Anisotropic Polyethylene Nanocrystals Labeled with a Single Fluorescent Dye Molecule : Towards Monitoring of Nanoparticle Orientation

Abstract

The three-dimensional orientation monitoring of anisotropic nanoparticles during dynamic processes is a fundamental issue. Herein we show that incorporation of a single fluorescent reporter molecule is a promising concept toward this goal. As a model system, shape anisotropic single lamella polyethylene (PE) nanocrystals bearing one single fluorescent reporter molecule were prepared via ring-opening metathesis polymerization (ROMP) of highly ring-strained trans-cyclooctene (trCOE) using a mixture of a dye-functionalized ruthenium-based initiator (1; perylene diimide (PDI) substituted Hoveyda–Grubbs second generation Ru alkylidene) and an appropriate excess of the unlabeled analogue (2; Hoveyda–Grubbs second generation Ru alkylidene) in aqueous microemulsion as a key step and subsequent exhaustive hydrogenation (>99.9%) of the main-chain unsaturated polymer in the nanoparticles to yield nanocrystals of high molecular weight, strictly linear PE (Mn = 8 × 105 g mol–1; Mw/Mn = 1.4). TEM and AFM show a particle thickness of ca. 12 nm with a lateral extension of typically 45 nm. Comparable initiation kinetics of both complexes 1 and 2, which is a key requirement for this approach, were revealed by fluorescence spectroscopy studies (ΔH‡ = 57.4 kJ mol–1, ΔS‡ = −73.0 J mol–1 K–1 for 1 vs ΔH‡ = 63.6 kJ mol–1, ΔS‡ = −80.8 J mol–1 K–1 for 2 for the initiation with n-butyl vinyl ether, respectively). The labeled nanocrystals were characterized by means of single molecule fluorescence spectroscopy. Orientational analysis via defocused wide-field fluorescence microscopy (DWFM) revealed a fixed orientation of the chromophores within the nanocrystals, with their long molecular axis predominantly oriented parallel to the polar axis of the nanoparticles.