Many fluorescent dyes or fluorochromes can specifically interact with certain target molecules, like most nucleic acid stains (DAPI, propidium iodide, ethidium bromide) but often they are covalently linked to other molecules in order to yield specific probes. In general, fluorescent probes are classified according to their excitation and emission characteristics, as well as their chemical and biological properties.
Fluorescent probes belonging to each of the important bio-molecular classes, including nucleic acids, polysaccharides, lipids and proteins have been generated as tools in cell biology, where they fulfil various tasks like detection of binding events, staining, localisation and tracing of target structures or measurement of ion concentrations to name just a few. Well known examples are fluorescently labelled antibodies or the biotin-streptavidin system.