Water-soluble N-heterocyclic carbene stabilized gold nanoparticles by top-down synthesis: performance in catalysis and photoacoustic imaging

N-heterocyclic carbenes (NHCs) have emerged as effective capping ligands for gold nanoparticles (AuNPs), offering improved stability and flexibility compared to sulfur-based alternatives. Two synthetic approaches, ‘bottom-up’ (BU) and ‘top-down’ (TD), have been described; however, comparative studies remain scarce. Herein, we report the TD-synthesized water-soluble NHC-stabilized AuNPs (NHC@AuNPs) for catalysis in water and biomedical applications, exhibiting exceptional stability in physiologically relevant conditions. Further, the catalytic ability of TD-NPs in model reactions was compared to their BU counterparts featuring the same NHC ligands. In detail, the catalysts were tested for the reduction of 4-nitrophenol and resazurin, as well as for the biomimetic oxidation of 3,3,5,5-tetramethylbenzidine (TMB) in aqueous conditions. The TD-NPs also showed distinct advantages over the BU NPs as contrast agents in photoacoustic imaging (PAI), further highlighting the differences between the two families of colloids.