Anal. Chem. 1997, 69, 2636-2639
Articles
Laser Desorption Fourier Transform Mass Spectrometry Exchange Studies of Air-Oxidized Alkanethiol Self-Assembled Monolayers on Gold Jill R. Scott,† Laura S. Baker,† W. Russell Everett,‡ Charles L. Wilkins,† and Ingrid Fritsch*,‡
Department of Chemistry, University of California, Riverside, California 92521, and Department of Chemistry and Biochemisty, University of Arkansas, Fayetteville, Arkansas, 72701
Exchange of self-assembled monolayers (SAMs) of dodecanethiol (C12H25SH) on gold with decanethiol (C10H21SH) was detected with laser-desorption Fourier transform mass spectrometry (LD-FTMS) in the negative ion mode. The amount of adsorption of C10H21SH is dependent on the extent of air oxidation of the C12H25SH SAM. In this study, a partially air oxidized C12H25SH SAM with a 2.2:1.5:1 ratio of dodecanethiolate (C12H25S-, m/z 201) to the presumed dodecanesulfinate (C12H25SO2-, m/z 233) to dodecanesulfonate (C12H25SO3-, m/z 249) on the surface is examined. After the sample is soaked in an ethanol solution of C10H21SH for 30 min, the LDFTMS spectrum shows that both the C12H25SO2- and the C21H25SO3- are gone, but a decanethiolate peak appears (C10H21S-, m/z 173) in the same ratio to C12H25S- (1.19) as the ratio of the combined oxidation products (1.16) prior to soaking. Subsequent air exposure (9 h) of this SAM composed of mixed thiolates gives a similar ratio (1.06) of the fully oxidized sulfonates of both species, which can be completely removed upon subsequent soaking in a solution of C12H25SH, resulting in a SAM of pure dodecanethiolate by FTMS analysis. Negligible exchange occurs when nonoxidized C12H25SH SAMs are soaked in decanethiol solution for 30 min. From these observations, we draw several conclusions. First, the results refute suspicions that sulfonates form in a laserinduced reaction of thiolates with O2 absorbed in the SAM during air exposure. Second, ionization efficiency of thiolates and sulfonates during LD-FTMS analysis is approximately equal. Third, previous literature reports of exchange experiments should be carefully scrutinized for possible evidence of the effects of air oxidation. Self-assembled monolayers (SAMs) of organothiols on gold have been the topic of extensive investigation because of their potential importance in a variety of applications, such as corrosion inhibition, electron transport studies, chemical sensors, and models for biomembranes. The utility of SAMs in any application is highly dependent on their stability. Consequently, several investigations have involved kinetics and thermodynamics of † ‡
University of California. University of Arkansas.
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adsorption of organothiols with gold and desorption kinetics of SAMs. Exchange of organothiols from solution for species on the surface is an indirect way to study these phenomena. Thus, Chidsey and co-workers,1 using a mixture of ferrocene-terminated and unsubstituted alkanethiols on gold, discovered that exchange of alkanethiol monolayers in ethanol solution was slow (on the order of days) and that a significant fraction of the monolayer did not exchange even this rapidly. This apparent existence of two thiol populations was interpreted in terms of surface defect and nondefect sites. Subsequent studies by Collard and Fox revealed similar exchange behavior.2 However, it has been well established by a variety of methods that air oxidation of organothiol SAMs3-7 occurs to form organosulfonates. These oxidized species quickly desorb from the surface when placed in a solubilizing solvent.8,9 In fact, it is said that this weak association with the surface is utilized in patterning of UV-irradiated SAMs, in which sulfonates generated in the irradiated portions are exchanged for organothiols in solution.10 Consequently, it is of great importance to understand the effects of simple non-UV air oxidation on exchange with organothiols in solution. Here, we report such exchange studies in which the surface is analyzed by 308 nm laser desorption Fourier transform mass spectrometry (LD-FTMS), a technique first applied to SAMs of organothiols on gold by Li et al. using a 193 nm laser.3 All the LD-FTMS spectra presented by those workers showed abundant alkanesulfonate peaks, indicative of air oxidation of the alkanethiol SAMs, although it was suggested that relative abundances of alkanethiol and alkanesulfonate peaks do no accurately reflect their actual abundances. This conclusion (1) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce J. Am. Chem. Soc. 1990, 112, 4301-4301. (2) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192-1197. (3) Li, Y.; Huang, J.; McIver, R. T., Jr.; Hemminger, J. C. J. Am. Chem. Soc. 1992, 114, 2428-2432. (4) Tarlov, M. J.; Newman, J. G. Langmuir 1992, 8, 1398-1405. (5) Burroughs, J. A.; Hanley, L. J. Am. Soc. Mass Spectrom. 1993, 4, 968-970. (6) Rieley, H.; Price, N. J.; White, R. G.; Blyth, R. I. R.; Robinson, A. W. Surf. Sci. 1995, 331-333, 189-195. (7) Rieley, H.; Price, N. J.; Smith, T. L.; Yang, S. J. Chem. Soc., Faraday Trans. 1996, 92, 3629-3634. (8) Tarlov, M. J.; Burgess, D. R. F., Jr.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305-5306. (9) Garrell, R. L.; Chadwick, J. E.; Severance, D. L.; McDonald, N. A.; Myles, D. C. J. Am. Chem. Soc. 1995, 117, 11563-11571. (10) Huang, J.; Dalhgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626628. S0003-2700(96)00964-X CCC: $14.00
© 1997 American Chemical Society
was based on the argument that the difference in the gas phase acidities of the two species would lead to preferential detection of alkanesulfonates. Furthermore, an XPS determination of oxygen indicated that at least one sample had less than 2% oxygen. In contrast, in the present study, using a 308 nm laser, we find approximately equal detection efficiency for the alkanethiolates and alkanesulfonates studied. LD-FTMS provides more definitive information about species on the surface than electrochemical reductive desorption, infrared spectroscopy, or ellipsometry. Not only can it detect both the thiolate and sulfonate species, but it does not require special “tags” to allow distinction between the two types of molecules that are exchanged. For example, groups that have differing redox potentials are needed for electrochemical monitoring,1,2 and functionalities such as cyano or carbonyl are required for infrared spectrometry.11 It is difficult to modify molecules in these ways without causing significant changes in structure and packing of the SAMs that they form. Scintillation studies reported by Schlenoff et al.12 utilize different isotopes of sulfur without using bulky tags. However, the disadvantage of such studies is that they cannot easily detect chemical changes within the labeled molecule. LD-FTMS analysis provides the requisite sensitivity and specificity. The present exchange studies using LDFTMS involve only long alkane chain thiolssdecanethiol (C10H21SH) and dodecanethiol (C12H25SH), which differ in length by two carbons.
EXPERIMENTAL SECTION Samples of SAMs of C12H25SH were prepared by self-assembly from a 0.001 M solution of dodecanethiol (Aldrich) in ethanol onto 2000 Å of gold with a 50 Å chromium adhesion layer sputtered on glass disks (11 mm diameter) cut from microscope slides.13,14 The docanethiol used to prepare the derivatizing solution was filtered through alumina immediately prior to preparation of the solution using a freshly opened bottle of anhydrous reagent alcohol (J.T. Baker), which had been purged by bubbling argon through it for 10 min in an argon-filled glovebag. The gold-coated disks were cleaned by immersion in Piranha solution (mixture of 30% hydrogen peroxide solution and concentrated (95%) sulfuric acid in the ratio 3:7) for 20 min (Caution! Piranha solution is a very strong oxidant and can spontaneously detonate upon contact with organic material!), followed by rinsing with copious amounts of Milli-Q deionized water. These disks were allowed to dry and were subsequently immersed in the dodecanethiol solution for 12-14 h to prepare the SAMs. Subsequently, an air-free environment in Ar-purged glovebags was rigorously enforced at all times, except during air exposure to oxidize samples. The dodecanethiol SAMs were rinsed with anhydrous ethanol with a stream of argon in the glovebag. Exchange experiments and analyses were conducted in the dark. Air-free transfer of samples from the glovebag to the FTMS was carried out by directly attaching the glovebag to the sample entry port of the instrument. LD-FTMS analyses were performed using a Lambda Physik excimer laser operating at 308 nm (XeCl), a vacuum system with 10-8 Torr (11) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337-342. (12) Schlenoff, J. B.; Li, M.; Ly, H.; Collard, D. M.; Fox, M. A. J. Am. Chem. Soc. 1995, 117, 12528-12536. (13) Everett, W. R.; Welch, T. L.; Reed, L.; Fritsch-Faules, I. Anal. Chem. 1995, 67, 292-298. (14) Everett, W. R.; Fritsch-Faules, I. Anal. Chim. Acta 1995, 307, 253-268.
Figure 1. Typical LD-FTMS spectra of a nonoxidized SAM of C12H25SH: (a) initial spectrum and (b) spectrum after soaking the SAM in 1 mM C10H21SH in ethanol for 30 min. The peak at m/z 197 is due to Au-.
background pressure, and a 3 T FT/MS Corp. (Madison, WI) single-cell FTMS. Each spectrum results from a single laser shot with power at approximately the threshold for gold ablation (∼2 × 106 W/cm2, 28 ns with a 0.6 mJ pulse). RESULTS AND DISCUSSION Figure 1 shows a negative ion LD-FTMS spectrum obtained following a control experiment in which exchange of C10H21SH was attempted with a nonoxidized C12H25SH SAM. As shown in Figure 1a, C12H25S- (m/z 201) is by far the predominant species present in the starting sample. After a 30 min soak in 1 mM C10H21SH in ethanol, only a small amount of C10H21S- (m/z 173) can be detected (Figure 1b), which we attribute to the presence of a very small quantity of a partially oxidized species (m/z 233, which exchanges) in the starting sample. This observation is consistent with previous reports that exchange is slow.1,2 Thus, we conclude that, if thiolates on the surface and thiols in ethanol solution exchange, it is very slow and does not occur to a significant extent on the time scale of the present experiments. Note also that the spectrum in Figure 1b shows a prominent peak at m/z 197 due to Au- as a result of that particular laser shot having power slightly greater than the gold ablation threshold (due to shot-to-shot variation). As a consequence of these fluctuations, the relative abundance of gold ions in the LD-FTMS spectra varies. However, control experiments show that this variation has no significant effect on the abundances of the organic ions relative to one another (see Supporting Information for examples of these control spectra). Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
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Figure 2. Typical LD-FTMS spectra of a single sample of a C12H25SAM that results from the following sequence of events: (a) as received at UC Riverside from Arkansas, ratio of (m/z 233 + m/z 249):m/z 201 ) 1.16; (b) after soaking the SAM in 1 mM C10H21SH in ethanol, ratio of m/z 173:m/z 201 ) 1.19; (c) after 9 h of additional air exposure, ratio of m/z 221:m/z 249 ) 1.06; and (d) after soaking the SAM in 1 mM C12H25SH in ethanol. The m/z 197 peak in all spectra is due to Au-.
Figure 2 shows mass spectra of exchange experiments that involve both C10H21SH and C12H25SH with various extents of airexposed SAMs. A C12H25SH SAM partially oxidized during transfer from Arkansas to California (Figure 2a), with a 2.2:1.5:1 ratio of dodecanethiolate (C12H25S-, m/z 201) to the presumed dodecanesulfinate (C12H25SO2-, m/z 233) to dodecanesulfonate (C12H25SO3-, m/z 249), was analyzed by LD-FTMS to determine the relative abundances of thiolate and oxidation products. The ratio of combined oxidation products to thiolate was 1.16. Following this analysis, the sample was removed from the FTMS and soaked in 1 mM C10H21SH solution in ethanol for 30 min, rinsed with ethanol, and analyzed again (Figure 2b). The m/z 233 and 249 peaks are now absent, but a new peak at m/z 173, corresponding to C10H21S-, has appeared. In this spectrum, the ratio of the new C10H21S- to C12H25S- is 1.19. This result clearly shows that the sulfinate and sulfonate exchange with thiols in solution but that the thiolates exchange slowly, if at all. Another important point is that this and the control experiment establish that the ionization efficiency of thiolates and sulfonates in LDFTMS is approximately the same. This conclusion contradicts previous literature reports which propose that ionization efficiency of sulfonates is much higher than that of thiolates.3,5,15 However, those previous conclusions are based on briefly mentioned X-ray photoelectron spectroscopy (XPS) measurements of oxygen3 and observation of lower than expected abundances of thiolates relative to sulfonates when SIMS15 and ion trap laser desorption analysis5 were employed. (15) Offord, D. A.; John, C. M.; Griffin, J. H. Langmuir 1994, 10, 761-766.
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Next, the mixed SAM with the ratio of C10H21S- to C12H25Sor 1.19 was exposed to air in the dark for 9 h, resulting in complete oxidation to the corresponding sulfonates, C10H21SO3- (m/z 221) and C12H25SO3- (m/z 249), detected in a 1.06 peak ratio by LDFTMS (Figure 2c). This result shows that a chain length difference of two carbons does not yield a significant different ionization efficiency for either thiolate or sulfonate species. Subsequent soaking for 30 min in 1 mM C12H25SH in ethanol, followed by rinsing, completely replaces both sulfonates with C12H25S- (Figure 2d). From these observations, we draw several conclusions. First, the results refute suspicions that sulfonates form due to laserinduced reaction of thiolates with O2 absorbed in the SAM during air exposure. If that were so, only the portion of the samples exposed to the laser during analysis would have formed sulfonates. Thus, roughly quantitative exchange of sulfonates for the entire surface, as seen here, would not occur. Accordingly, the laser cannot be the initiator of the sulfonate formation. Second, ionization efficiency of thiolates and sulfonates during LD-FTMS analysis is approximately equal. Third, previous literature reports of exchange experiments should be carefully scrutinized for possible evidence of the effects of air oxidation. ACKNOWLEDGMENT This work was supported by the National Science Foundation under Grant CHE-92-01277 (C.L.W.), Grant CHE-93-08946 (I.F.), and NSF Career Award CHE-96-24114 (I.F.). W.R.E. is grateful
for an Electrochemical Society Energy Research summer fellowship, sponsored by the U.S. DOE. SUPPORTING INFORMATION AVAILABLE Spectra of nonoxidized C12H25SH SAM on evaporated gold before and after soaking in 1 mM C10H21SH in ethanol, demonstrating a replicate analysis of the type included in Figure 1 in the paper; spectra of nonoxidized C12H25SH SAM on sputtered gold before and after soaking in 1 mM C10H21SH in ethanol, demonstrating a second replicate analysis of the type included in Figure 1 in the paper; spectra of nonoxidized C12H25SH SAM on evaporated gold before and after soaking in 1 mM C16H33SH in ethanol, demonstrating that this longer chain alkanethiol does not
exchange with the nonoxidized C12H25SH SAM; two sets of spectra illustrating internal consistency, for two different samples, of the ratio of oxidized to unoxidized species of C12H25SH SAM with slightly varying laser fluence, showing that laser fluence has little effect on either the ratio of oxidized to nonoxidized species or the ratio of sulfinate (m/z 233) to sulfonate (m/z 249) (13 pages). Ordering information is given on any current masthead page. Received for review September 20, 1996. April 15, 1997.X
Accepted
AC9609642 X
Abstract published in Advance ACS Abstracts, June 1, 1997.
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