ACTION OF COLD CHLORIDE (GOLD TONING) ON SILVER-ENHANCED 1 NM GOLD MARKERS

In conventional immunoelectron microscopy (IEM), very small colloidal gold particles (0.8-3 nm), or the gold compound Nanogold(1.4 nm) are s ilver-enhanced for easy detection. However, silver enhancement has dra wbacks. First, the silver layer is dissolved during fixation with osmi um tetroxide, even if the concentration and incubation time are strong ly reduced during pre-embedding labeling experiments in transmission e lectron microscopic (TEM) and scanning electron microscopic (SEM) stud ies. Second, aker exposure to the electron beam the silver layer may m igrate on the section or the whole particles may disappear. Sometimes silver migration can be observed even without irradiation. This effect strongly hampers reinvestigation of previously inspected areas, after some time of storage. In both cases, gold chloride treatment after si lver enhancement is sufficient to completely protect the silver-enhanc ed 1 nm gold markers. Gold chloride treatment is part of the so-called ''gold toning'' procedure, which is a method used to substitute and/o r cover the silver by a layer of gold. It can be applied in TEM and SE M experiments. As a serious drawback, gold chloride treatment slightly reduces the size of both unenhanced and silver-enhanced gold particle s and can lead to disintegrated silver/gold particles. Therefore, this technique is useful for pre-embedding IEM, on-(resin)section, and ult rathin cryosection labeling experiments. However it appears to be unsu itable for double-labeling studies using different gold sizes, for qua ntitation experiments, and in SEM. Microsc. Res. Tech. 42:59-65, 1998. (C) 1998 Wiley-Liss, Inc.