The number of biological and medical applications of environmentally sensitive dyes -- fluorescent probes -- has been explosively increasing in the last few years. The application areas include mostly measurements of membrane viscosity, cytosol fluidity, membrane potential (distributive or electrochromic dyes) and mapping of intracellular ionic composition. Furthermore, fluorescent molecules or groups may specifically mark almost any type of binding site or any selected class of macromolecules. Nonfluorescent molecules such as enzyme substrates, lipids, nucleosides can be modified into fluorescent ones without substantially affecting their biological activity. The most promising field is the combination of fluorescent probes and fluorescence microscopy yielding time-resolved spectrally resolved two-dimensional or three-dimensional images.
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