N-Heterocyclic Carbene-Based Polymer Coating of Gold Nanoparticles and Luminescent Quantum Dots

Since they were first synthesized by Arduengo and coworkers in 1991, N-heterocyclic carbenes (NHCs) have generated much interest for use as versatile metal-coordinating groups. NHC-presenting molecules exhibit strong coordination interactions with transition metal ions and surfaces by sharing their non-bonding electron pairs with the σ-accepting orbitals of the metal ions. NHC-appended molecules have recently been actively exploited as potentially effective ligands for the surface passivation of various colloidal nanomaterials.<br/>We characterize the coordination interactions between a few representative colloidal nanocrystals, including gold nanoparticles (AuNPs) and luminescent quantum dots (QDs), and one NHC-based polymer ligand. The latter presents multiple NHC groups and several short poly (ethylene glycol), PEG, chains as solubilizing blocks. We find that our NHC-decorated ligands rapidly coordinate onto both sets of nanocrystals, requiring ~ 5-10 min for complete ligand substitution and phase transfer. These Lewis base groups ideally match the soft Lewis acid character of transition metal colloids, promoting strong coordination bonding through soft-to-soft interactions. We combine NMR spectroscopy, fluorescence spectroscopy, high-resolution transmission electron microscopy supplemented with dynamic light scattering to characterize the nature of the binding interactions. Furthermore, the long-term stability of the NHC-stabilized nanocolloids have been tested after phase transfer to water, a highly challenging chemical venue for such groups, due to the moisture sensitive nature of NHC molecules. Data show that our NHC-stabilized nanocolloids exhibit long-term colloidal stability in buffer media while preserving their optical and fluorescing properties for at least one year of storage. We will discuss the ligand design and characterization of the polymer-stabilized nanocrystals under various conditions, with a particular focus on the beneficial effects of multi-coordination interactions.