Nonconventional Fluorescent Hyperbranched Polymer Dots as Skin Nanocarriers Constructed from an Olefinic Aliphatic AB₂-Type Monomer

We constructed an olefinic aliphatic AB2 monomer (π-AB2) for the synthesis of a nonconventional fluorescent hyperbranched polymers (π-HBPs) through self-polymerization. We obtained π-AB2 from N-(3-aminopropyl)diethanolamine and maleic anhydride in three steps with a total yield of 79%. The π-HBPs possessed hyperbranched poly(amido acid) structures with internal alkene moieties, branched amino groups, and carboxyl terminal units. With their nonconjugated hyperbranched structures, the π-HBPs functioned as water-soluble polymer dots. Inter- and intramolecular hydrogen bonding in the π-HBPs resulted in a physically cross-linked structures displaying nonconventional fluorescence. This fluorescence arose from spatial conjugation at the molecular level, leading to clustering-induced emission with the lone pairs of electrons and the π-electrons interacting in a narrow confinement space. The emission of π-HBP 03 occurred at 460 nm upon excitation of 390 nm, with quantum yields of 13.5 and 7.5% at pH 7 and 3, respectively. From an examination of the relationship between chemical structure and photoluminescence, we found that the fluorescence emissions of the HBP without internal alkene moieties and π-HBP 03 (with internal alkene moieties) were 395 and 460 nm, respectively (a red-shift of 65 nm when removing the olefinic units). The self-assembly of π-HBP 03 in aqueous solution produced nanosized (ca. 9.1 nm) particles, as determined through dynamic light scattering. π-HBP 03 is noncytotoxic, with a cell viability of >90% at 1500 μg mL–1. When applied to percutaneous absorption mouse skin, the penetration ability of the π-HBPs increased in various time profiles according to the cluster size and nanostructure of the polymer dots. When we incubated HaCaT cells with π-HBP 03 for 24 h, confocal microscopy revealed that π-HBP 03 became located within the cells. Furthermore, assays revealed that π-HBP 03 displayed effective antioxidant abilities when employed at concentrations of 0.6 and 1.5 mg mL–1.