Efficient dendrimers based on naphthalene indenofluorene for two-photon fluorescent imaging in living cells and tissues

Two novel dendrimer molecules (CzNIF and AnNIF) based on naphthalene indenofluorene chromophores were synthesized. The photophysical studies demonstrated that CzNIF exhibited typical charge transfer characteristics, and AnNIF showed local excited state-dominant properties. The two target dendrimers exhibited higher photoluminescence quantum yields in solution (ΦPL, 55.9% for CzNIF in toluene, 84.4% for AnNIF in dimethylformamide) and large Stokes shifts (142 nm for CzNIF, 139 nm for AnNIF). AnNIF exhibited intense two-photon excited fluorescence, with a large two-photon absorption cross-section (δ2) of 5437 GM in the range of 700–760 nm in dichloromethane solution. Water-dispersible nanoparticles (NPs) based on CzNIF and AnNIF were also fabricated. The AnNIF NPs exhibited a moderate ΦPL of 33% and a higher δ2 of 1.16 × 105 GM in water upon excitation by a 740 nm fs laser. Cytotoxicity experiments and flow cytometric analysis indicated that the two types of NPs had good biocompatibility for living cells and facilitated HeLa cell proliferation, especially AnNIF NPs. Studies with one- and two-photon fluorescence microscopy showed that the target NPs exhibited good photostability and could penetrate cells and enter the cytoplasm. Furthermore, the AnNIF NPs were successfully applied in two-photon fluorescence imaging of hepatic tissues and blood vessels of mouse ear and showed good deep-tissue imaging with a depth of 284 μm and 200 μm, respectively, illustrating that the target dendrimer molecule NPs possess great potential as excellent two-photon excitation fluorescence agents in various biological applications.