Oriented Bioconjugation of Unmodified Antibodies to Quantum Dots Capped with Copolymeric Ligands as Versatile Cellular Imaging Tools

Distinctive optical properties of inorganic quantum dot (QD) nanoparticles promise highly valuable probes for fluorescence-based detection methods, particularly for in vivo diagnostics, cell phenotyping via multiple markers or single molecule tracking. However, despite high hopes, this promise has not been fully realized yet, mainly due to difficulties at producing stable, nontoxic QD bioconjugates of negligible nonspecific binding. Here, a universal platform for antibody binding to QDs is presented that builds upon the controlled functionalization of CdSe/CdS/ZnS nanoparticles capped with a multidentate dithiol/zwitterion copolymer ligand. In a change-of-paradigm approach, thiol groups are concomitantly used as anchoring and bioconjugation units to covalently bind up to 10 protein A molecules per QD while preserving their long-term colloidal stability. Protein A conjugated to QDs then enables the oriented, stoichiometrically controlled immobilization of whole, unmodified antibodies by simple incubation. This QD-protein A immobilization platform displays remarkable antibody functionality retention after binding, usually a compromised property in antibody conjugation to surfaces. Typical QD-protein A-antibody assemblies contain about three fully functional antibodies. Validation experiments show that these nanobioconjugates overcome current limitations since they retain their colloidal stability and antibody functionality over 6 months, exhibit low nonspecific interactions with live cells and have very low toxicity: after 48 h incubation with 1 μM QD bioconjugates, HeLa cells retain more than 80% of their cellular metabolism. Finally, these QD nanobioconjugates possess a high specificity for extra- and intracellular targets in live and fixed cells. The dithiol/zwitterion QD-protein A nanoconjugates have thus a latent potential to become an off-the-shelf tool destined to unresolved biological questions.