Water-soluble and highly emissive near-infrared nano-probes by co-assembly of ionic amphiphiles: towards application in cell imaging

Water-soluble near-infrared (NIR) fluorescent dyes are extremely valuable in cell imaging. We here designed and synthesized an amphiphilic fluorescent dye (denoted by PBI-TPE-11), a bolaamphiphile bearing a conjugation of tetraphenylethylene and perylene bisimide in the middle and two aliphatic pyridinium at both ends. PBI-TPE-11 self-assembled into flake-like nanostructures in aqueous solution, and showed very weak fluorescent emission from 600 to 830 nm, covering the NIR region. This result seems discrepant with that previously reported in the literature, where the conjugation of PBI and TPE was proven an aggregation induced emission enhancing agent. Very interestingly, both the morphology and the emission intensity were altered by addition of sodium dodecyl benzene sulfonate (SDBS). Co-assembly of PBI-TPE-11 and SDBS formed nanowires, observed by atomic force microscope. Moreover, the emission of the co-assemblies was much stronger than the assemblies of neat PBI-TPE-11. An exciting quantum yield (QY) of 15% was obtained for the co-assemblies, while pure PBI-TPE-11 showed a QY of merely 0.2%. Finally, the co-assemblies were successfully applied in labeling the HeLa cells, and high viability and high contrast fluorescent images were achieved.