An Introduction to Gold Labeling
When a doctor or a scientist diagnoses a condition or an infection in the human body, they do so by identifying certain molecules, such as proteins or hormones, which are only there when that condition occurs. To find these molecules, they use other proteins, called "antibodies," which the body makes in order to defend itself against anything it does not recognize. The antibodies are attached to a reporter group - something which can easily be seen or detected. When the antibody binds to its target, the attached "label" goes with it, and by visualizing the label, we know where the target protein or hormone which we are trying to detect-called the "antigen" - is.
A number of different reporter groups are used, including radioactive nuclides which expose a film held over the sample (autoradiography), enzymes which convert a substrate into a highly colored product (ELISA) or which digest a substrate to give a photon of light to expose a film (chemiluminescence).
In the pregnancy test, and many other diagnostic tests, the label is a small gold particle, called a colloid, which is between 1 and 30 nm in diameter adsorbed or "stuck" electrostatically to the antibody. These can be seen individually by electron microscopy because they are much more electron-dense than cells or tissue; when they are used to detect antigens on membranes or in gels, they give a red color which can sometimes be seen, or they can be developed with a process called silver enhancement to give a dense black signal. This is among the most sensitive methods for detecting infectious organisms, and its use both as a research reagent in electron mcroscopy and as a diagnostic reagent is increasing steadily. One of our fellow tenants at the Long Island High Technology Incubator developed a simple test for the bacterium h. pylori, which causes peptic ulcers and possibly cancer, which uses colloidal gold-labeled proteins to detect the presence of human antibodies against the bacterium.
Colloidal gold particles are prepared by reducing a solution of a simple gold salt, such as potassium tetrachloraurate: small metal particles are formed which have a negative charge (the negative charge is believed to arise from dichlorogold (I) ions on their outer surface). Their size may be controlled by aditives, such as tannic acid. Proteins, which usually have a positive charge in solution near neutral pH, are attracted to the negatively charged surface and adhere to it. Immunogold probes are prepared by adsorbing antibodies to the gold particle surface; these are proteins which bind to a specific target, such as a tumor cell surface protein; they take the attached gold particle with them, and "label" the antigen.
Although colloidal gold is effective, even better results can be obtained with metal cluster complexes. Next, see how...