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Photoinduced Polar Transition of Substrate Surfaces by Photodegradable Cationic Adsorbate Monolayers Masaru Nakagawa,*,†,‡ Nozomi Nawa,† Takahiro Seki,§ and Tomokazu Iyoda†,‡ Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, and CREST, JST, 4-1-8 Honmachi, Kawaguchi, Saitama 332-0012, Japan Received May 6, 2003. In Final Form: July 30, 2003 We describe a novel method to prepare adsorption templates for colloidal particle partterning on silica and poly(ethylene terephthalate) (PET) plates using environment-friendly water media by convenient photolithography of an adsorbed monolayer formed from a new photodegradable multivalent cationic adsorbate. The photodegradable decaphenylcyclopentasilane derivative possessing quaternary pyridinium groups was adsorbed by a negatively charged silica or PET surface from the aqueous solutions to form a photodegradable cationic adsorbed monolayer exhibiting desorption resistance toward deionized water. Exposure to UV light emitting from a widely used low-pressure mercury lamp resulted in photodegradation of the photoreactive cyclopentasilane skeleton in the adsorbate molecule and reduced markedly the desorption resistance of the photodegraded adsorbate toward deionized water because of a photochemical decrease in adsorption sites per molecule. The photodegraded adsorbate desorption from the substrate surface was confirmed by UV-visible absorption spectroscopy and contact-angle and zeta-potential measurements. As a result, it was found that the photodegradable cationic adsorbed monolayer exhibiting a positive zetapotential value was capable of inducing a polar transition to a negative zeta-potential value near the initial substrate surface by UV exposure. Taking into account the overall facts, we successfully prepared a photopatterned cationic adsorbed monolayer of the quaternized cyclopentasilane derivative on a silica or PET plate by imagewise UV exposure and multiple rinses with deionized water. The substrate surface possessing a photopatterned surface-charge heterogeneity was available to adhesive templates for the site-selective surface adsorption of carboxy- and amino-modified polystyrene spheres charged with negative and positive signs, respectively. Furthermore, a new approach to fabricate binary particle arrays consisting of both the carboxy- and amino-modified spheres on the substrate surface was demonstrated by controlling the electrostatic interaction among the particles, the UV-exposed surface, and the unexposed surface as a function of pH values.
Introduction Nanotechnology progress has been increasing the necessity of molecular surfaces tailored to promote or resist the adsorption of nano- and micrometer-sized tiny substances. The principal methodology to control the substance adsorption is to make use of interactions between surfaces of a tiny substance and a substrate. One approach from a standpoint of the substrate surface is to use an organic self-assembled monolayer (SAM).1 The outermost SAM consisting of assemblages formed from the smallest functional molecules on the substrate surface is capable of controlling the physicochemical surface properties of adhesion, wettability, friction, chemical reactivity, and so forth. In particular, micro- and nano-patterned SAMs have been increasing interest in electronic, photonic, and biological device fabrication because metal deposition and plating,2 polymer and biomacromolecule anchoring,3,4 tiny particle adsorption,5-8 and neuron cell deposition and growth2,9 on specifically chosen surface regions have been demonstrated. The patterned SAM is fabricated mainly * To whom correspondence should be addressed. E-mail:
[email protected]. † Tokyo Institute of Technology. ‡ CREST, JST. § Present address: Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 4648603, Japan. (1) Ulman, A. Introduction to Thin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic: Boston, 1991. (2) (a) Dulcey, C. S.; Georger, J. H., Jr.; Krauthamer, V.; Stenger, D. A.; Fare, T. J.; Calvert, J. M. Science 1991, 252, 551. (b) Dressick, W. J.; Calvert, J. M. Jpn. J. Appl. Phys., Part 1 1993, 32 (12B), 5829.
on silica or gold by UV-light2 and electron-beam5a,10 lithography, soft lithography of commonly accepted microcontact printing (µCP),11 and scanning probe microscope lithography12 of the SAM. However, the photolithography using UV light is underutilized among these patterning methods, although (3) For polymer anchoring: (a) Prucker, O.; Schimmel, M.; Tovar, G.; Knoll, W.; Ru¨he, J. Adv. Mater. 1998, 10, 1073. (b) Biesalski, M.; Ru¨he, J. Macromolecules 1999, 32, 2309. (c) Peng, B.; Ru¨he, J.; Johannsmann, D. Adv. Mater. 2000, 12, 821. (d) Prucker, O.; Naumann, C. A.; Ru¨he, J.; Knoll, W.; Frank, C. W. J. Am. Chem. Soc. 1999, 121, 8766. (e) Ingall, M. D. K.; Honeyman, C. H.; Mercure, J. V.; Bianoni, P. A.; Kunz, R. R. J. Am. Chem. Soc. 1999, 121, 3607. (f) Rozsnyai, L. F.; Wrighton, M. S. Langmuir 1995, 11, 3913. (g) Clark, S. L.; Montague, M. F.; Hammond, P. T. Macromolecules 1997, 30, 7237. (h) Wilhelm, L. Y.; Huck, T. S.; Zhao, X.-M.; Whitesides, G. M. Langmuir 1999, 15, 1208. (i) Lackowski, W. M.; Ghosh, P.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 1419. (4) For biomolecule anchoring: (a) Ichinose, N.; Sugimura, H.; Uchida, T.; Shimo, N.; Masuhara, H.; Chem. Lett. 1993, 1961. (b) Morgan, H.; Pritchard, D. J.; Cooper, J. M. Biosens. Bioelectron. 1995, 10, 841. (c) Pritchard, D. J.; Morgan, H.; Cooper, J. M.; Angew. Chem., Int. Ed. Engl. 1995, 34, 91. (d) Fodor, S. P. A.; Read, J. L.; Pirrung, M. C.; Stryer, L.; Liu, A. T.; Solas, D. Science 1991, 251, 767. (e) Pease, A. C.; Solas, D.; Sullivan, E. J.; Cronin, M. T.; Holmes, C. P.; Foder, S. P. A. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 5022. (5) For polymer particles: (a) Harnett, C. K.; Satyalakshmi, K. M.; Craighead, H. G. Langmuir 2001, 17, 178. (b) Nakagawa, M.; Ichimura, K. Colloids Surf., A 2002, 204, 1. (6) For semiconductor particles: Vossmeyer, T.; Jia, S.; Delonno, E.; Diehl, M. R.; Kim, S. H.; Peng, X.; Alivisatos, A. P.; Heath, J. R. J. Appl. Phys. 1998, 84, 3664. (7) For metal particles: (a) Vossmeyer, T.; Deiono, E.; Health, J. R. Angew. Chem., Int. Ed. Engl. 1997, 36, 1080. (b) Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 5349. (c) Liu, J.-F.; Zhang, L.-G.; Gu, N.; Ren, J.-Y.; Wu, Y.-P.; Lu, Z.-H.; Mao, P.-S.; Chen, D.-Y. Thin Solid Films 1998, 327-329, 176.
10.1021/la0347758 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/13/2003
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reliable mass production of tailor-made photopatterned surfaces is possible. This situation probably arises from the following. First, an expensive vacuum UV-light source is required for patterning SAMs of organosilanes2,13 and organothiols14 possessing a long alkyl or perfluoroalkyl tail, for example, octadecytrimethoxylsilane, 2-(perfluorohexyl)ethyltriethoxysilane, octadecanethiol, and so on. The exposure tool is not so readily available. Second, photoreactive SAMs bringing about the photoinduced deprotection of a nitrobenzyl group4e-d,5b,7a or photoactivation of a diazonaphthoquinone moiety15 by convenient UV-light exposure are prepared only by a liquid-phase adsorption method using organic solvents. Because the SAM-forming molecules possess a large hydrophilic photoreactive group, a convenient chemical vapor adsorption method16 and a liquid-phase adsorption method using environmentally benign water are unavailable. Moreover, the SAM-forming molecules are not so readily available as a result of the complicated organic synthesis. Third, SAMs derived from organosilanes and organothiolates are formed on a specific surface so that the limitation of the substrate choice prevents us from utilizing the photopatternable SAMs. To overcome these drawbacks, photopatternable SAMs should be developed that are compatible with a wider variety of substrates, milder solvents, and UV wavelengths available from inexpensive lowpressure Hg lamps. Furthermore, monolayer-forming molecules other than organosilanes and organothiols should be developed. We have reported that crown conformers of calix[4]resorcinearene derivatives with eight polar head carboxy groups are adsorbed strongly by a polar aminosilylated silica surface from their dilute solutions to give densely packed SAMs through multipoint adsorption.17 The SAMs exhibit excellent desorption resistance toward polar organic solvents and even water, enough to control reversible liquid motion18 and liquid-crystal alignment19 (8) For inorganic particles: (a) Ha, K.; Lee, Y. J.; Chun, Y. S.; Park, Y. S.; Lee, G. S.; Yoon, K. B. Adv. Mater. 2001, 13, 594. (b) Masuda, Y.; Seo, W.; Koumoto, K. Thin Solid Films 2001, 382, 183. (c) Sugimura, H.; Hozumi, A.; Kameyama, T.; Takai, O. Adv. Mater. 2001, 13, 667. (d) Gu, Zhong; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2002, 41, 2068. (9) (a) Stenger, D. A.; Georger, J. H.; Dulcey, C. S.; Hickman, J. J.; Rudolph, A. S.; Nielsen, T. B.; McCort, S. M.; Calvert, J. M. J. Am. Chem. Soc. 1992, 114, 8435. (b) Ravenscroft, M. S.; Bateman, K. E.; Shaffer, K. M.; Schessler, H. M.; Jung, D. R.; Schneider, T. W.; Montgomery, C. B.; Custer, T. L.; Schaffner, A. E.; Liu, Q. Y.; Li, Y. X.; Barker, J. L.; Hickman, J. J. J. Am. Chem. Soc. 1998, 120, 12169. (c) Matsuzawa, M.; Tokumitsu, S.; Knoll, W.; Sasabe, H. Langmuir 1998, 14, 5133. (10) Geyer, W.; Stadler, V.; Eck, W.; Golzhauser, A.; Grunze, M.; Sauer, M.; Weimann, T.; Hinze, P. J. Vac. Sci. Technol., B 2001, 19, 2732. (11) (a) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (b) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 570. (c) Whitesides, G. M.; Ostuni, E.; Takayama, S.; Jiang, X.; Ingber, D. E. Annu. Rev. Biomed. Eng. 2001, 3, 335. (d) Crooks, R. M. ChemPhysChem 2001, 2, 644. (12) Liu, G.-Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457. (13) (a) Sugimura, H.; Nakagiri, N. Appl. Phys. 1998, A66, S427. (b) Sugimura, H.; Ushiyama, K.; Hozumi, A.; Takai, O.; Langmuir 2000, 16, 885. (14) (a) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342. (b) Tarlov, M. J.; Burgess, D. R. F., Jr.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (c) Zhang, Y.; Terrill, R. H.; Tanzer, T. A.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 2654. (15) Willard, N. P.; Camps, I. G. J. Eur. Pat. 0587231, 1994. (16) (a) Tada, H.; Nagayama, H. Langmuir, 1994, 10, 1472. (b) Shimoda, T.; Miyashita, S.; Takai, O. Sugimura, H. Jpn. Patent 20000282240, 2000. (17) Oh, S.-K.; Nakagawa, M.; Ichimura, K. Chem. Lett. 1999, 349. (18) (a) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288, 1624. (b) Oh, S.-K.; Nakagawa, M.; Ichimura, K. J. Mater. Chem. 2002, 12, 2262. (19) Oh, S.-K.; Nakagawa, M.; Ichimura, K. J. Mater. Chem. 2001, 11, 1563.
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on the SAM surface by photoirradiation. In addition, we found that a cationic macrocycle possessing four pyridinium groups is firmly adsorbed by a silica surface so as to bring about no desorption toward deionized water.20 It is evident from these results that adsorbate desorption from a substrate surface is significantly suppressed by plural noncovalent bonds through multipoint adsorption, which markedly enhances the adsorption stability. Taking the interfacial phenomena into consideration, we have developed a novel method for negative-type photopatterning of an adsorbed monolayer formed from a tetracation possessing two [2+2] photodimerizable 1,2-bis(4benzylpyridinio)ethylene moieties.21 When the positively charged tetracationic molecule adsorbed by a negatively charged silica surface increases the number of electrostatic adsorption sites per molecule as the result of increasing the molecular weight by the photodimerization process, the adsorption stability of the UV-exposed cationic molecule is markedly enhanced toward aqueous solutions containing an electrolyte such as NaCl. Consequently, the photopatterned adsorbed monolayer exhibiting surfacecharge heterogeneity on a silica surface is successfully obtained by imagewise UV exposure, followed by siteselective desorption of the unexposed tetracationic adsorbate by multiple rinses with a NaCl-containing aqueous solution. The negative-type photopatterning method has two distinctive advantages of (1) the availability of a widely used mercury lamp and (2) the use of environmentally benign water media during the adsorbed monolayer fabrication and photopatterning. However, our recent research has revealed that negatively charged polystyrene spheres are hardly adsorbed in accordance with a latent image of the photopatterned monolayer. It was conceivable that this situation arises from a flat-laid adsorption manner of the rigid rodlike tetracationic molecule on the silica surface so as to compensate negative charges of dissociated surface silanol groups because the silica plate adsorbing the tetracationic molecule exhibited almost neutral zeta-potential values.22 To realize site-selective surface adsorption of such colloidal particles, it is desirable that a positively charged monolayer adsorbed electrostatically on a negatively charged surface is capable of inducing a polar transition at specially chosen areas by photochemical means. In this paper, we describe the photoinduced polar transition of silica and poly(ethylene terephthalate) (PET) surfaces achieved by a photodegradable multivalent cationic adsorbate as shown in Figure 1a. The photodegradable adsorbate was designed to carry out positivetype photopatterning of the adsorbed monolayer on a negatively charged substrate surface on account of the following. First, the cationic adsorbate contains a cyclopentasilane ring as a photodegradable unit and quaternary pyridinium groups as cationic adsorption sites. It is anticipated that the photodegradable cylopentasilane unit is decomposed to smaller photoproducts by photocleavage of Si-Si bonds brought about by UV exposure at 254 nm.23 Second, the cationic molecule can be derived readily through chloromethylation and quaternization from decaphenylcyclopentasilane, which is cheaply obtainable as an industrial subproduct during preparation of poly(diphenylsilane). Third, the roles of the cationic pyridinum units in the adsorbed monolayer are anticipated to (20) Nakagawa, M.; Ichimura, K. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 2000, 345, 275. (21) Nakagawa, M.; Oh, S.-K.; Ichimura, K. Adv. Mater. 2000, 12, 403. (22) Nawa, N.; Nakagawa, M.; Seki, T.; Iyoda, T. Trans. Mater. Res. Soc. Jpn., in press. (23) Miller, R. D.; Michl, L. Chem. Rev. 1989, 89, 1359.
Photoinduced Polar Transition of Substrate Surfaces
Figure 1. Chemical structure of a multivalent cationic molecule with pyridinium groups used in this study (a) and positive-type photopatterning of a photodegradable cationic adsorbed monolayer on a substrate surface (b).
separate into adsorption sites tethered to a negatively charged surface and adsorption-free sites to promote surface adsorption of negatively charged polystyrene spheres. Here, we report on the adsorbed monolayer fabrication, photochemical behaviors, positive-type photopatterning procedures outlined in Figure 1b, and adsorption control of polystyrene particles charged with a negative or positive sign on the photopatterned monolayered surface. Experimental Section Materials. Decaphenylcyclopentasilane is a generous gift from Osaka Gas Co., Ltd. Deionized water of 18.0 MΩ‚cm after purification with a Millipore Milli-Q system was used. Fluorescent polystyrene particles were purchased from Molecular Probes, Inc. Carboxyl-modified Fluorosphere (F-8811) and amino-modified Fluorosphere (F-8761) 200 nm in diameter were used. Other reagents were of reagent grade and used as received. Preparation of a Photodegradable Multivalent Cationic Adsorbate. A photodegradable multivalent cationic adsorbate having quaternary pyridinium groups was synthesized from decaphenylcyclopentasilane through chloromethylation and quaternization. The chloromethylation was carried out in a manner similar to that of polystyrene24 and poly(methyl phenethyl silane)25 reported by Ban and Sukegawa and Seki et al. To a solution of decaphenylcyclopentasilane (10.0 g, 11.0 mmol) and chloromethyl methyl ether (128 g, 1.59 mol) in dehydrated chloroform (100 mL) cooled with an ice bath was added SnCl4 (14.3 g, 54.9 mmol) slowly under an argon atmosphere. The reaction mixture was stirred at 5 °C for 17 h. The mixture was washed with an aqueous NaHCO3 solution and deionized water and dried over anhydrous CaCl2. After concentration, the residue was subjected to column chromatography on silica gel using ethyl acetate and hexane (1:3 v/v) as the eluent. Chloromethylated decaphenylcyclopentasilane was obtained as a white powder of 11.8 g in 79.4% yield [1H NMR (200 MHz, DMSO-d6) δ: 4.2-4.8 (18H, broad, CH2Cl), 6.6-7.6 (42H, broad, aromatic H)]. The elemental analysis of the Cl atom (Cl, 23.51%) suggested that the chloromethylated decaphenylcyclopentasilane had approximately nine chloromethyl groups. Subsequently, the chloromethylated decaphenylcyclopentasilane (5.00 g) was reacted with excess amounts of dehydrated pyridine (120 g) in the presence of dehydrated THF (50 mL) at 30 °C for 7 days. The reaction mixture was poured into diethyl ether, and the resulting precipitate was filtered, washed with diethyl ether, and dried to give 7.32 g of cationic cyclopentasilane derivative as a deliques(24) Ban, H.; Sukegawa, K. Macromolecules 1987, 20, 1775. (25) Seki, T.; Tanigaki, N.; Yase, K.; Kaito, A.; Tamaki, T.; Ueno, K.; Tanaka, Y. Macromolecules 1995, 28, 16.
Langmuir, Vol. 19, No. 21, 2003 8771 cent white powder. The structural analysis with 1H NMR and Fourier transform infrared (FT-IR) spectroscopy was carried out after the anion exchange from hydrophilic chloride to hydrophobic hexafluorophosphate.26 1H NMR (200 MHz, acetone-d6) δ: 5.86.1 (18H, broad, CH2N), 6.8-9.2 (81H, broad, aromatic H). It was evident from the 1H NMR and FT-IR measurements that the quaternization reaction with pyridine fully proceeded and the multivalent cationic adsorbate had nine quaternary pyridinium groups on average. Preparation of a Cationic Adsorbed Monolayer on a Substrate Surface. A fused silica plate (10 × 30 × 1 mm) or a PET film (10 × 30 × 0.2 mm) was used as a substrate for adsorbed monolayer formation. The substrate was cleaned by exposure to UV ozone for 2 h using a Nippon Laser & Electronics ozone cleaner. The cleaned substrate was immediately immersed in an aqueous solution containing 3 wt % adsorbate at 50 °C for 1 h, subsequently rinsed with deionized water thoroughly, and dried by N2 gas to give a substrate adsorbing the photodegradable cationic adsorbate. Photoirradiation. A low-pressure Hg lamp was used as an UV light source. The silica or PET substrate adsorbing the photodegradable organic cation was exposed to UV light for 1-120 min. The UV-exposed substrate was rinsed with deionized water thoroughly and dried by N2 gas. Contact-angle, zeta-potential, and UV-visible spectral measurements of thus obtained substrates were carried out. Photopatterning of an adsorbed monolayer film of the organic cation was performed using a copper transmission electron microscopy (TEM) grid as a photomask. The grid with pinholes 100 µm in diameter was placed on the top of the substrate. The substrate was exposed through the photomask for 20 min, rinsed, and dried in the same manner. Surface Adsorption of Fluorescent Polystyrene Spheres on Photopatterned Monolayers. Aqueous solutions showing different pH values were prepared by adding a dilute solution containing either HCl or NaOH to deionized water. Two kinds of fluorescent carboxy- and amino-modified polystyrene spheres were diluted with the aqueous solutions, until aqueous dispersions of 0.4 wt % particle concentration were obtained. The pH values of the diluted particle dispersions were monitored just before use. The substrate after the photopatterning of the cationic adsorbed monolayer was immersed for 90 s in the particle dispersions, followed by multiple rinses with deionized water and drying by N2 gas. The substrate-adsorbing particles were observed with a fluorescence microscope. Physical Measurements. UV-visible absorption spectra of a monolayered film formed on a silica plate were taken on a JASCO MAC-1 weak absorption spectrophotometer. Sessile contact angles were measured using a Kyowa Kaimen Kagaku CA-X contact-angle meter. Fluorescent microscope observation was carried out using an Olympus BX60 optical microscope in the fluorescent mode in which a Flovel HCC600 color chargecoupled device camera was equipped. The pH values of the aqueous solutions and particle dispersions were measured with a Horiba B-212 pH meter. Zeta-Potential Measurements. The zeta potentials of the fluorescent polystyrene particles and the silica and PET substrates before and after the surface adsorption of the photodegradable cationic adsorbate were measured at 25 ( 1 °C with an Otsuka Electronics ELS-8000 electrophoretic light scattering spectrophotometer. HPC-coated (hydroxyl-propyl-cellulosecoated) polystyrene particles 520 nm in diameter were purchased from Otsuka Electronics and used as standard probing particles for the zeta-potential determination of substrates. The commercially available HPC-coated particles dispersed in water were diluted 500 times by adding a mixture of a 10 mmol dm-3 NaCl aqueous solution with either a 10 mmol dm-3 HCl aqueous solution or a 10 mmol dm-3 NaOH aqueous solution. The pH values of the diluted dispersions, keeping the ionic strength constant, were measured before use.
Results and Discussion Adsorbed Monolayer Formation of a Photodegradable Multivalent Cationic Adsorbate on a Silica (26) Nakagawa, M.; Rikukawa, M.; Watanabe, M.; Sanui, K.; Ogata, N. Bull. Chem. Soc. Jpn. 1997, 70, 737.
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Plate. When a negatively charged substrate such as a cleaned silica, metal oxide, or polymer plate is immersed in aqueous solutions containing low-molecular-weight organic molecules possessing a single positive charge, such as surfactants composed of a hydrophilic polar head and a hydrophobic alkyl tail, an adsorbed monolayer is readily formed from the organic molecules at an interface between the solution and the solid substrate.27 The adsorbed monolayer on the substrate surface is not stable enough to maintain the monolayer morphology upon rinsing with adsorbate-free deionized water, and the monolayerforming adsorbates are almost desorbed from the substrate surface. On the other hand, electrostatically adsorbed monolayers formed from multivalent cationic molecules,20,21 macrocyclic amphiphiles,17-19,28 and polyelectrolytes29 possessing plural adsorption sites exhibit high desorption resistance toward deionized water as a result of a stabilized multipoint adsorption manner on an oppositely charged substrate surface. These interfacial phenomena led us to study a photodegradable multivalent cationic molecule assembled on a negatively charged silica surface, under the assumption that photochemical fragmentation of the multivalent cationic molecule can lead to a decrease in the number of cationic adsorption sites per molecule, to result in desorption of the photoirradiated molecular fragments from the substrate surface. To demonstrate the photochemical events, we designed and synthesized a water-soluble multivalent cationic adsorbate consisting of a cyclopentasilane unit as a photodegradable site and quaternary pyridinium units as plural adsorption sites. The adsorbed monolayer formation of the photodegradable multivalent cationic adsorbate was carried out simply by immersing a cleaned silica plate in adsorbate-containing aqueous solutions with various concentrations in the range of 0.01-5.0 wt %, followed by multiple rinses with deionized water and drying. Adsorption of the cationic adsorbate on a silica plate was unequivocally confirmed by UV-visible absorption spectra showing a maximum adsorption band at 260 nm arising from pyridinium and cyclopentasilane moieties. Using the absorbance observed for the adsorbed monolayer at 260 nm, we plotted an adsorption isotherm of the cationic adsorbate toward a silica plate as a function of the adsorbate concentrations. As a result, the isotherm profile was a typical Langmuirtype, exhibiting that saturated adsorption of the cationic adsorbate took place at concentrations over 3 wt %. The immersion time and the temperature of the aqueous solutions were almost independent of the isotherm profiles. In addition, it was found from the UV-visible spectral measurements that no desorption of the photodegradable cationic adsorbate from a silica plate took place even after multiple rinses with deionized water. Assuming that the absorption coefficient of the cationic adsorbate in water is consistent with that on a silica plate, we estimated an occupied molecular area of the photodegradable cationic adsorbate on a silica plate in the case of saturated adsorption by the UV-visible spectral measurements. It was estimated from the molecular absorption coefficient at 260 nm in water, 260nm ) 6.1 × (27) Fujii, M.; Li, B.; Fukuda, K.; Kato, T.; Seimiya, T. Langmuir 1999, 15, 3689. (28) (a) Ichimura, K.; Fukushima, N.; Fujimaki, M.; Kawahara, S.; Matsuzawa, Y.; Hayashi, Y.; Kudo, K. Langmuir 1997, 13, 6780. (b) Fujimaki, M.; Kawahara, S.; Matsuzawa, Y.; Kurita, E.; Hayashi, Y.; Ichimura, K. Langmuir 1998, 14, 4495. (c) Ichimura, K.; Oh, S.-K.; Fujimaki, M.; Matsuzawa, Y.; Nakagawa, M. J. Inclusion Phenom. Macrocyclic Chem. 1999, 35, 173. (29) (a) Decher, G.; Hong, J.-H.; Macromol. Chem. Macromol. Symp. 1991, 46, 321. (b) Decher, G. Science 1997, 277, 1232. (c) Schwarz, S.; Eichhorn, K.-J.; Wischerhoff, E.; Laschewsky, A. Colloids Surf., A 1999, 159, 491.
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Figure 2. UV-visible adsorption spectral changes of the photodegradable cationic adsorbed monolayer on a silica plate upon irradiation with UV light from a low-pressure Hg lamp.
104 dm3 mol-1 cm-1, that the occupied molecular area was 3.0 ( 0.2 nm2, which was not far from the molecular area of 3.8 ( 0.2 nm2 estimated by the Corey-Pauling-Kolton model. These results clearly supported that the adsorbed monolayer exhibiting high desorption resistance toward deionized water was formed from the photodegradable multivalent cationic adsorbate on the silica surface by the simple immersion. From these results, we determined that the experimental condition to form the adsorbed monolayer on a silica plate is to immerse a cleaned silica plate in an aqueous solution of 3 wt % adsorbate at 50 °C for 1 h. Photochemistry of the Cationic Adsorbed Monolayer. Figure 2 shows the UV-visible spectral changes of the multivalent cationic adsorbate on a silica plate upon irradiation with UV light. When the monolayered film was exposed to UV light, broad absorption bands below about 300 nm attributable to a photoactive cyclopentasilane moiety and phenyl and pyridinium moieties were photobleached more with an increase in the UV-exposure time. A further UV-visible spectral change was not observed after the UV irradiation over 20 min. These spectral changes clearly indicated that the photoactive cyclopentasilane moiety was decomposed to disilane derivatives that could not absorb the actinic light of 254 nm any longer, as reported in the photochemistry of organic oligosilane derivatives.23 A noticeable feature is that the absorption bands attributable to the pyridinium and phenyl moieties also disappeared upon exposure to UV light. The photochemical events imply that cleavage of a Si-C covalent bond between a cyclopentasilane ring and phenyl groups also proceeded after UV exposure in a manner like that observed for the photochemistry of SAMs formed from phenyltrichlorosilane reported by Calvert et al.30 Further physical characterization of the adsorbed cationic monolayer before and after the UV irradiation was investigated by contact-angle and zeta-potential measurements. The sessile contact-angle measurements were carried out using deionized water as the probing liquid. A hydrophilic cleaned silica plate showing a contact angle of
