Fluorescent nanodiamonds (FND) as specifically targeted probes to image the interactions of transferrins (Tf) with their receptors that were over-expressed on HeLa cells were studied in this work. The 100-nm diamonds were irradiated with a 2.5-MeV ion beam and thermally annealed to emit bright fluorescence in the region 550-800 nm upon excitation at 514.5 nm; accordingly, the interference of cellular autofluorescence at 520-650 nm was eliminated significantly on detecting fluorescence at > 660 nm. FND-Tf bioconjugates were produced via the formation of an amide bond between carboxylated FND and transferrin and used to target transferrin receptors (TfR) on HeLa cells for activity testing. The receptor-mediated uptake of FND-Tf bioconjugates into HeLa cells was confirmed from confocal fluorescence images; in contrast, the uptake of FND-Tf bioconjugates on presaturated receptors or carboxylated FND on HeLa cells was unobserved. We measured also the fluorescence spectra and lifetimes of carboxylated FND, FND-Tf bioconjugates and FND-Tf-TfR complexes for characterization. Their measured lifetimes are 11.8 ± 0.1, 11.6 ± 0.1 and 12.1 ± 0.1 ns, respectively, and fluorescence spectra are comparable except that the two zero-phonon lines of FND-Tf-TfR complexes become indistinct. Both results show that the surface effects of chemical interactions on the emission center of FND are negligible.
- Confocal fluorescence images
- Fluorescence lifetime