Multidentate Polymer Coatings for Compact and Homogeneous Quantum Dots with Efficient Bioconjugation

Abstract

Quantum dots are fluorescent nanoparticles used to detect and image proteins and nucleic acids. Compared with organic dyes and fluorescent proteins, these nanocrystals have enhanced brightness, photostability, and wavelength tunability, but their larger size limits their use. Recently, multidentate polymer coatings have yielded stable quantum dots with small hydrodynamic dimensions (≤10 nm) due to high-affinity, compact wrapping around the nanocrystal. However, this coating technology has not been widely adopted because the resulting particles are frequently heterogeneous and clustered, and conjugation to biological molecules is difficult to control. In this article we develop new polymeric ligands and optimize coating and bioconjugation methodologies for core/shell CdSe/Cd(x)Zn(1-x)S quantum dots to generate homogeneous and compact products. We demonstrate that "ligand stripping" to rapidly displace nonpolar ligands with hydroxide ions allows homogeneous assembly with multidentate polymers at high temperature. The resulting aqueous nanocrystals are 7-12 nm in hydrodynamic diameter, have quantum yields similar to those in organic solvents, and strongly resist nonspecific interactions due to short oligoethylene glycol surfaces. Compared with a host of other methods, this technique is superior for eliminating small aggregates identified through chromatographic and single-molecule analysis. We also demonstrate high-efficiency bioconjugation through azide-alkyne click chemistry and self-assembly with hexa-histidine-tagged proteins that eliminate the need for product purification. The conjugates retain specificity of the attached biomolecules and are exceptional probes for immunofluorescence and single-molecule dynamic imaging. These results are expected to enable broad utilization of compact, biofunctional quantum dots for studying crowded macromolecular environments such as the neuronal synapse and cellular cytoplasm.

Quantum dot surface engineering: Toward inert fluorophores with compact size and bright, stable emission

“…We recently screened a variety of intermediate ligands and found that weakly bound X-type ions (hydroxide) [47] are optimal for yielding compact and homogeneous products (Fig. 8d) due to their high charge that stabilizes the particles in polar solvents and allows rapid displacement by multidentate polymers [189]. By heating at high temperature (110°C), small QD clusters are dissociated, the polymer is densely packed, and QY can be high, generating homogeneously coated nanocrystals with a hydrodynamic size of 7-12 nm with exceptional stable in aqueous solution in ambient conditions for more than one year without noticeable size changes [184, 214].…”

“…Norbornene-tetrazine cycloaddition reactions [217, 218] and strain-promoted azide-cyclooctyne cycloaddition reactions [189, 273] have been found to be fully compatible with QDs. These reactions are often quantitative in yield and therefore do not require purification, in contrast to conventional amide-generating coupling for which extensive purification steps are required.…”

“…(c) Examples of polymeric ligands, distinguished by the types of binding groups and whether they are random copolymers (L5-L7) or block copolymers (L8) [184-188]. (d) Coating methodologies for multidentate polymeric ligands require an intermediate ligand exchange step to render the nanoparticle hydrophilic with monodentate ligands (pink circles) prior to coating with polymeric ligands to ensure homogeneous multidentate attachment and compact binding, as assessed by gel chromatograms at right, showing hydrodynamic sizes in the range of 7-12 nm for three different particle core sizes (reproduced from reference [189] with permission).…”

Quantum dots–DNA bioconjugates: synthesis to applications

“…Several designs of these ligands have been tuned to increase bond strength, stability and solution properties of QDs. Similarly the procedure for ligand exchange may additionally influence the polydispersity of the water-solubilized QDs [17,18].…”

Illuminating life's building blocks

“…Selvin's lab has developed its own version of quantum dots that are smaller, closer to 9 nm in diameter 7 . That size reduction helps him to slip quantum dots into the approximately 20-40 nm gap between nerve cells, the synapse, where signalling molecules pass on messages to nearby nerve cells.…”