text: Fluorophores can be attached to proteins via specific functional groups, such as:
  * amino groups (e.g. via succinimide or Isothiocyanate);
  * carboxyl groups (e.g. via activation with carbodiimide and subsequent coupling with amine);
  * thiol (e.g. via maleimide or iodoacetamides);
  * azide (e.g. via click chemistry with terminal alkyne);
  or non-specificately (glutaraldehyde) or non-covalently (e.g. via hydrophobicity, etc.).
  These fluorophores are either small molecules, protein or quantum dots.
  Organic fluorophores fluoresce thanks to delocalized electrons which can jump a band and stabilize the energy absorbed, hence most fluorophores are conjugated systems. Several families exist and their excitations range from the infrared to the ultraviolet. <br>Lanthanides (chelated) are uniquely fluorescent metals, which emit thanks to transitions involving 4f orbits, which are forbidden, hence they have very low absorption coefficients and slow emissions, requiring excitation through fluorescent organic chelators (e.g. dipicolinate-based Terbium (III) chelators). <br>A third class of small molecule fluorophore is that of the transition metal-ligand complexes, which display molecular fluorescence from a metal-to-ligand charge transfer state which is partially forbidden, these are generally complexes of Ruthenium, Rhenium or Osmium.