N-Acetylalanine Monolayers at the Silver Surface Investigated by

Aug 27, 2008 - Wan-Joong Kim , Sanghee Kim , Bong Soo Lee , Ansoon Kim , Chil Seong Ah , Chul Huh , Gun Yong Sung and Wan Soo Yun. Langmuir 2009 ...
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J. Phys. Chem. C 2008, 112, 15022–15027

N-Acetylalanine Monolayers at the Silver Surface Investigated by Surface Enhanced Raman Scattering Spectroscopy and X-ray Photoelectron Spectroscopy: Effect of Metallic Ions Haifeng Yang,* Xuan Zhu, Wei Song, Yiping Sun, Guoping Duan, Xia Zhao, and Zongrang Zhang Department of Chemistry, Shanghai Normal UniVersity, 100 Guilin Road, Shanghai 200234, People’s Republic of China ReceiVed: May 14, 2008; ReVised Manuscript ReceiVed: July 21, 2008

Surface-enhanced Raman scattering (SERS) spectroscopy was applied to observe the process and structure of N-acetylalanine molecules self-assembled at the silver surface. SERS data show that the N-acetylalanine molecules adsorb at the silver surface through the amino group and the entire carboxyl group in a perpendicular way after a self-reorganization. Results of SERS and X-ray photoelectron spectroscopic experiments indicate that the Ce3+ affects the structure of monolayers because of the binding reaction between the Ce3+ and the carbonyl along with the amino groups in N-acetylalanine. The Ce3+ capped by N-acetylalanine monolayers can be removed by adding EDTA solution and the original structure of monolayers is recovered. However, the impact of K+ on the structure of N-acetylalanine monolayers is slight and can be ignored. Introduction The structural constraints of proteins and their role in controlling biological function have attracted increasing interest. Hence, various proteins were modified onto the metallic surface in many scientific and technological areas involving biocatalysis, biosensors, biocompatible membranes, and biocorrosion.1,2 In particular, amino acid monolayers, which provide a controllable, well-ordered, and functional interface as an expectation, could serve as a simple and important biomimic layer to observe interaction between metallic ions and the cell membrane. Numerous studies have been documented about the interaction between metal ions and amino acid to deepen the understanding for the interfacial structure of protein-metal. Therefore, metal-amino acid complexes have been synthesized and characterized by elemental analysis, FTIR spectroscopy, TG analysis, single-crystal X-ray diffraction, and Rayleigh lightscattering.3-6 Theoretical methods were also performed to form recognition patterns for these complexes.7-13 However, the above experiments were carried out in the solutions, which required a high concentration of species. As a result, the obtained results from the solutions were different from those of the real situations of proteins on metallic surfaces. The direct observation of adsorption behaviors of metallic ions on protein surfaces at the monolayer level should call for highly sensitive surface analytical techniques. The surface-enhanced Raman scattering (SERS) technique is one of the most sensitive methods for the detection of molecules adsorbed on rough metal surfaces.14-17 Enhancements of signal on the order of 104-106 are routinely observed, and in some systems enhancements of up to 1014 can be obtained.18 Such an enhancement factor makes this technique readily applicable to the study of submonolayer quantities of analytes. The high sensitivity and the inhibition from the fluorescence to the samples, especially to many biological molecules, make this technique even more popular.19-24 The SERS data can provide useful information regarding the structural features of some * Corresponding author. Phone/Fax: +86-21-6432-1648. E-mail: [email protected].

TABLE 1: Experimental and Theoretical Assignments of the SERS Spectra of N-Acetylalanine According to BLYP/ 6-311G Calculated Frequenciesa SERS bands (cm-1) A-A 381 391 600 712 852

A-A- A-A- A-A-Ce3+- calcd EDTA (cm-1) K+ Ce3+ 381 392 599 712 853

1006 1005 1130 1130 1157 1158 1238 1275 1312 1389

1237 1274 1311 1388

1452 1453 1560 1558 1587 1588

382 391 600 712 854 932 1005 1129 1157 1196 1238 1273 1317 1388 1418 1442 1453 1558 1587

382 392 599 712 852 1005 1130 1158 1240 1275 1311 1388 1455 1558 1589

386 407 601 716 829 948 1031 1122 1157 1209 1242 1297 1305 1395 1423 1440 1459 1562

assignment CH3 def., COO- def. CH3 def., COO- def. NH wag., CH3 def. COO- def. CsCOO- str. HNCdO symmetric str. NH wag., CH3def. CRCβ str. COO- str., CH3 def. CN def., CH3 str. CH3 def., COO- def. NH wag., CN str. NH wag., CN str. COO- symmetric str. CH3 i-p def. HNCdO ip def., CH3 ip def. CH3 def. NH wag., CN str. COO- asymmetric str.39

a str. stretching; wag. wagging; def. deforming: i-p in-plane. A-A: silver surface modified with N-acetylalanine. A-A-K+: silver surface covered with N-acetylalanine monolayers immersed in KNO3 (10 mM) solution. A-A-Ce3+: silver surface covered with N-acetylalanine monolayers immersed in Ce(NO3)3 (10 mM) solution.

amino acids, peptides, and proteins adsorbed on varying metallic surfaces.25-28 As mentioned above, amino acid or dipeptide bound to surface can serve as a simple system, which can be assisted in better interpreting the more complex systems of polypeptides and proteins. In this paper, N-acetylalanine as a model was selfassembled at the silver surface and its adsorption structure was analyzed by SERS spectroscopy. Considering the facility for chelating with amino acid and its potential medical application,12 the effect of Ce3+ on the structure of N-acetylalanine monolayers

10.1021/jp8042544 CCC: $40.75  2008 American Chemical Society Published on Web 08/27/2008

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Figure 1. SERS of N-acetylalanine (10 mM) adsorbed at the silver substrate (a) at the beginning, (b) after 0.5 h, and (c) after 3 h.

Figure 2. Schematic process of N-acetylalanine self-assembled at the silver surface.

was focused and investigated by SERS spectroscopy and X-ray photoelectron spectroscopy. Experimental Section Materials. N-Acetylalanine was purchased from Alfa Aesar and used without further purification. Solutions were freshly made by dissolving N-acetylalanine in Milli-Q water at a concentration of 10 mM. The dissolution was promoted by exposing the fresh solution to an ultrasonic bath for 20 min. Ce(NO3)3, KNO3, EDTA, and ethanol were of analytical grade. All solutions were dissolved by Milli-Q water at a concentration of 10 mM. Procedures. Before the Raman scattering measurement, the polycrystalline Ag electrode (99.99% pure) was polished successively with emery paper and 0.3 µm alumina. After that the electrode was rinsed in succession with copious amounts of Milli-Q water and pure ethanol and treated in an ultrasonic bath to remove any excess of alumina and any oxide of carbon produced during the process of polishing. It was then roughened by the oxidation-reduction cycle (ORC) method to obtain the SERS substrate.29 Electrochemical experiments were conducted in a CHI 650 electrochemical workstation. A Pt wire served as the counter electrode and all potentials are quoted versus a

standard calomel electrode (SCE). After washing with Milli-Q water, the Ag electrode was put into a homemade electrochemical cell with a quartz window at the top. After the cell was fixed on the XY stage below the sampling objective of the Raman spectrometer, the fresh N-acetylalanine solution (10 mM) was injected into the cell for self-assembly on the roughened Ag surface. The self-assembly process was monitored by time-dependent SERS spectra with 5-min intervals. For examining the influence of monolayers by metal ions, the roughened electrode was soaked in the N-acetylalanine solution (10 mM) for 3 h to form monolayers. Apparatus. A Jobin Yvon micro-Raman spectroscope (Super LabRam) was employed to obtain the Raman scattering signal, which was equipped with an integral Olympus BX40 microscope with 50× objective (8 mm), a holographic grating (1800 g/mm), a notch filter to cut off the Rayleigh line, and a semiconductorcooler, 1024 × 256 pixels, charge-coupled device detector. A He-Ne laser at 632.8 nm with a power of ca. 5 mW was used as the excitation source. A filter was employed to decrease the power of the laser to ca. 2 mW for the SERS experiment. Each spectrum was obtained by using three accumulations, and the acquisition time in each case was typically 8 s.

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Yang et al.

Figure 3. SERS spectra of the N-acetylalanine monolayers covered with Milli-Q water (a) and immersed in 10 mM Ce3+ solution and (b) and 10 mM K+ solution (c).

X-ray photoelectron spectroscopy analysis was performed on a PHI 5000 VersaProbe instrument (JEOL Ltd.) with Al as the exciting source. The samples were prepared by immerging clean Ag microsubstrates into the solutions for 3 h, and then allowing them to dry in air. Results and Discussion SERS Spectra for Self-Assembly of N-Acetylalanine. To form the monolayers, a roughened silver electrode was immersed in 10 mM N-acetylalanine solution. Time-dependent SERS spectra were recorded to monitor the self-assembled process and shown in Figure 1. To facilitate interpretation, the SERS bands of N-acetylalanine along with their assignments performed with the help of the theoretical calculation using DFT at the BLYP-6311G level30 are summarized in Table 1. It should be mentioned that the results of quantum chemical calculation at the BLYP-6311G level match the experimental data.31-38 At the very beginning, in the spectrum of Figure 1a, the strongest peak at 1005 cm-1 can be ascribed to NH wagging. The peaks with medium intensity at 1391, 1236, and 385 cm-1 are all related to the vibrations of the CO2- group. On the basis of the SERS mechanism and the surface selection rules, it is well-known that the vibrational modes of groups that attach to or are very close to the surface should be more enhanced in the SERS spectrum and the vibrational modes with parallel polarizability components with respect to the surface will not be enhanced. Therefore, these SERS features hint, at the initial stage of adsorption, that N-acetylalanine molecules might attach to the silver surface through the NH group together with one oxygen atom in the CO2- group. In Figure 1b acquired after self-assembling for 30 min, comparing with the Figure 1a, one notices that the intensity ratio of the peak at 1390 cm-1 to the peak at 1005 cm-1 is

enhanced from about 0.5 to 1 and the peak at 1390 cm-1 also becomes a prominent one. The SERS band at 385 cm-1 due to the CO2- group in Figure 1a separates into two bands at 382 and 391 cm-1. Simultaneously, an asymmetric stretching of the CO2- group is an obvious observation at 1589 cm-1. A novel band at 1560 cm-1 from cocontributions of the NH wagging and the CN stretching appears. All the above observations indicate that the adsorption fashions of CO2- and NH groups experience a reorganization process with time. After 3 h, the obtained SERS spectra of N-acetylalanine molecules at the silver surface remain stationary, meaning the adsorption becomes stable. In Figure 1c as an example, the intense peak at 1005 cm-1 of the NH wagging is still pronounced and the CO2- symmetric stretching at 1391 cm-1 becomes the strongest band. The CO2- deforming vibrations at 382 and 391 cm-1 are clearly visible. This varying of SERS bands suggests the final adsorption mode of N-acetylalanine molecules to construct monolayers via the NH group and the entire CO2group in light of Raman Selective Rules for surface. A scheme of the self-assembled procedure is depicted in Figure 2. The Reaction between N-Acetylalanine and Metallic Ions. (1) SERS Spectra. As deduced above, the formation of stable N-acetylalanine monolayers at the silver surface adopts coadsorption of CO2- and C-NH groups, leading the CdO group being parallel to and slightly away from the surface. Cerium ion (Ce3+) is introduced to investigate its effect on the monolayers, and is expected to react easily with carbonyl and amino in the N-acetylalanine molecule. Spectra a-c in Figure 3 display the SERS spectra of N-acetylalanine monolayers covered with Milli-Q water, 10 mM Ce(NO3)3 solution, and 10 mM KNO3 solution as a reference, respectively. It is found from Figure 3b that Ce3+ obviously impacts on the SERS spectral profile of N-acetylalanine monolayers. In the case of

N-Acetylalanine Monolayers at the Silver Surface

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Figure 4. XPS spectra of the Ag microsubstrate coated with 10 mM N-acetylalanine solution (A) and after immersing in 10 mM Ce3+ solution (B). The insets were from 875 to 915 eV.

Figure 5. SERS spectra of the N-acetylalanine monolayers capped by Ce3+ at the silver surface (a) and after treatment with 10 mM EDTA solution (b).

K+, the gained SERS spectrum (Figure 3c) shows litter change compared to Figure 3a. This means that the monolayers of N-acetylalanine at the silver surface are not influenced by K+ or the NO3- group, 40 which is confirmed by the corresponding XPS data. In Figure 3b, the novel peaks presenting at 932 and 1442 cm-1 are attributed to the symmetrical stretching and in-plane deforming of the free CdO group next to N(H). Two peaks appearing at 1196 and 1418 cm-1 are correlated with CsN deforming and stretching of CH3 connecting to the CdO group. Additionally, Ce3+ affects the 1312 cm-1 band of CsN vibration, showing a blue shift of 5 cm-1 to 1317 cm-1. The peaks relative to stretching of CN and wagging of NH at 1273

and 1558 cm-1 become broader with increases of their intensities. The aforementioned observations of spectral alternation indicate that the Ce3+ might interact with the CdO group and the N atom, leading to the CdO group accessing toward the silver surface and the related vibrations being enhanced by the surface to be seen. Meanwhile, the SERS bands from CO2vibrations at 1388 and 1587 cm-1 are unchangeable. The above spectral investigations show that the binding of the Ce3+ ion onto the monolayers induced the reorganization of the adsorption mode of N-acetylalanine at the silver surface. (2) XPS Data. X-ray photoelectron spectroscopy (XPS) is a nondestructive surface analytical technique that identifies the chemical elements in the top few atomic layers of a surface. It

15026 J. Phys. Chem. C, Vol. 112, No. 38, 2008 can provide information about elemental composition, chemical state and electronic state of the elements, surface adsorption species, binding energy, and bond binding.41-47 Herein, XPS was also employed to further observe the interaction between the Ce3+ and N-acetylalanine monolayers. The results of XPS are shown in Figure 4. The peaks observed obviously at 368.4 and 374.4 eV are attributed to Ag 3d3 and Ag 3d5, which result from the substrates of the polycrystalline Ag. The peaks of C 1s, O 1s, and N 1s that build up N-acetylalanine monolayers are located at 285, 531, and 400 eV, respectively. After careful comparison of panels A and B in Figure 4, some new peaks appearing at 886 and 905 eV are determined to be due to Ce 3d5 and Ce 3d3. The weak intensity of signals is in agreement with the fact that quite a small number of cerium atoms are capped by N-acetylalanine monolayers. XPS experimental results confirm well the interpretation for SERS observation of interaction between Ce3+ and N-acetylalanine monolayers. Recover of the Destruct Monolayers by EDTA. The ethylene diamine tetraacetic acid (EDTA) molecule contains six active points involving two amino groups together with four carboxyl groups, which can chelate with most metallic ions but alkali metals. Due to the forming constant of EDTA with Ce3+ of about 15.98 at room temperature and the product being in the soluble steady-going complex, it is supposed that the N-acetylalanine monolayers capped by Ce3+ were soaked in EDTA solution and Ce3+ should be removed from the surface. Figure 5 shows the SERS spectrum recorded from the silver surface with the N-acetylalanine monolayers attached Ce3+ after it is treated with 10 mM EDTA solution, along with the spectrum of the untreated one as a comparison. It could be seen in Figure 5a that the peak at 1316 cm-1 returns to 1311 cm-1 as found in the SERS spectrum of Figure 3a obtained from the N-acetylalanine monolayers/Ag. After EDTA treatment, some peaks at 932, 1196, 1416, and 1442 cm-1 rising from Ce3+ bound to N-acetylalanine monolayers disappear in Figure 5b. In addition, the profile of other peaks such as 1273 and 1558 cm-1 are refreshed. The SERS results indicate that, amazingly, after removing Ce3+ from N-acetylalanine monolayers with the help of EDTA, the altered monolayers should be recovered. A reversible process impact of Ce3+ on N-acetylalanine monolayers is also supported by XPS experiment. Conclusion In this paper, time-dependent SERS spectra imply that the N-acetylalanine molecules finally self-assemble at the silver surface through the amino group and the entire carboxyl after a reorganizing process. With the aid of EDTA, a reversible process of influence of Ce3+ on the N-acetylalanine monolayers at the silver surface is observed and confirmed by SERS spectroscopy and XPS measurement. Acknowledgment. This work was supported by the Natural Science Foundation in Shanghai (Grants No. 06JC14094), the Foundation of Shanghai Higher Education (Grants No. 06ZZ18), the Opening Foundation of State Key Laboratory for Chemo/ Biosensing and Chemometrics (Grants No. 2006012), the Foundation of Shanghai Normal University (Grants No. DYL200703), and the National Basic Research Program of China (No. 2008CB617504). We also express our thanks to Dr. Rong Wang for his help with the XPS measurement. Supporting Information Available: XPS graphs of Nacetylalanine monolayers (i) after immersing in 10 mM K+

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