Structural and Charge Sensitivity of Surface-Enhanced Raman

Article pubs.acs.org/JPCC

Structural and Charge Sensitivity of Surface-Enhanced Raman Spectroscopy of Adenine on Silver Surface: A Quantum Chemical Study Rong Huang,† Hong-Tao Yang,† Li Cui,‡ De-Yin Wu,*,† Bin Ren,† and Zhong-Qun Tian† †

State Key Laboratory of Physical Chemistry of Solid Surfaces and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China ‡ Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, Fujian, China S Supporting Information *

ABSTRACT: The interaction of adenine with silver surfaces has been investigated using density functional method. Two isomers of N9H and N7H were included to model surface species. Considering the complexity of silver surfaces in surface-enhanced Raman spectroscopy, neutral and positive silver clusters were used to mimic the substrate. Following the bonding principle, we consider adenine-approached silver clusters in different configurations and their relation to the Raman spectra. For neutral adenine Agn (n = 4, 7, and 9) complexes, N9H−Agn complexes are more stable than N7H−Agn ones. The corresponding Raman spectra strongly depended on the structure of adenine and the adsorption sites. Moreover, we find N7H interacts with one positively charged silver cluster via N3 and N9 at the same time as the most stable surface complex, which can reproduce the experimental surface Raman spectra of adenine well on silver surfaces.

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INTRODUCTION Adenine is an interesting molecule in the field of surfaceenhanced Raman spectroscopy (SERS). It is the first small organic molecule used in single-molecule SERS measurements and it dominates the SERS of DNA.1−3 Since its SERS spectrum was reported for the first time by Koglin and his coworkers,4−9 a number of studies showed that the SERS spectra of adenine are extremely dependent on the experimental conditions, such as the preparation of SERS substrates, the pH values of the electrolyte solution, surface concentrations, and applied potentials.10−15 Inversely, the surface state of these nanostructure can also be deduced from the different SERS spectra of adenine.16 The attractive topic causes many scientists to explore the interaction of adenine and its derivatives on coinage metal surfaces. Koglin et al. studied the adsorption behaviors of adenine and its derivatives on silver electrodes as well as on silver colloids with the laser wavelength at 514.5 nm. They proposed adenine approaching to the surface via N7 and the amino group.4−9 After then, numerous SERS studies thought that adenine adsorbed on silver surfaces through the N7 site together with the external amino group.10,17−20 However, the other adsorption configurations were also proposed continuously to interpret the SERS of adenine. For example, Moskovits et al. thought adenine adsorbed in a flat configuration through the external amino group on rough silver surfaces because most of the enhanced peaks came from the vibrations related to the amino group.21 Kim et al. thought adenine interacted with the silver surface by the N1 site due to the highly negative charge density of this site.22 Halas and co-workers thought that the © 2013 American Chemical Society

adsorption configuration at the N3 site was responsible for the SERS spectra of adenine and different DNA oligomers containing adenine.23 Apart from the SERS results, the surface-enhance infrared spectroscopy (SEIRS) proposed N7 and the amino group to be the adsorption site when adenine adsorbed on gold surface for the vibration of the amino group that was selectively enhanced.24,25 The results from reflection absorption infrared spectroscopy proposed that adenine approaches a copper surface with N9 and an additional interaction with N3.26 Scanning tunneling microscope (STM) results proposed a flat configuration when adenine adsorbed on gold surfaces.27 Another interesting result was proposed by Feyer et al. by using core-level photoemission and X-ray absorption. The results showed at a low coverage that adenine was almost parallel to the copper surface through the interaction of the amino group and N7, while at a high coverage it approached to the surface with N1 and the amino group in a perpendicular way.28 Until now, it is still difficult to definitely infer the adsorption orientation of adenine on coinage metal surfaces experimentally from SERS spectra. For there are five nitrogen atoms in adenine, which can be marked by N1, N3, N7, N9, and N10 and the isomerization effect, as shown in Figure 1. N9H and N7H are the two most stable isomers; the relative energy is about 7−8 kcal/mol in gas phase.29−31 N9H is the one in which Received: July 31, 2013 Revised: October 15, 2013 Published: October 15, 2013 23730

dx.doi.org/10.1021/jp407615r | J. Phys. Chem. C 2013, 117, 23730−23737

The Journal of Physical Chemistry C

Article

analogies to SERS active centers. Our results show that the surface Raman spectra of adenine are sensitive to the binding sites of adenine itself approaching the surfaces and the nature of metal materials. The results also indicate that N7H interaction with one positively charged silver cluster through N3 and N9 at the same time can reproduce the experimental results well.

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COMPUTATIONAL DETAILS When considering chemical bonding interaction or charge transfer process in SERS, a model of molecule with metallic clusters is proven to be a good method.36−38 The metallic cluster model was used to describe the interaction between adenine and the active sites on metal surfaces. Observing the structure of adenine, it may approach the surface with different orientations, such as a flat orientation through the orbital of the pyrimidine ring or the imidazole ring or the perpendicular orientation through the lone-paired orbital at one of the N1, N3, and N7 sites. In the early studies, the lone-paired orbital localized at the amino group was suggested to take part in the surface adsorption interaction; however, according to the theoretical result of Soto-Verdugo et al.,39 there is no interaction between adenine ring and a silver cluster and there is a very weak binding of the amino group with the silver cluster. Moreover, it is worth to noting that the binding interaction through the amine group will result in the significant enhancement of its wagging vibration.36 This could not be observed at the SERS experiments of adenine. So we only consider the interaction between the lone-paired orbitals on nitrogen with sp2 hybridization and silver clusters. The counterpoise correction of the basis set superposition error (BSSE)40 was considered, which corrected the overestimation of the binding energy in Table 1S (see Supporting Information). Furthermore, adenine easily isomerizes from the N9H to the low-energy isomer N7H. In our recent study, the latter isomer can be stabilized from the interaction with silver ion.35 So we considered the two adenine tautomers N9H and N7H, which were thought to coexist in aqueous solution. Meanwhile, Agn (n = 4, 7, and 9) of neutral and one positive charge are included in the calculations; the reason to choose Agn (n = 4, 7, and 9) will be explained in detail in the discussion part. Density functional calculations were carried out with Becke’s three-parameters exchange and Lee−Yang−Parr correlation hybrid functionals (B3LYP)41−43 in Gaussian 09.44 The convergence criterion for the optimized structures is within the energy difference of 10−8 au. The basis sets for C, N, and H atoms of investigated molecules were 6-311+G(d, p).45,46 For Ag atoms, the valence electrons and the inner shells were described by using the basis set, LANL2DZ, and the corresponding relativistic effective core potentials, respectively.47,48 The scaled quantum mechanics force field (SQMF) procedure was used to assign all the fundamental vibrational bands.49 Scale 2.0 program was used to get the potential energy distribution.50 We chose the scaling factors of 0.935 for the stretching vibrations of the N−H and C−H bonds, as well as 0.975 for the other internal coordinators to the force constant matrix calculated at the B3LYP/6-311+G(d, p) level. Considering the interaction of adenine with silver clusters, these scaling factors were used to correct the incomplete property of theoretical approaches and basis sets, as well as the neglect of anharmonicity. Absolute Raman intensities were calculated on top of the differential Raman scattering cross section (DRSC). In order to

Figure 1. The lone-paired orbitals of N9H and N7H as well as the corresponding energies (unit in au).

the hydrogen atom is located at the N9 position while N7H is located at the N7 position. Apart from the complexity of adenine, the surface state of metal nanostructures is also hard to be known well. It becomes quite difficult to infer the surface binding sites and possible adsorption orientations. In order to explore stable adsorption configurations of adenine on a metal surface, it needs to theoretically design and analyze various possible binding modes for adenine adsorptions. Theoretically, density functional calculations have been used to investigate the relationships of surface adsorption configurations and simulated Raman spectra of adenine on silver surfaces. Giese et al. provided the detailed analysis of SERS spectra of adenine adsorbed on different silver substrates.11 They thought that adenine adsorbs on silver in a titled configuration though the N7 site and the external amino group. It is worth noting that their DFT analysis did not consider the interaction between adenine and silver surfaces. Watanabe et al. tried to consider the interaction by adopting adenine with one silver atom complexes and thought N3 was the most possible interaction site.32 Jensen used an Ag20 clusters to mimic a silver surface to consider the interaction via N7 and to simulate Raman spectra of adsorbed adenine in static as well as dynamic Raman scattering processes.33 The calculated results indicated that there exist some differences between the simulated and measured Raman spectra. Lang et. al simulated the Raman spectra of two adenine isomers interacting with Ag20 at different surface sites. They thought that adenine interacts with the silver surface via N3 because it can reproduce the measured SERS spectra in silver colloids.34 But in fact, apart from the breathing mode, the other bands can not correspond to the experimental SERS peaks. Recently, adenine−Ag+ complexes were involved to explain the SERS of adenine on silver surfaces.12−15,35 Here, it was suggested that positively charged silver clusters may be responsible to surface Raman signals, as mentioned in early studies by Koglin and Otto. There are possible positive charged clusters in the surface of silver.9,16 Also, in our previous work about aniline with Ag surfaces it was proven that aniline with cationic Ag clusters can describe the interaction well.36 The goal of the present work is to obtain a deeper insight into the adsorption configuration and the chemical enhancement effect of adenine adsorbed on neutral and one positive charged silver clusters with different sizes. These clusters are 23731



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Figure 2. The lowest four LUMO orbitals of the neutral (left) and positively charged silver clusters (right). The corresponding energies (in au) are labeled beside. The energies in red are corresponding to those orbitals possibly involved in the binding to adenine.

consist of 5s atomic orbitals in silver clusters, as shown in Figure 2. For Ag7 and Ag9, the cases are very similar to each other. Therefore, this brings us an easy analysis method to understand the binding interaction between adenine and silver clusters. Comparing the orbitals of neutral and positive silver clusters, we can find the energies of orbitals decreasing obviously; this can be understood due to a large electron affinity energy for the strong electrostatic attraction. The shapes of the lowest four LUMO orbitals are almost kept the same except the Ag4. Although the LUMO of Ag4 changed when losing one electron, the adenine approaching to Ag4+ along the short Ag−Ag bond is more stable than along the long Ag−Ag bond. Binding Interaction in Neutral and Positive Adenine− Agn (n = 4, 7, and 9) Complexes. On the basis of the discussion above, we constructed different configurations of adenine with silver clusters and got the bonding orbitals. It consists of the s orbital of silver clusters and the lone pairs of adenine. Now we focused on the influence of the binding interaction on relative stability of different adenine−silver cluster complexes. The relative energies of various adenine−Ag4 complexes in gas phase have been given in Table 1. For N9H and N7H with

make direct comparison with the SERS experiments, the simulated Raman spectra were presented in terms of the Lorentzian expansion of the DRSC magnitudes from the Raman scattering factors (RSF) under the double-harmonic approximation.

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RESULTS AND DISCUSSION

Molecular Orbitals of Adenine and Silver Clusters (n = 4, 7, and 9). To understand the binding interaction between adenine and a silver cluster, we consider the binding interaction on the view of the general bonding rule, such as the energy and symmetry matches as well as the largest overlap between the interacting orbitals. Similar to the case meeting in pyridine binding to silver clusters,38 the lone-paired orbitals of nitrogen atoms with the sp2 hybridization can be considered as an electron donor taking participate in the bonding. The unoccupied orbital in silver clusters were considered as an acceptor. So in the next part, we are focusing on the lone-paired orbitals of adenine and the (lowest unoccupied molecular orbital) LUMO orbitals of silver clusters. Although there are more than 10 tautomers in adenine, our recent study indicated that adenine exists as a mixture of N9H and N7H isomers with the solvation model.35 Both N9H and N7H tautomers have three lone-paired orbitals. Figure 1 presents the lone-paired orbitals of N9H and N7H tautomers as well as the corresponding energies. The shown orbital energies of the two isomers are near to each other. Comparison of their electron density plots indicates that the lone-paired orbitals obviously exist as a mixture due to their close energies. Figure 2 presents the frontier molecular orbitals of the neutral and one positively charged Agn clusters (n = 4, 7, and 9). We chose the sizes of silver clusters, Ag4, Ag7, and Ag9 in the present model calculations. This is because their geometric structures of metallic clusters are good because they keep the same structures regardless if they have neutral or positive charges.51−54 Some parameters of the optimized clusters are indicated in the picture. For neutral Ag4, the LUMO orbital has a large population along the short Ag−Ag bond. This suggests that when we build the adenine−Ag4 complex the binding geometry should adopt a configuration with the lone pair along the short Ag−Ag bond axis. In this case, the largest overlap bonding and the local orbital symmetry can be well satisfied. Meanwhile, we noted that the frontier molecular orbitals

Table 1. Relative Energies (kcal/mol) of Various Adenine− Ag4 and Adenine−Ag4+ Complexes Calculated at the B3LYP/6-311+G(d,p) (C, N, and H)/LANL2DZ(Ag) Level Species

N1

N3

N7

N9H−Ag4a N7H−Ag4a N9H−Ag4+b N7H−Ag4+b

1.25 9.77 7.90 18.23

0.00 10.62 8.02 0.00c

1.78

N9 10.80

7.86

a

The energies are compared to N9H−Ag4 via N3 site. bCompared to N7H−Ag4+ via N3 cAdenine approaches Ag4+ via N3 and N9 at the same time. a = cos 144°, b = cos 72°.

Ag4, there are three sites that can be included with the silver cluster, respectively. For neutral adenine−silver complexes, though N9H interacting with Ag4 via N3 is the most stable one, there are small differences among the selected configurations for N9H. The N7H approach to Ag4 via N1 is about 1 kcal/mol more stable than the other two among the three N7H−Ag4 23732



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configurations and the related Raman spectra that can give fingerprint information. We have tried to construct two kinds of configurations; one is N9H in the same plane with a silver cluster and the other is N9H perpendicular with a silver cluster. Figure 4 shows the

complexes. However, the N9H−Ag4 complexes are more stable in the relative energy, about 9−10 kcal/mol than N7H−Ag4 ones, which mainly comes from the energy difference between N9H and N7H.30,31,35,55 The binding energies of these complexes were given in Table 1S (see Supporting Information); we can find that the binding energies of adenine−Ag4 are near to each other and the binding energies are much stronger of adenine−Ag4+ than adenine−Ag4. For positively charged complexes, it is another circumstance. N7H interacting with Ag4+ through N3 and N9 is the most stable one, which is similar to the case of adenine−silver ion complexes.35 The binding energy is 43.64 kcal/mol, which is the strongest one among all the adenine−Ag4 and adenine− Ag4+ complexes. After having the relative energies, we will get insight into the dependence of Raman spectra on the different configurations. The Influence of Different Silver Cluster Sizes on Raman Spectra. To simplify the problem, we first check the influence of the cluster sizes. Take N9H with different silver clusters at N7 site for example; Figure 3 presents the simulated

Figure 4. Simulated Raman spectra of N9H−Ag4 complexes with different approaching sites. The sites are (a) N1, (b) N3, and (c) N7. The other parameters are the same as Figure 3

optimized structures and simulated spectra of N9H with Ag4 at different sites. N9H is almost in the same plane with the silver cluster. The intensities of their spectra are close. Compared with the spectra of N9H−Ag4 at the N1 site, the N9H interacting with the N3 site exhibits two significant changes. First, the vibrational frequency of the ring breathing mode is blue shifted to 723 cm−1. This is because the breathing mode is from the N3−C4 stretching and the five-membered ring deformation.35 When the silver cluster interacts with adenine via N3, it directly restricts the N3−C4 stretching and the fivemembered ring deformation; the ring breathing mode blue shifts obviously. The lower wavenumber band takes a red shift to 600 cm−1, while the band at about 540 cm−1 obviously becomes a broad and weak band compared with that in N9H− Agn versus the N1 site. Second, there are two isolated bands at 1357 and 1412 cm−1 as well as a set of twins bands at about 1240 cm−1 in the region of 1200 to 1450 cm−1. The bands at near 1490 cm−1 display the middle intense twin bands. These features of the latter bands are obviously different from that in N9H−Agn versus the N1 site. Many previous studies suggested adenine via N7 adsorbed on silver surfaces. Our simulated Raman spectra of N9H interacting with Ag4 clusters via N7 site was presented in Figure 4c. The ring breathing mode still has the strongest Raman signal and located in 718 cm−1. The other two stronger peaks appear at 1249 and 1501 cm−1, both can be attributed to a mixed vibration of the N7−C8 stretching and the in-plane C8−H bending. The band at 1344 cm−1 is a middle intense peak. This is different from the experimental observation that the band always has the strongest Raman peak at 1330 cm−1 in the range of 1200 to 1500 cm−1 for adenine adsorbed on various SERS substrates.5,10−12,32

Figure 3. Simulated Raman spectra of N9H−Agn complexes via N7 at the B3LYP/6-311+G(d,p) (C, N, and H)/LANL2DZ(Ag) level, (a) n = 4, (b) n = 7, and (c) n = 9. The structures are given below. The incident wavelength of 514.5 nm was used here with a Lorentzian line width of 10 cm−1.

Raman spectra and the corresponding structures. The bond lengths of N7 to silver clusters are 2.306, 2.437, and 2.354 Å for Ag4, Ag7, and Ag9 respectively. The results indicate that a stronger interaction of N9H with Ag4 occurs. The results are also in accordance with the previous calculation of silver clusters with DNA bases.54 Through the differences in binding energies, comparison of these Raman spectra clearly shows that the profiles of Raman spectra for different metallic clusters at the same interacting site give very similar Raman shifts and relative intensities. The other sites with different silver clusters are shown in Figure 1S (see Supporting Information); they also show similarity. By taking the similarity into consideration, we thought that the size of the silver cluster does not influence significantly the Raman spectra of adsorbed adenine. Neutral N9H−Agn Clusters (n = 4, 7, and 9). Although N9H interacting with different neutral silver clusters through the N3 site is the most stable among three possible adsorption forms, their energy differences are quite small. This suggested to us to further investigate the relationship between different 23733



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> N1 > N9. Since the Raman intensity is related to the change of polarizability along the corresponding vibrational coordinates, the perpendicular one has a larger value.38 Similar to the case of N9H−silver complexes, the Raman spectra of N7H with silver cluster at different sites are not like each other. Compared to N9H with silver clusters, the Raman shift of the breathing mode almost keeps the same, and the profile of Raman spectra are cleaner. The breathing modes of the three configurations are 716, 723, and 717 cm−1 for N1, N3, as well as N9, respectively. This can be easily explained due to the adsorption site effect that has been shown in the N9H− silver complexes. Apart from the most characteristic breathing mode, for N7H−Ag4 at N1 there is a strong band at 556 cm−1 that comes from the wagging vibration of the amino group; the enhanced intensity is because the interaction between N7H and silver cluster leads to a wagging vibration of the amino group. The phenomenon has been explained in detail in our previous study.36 Another strong band is at 1374 cm−1 that is attributed to the stretchings of the C4−C5 and C4−N9 bonds. For N7H with the silver cluster through the N3 site, there is another strong band at 1575 cm−1, which mainly relates to the stretching vibration of N3−C4 and C4−C5 bonds. At N9, apart from the band at 716 cm−1, there are two strong bands located at 1358 cm−1 from the C2−N3 and C8−N9 stretchings and 1374 cm−1 from the C4−C5 stretching and the C2−H in-plane bending. In summary, the change of the hydrogen-attached location at N7H leads to a large structural difference of N7H−Ag4 to N9H−Ag4 complexes. Moreover, the Raman spectra of N7H− Ag4 complexes strongly depend on the interaction pattern. N7H−Agn+ Clusters. Since the complexity of SERS, the adsorption configuration of adenine on metal nanostructures is hard to confirm. Moreover, silver is easy to oxidize. As mentioned several times in the work of Koglin and the work of Otto, there often exist positively charged silver clusters on silver electrode surfaces in the SERS measurements.16 Recently, people think there are silver ions on silver surfaces confirmed by using X-ray photoelectron spectroscopy (XPS) for silver nanoparticles.15,57 Inspired from the previous studies of adenine with silver ions, we consider it possible for the interaction of N7H with a silver cluster that contains a positive charge. Analogous to neutral silver clusters, we constructed the structures following the bonding principle and get five stable positive charged N9H−Ag4+ and N7H−Ag4+ complexes. The relative energies are shown in Table 1. Considering the large energy differences and our previous results of adenine−silver ion complexes, in the following part we focus on the discussion of N7H−Agn+ via N3 and N9 at the same time. Figure 6 presents the comparison of the experimental SERS spectrum of adenine with the incident wavelength of 532 nm and simulated Raman spectra of N7H−Ag4+ in which adenine approaches the silver cluster by N3 and N9 at the same time. The experimental details are shown in Supporting Information. Moreover, the simulated Raman spectra of N7H−Agn+ (n = 4, 7, and 9) complexes are also given in Figure 2S (see Supporting Information). In a comparison of adenine with Ag4+, Ag7+, and Ag9+ complexes, the size of silver clusters does not influence the Raman spectra. We can see the two peaks at 721 and 1338 cm−1 dominate the simulated Raman spectrum in Figure 6. They can be attributed to the breathing mode and the C2−N3 as well as C8−N9 stretching as shown in Table 2. It is consistent with the observed SERS spectra on silver surface,

For N9H, our calculated results show that the three adsorption configurations along the nitrogen sites, such as the N1, N3, and N7 sites, display significantly different features in Raman spectra. This suggests that Raman spectroscopy can be used to identify surface adsorption of adenine on silver surfaces. However, these simulated Raman spectra of N9H−Ag 4 configurations cannot interpret the observed Raman reported in literatures. Recent studies proposed that adsorbed adenine can tautomerize due to the surface adsorption or the binding interaction with silver cations.26,35 Therefore, in the next section we will discuss the interaction between N7H and silver cluster, moreover, the simulated Raman spectra of N7H with silver cluster complexes will be explained in detail. Neutral N7H−Agn Clusters. For N7H, we consider three sites, N1, N3, and N9 atoms of adenine. The N7H is the second most stable isomer of adenine in gas phase and aqueous solution.30,31,35 Since its dipole moment is obviously larger than that of N9H, the polarization effect of the polar solvent like water can significantly stabilize the N7H isomer. In our previous paper, we predicted the N7H isomer preferring the formation of [N7H−Ag]22+ ionic complex.35 This agrees the Xray measurement crystal structure and the analysis of mass spectroscopic results.29,56 This is also in agreement with the adsorption configuration proposed based on the high resolution electron energy loss spectroscopy, where adenine adsorbed on copper surfaces versus N3 and N9 sites.26 Figure 5 shows the optimized structures and the simulated Raman spectra of N7H−Ag4 complexes at N1, N3, and N9

Figure 5. Simulated Raman spectra of N7H−Ag4 complexes. The sites are (a) N1, (b) N3, and (c) N9. The other parameters used in the simulated Raman spectra are the same as Figure 3.

sites, respectively. Observing the structures for N7H−Ag4 at N9, one can see that N7H is almost in the same plane with the silver cluster. While for N7H−Ag4 at N1 and N3, N7H are not in same plane, especially for N3; they almost interact in a perpendicular way. The torsion angles are 6.3, 25.3, and 93.6° for N9, N1, as well as N3, respectively. The differences in their structures show that their intensities are in a different order: N3 23734



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influence the structure of adenine, which will lead to changes of the vibrational spectra, so it is important to consider the influence of silver surfaces when one assigns the simulated spectra of adenine. The potential energy distribution given in Table 2 was based on the complex of adenine with Ag4+ while most of the previous distributions were based on adenine only.11,18 Besides the two characteristic bands around 730 and 1330 cm−1, if we observe the normal and surface-enhanced Raman spectra of adenine, when adenine adsorbed on silver surfaces in the region below 700 cm−1, there is no obvious change. While in the region of 700 to 1000 cm−1, apart from the blue shift of the breathing mode, the peak of 940 cm−1 in normal Raman blue shifts to 960 cm−1. This mode was contributed by the deformation of the five-member ring, and it blue shifts because the ring vibration was blocked by the adsorption. This mode was predicted at 957 cm−1 in our calculation. Moreover, in the region 1400 to 1600 cm−1 there appear three new peaks with vibrational frequencies at 1400, 1520, and 1570 cm−1. These peaks are all predicted well in our simulated Raman spectrum of N7H−Ag4+ via N3 and N9 sites. They are located at 1391, 1513, and 1578 cm−1, shifting from the peaks at 1371, 1508, and 1568 cm−1 in N7H.35 The 1391 cm−1 peak was mainly coming from the stretching of C4−C5 and in-plane bending of C2−H. The 1513 cm−1 peak was due to the stretching of C8− N9 and the in-plane C8−H bending. The 1578 cm−1 peak was

Figure 6. Comparison of (a) the experimental SERS spectrum of adenine and (b) simulated Raman spectrum of N7H−Ag4+ complexes. The corresponding structure is shown on the right. The other parameters of the calculation are the same as Figure 3.

which mainly have two bands located at 732 and 1330 cm−1. In order to make a good comparison with the experiments, we summarized some of the experimental results of adenine on different silver surfaces including silver colloids and silver electrodes, which are usually used as the SERS substrates, in Table 2.4,11,18,34 Moreover, as mentioned in the work of Watanabe et al.,32 the interaction of adenine with silver surfaces

Table 2. Comparsion Between the Experimental SERS of Adenine and Theoretically Scaled Vibrational Frequencies (ω, cm−1), Raman Activity (AR, Å4/amu), as Well as Assignments of N7H−Ag4+ Complex at the B3LYP/6-311+G(d,p) (C, N, and H)/ LANL2DZ(Ag) Levelabc freq

AR

this work 1652 1608 1578 1513 1482 1443 1391 1363 1337 1325 1252 1215 1150 1106 994 971 956 901 876 778 721 689 629 608 554 543 523

exp this work

17.0 0.6 37.8 4.7 9.2 7.4 43.4 11.3 66.1 11.3 15.6 21.9 1.4 11.4 17.6 0.1 5.5 4.9 0.2 0.2 30.6 0.1 0.1 9.0 1.9 1.0 2.0

ref 4

PED (%)

ref 11

ref 18

1463

1458

1651 1606 1574 1518 1467

1396 1372 1337

1397 1372 1336

1402

1396

1323

1327

1246

1244

1241

1249

1113 1013

1137 1025

1027

1119 1030

958

961 894

965 922

958

1650 1572 1515 1478 1401 1330 1246 1227

ref 34

1568 1517

732

736

733

735 689

730

624

633

626 563 543

623 576 542

619

540

βNH2 sci (36), vC6N10 (21), vC5C6 (17) βNH2 sci (47), vC5C6 (14) vN3C4 (33), vC4C5 (14), vN1C6 (11) vC8N9 (32), βC8H (18), vC6N10 (11) βC2H (34), vN1C6 (16), vC2N3 (12) βN7H(36), vN7C8 (30), vC6N10 (11) vC4C5 (31), βC2H (15) vN1C2(36),βC2H (18), vC6N10 (12) vC2N3 (22), vC8N9 (18), βC2H (13) vN3C4 (24), vC4N9 (23), vN1C6 (12) vC2N3 (38), βC8H (22), vC8N9 (18) vC5N7 (24), NH2 rock (18), βC8H (15) vN7C8 (43), βN7H (18), vC4N9 (10) βC8H (18), βN7H (12), vC4N9 (11), βr2 (11) NH2 rock (46), vN1C6 (27) γC2H (100) βr1 (51), βr2 (21), vC4C5 (11) βR3 (42), βR2 (15) γC8H (100) τR3 (50), τr1 (24), γNH2 (15) βR1 (20), vN3C4 (18) γ NH2 (48), τr1 (24), τR1(10) τr2 (71) vC5C6 (23), βR1 (21), βR2 (12) τR2 (32), τR3 (24), τr1 (17), γNH2(15) βtri2 (33), βR1 (20), βNH2 (17) βR2 (53), βNH2 (10)

a

v, stretching; β bending; rock, rocking; τ, torsion; r, five-membered ring; R, six-membered ring. bβr1=β1+ a(β2+β5)+ b(β3+β4);βr2=(ab)(β2−β5)+(1−a)(β3+β4). cβR1=2α1−α2−α3 + 2α4−α5−α6;βR2=α2−α3+α5−α6;βR3=α1−α2+α3−α4+α5−α6. 23735



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(3) Barhoumi, A.; Zhang, D.; Tam, F.; Halas, N. J. Surface-Enhanced Raman Spectroscopy of DNA. J. Am. Chem. Soc. 2008, 130, 5523− 5529. (4) Ervin, K. M.; Koglin, E.; Sequaris, J. M.; Valenta, P.; Nernberg, H. W. Surface enhanced Raman Spectra of Nucleic Acid Components Adsorbed at a Silver Electrode. J. Electroanal. Chem. 1980, 114, 179− 194. (5) Koglin, E.; Sequaris, J. M.; Valenta, P. Surface Raman Spectra of Nucleic Acid Components Adsorbed at a Silver Electrode. J. Mol. Struct. 1980, 60, 421−425. (6) Koglin, E.; Sequaris, J. M.; Valenta, P. Surface Enhanced Raman Spectroscopy of Nucleic Acid Bases on Ag Electrodes. J. Mol. Struct. 1982, 79, 185−189. (7) Koglin, E.; Sequaris, J. M.; Fritz, J. C.; Valenta, P. Surface Enhanced Raman Scattering (SERS) of Nucleic Acid Bases Adsorbed on Silver Colloids. J. Mol. Struct. 1984, 114, 219−223. (8) Koglin, E.; Lewinsky, H. H.; Séquaris, J.-M. The Short-range Sensitivity of SERS: Results for Biomolecules. Surf. Sci. 1985, 158, 370−380. (9) Koglin, E.; Séquaris, J.-M. Surface Enhanced Raman Scattering of Biomolecules. In Analytical Problems; Springer: Berlin/Heidelberg: 1986; Vol. 134, pp 1−57. (10) Watanabe, T.; Kawanami, O.; Katoh, H.; Honda, K.; Nishimura, Y.; Tsuboi, M. SERS Study of Molecular Adsorption: Some Nucleic Acid Bases on Ag Electrodes. Surf. Sci. 1985, 158, 341−351. (11) Giese, B.; McNaughton, D. Surface-Enhanced Raman Spectroscopic and Density Functional Theory Study of Adenine Adsorption to Silver Surfaces. J. Phys. Chem. B 2002, 106, 101−112. (12) Muniz-Miranda, M.; Gellini, C.; Pagliai, M.; Innocenti, Massimo; Salvi, P. R.; Schettino, V. SERS and Computational Studies on MicroRNA Chains Adsorbed on Silver Surfaces. J. Phys. Chem. C 2010, 114, 13730−13735. (13) Papadopoulou, E.; Bell, S. E. J. Surface Enhanced Raman Evidence for Ag+ Complexes of Adenine, Doxyadenosine and 5′dAMP Formed in Silver Colloids. Analyst. 2010, 135, 3034−3037. (14) Papadopoulou, E.; Bell, S. E. J. Structure of Adenine on Metal Nanoparticles: pH Equilibria and Formation of Ag+ Complexes Detected by Surface-Enhanced Raman Spectroscopy. J. Phys. Chem. C 2010, 114, 22644−22651. (15) Pagliai, M.; Caporali, S.; Muniz-Miranda, M.; Pratesi, G.; Schettino, V. SERS, XPS, and DFT Study of Adenine Adsorption on Silver and Gold Surfaces. J. Phys. Chem. Lett. 2012, 3, 242−245. (16) Otto, C.; Hoeben, F. P.; Greve, J. Surface-enhanced Raman Scattering of the Complexes of Silver with Adenine and dAMP. J. Raman Spectrosc. 1991, 22, 791−796. (17) Otto, C.; De Mul, F. F. M.; Huizinga, A.; Greve, J. Surface Enhanced Raman Scattering of Derivatives of Adenine: the Importance of the External Amino Group in Adenine for Surface Binding. J. Phys. Chem. 1988, 92, 1239−1244. (18) Li, J.; Fang, Y. An Investigation of the Surface Enhanced Raman Scattering (SERS) from a New Substrate of Silver-modified Silver Electrode by Magnetron Sputtering. Spectrochim. Acta., Part A 2007, 66, 994−1000. (19) Feng, F.; Zhi, G.; Jia, H. S.; Cheng, L.; Tian, Y. T.; Li, X. J. SERS Detection of Low-concentration Adenine by a Patterned Silver Structure Immersion Plated on a Silicon Nanoporous Pillar Array. Nanotechnology 2009, 20, 295501. (20) Potara, M.; Baia, M.; Farcau, C.; Astilean, S. Chitosan-coated Anisotropic Silver Nanoparticles as a SERS Substrate for Singlemolecule Detection. Nanotechnology 2012, 23, 055501. (21) Suh, J. S.; Moskovits, M. Surface-Enhanced Raman Spectroscopy of Amino Acids and Nucleotide Bases Adsorbed on Silver. J. Am. Chem. Soc. 1986, 108, 4711−4718. (22) Kim, S. K.; Joo, T. H.; Suh, S. W.; Kim, M. S. Surface-enhanced Raman Scattering (SERS) of Nucleic Acid Components in Silver Sol: Adenine Series. J. Raman Spectrosc. 1986, 17, 381−386. (23) Kundu, J.; Neumann, O.; Janesko, B. G.; Zhang, D.; Lal, S.; Barhoumi, A.; Scuseria, G. E.; Halas, N. J. Adenine− and Adenosine Monophosphate (AMP)−Gold Binding Interactions Studied by

assigned to the stretching of N3−C4 and C4−C5 bonds. On the basis of the analysis of the difference between normal Raman and SERS of adenine on silver surfaces, the simulated Raman spectrum of N7H−Ag4+ via N3 and N9 sites can explain the observed changes, and it is also the most stable species among all the adenine−Agn+ complexes. So we believe that the proposed configuration can explain most of the present surfaceenhanced Raman spectra of adenine.

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CONCLUSION We aimed to illustrate a clear adsorption picture of adenine which is a special molecule in SERS on silver surfaces in this work. Accordingly, we constructed different adenine−Agn and adenine−Agn+ (n = 4, 7, and 9) configurations on the basis of the bonding principle to check their stability and the influence of the interaction on the SERS of adsorbed adenine. The results showed that N9H−Agn complexes are more stable than N7H− Agn ones. Our results also showed that the tautomerization as well as the site of adenine approaching to the Agn obviously influences the Raman spectra of adenine. For adenine−Agn+ complexes, the N7H interacting with cationic silver clusters via N3 and N9 is the most stable one, and it can reproduce well most of the experimental surface-enhanced Raman spectra of adenine on silver surfaces. Here, we provide another way to understand the SERS spectra of adenine on silver surfaces, which the tautomerization and cationic silver surfaces are important. Even though there is good agreement of the present simulated Raman spectrum with the SERS of adenine, how to explain the single molecular sensitivity of adenine and the diversity of experimental results are still to be solved.

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ASSOCIATED CONTENT

S Supporting Information *

Further discussions on the bonding energies of different adenine−silver complexes, the influence of different silver clusters on the Raman spectra of adenine, and the experimental details are given. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of this work by the NSF of China (Nos. 20973143, 21021002, and 91027009), National Basic Research Programs (Nos. 2009CB930703) and Xiamen University (Nos. 2010121020). D.Y.W is grateful for the support from HPC of Xiamen University.

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REFERENCES

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Structural and Charge Sensitivity of Surface-Enhanced Raman

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