Langmuir 1998, 14, 4795-4801
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Static Secondary Ion Mass Spectrometry Studies of Self-Assembled Monolayers: Influence of Adsorbate Chain Length and Terminal Functional Group on Rates of Photooxidation of Alkanethiols on Gold Elaine Cooper and Graham J. Leggett* Department of Materials Engineering and Materials Design, The University of Nottingham, University Park, Nottingham NG7 2RD, U.K. Received March 2, 1998. In Final Form: June 16, 1998 The rates of photooxidation of self-assembled monolayers (SAMs) of alkanethiols containing methyl, hydroxyl, and carboxylic acid terminal groups have been studied using static secondary ion mass spectrometry (SIMS). SIMS has revealed a wealth of structure-specific peaks that enable the progress of photochemical reactions within the SAM to be monitored accurately and effectively. The addition of eight methylene groups to the alkyl chain leads to a reduction in the rate of reaction by a factor of approximately 4. The nature of the adsorbate terminal group exerts a profound influence over the reactivity of the SAM, with rate constants for the photooxidation reaction being in the ratio 4:2:1 for CH3:OH:COOH, for both long and short chain alkanethiols. The reduction in the rate constant observed for SAMs with polar terminal groups is attributed to the existence of strong hydrogen bonding interactions that stabilize the SAM, either by increasing adsorbate order or by reducing the rate of diffusion of active oxygen species to the S-Au interface. Tail group interactions may be more generally important in determining the structures and reactivities of SAMs.
Introduction Self-assembled monolayers offer remarkable prospects for the production of novel organic thin film materials for applications in fields ranging from medicine1-7 to microelectronic device fabrication.8-13 In many of these applications, the spatial control of SAM composition is an important desideratum, and a range of techniques has been developed for the fabrication of micron-scale and even nanoscale patterns in SAMs. Of particular importance are the techniques of microcontact printing, developed by Whitesides and co-workers,9,14-18 and photoli* Corresponding author. Now at: Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, U.K. E-mail:
[email protected]. (1) Lopez, G. P.; Albers, M. W.; Schreiber, S. L.; Carroll, R.; Peralta, E.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 5877. (2) Singhvi, R.; Kumar, A.; Lopez, G. P.; Stephanopoulos, G. N.; Wang, D. I. C.; Whitesides, G. M.; Ingber, D. E. Science 1994, 264, 696. (3) Hickman, J. J.; Bhatia, S. K.; Quong, J. N.; Shoen, P.; Stenger, D. A.; Pike, C.; Cotman, C. W. J. Vac. Sci. Technol. A 1994, 12, 607. (4) Mrksich, M.; Whitesides, G. M. Annu. Rev. Biophys. Biomol. Struct. 1996, 25, 55. (5) Mrksich, M.; Dike, L. E.; Tien, J.; Ingber, D. E.; Whitesides, G. M. Exp. Cell. Res. 1997, 235, 305. (6) Cooper, E.; Wiggs, R.; Hutt, D. A.; Parker, L.; Leggett, G. J.; Parker, T. L. J. Mater. Chem. 1997, 7, 435. (7) Tidwell, C. D.; Ertel, S. I.; Ratner, B. D.; Tarasevich, B.; Atre, S.; Allara, D. L. Langmuir 1997, 13, 3404. (8) Frisbie, C. D.; Martin, J. R.; Duff, R. R.; Wrighton, M. S. J. Am. Chem. Soc. 1992, 114, 4, 7142. (9) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (10) Lercel, M. J.; Tiberio, R. C.; Chapman, P. F.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol. B 1993, 11, 2823. (11) Lercel, M. J.; Redinbo, G. F.; Pardo, F. D.; Rooks, M.; Tiberio, R. C.; Simpson, P.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol. B 1994, 12, 3663. (12) Lercel, M. J.; Redinbo, G. F.; Craighead, H. G.; Sheen, C. W.; Allara, D. L. Appl. Phys. Lett. 1994, 65, 974. (13) Gardner, T. J.; Frisbie, C. D.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 6927. (14) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. Adv. Mater. 1994, 6, 600.
thography, developed by Tarlov and co-workers19,20 and Huang et al.21 Photolithographic processes have been utilized in the authors’ laboratory to create effective patterns from a range of simple thiols adsorbed onto polycrystalline gold, which may be employed to guide cellular attachment and growth.6 Detailed studies of the kinetics of photooxidation reactions are important because they enable optimization of the conditions utilized in microfabrication processes. More importantly, however, such studies offer insights into the factors that determine SAM stability and reactivity more generally. SAM stability has been an issue of some debate, because in order to exploit the commercial potential of these materials, it may be necessary that they exhibit lifetimes of days or even weeks. Oxidative reactions are likely to be a key class of degradative process, and it is therefore important to evaluate structural characteristics that may prolong the lifetimes of SAMs. We have performed detailed studies of the mechanism of photooxidation of SAMs of alkanethiols adsorbed on Au22 and Ag,23,24 using X-ray photoelectron spectroscopy (XPS),22,23 secondary ion mass spectrometry (SIMS),23 and surface-extended X-ray absorption fine structure (SEX(15) Kim, E.; Kumar, A.; Whitesides, G. M. J. Electrochem. Soc. 1995, 142, 628. (16) Wilbur, J. L.; Biebuyck, H. A.; MacDonald, J. C.; Whitesides, G. M. Langmuir 1995, 11, 1, 825. (17) Xia, Y.; Kim, E.; Whitesides, G. M. J. Electrochem. Soc. 1996, 143, 1070. (18) Mrksich, M.; Chen, C. S.; Xia, Y.; Dike, L. E.; Ingber, D. E.; Whitesides, G. M. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 10775. (19) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (20) Gillen, G.; Bennett, J.; Tarlov, M. J.; Burgess, D. R. F. Anal. Chem. 1994, 66, 2170. (21) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (22) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657. (23) Hutt, D. A.; Cooper, E.; Leggett, G. J. J. Phys. Chem. B 1998, 102, 174. (24) Hutt, D. A.; Cooper, E.; Leggett, G. J. Surf. Sci. 1998, 397, 154.
S0743-7463(98)00256-X CCC: $15.00 © 1998 American Chemical Society Published on Web 07/30/1998
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AFS),24 in order not only to understand the basic processes underlying SAM photopatterning better, but also to understand the relationship between SAM structure and reactivity. It has been shown that the mechanisms of photooxidation of SAMs of alkanethiols on Au and Ag are substantially different,23,25 with the rate-determining step on Au being penetration of active oxygen species to the Au-S interface,22 while reactivity on Ag is dominated by S-C bond scission leading to the formation of inorganic sulfur species.23-25 On Au, the principal oxidation product is an alkanesulfonate,22,26 whereas on Ag, the alkanesulfonate results from reaction of approximately one-third of the monolayer with oxygen while the remaining adsorbates undergo S-C scission.23-25 These differences in reactivity are most likely the result of differences in the S-metal bond on the two substrates. Tarlov and co-workers have previously used SIMS to investigate photopatterned SAMs.19,20 However, in general, SIMS has to date been relatively little used to characterize the reactivity of SAMs. In an earlier study in the authors’ laboratory, it was shown that SIMS provided an effective and highly specific probe for structural changes induced by the photooxidation of SAMs on silver.23 In particular, SIMS exhibited much higher molecular specificity: while XPS gave only a measure of the amount of oxidized sulfur (for SAMs on Ag, a significant amount of this was in the form of inorganic oxysulfur species), SIMS provided data on the rate of formation of alkanesulfonate species specifically. Moreover, at the early stages of oxidation, the enhanced sensitivity of SIMS provides the capability of monitoring the oxidation reaction at a stage when changes in the XPS spectra are difficult to detect. In the present study, we have focused on the role of tail group interactions for adsorbates with two different chain lengths. We wished to examine the effect on the rates of photooxidation of SAMs caused by the presence of terminal groups that are capable of forming hydrogen bonds. Static secondary ion mass spectrometry (SIMS) has been employed to study the kinetics of SAM photooxidation. We have found that SIMS spectra contain highly specific species that enable photochemical reactions within the SAM to be monitored quantitatively, both easily and accurately. We have compared the reactivity of short and long chain SAMs of alkanethiols on gold that possess methyl, hydroxyl and carboxylic acid terminal groups. These studies have revealed striking variations in the rates of photooxidation of SAMs with different terminal group functionalities but similar molecular lengths, indicating that tail group interactions may have a profound effect on the stabilities and reactivites of self-assembled monolayers. Experimental Section SAMs were prepared by immersion of gold-coated chromiumprimed glass slides in dilute ethanolic solutions of alkanethiols. The coverslips and glassware used in the sample preparation were cleaned prior to use by soaking in hot “Piranha” solution for 30 min, rinsed with copious amounts of distilled water, and dried in an oven at 70 °C. Metal deposition was carried out by thermal evaporation from resistively heated Mo boats in a General Engineering bell jar vacuum system with a base pressure of ∼1 × 10-6 Torr. A thin adhesive layer of chromium (99.99+%, Goodfellow Metals) was deposited first at a rate of 0.1 Å s-1 to a thickness of 30 Å. Gold was then deposited to a thickness of 300 Å at a rate of 0.5 Å s-1. After the substrates were allowed (25) Lewis, M.; Tarlov, M. J.; Carron, K. J. Am. Chem. Soc. 1995, 117, 9574. (26) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342.
Cooper and Leggett to cool, the bell jar was vented with N2, and the slides were removed from the evaporator and immediately immersed in 1 mM solutions of the thiols in degassed ethanol (99% purity) for approximately 18 h. Following removal from the thiol solution, the substrates were rinsed with degassed ethanol and dried in a stream of nitrogen. The methyl-terminated thiols, butanethiol (BT) and dodecanethiol (DDT), were purchased from Fluka, with a purity of 96 ( 1%, and used as received. 3-Mercaptopropanoic acid (MPA) (99+% purity) and mercapto-1-propanol (MPL) (95%) were purchased from Aldrich and also used without further purification. 11-Mercapto-1-undecanol (MUL) and mercaptoundecanoic acid (MUA) were prepared in the authors’ laboratory utilizing a method adapted from the one published previously by Bain et al.27 Small pieces (approximately 10 mm × 10 mm) were cut from the glass slides using a diamond scribe and either characterized by static SIMS or photooxidized using light from a medium pressure mercury arc lamp placed approximately 100 mm from the samples. Secondary ion mass spectrometry (SIMS) was performed on a system equipped with a VG MM 12-12 quadrupole mass analyzer and a VG liquid gallium metal ion gun (MIG). The primary ions were accelerated through a potential of 10 kV, focused into a spot less than 1 mm in diameter and rastered at TV rate across a region of area 25 mm2. The primary ion current used was ca. 0.8 nA so that the current density at the sample surface was ca. 3.2 nA cm-2. The primary dose was not allowed to exceed 5 × 1012 ions cm-2, to remain within the static regime. The intensities of key molecular peaks were routinely monitored during analysis and were found not to vary significantly in intensity before and after collection of a spectrum, confirming that operation was within the static regime.
Results Characterization of Unoxidized SAMs. Negative ion spectra were recorded for all six SAMs studied, and are shown in Figure 1. Previous studies have shown that the positive ion spectra of SAMs contain few structurally specific molecular species, and consequently, attention was focused on the collection of negative ion spectra.28-30 The negative ion SIMS spectra of all of the SAMs studied exhibited peaks at m/z 197 (Au-), 229 (AuS-), and 261 (AuS2-), and at m/z 394 (Au2-), 426 (Au2S-), and 458 (Au2S2-). All three short chain thiols exhibited significant peaks corresponding to Au(M - H)S- (at m/z 318 for BT, 320 for MPL and 334 for MPA), where M is an intact adsorbate molecule, together with (M - H)- peaks (at m/z 89 for BT, 91 for MPL, and 105 for MPA). An (M - 3H)- peak was also observed for MPA, at m/z 103. The spectra of the long chain thiols also exhibited similar peaks, but with reduced intensities. The spectrum of DDT exhibited weak peaks due to (M - H)- (m/z 201) and Au(M - H)S- (m/z 430). The spectrum of MUL exhibited a small peak due to (M - H)- at m/z 203 and a small peak due to Au(M H)S- at m/z 432. However, these were not prominent peaks. The corresponding peaks were even weaker in the spectrum of MUA. The (M - H)- peak at m/z 217 was of negligible intensity, as was the Au(M - H)S- peak. The spectra of BT and MPL exhibited significant AuMpeaks, at m/z 287 and 289, respectively, but the spectrum of MPA exhibited only a weak AuM- peak. The long chain thiols again exhibited a reduced intensity for this species, at m/z 399 for DDT and m/z 401 for MUL, with MUA not yielding a significant intensity for this peak. (27) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (28) Tarlov, M. J.; Newman, J. G. Langmuir 1992, 8, 1398. (29) Leggett, G. J.; Davies, M. C.; Jackson, D. E.; Tendler, S. J. B. J. Chem. Soc., Faraday Trans. 1993, 89, 179. (30) Leggett, G. J.; Davies, M. C.; Jackson, D. E.; Tendler, S. J. B. J. Phys. Chem. 1993, 97, 7, 5348.
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Figure 1. Static SIMS spectra of unoxidized SAMs of (a) BT, (b) DDT, (c) MPL, (d) MUL, (e) MPA, and (f) MUA.
The spectra of the methyl, and alcohol-terminated thiols exhibited peaks at m/z 275 and 289, corresponding to AuS2CH2- and AuS2CH2CH2-, respectively. However, these peaks were not observed with any significant intensity in the spectra of MPA and MUA. The MPA spectrum exhibited a peak at m/z 89, thought to correspond to (M - OH)-, but no such peak was observed in the spectrum of MUA, which instead exhibited a range of peaks (at m/z 99, 113, 127, 141, 155, 169, and 183) that correspond to chain fragments that retain the carboxylic acid group, presumably as the charge site, and that have the general formula CH2dCH(CH2)nCOO-. No corresponding ions were observed in the spectra of any of the other SAMs, however. Au(M - H)2- peaks were observed in the spectra of BT, MPL, and MPA, at m/z 375, 379, and 407, respectively. These were prominent peaks in the case of BT and MPL, while the Au(M - H)2- peak in the MPA spectrum was comparatively less intense. At lower mass, a variety of peaks were observed that provided relatively little structural information. These included the elemental peaks and small fragments (CH-, O-, OH-, C2H-, S-, SH-) observed in all of the spectra and, occasionally, small peaks due to SO3- and HSO4- at m/z 80 and 97, respectively. These oxysulfur peaks are thought to be due to the presence of small quantities of contaminants in the samples, for they were not generally accompanied by the pronounced peaks due to molecular oxidation products that were characteristic of the spectra of the photooxidized SAMs (see below). SAM Photooxidation. Certain changes were observed in the spectra of all of the SAMs as photooxidation proceeded. To illustrate these general trends, Figure 2 shows a sequence of spectra recorded for BT samples
exposed to UV radiation for periods of 10, 20, 30, and 60 min. After 10 min exposure, pronounced peaks were observed at m/z 80 and 97, due to SO3- and HSO4-, respectively. A new peak was also observed at m/z 137, due to the sulfonate of BT, CH3(CH2)3SO3-. Henceforth this species will be denoted MSO3-, although strictly it may be more accurate to use the less convenient designation (M - H + O3)-. The intense molecular species observed at m/z 287, 318, and 375 (AuM-, Au(M - H)Sand Au(M - H)2-, respectively) in the spectrum of the unoxidized material (see Figure 1) were also much reduced in intensity, and new peaks were observed at m/z 293/ 295. The identities of these peaks are not clear, although they were observed with reduced intensity in the spectra of DDT, MPL, and MUL following photooxidation. The molecular ion at m/z 89 was reduced in intensity, and a new peak appeared at m/z 87. Again, the identity of this species is not clear, although it was observed in the spectra of MPL and MPA following photooxidation and is thus probably generally characteristic of oxidized short chain thiols. As photooxidation proceeded, the intensity of the MSO3- species increased, while the molecular species characteristic of the unoxidized material declined. After 20 min, new peaks could clearly be discerned at m/z 195, 311, and 391. While the identity of the ion at m/z 311 was not clear, the ions at m/z 195 and 391 could correspond to (M - H)2OH- and Au(M - H)2O-, respectively. The observation of such partial oxidation products would be surprising, and the tentative nature of these assignments must be emphasized. After 30 min, the molecular species characteristic of the unoxidized SAM had disappeared. The relative intensities of the major peaks in the spectrum remained approximately constant over longer exposure times,
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Figure 2. Static SIMS spectra recorded following exposure of BT samples to UV light for 10, 20, 30, and 60 min.
Figure 3. Static SIMS spectra of fully photooxidized SAMs of (a) DDT, (b) MPL, (c) MUL, (d) MPA, and (e) MUA.
although the peak at m/z 195 declined relative to the m/z 197 peak (Au-). The complete disappearance of the molecular species that are characteristic of the unoxidized thiol provides one useful indicator that photooxidation of the SAM is complete. Other indicators include the variation of the intensities of the Au2S- and Au2S2- peaks; these declined to zero on complete oxidation of all of the SAMs. Spectra were recorded for DDT, MPL, MUL, MPA, and MUA SAMs following complete oxidation and are shown in Figure 3. All of these spectra exhibit intense peaks due to SO3- and HSO4-. The molecular species that characterized the unoxidized SAM were absent from the spectra of photooxidized DDT, and a pronounced MSO3- peak was observed at m/z 249 instead. However, new peaks were also observed at m/z 263, 277, and 293. These may be due to ions that contain
the molecular sulfonate and additional fragments. Given that the spectra of unoxidized SAMs did not show any evidence of such peaks, it is most reasonable to attribute them to photochemical reactivity between either neighboring adsorbates or adsorbates and contaminants. In the case of DDT, peaks at m/z 263 and 277 may represent (MSO3 + CH2)- and (MSO3 + C2H4)-, respectively, while the peak at m/z 293 may be due to the subsequent addition of O. The spectrum of photooxidized MUL also exhibited peaks at larger m/z values than that of the MSO3- species, including m/z 265, 279, and 293. Again, these may be due to photochemical reactions that lead to the addition of methylene groups to the molecular sulfonate, or some other more complex process. Photooxidized MUA exhibited little evidence of such photochemical reactions, but all three short chain thiols exhibited peaks at higher m/z than their
Static SIMS Studies of Self-Assembled Monolayers
molecular sulfonate peaks (for example, at m/z 151 for BT, equivalent to (MSO3 + CH2)-). However, evidence was not observed for cross-linking between adsorbate molecules. In separate studies of electron beam irradiation of SAMs, peaks were observed, following high exposures, in the m/z range 100-200, which corresponded to polycyclic aromatic ions.31 In the case of shorter adsorbates such as octanethiol, these ions contained a greater number of carbon atoms than the intact adsorbate, and they were attributed to the formation of cross-linked material. Such peaks were not observed in the present study, even after long UV exposures, and this is interpreted as indicating that cross-linking was not occurring as a result of photochemical processes, even though there appears to be evidence of some intermolecular reactivity, as described above. All three long chain SAMs exhibited a series of peaks attributed to ions with the general formula CH3(CH2)nSO3following photooxidation, at m/z 109, 123, 137, 151, 165, 179, 193, and 207. However, these were particularly pronounced in the spectra of photooxidized MUL and MUA. Unoxidized MUA exhibited a range of ions between m/z 100 and 200 that contained the terminal group and fragments of the adsorbate alkyl chain, while the methyland hydroxyl-terminated adsorbates did not exhibit any such ions, presumably because these terminal groups did not possess the ability of the carboxylate ion to stabilize negative charge. However, the sulfonate group is still more capable than the carboxylate group of stabilizing negative charge and thus the primary charge site shifts to the other end of the molecule following photooxidation. The presence of the sulfonate group in the photooxidized MUL and DDT SAMs clearly presents a means by which large negatively charged fragments may be stabilized, which is absent in the unoxidized SAMs. Hence large molecular fragments are only observed with appreciable intensity following photooxidation. The SIMS spectra of partially oxidized SAMs of MUA exhibited peaks due to both CH3(CH2)nSO3- and CH2dCH(CH2)nCOO- species, although the former were typically more prominent because of their greater ionization probabilities. The spectrum of photooxidized MUL, like those of DDT, BT, and MPL, exhibited a prominent MSO3- peak, together with a peak at m/z 235 corresponding to CH3(CH2)10SO3(i.e., the MSO3- species less the hydroxyl oxygen atom). However, the MSO3- peak (m/z 265) was only very weak in the spectrum of photooxidized MUA. Instead, the most prominent photooxidation product above m/z 200 was the CH3(CH2)9SO3- species at m/z 221. While an MSO3- peak was clearly observed in the spectrum of photooxidized MPA, it was nevertheless weaker than the corresponding peaks in the spectra of photooxidized BT and MPL. Kinetics of Photooxidation. A quantitative investigation of the kinetics of SAM photooxidation was performed. The quantification of SIMS data has been the subject of some debate, and it has been widely held that such phenomena as the matrix effect, where a given ion exhibits different secondary ion yields when sputtered from different chemical environments, make quantification difficult. While this is to some extent true for the measurement of absolute intensities in SIMS, the use of relative intensities is much more reliable and goes back to the work of Vickerman and co-workers.32,33 Using this approach, the present authors have demonstrated a close correlation between the relative intensities of key ions in (31) Hutt, D. A.; Leggett, G. J. Unpublished data, University of Nottingham, 1997. (32) Bordoli, R. S.; Vickerman, J. C. Surf. Sci. 1979, 85, 244. (33) Brown, A.; Vickerman, J. C. Vacuum 1981, 31, 429.
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the SIMS spectra of SAMs on Ag and their extent of oxidation as determined by XPS.23 The selection of ions of identical charge and similar masses for the quantitative analysis appears to result in the effective elimination of any matrix effect.34 Because there were clear changes in the intensities of diagnostic molecular species in the SIMS spectra of all six SAMs, which were related to the degree of photooxidation, SIMS provided a valuable means by which the rates of specific chemical transformations could be monitored. The molecular specificity of SIMS gives it a distinct advantage over XPS in studies of SAM reactivity, because of ambiguities in the photoelectron binding energies. In particular, not only is the S 2p peak comparatively weak, but it is difficult to distinguish readily between S that is contained in adsorbate species and inorganic sulfur products that result from photochemical reactions.23 By monitoring the rate of increase in the relative intensities of molecular products of the photooxidation reaction, it is possible to determine the rate of photooxidation of the adsorbates with a high degree of accuracy. Assuming that the photooxidation reaction takes the form
Y(CH2)nSAu + 3/2O2 f Y(CH2)nSO3- + Au+ (1) where Y is the terminal group (CH3, OH, or COOH), then it is possible to monitor the rate of the reaction by plotting the ratio of the intensities of an ion characteristic of the reaction product and an ion characteristic of the unoxidized adsorbate. In the case of BT, MPL, and MPA, the choice was straightforward: all three yielded pronounced peaks attributed to Au(M - H)S- prior to oxidation, and due to MSO3- after oxidation. For the three long chain SAMs, however, the Au(M - H)S- peak was comparatively weak, and it was therefore necessary to employ the AuS- peak to represent the unoxidized SAM instead. This peak was conveniently close in mass to the sulfonate peak for DDT and was observed to decline in parallel with the Au(M H)S- peak in the spectra of BT, MPL, and MPA. MUA did not yield a strong sulfonate peak either, however, and so the CH3(CH2)9SO3- species (m/z 221) was used instead. For MUL, the m/z 221 peak was the strongest above m/z 200 and the intensity of this ion was thus also used to calculate the extent of oxidation of MUL SAMs. The extent of oxidation χt was calculated from the ratios
[MSO3-]/([MSO3-] + [Au(M - H)S-])
(2)
for BT, MPL, and MPA,
[MSO3-]/([MSO3-] + [AuS-])
(3)
for DDT, and
[CH3(CH2)9SO3-]/([CH3(CH2)9SO3-] + [AuS-]) (4) for MUL and MUA. In each of the expressions above, [X] denotes the area of the peak representing species X in the SIMS spectrum. The extent of photooxidation is plotted for all six SAMs as a function of the time of exposure to the UV lamp in Figure 4. It can be seen that the rate of oxidation increases with decreasing chain length: BT, MPL, and MPA oxidize significantly faster than their respective long chain analogues, DDT, MUL, and MUA. Furthermore, for a given alkyl chain length, the nature of the adsorbate tail (34) Leggett, G. J.; Vickerman, J. C. Chem. Soc. Ann. Rep. C 1991, 77.
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Figure 4. Variation in the extent of oxidation at time t, χt, with the time of exposure of SAMs to UV light. Table 1. Rate Constants (k) and Half-Lives (t1/2)for SAM Photooxidation SAM
103 k/ min-1
t1/2/min
SAM
103 k/ min-1
t1/2/min
BT MPL MPA
80.0 37.7 19.2
8.7 18.4 36.1
DDT MUL MUA
20.8 9.9 5.5
33.3 70.2 126.7
group exerts a significant influence on the rate of photooxidation, with the methyl-terminated SAMs oxidizing fastest, followed by the hydroxyl-terminated SAMs and then the carboxylic acid-terminated SAMs. To quantify these effects, a simple kinetic analysis was performed. In an earlier work we showed that the photooxidation of methyl-terminated thiols obeys firstorder kinetics. Assuming that the same treatment may be applied here, the following rate law should apply:
lnχt′ ) lnχo′ - kt
(5)
where χt′ is the fraction of unoxidized thiol at time t and k is the rate constant. If χt′ is given by the ratios
[Au(M - H)S-]/([MSO3-] + [Au(M - H)S-]) (6) for BT, MPL, and MPA,
[AuS-]/([MSO3-] + [AuS-])
(7)
[AuS-]/([CH3(CH2)9SO3-] + [AuS-])
(8)
for DDT and
for MUL and MUA, then χo′ will be equal to 1 and ln χ∞′ will be equal to zero. Thus plots of the logarithms of (6 - 8) against time of exposure to UV radiation should yield straight lines, with gradients equal to -k. It was found that linear regression did indeed yield a straight line fit with a high level of confidence for all six SAMs, and the rate constants are shown in Table 1, together with halflives for the photooxidation reactions. It may be seen that for a given chain length, the rate constant for photooxidation of the hydroxyl-terminated thiol is approximately half that for photooxidation of the methylterminated thiol, and about twice that for photooxidation of the carboxylic acid-terminated thiol. The rate of reaction decreases by approximately a factor of 4 on increasing the chain length of the adsorbate by eight methylene groups, in approximate agreement with the results of earlier studies by XPS.22 Discussion The present study has confirmed the finding of an earlier study by X-ray photoelectron spectroscopy (XPS) that the
Figure 5. Schematic diagram illustrating the proposed mechanism of photooxidation of SAMs and the influence of adsorbate chain length. Long chain SAMs (top) are ordered, and access to the Au-S bond for reactive oxygen species is limited.Oxidation is therefore restricted to domain boundaries, step edges, and other discontinuities. For short chain SAMs, exhibiting lower levels of order (bottom), access is more open and the rate of reaction is higher.
rate of photooxidation of a SAM is profoundly influenced by the chain length of the adsorbate molecules.22 The present data indicate that the rate of oxidation of a SAM with any given terminal group is reduced by a factor of approximately 4 on increasing the chain length by eight methylene groups. The magnitude of the change in photooxidation rate determined here is broadly in agreement with that in the previous study. The dependence of the photooxidation rate on adsorbate chain length is explained in terms of adsorbate ordering (the mechanism proposed is illustrated schematically in Figure 5). In the present study, we have also found that for both long and short chain adsorbates, hydroxyl-terminated SAMs oxidize approximately half as fast as methylterminated SAMs, and carboxylic acid-terminated SAMs oxidized half as fast as hydroxyl-terminated ones. Thus MPA oxidized fractionally more slowly than DDT, a significantly longer adsorbate, and MUA oxidized very slowly indeed. The most probable explanation for this observation is that intermolecular interactions, between the terminal groups of adsorbate molecules, may contribute substantially to the stabilization of the SAM. The magnitude of the stabilization conferred by carboxylic acid terminal groups appears, on the basis of our data, to be significantly greater than that provided by hydrogen bonding between terminal hydroxyl groups. There is an alternative explanation for the slower oxidation of the SAMs with polar terminal groups: that they have higher surface energies and attract a layer of contamination that protects the SAM from photooxidation. It has been shown that this contamination layer is thick enough to provide contrast between polar and methylterminated regions in SEM images of patterned SAMs.35,36 However, the magnitude of the contrast difference between hydroxyl- and carboxylic acid-terminated SAMs in SEM images is significantly less than the difference in the contrast of methyl-terminated regions and regions functionalized with either of the polar terminal groups.31 This is consistent with the expected difference in surface energies, reflected in the water contact angles typically recorded for methyl-, hydroxyl-, and carboxylic acidterminated SAMs (found to be similar and
