Combining Ligand Design with Photoligation to Provide Compact, Colloidally Stable, and Easy to Conjugate Quantum Dots

Abstract

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.

Colloidal nanoparticles as advanced biological sensors

“…Short ethylene glycol segments can also be included to control nonspecific adsorption (46). Zwitterionic ligands yield exceptionally stable nanoparticles and are the state of the art for compactness and fouling resistance, particularly when combined with novel phase transfer routes like photoligation (47).…”

99mTc radiolabelling of Fe3O4–Au core–shell and Au–Fe3O4dumbbell-like nanoparticles

“…26 After deprotection of the boc-group, the linker was conjugated to mesyl-LA. 27 Ligand 2 was directly obtained after this step, whereas 3 and 4 required basic hydrolysis of the ethyl ester (Scheme 1). Since we did not intend to coat the NPs quantitatively with the chelator containing ligands, we synthesised compound 1 as a basic coating ligand with a terminal methyl ether, which is known to provide high colloidal stability.…”

Quantifying the density of surface capping ligands on semiconductor quantum dots

“…[30,41,42] Each ligand contained one lipoic acid, a short poly(ethylene glycol) chain as a spacer and a terminal group made of -OCH 3, -CHO or -N 3 . The synthetic routes to prepare LA-PEG 750 -OCH 3 and LA-PEG 1000 -N 3 have been detailed in previous literature (see Figure 1 for ligand structures).…”