TABLE 2: Vibration Wavenumbers (cm-1) of IRRAS-s, Surface Active IRRAS-p, and ATR Signals of Adsorbed Adenine (Assignments According to Reference 41)

. IRRAS-s (D2O) IRRAS-p/ATR (D2O)

assignment

ATR (H2O)

assignment

1677

1d0 sciss NH2, str C6-N10, C5-C6 (2-d0, 3-d0) (str N3-C4, N1-C6, C5-N7, N7-C8, bend N9-H, sciss NH2)

1453

5-d0 str C2-N3, N1-C6, bend C2-H, sciss NH2

1616

1643

1-d3 str. C6-N10, C5-C6 C4-C5, C2-N3, N1-C2

1577 1543 1516

1517

1474

1449

4-d3 str N7-C8, C2-N3 C6-N1, bend C8-H, C2-H 5-d3 str. C4-N9, C4-C5, C6-N10, N7-C8, bend C2-H, sciss ND2

1424 1373

1378

7-d3 str C5-N7, N1-C2, C4-C5, C6-N10, sciss ND2

1378

7-d0 bend C2-H, N9-H, str C8-N9, C4-N9

1327 1311

1317

9-d3 str. C8-N9, N1-C2, C5-N7, bend C8-H

1317

10-d0 bend C8-H, N9-H, str N7-C8

adsorbed on a gold electrode by the S atom bonded to the C8 atom (not by the NH2 group), show a strong enhancement of the scissoring NH2 mode but not of the mean ring vibration mode around 1610-1640 cm-1.86 The simultaneous coordination by some of the nitrogen atoms of the ring can also contribute to the blue shift of the band at 1640 cm-1. For steric reasons, once the coordination from the NH2 group is accepted, only the N1 or the N7 atoms can act as coordination sites. However the simultaneous coordination via N10 and N1 implies a substantially upright orientation of the molecule plane so most of the in-plane vibration modes should be surface active. On the contrary, the coordination by N10 and N7 is compatible with a slightly tilted orientation of the ring plane, which could explain that only a few vibration modes are active. Note that in Table 2 most of the active bands have contributions of the C6-N10 or the scissoring modes, so the molecule plane does not need to be strongly tilted to explain the surface activity of these modes. Moreover, all the active bands shown in Table 2 are associated to transition moments in the molecular y-axis so a tilted orientation of this plane with the N10 and N7 atoms closer to the metal surface plane is compatible with the data in Table 2. Desorption dynamic experiments of monolayers of adenine on polycrystalline gold thin films display at least two adsorption sites.87 On the basis of their IR spectra during desorption experiments and the band assignments in refs 54 and 41, the authors also concluded the adenine-gold interaction by the N10 but they could not explain the changes observed in the spectra during desorption experiments by reorientation of the molecular plane. The changes could be due to simultaneous coordination by the N7 atom, which will not represent significant distortions in the molecular

orientation. The enhancement of the bands at 1380 cm-1, assigned mainly to the N7sC5 stretching mode, and at 1449 cm-1 with contribution of the N7-C8 stretching mode supports the idea of a simultaneous coordination by N10 and N7 atoms. On the basis of these arguments and the antecedents described above the simultaneous coordination by the N10 and N7 atoms of adenine molecule to the metal is proposed here. In Figure 7, a tentative image of the tilted orientation of the molecular plane and the two coordination sites has been drawn. Potential-Dependent Behavior. The band assignments and adsorption model discussed above apply irrespective of the electrode potential, because the same features are observed in the spectra registered at all the potentials at which adenine is adsorbed. Therefore, potential-induced reorientation of the molecule, as observed for the adenine part in NAD and NAD+ adsorption on gold electrodes,60,70 can be ruled out. Probably, reorientation in the case of NAD and NAD+ molecules is induced by the adsorption of the nicotinamide part of the molecule. An interesting point is the fact that two adsorption peaks are observed in the voltammetric curves obtained with both Au(111) and Au(100) electrodes, which were suggested to be related to the adsorption on reconstructed or unreconstructed surfaces.1 The separation between both peaks is higher in the case of the Au(100) electrode (see Figure 1). However, the analysis of the spectra signals as a function of the electrode potential does not show any indication of changes in orientation or coordination sites with potential either with Au(111) or Au(100) electrodes, so the adsorption on both reconstructed and unreconstructed surfaces must be the same. Nevertheless, the electrode potential affects the intensity of adenine bands paralleling the potential

18792


Rodes et al.

Figure 7. Schematic drawing of the adsorption model.

Figure 9. Plots of band frequencies of the main band of adsorbed adenine as measured in the potential-difference spectra collected with Au(111) (a), Au(100) (b), and gold thin-film (c,d) electrodes in contact with a 1 mM adenine + 0.5 M NaF solution prepared in (a-c) deuterium oxide or (d) water.

Figure 8. (A,B) Plots of the integrated intensity of the main band for adsorbed adenine as measured in the potential-difference spectra collected with (A) Au(111) and (B) Au(100) electrodes. Open and solid symbols correspond, respectively, to data measured during the adsorption and desorption of adenine. (C) Comparison of the integrated intensities for the main band of adsorbed adenine as measured in the potential-difference spectra collected with Au(111) (a) and gold thinfilm (b) electrodes. Working solution: 1 mM adenine + 0.5 M NaF solution prepared in deuterium oxide.

dependence of adenine coverage. The plots of the integrated intensity of the main adsorbate band (around 1630 cm-1) in the external reflection experiments for the Au(111) and Au(100) electrodes (Figure 8A and B) show increasing intensity values for potentials up to -0.6 and 0 V, respectively. In addition to the differences for the onset potential for adenine adsorption, another important difference between the Au(111) and Au(100) electrodes concerns the hysteresis in the intensity curves corresponding to the positive- (adsorption) and negative-going (desorption) sweeps. No hysteresis is observed in the case of the Au(111) electrode. Conversely, the intensity curve for

adenine adsorption on Au(100) is shifted toward less negative potentials compared to the desorption curve. This behavior can be related to the kinetics of the adsorption, being slower for adenine adsorption on the reconstructed domains appearing at the lower potential limit because adsorption may induce the simultaneous lifting of the reconstruction of the gold surface.1 Obviously, the effect is more evident in the case of the Au(100) electrode. Figure 8C allows the comparison of the intensity curves obtained during adenine desorption on the Au(111) and the gold thin-film electrodes. As expected from the preferential (111) orientation of the latter after the electrochemical annealing, these two curves are mainly coincident in the potential range between -1.00 and -0.00 V. Changes in the intensity of the main adsorbate band reported in Figure 8 are paralleled by changes in the band frequency for the Au(100), Au(111) and gold thin-film electrodes. The corresponding band frequency values have been plotted in Figure 9 as a function of the electrode potential. These plots show a shift of the band frequency toward higher wavenumbers with increasing electrode potential. At higher potentials, the band frequency remains almost constant. The observed frequency shift, which parallels the increase of adsorbate coverage in the latter potential range (see the cyclic voltammograms in Figure 1 and the band intensity plots in Figure 7) can be ascribed to increasing dipole coupling due to lateral interactions between neighbor adsorbates.88 The Stark tuning effect could also contribute to the observed blue shift of the adsorbate band frequency.7,8 Therefore the same adsorption process takes place irrespective of the crystallographic or the reconstruction state of the gold electrode surfaces. Conclusions The spectroelectrochemical data reported in this work provide new information on the adsorption processes taking place at gold electrodes in contact with neutral adenine-containing solutions. Whereas the voltammetric results clearly indicate the existence of specific adsorption, the in situ infrared spectra, especially the strong ring stretching band around 1640 cm-1, allows the identification of the adsorbate formed at the gold electrode surface in the presence of adenine. The high surface sensitivity of ATR-SEIRAS enabled the identification of ad-

Adenine Adsorption at Single Crystal and Thin-Film Gold Electrodes ditional absorption bands in the spectra with the gold film, in the region between 1600 and 1100 cm-1. The ATR-SEIRAS spectra taken in H2O solutions allowed also concluding the surface activity of the amino group. An adsorption model is proposed in which the molecule plane is slightly tilted on the electrode surface. The molecule seems to coordinate to the gold substrate via the NH2 group in an sp3-like configuration, and most likely with contribution of the N7 ring atom. The model applies irrespective of the surface crystallographic orientation. On the other hand, potential-induced reorientations are not detected and the changes in the principal band intensity observed upon scanning the potential in opposite directions are attributed to differences in the rate of adsorption, depending on the reconstructed or unreconstructed state of the electrode surface. Acknowledgment. Financial support from Ministerio de Educacioń y Ciencia (Spain) (Projects CTQ2004-06645/BQU, CTQ2006-09868/BQU, and CTQ2006-04071/BQU, Fondos FEDER), Junta de Andalucia (Grupo FQM202), Generalitat Valenciana (ACOMP07-048), and the Universities of Alicante and Sevilla is greatly acknowledged. The authors also thank Dr. Jose´ Manuel Delgado for the preparation of the gold thin film electrodes and the SS.TT.I. of the University of Alicante for allowing the use of the sputtering facility. References and Notes (1) Prado, C.; Prieto, F.; Rueda, M.; Feliu, J.; Aldaz, A. Electrochim. Acta 2007, 52, 3168. (2) Brabec, V.; Kim, M. H.; Christian, S. D.; Dryhurst, G. J. Electroanal. Chem. 1979, 100, 111. (3) Fontanesi, C. J. Chem. Soc., Faraday Trans. 1994, 90 (19), 2925. (4) Katz, M.; Cumming, T. E.; Elving, P. J. Ber. Bunsen-Ges. Phys. Chem. 1979, 83, 614. (5) Kirste, S.; Donner, C. Phys. Chem. Chem. Phys. 2001, 3, 4384. (6) Prado, C.; Navarro, I.; Rueda, M.; François, H.; Buess-Herman, C. J. Electroanal. Chem. 2001, 500, 356. (7) Nichols, R. J. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH: Weinheim, 1992; Chapter 7. (8) Iwasita, T.; Nart, F. C. In AdVances in Electrochemical Science and Engineering; Gerischer, H., Tobias C. W., Eds.; VCH: Weinheim, 1995; Vol. 4, Ch. 3. (9) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861. (10) Rodes, A.; Pe´rez, J. M.; Aldaz, A. In Handbook of Fuel Cells. Fundamentals, Technology and Applications; Vielstich, W., Gasteiger, H. A., Lamm A., Eds.; John Wiley & Sons Ltd.: Chichester, 2003; Vol. 2, Chapter 16. (11) Alkire, R.; Kolb. D. M. In Diffraction and Spectroscopic Methods in Electrochemistry, AdVances in Electrochemical Science and Engineering; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: Weinheim, 2006.; Vol. 9 (12) Berna´, A.; Delgado, J. M.; Orts, J. M.; Rodes, A.; Feliu, J. M. Langmuir 2006, 22, 7192. (13) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664. (14) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951. (15) Wandlowski, T.; Ataka, K.; Pronkin, S.; Diesing, D. Electrochim. Acta 2004, 49, 1233. (16) Osawa, M. In Near Field Optics and Surface Plasmon Polaritons; Kawata, S., Ed.; Springer-Verlag: Berlin, 2001. (17) Osawa, M. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons: New York, 2002; Vol. 1. (18) Aroca, R. F.; Ross, D. J.; Domingo, C. Appl. Spectrosc. 2004, 58, 324A. (19) Noda, H.; Wan, L. J.; Osawa, M. Phys. Chem. Chem. Phys. 2001, 3, 3336. (20) Futamata, M. Chem. Phys. Lett. 2000, 317, 304. (21) Futamata, M. J. Electroanal. Chem. 2003, 550-551, 93. (22) Pronkin, S.; Wandlowski, T. J. Electroanal. Chem. 2003, 550551, 131. (23) Han, B.; Li, Z.; Pronkin, S.; Wandlowski, T. Can. J. Chem. 2004, 82, 1481. (24) Han, B.; Li, Z.; Wandlowski, T.; Blaszczyk, A.; Mayor, M. J. Phys. Chem. C 2007, 111 (37), 13855. (25) Rosendahl, S. M.; Danger, B. R.; Vivek, J. P.; Burgess, I. J. Langmuir 2009, 25 (4), 2241.


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Adenine Adsorption at Single Crystal and Thin-Film Gold Electrodes

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