SELF-ASSEMBLED INVERTED MICELLES PREPARED FROM A DENDRIMER TEMPLATE : PHASE TRANSFER OF ENCAPSULATED GUESTS

Here we describe the spontaneous assembly of fatty acids onto the surface of amine-terminated, poly(amidoamine) (PAMAM) dendrimers (Scheme 1). This process, which is driven by acidbase chemistry and ion pairing, results in the extraction of dendrimers from aqueous solutions to nonpolar phases. Dendrimer-encapsulated guest molecules, such as the hydrophilic dyes and catalytically active metal nanoparticles described here, can thus be easily solubilized in organic solvents. Dendrimers are monodisperse, hyperbranched polymers possessing a very high concentration of surface functional groups.1 Many of the properties of dendrimers, including their solubility, are strongly influenced by the nature of these terminal functionalities.2,3 For example, dendrimers terminated in hydrophobic groups are soluble in nonpolar solvents, while those having hydrophilic groups are soluble in polar solvents such as water and low-molecular-weight alcohols.4-6 Terminal groups are normally covalently bonded to the body of dendrimers, but there have been a few reports of electrostatic binding of charged molecules to the surface of dendrimer polyions.1,7-10 We now demonstrate that complete surface modification of dendrimers can be accomplished by spontaneous, acid-base self-assembly, which eliminates the need for chemical synthesis and purification. A similar approach has previously been reported for solubilization of ionic polymers11,12 and DNA13,14 in organic solvents. The electrostatic self-assembly process we report is reversible, and therefore the dendrimers and whatever guests they may contain can be easily shuttled between hydrophilic and hydrophobic phases by adjustment of the pH of the aqueous phase. Fourth-generation, amine-terminated PAMAM dendrimers (G4NH2) readily dissolve in toluene or heptane containing dodecanoic acid. The amount of G4-NH2 that can be dissolved in 1% dodecanoic acid/toluene corresponds to about 1 dendrimer per 70-80 molecules of acid15 which suggests an approximately 1:1 stoichiometry between the fatty acid and each of the 64 terminal amine groups present on the dendrimers. Transmission FT-IR spectroscopy of this toluene solution (Figure 1) indicates that solubilization is accompanied by proton transfer from the acid to the terminal amine groups of the dendrimers and that ionization is essentially complete. Evidence for this comes from the nearly complete disappearance of the dodecanoic acid carboxyl peak at 1710 cm-1 (part a of Figure 1) and the appearance of the asymmetric carboxylate peak at 1557 cm-1 (part c of Figure 1) upon addition of dendrimer.16 We conclude that the acid molecules arrange themselves around the dendrimer in a composite structure that resembles an inverted micelle having a hydrophilic dendritic interior and a hydrophobic alkyl-chain-dominated exterior that lends solubility to the ensemble (Scheme 1). In the presence of large excesses of dodecanoic acid, proton transfer extends to the tertiary amine groups within the dendrimer interior (see Supporting Information). Dendrimers with covalently grafted hydrophobic terminal groups have previously been shown to dissolve in nonpolar solvents and solubilize encapsulated guest molecules.3,4,6 The present system, however, offers two significant advantages. First, commercially available dendrimers can be used directly to prepare solutions in nonpolar solvents without the need for chemical synthesis or separation. Second, since acid-base interactions are reversible, addition of HCl to the aqueous phase or simple dilution of the organic phase with pure solvent leads to the transfer of G4-NH2 back into aqueous layer. We report here two examples * To whom correspondence should be addressed. Telephone: (409) 8455629. Fax: (409) 845-1399. E-mail: crooks@tamu.edu. (1) For a recent review, see Zeng, F. W.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681. (2) Hawker, C. J.; Chu, F. Macromolecules 1996, 29, 4370. (3) Cooper, A. I.; Londono, J. D.; Wignall, G.; McClain, J. B.; Samulski, E. T.; Lin, J. S.; Dobrynin, A.; Rubinstein, M.; Burke, A. L. C.; Frechet, J. M. J.; DeSimone, J. M. Nature 1997, 389, 368. (4) Jansen, J. F. G. A.; Meijer, E. W. Macromol. Symp. 1996, 27. (5) Schenning, A. P. H. J.; Elissen-Roman, C.; Weener, J.-W.; Baars, M. W. P. L.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199. (6) Sayed-Sweet, Y.; Hedstrand, D. M.; Spinder, R.; Tomalia, D. A. J. Mater. Chem. 1997, 7, 1199. (7) Li, Y.; Dubin, P. L.; Spindler, R.; Tomalia, D. A. Macromolecules 1995, 28, 8426. (8) Jockusch, S.; Turro, N. J.; Tomalia, D. A. Macromolecules 1995, 28, 8, 7416. (9) Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1990, 112, 8515. (10) Watkins, D. M.; Sayed-Sweet, Y.; Klimash, J. W.; Turro, N. J.; Tomalia, D. A. Langmuir 1997, 13, 3136. (11) Kabanov, A. V.; Sergeev, V. G.; Foster, M. S.; Kasaikin, V. A.; Levashov, A. V.; Kabanov, V. A. Macromolecules 1995, 28, 3657. (12) Bakeev, K. N.; Shu, Y. M.; Zezin, A. B.; Kabanov, V. A.; Lezov, A. V.; Mel’nikov, A. B.; Kolomiets, I. P.; Rjumtsev, E. I.; MacKnight, W. J. Macromolecules 1996, 29, 1320. (13) Sergeyev, V. G.; Mikhailenko, S. V.; Pyshkina, O. A.; Yaminsky, I. V.; Yoshikawa, K. J. Am. Chem. Soc. 1999, 121, 1780. (14) Mel’nikov, S. M.; Lindman, B. Langmuir 1999, 15, 1923. (15) This value was calculated from the maximum amount of neat G4NH2 which completely dissolves in 1% dodecanoic acid/toluene mixture after 5 min of sonication. (16) The νa(CO2) peak at 1557 cm-1 overlaps with the amide II band. In a similar system with the PAMAM dendrimer replaced by a poly(propyleneimine) Cascade dendrimer which does not contain any amide bonds, we have clearly observed the appearance of the νa(CO2) peak. Scheme 1 4910 J. Am. Chem. Soc. 1999, 121, 4910-4911