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Influence of gelator structures on formation and stability of supramolecular hydrogels
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Chinese Chemical Letters 18 (2007) 1548–1550 www.elsevier.com/locate/cclet
Influence of gelator structures on formation and stability of supramolecular hydrogels Shunji Kono a,b, Yu Jiang Wang a, Li Ming Tang a,* a b
Laboratory of Advanced Materials, Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China Department of Organic & Polymeric Materials, Graduate School of Science & Engineering, Tokyo Institute of Technology, 2-12-1-H120, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 23 July 2007
Abstract A supramolecular hydrogel (defined as G1) formed from 1,2,4,5-benzene tetracarboxylic acid (BTCA) and 2-amino-3hydroxypyridine possessed higher Tgel than that of another hydrogel (defined as G2) formed from BTCA and 3-hydroxypyridine. Based on the analysis of their xerogels by 1H NMR, IR and XRD, the higher stability of G1 was attributed to the formation of stronger hydrogen binding enhanced by the ortho amino group of 2-amino-3-hydroxypyridine. # 2007 Li Ming Tang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Supramolecular hydrogel; Gelation; Hydrogen bond
Supramolecular hydrogels have received considerably attention these years due to their academic interests and many potential applications in cosmetics, medical science, tissue engineering and so on [1–8]. These gels possess the similar properties as polymer gels but with much stronger temperature dependent characters. Due to the capability to form stable hydrogen bond, carboxyl and pyridyl contained compounds have been designed as new gelators for such gels [9–12]. For example, a hydrogel based on 1,2,4,5-benzenetetracarboxylic acid (BTCA) and 4-hydroxypyridine displays many unique performances, including high gel stability [9]. To understand the correlation between gelator structure and gel property, in this paper, we made an investigation on two similar hydrogels, one (G1) is based on BTCA and 2-amino-3-hydroxy pyridine and the other (G2) is based on BTCA and 3-hydroxypyridine. With G1 as an example, the gel was prepared by two steps. Firstly, suitable amount of BTCA and 2-amino-3hydroxypyridine were reacted in DMSO at 90 8C for 1 h. After purification, a white powder-like gelator was obtained in 85% yield. Secondly, the gel was formed spontaneously by cooling the hot aqueous solution of the gelator at room temperature. After drying the gel under vacuum at 25 8C, the remaining dried gel(xerogel) was obtained as small sized fibers. By the similar procedure, hydrogel G2 and its xerogel were also prepared. To understand the actual composition, the xerogels were directly analyzed by 1H NMR [13]. From the results in Fig. 1, the ratio of BTCA and the pyridyl compound in the xerogel can be calculated based on the intensities of the corresponding proton peaks. From the result in Table 1, the ratio of BTCA and the pyridyl compound is about the same for G1 and G2. It indicates that one BTCA molecule connects with two molecules of the pyridyl compounds in the
* Corresponding author. E-mail address: [email protected] (L.M. Tang). 1001-8417/$ – see front matter # 2007 Li Ming Tang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2007.10.025
S. Kono et al. / Chinese Chemical Letters 18 (2007) 1548–1550
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Fig. 1. 1H NMR spectra of G1, G2 and BTCA measured in DMSO-d6.
Table 1 Influence of assembling conditions on formation of hydrogels Gel
Ratio of BTCA and pyridyl compounda
Gelation timeb (min.)
MGCb (mg/mL)
Maximal Tgel (8C)
G1 G2
1/2.1 1/2.0
7 45
17 18
65 36
a b
Calculated from 1H NMR. Gelation time and MGC were both recorded at concentration of 20 mg/mL.
xerogels. In addition, the BTCA aromatic protons of G1 and G2 show distinct chemical shift to lower magnetic field compared with those of BTCA, i.e., the peak shifts from 7.93 ppm (for BTCA) to 8.48 ppm (for G1) and 8.07 ppm (for G2). Once the carboxyl group forms salt with a proton, the electron cloud density of the aromatic ring decreases and the deshielding effect makes the chemical shift moving to the lower magnetic field. In G1, the carboxyl group can form two of hydrogen bonds with both the pyridyl and amino groups. But for G2, only one of hydrogen bond is formed from carboxyl and pyridyl groups. Therefore the aromatic proton peak of G1 shifts to lower magnetic field than that of G2. To understand the hydrogen bonds formed in different gels, the IR spectra of the two xerogels are compared in Fig. 2. For G1 sample, the strong peak presented at 3371 cm 1 indicates the formation of hydrogen bond between NH2 and –COOH [14]. The broadened peak at 2800 cm 1 indicates the formation of hydrogen bond between -OH and –COOH [9]. Both of the two peaks at 2500 and 1900 cm 1 indicate the formation of hydrogen bond between -COOH and pyridyl groups [9,12,15,16]. Therefore there are a few kinds of hydrogen bonds in the xerogel. But for G2 sample, there is no peak at 3371 cm 1 and the peak at 1900 cm 1 is almost not observed. These results further prove that the formation of hydrogen bond is enhanced by using 2-amino-3-hydroxy pyridine rather than 3-hydroxy pyridine.
Fig. 2. IR spectra of G1 and G2.
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S. Kono et al. / Chinese Chemical Letters 18 (2007) 1548–1550
Fig. 3. X-ray diffraction diagrams for G1 xerogel by Bruker D8 (Advance), and the calculated length of G1 repeating unit by MOPAC program in Chem3D.
Based on the above results, a possible connection fashion for G1 was proposed in Fig. 3. The X-ray powder diffraction (XRD) spectrum of the xerogel of Gl is also shown in Fig. 3. There is the smallest angle peak at 7.748 (2u) corresponding to a d spacing of 1.15 nm, and such d spacing shows good agreement with the calculated length (1.14 nm) of the two molecules. The peak at 168 (2u), which corresponds to a d spacing of 0.55 nm, represents the p-p conjugated effect between aromatic rings. According to our recent article, phenolic hydroxyl group can form hydrogen bond with carboxyl group [9]. Therefore under gelation condition, the building units self-assemble into fibrous backbones driving by such force. The exterior hydrophilic groups of the aggregating fibers provide attraction to water molecules and facilitate the formation of the gel [12]. The gel performances of the two gels are summarized in Table 1. It is noticed that G1 displays much shorter gelation time, about the same minimal gelation concentration (MGC) and much higher maximal Tgel compared with G2. From the structural results, the higher stability of G1 can be attributed to the formation of more stable hydrogen bond by using 2-amino-3-hydroxy pyridine as the building component. In summary, this research indicates that the gel stability can be improved via adjusting the structure of the building units, which is helpful for designing new hydrogels. Acknowledgment The financial support from the National Natural Science Foundation of China (No. 20574041) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
N.A. Peppas, Y. Huang, M. Tottes-Lugo, J.H. Ward, J. Zhang, Annu. Rev. Biomed. Eng. 2 (2000) 9. B. Xing, C.W. Yu, K.H. Chow, P.L. Ho, D. Fu, B. Xu, J. Am. Chem. Soc. 124 (2002) 14846. J.C. Tiller, Angew. Chem., Int. Ed. 42 (2003) 3072. K.E. Uhrich, S.M. Cannizzaro, R.S. Langer, K.M. Shakesheff, Chem. Rev. 99 (1999) 3181. M. Suzuki, T. Sato, A. Kurose, H. Shirai, K. Hanabusa, Tetrahedron Lett. 46 (2005) 2741. L.A. Estroff, A.D. Hamilton, Chem. Rev. 104 (2004) 1201. K.Y. Lee, D.J. Mooney, Chem. Rev. 101 (2001) 1869. S.V. Vinogradov, T.K. Bronich, A.V. Kabanov, Adv. Drug Deliv. Rev. 54 (2002) 135. Y.J. Wang, L.M. Tang, Y. Wang, Chem. Lett. 35 (2006) 548. P. Dastidar, S. Okabe, K. Nakano, K. Iida, M. Miyata, N. Tohnai, M. Shibayama, J. Chem. Mater. 17 (2005) 741. D.R. Trivedi, A. Ballabh, J. Mater. Chem. 15 (2005) 2606. J.W. Wu, L.M. Tang, K. Chen, L. Yan, F. Li, Y.J. Wang, J. Colloid Interf. Sci. 30 (2007) 280. Data of G1: 1H-NMR (d/ppm): 6.60 (t, 1H, 5-pyridyl H), 6.20-7.60 (b, 2H, amino H), 7.10 (d, 1H, 3-pylidil H), 7.40 (d, 1H, 5-pyridyl H), 8.50 (s, 2H Ar H). IR (KBr, g, cm-1): 3371, 3305, 3100-2500, 1886, 1674, 1527 and 1357. Data of G2: 1H-NMR (d/ppm): 14.40 (b, 1H, -COOH), 10.30 (b, 1H, -OH), 8.04 (s, 2H, BTA Ar–H), 8.15 (s, 2H, 2-pyridyl 2-hydroxy H), 7.27-7.29 (m, 2H, 4-pyridyl H), 7.23-7.25 (m, 2H, 3-pyridyl H), 8.05-8.06 (t, 2H, 2-pyridyl 4-hydroxy H). FT-IR (KBr, cm-1): 3315, 3259, 2505, 1915, 1695. [14] V.E. Borisenko, E.E. Kolmakov, A. Koll, A.G. Rjasnyi, J. Mol. Struct. 828 (2007) 116. [15] S.L. Johnson, K.A. Rumon, J. Phys. Chem. 69 (1965) 74. [16] C.B. St. Pourcain, A.C. Griffin, Macromolecules 28 (1995) 4116.
Chinese Chemical Letters 18 (2007) 1548–1550 www.elsevier.com/locate/cclet
Influence of gelator structures on formation and stability of supramolecular hydrogels Shunji Kono a,b, Yu Jiang Wang a, Li Ming Tang a,* a b
Laboratory of Advanced Materials, Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China Department of Organic & Polymeric Materials, Graduate School of Science & Engineering, Tokyo Institute of Technology, 2-12-1-H120, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 23 July 2007
Abstract A supramolecular hydrogel (defined as G1) formed from 1,2,4,5-benzene tetracarboxylic acid (BTCA) and 2-amino-3hydroxypyridine possessed higher Tgel than that of another hydrogel (defined as G2) formed from BTCA and 3-hydroxypyridine. Based on the analysis of their xerogels by 1H NMR, IR and XRD, the higher stability of G1 was attributed to the formation of stronger hydrogen binding enhanced by the ortho amino group of 2-amino-3-hydroxypyridine. # 2007 Li Ming Tang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Supramolecular hydrogel; Gelation; Hydrogen bond
Supramolecular hydrogels have received considerably attention these years due to their academic interests and many potential applications in cosmetics, medical science, tissue engineering and so on [1–8]. These gels possess the similar properties as polymer gels but with much stronger temperature dependent characters. Due to the capability to form stable hydrogen bond, carboxyl and pyridyl contained compounds have been designed as new gelators for such gels [9–12]. For example, a hydrogel based on 1,2,4,5-benzenetetracarboxylic acid (BTCA) and 4-hydroxypyridine displays many unique performances, including high gel stability [9]. To understand the correlation between gelator structure and gel property, in this paper, we made an investigation on two similar hydrogels, one (G1) is based on BTCA and 2-amino-3-hydroxy pyridine and the other (G2) is based on BTCA and 3-hydroxypyridine. With G1 as an example, the gel was prepared by two steps. Firstly, suitable amount of BTCA and 2-amino-3hydroxypyridine were reacted in DMSO at 90 8C for 1 h. After purification, a white powder-like gelator was obtained in 85% yield. Secondly, the gel was formed spontaneously by cooling the hot aqueous solution of the gelator at room temperature. After drying the gel under vacuum at 25 8C, the remaining dried gel(xerogel) was obtained as small sized fibers. By the similar procedure, hydrogel G2 and its xerogel were also prepared. To understand the actual composition, the xerogels were directly analyzed by 1H NMR [13]. From the results in Fig. 1, the ratio of BTCA and the pyridyl compound in the xerogel can be calculated based on the intensities of the corresponding proton peaks. From the result in Table 1, the ratio of BTCA and the pyridyl compound is about the same for G1 and G2. It indicates that one BTCA molecule connects with two molecules of the pyridyl compounds in the
* Corresponding author. E-mail address: [email protected] (L.M. Tang). 1001-8417/$ – see front matter # 2007 Li Ming Tang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2007.10.025
S. Kono et al. / Chinese Chemical Letters 18 (2007) 1548–1550
1549
Fig. 1. 1H NMR spectra of G1, G2 and BTCA measured in DMSO-d6.
Table 1 Influence of assembling conditions on formation of hydrogels Gel
Ratio of BTCA and pyridyl compounda
Gelation timeb (min.)
MGCb (mg/mL)
Maximal Tgel (8C)
G1 G2
1/2.1 1/2.0
7 45
17 18
65 36
a b
Calculated from 1H NMR. Gelation time and MGC were both recorded at concentration of 20 mg/mL.
xerogels. In addition, the BTCA aromatic protons of G1 and G2 show distinct chemical shift to lower magnetic field compared with those of BTCA, i.e., the peak shifts from 7.93 ppm (for BTCA) to 8.48 ppm (for G1) and 8.07 ppm (for G2). Once the carboxyl group forms salt with a proton, the electron cloud density of the aromatic ring decreases and the deshielding effect makes the chemical shift moving to the lower magnetic field. In G1, the carboxyl group can form two of hydrogen bonds with both the pyridyl and amino groups. But for G2, only one of hydrogen bond is formed from carboxyl and pyridyl groups. Therefore the aromatic proton peak of G1 shifts to lower magnetic field than that of G2. To understand the hydrogen bonds formed in different gels, the IR spectra of the two xerogels are compared in Fig. 2. For G1 sample, the strong peak presented at 3371 cm 1 indicates the formation of hydrogen bond between NH2 and –COOH [14]. The broadened peak at 2800 cm 1 indicates the formation of hydrogen bond between -OH and –COOH [9]. Both of the two peaks at 2500 and 1900 cm 1 indicate the formation of hydrogen bond between -COOH and pyridyl groups [9,12,15,16]. Therefore there are a few kinds of hydrogen bonds in the xerogel. But for G2 sample, there is no peak at 3371 cm 1 and the peak at 1900 cm 1 is almost not observed. These results further prove that the formation of hydrogen bond is enhanced by using 2-amino-3-hydroxy pyridine rather than 3-hydroxy pyridine.
Fig. 2. IR spectra of G1 and G2.
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S. Kono et al. / Chinese Chemical Letters 18 (2007) 1548–1550
Fig. 3. X-ray diffraction diagrams for G1 xerogel by Bruker D8 (Advance), and the calculated length of G1 repeating unit by MOPAC program in Chem3D.
Based on the above results, a possible connection fashion for G1 was proposed in Fig. 3. The X-ray powder diffraction (XRD) spectrum of the xerogel of Gl is also shown in Fig. 3. There is the smallest angle peak at 7.748 (2u) corresponding to a d spacing of 1.15 nm, and such d spacing shows good agreement with the calculated length (1.14 nm) of the two molecules. The peak at 168 (2u), which corresponds to a d spacing of 0.55 nm, represents the p-p conjugated effect between aromatic rings. According to our recent article, phenolic hydroxyl group can form hydrogen bond with carboxyl group [9]. Therefore under gelation condition, the building units self-assemble into fibrous backbones driving by such force. The exterior hydrophilic groups of the aggregating fibers provide attraction to water molecules and facilitate the formation of the gel [12]. The gel performances of the two gels are summarized in Table 1. It is noticed that G1 displays much shorter gelation time, about the same minimal gelation concentration (MGC) and much higher maximal Tgel compared with G2. From the structural results, the higher stability of G1 can be attributed to the formation of more stable hydrogen bond by using 2-amino-3-hydroxy pyridine as the building component. In summary, this research indicates that the gel stability can be improved via adjusting the structure of the building units, which is helpful for designing new hydrogels. Acknowledgment The financial support from the National Natural Science Foundation of China (No. 20574041) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
N.A. Peppas, Y. Huang, M. Tottes-Lugo, J.H. Ward, J. Zhang, Annu. Rev. Biomed. Eng. 2 (2000) 9. B. Xing, C.W. Yu, K.H. Chow, P.L. Ho, D. Fu, B. Xu, J. Am. Chem. Soc. 124 (2002) 14846. J.C. Tiller, Angew. Chem., Int. Ed. 42 (2003) 3072. K.E. Uhrich, S.M. Cannizzaro, R.S. Langer, K.M. Shakesheff, Chem. Rev. 99 (1999) 3181. M. Suzuki, T. Sato, A. Kurose, H. Shirai, K. Hanabusa, Tetrahedron Lett. 46 (2005) 2741. L.A. Estroff, A.D. Hamilton, Chem. Rev. 104 (2004) 1201. K.Y. Lee, D.J. Mooney, Chem. Rev. 101 (2001) 1869. S.V. Vinogradov, T.K. Bronich, A.V. Kabanov, Adv. Drug Deliv. Rev. 54 (2002) 135. Y.J. Wang, L.M. Tang, Y. Wang, Chem. Lett. 35 (2006) 548. P. Dastidar, S. Okabe, K. Nakano, K. Iida, M. Miyata, N. Tohnai, M. Shibayama, J. Chem. Mater. 17 (2005) 741. D.R. Trivedi, A. Ballabh, J. Mater. Chem. 15 (2005) 2606. J.W. Wu, L.M. Tang, K. Chen, L. Yan, F. Li, Y.J. Wang, J. Colloid Interf. Sci. 30 (2007) 280. Data of G1: 1H-NMR (d/ppm): 6.60 (t, 1H, 5-pyridyl H), 6.20-7.60 (b, 2H, amino H), 7.10 (d, 1H, 3-pylidil H), 7.40 (d, 1H, 5-pyridyl H), 8.50 (s, 2H Ar H). IR (KBr, g, cm-1): 3371, 3305, 3100-2500, 1886, 1674, 1527 and 1357. Data of G2: 1H-NMR (d/ppm): 14.40 (b, 1H, -COOH), 10.30 (b, 1H, -OH), 8.04 (s, 2H, BTA Ar–H), 8.15 (s, 2H, 2-pyridyl 2-hydroxy H), 7.27-7.29 (m, 2H, 4-pyridyl H), 7.23-7.25 (m, 2H, 3-pyridyl H), 8.05-8.06 (t, 2H, 2-pyridyl 4-hydroxy H). FT-IR (KBr, cm-1): 3315, 3259, 2505, 1915, 1695. [14] V.E. Borisenko, E.E. Kolmakov, A. Koll, A.G. Rjasnyi, J. Mol. Struct. 828 (2007) 116. [15] S.L. Johnson, K.A. Rumon, J. Phys. Chem. 69 (1965) 74. [16] C.B. St. Pourcain, A.C. Griffin, Macromolecules 28 (1995) 4116.