Small-molecule ligands strongly affect the Förster resonance energy transfer between a quantum dot and a fluorescent protein

We report herein the study of Forster resonance energy transfer (FRET) between a CdSe/ZnS core/shell quantum dot (QD) capped with three different small-molecule ligands, 3-mercaptopropionic acid (MPA), glutathione (GSH), and dihydrolipoic acid (DHLA), and a hexa-histidine (His6)-tagged fluorescent protein, mCherry (FP). The Forster radius (R0) and the corresponding donor–acceptor distances (r) for each of the QD–FP FRET systems were evaluated by using the Forster dipole–dipole interaction formula. Interestingly, both the FRET efficiency (E) and r were found to be strongly dependent on the capping small-molecule ligands on the QD surface, where E ≈ 85% was obtained at a FP : QD copy number of 2 : 1 for the MPA capped QD, while that for the DHLA capped QD was 1), suggesting that the QD–His6-tagged biomolecule self-assembly is a facile, effective approach for making compact QD-bioconjugates which may have a wide range of sensing and biomedical applications.