ABSTRACT
The applications of colloidal gold have been described elsewhere [4, 5]; this chapter focuses on the use of covalently linked cluster complexes of gold and other metals. A gold cluster complex is a discrete molecular coordination compound comprising a central core, or “cluster” of electron-dense metal atoms, ligated by a shell of small organic molecules (ligands) that are linked to the metal atoms on the surface of the core. This structure gives clusters several important advantages as labels. The capping of the metal surface by ligands prevents nonspecific binding to cell and tissue components, which can occur with colloidal gold [6]. Cluster compounds are more stable and can be used under a wider range of conditions. Unlike colloidal gold, clusters do not require additional macromolecules such as bovine serum albumin or polyethylene glycol for stabilization, and the total size of the label is therefore significantly smaller. Because the clusters considered in this chapter are generally less than 3 nm in diameter, this allows the preparation of probes that are much smaller than conventional immunocolloids, and cluster labeling can take advantage of the higher resolution and penetration available with smaller conjugates. Most importantly,
while colloidal gold is adsorbed to its conjugate probe, clusters are conjugated by chemically specific covalent cross-linking. Therefore, the range of possible conjugate targeting agents includes any probe containing an appropriate reactive group. Clusters conjugates have been prepared with a wide variety of molecules that do not form colloidal gold conjugates, including lipids, oligonucleotides, peptides, and other small molecules [7].
