Highly Luminescent Positively Charged Quantum Dots Interacting with Proteins and Cells^†

We have studied interactions between positively charged MUTAB-stabilized quantum dots (QDs) and model proteins, serum and live cells using fluorescence correlation spectroscopy (FCS), dynamic light scattering (DLS), time-resolved photoluminescence (PL) and live-cell fluorescence imaging. Using human serum albumin (HSA) as a model protein, we measured the growth of a protein adsorption layer (“protein corona”) via time-resolved FCS. Corona formation was characterized by an apparent equilibrium dissociation coefficient, K_D ≈ 10 μM. HSA adlayer growth was surprisingly slow (timescale ca. 30 min), in stark contrast to many similar measurements with HSA and other proteins and different NPs. Time-resolved PL data revealed a characteristic quenching behavior depending on the QD surface coverage with HSA. Taken together, we found that MUTAB-QDs initially bind HSA molecules weakly (K_D ≈ 700 μM); however, the affinity is enhanced over time, presumably due to proton injection into the MUTAB layer by HSA triggering ligand dissociation. This process was also observed with human blood serum, showing equal kinetics for comparable HSA concentration. Moreover, imaging experiments with cultured human cells (HeLa) revealed that MUTAB-QDs bind to the cell membrane and perforate it. This process is reduced upon pre-adsorption of proteins on the MUTAB-QD surfaces.