Probe size and bound label conformation in colloidal gold-ligand labels and gold-immunolabels.

Colloidal gold can be produced in sizes ranging from 1.0nm to 150nm. All sizes of gold can be conjugated, principally by hydrophobic bonding, to a variety of molecules including ligands, enzymes and antibodies, as well as lectins and polysaccharides. The activity of most of these biological molecules is retained on conjugation with gold particles irregardless of size range, although the ratio of protein surface area to gold particle surface area varies widely depending on particle and protein size. We have employed low voltage high resolution scanning electron microscopy to compare, microscopically, the shapes of biological molecules unbound, bound to very small (3nm) gold particles, and bound to larger (18nm-30nm) gold particles. When very small gold particles are conjugated to large protein molecules, several particles bind along the length of each molecule, while smaller protein molecules often wrap around a single small gold particle. With larger gold particles, several biological molecules bind to a single gold particle. In addition, the shape of protein molecules bound to larger gold particles differs from that of molecules bound to small gold particles.