We have designed and synthesized a series of modular ligands based on poly(ethylene glycol) (PEG) coupled with functional terminal groups to promote biocompatibility of water-soluble quantum dots (QDs). Each hydrophilic ligand is comprised of three modules: a PEG single chain to promote hydrophilicity, dihydrolipoic acid (DHLA) unit connected to one end of the PEG chain for strong anchoring on the QD surface, and potential biological functional groups (biotin, amino, and carboxyl groups) at the other end of the PEG. Water-soluble QDs capped with one type or mixtures of the functional ligands were prepared via cap exchange with the native hydrophobic caps. Homogeneous QD solutions that are stable over extended periods of time and over broad pH range were prepared. Surface binding assay showed that DHLA-PEG-biotin-functionalized QDs strongly interacted with NeutrAvidin-modified surfaces. The new functional surface ligands studied here provide not only stable and highly water-soluble QDs but also simple and easy access to various biological entities.
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