© Copyright 2005 American Chemical Society
MARCH 29, 2005 VOLUME 21, NUMBER 7
Letters Improved Method for the Preparation of Carboxylic Acid and Amine Terminated Self-Assembled Monolayers of Alkanethiolates Hua Wang, Shengfu Chen, Lingyan Li, and Shaoyi Jiang* Department of Chemical Engineering, University of Washington, Seattle, Washington 98195 Received December 22, 2004. In Final Form: February 13, 2005 We present an improved method to prepare carboxylic acid (COOH) and amine (NH2) terminated selfassembled monolayers (SAMs) of alkanethiolates. In this method, a small amount of CF3COOH (for COOHSAM) or N(CH2CH3)3 (for NH2-SAM) is added into the ethanolic solution of alkanethiols during SAM formation. The freshly formed COOH- and NH2-SAMs are then rinsed with an ethanolic solution of NH4OH or CH3COOH, respectively. Both SAMs prepared with the improved method show better quality in terms of surface chemical composition, roughness, and wettability as measured by X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle, respectively. The formation of better SAMs can be attributed to the disruption of interplane hydrogen bonds.
Introduction Self-assembled monolayers (SAMs) of alkanethiolates have been widely used as platforms for the studies of protein adsorption and cell adhesion due to their simplicity and flexibility.1-3 They are used to control nanoscale surface properties, such as surface charge, wettability, and topography.4-8 Methyl (CH3) and hydroxyl (OH) terminated SAMs are used to model hydrophobic and hydrophilic neutral surfaces, while carboxylic acid (COOH) and amine (NH2) terminated SAMs are used to mimic negatively and positively charged surfaces, respectively. Thus, the quality of these SAMs is very important. * To whom correspondence should be addressed. (1) Ostuni, E.; Yan, L.; Whitesides, G. Colloids Surf., B 1999, 15, 3. (2) Wink, T.; vanZuilen, S.; Bult, A.; vanBennekom, W. Analyst 1997, 122, R43. (3) Xia, Y.; Gates, B.; Yin, Y. In Biochip Technology; Cheng, J., Kricka, L., Eds.; Harwood Academic Publishers: New York, 2001; p 39. (4) Ito, Y. Biomaterials 1999, 20, 2333. (5) Kapur, R.; Rudolph, A. Exp. Cell Res. 1998, 244, 275. (6) Mrksich, M.; Whitesides, G. Annu. Rev. Biophys. Biomol. Struct. 1996, 25, 55. (7) Wang, H.; Castner, D.; Ratner, B.; Jiang, S. Langmuir 2004, 20, 1877. (8) Garcia, A.; Vega, M.; Boettiger, D. Mol. Biol. Cell 1999, 10, 785.
Unlike CH3- and OH-SAMs, the quality of COOHand NH2-SAMs is harder to control. For COOH-SAM, a wide range of contact angles have been reported, such as 70°.11 For NH2-SAM, disordered structure was found with exposure of alkyl chains.12 In our previous measurements by X-ray photoelectron spectroscopy (XPS),13 unbound thiol molecules were detected on the surfaces of both COOH- and NH2-SAMs. Furthermore, an oxidized sulfur species was detected in NH2SAM, which was consistent with the Fourier transform infrared spectroscopy (FTIR) data from Wallwork et al.12 that sulfonic acid groups were present in NH2-SAM. Generally, COOH- and NH2-SAMs are prepared by the adsorption of alkanethiols onto a gold surface from an ethanolic solution, followed by rinsing with ethanol and drying under a N2 stream. It is expected that during SAM (9) Bain, C.; Whitesides, G. Angew. Chem., Int. Ed. Engl. 1989, 28, 506. (10) Michael, K.; Vernekar, V.; Keselowsky, B.; Meredith, J.; Latour, R.; Garcia, A. Langmuir 2003, 19, 8033. (11) Wang, M.; Palmer, L.; Schwartz, J.; Razatos, A. Langmuir 2004, 20, 7753. (12) Wallwork, M.; Smith, D.; Zhang, J. Langmuir 2001, 17, 1126. (13) (a) Li, L.; Chen, S.; Jiang, S. Langmuir 2003, 19, 2974. (b) Li, L.; Chen, S.; Jiang, S. Langmuir 2003, 19, 3266.
10.1021/la046810w CCC: $30.25 © 2005 American Chemical Society Published on Web 03/04/2005
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Figure 1. High-resolution XPS S(2p) spectra for (a) a COOH-SAM prepared in ethanol with 2% CF3COOH; (b) a COOH-SAM prepared in ethanol; (c) a NH2-SAM prepared in ethanol with 3% N(CH2CH3)3; and (d) a NH2-SAM prepared in ethanol.
formation, the interplane hydrogen bonds formed between terminal groups of alkanethiolates on gold and free alkanethiols in the bulk could lead to a partial second layer of alkanethiols on the top of SAMs of alkanethiolates. In the present study, we report an improved method to prepare COOH- and NH2-SAMs, in which the formation of interplane hydrogen bonds was disrupted by small molecules capable of forming hydrogen bonds in the ethanolic solution of alkanethiols. These SAMs were characterized for their surface chemical composition and film thickness, roughness, and wettability by XPS, atomic force microscopy (AFM), and contact angle, respectively. Experimental Section Gold (Alfa Aesar, Ward Hill, MA) was deposited onto freshly cleaved mica substrates (Mica New York Corp.) in a high-vacuum thermal evaporator (BOC Edwards Auto306). Typical evaporation rates were 0.3 nm/s. The thickness of the gold films was around 200 nm.14 11-Mercaptoundecanoic acid [HS(CH2)11COOH] was purchased from ProChimia (Sopot, Poland) while 11-amino-1undecanethiol [HS(CH2)11NH2] was purchased from Dojindo Molecular Technologies, Inc. (Gaithersburg, MD). COOH- and NH2-SAMs were formed by overnight soaking of UV ozonecleaned, gold-coated substrates in a 0.5 mM ethanolic solution of HS(CH2)11COOH with 2% (v/v) CF3COOH and of HS(CH2)11NH2 with 3% (v/v) N(CH2CH3)3, respectively. COOH- and NH2SAMs were then rinsed sequentially with ethanol, an ethanolic solution of NH4OH [10% (v/v), for COOH-SAM] or CH3COOH [10% (v/v), for NH2-SAM], and ethanol followed by drying in a stream of N2. Negative control samples were prepared by immersing gold-coated substrates in 0.5 mM ethanolic solutions of HS(CH2)11COOH or HS(CH2)11NH2 overnight, followed by rinsing with ethanol and drying in a stream of N2. XPS measurements were conducted using a Surface Science Instrument X-Probe spectrometer (Mountain View, CA) equipped with a monochromatic Al KR source (KE ) 1486.6 eV), a hemispherical analyzer, and a multichannel detector. The elemental composition was determined from spectra acquired at (14) Chen, S.; Liu, L.; Zhou, J.; Jiang, S. Langmuir 2003, 19, 2859.
a pass energy of 150 eV. All XPS data were acquired at a nominal photoelectron takeoff angle of 55°. SSI data analysis software was used to calculate the elemental compositions from the peak areas.15 All AFM images were acquired using a multimode Nanoscope IV (Digital Instruments, Santa Barbara, CA) equipped with a 10-µm E scanner. Commercial Si cantilevers (TESP, DI) with resonant frequencies of ∼270 kHz and tip apex radii of 5-10 nm were used. Images were recorded in height and amplitude modes. Nanoscope IV software was used to flatten and ascertain the height profile along a line across the image. The static contact angle of a 2-µL drop of distilled water applied to each SAM was measured using a RameHart 100 goniometer under ambient laboratory conditions.16 The measurements were repeated three times for each SAM sample.
Results and Discussion Table 1 represents the XPS atomic percentages of COOH- and NH2-SAMs prepared under different conditions. As compared with those prepared in ethanol, both SAMs prepared with the improved method show an obvious increase in their sulfur and gold compositions. This indicates that the film thicknesses of both SAMs prepared with the improved method are decreased because a thicker film will have less signal from the embedded sulfur and gold detected in XPS due to attenuation. Graham and Ratner17 found that the C/Au ratio from XPS for CH3-SAM follows a linear relationship as a function of the chain length of CH3-SAM (i.e., film thickness). Because this phenomenon is based on the attenuation of the gold signal by the overlying SAM, for NH2-SAM, the terminal N atom should also be counted into the chain length. Thus, the (C + N)/Au ratio should follow a linear relationship as a function of the chain length of NH2SAM. That is, a NH2-SAM with an 11-carbon chain should correspond to a CH3-SAM with a 12-carbon chain (C12), (15) Castner, D.; Hinds, K.; Grainger, D. Langmuir 1996, 12, 5083. (16) Veiseh, M.; Zareie, M.; Zhang, M. Langmuir 2002, 18, 6671. (17) Graham, D.; Ratner, B. Langmuir, in press.
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Figure 2. AFM images and cross section analysis for (a) a COOH-SAM prepared in ethanol with 2% CF3COOH; (b) a COOH-SAM prepared in ethanol; (c) a NH2-SAM prepared in ethanol with 3% N(CH2CH3)3; and (d) a NH2-SAM prepared in ethanol. Table 1. XPS Atomic Percentage of COOH- and NH2-SAMs Prepared under Different Conditions (Mean ( Standard Deviation) C11COOH (2% CF3COOH) Au(4f) S(2p) C(1s) N(1s) O(1s) {C(1s) + N(1s) [or O(1s)]}/Au(4f) C/Au ratio of CH3-SAM of equivalent chain length17
C11COOH (ethanol)
C11NH2 (3% N(CH2CH3)3)
C11NH2 (ethanol)
31.9 (1.2) 2.7 (0.4) 55.8 (1.6)
27.7 (2.5) 2.2 (0.7) 59.8 (3.5)
9.8 (0.6)
10.6 (0.6)
30.2 (0.6) 2.7 (0.1) 57.9 (0.9) 3.6 (0.5) 5.5 (0.2)
28.0 (0.6) 2.2 (0.2) 59.7 (0.8) 3.7 (0.4) 6.3 (0.3)
2.06
2.53 2.16 (C13)
with a (C + N)/Au ratio of 1.93 ( 0.10.17 Similarly, for a COOH-SAM with an 11-carbon chain, due to the bond angle of the COOH group, it should correspond to C13, with a (C + O)/Au ratio of 2.16.17 It can be seen from Table 1 that the film thickness of each SAM prepared with the improved method corresponds very well to the thickness of a monolayer based on the relationship established by Graham and Ratner. However, both SAMs prepared in ethanol appear to be thicker than a monolayer. From the high-resolution XPS S(2p) spectra (Figure 1), it can be seen that the COOH-SAM prepared in ethanol shows unbound thiols [S(2p3/2) binding energy of ∼164 eV] in addition to bound thiolates [S(2p3/2) binding energy
2.04
2.27 1.93 ( 0.10 (C12)
of ∼162 eV].15 The COOH-SAM prepared with the improved method shows only bound thiolate, indicating the elimination of the nonspecific adsorption of COOHalkanethiols. The NH2-SAM prepared in ethanol shows not only bound thiolates and unbound thiols but also oxidized sulfur species [S(2p3/2) binding energy > 166 eV), which could be sulfonic acid (SO32-) groups.12 The NH2SAM prepared with the improved method shows no unbound thiols and a significant reduction in the oxidized sulfur species. AFM was also used to characterize SAMs prepared under different conditions (Figure 2). AFM images show reduction in surface roughness or removal of the partial
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Table 2. Water Contact Angles of COOH- and NH2-SAMs Prepared under Different Conditionsa SAM (preparation conditions)
contact angle (standard deviations) (deg)
C11COOH (2% CF3COOH) C11COOH (ethanol) C11NH2 (3% N(CH2CH3)3) C11NH2 (ethanol)
