N-Heterocyclic Carbene-Stabilized Gold Nanoparticles: Mono-Versus Multidentate Ligands

Over the past decade, N-heterocyclic carbenes (NHCs) have attracted remarkable attention as metal-coordinating ligands because of their ability to strongly interact with transition metal complexes and surfaces. We investigate the coordination interaction between colloidal gold nanoparticles (AuNPs) and three sets of hydrophilic NHC-based ligands: an amine-modified small molecule, a monomeric NHC appended with a poly(ethylene glycol) (PEG) block, and a modified poly(isobutylene-alt-maleic anhydride), PIMA, that simultaneously presents multiple NHC groups and several short PEG chains. In this report, we find that all three ligands can rapidly coordinate onto AuNPs, as characterized using a combination of NMR spectroscopy, high-resolution transmission electron microscopy, and dynamic light scattering. These measurements have been supplemented with colloidal stability tests as well as competition from dithiothreitol molecules. Overall, we find that multidentate NHC polymer coating exhibits the highest affinity to AuNPs, which manifests in long-term colloidal stability in buffer media, absence of any aggregation, and better resistance to competition from reducing molecules. We further exploit these data to infer additional insights into the interaction and coordination of NHC molecules with Au surfaces.