Functionalization of nanocrystals is essential for their practical application, but synthesis on their surfaces is limited by incompatibilities with certain key reagents, as well as a poor understanding of how the organic passivation layer protects the nanocrystal. The copper-catalyzed azide–alkyne cycloaddition is among the most useful methods for ligating molecules to surfaces, but it has been largely useless for semiconductor quantum dots (QDs) because Cu ions quickly and irreversibly quench QD fluorescence. To discover nonquenching synthetic conditions for Cu-catalyzed click reactions on QD surfaces, we developed a combinatorial fluorescence assay to screen over 2000 reaction conditions to maximize cycloaddition efficiency while minimizing QD quenching. We identify conditions for complete coupling without significant quenching, which are compatible with common QD polymer surfaces and various azide/alkyne pairs. Based on insight from the combinatorial screen and mechanistic studies of Cu coordination and quenching, we find that both Cu+ and Cu2+ can quench quantum dot fluorescence, each with a different mechanism: Cu+ ions can pass through the organic barrier to quench QDs irreversibly, while Cu2+ ions are tightly coordinated to polymer surface carboxylates and can act as charge traps. Addition of excess QD ligands such as oleic acid reduces but does not eliminate Cu+ quenching, suggesting that even well-packed organic passivation layers are at least somewhat porous. Synthetically, full click coupling is possible with superstoichiometric concentrations of Cu accompanied by an excess of Cu+ ligands that selectively compete the Cu from the QD surface but allow it to remain catalytically active. Applied to the conjugation of am ion channel-specific peptidyl toxin to CdSe/ZnS QDs, we synthesize unquenched QD conjugates and image their specific and voltage-dependent affinity for ion channels in live cells.

Challenging the polymer barricades around quantum dots: keeping copper ions away from the surface during click reactions

2019 Spring National Meeting

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