Single-Pot Biofabrication of Zinc Sulfide Immuno-Quantum Dots

Quantum dots (QDs) are a powerful alternative to organic dyes and fluorescent proteins for biological and biomedical applications. These semiconductor nanocrystals are traditionally synthesized above 200 °C in organic solvents using toxic and costly precursors, and further steps are required to conjugate them to a biological ligand. Here, we describe a simple, aqueous route for the one-pot synthesis of antibody-derivatized zinc sulfide (ZnS) immuno-QDs. In this strategy, easily expressed and purified fusion proteins perform the dual function of nanocrystal mineralizers through ZnS binding sequences identified by cell surface display and adaptors for immunoglobin G (IgG) conjugation through a tandem repeat of the B domain of Staphylococcus aureus protein A. Although ≈4.3 nm ZnS wurtzite cores could be biomineralized from either zinc chloride or zinc acetate precursors, only the latter salt gives rise to protein-coated QDs with long shelf life and narrow hydrodynamic diameters (8.8 ± 1.4 nm). The biofabricated QDs have a quantum yield of 2.5% and blue-green ensemble emission with contributions from the band-edge at 340 nm and from trap states at 460 and 665 nm that are influenced by the identity of the protein shell. Murine IgG1 antibodies exhibit high affinity (Kd = 60 nM) for the protein shell, and stable immuno-QDs with a hydrodynamic diameter of 14.1 ± 1.3 nm are readily obtained by mixing biofabricated nanocrystals with human IgG.