DOI: 10.1021/la804235e 8639 Langmuir 2009, 25(15), 8639–8648 Published on Web 04/01/2009 pubs.acs.org/Langmuir © 2009 American Chemical Society Organogelation and Hydrogelation of Low-Molecular-Weight Amphiphilic Dipeptides: pH Responsiveness in Phase-Selective Gelation and Dye Removal † Tanmoy Kar, Sisir Debnath, Dibyendu Das, Anshupriya Shome, and Prasanta Kumar Das* Department of Biological Chemistry, Indian Association for the Cultivation of Science Jadavpur, Kolkata - 700 032, India Received December 23, 2008. Revised Manuscript Received February 24, 2009 The search for efficient low-molecular-weight gelators (LMWGs) with possible structure-activity correlation is on the rise. The present work reports a novel set of amphiphilic dipeptide-based carboxylic acids capable of efficiently gelating organic solvents. More interestingly, their sodium salts showed enhanced efficiency in organogelation with the additional ability to gelate water. Electrostatic interactions present in the aggregation of the sodium carboxylates of amphiphilic dipeptides seem to be important because some of the nongelator carboxylic acids turned out to be excellent gelators upon salt formation. The combinations and sequence of the amino acids in the dipeptide moiety were systematically altered to understand the collective importance of the nonpolar aliphatic/aromatic substitution in amino acids in the self-assembling behavior of amphiphiles. Almost a 20-fold enhancement in the gelation ability was observed on reversing the sequence of the amino acid residues, and in some cases, nongelators were transformed to efficient gelators. Spectroscopic and microscopic studies of these thermoreversible organo/hydrogels revealed that balanced participation of the noncovalent interactions including hydrogen bonding and van der Waals interactions are crucial for organo/hydrogelation. These dipeptides selectively gelate organic solvents from their mixtures with water, and the xerogels prepared from these organogels showed time-dependent adsorption of dyes such as crystal violet. The most remarkable feature of these gelators is the pH responsiveness, which was aptly utilized for the pH-dependent phase- selective gelation of either solvent in a biphasic mixture of oil and water. The dissimilar gelation ability of the acid and its salt originating from the pH responsiveness of the amphiphilic dipeptide was employed in the instant removal of large amounts of dyes for wastewater treatment. Introduction Gels, a class of soft materials, are gaining a huge amount of interest owing to their versatile applications in fields such as drug delivery, tissue engineering, cosmetics, optical sensors, templated materials, enzyme-immobilization matrices, and so on. 1-4 This surge has amplified the need for the rational design and synthesis of molecules having versatile gelation ability. In this regard, low- molecular-weight gelators (LMWGs) have become very impor- tant as a result of their supramolecular 3D networks that immobilize a variety of solvents. 5-14 Noncovalent interactions † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue. *To whom correspondence should be addressed. Fax: +(91)-33-24732805. E-mail: bcpkd@iacs.res.in. (1) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (b) George, M.; Weiss, R. G. Acc. Chem. Res. 2006, 39, 489. (c) Mitra, R. N.; Shome, A.; Paul, P.; Das, P. K. Org. Biomol. Chem. 2009, 7, 94. (2) (a) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821. (b) Jung, J. H.; Kobayashi, H.; Masuda, M.; Shimizu, T.; Shinkai, S. J. Am. Chem. Soc. 2001, 123, 8785. (c) Sone, E. D.; Zubarev, E. R.; Stupp, S. I. Angew. Chem., Int. Ed. 2002, 41, 1705. (d) Mitra, R. N.; Das, P. K. J. Phys. Chem. C 2008, 112, 8159. (3) (a) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shinkai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550. (b) Kobayashi, S.; Hanabusa, K.; Hamasaki, N.; Kimura, M.; Shirai, H.; Shinkai, S. Chem. Mater. 2000, 12, 1523. (c) Schoonbeek, F. S.; van Esch, J. H.; Wagewijs, B.; Rep, D. B. A.; de Haas, M. P.; Klapwijk, T. M.; Kellog, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 1999, 38, 1393. (d) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (e) Bhattacharya, S.; Ghosh, Y. K. Chem. Commun. 2001, 185. (4) (a) Bieser, A. M.; Tiller, J. C. J. Phys. Chem. B 2007, 111, 13180. (b) Bieser, A. M.; Tiller, J. C. Supramol. Chem. 2008, 20, 363. (c) Mohmeyer, N.; Wang, P.; Schmidt, H. W.; Zakeeruddin, S. M.; Graetzel, M. J. Mater. Chem. 2004, 14, 1905. (d) Kameta, N.; Minamikawa, H.; Masuda, M.; Mizuno, G.; Shimizu, T. Soft Matter 2008, 4, 1681. (e) Jung, J. H.; Rim, J. A.; Cho, E. J.; Lee, S. J.; Jeong, Y.II; Kameda, N.; Masuda, M.; Shimizu, T. Tetrahedron 2007, 63, 7449. (5) (a) Estroff, L. A.; Hamilton, A. D. Angew. Chem., Int. Ed. 2000, 39, 3447. (b) Hanabusa, K.; Itoh, A.; Kimura, M.; Shirai, H. Chem. Lett. 1999, 767. (c) Mitra, R. N.; Das, D.; Roy, S.; Das, P. K. J. Phys. Chem. B 2007, 111, 14107. (d) Luo, X.; Lin, B.; Liang, Y. Chem. Commun. 2001, 1556. (e) Dasgupta, A.; Mitra, R. N.; Roy, S.; Das, P. K. Chem. Asian J. 2006, 1, 780. (6) (a) Stock, H. T.; Turner, N. J.; McCague, R. J. Chem. Soc., Chem. Commun. 1995, 2063. (b) Roy, S.; Dasgupta, A.; Das, P. K. Langmuir 2007, 23, 11769. (c) Suzuki, M.; Owa, S.; Kimura, M.; Kurose, A.; Shiraib, H.; Hanabusa, K. Tetrahedron Lett. 2005, 46, 303. (d) Roy, S.; Das, D.; Dasgupta, A.; Mitra, R. N.; Das, P. K. Langmuir 2005, 21, 10398. (e) Roy, S.; Das, P. K. Biotechnol. Bioeng. 2008, 100, 756. (7) (a) Friggeri, A.; Gronwald, O.; van Bommel, K. J. C.; Shinkai, S.; Reinhoudt, D. N. J. Am. Chem. Soc. 2002, 124, 10754. (b) Kiyonaka, S.; Shinkai, S.; Hamachi, I. Chem. Eur. J. 2003, 9, 976. (8) (a) Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (b) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664. (9) (a) de Loos, M.; van Esch, J. H.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 2001, 40, 613. (b) George, M.; Weiss, R. G. Langmuir 2002, 18, 7124. (c) van Gorp, J. J.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 2002, 124, 14759. (d) Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148. (e) Wang, G.; Hamilton, A. D. Chem. Eur. J. 2002, 8, 1954. (f) Mohmeyer, N.; Kuang, D.; Wang, P.; Schmidt, H. W.; Zakeeruddin, S. M.; Gratzel, M. J. Mater. Chem. 2006, 16, 2978. (g) Bieser, A. M.; Tiller, J. C. Chem. Commun. 2005, 3942. (10) (a) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201. (b) Lee, K. Y.; Mooney, D. J. Chem. Rev. 2001, 101, 1869. (c) Tiller, J. C. Angew. Chem., Int. Ed. 2003, 42, 3072. (d) van Bommel, K. J. C.; van der Pol, C.; Muizebelt, I.; Friggeri, A.; Meetsma, A.; Feringa, B. L.; van Esch, J. Angew. Chem., Int. Ed. 2004, 43, 1663. (e) Bhattacharya, S.; Srivastava, A.; Pal, A. Angew. Chem., Int. Ed. 2006, 45, 2934. (f) Vemula, P. K.; John, G. Chem. Commun. 2006, 2218. (11) (a) von Lipowitz, A. Ann. Chem. Pharm. 1841, 38, 348. (b) Gortner, R. A.; Hoffman, W. F. J. Am. Chem. Soc. 1921, 43, 2199. (c) K :: ohler, K.; F :: orster, G.; Hauser, A.; Dobner, B.; Heiser, U. F.; Ziethe, F.; Richter, W.; Steiniger, F.; Drechsler, M.; Stettin, H.; Blume, A. J. Am. Chem. Soc. 2004, 126, 16804. (d) Debnath, S.; Shome, A.; Dutta, S.; Das, P. K. Chem. Eur. J. 2008, 14, 6870. (e) Herres, A.; van der Pol, C.; Stuart, M.; Friggeri, A.; Feringa, B. L.; van Esch, J. J. Am. Chem. Soc. 2003, 125, 14252. (f) Estroff, L. A.; Leiserowitz, L.; Addadi, L.; Weiner, S.; Hamilton, A. D. Adv. Mater. 2003, 15, 38. (12) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263, and references therein. (13) (a) Yang, Z.; Xu, B. Chem. Commun. 2004, 2424. (b) Kohler, K.; Forster, G.; Hauser, A.; Dobner, B.; Heiser, U. F.; Ziethe, F.; Richter, W.; Steiniger, F.; Drechsler, M.; Stettin, H.; Blume, A. Angew. Chem., Int. Ed. 2004, 43, 245. (c) Wang, G.; Hamilton, A. D. Chem. Commun. 2003, 310. (d) Friggeri, A.; Feringa, B. L.; van Esch, J. J. Controlled Release 2004, 97, 241. (e) John, G.; Vemula, P. K. Soft Matter 2006, 2, 909. (14) (a) Xing, B.; Yu, C. W.; Chow, K. H.; Ho, P. L.; Fu, D.; Xu, B. J. Am. Chem. Soc. 2002, 124, 14846. (b) Shome, A.; Debnath, S.; Das, P. K. Langmuir 2008, 24, 4280.