pubs.acs.org/Langmuir © 2010 American Chemical Society
Superamphiphiles Based on Charge Transfer Complex: Controllable Hierarchical Self-Assembly of Nanoribbons Chao Wang, Yinsheng Guo, Zhiqiang Wang, and Xi Zhang* Key Lab of Organic Optoelectronics & Molecular Engineering Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China Received June 26, 2010. Revised Manuscript Received July 24, 2010 We have demonstrated a hierarchical self-assembly of the nanoribbons on the basis of the concept of a superamphiphile. A viologen-containing surfactant (RV) and a water-soluable electron donor, 6,8-dihydroxypyrene-1,3-disulfonic acid disodium (DHP), are mixed in water to form a charge transfer complex (RV-DHP), functionalizing as a superamphiphile. RV-DHP can self-assemble in water to form single-layer nanoribbon at pH 9. Moreover, upon pH stimulus, the self-assembling nanostructure can be tunable reversibly between single-layer and multilayer nanoribbons. This study represents a new example of hierarchical self-assembly of one-dimensional nanostructures, which may find potential applications in the area of smart nanodevices.
Introduction Superamphiphiles or supramolecular amphiphiles refer to amphiphiles that are synthesized on the basis of noncovalent interactions.1 Because of the controllable nature of noncovalent interactions, the amphiphilicity of the superamphiphiles can be tunable between hydrophilic and hydrophobic, thus allowing for controlled self-assembly.2 By rational design of superamphiphiles, we can construct various nanostructures with desired functions.3 Among the self-assembling nanostructures, one-dimensional nanostructures have stimulated special interest for their potential applications as nanowires in nanodevices.4 The fabrication of stimuli-responsive one-dimensional nanostructures is likely to be further extended to the area of smart materials. Superamphiphiles *To whom correspondence should be addressed. Fax: 86-10-62771149. E-mail:
[email protected]. (1) (a) Zhang, X.; Wang, C. Chem. Soc. Rev. 2010, in press. (b) Wang, Y. P.; Xu, H. P.; Zhang, X. Adv. Mater. 2009, 21, 2849. (c) Wang, C.; Yin, S. C.; Chen, S. L.; Xu, H. P.; Wang, Z. Q.; Zhang, X. Angew. Chem., Int. Ed. 2008, 47, 9049. (d) Hermans, T. M.; Broeren, M. A. C.; Gomopoulos, N.; Smeijers, A. F.; Mezari, B.; Van Leeuwen, E. N. M.; Vos, M. R. J.; Magusin, P. C. M. M.; Hilbers, P. A.; Van Genderen, M. H. P.; Sommerdijk, N. A.; Fytas, J. M. G.; Meijer, E. W. J. Am. Chem. Soc. 2007, 129, 15631. (e) Jeon, Y. J.; Bharadwaj, P. K.; Choi, S.; Lee, J. W.; Kim, K. Angew. Chem., Int. Ed. 2002, 41, 4474. (f) Zhang, X.; Chen, Z. J.; W€urthner, F. J. Am. Chem. Soc. 2007, 129, 4866. (g) Cui, S.; Liu, H. B.; Gan, L. B.; Li, Y. L.; Zhu, D. B. Adv. Mater. 2006, 18, 2918. (h) Qiao, Y.; Lin, Y.; Wang, Y.; Yang, Z.; Liu, J.; Zhou, J.; Yan, Y.; Huang, J. Nano Lett. 2009, 9, 4500. (2) (a) Wang, Y. P.; Ma, N.; Wang, Z. Q.; Zhang, X. Angew. Chem., Int. Ed. 2007, 46, 2823. (b) Wang, C.; Guo, Y. S.; Wang, Y. P.; Xu, H. P.; Zhang, X. Chem. Commun. 2009, 5380. (3) (a) Ling, X. Y.; Phang, I. Y.; Maijenburg, W.; Sch€onherr, H.; Reinhoudt, D. N.; Vancso, G. J.; Huskens, J. Angew. Chem., Int. Ed. 2009, 48, 1001. (b) Zou, J.; Tao, F.; Jiang, M. Langmuir 2007, 23, 12791. (c) Jeon, Y. J.; Bharadwaj, P. K.; Choi., S.; Lee, J. K.; Kim, K. Angew. Chem., Int. Ed. 2002, 41, 4474. (d) Wang, Y. P.; Han, P.; Xu, H. P.; Wang, Z. Q.; Zhang, X.; Kabanov, A. V. Langmuir 2010, 26, 709. (4) (a) Song, B.; Chen, S. L.; Wang, Z. Q.; Zhang, X. Adv. Mater. 2007, 19, 416. (b) Chen, Y. L.; Zhu, B.; Zhang, F.; Han, Y.; Bo, Z. S. Angew. Chem., Int. Ed. 2008, 47, 6015. (c) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Rev. 2005, 105, 1491. (d) Shiraki, T.; Morikawa, M.; Kimizuka, N. Angew. Chem., Int. Ed. 2008, 47, 106. (e) Palmer, L. C.; Stupp, S. I. Acc. Chem. Res. 2008, 41, 1674. (f) Cho, B. K.; Kim, H. J.; Chung, Y. W.; Lee, B. I.; Lee, M. Adv. Polym. Sci. 2008, 219, 69. (g) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644. (h) Lenhert, S.; Zhang, L.; Mueller, J.; Wiesmann, H. P.; Erker, G.; Fuchs, H.; Chi, L. Adv. Mater. 2004, 16, 619. (i) Wan, S.; Guo, J.; Kim, J.; Ihee, H.; Jiang, D. L. Angew. Chem., Int. Ed. 2008, 47, 8826. (5) (a) Wang, C.; Guo, Y. S.; Wang, Y. P.; Xu, H. P.; Zhang, X. Angew. Chem., Int. Ed. 2009, 48, 8962. (b) Versluis, F.; Tomatsu, I.; Kehr, S.; Fregonese, C.; Tepper, A. W. J. W.; Stuart, M. C. A.; Ravoo, B. J.; Koning, R. I.; Kros, A. J. Am. Chem. Soc. 2009, 131, 13186.
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have proved to be good candidates for fabricating stimuli-responsive one-dimensional nanostructures, such as pH-responsive nanofibers and photoresponsive nanofibers.5 An interesting feature of one-dimensional nanostructures is that they can be used as building blocks to hierarchically selfassemble into architectures at a higher level, which may exhibit unique properties and functions that are not displayed by their individual components.6 The hierarchical self-assembling process of one-dimensional nanostructures has been highlighted frequently in nature for its important role in the areas of muscular energy conversion and neurodegenerative diseases.7 Moreover, the mimicking of this process is likely to spur the development of smart nanodevices with tunable currents paths.8 However, in artificial systems, the controllable hierarchical self-assembly of one-dimensional nanostructures remains rarely realized.9 Herein, in this paper, by the rational design of the superamphiphiles, we successfully demonstrate the hierarchical self-assembly of one-dimensional nanoribbons controlled by pH stimulus. The realization of hierarchical self-assembly relies on the rational control of noncovalent interactions. We have previously reported that in the superamphiphile, the cooperation of electrostatic interactions, charge transfer interactions, and hydrophobic interactions can produce a high binding interaction along the axis, inducing the formation of ultralong one-dimensional nanostructures.5a At this stage, if an additional hydrogen bonding perpendicular to the axis is introduced, the one-dimensional nanostructures are expected to further self-assemble into superstructures at (6) (a) Elemans, J. A. A. W.; Rowan, A. E.; Nolte, R. J. M. J. Mater. Chem. 2003, 13, 2661. (b) Choi, I. S.; Bowden, N.; Whitesides, G. M. Angew. Chem., Int. Ed. 1999, 38, 3078. (c) Engelkamp, H.; Middelbeek, S.; Nolte, R. J. M. Science 1999, 284, 785. (d) Wang, X.; Guerin, G.; Wang, H.; Wang, Y.; Manners, I.; Winnik, M. A. Science 2007, 317, 644. (e) Ko, Y. H.; Kim, E.; Hwang, I.; Kim, K. Chem. Commun. 2007, 1305. (7) (a) Tsai, C. J.; Ma, B. Y.; Kumar, S.; Wolfson, H.; Nussinov, R. Crit. Rev. Biochem. Mol. 2001, 36, 399. (b) Prockop, D. J.; Fertala, A. J. Struct. Biol. 1998, 122, 111. (c) Weiner, S.; Wagner, H. D. Annu. Rev. Mater. Sci. 1998, 28, 271. (8) (a) Nam, K. T.; Kim, D.-W.; Yoo, P. J.; Chiang, C.-Y.; Meethong, N.; Hammond, P. T.; Chiang, Y.-M.; Belcher, A. M. Science 2006, 312, 885. (b) Aldaye, F. A.; Palmer, A. L.; Sleiman, H. F. Science 2008, 321, 1795. (9) (a) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Science 2001, 294, 1684. (b) G€adt, T.; Leong, N. S.; Cambridge, G.; Winnik, M. A.; Manners, I. Nat. Mater. 2009, 8, 144. (c) Lee, E.; Kim, J.-K.; Lee, M. Angew. Chem., Int. Ed. 2008, 47, 6375. (d) Yu, T. B.; Bai, J. Z.; Guan, Z. B. Angew. Chem., Int. Ed. 2009, 48, 1097.
Published on Web 08/19/2010
DOI: 10.1021/la102586b
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Wang et al. Scheme 1. Schematic Illustration of the Hierarchical Self-Assembly of Nanoribbons
Figure 1. (a) UV-vis and (b) fluorescence emission spectra of the RV-DHP complex at different pH values.
UV-Vis Spectra and Fluorescence Emission Spectra.
higher levels. For this purpose, the formation of the superamphiphile is based on the formation of a charge transfer complex between the electron-deficient viologen moiety and the electronrich 6,8-dihydroxypyrene-1,3-disulfonic acid disodium (DHP) molecules (Scheme 1). Apart from its electron-rich and chargenegative nature, DHP also contains hydrogen-bonding sites and is therefore chosen as one component of the superamphiphile. The other component is a surfactant (RV) containing the electrondeficient viologen moiety. Driven by the electrostatic interactions and charge transfer interactions between viologen and DHP, RV and DHP can form a water-soluble charge transfer complex (RV-DHP). In addition, the amphiphilic RV-DHP complex functions as a superamphiphile. Because of the existence of two phenol groups on the same side of DHP molecules, it can be expected that hydrogen bonding will form between two DHP moieties of the superamphiphiles. Notably, by changing the pH in the solution, the phenol groups are tunable between ionized sate and neutral state, further manipulating the intermolecular hydrogen bondings.
Experimental Section Materials Preparation. DHP was commercially available from Fluka without further purifications. One gram of 4-40 -bipyridine and excessive 1-bromotetradecane were dissolved in the mixture of CH3CN and chloroform. The solution was kept stirring at 60 °C for one night. After the solvent was removed, the product was dissolved in chloroform and recrystallized in petroleum ether to yield yellow solid. The solid was further mixed with excessive CH3I in CH3CN, and the mixture was kept stirred and refluxed for one night. After the evaporation of CH3CN, the solid was dissolved in methanol and recrystallized in diethyl ether to get the final product. Yield, 78%. 1H NMR spectra were recorded on a JEOL JNM-ECA300 apparatus (300 MHz, DMSO, 25 °C, TMS): δ = 9.41 (d, 2H, 3JH-H = 6.2 Hz), 9.29 (d, 2H, 3JH-H = 6.2 Hz), 4.69 (t, 2H), 4.45 (s, 3H, 3JH-H = 7.2 Hz), 2.0 (m, 2H), 1.24 (m, 24H), 0.83 (t, 3H, 3JH-H = 14.4 Hz). 13C NMR spectra were recorded on a JEOL JNM-ECA300 apparatus (300 MHz, DMSO, 25 °C): δ = 149.0, 148.7, 147.2, 146.3, 127.1, 126.6, 61.4, 48.6, 31.8, 31.3, 29.5, 29.4, 29.2, 29.0, 26.0, 26.0, 22.6, 14.5. Mass spectrometry (ESI, m/z) calculated for C25H40N22þ, 184.16; found, 184.28. The solution was prepared by mixing equimolar amounts of DHP and RV in different phosphate buffer solutions. The concentration of the complex was 1 10-4 M. 1 H NMR and ESI-MS. NMR spectra were recorded on a JEOL JNM-ECA600 apparatus. ESI-MS spectra were recorded on a PE Sciex API 3000 apparatus. 14510 DOI: 10.1021/la102586b
UV-vis spectra were obtained using a HITACHI U-3010 spectrophotometer. Fluorescence spectra were obtained using a HITACHI F-7000 apparatus at a slit of 5.0 and at a scanning rate of 240 nm/min. The excitation wavelength was 310 nm. TEM Experiments. TEM observation was performed on a JEMO 2010 Electron microscopy operating at an acceleration voltage of 120 kV. The samples were prepared by drop coating the aqueous solution on the carbon-coated copper grid and then negatively staining it with phosphotungstic acid solution. AFM Experiments. AFM images were taken with a commercial multimode Nanoscope IV AFM with tapping-mode scanning from VEECO. RTESP silicon cantilevers were purchased from the same company. The sample was prepared on clean silicon slides.
Results and Disccusion pH Responsiveness of the Charge Transfer Complex. Because the pKa values of the first and the second stage ionization of phenol groups are about 8 and 9, respectively,10 the solution of the RV-DHP complex at pH 8 and pH 9 was prepared. As shown in Scheme 1, it is anticipated that the hydrogen bonding can be formed at pH 8. When the pH is tuned to 9.0, however, the hydrogen bonding will be weakened drastically, accompanied with the appearance of repulsive electrostatic interactions between the ionized phenol groups. The influence of pH changes can be detected by UV-vis absorption and emission spectra. As shown in Figure 1, when the pH in solution is changed from 8.0 to 9.0, a red shift can be observed on the absorption spectra of RV-DHP, which results from the different ionized state of the DHP molecules. At the same time, a decrease on the emission intensity of RV-DHP complex can be observed. One possible reason is that the electrostatic interaction between viologen and DHP is enhanced at a higher pH, which induces a tighter bonding between viologen and DHP, further decreasing the emission intensity. It should be noted that for RV or DHP alone, no peak shift can be observed on UV-vis spectra at different pH, further confirming that the spectra changes result from the pH-responsive feature of the charge transfer complex. Formation of Single-Layer Nanoribbon at pH 9. The RVDHP complex is expected to self-assemble in aqueous solution due to its amphiphilic nature. To reveal the self-assembling nanostructure of the aggregates, transmission electron microscopy (TEM) and atomic force microscopy (AFM) experiments were performed on samples of RV-DHP at pH 9. As shown in Figure 2a, typical nanoribbons were observed. The width of the nanoribbon is about 20 nm, and the length reaches tens of micrometers. The formation of this ultralong one-dimensional nanostructure can be attributed to the cooperation of electrostatic interactions, charge transfer interactions, and hydrophobic interactions. The ribbonlike nanostructure is further confirmed by AFM (Figure S1 (10) (a) Kermis, H. R.; Kostov, Y.; Rao, G. Analyst 2003, 128, 1181. (b) Zhu, L.; Nakayama, T.; Tomimoto, H.; Shingaya, Y.; Huang, Q. Nanotechnology 2009, 20, 325501.
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Figure 3. (a and b) TEM images of the intermediate structures of RV-DHP from pH 8 to pH 9.
Figure 2. (a) TEM images of the aggregates of RV-DHP at pH 9. (b) Distribution of the hydrodynamic diameter (DH) of RV-DHP at pH 9 and pH 8. (c and d) Magnified TEM image of the aggregates at pH 8.
in the Supporting Information), and the nanoribbons give a uniform height of 6.0 nm. This dimension is in reasonable agreement with twice the fully extended molecular length of RV (∼3 nm by energy minimization modeling), thus indicating a single bilayer packing with the alkyl chains inside in the cross-section. Self-Assembling Behavior of RV-DHP at pH 8. When the pH is tuned from 9 to 8 by adding some hydrochloric acid to the buffer solution, the phenol group on the nanoribbons simultaneously changes from an ionized state to the neutral state. As a result, the hydrogen bondings will appear, while the repulsive electrostatic interactions will be weakened drastically. Remarkably, dynamic light scattering (DLS) experiments show that the average hydrodynamic diameter (DH) increased from about 30 to 300 nm (Figure 2b). These results indicate that the size of the aggregates increases significantly when the pH changes from 9 to 8. To clarify the structural changes upon the pH stimulus, TEM and AFM experiments of the samples of RV-DHP at pH 8 were carried out. The TEM micrographs reveal that ribbonlike aggregates with much larger sizes are formed (Figure 2c). The lengths of the nanoribbons are still up to tens of micrometers, but the width reaches 60-100 nm. Close examination of the magnified image reveals that the ribbons are multilayered, consisting of laterally stacked elementary single-layer one-dimensional nanostructures (Figure 2d). The density profile perpendicular to the long axis of the ribbon showed an interlayer distance of about 6-7 nm, which is consistent with the height of the previous single-layer nanoribbon. AFM experiments further confirm the formation of the multilayer nanoribbons (Figure S2 in the Supporting Information). AFM Observations of the Multilayer Nanoribbons of RVDHP Complex at pH 8. The multilayer feature can also be observed from the AFM images (Figure S2 in the Supporting Information). The height image shows that the height of the nanoribbon is about 20 nm, consistent with the width of the single-layer nanoribbon. These findings indicate that the single-layer nanoribbons have laterally stacked into multilayer nanoribbons when the pH changes from 9 to 8. The trend is in agreement with the results obtained by DLS measurements. It should be noted that when pH changes back to 9 using sodium hydroxide, the multilayer nanoribbon will dissociate into single-layer nanoribbon again, indicating that the hierarchical self-assembling process is reversible. It takes several hours for the complete transformation between the single-layer and the multilayer nanoribbons. Self-Assembling Behavior of RV-DHP Complex at pH 7 and pH 10. As a control experiment, we have studied the Langmuir 2010, 26(18), 14509–14511
self-assembling behaviors of RV-DHP at pH 7 and pH 10 as well. The pH 7 RV-DHP solution was obtained by adding concentrated HCl to the pH 8 RV-DHP solution. Because of the charge neutralization, the RV-DHP complex will precipitate from water; therefore, no aggregates can be obtained. At pH 10, the size and shape of the aggregates are nearly the same with those of pH 9 (Figure S3 in the Supporting Information), indicating that the self-assembling nanostructures remain unchanged when the pH is higher than 9. Intermediate Structures during the Hierarchical SelfAssembly Process. We have captured the intermediate structure using TEM (Figure 3) to further confirm the hierarchical selfassembly process of the nanoribbons, since the complete transformation between the single-layer and the multilayer nanoribbons needs several hours. It can be observed that the single-layer nanoribbons are gradually flaked off the multilayer nanoribbons exactly along the edge of the layers, indicating that the multilayer nanoribbons are indeed formed by the lateral packing of singlelayer nanoribbon. The hierarchical self-assembling process of the nanoribbons results from the cooperation of different noncovalent interactions, including hydrogen bonding, electrostatic interaction, and hydrophobic interaction. When the pH is changed from 9 to 8, the electrostatic repulsion is weakened, and the hydrogen bonding is enhanced; therefore, an extra interaction perpendicular to the axis is induced, resulting in the hierarchical self-assembly of nanoribbons.
Conclusion In conclusion, we have developed furthermore the concept of superamphiphile and demonstrated a successful example of controllable hierarchical self-assembly of nanoribbons. By the rational control of noncovalent interactions, the self-assembling nanostructure can be tunable reversibly between single-layer and multilayer nanoribbons controlled by pH stimulus. The study has shown that a superamphiphile based on a charge transfer complex is a general way of fabricating one-dimensional nanostructures. Because the hierarchical self-assembly of one-dimensional nanostructures plays a vital role for the energy conversion in human bodies, our results may spur the further development of artificial muscles and smart electronic nanodevices. Acknowledgment. This work was financially supported by the National Basic Research Program (2007CB808000), NSFC (50973051 and 20974059), NSFC-DFG joint grant (TRR 61), and Tsinghua University Initiative Scientific Research Program (2009THZ02230). We thank Prof. Huaping Xu for his helpful disccusion. Supporting Information Available: AFM images of the height of the nanoribbon at pH 9 and AFM images of the multilayer nanoribbon of RV-DHP at pH 8. This material is available free of charge via the Internet at http://pubs.acs.org. DOI: 10.1021/la102586b
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