pH responsive histidin-2-ylidene stabilized gold nanoparticles.

N-Heterocyclic carbene-stabilized metal nanoparticles have drawn much attention over the last decade due their strong carbon metal bond. Although several reports show increased stability of such N-heterocyclic carbene-stabilized metal nanoparticles, only limited examples of water-soluble N-heterocyclic carbene stabilized metal nanoparticles are known to date. However, water dispersibility and stability in biologically relevant solvents would be a prerequisite for any biological applications. Drawing from the natural amino acid chiral pool, L-histidine was utilized for preparing chiral NHC ligands in the synthesis of water soluble NHC-stabilized gold nanoparticles. For this purpose, N-acetyl-L-histidine ethyl ester was converted into its imidazolium salt either using methyl iodide or 2-iodopropane as alkylation agent. Subsequent reaction of the imidazolium salt with [Au(SMe)Cl] yielded the corresponding organometallic gold chloride complex. Histidine-2-ylidene stabilized gold nanoparticles were first generated in organic solvents; the histidine derived capping ligand bore ethyl ester moieties which were saponified, affording water soluble pH-responsive NHC-stabilized gold nanoparticles. These gold nanoparticles show remarkable stability in aqueous solutions, with gold nanoparticle solutions remaining stable after months of storage.