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Multitechnique Surface Characterization of Derivatization Efficiencies for Hydroxyl-Terminated Self-Assembled Monolayers Sheng Pan, David G. Castner, and Buddy D. Ratner* Departments of Chemical Engineering and Bioengineering, Box 351720, University of Washington, Seattle, Washington 98195 Received November 5, 1997. In Final Form: February 23, 1998 Surface derivatization with trifluoroacetic anhydride (TFAA) on self-assembled monolayers of 16mercapto-1-hexadecanol (MHD) was studied to develop selective and well-defined methods for the surface modification of biomaterials. The terminal hydroxyl groups of MHD were functionalized by both solventand vapor-phase reactions. The reaction kinetics, accessibility, and reactivity of SAMs with different surface coverages and film structures were investigated via complementary surface analytical techniques (external reflection-absorption FTIR, XPS, and TOF-SIMS). The TFAA derivatization reaction exhibited rapid kinetics on the hydroxyl-terminated SAMs. The data from each of the surface analytical techniques consistently indicated a nearly complete surface reaction. This suggests that the trifluoromethyl ester group did not introduce significant steric hindrance. The high accessibility and reactivity of TFAA derivatization on hydroxyl SAMs is related here to earlier studies on polymer systems.
Introduction Self-assembled monolayers (SAMs) with different terminal groups have potential applications in several areas of bioengineering, including biosensors, biomimetic processes, and biomaterials, due to their well-defined, organized structure and stability. The outermost functional groups of a SAM can be chemically modified to create well-defined surfaces for the study of interactions with biomolecules and cells. An understanding of the accessibility and reactivity of the outermost groups is a critical step in developing systematic control of the chemical composition of the surfaces. Furthermore, defect structures in materials may influence biological reactions. Studies with molecular film structures ranging from almost perfectly ordered to highly disordered will help us to assess the role of defects in biological reactions. Chemical derivatization techniques were developed by a number of researchers to selectively label polymer surface functional groups of interest, for analysis by methods such as X-ray photoelectron spectroscopy (XPS)1-4 and mass spectrometry.5,6 The application of chemical derivatization techniques to build SAM multilayer structures with novel properties is a new and exciting area with growing interest. Sun et al. reported probably the first vapor-phase derivatization study on amine- and hydroxyl-terminated SAMs using dimethylchlorosilane.7 Other SAM derivatization studies include amine and * To whom correspondence should be directed. Telephone: (206)685-1005. Fax: (206)616-9763. E-mail:
[email protected]. washington.edu. (1) Batich, C. D.; Wendt, R. C. Am. Chem. Soc. Symp. Ser. 1981, 162, 221. (2) Batich, C. D. Appl. Surf. Sci. 1988, 32, 57. (3) Chilkoti, A.; Ratner, B. D.; Briggs, D. Chem. Mater. 1991, 3, 51. (4) Chilkoti, A.; Ratner, B. D. Surf. Interface Anal. 1991, 17, 567. (5) Anderegg, R. J. Mass Spectrom. Rev. 1988, 7, 395. (6) Saxby, M. J. Org. Mass Spectrom. 1970, 4, 133. (7) Sun, L.; Thomas, R. C.; Crooks, R. M. J. Am. Chem. Soc. 1991, 113, 8550. (8) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (9) Obeng, Y. S.; Laing, M. E.; Friedli, A. C.; Yang, H. C.; Wang, D.; Thulstrup, E. W.; Bard, A. J.; Michl, J. J. Am. Chem. Soc. 1992, 114, 9943.
alcohol derivatization on 11-mercaptodecanoic acid SAMs,8 ruthenium pentamine functionalization of rigid-rod dithiol SAMs,9 and photoattachment of amines to di-11-(4azidobenzoate)-1-undecyl disulfide SAMs.10 More detailed investigations of SAM derivatization focusing on surface accessibility and reactivity were published recently. Himmel et al.11 presented an ultrahigh vacuum (UHV) surface derivatization study of phenyl isocyanate on OH and COOH SAMs by the chemical vapor deposition (CVD) technique. They achieved an 87% reaction yield on OH SAMs by condensing a phenyl isocyanate multilayer on top of the SAMs at low temperature (130 K) and then warming the samples to 290 K. Their observation is consistent with the space requirement of phenyl rings with respect to the packing of alkyl chains in SAMs, according to previous theoretical studies on SAMs with phenyl sufone groups.12,13 Bertilsson and Liedberg14 studied the distribution of OH groups in a mixed SAM of 16-mercapto-1-hexadecanol (MHD) and n-alkanethiols by reacting it with trifluoroacetic anhydride (TFAA) in tetrahydrofuran (THF) solution. Although there was no evidence of underivatized OH groups detected by IR, the yield of the reaction was estimated to be 80-90% due to concern about possible steric effects. Later, Hutt and Leggett15 presented XPS and contact angle studies of vapor-phase derivatization on monolayers of mercaptopropanol and mercaptopropanoic acid. For TFAA vapor-phase derivatization, although the XPS data indicated a nearly complete surface reaction, they speculated there were not sufficient degrees of freedom for the CF3 groups to pack at the same density as that of the underlying monolayer, based on the packing density of perfluorinated SAMs. (10) Wollman, E. W.; Kang, D.; Frisbie, D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395. (11) Himmel, H.-J.; Weiss, K.; Jager, B.; Dannenberger, O.; Grunze, M.; Woll, C. Langmuir 1997, 13, 4943. (12) Shnidman, Y.; Ulman, A.; Eilers, J. E. Langmuir 1993, 9, 1071. (13) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991; p 237. (14) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141. (15) Hutt, D. A.; Leggett, G. J. Langmuir 1997, 13, 2740.
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The controversy between experimental data and the theoretical interpretation regarding TFAA derivatization efficiency on SAMs leads us to believe that a detailed investigation of TFAA derivatization on SAMs is timely. On polymer surfaces, the yield of TFAA derivatizations on OH and epoxide groups can reach nearly 100%.3,4 In this study, TFAA was used as a model compound to derivatize a self-assembled monolayer of MHD on a polycrystalline gold surface in both solvent and vapor phases. The surface reaction kinetics, accessibility, and reactivity of the outermost functional groups, as well as the stability of the SAM systems under reaction environments, were investigated by complementary surface analytical techniques. Experimental Section Substrate Preparation, SAM Formation, and Surface Derivatization. Gold substrates were prepared as follows. Clean glass (VWR microslides) and polished Si wafers (Silicon Quest International) were first coated with a 50-Å layer of chromium to improve adhesion of gold to the substrates. Then 2000 Å of high-purity gold (99.999%, Alfa) was deposited on the top of the chromium layer. Electron-beam evaporation was used to deposit both chromium and gold. The solutions of 16-mercapto-1-hexadecanol (MHD, HS(CH2)16OH) (99.2%, Pharmacia) were prepared with absolute ethanol (200 proof, McCormick). The SAMs were formed by immersing gold substrates into thiol solutions of appropriate concentrations (0.01 and 1.00 mM) for specific time periods at room temperature. The immersion time varied from a few seconds to 4 days. After removal, the SAM samples were rinsed extensively with absolute ethanol, ultrasonically cleaned in ethanol for 2 min, and then thoroughly rinsed with ethanol again. The ultrasonication step appears to be necessary in order to remove small quantities of physisorbed thiols.16 Following cleaning, the samples were blown dry with nitrogen. The OH groups of MHD can be functionalized by reacting them with TFAA. For the solvent-phase reaction, the hydroxyl SAMs were derivatized in a tetrahydrofuran (THF) (99.9%, H2O < 0.01%, J. T. Baker) solution containing 0.1 mM trifluoroacetic anhydride (TFAA) (99+%, Aldrich) and 0.1 mM triethylamine (Et3N) (98%, J. T. Baker) under nitrogen purge. For the vaporphase reaction, the hydroxyl SAMs were exposed to TFAA vapor in a sealed reactor under ambient conditions. Surface Characterization. X-ray Photoelectron Spectroscopy. XPS experiments were performed on a SSX-100 (Surface Science Instruments) system with a Al KR X-ray source (hν ) 1486.6 eV), a quartz monochromator, a hemispherical analyzer, and a position-sensitive detector. The binding energy (BE) scales for the monolayer on gold, before and after TFAA reaction, were referenced by setting the Au4f/7/2 BE to 84.0 eV. The survey and high-resolution spectra were acquired at pass energies of 150 and 50 eV, respectively. All XPS data were acquired at a nominal photoelectron takeoff angle of 55°, where the takeoff angle is defined as the angle between the surface normal and the axis of the analyzer lens. SSI data analysis software was used to calculate the elemental composition from peak areas and to peak fit the high-resolution spectra. The accuracy and precision of XPS compositional data were estimated to be (3% or better for all elements in the monolayer except S. Time-of-Flight Secondary Ion Mass Spectrometry. The timeof-flight secondary ion mass spectrometry (TOF-SIMS) experiments were performed on a model 7200 Physical Electronics PHI instrument (Eden Prairie, MN) equipped with a pulsed 8-keV Cs+ ion source and a two-stage reflectron time-of-flight mass analyzer. Data were acquired over a mass range from m/z ) 0 to 2000 for both positive and negative secondary ions. The ion source was operated with a current of 1.5 pA and a pulse width of 1.2 ns. The total ion dose used to acquire each spectrum was maintained below 2 × 1012 ions/cm2. The area of analysis for each spectrum was 0.01 mm2. The mass resolution (M/∆M) for both positive and negative secondary ions was typically between (16) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882.
Figure 1. External reflection-adsorption FTIR spectra of TFAA solvent-phase derivatization kinetics on a self-assembled monolayer of 16-mercapto-1-hexadecanol (MHD). 6000 and 8000. The mass scale for the negative secondary ions was calibrated using peaks originating from C2H-, SH-, Au2S-, and Au2[M-H] -. The mass scale for the positive secondary ions was calibrated using the C3H5+, C2H3+, and CH3+ peaks. The difference between the theoretical and observed masses for both positive and negative calibration ions was less than 10 ppm. FTIR. External reflection-adsorption FTIR measurements were performed on a Digilab FTS-60A spectrometer in 80° grazing angle reflection mode. The incidence light was p-polarized. The system was purged with water and carbon dioxide-free air. A liquid-nitrogen-cooled MCT detector was used. All data were taken at 4-cm-1 resolution with the accumulation of 1024 scans. The IR peak intensity measurement was estimated to be subject to 4% to 7% error.17,18 Where multiple IR spectra were plotted on the same graph, the intensity scales were the same. Contact Angle Measurements. Static advancing water contact angles were measured using a Rame-Hart A-100 goniometer under ambient conditions. To achieve this measurement, a drop of water with a fixed volume was added slowly to the surface with a flat tip hypodermic needle attached to a syringe. Once observable drift in the drop shape stopped, the static advancing contact angle was measured without removing the needle from the drop. The data were obtained by taking the average of three samples. For each sample, five different spots were measured.
Results Reaction Kinetics. Equilibrium self-assembled monolayers of MHD (24 h in 1 mM solution) were derivatized with TFAA in THF for different lengths of time. The SAMs were characterized by external reflection-adsorption FTIR, XPS, and TOF-SIMS to ensure they had formed a closed packed, highly ordered monolayer on the gold surface prior to the surface reaction. Figure 1 shows the external reflection-adsorption FTIR spectra at different (17) Porter, M. D.; Bright, T. B.; Allara, D. L.; Kuwana, T. Anal. Chem. 1986, 58, 2461. (18) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927.
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Figure 2. IR intensities of the OH group and the CF3 group as a function of TFAA surface reaction time (solvent-phase derivatization).
reaction times during the solvent-phase derivatization. The adsorption bands appearing at 2918 and 2850 cm-1 represent the C-H asymmetric (νa) stretching mode and the C-H symmetric (νs) stretching mode, respectively.19 The band at 2878 cm-1 was assigned by Nuzzo and coworkers to the C-H stretching mode associated with the CH2 group adjacent to the OH group.20 At high wavenumbers, the broad absorption band appearing in the 3500-3100-cm-1 region is characteristic for intermolecularly hydrogen-bonded O-H.19 The reacted trifluoromethyl ester group has several characteristic C-F stretching bands in the 1300-1100-cm-1 region and a CdO stretching band at 1806 cm-1.19 The strongest bands at 1243, 1192, and 1155 cm-1 are assigned to the ν(FCF2), ν(CF3), and ν(CF2) stretching modes.14,21 The intensities of the C-F and O-H stretching bands are plotted as a function of reaction time in Figure 2 and indicate that the TFAA derivatization reaction is rapid. The intensity of the OH group decreases as reaction time increases and is no longer observable by FTIR after 15 min. The static contact angle (H2O) data also indicate a significant decrease of wettability. Contact angles rose from 10 ( 0.6° for MHD SAMs to 101 ( 0.7° after derivatization for 30 min. The frequency and intensity of the C-H stretching mode at different reaction stages as indications of crystallinity and molecular orientation, respectively,22-25 are essentially the same as those for the MHD monolayers before reaction. Vapor-phase reaction is more rapid than that in the solvent phase. Our IR data indicated the vapor phase reaction went to completion within 10 min, since all spectra after 10 min of reaction were identical. Reactivity and Asscessibility. Figure 3 shows highresolution C 1s XPS spectra for MHD before and after 1 h of derivatization with TFAA in solvent and vapor phases. For MHD SAMs, the C 1s spectra can be resolved into two peaks: one at 285.1 eV for CHx species and another at 286.5 eV for CsOH species. The C 1s spectral envelope of a TFAA-derivatized MHD monolayer required a four(19) Socrates, G. Infrared Characteristic Group Frequencies Tables and Charts, 2nd ed.; John Wiley & Sons: Chichester, U.K., 1994; p 62. (20) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (21) Redington, R. L. Spectrochim. Acta 1975, 31A, 1699. (22) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700. (23) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (24) Snyder, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M. J. Phys. Chem. 1986, 90, 5623. (25) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52.
Figure 3. XPS C 1s spectra of MHD monolayers before and after derivatization with TFAA for 1 h in both the liquid and vapor phases. Table 1. Theoretically and Experimentally Determined Composition (Atomic Ratio) of TFAA-Derivatized MHD 100% reaction solvent-phase vapor-phase (theoretical) derivatization derivatization F/O CsO/OsCdO/CF3
1.5:1 1:1:1
1.5:1 1.1:1:1
1.5:1 1.1:1:1
peak fit. The new peaks at 290.0 and 293.2 eV can be attributed to the ester carbon (OsCdO) and CF3 groups, respectively. The yield of the reaction can be calculated from the F/O atomic ratio and the ratio of the CsO, OsCdO, and CF3 species in the high-resolution C 1s spectra. The results of XPS analyses of the TFAA derivatization on MHD monolayers are summarized in Table 1. For both solvent- and vapor-phase derivatizations, the F/O atomic ratio and the high-resolution C 1s results indicate a nearly complete functionalization of surface hydroxyl groups. Furthermore, examination of C-H stretching modes in the FTIR spectra before and after reaction revealed that the alkyl chain appeared to be unaffected by TFAA functionalization. The strongest evidence for complete derivatization was provided by TOF-SIMS due to its high sensitivity and molecular specificity. Specific molecular ions can be observed from the negative-ion spectra of MHD SAMs before and after TFAA reaction (Figure 4). Before the reaction, several molecular ions related to MHD SAMs such as AuM-, AuSM-, Au2[M-H]-, and Au[M-H]2- (M ) HS(CH2)16OH) were observed in the negative-ion mass spectrum. After both solvent- and vapor-phase derivatization, the TOF-SIMS spectra show the absence of the AuM-, AuSM-, Au2[M-H]-, and Au[M-H]2- signals (M ) HS(CH2)16OH) and the appearance of the Au2[M′-H]signal (M′ ) HS(CH2)16OCOCF3). Since TOF-SIMS is a
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Figure 4. TOF-SIMS negative-ion mass spectra of MHD monolayers before and after derivatization with TFAA for 1 h in both the liquid and vapor phases.
much more sensitive technique than XPS and FTIR, the TOF-SIMS observations lead us to believe that MHD SAMs were almost completely derivatized by TFAA. TFAA Derivatization of Incomplete SAMs. Incomplete SAMs were created by immersing gold substrates in 0.01 mM MHD solutions for different lengths of time. Figure 5 shows the FTIR spectra of a series of SAMs with different surface coverages, before and after TFAA derivatization. The intensity of the C-H stretching mode is a function of surface concentration as well as molecular orientation. The C-H dipole can only couple to the external reflection IR beam if it has a component oriented along the surface normal based on the IR surface selection rule.26 In Figure 6, the initial increase of the IR intensities with immersion time is primarily due to the increase of surface concentra-
tion. As surface coverage increases to a certain level, the decrease of IR intensity suggests a reorientation of molecules to a position where molecules are tilted 30-40° from the surface normal.27,28 Furthermore, the frequency of the C-H stretching can be monitored to gain insight into the chain conformation. A downward shift of C-H asymmetric (νa) stretching from 2924 to 2917 cm-1 suggests an increase of the crystallinity and ordering of the monolayer structure as the surface coverage increases.23-25 The accessibility and reactivity of SAMs with different structures and orientations can be assessed by TFAA derivatization. It appears that the surface funtionalization by TFAA is complete for SAMs with different surface coverages and structures (Figure 5). The observation was
(26) Golden, W. G. Fourier Transform infrared reflection-absorption spectroscopy. In Fourier Transform Infrared Spectroscopy: Applications to Chemical Systems; Ferraro, J. R., Basile, L. J., Eds.; Academic Press: Orlando, FL, 1985; Vol. 4, p 315.
(27) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (28) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
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Figure 6. IR intensities of the C-H asymmetric stretching mode (∼2918 cm-1) of MHD SAMs with different surface coverages and structures.
further confirmed by TOF-SIMS investigation on incomplete MHD SAMs before and after TFAA derivatization. Discussion TFAA surface derivatizaion on MHD SAMs proceeded rapidly to completion. For solvent-phase derivatization, an earlier radiolabel (35S) study suggested that SAMs immersed in pure solvents for a long period of time might exhibit significant desorption of thiol molecules from the gold surface.29 However, for short periods of time the desorption was not significant. Our XPS data did not indicate a detectable desorption of thiol molecules after immersion in THF for 1 h. Also, the C/Au and O/Au ratios, which are direct measures of surface coverage, did not show significant differences between the solvent-phaseand vapor-phase-derivatized samples. The reaction in the vapor phase is faster than that in the solvent phase, possibly due to a higher concentration of TFAA in the vapor. TFAA has a boiling point of 39.5-40 °C, which leads to rapid vaporization at room temperature. Structural studies of SAMs indicated that long chain n-alkanethiols (CH3(CH2)21SH) adopt a commensurate x3 × x3R30° hexagonal overlayer lattice on Au(111) which places all adsorbates at identical sites.30-37 This structure produces a nearest-neighbor distance between S head groups of 5.0 Å with a 30° tilt angle from the surface normal to allow the molecules to maximize the van der Waals interactions.27,28,38,39 On Au(110) and Au(100) different assembled alkanethiol structures were found.32,34,40 For Au(110), the long chain alkanethiols remove the “missing” row reconstruction of the Au surface and form a com-
Figure 5. External reflection-adsorption FTIR spectra of MHD monolayers with different surface coverages and film structures (created by immersion in 0.01 mM thiol solutions for different lengths of time) before and after TFAA derivatization for 1 h in THF.
(29) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (30) Liu, G.; Salmeron, M. B. Langmuir 1994, 10, 367. (31) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (32) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (33) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421. (34) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (35) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-y.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493. (36) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (37) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (38) Jaschke, M.; Schonherr, H.; Wolf, H.; Butt, H.-J.; Bamberg, E.; Besocke, M. K.; Ringsdorf, H. J. Phys. Chem. 1996, 100, 2290. (39) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 1147. (40) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234.
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mensurate c(2×2) lattice.40 Alkanethiol molecules on Au(100) have the highest density among all three systems.32,40 The areas per molecule of long chain n-alkanethiols (docosyl mercaptan) on Au(111), Au(110), and Au(100) gold single-crystal surfaces are 21.7, 23.6, and 17.8 Å2, respectively.35 In a recent TFAA derivatization study,15 the theoretical expectation of steric hindrance of CF3 was not consistent with the experimental data, which showed nearly 100% reaction yield. The main argument for the possible steric hindrance of CF3 groups was based on the packing density of perfluorinated SAMs, which is less than that of methylene chain SAMs. The fluorinated thiol molecules (CF3(CF2)n(CH2)2SH) form an incommensurate hexagonal lattice with a lattice constant of 5.7 Å on Au(111).41 The area per molecule of perfluorinated SAMs is 29 Å2/molecule with a 20° tilt angle.42 Thus, the packing density of perfluorinated SAMs is about 33% less than those of alkanethiol SAMs. However, the structure and packing density of a SAM system depend not only on the geometry of thiol molecules but also on the binding energy, the specific binding site on gold, and the possible interaction between tail groups. It is inaccurate to use the lower packing density of perfluorinated SAMs to imply a steric hindrance in TFAA derivatization on other types of SAM systems. Given the packing structure of MHD SAMs, the most important factor to consider for examining steric effects of TFAA derivatization is the geometric size of the trifluoromethyl ester group. The van der Waals volume of CF3 is 31.17 Å3, calculated by numerical integration of the van der Waals envelope.43 Therefore, the crosssectional area of CF3 can be estimated as 12 Å2 by using the hard-sphere approximation. As a comparison, the van der Waals volume of CH3 is 16.15 Å3,43 and the crosssectional area is 7.7 Å2 using the same approach. On the basis of the structure and the packing density of MHD SAMs and the size of the trifluoromethyl ester group, the steric effect of CF3 does not appear to be as significant as that suggested in refs 14 and 15. Our experimental data (41) Liu, G.-Y.; Fenter, P.; Chidsey, C. E. S.; Ogletree, D. F.; Eisenberger, P.; Salmeron, M. J. J. Chem. Phys. 1994, 101, 4301. (42) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507. (43) Motoc, I.; Marshall, G. R. Chem. Phys. Lett. 1985, 116, 415.
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support steric arguments that the OH-terminated groups in MHD SAMs can be completely functionalized. On the basis of the IR data, the SAM systems with partial monolayer coverage may contain a significant concentration of alkyl chains with gauche conformations and may have a higher tilt angle from the surface normal. In contrast, for the SAMs with complete monolayer coverage, the alkyl chains are primarily in an all-trans conformation with a close packed, highly ordered structure. The successful TFAA derivatization on partial SAMs indicates good accessibility of OH groups regardless of different surface coverages, packing densities, and orientations of the molecular axis. The exposure of OH groups on the surface may suggest an outward orientation of the OH due to the formation of a H-bonded network on the surface and the possible interactions with the solvent (ethanol). By examining the frequencies of C-H stretching before and after reaction, the introduction of the trifluoromethyl ester group does not appear to have a significant effect on the film structure of incomplete SAMs. Conclusions TFAA surface derivatization of self-assembled monolayers of 16-mercapto-1-hexadecanol demonstrated rapid reaction kinetics in both the solvent and vapor phases. MHD SAMs with different surface coverages, packing densities, and molecular orientations were completely functionalized, indicating the OH groups were accessible and reactive in all cases. For complete monolayer SAMs with a closed packed, crystal-like structure, surface analytical data from a number of techniques support the argument that the steric hindrance of the CF3 group is not significant. Acknowledgment. The authors gratefully acknowledge support from NIH Grant RR01296 and NSF Engineering Research Center EEC-9529161 (UWEB). We thank Mimi Mar for the preparation of gold substrates and Pharmacia Corporation for providing the gift of 16-mercapto-1-hexadecanol. LA971212L
