Multidentate Hybrid Ligand for Efficient and Robust Hydrophilic Quantum Dots

Colloidal quantum dots (QDs) are promising nanomaterials in various fields thanks to high photoluminescence quantum yield (PLQY), band gap tunability and spectrally pure emission profile. To utilize QDs to various applications, surface functionalization, exchanging original surface ligands with functional moieties, is typically demanded for solvent exchange, orthogonal processing capability, long-term colloidal stability, morphology control, and so on. However, typical ligand exchange protocols are likely to deteriorate innate properties of QDs owing to surface etching by excessive use of introducing ligands, for instance, a reduction in PLQY and colloidal stability during/after the fabrication of aqueous QDs. Adopting multidentate bindings in the functional molecules are one of promising approaches for resolving such troubles as an increased binding affinity allows for effective ligand exchange with minimal amount of ligands. However, the limited number of multidentate ligands and complex synthesis routes for granting multidentate nature obstruct their pervasive use.<br/>Herein, we propose the facile synthesis of multidentate hybrid composed of zinc thiolate backbone and functional hydrocarbon branches. For demonstration, zinc chloride (ZnCl₂) and 3-mercaptopropionic acid (MPA) were chosen for preparing hydrophilic hybrid ligands. ¹H nuclear magnetic resonance spectroscopy and Fourier transformed infrared spectroscopy revealed two different configurations of linear and toroidal structure. Their multidentate binding on QDs was confirmed in two-dimensional ¹H nuclear Overhauser effect spectroscopy displaying clear negative cross peaks from propionate branches bound on surface. Titration of CdSe QDs using the hybrid ligands revealed enhanced binding affinity of the hybrid ligands, enabling effective aqueous phase transfer with ~70-fold fewer binding groups compared to MPA. The aqueous QDs using the hybrid ligands featured sustained PLQY and photochemical stability; the hybrid-capped InP/ZnSe/ZnS QDs displayed ~90% of PLQY that is the record efficient to our best knowledge. Prolonged shelf-life and resistance to ultraviolet irradiation were also achieved. We attribute these enhancements to the robust, thick hybrid ligand shell prohibiting invasion of oxygen and water molecules. We believe that our hybrid ligands are expected to serve as useful building blocks for nanocrystal functionalization.