Preparation of QD conjugates. Organic-soluble, CdSe/ZnS core-shell
nanocrystals21,22 were isolated from hexanes and ligand solution with an
equal volume of methanol, rinsed with methanol, and redispersed in CHCl3.
These materials were mixed with neutralized amphiphilic polymer (40%
octylamine-modified polyacrylic acid, 2,000 units/QD) in CHCl3, and the
solvent evaporated. The dry film was redispersed in water and purified from
excess polymer by gel filtration. The surface coating was cross-linked further
by EDC (1-ethyl-3-(3-dimethylamino propyl)carbodiimide)-mediated coupling to lysine (or polyethylene glycol–lysine), and these materials were then
coupled to streptavidin or antibodies by an EDC-mediated coupling reaction in 10 mM borate buffer, pH 8.0. Unless otherwise noted, QD bioconjugates were diluted for use in 10 mM borate buffer, pH 8.2. QD 535, QD 560,
QD 608, and QD 630 were used for this report.