Self-Assembled Monolayers of Cystamine and Cysteamine on Gold

the XPS experiments reveal that a shoulder on the S 2p3/2 peak (situated at 162.1 eV) develops at 161.3. eV upon increasing the adsorption time from m...
19 downloads 0 Views 107KB Size
Download PDF
6370

Langmuir 1999, 15, 6370-6378

Self-Assembled Monolayers of Cystamine and Cysteamine on Gold Studied by XPS and Voltammetry Mikael Wirde* and Ulrik Gelius Department of Physics, Uppsala University, P.O. Box 530, SE-751 21 Uppsala, Sweden

Leif Nyholm Department of Analytical Chemistry, Uppsala University, P.O. Box 531, SE-751 21 Uppsala, Sweden Received March 18, 1999 The formation of self-assembled chemisorbed layers of cystamine, cysteamine, and 4-aminothiophenol on gold has been studied by XPS and voltammetry. These compounds, often used in the preparation of biosensors and modified electrodes, are shown to yield surface coverages of approximately 80% of that of a octadecanethiol monolayer within 5 min in millimolar aqueous and ethanolic solutions. The results of the XPS experiments reveal that a shoulder on the S 2p3/2 peak (situated at 162.1 eV) develops at 161.3 eV upon increasing the adsorption time from minutes to 1 week and that the initial rate of formation of the shoulder is higher for cystamine than for cysteamine. This shoulder is believed to be due to the presence of a sulfur species with a higher coordination number with respect to gold. Increased adsorption times also give rise to increased amounts of oxidized carbon and sulfur in the films. The oxidation of the sulfur in the thiols results in a detachment of the molecules from the gold surface, as indicated by XPS experiments with different takeoff angles. The main N 1s peak for cystamine is shifted toward higher binding energies for increasing adsorption times while two prominent nitrogen peaks are generally seen for cysteamine. For cysteamine, increasing adsorption times result in an increase of the main nitrogen component at the higher binding energy, yielding an apparent shift in the nitrogen peak with time similar to that seen for cystamine. Possible explanations for these experimental findings are discussed. Cystamine, cysteamine, and 4-aminothiophenol films on gold are shown to be irreversibly oxidized in the gold oxide formation region. On the basis of evaluation of the oxidation charge, surface coverages of approximately 1 × 10-9 mol/cm2 were obtained for adsorption times between 5 min and 1 week.

Introduction The adsorption of thiols and disulfides on gold surfaces has recently attracted considerable interest, as it has been shown that such adsorption can result in the formation of well-organized self-assembled monolayers.1-7 Such monolayers may, for example, be used for the modification of the wettability and optical properties of a gold surface or as a means of immobilizing electroactive molecules or enzymes on gold electrodes. In the latter cases, cystamine and cysteamine have frequently been employed as bifunctional building blocks, where the sulfur atoms of the molecules bind to the gold surface while the amino groups may be employed for the attachment of other groups to the self-assembled thiol layer.8-20 Cystamine monolayers (1) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (2) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (3) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (4) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (5) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (6) Ulman, A. An introduction to ultrathin organic films: From Langmuir-Blodgett to self-assembly; Academic press: New York, 1991. (7) Evans, S. D.; Ulman, A. Chem. Phys. Lett. 1990, 170, 462. (8) Willner, I.; Riklin, A.; Shoham, B.; Rivenzon, D.; Katz, E. Adv. Mater. 1993, 5, 912. (9) Riklin, A.; Willner, I. Anal. Chem. 1995, 67, 4118. (10) Katz, E.; Lion-Dagan, M.; Willner, I. J. Electroanal. Chem. 1996, 408, 107. (11) Arias, F.; Godinez, L. A.; Wilson, S. R.; Kaifer, A. E.; Echegoyen, L. J. Am. Chem. Soc. 1996, 118, 6086. (12) Willner, I.; Doron, A.; Katz, E.; Levi, S. Langmuir 1996, 12, 946. (13) Hill, H. A. O.; Lawrance, G. A. J. Electroanal. Chem. 1989, 270, 309. (14) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313.

have consequently been used to immobilize, for example, microperoxidase,21,22 glutathione reductase,23 fructose dehydrogenase,24 glucose dehydrogenase apoenzyme,25 and glucose oxidase8,19 on gold electrodes. A glucose sensor based on a self-assembled monolayer of 4-aminothiophenol has also been described.26 The application of self-assembled monolayers of thiols for the design of modified electrodes and biosensors was recently reviewed by Mandler and Turyan27 and Aizawa et al.28 While self-assembled monolayers of alkanethiols containing more than 10 carbon atoms have been studied extensively with a multitude of spectroscopic and electrochemical techniques,2-7 only a limited number of studies (15) Katz, E.; Lo¨tzbeyer, T.; Schlereth, D. D.; Schuhmann, W.; Schmidt, H. L. J. Electroanal. Chem. 1994, 373, 189. (16) Katz, E.; Schlereth, D. D.; Schmidt, H. L. J. Electroanal. Chem. 1994, 367, 59. (17) Ha¨ussling, L.; Ringsdorf, H.; Schmitt, F. J.; Knoll, W. Langmuir 1991, 7, 1837. (18) Willner, I.; Riklin, A. Anal. Chem. 1994, 66, 1535. (19) Willner, I.; Heleg-Shabtai, V.; Blonder, R.; Katz, E.; Tao, G. J. Am. Chem. Soc. 1996, 118, 10321. (20) Millot, M. C.; Martin, F.; Bousquet, D.; Sebille, B.; Levy, Y. Sens. Actuators 1995, B29, 268. (21) Lo¨tzbeyer, T.; Schuhmann, W.; Katz, E.; Falter, J.; Schmidt, H. L. J. Electroanal. Chem. 1994, 377, 291. (22) Moore, A. N. J.; Katz, E.; Willner, I. J. Electroanal. Chem. 1996, 417, 189. (23) Katz, E.; Riklin, A.; Willner, I. J. Electroanal. Chem. 1993, 354, 129. (24) Kinnear, K. T.; Monbouquette, H. G. Anal. Chem. 1997, 69, 1771. (25) Katz, E.; Schlereth, D. D.; Schmidt, H. L.; Olsthoorn, A. J. J. J. Electroanal. Chem. 1994, 368, 165. (26) Kajiya, Y.; Okamoto, T.; Yoneyama, H. Chem. Lett. 1993, 2107. (27) Mandler, D.; Turyan, I. Electroanalysis 1996, 8, 207. (28) Aizawa, M.; Nishiguchi, K.; Imamura, M.; Kobatake, E.; Haruyama, T.; Ikariyama, Y. Sens. Actuators, B 1995, 24-25, 1.

10.1021/la9903245 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/02/1999

SAMs of Cystamine and Cysteamine on Gold

dedicated to self-assembled monolayers of cystamine and cysteamine can be found in the literature. One reason for this is, most likely, the observation that thiols and disulfides with short carbon chains give rise to less wellorganized monolayers compared to those obtained with the corresponding molecules having longer hydrocarbon chains.1,29 The presence of ionic functional groups (such as amines) has also been reported to destabilize selfassembled monolayers in solvents with high dielectric constants.30 Hydrogen bonding between amino groups and polar coadsorbates has, for example, been found to influence the structure of the self-assembled monolayers of mercaptododecylamine.31 Wagner et al.29 studied the adsorption of cystamine on gold surfaces using STM and ellipsometry and reported that a monolayer was formed, although the length of cystamine (ca. 4 Å) was not sufficient for adequate interchain stabilization. Caldwell et al.32 observed S 2p and N 1s lines in the XPS spectra after the adsorption of cystamine on a gold surface, indicating the presence of an adsorbed layer, but did not discuss this layer further. The adsorption of cysteamine and cystamine on gold and platinum electrodes was studied indirectly by Katz and Solovev33 by studying the cyclic voltammetry of naphthoquinones attached to the adsorbed thiol layer. It was reported that the adsorption of cystamine was considerably slower than that of cysteamine, probably due to the slow cleavage of the sulfur-sulfur bond of the disulfide. The present paper deals with a characterization of gold surfaces, coated with a self-assembled layer of cystamine, cysteamine, or 4-aminothiophenol, by means of XPS and voltammetry. The influence of the adsorption time, takeoff angle, and choice of solvent on the shape of the S 2p, N 1s, and C 1s spectra, as well as the voltammograms has been investigated. Experimental Section Cystamine dihydrochloride, (SCH2CH2NH2)2‚2HCl (>98%), cysteamine hydrochloride, HSCH2CH2NH2‚HCl (>99%), and 4-aminothiophenol, HSPhNH2 (90-95%), were obtained from Fluka (Buchs, Switzerland) and were used as received. Octadecanethiol (>98%) was supplied by Merck (Darmstadt, Germany). Ethanol (99.5%) was obtained from Kemetyl (Stockholm, Sweden). The water was of Milli-Q quality (Millipore, Bedford, MA). Chlorobenzene (>99.5%), hydrogen peroxide (>30%), ammonia (25%), and 2-propanol (p.a.) were obtained from KEBO (Stockholm, Sweden). All other chemicals were of pro analysis quality. The gold substrates, with a standard dimension of 1 cm × 1 cm, were kindly provided by Dr. Bo Liedberg at Linko¨ping University, Sweden. The gold substrates were prepared by vacuum deposition of a thin (10 Å) adhesion layer of chromium, followed by a 200 nm film of gold, onto clean glass substrates. All substrates were subsequently cleaned by ultrasonication in chlorobenzene, followed by immersion in a 5:1:1 H2O/NH3/H2O2 solution at 80 °C for 3 min. This cleaning procedure was found to be successful by comparing XPS survey spectra recorded before and after the cleaning as a reduction of the sulfur and nitrogen intensities to insignificant levels was noted. Any remaining carbon and oxygen was attributed to contamination arising from the few minutes that the sample was kept in the atmosphere prior to mounting in the XPS spectrometer for analysis. (29) Wagner, P.; Hegner, M.; Gu¨ntherodt, H. J.; Semenza, G. Langmuir 1995, 11, 3867. (30) Doblhofer, K.; Figura, J.; Fuhrhop, J. H. Langmuir 1992, 8, 1811. (31) Sprik, M.; Delamarche, E.; Michel, B.; Ro¨thlisberger, U.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 4116. (32) Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J. Langmuir 1993, 9, 1945. (33) Katz, E. Y.; Solovev, A. A. J. Electroanal. Chem. 1990, 291, 171.

Langmuir, Vol. 15, No. 19, 1999 6371 After cleaning, the substrates were rinsed with the solvent and then immediately immersed in the solutions of study. Water and ethanol were used as solvents, and three different molecules were studied, 4-aminothiophenol, cystamine, and cysteamine, with special emphasis on cystamine and cysteamine. All solutions of the compounds had a concentration of 1.2-2.9 mM and were prepared by dissolving the compounds in water. Solutions of cystamine in ethanol were additionally prepared to investigate the difference in adsorption behavior between solvents of different dipole moment. All measurements were performed ex situ. The post-treatments of the samples differed in the XPS and electrochemistry studies due to the different experimental requirements of the two techniques. For XPS, the samples were ultrasonicated with chlorobenzene and then rinsed with 2-propanol and water followed by drying with argon. To investigate the influence of the 2-propanol wash on the contamination layer, experiments were also carried out in which the 2-propanol rinse was omitted. The electrochemistry samples were rinsed and ultrasonicated in ethanol or water prior to the recording of the voltammograms. The XPS experiments were performed on a Scienta ESCA-300 spectrometer using monochromatic Al KR radiation.34 By moving the sample, nine different positions were analyzed on each sample. The pass energy was kept at 300 eV, and the entrance slit was 0.8 mm wide. The S 2p, N 1s, C 1s, and Au 4f regions were recorded at each analysis position using a takeoff angle (TOA) of 90°. Some experiments were also performed in which the O1s region was studied. To study the spatial distribution of the elements in the film, experiments were also carried out with a TOA value of 10°. The Au 4f peak at 84.0 eV was used to check the binding energy scale of the instrument, and all spectra were also referenced to this peak. A survey spectrum was recorded for each sample to assess the amount of contaminants. The S 2p and N 1s spectra were curve fitted using Voigt-like functions.35 The sulfur doublets were fitted using an intensity ratio and energy separation of 2:1 and 1.19 eV, respectively. The homogeneity of the films was evaluated by comparing XPS spectra from nine different spots on each sample. The staircase voltammetric experiments were carried out with laboratory-designed voltammetric equipment. Rinsed (1 cm × 1 cm) gold pieces or gold pieces coated with cystamine, cysteamine, or 4-aminothiophenol were used as working electrodes while the reference electrode was a saturated Ag/AgCl reference electrode. A 3 mm gold electrode was also employed as the working electrode in some experiments. The reference electrode was always used in combination with a bridge containing the supporting electrolyte. A stainless steel electrode was used as the counter electrode. The supporting electrolyte consisted of a pH 6.8 50 mM phosphate buffer.

Results and Discussion XPS. Figure 1, which presents N 1s and S 2p data for cystamine, cysteamine, and 4-aminothiophenol, provides evidence that adsorbed layers are formed on gold for all three molecules studied. The figure depicts the results after a 1 h immersion in water solutions. In the S 2p spectra, three different components can be identified. The main doublet (S1) with the 3/2-peak at 162.1 eV is attributed to a normal S-Au bond while the second component (S2), attributed to disulfide or possibly unbound sulfur,36 is located at about 163-164 eV. This component can most easily be seen as a deviation from the 2:1 ratio normally seen in the S 2p doublet and is most noticeable in the 4-aminothiophenol spectrum. On the low binding energy side (shifted about 0.8 eV to 161.3 eV), a shoulder (denoted S3) can be seen for cystamine and 4-aminothiophenol. The S3 component, which also has been (34) Gelius, U.; Wannberg, B.; Baltzer, P.; Fellner-Feldegg, H.; Carlsson, G.; Johansson, C.-G.; Larsson, J.; Mu¨nger, P.; Vegerfors, G. J. Electron Spectrosc. Relat. Phenom. 1990, 52, 747. (35) Wertheim, G. K.; Butler, M. A.; West, K. W.; Buchanan, D. N. E. Rev. Sci. Instrum. 1974, 45, 1369. (36) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083.

6372 Langmuir, Vol. 15, No. 19, 1999

Figure 1. (a) N 1s and (b) S 2p XPS spectra for self-assembled monolayers of cysteamine, cystamine, and 4-aminothiophenol (4-ATP). Adsorption time, 1 h. Solutions: 1.2 mM cysteamine in water, 1.2 mM cystamine in water, and 2.4 mM 4-ATP in ethanol. TOA ) 90°. The solid lines represent smoothed spectra. The shifted doublet seen as a shoulder on the low-bindingenergy side of the S 2p spectra has been denoted S3.

Figure 2. Influence of the adsorption time on the S 2p spectra for self-assembled layers of (a) cysteamine and (b) cystamine prepared from 1.2 mM solutions in water. TOA ) 90°. The solid lines represent smoothed spectra.

seen with monolayers of alkanethiols,37 will be discussed further below. The cysteamine N 1s spectrum in Figure 1a shows two strong components at approximately 399.2 and 400.3 eV, respectively, and a weak component at approximately 401.7 eV. The N 1s peak for cystamine at approximately 399.2 eV is asymmetric, suggesting the presence of both these higher binding energy components also for this compound. For 4-aminothiophenol, the corresponding third component seems to be absent while the second component is much reduced, resulting in essentially a single peak at approximately 399.3 eV. As will be shown below, the relative intensity of the two prominent nitrogen components for cysteamine and the position of the cystamine peak depend on the adsorption time. Figure 2 shows the S 2p region of the XPS spectra for cysteamine and cystamine as a function of adsorption time. It is seen that increasing adsorption times from 5 min to 1 week results in an increase in the total S 2p intensity as well as in the relative intensity of the S3 component. Apparently, the initial development of S3 occurs at a higher rate for cystamine than for cysteamine. As is seen from the increased intensity with time at higher binding energies than approximately 164 eV in Figure 2, increasing adsorption times also result in increased amounts of oxidized sulfur species in the films. The formation of oxidized

Wirde et al.

Figure 3. Influence of the adsorption time on the N 1s spectra for self-assembled layers of (a) cysteamine and (b) cystamine prepared from 1.2 mM solutions of cysteamine and cystamine in water, respectively. TOA ) 90°. The solid lines represent smoothed spectra.

sulfur species, such as sulfinates and sulfonates, has previously been found for self-assembled monolayers of thiols with longer chain lengths.38,39 In a study of 4-aminothiophenol SAMs, Lukkari et al.,40 for example, found approximately 16% oxygen-containing sulfur species (identified as sulfonates). As the oxidized sulfur species most likely are negatively charged, these species can be expected to remain in the film even after washing of the film due to interactions with, for example, charged amino groups. Figure 3 shows the time dependence of the N 1s peaks for cysteamine and cystamine. In agreement with the S 2p results in Figure 2 (which stem from the same experiment), the magnitude of the N 1s peaks increases with the adsorption time. Although the increase in the cysteamine nitrogen signal with the adsorption time is not as prominent as that for cystamine, it is clear that there is a corresponding effect. The increase is most likely correlated with the increase in the sulfur signal, yielding the logical conclusion that the amount of adsorbed molecules increases with adsorption time. Two other effects can also be noted in Figure 3. First, a prominent secondary peak develops on the high-binding-energy side of the N 1s structure of cysteamine when the adsorption time is increased. This results in an apparent shift in the cysteamine peak from approximately 399.2 to 400.0 eV when increasing the adsorption time from 5 min to 1 week. There is also a corresponding shift in the cystamine N 1s peak from approximately 399.1 to 400.0 eV with increasing adsorption time. To investigate the influence of the solvent on the XPS results, experiments were also carried out with ethanolic cystamine solutions using adsorption times of 5 min, 1 h, and 1 day, respectively. It was found that the results were analogous to those obtained in water solutions, indicating that the development of the S3 sulfur component and the characteristic intensity dependence of the low- and highbinding-energy nitrogen peaks on the adsorption time are present in both aqueous and ethanolic solutions. Although the sulfur S3 component can be seen for both cystamine and cysteamine, the initial rate by which this (37) Wirde, M.; Gelius, U.; Dunbar, T.; Allara, D. L. Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 131, 245. (38) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147. (39) Lee, M.-T.; Hsueh, C.-C.; Freund, M. S.; Ferguson, G. S. Langmuir 1998, 14, 6419. (40) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonqvist, T.; Kankare, J. Langmuir 1998, 14, 1705.

SAMs of Cystamine and Cysteamine on Gold

component develops differs for the two compounds (see Figure 2). This was most clearly seen when studying the values for the normalized area of the S3 component as a function of adsorption time. For an adsorption time of 5 min, the S3 component for cystamine in water corresponded to approximately 17% of the total sulfur area while the corresponding value for cysteamine was approximately 11%. After 1 h, the corresponding values were approximately 28% and 11% compared to approximately 39% and 22% after adsorption for 1 day. After 1 week, the relative contribution of the S3 component was, however, found to be 40% for both cystamine and cysteamine. The latter values show that the same relative amount of the S3 component was obtained for both compounds at sufficiently long adsorption times. For cystamine, there was no significant difference between the time dependence of the intensity of S3 for adsorption from water and ethanolic solutions, again indicating that the nature of the solvent was of minor importance for the development of S3. As mentioned in the Experimental Section, the homogeneity of all films discussed in this work was evaluated by comparing XPS spectra from nine different spots on each sample. No heterogeneity of any of the films was detected with this method. Contamination of the samples was found in the form of oxygen and excess carbon, contaminants normally seen on hydrophilic samples exposed to air. It has, for example, been found that adsorption of cystamine to form a SAM leads to a hydrophilic surface with the amino end groups covered with coadsorbates.29 It has also been reported that polar self-assembled monolayers are always coated with a water film, although this effect was found to be small for aminoterminated monolayers.31 Since cystamine and cysteamine were obtained as salts with chloride as the counterion, special attention was paid to the Cl 2p region in the survey spectra. As no chlorine was seen in these spectra, these ions were assumed to have been washed away during the post-treatment rinsing. The 2-propanol wash used in the present study was not found to introduce significantly increased amounts of contamination carbon on the gold surfaces. Trace amounts (normally less than 3% of a monolayer) of copper were seen on some samples. Thiols are known to adsorb on copper surfaces,41,42 and copper ions are generally present as an impurity in solutions of thiols and disulfides. There was, however, no correlation between the copper content and the surface coverage and adsorption time dependencies described in this work or between the copper content and the nature of the solvent. The cystamine C 1s spectrum as a function of the adsorption time is shown in Figure 4. The large peak at approximately 285 eV was also seen for the cleaned gold surfaces which had not been subjected to the solutions containing cystamine, cysteamine, or 4-aminothiophenol and hence most likely stems from the usual adventitious carbon contamination of the gold surface. The use of increased adsorption times results in the development of a shoulder on the large peak at a binding energy of approximately 286.5 eV, as well as an increased intensity for the components with binding energies up to approximately 290 eV. As more cystamine is adsorbed on the gold surface for longer adsorption times, it is reasonable to assume that the increase in the amount of the components with binding energies larger than approximately 286 eV is correlated with the increase in the amount (41) Freeman, T. L.; Evans, S. D.; Ulman, A. Langmuir 1995, 11, 4411. (42) 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.

Langmuir, Vol. 15, No. 19, 1999 6373

Figure 4. Influence of the adsorption time on the C 1s spectra for self-assembled layers of cystamine prepared from 2.7 mM solutions of cystamine in water. TOA ) 90°. The solid lines represent smoothed spectra.

of cystamine in the film. One possible species contributing to this increase could be hydrogen carbonate present in the film as a counterion to the charged amino groups. Another explanation for the increase in this binding energy region could be the formation of carbamate as a result of a reaction between carbon dioxide and the cystamine amino groups.31,43-45 The extent of such a reaction is, however, difficult to predict given the lack of information available on this kind of reaction involving amino-groupterminated self-assembled monolayers and the stability of such carbamates during XPS measurements. Sprik et al.,31 for example, studied amino-terminated SAMs obtained from mercaptododecylamine disulfide by STM, assuming that their STM images were not dominated by carbamates. The O 1s spectrum for cystamine obtained with an adsorption time of 1 week featured two main peaks at approximately 531.7 and 533.1 eV, respectively. While the former component was ascribed to hydrogen carbonate and/or sulfonate, the latter component was most likely due to contamination from, for example, water. In an attempt to try to obtain information about the positioning of the different elements in a cystamine film, experiments were performed with two different TOA values for different adsorption times. The results of some of these experiments can be found in Figure 5. For sulfur, there was a significant decrease in the total intensity when using a TOA value of 10° compared to 90°. The smaller corresponding decreases for nitrogen and carbon suggest that the majority of sulfur atoms in the film are located closer to the gold surface than the nitrogen and carbon atoms. This is in agreement with an adsorption of the cystamine molecules with the thiol group attached to the gold surface and the amino groups pointing away from the gold surface. The 10° and 90° spectra for nitrogen and carbon in Figure 5 show that the carbon atoms are mainly located on top of the nitrogen atoms, as would be expected if the majority of carbon atoms are situated in a contamination layer. (43) Caplow, M. J. Am. Chem. Soc. 1968, 90, 6795. (44) Chen, J.-G.; Sandberg, M.; Weber, S. G. J. Am. Chem. Soc. 1993, 115, 7343. (45) Versteeg, G. F.; Van Dijck, L. A. J.; Van Swaaij, W. P. M. Chem. Eng. Commun. 1996, 144, 113.

6374 Langmuir, Vol. 15, No. 19, 1999

Wirde et al. Table 1. Normalized Intensity Ratios Calculated from Cystamine Spectra Obtained for Different Takeoff Angles and Adsorption Times Using a 2.7 mM Solution of Cystamine in Watera time 5 min 5 min 1h 1h 24 h 24 h 1 week 1 week

TOA S/Au S*/Au S/N 90° 10° 90° 10° 90° 10° 90° 10°

0.034 0.16 0.041 0.22 0.047 0.28 0.051 0.33

0.032 0.14 0.039 0.19 0.042 0.20 0.046 0.24

0.90 0.73 0.87 0.75 0.90 0.79 0.74 0.65

S/C

N/C

O/S

O/N O/C

0.10 0.06 0.11 0.06 0.11 0.07 0.10 0.06

0.11 0.09 0.13 0.08 0.12 0.09 0.14 0.10

1.97 2.28 1.94 2.44 2.40 2.94 3.26 3.83

1.78 1.68 1.68 1.82 2.15 2.33 2.40 2.49

0.20 0.15 0.21 0.15 0.27 0.20 0.33 0.25

a The S* values were obtained by subtracting the contribution from the oxidized sulfur species from the total sulfur areas.

Figure 5. (a) N 1s, (b) C 1s, and (c) S 2p XPS spectra obtained with TOA ) 90° and 10°, respectively, for a self-assembled monolayer of cystamine: 2.7 mM cystamine solution in water; adsorption time, 1 week.

To obtain values of the surface coverages and to assess the extent of the contamination, a quantitative analysis was performed using the areas of the O 1s, N 1s, C 1s, and S 2p lines. In the theoretical calculations, it was assumed that the cystamine molecules were adsorbed as all-trans thiolates with the same molecular geometry as that of a regular octadecanethiol (i.e. C18) SAM. Although this is not generally true, gauche defects are not expected to produce any significant changes in the calculations below. The expression for the XPS intensity from a certain core electron in atom A is given by

IA )

(

FA dσA sin θ dΩ

NkχA,K + K-1

∑

k)1

(∑

))

K-1

∆xn(τ)

n)k

λA,n sin θ

NkχA,k exp -

(1)

for a film with K layers. FA is a common intensity factor arising from, for example, instrument parameters, Nk is the atomic density in each layer of the film, dσA/dΩ is the differential photoelectric cross section of the core electron, and θ is the TOA. The parameter χΑ,k is the stoichiometric fraction of atom A in layer k. χΑ,k is unity for each layer containing only A atoms and zero for the other layers. Furthermore, ∆xn(τ) is the distance between two neighboring layers n and n + 1 while λΑ,n is the electron attenuation length in layer n. The tilt angle of the chains is given by τ. To simplify the calculations, it was assumed that there is no variation of Nk and λΑ,n through the film (i.e. Nk ) N and λΑ,n ) λΑ for all k and n, respectively). The product FA(dσA/dΩ) is treated by introducing a kind of sensitivity factor (SA), which is assumed to be proportional to FA(dσA/dΩ). It is convenient to introduce the normalized intensity, given by IAnorm ) IA/SA. The ratio of the normalized intensities IAnorm and IBnorm is then denoted RA/B. RA/B more closely reflects the total stoichiometric ratios χΑ/χΒ but still has to be corrected for the effect of the electron attenuation. The theoretical calculations were performed only for θ ) 90°. Values of λA were calculated from the empirical parametrization given by Laibinis et al.,46 giving λO1s ) 30 Å, λN1s ) 33 Å, λC1s ) 35 Å, and λS2p ) 38 Å for a closely packed monolayer. The layer distances were chosen from a generally accepted model where the alkyl chains tilt at (46) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017.

τ ) 30°, giving ∆xn(30°) ) 1.10 Å.47,48 A cystamine film would then yield RS/N ) 0.91 and RS/C ) 0.48 due to the attenuation of the S 2p electrons through the film. Table 1 shows a number of normalized intensity ratios for cystamine. These ratios were obtained by using sensitivity factors equal to the cross sections given by Scofield.49 The use of these values as sensitivity factors for the elements of interest here is based on our long experience of analyzing various organic molecules, and they usually give errors below 10%. In accordance with the calculation, the experimental S/N normalized intensity ratios for TOA ) 90° are close to 0.9 for all adsorption times, except for 1 week, where this ratio is 0.74. At a TOA of 10°, the values are around 0.75. These results clearly show that the cystamine molecules adsorb with the amino groups above the thiolate head groups. However, the S/C ratios are around 0.1, much lower than the expected RS/C value of 0.48, suggesting the aforementioned presence of a carbon-containing contamination layer. The presence of oxygen contamination is also clear from Table 1. The general trend in the O/S, O/N, and O/C ratios is that the oxygen concentration increases with adsorption time. The adsorption time dependence of the C 1s spectra (see Figure 4) indicates that this increase in the oxygen content in the film is mainly due to an increase in the concentration of molecules containing both oxygen and carbon, such as hydrogen carbonate, as already discussed. The oxidation of cystamine to yield oxidized species such as sulfinates and sulfonates will also result in an increase in the oxygen concentration in the film. Another, less likely, explanation could be the formation of carbamates. The O/C ratios for 10° are lower than those for 90°, whereas both the O/S and O/N ratios are higher at 10°, indicating that the oxygen is located below the majority of the carbon but above the cystamine film. Simulating the hydrocarbon contamination by adding six hydrocarbon layers to the theoretical model and two to three oxygen layers between the cystamine monolayer and the hydrocarbon layers yields an RS/C of 0.10, well in agreement with Table 1. In addition, RO/S was calculated to be 2.18 and 3.30 with a model containing two and three oxygen layers, respectively. This is in fair agreement with the experimental values. The increase in the S/Au ratio with adsorption time clearly shows that a longer adsorption time results in a higher surface coverage of cystamine. Assuming that a full monolayer was achieved for an adsorption time of 1 week, the 90° S/Au values indicate a surface coverage of (47) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (48) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (49) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1979, 8, 129.

SAMs of Cystamine and Cysteamine on Gold

Figure 6. Comparison of cystamine S 2p XPS spectra obtained with adsorption times of 5 min and 1 week (dotted lines) with that of an octadecanethiol monolayer obtained with the adsorption time 2 days (solid line). Solutions: 2.7 mM cystamine solution in water and 1.0 mM octadecanethiol in ethanol, respectively. TOA ) 90°.

approximately 70% after an adsorption time of only 5 min. It is also seen that the amount of oxidized sulfur species in the film increases with the adsorption times. This trend is particularly clear for TOA ) 10°. The surface coverages can also be compared to that of a well-defined self-assembled monolayer of octadecanethiol on gold. Using eq 1 and assuming the contamination layers described above for cystamine and a contaminationfree film for the C18 thiol, the S 2p intensity of a cystamine monolayer is expected to be about 20% higher (22% and 19% for two and three oxygen layers, respectively) than that for the C18 thiol monolayer. Figure 6 shows smoothed spectra obtained for a cystamine and octadecanethiol SAM, respectively, scaled to make the gold background equal for both spectra. After 5 min adsorption, before any noticeable amount of S3 can be seen, the S 2p intensities appear practically equal for both molecules. This indicates an initial lower coverage for cystamine than that for a well-established monolayer of the C18 thiol. A corresponding comparison for the 1 week cystamine sample, on the other hand, shows that the cystamine total sulfur intensity is higher than that of the C18 thiol. This difference is mainly due to the development of the S3 component at lower binding energies and the increased amounts of oxidized sulfur species in the film. On the basis of the results presented in Figure 6 and the expected 20% higher intensity for a cystamine film compared to that of the C18 thiol, it seems reasonable to conclude that the surface coverage for cystamine after adsorption for 1 week is closer to that of a single monolayer than that of a bilayer or a multilayer. The finding that a relatively high surface coverage (i.e. 70% of the 1 week coverage or 80% of that of a octadecanethiol monolayer) is reached after a relatively short adsorption time (i.e. 5 min) while the attainment of a full monolayer requires much longer adsorption times is in general agreement with results obtained for other thiols.2,50,51 Hu and Bard50 found a surface coverage of 60% within 15 min for a carboxylic-acid-terminated thiol while a full surface coverage was found after 2-3 h. The rate (50) Hu, K.; Bard, A. J. Langmuir 1998, 14, 4790. (51) Pan, W.; Durning, C. J.; Turro, N. J. Langmuir 1996, 12, 4469.

Langmuir, Vol. 15, No. 19, 1999 6375

of the adsorption of the charged thiol was proposed to be slower than that for the adsorption of alkanethiols due to repulsive interactions. Two separate time domains, including a fast and a slow process, were also reported by Pan et al.51 for the adsorption of 1-dodecanethiol and 11mercaptoundecanol on gold. Further analysis of the XPS spectra recorded at TOA values of 10° and 90° from the self-assembled cystamine layer indicate that the relative contribution from the S3 component to the total sulfur intensity was very similar for the two angles. This suggests that the S3 sulfur atoms are present at the gold surface. The components corresponding to oxidized sulfur were, on the other hand, situated further away from the gold surface, as the relative contribution from these components to the total sulfur intensity was about twice as large for a TOA value of 10° than for 90°. This implies that the oxidized sulfur species are no longer directly attached to the gold surface but are situated above the nitrogens in the film, below the hydrocarbon contamination layer. Our results are hence in agreement with those reported by Lee et al.,39 who recently showed that the oxidized sulfur species readily desorb from the gold surface. For nitrogen, the increase in the N 1s 90° intensity with formation time was more pronounced than that for 10°, in agreement with the hypothesis that the nitrogen atoms are situated below a carbon-containing contamination layer. The results also show that the nitrogen atoms are positioned above the sulfur atoms for all adsorption times. The intensity for the nitrogen atoms corresponding to a binding energy at about 400 eV was seen to increase with the adsorption time for both TOA values (see also Figure 3). The relative contribution of the high-energy nitrogen component compared that of to the low-energy component was, however, approximately 20% higher for a TOA value of 10° than for 90° for all the cystamine adsorption times. This suggests that the different nitrogen components are due to different orientations of the nitrogen atoms in the film and that the higher binding energy components are located somewhat further away from the gold surface than the components corresponding to lower binding energies. The relative intensities of the carbon components with binding energies higher than approximately 286 eV were very similar for TOAs of 10 and 90° for all the cystamine adsorption times. In both cases, the relative intensity increased with adsorption time. As the cystamine carbons should be situated at a binding energy of approximately 286 eV, it is reasonable to assume that the increase in the relative intensity of the components with binding energies higher than approximately 286 eV was correlated with the increasing surface coverage of cystamine. The interpretation of the carbon spectra is, however, complicated by the fact that the majority of the carbon atoms in the film clearly are due to contamination. For oxygen, it was found that the relative intensity of the component with a binding energy of approximately 533.1 eV was approximately 20% higher for a TOA value of 10° than for 90° which suggests that these oxygens are situated in the lower region of the contamination layer, as assumed in the estimation of the surface coverage for cystamine. The results also indicate that the oxygen component at approximately 531.7 eV, which probably is due to carbonate and/or sulfonate, corresponds to oxygens present slightly closer to the cystamine layer. One of the key findings of this study is the development of the S3 sulfur species on the low-binding-energy side of the S 2p doublet with increasing adsorption times. The experimental results indicate that this species is situated close to the gold surface and that its initial rate of

6376 Langmuir, Vol. 15, No. 19, 1999

development is higher for cystamine than for cysteamine. The estimation of the surface coverages for cystamine shows that formation of a bilayer or multilayer of cystamine is unlikely under the present experimental conditions. Such unbound sulfur species would also have binding energies higher than that for the S1 sulfur component36 in Figure 1. Moreover, the relative intensity of such species would increase when going from a TOA value of 90° to 10°. According to the S 2p curve fits, the S1 intensity appears to decrease with time along with the increase of the S3 component, which suggests a reorganization of the monolayer. As there is a concurrent increase in the total S 2p area, the S3 peak can, however, not entirely be attributed to a redistribution within the film. This, however, requires the presence of different binding sites or adsorption orientations (e.g. molecules either standing up or lying down flat on the surface), possibly in combination with a reorganization of the top gold layer. The shift of the S3 component is about 0.8 eV. Previous reports of sulfur adsorbed on gold have indicated shifts of 0.2-0.3 eV for the difference between terrace and step sites.52 The experimentally found value of approximately 0.8 eV thus seems too large to allow the S3 component to be attributed to differences in binding sites. Given the low binding energy and angular dependence of the S3 component, we propose that the S3 species corresponds to sulfur atoms with a higher degree of coordination to gold atoms than that for the initially adsorbed thiols. These species may appear, for example, as a result of a reorganization of the gold surface with time. Our data suggest that such a reorganization of the gold surface is required to allow a surface coverage corresponding to that for, for example, a C18 thiol monolayer. It is possible that this phenomenon is related to the findings that approximately 25% of one monolayer of gold can be lost during electrochemical experiments in the presence of cysteine.53 The latter loss was found to be correlated with the reduction of surface gold oxide and was claimed to be due to formation of strong coordination bonds between cysteine and Au(I) formed as an intermediate product in the reduction of AuO to Au. The faster development of S3 for cystamine than for cysteamine found in the present study suggests that such a restructuring process may benefit from the presence of two closely positioned thiol groups on the gold surface. As each cystamine molecule gives rise to two thiols after the breakage of the S-S bond, the probability of having two sulfur atoms positioned relatively close to each other on the gold surface should be higher for cystamine than for cysteamine. Sprik et al.31 have proposed that the adsorption kinetics of depositing disulfides on gold prior to the breaking of the S-S bond might favor a particular modulation of the basic hexagonal structure of the layer. Another key finding of the present work is the shift in the nitrogen peak with increasing adsorption times seen for cystamine and cysteamine. For cysteamine, this is seen as a gradual exchange of intensity between the two main components, while for cystamine the nitrogen peak is shifted in the high-binding-energy direction. Our results indicate that these effects are unlikely to be due to a loosely bound layer of molecules on top of the monolayer or a process involving an initial adsorption of molecules lying down on the gold surface. Although there are oxidized sulfur species in the film which most likely still have their amino groups attached, the amounts of these species are (52) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (53) Tu¨do¨s, A. J.; Johnson, D. C. Anal. Chem. 1995, 67, 557.

Wirde et al.

too small to offer a direct explanation of the changes seen in the nitrogen spectra. Another explanation which we have considered and discarded is the presence of both uncharged and charged amino groups in the film, as suggested by Lukkari et al.40 It is, however, not immediately clear to us why the amount of charged amino groups in the film would depend on the adsorption time. Adopting this model, our results would, for example, imply an almost complete conversion of initially uncharged amino groups to charged amino groups for cystamine with increasing adsorption time. The similarity found between the nitrogen binding energies for 4-aminothiophenol and cystamine, two compounds with pKa values as different as 4.3 and 10.8, respectively,10 also makes this explanation less likely, although it should be stressed that these pKa values do not refer to surfaceconfined molecules. The pKa of 4-aminothiophenol adsorbed on gold has been determined to be 6.9 ( 0.5 by differential capacitance measurements.54 Analogously, Hu and Bard50 found that surface-confined carboxyl acid groups had a surface pKa of 7.7 and were fully dissociated first at a pH near 10. On the basis of the present results, a more likely explanation for the dependence of the nitrogen spectra on the adsorption time could involve surface coverage (and thus adsorption time) dependent interactions between the amino group nitrogens and the surrounding atoms. These changes in the interactions with time could be coupled to the increasing amounts of oxygen, oxidized carbon, and sulfur species in the film and/or the reorganization of the gold surface, as reflected from the S3 sulfur component, with increasing adsorption time. Formation of carbamate as a result of a reaction between carbon dioxide and the amino groups may also contribute to this effect. It has been reported29 that adsorption of cystamine to form a SAM leads to a hydrophilic surface with amino end groups covered with coadsorbates which most likely yield hydrogen-bonding-induced surface reconstructions. It seems likely that the probability for such a reorganization of the film would depend on the surface coverage. This likelihood probably also depends on the nature of the ionic functional group, as bulky ionic groups have been reported to yield a roughened surface in which the ionic centers no longer are located in one plane parallel to the metallic substrate.30 It has been reported31 that the aggregation of hydrogenbonded clusters at the outer surface of an aminoterminated monolayer is a very slow process that continues after the tilted monolayer of alkyl chains has formed and that the only noticeable effect on the alkyl layer is a modest increase in the number of gauche defects for the first few carbon bonds attached to the functional group. Electrochemistry. The electrochemistry of cystamine was studied in a 50 mM aqueous phosphate buffer of pH 6.8 by recording cyclic voltammograms in the potential region between +1500 and -1000 mV, using a 3 mm gold electrode and a 2.4 mM cystamine solution. In addition to the irreversible reduction of cystamine at about -900 mV previously described,11,33 the present experimental results confirmed that cystamine is oxidized in the gold oxide formation potential region, in agreement with previous findings.55 Similar results were also obtained after washing and transferring the electrode, previously exposed to an aqueous 50 mM cystamine solution for 1 h, to a pure buffer solution (see Figure 7) and for approximately 1 cm2 gold pieces that had been exposed to a 1.17 mM cystamine solution in water for 1 h and 1 day, respectively, as well as for 1 week. (54) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385. (55) Owens, G. S.; LaCourse, W. R. Curr. Sep. 1996, 14, 82.

SAMs of Cystamine and Cysteamine on Gold

Langmuir, Vol. 15, No. 19, 1999 6377 Table 2. Estimated Surface Coverages Obtained by Integration of the Cyclic Voltammetric Oxidation Peaks Recorded with the Modified Gold Electrodesa Surface coverage/(mol/cm2) adsorption 4-aminothiophenol cysteamine cystamine cystamine time (ethanol) (water) (water) (ethanol) 5 min 1h 24 h 1 week

n.d. 9.9 × 10-10 1.6 × 10-9 n.d.

1 × 10-9 6 × 10-10 9 × 10-10 n.d.

n.d. 9 × 10-10 1 × 10-9 1 × 10-9

n.d. 8 × 10-10 1 × 10-9 n.d.

a n.d. ) not determined. Solutions: 2.4 mM 4-aminothiophenol in ethanol, 1.2 mM cysteamine in water, 1.2 mM cystamine in water, and 2.4 mM cystamine in ethanol, respectively.

Figure 7. Cyclic voltammograms recorded in a 50 mM phosphate buffer of pH 6.8 for a 3 mm gold electrode prior to (dotted line) and after (solid line) exposure to a 50 mM aqueous solution of cystamine in water for 1 h. The scan direction was positive, and the scans were initiated at a potential of 0 mV versus Ag/AgCl.

on the basis of the charge associated with the oxidation of the adsorbed layers. Such estimations of the surface coverages were hence made on the basis of the difference between the first and second cycle oxidation charges in the potential region from +400 to 1500 mV, assuming a 3e- oxidation48 of the cystamine and cysteamine films and a 2e- oxidation of the 4-aminothiophenol layer,58,59 respectively. An alternative oxidation mechanism for 4-aminothiophenol, involving a 1e-, 1H+ reaction for pH values lower than that used in the present study, has been proposed by Lukkari et al.40 In the present evaluations, the second cycle oxidation charges were always found to match those of the corresponding gold oxide reduction peaks, confirming that the second cycle oxidation charge may be used to compensate for the charge due to the formation of gold oxide. In these estimations, no correction for the surface roughness of the gold electrodes was made. As seen in Table 2, the results show that surface coverages of approximately 1 × 10-9 mol/cm2 were obtained on the 1 cm × 1 cm gold pieces upon their exposure to millimolar solutions of 4-aminothiophenol, cysteamine, and cystamine for adsorption times between 5 min and 1 week. The value of about 1 × 10-9 mol/cm2 experimentally found for cystamine can be compared with that of 8 × 10-10 mol/cm2 reported for a perfectly packed cystamine monolayer.11 Unlike the XPS results, the values in Table 2 do not seem to indicate any significant dependence of the adsorbed amount on the adsorption time. This dependence is most likely masked by the uncertainties associated with the voltammetric estimations of the surface coverages. As also seen in Table 2, there was no significant difference between the results obtained for cysteamine and cystamine, or for cystamine dissolved in water and ethanol, respectively.

In all cases, the first cycle oxidation charge in the gold oxide region was significantly larger than the second cycle charge while the gold reduction peak areas for the first and second cycles were approximately equal. The latter indicates an irreversible oxidation of the self-assembled monolayer of cystamine on the first scan. This is also supported by the finding that the size of the gold oxide reduction peak at approximately +460 mV was practically identical to that obtained in the pure buffer solution with a clean gold electrode (see Figure 7). There was also no significant difference between the second cycle responses obtained with a cystamine-coated electrode and the responses obtained with a clean gold electrode. The oxidation of the cystamine layer probably involves an oxidation of the SH groups of the thiols, formed upon the adsorption of cystamine, to yield either SO3- ions52 or SO2groups,56 which desorb from the gold surface. The voltammetric behavior of cystamine films formed from ethanolic solutions was very similar to that for the films in aqueous solutions. There was also no difference in the electrochemical response for films formed by the exposure of gold pieces to solutions containing cystamine or cysteamine. The latter finding, which indicates that the same adsorbed species is formed in both cases, is not unexpected, as the adsorption of cystamine on gold has been reported to involve a rupture of the disulfide bond,29,33 in full agreement with the present XPS results. Gold pieces exposed to a solution of 2.4 mM 4-aminothiophenol in ethanol for 1 h and 1 day, respectively, also gave rise to voltammograms with larger first cycle oxidation peak charges in analogy with the cystamine and cysteamine experiments. In this case, the first cycle oxidation was, however, initiated at more positive potentials (i.e. at approximately +800 mV) than those for cystamine (or cysteamine)-modified electrodes. Unlike cystamine, the oxidation of 4-aminothiophenol has been found to involve an oxidation of the amino group rather than the thiol group.40,57 The oxidation has been reported to involve an electrochemical-chemical-electrochemical (ECE) reaction including an oxidation of the amino group to a radical cation that undergoes a dimerization and hydrolysis to give rise to quinonic species.58 The amount of cystamine, cysteamine, and 4-aminothiophenol adsorbed on a gold electrode can be estimated

The XPS and voltammetric results verify that an adsorbed layer of cystamine, cysteamine, and 4-aminothiophenol is formed on gold surfaces. An approximate surface coverage of 80% of that for a C18 thiol monolayer was found for cystamine after an adsorption time of only 5 mins. Longer adsorption times were found to result in higher surface coverages approaching that of the C18 thiol monolayer and also gave rise to changes in the XPS spectra. For both cystamine and cysteamine, a shoulder on the 162.1 eV S 2p3/2 peak developed at 161.3 eV upon increasing the adsorption time from 5 minutes to 1 week. The initial rate of formation of this shoulder, which reached a relative intensity of 40% after a 1 week adsorption, was higher for cystamine than for cysteamine. The latter could indicate that the development of the shoulder is coupled

(56) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (57) Mohri, N.; Matsushita, S.; Inoue, M. Langmuir 1998, 14, 2343.

(58) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688. (59) Parker, V. D. In Organic Electrochemistry; Baizer, M. M., Ed.; Marcel Dekker: New York, 1973; Chapter XIV.

Conclusions

6378 Langmuir, Vol. 15, No. 19, 1999

to the probability of having two sulfur atoms positioned close to each other on the gold surface. It is suggested that the 161.3 eV S 2p3/2 peak is due to sulfur atoms in the gold surface having a higher coordination number than normal thiolates, for example, as a result of a reorganization of the gold surface. For increased adsorption times, changes were also seen in the N 1s region for both cystamine and cysteamine. The relative intensity of the high-bindingenergy N 1s component for cysteamine increased while the N 1s peak for cystamine was shifted toward higher binding energies. Results from, for example, XPS measurements with different takeoff angles suggest that these changes in the N 1s region for cystamine and cysteamine could reflect changes in the interactions between the nitrogen atoms and surrounding atoms as a result of an increased surface coverage and/or increased concentrations of oxygen, oxidized carbon, and sulfur species in the film. It can also be concluded that increased adsorption times result in increased concentrations of oxidized sulfur

Wirde et al.

species (e.g. sulfinates and sulfonates) in the film. These negatively charged species are no longer in direct contact with the gold surface and probably remain in the film as a result of electrostatic interaction with the positively charged amino groups in the film. The results of the electrochemical experiments reveal that cystamine, cysteamine, and 4-aminothiophenol films on gold are oxidized irreversibly in the gold oxide formation region and that surface coverages of approximately 1 × 10-9 mol/cm2 are obtained for adsorption times between 5 min and 1 week. Acknowledgment. Financial support from Swedish Natural Science Research Council (NFR) Grant No. K-AA/ KU 09368-320 is gratefully acknowledged. We also wish to thank Bo Liedberg, Linko¨ping, for providing us with gold substrates. LA9903245