Self-assembly of semiconductor quantum dots with porphyrin chromophores: Energy relaxation processes and biomedical applications

A "bottom-up" self-assembly strategy was used for the directed formation of two types of heterogeneous organic-inorganic nanoassemblies based on colloidal semiconductor quantum dots (QDs) and porphyrin molecules: (i) TOPO-capped CdSe/ZnS core/shell QDs attached via coordination interactions with tetra-pyridylporphyrins, H2P(3'-Py)4, in toluene, and (ii) AgInS/ZnS core/shell QDs stabilized by glutathione (GSH) electrostatically coupled with positively charged porphyrin molecules H2P(4'-MePy+)4 via Coulomb attraction in water. Using a quantitative experimental analysis of the QD photoluminescence quenching in QD-porphyrin nanoassemblies (steady-state, time-resolved, time correlated single photon counting mode (TCSPC) spectroscopy) as well as theoretical calculations and quantum chemical modeling, it is shown that upon attachment of porphyrin molecules to QDs surface-mediated processes dictate the probability of several competing non-radiative photophysical phenomena leading to static and dynamic quenching of the QD photoluminescence: (i) non-radiative energy transfer QDRTporphyrin of Foerster type; (ii) non-FRET processes including electron tunneling beyond the QD core under conditions of quantum confinement; (iii) the influence of the dynamics of capping ligand molecules leading to the formation of near band edge states (surface traps). The competition between FRET and non-FRET quenching processes drastically depends on the QD size as well as the solvent and the spatial displacement of porphyrin macrocycles on the QD surface. On the basis of the results obtained the nanoassemblies are demonstrated to be promising for various biomedical applications (drug delivery carriers, distant testing of local pH, singlet oxygen generation for the photodynamic treatment, etc.).