- Email: [email protected]
Copper(I) Multilayer Thin Films on Gold
Journal of Colloid and Interface Science 249, 301–306 (2002) doi:10.1006/jcis.2002.8265, available online at http://www.idealibrary.com on
Self-Assembly of L-Cysteine-Copper(II)/Copper(I) Multilayer Thin Films on Gold Wen-Wei Zhang, Chang-Sheng Lu, Yang Zou, Jing-Li Xie, Xiao-Ming Ren, Hui-Zhen Zhu, and Qing-Jin Meng1 Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, Nanjing University, No. 22 Hankou Road, Nanjing 210093, People’s Republic of China E-mail: [email protected] Received November 12, 2001; accepted January 24, 2002; published online April 10, 2002
Self-assembled multilayer thin films have been prepared on Au substrate by alternate surface derivatization with L-cysteine hydrochloride and cupric perchlorate. The layer-by-layer structure at each step of multilayer formation was investigated by X-ray photoelectron spectroscopy. The measurements indicate that there are two structure modes in the multilayers. One is that Cu2+ sandwiches between two amino acid groups. The other one is that Cu+ is bonded through disulfide and thiolate. This process is also confirmed by cyclic voltammetry of Cu ion at different self-assembled multilayers. Steps further on will lead to repeated multilayer films. C 2002 Elsevier Science (USA) Key Words: self-assembled multilayers; L-cysteine hydrochloride; coordination and reduction of Cu2+ ; XPS; cyclic voltammetry.
INTRODUCTION
EXPERIMENTAL
Self-assembled monolayers (SAMs) are molecular assemblies spontaneously formed by the immersion of an appropriate substrate into a solution of an active adsorbate, which provides an elegant way for forming well-defined organic assemblies with a wide range of surface functionalities (1). In the past two decades, SAMs have become an active research area for their fundamental importance in understanding interfacial properties as well as for their potential applications in molecular technologies. For examples, SAM films have been used to study important fundamental processes containing electron transfer (2), surface wetting (3), adhesion (4), catalysis (5), and molecular recognition (6). Such films have also been utilized to the construction of various interfaces for biological membranes (7), molecular electronics, nonlinear optics, optical switches, chemical sensors, and high-density memory devices (8). Extension of this approach to multilayers might enhance some properties, since new classes of materials possessing functional groups at controlled sites can be designed and prepared with molecular precision. So the synthesis and characterization of multilayer assemblies is generally more challenging than that of monolayers (9a). Multilayer 1
construction has been achieved by self-assembly of the layers through covalent bonds (10), ionic interaction (11), or metal coordination (9). In this paper, we choose L-cysteine hydrochloride possessing bifunctional ligands as the adsorbate to form the self-assembled monolayers and multilayers. In other words, L-cysteine hydrochloride has a thiol group on one side capable of chemisorbing on an Au surface or oxidation of Cu2+ , and it has an amino and a carboxylic acid group on the other side capable of ion binding. Self-assembled monolayers were formed by the chemisorption of the thiol group on an Au surface. Then multilayers were obtained by surface derivatization with Cu2+ and cysteines. The formation of multilayers was monitored by XPS. The investigation of CV further confirmed the structure of multilayers.
To whom correspondence should be addressed. Fax: 086-25-3317761. 301
Thin film preparation. Self-assembled monolayers and multilayers for XPS measurements were formed on gold substrates. Gold substrates with a predominant (111) texture were prepared by evaporating gold films of approximately 100 nm thickness on glass microscopy slides. Prior to gold deposition, the glass slides were cleaned in a fresh mixture of hot piraha solution (1 : 3 v/v H2 O2 (30%) : H2 SO4 (98%)) for 30 min, rinsed with water and ethanol, dried in an over at 140◦ C overnight, then primed with a ˚ Cr layer to promote the adhesion of gold. Before they were 50-A used for self-assembly, the gold substrates were immersed into hot chromic acid (saturated K2 Cr2 O7 in 90% H2 SO4 ) for a few seconds, rinsed with copious amounts of water and ethanol, followed by drying in a N2 gas stream. Then they were immediately immersed in a 5 mM ethanol solution of L-cysteine hydrochloride for about 20 h. Afterward, the substrates were removed from the solution, rinsed thoroughly with ethanol, water, and ethanol once more, and dried with nitrogen. The dry, self-assembled substrates were then dipped into a 20 mM aqueous solution of Cu(ClO4 )2 for about 2 h, followed by extensive rinsing with water and ethanol and immersion in a 5 mM solution of cystei hydrochloride in ethanol for about 20 h again. Alternate repetitions of these procedures led to the formation of multilayers. 0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
All rights reserved.
302
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SAMs for electrochemical measurements were prepared on gold electrodes. The gold electrodes were disks of diameter 0.3 mm prepared by sealing an annealed gold wire in a glass tube. The electrodes were first polished with mash sand paper (800 mesh) and then with chamois leather, followed by thoroughly rinsing with pure water. The gold electrodes were then electrochemically cleaned by cycling the electrode potential between −0.5 and +1.5 V vs Ag/AgCl in 0.5 M H2 SO4 (AR) for several minutes until a stable and standard voltammogram was obtained. Then they were rinsed with deioned water and ethanol, dried with argon gas, and immediately immersed in the deposition solution. The deposition solution and the process of multilayer formation for CV measurements are the same as those for XPS measurements. XPS. X-ray photoelectron spectra were obtained with an ESCALAB MK-II instrument (VG Company, England), which was equipped with a MgK α source, operating at 300 W. The base pressure in the chamber was better than 10−10 mbar. Electron binding energies were referenced to the Au 4 f 7/2 transition at 84.0 eV. Electrochemistry. Cyclic voltammetry experiments were carried out in a 0.1 M aqueous K2 SO4 solution using an EG & G model 273 potentiostat and a YEM TYPE-3036 X-Y recorder. The electrochemical measurements were performed in a threeelectrode cell. The modified Au electrode, Ag/AgCl, and Pt wire were used as working electrode, reference electrode, and counterelectrode, respectively. The electrolyte solution was degassed for 20 min with argon prior to each experiment.
FIG. 1. L-cysteine
XPS spectra of the (A) S2 p and (B) Cu2 p regions of a single layer of hydrochloride on Au after exposure to Cu2+ ions (Au-(L-Cys)-Cu).
RESULTS AND DISCUSSION
XPS measurements. L-Cysteine hydrochloride possesses bifunctional terminal groups on each side, the thiol (–SH) group on one side, the amino (–NH2 ) and the carboxyl (–COOH) groups on the other side, all these groups may chemisorb on metal surfaces (12). Cu2+ is known to oxidize thiols to disulfides (13) and coordinate to glycine (14). Therefore, it is possible to form assembled multilayers on metal surface based on the interaction of cysteine with Cu2+ . In trying to understand the structure of these multilayers, we have carried out careful XPS studies on the compounds formed at different steps, especially on the binding energies of S2 p and Cu2 p since different states of S and Cu have different electron binding energies. Figure 1 shows the XPS S2 p and Cu2 p spectra for the SAM of Au-cysteine after exposure to 20 mM solution of Cu(ClO4 )2 (Au-L-Cys-Cu). The sulfur peak consists of a spin split doublet of S2 p3/2 and S2 p1/2 , the binding energies of them correspond to 162.0 eV (fwhm = 1.2 eV) and 163.2 eV (fwhm = 1.1 eV), respectively, and the peak area ratio of S2 p3/2 to S2 p1/2 is nearly 2 : 1 (Fig. 1A). This demonstrates the formation of an Au–S bond in the mixed monolayer of Au-LCys-Cu (15), which is agreeable with Liedberg and co-workers (16). The XPS spectrum in the Cu2 p region is characterized at 934.2 eV (Cu2 p3/2 ) and 953.8 eV (Cu2 p1/2 ) (Fig. 1B). Furthermore, the satellites were clearly observed (shake-up lines, 938–
945 eV for Cu2 p3/2 , 962–964 eV for Cu2 p1/2 ). All above give evidence that Cu presents in the +2 oxidation state. The ratio found for S/Cu is 1/1.30, which is compared to an expected ratio of 1/1. Moreover, no Cl2 p XPS signals were measured at this stage. Therefore, at the first step, the thiol group covalently bonded to the gold surface, leaving the amino and the carboxyl groups free on its surface. Both the groups are subsequently capable of coordinating to Cu2+ . Then how will the surfacial Cu2+ react with additional L-cysteine hydrochloride? XPS spectra of S2 p and Cu2 p at this stage provide useful information. Figure 2 shows the XPS S2 p and Cu2 p spectra of multilayer Au-(L-Cys)–Cu-(L-Cys) which was obtained by immersion of the monolayer Au-(L-Cys)-Cu in a 5 mM ethanol solution of L-cysteine hydrochloride for 20 h once more. The fwhm of the sulfur peak of Au-(L-Cys)–Cu-(L-Cys) is broader than that of Au-(L-Cys)-Cu, which suggests that sulfur exists at different states when a second L-cysteine layer was formed. It is actually true that the sulfur peak at this stage consists of two spin split doublets (Fig. 2A): one occurs at 161.8 eV (S2 p3/2 , fwhm = 1.5 eV) and 163.0 eV (S2 p1/2 , fwhm = 1.5 eV), corresponding to the thiolate (Au–S bond) (15); the other occurs at 163.6 eV (S2 p3/2 , fwhm = 1.5 eV) and 164.7 eV (S2 p1/2 , fwhm = 1.5 eV), corresponding to the thiol group (S–H bond)
SELF-ASSEMBLY OF MULTILAYER THIN FILMS ON GOLD
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Cu-(L-Cys) also react with Cu2+ and the subsequent L-cysteine hydrochloride to form the respective disulfide in our system? The counterpart XPS spectra of S2 p and Cu2 p will shed light on the question. Figure 3 shows the XPS S2 p and Cu2 p spectra of multilayer Au-(L-Cys)–Cu-(L-Cys)–Cu-(L-Cys), which was obtained by immersion of Au-(L-Cys)–Cu-(L-Cys) in the 20 mM solution of Cu(ClO4 )2 and successively in the 5 mM solution of L-cysteine hydrochloride. The sulfur peak at this stage is also successfully reproduced by a combination of two sets of doublets (Fig. 3A): one occurs at 161.8 eV (S2 p3/2 , fwhm = 1.5 eV) and 163.1 eV (S2 p1/2 , fwhm = 1.5 eV), which corresponds to the thiolate (Au–S bond) (15), the other occurs at 162.2 eV (S2 p3/2 , fwhm = 1.3 eV) and 163.4 eV (S2 p1/2 , fwhm = 1.3 eV), which corresponds to the disulfide (Cu–S bond) (18). Both peak area ratios of S2 p3/2 to S2 p1/2 are nearly 2 : 1. No peak for thiol group (S–H) is observed. Moreover, compared with the Cu2 p XPS spectrum in Fig. 2B, a new peak at 931.6 eV (Cu2 p3/2 ), has appeared in Fig. 3B, which confirms the formation of the +1 oxidation state Cu+ at this stage. In addition to the +1 oxidation state Cu+ , Cu2+ also exists at 933.8 eV (Cu2 p3/2 ), together with its shake-up lines at 938–945 eV. The ratio of S/Cu is found to be 3.13/2, which is very close to the theoretical ratio S/Cu of 3/2
FIG. 2. XPS spectra of the (A) S2 p and (B) Cu2 p regions of multilayer Au-(L-Cys)–Cu-(L-Cys).
(15). Both peak area ratios of S2 p3/2 to S2 p1/2 are about 2 : 1. So it can be preliminarly concluded that the second L-cysteine layer is attached to the Cu2+ ion of Au-(L-Cys)-Cu through the coordination of the amino and the carboxyl groups. The Cu2 p XPS spectrum (Fig. 2B) provides more information about the film structure. After the monolayer of Au-(L-Cys)-Cu was immersed in the ethanol solution of L-cysteine hydrochloride for about 20 h, Cu still remained in the +2 oxidation state, with a Cu2 p3/2 binding energy of 933.8 eV and its shake-up lines in the range 938–945 eV, and Cu2 p1/2 binding energy of 954.1 eV and its shake-up lines in the range 961–964 eV. Therefore, it is true that the second L-cysteine layer coordinates to the surfacial Cu2+ of the monolayer Au-(L-Cys)-Cu through its amino and carboxyl group, leaving the thiol group free on the surface. So at this stage, thin films containing an Cu2+ site sandwiched between the first layer of cysteine and the second layer of cysteine were formed, and the Cu2+ ion was surrounded by two amino and two carbonyl groups to form a coordinated compound. The experimentally determined ratio of S/Cu is 2.20 : 1. It is near to the theoretical ratio S/Cu of 2/1 (see Fig. 4b). It is well known that Cu2+ can oxidize thiols to disulfides (13). Will the surfacial thiol group of multilayer Au-(L-Cys)–
FIG. 3. XPS spectra of the (A) S2 p and (B) Cu2 p regions of multilayer Au-(L-Cys)–Cu-(L-Cys)–Cu-(L-Cys).
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FIG. 4. Scheme of multilayer formation by alternate surface derivatization with L-cysteine hydrochloride and Cu2+ ions. (a) SAM of L-cysteine on Au (Au(L-Cys)). (b) Attachment of Cu2+ and a second L-cysteine to the surface of the SAM (Au-(L-Cys)–Cu-(L-Cys)). Coordination of the amino and the carboxyl groups of Cu2+ forms the complex. (c) Attachment of a second Cu2+ and a third L-cysteine layer to the surface of multilayer Au-(L-Cys)–Cu-(L-Cys) (Au-(L-Cys)–Cu(L-Cys)–Cu-(L-Cys). Reduction of Cu2+ leads to the formation of disulfide.
(see Fig. 4c). So it is clear that the surfacial free thiol group of multilayer Au-(L-Cys)–Cu-(L-Cys) reacts first with the attached Cu2+ and then with the subsequent L-cysteine hydrochloride to form the disulfides, leaving the amino and the carboxyl groups on the surface of multilayers again. The formation of disulfides is similar to the results of Bard and co-workers (19). In our study, L-cysteine hydrochloride was used as the bifunctional building unit, both the sublayer of the Cu2+ complex and the sublayer of the Cu+ -disulfide being alternately produced. Figure 4 shows the scheme of multilayer formation by alternate surface derivatization with L-cysteine hydrochloride and Cu2+ ions. It can be
FIG. 5. Typical XPS Cu2 p spectrum of a Cu monolayer adsorbed to a SAM of L-cysteine hydrochloride on Au after exposure to the X-ray beam for about 1 h.
inferred that steps further on will form repeated multilayer films containing alternate structures of the complex and disulfide. Moreover, it is worth noting that a reduction of Cu(II) will occur upon prolonged exposure of the monolayer or multilayer samples to the X-ray beam during XPS examination. Figure 5 shows a typical spectrum of the same monolayer as in Fig. 1B after exposure to the X-ray beam for about 1 h. It can be seen clearly that Cu(II) has almost been converted to Cu(I) since the shake-up lines almost disappear. So special care should be taken to avoid prolonged beam exposure of a single spot of our samples during XPS measurement. Cyclic voltammetry. In order to determine the oxidation state of Cu ions in the thin films, and to consummate the results of XPS measurements, the electrochemical behaviors of Cu ions both adsorbed to the SAM of L-cysteine hydrochloride (Au-(L-Cys)Cu) and adsorbed to the multilayer Au-(L-Cys)–Cu-(L-Cys) (Au(L-Cys)–Cu-(L-Cys)-Cu) were studied. Figure 6 shows the cyclic voltammogram of a SAM of L-cysteine hydrochloride on gold electrode after the adsorption of Cu2+ ions. The sweep range is 3.0 → −0.2 → 0.7 → 0.3 V. As expected, Cu is present in the +2 oxidation state. The cathodic peak at 160 mV represents the reduction of surface bound Cu2+ to Cu+ , and the subsequent anodic peak at 230 mV represents that the reduced Cu+ is reoxidized to Cu2+ . It is noticeable that during the course of the first cyclic voltammetric cycle a very little change in current or potential is obtained, which indicates that some rearrangement of the copper coordination in the SAM may take place (20). Afterward, a fairly stable response is recorded upon repetitive cycling,
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characterized by a reduction peak at 170 mV, representing the reduction of Cu2+ to Cu+ , and followed by a second anodic sweep of an oxidation peak at 355 mV. Repeated cycling afterward almost remain the same and does not lead to a significant loss of signal, which indicates that the multilayer is also fairly stable under this experimental condition. From the above results, it can be seen that the electrochemical studies not only complement the XPS experiments, but also provide an independent way of determining the oxidation state of Cu at different sites. SUMMARY
FIG. 6. Cyclic voltammogram of Cu ions adsorbed to a SAM of L-cysteine hydrochloride on a gold electrode (d = 0.3 mm) in 0.1 M aqueous K2 SO4 solution. The potential sweep begins at the open circuit potential of 300 mV vs Ag/AgCl in the direction of the arrow. The first, second, and fifth cycles are shown. Sweep rate was 100 mV s−1 .
which suggests that Cu remains strongly adsorbed in both oxidation states within the potential range of the measurement. On the other hand, the oxidation state of Cu in multilayer Au-(LCys)–Cu-(L-Cys)-Cu is in agreement with the respective results of XPS and the views of Bard and co-workers (19). Figure 7 shows the cyclic voltammogram of Cu ions in the multilayer Au-(L-Cys)–Cu-(L-Cys)-Cu on the gold electrode. The sweep range is 0.2 → 0.7 → −0.2 → 0.7 V. The first anodic sweep shows the oxidation of Cu+ to Cu2+ as a shoulder at 380 mV. This reveals that some of the Cu ions already exist in Cu+ in the multilayer Au-(L-Cys)–Cu-(L-Cys)-Cu. In accordance with XPS, formation of Cu+ attached to cysteine via sulfur during multilayer growth was accompanied by the oxidation of thiol groups of cysteine to disulfides. Subsequent cathodic sweep is
In this paper, we have shown that it is possible to prepare self-assembled multilayers on gold by alternate surface derivatization with L-cysteine hydrochloride and Cu2+ ion utilizing the chemisorptive properties of the thiol functionality and the coordinative and reductive properties of Cu2+ ion. XPS measurements show that Cu ions and cysteines in multilayer thin films exist in two different modes. One mode is that the Cu site is sandwiched between the two amino acid groups of two L-cysteine molecules, leaving the thiol group on its surface, and Cu is present in the +2 oxidation state; The other is that Cu ions are bonded through disulfide and thiolate, leaving the amino acid group on its surface and Cu is present in the +1 oxidation state. The two kinds of structures are arranged alternatively. The different oxidation states of Cu are also confirmed by cyclic voltammetry. Both self-assembled mono- and multilayer films are stable within the potential range of our experiments. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (No. 29771017, No. 29831010) and the State Education Commission of China. We are very grateful to Dr. Xiao-Shu Wang for her help in obtaining the XPS spectra.
REFERENCES
FIG. 7. Cyclic voltammogram of Cu ions in multilayer Au-(L-Cys)–Cu-(LCys)-Cu on a gold electrode (d = 0.3 mm) in 0.1 M aqueous K2 SO4 solution. The potential sweep begins at the open circuit potential of 200 mV vs Ag/AgCl in the direction of the arrow. Sweep rate was 100 mV s−1 .
1. Ulman, A., “An Introduction to Ultrathin Organic Films from LangmuirBlodgett to Self-Assembly.” Academic Press, San Diego, 1991. 2. Murray, R. W., “Molecular Design of Electrode Surface,” Techniques of Chemistry Series, Vol. 255, p. 1230. Wiley, New York, 1992. 3. Labinis, P. E., Nuzzo, R. G., and Whitesides, G. M., J. Phys. Chem. 96, 5097 (1992). 4. Chaudhury, M. K., and Whitesides, G. M., Science 255, 1230 (1992). 5. Somorjai, G. A., “Chemistry of Two Dimensions: Surface.” Cornell Univ. Press, Ithaca, NY, 1981. 6. (a) Kaussling, L., Michel, B., Ringsdorf, H., and Rohre, H., Angew. Chem. Int. Ed. Engl. 30, 569 (1991). (b) Kumar, A., Biebuyck, H. A., and Whitesides, G. M., Langmuir 10, 1498 (1994). (c) Schierbaum, K. D., Weiss, T., Thoden Van Velzen, E. U., Engbersen, J. F. J., Reinhoudt, D. N., and Gopel, W., Science 265, 1413 (1994). 7. Dubois, L. H., and Nuzzo, R. G., Ann. Rev. Phys. Chem. 43, 437 (1992). 8. (a) Ulman, A., Adv. Mater. 2, 573 (1990). (b) Lin, W., Wong, G. K., and Marks, T. J., J. Am. Chem. Soc. 118, 8034 (1996). (c) Maoz, R., Matlis, S., Dimasi, E., Ocko, M. B., and Sagiv, J., Nature 384, 150 (1996). 9. (a) Yang, J. C., Aoki, K., Hong., H.-J., Sackett, D. D., Arendt, M. F., Yau, S.-L., Bell, C. M., and Mallouk, T. E., J. Am. Chem. Soc. 115, 11,855 (1993).
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(b) Anat, H., Tamar, M., Hagai, C., Sophi, M., Jacqueline, L., Alexander, V., Abraham, S., and Israel, R., J. Am. Chem. Soc. 120, 13,469 (1998). Katz, H. E., Chem. Mater. 6, 2227 (1994). (a) Netzer, L., and Sagiv, J., J. Am. Chem. Soc. 105, 674 (1983). (b) Lee, H., Kepley, L. J., Hong, H.-G., and Mallouk, T. E., J. Am. Chem. Soc. 110, 618 (1988). (c) Li, D., Ratner, M. A., and Marks, T. J., J. Am. Chem. Soc. 112, 7389 (1990). (d) Ansell, M. A., Zeppenfeld, A. C., Yoshinotom, K., Logan, E. B., and Page, C., J. Chem. Mater. 8, 591 (1996). Ulman, A., Chem. Rev. 96, 1533 (1996). Michaelies, L., and Schubert, M. P., J. Am. Chem. Soc. 52, 4418 (1930). (a) Sen, D. N., Mizushima, S.-I., Curran, C., and Quagliano, J. V., J. Am. Chem. Soc. 77, 211 (1955). (b) Tomita, K., Bull. Chem. Soc. Jpn. 34, 280 (1961). (a) Hutchison, J. E., Postlethwaite, T. A., and Murray, R. W., Langmuir 9, 3277 (1993). (b) Fritz, M. C., Hahner, G., Spencer, N. D., Burli, R., and
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Vasella, A., Langmuir 12, 6074 (1996). (c) Nakanishi, T., Ohtani, B., and Uosaki, K., J. Phys. Chem. B 102, 1571 (1998). Uvdal, K., Bodo, P., and Liedberg, B., J. Colloid Interface Sci. 149, 162 (1992). (a) Moulder, J. F., Stickle, W. F., Sobol, P. E., and Bomben, K. D., “Handbook of X-Ray Photoelectron Spectroscopy.” Physical Electronics Eden Prairie, MN, 1995. (b) Freeman, T. L., Evans, S. D., and Ulman, A., Thin Solid Films 784 (1994). In order to get the S2 p binding energy corresponding to the disulfide (Cu– S bond), we prepared the disulfide by the reaction of C12 H25 SH with Cu(ClO4 )2 . XPS analysis of this disulfide showed that Cu is present in the +1 oxidation state, and the sulfur peak consists of a spin split doublet, with the S2 p3/2 and S2 p1/2 binding energies at 162.2 and 163.4 eV, respectively. Brust, M., Blass, P. M., and Bard, A. J., Langmuir 13, 5602 (1997). Arrigan, D. W.-M., and Bihan, L. L., Analyst 124, 1645 (1999).
Self-Assembly of L-Cysteine-Copper(II)/Copper(I) Multilayer Thin Films on Gold Wen-Wei Zhang, Chang-Sheng Lu, Yang Zou, Jing-Li Xie, Xiao-Ming Ren, Hui-Zhen Zhu, and Qing-Jin Meng1 Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, Nanjing University, No. 22 Hankou Road, Nanjing 210093, People’s Republic of China E-mail: [email protected] Received November 12, 2001; accepted January 24, 2002; published online April 10, 2002
Self-assembled multilayer thin films have been prepared on Au substrate by alternate surface derivatization with L-cysteine hydrochloride and cupric perchlorate. The layer-by-layer structure at each step of multilayer formation was investigated by X-ray photoelectron spectroscopy. The measurements indicate that there are two structure modes in the multilayers. One is that Cu2+ sandwiches between two amino acid groups. The other one is that Cu+ is bonded through disulfide and thiolate. This process is also confirmed by cyclic voltammetry of Cu ion at different self-assembled multilayers. Steps further on will lead to repeated multilayer films. C 2002 Elsevier Science (USA) Key Words: self-assembled multilayers; L-cysteine hydrochloride; coordination and reduction of Cu2+ ; XPS; cyclic voltammetry.
INTRODUCTION
EXPERIMENTAL
Self-assembled monolayers (SAMs) are molecular assemblies spontaneously formed by the immersion of an appropriate substrate into a solution of an active adsorbate, which provides an elegant way for forming well-defined organic assemblies with a wide range of surface functionalities (1). In the past two decades, SAMs have become an active research area for their fundamental importance in understanding interfacial properties as well as for their potential applications in molecular technologies. For examples, SAM films have been used to study important fundamental processes containing electron transfer (2), surface wetting (3), adhesion (4), catalysis (5), and molecular recognition (6). Such films have also been utilized to the construction of various interfaces for biological membranes (7), molecular electronics, nonlinear optics, optical switches, chemical sensors, and high-density memory devices (8). Extension of this approach to multilayers might enhance some properties, since new classes of materials possessing functional groups at controlled sites can be designed and prepared with molecular precision. So the synthesis and characterization of multilayer assemblies is generally more challenging than that of monolayers (9a). Multilayer 1
construction has been achieved by self-assembly of the layers through covalent bonds (10), ionic interaction (11), or metal coordination (9). In this paper, we choose L-cysteine hydrochloride possessing bifunctional ligands as the adsorbate to form the self-assembled monolayers and multilayers. In other words, L-cysteine hydrochloride has a thiol group on one side capable of chemisorbing on an Au surface or oxidation of Cu2+ , and it has an amino and a carboxylic acid group on the other side capable of ion binding. Self-assembled monolayers were formed by the chemisorption of the thiol group on an Au surface. Then multilayers were obtained by surface derivatization with Cu2+ and cysteines. The formation of multilayers was monitored by XPS. The investigation of CV further confirmed the structure of multilayers.
To whom correspondence should be addressed. Fax: 086-25-3317761. 301
Thin film preparation. Self-assembled monolayers and multilayers for XPS measurements were formed on gold substrates. Gold substrates with a predominant (111) texture were prepared by evaporating gold films of approximately 100 nm thickness on glass microscopy slides. Prior to gold deposition, the glass slides were cleaned in a fresh mixture of hot piraha solution (1 : 3 v/v H2 O2 (30%) : H2 SO4 (98%)) for 30 min, rinsed with water and ethanol, dried in an over at 140◦ C overnight, then primed with a ˚ Cr layer to promote the adhesion of gold. Before they were 50-A used for self-assembly, the gold substrates were immersed into hot chromic acid (saturated K2 Cr2 O7 in 90% H2 SO4 ) for a few seconds, rinsed with copious amounts of water and ethanol, followed by drying in a N2 gas stream. Then they were immediately immersed in a 5 mM ethanol solution of L-cysteine hydrochloride for about 20 h. Afterward, the substrates were removed from the solution, rinsed thoroughly with ethanol, water, and ethanol once more, and dried with nitrogen. The dry, self-assembled substrates were then dipped into a 20 mM aqueous solution of Cu(ClO4 )2 for about 2 h, followed by extensive rinsing with water and ethanol and immersion in a 5 mM solution of cystei hydrochloride in ethanol for about 20 h again. Alternate repetitions of these procedures led to the formation of multilayers. 0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
All rights reserved.
302
ZHANG ET AL.
SAMs for electrochemical measurements were prepared on gold electrodes. The gold electrodes were disks of diameter 0.3 mm prepared by sealing an annealed gold wire in a glass tube. The electrodes were first polished with mash sand paper (800 mesh) and then with chamois leather, followed by thoroughly rinsing with pure water. The gold electrodes were then electrochemically cleaned by cycling the electrode potential between −0.5 and +1.5 V vs Ag/AgCl in 0.5 M H2 SO4 (AR) for several minutes until a stable and standard voltammogram was obtained. Then they were rinsed with deioned water and ethanol, dried with argon gas, and immediately immersed in the deposition solution. The deposition solution and the process of multilayer formation for CV measurements are the same as those for XPS measurements. XPS. X-ray photoelectron spectra were obtained with an ESCALAB MK-II instrument (VG Company, England), which was equipped with a MgK α source, operating at 300 W. The base pressure in the chamber was better than 10−10 mbar. Electron binding energies were referenced to the Au 4 f 7/2 transition at 84.0 eV. Electrochemistry. Cyclic voltammetry experiments were carried out in a 0.1 M aqueous K2 SO4 solution using an EG & G model 273 potentiostat and a YEM TYPE-3036 X-Y recorder. The electrochemical measurements were performed in a threeelectrode cell. The modified Au electrode, Ag/AgCl, and Pt wire were used as working electrode, reference electrode, and counterelectrode, respectively. The electrolyte solution was degassed for 20 min with argon prior to each experiment.
FIG. 1. L-cysteine
XPS spectra of the (A) S2 p and (B) Cu2 p regions of a single layer of hydrochloride on Au after exposure to Cu2+ ions (Au-(L-Cys)-Cu).
RESULTS AND DISCUSSION
XPS measurements. L-Cysteine hydrochloride possesses bifunctional terminal groups on each side, the thiol (–SH) group on one side, the amino (–NH2 ) and the carboxyl (–COOH) groups on the other side, all these groups may chemisorb on metal surfaces (12). Cu2+ is known to oxidize thiols to disulfides (13) and coordinate to glycine (14). Therefore, it is possible to form assembled multilayers on metal surface based on the interaction of cysteine with Cu2+ . In trying to understand the structure of these multilayers, we have carried out careful XPS studies on the compounds formed at different steps, especially on the binding energies of S2 p and Cu2 p since different states of S and Cu have different electron binding energies. Figure 1 shows the XPS S2 p and Cu2 p spectra for the SAM of Au-cysteine after exposure to 20 mM solution of Cu(ClO4 )2 (Au-L-Cys-Cu). The sulfur peak consists of a spin split doublet of S2 p3/2 and S2 p1/2 , the binding energies of them correspond to 162.0 eV (fwhm = 1.2 eV) and 163.2 eV (fwhm = 1.1 eV), respectively, and the peak area ratio of S2 p3/2 to S2 p1/2 is nearly 2 : 1 (Fig. 1A). This demonstrates the formation of an Au–S bond in the mixed monolayer of Au-LCys-Cu (15), which is agreeable with Liedberg and co-workers (16). The XPS spectrum in the Cu2 p region is characterized at 934.2 eV (Cu2 p3/2 ) and 953.8 eV (Cu2 p1/2 ) (Fig. 1B). Furthermore, the satellites were clearly observed (shake-up lines, 938–
945 eV for Cu2 p3/2 , 962–964 eV for Cu2 p1/2 ). All above give evidence that Cu presents in the +2 oxidation state. The ratio found for S/Cu is 1/1.30, which is compared to an expected ratio of 1/1. Moreover, no Cl2 p XPS signals were measured at this stage. Therefore, at the first step, the thiol group covalently bonded to the gold surface, leaving the amino and the carboxyl groups free on its surface. Both the groups are subsequently capable of coordinating to Cu2+ . Then how will the surfacial Cu2+ react with additional L-cysteine hydrochloride? XPS spectra of S2 p and Cu2 p at this stage provide useful information. Figure 2 shows the XPS S2 p and Cu2 p spectra of multilayer Au-(L-Cys)–Cu-(L-Cys) which was obtained by immersion of the monolayer Au-(L-Cys)-Cu in a 5 mM ethanol solution of L-cysteine hydrochloride for 20 h once more. The fwhm of the sulfur peak of Au-(L-Cys)–Cu-(L-Cys) is broader than that of Au-(L-Cys)-Cu, which suggests that sulfur exists at different states when a second L-cysteine layer was formed. It is actually true that the sulfur peak at this stage consists of two spin split doublets (Fig. 2A): one occurs at 161.8 eV (S2 p3/2 , fwhm = 1.5 eV) and 163.0 eV (S2 p1/2 , fwhm = 1.5 eV), corresponding to the thiolate (Au–S bond) (15); the other occurs at 163.6 eV (S2 p3/2 , fwhm = 1.5 eV) and 164.7 eV (S2 p1/2 , fwhm = 1.5 eV), corresponding to the thiol group (S–H bond)
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Cu-(L-Cys) also react with Cu2+ and the subsequent L-cysteine hydrochloride to form the respective disulfide in our system? The counterpart XPS spectra of S2 p and Cu2 p will shed light on the question. Figure 3 shows the XPS S2 p and Cu2 p spectra of multilayer Au-(L-Cys)–Cu-(L-Cys)–Cu-(L-Cys), which was obtained by immersion of Au-(L-Cys)–Cu-(L-Cys) in the 20 mM solution of Cu(ClO4 )2 and successively in the 5 mM solution of L-cysteine hydrochloride. The sulfur peak at this stage is also successfully reproduced by a combination of two sets of doublets (Fig. 3A): one occurs at 161.8 eV (S2 p3/2 , fwhm = 1.5 eV) and 163.1 eV (S2 p1/2 , fwhm = 1.5 eV), which corresponds to the thiolate (Au–S bond) (15), the other occurs at 162.2 eV (S2 p3/2 , fwhm = 1.3 eV) and 163.4 eV (S2 p1/2 , fwhm = 1.3 eV), which corresponds to the disulfide (Cu–S bond) (18). Both peak area ratios of S2 p3/2 to S2 p1/2 are nearly 2 : 1. No peak for thiol group (S–H) is observed. Moreover, compared with the Cu2 p XPS spectrum in Fig. 2B, a new peak at 931.6 eV (Cu2 p3/2 ), has appeared in Fig. 3B, which confirms the formation of the +1 oxidation state Cu+ at this stage. In addition to the +1 oxidation state Cu+ , Cu2+ also exists at 933.8 eV (Cu2 p3/2 ), together with its shake-up lines at 938–945 eV. The ratio of S/Cu is found to be 3.13/2, which is very close to the theoretical ratio S/Cu of 3/2
FIG. 2. XPS spectra of the (A) S2 p and (B) Cu2 p regions of multilayer Au-(L-Cys)–Cu-(L-Cys).
(15). Both peak area ratios of S2 p3/2 to S2 p1/2 are about 2 : 1. So it can be preliminarly concluded that the second L-cysteine layer is attached to the Cu2+ ion of Au-(L-Cys)-Cu through the coordination of the amino and the carboxyl groups. The Cu2 p XPS spectrum (Fig. 2B) provides more information about the film structure. After the monolayer of Au-(L-Cys)-Cu was immersed in the ethanol solution of L-cysteine hydrochloride for about 20 h, Cu still remained in the +2 oxidation state, with a Cu2 p3/2 binding energy of 933.8 eV and its shake-up lines in the range 938–945 eV, and Cu2 p1/2 binding energy of 954.1 eV and its shake-up lines in the range 961–964 eV. Therefore, it is true that the second L-cysteine layer coordinates to the surfacial Cu2+ of the monolayer Au-(L-Cys)-Cu through its amino and carboxyl group, leaving the thiol group free on the surface. So at this stage, thin films containing an Cu2+ site sandwiched between the first layer of cysteine and the second layer of cysteine were formed, and the Cu2+ ion was surrounded by two amino and two carbonyl groups to form a coordinated compound. The experimentally determined ratio of S/Cu is 2.20 : 1. It is near to the theoretical ratio S/Cu of 2/1 (see Fig. 4b). It is well known that Cu2+ can oxidize thiols to disulfides (13). Will the surfacial thiol group of multilayer Au-(L-Cys)–
FIG. 3. XPS spectra of the (A) S2 p and (B) Cu2 p regions of multilayer Au-(L-Cys)–Cu-(L-Cys)–Cu-(L-Cys).
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FIG. 4. Scheme of multilayer formation by alternate surface derivatization with L-cysteine hydrochloride and Cu2+ ions. (a) SAM of L-cysteine on Au (Au(L-Cys)). (b) Attachment of Cu2+ and a second L-cysteine to the surface of the SAM (Au-(L-Cys)–Cu-(L-Cys)). Coordination of the amino and the carboxyl groups of Cu2+ forms the complex. (c) Attachment of a second Cu2+ and a third L-cysteine layer to the surface of multilayer Au-(L-Cys)–Cu-(L-Cys) (Au-(L-Cys)–Cu(L-Cys)–Cu-(L-Cys). Reduction of Cu2+ leads to the formation of disulfide.
(see Fig. 4c). So it is clear that the surfacial free thiol group of multilayer Au-(L-Cys)–Cu-(L-Cys) reacts first with the attached Cu2+ and then with the subsequent L-cysteine hydrochloride to form the disulfides, leaving the amino and the carboxyl groups on the surface of multilayers again. The formation of disulfides is similar to the results of Bard and co-workers (19). In our study, L-cysteine hydrochloride was used as the bifunctional building unit, both the sublayer of the Cu2+ complex and the sublayer of the Cu+ -disulfide being alternately produced. Figure 4 shows the scheme of multilayer formation by alternate surface derivatization with L-cysteine hydrochloride and Cu2+ ions. It can be
FIG. 5. Typical XPS Cu2 p spectrum of a Cu monolayer adsorbed to a SAM of L-cysteine hydrochloride on Au after exposure to the X-ray beam for about 1 h.
inferred that steps further on will form repeated multilayer films containing alternate structures of the complex and disulfide. Moreover, it is worth noting that a reduction of Cu(II) will occur upon prolonged exposure of the monolayer or multilayer samples to the X-ray beam during XPS examination. Figure 5 shows a typical spectrum of the same monolayer as in Fig. 1B after exposure to the X-ray beam for about 1 h. It can be seen clearly that Cu(II) has almost been converted to Cu(I) since the shake-up lines almost disappear. So special care should be taken to avoid prolonged beam exposure of a single spot of our samples during XPS measurement. Cyclic voltammetry. In order to determine the oxidation state of Cu ions in the thin films, and to consummate the results of XPS measurements, the electrochemical behaviors of Cu ions both adsorbed to the SAM of L-cysteine hydrochloride (Au-(L-Cys)Cu) and adsorbed to the multilayer Au-(L-Cys)–Cu-(L-Cys) (Au(L-Cys)–Cu-(L-Cys)-Cu) were studied. Figure 6 shows the cyclic voltammogram of a SAM of L-cysteine hydrochloride on gold electrode after the adsorption of Cu2+ ions. The sweep range is 3.0 → −0.2 → 0.7 → 0.3 V. As expected, Cu is present in the +2 oxidation state. The cathodic peak at 160 mV represents the reduction of surface bound Cu2+ to Cu+ , and the subsequent anodic peak at 230 mV represents that the reduced Cu+ is reoxidized to Cu2+ . It is noticeable that during the course of the first cyclic voltammetric cycle a very little change in current or potential is obtained, which indicates that some rearrangement of the copper coordination in the SAM may take place (20). Afterward, a fairly stable response is recorded upon repetitive cycling,
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characterized by a reduction peak at 170 mV, representing the reduction of Cu2+ to Cu+ , and followed by a second anodic sweep of an oxidation peak at 355 mV. Repeated cycling afterward almost remain the same and does not lead to a significant loss of signal, which indicates that the multilayer is also fairly stable under this experimental condition. From the above results, it can be seen that the electrochemical studies not only complement the XPS experiments, but also provide an independent way of determining the oxidation state of Cu at different sites. SUMMARY
FIG. 6. Cyclic voltammogram of Cu ions adsorbed to a SAM of L-cysteine hydrochloride on a gold electrode (d = 0.3 mm) in 0.1 M aqueous K2 SO4 solution. The potential sweep begins at the open circuit potential of 300 mV vs Ag/AgCl in the direction of the arrow. The first, second, and fifth cycles are shown. Sweep rate was 100 mV s−1 .
which suggests that Cu remains strongly adsorbed in both oxidation states within the potential range of the measurement. On the other hand, the oxidation state of Cu in multilayer Au-(LCys)–Cu-(L-Cys)-Cu is in agreement with the respective results of XPS and the views of Bard and co-workers (19). Figure 7 shows the cyclic voltammogram of Cu ions in the multilayer Au-(L-Cys)–Cu-(L-Cys)-Cu on the gold electrode. The sweep range is 0.2 → 0.7 → −0.2 → 0.7 V. The first anodic sweep shows the oxidation of Cu+ to Cu2+ as a shoulder at 380 mV. This reveals that some of the Cu ions already exist in Cu+ in the multilayer Au-(L-Cys)–Cu-(L-Cys)-Cu. In accordance with XPS, formation of Cu+ attached to cysteine via sulfur during multilayer growth was accompanied by the oxidation of thiol groups of cysteine to disulfides. Subsequent cathodic sweep is
In this paper, we have shown that it is possible to prepare self-assembled multilayers on gold by alternate surface derivatization with L-cysteine hydrochloride and Cu2+ ion utilizing the chemisorptive properties of the thiol functionality and the coordinative and reductive properties of Cu2+ ion. XPS measurements show that Cu ions and cysteines in multilayer thin films exist in two different modes. One mode is that the Cu site is sandwiched between the two amino acid groups of two L-cysteine molecules, leaving the thiol group on its surface, and Cu is present in the +2 oxidation state; The other is that Cu ions are bonded through disulfide and thiolate, leaving the amino acid group on its surface and Cu is present in the +1 oxidation state. The two kinds of structures are arranged alternatively. The different oxidation states of Cu are also confirmed by cyclic voltammetry. Both self-assembled mono- and multilayer films are stable within the potential range of our experiments. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (No. 29771017, No. 29831010) and the State Education Commission of China. We are very grateful to Dr. Xiao-Shu Wang for her help in obtaining the XPS spectra.
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FIG. 7. Cyclic voltammogram of Cu ions in multilayer Au-(L-Cys)–Cu-(LCys)-Cu on a gold electrode (d = 0.3 mm) in 0.1 M aqueous K2 SO4 solution. The potential sweep begins at the open circuit potential of 200 mV vs Ag/AgCl in the direction of the arrow. Sweep rate was 100 mV s−1 .
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