PEGylated conjugated polyelectrolytes containing 2,1,3-benzoxadiazole units for targeted cell imaging

Two amphiphilic conjugated polyelectrolytes (CPEs) were designed and synthesized via a post-polymerization strategy. A neutral precursor polymer of poly(9,9-bis(6'-azidohexyl)fluorene-alt-2,1,3-benzoxadiazole) (PFBD-N-3) was synthesized first through Suzuki polymerization, which was followed by click reaction to incorporate dense poly(ethylene glycol) (PEG) to afford PFBD-PEG(600)-COOH and PFBD-PEG(2000)-COOH, respectively. The CPEs show better solubility in water with increasing PEG chain length due to the increased hydrophilicity of longer PEG chains. Both CPEs formed nanoparticles (NPs) in water and were used as cellular probes for visualization of HT-29 cancer cells. As compared to PFBD-PEG(600)-COOH NPs, PFBD-PEG(2000)-COOH NPs were less efficiently taken up by cells due to the longer PEG side chains, which significantly inhibited nonspecific cellular uptake. Further conjugation of PFBD-PEG(2000)-COOH with c(RGDfK) yielded PFBD-PEG(2000)-RGD NPs as a specific cellular probe for targeted cancer cell imaging. Preliminary study revealed that PFBD-PEG(2000)-RGD NPs were favorable for targeted cellular imaging with low cytotoxicity, high selectivity and good photostability.