NANO LETTERS
Preparation of Fullerene-Shell Dendrimer-Core Nanoconjugates
2005 Vol. 5, No. 6 1171-1173
Anton W. Jensen,*,† Brijesh S. Maru,† Xi Zhang,† Dillip K. Mohanty,*,† Bradley D. Fahlman,† Douglas R. Swanson,‡ and Donald A. Tomalia*,†,‡ Department of Chemistry, Central Michigan UniVersity, Mount Pleasant, Michigan 48859, and Dendritic NanoTechnologies Inc., 2625 Denison DriVe, Suite B, Mount Pleasant, Michigan 48858 Received February 15, 2005; Revised Manuscript Received March 16, 2005
ABSTRACT Generation 4 amine-terminated polyamidoamine dendrimer (PAMAM G4) was allowed to react with an excess of buckminsterfullerene (C60) to form a nanoconjugate containing a PAMAM core and C60 shell. The PAMAM−C60 conjugate was characterized by MALDI-TOF, TGA, UV−vis, and IR spectroscopy. Approximately thirty shell fullerenes surround each dendrimer core. The conjugates catalyze photooxidation of thioanisole by generation of singlet oxygen (1O2). The oxidation reactions occur in both organic and aqueous solvents, but the reactivity is enhanced in aqueous solution, possibly due to a nanoreactor effect resulting from diffusion of hydrophobic reactant molecules into dendrimer cavities.1
Polyamidoamine (PAMAM) dendrimers and buckminsterfullerene (C60) are two of the most studied molecules of the past several years.2 In this communication we report the preparation of a material containing a PAMAM dendrimer core coated with a shell of C60 molecules (Scheme 1). Numerous PAMAM conjugates have been prepared for a variety of purposes.3 Recent examples include the preparation of PEG- [poly(ethylene glycol)], cyclodextrin-, and antibodydendrimer conjugates for DNA delivery and antibody directed targeting.3 The PAMAM-C60 conjugate is used to generate singlet oxygen (1O2), to carry out oxidation reactions.4 Upon photoexcitation to the singlet excited state, C60 undergoes intersystem crossing to the triplet excited state (3C60). From the triplet state, fullerenes convert ground-state triplet oxygen to 1O2. The quantum yield for this overall process is nearly 1, making fullerenes very efficient photosensitizers for the formation of 1O2.5 Fullerene-containing, heterogeneous, 1O2 producing catalysts have been prepared by adding C60 to the surface of spherical amine-coated polymer beads.6 A PAMAM (G4) dendrimer is essentially a nanobead7 covered with amine groups that has been successfully derivatized with various compounds.3,8 C60, an electrondeficient molecule, reacts readily with amine groups.9 Thus it was possible to attach fullerene molecules to the dendrimer surface (see Scheme 1) by slowly dripping a heterogeneous * Corresponding authors. A. Jensen:
[email protected], 989-7743125; D. K. Mohanty,
[email protected], 989-774-6445; D. A. Tomalia,
[email protected]. † Central Michigan University. ‡ Dendritic NanoTechnologies Inc. 10.1021/nl0502975 CCC: $30.25 Published on Web 05/18/2005
© 2005 American Chemical Society
solution of PAMAM (G4) dendrimer10 (finely dispersed in pyridine, about 40 mg in 200 mL) into a homogeneous solution containing a large excess of C60 in pyridine (700 mL, 0.8 mM). The mixture was then allowed to stir overnight before the removal of the pyridine by vacuum distillation.11 The resulting brownish-black solid was then extracted several times with benzene (∼1.6 L), and the benzene was then evaporated under high vacuum, to yield crude product. The catalyst is further purified by column chromatography, using toluene (to remove unreacted, purple C60) and then with pyridine (to isolate the brownish catalyst). The yield after chromatography was 89%. UV-vis analyses of the catalyst exhibited a change in the absorption spectra relative to underivatized C60 (most notably, the loss of the minimum between 400 and 500 nm). Comparison of IR spectra of the starting material and product shows an increase in the number of carbonyl stretches characteristic of a PAMAM dendrimer core (probably resulting from inhomogeneity of fullerene addition), coupled with a decrease in the absorbances due to the surface primary amine groups. Using the Mansfield-Tomalia-Rakesh equation for parking spheres on spheres,2a,12 the maximum number of C60 molecules that could be arranged around each dendrimer was determined to be about 131 (equation 1; r1/r2 > 1.20; r1 and r2 are the dendrimer and fullerene radii, which are approximately 25 and 5 angstroms respectively).13 This analysis is based only on the radii of the spheres. Nmax ) (2π/31/2)(r1/r2 + 1)2
(1)
However, due to sterics, it is highly unlikely that each amine
Scheme 1. Representative Core-Shell Architecture of the PAMAM-Fullerene Conjugate (Z represents peripheral NH2, NH, groups on the PAMAM surface)
Scheme 2.
Possible Reaction of Multiple PAMAM Amino Groups with One Fullerene
The proposed mechanism involves SET.9
can react with two fullerene molecules. Additionally, primary amines are significantly more reactive with fullerenes than secondary amines.9b Thus, the number of fullerene molecules on each dendrimer surface should not exceed the total number of available primary amine groups, 64 (a number significantly below the 131 molecules predicted by eq 1 above). Also, each C60 can react with more than one amine group (there are six independent pyracylene units per C60 molecule). Therefore, it is likely that once one fullerene binds to the surface of a dendrimer, it can react with at least one adjacent amine group (through an intramolecular addition) prior to the intermolecular addition of another C60 molecule (see Scheme 2). This reaction further depletes the number of fullerenes that could add to the surface. The average molecular weight of the product was determined by MALDI-TOF to be approximately 35700 ( 2700 g/mol.14 This corresponds to a C60/dendrimer molar ratio of approximately 30:1, which is almost exactly two terminal amine groups per fullerene. This strongly suggests that two amine groups bond to most fullerenes (see Scheme 2). Thermogravimetric analyses were used to further ascertain this value. Thermograms obtained from thermogravimetric (TG) analyses of C60, catalyst, and dendrimer showed 1172
significant difference in thermal stabilities of dendrimer and C60. For example, when dendrimer is heated at 2.5 °C/min up to 400 °C and then held at that temperature for 60 min, nearly all of the dendrimer mass is lost, with a char yield of less than 5%. However fullerene molecules subjected to the same process did not exhibit noticeable weight loss. On the other hand, when the conjugate, a hybrid of these two species, is heated under the same conditions, it exhibits intermediate weight loss behavior (approximately 36% mass loss). When the dendrimer char is taken into account, it is possible to conclude that, on the average, each catalyst is made up of approximately 60% fullerene and 40% dendrimer by mass. This corresponds to a 30:1 molar ratio, consistent with our findings from the MALDI-TOF measurements. The catalytic activity was investigated using the known photooxidation of thioanisole to methyl phenyl sulfoxide. Chloroform and water were chosen as the solvents. In chloroform thioanisole is soluble, while in water it is not. We reasoned that photooxidations in a heterogeneous mixture of thioanisole, water, and fullerene-dendrimer conjugate might drive the thioanisole into the relatively hydrophobic interior of the fullerene-dendrimer conjugate and enhance the rate of oxidation.15 Indeed, when water was used the oxidation was 25% complete after 2 h, while in chloroform the reaction was only 16% complete.16 Oxidation of water soluble compounds inside dendrimer cavities has been observed.1 The catalytic activity of the fullerene-dendrimer conjugate was compared to that of fullerene-coated polystyrene resin beads previously reported.6 Twice the amount of fullerenecoated polystyrene beads was required to achieve the same extent of thioanisole oxidation as fullerene-dendrimer conjugates under otherwise identical conditions. Control experiments performed by irradiating thioanisole in chloroform without any catalyst show a maximum of 6% oxidation over the same time period. In summary, fullerene-coated dendrimers have been prepared by the reaction of G4 PAMAM dendrimers with C60. The fullerene-dendrimer conjugates were characterized by Nano Lett., Vol. 5, No. 6, 2005
UV, IR, MALDI-TOF, and TGA. UV and TG analyses, as well as photooxidation data, confirm the presence of fullerenes. IR spectroscopy and TG analyses indicate the presence of PAMAM dendrimer. MALDI-TOF and TGA measurements suggest that the conjugate contains a 30:1 molar ratio of fullerene to dendrimer. Comparison of the rate of photooxidations in water and chloroform indicates that thioanisole diffuses into the dendrimer cavities during oxidation in water. Acknowledgment. We thank Dr. Steven McKnight and Dr. Bruce Fink at the U.S. Army Research Labs for funding most of this research (ARL contract numbers DAAD 1903-2-0012 and W911NF-04-2-0030). Supporting Information Available: The IR, MALDI, and UV spectra of the catalytic material; UV of C60; IR of PAMAM; and TGA of C60, PAMAM, and catalytic material are included. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Hecht, S.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2001, 123, 6959. (2) (a) Tomalia, D. A. Prog. Polym. Sci. 2005, 30, 294. (b) Tomalia, D. A. Aldrichimica Acta 2004, 37, 39. (c) Nakamura, E.; Isobe, H. Acc. Chem. Res. 2003, 36, 807. (d) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (e) Lee, K.; Song, H.; Park, J. T. Acc. Chem. Res. 2003, 36, 78. (f) Schuster, D. I.; Cheng, P.; Jarowski, P. D.; Guldi, D. M.; Luo, C.; Echegoyen, L.; Pyo, S.; Holzwarth, A. R.; Braslavsky, S. E.; Williams, R. M.; Klihm, G. J. Am. Chem. Soc. 2004, 126, 7257. (g) Tomalia, D. A.; Brothers, H. M., II; Piehler, L. T.; Dupont Durst, H.; Swanson, D. R. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5081. (3) (a) Luo, D.; Haverstick, K.; Belcheva, N.; Han, E.; Saltzman, W. M. Macromolecules 2002, 35, 3456. (b) Kihara, F.; Arima, H.; Tsutsumi, T.; Hirayama, F.; Uekama, K. Bioconjugate Chem. 2003, 14, 342. (c) Thomas, T. P.; Patri, A. K.; Myc, A.; Myaing, M. T.; Ye, J. Y.; Norris, T. B.; Baker, J. R. Biomacromolecules 2004, 5, 2269. (d) DeMattei, C. R.; Huang, B.; Tomalia, D. A. Nano Lett. 2004, 4, 771. (4) (a) Adam, W.; Bottke, N.; Engels, B.; Krebs, O. J. Am. Chem. Soc. 2001, 123, 5542. (b) Gollnick, K.; Kuhn, H. J. Ene-Reactions with
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(7) (8) (9)
(10) (11) (12) (13) (14)
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Singlet Oxygen in Organic Chemistry, Singlet Oxygen; Wasserman, H. H., Murray, R. W., Eds.; Academic Press: London, 1979; vol 40, p 287. (c) Bloodworth, A. J.; Eggelte, H. J. Chapter 4, Endoperoxides in Singlet Oxygen Volume II: Reaction Modes and Products, Part 1; Frimer, A. A., Ed.; CRC Press: Boca Raton, FL, 1985; p 93. Aborgast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Phys. Chem. 1991, 95, 11. (a) Jensen, A. W.; Daniels, C. J. Org. Chem. 2003, 68, 207. (b) Latassa, D.; Enger, O.; Thilgen, C.; Habicher, T.; Offermanns, H.; Deiderich, F. J. Mater. Chem. 2002, 12, 1993. Maiti, P. K.; Cagin, T.; Wang, G.; Goddard, W. A., III Macromolecules 2004, 37, 6236. Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (a) Hirsch, A.; Li, Q.; Wudl, F. Angew. Chem., Int. Ed. Engl. 1991, 30, 1309. (b) Geckeler, K. E.; Hirsch, A. J. Am. Chem. Soc. 1993, 115, 3850. The dendrimer was obtained from Dendritic NanoTechnologies Inc., 2625 Denison Drive, Suite B., Mount Pleasant, MI 48858. A variety of other solvents were unsatisfactory due to insolubility of either the hydrophilic dendrimer or the hydrophobic fullerene. Mansfield, M. L.; Rakesh, L.; Tomalia, D. A. J. Chem. Phys. 1996, 105, 3245. Various radii have been reported for PAMAM dendrimer G4. For a good summary see reference 7. The mass of isolated polymer appears to be very sensitive to the rate of addition of the dendrimer. Best results were obtained by adding the dendrimer dropwise overnight (15 s between each drop, from a buret). The light source was a 450 W medium-pressure Hg lamp. Samples were irradiated through Pyrex glass with a constant stream of O2 bubbling through the sample. Each sample initially contained 0.12 mL of thioanisole and 10 mg of conjugate in 3.0 mL solvent and was irradiated for 2 h. Irradiations in water were analyzed by 1H NMR after adding enough d6-DMSO to dissolve the insoluble thioanisole and methyl phenyl sulfoxide. After each initial irradiation the catalyst was recovered and the oxidation was repeated two more times to check the reusability of the material. In both water and chloroform the recovered catalyst was less reactive the second time but showed no decrease in activity between the second and third trials. This is likely due to some initial photodecomposition of the dendrimer conjugate during the first photoreaction. The aqueous oxidations were higher than those in chloroform for each trial.
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