Synthesis of a Large Water-Soluble Gold Cluster (Au32-36) and its Properties as a Potential Label in Electron Microscopy

Large water-soluble gold clusters are used for the labelling of biological material to visualize the posi­ tion of reactive sites by electron microscopy [1]. In most investigations of this type a commercial prod­ uct was used, for which little chemical information has been published (for review see [9]). Large gold clusters can be prepared using phosphines as lig­ ands [2, 3] or more easily with mercaptans in place of phosphines [4,5,11]. Mercaptans as ligands may interfere, however, if clusters are to be attached to the SH group of proteins, which is the most con­ venient way to label specific sites. For this reason, large gold clusters with phosphine ligands are still of interest. We found that a water-soluble cluster with a core of 32 to 36 gold atoms could be easily prepared using 3 as a ligand.