Non-blinking, highly luminescent, pH- and heavy-metal-ion-stable organic nanodots for bio-imaging

Heavy-metal-free, organic-small-molecule-based fluorescent nanodots (Sdots, 31-60 nm in diameter) based on 2,7-di(4-(diphenylamino)phenyl-2,1,3-benzothiadiazol-7-yl)-9,9'-spirobifluorene (Spiro-BTA) were prepared through a simple solution process. The Sdots show not only non-blinking and high-brightness fluorescence but also stability in various pH conditions and heavy metal ion solutions. More importantly, Spiro-BTA Sdots demonstrate obviously much higher brightness with very high signal-to-background ratio in Hela cell, compared with common CdSe/ZnS QDs. In addition, they exhibit large Stokes shifts, broad absorption spectra, and low toxicity to living cells which enable their applications as good fluorescence probes for bio-imaging. Further application of Sdots for folate receptor-mediated live-cell endocytosis was demonstrated by non-convalent modification with folic acid linked multidentate ligands.