A Modular Phase Transfer and Ligand Exchange Protocol for Quantum Dots

In this paper, we describe a quantum dot (qdot) phase tranfer proto using ligand exchange and the amino acid.: histidine. The phase transfer from nonpolar solvents to aqneous.,hufferkisihOrriogenecius, and no appredable precipitaficiriodctir TheinnOle'dilel-histidine was chosen in order to first displace the organic encapsulation and Second to provide a weakly, chemisorbing intermediate at the qdot ionic interface.This allows the histidine to act as an intermediate shell upon which further direct ligand exchange can occur. Since this intermediate encapsulation is easily displaced by an assortment of different molecules while in aqueous buffers, we refer to this approach as modular. Characterization via FTIR and NMR revealed the extent of ligand exchange, and provides insights into the interfacial binding mechanism. The colloidal stability and photostability of the qdots was probed via UV-vis and steady state fluorescence, which revealed promising quantum yield stability of greater than 1 year. The qdots have hydrodynamic diameters of <12 nm and surface charges dependent upon ligand type and coverage. The modularity of this approach is shown by tailoring the qdot surface charge via sequential ligand exchange using mixed monolayers of carboxylic acid and poly(ethylene glycol)-terminated thiols.