Conjugated Polymer Nanoparticles for Fluorescent Imaging of Live Cells

Conjugated polymers (CPs) are attractive fluorescent materials that have high potential for microscopic imaging. Superior photophysical properties such as high quantum yield, photostability, and extremely large two-photon (2P) absorptivity make CPs promising potential contrast agents for optical microscopy. Despite these promising characteristics, optical imaging applications of CPs in cellular or tissue systems have been largely limited by the intrinsic hydrophobic nature of conjugated backbones. The challenge is to preserve the attractive photophysical properties of CPs in aqueous phases suitable for biological applications. We have demonstrated that organic acid treatment of CPs followed by ultrafiltration produces water-dispersed conjugated polymer nanoparticles (CPNs) exhibiting high QYs, photostability, and non-toxicity to live cells. The controlled chain-chain interaction by organic acids allows stable nanoparticle formation, while the photophysical properties of CPs are relatively preserved in aqueous phases. Further fine tuning of the particle formation leads to dramatic reduction of the particle size below 10 nm. In addition, CPNs exhibit super bright 2P characteristics and photostability that are comparable to those of QDs. Non-toxicity of CPNs allows for long-term monitoring of endothelial cells in a tissue model, supporting CPN's potential in biological and biomedical applications.