Colloidal semiconductor quantum dots (QDs) are attractive materials for bio-imaging and sensing applications due to their favorable optical properties. To be compatible with biological environments, the QD surface must be passivated by appropriate coatings, such as hydrophilic polymers, that promote colloidal stability and help to maintain the quantum yield, while resisting nonspecific interactions. Multiply-binding copolymer ligands in particular have proven to be effective in achieving QDs that exhibit low nonspecific binding and low acute toxicities, however their resistance to interference from endogenous molecules is still of concern. We assessed the stability of core/shell CdSe/CdxZn1−xS QDs coated with imidazole-bearing polymeric ligands in the presence of L-glutathione (GSH) under neutral conditions by means of photoluminescence and absorption spectroscopy. In particular, we synthesized copolymer ligands with different molecular weights in block and random sequences containing imidazole and poly(ethylene glycol) side chains on a methacrylate backbone via reversible addition-fragmentation chain transfer (RAFT) polymerization. This allows us to evaluate the influence of length and monomer sequence on QD stability. Introduction of GSH, an endogenous monothiol, to the polymer-coated QDs led to a reproducible change in PL quantum yield. The PL change was greatly diminished for block copolymers versus random copolymers of similar molecular weight and composition. Additionally, higher ligand populations were found on initial purification of block copolymer-coated QDs versus the random copolymers. We demonstrate that GSH does not significantly displace the polymers from the QD surface, suggesting that PL changes are driven primarily through ligand association. The block copolymers are more resistant to glutathione association —and indeed stronger binders— than the randomly-ordered copolymers, with higher molecular weight contributing to increased stability in both cases. To the best of our knowledge, this is the first study to directly compare the relative stability of block and random polymeric imidazole ligands.

Multiply-binding polymeric imidazole ligands: Influence of molecular weight and monomer sequence on colloidal quantum dot stability

2019 Spring National Meeting

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