Calix[8]arene Functionalized Polyglycerol Nanogels for Encapsulation and Stabilization of Fluorescent Dyes
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Calix[8]arene Functionalized Polyglycerol Nanogels for Encapsulation and Stabilization of Fluorescent Dyes Dirk Steinhilber,1 Florian Paulus,1 Andrew T. Zill,2 Steven C. Zimmerman2 and Rainer Haag1 1
Freie Universität Berlin, Institut für Chemie und Biochemie, Takustraße 3, 14195 Berlin,
Germany 2
University of Illinois at Urbana Champaign, 600 S. Mathews Avenue, Urbana, Illinois 61801,
USA ABSTRACT In this paper we report on the preparation of calix[8]arene functionalized polyglycerol nanogels by miniemulsion polymerization. The gel macromonomers were prepared by anionic ring-opening multibranching polymerization of glycidol using calix[8]arene as initiator. 1,1 ,3,3,3 ,3 -hexamethyl-2,2 -indotricarbocyanine iodide (HITC) red fluorescent dye was used as a guest molecule. Photobleaching upon strong laser illumination was significantly reduced when the dye was encapsulated inside the nanogel. INTRODUCTION Multifunctional polymers, which include materials properties like biofunctionality, biodegradability, multivalency, and shape memory effects, are an exciting class of materials [1, 2]. The combination of a high density of endgroups and a compact, well defined molecule structure makes multifunctional dendritic architectures attractive for biomedical applications [3]. Due to their narrow degree of molecular weight dispersity, flexible design, and biocompatible nature, dendritic polyglycerols (dPGs) have a broad range of potential applications in medicine and pharmacology [4]. dPG is traditionally prepared by anionic ring-opening multibranching polymerization of glycidol under slow monomer addition [5]. Polymers with a number average molecular weight (Mn) of up to 20 kDa and narrow molecular weight dispersities (MWD) are obtained by the use of macroinitiators [6]. Recently, Brooks and co-workers reported a molecular weight extension of dPG up to 1 MDa, which corresponds to a hydrodynamic diameter of 20 nm.[7] Since polymeric nanoparticles with sizes between 25 and 100 nm are expected to show efficient endocytosis into cancer cells [8], we developed polyglycerol nanogels in this size range which were synthesized using miniemulsification methods. The crosslinking of both dPG macromonomers and commercial glycerol-based monomers in miniemulsion nanodroplets yielded biocompatible nanogels which show excellent cell penetration [9,10]. We also reported on the preparation of disulfide-containing polyglycerol nanogels which degrade in the reductive environment of cells. This responsiveness is beneficial for the controlled intracellular drug release, and the resultant low molecular weight polymer fragments can be cleared by the kidneys [11]. Additionally, we extended the size of dPGs to the micrometer scale by gelling dPG and, in part, PEG macromonomers in aqueous droplets. We used microfluidic emulsification to prepare
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micrometer-sized pre-particle droplets and cure these droplets by free-radical crosslinking polymerization of the dPG building blocks and the PEG macro-crosslinkers. Mild gelation conditions allowed us
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