Reversible Dispersion of Single-Walled Carbon Nanotubes Based on

Oct 6, 2010 - Revised Manuscript Received September 25, 2010. A CO2-responsive dispersant, N,N-dimethyl-N. 0. -(pyren-1-ylmethyl) acetimidamidinium ( ...
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Reversible Dispersion of Single-Walled Carbon Nanotubes Based on a CO2-Responsive Dispersant Yan Ding, Senlin Chen, Huaping Xu, Zhiqiang Wang, and Xi Zhang* Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China

Thien Huynh Ngo and Mario Smet Department of Chemistry, University of Leuven, Celestijnenlaan B-200F, 3001 Heverlee, Leuven, Belgium Received September 2, 2010. Revised Manuscript Received September 25, 2010 A CO2-responsive dispersant, N,N-dimethyl-N0 -(pyren-1-ylmethyl) acetimidamidinium (PyAHþ), which bears both a pyrene moiety and an amidinium cation, has been successfully synthesized. Through strong π-π interaction between the pyrene moiety and single-walled carbon nanotubes (SWNTs), we have demonstrated that PyAHþ can be modified onto SWNT surfaces to promote the dispersion of SWNTs in water. Furthermore, taking advantage of gas triggered interconversions between the amidinium cation and amidine, reversible control on the solubility of SWNTs has been achieved simply through alternated bubbling of CO2 and Ar. This work has demonstrated a new method for controlled dispersion and aggregation of SWNTs, and it may contribute to the development of gas responsive carbon materials.

Introduction Single-walled carbon nanotubes (SWNTs) have become extremely promising materials due to their unique structural, mechanical, and electronic properties.1-4 However, their application has been greatly limited by their poor solubility in any common solvent.5,6 To solve this problem, readily soluble dispersants are usually used to modify the surfaces of SWNTs,7-10 through either covalent11-14 or noncovalent15-21 approaches. Noncovalent modification is often preferred because it avoids *To whom correspondence should be addressed. E-mail: xi@ mail.tsinghua.edu.cn. Fax: þ86-010-62771149.

(1) Iijima, S. Nature 1991, 354, 56. (2) Haddon, R. C. Acc. Chem. Res. 2002, 35, 997. (3) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (4) Qu, L.; Dai, L. J. Am. Chem. Soc. 2005, 127, 10806. (5) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483. (6) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (7) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 106, 1105. (8) Zhao, Y.; Stoddart, J. F. Acc. Chem. Res. 2009, 42, 1161. (9) Britz, D. A.; Khlobystov, A. N. Chem. Soc. Rev. 2006, 35, 637. (10) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853. (11) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (12) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760. (13) Peng, H.; Alemany, L. B.; Margrave, J. L.; Khabashesku, V. N. J. Am. Chem. Soc. 2003, 125, 15174. (14) Wang, Y.; Iqbal, Z.; Mitra, S. J. Am. Chem. Soc. 2006, 128, 95. (15) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838. (16) Nakashima, N.; Tomonari, Y.; Murakami, H. Chem. Lett. 2002, 638. (17) Moore, V. C.; Strano, M. S.; Haroz, E. H.; Hauge, R. H.; Smalley, R. E.; Schmidt, J.; Talmon, Y. Nano Lett. 2003, 3, 1379. (18) Wang, D.; Ji, W.; Li, Z.; Chen, L. J. Am. Chem. Soc. 2006, 128, 6556. (19) Nakayama-Ratchford, N.; Bangsaruntip, S.; Sun, X.; Welsher, K.; Dai, H. J. Am. Chem. Soc. 2007, 129, 2448. (20) Ikeda, A.; Tanaka, Y.; Nobusawa, K.; Kikuchi, J. Langmuir 2007, 23, 10913. (21) Ogoshi, T.; Takashima, Y.; Yamaguchi, H.; Harada, A. J. Am. Chem. Soc. 2007, 129, 4878.

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disturbing the intrinsic properties of SWNTs. Furthermore, for potential application in more complicated systems, such as smart sensors and drug delivery, it is highly desirable to regulate the solubility of SWNTs.22 Therefore, different kinds of stimuliresponsive dispersants, which will undergo changes in structure or property in response to external stimuli, have been developed, including pH,23-25 temperature,25,26 light,27 and oxidationreduction28 responsive dispersants. Once modified onto SWNT surfaces, the stimuli response of the dispersants will alter the surface property of SWNTs, thus leading to the dispersion or aggregation of SWNTs. Herein, we report the use of a CO2-responsive dispersant, N,Ndimethyl-N0 -(pyren-1-ylmethyl) acetimidamidinium (denoted as PyAHþ, Scheme 1), to achieve the controlled dispersion and aggregation of SWNTs. The dispersant PyAHþ bears a pyrene moiety and an amidinium cation. Through the strong π-π interaction between the pyrene moiety and SWNTs, PyAHþ can be modified onto SWNT surfaces, so that an aqueous dispersion of SWNTs is obtained. Moreover, utilizing reversible interconversions between amidinium cation and amidine,29,30 which can be simply triggered by bubbling CO2 or Ar, we have been able to control the dispersion and aggregation of SWNTs in water reversibly (Scheme 1). Since the synthesis of PyAHþ only involves a onestep organic reaction and a combination of CO2 and Ar is enough for adjusting the solubility of SWNTs, this method is considered to be comparatively simple, inexpensive, and environmentally (22) Barone, P. W.; Strano, M. S. Angew. Chem., Int. Ed. 2006, 118, 8318. (23) Nepal, D.; Geckeler, K. E. Small 2006, 2, 406. (24) Grunlan, J. C.; Liu, L.; Kim, Y. S. Nano Lett. 2006, 6, 911. (25) Wang, D.; Chen, L. Nano Lett. 2007, 7, 1480. (26) Etika, K. C.; Jochum, F. D.; Theato, P.; Grunlan, J. C. J. Am. Chem. Soc. 2009, 131, 13598. (27) Chen, S.; Jiang, Y.; Wang, Z.; Zhang, X.; Dai, L.; Smet, M. Langmuir 2008, 24, 9233. (28) Nobusawa, K.; Ikeda, A.; Kikuchi, J.; Kawano, S.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2008, 47, 4577. (29) Liu, Y.; Jessop, P. G.; Cunningham, M.; Eckert, C. A.; Liotta, C. L. Science 2006, 313, 958. (30) Yu, T.; Cristiano, R.; Weiss, R. G. Chem. Soc. Rev. 2010, 39, 1435.

Published on Web 10/06/2010

DOI: 10.1021/la103519t

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Ding et al. Scheme 1. Controlled Dispersion and Aggregation of SWNTs with a CO2-Responsive Dispersant

Scheme 2. Interconversions between PyAHþ and PyA

friendly. Although many papers have been published concerning responsive dispersion of SWNTs, to the best of our knowledge this is the first report on gas controlled dispersion and aggregation of SWNTs.

Experimental Section Materials. SWNTs were kindly provided by Prof. Yongsheng Chen (College of Chemistry, Nankai University). Pyren-1-ylmethanamine hydrochloride and 1,1-dimethoxy-N,N-dimethylethanamine were purchased from Aldrich. Other chemicals were analytical-grade reagents and were used as received. Deionized water was used for all aqueous solutions. Synthesis of PyAHþ Hydrogencarbonate. A solution of pyren-1-ylmethanamine hydrochloride (100 mg, 0.37 mmol) and 1,1-dimethoxy-N,N-dimethylethanamine (3 mL) was stirred at 60 °C overnight under Ar atmosphere. After cooling down to room temperature, the solvent was evaporated under reduced pressure. A mixture of dichloromethane and water (1:1, 20 mL) was added. For convenience, dry ice was added to introduce CO2 and the solution was stirred for 15 min. The aqueous phase was separated, and water was added to the organic phase. Extra dry ice was added, and the procedure was repeated three times. To the combined aqueous solution, dichloromethane was added (10 mL) and Ar was purged through the solution (15 min). The organic phase was separated, extra dichloromethane was added to the aqueous phase, and the procedure was repeated three times. To the combined organic phase, heptane (10 mL) was added, and dichloromethane was evaporated under reduced pressure. The pure PyAHþ hydrogencarbonate (47.4 mg, 35%) was obtained as precipitate after filtration. The obtained PyAHþ salt can remain stable with presence of CO2 and H2O in air. The pure N,Ndimethyl-N0 -(pyren-1-ylmethyl) acetimidamide (denoted as PyA, Scheme 1, 25 mg, 22%) was obtained from the filtrate after crystallization in the fridge. PyAHþ Hydrogencarbonate. MS (CI) m/z (ESIþ) 363.7 (MþHþ); 1H NMR (600 MHz, CDCl3) δ 11.18 (sbr, 2H, Hacid), 8.42 (d, J = 9.1 Hz, 1H, Hpyrene), 8.15-8.06 (m, 4H, Hpyrene), 8.00-7.93 (m, 4H, Hpyrene), 5.39 (s, 2H, HCH2), 3.57 (s, 3H, HCH3), 3.10 (s, 3H, HCH3), 2.03 (s, 3H, HCH3); 13C NMR (150 MHz, CDCl3) δ 164.5, 131.3/131.2, 130.7, 129.4, 128.8 (CH), 128.3, 127.8 (CH), 127.3 (CH), 126.3 (CH), 125.5 (CH), 125.7 (CH), 125.0 (CH), 124.6, 122.4 (CH), 46.0 (CH2), 42.1 (CH3), 41.5 (CH3), 15.9 (CH3). PyA. MS (CI) m/z (ESIþ) 301.4 (MþHþ); 1H NMR (600 MHz, CDCl3) δ 8.42 (d, J = 9.4 Hz, 1H, Hpyrene), 8.17-8.10 (m, 4H, Hpyrene), 8.04-7.96 (m, 4H, Hpyrene), 5.21 (s, 2H, HCH2), 3.03 (s, 6H, HCH3), 2.03 (s, 3H, HCH3); 13C NMR (150 MHz, CDCl3) δ 16668 DOI: 10.1021/la103519t

136.6, 131.6, 131.1, 130.0, 128.5, 127.8 (CH), 127.1 (CH3), 126.6 (CH), 125.8 (CH), 125.3 (CH), 125.2/125.1, 125.0-124.9 (CH), 123.6 (CH), 51.6 (CH2), 38.4 (CH3), 13.3 (CH3). Sample Preparation. A solution of PyAHþ (5.5  10-4 mol/L) was obtained by dissolving 0.6 mg of PyAHþ hydrogencarbonate in 3 mL of CO2 saturated H2O. In particular, this solution of PyAHþ was diluted 10 times before it was used in UV-vis spectrum experiments. To prepare an aqueous dispersion of SWNTs, an aliquot of 0.6 mg of SWNTs was added into 3 mL of the prepared solution of PyAHþ, followed by sonication for 1 h at room temperature. The resultant suspension was then centrifuged at 10 000 rpm for 10 min to give a homogeneous dispersion of SWNTs. For thermal gravimetric analysis (TGA), a 50 mL dispersion of SWNTs was filtered through a microporous membrane (0.22 μm). PyAHþ-SWNTs left on the membrane were then washed by CO2 saturated H2O and vacuum-dried for 12 h. Due to removal of CO2 and H2O during vacuum drying, the sample we finally obtained was actually PyA-SWNTs. To prepare samples for atomic force microscopy (AFM) experiments, a driblet of the dispersion of SWNTs was dropped onto a freshly cleaved mica substrate, kept on the substrate for 5 min, and blotted. Then the substrate was slightly flushed with CO2 saturated H2O and dried with N2 gas. Characterization. NMR spectra were acquired on commercial instruments (Bruker Avance 600 MHz), and chemical shifts (δ) were reported in parts per million (ppm) referenced to tetramethylsilane (TMS) (1H) or the carbon signal of deuterated solvents (13C). Detailed 13C NMR peak assignments were obtained by careful analysis of DEPT, HMQC, and HMBC NMR spectra. Mass spectra were run using a HP5989A apparatus (EI, 70 eV ionization energy) with an Apollo 300 data system, a Kratos MS50TC instrument for exact mass measurements (performed in the EI mode at a resolution of 10000), a Micromass Quattro II apparatus (electrospray ionization (ESI) with MASSLYNX data system), or a Thermo Finnigan LCQ Advantage apparatus (APCI/ ESI, solvent CH3CN). UV spectra were obtained on a Hitachi U-3010 spectrophotometer. Fluorescence spectroscopy was performed on a Hitachi F-7000 spectrophotometer. ζ-potential measurement was carried out on a Malvern Nano ZS90 zetasizer. AFM images were taken on a commercial instrument Nanoscope IV instrument (Veeco Company) with Si cantilevers (200-300 kHz, Veeco Company). Experiments were conducted in air tapping mode at room temperature. TGA was conducted on a Mettler-Toledo TG/SDTA 851 thermal analyzer. Samples were heated in flowing N2 (50 mL/min) from room temperature to 750 °C at a rate of 10 °C/min. Langmuir 2010, 26(22), 16667–16671

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Figure 1. UV absorbance of the aqueous phase of the mixture consisting of H2O, CH2Cl2, and PyAHþ/PyA as (a) Ar or (b) CO2 was bubbled into the mixture.

Figure 2. Pictures of (a) aqueous solution of PyAHþ and (b) dispersion of SWNTs.

Results and Discussion The synthesized dispersant PyAHþ is responsive to CO2.29,30 It can reversibly convert into its neutral form PyA in response to removal of CO2 in aqueous solution (Scheme 2), which can be evidenced by UV spectroscopy. As a cation, PyAHþ has a higher solubility in H2O than in CH2Cl2, while neutral PyA prefers to be dissolved in CH2Cl2. So if PyAHþ is dissolved in a CO2 saturated mixture of H2O and CH2Cl2, most of the PyAHþ will be in the aqueous phase. And when Ar is bubbled into such a mixture to remove the CO2, PyAHþ will transform into PyA and therefore transfer from the H2O to the CH2Cl2 phase, which will lead to a decrease of pyrene groups in the aqueous phase. Based on that, UV spectroscopy was employed to follow the absorbance of the aqueous phase. In our experiment, 2.5 mL of CH2Cl2 was added into 2.5 mL of aqueous solution of PyAHþ. Then CO2 was bubbled into the mixture for 15 min to ensure an equilibrium distribution of PyAHþ in the two phases. After that, Ar was bubbled into the mixture, and UV spectra of the aqueous phase were measured at certain time intervals. As is shown in Figure 1a, characteristic peaks of pyrene exhibited lower and lower intensity as time was extended, indicating the conversion of PyAHþ into PyA. It took about 40 min Langmuir 2010, 26(22), 16667–16671

Figure 3. AFM image of dispersed SWNTs.

before the absorbance intensity reached a minimum. After that, we introduced CO2 into the mixture to study the reversibility of the conversion discussed above. According to Figure 1b, absorbance of the aqueous phase gradually mounted up as CO2 was bubbled into the mixture because the PyA in CH2Cl2 went back to H2O in the form of PyAHþ. Note that the reverse process proceeded much more quickly. It took only 4 min to reach equilibrium. SWNTs have successfully been dispersed in water using PyAHþ as a dispersant. To get an aqueous dispersion of SWNTs, an excess amount of SWNTs was added to a prepared aqueous solution of PyAHþ (Figure 2a), followed by sonication of the mixture for 1 h. The resultant suspension was then centrifuged to give a homogeneous dispersion of SWNTs (Figure 2b). The obtained dispersion can stand stable in a sealed vial for more than 2 weeks. AFM experiments gave clear images of both individually dispersed SWNTs and small SWNT bundles, which were 1-5 nm in diameter and hundreds of nanometers to several micrometers in length (Figure 3). Dispersion of SWNTs results from the formation of a PyAHþSWNT complex. SWNTs tend to aggregate into bundles because of the strong van der Waals interactions between them. However, when PyAHþ is modified onto SWNTs through π-π interaction, DOI: 10.1021/la103519t

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Figure 4. Fluorescence emission of aqueous solution of PyAHþ

(black, 5.5  10-4 mol/L) and dispersion of PyAHþ-SWNTs (red).

Figure 6. Pictures of (a) suspension with precipitates of aggregated SWNTs and (b) solution of SWNTs that were dispersed for the second time.

Figure 5. TGA data for original untreated SWNTs (black) and PyA-SWNTs (red).

the SWNT surfaces will get positively charged. The consequent electrostatic repulsion between SWNTs will counteract the van der Waals interaction and thus facilitate the dispersion of SWNTs. To confirm the complexation between SWNTs and PyAHþ, fluorescence spectroscopy was employed to compare the difference in fluorescence emission between the solution of PyAHþ and the aqueous dispersion of SWNTs. As is shown by Figure 4, a solution of PyAHþ displayed characteristic fluorescence emission of pyrene groups with high intensity. However, after SWNTs were dispersed in that solution, fluorescence of the resultant dispersion was substantially quenched. As was previously reported, pyrene would suffer fluorescence quenching when it was bound to SWNTs through π-π stacking due to the energy transfer from pyrene to SWNTs.31,32 Since the only difference between the solution of PyAHþ and the dispersion of SWNTs is the introduction of SWNTs, the quenching should be attributed to the complexation of the pyrene group of PyAHþ and SWNTs. Moreover, we also employed TGA to see whether PyAHþ was modified onto SWNTs and to calculate the content of PyAHþ in the complex of PyAHþ-SWNTs. For this purpose, a sample was prepared by filtration of the dispersion of SWNTs followed by (31) Qu, L.; Martin, R. B.; Huang, W.; Fu, K.; Zweifel, D.; Lin, Y.; Sun, Y.-P.; Bunker, C. E.; Harruff, B. A.; Gord, J. R.; Allard, L. F. J. Chem. Phys. 2002, 117, 8089. (32) Tomonari, Y.; Murakami, H.; Nakashima, N. Chem.—Eur. J. 2006, 12, 4027.

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vacuum drying. The latter treatment resulted in the removal of CO2 and H2O, giving a sample consisting of PyA-SWNTs. Another sample of the original untreated SWNTs was also studied in a control experiment. Thermal gravimetric data of both samples are shown in Figure 5. Compared to the original SWNTs, the prepared sample of PyA-SWNTs underwent more weight loss as temperature went up. It lost 83.6% of its initial weight by 650 °C while the original SWNTs lost 80.2%. The extra weight loss of PyA-SWNTs confirms the existence of the organic component (PyA) in it, which is additional evidence of the complexation between PyAHþ and SWNTs. According to the weight loss, the content of PyA in the PyA-SWNT complex is calculated to be 17.2%. In addition, the content of PyAHþ in PyAHþ-SWNTs is also 17.2% due to little difference between PyA and PyAHþ in molecular weight. In addition, ζ-potential measurements were conducted to characterize the surface charge of the PyAHþ-SWNT complex. The ζ-potential reflects the magnitude of the electrostatic repulsion between the PyAHþ-SWNTs and thus can indicate the stability of the dispersion of SWNTs.33,34 Measurement on the aqueous dispersion of SWNTs gave a ζ-potential of 55.1 mV, which is a medium value in agreement with the stability of the dispersion. However, the ζ-potential of the dispersion may not be equal to the ζ-potential of the PyAHþ-SWNT complex. Since PyAHþ is a surfactant, aggregates of PyAHþ may exist in the dispersion of SWNTs, which could also contribute to the ζ-potential value of the dispersion. ζ-potential measurement on a solution of PyAHþ showed that the ζ-potential of the aggregates formed by PyAHþ was as high as 60.4 mV. Therefore, to exclude that possibility, the PyAHþ-SWNTs in the dispersion were filtered out using a microporous membrane (0.22 μm), and the ζ-potential of the filtrate was measured. The result was only 4.91 mV, indicating that there were no aggregates of PyAHþ in the filtrate or in the dispersion of PyAHþ-SWNTs. Based on that, we can conclude that the ζ-potential of the PyAHþ-SWNT complex is 55.1 mV, which reflects medium stability of the dispersion we prepared. (33) White, B.; Banerjee, S.; O’Brien, S.; Turro, N. J.; Herman, I. P. J. Phys. Chem. C 2007, 111, 13684. (34) Sun, Z.; Nicolosi, V.; Rickard, D.; Bergin, S. D.; Aherne, D.; Coleman, J. N. J. Phys. Chem. C 2008, 112, 10692.

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positive surface charge of the SWNTs. Then CO2 was introduced into the obtained suspension for 20 min. After subsequent sonication of the suspension, the precipitated SWNTs were redispersed in water (Figure 6b). AFM images (Figure 7) showed that the SWNTs were again well dispersed, with similar size and shape to the initially dispersed SWNTs. Moreover, the ζ-potential of the redispersed SWNTs was also measured. The result, 54.7 mV, was very close to the 55.1 mV of the initially dispersed SWNTs, which points to nearly full recovery of the positive charge on the surface of the SWNTs. Those similarities in size and ζ-potential between the initially dispersed and redispersed SWNTs indicate good reversibility of the aggregation process, which can also be supported by UV-vis spectroscopy. Moreover, the dispersionaggregation-redispersion cycle can be repeated at least 10 times without observable changes.

Conclusion

Figure 7. AFM image of redispersed SWNTs.

After we dispersed SWNTs in water with PyAHþ, further experiments were carried out to achieve the gas controlled aggregation and dispersion of SWNTs. On the basis of the reversible interconversions between PyAHþ and PyA, the surface charge of the modified SWNTs can be altered by gas bubbling. First, Ar was bubbled into the dispersion of SWNTs to remove CO2. About 40 min later, the original dark color of the dispersion faded, and precipitates of aggregated SWNTs appeared (Figure 6a). The aggregation was also supported by scanning electron microscope (SEM) observation (data not shown). The aggregation took place because removal of CO2 resulted in the transformation of PyAHþ into PyA on the SWNTs and thus the elimination of the

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In summary, we have successfully synthesized a CO2-responsive dispersant PyAHþ and used it for dispersion of SWNTs in water. More interestingly, taking advantage of the gas triggered interconversions between PyAHþ and PyA, the solubility of SWNTs can be reversibly controlled in water just through bubbling CO2 and Ar. This work may contribute to the development of SWNT based switching devices and gas responsive materials. Acknowledgment. This work was supported by the National Basic Research Program (2007CB808000), National Natural Science Foundation of China (50973051, 20974059), Tsinghua University Initiative Scientific Research Program (2009THZ02230), and the Bilateral Grant BIL 09/08 between Belgium and China. The authors thank the IWT (Institute for the Promotion of Innovation through Science and Technology in Flanders) and the FWO (Fund for Scientific Research-Flanders) for a doctoral fellowship and financial support to T.H.N.

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