Bioconjugates Based on Semiconductor Quantum Dots and Porphyrin Ligands: Properties, Exciton Relaxation Pathways and Singlet Oxygen Generation Efficiency for Photodynamic Therapy Applications

摘要

At the moment, though several examples of employing QDs for the photodynamic therapy (PDT) have been described, their full capabilities have yet to be harnessed, and QD-photosensitizer conjugates are still an intriguing option for PDT applications. In this review, the basic photophysical properties of semiconductor nanocrystals are discussed and their potential conformances with the above criteria are highlighted. Special interest is devoted to the analysis of exciton relaxation pathways in self-assembled bioconjugates comprising a semiconductor CdSe/ZnS QDs coupled with porphyrin molecule (FRET and electron tunneling in the conditions of quantum confinement). Based on these results, available data are presented for intrinsic singlet oxygen generation by alone semiconductor QDs as well as for QD-porphyrin bioconjugates in solutions at ambient temperature. Within the accuracy of the independent experiments, FRET efficiencies found from the direct sensitization of porphyrin fluorescence in bioconjugates are in a good agreement with the corresponding values obtained via the direct ~1O2 generation measurements at low laser excitation, thus providing for the first time strong evidence that namely FRET process QD→porphyrin is a reason of singlet oxygen generation by bioconjugates. Finally, structural and photophysical properties of the above bioconjugates are discussed in the context of possible application in PDT by means of direct optical excitation by a core nanocrystal with subsequent excitation transfer to the molecule which in turn performs singlet oxygen generation in a traditional way as current PDT photosensitizers do, i.e., via the diffusionally controlled energy transfer from the triplet excited photosensitizer to molecular oxygen.