Colloidal Gold Markers and Probes for Routine Application in Microscopy

Colloidal gold can be used as an electron-dense cytochemical probe for direct or indirect labelling techniques. Gold markers can be prepared in a size range of 5-150 nm, and show size-dependent shape characteristics and absorption spectra. Size and shape distribution increases with mean particle diameter, with appreciable overlapping between populations of different mean size. Using radio isotope-binding assays, spectrophotometric analysis and an innovative rapid microtitration technique, the effect of pH, ionic strength and protein concentration on gold-protein interaction has been studied. Efficient adsorption of protein to gold occurs at, or near, the pI of the protein. The amount of protein needed to effect stabilization is both a function of pH and of ionic strength, but does not reflect the amount of protein binding for all proteins. There is evidence for multilamellar adsorption of proteins to gold, which is discussed in context of the bioactivity, and stability of the probe. A working protocol for the routine reproducible manufacture of protein-gold probes is given, making use of the microtitration assay.