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Creation of Novel Double-Helical Silica Nanotubes Using Binary Gel System Jong Hwa Jung,† Kaname Yoshida,‡ and Toshimi Shimizu*,†,‡ CREST, Japan Science and Technology Corporation (JST), Nanoarchitectonics Research Center (NARC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan, and Nanoarchitectonics Research Center (NARC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received June 28, 2002. In Final Form: September 11, 2002 Sugar-based gelator p-dodecanoyl-aminophenyl-β-D-glucopyranoside 1 has been shown to self-assemble in the presence of p-aminophenyl glucopyranoside 2 into double-helical fibers with diameters of 3-25 nm and lengths of several micrometers. The sol-gel polymerization of tetraethoxysilane (TEOS) was carried out using a binary gel system. The double-helical structure of the organic assemblies could be accurately transcribed into the silica structure. This result indicates that the amino group of the p-aminophenyl glucopyranoside acts as the efficient driving force to create novel double-helical silica nanotubes.
Introduction The direct synthesis of discrete inorganic architectures generally necessitates the use of dispersed organic supramolecular structures with commensurate size and dimensionality. Inorganic materials with well-defined three-dimensional morphologies have been synthesized using organic templates such as surfactants,1-8 polymers,9,10 organic crystals,11 and gelators,12-14 and they have potential applications as, for example, catalysis, chromatography, adsorbent, or controlled release materials. Most of the inorganic materials have been obtained as mesoporous-, vesicular-, or linear tube-type nanostructures.1-11 On the other hand, relatively little work on helical inorganic materials has been reported despite their utilization as asymmetric reaction catalysts,12,13 helical sensors,15,16 inorganic optical materials,17 and so forth. Although N-alkylaldonamide amphiphiles,18,19 and syn* Fax no.: +81-298-61-2659. E-mail:
[email protected]. † CREST, Japan Science and Technology Corporation. ‡ Nanoarchitectonics Research Center (NARC), National Institute of Advanced Industrial Science and Technology. (1) Kim, S. S.; Zhang, W.; Pinnavaia, T. J. Science 1998, 282, 1302. (2) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692. (3) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867. (4) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (5) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317. (6) Stupp, S. I.; Braun, P. V. Science 1997, 277, 1242. (7) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267. (8) Tanev, P. T.; Liang, Y.; Pinnavaia, T. P. J. Am. Chem. Soc. 1997, 119, 8616. (9) Ichinose, I.; Kunitake, T. Adv. Mater. 2002, 14, 344. (10) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (11) Miyai, F.; Davis, S. A.; Charmant, J. P. H.; Mann, S. Chem. Mater. 1999, 11, 3021. (12) Jung, J. H.; Ono, Y.; Hanabusa, K.; Shinkai, S. J. Am. Chem. Soc. 2000, 122, 5008. (13) Jung, J. H.; Kobayashi, H.; Masuda, M.; Shimizu, T.; Shinkai, S. J. Am. Chem. Soc. 2001, 123, 8785. (14) Kobayashi, S.; Hanabusa, K.; Hamasaki, N.; Kimura, M.; Shirai, H.; Shinkai, S. Chem. Mater. 2000, 12, 1523. (15) Bodenho¨fer, K.; Hierlemann, A.; Seemann, J.; Gauglitz, G.; Koppenhoefer, B.; Go¨pel, W. Nature 1997, 387, 577. (16) Cornell, B. A.; Braach-Maksvytis, V. L. B.; King, L. G.; Osman, P. D. J.; Raguse, B.; Wieczorek, L.; Pace, R. J. Nature 1997, 387, 580. (17) Hodgkinson, I.; Wu, Q. h. Adv. Mater. 2001, 13, 889. (18) Fuhrhop, J.-H.; Boettcher, C. J. Am. Chem. Soc. 1990, 112, 1768.
thetic nucleotide analogues19,20 in artificial systems as well as DNA, RNA, and polypeptides in nature, self-assemble into fascinating highly ordered superstructures such as single-, double-, and triple-helical structures, their transcription into inorganic materials has not yet been achieved.21 The reasons for this are 2-fold: (i) either the presence of cationic charges or efficient hydrogen-bond donating moieties are required in order to adsorb “anionic” inorganic precursor moieties onto the organic molecular assemblies, and (ii) the self-assembled morphology of organic molecules should be stably maintained during the sol-gel reaction. Obtaining the helical structures that satisfy the above requirements, however, can be accomplished using stable, artificial, organic, supramolecular self-assembled structures. We have prepared sugarbased gelator 1 as a template, which, together with aminophenyl glucopyranoside 2, can form a gel consisting of double-helical fibers that can be transcribed into doublehelical silica nanotubes. Here, we report on the preparation of the self-assembled organic double-helical superstructure and its sol-gel transcription to prepare the double-helical silica nanotube. Results and Discussion Compound 1 was synthesized according to the method reported previously.21 The gelator 1 was able to gelate 8 out of 10 of the organic solvents tested, such as chloroform, tetrahydrofuran, 1-butanol, 1-hexanol, and ethyl acetate. Of particular interest is its ability to gelate water in the presence of methanol or ethanol (∼1-50 wt %) at concentrations less than 0.1 wt %, corresponding to more than 50 000 molecules of water per gelator molecule.22 These results prove that 1 can act as an amphiphilic gelator both in water and in organic solvents. However, we have found that the presence of at least a moderate (19) Feiters, M. C.; Nolte, R. J. M. Advances in Supramolecular Chemistry: Chiral Self-Assembled Structures from Biomolecules and Synthetic Ananlogues; Gokel, G. W., Ed.; JAI Press Inc.: Stamford, CT, 2000. (20) Shimizu, T.; Iwaura, R.; Masuda, M.; Hanada, T.; Yase, K. J. Am. Chem. Soc. 2001, 123, 5947. (21) Sugiyasu, K.; Tamaru, S.; Takeuchi, M.; Bertheir, D.; Huc, I.; Oda, R.; Shinkai, S. Chem. Commun. 2002, 1212. However, there was no tubular structure of the double-helical silica obtained from gemini surfactants in this literature. (22) Jung, J. H.; John, G.; Masuda, M.; Yoshida, K.; Shinkai, S.; Shimizu, T. Langmuir 2001, 17, 7229.
10.1021/la020594e CCC: $22.00 © 2002 American Chemical Society Published on Web 10/16/2002
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Figure 1. (A and B) FE-SEM images of the xerogel prepared from the mixed gel of 1 and 2 (1/2 ) 1:1 w/w) in water-methanol (10:1 v/v). (C) A possible self-assembling model in the bilayered chiral fiber from the mixed gel of 1 and 2: the powder XRD experiment indicated that hydrogel 1 + 2 maintained an interdigitated bilayer structure with the alky chain titled. Arrows indicate the double-helical structures (parts A and B).
amount of cationic charges or efficient hydrogen-binding sites in the gel architecture is indispensable for sol-gel transcription into inorganic materials.12,13 Therefore, we performed the sol-gel transcription in the presence of aminophenyl glucopyanoside 2, the amine moiety of which can function as a binding site for hydrolyzed tetraethoxysilane (TEOS). Furthermore, this adduct should induce the formation of helical superstructure as a result of strong rigid and chiral packing of the sugar moieties through intermolecular hydrogen bonding interaction. After sample preparation by the freezing-pumping method,23 we observed the morphologies of the hydrogels obtained from 1 as well as from the mixture of 1 and 2 by scanning electron microscopy (SEM). In Figure 1A and B, the SEM pictures of the 1 + 2 mixed gel (1:1 w/w) show well-defined double-helical fiber structure with diameters of 3-25 nm (arrows in Figure 1A and B), lengths of several micrometers, and dimensions reminiscent of those of double-helical DNA and RNA structures. The gel of 1, on the other hand, only displayed a somewhat twisted fiber structure with diameters of 20-100 nm.22 These results suggest that the self-assembled superstructure in the gel of 1 + 2 was orientated into a more explicit chiral packing through the intermolecular hydrogen-bonding interactions of the sugar moieties as well as the π-π stacking of the phenyl moieties present in both 1 and 2. The mixed gels were also studied by differential scanning calorimetry (DSC). The Tsol-gel (sol-gel transition temperature) of gel (1.0 wt %) in a 1:1 mixture was 95.2 °C with a single peak. According to NMR experiment, 20% of p-aminophenylglucopyranose (2) in a 1:1 mixture was incorporated into (23) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, R. J. M.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 664.
Figure 2. CD spectra of hydrogels [0.1 wt %, water-methanol ) 1:1 (v/v)] of the 1 + 2 mixture at 25 °C: (A) 1/2 ) 1:0 (w/w); (B) 1/2 ) 1:1.5 (w/w); (C) 1/2 ) 1:4 (w/w).
gel fiber. These results indicate that aminophenylglucopyranoside (2) was truly incorporated into the gel phase of 1 (Figure 1C). To further study the chiral packing structure, we measured circular dichroism (CD) spectra of the gel 1, which gave a CD maximum at 230 nm. Addition of 2 to the gel of 1 resulted in a gradual increase in the CD intensity (Figure 2), proving that the microscopic chiral change upon going from the pure gel of 1 to the mixed gel can be reflected in the resultant macroscopic chiral structure. To transcribe the double-helical superstructures of the hydrogels into their analogous silica structures, we carried out the sol-gel polymerization of TEOS in a watermethanol (10:1 v/v) gel of 1 + 2.24 The structure of the silica nanotube obtained by sol-gel trnascription was observed by SEM and transmission electron microscopy (TEM). After calcination, the morphology of the silica (24) The experimental procedure is as follows: 1 + 2 (3.0-12.0 mg; 1/2 ) 1.0:0.1-4.0 w/w) was dissolved in water-methanol (10:1 v/v, 1000 mg), TEOS (20.0-50.0 mg) was added, and finally benzylamine (6.0-10.0 mg) was added as a catalyst for the sol-gel polymerization. The sample was left without stirring at ambient temperature for 7 days. Subsequently, the sample was heated at 200 °C for 2 h followed by 2 h at 500 °C, both under a nitrogen atmosphere, and it was then kept at 500 °C under aerobic conditions for 4 h.
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Figure 3. (A and B) FE-SEM and (C and D) TEM images of the double-helical silica nanotube obtained from the mixed gel of 1 and 2 (1:1 w/w) after calcination, and (E) schematic representation of the double-helical structure of the silica nanotubes through SEM and TEM observations. a and b indicate two silica nanotubes from which the double helixes are constructed (parts B and D).
product turned out to be the well defined double-helical nanotube possessing a diameter of 50-80 nm and a pitch of 50-60 nm (Figure 3A and B). As far as can be determined, all chiral structures possess a right-handed helical motif. Since the exciton-coupling band present in the CD spectrum of the 1 + 2 mixed gel also showed the right-handed helicity, this confirms that the microscopic helicity was reflected in the macroscopic helicity. All these results prove that the double-helical structure of the selfassembled gel fibers of 1 and 2 was successfully transcribed into the silica nanotube structure, most likely through hydrogen-bonding interactions between the amine moiety of 2 and negatively charged oligomeric silica particles. Additional investigation showed that with transcription of mixtures of 1 and 2 with weight ratios R [)2/(1 + 2)] between 0.2 and 0.9, at all times the right-handed doublehelical silica was obtained. For R-values below 0.2, the resultant silica only displayed the conventional granular structure. The results clearly support the view that (i) the presence of a minimal number of the hydrogen-binding sites is indispensable for transcription of the self-assembled structure into the silica structure, (ii) the helicity of silica can be accurately transcribed from that of the template, and (iii) this method will be applicable for the efficient transcription of self-assembled superstructures into inorganic materials. TEM pictures were taken after calcination of the transcribed hydrogels (Figure 3C and D) in order to further
confirm whether the self-assembled superstructure really acted as a template for the formation of the silica structure. The silica structure obtained from the mixed gel of 1 and 2 in water-methanol shows an inner hollow structure originating from the helical gel fibers. High magnification TEM images of the double-helical silica tubes display approximately a 5.0-nm nanospace between the helical strands (Figure 3D).25 Furthermore, there is almost no change between the silica before and after calcination, indicating that the double-helical structure of the silica indeed originates from the self-assembled organogel superstructure. Since electron energy-loss spectroscopy (ELLS) is capable of providing elemental and chemical information at high spatial resolution, some energy filters equipped on the TEM have been developed. Therefore, the functioning of the gel fibers as a template for sol-gel transcription was confirmed independently by elemental mapping with electron energy-loss spectroscopy,26 as shown in Figure 4. The material (Figure 4A) contains silicon (Figure 4B) and carbon (Figure 4C) components (25) Actually, most of the strand space is about 5 nm. A small amount (less than 3%) of the silica possessed around 20 nm between strands. To best help the reader understand, we have illustrated Figure 3D. (26) The EELS spectra were done with Zeiss LEO 912 OMEGA, using an accelerating voltage of 120 kV on the micro carbon grids. The elemental maps were obtained by subtracting the background intensities under core-loss edges using the three-window technique.
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Figure 4. TEM images with electron energy-loss spectroscopy (EELS) of the double-helical silica nanotube obtained from the mixed gel of 1 and 2 (1:1 w/w). (A) Zero-loss image, (B) silicon component, and (C) carbon component before calcination. (D) Zero-loss image, (E) silicon component, and (F) carbon component after calcination.
Figure 5. (A) Nitrogen adsorption-desorption isotherms and (B) DEF PSD curve for a calcined double-helical silica nanotube obtained from a hydrogel 1 + 2 mixture (1/2 w/w).
before calcination, whereas the targeting material (Figure 4D) exclusively showed a silicon component (Figure 4E) after calcination. This result implies again that TEOS was adsorbed on the surface of organic double-helical fibers as a result of hydrogen-bonding interactions. Furthermore,
all results obtained clearly support the view that the double-helical silica nanotube possesses a nanometer-sized cavity. To determine the size profile of the inner cavity of the silica nanotubes, we investigated the DFT pore size distribution (PSD) and surface area of the double-helical silica using the N2 adsorption-desorption isotherm.1,8 According to N2 adsorption-desorption experiments, the silica was found to possess 450-500 m2/g of surface area and pores with two distinct diameters (Figure 5). Judging from TEM and DFT observations, the first is 1.2-1.5 nm in diameter, which is similar to the diameters of singlewall carbon nanotubes, and most likely originates from the template gel fibers. Probably this inner diameter of silica with some shrinkage occurred during calcination, due to the inner diameter being slightly smaller than that of a template fiber. The second type of pore has an average size of 5.0 nm and originates from the space between the two separate helical silica strands, consistent with TEM observation. In conclusion, the present paper has demonstrated that a novel double-helical structure of the silica nanotube was produced using a binary sugar-based gel system. The amino group of the added p-aminophenylglucopyranoside in the gel fiber acted as the efficient driving force to produce the novel double-helical structure of the silica. We suggest that the sol-gel transcription of hierarchical self-assembled superstructures is generally applicable for the preparation of novel chiral inorganic materials. Not only can helical inorganic nanotubes have applications as asymmetric catalysts, but they also can be useful in tissue engineering, in biomaterial fabrication, in fabrication of molecular electronic devices, in fabrication of helical sensors, and as inorganic chiral optical materials for inorganic biaxial films, chiral reflectors, and filters. LA020594E
