Chemisorption of 2-Mercaptoethanol on Silver, Copper, and Gold

Chemisorption of 2-Mercaptoethanol on Silver, Copper, and Gold: Direct Raman Evidence of Acid-Induced Changes in Adsorption/Desorption Equilibria...
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Langmuir 2003, 19, 3805-3813

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Chemisorption of 2-Mercaptoethanol on Silver, Copper, and Gold: Direct Raman Evidence of Acid-Induced Changes in Adsorption/Desorption Equilibria Andrzej Kudelski* Warsaw University, Department of Chemistry, Pasteur 1, 02-093 Warsaw, Poland Received November 21, 2002. In Final Form: February 18, 2003 Molecules of 2-mercaptoethanol (ME) were spontaneously chemisorbed on silver, copper, and gold surfaces. Surface-enhanced Raman scattering investigation revealed that, as for unsubstituted alkanethiols, the average orientation of the “molecular chain” of ME is the most perpendicular to the metal surface for ME molecules adsorbed on silver. Immersion of an ME-modified electrode in diluted ME solution leads to quick desorption of a portion of the monolayer and an increase in the relative surface concentration of the gauche conformer. The time constant of this rearrangement (below 1 min) is more than 1 order of magnitude shorter than that of monolayers formed from analogous thiols (HS-(CH2)2-X) with X ) CH3, NH2, COOH, or SO3Na. The structure of the ME monolayer is highly pH-sensitive, but it is independent of the presence of neutral salts in solutions. In acidic solutions, the surface concentration of a gauche conformer considerably increases. Since protonation of a significant number of hydroxyl groups is unlikely under the conditions used, it is likely that in acidic solutions the kinetics of the desorption and the desorption/adsorption equilibrium are changed. It is probable that desorption of ME as thiol molecules is facilitated because some of the sulfur atoms of ME adsorbed as thiolate are protonated. We also found an analogous effect, although less pronounced, for monolayers self-assembled from propanethiol. This indicates that this mechanism, so far not considered, can also be important for some other (especially short-chain) thiols. In basic solutions, the concentration of a trans conformer increases and probably some of the hydroxyl groups dissociate. For all investigated solutions, the structure of the ME monolayer on gold was found to be less affected by its surroundings than that of monolayers on silver or copper.

Introduction Organic monolayers formed from ω-functionalized alkanethiols (HS-(CH2)n-X) on metal substrates have been the subject of intensive studies both out of the fundamental interests of surface chemistry and because of their technological significance.1-5 Thiols are among the most successful chemicals employed as an attachment to metals because they react chemically with gold, silver, and copper, thus forming very stable metal-sulfur bonds (in other words, thiols chemisorb at silver, gold, and copper as the corresponding thiolates).6-10 The attached X group controls the surface properties of the formed monolayer, which can be engineered for various purposes. The length of the alkyl chain is an important factor. For instance, shortchain thiols are more suitable to applications of selfassembled monolayers (SAMs) as electroanalytical4,11 and surface-enhanced Raman scattering (SERS)12,13 sensors. * Fax: +4822-8225996. E-mail: [email protected]. (1) Murray, R. W. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; J. Wiley: New York, 1992; p 1. (2) Mizutani, F.; Yabuki, S.; Sato, Y.; Sawaguchi, T.; Iijima, S. Electrochim. Acta 2000, 45, 2945. (3) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (4) Li, J.; Cheng, G.; Dong, S. J. Electroanal. Chem. 1996, 416, 97. (5) Tarabara, V. V.; Nabiev, I. R.; Feofanov, A. V. Langmuir 1998, 14, 1092. (6) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (7) Evans, S. D.; Ulman, A. Chem. Phys. Lett. 1990, 170, 462. (8) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 3629. (9) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (10) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (11) Kertesz, V.; Whittemore, N. A.; Inamati, G. B.; Manoharan, M.; Cook, P. D.; Baker, D. C.; Chambers, J. Q. Electroanalysis 2000, 12, 889. (12) Tsen, M.; Sun, L. Anal. Chim. Acta 1995, 307, 333. (13) Ye, Q.; Fang, J. X.; Sun, L. J. Phys. Chem. B 1997, 101, 8221.

Despite the tremendous interest in applications of monolayers formed from functionalized alkanethiols, there has been relatively little research directed toward the understanding of the influence of the surroundings on their structure. Such investigations can have special importance for monolayers formed from short-chain thiols, which are significantly more easily affected by the surrounding electrolyte solution than monolayers formed from longer chain thiols.14 In the present work, we report on changes in the structure of monolayers formed from 2-mercaptoethanol (HS-CH2-CH2-OH, abbreviated to ME) induced by the surrounding solutions. ME monolayers have been widely used to immobilize many types of different molecules on the metal surface.5,15-18 Since the structure of linking layers is known to influence the structure and properties of layers of attached molecules (for example, see adsorption of β-carotene on silver covered with ME monolayers5), we hope that our detailed studies of the structure of ME monolayers on silver, gold, and copper may be of practical importance in many applications when ME monolayers are used in an electrolyte solution. This report describes also the influence of acids on the structure of thiolate monolayers. How thiols bind to metal surfaces and how they desorb are long-standing open questions. The results of this work provide a better understanding of the adsorption/desorption equilibrium. (14) Schoenfisch, M. H.; Pemberton, J. E. Langmuir 1999, 15, 509. (15) Lee, I.; Lee, J. W.; Greenbaum, E. Phys. Rev. Lett. 1997, 79, 3294. (16) Wehling, B.; Hill, W.; Klockow, D. Chem. Phys. Lett. 1994, 225, 67. (17) Smejkal, P.; Vlckova, B.; Prochazka, M.; Mojzes, P.; Plfeger, J. Vib. Spectrosc. 1999, 19, 243. (18) Kuznetsov, B. A.; Byzova, N. A.; Shumakovich, G. P.; Mazhorova, L. E.; Mutuskin, A. A. Bioelectrochem. Bioenerg. 1996, 40, 249.

10.1021/la0209280 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/21/2003

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The structure of ME monolayers was explored with SERS, which seems to be one of the most sensitive tools for studying the orientation and conformation of adsorbed molecules.19-21 Moreover, water solutions used in these investigations are very weak Raman scatterers, allowing access to all spectral regions, including very valuable information in low-frequency regions that is not easily accessible with infrared (IR) measurements. Experimental Section All chemicals were purchased from commercial suppliers (Merck, Fluka) and were used as received. 2-Mercaptoethanol (Merck) and propanethiol (Merck) were of reagent grade, whereas all inorganic chemicals were of analytical reagent grade. Housedistilled water was further purified with a Millipore Milli-Q water system. Before Raman measurements, the silver, copper, and gold surfaces for the ME film deposition were electrochemically roughened to obtain a sufficiently enhanced intensity of the SERS bands. Electrochemical roughening was carried out in a conventional three-electrode cell with a large platinum sheet as the counter electrode and a saturated calomel electrode (SCE) as the reference (all potentials are quoted versus this electrode). The silver electrodes were roughened by three successive positivenegative cycles in a 0.1 M KCl aqueous solution from -0.3 to 0.3 to -0.3 V at a sweep rate of 5 mV s-1. Gold substrates were roughened in a 0.1 M KCl solution by 20 cycles for the potential changing from -0.6 to +1.25 to -0.6 V at a sweep rate of 0.2 V s-1. The copper electrodes were roughened by 10 successive positive-negative scans at a sweep rate of 20 mV s-1 from -0.55 to +0.05 to -0.55 V in a 0.2 M LiCl and 0.01 M CuCl2 solution. The working metal electrodes were removed at an open circuit potential and very carefully rinsed with water. Raman measurements were performed using either an ISA T64000 (Jobin Yvon) or a Labram 1000 (Dilor) spectrometer. Both Raman spectrometers were equipped with an Olympus BX40 microscope with a 50× long-distance objective (LMPLFL50×/0.50). A Laser-Tech model LJ-800 mixed argon/ krypton laser or a He-Ne laser provided red excitation radiation for T64000 and Labram 1000 spectrometers, respectively. The small difference in the wavelength of excitation radiations (647.1 and 632.8 nm for the krypton and the He-Ne lasers, respectively) did not cause noticeable differences in the Raman spectra. Both Raman spectrometers were equipped with liquid nitrogen cooled charge-coupled device (CCD) detectors.

Results and Discussion Structures of ME Monolayers on Silver, Copper, and Gold. Figure 1 shows Raman spectra of ME in the liquid state (spectrum a) and adsorbed at the silver, copper, and gold surfaces (spectra b, c, and d, respectively). The ME monolayers were formed by immersion of the electrochemically activated metal substrates in a 10 mM aqueous solution of ME for 2 h (control experiments showed that monolayers prepared for 24 h exhibited the same behavior as those formed for 2 h). SERS measurements were carried out for metal substrates immersed also in a 10 mM ME solution. The missing S-H stretching band at around 2566 cm-1 for the surface species indicates the expected covalent bonding of the sulfur atoms to the metal surfaces.5,16 The strongest Raman bands for adsorbed molecules are at 2922 cm-1, typical for the C-H stretching,8,9 1009 cm-1, due to out-of-phase C-C-O stretching,5,16 and about 630 and 720 cm-1, assigned to the C-S stretching.5,8,9,16,22 The ν(C-S) band at a lower (19) Pemberton, J. E. In The Handbook of Surface Imaging and Visualization; Hubbard, A. T., Ed.; CRC Press: New York, 1995; Chapter 47. (20) Pettinger, B. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH: New York, 1992; Chapter 6. (21) Otto, A. J. Raman Spectrosc. 1991, 22, 743. (22) Joo, T. H.; Kim, K.; Kim, M. S. J. Phys. Chem. 1986, 90, 5816.

Figure 1. Raman spectrum for (a) liquid ME and SERS spectra of ME monolayers formed on roughened (b) silver, (c) copper, and (d) gold. SERS measurements were carried out for metal substrates immersed in a 10 mM ME aqueous solution. Spectra are scaled and shifted for clarity. The inset shows possible structures of the S-C-C chain and the orientation of some C-H bonds: (T) trans conformation; (G) gauche conformation.

wavenumber (about 630 cm-1) is characteristic for the gauche conformation of the S-C-C chain, and the ν(C-S) band at a higher wavenumber (about 720 cm-1) is characteristic for the trans conformation.5,8,9,16,22 The inset in Figure 1 shows the gauche and trans structures of the alkyl chain. As can be seen from Figure 1, the ν(C-S) bands of the trans and gauche conformers observed in the Raman spectrum of pure ME in the liquid state at 758 and 660 cm-1 undergo significant downshifts in the SERS spectrum of adsorbed ME. The shift of the C-S bands of the surface species toward lower wavenumbers is related to a withdrawal of electron density from the C-S bond due to bonding to the metal surface5,8,16 and is probably also slightly related to a “relative” increase in the effective mass of the sulfur atom due to its interaction with the metal atom(s).5 Since the position of ν(C-S) bands depends on the C-C-S-metal torsion angle,5 the actual positions of these bands strongly depend on the overall ordering of the monolayer and are very sensitive to the state of the substrate and surroundings (e.g., see Figure 5). The intensity ratio of trans and gauche ν(C-S) bands for adsorbed ME is significantly larger than for the liquid ME. This is due to the ordering processes at the metal surface caused by the formation of relatively densely

Chemisorption of 2-Mercaptoethanol

packed monolayers (an ME molecule in the gauche conformation takes more “surface space” than the molecule in the trans conformation; see the inset in Figure 1). Comparison of a large number of spectra revealed that the intensity ratios of trans and gauche ν(C-S) bands are the highest for layers formed on silver, lower for gold, and the lowest for monolayers formed on copper. Such differences in structures are typical for thiolate selfassembled monolayers and have been previously observed by many groups9,23-34 (although, as far as we know, for molecules composed of only two carbon atoms analogous results are available only for HS-(CH2)2-NH2).34 Many previous measurements showed that the chain of adsorbed alkanethiols is significantly more tilted for molecules adsorbed on Au than on Ag supports.9,23-25 Since the basic spacings of the lattices of these two substrates are very similar (2.89 Å for Ag versus 2.88 Å for Au),35 the different orientation at Au and Ag must be caused by differences in the metal-S bonding at the two substrates (both the sulfur-metal binding energy and the character and geometry of the sulfur-substrate bonding).9,31 Despite differences in lattice spacings between Cu and Ag (2.56 Å for Cu versus 2.89 Å for Ag),35 the structure of various long-chain alkanethiolate layers formed on oxide-free copper was found to be relatively similar to the structure of monolayers formed on silver with the alkyl chains almost perpendicular to the metal surface.26-28 However, the monolayers formed on copper show a pronounced sensitivity to the details of the sample preparation and if copper substrates were slightly oxidized, a considerable amount of a gauche conformer was detected.26 It is likely that for ME molecules adsorbed on copper, in addition to analogous effects for unsubstituted alkanethiols, some of the ME molecules can interact with the metal surface also via their hydroxyl groups (copper has a large affinity for oxygen)36 in addition to the chemisorption via the thiol function. Such additional bonding forces the adsorbed molecule to adopt a gauche conformation of the alkyl chain (see the inset in Figure 1). There is also a possibility that not all of the oxygen present on the copper surface (e.g., as oxide)37 is removed during the monolayer incubation38 and the hydroxyl groups of some ME molecules interact with the “surface oxygen” through H-bonding, additionally stabilizing the gauche conformation. According to Tarabara et al.,5 the ratio of the SERS cross sections of trans and gauche ν(C-S) bands is equal (23) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter M. D. J. Am. Chem. Soc. 1991, 113, 2370. (24) Ehler, T. T.; Malmberg, M.; Noe, L. J. J. Phys. Chem. B 1997, 101, 1268. (25) Rong, H. T.; Frey, S.; Yang, Y. J.; Zharnikov, M.; Buck, M.; Wuhn, M.; Woll, C.; Helmchen, G. Langmuir 2001, 17, 1582. (26) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (27) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990. (28) Nemetz, A.; Fischer, T.; Ulman, A.; Knoll, W. J. Chem. Phys. 1993, 98, 5912. (29) Fenter, P.; Eisenberger, P.; Li, J.; Camillone, N., III; Bernasek, S.; Scoles, G.; Ramanarayanan, T. A.; Liang, K. S. Langmuir 1991, 7, 2013. (30) Chenakin, S. P. Vacuum 2002, 66, 157. (31) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y. J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359. (32) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 4500. (33) Chang, S. C.; Chao, I.; Tao, Y. T. J. Am. Chem. Soc. 1994, 116, 6792. (34) Michota, A.; Kudelski, A.; Bukowska, J. Surf. Sci. 2002, 502, 214. (35) Kittel, C. Solid State Physics, 5th ed.; Wiley: New York, 1976; p 32. (36) Chan, H. Y. H.; Takoudis, C. G.; Weaver, M. J. J. Phys. Chem. B 1999, 103, 357.

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to 0.36 ( 0.06; therefore, the ratio of the surface concentrations of the trans and gauche conformers of ME should be 2.8 times higher than the ratio of the corresponding ν(C-S) bands. However, to calculate the ratio of SERS cross sections Tarabara et al. assumed that the intensity of the out-of-phase C-C-O stretching band at 1009 cm-1 is independent of a gauche-trans equilibrium.5 According to our observations (see the discussion concerning Figure 5), such an assumption may not be correct; therefore, at this stage we are not able to determine the actual ratio of surface concentrations of trans and gauche conformers. Influence of the Bulk Concentration of ME. Figure 2 shows dependences of the ratio of the trans and gauche ν(C-S) bands of ME monolayers formed at Ag, Au, and Cu on the bulk concentration of ME in the surrounding solution.39 The measurements were carried out after 1-2 h of soaking of the metal substrate in the thiol solution (control experiments carried out on silver with the ME bulk concentration of 10-4, 10-5, 10-6, and 10-7 M showed that monolayers soaked for 24 h exhibited the same behavior as those kept for 1 h in the diluted ME solutions). The solution was changed from high to low concentration. Tarabara et al. showed that if ME was adsorbed on a silver sol from a very diluted solution the percentage of ME adsorbed in a trans conformation dropped by about 3 times.5 This rearrangement was observed for the bulk ME concentration in the region of 10-5-10-6 M.5 On the bulk silver substrate, a similar rearrangement can be seen for the ME concentration in the region of 10-4-10-6 M (see Figure 2). For ME monolayers on copper, one can also observe pronounced structure rearrangement for an ME bulk concentration in the range of 10-3-10-4 M, whereas for monolayers formed on gold the structural changes are significantly smaller. As can be seen from Figure 2 for monolayers formed at various metals, the rearrangements clearly occur at different ME bulk concentrations. In conclusion, to maintain a high surface concentration of ME molecules in a trans conformation, the monolayers on Cu, Ag, and Au should be formed from (and kept in) 10-3, 10-4, and 10-5 M ME solutions, respectively. The differences in the isotherms of ME adsorption on different metals must be caused by differences in the metal-S bonding at the three substrates. Kinetics of Monolayer Formation and ME Desorption. Figure 3 shows the evolution of the SERS spectrum during the formation of a self-assembled ME monolayer on Ag from a 10 mM ME aqueous solution (a solution typically used in various applications). As can be seen from Figure 3 during the first 1.5 min of adsorption, the intensity ratio of the ν(C-S) trans and gauche bands continuously increased. We also followed the evolution of the SERS spectrum during 24 h, and the later spectra do (37) In principle, one could consider that in addition to oxides the copper surface is also covered with cupric hydroxide. Undoubtedly hydroxide is present at copper surfaces immersed in alkaline solutions (ref 40) although this is not thermodynamically preferred, since the equilibrium potentials for the Cu/Cu2O and Cu2O/Cu(OH)2 couples at pH 14 are -0.356 and -0.08 V versus standard hydrogen electrode, respectively (Bertocci, U.; Turner, D. R. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1974; Vol. 2, p 383). In the neutral solution, however, we do not expect the presence of hydroxide on the copper surface. (38) We were able to observe a Raman spectrum of Cu2O only on copper substrates immersed in ME solutions for a few minutes. Therefore, the possible surface concentration of remaining oxide could be rather only very low. (39) In a solution with an ME concentration of 10-8 M, a significant amount of ME could be adsorbed on the metal surface or on the glass walls of the cell. Therefore, the actual ME concentration can be significantly different from 10-8 M. Hence the results obtained in this solution should be considered only qualitatively not quantitatively.

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Figure 2. Dependences of the ratio of the trans and gauche ν(C-S) bands of ME monolayers formed at silver ((), gold (2), and copper (9) on the bulk ME concentration in the surrounding solution. The measurements were carried out after 1-2 h of soaking of the metal substrate in the thiol solution. The solution was changed from high to low concentration. The data for each presented point were calculated from 10 measurements at various points of the sample. The error bars for silver and copper show the range within all 10 experimental data laid. For gold, the error bars are given in the Supporting Information to enhance the clarity of the presentation. Inset: fragments of respective SERS spectra in the ν(C-S) region for ME adsorbed on Ag. Bulk concentration of ME: (a) 10-2, (b) 10-3, (c) 10-4, (d) 10-5, (e) 10-6, (f) 10-7, and (g) 10-8 M. Spectra are scaled and shifted for clarity.

not show significant changes. The observed increase in the relative concentration of the trans conformer can be very easily explained since ME molecules in the trans conformation can be more densely packed. From data shown in Figure 3, one can estimate the time constant of this reaction as less than 30 s. We found that chemisorption on gold is also relatively quick; however, because of significantly lower SERS enhancement factors for gold, which caused the need for larger integration times, we were unable to follow spectral changes on similarly short time scales. Formation of an ME monolayer on copper is slower, and the changes in the spectrum of adsorbed ME molecules could be observed during more than 20 min of the incubation. As one can expect,36 electrochemically activated copper electrodes, used as substrates for the thiol chemisorption, were significantly oxidized (measurements at an electrochemically roughened copper electrode revealed Raman bands at 624 and 528 cm-1 due to copper-oxygen lattice vibrations;36,40 see the Raman spectrum in the Supporting Information). The oxides must (40) Niaura, G. Electrochim. Acta 2000, 45, 3507.

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Figure 3. Temporal evolution of SERS spectra during formation of an ME monolayer on silver from a 10 mM ME aqueous solution. The layer was grown for (a) 0.5 min, (b) 1 min, (c) 1.5 min, (d) 2 min, and (e) 10 min. Spectra are scaled and shifted for clarity. The inset shows the dependence of the intensity ratio of trans and gauche ν(C-S) bands as a function of incubation time: (9) denotes data calculated from spectra presented in this figure; (0) denotes other experimental points.

be removed during the formation of the thiolate monolayer either by reduction or displacement by the thiol. We found that Raman bands due to copper-oxygen vibrations disappeared relatively slowly (the band at 528 cm-1 was still noticeable after 5 min of incubation). Moreover, we suppose that even after the “disappearance” of oxide bands (or, in other words, when we were not able to observe these bands), some oxygen species are still present on the copper surface and influence the incubation process. Figure 4 shows the temporal evolution of the SERS spectra during storage of an ME monolayer on silver in pure water.41 As can be seen from Figure 4, after no more than 1 min the ME monolayer rearranges itself and a significant number of the adsorbed ME molecules undergo a change in conformation from trans to gauche (thus the ME layer adopts an average conformation typical for an extremely low bulk ME concentration; see Figure 2). Undoubtedly, some ME molecules are removed from the surface; therefore, the additional portion of the remaining molecules easily adopt the gauche conformation, which requires more space on the metal surface (see the inset in Figure 1). After an initial fast rearrangement, the ME monolayer is relatively stable during the investigated period of 24 h (see Figure 4). A similar effect was observed for ME layers on gold and copper kept in pure water and for layers on all investigated substrates (Ag, Au, and Cu)

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Figure 4. Temporal evolution of SERS spectra during desorption of ME molecules. (a) SERS spectrum of an ME monolayer on silver measured in a 10 mM ME aqueous solution. Then, the layer was washed in pure water, transferred into another cell (ref 41), and kept for (b) 1 min, (c) 2 min, (d) 60 min, and (e) 24 h in pure water. Spectra are shifted and normalized according to the intensity of the 720 cm-1 band. Inset: fragments of the respective SERS spectra in the ν(CH2) region.

kept in solutions of various chlorides, sulfates, and perchlorates (we investigated the concentration range up to 0.1 M). An example of the dependence on time for an ME monolayer on copper kept in a 0.1 M KCl aqueous solution41 is shown in Figure 5. Most probably there is a small electrolyte effect (see the next paragraph); however, because of some irreproducibility of the trans/gauche ratios for various places on the sample, the possible electrolyte effects are below the error limit in these experiments. The investigations of monolayers formed from analogous thiols (HS-(CH2)2-NH2,42 HS-(CH2)2-COOH,43 HS(41) During these experiments, we immersed roughened pieces of metals with the geometric surface area of about 0.2 cm2 into the cell containing 20 cm3 of pure water (or an electrolyte solution). Before transfer to the cell, the metal electrodes were carefully washed for 15 s in 50 cm3 of pure water (or a proper electrolyte solution). However, because of desorption some ME appears in the solution. According to Gui et al. (Gui, J. Y.; Stern, D. A.; Frank, D. G.; Lu, F.; Zapien, D. C.; Hubbard, A. T. Langmuir 1991, 7, 955), the molecular packing density of ME on Ag(111) is 0.70 nmol cm-2 (for our rough estimations we can assume this for all used surfaces). Roughly assuming that the real surface of the electrode is 1.5 times higher than the geometric area (see the comparison of two- and three-dimensional areas of some SERS-active samples in: Compagnini, G.; Pignataro, B.; Pelligra, B. Chem. Phys. Lett. 1997, 272, 453), the total desorption of the ME monolayer would result in an ME bulk concentration of about 10-8 M. However, we decided to not carry out measurements in flow conditions, where ME molecules could be actually removed from the surrounding solution, since the procedure described above is applied in the practical applications of ME monolayers.

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Figure 5. Temporal evolution of SERS spectra during desorption of ME molecules. (a) SERS spectrum of an ME monolayer on copper measured in a 10 mM ME + 0.1 M KCl aqueous solution. Then, the layer was washed in a 0.1 M KCl aqueous solution, transferred into another cell (ref 41), and kept for (b) 2 min and (c) 24 h in a 0.1 M KCl aqueous solution. Spectra were shifted and normalized according to the total intensity (both trans and gauche) of the ν(C-S) bands. Inset: fragments of respective SERS spectra in the ν(CH2) region.

(CH2)2-SO3Na,44 and HS-(CH2)2-CH3) showed that the desorption process is significantly slower in those cases. For example, for HS-(CH2)2-NH242 or HS-(CH2)2SO3Na44 large further changes in the monolayer structure could be observed even after 1 h of soaking. For HS(CH2)2-CH3, the rearrangement is slightly faster, but even in this case about 80% of the process occurs during about 10 min (see Figure 6), whereas for ME it occurs in less than 1 min. Hence the time constant of the desorption process for ME is about 1 or 2 orders of magnitude longer than that of the analogous alkanethiols. These differences are probably caused by the difference in the access of water molecules to the sulfur group. As one might expect, the intensity ratio of the gauche and trans ν(C-S) bands correlates with the intensities of other SERS bands. For example, as can be seen from Figure 5 (the presented spectra are normalized according to the total intensity of both ν(C-S)T and ν(C-S)G bands) a higher contribution of the ν(C-S)G component causes a higher intensity in the ν(C-H) region. The change in the relative intensity of these bands is attributed to the strong dependence of the enhancement factor on the orientation (42) Kudelski, A.; Hill, W. Langmuir 1999, 15, 3162. (43) Kudelski, A. Surf. Sci. 2002, 502, 219. (44) Kudelski, A. Langmuir 2002, 18, 4741.

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Figure 6. Temporal evolution of the intensity ratio of the ν(C-S) trans (at 701 cm-1) and gauche (at 630 cm-1) bands for a PTH monolayer during storage in pure water (ref 41). An PTH monolayer was formed by immersing the metal substrate in a 10 mM aqueous solution of PTH for 2 h. (() Denotes the average ratio of ν(C-S) trans and gauche bands for a PTH monolayer immersed in a 10 mM PTH solution. (The SERS spectrum of PTH adsorbed on Ag is given in the Supporting Information.)

of adsorbed molecules versus the metal surface. The surface selection rule of SERS says that adsorbate vibrations connected to molecular scattering components perpendicular to the metal surface are more strongly enhanced.20 As can be seen from the inset in Figure 1, the C-H bonds (and the “movements” of hydrogen) of the trans conformer are nearly parallel to the surface plane; therefore, the ν(C-H) band is significantly weaker for the trans conformer. One can also notice changes in the ratio of components of the Raman band at about 2900 cm-1. This is due to various “orientations” of polarizability tensors for various components.45 Unfortunately, the Fermi resonance interaction involving HCH bending overtones is dominant in determining the shape of the Raman C-H stretching spectrum;46 therefore, the analysis of the dependence of the enhancement factor of various components on the orientation of adsorbed molecules would be very complicated and would give rather complex correlations. The change in the intensity ratio of trans and gauche ν(C-S) bands also strongly influences the relative intensity of the bands at about 400 and 1010 cm-1. The intensities of these bands are roughly proportional to the intensity of the ν(C-S)T band (see Figures 4, 5, and 7). The band at 1010 cm-1 has been ascribed to the out-ofphase stretching C-C-O vibration, and the band at 400 cm-1 to the C-C-O bending.16 Undoubtedly, for the trans conformer the C-C-O bonds and the movements of oxygen and carbon atoms during out-of-phase C-C-O stretching are nearly perpendicular to the surface plane (see inset in Figure 1). Therefore, as one could expect, this band is strong for the trans conformer. Analogous relations between Raman intensity and the relative orientation and movements of the ends of the C-C-O chain also hold for (45) Pemberton, J. E.; Bryant, M. A.; Sobocinski, R. L.; Joa, S. L. J. Phys. Chem. 1992, 96, 3776. (46) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145.

Kudelski

the band at 403 cm-1 which is due to the C-C-O bending (see Figure 5). We also noticed the correlation between the other bands (e.g., at 924 cm-1) and trans and gauche ν(C-S) bands; however, due to the unknown potential energy distributions of these vibrations, we are unable to use these correlations to predict (or confirm) changes of the ME orientation. Of course, the above-discussed correlations could also be observed when comparing SERS spectra of monolayers on different substrates (see Figure 1) and in solutions with different pHs (see the next paragraph). Electrolyte- and pH-Induced Changes. The structure of monolayers formed from some short-chain thiols was found to be rather sensitive to the surroundings. For example, a small amount of halide anions in the surrounding solution caused the rearrangement of a large part of adsorbed cysteamine (HS-CH2-CH2-NH2) molecules.47 On the other hand, some monolayers, for example, those formed from propanethiol, are significantly less sensitive to the surrounding solution.14 We found that ME layers are relatively insensitive even to the concentrated solutions of typically used salts composed of K+, Na+, and Li+ cations and anions such as Cl-, SO42-, and ClO4-. For example, for ME layers soaked in a 10 mM ME solution saturated with KCl the increase in the relative surface concentration of ME molecules adsorbed in a gauche conformation was only about 35% (see Figure 7), whereas for cysteamine monolayers the increase in the relative surface concentration of a gauche conformer was 260% in 0.1 M KCl solution.47 Such high “insensitivity” to the presence of electrolyte is very convenient for practical reasons.48 The small electrolyteinduced increase in the relative surface concentration of a gauche conformer is probably caused by adsorption of some ions of the electrolyte on the metal surface. It is possible that the space above the adsorbed inorganic ions is taken by the terminal part of the bent ME molecules. Figure 8 shows SERS spectra of ME monolayers on silver immersed in basic (with KOH) and acidic (with HCl) 10 mM ME solutions. As can be seen, in these cases the structure of the ME monolayers is significantly different from the structure of a monolayer soaked in neutral solution. In 1 M KOH solution, the ratio of trans and gauche ν(C-S) bands is equal to 5 (see Figure 8, spectrum a), whereas in a 10 mM ME solution in pure water or in ME-containing salt solutions this ratio is not higher than 3 (see Figure 8, spectrum c, or Figures 7 and 2). In acidic solutions, on the other hand, one can observe a very large increase in the relative surface concentration of a gauche conformer (see Figure 8, spectra d and e). The same effects as described above have also been observed in analogous experiments with H2SO4 and NaOH solutions. For ME monolayers on gold, we observed a similar effect in acidic solutions (i.e., an increase in the relative surface con(47) Michota, A.; Kudelski, A.; Bukowska, J. J. Raman Spectrosc. 2001, 32, 345. (48) One of the most important applications of monolayers formed from short-chain-functionalized alkanethiols is the attachment on such modified metal surfaces of many different molecules that cannot be directly attached to an unmodified metal surface ((a) Gooding, J. J.; Pugliano, L.; Hibbert, D. B.; Erokhin, P. Electrochem. Commun. 2000, 2, 217. (b) Ostuni, E.; Yan, L.; Whitesides, G. M. Colloids Surf., B 1999, 15, 3. (c) Gobi, K. V.; Mizutani, F. J. Electroanal. Chem. 2000, 484, 172.). For example, separating trapped enzymes from the metal surface very often prevents metal-induced perturbation of electronic structure and/or conformation of adsorbed enzyme molecules. In most cases, linkage layers formed from thiols are used in an electrolyte solution and the structure of the linkage layers can significantly influence the structure and properties of layers of attached molecules (see ref 5 or: Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J. Langmuir 1993, 9, 1945).

Chemisorption of 2-Mercaptoethanol

Figure 7. SERS spectra of an ME layer on silver immersed in (a) a 10 mM aqueous solution of ME, (b) 1 M KCl + 10 mM ME, and (c) a 10 mM solution of ME saturated with KCl. Spectra are scaled and shifted for clarity.

centration of a gauche conformer). As can be seen from Figure 9, this effect is smaller than on silver although it is undoubtedly statistically important (we compared a series of 20 spectra measured at various points of the metal substrates). For ME monolayers on gold soaked in KOH solutions, we have not noticed any effect larger than the possible experimental error. In both HCl and KOH solutions, ME monolayers on copper showed a large pH dependence of the monolayer structure (see inset in Figure 9), similar to the dependence reported above for monolayers formed on silver. The comparison of the influence of acids and bases on the structure of ME monolayers (compare Figures 8 and 9) shows that the ME monolayers on gold are significantly less sensitive to the surroundings (and to the ME bulk concentration; see Figure 2) than the monolayers formed on silver and copper. It is worth emphasizing that the decrease in the intensity ratio of trans and gauche ν(C-S) bands is followed by a large increase in the intensity of the ν(C-H) band (see inset in Figure 8), as would be expected from the surface selection rules (see the discussion at the end of the previous section). The above-described effects must be caused by H+ and OH- ions since both K+ and Cl- ions in the neutral salt do not greatly influence the monolayer structure (see Figure 7). In basic solutions, the most probable explanation of the observed changes is deprotonation of the hydroxyl groups49,50 and formation of a more ordered structure by the terminal groups. The dissociation of a significant

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Figure 8. SERS spectra of ME monolayers on silver immersed in (a) 10 mM ME + 1 M KOH, (b) 10 mM ME + 0.1 M KOH, (c) 10 mM ME, (d) 10 mM ME + 0.1 M HCl, and (e) 10 mM ME + 1 M HCl solutions. Spectra are shifted and normalized according to the intensity of the 724 cm-1 band. Inset: fragments of the respective SERS spectra in the ν(C-H) region. The spectra presented in the inset are shifted but not additionally scaled (they are still normalized according to the intensity of the 724 cm-1 band).

amount of hydroxyl groups followed by the electrostatical attachment of cations to the -O- groups can result in a more uniform saltlike structure of the molecules’ terminal groups (in our case -OH/-O-), resembling the structure of the crystal. The other possibility is a rearrangement of the terminal groups due to rearrangement of the Hbonding network caused by the dissociation of a portion of the hydroxyl groups. We suppose that formation of a more “ordered” structure by the terminal groups could facilitate ME molecules adopting a trans conformation. It is worth emphasizing that a similar correlation between molecular conformation and the dissociation of the terminal group has been observed for cysteamine42 and thioglycolic acid.51 (49) The double-layer capacitance of ME monolayers on Au(111) is constant between pH ) 3 and 9 but increases abruptly in the alkali side beyond 12. According to Kakiuchi et al. (Kakiuchi, T.; Iida, M.; Imabayashi, S.; Niki, K. Langmuir 2000, 16, 5397), this increase may correspond to the commencement of the dissociation of the -OH group. (50) The acidic dissociation constant of ethanol in dilute aqueous solution has been studied using calorimetric and spectrophotometric methods (Jandik, P.; Meites, L.; Zuman, P. J. Phys. Chem. 1983, 87, 238). In 0.96 M NaOH + 0.086 M ethanol solution, the concentration ratio of dissociated and nondissociated ethanol was determined as equal to 10-0.547. Hence 22.1% of ethanol molecules are dissociated in such conditions.

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Figure 9. SERS spectra of an ME monolayer formed on gold. The monolayer was kept in (a) 10 mM ME, (b) 10 mM ME + 1 M KCl, (c) 10 mM ME + 1 M KOH, and (d) 10 mM ME + 1 M HCl solutions. Spectra were scaled and shifted to enhance clarity of presentation. The inset shows SERS spectra of an ME monolayer formed on copper and kept in (R) 10 mM ME + 1 M HCl and (β) 10 mM ME + 1 M KOH solutions.

Large changes in the monolayer structure are observed in acidic solutions (see Figures 8 and 9). In aqueous solutions with pH ) 0 or 1, protonation of a significant amount of an aliphatic alcohol is rather unlikely.52 A large proportion of the ME molecules are also not destroyed in the acidic solution, since neutralization of acidified 10 mM ME solution by addition of KOH leads to the equilibrium typical for a neutral 10 mM ME solution. Therefore, the most probable explanation of the observed phenomenon is facilitation of the ME desorption (it is probable that some sulfur atoms of ME adsorbed as thiolate are protonated and some ME is desorbed as thiol molecules). If the desorption process becomes relatively easier, the surface density of adsorbed species must go down.53 A lower surface density of adsorbed ME molecules leads to a higher ratio of surface concentrations of gauche and trans conformers (see the paragraph Kinetics of Monolayer Formation and ME Desorption). To verify that the hydroxyl groups of ME do not cause this effect, similar experiments were carried out in acidic (51) Krolikowska, A.; Kudelski, A.; Michota, A.; Bukowska, J. Surf. Sci., in press. (52) Hydroxyl groups attached to the aliphatic chains can serve as both proton acceptors and proton donors. Acidity of alcohols is illustrated in comment 50. Basicity of aliphatic alcohols is, however, significantly smaller. For example, the equilibrium constant K ) ([CH3OH2+] × [H2O])/([CH3OH] × [H3O+]) of the proton exchange reaction: CH3OH + H3O+ T H2O + CH3OH2+ is equal to 8.6 × 10-3 (King, E. J. In Physical Chemistry of Organic Solvent Systems; Covington, A. K., Dickinson, T., Eds; Plenum Press: New York, 1973; p 336). Therefore, in diluted methanol solution acidified to pH ) 1 the [CH3OH2+]/[CH3OH] ratio is only equal to 1.6 × 10-5 (and 1.6 × 10-4 for the 1 M acid solution also used in this work).

Kudelski

Figure 10. SERS spectra of a PTH monolayer formed on silver. The monolayer was kept in (a) 10 mM PTH, (b) 10 mM PTH + 1 M KCl, and (c) 10 mM PTH + 1 M HCl solutions. Spectra are scaled and shifted to enhance clarity of presentation.

solutions using an unsubstituted alkanethiol with the shortest possible hydrocarbon chain. For the more detailed investigations, propanethiol (PTH) was chosen since ethanethiol (ETH) does not form self-assembled monolayers (SERS spectra of ETH and PTH adsorbed on silver are given in the Supporting Information). Figure 10 shows typical SERS spectra of PTH monolayers on silver immersed in a 10 mM PTH aqueous solution or a 10 mM PTH + 1 M KCl or 10 mM PTH + 1 M HCl solution. Due to some irreproducibility of the substrates, measurements were repeated many times. It was found that at least in 70% of the cases the ratio of trans and gauche ν(C-S) bands is in the range of 7.0-7.7 for a monolayer in contact with pure PTH solution, 6.5-7.2 when KCl is added, and 4.0-4.7 after addition of HCl. This shows that one can undoubtedly observe a similar influence of the acids for PTH as for ME monolayers. The effect for PTH is probably (53) In principle, the adsorption/desorption equilibrium could also be changed by the protonation of HS groups of ME molecules in the solution. However, the proton affinities of H2S and H2O are very similar (Delbene, J. E.; Shavitt, I. J. Phys. Chem. 1990, 94, 5514). The same holds for methanol and methanethiol (Smith, B. J.; Radom, L. J. Am. Chem. Soc. 1993, 115, 4885). Therefore, one can assume that equilibrium constants of the proton exchange reaction (ref 52) for thiols and alcohols should be of roughly the same order of magnitude. As can be seen from Figure 2, to obtain the relative surface concentration of gauche conformer that was observed in the acidic solutions, the bulk concentration of unprotonated ME has to decrease at least 4 orders of magnitude. This would mean that the base dissociation constant for thiols should be about 8 orders of magnitude higher than for analogous alcohols (see comment 52). This is clearly highly unlikely. It is also worth emphasizing that thiols are significantly stronger acids than alcohols (Burk, P.; Koppel, I. A.; Rummel, A.; Trummal, A. J. Phys. Chem. A 2000, 104, 1602).

Chemisorption of 2-Mercaptoethanol

smaller due to the lower accessibility of the sulfur end to the solvent molecules for PTH than for ME monolayers. This probably also causes differences in the monolayer stability; see the previous paragraph. To verify the hypothesis concerning acid-induced thiol desorption, we decided to investigate the temporal evolution of SERS spectra of ME monolayers soaked in acidic solutions. We found that in 0.1 M HCl or H2SO4 solution the first stage of desorption is the same as in neutral solution (very quickly, a significant number of the adsorbed ME molecules rearrange from trans into gauche conformations). However, longer soaking in acidic solution leads to further significant decrease of the SERS intensity, whereas for ME monolayers kept in pure water41 or in a salt solution, a good quality SERS spectrum can be measured even after 24 h of soaking (see Figures 4 and 5). This means that a large portion of the ME molecules are further removed from the metal surface in the acidic solution. However, we should consider these results concerning the temporal evolution of the SERS spectra of ME monolayers soaked in acidic solutions only as a suggestion, because there is also a possibility that acids catalyze oxidation of desorbed ME by traces of oxygen or other impurities. In such a case, the bulk concentration of desorbed ME would be lower in acidic solutions, which can cause additional ME desorption. (The temporal evolution of a SERS spectrum during storage of an ME monolayer on gold in HCl solution is given in the Supporting Information.) Summary 2-Mercaptoethanol was chemisorbed on silver, copper, and gold surfaces, and the structures of formed monolayers were investigated by use of surface-enhanced Raman scattering. Raman investigations revealed that, as for unsubstituted alkanethiols, the monolayers formed on silver are, for the most part, composed of molecules in the trans conformation. Immersion of an ME monolayer in a solution of very low ME concentration leads to fast significant increase in the ratio of surface concentrations of gauche and trans conformers, due to a decrease in the total surface concentration of ME molecules and rearrangement of some of the adsorbed ME molecules from the trans to the gauche conformation. The process of rearrangement of the monolayer structure (and partial desorption of ME) is exceptionally fast (time constant below 1 min) in comparison with the rearrangement of the monolayers formed from analogous thiols. This is probably due to relatively easier access of the solvent to the sulfur moiety of ME than for other thiols. For ME monolayers kept in almost ME-free solutions, the percentage of trans conformation drops significantly. To maintain a higher surface concentration of ME molecules in the trans conformation, the monolayers should be used in ME-containing solutions with the ME

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concentration of at least 10-3, 10-4, and 10-5 M for monolayers on copper, silver, and gold substrates, respectively. The structure of ME monolayers is relatively insensitive to the presence of solutions of neutral salts; however, the solutions of acids and bases significantly influence the monolayer structure. It was found that in acidic solutions the surface concentration of a gauche conformer considerably increases, since the kinetics of desorption and the desorption/adsorption equilibrium are changed. We think that desorption is facilitated because some sulfur atoms of ME adsorbed as thiolate are protonated and some ME is desorbed as thiol molecules. An analogous effect was found for monolayers formed from propanethiol. This shows that the influence of acidic solutions on the monolayer structure can also be important for monolayers formed from other (especially short-chain) thiols. It is worth emphasizing that relatively concentrated acids (e.g., 0.1 M H2SO4) are sometimes used as the medium to investigate the structure of thiol monolayers.54,55 In basic solutions, the concentration of a trans conformer increases. We suppose that more ME molecules adopt a trans conformation due to formation of a more ordered structure by the terminal groups. It is probable that if a significant number of hydroxyl groups dissociates, a more uniform saltlike structure from -OH/-O- groups is formed, resembling the structure of the crystal. The other possibility is ordering of the terminal groups due to formation of a new H-bonding network. ME monolayers formed on Ag, Au, and Cu substrates significantly differ one from another in terms of the sensitivity to the surroundings. We found that ME monolayers on Au were less easily affected, which could be important in various applications. Acknowledgment. The author expresses his gratitude to Dr. Bruno Pettinger from the Fritz Haber Institut der Max-Planck-Gesellschaft (Berlin, Germany) for making the Raman spectrometer Labram/Dilor available. Helpful discussions with Professor Jolanta Bukowska and Dr. Sarah L. Horswell are also gratefully acknowledged. This work was financially supported by KBN Grant 7 T09A 112 21. Supporting Information Available: Error bars for Figure 2, the Raman spectrum measured at a SERS-active copper electrode, the SERS spectra of ethanethiol and propanethiol adsorbed at silver, and the temporal evolution of SERS spectra during storage of an ME monolayer on gold in a HCl solution. This material is available free of charge via the Internet at http://pubs.acs.org. LA0209280 (54) Esplandiu, M. J.; Hagenstrom, H.; Kolb, D. M. Langmuir 2001, 17, 828. (55) Hagenstrom, H.; Esplandiu, M. J.; Kolb, D. M. Langmuir 2001, 17, 839.