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Gold nanoparticles (AuNPs) have found use in broad range of applications such as in catalysis and nanomedicine. Despite the fact that thiol-based AuNPs have been widely studied, they suffer from relative instability in various conditions, such as high and low temperatures, pH variations, and are prone to oxidation. Over the last decade, N-heterocyclic carbenes (NHCs) have been under spotlight as suitable ligands to stabilize metal nanoparticles and surfaces. Although NHC-functionalized AuNPs have been shown to outperform their thiol-based analogs in terms of stability, their applications in nanomedicine have not been realized. Hybrid nanomaterials, such as AuNPs tagged with π-conjugated molecules with aggregation-induced emission (AIE) property, are promising candidates to develop fluorescent materials for cellular imaging. The combination of NHC-stabilized AuNPs with AIE to form stable, fluorescent hybrid AuNPs is of significant interest to open the door to develop new NHC-based nanomaterials. Herein, we report the synthesis and characterization of water-soluble fluorescent NHC-decorated AuNPs for potential applications in nanomedicine. Their stability in biologically relevant conditions is investigated. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]