1460
Langmuir 2000, 16, 1460-1462
Divergent Surface Functionalization Using Acid Fluoride-Functionalized Self-Assembled Monolayers
Scheme 1
Angelika Niemz, Eunhee Jeoung, Andrew K. Boal, Robert Deans, and Vincent M. Rotello* Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003 Received July 20, 1999
Introduction Self-assembled monolayers (SAMs) provide a powerful platform for nanotechnology,1 with numerous applications including chemo-2 and biosensors,3 creation of surfaces for controlled crystal growth,4 fabrication of biomimetic systems,5 and engineering of surface structure.6 Central to the realization of these applications is the preparation of chemically diverse SAMs. These diverse monolayers are generally formed by the convergent synthesis of monolayer systems through simultaneous adsorption of multiple thiols. Recently, however, there has been a trend to develop divergent synthetic routes to functionalized monolayers. This approach allows the rapid creation of diverse surfaces7 and is especially well suited to combinatorial strategies. A key requirement for this methodology is the availability of reactive terminal functionality that is stable during the deposition process yet reacts efficiently with modifying elements. Recently, various methods for carboxylic acid activation on self-assembled monolayers have been reported. These methods include the formation of acyl chloride-functionalized SAMs using thionyl chloride8 and the application of mixed anhydrides or interchain anhydrides9 and pentafluorophenyl esters.10 The instability of SAM surfaces to thionyl chloride and the steric limitations inherent in the anhydride and active ester methods limit the utility of these routes. In recent studies, carboxylic acid fluorides (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (b) Whitesides, G. M. Sci. Am. 1995, 273 (3), 146. (c) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) (a) Tokuhisa, H.; Crooks, R. M. Langmuir 1997, 13, 5608. (b) Dermody, D. L.; Crooks, R. M.; Kim, T. J. Am. Chem. Soc. 1996, 118, 11912. (c) Flink, S.; Boukamp, R. A.; van den Berg, A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 4652. (3) (a) Kinnear, K. T.; Monbouquette, H. G. Anal. Chem. 1997, 69, 1771. (b) Sigal, G. B.; Bamdad, C.; Barberis, A.; Strominger, J.; Whitesides, G. M. Anal. Chem. 1996, 68, 490. (c) Roberts, C.; Chen, C. S.; Mrksich, M.; Martichonok, V.; Ingber, D. E.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 6548. (d) Higashi, N.; Takahashi, M.; Niwa, M. Langmuir 1999, 15, 111. (4) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 2629. (5) (a) Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 1999, 121, 4914. (b) Fitzmaurice, D.; Rao, S. N.; Preece, J. A.; Stoddart, J. F.; Wenger, S.; Zaccheroni, N. Angew. Chem., Int. Ed. Engl. 1999, 38, 1147. (c) Tam-Chang, S.-W.; Mason, J.; Iverson, I.; Hwang, K.-O.; Leonard, C. J. Chem. Soc., Chem. Commun. 1999, 65. (6) Schonherr, H.; Kremer, F. J. B.; Kumar, S.; Rego, J. A.; Wolf, H.; Ringsdorf, H.; Laschke, M.; Butt, H. J.; Bamberg, E. J. Am. Chem. Soc. 1996, 118, 13051. (7) (a) Yousaf, M. N.; Mrksich, M. J. J. Am. Chem. Soc. 1999, 121, 4286. (b) Horton, R. C., Jr.; Herne, T. M.; Myles, D. C. J. Am. Chem. Soc. 1997, 119, 12980. (c) Yan, L.; Zhao, X. M.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 6179. (8) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (9) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704. (10) Lahiri, J.; Ostuni, E.; Whitesides, G. M. Langmuir 1999, 15, 2055.
have been shown to be stable to ambient conditions yet react cleanly and in high yield with primary amines with no added base or catalyst.11 Acid fluorides are sterically nondemanding and reasonably stable under ambient conditions, making them ideal intermediates for surface functionalization. We present here the application of this methodology to the divergent synthesis of functionalized monolayers. Experimental Section Nanopure H2O, USP-grade absolute ethanol, and reagent grade CH2Cl2 dried via distillation over CaH2 were used for washing and solution preparation. Octyl disulfide was obtained via air oxidation of octanethiol. N-(2-aminoethyl)ferroceneamide (5c) was synthesized by reacting ferrocene acid fluoride with ethylenediamine.12a Tetrabutylammonium perchlorate (TBAP, obtained from SACHEM, electrometric grade) was recrystallized twice from water and dried for several days under high vacuum. Other chemicals were reagent grade and were used without further purification. 11,11′-Dithiobis(undecanoic acid) (1) (Scheme 1). To a solution of 11-mercaptoundecanoic acid (200 mg, 0.92 mmol) in methanol (50 mL) was added potassium carbonate (500 mg, 3.6 mmol). The suspension was stirred overnight open to the air. Ice water (25 mL) and concentrated HCl (2 mL) were then added, and the mixture was extracted twice with EtOAc. The organic portion was collected, washed once with saturated aqueous NaCl, and dried over MgSO4. This gave the crude disulfide 1 (150 mg, 75% yield) as a colorless solid which was used without further purification. 11,11′-Dithiobis(undecanoic acid fluoride) (2) (Scheme 1). To a solution of acid disulfide 1 (100 mg, 0.23 mmol) in CH2Cl2 (10 mL) at 0 °C was added pyridine (0.2 mL, 200 mg, 2.5 mmol) followed by cyanuric fluoride (0.1 mL, 100 mg, 0.75 mmol). The mixture was stirred under argon for 6 h, ice was then added, and the suspension was filtered through Celite. The filtrate was washed with water and dried over MgSO4. Bis(acid fluoride) 2 (86 mg, 85% yield) was found to be >95% pure by 1H NMR and used as is to form SAMs. This material can be stored under argon in the freezer for several months without significant degradation. (11) (a) Carpino, L.; Sadat-Aalaee, D.; Chao, G.; DeSelms, R. J. Am. Chem. Soc. 1990, 112, 9651. (b) Kokotos, G.; Noula, C. J. Org. Chem. 1996, 61, 6994. (c) Galow, T. H.; Rodrigo, J.; Cleary, K.; Cooke, G.; Rotello, V. M. J. Org. Chem. 1999, 64 (4), 3745. (12) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (b) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152.
10.1021/la990972o CCC: $19.00 © 2000 American Chemical Society Published on Web 11/20/1999
Notes
Langmuir, Vol. 16, No. 3, 2000 1461 Scheme 2
Figure 1. F(1s) and N(1s) multiplex region for the XPS of surface 3 before (a) and after (b) derivatization with 5a.
Monolayer Preparation and Functionalization (Scheme 2). Gold substrates (50 Å Ti, 2000 Å Au on glass slides) were purchased from Evaporated Metal Films (Ithaca, NY). Prior to use, gold slides were cleaned for 20 min in hot piranha etch (H2SO4 concentrated + 30% H2O2, 4:1, 75 °C. CAUTION: strong oxidant), rinsed with H2O and EtOH, and dried under vacuum. Deposition and functionalization of the monolayers was carried out with degassed solutions in Schlenk-type flasks under argon. Deposition solutions contained a total disulfide concentration of 10-3 M in CH2Cl2, which for the mixed monolayer meant a 3:1 ratio of octyl disulfide (7.5 × 10-4 M) and the bis(acid fluoride) disulfide 1 (2.5 × 10-4 M). Monolayers were formed by immersion of the gold slides into the deposition solution for 12-24 h, followed by rinsing with copious amounts of CH2Cl2 and drying under vacuum. For further functionalization, the acid fluoride SAM was immersed for 12 h into solutions of dodecylamine (5a, 5 × 10-3 M, CH2Cl2), tryptamine (5b, 1.5 × 10-3 M, CH2Cl2), and N-(2-aminoethyl)ferroceneamide (5c, 1.5 × 10-3 M, CH2Cl2) to provide surfaces 4a, 4b, and 4c, respectively. The amidefunctionalized SAMs were rinsed with CH2Cl2 and dried under vacuum. X-ray Photoelectron Spectroscopy. XPS spectra were obtained on a Physical Electronics Inc. Model 5100 XPS spectrophotometer using the Mg KR source with an electron beam power of 400 W. Spectra were acquired at takeoff angles of 15° and 75° from the surface. High-resolution spectra were recorded with a 35.75 eV pass energy and acquisition times ranging from 3 to 6 min. Grazing Angle FT-IR Spectroscopy. Spectra were acquired in single reflection mode on a nitrogen-purged Perkin-Elmer 2000 FT-IR spectrophotometer equipped with the Graseby Specac variable-angle spectral reflectance accessory. The p-polarized light was reflected off the surface at an 84° angle. All spectra are reported in the absorption mode relative to a clean gold substrate. A narrow band MCT-detector (6500-700 cm-1) cooled with liquid nitrogen was used to detect the reflected light, with a spectral resolution of 2 cm-1. Cyclic Voltammetry. Experiments were carried out in dry, degassed CH2Cl2 containing 0.1 M TBAP as carrier electrolyte. A 20 mm2 exposed SAM area served as the working electrode, a Pt-spiral auxiliary electrode and a fritted Ag wire pseudoreference electrode completed the circuit. Prior to the actual
Figure 2. FTIR spectrum of surface 3 after reaction with 5a. experiment, the reference electrode was calibrated against ferrocene (10-3 M in 0.1 M TBAP, CH2Cl2).
Results and Discussion The presence of the acid fluoride functionality in monolayer 3 was confirmed using XPS spectroscopy. In the XPS spectra, a strong F (1s) peak was observed along with peaks assigned to C(1s), S(1s), and O(1s) (Figure 1a). Initial exploration of derivatization of surface 3 was undertaken by reaction with dodecylamine (5a) to provide surface 4a. This new surface was found to be highly hydrophobic with a stationary water contact angle of 89°, lower than would be expected for a crystalline long-chain alkanethiol monolayers12 but similar to values obtained for related mixed monolayers.9c,13 Since only 25% of the surface is functionalized with the long-chain amine, the upper region of the monolayer is loosely packed and noncrystalline, resulting in the lower contact angle. To quantify the extent of derivatization, XPS and IR spectra were obtained for these surfaces (Figures 1b and 2, respectively). In the XPS spectra, the disappearance of (13) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330.
1462
Langmuir, Vol. 16, No. 3, 2000
Notes
in Figure 3. These voltammograms, which show a peak current directly proportional to the scan rate, demonstrate immobilization of the ferrocene redox unit.14 The peakto-peak separation is larger than the theoretically expected 0 mV and increases with increasing scan rate. This behavior indicates that the electron transfer between ferrocene and the gold electrode, via the long-chain alkyl monolayer, represents a rate-limiting step.15 A certain portion of the peak-to-peak separation can also be attributed to uncompensated internal resistance, a common occurrence in organic media of high resistivity. In summary, we have prepared an acid fluorideterminated SAM on a gold surface. These SAMs were found to react cleanly and efficiently with a variety of primary amines to yield the corresponding amides as well as more functionally diverse monolayers. Application of this methodology to the creation of chemically diverse and combinatorially functionalized SAMs is underway and will be reported in due course. Figure 3. Cyclic voltammograms of the ferrocene-derivatized surface 4c as working electrode at (a) 200 mV/s, (b) 100 mV/s, and (c) 50 mV/s, using CH2Cl2 with 0.1 M tetrabutylammonium perchlorate as the electrolyte.
the F(1s) peak was observed and a new peak assigned to N(1s) appeared, demonstrating reaction of the acid fluoride with the amide. The FTIR spectrum showed intense peaks in the methylene stretch region as well as a carbonyl peak at 1540 cm-1, consistent with the formation of a longchain amide. To demonstrate the diversity of this method, surfaces 4b and 4c were also prepared. As before, very little or no F(1s) was observed in the XPS spectra of the postfunctionalized surfaces, indicating that this reaction can efficiently be extended to functionalized primary amines. We also explored the electrochemistry of surface 4c; a series of cyclic voltammograms of this surface is shown
Acknowledgment. This research was supported by the National Science Foundation (Grants CHE9905492 and DMR-9809365 (MERSEC instrumentation) and the Petroleum Research Fund of the ACS (PRF 33137-AC4,5). V.M.R. acknowledges support from the Alfred P. Sloan Foundation, Research Corporation, and the Camille and Henry Dreyfus Foundation. Supporting Information Available: Spectral data for disulfide 2 and XPS of surfaces 5a-5c (11 pages PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA990972O (14) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, pp 109335. (15) Weber, K.; Hockett, L.; Creager, S. J. Phys. Chem. B 1997, 101, 8286.
