A non-covalent complex of quantum dots and chlorin e(6): efficient energy transfer and remarkable stability in living cells revealed by FLIM

A Forster resonance energy transfer (FRET) system of semiconductor quantum dots and porphyrins represents a new promising photosensitizing tool for the photodynamic therapy of cancer. In this work, we demonstrate the ability of a non-covalent complex formed between commercial lipid-coated CdSe/ ZnS quantum dots (QD) bearing different terminal groups (carboxyl, amine or non-functionalized) and a second-generation photosensitizer, chlorin e(6) (Ce-6) to enter living HeLa cells with maintained integrity and perform FRET from two-photon excited QD to bound Ce-6 molecules. Spectroscopic changes, the highly efficient FRET, observed upon Ce-6 binding to QD, and remarkable stability of the QD-Ce-6 complex in different media suggest that Ce-6 penetrates inside the lipid coating close to the inorganic core of QD. Two-photon fluorescence lifetime imaging microscopy (FLIM) on living HeLa cells revealed that QD-Ce-6 complexes localize within the plasma membrane and intracellular compartments and preserve high FRET efficiency (similar to 50%). The latter was confirmed by recovery of QD emission lifetime after photobleaching of Ce-6. The intracellular distribution pattern and FRET efficiency of QD-Ce-6 complexes did not depend on the charge of QD terminal groups. Given the non-covalent nature of the complex, its exceptional stability in cellulo can be explained by a combination of hydrophobic interactions and coordination of carboxyl groups of Ce6 with the ZnS shell of QD. These findings suggest a simple route to the preparation of QD-photosensitizer complexes featuring efficient FRET and high stability in cellulo without using time-consuming conjugation protocols.