Freeze-Resistant Cadmium-Free Quantum Dots for Live-Cell Imaging

A new strategy has been developed to provide freeze-resistant, water-soluble, and cadmium-free quantum dots made of CuInS2/ZnS, which can easily be conjugated with biomolecules. A new class of multifunctional and tetra-coordinating ligands has been synthesized based on lipoic acid, starting from lysine. The synthesized ligands not only provide facile ligand exchange of CuInS2/ZnS quantum dots but also offer an exceptional colloidal stability upon freezing, robust photophysical properties in various aqueous media across a wide pH range, and buffer composition with a tunable surface charge for the quantum dots. The quantum dots after ligand exchange is easily amenable to covalent modification with biotin binding traptavidin protein exclusively by employing simple carbodimide coupling chemistry. The quantum dots also showed excellent biocompatibility and low nonspecific binding to the cells and, thus, are suitable for live-cell imaging of cell-surface receptors.