Miniemulsion polymerization-formulated poly(fluorene-alt-6-(2-ethylhexyl)-[1,2,5]thiadiazole[3,4-f]benzotriazole) for cancer cell imaging

Near-infrared (NIR) conjugated polymer nanoparticles (CPNPs) possesses advantages for bioimaging because of their merits in low autofluorescence interference, good biocompatibility, large light extinction coefficient, improved photostability, and facile functionalization capability. Different from conventional post-polymerization methods for fabricating NIR CPNPs, like nanoprecipitation, emulsion and solvent evaporation/miniemulsion, and self-assembly, herein, an efficient one-pot strategy by in-situ miniemulsion polymerization is demonstrated to synthesize CPNPs for NIR fluorescence imaging of cancer cells. Via adjusting the donor-acceptor strength, the new CP, poly(fluorene-alt-6-(2-ethylhexyl)-[1,2,5]thiadiazole[3,4-f]benzotriazole) (PFTA), show absorption and emission maximum at 648 nm and 730 nm, respectively, which is much more red-shifted than its counterpart, poly(fluorene-alt-benzo-2,1,3-thiadiazole) (PFBT). DFT calculation demonstrates that the repeating unit of PFTA has a smaller dihedral angel to 33.2° and much reduced energy gap to 2.46 eV, as compared to that of PFBT NPs. Importantly, through controlling surfactant concentrations from 40 g·L⁻¹ to 2.5 g·L⁻¹ during the miniemulsion polymerization, the particles size can be accordingly changed from even 88 to 210 nm. Furthermore, the particles show extinction coefficient of 2.82 × 10⁴ M⁻¹·cm⁻¹at 648 nm, photoluminescence quantum yields c.a. 2.4% and fluorescence lifetime c.a. 1.26 ns, respectively. Moreover, the PFTA NPs exhibit biocompatibility, good colloidal stability and bright fluorescence signals to clearly pinpoint breast cancer cells, with a trend that smaller NP size, higher cellular uptake. This work demonstrates the promise of miniemulsion polymerization as a facile method to fabricate NIR CPNPs for bioimaging.