Colloidal gold complexes are formed by non-covalent attachment of antibodies to the surface of colloidal gold particles. They are available either for transmission and scanning electron microscopy (EM Grade) or for light microscopy and immunoblotting (LM Grade). The EM Grade is distinguished from other commercial preparations by careful separation of monomeric particles from small aggregates using ultracentrifugation in density gradients. The resulting monomeric complexes are suspended in sterile buffer containing preservatives and stabilizers. All particle sizes in the EM Grade category also may be used for light microscopy and immunoblotting by those who prefer to use particles larger than 4 nm.

Since signal intensity is relatively independent of particle size when silver enhancement is used, we offer 4 nm particles for our LM Grade because this size may penetrate tissues better than larger particles. The 4 nm size also may be used for electron microscopy in studies that require smaller particles since they are relatively uniform in size (coefficient of variation less than or equal to 15%). However, since small aggregates are not removed from this grade by ultracentrifugation in density gradients, they should only be used for electron microscopy with the understanding that the possible presence of small aggregates may lead to misleading results.

A detailed protocol for silver enhancement is provided with all orders for LM Grade products. For convenience, commercial silver enhancement kits also can be used. All LM Grade colloidal gold-antibody complexes are freeze dried in buffer with stabilizers and sodium azide. After reconstitution, they may be frozen in aliquots for extended storage.