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Langmuir 1999, 15, 1024-1032
Influence of Tail-Group Hydrogen Bonding on the Stabilities of Self-Assembled Monolayers of Alkylthiols 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. and Department of Chemistry, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD, U.K. Received July 1, 1998. In Final Form: November 24, 1998 Self-assembled monolayers (SAMs) of alkylthiols with polar terminal groups on gold may be photopatterned to produce structures that are stable against displacement by methyl-terminated thiols with longer alkyl chains. For example, 3-mercaptopropanoic acid (MPA) patterns are not eroded by octadecanethiol (ODT). However, photopatterned SAMs of methyl-terminated thiols shorter than tetradecanethiol are eroded by exposure to solutions of thiols with polar terminal groups. For example, dodecanethiol (DDT) patterns are eroded by MPA. The erosion process commences at boundaries between masked and unmasked regions of the sample and continues rapidly to completion. At locations remote from such boundaries, the alkyl chains of the adsorbate molecules provide a steric barrier that protects the headgroup from attack by molecules in solution. However, at boundary regions, there is a breakdown in this steric protection that facilitates displacement of nonpolar adsorbates by molecules with hydrogen-bonding terminal groups, leading to the thermodynamically favored product. Formation of a complete monolayer in oxidized regions (by displacement of alkylsulfonates) halts the displacement process. The well-attested stability of SAMs under extreme conditions may be attributed to a conjunction of S-Au interactions with steric effects and intermolecular interactions. However, it is postulated that the significance of the latter two factors is somewhat greater than has often been thought previously. It is concluded that hydrogen bonding contributes greatly to the stabilities of SAMs formed from molecules with polar terminal groups.
Introduction There is now widespread interest in the exploitation of self-assembled monolayers (SAMs) for applications ranging from nanotechnology to fundamental surface science. The development of a range of methods for SAM patterning, including photolithography,1-4 micro contact printing,5-9 electron and ion beam lithography,10-12 and STM13 techniques, has been of particular importance because it has become possible to rapidly and conveniently produce micron- and submicron-scale arrays of thiols that are chemically stable and provide templates for subse* Author to whom correspondence should be addressed: e-mail,
[email protected]. † The University of Nottingham. ‡ University of Manchester Institute of Science and Technology. (1) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (2) Gillen, G.; Bennett, J.; Tarlov, M. J.; Burgess, D. R. F. Anal. Chem. 1994, 66, 2170. (3) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342. (4) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (5) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (6) Kim, E.; Kumar, A.; Whitesides, G. M. J. Electrochem. Soc. 1995, 142, 628. (7) Xia, Y.; Kim, E.; Whitesides, G. M. J. Electrochem. Soc. 1996, 143, 1070. (8) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. Adv. Mater. 1994, 6, 600. (9) Mrksich, M.; Whitesides, G. M. Trends Biotechnol. 1995, 13, 228. (10) Gillen, G.; Wight, S.; Bennett, J.; Tarlov, M. J. Appl. Phys. Lett. 1994, 65, 534. (11) 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. (12) Tiberio, R. C.; Craighead, H. G.; Lercel, M.; Lau, T.; Sheen, C. W.; Allara, D. L. Appl. Phys. Lett. 1993, 62, 476. (13) Lercel, M. J.; Redinbo, G. F.; Craighead, H. G.; Sheen, C. W.; Allara, D. L. Appl. Phys. Lett. 1994, 65, 974.
quent derivatization and etching. Whitesides and coworkers have provided many illustrations of the broad applicability and simplicity of micro contact printing, leading to the fabrication of a range of novel two- and three-dimensional structures.5-9 However, photolithography is also simply exploited for SAM patterning, and we have become interested in utilizing photopatterning techniques to prepare microstructured surface chemistries for fundamental studies of cell-material interactions14,15 directed toward developing an understanding of the surface chemical cues that guide cell attachment and growth on synthetic substrata. We have been performing a systematic investigation of SAM photo-oxidation16-19 and photopatterning, with the objective of identifying the key parameters that control rates of photo-oxidation and the effectiveness of pattern fabrication. The influence of adsorbate chain length has been of particular interest. In a recent study of SAMs on gold substrates, it was found that rates of photo-oxidation depend sharply on alkyl chain length.16 The rate constant for the oxidation of adsorbed thiols to alkyl sulfonates increased rapidly with decreasing chain length for molecules with more than 11 methylene units in the alkyl chain. Similar chain-length effects were observed in a recent study of the photo-oxidation of SAMs on silver,17 but the mechanism was found to be more complex, with an additional process (S-C bond scission) competing with (14) Hutt, D. A.; Cooper, E.; Parker, L.; Leggett, G. J.; Parker, T. L. Langmuir 1996, 12, 5494. (15) Cooper, E.; Wiggs, R.; Hutt, D. A.; Parker, L.; Leggett, G. J.; Parker, T. L. J. Mater. Chem. 1997, 7, 435. (16) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657. (17) Hutt, D. A.; Cooper, E.; Leggett, G. J. J. Phys. Chem. B 1998, 102, 174. (18) Hutt, D. A.; Cooper, E.; Leggett, G. J. Surf. Sci. 1998, 397, 154. (19) Cooper, E.; Leggett, G. J. Langmuir 1998, 14, 4795.
10.1021/la980795b CCC: $18.00 © 1999 American Chemical Society Published on Web 01/20/1999
Self-Assembled Monolayers of Alkylthiols on Gold
thiol oxidation, in line with earlier predictions of Lewis and Tarlov.20 On both metals, the rapid oxidation of shortchain thiols was attributed to their more open structures, facilitating more ready access of excited oxygen species to the S-metal bond.16,17,19 For molecules with longer alkyl chains, access to the S-metal bond is more restricted and attack is presumably limited to step edges, domain boundaries, and similar discontinuities and defects. The implication of these data is that short-chain thiols offer a processing advantage (i.e., they are significantly more rapidly patterned) provided that they possess adequate stability for their end function. This is clearly a crucial consideration, because it has long been known that SAM stability increases with increasing alkyl chain length. In a recent static SIMS study, we also showed that the nature of the adsorbate terminal group had a profound effect on the rate of reaction, with methyl-terminated SAMs oxidizing twice as fast as hydroxyl-terminated SAMs, and four times as fast as carboxylic acid-terminated SAMs.19 In an earlier study, we prepared patterned SAMs containing regions functionalized with 3-mercaptopropanoic acid (MPA) and octanethiol (OT) by, first, forming a monolayer of MPA; second, photo-oxidizing via irradiation with a UV lamp through a mask; and third, immersing the patterned sample in a solution of OT.14 Crisp patterns resulted, with edge definition similar to that of patterns of long-chain thiols prepared by other workers. Subsequent studies of cellular attachment revealed that these patterns exhibited good stability in culture and we were able to demonstrate the formation of cellular “wires” on narrow tracks of MPA.14,15 We wished to explore more systematically the effect of the alkyl chain length and terminal group on the quality of photopatterned SAMs formed in this way, to see whether short-chain SAMs could generally be effectively patterned. We have found that the nature of the tail functionality exerts a profound influence over the quality of the resulting pattern. Adsorbates that are capable of forming hydrogen bonds between their terminal groups appear to exhibit greater stability than corresponding methyl-terminated adsorbates: patterns formed from methyl-terminated thiols shorter than tetradecanethiol are eroded by thiols with hydroxyl and carboxylic acid terminal groups that contain as many as 10 fewer methylene groups. Hydrogen bonding between terminal groups in SAMs is thus thought to be highly effective and contributes greatly to the stabilization of hydroxylterminated and carboxylic acid terminated monolayers. These findings have important implications for our understanding of the factors that determine SAM stability and reactivity. Experimental Section
Langmuir, Vol. 15, No. 4, 1999 1025 from the evaporator and immediately immersed in 1 mM solutions of the thiols in degassed ethanol (99.999% 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, namely propanethiol (PT), butanethiol (BT), hexanethiol (HT), octanethiol (OT), decanethiol (DT), dodecanethiol (DDT), tetradecanethiol (TDT), hexadecanethiol (HDT), and octadecanethiol (ODT), were all 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-1undecanol (MUL) and mercaptoundecanoic acid (MUA) were synthesized in the authors’ laboratory using a method adapted from one described by Bain et al. in a previous publication.21 Preparation of Patterned Monolayers. The patterned monolayers were created using a variation of the photolithographic process of Tarlov et al.1,2 employing transmission electron microscope grids as masks to create micron-scale features on the surface. The masks employed were composed of bars of width 55 and 110 µm, contained within a ring of diameter 3.5 mm. A medium-pressure mercury arc lamp was used as the UV source, and the freshly prepared monolayers were placed approximately 100 mm from the lamp. The samples were exposed to the UV radiation for a length of time that was known to achieve complete photo-oxidation of the monolayer, as previously determined using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS), and then placed in a solution of a second thiol for some minutes. Following removal from the second thiol solution, the substrates were rinsed with degassed ethanol and dried in a stream of N2. UV irradiation of alkylthiol-on-gold SAMs in the presence of air results in the photo-oxidation of the alkylthiolate adsorbate complex to yield an alkylsulfonate. The weakness of the SO3-Au interaction facilitates the displacement of the alkylsulfonate by a second thiol adsorbed from solution, resulting in the formation of a new SAM on the UV-exposed areas. Characterization of SAMs. The patterned monolayers were characterised using scanning electron microscopy (SEM) and imaging SIMS. SEM has been shown to reveal contrast between regions of high and low surface energy due to attenuation of the secondary electron signal arising from the gold by the spontaneous adsorption of airborne molecules on the high-energy wetting regions of the surface. Thus, regions of dark contrast on the SEM image correspond to areas of high surface energy and regions of light contrast to low-energy regions. The SEM images were acquired in the secondary electron detection mode using a JEOL JSM-6400 scanning microscope, with a 35 kV primary electron beam and a current of 3 nA. The electron detector was operated with a collection voltage of +300 V. Imaging 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 and focused into a spot less than 1 µm in diameter. The primary ion current used was ca. 0.8 nA. In spectroscopic mode, the beam was rastered at TV rate across a 25 mm2 area, with the consequence 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 in spectroscopic mode, to remain within the static regime, although higher doses were employed in imaging mode.
Preparation of Monolayers. The single-component monolayers were prepared on gold films supported on chromium primed glass coverslips according to now standard procedures. The coverslips and glassware used in the sample preparation were first cleaned by soaking in hot “piranha” solution for 30 min, rinsed with copious amounts of distilled water, and dried in an oven at 70 °C. CAUTION: Piranha solution reacts violently with organic material. Metal deposition was carried out by thermal evaporation from restively 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 ( 10 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 to cool, the bell jar was vented with N2, the slides were removed
Previous work by Tarlov and co-workers1,2 and in this laboratory14,15 has shown that very crisp, well-defined patterns may be created when a hydrophilic monolayer is prepared, exposed to UV radiation through a mask, and dipped in solution of a methyl-terminated thiol. Figure 1 shows typical examples of SEM images recorded for monolayers patterned in this fashion. Micrographs are shown for MPA/OT and MPA/ODT patterned monolayers, where the first thiol A of each pair A/B is the one from
(20) Lewis, M.; Tarlov, M. J.; Carron, K. J. Am. Chem. Soc. 1995, 117, 9574.
(21) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
Results
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Figure 1. SEM images of patterned SAMs formed from the combinations MPA/OT (a) and MPA/ODT (b).
Figure 2. SIMS images formed by mapping the [AuS(SCH2CH2COOH)]- ion intensity from patterned SAMs formed from the combinations MPA/OT (a) and MPA/ODT (b).
which the initial SAM was formed, and the second thiol B is the one adsorbed following photopatterning. Light contrast is observed for the hydrophobic regions of the surface, and dark for the hydrophilic regions.22,23 The SEM images highlight the clear transition between hydrophobic and hydrophilic regions on the surface, with very sharp edges between the areas of different surface chemistry. This patterning procedure has been shown to be effective for a number of thiols with carboxylic acid and hydroxyl tail functionalities. In each case, the effectiveness of patterning was the same, irrespective of the precise nature of the initial hydrophilic monolayer or the alkyl chain length of the methyl-terminated thiol. While SEM provides a valuable guide to the geometrical effectiveness of pattern formation, it does not provide chemical specificity. Imaging SIMS was therefore employed to verify our interpretation of the SEM data. The (22) Lopez, G. P.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1513. (23) Wollman, E. W.; Frisbie, C. D.; Wrighton, M. S. Langmuir 1993, 9, 1517.
static SIMS spectra of MPA, OT, and ODT have been described in detail elsewhere.19,24-26 They include a number of molecular peaks that are diagnostic of the individual adsorbate molecules. Among the prominent peaks above m/z 200 in the SIMS spectrum of MPA, the peak at m/z 334 is particularly important because it corresponds to the [AuS(SCH2CH2COOH)]- ion, henceforth denoted by the abbreviation AuMS-. This ion contains an entire adsorbate molecule, linked to an Au and an S atom, and is therefore specific to SAMs of MPA.19 AuMS- species are also observed in the SIMS spectra of other SAMs. For example, for OT, the AuMS- peak appears at m/z 374. By mapping the intensity of a specific AuMSion, it is possible to map regions functionalized with a specific SAM chemistry. Figure 2 shows SIMS images of samples prepared by immersion of photopatterned MPA (24) Leggett, G. J.; Davies, M. C.; Jackson, D. E.; Tendler, S. J. B. J. Chem. Soc., Faraday Trans. 1993, 89, 179. (25) Leggett, G. J.; Davies, M. C.; Jackson, D. E.; Tendler, S. J. B. J. Phys. Chem. 1993, 97, 5348. (26) Tarlov, M. J.; Newman, J. G. Langmuir 1992, 8, 1398.
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Figure 3. SEM images of patterned SAMs formed from the combinations DDT/MPA (a) and ODT/MPA (b).
SAMs in solutions of OT and ODT. In each case, the intensity of the [AuS(SCH2CH2COOH)]- ion has been mapped. The high-intensity regions thus correspond to regions occupied by MPA and are coincident with the regions of low intensity in the SEM images, confirming our earlier interpretation. Patterns were also created in the reverse fashion to those described above, i.e., by starting with a hydrophobic, methyl-terminated monolayer, HS(CH2)nCH3, exposing it to UV irradiation, and dipping it in a solution of a hydroxylterminated or carboxylic acid terminated thiol. Figure 3 shows SEM images obtained when monolayers created from DDT and ODT were photopatterned and dipped in a solution of MPA. For DDT and shorter thiols, the pattern of the grid was eroded on dipping in a solution of MPA. The wide outer ring of the region originally patterned with DDT can just be observed, but the boundaries between the regions of different surface chemistry are blurred and the pattern within the ring cannot be seen. Only those patterns created using tetradecanethiol and longer adsorbates exhibited contrast between regions of different surface chemistry comparably sharp to that seen from the substrates patterned using a hydrophilic monolayer as the base substrate. In the case of the short-chain methyl-terminated thiols (n e 11), it is clear that, in addition to replacing thiols that have been photo-oxidized to weakly bound alkylsulfonates, MPA is also replacing some of those adsorbed thiols that have not been photooxidized. To confirm this interpretation of the SEM data, SIMS images were again recorded. Figure 4 shows [AuS(SCH2CH2COOH)]- and O- images of photopatterned DDT and HDT monolayers that have been immersed in solutions of MPA. For DDT, the MPA was found to adsorb onto regions of the surface that had been masked during photo-oxidation, but for HDT, no erosion occurred. Both images gave high contrast where the SEM images showed low intensity, confirming the conclusion drawn from the SEM data that the unoxidized regions of the methylterminated thiol SAMs, are eroded by MPA for DDT and shorter thiols but not for TDT, HDT, and ODT. The rate of erosion of the shorter-chained methylterminated molecules from patterns was explored by varying the time of exposure of the pattern to the MPA
solution. Figure 5 shows three SEM images of OT patterns that have been exposed to the MPA solution for three different time periods. The shortest time period corresponds to a sample inserted into the MPA solution only momentarily and then immediately retracted and rinsed with ethanol. It can be seen that even under these conditions, extensive erosion has occurred. The pattern is not further eroded after exposure of the OT pattern to the MPA solution for 24 h, indicating that the erosion process is rapid and ceases equally rapidly after the attainment of some kind of steady-state surface composition. Patterns formed in methyl-terminated SAMs were exposed to solutions of MUA to examine whether the additional methylene groups in this adsorbate’s alkyl chain provided a further driving force for erosion of patterns. However, the behavior observed was the same as that observed for MPA. To determine whether hydroxyl terminal groups produced different rates of erosion from carboxylic acid terminal groups, similar studies were performed using MPL and MUL. Again, no significant differences were observed. Figure 6 shows the SEM images recorded for OT and ODT patterns exposed to solutions of MPL and MUL. It can been seen from the images that similar behavior is observed for patterns created using both of the hydroxyl-terminated thiols and that the results are similar to those obtained using MPA and shown in Figure 3: the long-chain thiol pattern resists erosion, while the short-chain thiol pattern is eroded in a similar fashion. Finally, we examined combinations of alcohol- and acidterminated SAMs prepared either by adsorbing the alcohol or the acid-terminated thiol first, and then patterning and exposure to a solution of an acid- or alcohol-terminated thiol. No erosion was observed in either case. Recent results by Zhang et al.27 and by Norrod and Rowlen28 have suggested that ozone may be the active agent in SAM photo-oxidation. If this is the case, then diffusion of ozone under the mask may lead to erosion of “masked” material. We have already shown that SAM photo-oxidation rates are highly dependent on both the alkyl chain length and the nature of the terminal functional group of the adsorbate, and those adsorbates (27) Zhang, Y.; Terrill, R. H.; Tanzer, T. A.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 2654. (28) Norrod, K. L.; Rowlen, K. L. J. Am. Chem. Soc. 1998, 120, 2656.
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Figure 4. SIMS images formed by mapping the [AuS(SCH2CH2COOH)]- (a and b) and O- (c and d) intensities from patterned SAMs formed from the combinations DDT/MPA (a and c) and HDT/MPA (b and d).
that were found to be eroded in the present study were also those that exhibited the higher rates of photooxidation in the previous studies.16-19 To determine whether the erosion occurred during immersion in the solution of the second thiol, or as a result of such “undercutting” processes, SIMS images were recorded prior to immersion of the patterned second thiol solution. It was found that the pattern was clearly defined prior to immersion, with no evidence of oxidation occurring under the masks. Figure 7 shows an O- image of a DDT SAM, recorded directly following patterning. The pattern is clearly visible but shows no sign of erosion and there is no evidence of oxidation in the masked regions. Thus the erosion may be attributed entirely to interactions between unoxidized material and solution-phase thiols. To determine whether the solubility of the photooxidation product had an effect on the displacement process, we recorded SIMS spectra of photo-oxidized SAMs of methyl-terminated, hydroxyl-terminated and carboxylic acid-terminated adsorbates following immersion in thiol solutions. After 10 min, the immersion time employed in the majority of the preceding experiments, all the oxidized
thiols were found to be completely displaced by the solution-phase thiol (the corresponding sulfonate peaks disappeared from the spectrum) with the exception of oxidized MPA. For this one system, complete displacement did not occur until the sample had been immersed for some 6 h. The time period required for complete displacment of the oxidation product was found to be independent of the chain length of the solution-phase thiol, indicating that the slow removal of the sulfonate of MPA was due to its limited solubility in the liquid phase (possibly because it is highly polar). However, because of the close similarity of the behavior of all four thiols with hydrogen-bonding terminal groups in the displacement experiments, this limited solubility of the MPA sulfonate is not thought to be relevant to interprepation of the data presented here. Discussion Our data show that the photopatterning of SAMs composed of polar terminal groups, by irradiation with UV light through a suitable mask and subsequent immersion of the sample in a solution of a thiol with a methyl terminal group, leads to sharply resolved patterns.
Self-Assembled Monolayers of Alkylthiols on Gold
Langmuir, Vol. 15, No. 4, 1999 1029
Figure 5. SEM images of OT patterns exposed to solutions of MPA (a) momentarily, (b) for 10 min, and (c) for 24 h.
The edge resolution appears to be maintained even when the methyl-terminated adsorbate has an alkyl chain somewhat longer than that of the molecule with the polar tail group (for example, MPA patterns were not eroded by ODT). However, when the procedure was reversed, it was found that crisp well-resolved patterns could be achieved only for TDT and longer adsorbates; patterns of methylterminated adsorbates with shorter chain lengths were eroded by thiols with hydroxyl and carboxylic acid groups, leading to poor feature definition and almost complete loss of the smallest features created. While these results are purely qualitative, they nevertheless provide us with important insights into the parameters that control the stabilities of SAMs formed from the adsorption of alkylthiols onto gold. Hydrogen Bonding between Tail Groups Leads to Significant Additional Stabilization of SAMs. Erosion of the patterns created in methyl-terminated SAMs by photolithography implies the displacement of unoxidized thiol molecules by hydroxyl-terminated and
carboxylic acid-terminated thiols in solution. The simplest explanation for this effect is that the formation of hydrogen bonds between adsorbates can lead to an additional stabilization of the monolayer. While this is not unexpected, our data suggest that the magnitude of this additional stabilization is rather significant. In particular, it was found that DDT was eroded by both MPA and MPL, molecules which possessed nine fewer methylene groups, indicative of a substantial stabilization arising from hydrogen bonding interactions. Only methyl-terminated thiols as long as TDT were found to be stable against displacement by thiols with polar terminal groups. We therefore postulate that intermolecular interactions may make a significant contribution to the enthalpy of adsorption of alkylthiol SAMs on Au. The magnitude of the contribution from carboxylic acid groups may be loosely estimated from our data. TDT was just able to resist erosion by MPA, while DDT was not. Thus we may say that the carboxylic acid terminal group provides a stabilization greater than nine methylene groups (as-
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Figure 6. SEM images of patterned SAMs formed from the combinations OT/MPL (a), OT/MUL (b), ODT/MPL (c), and ODT/MUL (d).
suming that the stabilization provided by the methyl group is small)sa very significant effect indeed. Nuzzo et al. estimated the lower limit of the contribution to the enthalpy of adsorption of an alkylthiol on gold arising from interactions between methylene groups to be 0.8 kcal mol-1 for each additional chain methylene group.29 Combined with our data, this suggests a contribution of at least 7.2 kcal mol-1 from a carboxylic acid terminal group (given that one carboxylic acid group is equivalent to at least 9 methylene groups). This is within the range expected for solution-phase carboxylic acids, suggesting that the interactions between terminal groups in SAMs may be very effective. However, the figure of 7.2 kcal mol-1 is at best only a rough guide to the magnitude of the hydrogen bonding interaction; it must be borne in mind (29) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558.
that Nuzzo et al. emphasized the approximate nature of their calculations, and indicated that the actual stabilization energy per methylene group could in fact be significantly larger. Moreover, to complicate matters further, their figure was calculated for hexadecanethiol and, as we discuss below (in connection with the role of crystallinity), there may be additional stabilization for the longest thiol adsorbates. Nevertheless, it is clear from our data that hydrogen bonding can make a very substantial contribution to SAM stability. Similar results were observed when the two hydroxylterminated thiols were employed. While it would not necessarily be expected a priori that equivalent stabilization could be achieved with hydroxyl and carboxylic acid terminal groups, it is not possible to speculate confidently regarding the relative effects of the two types of functional groups. Although, superficially, our data suggest the
Self-Assembled Monolayers of Alkylthiols on Gold
Figure 7. O- SIMS image of a DDT sample following photopatterning but without immersion in a solution of a second thiol.
stabilization may be similar in magnitude (because neither MPA nor MPL erodes TDT patterns while both erode DDT patterns), the additional stability exhibited by TDT and longer methyl-terminated thiols may be larger than any provided by hydrogen bonding with the result that, while both carboxylic acid and hydroxyl group hydrogen bonding interactions provide sufficient additional stabilization to lead to the erosion of DDT patterns, neither may provide sufficient additional stabilization to cause erosion of TDT even though the magnitude of the stabilization provided is different (see discussion of crystallinity below). There is recent evidence from other sources that interchain interactions may in fact play a significant role in determining SAM structure and stability. Perhaps the best evidence for this hypothesis comes from the STM study by Poirier et al. of the hydration of monolayers of mercaptohexanol on Au(111),30 in which a structural transformation was observed to occur on exposure of the SAMs to water vapor. Poirier et al. concluded that interchain hydrogen bonding interactions played a very significant role in determining the structure of mercaptohexanol SAMs. Static SIMS data from the authors’ laboratory have also indicated that SAMs with hydrogen bonding terminal groups enjoy a considerably increased stability against photo-oxidation. Steric Effects Contribute Significantly to the Stabilization of SAMs on Gold. Notwithstanding the above conclusions, there is nevertheless considerable evidence that SAMs are extremely stable in aggressive environments. Whitesides and co-workers have performed elegant microfabrication experiments, in which SAMs have proved resistant to attack by etching agents such as aqua regia and potassium cyanide.5-9 Studies by Whitesides and co-workers31,32 and the present authors14,15 bear testimony to the stability of SAMs, including photopatterned materials, in cell culture conditions. Paradoxically, therefore, there is extensive evidence that SAMs are highly (30) Poirier, G. E.; Pylant, E. D.; White, J. M. J. Chem. Phys. 1996, 105, 2089. (31) 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. (32) Mrksich, M.; Dike, L. E.; Tien, J.; Ingber, D. E.; Whitesides, G. M. Exp. Cell Res. 1997, 235, 305.
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stable materials. While much of this stability may undoubtedly be attributed to the strength of the S-Au interaction, our data suggest that an additional factor may be significant: the alkyl chains in a complete SAM provide a steric barrier to attack by solution-phase species. While, with the exception of PT, contact-angle measurements show that methyl-terminated thiols show no tendency to be displaced by carboxylic acid terminated and hydroxyl-terminated thiols over time periods up to 24 h, methyl-terminated thiols up to DDT are eroded by those molecules following patterning. It is concluded that the erosion processes we have reported in the present study are facilitated by the breakdown in SAM coverage that occurs along boundaries between oxidized and intact material. Here there is a breakdown in steric protection and the thiol becomes susceptible to displacement by a solution-phase molecule. In an earlier study, it was found that freshly formed SAMs of BT were stable against displacement by solution phase MPL. However, following retention of the samples in a vacuum for 72 h, so that the low-coverage structure reported by Poirier and co-workers formed,33,34 the SAMs were completely displaced following immersion in a solution of MPL. We attributed this to the loss of the steric protection provided by the alkyl chains in the high-coverage structure; once the low-coverage structure has formed, with the alkyl chains aligned parallel, instead of perpendicular, to the sample surface, the headgroup is unprotected from solution-phase molecules. Following photopatterning, this steric protection offered against the approach of molecules perpendicular to the SAM surface does not provide complete protection because the creation of oxidized, disordered regions facilitates other forms of attack. We postulate that once the more readily displaced alkylsulfonates have begun to be removed from oxidized areas of the surface, the intact regions are eroded by solution-phase thiols with attack concentrating along boundaries between oxidized and unoxidized regions of the pattern (see Figure 8). Here, steric protection is minimized and solution phase thiols may be in equilibrium with intact thiols at the surface, with desorption possibly occurring via a bimolecular route that involves the exchange of a hydrogen atom between the headgroup of a solution-phase thiol and the headgroup of an adsorbate. As soon as a complete monolayer has formed in the oxidized regions, steric factors come into play, and no further displacement of intact thiols may occur. This would explain the lack of any time-dependence in the resolution of the resulting eroded patterns: the structures are the same after exposure to the solution-phase hydrogen bonding molecule for 1 min, 10 min, and 24 h. Thus, it seems that the oxidized regions are rapidly replaced with fresh thiols and the erosion process is subsequently halted, consistent with the extensive evidence published previously that SAM formation is typically complete after very short time periods (even though ordering and other structural rearrangements may occur more slowly). The Role of Crystallinity. Photopatterned TDT was found to be stable against erosion by solution-phase MPA, indicating that it possesses a sufficient number of methylene groups to offset the stabilizing effect of hydrogen bonding in MPA SAMs. However, when MPA patterns were exposed to ODT in solution, they were found to be stable against displacement, too. This was a surprising observation, because the enthalpy of adsorption of ODT, (33) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (34) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966.
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Figure 8. Schematic illustration showing a possible bimolecular mechanism for the erosion of adsorbed thiols following patterning.
∆Hads(ODT), should be greater than ∆Hads(TDT), and should consequently be larger than for ∆Hads(MPA). Furthermore, while MPA is not displaced by ODT, neither is ODT displaced by MPA. One explanation for these anomalies is that the displacement process is activated: for long-chain methylterminated adsorbates, there is an additional barrier to displacement of the adsorbate. Alternatively, there may be another factor that may contribute to the stabilization of the long chain thiols, for example, the work required to overcome the forces of intermolecular attraction within the surface layer. Significantly, the long-chain thiols studied here are probably in a two-dimensional crystal state, while the short-chain thiols (PT, BT, OT) are probably in a two-dimensional liquid state. Nuzzo et al. quoted a figure of 0.6 kcal mol-1 per methylene unit for the heat of fusion or vaporization of a normal alkane,29 and for TDT, this gives rise to an additional stabilization of 7.8 kcal mol-1. Thus the additional stabilization due to hydrogen bonding interactions on adsorption of a thiol with a polar terminal group may not, for adequately long alkyl chains, provide a sufficient energetic gain to overcome the barrier to desorption from the existing twodimensional crystalline array of methyl-terminated adsorbates. The resistance of MPA patterns to displacement by ODT is more difficult to analyze. There may be an additional barrier to desorption that arises from the formation of hydrogen bonding. Alternatively, it is possible that hydrogen bonding interactions are able to crystallize the monolayer at a lower temperature than is possible for the corresponding methyl-terminated adsorbate, with the result that a significant additional gain in the enthalpy of adsorption is realized. We have no empirical evidence to this effect, however, and it must remain speculation. Clear patterns were formed using TDT in combination with MUA and MUL. Superficially, this indicates that the stabilization provided by the polar groups in the longer adsorbates is somewhat less than that provided by the same functional groups in the shorter adsorbates, MPL and MPA. However, the presence of an activation barrier to displacement, or some additional contribution to the stabilization of TDT and longer adsorbates through the presence of crystallinity, would also be consistent with this observation. A remaining unanswered question concerns the ability of long-chain methyl-terminated thiols to erode short-chain methyl-terminated thiols. It would be valuable to compare the erosion of, for example, DDT and TDT by ODT. However, for methyl-terminated SAMs with similar chain
lengths, small contrast differences are observed by SEM. We attempted to use imaging SIMS but found that our instrument (which has only a quadrupole analyzer) lacked the sensitivity to form adequate images using the molecular peaks desorbed from these adsorbates. In future studies, it would be valuable to employ a ToF SIMS system to image patterns formed from methyl-terminated adsorbates. Conclusions Patterns formed from methyl-terminated SAMs shorter than TDT using photolithographic techniques are subject to erosion by solution-phase thiols possessing up to 9 fewer methylene groups and either a hydroxyl or a carboxylic acid terminal group. Patterns formed from hydroxylterminated and carboxylic acid-terminated monolayers are stable against erosion by methyl-terminated thiols that have significantly longer alkyl chains. Hydrogen bonding between terminal functional groups appears to contribute significantly to the stabilization of SAMs formed from hydroxyl-terminated and carboxylic acid-terminated monolayers. Erosion of photopatterned SAMs comes rapidly to completion, indicating that displacement processes are inhibited by steric protection once the alkylsulfonates have been displaced from the photo-oxidized regions of the surface. Thus the alkyl chains within a complete SAM present a steric barrier to displacement by thiols with a greater heat of adsorption. For long-chain methyl-terminated thiols, there appears to be an additional stabilization, possibly associated with the formation of a two-dimensional crystal structure. The displacement process appears to be activated and methylterminated thiols with long alkyl chains appear to be stable against displacement by hydrogen bonding molecules of similar chain length. In summary, SAM stability is the result of a combination of factors, including a sulfur-gold interaction strong enough to guide assembly, steric protection of the Au-S interface by adsorbate alkyl chains, and the existence of multiple intermolecular interactions. Terminal group interactions may make a significant contribution and provide a means by which SAM stability may be enhanced. Acknowledgment. The authors are grateful to the Leverhulme Trust for their financial support. G.J.L. thanks the Nuffield Foundation for a Science Research Fellowship. LA980795B
