Poly(N-isopropylacrylamide-co-1-vinylimidazole) Hydrogel Nanoparticles Prepared and Hydrophobically Modified in Liposome Reactors: Atomic Force Microscopy and Dynamic Light Scattering Study Sergey Kazakov, Marian Kaholek, Dina Kudasheva, Iwao Teraoka, Mary K. Cowman, and Kalle Levon* Department of Chemical Engineering, Chemistry, and Materials Science, Polytechnic University, Six MetroTech Center, Brooklyn, New York 11201 Received February 21, 2003 The poly(N-isopropylacrylamide-co-1-vinylimidazole) hydrogel nanoparticles (nanogels; 30-300 nm in diameter) with hydrophobic N-octadecylacrylamide chains exposed on their surfaces have been prepared by UV-induced polymerization. The internal reservoir of liposomes was used as a container for the water- soluble components of a gel-forming medium while the interlayer space of the liposomal lipid membrane was used as a container for the water-insoluble components. The abrupt collapsing of the nanogels was accompanied with their aggregation above the volume phase transition temperature, as was observed by dynamic light scattering (DLS). The combined DLS and atomic force microscopy (AFM) study shows that the obtained hydrogel nanoparticles were highly compatible with the lipid bilayer. This compatibility led to a self-initiated coating of the hydrogel particles by a lipid layer upon mixing of liposomes and nanogels. Swollen and collapsed states of the nanogels at different pH values were visualized by AFM. The flattening of the liposomes and nanogels was compared upon their deposition onto a mica surface in air. The quantitative characteristics of the nanogels in swollen and collapsed states were revealed by cross sectioning of AFM images. The findings indicate great potential for using the spherical nanoparticles (liposomes and nanogels) as starting materials for the fabrication of planar and hemispherical biophysical nanodevices. Introduction Nanometer-scale hydrogel/lipid bilayer systems are attractive objects for research and technical studies leading to novel sensors and other devices because the hydrogel/lipid bilayer assemblies combine properties of both materials. Spherical hydrogel particles in combina- tion with liposomes have already found a variety of biomedical applications (drug delivery, drug targeting, protein separation, enzyme immobilization, and so forth). 1-6 An appropriate assembly of a lipid bilayer on a spherical hydrogel surface can be considered as an artificial cell analogue, as a model system to study the functions of the membrane and membrane proteins, as an element of novel sensory devices, and as a platform for multivalent receptors. 7-11 Two configurations of the hydrogel/lipid bilayer as- sembly can be designed: spherical and planar. It seems attractive to use spherical nanoparticles (nanogels and liposomes) for the fabrication of miniature planar or hemispherical biomimetic nanodevices. Keeping this in mind, the compatibility of the materials is essential on the step of technological development, whereas the responsiveness of the nanoparticles to physical or chemical stimuli allows one to enhance the functionality of the hydrogel/lipid bilayer assembly. Liposomes have been used as a container for storage, for transport, for the controlled release of compounds, 12,13 or even for fabrication of hydrogel particles. 14-17 The protocol of preparation of the bare hydrogel particles employing the liposomal interior as a reactor for radical gelation has already been reported elsewhere. 16 N- Isopropylacrylamide (NIPA)-based polyelectrolyte gel particles with incorporated imidazolyl groups exhibit significant volume changes in response to the temperature and pH. 18 In this work, we focus on the properties of temperature- and pH-sensitive poly(N-isopropylacryla- mide-co-1-vinylimidazole) (PNIPA-VI) nanogels synthe- * Corresponding author. Phone: 1-718-260 3339. Fax: 1-718- 260 3125. E-mail: klevon@poly.edu. (1) (a) Ng, C. C.; Cheng, Y.-L.; Pennefather, P. S. Macromolecules 2001, 34, 5759-5765. (b) De Miguel, I.; Imbertie, L.; Rieumajou, V.; Major, M.; Kravtzoff, R.; Betbeder, D. Pharm. Res. 2000, 17, 817-824. (c) Kraft, M. L.; Moore, J. S. J. 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