Perylene diimide-tagged N-heterocyclic carbene-stabilized gold nanoparticles: How much ligand desorbs from surface in presence of thiols?

Abstract

N-Heterocyclic carbenes (NHCs) have recently emerged as viable alternatives to commonly used thiols to stabilize a variety of metal surfaces and nanoparticles. In this context, thanks to their biocompatibility and novel optical properties, NHC-stabilized gold nanoparticles (AuNPs) have been extensively studied. It has been shown that such materials exhibit improved stabilities in acidic and basic solutions, high temperatures, electrolyte solutions, cell culture media, and to some extent to nucleophilic thiols. Despite intense efforts, instability of NHC-functionalized AuNPs to thiols has been an ongoing challenge. In order to circumvent this problem, quantification of NHC desorption from nanoparticle surface by the invading thiols would constitute a necessary first step. To do this, we have first developed water-soluble azide decorated NHC-stabilized “clickable” AuNPs. Optically active perylene diimide (PDI)-tagged AuNP hybrids are then obtained by means of Cu-catalyzed alkyne–azide cycloaddition between these AuNPs and an alkyne-decorated PDI derivative. Investigation of photophysical properties of these AuNP/PDI hybrids revealed that the fluorescence of PDI molecules is effectively quenched by AuNPs in aqueous solution. The extent of NHC desorption from AuNP surface in presence of glutathione (4 mM), as a biologically relevant thiol, is then quantified by means of fluorescence emission restoration of PDI molecules upon detachment from AuNP surfaces. Our results demonstrate that while ∼20% of surface NHCs are displaced by glutathione within the first 24 h of their exposure to the thiol, ligand desorption reaches ∼45% after one week. We believe that these findings will provide more insight on true stability of NHC-stabilized materials.