One of the most common uses of dyes in biology and medicine is as staining agents. The role of such agents is in the selective coloration of certain biological features, which may be used to aid their characterisa­tion. These techniques sometimes involve well-established dyes, but fre­quently make use of coloured derivatives synthesised for a specific pur­pose. The most important dyes in current use as staining agents are strongly fluorescent because of the ease and sensitivity of detection and their ability to provide high specificity. The methods generally involve the selective binding of the dye molecules to substances, tissue, cells or micro-organisms and subsequent detection by fluorimetric or micro­scopic techniques. The mechanism of the binding may involve either physical adsorption or covalent bonding but in many cases it is not fully established. Fluorescent dyes are also used extensively in the detection and identification of biological substances. Examples of such analytical applications, which most commonly have a diagnostic purpose, include the detection of specific antibodies, the identification of nucleic acids, proteins, lipids and polysaccharides in cells, the identification of specific nucleotide sequences in DNA, the study of cell membranes and the early detection of cancer cells. The dyes are thus useful as probes for the investigation of biological activity and the mechanism of biological reac­tions. Research using such probes, particularly in the study of biomem­brane properties, has already led to the development of new diagnostic procedures for certain diseases. In addition to selectivity of binding, the fluorescent dyes used must be non-toxic, exhibit a high quantum yield and a specific emission spectrum which is sufficiently distinct from back­ground fluorescence. Since fluorescence in the blue-violet region is often inherent in biological systems, dyes which fluoresce in the orange, red and green regions are most usefully employed. Among the most widely used fluorochromes in biomedical applications are fluorescein, rhodamines, acridines and their derivatives.

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