In Situ Deprotection and Assembly of S-Tritylalkanethiols on Gold

Langmuir , 2004, 20 (21), pp 9144–9150. DOI: 10.1021/la049627b. Publication Date ... 2004 American Chemical Society. Cite this:Langmuir 20, 21, 9144...
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Langmuir 2004, 20, 9144-9150

In Situ Deprotection and Assembly of S-Tritylalkanethiols on Gold Yields Monolayers Comparable to Those Prepared Directly from Alkanethiols Christina E. Inman, Scott M. Reed,† and James E. Hutchison* Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403-1253 Received February 12, 2004. In Final Form: May 12, 2004

In this paper we describe a systematic study comparing the properties of self-assembled monolayers (SAMs) formed by in situ deprotection and assembly of S-triphenylmethyl (trityl)- and thiolacetate-protected alkanethiols to those of SAMs formed from the parent alkanethiols. The two in situ deprotections were carried out in trifluoroacetic acid and THF/ammonium hydroxide, respectively. Monolayers of octadecanethiol (ODT) and the peptide-containing alkanethiol 3-mercapto-N-n-pentadecylpropionamide (1ATC15) were assembled on gold using the two in situ methods and characterized by contact angle goniometry, X-ray photoelectron spectroscopy, polarization modulation infrared reflection absorption spectroscopy, and electrochemical characterization methods to assess how the monolayer properties compare to those of monolayers prepared by traditional methods. The results for the in situ deprotection of the trityl-protected molecules demonstrate that this method can afford high-quality monolayers that are nearly indistinguishable from those prepared directly from alkanethiols. The quality of the monolayers prepared using this method is shown to depend on the solubility of the trityl-protected compound in trifluoroacetic acid. The results for the in situ deprotection of acetyl-ODT indicate this method yields low-quality monolayers that contain mixtures of adsorbates bound as thiolates and thiolacetates. In situ trityl deprotection is a useful approach for monolayer formation that greatly simplifies the purification, handling, and assembly of thiol-containing monolayer precursors.

Introduction Self-assembled monolayers have a wide range of potential applications, including nonlinear optics,1 electrontransfer studies,2 chemical sensing,3 and corrosion passivation.4 Alkanethiols have historically been used for monolayer studies because of their strong affinity for metals such as gold5 and their ability to self-assemble into well-ordered monolayers.6-9 For most applications, however, the introduction of additional functionality within the chain or at its terminal position is necessary to tune the structure and stability of the monolayer as well as control its interactions with other species.10-13 As research groups explore the properties and use of monolayers formed from thiols with more complex structures and chemical functionality, the reactivity of the thiol * To whom correspondence should be addressed. E-mail: hutch@ uoregon.edu. † Present address: Department of Chemistry, Portland State University, Portland, OR 97207. (1) Nie, W. Adv. Mater. 1993, 5, 520-545. (2) Yu, H.-Z.; Zhang, H.-L.; Liu, Z.-F. Langmuir 1998, 14, 619-624. (3) Li, S.; Crooks, R. M. Langmuir 1993, 9, 1775-1780. (4) Zamborini, F. P.; Crooks, R. M. Langmuir 1998, 14, 3279-3286. (5) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (6) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (7) Bain, C. D.; Troughton, E. B.; Tao, T.-Y.; Evall, J.; Whitesides, G. M.; Nuzzon, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (8) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (9) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (10) Nuzzo, R. G. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 48274829. (11) Vijayamohanan, K.; Aslam, M. Appl. Biochem. Biotechnol. 2001, 96, 25-39. (12) Fendler, J. H. Chem. Mater. 2001, 13, 3196-3210. (13) Lee, Y.-S.; Mrksich, M. Trends Biotechnol. 2002, 20, S14-S18.

group presents difficulty in preparing and handling the monolayer precursors. If the functional group is thiol reactive, both intra- and intermolecular reactions may occur. The thiols can also react with each other in the presence of oxygen to form disulfides.14 While monolayers can be formed from disulfides, the assembly process has been reported to be much slower and the properties of the resulting monolayers are not always the same.15,16 Molecules possessing multiple thiol groups present an additional challenge because disulfide formation leads to cross-linking or polymerization.17 A final drawback of working with thiols and thiolacetates is their extremely unpleasant odor.18 Because many applications rely on the ability to form monolayers from thiols with both internal and ω functionality, routes to monolayer formation that minimize the time the molecule is in the free thiol state are especially desirable. Protecting groups are often used during the synthesis of thiol-containing molecules to avoid disulfide formation and prevent the thiol group from reacting with other sensitive functionalities.19 Protecting groups provide additional benefits by reducing odor and enhancing solubility throughout synthetic manipulations and purification (14) Tarbell, D. S. The Mechanism of Oxidation of Thiols to Disulfides. In Organic Sulfur Compounds, Kharasch, N., Ed.; Pergamon Press: New York, 1961; Vol. 1, pp 97-102. (15) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365-385. (16) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723-727. (17) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parik, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529-9534. (18) Chemical analysis has shown that volatile thiols and acetate derivatives of these thiols are major components in skunk defensive secretion. See: Wood, W. F. J. Chem. Ecol. 1990, 16, 2057-2065. (19) Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis, 2nd ed.; Wiley: New York, 1991.

10.1021/la049627b CCC: $27.50 © 2004 American Chemical Society Published on Web 09/08/2004

Assembly of S-Tritylalkanethiols on Gold Chart 1

steps. The trityl (triphenylmethane) protecting group is commonly used in the synthesis of thiol-containing monolayer precursors because it is stable in most chemical environments. The trityl group increases the solubility of the monolayer precursor during synthesis and eases purification because the trityl group aids in crystallization and is UV active for thin-layer chromatrography (TLC) visualization. Just prior to use, the protecting group is typically removed from the monolayer precursor and the adsorbate is purified. However, many monolayer molecules (e.g., those containing thiophenolic functions or molecules containing basic groups that facilitate disulfide formation) are so reactive that isolation of the pure material after deprotection is extremely difficult.20 Alternatives to the deprotection/purification approach described above are methods that generate the thiol directly in the assembly solution. Tour et al. have reported the use of in situ deprotection methods to form monolayers from self-reactive, conjugated aromatic thiols from acetylprotected molecules.17,21,22 Ellipsometry and X-ray photoelectron spectroscopy (XPS) measurements were used to confirm the formation of high-coverage monolayers. In some cases, the monolayers block up to 99% of the electrochemical response of the gold surface to potassium ferricyanide, suggesting a relative absence of defects within these monolayers. Much less is known about the in situ generation/assembly of alkanethiols. Lukkari et al. reported that alkyl (and aryl) thiosulfates (Bunte salts, R-SSO3-) assemble on gold through a Au-SR interaction; however, the assembly process is slower by several orders of magnitude, and the surface coverage is lower than that of the corresponding monolayers formed from free thiols.23 This approach is convenient, although best suited to applications that do not require high-coverage or wellordered monolayers. Thus, there is still a need for convenient methods for the preparation of well-ordered alkanethiol monolayers through in situ methods. In this study we compare the properties of monolayers formed from the in situ deprotection of trityl-protected octadecanethiol (ODT) and 3-mercapto-N-n-pentadecylpropionamide (1ATC15) and acetyl-protected octadecanethiol to those of monolayers prepared from purified octadecanethiol and 1ATC15 (Chart 1). We compare two methods of in situ deprotection for ODT. We also explore how in situ deprotection of S-trityl molecules impacts the formation of both straight-chain hydrocarbon and internally substituted alkanethiol monolayers. The rationale for selecting these two particular molecules for study is that monolayers formed from the purfied thiols of both (20) Cheng, L.; Yang, J.; Yao, Y.; Price, D. W.; Dirk, S. M.; Tour, J. M. Langmuir 2004, 20, 1335-1341. (21) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705-1707. (22) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721-2732. (23) Lukkar, J.; Meretoja, M.; Kartio, I.; Laajalehto, K.; Rajama¨ki, M.; Lindstro¨m, M.; Kankare, J. Langmuir 1999, 15, 3529-3537.

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have been well characterized.6,7,24-27 By employing a variety of monolayer characterization methods and comparing the results for monolayers formed from in situ methods to those for monolayers formed from the purified thiols, we found that the quality of the monolayers formed from in situ deprotection of trityl-protected adsorbates was strongly influenced by the solubility of the protected molecule in trifluoroacetic acid. As long as the molecule is soluble, the resulting monolayers are indistinguishable from monolayers formed from the purified thiol. In contrast, the in situ deprotection of acetyl-protected alkanethiol precursors resulted in low-quality monolayers. Experimental Section Chemicals and Synthesis. Triethylsilane was obtained from Lancaster. Trifluoroacetic acid was obtained from Applied Biosystems. EDCI (3-N ′-dimethylaminopropyl-N-ethylcarbodiimide) was obtained from Bachem. 1-octadecanethiol (98%) was obtained from Aldrich and recrystallized once from nitrogensparged ethanol prior to use. All other reagents were from Aldrich and used without further purification. Dichloromethane was distilled over calcium hydride prior to use. Tetrahydrofuran was distilled over potassium prior to use. Synthesis of Trityl-ODT. Triphenylmethanethiol (1.0330 g, 5.1 mmol) was dissolved in 50 mL of nitrogen-sparged, distilled THF in a 200 mL round-bottom flask equipped with a stirbar. Sodium hydride (0.12 g, 5.0 mmol) was added, turning the solution a golden yellow, followed by addition of 1-bromooctadecane (1.1523 g, 3.5 mmol). The reaction mixture was stirred at room temperature for 24 h. A 5 mL portion of deionized water was added to quench the excess NaH. The mixture was then reduced to 10 mL in vacuo, and 100 mL of dichloromethane were added. The product was washed with 3 × 100 mL of deionized water, stirred with activated charcoal to remove most of the yellow color, and then filtered. The solvent was removed in vacuo, resulting in a yellow solid. The crude product was recrystallized from 1:1 CH2Cl2/CH3OH to yield white, needle-shaped crystals (1.047 g, 49%). Mp: 70.5-71.9 °C. 1H NMR (300 MHz, CDCl3): δ 7.207.42 (m, 15H), 2.11 (t, 2H) 1.36 (m, 2H), 1.26 (m, 30H), 0.89 (t, 3H). Synthesis of Acetyl-ODT. Potassium carbonate (0.662 g, 4.8 mmol) was suspended in 80 mL of distilled THF. Thiolacetic acid (0.35 mL, 4.8 mmol) was added to the K2CO3 suspension with stirring, turning the solution pale yellow. Next 1-bromooctadecane (1.325 g, 3.97 mmol) was added, and the mixture was allowed to reflux for 24 h. The resulting tan mixture was concentrated by rotary evaporation and dissolved in 100 mL of dichloromethane. The product was washed with 3 × 50 mL of nitrogen-sparged deionized water, and the solvent was removed by rotary evaporation. The resulting brown solid was recrystallized twice from methanol to yield white crystals (0.5722 g, 44%). Mp: 39.6-40.8 °C. 1H NMR (300 MHz, CDCl3): δ 2.87 (t, 2H), 2.33 (s, 3H), 1.26 (m, 30H) 0.89 (t, 3H). Synthesis of Trityl-1ATC15. Trityl-1ATC15 was synthesized using a synthesis method developed for trityl-1ATC9,28 where the primary amine is coupled to 3-(tritylsulfanyl)propionic acid. The crude product was recrystallized from ethanol and water, resulting in a white, crystalline product (96.5% yield). Mp: 86.688.2 °C. 1H NMR (300 MHz, CDCl3): δ 7.20-7.42 (m, 15H), 5.51 (s, 1H), 3.16 (q, 2H), 2.51 (t, 2H), 2.01 (t, 2H), 1.42 (m, 2H), 1.25 (m, 24H), 0.89 (t, 3H). Deprotection and Purification of 1ATC15. Trifluoroacetic acid (TFA) (2 mL) was added to 0.216 g of trityl-1ATC15, resulting in a bright yellow solution. Twenty drops of triethylsilane (TES) were added until the yellow color disappeared. The solvent was (24) Evans, S. D.; Goppert-Berarducci, K. E.; Urankar, E.; Gerenser, L. J.; Ulman, A.; Snyder, R. G. Langmuir 1991, 7, 2700-2709. (25) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239-5243. (26) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 53195327. (27) Clegg, R. S.; Reed, S. M.; Smith, R. K.; Barron, B. L.; Rear, J. A.; Hutchison, J. E. Langmuir 1999, 15, 8876-8883. (28) Reed, S. M. Ph.D. Dissertation, Department of Chemistry, University of Oregon, 2001; p 65.

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removed in vacuo, and the resulting product was purified via silica rotary chromatography using a 2 mm chromatotron plate with ethyl acetate as the eluent, giving a pure, white product (0.1104 g, 90%). 1H NMR (300 MHz, CDCl3): δ 5.51 (s, 1H), 3.270 (q, 2H), 2.82 (q, 2H), 2.48 (t, 2H), 1.50 (m, 2H), 1.26 (m, 24H), 0.89 (t, 3H). Substrate and Monolayer Formation. Gold films were deposited on piranha solution-cleaned (5:1 H2SO4/H2O2; caution! piranha solution reacts violently with organic material) glass slides by evaporating a 75 Å chromium adhesion layer onto the clean slides followed by the evaporation of 1500-2000 Å of gold. The slides were stored in a Coplin staining dish in absolute ethanol. Just prior to use, the gold substrates were cleaned by ozonolysis for 10 min in a UV cleaner, rinsed with copious amounts of Nanopure water followed by absolute ethanol, and dried with argon. In Situ Deprotection and Monolayer Formation of Trityl-ODT. A number of in situ deprotection schemes were explored in this study. Initially, trityl-ODT was finely ground using a mortar and pestle and transferred to a 2 oz. piranha solution-cleaned glass jar. Trifluoroacetic acid (2 mL) was added followed by dropwise addition of triethylsilane until the yellow color disappeared. Nitrogen-sparged, absolute ethanol was added to dilute the mixture to 1 mM octadecanethiol, and the solution was heated under nitrogen until all the solid dissolved. The solution was removed from the heat, and a clean gold substrate was added to soak for at least 24 h before characterization. For some electrochemical studies (see Optimization of In Situ Deprotection of Trityl-ODT), monolayers were prepared by adding 2 mL of trifluoroacetic acid to 0.020 g of trityl-ODT in a piranha solution-cleaned glass jar. The mixture was sonicated for 1 min, and triethylsilane was then added dropwise until the yellow color disappeared. Nitrogen-sparged, absolute ethanol (68 mL) was added, and the mixture was filtered to remove any unreacted trityl-ODT. A clean gold substrate was added to soak for at least 24 h prior to characterization. In Situ Deprotection and Monolayer Formation of Trityl1ATC15. Trifluoroacetic acid (2 mL) was added to trityl-1ATC15 (0.039 g, 0.07 mmol) in a 2 oz. piranha solution-cleaned glass jar, resulting in a bright yellow solution. Triethylsilane was added dropwise until the color disappeared. The mixture was then diluted to 1 mM 1ATC15 with nitrogen-sparged, absolute ethanol. A clean gold substrate was immediately added and allowed to soak for at least 24 h before characterization. In situ Deprotection and Monolayer Formation of Acetyl-ODT. This procedure was performed analogous to the one described by Tour et al. for aromatic thiols.17 Acetyl-ODT (0.023 g, 0.07 mmol) was dissolved in 70 mL of nitrogen-sparged, distilled THF to make a 1 mM solution. Ammonium hydroxide (30%, 115 µL) was added. The solution was stirred briefly. A clean gold substrate was immediately added and allowed to soak for at least 24 h before characterization. Characterization. Monolayers were characterized using a combination of contact angle goniometry, X-ray photoelectron spectroscopy, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and electrochemistry. Contact Angle Goniometry. Contact angle measurements were made on the gold substrates following monolayer formation using both a dynamic drop technique and a stationary drop technique.7 Briefly, in the dynamic drop technique, a 1-2 µL drop of Nanopure water is suspended from a syringe and the sample stage containing the gold substrate is raised until the drop just touches the surface of interest. The advancing contact angle is defined to be the maximum contact angle that can be achieved while increasing the size of the drop with water from the syringe. For the stationary drop technique, a 1-2 µL drop of Nanopure water was suspended over the surface of interest. The surface was raised until it made contact with the drop and then slowly lowered, taking the drop with it. The contact angle was then measured with a low-power microscope equipped with a protractor reticule. For each substrate, the angles of at least 10 drops were measured and averaged. X-ray Photoelectron Spectroscopy. Experiments were performed on a Kratos HSi analytical spectrometer using monochromatic Al KR radiation (13.5 kV, 15 mA) with a pass energy of 20 and 0.1 eV step size.

Inman et al. Polarization Modulation Infrared Reflection Absorption Spectroscopy. Studies were performed on a Nicolet Magna-IR 550 spectrometer with dual-channel input and equipped with a photoelastic modulation (PEM) accessory (ThermoNicolet, Madison, WI) using 1024 signal-averaged scans with a mirror velocity of 0.9494 cm/s and a resolution of 2 cm-1. The PEM module consists of beam steering and focusing optics, a wire grid polarizer, a PEM head and controller assembly (Hinds Instruments, Hillsboro, OR), an MCT-A liquid nitrogen cooled detector (ThermoNicolet, Madison, WI), and an SSD demodulator (GWC Instruments, Madison, WI). For PM-IRRAS, no background spectrum collection is necessary.29 Baseline normalization30 was performed using Igor Pro software (Wavemetrics, Lake Oswego, OR). Electrochemistry. Double-layer capacitance (DLC)6 and electrochemical blocking effect (EBE)6 experiments were performed using a BAS 100 B electrochemical workstation, with a bare gold or SAM-derivatized working electrode, a platinum counter electrode, and a saturated calomel reference electrode (SCE). The electrolyte was 1.0 M KCl(aq) and the analyte for the electrochemical blocking studies was 1.0 mM K3Fe(CN)6. In both experiments, the potential was swept from +450 to -200 mV at a rate of 100 mV/s.

Results and Discussion Here, our approaches to in situ deprotection of alkanethiols are described and the resulting monolayers compared. We probed the differences in octadecanethiol and 1ATC15 monolayers formed from purified thiols, and from the in situ deprotection of trityl-protected and acetylprotected thiols. To make a clear assessment of any structural differences, we used a combination of surface characterization methods. In Situ Deprotection Strategy. The traditional method used to form thiol monolayers on gold consists of dissolving the purified adsorbate molecule in an ethanol solution, followed by addition of the substrate. In contrast, in situ methods allow the monolayer to be formed without prior purification of the adsorbate, reducing the time that the molecule is in the free thiol state and thus decreasing the likelihood of disulfide formation or other deleterious side reactions. There are a number of important requirements for an effective in situ method: the deprotection chemistry should be efficient and selective, the protecting group must not interfere (through sterics or chemical reaction) with the assembly process, and the deprotection reagents should not contaminate the monolayer or interfere with the assembly process. As we will show below, it is also important that the protected form of the alkanethiol (e.g., in the case of the acetyl-protected thiol) does not compete with the deprotected thiol in assembly on the surface. With these requirements in mind, we explored two different in situ deprotection approaches to monolayer formation. We examined the in situ deprotection method developed by Tour et al. for aromatic thiols,17,21,22 where an acetyl-protected thiol is dissolved in tetrahydrofuran and deprotected using ammonium hydroxide. We also examined a new in situ deprotection method for trityl-protected thiols. The deprotection chemistry of the trityl protecting group is rapid and efficient, having been developed and used extensively in the synthesis of thiol-containing molecules.19 The in situ deprotection technique is convenient as well: the tritylprotected thiol is simply dissolved in a small amount of (29) Frey, B. L.; Corn, R. M.; Weibel, S. C. Polarization-modulation Approaches to Reflection-Absorption Spectroscopy. In Handbook of Vibrational Spectroscopy; Chambers, J., Griffiths, P., Eds.; Wiley: New York, 2001; Vol. 2, pp 1042-1056. (30) Baseline normalization was performed using a procedure written for IgorPro by Robert Corn’s group. For more information, see http:// corninfo.chem.wisc.edu.

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Table 1. Comparison of Monolayers Formed from Purified Thiol and in Situ Deprotection Techniques by Contact Angle Goniometry and XPS monolayer molecule ODT in situ trityl-ODT in situ acetyl-ODT 1ATC15 in situ 1ATC15

contact angle (deg) advancing static 118.6 ( 1.1b 117.9 ( 0.9 67 ( 3 119.0 ( 2d 119.0 ( 0.7

111.8 ( 2.4c 110.8 ( 0.9 57 ( 3 115 ( 2 114 ( 2

sulfur 2p3/2 binding energya (eV) 158.5 158.6 157.8 158.5 158.4

a The data represent an average of three different samples. Literature value7 118 ( 2°. c Literature value7 112 ( 2°. d Literature value25 119 ( 2°.

b

trifluoroacetic acid, triethylsilane is added to trap the produced trityl cation, and the resulting solution is diluted with nitrogen-sparged ethanol before the substrate is added. Next we compare the monolayers resulting from the two in situ techniques to monolayers formed from the purified adsorbate. Characterization of Monolayers. Contact angle goniometry was used to probe the general degree of order within the surface methyl groups.7 X-ray photoelectron spectroscopy was used to probe any differences in the nature of the chemical interaction between the adsorbate and the gold surface. PM-IRRAS provided information about the overall monolayer order as judged by the packing of the alkyl chains and the extent of hydrogen bonding between amide groups.6 Electrochemical techniques (double-layer capacitance and electrochemical blocking) were used to assess the extent and nature of any monolayer defects. Contact Angle Goniometry. Contact angle goniometry was used as an initial assessment of monolayer formation and to probe the overall order of the surface methyl groups for monolayers formed from each of the assembly methods. Representative values of advancing and static contact angles for monolayers formed from each method are detailed in Table 1. The advancing contact angle measurements for ODT and 1ATC15 monolayers formed from the in situ trityl deprotection (117.9 ( 0.9° for trityl-ODT and 119.0 ( 0.7° for trityl-1ATC15) are within the experimental uncertainty of the contact angles of monolayers made from the purified thiols (118.6 ( 1.1° for ODT and 119.0 ( 2.0° for 1ATC15) and consistent with values reported in the literature. The literature values for ODT and 1ATC15 advancing contact angles are 118 ( 2° and 119 ( 2°, respectively.7,25 The static contact angles for the in situ deprotection of trityl monolayers are also consistent between the methods and with those reported in the literature. The contact angle measurements for the in situ deprotection of acetyl-ODT are significantly lower than for monolayers formed using either of the other two methods, indicating that this technique results in lower coverage and/or less ordered monolayers. Because there is the potential for acetyl-protected thiols to assemble on gold,17,31 we measured the contact angles of monolayers formed from acetyl-ODT in THF. Monolayers formed from pure acetyl-ODT gave a static contact angle of 97.9 ( 1.1° and an advancing contact angle of 104.3 ( 1.5°. These data suggest that the in situ deprotection/assembly of acetylODT may be complicated by a competing assembly of the acetyl-protected thiol. The resulting monolayers are of lower quality than monolayers made from either purified ODT or purified acetyl-ODT. (31) Kang, Y.; Won, D.-J.; Kim, S. R.; Seo, K.; Choi, H.-S.; Lee, G.; Noh, Z.; Lee, T. S.; Lee, C. Mater. Sci. Eng., C 2004, 24, 43-46.

Figure 1. Comparison of infrared spectra for ODT monolayers formed using two in situ techniques to those for monolayers formed from purified ODT. Two representative spectra for each technique are plotted to demonstrate the sample-to-sample variability. Monolayers formed from the in situ deprotection of trityl-ODT (top) demonstrate only slightly broader peaks compared to monolayers formed from purified thiol. In situ deprotection of acetyl-ODT (bottom) results in monolayers with significantly broadened and shifted peaks.

X-ray Photoelectron Spectroscopy. XPS was used to probe the chemical state of sulfur in the monolayers resulting from each method. Table 1 compares the sulfur 2p3/2 peak positions for ODT and 1ATC15 formed from the purified thiol and in situ deprotection methods. The signals for sulfur are compared for monolayers formed from purified thiols with those for monolayers formed from in situ trityl deprotection. In both cases, only one state for sulfur is observed and the peak positions are within experimental uncertainty. This indicates that the sulfur atoms are in the same chemical environment in both sets of monolayers. The sulfur peak shape and position for monolayers formed from in situ acetyl deprotection are significantly different from those observed for purified ODT monolayers. The sulfur peaks were broadened (fwhm ) 1.6 eV versus 0.8 eV for purified ODT monolayers), with the main peak shifted by -0.8 eV when compared to that of purified ODT monolayers. The spectrum for acetyl-ODT monolayers shows two types of carbon peaks and a significant oxygen peak, whereas the spectrum for monolayers formed from purified ODT shows only one type of carbon peak and no oxygen peak. Taken together, these data suggest that the monolayers formed by in situ acetyl deprotection likely contain a mixture of thiolate and thiolacetate adsorbates, indicating that the deprotection was incomplete. Polarization Modulation Infrared Reflection Absorption Spectroscopy. PM-IRRAS is commonly used to determine the order and molecular orientation of monolayers adsorbed on a metal surface. Here, we compare the peak positions, peak widths, and relative intensities in the IR spectra for ODT (Figure 1) and 1ATC15 (Figure 2) monolayers formed from both in situ and traditional methods. The peak positions, peak widths, and relative intensities in both the alkyl and amide regions for 1ATC15 mono-

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Table 2. Double-Layer Capacitance and Electrochemical Blocking for the Monolayers Formed from the Purified Thiol and In Situ Deprotection Techniques

a

monolayer molecule

Cdla (µF/cm2)

%BEa

ODT in situ trityl-ODT in situ acetyl-ODT

0.93 ( 0.19 1.3 ( 0.4 22 ( 8

99.96 ( 0.02 95 ( 7 65 ( 30

monolayer molecule

Cdla (µF/cm2)

%BEa

1ATC15 in situ 1ATC15

1.12 ( 0.19 1.08 ( 0.11

99.95 ( 0.08 99.97 ( 0.03

The data presented represent an average of 10 different monolayer substrates.

Figure 2. Comparison of infrared spectra for monolayers formed from in situ deprotection of trityl-1ATC15 to those for monolayers formed from purified 1ATC15. Two representative spectra for each technique are plotted to demonstrate the sample-to-sample variability. The PM-IRRAS spectra for monolayers formed from the in situ deprotection of trityl1ATC15 are indistinguishable from those for monolayers formed from the purified thiol both in the alkyl region (top) and in the amide region (bottom).

layers formed from purified thiol and in situ trityl deprotection are indistinguishable by PM-IRRAS (see Figure 2). This suggests a similar, high degree of order within the hydrocarbon and amide regions for monolayers prepared by both methods, consistent with our previous work that demonstrated that well-ordered monolayers can be formed from alkanethiols with internal amide functionality.25 In the case of ODT, the peak positions in the spectra for in situ trityl-ODT monolayers are indistinguishable from the alkyl peak positions observed for purified ODT monolayers (see Figure 1, top), but the peaks are slightly broader. The spectra for in situ trityl-ODT monolayers also demonstrate slightly more sample to sample variability than those for monolayers formed from purified ODT. Finally, the intensity (relative to the other CH peaks) of the CH2(sym) peak at 2850 cm-1 for the in situ tritylODT monolayers is generally larger than for monolayers formed from purified ODT. In their studies of alkanethiol monolayers on gold, Porter et al. determined that the introduction of a small amount of CH2 group disorder would result in an increase in the CH2(sym) peak intensity.6 Taken together, the IR data suggest that monolayers formed from the in situ deprotection of ODT

are slightly less ordered than monolayers formed from the purified thiol. In the PM-IRRAS spectrum for in situ acetyl-ODT, only the CH2(asym) peak at 2922 cm-1 and the CH2(sym) peak at 2852 cm-1 are present, while the CH3(sym) peak at 2878 cm-1 and the CH3(asym) peak at 2964 cm-1 normally observed for ODT monolayers are absent. The IR data suggest that the monolayer has a different orientation than the ODT monolayers resulting from the other two methods. The peaks that are present in the in situ acetylODT spectrum demonstrate significant broadening and are blue shifted by 3-5 cm-1 when compared to the alkyl peaks observed for monolayers formed using either of the other two methods. Thus, in situ acetyl-ODT monolayers are poorly ordered, as shown by IR data. Electrochemical Characterization. Both DLC and EBE experiments were performed to probe the nature of defects within the monolayers formed using each technique. Double-layer capacitance provides information on the average structure of the assemblies. If a monolayer is impermeable to electrolyte penetration, it behaves like an ideal capacitor, where a small capacitance (Cdl ) ∼1 F/cm2) indicates a well-ordered monolayer.6,32,33 EBE is a highly sensitive probe of defects, where a large percent blocking effect (%BE) (>99.9%) indicates a defect-free film. For a more detailed description of how double-layer capacitance and electrochemical blocking can be used to characterize alkanethiol monolayers, see Porter et al.6 Figures 3 and 4 present representative electrochemical data for ODT and 1ATC15 monolayers, respectively, for qualitative comparison of the films. The general shapes of the double-layer capacitance and electrochemical blocking curves and the current densities are in good agreement between monolayers formed from the purified thiol and those formed from the in situ deprotection of trityl-ODT and trityl-1ATC15. Both of these methods yield monolayers that are comparable to the high-quality monolayers reported in the literature.6,25 Table 2 contains the calculated Cdl and %BE for quantitative comparison of the monolayers. The average Cdl and %BE are both within 1 standard deviation of each other for monolayers formed from purified thiols and those formed from the in situ deprotection of both trityl-ODT and trityl-1ATC15. However, the standard deviation of the blocking effect for in situ trityl-ODT monolayers is 3 orders of magnitude greater than for monolayers formed from purified ODT. These data suggest that the in situ technique results in ODT monolayers with higher variability, as was also observed in PM-IRRAS. Figure 3A demonstrates that in situ acetyl-ODT monolayers pass substantially more faradic current than purified ODT monolayers, indicating that the in situ acetyl-ODT monolayers are more permeable to electrolyte, and therefore have more defects. The shape of the acetylODT plot in Figure 3B, along with the significant current flow, indicates that the monolayers resulting from this method have defects on the micrometer scale or greater.6 (32) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. (33) Zehner, R. W.; Sita, L. R. Langmuir 1997, 13, 2973-2979.

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Figure 3. Double-layer capacitance and electrochemical blocking results used to compare the preparation methods for ODT monolayers: double-layer capacitance (A) and electrochemical blocking (B) comparison of ODT and in situ acetyl-ODT and doublelayer capacitance (C) and electrochemical blocking (D) comparison of ODT and in situ trityl-ODT. Double-layer capacitance data indicate that in situ acetyl-ODT monolayers are far less ordered than monolayers formed from purified ODT. The shape of the electrochemical blocking curve for in situ acetyl-ODT monolayers suggests the presence of large defects within the monolayer. Both electrochemical characterization methods suggest that monolayers formed from purified thiols and in situ ODT are of comparable quality. The scan rate was 100 mV/s. The electrolyte was 1.0 M KCl for double-layer capacitance and 1.0 mM K3Fe(CN)6 in 1.0 M KCl for electrochemical blocking.

Figure 4. Double-layer capacitance (A) and electrochemical blocking (B) results used to compare the preparation methods for 1ATC15 monolayers. These techniques probe the order of a monolayer as well as the types of defects present. Doublelayer capacitance indicates monolayers formed from purified 1ATC15 and in situ trityl-1ATC15 have similar order, and electrochemical blocking indicates that monolayers formed from both methods are relatively defect free; however, the in situ trityl-1ATC15 monolayers have fewer defects. The scan rate was 100 mV/s. The electrolyte was 1.0 M KCl for double-layer capacitance and 1.0 mM K3Fe(CN)6 in 1.0 M KCl for electrochemical blocking.

Cdl and %BE for monolayers formed from the in situ deprotection of acetyl-ODT were not within experimental uncertainty of those for monolayers formed from purified ODT, indicating that this technique results in films with a large number of defects.

Optimization of In Situ Deprotection of TritylODT. While all of the characterization methods used in this study showed a substantial difference between monolayers formed from in situ deprotection of acetylODT and those formed from purified ODT, differences in monolayers formed from in situ deprotection of trityl-ODT and from purified ODT were generally within experimental uncertainty. Both PM-IRRAS and electrochemical characterization, however, show that there is more variability in monolayers formed from the in situ deprotection technique for the trityl-ODT precursor than monolayers formed from the pure thiols. In contrast, there was no difference in 1ATC15 monolayers formed using either method. One difference between the in situ deprotection of the trityl monolayer precursors is that trityl1ATC15 is readily soluble in TFA, while trityl-ODT is not. The lower solubility may result in incomplete deprotection of the precursor. Because in situ deprotection of trityl-ODT is likely incomplete, we performed a control experiment to determine if the protected molecule could interact with the gold surface. After soaking a clean gold substrate in a 1 mM solution of trityl-ODT for 24 h, we measured a contact angle of 92 ( 5°, suggesting that trityl-ODT adsorbs on the surface. The IR spectrum for the resulting film displays peaks that are significantly broader and shifted relative to the peaks observed for ODT monolayers formed from either the purified thiol or the in situ deprotection of tritylODT (see the Supporting Information for the spectrum). The double-layer capacitance data for the film (6 µF/cm2) and the electrochemical blocking (84%), taken with the contact angle and PM-IRRAS data, suggest that a monolayer is formed when a gold substrate is exposed to a tritylODT solution, although the resulting monolayer is of poor quality. On the basis of these observations, we made a number of attempts to increase the solubility of the trityl-ODT to ensure complete deprotection. We began by increasing the volume of ethanol, by attempting to make a 1 mM

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ethanolic solution of trityl-ODT before adding the TFA and TES for deprotection. The trityl-ODT molecule, however, was not completely soluble in ethanol, and some solid trityl-ODT remained in solution after addition of TFA and TES. This approach resulted in monolayers with more variability than those formed using the original deprotection approach (which may have been due to incomplete deprotection or the presence of the trityl-ODT precursor). Next we tried dissolving the trityl-ODT in 1 mL of methylene chloride and then deprotecting with TFA and TES, followed by addition of enough ethanol to make a 1 mM solution. While the trityl-ODT was completely deprotected by this method, the resulting ODT monolayers demonstrated greater variability than those formed using the initial in situ method. In this case, the monolayer variability may be due to the presence of methylene chloride.34,35 We also attempted removal of the methylene chloride following deprotection with nitrogen flow and/or vacuum removal before the ethanol was added to make up the soaking solution, but the resulting monolayers were still highly variable. Finally, we tried to increase the amount of the tritylODT solubilized through sonication. In this method, tritylODT was sonicated for 1 min in TFA to dissolve the tritylODT as much as possible. TES and-nitrogen sparged ethanol were added, and the solution was filtered to remove any remaining trityl-ODT before the gold substrates were added. Using this monolayer formation method for in situ trityl monolayers, the double-layer capacitance and electrochemical blocking data for the resulting monolayers were much more consistent. The average Cdl and %BE for the resulting monolayers were 1.15 ( 0.23 µF/cm2 and 99.94 ( 0.05%, respectively. Conclusions On the basis of the data obtained for 1ATC15, in situ deprotection of trityl molecules that are readily soluble in TFA yields monolayers that are of quality comparable to that of monolayers formed from the purified thiol. When the trityl precursor is not completely soluble in TFA (e.g., in the case of trityl-ODT), the resulting monolayers are of high quality, but demonstrate slightly more sampleto-sample variability than monolayers formed from purified ODT. This variability may be due to interactions of the trityl-ODT molecule with the surface. When a highquality monolayer is not required, the trityl-protected thiol can be used to bind the adsorbate to a gold surface without any deprotection. The in situ deprotection of acetyl-ODT results in poorly ordered monolayers with a large number of defects. The poor quality of the monolayers formed from in situ deprotection of acetyl-ODT may be due to a number of factors. The XPS data suggest that some acetyl-ODT may be bound to the surface as well as ODT. The presence of ammonium hydroxide in the deprotection mixture may promote disulfide formation in the solution, further adding (34) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (35) Yan, D.; Jennings, G. K.; Weinstein, R. D Ind. Eng. Chem. Res. 2002, 41, 4528-4533.

Inman et al.

to the heterogenenity of the adsorbate mixture. It is also possible that using tetrahydrofuran rather than ethanol adversely impacts the order of the resulting monolayer, as solvent has been shown to impact monolayer adsorption kinetics36,37 as well as monolayer organization.34,35 While contact angle goniometry and XPS show no significant difference in monolayers formed from purified ODT and the in situ deprotected trityl molecule, electrochemical characterization, a technique that is better able to probe individual defects, shows a statistical difference between monolayers formed from the two methods. The PM-IRRAS data for trityl-ODT also demonstrated more variability and a slight broadening of peaks when compared to those for purified ODT monolayers. These data, along with the results for the in situ deprotection of acetylODT, underscore the importance of using a variety of characterization methods to assess monolayer quality. On the basis of the data reported here, it is clear that the base-catalyzed deprotection of acetyl-protected ODT results in low-quality alkanethiol monolayers that are most likely a mixture of acetyl, thiol, and/or disulfide components. In situations where a trityl-protected molecule is soluble in TFA or not accessible by traditional purification techniques, in situ deprotection should be an excellent technique to form well-ordered monolayers. Because in situ deprotection enables access to a wide variety of monolayer molecules that are not currently available via traditional purification methods, this technique may be used to explore monolayer molecules with a wide range of functionalities. By increasing the array of accessible monolayer components, the technique can aid in the exploration of how molecular level interactions within a monolayer lead to supramolecular structure and stability, an understanding that is critical for the rational design of materials formed by self-assembly. This approach may be especially useful for systems such as the one described by Pollack et al., where a sequential deprotection approach was used to orient asymmetric molecules for electronic characterization.38 Acknowledgment. We gratefully acknowledge support from the National Science Foundation (CAREER award; Grant CHE 9702726), a Department of Education GAANN Fellowship (C.E.I.), a National Science Foundation IGERT (DGE-0114419) Fellowship (C.E.I.), and a University of Oregon Doctoral Research Fellowship (S.M.R.). Supporting Information Available: XPS spectra for ODT and 1ATC15 monolayers formed from both purified thiols and in situ deprotection (Figures S1-S5) and a comparison of the PM-IRRAS spectra for monolayers formed from trityl-ODT and purified ODT (Figure S6) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA049627B (36) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731-4740. (37) Bensebaa, F.; Voicu, R.; Huron, L.; Ellis, T. H. Langmuir 1997, 13, 5335-5340. (38) Pollack, S. K.; Naciri, J.; Mastrangelo, J.; Patterson, C. H.; Torres, J.; Moore, M.; Shashidhar, R.; Kushmerick, J. G. Langmuir 2004, 20, 1838-1842.