The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

This Account reports the synthesis and characterization of dendrimer-encapsulated metal nanoparticles and their applications to catalysis. These materials are prepared by sequestering metal ions within dendrimers followed by chemical reduction to yield the corresponding zerovalent metal nanoparticle. The size of such particles depends on the number of metal ions initially loaded into the dendrimer. Intradendrimer hydrogenation and carbon−carbon coupling reactions in water, organic solvents, biphasic fluorous/organic solvents, and supercritical CO2 are also described.

http://rcrooks.cm.utexas.edu/research/resources/Publications/rmc113.pdf

Dendrimer-Encapsulated Metal Nanoparticles: Synthesis, Characterization, and Applications to Catalysis

Poly(amidoamine) (PAMAM) dendrimers: from biomimicry to drug delivery and biomedical applications

Birth of a new macromolecular architecture: dendrimers as quantized building blocks for nanoscale synthetic polymer chemistry

Carbon dioxide is often promoted as a sustainable solvent, as CO 2 is non-flammable, exhibits a relatively low toxicity and is naturally abundant. However, injudicious use of carbon dioxide in a process or product can reduce rather than enhance overall sustainability. This review specifically examines the use of CO 2 to create greener processes and products, with a focus on research and commercialization efforts performed since 1995. The literature reveals that use of CO 2 has permeated almost all facets of the chemical industry and that careful application of CO 2 technology can result in products (and processes) that are cleaner, less expensive and of higher quality.

Supercritical and near-critical CO2 in green chemical synthesis and processing

This chapter describes composite materials composed of dendrimers and metals or semiconductors. Three types of dendrimer/metal-ion composites are discussed: dendrimers containing structural metal ions, nonstructural exterior metal ions, and nonstructural interior metal ions. Nonstructural interior metal ions can be reduced to yield dendrimer-encapsulated metal and semiconductor nanoparticles. These materials are the principal focus of this chapter. Poly(amidoamine) (PAMAM) and poly(propylene imine) dendrimers, which are the two commercially available families of dendrimers, are in many cases monodisperse in size. Accordingly, they have a generation-dependent number of interior tertiary amines. These are able to complex a range of metal ions including Cu2+, Pd2+, and Pt2+. The maximum number of metal ions that can be sorbed within the dendrimer interior depends on the metal ion, the dendrimer type, and the dendrimer generation. For example, a generation six PAMAM dendrimer can contain up to 64 Cu2+ ions. Nonstructural interior ions can be chemically reduced to yield dendrimer-encapsulated metal nanoparticles. Because each dendrimer contains a specific number of ions, the resulting metal nanoparticles are in many cases of nearly monodisperse size. Nanoparticles within dendrimers are stabilized by the dendrimer framework; that is, the dendrimer first acts as a molecular template to prepare the metal nanoparticles and then as a stabilizer to prevent agglomeration. These composites are useful for a range of catalytic applications including hydrogenations and Heck chemistry. The unique properties of the interior dendrimer microenvironment can result in formation of products not observed in the absence of the dendrimer. Moreover the exterior dendrimer branches act as a selective gate that controls access to the interior nanoparticle, which results in selective catalysis. In addition to single-metal nanoparticles, it is also possible to prepare bimetallic nanoclusters and dendrimer-encapsulated semiconductor nanoparticles, such as CdS, using this same general approach.

Dendrimer-Encapsulated Metals and Semiconductors: Synthesis, Characterization, and Applications

We report the preparation and characterization of dendrimerencapsulated CdS nanoparticles (DE-CdS). These luminescent assemblies are prepared using dendrimers as both nanoreactor and stabilizer. That is, the dendrimer first acts as a nanoreactor that sequesters Cd 2+ ions, and then after reaction with S 2it stabilizes the resulting CdS nanoparticles by preventing agglomeration. Because the size of the nanoparticle (or quantum dot, QD) is related to the size (or generation, G) of the dendrimer template used to prepare it, and because the optical properties of QDs are size-dependent, the absorptive and emissive properties of the QDs are a function of the generation of the dendrimer template used to prepare them. Additionally, the numerous reactive groups on the surface of the dendrimer can be used to render the composites soluble in essentially any solvent, including water, organic solvents, 1 fluorous phases, 2 and even supercritical fluids,3 and as synthetic handles for attachment of ligands to direct binding of the composites to surfaces, 4 biological ligands, 5 DNA,6,7 and other targets. New methods for preparing useful luminescent QDs have recently been described. Three particularly significant findings are relevant to our work. First, means for closely controlling the size of CdS and CdSe QDs have been discovered. For example, the high-temperature organometallic route reported by Bawendi provides a high degree of control over both particle size and monodispersity. 8 Second, it was found that “capped” or coreshell (e.g., (CdSe)ZnS) structures result in enhanced stability and much higher quantum efficiency compared to uncapped CdS or CdSe nanocrystals. 9-11 Finally, methods for attaching biologically sensitive structures to the semiconductor surface have been reported. 12,13 These highly luminescent nanoparticles have photophysical properties superior to organic dyes (increased quantum yield and photostability), but the high temperature required to synthesize them can be problematic for some applications. It has also been found that attachment of biologically active groups requires multiple steps, and that the resulting composites have lower quantum yields than unmodified QDs. Murphy was the first to report using dendrimers to control the size of QDs. 14-17 These materials have been shown to be agglomerates of spatially segregated QDs stabilized by multiple dendrimers. That is, in these interdendrimer composites, many dendrimers surround multiple QDs to yield large, aggregated structures. 14 This geometry is very different than the nonaggregated CdS nanoparticles that are sequestered within individual dendrimers that we describe here. Both families of materials have their virtues, but the latter afford control over absorbance and emission characteristics through dendrimer-mediated, size-induced quantum confinement effects. DE-CdS composites also lend themselves to straightforward chemical modification of the dendrimer periphery. The fact that they are discreet nanoscopic entities is also critical to their future use as luminescent labels for biochemical applications. The preparation of DE-CdS composites is similar to that of the dendrimer-encapsulated metal nanoparticles we have reported previously. 18-20 The latter are prepared by extraction of metal ions into the interior of dendrimers, followed by chemical reduction. DE-CdS composites are prepared by alternating dropwise addition of Cd2+ and S2(16 mM Cd(NO3)2 and 16 mM Na2S in methanol) to a solution containing hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers of various generation (Gn-OH). The final concentrations of Gn-OH (given in parentheses) and Cd 2+ and S2are as follows: G4-OH (0.1 mM), 1.6 mM; G6-OH (0.05 mM), 3.2 mM; and G8-OH (0.012 mM), 3.1 mM. These relative dendrimer/ion ratios were chosen to maintain a constant ratio of the ion concentration to the number of dendritic tertiary amines present in the outermost shell of the dendrimer. The G6-OH and G8-OH DE-CdS nanocomposites require no purification after preparation, while the G4-OH-encapsulated particles require aqueous washing or a size-selective photoetching21 step to remove bulk CdS stabilized by multiple dendrimers.14,16,17 Figure 1 shows absorption and luminescence spectra of DECdS assemblies prepared using three different generations of dendrimers (G4, G6, and G8). For example, G4-OH DE-CdS (spectrum a) results in a sharp absorbance band at 280 nm and an emission peak at 320 nm. When higher generation dendrimers are used as templates, larger CdS QDs result. QDs prepared in G6-OH templates yield absorbance and emission maxima at 350 and 470 nm, respectively (spectrum b), while a CdS QD encapsulated within a G8-OH dendrimer absorbs and emits at 370 and 510 nm, respectively (spectrum c). The apparent diameters of the DE-CdS composites, as inferred from their optical absorbance spectra, 22,23 are 1.3 nm for G4-OH/CdS, 1.8 nm for G6-OH/CdS, and 2.3 nm for G8-OH/CdS. These particle sizes are consistent with those of the dendrimer-encapsulated metal nanoparticles we reported previously. 18,24 This high degree of * To whom correspondence should be addressed. (1) Chechik, V.; Zhao, M.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 4910-4911. (2) Chechik, V.; Crooks, R. M. J. Am. Chem. Soc. 2000, 122, 1243-1244. (3) Yeung, L. K.; Crooks, R. M.J. Am. Chem. Soc. Submitted for publication. (4) Chechik, V.; Crooks, R. M. Langmuir1999, 15, 6364-6369. (5) Wilbur, D. S.; Pathare, P. M.; Hamlin, D. K.; Buhler, K. R.; Vessella, R. L. Bioconj. Chem. 1998, 9, 813-825. (6) Kukowska-Latallo, J. F.; Bielinska, A. U.; Johnson, J.; Spindler, R.; Tomalia, D. A.; Baker, J. R. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 48974902. (7) Bielinska, A. U.; Chen, C.; Johnson, J.; Baker, J. R. Bioconj. Chem. 1999, 10, 843-850. (8) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J Am. Chem. Soc. 1993, 115, 8706-8715. (9) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649-5655. (10) Hines, M. A.; Guyot-Sionnest, P. J Phys. Chem. 1996, 100, 468471. (11) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattousi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463-9475. (12) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (13) Chan, W. C. W.; Nie, S. Science1998, 281, 2016-2018. (14) Sooklal, K.; Hanus, L. H.; Ploehn, H. J.; Murphy, C. J. AdV. Mater. 1998, 10, 1083-1097. (15) Lackowics, J. R.; Gryczynski, I.; Gryczynski, Z.; Murphy, C. J. J. Phys. Chem. B 1999, 103, 7613-7620. (16) Huang, J.; Sooklal, K.; Murphy, C. J.; Ploehn, H. J. Chem. Mater. 1999, 11, 3595-3601. (17) Hanus, L. H.; Sooklal, K.; Murphy, C. J.; Ploehn, H. J. Langmuir 2000, 16, 2621-2626. (18) Zhao, M.; Sun, L.; Crooks, R. M. J Am. Chem. Soc. 1998, 120, 48774878. (19) Zhao, M.; Crooks, R. M. AdV. Mater. 1999, 11, 217-220. (20) Zhao, M.; Crooks, R. M. Chem. Mater. 1999, 11, 3379-3385. (21) Torimoto, T.; Konati, H.; Sakata, T.; Mori, H.; Yoneyama, H. Chem. Lett. 1999, 5, 379-380. (22) Vossmeyer, T.; Katsikas, L.; Girsig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. 1994, 98, 76657673. (23) Vossmeyer, T.; Reck, G.; Katsikas, L.; Haupt, E. T. K.; Schulz, B.; Weller, H. Science1995, 267, 1476-1479. (24) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1999, 38, 364366. 12886 J. Am. Chem. Soc. 2000,122,12886-12887

Preparation and Characterization of Dendrimer-Encapsulated CdS Semiconductor Quantum Dots

We report the preparation and characterization of viologen-functionalized generation 2, 4, and 6 poly(amidoamine) (PAMAM) dendrimers. An amidation reaction between the succinimide ester of 1-ethyl-1‘-(3-propionic acid)-4,4‘-bipyridylium dibromide and the dendrimer primary amines resulted in 13−34% end group functionalization. The water-soluble dendrimers were examined by 1H NMR, MALDI-TOF MS, and UV−vis spectroscopy to determine the extent of functionalization. UV−vis spectra, obtained following chemical reduction of the viologenated dendrimers, indicated that the viologen radical cations were largely dimerized, which demonstrates the close proximity of the terminal viologens. Dynamic light scattering (DLS) measurements indicate that the oxidized viologenated-dendrimers are predominately unaggregated in aqueous electrolyte solutions. Diffusion coefficients determined by chronoamperometry for the three generations of viologenated PAMAM dendrimer are within 15% of values calculated using the Stokes−Einstein...

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Buford I. Lemon

Papers

Dendrimer-Encapsulated Metals and Semiconductors: Synthesis, Characterization, and Applications

Preparation and Characterization of Dendrimer-Encapsulated CdS Semiconductor Quantum Dots

Electrochemical and Spectroscopic Characterization of Viologen-Functionalized Poly(Amidoamine) Dendrimers†

Synthesis, Characterization, and Applications of Dendrimer-Encapsulated Metal and Semiconductor Nanoparticles