Conjugated Polymer Nanoparticles by Metathesis Polyondensation in Aqueous Miniemulsion and their Fluorescence Detection in Cells

Abstract

Conjugated polymers have been studied intensly over the past three decades, due to their conductive and luminescent properties. In the form of nanoparticles, however, luminescent polymers have found interest only most recently, and at the beginning of this thesis only a handful of publications existed on such particles. They are of interest for various reasons, such as the very high luminescence brightness of individual nanoparticles, or the possibility of overcoming the notorious issue of processing insoluble, intractable polymers via particulate systems. From the various available methods for that purpose, the synthesis of conjugated polymer nanoparticles from the appropriate monomers by polymerization in highly disperse aqueous systems is attractive, as it is not restricted to soluble polymer derivatives and in principal can afford a broader scope of nanoparticle structures. However, the synthesis of conjugated polymers commonly involves transition metal catalyzed step-growth reactions, which are prone to catalyst decomposition by moisture and air.The synthesis of poly(p-phenylene vinylene) nanoparticle dispersions was investigated by acyclic diene metathesis (ADMET) coordination polymerization of appropriate divinylbenzenes in aqueous miniemulsion. Appropriate substituted divinylbenzenes were prepared for this purpose in high purities suited for step-growth polymerization following literature protocols. Ruthenium alkylidene catalyst precursors were found to oligomerize divinylbenzenes to the respective oligo(phenylene vinylenes) under aqueous miniemulsion polymerization conditions, resulting in brightly colored nanoparticle dispersions with particle sizes ranging from 100 to 300 nm. The limited molecular weights may be due partially to catalyst decomposition under these reaction conditions, however, comparative studies in non-aqueous systems under inert conditions did not yield substantially higher molecular weights. Dialkoxy- and dialkyl-substituted divinylbenzenes were converted to the corresponding red and green polymer nanoparticles consisting of substituted oligo(phenylene vinylene)s with average degrees of polymerization of DPn = 8. Of the two catalyst precursors studies, both, the Grubbs second generation and the Hoveyda-Grubbs second generation metathesis catalyst, were able to polymerize dialkoxy-substituted monomers, whereas only the former was able to polymerize dialkyl-substituted monomers in emulsion. This is attributed to the lack of stabilization of the catalyst resting state towards the aqueous environment, due to the lack of a coordinating other moiety from the monomer. The obtained highly fluorescent conjugated polymer nanoparticle dispersions were found to be colloidally stable and to be processable by simple ink-jet printing techniques into lateral structures. This work represented the first demonstration of the synthesis of polymer nanoparticles by step-growth coordination polymerization, in the form of aqueous dispersions.It has previously been shown that conjugated polymer nanoparticles are excellent fluorescent probes for cell imaging due to their extraordinary brightness and photostability. Extremely high two-photon action cross sections have been reported for conjugated polymer nanoparticles generated by postpolymerization techniques. Against that background, conjugated polymer nanoparticles from Sonogashira coupling in aqueous miniemulsion, which originated from another PhD project within the research group, were extensively studied for their suitability as fluorescent probes in live cell imaging and for their two-photon action cross sections. Covalently incorporated dyes allowed for the bathochromic tuning of the emission color while keeping the absorption band constant. The nanoparticles were found to be taken up by living HeLa cells accumulating in the cytosol without showing any conceivable cytotoxicity. Visual differentiation of nanoparticle species in mixtures at single-wavelength excitation enabled multicolor fluorescence images. A differentiation of the location of different particle species in cells could be demonstrated with both linear and non-linear excitation. Systems emitting multiple colors at single-wavelength excitation are of general relevance for multiplex bioanalysis and biomedical imaging. Such a system on the basis of conjugated polymer nanoparticles was demonstrated for the first time. The two-photon action cross sections of conjugated polymer nanoparticles from Sonogashira coupling in aqueous miniemulsion were found to range from 106 to 107 Göppert-Meyer units, which is several orders of magnitude brighter than inorganic quantum dots or conventional organic fluorophores. Conjugated polymer nanoparticles from miniemulsion polymerization thus proved to be excellent highly fluorescent photostable probes for cell imaging with extraordinary high two-photon action cross sections enabling multicolor and multiphoton bioimaging of living systems.