13192
J. Phys. Chem. C 2007, 111, 13192-13196
Adsorption of Cysteine on Cu(110) Studied by Core-Level Photoelectron Spectroscopy Jeong Won Kim,*,† Han-Na Hwang,‡ and Chan-Cuk Hwang‡ Korea Research Institute of Standards and Science, 1 Doryong-dong, Daejon 305-340, Korea, and Beamline Research DiVision, Pohang Accelerator Laboratory, Pohang UniVersity of Science and Technology, Pohang, Kyungbuk 790-784, Korea ReceiVed: March 30, 2007; In Final Form: July 3, 2007
Adsorption and ordering of cysteine molecules on Cu(110) surface have been studied by photoelectron spectroscopy using synchrotron radiation and low-energy electron diffraction (LEED). Analysis of S 2p, N 1s, O 1s, and C 1s core levels as a function of the cysteine coverage on Cu(110) reveals quite different behavior from the previously studied case on a Au(110) surface. At low coverages there evolve sequentially two types of thiolates with the S 2p binding energy difference of 0.85 eV. The relative population of the two thiolates induces a structural change in the ordering from 2 × 1 to c(2 × 2). Only above half the saturation coverage the LEED pattern becomes diffuse and a configurational change of the molecule by protonation of the functional groups occurs. Such an unusual evolution of the cysteine adsorption on the Cu(110) surface is discussed in relation to chemical reactions and possible molecular rearrangement on the surface.
1. Introduction Functionalization of biomolecules is a growing field of fusion technologies emerging recently. The manipulation of the interfaces between biological and inorganic systems at the molecular level is a desirable goal for the fundamental understanding of the interfacial phenomena through biological interactions and for the potential applications to nanobiotechnology and biomedical sciences. Self-assembly on surfaces, for example, draws great attention both in the field of basic surface chemistry and in the application to the surface modification industry.1 The immobilization of functional molecules such as thiols on a surface and subsequent chemical reactions on those became a popular step to build biochips, sensors, lubricants, etc. Protein can also have such a potential application, comprising various amino acids which play an important role in the formation of chemical bonds with inorganic templates. Among common amino acids, a sulfur atom is present in two amino acids; cysteine and methionine. Especially, the sulfhydryl group in the cysteine is highly reactive and plays a key role in protein shaping by disulfide bonds. On a noble metal such as a Au(110) surface, the cysteine molecules make a short-range ordering which is believed as a consequence of homochiral dimer interaction2 and unidirectional molecular growth.3,4 On a transition metal surface like Cu(110), however, cysteine makes a long-range ordered pattern,5 which might hint at a different interaction on the molecule/metal interface. Furthermore, simple alkanethiols decompose easily to thiolates on the Cu(110) surface by S-H bond scission even at low temperatures,6 whereas those are usually intact on a Au(111) surface.7 Even though the adsorption of cysteine on Cu surfaces from aqueous solution and gas-phase evaporation has been studied by X-ray photoelectron spectroscopy (XPS) before, poor resolution did not only hamper detailed analysis but there has been no study on a coverage dependence in the submonolayer level either.8,9 * Corresponding author. E-mail:
[email protected]. † Korea Research Institute of Standards and Science. ‡ Pohang University of Science and Technology.
Here we report on a detailed study of the initial stage of cysteine adsorption on Cu(110) by high-resolution XPS using a synchrotron radiation light source and compare it with recent results on Au(110) surfaces.10,11 Since each atomic core level in cysteine shows characteristic chemical shifts, the changes in chemical configuration upon cysteine adsorption on Cu(110) can be inferred at various coverages. At low coverages, there occurs the deprotonation of thiol and carboxylic groups by the electron transfer from the substrate Cu atoms. The adsorptioninduced orderings of cysteine on the Cu(110) surface have been observed by low-energy electron diffraction (LEED) with combination of XPS. The relative population change of two types of thiolates with the coverage brings out a series of ordered phases of 2 × 1 and c(2 × 2). At high coverages, as the deprotonation decreases, the cysteine begins to form a second layer above the first layer. 2. Experimental Section A Cu(110) crystal was cleaned by repeated cycles of mild Ar+ sputtering (700 eV) and annealing to 400 °C. The cleanliness and surface ordering of the initial substrate were confirmed repeatedly by the absence of impurities such as S and C in the XPS spectrum and a sharp 1 × 1 LEED pattern. L-Cysteine (Aldrich, >97% purity) without further purification was evaporated from a water-cooled Knudsen cell at ∼80 °C in ultrahigh vacuum onto the clean Cu(110) surface at room temperature. All measurements were performed at beamline 7B1 at PAL (Pohang Acceleration Laboratory) in Pohang. The photoelectron signals were recorded with a PHOIBOS 150 electron energy analyzer equipped with a two-dimensional charge-coupled device (2D CCD) detector (Specs GmbH), collecting the photoelectrons from the surface normal. The S 2p, N 1s, O 1s, and C 1s core levels from the adsorbed cysteine molecule were recorded at various photon energies from 270 to 600 eV which provide surface-sensitive information for each core level. The total energy resolution ranged from 0.4 to 0.7 eV depending upon the used photon energies. During the scan, we have observed no spectral change which was caused by
10.1021/jp072496d CCC: $37.00 © 2007 American Chemical Society Published on Web 08/14/2007
Adsorption of Cysteine on Cu(110)
Figure 1. S 2p core-level spectra and peak analysis upon the cysteine adsorption on the Cu(110) surface.
X-ray beam irradiation elsewhere.10-12 The photon energies were calibrated each time by measuring signals separately from a clean Au film with the Au 4f7/2 reference binding energy of 83.97 eV. 3. Results and Discussion 3.1. S 2p Core-Level Spectra. Figure 1 shows a series of S 2p core-level spectra upon increasing the cysteine coverage on a Cu(110) surface. The coverage (θ) here means a relative value with respect to a saturated amount at room temperature. A few selected spectra are fitted using Voigt functions after Shirleytype background subtraction. Each S 2p doublet has a spinorbit splitting of 1.18 eV with a branching ratio of 2:1. The full widths at half-maximum (fwhm) of the S 2p doublets are varied in the range of 0.6 to 1.0 depending upon the different chemical species on the surface. At a very early stage (θ < 0.12) of cysteine deposition, a single peak is observed at the S 2p3/2 binding energy of 161.31 eV (S1). When the cysteine coverage is increased, the peak is shifted slightly to 161.48 eV and another peak emerges subsequently at 162.33 ( 0.1 eV (S2). At θ ) 0.48, the intensities of the S1 and S2 components with the binding energy difference of 0.85 eV become almost equivalent. As the coverage increases further, the relative intensity of the S2 peak increases and finally another broad (fwhm ∼ 1.0 eV) peak S3 at 164.34 ( 0.1 eV clearly appears at the saturation coverage. To trace the above changes in the S 2p with the cysteine adsorption, we compare the present peak positions with previously observed values for relevant systems as listed in Table 1. The binding energies for the two peaks (S1 and S2) are both in the range for the thiolates formed by S-H bond scission. The coexistence of the two different thiolates on the Cu(110) surface
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13193 is in sharp contrast to the case on the Au(110) surface where there is no other S 2p component but at 161.95 eV over the wide range of cysteine coverages.10 On the other hand, the peak position of the final species S3 is in good agreement with those for the second layer species (163.4-164.3 eV for cysteine in Table 1) which usually retains the S-H bond. To see any changes by heat treatment, a mild annealing of the cysteinesaturated surface at around 100 °C for a few minutes is performed. As shown on the top spectrum in Figure 1, the only S1 peak becomes dominant, while the S2 peak which is presumed to be the second thiolate feature disappears. The annealing procedure probably causes desorption of weakly bound second layer molecules and diffusion of remaining molecules to a stable site. In other words, all other components S2 and S3 are desorbed to vacuum by thermal energy or are converted to the S1 which is believed to be the most stable species on the Cu(110) at low coverages. One more interesting observation is that the initial peak position for the S1 at θ ) 0.12 is the same as the one for the S atom segregated from the bulk (bottom-most spectrum in Figure 1), which is commonly detected on a Cu surface when annealed at high temperatures. This means that the S1 species is very similar to an atomic S state on the Cu(110) surface in terms of chemical environment reflected by the same core-level binding energy. Within a reasonable speculation for this, we suspect a partial molecular decomposition of the cysteine at the very early stage of adsorption, which is to be discussed below again. 3.2. N 1s and O 1s Core-Level Spectra. In Figure 2, parts a and b, we have taken N 1s and O 1s spectra, respectively, as the cysteine coverage increases. At a low-coverage regime, the N 1s shows only a single peak at 399.6 ( 0.2 (N1), and the second peak at 401.7 ( 0.2 eV (N2) appears only above half the saturation coverage. Eventually the N2 becomes dominant at high coverages, while the N1 intensity becomes saturated or converted partially into the N2. This behavior is totally different from the case on the Au(110) surface where two peaks at 399.5 and 401.4 eV are resolved from the beginning.10 Referring to previous results in Table 1, the peak positions for the N1 and N2 are well fitted to NH2 and NH3+ configurations, respectively.9-11 Thus, the amino group undergoes a configurational change by protonation on the Cu(110) surface only at high coverages. However, as exhibited on the top spectrum of Figure 2, the mild annealing process takes those peaks back to a single feature of which position coincides with that of the N1 peak. The O 1s core levels in Figure 2b show almost a single feature at 531.5 ( 0.2 eV at low coverages, but an asymmetric tail on the higher binding energy side grows as the coverage increases. The fwhm of the O 1s peak gradually increases from 1.5 to 2.1 eV with the coverage, even though a clear peak separation is not resolved. This is also quite different from the Au(110) surface where they fitted the O 1s with three components from the beginning.10 From the results for adsorbed amino acids, the symmetric O 1s peak and its position at the initial stage correspond to those for COO- formation generated by the loss of the acidic hydrogen in the carboxylic group of the cysteine.9,13 On the other hand, the higher binding energy tail at high coverages is an indication that more than two different kinds of O atoms coexist on the Cu(110) surface. This takes place usually by the protonation of the carboxylate (COOH). However, a mild annealing eliminates the higher binding energy tail and makes almost a single feature at 531.1 eV which is adversely attributed to the COO- formation back again. 3.3. C 1s Core-Level Spectra. Figure 3 shows C 1s corelevel spectra measured under the same sequence as in Figures
13194 J. Phys. Chem. C, Vol. 111, No. 35, 2007
Kim et al.
TABLE 1: List of XPS Core-Level Binding Energies (eV) for Related Systems and Present Results S 2p3/2 cysteine/Cu(110) [ref 8] present results cysteine/Au(110) [ref 10] S-, NH2, COOH multilayer CH3SH/Cu(110) [ref 6] CH3SCH3SH multilayer cysteine/Pt(111) [ref 12] glycine/Cu(110) [ref 13] -H2NCH2COOmethionine/Cu(110) [ref 14]
N 1s
O 1s
162.0, 163.9 161.48, 162.33
285.1, 286.1, 288.2 285.1, 286.0, 288.1
C 1s
399.9 399.6, 401.4
531.6 531.5
161.95 164.3
284.8, 286.3, 288.2, 289.2
399.5 401.5 (NH3+)
531.2
162.4 164.4 165.2 162.2, 163.4
284.1 285.5 286.3 285.0, 286.6, 289.0
400.8, 402.3
532.3
286.23, 288.25
399.85
531.57
161.6, 163.7
1 and 2. The bottom spectrum at the cysteine coverage of θ ) 0.12 is fitted also using Voigt functions after Shirley-type background subtraction. There are four different C 1s peaks fitted at the binding energies of 283.9 (C1), 285.1 (C2), 286.0 (C3), and 288.1 eV (C4). From the cysteine molecule, one can observe three different types of C atomic core levels corre-
Figure 2. N 1s (a) and O 1s (b) core-level spectra upon cysteine adsorption on the Cu(110) surface.
Figure 3. C 1s core-level spectra taken upon the cysteine adsorption on the Cu(110) surface.
sponding to the carbons next to thiol and amino groups, and in the carboxylic group, respectively. On the basis of the electronegativity difference and charge transfer between those atoms in the cysteine, the binding energies for the three components C2-C4 are ascribed to C atoms bonded to S, N, and O atoms. Indeed, the peak positions for the C2-C4 components are in good agreement with the previous results for the cysteine adsorption as listed in Table 1. On the other hand, the C1 component at the lowest binding energy has no correspondent but dissociated hydrocarbons on the surface. The occurrence of the C1 species agrees with the above S 2p core-level result which shows the atomic S-like peak at the very initial stage of adsorption. This can happen through the partial decomposition of the cysteine into S atom and its residue by S-C bond cleavage, which leads to a proton transfer for the formation of -CH3CH(NH2)COO- on the surface. Indeed the Cu(110) surface seems to induce a rigorous reaction with sulfurcontaining molecules at a very early stage of adsorption, which was revealed before by the result of alkanethiols/Cu(110).6 One reason for the strong initial reaction of Cu versus Au surfaces may be the d10s1 electron configuration with a significantly lower ionization energy of the former (745.5 vs 890.1 kJ/mol), i.e., more free electrons at the surface. Since the C atoms themselves basically do not hold any functionality, a distinct spectral change or any new peak in the C 1s with coverage is not observed in Figure 3. However, substantial changes in the peak intensities and positions occur between θ ) 0.48 and 0.61. The intensities of the C2 and C4 peaks, which are remarkably small at low coverages, begin to increase. In addition, the C3 and C4 peaks are shifted to high binding energies parallel with a high binding energy shoulder next to the C4 peak. First, the peak shifts around half the saturation coverage can be understood by the relation to configurational changes of the functional groups as observed above on the N 1s and O 1s core levels. For instance, the protonation of the amino (NH2 f NH3) and carboxylic (COOf COOH) groups, respectively, adds the higher binding C 1s components next to the C3 and C4 peaks. Second, the major reason of the change in the relative intensity ratio for the three peaks C2-C4 is the influence of a photoelectron diffraction effect. Since a photon energy of 370 eV from synchrotron radiation is used, the multiple-scattering diffraction effect at low electron kinetic energies is strong. The molecular ordering and its specific orientation at low coverages drive the photoelectrons emitted from the surface to be influenced much more by the photoelectron diffraction effect. That is why the intensity ratio of the C2/C3/C4 is far from the ideal value of 1:1:1 at low coverages. However, at high coverages, the intensity ratio approaches the ideal value of 1:1:1, which means that additional molecules are adsorbed more or less in a disordered fashion
Adsorption of Cysteine on Cu(110)
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13195
Figure 4. LEED patterns for (a) clean Cu(110) surface with two arrows indicating a unit cell, (b) a faint 2 × 1 structure upon cysteine adsorption at θ < 0.25, and (c) a c(2 × 2) at θ ∼ 0.5. The circles in (b) and (c) designate the positions of the fractional spots for each reconstruction.
afterward. Upon mild annealing of the saturated molecular layer, a large portion of the high binding energy tails is reduced and the peaks are shifted almost back to the original positions, due to the elimination of the second layer component and the diffusion of the molecules, as shown on the top spectrum in Figure 3. However, as the top spectrum does not match any of the spectra below, the annealing procedure seems to produce a simple configuration but different from any of room-temperature adsorbed surfaces. 3.4. LEED Patterns. Figure 4 shows an evolution of LEED patterns observed upon cysteine adsorption. The initial surface, clean Cu(110), reveals a distinct 1 × 1 pattern in Figure 4a. At low coverages, a faint 2 × 1 LEED pattern with half-order fractional spots emerges in Figure 4b. Around a half-coverage of the cysteine saturation, the LEED exhibits a c(2 × 2) pattern with a little streak around the fractional spots in the [001] direction (Figure 4c). Those fractional LEED spots upon cysteine adsorption are rather weak and streaky all through the coverages. Further deposition makes the overall LEED pattern diffuse and eliminates the fractional spots, which hints at the disordered molecular adsorption at high coverages. The sharpness and brightness of the LEED patterns in Figure 4 do not show much change upon annealing the substrate but depend only on the cysteine coverage. This behavior is totally different from the chiral orderings shown by tartaric acid and alanine on Cu(110) surfaces,14,15 where various long-range molecular orderings are induced depending upon the substrate temperature. It is noticeable that the gradual change of the LEED pattern from 2 × 1 to c(2 × 2) with the cysteine coverage seems to be related to the relative population change of the two types of thiolates, as shown by the intensity change of the S1 and S2 peaks in Figure 1. Such two different types of thiolates from alkanethiols with a binding energy difference of 0.9 eV discriminated by annealing temperatures on Cu(111) were reported before.16 They suggested that the binding energy separation of the two different thiolates is due to two different S adsorption sites on the underlying Cu surface. Thus, it is expected that the S1 and S2 species possess different bonding sites on the Cu(110) surfaces as well, even
though the exact assignment of the each adsorption site is beyond the present study. 3.5. Chemical Reactions on the Surface. Distinct changes in the S 2p, N 1s, and O 1s core levels with cysteine coverage indicate a series of chemical changes occurring on the Cu(110) surface as listed in Table 2. The cysteine arrives from the gas phase in its molecular form, i.e., HSCH2CH(NH2)COOH. At very low coverages catalytic C-S bond scission occurs with simultaneous hydrogen transfer to the CH2 carbon, i.e., formation of atomic S and CH3CH(NH2)COO- and their adsorption. The next cysteine molecule gets adsorbed by heterolytic S-H and O-H bond scission by surface electrons as -SCH2CH(NH2)COO- or -SCH3CH(NH2)COOH to the substrate. That is, the electrons from the metal surface deprotonate both SH and OH groups at binding. Later on such deprotonation decreases, and sequential adsorption such as -SCH2CH(NH2)COOH, -SCH2CH(NH3)COO-, or -SCH2CH(NH3)COOH also occurs with rising probability of amino group protonation at θ > 0.5. Finally, at high coverages the cysteine binds at a second layer by the form of HSCH2CH(NH3)COOor HSCH2CH(NH3)COOH as revealed by the sudden appearance of the S3 peak in Figure 1. The identification of the possible chemical species on the Cu(110) surface and their peak occurrence with raising the cysteine coverage are summarized in Table 2. Of course, those chemical changes are accompanied by a relevant molecular rearrangement on the surface. Those pictures of cysteine reactions are very similar to what was predicted by Uvdal et al. who proposed different adsorption geometries on Au and Cu surfaces.9 Comparing their and our results, the difference between the Au and Cu substrates is that the adsorption of the cysteine on Cu(110) occurs by sequential reactions, whereas that on Au(110) occurs by a concerted type of mixed reactions from the early stage. This difference probably comes from the d10s1 electron configuration of the two metallic elements in that Cu, because of much lower ionization energy on its surface, has more available free electrons donated to the cysteine molecules for deprotonation at low coverages. 4. Conclusions We have observed sequential changes in atomic core levels and LEED patterns as a function of cysteine coverage on a Cu(110) surface. The evolution of core-level spectra shows quite different behavior from other noble metal surfaces. The Cu(110) surface provides the adsorbed cysteine sufficient electrons to deprotonate the thiol and carboxylic groups at low coverages. In particular, the cysteine adsorption brings out ordered phases of 2 × 1 and c(2 × 2) which are influenced by the relative population of the two different thiolate species. The molecular ordering for the c(2 × 2) structure seems to be induced by the equal population of the two different S bonding
TABLE 2: Series of Chemical Species to Be Formed on a Cu(110) Surface with Coveragea
a
Θ
species
0
