Since their introduction in 1991 by Arduengo, N-heterocyclic carbenes (NHCs) have attracted much attention as versatile metal-coordinating groups. Given its inherent σ-donating nature, an NHC group is considered a soft Lewis base and thus exhibits strong coordination (via soft-to-soft) with transition metals, which present soft Lewis acidic character. This mode of interaction has been actively investigated for surface passivating nanocrystal surfaces.<br/>In this contribution, we probe the coordination interactions between colloidal gold nanoparticles (AuNPs) and three structurally distinct NHC-based ligands: two monodentate and one multidentate ligands. The latter combines multiple NHC groups and several poly (ethylene glycol) (PEG) solubilizing moieties. We combine NMR spectroscopy, high-resolution transmission electron microscopy and dynamic light scattering to characterize the nature of the binding interactions. We find that rapid competitive ligation onto the AuNPs (often requiring several minutes of mixing) takes place regardless of the exact structure of the ligand. Furthermore, the long term stability of the NHC-stabilized AuNPs have been tested after phase transfer to water, a highly challenging chemical venue for such groups. Data show that our NHC-polymers exhibit long-term colloidal stability in buffer media with no sign of degradation or aggregation build up for at least one year. We also compared dithiothreitol (DTT) competition tests applied side-by-side to dispersions of NHC-stabilized AuNPs and NPs protected by a dihydrolipoic acid-modified polymer; this allowed us to propose a rationale for how NHC binding onto the NP surfaces compares to those exhibited by thiol-ligands. Finally, the benefits of the multidentate NHC-polymer ligand design has also been demonstrated for the coating of other transition metal core colloids (e.g., magnetic iron oxide and silver NPs).

N-heterocyclic carbene-stabilized gold nanoparticles: Effects of the ligand coordination

ACS Fall 2021

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