Multiply-Binding Polymeric Imidazole Ligands: Influence of Molecular Weight and Monomer Sequence on Colloidal Quantum Dot Stability

Abstract

This study addresses the colloidal stability of polymer-coated core/shell CdSe/Cd x Zn 1−x S quantum dots (QDs) in the presence of L-glutathione (GSH) under neutral conditions by means of photoluminescence (PL) and absorption spectroscopy. A reversible addition−fragmentation chain-transfer-mediated synthesis was employed to produce co-polymer ligands with different molecular weights in block and random sequences. Poly(ethylene glycol) sidechains were incorporated on a methacrylate backbone for solubility in phosphate-buffered saline. Imidazole-bearing histamine sidechains were installed postsynthetically to facilitate binding to the QD surface. Introduction of L-glutathione, an endogenous monothiol, to aqueous solutions of the polymer-coated QDs led to an increase in PL quantum yield, indicating an interaction between the QD and glutathione. The PL change was greatly diminished for block co-polymers versus random co-polymers of similar molecular weight and composition. Additionally, higher ligand populations were found upon initial purification of block co-polymer-coated QDs versus the random co-polymers. We demonstrate that glutathione does not significantly displace the polymer ligands 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 co-polymers, with higher-molecular-weight contributing to improved 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 and should aid in developing more stable ligand-exchanged QDs for bioimaging and biosensing, especially for in vivo and intracellular applications.

Compact, “Clickable” Quantum Dots Photoligated with Multifunctional Zwitterionic Polymers for Immunofluorescence and In Vivo Imaging

“…In earlier studies, polymer ligands appended with thiol anchors have been extensively exploited, since these groups exhibit strong affinity to QD surfaces. ,− However, thiol groups tend to negatively affect the fluorescence properties of QDs. , In addition, this mode of surface coordination can be weakened by oxidation of the thiols . To overcome some of these drawbacks, imidazole-based polymer ligands have been proposed as an alternative to thiolated ones. − Imidazole is not affected by issues of photo-oxidation, and it can potentially enhance the QD fluorescence. ,, Nonetheless, the coordination strength of imidazoles is weaker than that of thiols, particularly in acidic conditions (pH < 6) . Building on these observations, our group explored the use of polymer ligands with mixed coordination where two types of anchoring groups, lipoic acid and imidazole, are combined within the same ligand structure to bind on the QDs .…”

Nanoengineering with RAFT polymers: from nanocomposite design to applications

“…One way of circumventing this issue is to use macromolecular ligands, which can be easily prepared via RAFT polymerization. Here, a multiply-binding copolymer design has been proven to be very effective: one type of monomer contains an anchoring group for QD binding, such as thiol, 164 carboxyl, 146,147 imidazole 112,188,189 and 8-hydroxyquinoline 241 moieties. Another part of the polymer adjusts dispersibility and provides additional properties.…”

“…Again, RAFT polymerization provides a very straightforward synthesis of functional polymers containing PEG derivatives with suitable anchoring groups for diverse nanomaterials. 111,146,181,188,189,238 Zwitterionic polymers possess equal cationic and anionic moieties on the chain: the polymer maintains charge neutrality while carrying high ion density. This unique property makes them highly resistant to nonspecific protein adsorption due to their strong hydration ability resulting from the high charge density.…”

“…Combing both hydrophilic block and anchoring blocks provides long-term colloidal stability across a wide pH-range and against contaminations. 112,161,188,189 Lequeux and co-workers fabricated zwitterionic vinylimidazole BCPs using RAFT polymerization to decorate the surface of QDs (Fig. 6A).…”

“…Also, higher molecular weights contributed to higher colloidal stability of QDs for both types of polymers. 189 3.1.2. Thermoresponsive polymer shells.…”