Highly Emissive BODIPY-Grafted Polymer Dots for Cellular and Tissular STED Imaging

Stimulated emission depletion (STED) microscopy requires fluorescent probes to exhibit high brightness, good photostability, a sensitive optical depletion response, and narrow spectral features. There are great interests in using polymer dots (Pdots) for STED imaging due to their exceptional brightness and photobleaching resistance. However, the conventional Pdots either suffer from broad spectra or an unsatisfactory STED response. Herein, we developed a general method for obtaining Pdots with desirable optical properties for STED microscopy. Specifically, boron-dipyrromethene (BODIPY) chromophores were grafted on to a polystyrene backbone to obtain polymers with narrow spectral profiles. The grafting ratio was precisely controlled to minimize aggregation-induced quenching. Conjugating BODIPYs to side chains reduced interactions between the chromophores, resulting in a long excited state lifetime, which is critical for obtaining complete fluorescence depletion. Using this strategy, we synthesized three-color Pdots with narrow spectra features. Compared to directly encapsulating BODIPYs into nanoparticles, our strategy achieved 2-10 times higher single-particle brightness. We used Pdots for single-particle, cellular, and tissular STED imaging. The Pdots showed high spatial resolutions and could clearly resolve subdiffraction-limit structures in cells and tissue sections, indicating great application potential in in vitro diagnostics and biomedical imaging applications.