Fluorophore

A fluorophore, in analogy to a chromophore, is a component of a molecule which causes a molecule to be fluorescent. It is a functional group in a molecule which will absorb energy of a specific wavelength and re-emit energy at a different (but equally specific) wavelength. The amount and wavelength of the emitted energy depend on both the fluorophore and the chemical environment of the fluorophore. This technology has particular importance in the field of biochemistry and protein studies, eg. in immunofluorescence and immunohistochemistry.

Fluorescein isothiocyanate, a reactive derivative of fluorescein, has been one of the most common fluorophores chemically attached to other, non-fluorescent molecules to create new and fluorescent molecules for a variety of applications. Other historically common fluorophores are derivatives of rhodamine, coumarin and cyanine.

A newer generation of fluorophores such as the Alexa Fluors and the DyLight Fluors are generally more photostable, brighter, and less pH-sensitive than other standard dyes of comparable excitation and emission.

Size
The size of the fluorophore might sterically hinder the tagged molecule:
 * quantum dot: 2-10 nm (diameter), 100-100,000 atoms
 * Green fluorescent protein (GFP) 26 kDa
 * luciferin: about 20 atoms