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Inorganic colloidal nanocrystals (including luminescent quantum dots, QDs, and metal nanoparticles and clusters) provide flexible platforms for arraying various functional molecules ranging from proteins and peptides to optically active molecules. We have investigated the use of redox active complexes and metal clusters to alter the optical and spectroscopic properties of luminescent QDs. In one example, we focused on the charge transfer interactions between ZnS-overcoated CdSe QDs and molecular dopamine; QDs surface-capped with poly(ethylene glycol)-appended dihydrolipoic acid (DHLA-PEG) were covalently coupled to dopamine-isothiocyanate, with control over the valence and separation distance between the dot center and location of the complexes. We measured pronounced pH-dependent PL losses for these assemblies, and found that: (A) the quenching strongly depended on the number of dopamines per QD-conjugate; (B) the PL quenching efficiency was substantially increased in alkaline buffers; (C) the pH-dependent PL losses were also strongly influenced by the presence (or absence) of oxygen in the medium. We attribute these results to dual interactions between the QDs and two forms of dopamine: the reduced catechol and oxidized quinone. These produce valence-, pH- and oxygen-dependent PL quenching combined with shortening of the exciton lifetime. These findings were further supported by transient absorption measurements where changes in the electron and hole intraband relaxation rates were measured when the pH of the medium was changed from acidic to alkaline. |
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