Stable, small, specific, low-valency quantum dots for single-molecule imaging.

We have developed a strategy for synthesizing immediately activable, water-soluble, compact (∼10-12 nm hydrodynamic diameter) quantum dots with a small number of stable and controllable conjugation handles for long distance delivery and subsequent biomolecule conjugation. Upon covalent conjugation with engineered monovalent streptavidin, the sample results in a population consisting of low-valency quantum dots. Alternatively, we have synthesized quantum dots with a small number of biotin molecules that can self-assemble with engineered divalent streptavidin via high-affinity biotin-streptavidin interactions. Being compact, stable and highly specific against biotinylated proteins of interest, these low-valency quantum dots are ideal for labeling and tracking single molecules on the cell surface with high spatiotemporal resolution for different biological systems and applications.