Tuning the Redox Coupling between Quantum Dots and Dopamine in Hybrid Nanoscale Assemblies

We explored the charge transfer interactions between CdSe–ZnS core–shell quantum dots (QDs) and the redox active neurotransmitter dopamine, using covalently assembled QD–dopamine conjugates. We combined steady-state fluorescence, time-resolved fluorescence, and transient absorption bleach measurements to probe the effects of changing the QD size (thus the QD energy levels) and the conjugate valence on the rate of QD photoluminescence quenching when the pH of the medium was adjusted from acidic to alkaline. We measured substantially larger quenching efficiencies, combined with more pronounced shortening of the carrier dynamics of these assemblies for smaller size QDs and in alkaline pH. Moreover, we found that changes in the QD size alter the electron and hole relaxation of photoexcited QDs but with different extents. For instance, a pronounced change in the hole relaxation was measured in alkaline buffers. Moreover, the hole relaxation was faster for conjugates of green-emitting QDs as compared to their red-emitting counterparts. We attribute these results to the more favorable electron transfer rates from the reduced form of the dopamine to the valence band of the QDs, a process that becomes more efficient for green-emitting QDs. The latter benefits from lower oxidation potential and larger energy mismatch with the green QDs in alkaline buffers. In comparison, the effects of pH changes on the rates of electron transfer from excited QDs to dopamine are less affected by the QD size. These findings reflect the importance of the energy mismatch between the QD energy levels and the redox levels of dopamine, and shed light onto the complex interactions involved in these assemblies. Such conjugates also provide promising sensing and imaging tools for use in in vivo experiments.