Synthesis of water-soluble dye-cored poly(amidoamine) dendrimers for long-term live cell imaging

Hydrophilic dendrimers, especially poly(amidoamine) (PAMAM) dendrimers are widely applied in modifying fluorescent dyes to endow them with water solubility and biocompatibility for biologic fluorescence imaging. Common preparation strategies of fluorescent dendrimers including encapsulating dyes or attaching dyes at periphery of dendrimers might cause uncertain constituent and lower biocompatibility. Here, we have developed a series of water-soluble fluorescent dendrimers with dye as core and fan-shaped PAMAM as arms. Carboxylated perylene bisimides (PBI) dye and squarylium indocyanine (SICy) dye were conjugated with PAMAM dendrons by amidation to obtain a series of fluorescent dendrimers with enhanced water-solubility. Two PBI based dendrimers (PBI-G2.5 and PBI-G1.5) were chosen as model compounds for further optical, self-assembly and biological studies. In aqueous environment, PBI-G2.5 exhibited strong fluorescence, small size (similar to 30 nm) and slightly positive surface charge (similar to 2.46 mV), which are ideal for biomedical applications. In vitro assays demonstrated that PBI-G2.5 nanoparticles accumulated in the cytoplasm of HeLa cells with rapid cellular uptake. The strong fluorescence in HeLa cells remained for over 48 h. To conclude, our study provides an effective strategy for preparing water-soluble fluorescent dendrimers towards long-term live cell imaging.