Colloidal gold-antibody conjugates are easy to prepare and are an excellent choice for microscopic applications. Colloidal gold is an aqueous suspension of nanometer-sized particles of gold. Typically, chloroauric acid, HAuCl4, is reduced with dilute solutions of sodium citrate, as described here. This will cause the gold to form small aggregates that will associate with proteins. Gold particles of specific sizes can be isolated and differentiated microscopically, allowing these particles to be used for multiple-label experiments. Colloidal gold-labeled antibodies are widely used in electron microscopy (EM), and can be used for light microscopy but require additional steps (silver enhancement).
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