Hydrodynamic dimensions, electrophoretic mobility, and stability of hydrophilic quantum dots

Luminescent semiconductor quantum dots (QDs) have great potential for use in biological assays and imaging. These nanocrystals are capped with surface ligands (bifunctional molecules, amphiphilic polymers, phospholipids, etc.) that render them hydrophilic and provide them with functional properties. These coatings alters their hydrodynamic radii and surface charge, which can drastically affect properties such as diffusion within the cell cytoplasm. Heavy atom techniques such as transmission electron microscopy and X-ray scattering probe the inorganic core and do not take into account the ligand coating. Herein we use dynamic light scattering to characterize the hydrodynamic radius (R(H)) of CdSe-ZnS QDs capped with various hydrophilic surface coatings (including dihydrolipoic acid and amphiphilic polymers) and self-assembled QD-protein bioconjugates. Experiments were complemented with measurements of the geometric size and zeta potential using agarose gel electrophoresis and laser Doppler velocimetry. We find that the effects of surface ligands on the hydrodynamic radius and on the nanoparticle mobility are complex and strongly depend on a combination of the inorganic core size and nature and lateral extension of the hydrophilic surface coating. These properties are critical for the design of QD-based biosensing assays as well as QD bioconjugate diffusion in live cells.