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Langmuir 2006, 22, 5604-5610
A General Method for the Controlled Embedding of Nanoparticles in Silica Colloids Christina Graf,* Sofia Dembski, Andreas Hofmann, and Eckart Ru¨hl*,† Institut fu¨r Physikalische Chemie, UniVersita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany ReceiVed January 14, 2006. In Final Form: April 12, 2006 A novel method for the controlled embedding of multiple nanoparticles of various materials, such as gold nanoparticles, quantum dots, and magnetic nanoparticles, in silica colloids is presented. After adsorption of the amphiphilic polymer poly(vinylpyrrolidone) on hydrophobic or hydrophilic stabilized nanoparticles, these are adsorbed on silica spheres and covered by variable-thickness silica shells. This silica coating protects the embedded nanoparticles against chemical transformations, which is of crucial importance for the biocompatibility of particles containing toxic elements. Moreover, it is found that the optical properties of the nanoparticles are retained. Possible applications of multicore particles are briefly discussed.
Introduction Nanoparticles have unique, size-dependent properties. Therefore, they are of increasing interest for both fundamental research and numerous applications. Small noble metal nanoparticles exhibit characteristic colors due to the excitation of surface plasmon modes, which can be tuned by their size, shape, and the local environment. Semiconductor nanoparticles (quantum dots, QD) are of specific interest because of their specific optical and electronic properties.1-3 This is accompanied by a sharp, tunable emission band, where strong resistance to photobleaching is observed. As a consequence of these distinct properties, QD have the potential for numerous applications including biolabeling or they may be used as tracers in diffusion studies. Superparamagnetic nanoparticles have been studied with respect to biomedical applications,4-6 such as MRI contrast enhancement,7 magnetic immobilization, and drug targeting.8 Many of the promising applications of nanoparticles require that they are transferred in a suitable medium, for instance, in aqueous solution allowing for applications in life sciences. Highquality semiconductor nanoparticles are usually stabilized by organic ligands that render them hydrophobic. However, they are often unstable with respect to oxidation and they are cytotoxic.9 Other syntheses yield hydrophilically stabilized particles, e.g., citrate-stabilized gold particles that cannot be transferred in apolar media.10 Therefore, several approaches have been developed where nanoparticles are coated by protective shells so they can be transferred in other media.10,11 Usually, such syntheses are * To whom correspondence should be addressed. E-mail: cgraf@ phys-chemie.uni-wuerzburg.de (C.G.);
[email protected] (E.R.). † Present address: Physikalische und Theoretische Chemie, Institut fu ¨r Chemie und Biochemie, Freie Universita¨t Berlin, Takustr. 3, D-14195 Berlin, Germany. (1) Empedocles, S. A.; Norris, D. J.; Bawendi, M. G. Phys. ReV. Lett. 1996, 77, 3873-3876. (2) Norris, D. J.; Bawendi, M. G. Phys. ReV. B 1996, 53, 16338. (3) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354 (4) Meldrum, F. C.; Heywood, B. R.; Mann, S. Science 1992, 257, 522. (5) Tanaka, T.; Matsunaga, T. Anal. Chem. 2000, 72, 3518. (6) Matsunaga, T.; Kawasaki, M.; Yu, X.; Tsujimura, N.; Nakamura, N. l. Anal. Chem. 1996, 68, 3551. (7) Harisinghani, M. G.; Barentsz, J.; Hahn, P. F.; Deserno, W. M.; Tabatabaei, S.; Hulsbergen van de Kaa, C.; de la Rosette, J.; Weissleder, R. New Eng. J. Med. 2003, 348, 2491. (8) Nakamura, N.; Burgess, J. G.; Yagiuda, K.; Kudo, S.; Sakaguchi, T.; Matsunaga, T. Anal. Chem. 1993, 65, 2030. (9) Derfus, A. M.; Chan, W. C. W.; Bhatia, S. N. Nano Lett. 2004, 4, 11 (10) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329.
specific to the class of materials of interest, so that these are not suitable for generally applicable synthesis routes. The regime of the strongest size effects in optical properties is confined to 2-40 nm particles. By adsorbing or embedding such particles in larger matrix spheres, larger particles can be prepared that still exhibit optical properties of small nanospheres, but these are considerably easier to handle. This, however, requires a well-controlled preparation method, especially the prevention of uncontrolled aggregation and clustering, so that specific properties of the small nanoparticles are retained. Moreover, it should be also possible to enhance specific properties in this way, for instance, by embedding fluorescing particles in a wellcontrolled radial distance to, e.g., a metal particle in one larger colloidal particle and enhancing in this way their fluorescence quantum yield as it has been already shown for layered nanoparticle systems.12 This concept may also be used as a betterdefined way for the combination of the properties of two species in one matrix colloid, such as magnetic and strongly luminescent properties. Up to now, synthetic routes have been used for this purpose where the matrix colloid formation and the nanoparticle inclusion happen simultaneously. Therefore, the interparticle distances of the embedded nanoparticles can be hardly controlled.13-15 Moreover, in the embedding process which uses a defined matrix particle as its core, it appears also to be possible to control the location of photoluminescing nanoparticles on the nanometer scale, which allows one to study the local density of states with respect to photonic applications. Further, the incorporation of luminescent semiconductor nanocrystals into photonic crystals has received considerable attention in the past years as a promising pathway to novel light sources with controllable spontaneous emission.16 Such crystals can be prepared from monodisperse latex or silica spheres, either bare, doped, or covered by a shell. Therefore, the synthesis of semiconductor nanoparticle-doped colloidal particles that can (11) Gerion, D.; Pinaud, F.; Williams, S. C.; Parak, W. J.; Zanchet, D.; Weiss, S.; Alivisatos, A. P. J. Phys. Chem. B 2001, 105, 8861. (12) Kulakovich, O.; Strekal, N.; Yaroshevich, A.; Maskevich, S.; Gaponenko, S.; Nabiev, I.; Woggon, U.; Artemyev, M. Nano Lett. 2002, 2, 1449. (13) Kim, J.; Lee, J. E.; Lee, J.; Yu, J. H.; Kim, B. C.; An, K.; Hwang, Y.; Shin, C.-H.; Park, J.-G.; Kim, J.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 688. (14) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.; Kundaliya, D.; Ying, J. Y. J. Am. Chem. Soc. 2005, 127, 4990. (15) Mu¨ller-Schulte, D.; Schmitz-Rode, T.; Borm, P. J. Magn. Magn. Mater. 2005, 293, 135. (16) Rogach, A. L.; Kotov, N. A.; Koktysh, D. S.; Susha, A. S.; Caruso, F. Colloids Surf., A 2002, 202, 135.
10.1021/la060136w CCC: $33.50 © 2006 American Chemical Society Published on Web 05/24/2006
Embedding of Nanoparticles in Silica Coloids
serve as building blocks for such crystals is an important issue in colloidal science. Particles containing multiple nanoparticles can be prepared by aggregation processes, such as trapping of nanoparticles during a sol gel process,17 micelle formation,18 or by controlled adsorption of the nanoparticles on a preformed supporting core19 and further coating with a protective layer.20,21 Aggregation processes are often poorly controllable, and therefore, they yield particles where neither the polydispersity of the colloidal particles nor the radial position of the nanoparticles in the matrix particle can be well controlled. On the other hand, the second method using a preformed colloidal core is easier to control. However, this often requires a functionalization step to attach the nanoparticles on the core colloid or to transfer the nanoparticles into a more polar medium where the reaction takes place. Both can be achieved by a ligand-exchange reaction. The success of such a reaction often depends significantly on the specific surface properties of the nanoparticle. Consequently, these reactions are often not well reproducible and they are often limited to certain material systems. Therefore, the direct adsorption of an appropriate polymer, such as poly(vinylpyrrolidone) (PVP), on nanoparticles leading to changes in surface properties and particle stability is a simple alternative to these exchange reactions.22 The amphiphilic, nonionic polymer PVP is widely used in science and technology and adsorbs readily to a broad range of different materials including metals (e.g., gold, silver, iron)22 and metal oxides (kaolinite, titanium dioxide, iron oxide, alumina).23 PVP stabilizes colloidal particles in water and many nonaqueous solvents. An exchange of the ligands is in this case not required. Silica is an attractive material for the embedding of nanoparticles because it can be easily functionalized. In this way, a wide variety of coating procedures has been developed for silica colloids that allow one to disperse them in various solvents ranging from very polar to apolar ones.24 Moreover, several routes for biofunctionalization of silica colloids are reported.25,26 Silica colloids are prepared with a very low polydispersity (
