Langmuir 2006, 22, 3715-3720
3715
Investigation into pH-Responsive Self-Assembled Monolayers of Acylated Anthranilate-Terminated Alkanethiol on a Gold Surface Yugui Jiang, Zhiqiang Wang, Huaping Xu, Huan Chen, and Xi Zhang* Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, PR China
Mario Smet and Wim Dehaen Department of Chemistry, UniVersity of LeuVen, Celestijnenlaan 200F, B-3001 LeuVen, Belgium
Yoshiaki Hirano and Yukihiro Ozaki Department of Chemistry, School of Science, Kwansei-Gakuin UniVersity, Sanda, 669-1337, Japan ReceiVed December 19, 2005. In Final Form: February 22, 2006 This article describes the preparation of pH-responsive self-assembled monolayers (SAMs) of acylated anthranilateterminated alkanethiol. These monolayers are formed by chemisorption of the alkanethiol molecules onto a gold surface, resulting in different wetting properties of the surfaces depending upon the pH. By using various characterization techniques (e.g., infrared spectroscopy, cyclic voltammetry, contact angle measurements, and surface energy analysis), we have found that the changes in the wetting properties originate from the different surface structures of the monolayers in different pH environments. From surface energy analysis, we found that the disperse components of the surface energy on such SAMs predominate after treatment with pH 1 water, whereas the polar components of the surface energy on such SAMs predominate after treatment with pH 13 water. It is greatly anticipated that this line of research will provide new insight into the mechanism behind pH-responsive properties, facilitating the design and synthesis of new surface-active molecules for the fabrication of pH-responsive functional surfaces.
1. Introduction Self-assembled monolayers (SAMs) 1-14 are molecular assemblies that are formed spontaneously by the immersion of an appropriate substrate into a solution of an active surfactant. SAMs of sulfur-containing molecules on gold substrates, first introduced by Nuzzo et al.,15 have proved to be a convenient way to produce surfaces with specific chemical functionalities that allow for the precise tuning of surface properties. SAMs have shown many promising applications in lubrication, corrosion protection, photolithography, electrical resists, and sensing systems.16-19 * Corresponding author. E-mail:
[email protected]. (1) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (2) Sek, S.; Palys, B.; Bilewicz, R. J. Phys. Chem. B 2002, 106, 5907. (3) Nielsen, J. U.; Esplandiu, M. J.; Kolb, D. M. Langmuir 2001, 17, 3454. (4) Brewer, S. H.; Allen, A. M.; Lappi, S. E.; Chasse, T. L.; Briggman, K. A.; Gorman, C. B.; Franzen, S. Langmuir 2004, 20, 5512. (5) Garg, N.; Carrasquillo-Molina, E.; Lee, T. R. Langmuir 2002, 18, 2717. (6) Clegg, R. S.; Reed, S. M.; Smith, R. K.; Barron, B. L.; Rear, J. A.; Hutchison, J. E. Langmuir 1999, 15, 8876. (7) Chang, S.-C.; Chao, I.; Tao, Y.-T. J. Am. Chem. Soc. 1994, 116, 6792. (8) Akiyama, H.; Tamada, K.; Nagasawa, J.; Abe, K.; Tamaki, T. J. Phys. Chem. B 2003, 107, 130. (9) Yu, H.-Z.; Ye, S.; Zhang, H.-L.; Uosaki, K.; Liu, Z.-F. Langmuir 2000, 16, 6948. (10) Zhong, C.-J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518. (11) Chailapakul, O.; Sun, L.; Xu, C.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459. (12) Wang, R.; Iyoda, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. Langmuir 1997, 13, 4644. (13) Zhang, M.; Anderson, M. R. Langmuir 1994, 10, 2807. (14) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (15) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (16) Doron, A.; Katz, E.; Tao, G.; Willner, I. Langmuir 1997, 13, 1783. (17) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. AdV. Mater. 2000, 12, 1315. (18) Ulman, A. Chem. ReV. 1996, 96, 1533. (19) Schoenherr, H.; Vancso, G. J.; Huisman, B. H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 1997, 13, 1567.
Recently, many efforts have been made to introduce functionalities into SAMs to broaden their usefulness. One of the most effective and popular strategies for the functionalization of SAMs is the modification of the terminal groups at the outer surface of the SAMs (i.e., the use of ω-substituted thiol monolayers20 such as the HS-(CH2)n-X type chemisorbed on Au (111)). Such substituted SAMs can modify both the chemical and physical properties of the substrates, providing a variety of chemically tailored surfaces. Moreover, it has long been recognized that the ω terminus can provide a convenient attachment point for subsequent chemistry. In this way, derivative surfaces or multilayer assemblies may be produced.21 Stimuli-responsive surfaces have properties that can be changed depending on the external stimuli of the environment. Various stimuli-responsive surfaces can easily be made by introducing environmental stimuli-responsive or “smart” materials onto surfaces. Different stimuli (e.g., electric field,22 thermal treatment,23 redox electrochemistry,24 photochemistry,25 etc.) can be used to tune the physicochemical properties of the material. Among them, changing the pH is a promising stimulus considering the numerous pH gradients that exist in both normal and physiological states. Smart materials that respond to pH by altering structures, barrier properties, and surface properties have enormous potential in chemical sensors,26-28 membrane separa(20) Li, L.; Chen, S.; Jiang, S. Langmuir 2003, 19, 2974. (21) Evans, S. D.; Ulman, A.; Goppert-Berarducci, K. E.; Gerenser, L. J. J. Am. Chem. Soc. 1991, 113, 5866. (22) Lahann, J.; Mitragotri, S.; Tran, T.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (23) Crevoisier, D.; Fabre, P.; Corpart, J.; Leibler, L. Science 1999, 285, 1246. (24) Wang, X.; Kharitonov, A. B.; Katz, E.; Willner, I. Chem. Commun. 2003, 1542. (25) Wang, X.; Zeevi, S.; Kharitonov, A. B.; Katz, E.; Willner, I. Phys. Chem. Chem. Phys. 2003, 5, 4236.
10.1021/la053425d CCC: $33.50 © 2006 American Chemical Society Published on Web 03/21/2006
3716 Langmuir, Vol. 22, No. 8, 2006 Scheme 1. Chemical Structure of 2-(11-Mercaptoundecanamido) Benzoic Acid (MUABA)
tions,29,30 and dynamic surfaces.31,32 Taking a pH-responsive membrane,30,33 for example, Hester and co-workers reported the fabrication of a kind of pH-responsive material that can extend or contract to close or open pores in the presence of target molecules and form the basis of smart membranes.29 Such membranes are useful for controlled-delivery systems34 and the selective separation of proteins.35 For pH-responsive surfaces, changes in pH can have an impact on physicochemical properties such as film resistance36 and especially the wettability. One of many attractive features is the remarkable nanoscale control over the properties of such surfaces by varying the pH conditions. In our previous study, we reported surfaces with pH-responsive properties prepared by the modification of the rough surface of gold nanostructures with 2-(11-mercaptoundecanamido) benzoic acid (MUABA).37 The rough surface can exhibit pH-responsive properties changing from near superhydrophobicity to superhydrophilicity. However, this makes detailed surface characterization impossible. Because the pH-responsive films have broad potential in many applications in sensing and separations, to understand and to tailor such properties of the surfaces it is essential to acquire better knowledge of such monolayers to enable new insights into how such films affect the pH-responsive properties. Because the self-assembly technique is one of the simplest and most effective methods of preparing thin films, herein we investigate the application of pH-responsive monolayers assembled on a planar gold surface as functionalized interfaces and study the behavior of such pH-responsive SAMs using infrared spectroscopy, cyclic voltammetry, contact angle measurements, and surface energy analysis. We demonstrate that better understanding of the monolayer behavior under different pH conditions is the key to controlling the macroscopic physical properties of the surfaces (e.g., surface wettability) that are very interesting and fundamentally important to the above-mentioned applications. 2. Experimental Section Materials. The pH-responsive molecule 2-(11-mercaptoundecanamido) benzoic acid (MUABA) shown in Scheme 1 was synthesized as published previously.37 The molecular formula of MUABA is HS-C10H20-CO-NH-C6H4-o-COOH, and this compound was purified using silica gel chromatography and characterized by 1H NMR, 13C NMR, and ESI-MS. Potassium ferricyanide was purchased from the Tianjing Chemical Reagent Company. Other (26) Richter, A.; Bund, A.; Keller, M.; Arndt, K.-F. Sens. Actuators, B 2004, 99, 579. (27) Gerlach, G.; Guenther, M.; Suchaneck, G.; Sorber, J.; Arndt, K.-F.; Richter, A. Macromol. Symp. 2004, 210, 403. (28) Lakard, B.; Herlem, G.; de Labachelerie, M.; Daniau, W.; Martin, G.; Jeannot, J.-C.; Robert, L.; Fahys, B. Biosens. Bioelectron. 2004, 19, 595. (29) Hester, J. F.; Olugebefola, S. C.; Mayes, A. M. J. Membr. Sci. 2002, 208, 375. (30) Ito, Y.; Park, Y. S.; Imanishi, Y. Langmuir 2000, 16, 5376. (31) Zhu, X.; DeGraaf, J.; Winnik, F. M.; Leckband, D. Langmuir 2004, 20, 1459. (32) Wilson, M. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 8718. (33) Hou, Z.; Abbott, N. L.; Stroeve, P. Langmuir 2000, 16, 2401. (34) Saito, K.; Ishizuka, S.; Higa, M.; Tanioka, A. Polymer 1996, 37, 2493. (35) Li, Y.; Spencer, H. G. ACS Symp. Ser. 1994, 540, 297. (36) Bai, D.; Habersberger, B. M.; Jennings G. K. J. Am. Chem. Soc. 2005, 127, 16486. (37) Jiang, Y.; Wang, Z.; Yu, X.; Shi, F.; Xu, H.; Zhang, X.; Smet, M.; Dehaen, W. Langmuir 2005, 21, 1986.
Jiang et al. chemicals were analytical-grade reagents and were used as received. All aqueous solutions were prepared with deionized water. Preparation of the Gold Substrates. After thorough cleaning procedures, quartz or glass substrates were precoated with 6 nm of chromium followed by 130 nm of high-purity gold, evaporated at a pressure of 5 × 10-6 mbar from a resistively heated tungsten and molybdenum boat, respectively.38 After the substrates returned to room temperature slowly, the chamber was back-filled with highpurity nitrogen. Hence, the gold-coated substrates were fabricated and ready for the preparation of SAMs. The resulting gold substrates for scanning tunneling microscopy (STM) measurements were further flame annealed for about 1 min.39 This treatment often produces large areas of Au (111) terraces extending over thousands of angstroms. Preparation of SAMs on Gold Surfaces. Before adsorption of the thiol monolayer, the gold substrates were cleaned by immersing them in piranha solution (70:30 vol/vol 98% sulfuric acid/30 wt % H2O2) for 30 min. Caution: piranha solution is highly corrosiVe and reacts Violently with organic materials. It should be handled with great care. Upon removal, these cleaned gold slides were rinsed with large amounts of deionized water and ethanol, sonicated in ethanol40 for 1 h, rinsed with a large amount of ethanol, and then dried in a stream of high-purity nitrogen immediately prior to the formation of thiol monolayers. The adsorption of thiols on gold from their solution in ethanol was performed according to the procedures reported previously.37,38,41,42 The gold substrates were dipped into the ethanolic solution of MUABA (1 mM) at room temperature overnight. Then, the SAMs on gold substrates were rinsed extensively with ethanol and dried in a stream of dry nitrogen before characterization. General Techniques. XPS Measurements. X-ray photoelectron spectroscopy (XPS) was performed on a PHI-5300 spectrometer with an Al KR X-ray source (1486.6 eV). The base pressure in the XPS analysis chamber during spectral acquisition was 2 × 10-8 Torr. The spectra were collected using a pass energy of 35.75 eV with MUABA SAM-covered gold slides as the sample substrate and were calibrated using the binding energy of C 1s as the reference. STM Measurements. STM measurements were carried out with a commercial instrument (Digital Instruments, Multimode Nanoscope IV) at room temperature in air. STM tips were prepared from Pt-Ir (90/10) wire (0.25 mm diameter) by the mechanical cutting method. STM images were taken in constant-current mode at a scan rate of 1 Hz and a resolution of 512 pixels × 512 pixels. Tunneling parameters of around 500 mV (bias voltage) and 500 pA (current setpoint) were used for taking images unless indicated otherwise. Electrochemical Measurements. Electrochemical measurements were performed using a potentiostat (Autolab PGSTAT12, Netherlands). Electrochemistry was carried out in a conventional threeelectrode glass electrochemical cell. A bare gold electrode (model CHI 101, 2 mm diameter) was used as a working electrode. An auxiliary electrode was platinum, and the reference electrode was a Ag/AgCl (saturated KCl) electrode. The experiments were performed at ambient temperature. The gold electrode underwent the following pretreatments to get a mirrorlike surface. First, the gold electrode was mechanically polished with 1 µm, 0.3 µm, and 0.05 µm R-Al2O3 and washed ultrasonically with deionized water. Next, it was electrochemically scanned in a 2 mM K3[Fe(CN)6] + 0.1 M KCl solution by potential scanning between -0.05 and 0.55 V until a reproducible cyclic voltammogram was obtained and was then completely rinsed with deionized water and tetrahydrofuran. Finally, it was dried with high-purity nitrogen before monolayer adsorption. The monolayer was formed by placing the bare gold (38) Dong, B.; Huo, F.; Zhang, L.; Yang, X.; Wang, Z.; Zhang, X.; Gong, S.; Li, J. Chem.sEur. J. 2003, 9, 2331. (39) Haiss, W.; Lackey, D.; Sass, J. K.; Besocke, K. H. J. Chem. Phys. 1991, 95, 2193. (40) Tsai, M.-Y.; Lin, J.-C. J. Colloid Interface Sci. 2001, 238, 259. (41) Yu, X.; Wang, Z.; Jiang, Y.; Shi, F.; Zhang, X. AdV. Mater. 2005, 17, 1289. (42) Zhang, L.; Huo, F.; Wang, Z.; Wu, L.; Zhang, X.; Ho¨ppener, S.; Chi, L.; Fuchs, H.; Zhao, J.; Niu, L.; Dong, S. Langmuir 2000, 16, 3813.
Anthranilate-Terminated Alkanethiol on a Au Surface electrode in a 1 mM solution of MUABA in ethanol overnight at room temperature. After that, the gold electrode modified with MUABA SAMs was rinsed with absolute ethanol and dried with high-purity nitrogen before further characterization. IR Measurements. Fourier transform infrared (FTIR) transmission and reflection-absorption (RAS) spectra were collected at a spectral resolution of 4 cm-1 with an IFS-66v/S FTIR spectrometer (Bruker) in vacuum at around 1 to 2 mbar of pressure. A KBr beam splitter was used for mid-IR. All of the FTIR transmission spectra were recorded by coadding 256 scans. IR-RAS was performed with p-polarized light incidence at 80° relative to the surface normal using a Bruker accessory. The reflected light was detected with a liquid-nitrogen-cooled MCT detector. A cleaned gold-coated wafer was used to obtain a background spectrum for IR-RAS spectra. To measure IR-RAS spectra, 4096 signal-averaged scans were accumulated in the spectral range of 4000-800 cm-1. The spectra were baseline corrected using a commercial software package. Contact Angle Measurements. Water contact angle values were acquired at room temperature under ambient conditions using an optical contact angle measuring device (OCA 20, Dataphysics Instruments GmbH) by a sessile drop measuring method, which is a static contact angle assessment. Halogen lighting with continuous adjustable intensity without hysteresis for homogeneous back lighting was used to image the water droplet, whereas a 0.7-4.5-fold magnification CCD-camera video system with a resolution of 768 pixels × 576 pixels was used to monitor and record the data. Ellipse fitting was selected as the default calculation method. The main test liquid chosen to evaluate hydrophobicity was deionized water. In the case of MUABA SAM surfaces, in each measurement a 2 µL droplet was dispensed onto the substrates under investigation. Description of the Method Used to Determine Surface Energy.43 By using the Owens, Wendt, Rabel, and Kaelble (OWRK) method to determine surface energy with suitable liquid physical property databases, the procedure is simplified with a suitable computer program (Supporting Information). Given the contact angles under two different testing liquids and the surface tension data of these liquids, the surface energy can be calculated. Herein, the surface energy measurements under different surface conditions were probed with water and ethylene glycol as the two kinds of testing liquids. In the present experiment, we used the following values of surface tension and its components for test liquids: the surface tension values for water and ethylene glycol are 72.8 and 47.7 mN/m, respectively. The dispersive and polar parts of the surface tension for water are 21.8 and 51.0 mN/m, respectively. The dispersive and polar parts of the surface tension for ethylene glycol are 30.9 and 16.8 mN/m, respectively. (For further details, please see Supporting Information.)
3. Results and Discussion The formation of thiol SAMs is known to result mainly from the chemisorption between surface gold atoms and sulfur atoms of molecules and the interchain interaction between alkyl chains. The idealized model structure of ω-substituted alkanethiol SAMs with amide moieties is shown in Figure 1. The most important feature in Figure 1 is the orientation of the chains that exists to accommodate the trans conformation of the amide and the organization of the alkyl chains with interchain hydrogen bonding interactions among adjacent amide groups. We have employed different surface characterization techniques to study the surface structure in an attempt to bridge the gap between the microscopic surface structure and macroscopic wetting properties. To confirm the chemisorption between the surface-active MUABA molecules and gold substrates, we first used XPS to investigate the binding energy and the elemental composition of the MUABA SAMs. As shown in Table 1, the binding energies of C 1s, N 1s, O 1s, S 2p, and Au 4f7/2 were found to be 285.00, 400.05, 532.13, 162.02, and 84.09 eV (Supporting Information), respectively. It has been reported that the S 2p peak is centered (43) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741.
Langmuir, Vol. 22, No. 8, 2006 3717
Figure 1. Idealized model of a monolayer of ω-substituted alkanethiols with amide moieties adsorbed on the Au (111) surface. Interchain hydrogen bonding occurs between the adjacent amide groups. Here, X denotes the -C6H5-o-COOH group. Table 1. XPS Peak Assignments and Atomic Composition of MUABA SAMs on Gold C1s N1s O1s S2p Au4f7/2
BEa
obsd atom %b
pred atom %b
285.00 400.05 532.13 162.02 84.09
75.07 4.20 17.52 3.21
78.26 4.35 13.04 4.35
a BE ) binding energy in eV. b Atomic composition in atom %. Abbreviations: obsd atom % ) observed atomic composition by XPS; pred atom % ) predicted from the molecular formula of the adsorbate molecules.
Figure 2. XPS survey and S 2p spectra of MUABA monolayers adsorbed on a gold surface. High-resolution spectra of the S 2p regions are presented and referenced to the binding energy of C 1s. The binding energies of C 1s, N 1s, O 1s, S 2p, and Au 4f7/2 are given in Table 1.
at 162 eV for bound thiolate on gold surfaces and at 163 to 164 eV for unbound thiol species.44,45 The XPS peak of S 2p with a binding energy of 162.02 eV shown in Figure 2 clearly indicates that the MUABA molecules are chemisorbed onto the gold substrates, with the formation of a Au-S bond. Peaks corresponding to free thiols were not observed, indicating that the physisorption of MUABA and multilayer formation do not occur. We have used XPS to provide further quantitative information on the surface concentration (Table 1). From the integrated peak area of the signals of the different elements in XPS spectra, the atomic composition of the MUABA SAMs on gold surfaces could be measured. The results were in good agreement with the theoretical data calculated from the MUABA molecular formula, as shown in Scheme 1. This observation provides further evidence for the chemisorption of MUABA molecules on gold substrates. Because STM can provide a direct image of monolayers on the microscopic scale, the MUABA SAMs on gold surfaces were studied with this technique. Figure 3a shows a typical surface (44) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (45) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147.
3718 Langmuir, Vol. 22, No. 8, 2006
Figure 3. (Left) STM image (305.6 × 305.6 nm2) of the MUABA SAMs on a Au (111) surface. (Right) STM image (92.99 × 92.99 nm2) of the MUABA SAMs on a Au (111) surface. The images were taken in constant-current mode with a data scale bar of 2 nm.
morphology of a Au (111) surface covered with a SAM of MUABA prepared by immersing bare gold substrates in a 1 mM MUABA solution overnight. As shown by the STM observation, the MUABA does form a SAM on the gold surfaces but with many pit-like defects. The pit-like defects that usually exist in SAMs are likely to be defects in the Au surface layer because the average depth of these pit-like defects is about 0.24 nm according to STM section analysis (Supporting Information), which is similar to a Au (111) single-atom step height of 0.24 nm.46,47 These typical pits, so-called Au vacancy islands,48 were revealed by numerous STM studies, which are likely to form during the assembly of MUABA monolayers on Au (111) surfaces. The electrochemical properties for MUABA SAMs on gold were studied by cyclic voltammetry (CV) using the Fe(CN)63-/ Fe(CN)64- system as a redox probe. Figure 4 shows the cyclic voltammetric responses of the bare and MUABA SAM modified gold electrodes in the presence of Fe(CN)63-. In the case of a bare gold electrode (Figure 4A), a pair of easily reversible waves with a small peak potential separation of 0.076 V were observed at a scan rate of 0.10 V s-1. Figure 4B shows cyclic voltammograms of MUABA SAM modified gold electrodes in a solution of 2 mM K3[Fe(CN)6] + 0.1 M KCl. Upon monolayer formation on a gold surface, we observed a quick decrease in the current response. It can be seen that very few redox peaks were obtained at the MUABA monolayer modified electrode in the potential range from -0.05 to 0.55 V, indicting that the SAMs inhibit the electron-transfer process between the probe and the gold electrode. Therefore, the CV data suggest that a compact MUABA monolayer with a strong blocking effect of electron transfer was formed on the gold surfaces. To provide more detailed information about the molecular structure of the MUABA SAMs, we used both IR transmission and RAS spectroscopy. The assignments of the MUABA bands are summarized in Table 2. The CH2 bands of the alkyl chain of MUABA appear at 2921.8 and 2850.4 cm-1 in the transmission spectrum (Figure 5a), indicating that the alkyl chains have one or two gauche structures. The band at 1679.8 cm-1 is due to the amide I mode, and its shoulder at 1697.1 cm-1 is attributed to the CdO stretching mode of the carboxyl groups, suggesting that the carboxyl groups are involved in hydrogen bonding in the bulk state of MUABA50 (i.e., there is no band that can be assigned to the CdO stretching mode of free carboxyl groups). It is well known that the peak positions of the symmetric and (46) Poirier, G. E. Chem. ReV. 1997, 97, 1117. (47) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll, Ch.; Grunze, M. Langmuir 1993, 9, 4. (48) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826.
Jiang et al.
Figure 4. Cyclic voltammograms of (A) bare gold electrodes (-) and (B) MUABA SAM-modified gold electrodes (- - -) in a solution of 2 mM K3[Fe(CN)6] + 0.1 M KCl at a scan rate of 0.10 V/s.
the antisymmetric CH2 stretching vibrations can be used as sensitive indicators of the alkyl chain ordering51-54 because the frequencies of the CH2 stretching bands are sensitive to the conformation of a hydrocarbon chain. Low frequencies (∼2918 and ∼2848 cm-1) are characteristic of a highly ordered (transzigzag) alkyl chain, whereas their upward shifts are indicative of an increase in conformational disorder (i.e., the presence of gauche conformers) in the hydrocarbon chain.55,56 The IR-RAS spectrum of the MUABA cast film (Figure 5b) shows CH2 stretching bands at 2927.6 and 2854.3 cm-1, suggesting that this film contains poorly ordered alkyl chains. The peak at 1693.3 cm-1 is more intense compared to the peak at 1697.1 cm-1 in the transmission spectrum, where the interaction between -COOH and the gold surface may be involved, probably because of the fact that the carboxyl groups are more perpendicular with respect to the gold surface according to the surface selection rules of reflection IR spectroscopy. The assignments of the monolayer IR-RAS data are comparable with the IR transmission and RAS data of a MUABA cast film shown in Table 2. The band at 1693.3 cm-1 is due to the hydrogen-bonded CdO in the carboxyl groups (Figure 6a). The amide I is weak, which is due to the almost-parallel-oriented amide CdO groups of MUABA SAMs with respect to the gold surface. From the above discussion and the following IR data of the MUABA SAM treated with pH 1 water, we can see that in such an untreated monolayer the amide CdO is likely to form hydrogen bonds with the adjacent amide NH groups and also with the -OH of the carboxyl groups (both intramolecular and intermolecular hydrogen bonding can be involved), as after the treatment of the MUABA SAMs with pH 1 water the amide II band shows an upward shift with the increasing fraction of the free form CdO in the carboxyl groups (vide post). The intermolecular hydrogen bonding among the outside -COOH groups of MUABA may also be formed to some extent.50 Upon treatment of the MUABA SAMs with pH 1 water, the CH2 antisymmetric and symmetric stretching bands appear at (49) The untreated MUABA SAMs on gold substrates are somewhat sensitive to the surrounding conditions. The peak positions of the CH2 antisymmetrical and symmetrical stretching vibrations show poor reproducibility. The poor reproducibility may be due to changes in the SAMs thickness and roughness and differences in quality of Au (111) face. (50) Arnold, R.; Azzam, W.; Terfort, A.; Wo¨ll, C. Langmuir 2002, 18, 3980. (51) Duan, L.; Garrett, S. J. Langmuir 2001, 17, 2986. (52) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (53) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (54) Zhao, B.; Li, H.; Zhang, X.; Shen, J.; Ozaki, Y. J. Phys. Chem. B 1998, 102, 6515. (55) Umemura, J.; Cameron, D. G.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 602, 32. (56) Sapper, H.; Cameron, D. G.; Mantsch, H. H. Can. J. Chem. 1981, 59, 2543.
Anthranilate-Terminated Alkanethiol on a Au Surface
Langmuir, Vol. 22, No. 8, 2006 3719
Table 2. Frequencies (cm-1) and Assignments of IR Bands in Transmission and RAS Spectra of MUABA in Cast Films and SAMsa transmission
IR-RAS
cast film 3330.6 2921.8 2850.4 1697.1 sh 1679.8 1585.3 1529.4 1450.3 1263.2
cast film
pH 1 treated SAM
untreated SAM
pH 13 treated SAM
b
b
b
b
2927.6 2854.3 1693.3
c
2925.6 2854.3 1697.1
2923.7 2852.4
c
1693.3
1587.2 1529.4 1452.2
c
1452.2
1743.4 1660.5 w 1591.1 1535.1 1452.2
b
b
b
1658.6 w 1593.0
1660.5 1596.9 1525.5 1454.1 b
assignments N-H str CH2 antisym str CH2 sym str CdO str in COOH (with/without H-bonding) amide I benzene
ring amide II
benzene
ring amide III
a Abbreviations: antisym ) antisymmetric, sym ) symmetric, str ) stretch, sh ) shoulder, and w ) weak. Transmission and IR-RAS spectra were measured on CaF2 slides and on gold substrates, respectively. b These regions overlap with other peak, causing difficulties for peak assignments. c The reproducibility of the peak position is not very good. (See ref 49 for more details).
Figure 5. (a) FTIR transmission spectrum of a MUABA cast film on a CaF2 slide. (b) Infrared reflection absorption spectrum (IRRAS) of a MUABA cast film on a gold surface.
2925.6 and 2854.3 cm-1, respectively (Supporting Information). This suggests that the hydrocarbon chains in these MUABA SAMs are poorly ordered (i.e., the gauche conformations of alkyl chains are heavily weighted in this case). In the IR-RAS data of the MUABA SAMs on gold substrates, after the treatment with pH 1 water, bands at 1743.4 and 1697.1 cm-1 are observed that can be assigned to free (fully protonated carboxyl groups) and hydrogen-bonded CdO of the carboxyl groups, respectively, revealing the coexistence of hydrogen-bonded and free forms (Figure 6b). Amide I is weak, which gives a hint that the paralleloriented amide CdO groups probably do not change very much after the treatment of the MUABA SAMs with pH 1 water. The fact that the hydrogen-bonded CdO in the carboxyl groups (signal at 1697.1 cm-1) also shows an upward shift compared with that of the untreated SAMs (Figure 6a, signal at 1693.3 cm-1) indicates that the hydrogen bonding in which these carbonyl groups are involved is weakened after treatment with pH 1 water. A band due to amide II appears at 1529.4 cm-1 in the IR-RAS spectrum of the MUABA cast film (Figure 5a), whereas the monolayer spectrum shows an amide II band at 1535.1 cm-1. These observations also suggest that the amide II band is involved in the hydrogen bonding. The amide II band at 1535.1 cm-1 also shows an upward shift compared with that of the untreated MUABA SAMs, and thus the amide II upward shift may be due to the increasing fraction of NH groups involved in hydrogen bonding. This is in agreement with the observed increasing fraction of the free-form CdO in the carboxyl groups, which is likely to coincide with the release of more amide CdO groups (out of the hydrogen bonding with the -OH of the carboxyl groups) that are available to form hydrogen bonds with the adjacent amide NH groups. Also, the intermolecular hydrogen
bonding among the outside -COOH groups may be weakened to some extent because of full protonation. The structure of the alkyl chains was found to be altered with the great changes in the hydrogen bonding environments when the MUABA SAMs were treated with pH 13 water (Figure 6c). After this treatment, there are no -COOH groups in the MUABA SAM anymore because the -COOH groups are deprotonated, resulting in decreased hydrogen bonding among CdO in the carboxyl groups. As shown in the Supporting Information, CH2 antisymmetric and symmetric stretching bands of MUABA SAMs were observed at 2923.7 and 2852.4 cm-1, respectively. This suggests that the hydrocarbon chains of MUABA SAMs are more ordered than those after the treatment with pH 1 water. The observed band at 1660.5 cm-1 is due to the amide I mode. The existence of hydrogen bonding between the adjacent amide groups is further confirmed by the frequency of amide I (1660.5 cm-1), which is largely due to a CdO stretching mode. The corresponding band appears at 1679.8 cm-1 in the IR transmission spectrum of the MUABA cast film. The intensity of the amide CdO stretching band is also relatively strong (Figure 6c), indicating that the CdO groups are more perpendicular with respect to the gold surface according to the surface selection rules of reflection IR spectroscopy.57 The amide II band at 1525.5 cm-1 shows a downward shift as compared to the corresponding band at 1535.1 cm-1 of the pH 1 water-treated MUABA SAMs, suggesting that the amide NH groups are less involved in hydrogen bonding, probably because of the spatial hindrance of the large acylated anthranilate groups after treatment with pH 13 water. The downward shift of amide II may be due to the change in the alkyl conformation. We finally employed wettability studies to investigate the changes in the surface structure under different pH conditions. We wonder if the functional groups at the MUABA SAMwater interface are conformationally mobile with respect to the interface, which can influence the wettability properties. The contact angle measurement data are summarized in Figure 7. For comparison, we applied treatments similar to those in the IRRAS experiments. After treatment with pH 1 and 13 water, the MUABA SAMs showed clearly different wetting properties. The MUABA SAMs showed more hydrophobic behavior after treatment with pH 1 water and more hydrophilic behavior after treatment with pH 13 water (Figure 7). It is very likely that the observed changes in wettability are due to the exposure of rather (57) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62.
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Figure 6. IR-RAS spectra in the region of 1900-1300 cm-1 of MUABA SAMs on gold. (a) Untreated SAMs, (b) SAMs treated with pH 1 water, and (c) SAMs treated with pH 13 water.
Figure 7. Water contact angle measurements on the MUABAmodified gold surfaces as a function of the deposition time of SAM formation: b, untreated MUABA SAMs/Au; 2, MUABA SAMs/ Au treated with pH 1 water; 9, MUABA SAMs/Au treated with pH 13 water (droplet size, 2 µL).
observations suggest that the functional groups at the MUABA SAM-water interface are conformationally mobile with respect to the interface and that this mobility can strongly influence the macroscopic properties of the interface such as the wettability. Whitesides et al.58 found that the wetting properties are dominated by very short-range interactions (
