Department of Chemistry, 77 Massachusetts Avenue, Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts 02139, USA.
We describe a method to generate monovalent quantum dots (QDs) using agarose gel electrophoresis. We passivated QDs with a carboxy-terminated polyethylene-glycol ligand, yielding particles with half the diameter of commercial QDs, which we conjugated to a single copy of a high-affinity targeting moiety (monovalent streptavidin or antibody to carcinoembryonic antigen) to label cell-surface proteins. The small size improved access of QD-labeled glutamate receptors to neuronal synapses, and monovalency prevented EphA3 tyrosine kinase activation.