Combining ligand design with photoligation to provide compact, colloidally stable, and easy to conjugate quantum dots

We describe the design and synthesis of two compact multicoordinating (lipoic acid-appended) zwitterion ligands for the capping of luminescent quantum dots, QDs. This design is combined with a novel and easy to implement photoligation strategy to promote the in situ ligand exchange and transfer of the QDs to buffer media. This method involves the irradiation of the native hydrophobic nanocrystals in the presence of the ligands, which promotes in situ cap exchange and phase transfer of the QDs, eliminating the need for a chemical reduction of the dithiolane groups. Applied to the present LA-zwitterion ligands, this route has provided QDs with high photoluminescence yields and excellent colloidal stability over a broad range of conditions, including acidic and basic pH, in the presence of growth media and excess salt conditions. The small lateral extension of the capping layer allowed easy conjugation of the QDs to globular proteins expressing a terminal polyhistidine tag, where binding is promoted by metal-affinity interactions between the accessible Zn-rich surface and imidazoles in the terminal tag of the proteins. The ability to carry out conjugation in acidic as well as basic conditions opens up the possibility to use such self-assembled QD-protein conjugates in various biological applications.