Colloidal NCs consist of an inorganic particle and an organic coating that determines
their solubility, functionality, and influences their photophysics. In order for these NCs to be
biocompatible, they must be water-soluble, nontoxic to the cell, and offer conjugation
chemistries for attaching recognition molecules to their surfaces. In addition they should
efficiently target to biomolecules of interest, be chemically stable, and preserve their high
photostability. The requirements for their application in single-molecule biological studies are
even more stringent: fluorescent NCs should be monodisperse, have relatively small size (to limit
steric hindrance), reduced blinking, large saturation intensity, and high quantum yield (QY).