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Langmuir 1998, 14, 1664-1673
Two-Laser Mass Spectrometry of Thiolate, Disulfide, and Sulfide Self-Assembled Monolayers Jennifer L. Trevor,†,‡ Keith R. Lykke,‡,§ Michael J. Pellin,‡ and Luke Hanley*,† Materials Science and Chemistry Divisions, Argonne National Laboratory, Argonne, Illinois 60439 Received October 16, 1997. In Final Form: December 22, 1997 Self-assembled monolayers (SAMs) of thiolates, disulfides (RSSR+), and sulfides were studied on Au by N2 laser desorption followed by vacuum ultraviolet (VUV) (118-nm) photoionization of secondary neutrals in a time-of-flight mass spectrometer. Dimers (RSSR+) dominated the photoionization mass spectrum from all chain lengths of alkanethiolates and disulfides studied. Nonmethyl-terminated alkanethiolates with X ) (OH and COOH) were detected as dimers without loss of the terminal group. Phenyl-SAMs with X ) (H, OH, OCH3, Cl, and NO2) were detected as both monomers and dimers. Thiocholesterol SAMs were detected solely as monomers. The data suggest that dimerization occurs as a result of the recombination of surface thiolates during desorption. The alkane sulfides were detected intact, but with additional monomer and dimer species present in the spectra. The appearance of dimers is not a strong function of adsorbate structure or ordering and therefore cannot be taken as evidence for or against the recently proposed model of thiolate dimers on Au surfaces. Two receptor adsorbates, resorcin[4]arene tetrasulfide and β-cyclodextrin sulfide were examined by two-laser mass spectrometry (L2MS), but only the former gave identifiable high mass peaks. Mixed thiolate and disulfide monolayers generated both pure and mixed dimers, providing information on nearest neighbor interactions. The mixed disulfide results indicate there is a common adsorption state for thiolates and disulfides. The laser desorption and VUV photoionization cross sections for these various organosulfur SAMs were found to be similar. L2MS with VUV photoionization was nonselective in its detection of these organosulfur species and produced mass spectra with little fragmentation.
1. Introduction Organosulfur self-assembled monolayers (SAMs) are uniquely suited to serve as model organic surfaces for understanding interfacial phenomenon. Extensive experimental work has shown that it is relatively easy to prepare stable, densely packed, and well-ordered SAMs on gold surfaces.1 The terminal group of SAMs can be chemically modified or mixed monolayers can be prepared to alter the macroscopic surface properties in a controllable fashion.2 Organosulfur self-assembly on Au has been attractive primarily because of the strong affinity of the Au-S bond relative to other functional groups and because Au does not form a very stable oxide. Ag, Cu, Pt, and other metal surfaces also form SAMs, but with varying degrees of order, density, and stability.3 A wide variety of experimental methods have been used to characterize SAMs. Chemisorption of alkanethiols on Au is believed to occur by oxidative addition to produce an alkanethiolate.4 Similarly, disulfides undergo S-S bond dissociation, adsorbing on the surface as two separate alkanethiolate species. X-ray photoelectron spectroscopy * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607. ‡ Materials Science and Chemistry Division, Argonne National Laboratory, Argonne, IL 60439. § Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. (1) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (2) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8, 719-729. (3) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 1252812536. (4) Xu, J.; Li, H.-L. J. Colloid Interface Sci. 1995, 176, 138-149.
was unable to distinguish between monolayers formed from thiols and disulfides.5 Furthermore, exchange experiments with competing thiolates have displaced asymmetric disulfides, giving evidence of S-S bond cleavage.6 It is uncertain as to whether sulfides cleave upon surface adsorption. Porter and co-workers have postulated a C-S cleavage, whereas others have contended that no cleavage occurs.7 The ordering of thiolate and disulfide SAMs on gold has also been studied extensively. Tunneling microscopy and diffraction methods have found that SAMs display complex phase behavior that depends upon coverage, temperature, chain length, and method of preparation.1,8,9 Many desorption mass spectrometric (MS) methods have been used to analyze SAMs and provide information on their adsorption state. The similarity of the desorption products from thiolate and disulfide SAMs on Au have been explained by their identical adsorbed states.3 Sulfides are believed to desorb intact, and hence the adsorbed state has been projected to also be an intact sulfide.10 Negative ion secondary ion mass spectrometry has been useful in the analysis of thiolate, disulfide, and sulfide (5) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989; 5, 723-727. (6) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 17661770. (7) Zhong, C.-J.; Porter, M. D. J. Am. Chem. Soc. 1994, 116, 1161611617. (8) van Velzen, E. U. T.; Engbersen, J. F. J.; de Lange, P. J.; Mahy, J. W. G.; Reinhoudt, D. N. J. Am. Chem. Soc. 1995, 117, 6853-6862. (9) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (10) Beulen, M. W. J.; Huisman, B.-H.; van der Heijden, P. A.; van Veggel, F. C. J. M.; Simons, M. G.; Biemond, E. M. E. F.; de Lange, P. J.; Reinhoudt, D. N. Langmuir 1996, 12, 6170-6172.
S0743-7463(97)01136-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/06/1998
L2MS of Thiolate, Disulfide, and Sulfide SAMs
SAMs, and information has been gained regarding adsorption behavior.3,11 Single-laser desorption mass spectrometry (LDMS) has also been used effectively to analyze thiolate SAMs. Negative ion LDMS of thiolates has demonstrated that sulfinates and sulfonates form by photooxidation of the alkanethiolate SAMs on Au.12 Fast atom bombardment and matrix-assisted laser desorption/ ionization (MALDI) have been used to detect SAMs, but both require the addition of a matrix compound on the surface which may lead to the modification of the SAM.10,13 However, novel host-guest type SAMs have been successfully detected using MALDI.14 Temperature-programmed desorption (TPD) has detected recombined thiolate and disulfide SAMs. Initial TPD work by Nuzzo et al. detected the parent ion from a monomer and dimerization of the individual thiolates from a cleaved disulfide.15 More recent TPD measurements of thiols and disulfides have detected a dimer species following desorption.16,17 All of the aforementioned surface mass spectrometries have provided useful chemical information on SAMs. However, they all suffer to varying degrees from fragmentation of the desorbed species, selective detection, and/ or low signal-to-noise ratios. Time-of-flight (TOF) twolaser mass spectrometry (L2MS) with vacuum ultraviolet (VUV) photoionization has the advantage of high detection sensitivity and nonselectivity.18,19 We demonstrate here that desorption with a nitrogen laser (337-nm) is similar to TPD in that it can induce a thermal desorption of adsorbates. Ultrafast desorption of intact species is usually preferred over thermal decomposition in LD, whereas decomposition is often preferred in TPD.20 Spatial chemical information about the surface adsorbates can also be obtained by LD if the desorption laser is focused into a micron-size spot. There are several additional benefits that accrue to L2MS when VUV photoionization is used. Our preliminary results showed that laser desorption/VUV photoionization causes very little fragmentation of adsorbed species, simplifying spectral interpretation.18,19 The separation of the desorption and ionization steps eliminates interfering matrix effects in both steps.21 The desorbed neutral yields are less sensitive to the substrate properties, and the measurement is more reflective of the true concentration of the surface constituents. Finally, VUV radiation causes nonselective single-photon ionization for a wide range of desorbed species. Laser-induced photoelectron ionization of laserdesorbed neutrals has also been employed as a nonselective method of surface analysis.22 (11) Hagenhoff, B.; Benninghoven, A.; Spinke, J.; Liley, M.; Knoll, W. Langmuir 1993, 9, 1622-1624. (12) (a) Li, Y.; Huang, J.; McIver, R. T. J.; Hemminger, J. C. J. Am. Chem. Soc. 1992, 114, 2428-2432. (b) Scott, J. R.; Baker, L. S.; Everett, W. R.; Wilkins, C. L.; Fritsch, I. Anal. Chem. 1997, 69, 2636-2639. (13) Mouradian, S.; Nelson, C. M.; Smith, L. M. J. Am. Chem. Soc. 1996, 118, 8639-8645. (14) Henke, C.; Steinem, C.; Janshoff, A.; Steffan, G.; Luftmann, H.; Sieber, M.; Galla, H.-J. Anal. Chem. 1996, 68, 3158-3165. (15) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733-740. (16) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys., Part 2 1996, 35, L799-L802. (17) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys., Part 1 1996, 35, 5866-5872. (18) Trevor, J. L.; Hanley, L.; Lykke, K. R. Rapid Commun. Mass Spectrom. 1997, 11, 587-589. (19) Trevor, J. L.; Mencer, D. E.; Lykke, K. R.; Pellin, M. J.; Hanley, L. Anal. Chem. 1997, 69, 4331-4338. (20) Zenobi, R. Chimia 1994, 48, 64-71. (21) Savina, M. R.; Lykke, K. R. Trends Anal. Chem. 1997, 16, 242252. (22) Schriemer, D. C.; Li, L. Anal. Chem. 1996, 68, 250-256.
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Figure 1. Scheme for possible desorption products for thiols, disulfides, and sulfides from various surfaces.
In this paper, we demonstrate the utility of laser desorption/VUV photoionization mass spectrometry for molecular surface analysis. We demonstrate this L2MS method by analyzing a variety of thiolate, disulfide, and sulfide SAMs on Au with various terminal groups and chain lengths. We find that monolayers formed from thiols and disulfides desorb as dimers (or disulfides) whereas sulfide monolayers desorb as sulfides, as indicated schematically in Figure 1. We also show that desorption dimerization of thiolates and disulfides can give additional information about nearest neighbors. 2. Experimental Details The following portions of the experimental section encompass the details of the apparatus, microscope, vacuum ultraviolet (VUV) generation, and sample preparation. The TOF Apparatus section introduces the vacuum system, ion optics, sample transfer, and desorption laser facilities. Further information on the operation of the Schwarzschild microscope is included in the second section while the third part contains the details of VUV generation. Finally, the fourth section explains the sample preparation. All experimental work was performed at Argonne National Laboratory. A. TOF Apparatus. All experiments were conducted on a linear 1.3-m TOF mass spectrometer constructed in our laboratory that has been described in detail previously.18,21 Samples were inserted into the chamber (base pressure ∼10-8 Torr) by a sample transfer stage that facilitated the rapid exchange of samples. Two Wiley McLaren23 extraction grids that served to increase the mass resolution were positioned behind the target which was held at 8 kV. The ions were directed to an 18-mm diameter dual microchannel plate detector by horizontal and vertical deflector plates and an einzel lens in the TOF tube. The detector output was amplified and sent to a 300-MHz digital storage oscilloscope, which was triggered by a photodiode monitoring the ionization laser. The raw data was then sent to a desktop computer via a GPIB interface. A Schwarzschild microscope21 focused the desorption laser into the chamber, enabling molecules to be desorbed from ∼1-µm-diameter spots on the sample. A motor-driven mirror mount preceding the microscope was used for scanning the desorption beam across the surface. For all experiments reported in combination with VUV postionization, an ∼4-ns pulse length nitrogen laser (337nm) was used for desorption. The output was attenuated with a neutral density filter wheel. The estimated intensity at the surface was
