Comparison and Stability of CdSe Nanocrystals Covered with Amphiphilic Poly(Amidoamine) Dendrimers Chunxin Zhang, † Stephen O’Brien, ‡ and Lajos Balogh* ,† Center for Biologic Nanotechnology, UniVersity of Michigan, Ann Arbor, Michigan 48109, and Department of Applied Physics, Columbia UniVersity, New York, New York 10027 ReceiVed: NoVember 15, 2001; In Final Form: June 20, 2002 This paper describes the surface modification of CdSe nanoparticles (NPs) with amphiphilic and flexible poly(amidoamine) (PAMAM) dendrimers carrying different numbers of hydrophobic aliphatic chains. Although hydrophilic full generation dendrimers with primary amine termini caused aggregation of the CdSe particles, the amphiphilic dendrimers proved to be useful caps for the CdSe nanocrystals and solubilized them in chloroform. Complete exchange of the original TOPO/TOP caps was achieved through a CdSe/pyridine intermediate. It was found that both amine groups (which interact with the nanoparticle surface) and hydrophobic chains (which provide the particles with solubility in the solvent) were necessary to stabilize the CdSe NPs. The PAMAM covered CdSe nanocrystals were characterized using NMR, UV-visible absorption, photo- luminescence (PL), and TEM. UV-vis absorption and PL of such CdSe/PAMAM systems were studied during a two month period, and it was found that PAMAM derivatives with only secondary and tertiary amines provided a better protection for the nanocrystals than those with primary amines. Introduction Research on semiconductor nanoparticles has been expanded tremendously during the past couple of decades. 1-4 Semicon- ductor nanocrystals possess unique optical and electronic properties because of size quantization effect on the nanometer scale. Such NPs are usually capped with organic molecules containing an electron donating ligand, such as trioctylphosphine oxide (TOPO) or pyridine, to provide chemical and electric passivation. 5,6 Surface modification of NPs is an important issue because surface ligands not only determine the stability of the nanoparticles but also influence their optical properties and control their compatibility with the actual physical environment. Although TOPO/TOP and long chain aliphatic primary amine passivated CdSe QDs are relatively stable when properly stored, they cannot be supplied with additional functionalities. CdSe/ TOPO preparations also usually contain a high excess of TOPO, which makes them impossible to use them directly in biologic systems. The mechanisms of this “capping” appear to be similar to metal ion complexation by organic molecules, i.e., a donor- acceptor type interaction between the NP surface and the ligand site of the organic molecule. If surface caps are bound covalently, dry nanoparticles remain soluble when only solvent is added. However, in case of a dynamic equilibrium, an excess of organic molecules must be present in the solution. On the basis of analogy with metal ion chelation, we assumed that multiligand polymers should form more stable surface caps than molecules with a single ligand if bound by noncovalent interactions. In the meantime, enormous research efforts have been devoted to the synthesis and characterization of dendrimers as well. Polymers with dendritic architectures have large numbers of functional terminal groups, which are exposed on their surface when in solution. We have selected poly(amidoamine) PAMAM dendrimers 7-11 to carry electron donating amine ligands as they have a predetermined size with narrow molecular weight distributions and their molecular weight can be varied system- atically from generation to generation. Physical properties of PAMAM dendrimers, such as solubility and interactions with the surrounding environment, 12-15 can be altered by changing the characteristics of terminal groups. Introduction of additional surface functionalities should be relatively simple by using dendritic polymers because of their multifunctionality. PAMAMs have been used as templates for semiconductor particles 16-21 and metal nanoparticles 22-26 where hybrid nano- composites were formed in situ in the presence of dendrimers. In a study on CdS nanocomposites, 18 it was suggested that PAMAM dendrimers retard the growth of the nanoparticles thereby producing small CdS nanocrystals, although these composite particles proved to be unstable and underwent further aggregation to form micrometer scale flocs. In our work, instead of using dendrimers as templates, we introduce dendrimer caps on preformed CdSe nanocrystals synthesized by a well-known wet chemistry procedure in TOPO/ TOP. 27,28 Using dendrimers as surface ligands, we wish to develop a universal method for adjusting the nanoparticle surface in a controlled manner and for adding further functionality to the nanoparticles. TOPO covered particles proved to be stable and have a long shelf life if stored properly. However, it is not possible to add further moieties to the TOPO passivated NP surface that would empower us to target specific cell-receptors and use the particles for biomedical research. Dendritic mol- ecules, with an exact number of functional groups, should provide excellent model compounds to study surface passivation of NPs. These multifunctional dendrimer caps possess “heads” * To whom correspondence should be addressed. Lajos Balogh, Center for Biologic Nanotechnology, Department of Internal Medicine, School of Medicine, and, Macromolecular Science and Engineering, Colleges of Engineering and Literature, Science and The Arts, 4010A Kresge Res. Bldg. II, 200 Zina Pitcher Pl., University of Michigan, Ann Arbor 48109-0533. Phone: (734) 615-0623. Fax: (734) 615-0621. E-mail: baloghl@umich.edu. † University of Michigan. ‡ Columbia University. 10316 J. Phys. Chem. B 2002, 106, 10316-10321 10.1021/jp014241k CCC: $22.00 © 2002 American Chemical Society Published on Web 09/12/2002