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Langmuir 2006, 22, 360-362
Fabrication of Fluorescent Nanotubes Based on Layer-by-Layer Assembly via Covalent Bond Ying Tian, Qiang He, Cheng Tao, and Junbai Li* International Joint Lab, Key Lab of Colloid and Interface Science, The Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed September 11, 2005. In Final Form: October 5, 2005 A pressure-filter-template approach was employed to prepare fluorescent nanotubes of polyethyleneimine (PEI) and 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) through covalent combination in the porous of alumina template based on the layer-by-layer (LbL) assembly technique. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images exhibited a tubular structure of the obtained samples. The wall thickness of the nanotubes is well controlled by varying the assembled cycle number, which is detected by UV-visible spectroscopy. Fourier transform infrared (FT-IR) spectroscopy confirmed the formation of covalent bonds between PEI and PTCDA in nanotubes.
Introduction Considerable attention is currently being paid to nanostructured materials such as wires, rods, belts, and tubes.1,2 Nanotubes made of carbon and metals are investigated because of their unique electronic, optical, and mechanical properties and their potential applications as catalysis carriers.3,4 Several approaches have been developed to prepare various nanotubes.5 One of the most efficient ways to fabricate nanotubes is the template method,6 which allows controlling the outer diameter and the length of the nanotubes. Also, different types of functional groups can be linked to the porous wall surfaces, resulting in multicompound materials useable as sensor carriers as well as reactive media for optoelectronic materials.7,8 Recently, our group introduced a pressure-filter-template technique for the preparation of polyelectrolyte multilayer nanotubes.9 It successfully combines the pressure-filter-template approach with the LbL technique, which uses the electrostatic attractions between oppositely charged species. Thus, highly flexible polyelectrolyte complex nanotubes can be fabricated10 with the possibility to control the wall thickness and component and functionalization of the nanotubes.11 On the basis of the combination of LbL assembly and the template technique, several groups have prepared various functional nanotubes. However, among the interactions between molecules, the covalent bond possesses the strongest binding energy compared with other molecular interactions, such as electrostatic attraction, ligand bond, and so on. Thus, the complex nanotubes * Corresponding author. Institute of Chemistry, Chinese Academy of Sciences, Zhong Guan Cun, Bei Yi Jie 2, Beijing 100080, China. E-mail:
[email protected]. Telephone: +86 10 82614087. Fax: +86 10 82612629. (1) Iijma, S. Nature 1991, 354, 56. (2) Shen, S. C.; Hidajat, K.; Yu, L. E.; Kawi, S. AdV. Mater. 2004, 16, 541545. (3) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N.; Nevanen, T. K.; Soderlund, H.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11864-11865. (4) Klefenz, H. Eng. Life Sci. 2004, 4, 211-218. (5) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew. Chem., Int. Ed. 2001, 40, 988-1011. (6) Masuda, H.; Fukuda, K. Science 1995, 268, 1466. (7) Christopher, A. D.; Jamesour, M. T. J. Am. Chem. Soc. 2003, 125, 11561157. (8) Jenekhe, S. A.; Reichmanis, E.; Ward, M. D. Chem. Mater. 2004, 16, 25-26. (9) Caruso, F.; Trau, D.; Mohwald, H.; Renneberg, R. Langmuir 2000, 16, 1485-1488. (10) Ai, S. F.; Lu, G.; He, Q.; Li, J. B. J. Am. Chem. Soc. 2003, 125, 11140. (11) Hou, S.; Harrell, C. C.; Trofin, L.; Kohli, P.; Martin, C. R. J. Am. Chem. Soc. 2004, 126, 5674-5675.
fabricated by covalent LbL assembly should be more stable than others. In addition, the complex nanotubes of many small functional molecules cannot be prepared through the usual methods. Furthermore, 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) as a perylene derivative is a typical functional molecule with the feature of photoelectronic responses. It has been widely applied in electronic and optoelectronic devices, such as light-emitting diodes, and for the fabrication of waveguides.12,13 High photostability of PTCDA is applied in dye-sensitized solar cells.14-17 Assembly of PTCDA in the wall of nanotubes may result in the synthesized nanotubes with the optoelectronic response or visible features. Many assembled nanotubes are expected to be applied in the living system, and the fluorescent nanotubes allow us to determine their position. Herein we present a method to fabricate complex nanotubes of polyethyleneimine (PEI) and 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) through the alternative deposition of polymers and small functional molecules via covalent bonds. The polymer serves as a stabilizing backbone, and the small functional molecules are attached to the polymer layer through additive chemical reaction. Scheme 1 depicts the chemical reaction process for synthesizing the LbL nanotubes. Experimental Section Chemicals. Alumni membranes with a pore diameter of 200 nm and a membrane thickness of 60 µm were obtained from Whatman. Poly(ethylenimine) (PEI, Mw ) 5,000-10,000) was obtained from ICN Co., Ltd. and 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) was purchased from Aldrich, which have been used without further treatment. The PET and PTCDA were 0.05 wt % dissolved in isopropyl alcohol and o-phenol solution, respectively. Nanotube Preparation. The substrate was first immersed in a PEI isopropyl alcohol solution for 10 min, then the slide was rinsed with isopropyl alcohol and dried under N2. Next, the slide was transferred into a PTCDA o-phenol solution for 10 min. The multilayer films can be expressed as (PEI/PTCDA)n, where n is the number of deposition cycles. For the preparation of complex (12) ] Burrow, P. E.; Forrest, S. R. Appl. Phys. Lett. 1993, 62, 3102. (13) Djuristic, A. B.; Fritz, T.; Leo, K. Opt. Commun. 2000, 183, 123. (14) Ford, W. E.; Kamat, P. V. J. Phys. Chem. 1987, 91, 6373. (15) Nguyen, K. C.; Weiss, D. S. J. Imaging Technol. 1989, 15, 158. (16) Forrs, S. R. J. Appl. Phys. 1984, 55, 1492. (17) O′Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.; Gaines, G. L.; Wasielewski, M. R. Science 1992, 257, 63-65.
10.1021/la0524768 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/10/2005
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Scheme 1. Chemical Reactions Procedures between PEI and PTCDAa
a
Reaction has taken place in the inner wall of an alumina template
Figure 2. Infrared detection of PEI, PTCDA, and (PEI/PTCDA)10 nanotubes, respectively. The peaks at 1635.5 and 1566.1 cm-1 represent the newly formed covalent bond, -CO-NH-, from (PEI/ PTCDA)10.
Results and Discussion
Figure 1. (A) SEM image of (PEI/PTCDA)6 nanotubes with the diameter about 330 nm, corresponding to the diameter of the template pores. The cycle number is n ) 6, with the wall thickness 100 ( 10 nm. (B) TEM images of (PEI/PTCDA)6 nanotubes. (C) SEM image of (PEI/PTCDA)3. (D) Selected section of a broken nanotube (C) with a thickness of about 50 ( 5 nm.
nanotubes, the alumni membranes were first coated with PEI by filtering the PEI solution through the pores of the membrane. This aids subsequent multilayer buildup. Then the PTCDA solution was deposited by the same procedure through covalent bonding. After several cycles, multilayer films were formed on the pore inner walls of the alumni membrane. Finally, we removed the deposited film on the top and bottom surface of the membrane by wiping the surfaces with carborundum sand paper. Once the alumni membrane was dissolved by 1 M NaOH aqueous solution, the nanotubes were liberated into the solution (Scheme 1). Characterization. The quartz slide was used for UV-visible analysis, and the CaF2 plate was used for FT-IR spectroscopy measurement. The scanning electron microscopy (SEM) micrographs were acquired on an S-4300 (HITACHI, Japan), and the transmission electron microscopy (TEM) measurements were performed by 200CX (JEM, Japan). Fourier transform infrared (FT-IR) spectra were recorded on a TENSOR 27 instrument (BRUKER, Germany). The UV-vis spectra were carried out on a HITACHI U-3010 UV-vis spectrometer. Confocal laser scanning microscopy (CLSM) images were taken with an Olympus FV500 confocal system.
The scanning electron microscopy (SEM) image of Figure 1a shows that the uniform (PEI/PTCDA)n nanotubes were successfully synthesized. The diameter of nanotubes is similar to the pore size of the alumina template (350 ( 50 nm), and the length is according to the thickness of the template membrane (60 µm). The nanotubes obtained are also flexible. The wall thickness of (PEI/PTCDA)6 nanotubes is 100 ( 10 nm, so each bilayer is about 16 nm. The nanotubes were also investigated by transmission electron microscopy (TEM), as shown in Figure 1b. The nanotubes have similar uniform size and shape as that measured by SEM. The nanotubes of three cycles are about 50 ( 5 nm (Figure 1c), just the half of the six circles, thus the wall thickness is strictly linear with the increase of the cycle number. IR spectroscopy (Figure 2) on a (PEI/PTCDA)10 nanotube sample confirmed the formation of amide bonds. The appearance of an absorption band around 2800 cm-1 indicates the presence of PEI on carboxylic acid. The signals at 1635.5 and 1566.1 cm-1 are attributed to amide I and II vibrations, respectively. A PEI film that was physisorbed to a carboxylic-acid-terminated SAM did not exhibit these characteristic absorptions. It is the characteristic adsorption of the C-N bond from the group of -CO-NH- and confirms the formation of covalent amide bonds between PEI and PTCDA. It is expected that the absorption intensity of UV spectra in the assembled PEI/PTCDA multilayer increases with the assembled layers. The UV adsorption measurements for every assembled (PEI/PTCDA) layer at a quartz substrate reveal a sequence of UV spectra from samples after the deposition of one additional bilayer. The main absorption peak appears at 385 nm.
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Figure 3. (A) UV spectra of (PEI/PTCDA)n (n ) 14-20) multilayers. (B) UV absorption intensity increased with the number of assembled layer at 385 nm.
variations. Figure 4a and b shows that the obtained nanotubes have a very high flexibility and stability. The length of the nanotubes is about 60 µm (Figure 4c), similar to that obtained by SEM. The intensity distribution of the fluorescent nanotubes is indicated by selecting an individual tube (Figure 4d). It shows that the fluorescent component of PTCDA mainly remains in the wall of the nanotubes.
Conclusion
Figure 4. Slim and bright lines representing the (PEI/PTCDA)6 nanotubes. (A) and (B) exhibit the fluorescent nanotubes wrapped along different direction. (C) Selected single nanotube was used for the intensity measurement, the tube length is similar to the thickness of the template membrane, 60 µm. (D) Intensity profile of a fluorescent nanotube.
At this point, the absorption strength is linear to the number of the deposition (Figure 3b). Figure 4 displays a mass of the resulting fluorescent (PEI/ PTCDA)6 nanotubes, which are entwisted with each other. It should be noted that such assembled nanotubes can reserve their fluorescent properties for up to 10 months without morphology
In conclusion, we have demonstrated that small functional molecules such as PTCDA can be part of a bicomponent film that forms the walls of nanotubes. The small molecules are covalently linked to the other component, a polymer (PEI) which serves as a mechanical backbone. The electrooptical properties of the small molecule are retained after the wall assembly, rendering the nanotubes fluorescent. Such nanotubes prepared with a combination of the pressure-filter-template approach and the LbL technique allows a control of the wall geometry and composition. The successful fabrication of the fluorescent complex nanotubes may have potential applications in catalysis or drug delivery and the design of optical devices and sensors. Acknowledgment. This work was financially supported by the National Nature Science Foundation of China (project nos. 20404015 and 20471063), the Chinese Academy of Sciences, as well as the German Max Planck Society collaborated project. We wish to acknowledge Hans Riegler for critically reading the manuscript. LA0524768
