To be of use as immunocytochemical markers, the gold spheres in the sol have to be bound to specific ligands such as proteins (e.g., immunoglobulins) and lectins. The binding is by straightforward electrostatic absorption relying on the negative charge of the gold interacting with the positive charge of the protein and the complex obtained is highly stable. Moreover, the molecules bound to the gold retain their biological properties, an indispensable feature for immunocytochemistry. Unbound gold sol is unstable and changes color from red to blue in the presence of salts (e.g., sodium chloride). This can be used as a test for finding the amount of protein required to saturate (i.e., stabilize) a given quantity of gold sol. The protein-gold complex is isolated from excess protein and gold particle aggregates by density centrifugation. The complexes thus retrieved are stable for at least a year when refrigerated. Samples may be stored for even longer periods in 50% glycerol at -20°C.
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