Self-Assembled Quantum Dot-Bioconjugates: Characterization and Use for Sensing Proteolytic Activity

摘要

We present a characterization of the metal-affinity driven self-assembly between luminescent CdSe-ZnS core-shell semiconductor quantum dots (QDs) and either peptides or proteins appended with various length terminal polyhistidine tags. We first monitor the kinetics of self-assembly between surface-immobilized QDs and proteins/peptides under flow conditions (immobilized). To accomplish this, the QDs were immobilized onto functionalized substrates and then exposed to dye-labeled peptides/proteins. By using evanescent wave excitation of the substrate, self-assembly was assessed by monitoring the time-dependent changes in the dye fluorescence. This configuration was complemented with experiments using freely diffusing QDs and proteins/peptides (solution-phase) via energy transfer between QDs and dye-labeled proteins/peptides. Cumulatively, these measurements allowed determination of kinetic parameters, including association and dissociation rates (k_(on) and k_(off)) and the binding constant (K_d). We find that self-assembly is rapid with an equilibrium constant K_d~(-1) in the low nM. We next demonstrate the importance of understanding this self-assembly by creating QD-peptide bioconjugates which we employ as substrates to monitor the cleavage activity of proteolytic enzymes. This confirms that metal-affinity interactions can provide QD-bioconjugates that are functional and stable.