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Design of a Chiral Surface by Modifying an Anionically Charged Single-Layered Inorganic Compound with Metal Complexes Shin Takahashi,† Rina Tanaka,† Noboru Wakabayashi,† Masahiro Taniguchi,‡ and Akihiko Yamagishi*,§,| Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan, Department of Materials Science and Engineering, Kanazawa Institute of Technology, Ishikawa 921-8501, Japan, Graduate School of Science, Department of Earth and Planetary Science, The University of Tokyo, Tokyo 113-0033, Japan, and CREST, Japan Science and Technology Corporation, Tokyo 102-8666, Japan Received January 6, 2003. In Final Form: May 12, 2003 We have developed a novel method of modifying a solid surface with a single-layered film of anionic phyllosilicate as an anchor of cationic functional molecules. According to the method, a hybrid film of octadecylammonium and montmorillonite layer was deposited onto a Si substrate by the LangmuirBlodgett method. Thereafter the Si substrate was immersed in methanol to remove an organic layer, leaving a single-layered montmorillonite film. As a final step, a cationic chiral molecule, ∆-[Ru(phen)3]2+ (phen ) 1,10-phenanthroline), was self-assembled from its aqueous solution onto the single-layered montmorillonite film. The thickness of a single-layered montmorillonite film was estimated to be ca. 1.51.6 nm by atomic force microscopy observation. The thickness of the film increased to ca. 2.6-2.7 nm after the self-assembly treatment. The results were consistent with the analyses of interference oscillation patterns of specular X-ray reflectivity (SXR) curves. From the analyses of SXR curves, the thickness of the adlayer of ∆-[Ru(phen)3]2+ was estimated to be ca. 1.5-1.6 nm. On the basis of this, it was concluded that ∆-[Ru(phen)3]2+ formed an adlayer within a monolayer scale. The present method may open a way to modify a solid surface by use of a water-soluble functional molecule with neither cross-linking group nor amphiphilic property.
1. Introduction Modification of a solid surface by functional films has attracted wide attention in physical, biological, and chemical research areas.1-15 It provides us with not only fundamental understanding of mechanism on molecular recognition by a surface but also various practical applications for functional devices. In the present paper, a novel method of creating a chiral solid surface has been presented by extending the previous method of preparing * To whom correspondence may be addressed. Fax: +81-3-58414553. E-mail:
[email protected]. † Hokkaido University. ‡ Kanazawa Institute of Technology. § The University of Tokyo. | CREST, Japan Science and Technology Corporation. (1) Rogozhin, S. V.; Davancov, V. A. Chem. Commun. 1971, 490. (2) Yamagishi, A.; Aramata A. J. Electroanal. Chem. 1985, 191, 449. (3) Yao, K.; Nshimura, S.; Ma, T.; Okamoto, K.; Inoue, K.; Abe, E.; Tateyama, H.; Yamagishi, A. J Electroanal. Chem. 2001, 510, 144. (4) Inose, Y.; Moniwa, S.; Aramata, A.; Yamagishi, A.; Kyaw-Naing Chem. Commun. 1997, 111. (5) Ohtani, B.; Shintani, A.; Uosaki, K. J. Am. Chem. Soc. 1999, 121, 6515. (6) Yoshida, M.; Hatate, Y.; Uezu, K.; Goto, M.; Furusaki, S. Colloids Surf. 2000, 169, 259. (7) Watanabe, T.; Okawa, Y.; Tsuzuki, H.; Yoshida, S.; Nihei, Y. Chem. Lett. 1988, 1183. (8) Tatsuma, T.; Tsuzuki, H.; Okawa, Y.; Yoshida, S.; Watanabe, T. Thin Solid Films 1991, 202, 145. (9) Miller, R. D.; Michl, J. Chem. Rev. 1989, 89, 1359. (10) Rojas, M. T.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 5883. (11) Ullman, A. Chem. Rev. 1996, 96, 1533. (12) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (13) Aoki, A.; Abe, Y.; Miyashita, T. Langmuir 1999, 15, 1463. (14) Ansell, M. A.; Cogan, E. B.; Page, C. J. Langmuir 2000, 16, 1172. (15) Umemura, Y.; Yamagishi, A.; Schoonheydt, R.; Persoons, A.; Schryver, F. Langmuir 2001, 17, 449.
an organic-inorganic hybrid film.16 A surface modified with optically active compounds is a challenging subject, since it leads to the recognition of molecular chirality and asymmetric synthesis.1-6 For instance, a silica or an organic polymer modified with optically active compounds has been widely used as a stationary phase of column chromatography for optical resolution.1 An electrode modified with chiral metal complexes intercalated in a layered inorganic material exhibits chiral discrimination.2,3 For understanding a detailed mechanism of chiral recognition, a well-ordered solid surface modified with functional molecules will be required. Self-assembly (SA) and Langmuir-Blodgett (LB) techniques have been widely used for preparation of such a modified solid surface. In these methods, a thin molecular film is formed on a solid surface by immobilizing molecules through cross-linking groups such as thiol or silanol groups or depositing a floating monolayer of amphiphilic molecules with long alkyl chains. Generally speaking, molecules with such functional groups are difficult to synthesize. Besides, the presence of thiol or alkyl moieties may act as an obstacle against molecular recognition. Recently we reported the characterization of an organicinorganic hybrid LB film consisting of octadecylammonium hydrochloride and inorganic-layered silicate by means of grazing incidence in-plane X-ray diffraction (in-plane XRD) and specular X-ray reflectivity (SXR) measurements.16 Sodium montmorillonite was used as an inorganic-layered silicate. The material is a naturally occurring layered silicate with cation exchanging properties. One layer (16) Takahashi, S.; Taniguchi, M.; Omote, K.; Wakabayshi, N.; Tanaka, R.; Yamagishi, A. Chem. Phys. Lett. 2002, 352, 213.
10.1021/la034021t CCC: $25.00 © 2003 American Chemical Society Published on Web 06/18/2003
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Figure 1. Schematic representation of preparation of a singlelayered inorganic film and self-assembly of ∆-[Ru(phen)3]2+. The thickness of a silicon wafer is tentatively drawn to be in the same order as that of an organic layer.
consists of two silica tetrahedral sheets sandwiching an alumina octahedral sheet with the total thickness of ca. 0.95 nm.17 A uniformly layered structure as well as parallel orientation of the (001) plane of montmorillonite crystal to a film surface was confirmed. The present study reports that an organic part of the above hybrid LB film can be removed by being immersed in a methanol solution, leaving a single-layered montmorillonite film. Furthermore a produced single-layered montmorillonite film is used as an adsorbent anchoring cationic functional molecules. ∆-[Ru(phen)3]2+ (phen ) 1,10-phenanthroline) was used as an example of a functional chiral molecule. The molecule is known to have a chiral recognition ability as well as photochemical properties.2,3 The present work provides a novel method of preparing a chiral surface with water-soluble functional molecules. 2. Experimental Section Materials. Sodium montmorillonite (Kunipia P) was purchased from Kunimine Ind. Co. (Japan). The material is stated to have the elemental composition of (Si7.20Al0.80)(Al5.97Mg0.03)O20(OH)4(Na0.76K0.02Ca0.02). According to the manufacturer, isomorphous substitution occurs dominantly in the tetrahedral layer (96%). Cation-exchange capacityis stated to be 117 mequiv/100 g. Octadecylammonium chloride (ODAH+Cl-) was purified by recrystallization from methanol. ∆-[Ru(phen)3](ClO4)2 was obtained by resolving its racemic mixture with d-(+)-potassium antimony tartrate and purified by being eluted on a 4 mm (i.d.) × 25 cm column of Capcell Pack (Shiseido Co. Ltd. (Japan)) with methanol. A single-side mirrorlike polished Si(100) wafer (a gift from Shin-Etsu Chemical Co., Japan) was used throughout the experiments. A hydrophilic Si substrate was prepared by being immersed in a mixed solution of 30% hydrogen peroxide and 25% ammonia water (1:1 v/v) at 80 °C for 2 h and rinsed with pure water. Sample Preparation. A scheme for preparation of a singlelayered montmorillonite film adsorbing ∆-[Ru(phen)3]2+ cations is shown in Figure 1. A 10:1 (v/v) chloroform/methanol mixture of (ODAH+Cl-) was spread onto an aqueous dispersion of 0.001 g L-1 sodium montmorillonite. Ninety minutes after the solution was spread, a surface was compressed at a rate of 10 cm2 min-1 up to 15 mN m-1. The floating monolayer was transferred onto a hydrophilic Si substrate by the vertical dipping method at a surface pressure of 15 mN m-1 and dipping speed of 1 mm min-1. The substrate was dried in a desiccator at least for 12 h. In this way, a Si substrate modified with a hybrid film of montmorillonite and ODAH+Cl- was prepared. As a next step, the above Si substrate was immersed in methanol for 8-12 h to remove an organic part (ODAH+Cl-). Thereafter the substrate was rinsed with ultrapure water. In (17) Brindley, G. W.; Brown G. Crystal structures of Clay Minerals and Their X-ray Identification; Mineralogical Society: London, 1984; Chapter 3.
Figure 2. FTIR transmission spectra of organic-inorganic hybrid films deposited onto a Si substrate in the region of symmetric and asymmetric CH2 stretching vibrations. Solid (a) and broken (b) curves correspond to the samples before and after methanol treatment, respectively. this way, a Si substrate modified with a single-layered montmorillonite film was prepared. The substrate was immersed in an aqueous solution of (1.1-1.2) × 10-4 M ∆-[Ru(phen)3](ClO4)2 for 2-6 h. During this procedure, ∆-[Ru(phen)3]2+ was selfassembled onto a monrmorillonite film to form an adlayer. The substrate was rinsed with pure water to remove excess ∆-[Ru(phen)3](ClO4)2. The resultant film will be denoted as a ∆-[Ru(phen)3]2+-montmorillonite film. All procedures were done at 20 °C. Structural Characterization of a Film. The structure of a film was evaluated by means of FTIR, atomic force microscopy (AFM), and specular X-ray reflectivity (SXR) measurements. FTIR measurements were carried out at a resolution of 8 cm-1 with a JIR7000 spectrometer (JEOL Co., Japan) equipped with a MCT detector. The surface morphology of a film was studied with a Nanoscope III (Digital Instruments) AFM apparatus by tapping mode in the scan range of 2 µm × 2 µm. SXR measurements were carried out by a RINT 2100 in-plane X-ray diffractometer (Rigaku Co., Japan) which has an X-ray source (Cu KR 1.54 Å/2 kW) equipped with a graded d-spacing parabolic multilayered mirror.18 The SXR measurements were done in specular reflection mode; i.e., an incident angle was equal to an exit angle. In the present work, glancing incidence angle R was used. SXR curves were analyzed by fitting program RGXR (Rigaku), which is based on the X-ray reflection theory by Parratt19 with a roughness model developed by Croce and Nevot.20
Results and Discussion Atomic Force Microscopy (AFM) Observation and FTIR Measurement. Spectra a and b of Figure 2 show FTIR transmission spectra of an organic-inorganic hybrid LB film in the region of CH2 symmetric and asymmetric stretching vibration modes before and after methanol treatment, respectively. The disappearance of CH2 stretching vibration bands (Figure 2b) clearly demonstrates the detachment of an organic part (ODAH+) from a hybrid LB film during the methanol treatment. As a result, a singlelayered montmorillonite film was obtained. As for the mechanism of the removal of an organic part, it was speculated that ODAH+ cations were dissociated partly by being replaced by a trace of protons in methanol and partly as neutral molecules of octadecylamine. (18) Omote, K.; Harada, J. Adv. X-ray Anal. 2000, 43, 192. (19) Parratt, L. G. Phys. Rev. 1954, 95, 359. (20) Nevot, L.; Cross P. Rev. Phys. Appl. 1980, 15, 761.
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Figure 3. AFM images (2.0 µm × 2.0 µm) (upper) and crosssectional profiles (lower) of single-layered inorganic films (a) before and (b) after the self-assembly of ∆-[Ru(phen)3]2+, respectively.
Parts a and b of Figure 3 show AFM images with the cross-sectional profiles on a single-layered montmorillonite film and a ∆-[Ru(phen)3]2+-montmorillonite film, respectively. As seen in Figure 3a, it was confirmed that a singlelayered montmorillonite film remained on a substrate after the detachment process of the organic part. The fact that a montmorillonite layer was so strongly fixed on a hydrophilic Si substrate was explained on the basis of the following two reasons: One is an electrostatic force between a negatively charged clay layer and a positively charged Si substrate due to protonated OH groups on a surface. This interaction might be even more efficient since a clay particle takes a platelike shape and is in close contact with an atomically flat surface. The other reason is a kinetic aspect that water molecules penetrated a narrow region between a clay layer and a Si substrate very slowly. A large amount of time was required until a clay layer was removed from the surface and into the water medium. The surface of an individual montmorillonite particle seemed to be quite smooth. The thickness of each montmorillonite layer as measured from a Si substrate surface was estimated to be ca. 1.5 ( 0.2 nm. Figure 3b shows the AFM image after a Si substrate modified with a single-layered montmorillonite film was immersed in an aqueous ∆-[Ru(phen)3]2+ solution for 4 h to form a ∆-[Ru(phen)3]2+-montmorillonite film. The film thickness was estimated to be ca. 2.7 ( 0.2 nm. The increase of film thickness suggested that ∆-[Ru(phen)3]2+ formed an adlayer on a montmorillonite layer. It should be noted that these AFM images did not reflect total surface properties, but they gave information on a limited region of a film surface (2 µm × 2 µm). The results are compared with the surface properties averaged over a macroscopic scale as were evaluated by the SXR measurements. Specular X-ray Reflectivity (SXR) Measurements. SXR profiles for a single-layered montmorillonite film and a ∆-[Ru(phen)3]2+-montmorillonite film on Si substrates are shown in parts a and b of Figure 4, respectively. The layer-by-layer structure of a film was confirmed by the appearance of clear interference oscillation patterns (Kiessig fringes) on both SXR curves. The observed fringes were analyzed on the basis of the structural models derived from the above AFM observations. Fitting was made with three parameters or layer density, layer thickness, and surface roughness of each layer, while those of thickness
Takahashi et al.
Figure 4. SXR profiles of (a) a single-layered inorganic film and (b) a ∆-[Ru(phen)3]2+ montmorillonite hybrid film. Empty circles and solid curves denote experimental data and fitting curves, respectively. The fitting parameters are given in Table 1. Table 1. Fitting Parameters for the Single-Layered Anionic Silicate Film Deposited on Silicon Substrate and ∆-[Ru(phen)3]2+ Film Self-Assembled on the Single-Layered Anionic Silicate Film layer
density (g cm-3)
thickness (nm)
roughness (nm)
(a) Single-Layered Anionic Silicate Film Deposited on Silicon Substrate inorganic layer 1.51 1.52 oxide 2.09 0.58 silicon 2.33
0 0.17 0.18
(b) ∆-[Ru(phen)3]2+ Film Self-Assembled ∆-[Ru(phen)3]2+ 1.35 1.53 inorganic layer 1.82 1.29 oxide 2.24 0.76 silicon 2.33
0 0.30 0.27 0
and density of the Si substrate were fixed at certain values. A SiO2 amorphous layer was assumed to be present on a Si substrate. The fitting parameters are given in Table 1. On analysis of the SXR curve in Figure 4a, the thickness of a single-layered montmorillonite film was determined to be 1.52 nm, which is in good agreement with the results from AFM measurements as described in the previous section. As supported by the previous report on grazing incidence in-plane X-ray diffraction measurements,16 the film is characterized by uniform layer structure and parallel orientation of the (001) plane of montmorillonite crystal.16 It should be mentioned that the observed thickness of a single-layered montmorillonite layer (1.52 nm) is larger than the reported thickness of a single layer of montmorillonite (0.95 nm).17 Our interpretation is that solvated Na+ ions were located in a region between a clay layer and a Si substrate. These ions were present as the countercations of original montmorillonite and stayed there by partly compensating the negative charge of a clay layer at the state of an ODAH+-montmorillonite hybrid film. Figure 4b is the SXR profile of a ∆-[Ru(phen)3]2+montmorillonite film on a Si substrate. It shows an interference oscillation pattern different from that of a single-layered montmorillonite film, implying that [Ru(phen)3]2+ complexes formed an adlayer on a single-layered
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montmorillonite. The thicknesses of a single-layered montmorillonite and a ∆-[Ru(phen)3]2+ adlayer were determined to be 1.29 and 1.53 nm, respectively. The results are consistent with a structural model as illustrated in Figure 1. The total thickness of 2.82 nm was nearly identical to the result from the AFM observation on a ∆-[Ru(phen)3]2+-montmorillonite film (ca. 2.7 ( 0.2 nm). Assuming that a ∆-[Ru(phen)3]2+ molecule is a sphere with a radius of 0.6 nm, the adlayer is concluded to be a monolayer of ∆-[Ru(phen)3]2+ ions. Notably the thickness of a montmorillonite layer in the ∆-[Ru(phen)3]2+-montmorillonite film was found to decrease by 0.23 nm in comparison with that of a single-layered montmorillonite. Examining a fluctuation in the estimate of each layer thickness, we conclude that the thickness of a singlelayered montmorillonite film was within the region of 1.51.7 nm, while that of a ∆-[Ru(phen)3]2+-montmorillonite film fell into 1.1-1.5 nm. In addition, the thickness of a ∆-[Ru(phen)3]2+ layer was constant within 1.5 ( 0.1 nm. From the present result, the observed decrease of thickness of a single-layered montmorillonite through selfassembling treatment may be ascribed to the elimination of hydrated Na+ cations on the complete compensation of a negative charge of a montmorillonite layer by adsorption of ∆-[Ru(phen)3]2+. To confirm this model, elementary analysis such as an angle-resolved X-ray photoelectron spectroscopy is required. Conclusion We have proposed a new technique for fabricating a functional film consisting of water-soluble cationic func-
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tional molecules and an anionically charged montmorillonite layer on a solid surface. The resultant functional film consisted of a ∆-[Ru(phen)3]2+ layer anchored on a clay layer. Its layer structure was evaluated by AFM and SXR measurements. AFM images showed the formation of a uniform and highly oriented single-layered montmorillonite film when a hybrid Langmuir-Blodgett film of octadecylammonium hydrochloride and montmorillonite was immersed in methanol for 8-10 h. ∆-[Ru(phen)3]2+ was found to be self-assembled onto a montmorillonite surface from its aqueous solution. From the analyses of the interference oscillation patterns of SXR curves, the thickness of a ∆-[Ru(phen)3]2+ layer was determined to be ca. 1.5 ( 0.1 nm, while that of a single-layered montmorillonite below the [Ru(phen)3]2+ layer was within the region of 1.1-1.5 nm. The present technique will be widely available for modification of a solid surface by use of watersoluble functional molecules. Acknowledgment. Special thanks are due to the ShinEtsu Chemical Corporation for gift of Si wafers. This work was partly supported by Grants in Aid for Scientific Research from New Energy and Industrial Technology Development Organization (NEDO), a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government, Japan, and CREST of JST (Japan Science and Technology Corporation), to whom the author’s thanks are due. LA034021T
