Optically Detectable Colloidal Metal Labels: Properties, Methods, and Biomedical Applications

The optical detection of sub-wavelength sized metal particles has its scientific roots nearly a hundred years ago when Zigmondy first reported the observation of individual metal colloids under a microscope However, it was not until the early 1980s that gold colloid (5 nm - 20 nm in diameter) that had antibodies attached to the particle surface were used to specially target immunogenic cellular proteins. The precise location of these electron dense markers labels (immunogold) can be visualized using Transmission Electron Microscopy (TEM) (reviewed by Hayat). Alternatively, the labels can be viewed with a standard optical microscope by increasing their size with an Immuno-Gold Silver Staining (IGSS) procedure that deposits additional silver ions on the colloidal gold nucleation sites to form bulk silver. Another procedure commonly used for RNA in-situ studies is autoradiography, a process where silver ions nucleated as a result of radioactive decay from a 3H or 35S labeled nucleic acid probe form colloids in a silver emulsion film and are detected using optical microscopy techniques.