Colloidal gold reagents 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 colloidal gold-protein complexes are suspended in sterile-filtered buffer containing stabilizers and a preservative. All particle sizes in the EM Grade category may also 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 (LM Grade) because this size may penetrate tissues better than larger particles. The 4 nm size 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%), though small aggregates are not removed from this grade. Therefore, our 4 nm particles should only be used for electron microscopy with the understanding that the presence of small aggregates may give misleading results.
A detailed protocol for silver enhancement is provided with all orders for LM Grade products. All reagents involved in the light-insensitive silver enhancement reaction can be prepared easily in the laboratory. However, those who wish to use commercially available silver enhancement kits can continue to do so. All LM Grade colloidal gold-protein complexes are freeze-dried in buffer with stabilizers and a preservative. After reconstitution, they may be frozen in aliquots for extended storage.