Langmuir 2008, 24, 5229-5232
5229
Stimuli-Responsive Supramolecular Nanostructure from Amphiphilic Calix[4]arene and Its Three-Dimensional Dendritic Silver Nanostructure Eun Jin Cho,† Jeong Ku Kang,† Won Seok Han,‡ and Jong Hwa Jung*,‡ Department of Materials Science and Engineering, KAIST, Daejeon 305-701, South Korea, and Department of Chemistry and Research Institute of Natural Science, Gyeongsang National UniVersity, Jinju 660-701, South Korea ReceiVed January 22, 2008. ReVised Manuscript ReceiVed March 7, 2008 We synthesized a tetrameric amphiphilic molecule (1) based on a calix[4]arene building block that self-assembled into different tunable and stable aggregation structures at different pH values in aqueous solution. The amphiphilic calix[4]arene molecule (1) formed a spherical structure at pH 3. However, 1 formed a hollow necklacelike structure of 500 nm diameter at pH 7. These results indicate that the self-assembled morphologies of 1 are strongly dependent on pH values. In addition, a 3D dendritic silver nanostructure was obtained by the self-assembly of 1 at pH 7.
I. Introduction In the fields of chemical biology and materials science, the controlled self-assembly of specific amphiphilic molecules is an important process that provides the spontaneous generation of a well-defined, discrete aggregate structure from molecular components under thermodynamic equilibrium.1 Amphiphilic molecules consisting of hydrophilic and lipophilic block segments have been proven to be promising scaffolds for self-assembled structures on the nanometer scale.2–4 Mechanisms that control the morphology of the self-assembled nanostructures are important aspects of this research, especially as it applies to morphological control in response to the introduction of hydrophilic and lipophilic segments or to external stimuli such as pH and temperature.5–7 To generate precisely controlled and well-defined nanostructures through the use of external stimuli such as pH, however, the elaborate design of corresponding building blocks * To whom correspondence should be addressed. E-mail: jonghwa@ gnu.ac.kr. Tel: +82-55-751-6027. Fax: +82-55-758-6027. † KAIST. ‡ Gyeongsang National University. (1) (a) Elemans, J. A. A. W.; Rowan, A. E.; Nolte, R. J. M. J. Mater. Chem. 2003, 13, 2661–2670. (b) Antonietti, M.; Fo¨rster, S. AdV. Mater. 2003, 15, 1323– 1333. (c) Lee, M.; Cho, B.-K.; Zin, W.-C. Chem. ReV. 2001, 101, 3869–3892. (2) (a) Vriezema, D. M.; Hoogboom, J. H.; Velonia, K.; Takazawa, K.; Christianen, P. C. M.; Mann, J. C.; Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 2003, 42, 772–776. (b) Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944–1947. (3) (a) Jeon, Y. J.; Bharadwaj, P. K.; Choi, S. W.; Lee, J. W.; Kim, K. Angew. Chem., Int. Ed. 2002, 41, 4474–4476. (b) Larpent, C.; Laplace, A.; Zemb, T. Angew. Chem., Int. Ed. 2004, 43, 3163–3167. (4) (a) Tanaka, Y.; Miyachi, M.; Kobuke, Y. Angew. Chem., Int. Ed. 1999, 38, 504–506. (b) Kellermann, M.; Bauer, W.; Hirsch, A.; Schade, B.; Ludwig, K.; Bo¨ttcher, C. Angew. Chem., Int. Ed. 2004, 43, 2959–2962. (c) Hayashida, O.; Mizuki, K.; Akagi, K.; Matsuo, A.; Kanamori, T.; Nakai, T.; Sando, S.; Aoyama, Y. J. Am. Chem. Soc. 2003, 125, 594–601. (5) (a) Guo, X.; Szoka, F. C. Acc. Chem. Res. 2003, 36, 335–341. (b) Sumida, Y.; Masuyama, A.; Takasu, M.; Kida, T.; Nakatsuji, Y.; Ikeda, I.; Nojima, M. Langmuir 2001, 17, 609–612. (6) (a) Johnsson, M.; Wagenaar, A.; Engberts, J. B. F. N. J. Am. Chem. Soc. 2003, 125, 757–760. (b) Zhu, J.; Munn, R. J.; Nantz, M. H. J. Am. Chem. Soc. 2000, 122, 2645–2646. (c) Drummond, D. C.; Zignani, M.; Leroux, J.-C. Prog. Lipid Res. 2000, 39, 409–460. (7) (a) Checot, F.; Lecommandoux, S.; Gnanou, Y.; Klok, H.-A. Angew. Chem., Int. Ed. 2002, 41, 1339–1343. (b) Lee, M.; Lee, S.-J. J. Am. Chem. Soc. 2004, 126, 12724–12725. (8) Gutsche, C. D. Synthesis of Calixarene and Thiacalixarenes. In Calixarenes 2001; Asfai, Z., Bo¨hmer, V., Harrowfield, J., Vicens, J., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2001.
is required because the information determining their specific assembly should be encoded in their molecular architecture. Of the many functional building blocks, the calix[4]arene moiety is particularly useful as a rigid segment to form a well-defined nanometer-sized structure.9,10 Lee’s group reported the stimuliresponsive supramolecular nanocapsules from an amphiphilic calixarene assembly.7b Martin and co-workers discovered a similar phenomenon in the pH-dependent self-assembly of semifluorinated calix[4]arenes.11 Although many studies of calix[4]arenes have been reported, the pH-dependent morphology tuning of calix[4]arenes remains relatively unexplored. Accordingly, we have synthesized an amphiphilic tetramer based on a calix[4]arene building block consisting of four lipophilic decyl chains at the lower rim and four alanine hydrophilic chains at the upper rim that can endow aggregates with enhanced stability and pHresponsive character. Specifically, the alanine moieties were introduced to allow for an efficient intermolecular hydrogenbonding interaction.
We report here the self-assembled morphology control of 1 at pH 3, 7, and 9 in aqueous solution and its metallization into a form displaying silver-covered dendrites, as demonstrated by field-emission scanning electron microscopy (FE-SEM), energy-
10.1021/la800208w CCC: $40.75 2008 American Chemical Society Published on Web 04/12/2008
5230 Langmuir, Vol. 24, No. 10, 2008
Letters
Figure 1. EF-TEM images of self-assembled 1 formed at (a) pH 3 and (b) pH 7 in aqueous solution.
filtered transmission electron microscopy (EF-TEM), circular dichroism (CD), and FTIR spectroscopy studies.
II. Experimental Section Apparatus for Spectroscopy Measurement. 1H and 13C NMR spectra were measured on a Bruker ARX 300 apparatus. IR spectra were obtained in KBr pellets using a Shimadzu 8100 FTIR spectrometer, and an MS spectrum was obtained from a JEOL JMS-700 mass spectrometer. The optical absorption spectra of the samples were obtained at room temperature in the 190-750 nm wavelength region using a UV-vis spectrophotometer (JASCO V-530). TEM and SEM Observations. For transmission electron microscopy (TEM), a piece of the gel was placed on a carbon-coated copper grid (400 mesh) and removed after 1 min, leaving some small patches of the sample on the grid. Then, they were dried for 1 h at low pressure. The specimens were examined with a Hitachi H-7100 using an accelerating voltage of 100 kV and a 16 mm working distance. Scanning electron microscopy (SEM) was carried out on a Hitachi S-4500. The accelerating voltage of the SEM was 5-15 kV, and the emission current was 10 µA. CD Measurement. Circular dichroism (CD) studies were performed on a JASCO J-715 spectropolarimeter operating between 190 and 500 nm. The samples (5.0 × 10-3 M) were placed in waterjacketed quartz cells with a path length of 1.0 mm. The spectrometer was calibrated with ammonium-d-camphorsulfonate ([θ]291 ) 7910 cmnbsp; cm2 dmol-1) and D-pantonyllactone ([θ]219 ) -16 140 in water, [θ]223 ) -12 420 in methanol). XRD Measurement. The XRD of a freeze-dried sample was measured with a Rigaku diffractometer (type 4037) using graded d-space elliptical side-by-side multilayer optics, monochromated Cu KR radiation (40 kV, 30 mA), and an imaging plate (R-Axis IV). The typical exposure time was 10 min with a150 mm camera length. Dried samples were vacuum dried to constant weight and then put into capillary tubes. Preparation of Self-Assembled 1 The self-assembled nanostructures of amphiphile 1 was fabricated by the following methods. The amphiphile (10 mg) was dispersed in water (1.0 mL). Then, heating the mixture to 95 °C for 30 min was sufficient to obtain a homogeneous transparent solution. The aqueous solution was allowed to cool to room temperature. Finally, we observed the morphologies of self-assembled 1. The pH of the solution was adjusted by using HCl or NaOH.
III. Results and Discussion Alanine-functionalized calix[4]arene 1 was synthesized by the four steps as shown in Scheme S1 in Supporting Informa(9) (a) Nakai, T.; Kanamori, T.; Sando, S.; Aoyama, Y. J. Am. Chem. Soc. 2003, 125, 8465–8475. (b) Hayashida, O.; Mizuki, K.; Akagi, K.; Matsuo, A.; Kanamori, T.; Nakai, T.; Sando, S.; Aoyama, Y. J. Am. Chem. Soc. 2003, 125, 594–601. (c) Aoyama, Y. Chem.sEur. J. 2004, 10, 588–593. (10) (a) Podoprygorina, G.; Zhang, J.; Brusko, V.; Bolte, M.; Janshoff, A.; Bo¨hmer, V. Org. Lett. 2003, 5, 5071–5074. (b) John, G.; Mason, M.; Ajayan, P. M.; dordick, J. S. J. Am. Chem. Soc. 2004, 126, 15012–15013. (c) Kim, J. S.; Yang, S. H.; Rim, J. A.; Kim, J. Y.; Vicens, J.; Shinkai, S. Tetrahedron Lett. 2001, 42, 8047–8050. (11) Martin, O. M.; Mecozzi, S. Tetrahedron 2007, 63, 5539–5547.
Figure 2. XRD patterns of the product obtained by AgNO3 in the presence of self-assembled 1 (a) before and (b) after the addition of ascorbic acid.
tion. In the first step, compound 2 was treated with chloromethyl octyl ether to give tetraphenyl methylene chloride 3. In the second step, the alanine ethyl ether was added to a solution of the corresponding compound 4. After the alkylation of 4, treatment with 5 and NaOH afforded the desired product 1 as a white powder. To obtain visual insight into the aggregation mode, we observed the supramolecular architectures of self-assembled 1 at different pH values by EF-TEM. The EF-TEM images of self-assembled 1 were obtained at pH 3, 7, and 9 in aqueous solution (Figure 1). The self-assembled samples obtained at pH 3 revealed a spherical structure 200-250 nm in diameter, but we did not observe the morphology of 1 at pH 9. It is also known that aggregate size can be regulated by systematic variation in the hydrophilic chain length of the molecule.7b The amphiphilic calix[4]arene molecules with a small hydrophilic part assemble into well-defined and tunable vesicles that decrease significantly in diameter with increasing hydrophilic chain length. Further increasing the chain length may be induced by the collapse of the vesicles into spherical micelles.7b More importantly, a significant change in the aggregate architecture with pH was observed for the amphiphilic calix[4]arene 1. The self-assembled 1 at pH 7 presented a necklacelike structure composed of spheres that were ca. 500 nm in diameter and connected to one another. (A detailed explanation of the necklacelike structure obtained at pH 7 will follow.) Also, both ends of the necklacelike structure were closed. Although these changes are not kinetically very fast and there is not a complete shift from one aggregation form to another, these results indicate that the morphologies of selfassembled 1 are strongly dependent on the pH of the solution. To confirm the influence of pH on the morphology of the novel nanostructures, we obtained the IR spectra of self-assembled 1 at pH 3, 7, and 9 (Figure S1). Clearly, the hollow necklacelike structure formed at neutral pH with strong
Letters
Langmuir, Vol. 24, No. 10, 2008 5231
Figure 3. (A) SEM image of the 3D dendritic silver nanostructure after reduction. (B) Elemetal mapping of the 3D dendritic silver after reduction: (a) bright field, (b) Ag component, (c) C component, and (d) O component.
hydrogen bonding between the carbonyl groups and the highly organized, closely packed hydrocarbon chains exhibit sharp IR bands, as compared to the absorption band of the monomer species, which was produced in aqueous solution at pH 9. Prominent differences were observed in a few regions of the IR spectrum. First, hollow structure 1 obtained at pH 7 displayed strong bands at 1734 and 1503 cm-1 corresponding to the COOH and COO- species, respectively. In contrast, the spectrum of spherical structure 1 obtained at pH 3 showed only a single strong band at 1734 cm-1 for COOH. An alkaline solution of 1 (pH 9) shows a single band for the ionized COO- species at 1503 cm-1, which displayed no solidified morphology. These results clearly indicate that in the presence of both the unionized and ionized species of the carboxyl group at pH 7 the intermolecular hydrogen bonding interactions from sphere to sphere may play a critical role in the formation of the high-axial-ratio necklacelike structure (Figures S1b and S2). To investigate the effects of chirality, we examined the CD and UV-vis spectra of aqueous solutions of self-assembled 1 at pH 3, 7, and 9 (Figure S3). In the CD spectra, λθ)0 for self-assembled 1 appeared at ∼220 nm, which originated from the amino acid group of the alanine moiety by the n-π* transition.12 Also, the λmax value in the UV absorption spectrum of 1 was observed at ∼220 nm (Figure S4). Thus, it is clear that CD spectra of self-assembled 1 were induced by the exciton coupling effect. The CD spectra of self-assembled 1 exhibit a positive sign for the first Cotton effect (Figure S3), indicating that the dipole moments are oriented in a clockwise direction in the aggregate of 1. In addition, the result indicates that molecules of 1 are helically organized. The CD signal intensity of selfassembled 1 prepared at pH 7 was higher than those prepared at pH 3 and 9. The CD results provide direct evidence of more helically well organized chiral molecular architecture in a necklacelike structure and that the molecular packing of selfassembled 1 species prepared at pH 3 and 9 are loosely packed chiral structures. We also conducted metallization studies using self-assembled 1. In an optimized experiment, self-assembled 1 (10 mg) was prepared in a pH 7 aqueous solution (10 mL). AgNO3 (0.05 M), as the source of silver ion, and ascorbic acid (0.01 M) were prepared in distilled water (1 mL of each solution). To the aqueous solution of self-assembled 1 was added the AgNO3 solution (50 µL). The reaction mixture was stirred for 30 min at room
temperature. Then, ascorbic acid (0.01 M), which served as the reducing agent, was added to the reaction mixture (10 mL). The reaction mixture exhibited a brown color, indicating the formation of silver. The final product was isolated and washed several times with distilled water and ethanol to remove the supernatant completely. Finally, the product was redispersed in ethanol for further instrumental analysis. Figure 2 shows the XRD pattern of the product obtained by the addition of AgNO3 in the presence of self-assembled 1 as a stabilizer before and after the addition of ascorbic acid. The reflection peaks can be indexed to the corresponding (111), (102), (020), (211), (113), (112), (220), (213), (114), (024), (215), and (126) faces before the addition of ascorbic acid, which correspond to the reflection of face-centered cubic (fcc) crystalline AgNO3 (JCPDS 43-0649). However, after treatment with ascorbic acid, only four peaks appeared at 38, 44, 64, and 77 for (111), (200), (220), and (311), respectively. All of these distinct diffraction peaks correspond to the reflection of face-centered cubic (fcc) crystalline silver without any impurity peaks (JCPDS 65-2871). These results indicate that crystalline AgNO3 was changed into crystalline silver with a typical fcc structure by the reducing agent. In addition, EDX measurement indicated that the product displays a strong peak for silver that does not exist in the original self-assembled 1 (Figure S5). Figure 3A shows SEM images of the 3D dendritic silver structure after reduction. Each dendrite consists of a long central backbone with very sharp secondary branches that preferentially grow along two definite directions rather than randomly. Surprisingly, the secondary branches that emerge at 60° in relation to the central backbone have uniform spacing and are parallel to each other. These secondary branches closely resemble needlelike crystals, which are the extreme case of anisotropic growth. The 3D dendritic siliver structure may be induced by diffusion-limited aggregation, which produces complex random dendritic structures.13 Calix[4]arene 1 probably acted as a stabilizer and shape controller in the diffusion-limited aggregation process.14 Furthermore, we performed elemental mapping with EELS as shown in Figure 3B. The 3D dendritic structure (Figure 3B, a) contained silver (Figure 3B, b), carbon (Figure 3B, c), and oxygen (Figure 3B, d) components. These findings strongly support the view that silver was deposited onto the surface of self-assembled 1.
(12) Sreerama, N.; Woody, R. W. In Circular Dichroism: Principles and Applications; Berova, N., Nakanishi, K., Woody, R. W., Eds.; Wiley-VCH: New York, 2000; pp 601-620.
(13) Meakin, P. Phys. ReV. Lett. 1983, 51, 1119–1126. (14) Lee, J.-H.; Kamada, K.; Enomoto, N.; Hojo, J. Chem. Lett. 2007, 36, 728–729.
5232 Langmuir, Vol. 24, No. 10, 2008
Letters
Conclusions
amphiphilic calix[4]arene derivative is useful as a stabilizer to obtain new inorganic materials.
In summary, amphiphilic calix[4]arene derivative 1 with four attached alanine moieties was found to form spherical or hollow necklacelike self-aggregates in aqueous solutions at pH values of 3 and 7 respectively, as confirmed by EF-TEM and FE-SEM studies. The morphology of amphiphilic calix[4]arene 1 is strongly dependent on pH. In addition, a 3D dendritic silver-coated nanostructure could be obtained by using self-assembled 1. From a technological point of view, this well-controlled nanostructure may have important applications in microelectronic devices or nanometer-scale electrodes. This finding indicates that the
Acknowledgment. This work was supported in part by the KOSEF (R01-2007-000-20299-0) and KRF (KRF-2005-005J09703). Supporting Information Available: Synthesis route, proposed structure, CD, and UV spectrum of self-assembled 1 and EDX data. This material is available free of charge via the Internet at http:// pubs.acs.org. LA800208W
