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Adsorption Configuration for Cysteine on Ge(100): Coverage-Dependent Surface Reorientation Sunmin Park, Sena Yang, Nari Shin, Eunbyoul Lee, and Hangil Lee* Department of Chemistry, Sookmyung Women’s UniVersity, Seoul 140-742, Republic of Korea ReceiVed: May 3, 2010; ReVised Manuscript ReceiVed: July 28, 2010
The adsorption structure and surface reorientation of cysteine molecules on a Ge(100) surface were studied using high-resolution core-level photoemission spectroscopy and low-energy electron diffraction (LEED) to track the variation in adsorption structure as a function of cysteine coverage. Analysis of the S 2p, C 1s, N 1s, and O 1s core-level spectra revealed quite different adsorption behaviors as a function of cysteine coverage. At 0.4 ML (below half a monolayer), a single S 2p peak and a single N 1s peak were observed, consistent with an adsorption structure that contained one type of thiol conformation and a neutrally charged amino moiety. At 0.60 ML (over half a monolayer), two S 2p peaks emerged with a binding energy difference of 0.91 eV, indicative of two types of thiols, and two N 1s peaks were observed, which were consistent with the presence of both neutrally charged NH2 and positively charged NH2+ moieties. The relative populations of the two thiols induced a structural change in the ordering, from a 2 × 1 to a 1 × 1 of reorientation of the cysteine molecule adsorbed on the Ge(100) surface. At a higher coverage (over 1.0 ML), the LEED pattern became diffuse, and a configurational change of the molecule resulted from protonation of the NH2 group. We systematically elucidated the evolution of cysteine adsorption structures on Ge(100) surfaces as a function of coverage density. I. Introduction Studies of the amino acid adsorption structures on solid surfaces are of great importance for industrial applications, such as the development of biosensors; these studies are also important because an understanding of the adsorption properties of molecules with multifunctional groups and the behavior of the building blocks of biological molecules hold inherent academic value.1-6 Each amino acid consists of a carboxyl group (-COOH), an amino group (-NH2), a hydrogen atom (-H), and a side chain (the R group) attached to the R-carbon. A variety of functional groups may be used to form the R groups in amino acids. The variety of intra- or intermolecular forces between the functional groups and surface features produces extensive variability in the adsorption structures of amino acids on solid surfaces.7,8 The functionalization of biomolecules is a fascinating field. Elucidating the molecular level behavior of biomolecules at an interface with a solid surface is desirable because such interfacial phenomena lie at the heart of biological interactions and, hence, have implications for industrial applications in nanobiotechnology and biomedical sciences.1,4,9-11 The increasing interest in biomolecules has driven studies of functional molecules (e.g., heteromolecules, including sulfur, nitrogen, and oxygen) on surfaces as a strategy for biochip or sensor design. Polypeptides and proteins have potential for such applications because they are composed of amino acids that can each form chemical bonds with a solid surface.12,13 Here, we focus on cysteine adsorbed onto a Ge(100) surface. The lower reactivity of Ge(100) surfaces, relative to Si(100) surfaces, reduces the number (variability) of cysteine adsorption structures on the surface. Two of the common amino acids contain a sulfur atom: cysteine and methionine. Because the sulfhydryl group in cysteine is * To whom correspondence should be addressed. E-mail: easyscan@ sookmyung.ac.kr. Tel: +82-2-710-9409. Fax: +82-2-2077-7321.
Figure 1. Structure of the cysteine molecule. Each yellow, gray, blue, and red colored ball indicates sulfur (S), carbon (C), nitrogen (N), and oxygen (O), respectively.
highly reactive, cysteine is expected to dissociate and form a thiol. As a result, cysteine can play a key role in protein structures through the formation of disulfide bonds.14 In the present study, we have elucidated the precise adsorption structures of cysteine molecules on Ge(100) surfaces as a function of coverage. In particular, we scrutinized the behavior of sulfur in cysteine using high-resolution core-level photoemission spectroscopy (HRCLPES) and low-energy electron diffraction (LEED). To our knowledge, no systematic experimental study has been performed to examine the adsorption structures of cysteine on Ge(100). II. Experimental Section The Ge(100) surface (p-type, R ) 0.10-0.39 Ω) was cleaned by several cycles of sputtering with 1 keV Ar+ ions for 20 min at 700 K, followed by annealing at 900 K for 10 min. The cleanliness of the Ge(100) 2 × 1 surface was checked by lowenergy electron diffraction (LEED). L-cysteine (L-HSCH2CH(NH2)COOH, 99% purity; see Figure 1) was purchased from Aldrich and further purified through several sublimation and pumping cycles to remove all dissolved gases prior to exposure to the Ge(100) surface. To obtain the appropriate vapor pressure
10.1021/jp104029v 2010 American Chemical Society Published on Web 08/10/2010
Adsorption Configuration for Cysteine on Ge(100)
J. Phys. Chem. C, Vol. 114, No. 34, 2010 14529
Figure 2. LEED patterns obtained at 120 eV for (a) clean Ge(100) 2 × 1, (b) 0.40 ML cysteine on Ge(100) with a 2 × 1 reorientation, and (c) 0.60 ML cysteine on Ge(100) with a 1 × 1 reorientation.
for dosing, the dosing line was heated at 320 K during cysteine deposition. Moreover, we confirm its condition using temperature-programmed desorption (TPD) before we perform experiments of HRCLPES to clarify deposition of cysteine molecules without bond breaking, and then we deposited cysteine molecules every 30 s to confirm a constant deposition rate. We defined 1.0 ML (monolayer) cysteine coverage as the maximum density of cysteine that can be deposited on the Ge(100) surface without yielding the S 2p HRCLPES peak characteristic of multilayer formation. HRCLPES spectra were measured at the 7B1 beamline of the Pohang Accelerator Laboratory. The S 2p, C 1s, N 1s, and O 1s core-level spectra were obtained using a PHOIBOS 150 electron energy analyzer equipped with a two-dimensional charge-coupled device (2D CCD) detector (Specs, GmbH) using photon energies of 230, 340, 460, and 588 eV to enhance the surface sensitivity. Four binding energies in the core-level spectra were calibrated with respect to the binding energy of the Au 4f core-level spectrum (84.0 eV) for the same photon energy. The base pressure of the chamber was maintained below 9.5 × 10-11 Torr. All spectra were recorded in the normal emission mode. The photoemission spectra were carefully analyzed using a standard nonlinear least-squares fitting procedure with Voigt functions.15 III. Results and Discussion We will first discuss the change of orientation of a cysteine molecule adsorbed on the Ge(100) surface, as a function of cysteine coverage, using LEED. After confirming that the clean Ge(100) substrate gave rise to a sharp 2 × 1 pattern (Figure 2a), we deposited 0.40 ML cysteine on the Ge(100) surface and obtained the LEED image shown in Figure 2b. At this coverage, we observed a faint 2 × 1 LEED pattern, indicating that the Ge(100) surface was not fully covered with cysteine molecules. The observation of a 2 × 1 pattern, albeit with broader spots, indicated that at a coverage of 0.40 ML, the cysteine molecules did not induce the change of orientation of the cysteine molecule adsorbed on the Ge(100) surface. Figure 2c shows the LEED pattern obtained after deposition of 0.60 ML cysteine (over 0.5 ML). Interestingly, this image suggested a 1 × 1 reorientation. These findings indicated a change in the adsorption and electronic structures upon increasing coverage from 0.40 to 0.60 ML cysteine. For cysteine coverages g 1.00 ML (saturation coverage), only a blurred 1 × 1 image was observed, indicative of a multilayered or disordered structure (not shown here). This structure will be discussed in the context of the photoemission data. The gradual change of the LEED pattern, from 2 × 1 to 1 × 1 with increasing cysteine coverage, may have been related to a change in the relative populations of the two types of thiols. This hypothesis will be discussed below, after presenting the results of the HRCLPES experiments. HRCLPES experiments were performed to elucidate the origin of variation in adsorption
Figure 3. C 1s (top panels) and S 2p (bottom panels) core-level spectra as a function of cysteine coverage. (a, d) 0.40 ML coverage, (b, e) 0.60 ML coverage, and (c, f) 1.20 ML cysteine adsorbed on Ge(100) at 300 K. The filled circles are experimental values, and the solid lines represent the results of peak fitting.
(or electronic) structure and to correlate the adsorption structure with the bonding state of the sulfur atom in cysteine. We first acquired the C 1s (top panels) (Figure 3) and S 2p (bottom panels) core-level spectra at three surface coverages (0.40, 0.60, and 1.20 ML) after checking the cleanliness of the substrate. Figure 3a shows the C 1s core-level spectrum obtained after deposition of 0.40 ML cysteine on the Ge(100) surface at 300 K. This spectrum shows four distinct bonding features. First, considering the electronegativity and peak intensity, we assigned three of the bonding features, designated as C1, C2, and C3, to -CH2-SH (284.4 eV, C1), -C-NH2 (285.6 eV, C2), and -COO- (288.4 eV, C3) because they involved bonding to S, N, and O, respectively. However, the fourth bonding feature in the spectrum (283.7 eV, designated C4) had no corresponding bond character and was assigned to a dissociated hydrocarbon species on the surface on the basis of the binding energy.14 Interestingly, the evolution of the C4 species was well matched with the bonding feature of the atomic S-like peak at the initial stages of adsorption. Thus, the C4 bonding feature could be attributed to the partial decomposition of the cysteine into an S atom and its residue by S-C bond cleavage. Evidence supporting this peak assignment will be presented below in the analysis of the S 2p core-level spectra. Figure 3d shows the S 2p core-level spectrum obtained after deposition of 0.40 ML cysteine on the Ge(100) surface. In this spectrum, an S 2p doublet was observed with a spin-orbit splitting of 1.18 eV, a branching ratio of 2:1, and a full width at half-maximum (fwhm) of 0.60 eV. At this coverage (0.40 ML), a single S peak was observed at the S 2p3/2 binding energy of 161.9 eV (S1). This binding energy for the S1 bonding feature was consistent with a thiol (SH) formed by S-H bond scission.16 Hence, we speculated that the cysteine molecules had adsorbed onto the Ge(100) surface via a S-Ge bonding configuration.
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Figure 4. N 1s (top panels) and O 1s (bottom panels) core-level spectra as a function of cysteine coverage. (a, d) 0.40 ML, (b, e) 0.60 ML, and (c, f) 1.20 ML cysteine adsorbed on Ge(100) at 300 K. The filled circles are experimental values, and the solid lines represent the results of peak fitting.
Upon increasing the cysteine molecule coverage to 0.60 ML (Figure 3b,e), we observed features that differed from those described above for 0.40 ML cysteine on Ge(100). As shown in Figure 3b, a new peak emerged (marked as C5) at 286.5 eV. The binding energy suggested two possibilities for the C5 bonding feature: a new C-N related feature or a new C-S related feature. Concurrently, we obtained the S 2p core-level spectrum at a cysteine coverage of 0.60 ML (Figure 3e). This spectrum contained an additional S 2p peak (marked S2) at 162.9 eV that was not present in the 0.40 ML spectrum. Considering the binding energy of S2, we can scrutinize that the S2 peak also is another type of thiol. Moreover, we could clearly exclude the possibility that the C5 bonding feature was related to the S2 bonding feature because the latter feature fell in the range of a thiol (SH) formed by S-H bond scission. Hence, we considered the C5 bonding feature as having arisen from the bond between C and N, as will be discussed below in relation to the N 1s core-level spectra. Next, we increased the cysteine coverage over saturation coverage (1.20 ML) to confirm the multilayer effect. Figure 3c,f shows the C 1s and S 2p core-level spectra obtained at this coverage. Compared to the 0.60 ML spectrum, the only differences were that the intensities of the C1, C2, C3, and C5 bonding features increased slightly, whereas the intensity of C4 decreased slightly. Interestingly, the S 2p spectrum exhibited an additional broad (fwhm ) 1.1 eV) peak at 163.9 eV, designated as S3. First, because no changes in the C 1s peaks were observed, we inferred that the S3 bonding feature was not directly related to carbon bonding. Hence, we considered two possible sources of the S3 feature: (1) it arose from the multilayer structure, and (2) it was derived from bond formation between the Ge(100) surface and sulfur. Given that the binding energy of S3 was 163.9 eV, we excluded the possibility of Ge-S
Park et al. (sulfide) bonding. Thus, we concluded that the S3 bonding feature originated from a multilayer effect. The variations in the C 1s and S 2p peaks as a function of cysteine coverage led us to conclude that the peaks S1 and S2 were due to two types of thiols formed by S-H bond scission, whereas the S3 peak arose from a second layer of cysteine (163.4-164.3 eV), which usually retained the S-H bond.16 By a similar process, we obtained the N 1s core-level spectra as a function of cysteine coverage on Ge(100) at 300 K after confirming that the Ge(100) surface was clean. Figure 4a shows the N 1s core-level spectrum obtained after deposition of 0.40 ML cysteine on Ge(100). A single N peak was observed at 398.7 eV (marked as N1). This binding energy indicated that the nitrogen in the amine group adsorbed on the Ge(100) surface was in a neutrally charged state.17 At a cysteine coverage of 0.60 ML (Figure 4b), an additional distinct N 1s peak was observed at 400.1 eV (marked as N2), indicative of a charged nitrogen species. Interestingly, in the C 1s core-level spectrum at 0.60 ML coverage (Figure 3b), we observed an additional peak (C5) and determined that this feature was a C-N related, rather than a C-S related, bonding feature on the basis of the binding energy. As a result, we concluded that the C5 feature was correlated with the N2 feature. Upon increasing the cysteine coverage to 1.20 ML (Figure 4c), we observed an enhancement of the N2 peak. Given that the intensity of the C5 feature also increased upon increasing coverage from 0.60 to 1.20 ML (see Figure 3c), we inferred that the C5 and N2 features were related. Moreover, the spectra indicated that the N2 bonding feature became dominant at high coverages, whereas the N1 intensity remained constant or converted partially into N2. Considering the binding energies, the peak positions for the N1 and N2 bonding features corresponded well to the NH2 and NH2+ configurations, respectively.16,18 Moreover, we can speculate that N2 can be related to the mixture of -NH2+ and NH3+ (multilayer) configurations in Figure 4c. Thus, the spectral data indicated that the amine group in cysteine on the Ge(100) surface underwent a configurational change upon protonation only at high coverage, similar to the behavior of cysteine on a Cu(110) substrate.14 Finally, we examined the changes in the O 1s core-level spectra recorded for samples with varying cysteine coverage at 300 K. On the basis of the electronegativity, we divided the spectral features into two bonding states: feature O1 (531.1 eV) and feature O2 (532.2 eV), as observed in the -C-OH and -CdO bonding features, respectively, of glycine, alanine, and valine.19-21 A comparison of the spectra recorded at coverages of 0.40, 0.60, and 1.20 ML (Figure 4d-f) disclosed a negligible dependence on cysteine coverage, except for a small chemical shift due to the incremental increase in cysteine coverage. Therefore, we concluded that the two types of oxygen in cysteine did not participate. In summary, we found that, at low coverage, the cysteine (HSCH2CH(NH2)COOH) adsorbed via catalytic C-S bond scission with hydrogen transfer to the CH2 carbon to form atomic S and CH3CH(NH2)COO-. As the cysteine coverage increased
TABLE 1: Change in Binding Energy for Four Core-Level Spectra of Cysteine Adsorbed on Ge(100) at 300 K as a Function of Coverage C 1s (eV)
S 2p
N 1s
O 1s (eV)
coverage (ML)
C1/C2/C3/C4/C5
S1/S2/S3
N1/N2
O1/O2
0.40 0.60 1.20
284.3/285.6/288.4/283.7/(N/A) 284.4/285.6/288.5/283.7/286.5 284.6/285.7/288.6/283.8/286.5
161.9/(N/A)/(N/A) 162.0/162.9/(N/A) 162.1/163.0/163.9
398.7/(N/A) 398.8/400.1 398.8/400.2
531.1/532.2 531.1/532.2 531.2/532.3
Adsorption Configuration for Cysteine on Ge(100) to 0.40 ML, we observed that the next cysteine molecule adsorbed via heterolytic S-H and O-H bond scission by surface electrons to form -SCH3CH(NH2)COOH on the substrate. At 0.60 ML, the adsorption produced -SCH3CH(NH2+)COOH- with increasing amino group protonation. Finally, at 1.20 ML, the cysteine bound to form a second layer through formation of HSCH2CH(NH3+)COOH, which was indicated by the appearance of the S3 peak shown in Figure 3. The variations in binding energy as a function of coverage on the Ge(100) surface at 300 K, obtained from the core-level spectra, are summarized in Table 1. IV. Conclusions We used HRCLPES and LEED to investigate the bonding states of cysteine adsorbed onto Ge(100) surfaces at 300 K. Our results indicated that both the amine and the carboxyl groups concurrently participated in cysteine adsorption on Ge(100) surfaces. First, we observed sequential changes in the atomic core level and LEED patterns as a function of cysteine coverage on the Ge(100) surfaces. In particular, cysteine adsorption induced a change in the surface reorientation, from 2 × 1 to 1 × 1, which was influenced by two distinct thiol species (S1 and S2). The findings suggested that the molecular ordering for the 1 × 1 structure was induced by two different S-bonding sites. On the basis of the HRCLPES analysis, we speculated that the observed behavior was derived from the configurational changes in amino groups that arose from the protonation of NH2 to form NH2+. At a higher coverage (1.20 ML), an additional higher binding peak appeared in the S 2p spectrum, indicating that a second cysteine layer formed on top of the first layer at this coverage.
J. Phys. Chem. C, Vol. 114, No. 34, 2010 14531 Acknowledgment. This work was supported by the Sookmyung Women’s University Research Grants 2009. References and Notes (1) Filler, M. A.; Bent, S. F. Prog. Surf. Sci. 2003, 73, 1. (2) Whaley, S. R.; English, D. S.; Hu, E. L.; Barbara, P. F.; Belcher, A. M. Nature 2000, 405, 665–668. (3) Goede, K.; Busch, P.; Grundmann, M. Nano Lett. 2004, 4, 2115. (4) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1. (5) Chen, Q.; Richardson, N. V. Nat. Mater. 2003, 2, 324. (6) Otero, R.; Scho¨ck, M.; Molina, L. M.; Laegsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Angew. Chem., Int. Ed. 2004, 44, 2. (7) Wolkow, R. A. Annu. ReV. Phys. Chem. 1999, 50, 413. (8) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell, J. N. Acc. Chem. Res. 2000, 33, 617. (9) Sarikaya, M.; Tamerler, C.; Jen, A. K. Y.; Schulten, K.; Baneyx, F. Nat. Mater. 2003, 2, 577. (10) Preuss, M.; Schmidt, W. G.; Bechstedt, F. Phys. ReV. Lett. 2005, 94, 236102. (11) Ghiringhelli, L. M.; Schravendijk, P.; Sitted, L. D. Phys. ReV. B 2006, 74, 035437. (12) Fink, A.; Menzel, D.; Widdra, W. J. Phys. Chem. B 2001, 105, 3828. (13) Jeon, S. M.; Jung, S. J.; Lim, D. K.; Kim, H.-D.; Lee, H.; Kim, S. J. Am. Chem. Soc. 2006, 128, 6296. (14) Kim, J. W.; Hwang, H.-N.; Hwang, C.-C. J. Phys. Chem. C 2007, 111, 13192. (15) Schreier, F. J. Quant. Spectrosc. Radiat. Transfer 1992, 48, 743. (16) Gonella, G.; Terreni, S.; Cvetko, D.; Cossaro, A.; Mattera, L.; Cavalleri, O.; Rolandi, R.; Morgante, A.; Floreano, L.; Canepa, M. J. Phys. Chem. B 2005, 109, 18003. (17) Chen, X. H.; Ranke, W. Surf. Sci. 1992, 262, 294. (18) Cavalleri, O.; Gonella, G.; Terreni, S.; Vignolo, M.; Pelori, P.; Floreano, L.; Morgante, A.; Anepa, M.; Rolandi, R. J. Phys.: Condens. Matter 2004, 16, S2477. (19) Youn, Y.-S.; Jung, S.-J.; Lee, H.; Kim, S. Langmuir 2009, 25, 7438. (20) Lee, H.; Youn, Y.-S.; Kim, S. Langmuir 2009, 25, 12574. (21) Youn, Y.-S.; Lee, H.; Kim, S. Chem. Phys. Chem. 2010, 11, 354.
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