Fluorescently Labeled Branched Copolymer Nanoparticles for In Situ Characterization of Nanovectors and Imaging of Cargo Release

Polymeric nanoparticles (NPs) continue to assert their high potential as efficient nanovectors for nanomedicine and imaging agents. Among them, biocompatible polyester block copolymers (BCP) possess appealing features as their physicochemical properties can be readily modified by the composition of their blocks. Although the fluorescent labeling of polymers is often used to track the formed NPs in bioimaging, it is rarely used to characterize their physicochemical properties. In this work, we assumed that the branching degree of the hydrophobic block copolymer might play an important role in the properties of the biocompatible NPs and thus could help in controlling their cargo encapsulation and release. To this end, clickable PEG-PCL block copolymers with various branching degrees were synthesized. Owing to their covalent fluorescent labeling using strain-promoted azide-alkyne cycloaddition, several parameters like the critical aggregation concentration of the BCPs as well as the colloidal stability, the core polarity, and stealth of the NPs have been studied. Taking advantage of the fluorescence labeling of the NPs’ core, their ability to encapsulate and release a fluorescent cargo (Rhodamine C18) was assessed by Förster resonance energy transfer (FRET). The observed differences in the release profile were confirmed in cells where the fluorescent NPs and their cargo were tracked. Finally, the cargo release of endocytosed NPs was imaged and assessed by FRET ratiometric imaging. This study proved that the covalent fluorescent labeling of the BCPs is an efficient tool, offering various methods to characterize and assess the effects of polymers’ modifications on the NPs’ properties.