9374
Langmuir 2005, 21, 9374-9380
Photocleavable Microcapsules Built from Photoreactive Nanospheres Xiaofeng Yuan, Karl Fischer, and Wolfgang Scha¨rtl* Institut fu¨ r Physikalische Chemie der Johannes-Gutenberg-Universita¨ t Mainz, Welderweg 11, 55099 Mainz, Germany Received June 6, 2005. In Final Form: July 27, 2005 We show how photo-cross-linking of nanoparticles within the micrometer-sized thin oil shell of wateroil-water emulsion droplets leads to a new species of optically addressable microcontainers. The inner water droplet of these emulsions may contain drugs, dyes, or other water-soluble components, leading to filled containers. The thickness, mechanical stability, and light resistance of the container walls can be controlled in a simple way by the amount and adjustable photoreactivity of the nanoparticles. Importantly, the chemical bonds between the nanoparticles constituting the microcapsule shell can be cleaved photochemically by irradiation with UV light. This optically controlled destruction of our microcontainers opens up a pathway to controlled release of the enclosed components, as will be illustrated by the example of enclosed cyclodextrin molecules.
1. Introduction The encapsulation of chemical substances in microcontainers and their controlled release are highly challenging and very important topics.1-5 Such systems may open new pathways to chemical reactions on a nanoscale level and also are extremely relevant to health care (drug targeting and controlled drug release). Different types of micrometer-sized containers have been prepared using microscopic templates to form the shell of a microcapsule. Here, suitable templates may vary from monodisperse polymeric microparticles to micrometer-sized oil or water droplets, and the shell itself may consist of polymer chains or spherical colloidal particles. For example, following the principle of layer-by-layer assembly of oppositely charged polyelectrolyte chains developed by Decher 10 years ago,6 Mo¨hwald, Caruso, and Sukhurukov have prepared and investigated a large variety of layer-by-layer microcapsules7-26 by assembling the polymer chains onto col* Corresponding author. E-mail:
[email protected]. (1) Hino, T.; Shimabayashi, S.; Tanaka, M.; Nakano, M.; Okochi, H. J. Microencapsulation 2001, 18, 19. (2) Thomas, J. A.; Seton, L.; Davey, R. J.; DeWolf, C. E. Chem. Commun. 2002, 1072. (3) Kiyoyama, S.; Shiomori, K.; Kawano, Y.; Hatate, Y. J. Microencapsulation 2003, 20, 497. (4) Mal, N.; Fujiwara, M.; Tanaka, Y. Nature 2003, 421, 350. (5) Sukhorukov, G. B.; Rogach, A. L.; Zebli, B.; Liedl, T.; Skirtach, A. G.; Ko¨hler, K.; Antipov, A. A.; Gaponik, N.; Susha, A. S.; Winterhalter, M.; Parak, W. J. Small 2005, 1, 194. (6) Decher, G. Science 1997, 227, 1232. (7) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (8) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317. (9) Sukhorukov, G. B.; Donath, E.; Moya, S.; Susha, A. S.; Voigt, A.; Hartmann, J.; Mohwald, H. J. Microencapsulation 2000, 17, 177-185. (10) Voigt, A.; Buske, N.; Sukhorukov, G. B.; Antipov, A. A.; Leporatti, S.; Lichtenfeld, H.; Baumler, H.; Donath, E.; Mohwald, H. J. Magn. Magn. Mater. 2001, 225, 59-66. (11) Gittins, D. J.; Susha, A.; Schoeler, B.; Caruso, F. Adv. Mater. 2002, 14, 508. (12) Radtchenko, I. L.; Giersig, M.; Sukhorukov, G. B. Langmuir 2002, 18, 8204. (13) Colloids and Colloid Assemblies: Synthesis, Modification, Organization and Utilization of Colloidal Particles; Wiley-VCH: Weinheim, Germany, 2003. (14) Caruso, F.; Sukhorukov, G. Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, Germany, 2003. (15) Glinel, K.; Sukhorukov, G. B.; Mohwald, H.; Khrenov, V.; Tauer, K. Macromol. Chem. Phys. 2003, 204, 1784.
loidal particles as templates and subsequently removing the template. An alternative system consists of colloidosomes, microscopic capsules prepared by self-assembly of colloidal nanoparticles at the oil-water interface of emulsion droplets27 or within the oil layer of a water/oil/ water double emulsion.28 Employing a flocculation process, Murthy et al. have prepared a very interesting new system: water-filled silica hollow microspheres obtained upon addition of negatively charged silica nanoparticles to a suspension of poly(L-lysine)-gold nanoparticle assemblies.29 Besides the development of various microcapsular systems during the past decade, however, there are still only a few examples of microcapsules whose properties (mechanical stability, release rate, etc.) can be tuned by external stimuli such as light both in a simple and a controlled way. As one example, Caruso and coworkers developed a very nice microcapsular system that, because of light-absorbing gold nanoparticles incorporated within the polyelectrolyte layers constituting the microcapsule shell, is optically addressable for controlled drug delivery.19 Reversible photoresponsive systems have been (16) Glinel, K.; Sukhorukov, G. B.; Mohwald, H.; Khrenov, V.; Tauer, K. Macromol. Chem. Phys. 2003, 204, 1784-1790. (17) Antipov, A. A.; Sukhorukov, G. B. Adv. Colloid Interface Sci. 2004, 111, 49-61. (18) Dejugnat, C.; Sukhorukov, G. B. Langmuir 2004, 20, 72657269. (19) Radt, B.; Smith, T. A.; Caruso, F. Adv. Mater. 2004, 16, 2184. (20) Shchukin, D. G.; Shutava, T.; Shchukina, E.; Sukhorukov, G. B.; Lvov, Y. M. Chem. Mater. 2004, 16, 3446-3451. (21) Sukhorukov, G. B.; Fery, A.; Brumen, M.; Mohwald, H. Phys. Chem. Chem. Phys. 2004, 6, 4078-4089. (22) Volodkin, D. V.; Petrov, A. I.; Prevot, M.; Sukhorukov, G. B. Langmuir 2004, 20, 3398-3406. (23) Dejugnat, C.; Halozan, D.; Sukhorukov, G. B. Macromol. Rapid Commun. 2005, 26, 961-967. (24) Dong, W. F.; Ferri, J. K.; Adalsteinsson, T.; Schonhoff, M.; Sukhorukov, G. B.; Mohwald, H. Chem. Mater. 2005, 17, 2603-2611. (25) Heuberger, R.; Sukhorukov, G.; Voros, J.; Textor, M.; Mohwald, H. Adv. Funct. Mater. 2005, 15, 357-366. (26) Sukhorukov, G. B.; Rogach, A. L.; Zebli, B.; Liedl, T.; Skirtach, A. G.; Kohler, K.; Antipov, A. A.; Gaponik, N.; Susha, A. S.; Winterhalter, M.; Parak, W. J. Small 2005, 1, 194-200. (27) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (28) Fujiwara, M.; Shiokawa, K.; Tanaka, Y.; Nakahara, Y. Chem. Mater. 2004, 16, 5420. (29) Murthy, V. S.; Cha, J. S.; Stucky, G. D.; Wong, M. S. J. Am. Chem. Soc. 2004, 126, 5292.
10.1021/la051491+ CCC: $30.25 © 2005 American Chemical Society Published on Web 08/23/2005
Microcapsules Built from Photoreactive Nanospheres
Langmuir, Vol. 21, No. 20, 2005 9375 Scheme 1. Reversible 2 + 2 Photoaddition of Nitrocinnamatea
a Note that about 150 of these photoreactive dye molecules are chemically attached to 1 polyorganosiloxane nanoparticle via an ester bond as indicated.
2.1. Preparation of Photoreactive Nanoparticles. Chlorobenzyl-functionalized poly-(organosiloxane) nanoparticles of
average diameter 20 nm and size polydispersity 90% of the cyclodextrin molecules were encapsulated successfully and no cyclodextrin was leaking out of the microcapsules before photocleavage (solid line and dashed line in Figure 10, respectively). According to the fluorescence intensity resulting from microcapsule dispersions after photocleavage (dashed-dotted line) in comparison to that resulting from an aqueous solution of nonencapsulated freely diffusing cyclodextrin molecules (dotted line), about 50% of the encapsulated cyclodextrin molecules were released after photocleavage. There could be several reasons that the release is smaller than 100%: for the thick-shelled microcapsules used for these release experiments (shell thickness of 40 layers of cross-linked nanoparticles), some capsules may still remain intact after photocleavage. Also, some cyclodextrin molecules may simply absorb onto the large microcapsule shell fragments. In addition, photocleavage of the microcapsules leads to the precipitation of micrometer-sized capsule shell fragments (see above), and some cyclodextrin molecules might also get entrapped within this insoluble precipitate. Whatever the reason, we still consider a release of about 50% of the encapsulated molecules to be highly satisfactory, supporting our claim that our new microcapsules may indeed provide an attractive new system for potential applications in the field of optically controlled release. A systematic study of the amount of released cyclodextrin as a function of irradiation time and capsule architecture (variation of shell thickness and of the number of cinnamate labels per nanoparticle) will be presented in a subsequent publication. Conclusions We have shown how the photo-cross-linking of nanoparticles within water-oil-water emulsion droplets leads (43) Trkula, M.; Keller, R. A. Anal. Chem. 1985, 57, 1663. (44) Nithipatikom, K.; McGown, L. B. Anal. Chem. 1986, 58, 3145.
9380
Langmuir, Vol. 21, No. 20, 2005
Yuan et al.
Figure 11. Fluorescence micrographs of W/O/W emulsion droplets prepared according to our standard procedure (left) and after homogenization with an Avestin C5 pressure homogenizer (right). Scale bars 10 µm.
to photocleavable microcontainers. The inner water droplet may contain water-soluble molecules, resulting in filled containers as shown for the example cyclodextrin. As in the case of the layer-by-layer microcapsules developed previously by Caruso, Mo¨hwald, and Sukhurukov et al., the thickness, mechanical stability, and light resistance of the container walls can also be controlled for our new system simply by regulating the number of nanoparticles and/or the number of photoreactive dye labels per nanoparticle. An additional major advantage is that filling our microcapsules with water-soluble substrate molecules is extremely simple using a solution of the guest molecules as the inner water phase of the W/O/W emulsion. In addition and importantly, the first tests (see Figure 11) have shown that by using a pressure homogenizer (Avestin
C5) not only W/O/W microdroplets of reduced polydispersity but also much smaller microdroplets and therefore corresponding smaller microcapsules can be prepared. Acknowledgment. This work has been financially supported by DFG grant SCHA620/4-1. We thank Mr. Renguo Xie for preparing the highly fluorescent CdSe nanocrystals, Professor Dave Weitz (DEAS, Harvard University) for helpful discussions, and Dr. Jishan Wu and Mr. Gunnar Glasser (both at the Max Planck Institute fu¨r Polymerforschung, Mainz, Germany) for the SEM images. LA051491+
